docs/en/faq.wml
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 ## translation metadata
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 # Revision: $Revision$
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 # Translation-Priority: 2-medium
 
 #include "head.wmi" TITLE="Tor Project: FAQ" CHARSET="UTF-8"
 <div id="content" class="clearfix">
   <div id="breadcrumbs">
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     <a href="<page index>">Home &raquo; </a>
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     <a href="<page docs/documentation>">Documentation &raquo; </a>
     <a href="<page docs/faq>">FAQ</a>
   </div>
   <div id="maincol">
     <!-- PUT CONTENT AFTER THIS TAG -->
     <h1>Tor FAQ</h1>
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     <hr>
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     <a id="general"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#general">General
     questions:</a></h4>
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     <ul>
     <li><a href="#WhatIsTor">What is Tor?</a></li>
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     <li><a href="#Torisdifferent">How is Tor different from other proxies?
     </a></li>
     <li><a href="#CompatibleApplications">What programs can I use with Tor?
     </a></li>
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     <li><a href="#WhyCalledTor">Why is it called Tor?</a></li>
     <li><a href="#Backdoor">Is there a backdoor in Tor?</a></li>
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     <li><a href="#DistributingTor">Can I distribute Tor?</a></li>
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     <li><a href="#SupportMail">How can I get support?</a></li>
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     <li><a href="#Forum">Is there a Tor forum?</a></li>
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     <li><a href="#WhySlow">Why is Tor so slow?</a></li>
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     <li><a href="#FileSharing">How can I share files anonymously through Tor?
     </a></li>
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     <li><a href="#Funding">What would The Tor Project do with more funding?
     </a></li>
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     <li><a href="#IsItWorking">How can I tell if Tor is working, and that my
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     connections really are anonymized?</a></li>
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     <li><a href="#Mobile">Can I use Tor on my phone or mobile device?</a></li>
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     <li><a href="#OutboundPorts">Which outbound ports must be open when using
     Tor as a client?</a></li>
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     <li><a href="#FTP">How do I use my browser for ftp with Tor?</a></li>
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     <li><a href="#NoDataScrubbing">Does Tor remove personal information from
     the data my application sends?</a></li>
     <li><a href="#Metrics">How many people use Tor? How many relays or exit
     nodes are there?</a></li>
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     </ul>
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     <a id="comp-install"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#comp-install">
     Compilation and Installation:</a></h4>
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     <ul>
     <li><a href="#HowUninstallTor">How do I uninstall Tor?</a></li>
     <li><a href="#PGPSigs">What are these "sig" files on the download
     page?</a></li>
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     <li><a href="#GetTor">Your website is blocked in my country. How
     do I download Tor?</a></li>
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     <li><a href="#VirusFalsePositives">Why does my Tor executable appear to
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     have a virus or spyware?</a></li>
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     <li><a href="#tarballs">How do I open a .tar.gz or .tar.xz file?</a></li>
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     <li><a href="#LiveCD">Is there a LiveCD or other bundle that includes Tor?
     </a></li>
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     </ul>
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     <a id="tbb"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#tbb">Tor Browser
     (general):</a></h4>
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     <ul>
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     <li><a href="#TBBFlash">Why can't I view videos on YouTube and other
     Flash-based sites?</a></li>
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     <li><a href="#Ubuntu">I'm using Ubuntu, and I can't start Tor Browser.
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     </a></li>
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     <li><a href="#SophosOnMac">I'm using the Sophos anti-virus
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     software on my Mac, and Tor starts but I can't browse anywhere.</a></li>
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     <li><a href="#XPCOMError">When I start Tor Browser I get an  error message:
     "Cannot load XPCOM".</a></li>
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     <li><a href="#TBBOtherExtensions">Can I install other Firefox
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     extensions? Which extensions should I avoid using?</a></li>
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     <li><a href="#TBBJavaScriptEnabled">Why is NoScript configured to allow
     JavaScript by default in Tor Browser?  Isn't that unsafe?</a></li>
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     <li><a href="#TBBOtherBrowser">I want to use Chrome/IE/Opera/etc
     with Tor.</a></li>
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     <li><a href="#GoogleCAPTCHA">Google makes me solve a CAPTCHA or tells
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     me I have spyware installed.</a></li>
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     <li><a href="#ForeignLanguages">Why does Google show up in foreign
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     languages?</a></li>
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     <li><a href="#GmailWarning">Gmail warns me that my account may have
     been compromised.</a></li>
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     <li><a href="#NeedToUseAProxy">My internet connection requires an HTTP
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     or SOCKS Proxy</a></li>
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     <li><a href="#TBBSocksPort">I want to
     run another application through Tor.</a></li>
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     <li><a href="#CantSetProxy">What should I do if I can't set a proxy
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     with my application?</a></li>
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     </ul>
 
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     <a id="tbb-3plus"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#tbb-3plus">Tor
     Browser (3.x and later):</a></h4>
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     <ul>
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     <li><a href="#DisableJS">How do I disable JavaScript?</a></li>
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     <li><a href="#VerifyDownload">How do I verify the download
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     (sha256sums.txt)?</a></li>
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     <li><a href="#NewIdentityClosingTabs">Why does "New Identity" close
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     all my open tabs?</a></li>
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     <li><a href="#ConfigureRelayOrBridge">How do I configure Tor as a relay
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     or bridge?</a></li>
     <li><a href="#Timestamps">Why are the file timestamps from 2000?</a></li>
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     <li><a href="#TBBSourceCode">Where is the source code for Tor Browser?
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     How do I verify a build?</a></li>
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     </ul>
 
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     <a id="advanced"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#advanced">Advanced
     Tor usage:</a></h4>
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     <ul>
     <li><a href="#torrc">I'm supposed to "edit my torrc". What does
     that mean?</a></li>
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     <li><a href="#datadir">Where's tor's data directory?</a></li>
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     <li><a href="#Logs">How do I set up logging, or see Tor's
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     logs?</a></li>
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     <li><a href="#LogLevel">What log level should I use?</a></li>
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     <li><a href="#DoesntWork">Tor is running, but it's not working
     correctly.</a></li>
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     <li><a href="#TorCrash">My Tor keeps crashing.</a></li>
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     <li><a href="#ChooseEntryExit">Can I control which nodes (or country)
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     are used for entry/exit?</a></li>
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     <li><a href="#FirewallPorts">My firewall only allows a few outgoing
     ports.</a></li>
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     <li><a href="#DefaultExitPorts">Is there a list of default exit ports?
     </a></li>
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     <li><a href="#WarningsAboutSOCKSandDNSInformationLeaks">I keep seeing
     these warnings about SOCKS and DNS information leaks. Should I
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     worry?</a></li>
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     <li><a href="#SocksAndDNS">How do I check if my application that uses
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     SOCKS is leaking DNS requests?</a></li>
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     <li><a href="#TorClientOnADifferentComputerThanMyApplications">I want to
     run my Tor client on a
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     different computer than my applications.</a></li>
     <li><a href="#ServerClient">Can I install Tor on a central server, and
     have my clients connect to it?</a></li>
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     <li><a href="#IPv6">How do I use Tor from an IPv6 only host/computer?</a></li>
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     </ul>
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     <a id="relay"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#relay">Running a
     Tor relay:</a></h4>
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     <ul>
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     <li><a href="#HowDoIDecide">How do I decide if I should run a relay?
     </a></li>
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     <li><a href="#MostNeededRelayType">What type of relays are most needed?
     </a></li>
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     <li><a href="#WhyIsntMyRelayBeingUsedMore">Why isn't my relay being
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     used more?</a></li>
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     <li><a href="#IDontHaveAStaticIP">Can I run a Tor relay using a dynamic IP
     address?</a></li>
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     <li><a href="#IPv6Relay">Can I use IPv6 on my relay?</a></li>
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     <li><a href="#PortscannedMore">Why do I get portscanned more often
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     when I run a Tor relay?</a></li>
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     <li><a href="#HighCapacityConnection">How can I get Tor to fully
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     make use of my high capacity connection?</a></li>
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     <li><a href="#RelayFlexible">How stable does my relay need to be?</a></li>
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     <li><a href="#BandwidthShaping">What bandwidth shaping options are
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     available to Tor relays?</a></li>
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     <li><a href="#LimitTotalBandwidth">How can I limit the total amount
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     of bandwidth used by my Tor relay?</a></li>
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     <li><a href="#RelayWritesMoreThanItReads">Why does my relay write
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     more bytes onto the network than it reads?</a></li>
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     <li><a href="#Hibernation">Why can I not browse anymore after
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     limiting bandwidth on my Tor relay?</a></li>
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     <li><a href="#ExitPolicies">I'd run a relay, but I don't want to deal
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     with abuse issues.</a></li>
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     <li><a href="#PackagedTor">Should I install Tor from my package manager,
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     or build from source?</a></li>
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     <li><a href="#WhatIsTheBadExitFlag">What is the BadExit flag?</a></li>
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     <li><a href="#IGotTheBadExitFlagWhyDidThatHappen">I got the BadExit flag.
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     Why did that happen?</a></li>
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     <li><a href="#MyRelayRecentlyGotTheGuardFlagAndTrafficDroppedByHalf">My
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     relay recently got the Guard flag and traffic dropped by half.</a></li>
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     <li><a href="#RelayOrBridge">Should I be a normal relay or bridge
     relay?</a></li>
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     <li><a href="#UpgradeOrMove">I want to upgrade/move my relay. How do I
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     keep the same key?</a></li>
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     <li><a href="#OfflineED25519">How do offline ed25519 identity keys work?
     What do I need to know?</a></li>
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     <li><a href="#MultipleRelays">I want to run more than one relay.</a></li>
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     <li><a href="#NTService">How do I run my Tor relay as an NT service?
     </a></li>
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     <li><a href="#VirtualServer">Can I run a Tor relay from my virtual server
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     account?</a></li>
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     <li><a href="#WrongIP">My relay is picking the wrong IP address.</a></li>
     <li><a href="#BehindANAT">I'm behind a NAT/Firewall</a></li>
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     <li><a href="#OutgoingFirewall">How should I configure the outgoing filters
     on my relay?</a></li>
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     <li><a href="#RelayMemory">Why is my Tor relay using so much memory?
     </a></li>
     <li><a href="#BetterAnonymity">Do I get better anonymity if I run a relay?
     </a></li>
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     <li><a href="#FacingLegalTrouble">I'm facing legal trouble. How do I
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     prove that my server was a Tor relay at a given time?</a></li>
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     <li><a href="#RelayDonations">Can I donate for a relay rather than
     run my own?</a></li>
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     </ul>
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     <a id="onion-services"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#onion-services">
     Tor onion services:</a></h4>
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     <ul>
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     <li><a href="#AccessOnionServices">How do I access onion services?</a></li>
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     <li><a href="#ProvideAnOnionService">How do I provide an onion service?
     </a></li>
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     </ul>
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     <a id="dev"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#dev">Development:
     </a></h4>
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     <ul>
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     <li><a href="#VersionNumbers">What do these weird version numbers
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     mean?</a></li>
     <li><a href="#PrivateTorNetwork">How do I set up my own private
     Tor network?</a></li>
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     <li><a href="#UseTorWithJava">How can I make my Java program use the
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     Tor network?</a></li>
     <li><a href="#WhatIsLibevent">What is Libevent?</a></li>
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     <li><a href="#MyNewFeature">What do I need to do to get a new feature
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     into Tor?</a></li>
     </ul>
 
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     <a id="anonsec"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#anonsec">Anonymity
     and Security:</a></h4>
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     <ul>
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     <li><a href="#WhatProtectionsDoesTorProvide">What protections does Tor
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     provide?</a></li>
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     <li><a href="#CanExitNodesEavesdrop">Can exit nodes eavesdrop on
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     communications? Isn't that bad? </a></li>
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     <li><a href="#AmITotallyAnonymous">So I'm totally anonymous if I use
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     Tor?</a></li>
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     <li><a href="#KeyManagement">Tell me about all the keys Tor uses.</a></li>
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     <li><a href="#EntryGuards">What are Entry Guards?</a></li>
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     <li><a href="#ChangePaths">How often does Tor change its paths?</a></li>
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     <li><a href="#CellSize">Tor uses hundreds of bytes for every IRC line. I
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     can't afford that!</a></li>
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     <li><a href="#OutboundConnections">Why does netstat show these outbound
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     connections?</a></li>
     <li><a href="#PowerfulBlockers">What about powerful blocking mechanisms
     </a></li>
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     <li><a href="#RemotePhysicalDeviceFingerprinting">Does Tor resist
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     "remote physical device fingerprinting"?</a></li>
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     <li><a href="#IsTorLikeAVPN">Is Tor like a VPN?</a></li>
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     <li><a href="#Proxychains">Aren't 10 proxies (proxychains) better than
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     Tor with only 3 hops?</a></li>
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     <li><a href="#AttacksOnOnionRouting">What attacks remain against onion
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     routing?</a></li>
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     <li><a href="#LearnMoreAboutAnonymity">Where can I learn more about
     anonymity?</a></li>
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     </ul>
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     <a id="altdesigns"></a>
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     <h4 style="margin-bottom: 18px"><a class="anchor" href="#altdesigns">
     Alternate designs that we don't do (yet):</a></h4>
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     <ul>
     <li><a href="#EverybodyARelay">You should make every Tor user be a
     relay.</a></li>
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     <li><a href="#TransportIPnotTCP">You should transport all IP packets,
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     not just TCP packets.</a></li>
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     <li><a href="#HideExits">You should hide the list of Tor relays,
     so people can't block the exits.</a></li>
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     <li><a href="#ChoosePathLength">You should let people choose their path
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     length.</a></li>
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     <li><a href="#ChoosePathCountries">You should change path selection to avoid
     entering and exiting from the same country.</a></li>
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     <li><a href="#SplitEachConnection">You should split each connection over
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     many paths.</a></li>
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     <li><a href="#MigrateApplicationStreamsAcrossCircuits">You should migrate
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     application streams across circuits.</a></li>
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     <li><a href="#LetTheNetworkPickThePath">You should let the network pick
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     the path, not the client.</a></li>
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     <li><a href="#UnallocatedNetBlocks">Your default exit policy should block
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     unallocated net blocks too.</a></li>
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     <li><a href="#BlockWebsites">Exit policies should be able to block
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     websites, not just IP addresses.</a></li>
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     <li><a href="#BlockContent">You should change Tor to prevent users from
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     posting certain content.</a></li>
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     <li><a href="#SendPadding">You should send padding so it's more secure.
     </a></li>
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     <li><a href="#Steganography">You should use steganography to hide Tor
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     traffic.</a></li>
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     </ul>
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     <p>For other questions not yet on this version of the FAQ, see the
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     <a href="<wikifaq>">wiki FAQ</a> for now.
     </p>
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     <hr>
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     <a id="General"></a>
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     <h2><a class="anchor" href="#General">General:</a></h2>
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     <a id="WhatIsTor"></a>
     <h3><a class="anchor" href="#WhatIsTor">What is Tor?</a></h3>
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     <p>
     The name "Tor" can refer to several different components.
     </p>
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     <p>
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     Tor is a program you can run on your computer that helps keep
     you safe on the Internet. It protects you by bouncing your communications
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     around a distributed network of relays run by volunteers all around
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     the world: it prevents somebody watching your Internet connection from
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     learning what sites you visit, and it prevents the sites you visit
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     from learning your physical location.
     This set of volunteer relays is called the <b>Tor network</b>.
     The way most people use Tor is with <b>Tor Browser</b>,
     which is a version of Firefox that fixes many privacy issues.
     You can read more about how Tor works on the <a href="<page
     about/overview>">overview page</a>.
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     </p>
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     <p>
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     The <b>Tor Project</b> is a non-profit (charity) organization that
     maintains and develops the Tor software.
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     </p>
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     <hr>
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     <a id="Torisdifferent"></a>
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     <h3><a class="anchor" href="#Torisdifferent">How is Tor different from other
     proxies?</a></h3>
     <p>
     A typical proxy provider sets up a server somewhere on the Internet and
     allows you to use it to relay your traffic. This creates a simple, easy to
     maintain architecture. The users all enter and leave through the same server.
     The provider may charge for use of the proxy, or fund their costs through
     advertisements on the server.  In the simplest configuration, you don't have
     to install anything.  You just have to point your browser at their proxy
     server.
     Simple proxy providers are fine solutions if you do not want protections for
     your privacy and anonymity online and you trust the provider to not do bad
     things.  Some simple proxy providers use SSL to secure your connection
     to them, which protects you against local eavesdroppers, such as those at a
     cafe with free wifi Internet.
     </p>
     <p>
     Simple proxy providers also create a single point of failure. The provider
     knows both who you are and what you browse on the Internet. They can see
     your traffic as it passes through their server.  In some cases, they can
     even see inside your encrypted traffic as they relay it to your banking
     site or to ecommerce stores.
     You have to trust the provider isn't watching your traffic, injecting their
     own advertisements into your traffic stream, or recording your personal
     details.
     </p>
     <p>
     Tor passes your traffic through at least 3 different servers before sending
     it on to the destination. Because there's a separate layer of encryption for
     each of the three relays, somebody watching your Internet connection can't
     modify, or read, what you are sending into the Tor network. Your traffic is
     encrypted between the Tor client (on your computer) and where it pops out
     somewhere else in the world.
     </p>
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     <p>
     <dl>
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     <dt>Doesn't the first server see who I am?</dt>
     <dd>Possibly. A bad first of three servers can see encrypted Tor traffic
     coming from your computer. It still doesn't know who you are and what you
     are doing over Tor.  It merely sees "This IP address is using Tor".  Tor is
     not illegal anywhere in the world, so using Tor by itself is fine.  You are
     still protected from this node figuring out both who you are and where you
     are going on the Internet.
     </dd>
     <dt>Can't the third server see my traffic?</dt>
     <dd>Possibly.  A bad third of three servers can see the traffic you sent
     into Tor.  It won't know who sent this traffic.  If you're using encryption
     (like HTTPS), it will only know the destination. See
     <a href="https://www.eff.org/pages/tor-and-https">this visualization of Tor
     and HTTPS</a> to understand how Tor and HTTPS interact.
     </dd>
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     </dl>
     </p>
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     <hr>
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     <a id="CompatibleApplications"></a>
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     <h3><a class="anchor" href="#CompatibleApplications">What programs can I
     use with Tor?</a></h3>
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     <p>
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     Most people use Tor Browser,
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     which includes everything you need to browse the web safely using
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     Tor. Using other browsers is <a href="#TBBOtherBrowser">dangerous
     and not recommended</a>.
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     </p>
     <p>
     There are plenty of other programs you can use with Tor,
     but we haven't researched the application-level anonymity
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     issues on all of them well enough to be able to recommend a safe
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     configuration. Our wiki has a community-maintained list of
     instructions for <a
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     href="<wiki>doc/TorifyHOWTO">Torifying
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     specific applications</a>.
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     Please add to these lists and help us keep them accurate!
     </p>
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     <hr>
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     <a id="WhyCalledTor"></a>
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     <h3><a class="anchor" href="#WhyCalledTor">Why is it called Tor?</a></h3>
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     <p>
     Because Tor is the onion routing network. When we were starting the
     new next-generation design and implementation of onion routing in
     2001-2002, we would tell people we were working on onion routing,
     and they would say "Neat. Which one?" Even if onion routing has
     become a standard household term, Tor was born out of the actual <a
     href="http://www.onion-router.net/">onion routing project</a> run by
     the Naval Research Lab.
     </p>
0b68c29c
 
2a9aaa80
     <p>
     (It's also got a fine translation from German and Turkish.)
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     Note: even though it originally came from an acronym, Tor is not spelled
2a9aaa80
     "TOR". Only the first letter is capitalized. In fact, we can usually
5e8d08ab
     spot people who haven't read any of our website (and have instead learned
     everything they know about Tor from news articles) by the fact that they
2a9aaa80
     spell it wrong.
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
2a9aaa80
     <a id="Backdoor"></a>
5e8d08ab
     <h3><a class="anchor" href="#Backdoor">Is there a backdoor in Tor?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
1c8ac595
     There is absolutely no backdoor in Tor.  We know some smart lawyers
     who say that it's unlikely that anybody will try to make us add one
     in our jurisdiction (U.S.). If they do ask us, we will fight them,
     and (the lawyers say) probably win.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     We will
     <a href="https://media.ccc.de/v/31c3_-_6251_-_en_-_saal_1_-_201412301400_-_state_of_the_onion_-_jacob_-_arma">
     never</a> put a backdoor in Tor.
     We think that putting a backdoor in Tor would be tremendously irresponsible
     to our users, and a bad precedent for security software in general. If we
     ever put a deliberate backdoor in our security software, it would ruin our
     professional reputations.
     Nobody would trust our software ever again &mdash; for excellent reason!
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     But that said, there are still plenty of subtle attacks people might try.
     Somebody might impersonate us, or break into our computers, or something
     like that. Tor is open source, and you should always check the source (or
     at least the diffs since the last release) for suspicious things. If we (or
     the distributors) don't give you source, that's a sure sign something funny
     might be going on. You should also check the
     <a href="<page docs/verifying-signatures>">PGP signatures</a> on the
     releases, to make sure nobody messed with the distribution sites.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     Also, there might be accidental bugs in Tor that could affect your
37855015
     anonymity. We periodically find and fix anonymity-related bugs, so
1c8ac595
     make sure you keep your Tor versions up-to-date.
2a9aaa80
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
2a9aaa80
     <a id="DistributingTor"></a>
5e8d08ab
     <h3><a class="anchor" href="#DistributingTor">Can I distribute Tor?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
     Yes.
     </p>
0b68c29c
 
2a9aaa80
     <p>
f86c4fee
     The Tor software is <a href="https://www.fsf.org/">free software</a>. This
     means we give you the rights to redistribute the Tor software, either
     modified or unmodified, either for a fee or gratis. You don't have to
2a9aaa80
     ask us for specific permission.
     </p>
0b68c29c
 
2a9aaa80
     <p>
f86c4fee
     However, if you want to redistribute the Tor software you must follow our
2a9aaa80
     <a href="<gitblob>LICENSE">LICENSE</a>.
f86c4fee
     Essentially this means that you need to include our LICENSE file along
2a9aaa80
     with whatever part of the Tor software you're distributing.
     </p>
0b68c29c
 
2a9aaa80
     <p>
f86c4fee
     Most people who ask us this question don't want to distribute just the
db1b8809
     Tor software, though. They want to distribute the <a
11ab6cce
     href="<page projects/torbrowser>">Tor Browser</a>. This includes <a
f86c4fee
     href="https://www.mozilla.org/en-US/firefox/organizations/">Firefox
2b63e88f
     Extended Support Release</a>, and the NoScript and HTTPS-Everywhere
     extensions. You will need to follow the license for those programs as
     well. Both of those Firefox extensions are distributed under
f86c4fee
     the <a href="https://www.fsf.org/licensing/licenses/gpl.html">GNU General
2b63e88f
     Public License</a>, while Firefox ESR is released under the Mozilla Public
     License. The simplest way to obey their licenses is to include the source
     code for these programs everywhere you include the bundles themselves.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
f86c4fee
     Also, you should make sure not to confuse your readers about what Tor is,
     who makes it, and what properties it provides (and doesn't provide). See
     our <a href="<page docs/trademark-faq>">trademark FAQ</a> for details.
2a9aaa80
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
2a9aaa80
     <a id="SupportMail"></a>
5e8d08ab
     <h3><a class="anchor" href="#SupportMail">How can I get support?</a></h3>
7841030e
 
1660b67d
     <p>See the <a href="<page about/contact>#support">Support section
     on the contact page</a>.
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
c5a65f0d
     <a id="Forum"></a>
     <h3><a class="anchor" href="#Forum">Is there a Tor forum?</a></h3>
 
13b97478
     <p>We have a <a href="https://tor.stackexchange.com/">StackExchange
ff62bd3e
     page</a> that is currently in public beta.
c5a65f0d
     </p>
 
     <hr>
 
2a9aaa80
     <a id="WhySlow"></a>
     <h3><a class="anchor" href="#WhySlow">Why is Tor so slow?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
     There are many reasons why the Tor network is currently slow.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     Before we answer, though, you should realize that Tor is never going to
     be blazing fast. Your traffic is bouncing through volunteers' computers
     in various parts of the world, and some bottlenecks and network latency
2a9aaa80
     will always be present. You shouldn't expect to see university-style
     bandwidth through Tor.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     But that doesn't mean that it can't be improved. The current Tor network
     is quite small compared to the number of people trying to use it, and
     many of these users don't understand or care that Tor can't currently
2a9aaa80
     handle file-sharing traffic load.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     For the much more in-depth answer, see
     <a href="<blog>why-tor-is-slow">Roger's blog post on the topic</a>,
     which includes both a detailed PDF and a video to go with it.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     What can you do to help?
     </p>
0b68c29c
 
2a9aaa80
     <ul>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     <a href="<page docs/tor-doc-relay>">Configure your Tor to relay traffic
     for others</a>. Help make the Tor network large enough that we can handle
2a9aaa80
     all the users who want privacy and security on the Internet.
     </li>
0b68c29c
 
2a9aaa80
     <li>
c261c7ec
     Help us make Tor more usable. We
2a9aaa80
     especially need people to help make it easier to configure your Tor
     as a relay. Also, we need help with clear simple documentation to
     walk people through setting it up.
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     There are some bottlenecks in the current Tor network. Help us design
     experiments to track down and demonstrate where the problems are, and
2a9aaa80
     then we can focus better on fixing them.
     </li>
0b68c29c
 
2a9aaa80
     <li>
     Tor needs some architectural changes too. One important change is to
5e8d08ab
     start providing <a href="#EverybodyARelay">better service to people who
     relay traffic</a>. We're working on this, and we'll finish faster if we
2a9aaa80
     get to spend more time on it.
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Help do other things so we can do the hard stuff. Please take a moment
     to figure out what your skills and interests are, and then
     <a href="<page getinvolved/volunteer>">look at our volunteer page</a>.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Help find sponsors for Tor. Do you work at a company or government agency
2a9aaa80
     that uses Tor or has a use for Internet privacy, e.g. to browse the
5e8d08ab
     competition's websites discreetly, or to connect back to the home servers
     when on the road without revealing affiliations? If your organization has
     an interest in keeping the Tor network working, please contact them about
     supporting Tor. Without sponsors, Tor is going to become even slower.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
     If you can't help out with any of the above, you can still help out
5e8d08ab
     individually by <a href="<page donate/donate>">donating a bit of money to
     the cause</a>. It adds up!
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     </ul>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
91d870fd
     <a id="FileSharing"></a>
13b97478
     <h3><a class="anchor" href="#FileSharing">How can I share files
f30e672d
     anonymously through Tor?</a></h3>
91d870fd
 
     <p>
13b97478
     File sharing (peer-to-peer/P2P) is widely unwanted in the Tor network,
     and exit nodes are configured to block file sharing traffic by default.
     Tor is not really designed for it, and file sharing through Tor slows
5e8d08ab
     down everyone's browsing. Also, Bittorrent over Tor
     <a href="https://blog.torproject.org/blog/bittorrent-over-tor-isnt-good-idea">
f30e672d
     is not anonymous</a>!
91d870fd
     </p>
 
     <hr>
 
2a9aaa80
     <a id="Funding"></a>
5e8d08ab
     <h3><a class="anchor" href="#Funding">What would The Tor Project do with
     more funding?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     The Tor network's
     <a href="https://metrics.torproject.org/networksize.html">several thousand
     </a> relays push <a href="https://metrics.torproject.org/bandwidth.html">
     around 100 Gbps on average</a>. We have
     <a href="https://metrics.torproject.org/userstats-relay-country.html">
     millions of daily users</a>. But the Tor network is not yet self-sustaining.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     There are six main development/maintenance pushes that need attention:
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <ul>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Scalability: We need to keep scaling and decentralizing the Tor architecture
     so it can handle thousands of relays and millions of users. The upcoming
     stable release is a major improvement, but there's lots more to be done next
     in terms of keeping Tor fast and stable.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     User support: With this many users, a lot of people are asking questions
     all the time, offering to help out with things, and so on. We need good
     clean docs, and we need to spend some effort coordinating volunteers.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Relay support: the Tor network is run by volunteers, but they still need
2a9aaa80
     attention with prompt bug fixes, explanations when things go wrong,
5e8d08ab
     reminders to upgrade, and so on. The network itself is a commons, and
     somebody needs to spend some energy making sure the relay operators stay
     happy. We also need to work on stability on some platforms &mdash; e.g.,
db82b71b
     Tor relays have problems on Win XP currently.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Usability: Beyond documentation, we also need to work on usability of the
     software itself. This includes installers, clean GUIs, easy configuration
     to interface with other applications, and generally automating all of
c261c7ec
     the difficult and confusing steps inside Tor.
     Usability for privacy software has never been easy.
2a9aaa80
     </li>
0b68c29c
 
2a9aaa80
     <li>
     Incentives: We need to work on ways to encourage people to configure
     their Tors as relays and exit nodes rather than just clients.
5e8d08ab
     <a href="#EverybodyARelay">We need to make it easy to become a relay,
2a9aaa80
     and we need to give people incentives to do it.</a>
     </li>
0b68c29c
 
2a9aaa80
     <li>
5e8d08ab
     Research: The anonymous communications field is full of surprises and
     gotchas. In our copious free time, we also help run top anonymity and
     privacy conferences like <a href="http://petsymposium.org/">PETS</a>.
     We've identified a set of critical
     <a href="<page getinvolved/volunteer>#Research">Tor research questions</a>
     that will help us figure out how to make Tor secure against the variety of
     attacks out there. Of course, there are more research questions waiting
2a9aaa80
     behind these.
     </li>
0b68c29c
 
2a9aaa80
     </ul>
0b68c29c
 
2a9aaa80
     <p>
     We're continuing to move forward on all of these, but at this rate
5e8d08ab
     <a href="#WhySlow">the Tor network is growing faster than the developers
2a9aaa80
     can keep up</a>.
5e8d08ab
     Now would be an excellent time to add a few more developers to the effort
2a9aaa80
     so we can continue to grow the network.
     </p>
0b68c29c
 
2a9aaa80
     <p>
     We are also excited about tackling related problems, such as
     censorship-resistance.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     We are proud to have
     <a href="<page about/sponsors>">sponsorship and support</a> from the Omidyar
     Network, the International Broadcasting Bureau, Bell Security Solutions,
     the Electronic Frontier Foundation, several government agencies and research
     groups, and hundreds of private contributors.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     However, this support is not enough to keep Tor abreast of changes in the
     Internet privacy landscape. Please <a href="<page donate/donate>">donate</a>
     to the project, or <a href="<page about/contact>">contact</a> our executive
2a9aaa80
     director for information on making grants or major donations.
     </p>
0b68c29c
 
73bd2ad3
     <hr>
 
 
     <a id="Mobile"></a>
2b63e88f
     <h3><a class="anchor" href="#Mobile">Can I use Tor on my phone or mobile
73bd2ad3
     device?</a></h3>
 
     <p>
2b63e88f
     Tor on Android devices is maintained by the <a
     href="https://guardianproject.info">Guardian Project</a>. Currently, there
     is no supported way of using Tor on iOS; the Guardian Project is
73bd2ad3
     working to make this a reality in the future.
     </p>
 
611c066e
     <hr>
 
b75b913d
     <a id="OutboundPorts"></a>
5e8d08ab
     <h3><a class="anchor" href="#OutboundPorts">Which outbound ports must be
     open when using Tor as a client?</a></h3>
91d870fd
     <p>
13b97478
     Tor may attempt to connect to any port that is advertised in the
     directory as an ORPort (for making Tor connections) or a DirPort (for
f1aa31be
     fetching updates to the directory). There are a variety of these ports:
     many of them are running on 80, 443, 9001, and 9030, but many use other
     ports too.
91d870fd
     </p>
     <p>
5e8d08ab
     When using Tor as a client, you could probably get away with opening only
     those four ports. Since Tor does all its connections in the background, it
     will retry ones that fail, and hopefully you'll never have to know that it
     failed, as long as it finds a working one often enough. However, to get the
     most diversity in your entry nodes &mdash; and thus the most security
f1aa31be
     &mdash; as well as the most robustness in your connectivity, you'll
     want to let it connect to all of them.
     See the FAQ entry on <a href="#FirewallPorts">firewalled ports</a> if
     you want to explicitly tell your Tor client which ports are reachable
     for you.
     </p>
2b63e88f
 
91d870fd
     <hr>
2b63e88f
 
91d870fd
     <a id="IsItWorking"></a>
13b97478
     <h3><a class="anchor" href="#IsItWorking">How can I tell if Tor is
f30e672d
     working, and that my connections really are anonymized?</a></h3>
91d870fd
 
     <p>
13b97478
     There are sites you can visit that will tell you if you appear to be
5e8d08ab
     coming through the Tor network. Try the
     <a href="https://check.torproject.org">Tor Check</a> site and see whether
     it thinks you are using Tor or not.
91d870fd
     </p>
2b63e88f
 
91d870fd
     <hr>
2b63e88f
 
91d870fd
     <a id="FTP"></a>
     <h3><a class="anchor" href="#FTP">How do I use my browser for ftp with Tor?
     </a></h3>
 
f30e672d
     <p>
32700e09
     Use <a href="https://torproject.org/projects/torbrowser.html">Tor
     Browser</a>. If you want a separate application for an
13b97478
     ftp client, we've heard good things about  FileZilla for Windows. You can
     configure it to point to Tor as a "socks4a" proxy on "localhost" port
     "9050".
f30e672d
     </p>
34bb6a9b
 
91d870fd
     <hr>
2b63e88f
 
34bb6a9b
     <a id="NoDataScrubbing"></a>
13b97478
     <h3><a class="anchor" href="#NoDataScrubbing">Does Tor remove personal
34bb6a9b
     information from the data my application sends?</a></h3>
 
13b97478
     <p>No, it doesn't. You need to use a separate program that understands
     your application and protocol and knows how to clean or "scrub" the data
5bd3add6
     it sends. Tor Browser tries to keep application-level data,
     like the user-agent string, uniform for all users. Tor Browser can't
13b97478
     do anything about text that you type into forms, though. <a
cf814f9e
     href="<page download/download-easy>#warning">Be
34bb6a9b
     careful and be smart.</a>
     </p>
 
     <hr>
 
7841030e
     <a id="Metrics"></a>
37855015
     <h3><a class="anchor" href="#Metrics">How many people use Tor? How
f30e672d
     many relays or exit nodes are there?</a></h3>
7841030e
 
91d870fd
     <p>
     All this and more about measuring Tor can be found at the <a
dc7ed63e
     href="https://metrics.torproject.org/">Tor Metrics Portal</a>.</p>
ed5ac546
     <hr>
0b68c29c
 
6f107e8d
     <a id="CompilationAndInstallation"></a>
5e8d08ab
     <h2><a class="anchor" href="#CompilationAndInstallation">Compilation And
     Installation:</a></h2>
6f107e8d
 
2a9aaa80
     <a id="HowUninstallTor"></a>
5e8d08ab
     <h3><a class="anchor" href="#HowUninstallTor">How do I uninstall Tor?
     </a></h3>
37855015
 
     <p>
5e8d08ab
     Tor Browser does not install itself in the classic sense of applications.
     You just simply delete the folder or directory named "Tor Browser" and it
     is removed from your system.
37855015
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     If this is not related to Tor Browser, uninstallation depends entirely on
     how you installed it and which operating system you have. If you installed
     a package, then hopefully your package has a way to uninstall itself.
     The Windows packages include uninstallers.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     For Mac OS X, follow the
     <a href="<page docs/tor-doc-osx>#uninstall">uninstall directions</a>.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     If you installed by source, I'm afraid there is no easy uninstall method.
     But on the bright side, by default it only installs into /usr/local/ and it
     should be pretty easy to notice things there.
2a9aaa80
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
2a9aaa80
     <a id="PGPSigs"></a>
5e8d08ab
     <h3><a class="anchor" href="#PGPSigs">What are these "sig" files on the
     download page?</a></h3>
 
     <p>
     These are PGP signatures, so you can verify that the file you've downloaded
     is exactly the one that we intended you to get.
     </p>
 
     <p>
     Please read the
     <a href="<page docs/verifying-signatures>">verifying signatures</a>
     page for details.
     </p>
 
     <hr>
 
     <a id="GetTor"></a>
     <h3><a class="anchor" href="#GetTor">Your website is blocked in my country.
     How do I download Tor?</a></h3>
 
     <p>
     Some government or corporate firewalls censor connections to Tor's website.
     In those cases, you have three options. First, get it from a friend &mdash;
     <a href="<page projects/torbrowser>">Tor Browser</a> fits nicely on a USB
     key. Second, find the
     <a href="https://encrypted.google.com/search?q=tor+mirrors">google cache</a>
     for the <a href="<page getinvolved/mirrors>">Tor mirrors</a> page and see if
     any of those copies of our website work for you. Third, you can download Tor
     Browser via email: log in to your email account and send an email to
     '<tt>gettor@torproject.org</tt>' with one of the following words in the
     body of the message: <tt>windows</tt>, <tt>osx</tt> or <tt>linux</tt>
     (case insensitive).
     You will receive a reply with links from popular cloud services to download
     Tor Browser for Windows, Mac OS X or Linux, depending on the option you
     chose. Currently, the only cloud service supported is Dropbox. If you send
     a blank message or anything different from the options mentioned, you will
     receive a help message with detailed instructions to ask for Tor Browser
     via email. Please note that you can use this service from any email address:
     gmail, yahoo, hotmail, riseup, etc. The only restriction is that you can do
     a maximum of three requests in a row, after that you'll have to wait 20
     minutes to use it again. See the
     <a href="../projects/gettor.html">GetTor</a> section for more information.
     </p>
 
     <p>
     Be sure to <a href="<page docs/verifying-signatures>">verify the
     signature</a> of any package you download, especially when you get it from
     somewhere other than our official HTTPS website.
     </p>
465835b2
 
ed5ac546
     <hr>
0b68c29c
 
082877a6
     <a id="VirusFalsePositives"></a>
5fe101b2
     <h3><a class="anchor" href="#VirusFalsePositives">Why does my
     Tor executable appear to have a virus or spyware?</a></h3>
082877a6
     <p>
13b97478
     Sometimes, overzealous Windows virus and spyware detectors trigger on
     some parts of the Tor Windows binary. Our best guess is that these are
     false positives — after all, the anti-virus and anti-spyware business is
     just a guessing game anyway. You should contact your vendor and explain
     that you have a program that seems to be triggering false positives. Or
082877a6
     pick a better vendor.
     </p>
5e8d08ab
 
13b97478
     <p>In the meantime, we encourage you to not just take our word for it.
     Our job is to provide the source; if you're concerned, please do
082877a6
     recompile it yourself.</p>
 
     <hr>
 
6f107e8d
     <a id="tarballs"></a>
13b97478
     <h3><a class="anchor" href="#tarballs">How do I open a .tar.gz
c70b2a5f
     or .tar.xz file?</a></h3>
991262e9
 
     <p>
     Tar is a common archive utility for Unix and Linux systems. If your
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     system has a mouse, you can usually open them by double clicking.
     Otherwise open a command prompt and execute</p>
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     <pre>tar xzf &lt;FILENAME&gt;.tar.gz</pre>
     or
     <pre>tar xJf &lt;FILENAME&gt;.tar.xz</pre>
 
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     <p>
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     as documented on tar's man page.
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     </p>
 
     <hr>
 
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     <a id="LiveCD"></a>
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     <h3><a class="anchor" href="#LiveCD">Is there a LiveCD or other bundle that
     includes Tor?</a></h3>
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     <p>
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     Yes.  Use <a href="https://tails.boum.org/">The Amnesic Incognito
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     Live System</a> or <a href="<page projects/torbrowser>">Tor Browser</a>.
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     </p>
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     <hr>
 
     <a id="TBBGeneral"></a>
     <h2><a class="anchor" href="#TBBGeneral">Tor Browser (general):</a></h2>
 
     <a id="TBBFlash"></a>
     <h3><a class="anchor" href="#TBBFlash">Why can't I view videos on some
     Flash-based sites?</a></h3>
 
     <p>
     Some sites require third party browser plugins such as Flash.
     Plugins operate independently from Firefox and can perform activity on your
     computer that ruins your anonymity. This includes but is not limited to:
     completely disregarding proxy settings, querying your
     <a href="http://forums.sun.com/thread.jspa?threadID=5162138&amp;messageID=9618376">
     local IP address</a>, and
     <a href="http://epic.org/privacy/cookies/flash.html">storing their
     owncookies</a>. It is possible to use a LiveCD solution such as or
     <a href="https://tails.boum.org/">The Amnesic Incognito Live System</a>
     that creates a secure, transparent proxy to protect you from proxy bypass,
     however issues with local IP address discovery and Flash cookies still remain.
     </p>
 
     <hr>
 
     <a id="Ubuntu"></a>
     <h3><a class="anchor" href="#Ubuntu">I'm using Ubuntu and I can't start Tor
     Browser.</a></h3>
     <p>
     You'll need to tell Ubuntu that you want the ability to execute shell
     scripts from the graphical interface. Open "Files" (Unity's explorer), open
     Preferences-> Behavior Tab -> Set "Run executable text files when they are
     opened" to "Ask every time", then OK.
     </p>
 
     <p>You can also start Tor Browser from the command line by running </p>
 
     <pre>./start-tor-browser</pre>
 
     <p>
     from inside the Tor Browser directory.
     </p>
 
     <hr>
 
     <a id="SophosOnMac"></a>
     <h3><a class="anchor" href="#SophosOnMac">I'm using the Sophos anti-virus
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     software on my Mac, and Tor starts but I can't browse anywhere.</a></h3>
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     <p>
     You'll need to modify Sophos anti-virus so that Tor can connect to the
     internet. Go to Preferences -> Web Protection -> General, and turn off
     the protections for "Malicious websites" and "Malicious downloads".
     </p>
 
     <p>
     We encourage affected Sophos users to contact Sophos support about
     this issue.
     </p>
 
     <hr>
 
     <a id="XPCOMError"></a>
     <h3><a class="anchor" href="#XPCOMError">When I start Tor Browser I get an
     error message: "Cannot load XPCOM".</a></h3>
 
     <p>
     This <a href="https://trac.torproject.org/projects/tor/ticket/10789">
     problem</a> is specifically caused by the Webroot SecureAnywhere Antivirus
     software.
     From the Webroot control panel, go to Identity Protection &rarr; Application
     Protection, and set all the files in your Tor Browser folder to 'Allow'.
     We encourage affected Webroot users to contact Webroot support about this
     issue.
     </p>
 
     <hr>
 
     <a id="TBBOtherExtensions"></a>
     <h3><a class="anchor" href="#TBBOtherExtensions">Can I install other
     Firefox extensions?</a></h3>
 
     <p>
     Tor Browser is free software, so there is nothing preventing you from
     modifying it any way you like. However, we do not recommend installing any
     additional Firefox add-ons with Tor Browser. Add-ons can break
     your anonymity in a number of ways, including browser fingerprinting and
     bypassing proxy settings.
     </p>
 
     <p>
     Some people have suggested we include ad-blocking software or anti-tracking
     software with Tor Browser. Right now, we do not think that's such a good
     idea. Tor Browser aims to provide sufficient privacy that additional add-ons
     to stop ads and trackers are not necessary. Using add-ons like these may
     cause some sites to break, which
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     <a href="/projects/torbrowser/design/#philosophy">
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     we don't want to do</a>. Additionally, maintaining a list of "bad" sites
     that should be black-listed provides another opportunity to uniquely
     fingerprint users.
     </p>
 
     <hr>
 
     <a id="TBBJavaScriptEnabled"></a>
     <a id="TBBCanIBlockJS"></a>
     <h3><a class="anchor" href="#TBBJavaScriptEnabled">Why is NoScript
     configured to allow JavaScript by default in Tor Browser?
     Isn't that unsafe?</a></h3>
 
     <p>
     We configure NoScript to allow JavaScript by default in Tor Browser because
     many websites will not work with JavaScript disabled. Most users would give
     up on Tor entirely if a website they want to use requires JavaScript,
     because they would not know how to allow a website to use JavaScript
     (or that enabling JavaScript might make a website work).
     </p>
 
     <p>
     There's a tradeoff here. On the one hand, we should leave JavaScript
     enabled by default so websites work the way users expect. On the other hand,
     we should disable JavaScript by default to better protect against browser
     vulnerabilities (
     <a href="https://blog.torproject.org/blog/tor-security-advisory-old-tor-browser-bundles-vulnerable">
     not just a theoretical concern!</a>). But there's a third issue: websites
     can easily determine whether you have allowed JavaScript for them, and if
     you disable JavaScript by default but then allow a few websites to run
     scripts (the way most people use NoScript), then your choice of whitelisted
     websites acts as a sort of cookie that makes you recognizable (and
     distinguishable), thus harming your anonymity.
     </p>
 
     <p>
     Ultimately, we want the default Tor bundles to use a combination of
     firewalls (like the iptables rules in
     <a href="https://tails.boum.org/">Tails</a>) and
     <a href="https://trac.torproject.org/projects/tor/ticket/7680">sandboxes</a>
     to make JavaScript not so scary. In the shorter term, TBB 3.0 will hopefully
     <a href="https://trac.torproject.org/projects/tor/ticket/9387">allow users
     to choose their JavaScript settings more easily</a> &mdash; but the
     partitioning concern will remain.
     </p>
 
     <p>
     Until we get there, feel free to leave JavaScript on or off depending
     on your security, anonymity, and usability priorities.
     </p>
 
     <hr>
 
     <a id="TBBOtherBrowser"></a>
     <h3><a class="anchor" href="#TBBOtherBrowser">I want to use
     Chrome/IE/Opera/etc with Tor.</a></h3>
 
     <p>
     In short, using any browser besides Tor Browser with Tor is a really bad idea.
     </p>
 
     <p>
     Our efforts to work with the Chrome team to
     <a href="https://blog.torproject.org/blog/google-chrome-incognito-mode-tor-and-fingerprinting">
     add missing APIs</a> were unsuccessful, unfortunately. Currently, it is
     impossible to use other browsers and get the same level of protections as
     when using Tor Browser.
     </p>
 
     <hr>
 
     <a id="GoogleCAPTCHA"></a>
     <h3><a class="anchor" href="#GoogleCAPTCHA">Google makes me solve a CAPTCHA
     or tells me I have spyware installed.</a></h3>
 
     <p>
     This is a known and intermittent problem; it does not mean that Google
     considers Tor to be spyware.
     </p>
 
     <p>
     When you use Tor, you are sending queries through exit relays that are
     also shared by thousands of other users. Tor users typically see this
     message when many Tor users are querying Google in a short period of time.
     Google interprets the high volume of traffic from a single IP address
     (the exit relay you happened to pick) as somebody trying to "crawl" their
     website, so it slows down traffic from that IP address for a short time.
     </p>
 
     <p>
     An alternate explanation is that Google tries to detect certain kinds of
     spyware or viruses that send distinctive queries to Google Search. It notes
     the IP addresses from which those queries are received (not realizing that
     they are Tor exit relays), and tries to warn any connections coming from
     those IP addresses that recent queries indicate an infection.
     </p>
 
     <p>
     To our knowledge, Google is not doing anything intentionally specifically
     to deter or block Tor use. The error message about an infected machine
     should clear up again after a short time.
     </p>
 
     <hr />
 
     <a id="ForeignLanguages"></a>
     <h3><a class="anchor" href="#ForeignLanguages">
     Why does Google show up in foreign languages?</a></h3>
 
     <p>
     Google uses "geolocation" to determine where in the world you are, so it
     can give you a personalized experience. This includes using the language
     it thinks you prefer, and it also includes giving you different results
     on your queries.
     </p>
 
     <p>
     If you really want to see Google in English you can click the link that
     provides that. But we consider this a feature with Tor, not a bug --- the
     Internet is not flat, and it in fact does look different depending on
     where you are. This feature reminds people of this fact.
     </p>
 
     <p>
     Note that Google search URLs take name/value pairs as arguments and one
     of those names is "hl". If you set "hl" to "en" then Google will return
     search results in English regardless of what Google server you have been
     sent to. On a query this looks like:
     </p>
 
     <pre>https://encrypted.google.com/search?q=online%20anonymity&hl=en</pre>
 
     <p>
     Another method is to simply use your country code for accessing Google.
     This can be google.be, google.de, google.us and so on.
     </p>
 
     <hr />
 
     <a id="GmailWarning"></a>
     <h3><a class="anchor" href="#GmailWarning">Gmail warns me that my account
     may have been compromised.</a></h3>
 
     <p>
     Sometimes, after you've used Gmail over Tor, Google presents a pop-up
     notification that your account may have been compromised.
     The notification window lists a series of IP addresses and locations
     throughout the world recently used to access your account.
     </p>
 
     <p>
     In general this is a false alarm: Google saw a bunch of logins from
     different places, as a result of running the service via Tor, and decided
     it was a good idea to confirm the account was being accessed by it's
     rightful owner.
     </p>
 
     <p>
     Even though this may be a biproduct of using the service via tor, that
     doesn't mean you can entirely ignore the warning. It is <i>probably</i> a
     false positive, but it might not be since it is possible for someone to
     hijack your Google cookie.
     </p>
 
     <p>
     Cookie hijacking is possible by either physical access to your computer or
     by watching your network traffic.  In theory only physical access should
     compromise your system because Gmail and similar services should only send
     the cookie over an SSL link. In practice, alas, it's
     <a href="http://fscked.org/blog/fully-automated-active-https-cookie-hijacking">
     way more complex than that</a>.
     </p>
 
     <p>
     And if somebody <i>did</i> steal your google cookie, they might end up
     logging in from unusual places (though of course they also might not). So
     the summary is that since you're using Tor, this security measure that
     Google uses isn't so useful for you, because it's full of false positives.
     You'll have to use other approaches, like seeing if anything looks weird on
     the account, or looking at the timestamps for recent logins and wondering
     if you actually logged in at those times.
     </p>
 
     <hr>
 
     <a id="NeedToUseAProxy"></a>
     <h3><a class="anchor" href="#NeedToUseAProxy">My internet connection
     requires an HTTP or SOCKS Proxy</a></h3>
 
     <p>
     You can set Proxy IP address, port, and authentication information in
     Tor Browser's Network Settings. If you're using Tor another way, check out
     the HTTPProxy and HTTPSProxy config options in the
     <a href="<page docs/tor-manual>">man page</a>, and modify your torrc file
     accordingly. You will need an HTTP proxy for doing GET requests to fetch
     the Tor directory, and you will need an HTTPS proxy for doing CONNECT
     requests to get to Tor relays. (It's fine if they're the same proxy.)
     Tor also recognizes the torrc options Socks4Proxy and Socks5Proxy.
     </p>
 
     <p>
     Also read up on the HTTPProxyAuthenticator and HTTPSProxyAuthenticator
     options if your proxy requires auth. We only support basic auth currently,
     but if you need NTLM authentication, you may find
     <a href="http://archives.seul.org/or/talk/Jun-2005/msg00223.html">this post
     in the archives</a> useful.
     </p>
 
     <p>
     If your proxies only allow you to connect to certain ports, look at the
     entry on <a href="#FirewallPorts">Firewalled clients</a> for how
     to restrict what ports your Tor will try to access.
     </p>
 
     <hr>
 
     <a id="TBBSocksPort"></a>
     <h3><a class="anchor" href="#TBBSocksPort">I want to run another
     application through Tor.</a></h3>
 
     <p>
     If you are trying to use some external application with Tor, step zero
     should be to <a href="<page download/download>#warning">reread the set
     of warnings</a> for ways you can screw up. Step one should be to try
     to use a SOCKS proxy rather than an HTTP proxy.
     Typically Tor listens for SOCKS connections on port 9050. Tor Browser
     listens on port 9150.
     </p>
 
     <p>
     If your application doesn't support SOCKS proxies, feel free to install
     <a href="http://www.privoxy.org/">privoxy</a>.
     However, please realize that this approach is not recommended for novice
     users. Privoxy has an
     <a href="http://www.privoxy.org/faq/misc.html#TOR">
     example configuration</a> of Tor and Privoxy.
     </p>
 
     <p>
     If you're unable to use the application's native proxy settings, all hope is
     not lost. See <a href="#CantSetProxy">below</a>.
     </p>
 
     <hr>
 
     <a id="CantSetProxy"></a>
     <h3><a class="anchor" href="#CantSetProxy">What should I do if I can't
     set a proxy with my application?</a></h3>
 
     <p>
     On Unix, we recommend you give
     <a href="https://github.com/dgoulet/torsocks/">torsocks</a> a try.
     Alternative proxifying tools like
     <a href="http://www.dest-unreach.org/socat/">socat</a> and
     <a href="http://proxychains.sourceforge.net/">proxychains</a> are also
     available.</p>
 
     <p>
     The Windows way to force applications through Tor is less clear.
     <a href="http://freecap.ru/eng/">Some</a>
     <a href="http://www.freehaven.net/~aphex/torcap/">tools</a> have been
     <a href="http://www.crowdstrike.com/community-tools/index.html#tool-79">
     proposed</a>, but we'd also like to see further testing done here.
     </p>
 
     <hr>
 
     <a id="TBB3.x"></a>
     <h2><a class="anchor" href="#TBB3.x">Tor Browser (3.x and later):</a></h2>
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     <a id="WhereDidVidaliaGo"></a>
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     <h3><a class="anchor" href="#WhereDidVidaliaGo">Where did the world map
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     (Vidalia) go?</a></h3>
 
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     <p>Vidalia has been replaced with Tor Launcher, which is a Firefox
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     extension that provides similar functionality.</p>
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     <hr>
 
     <a id="DisableJS"></a>
     <h3><a class="anchor" href="#DisableJS">How do I disable JavaScript?</a>
     </h3>
 
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     <p>Alas, Mozilla decided to get rid of the config checkbox for JavaScript
     from earlier Firefox versions. And since TBB 3.5 is based on Firefox 24
     (FF17 is unmaintained), that means TBB 3.5 doesn't have the config
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     checkbox anymore either, which is unfortunate.</p>
 
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     <p>The simplest way to disable JavaScript in TBB 3.5 is to click on the
     Noscript "S" (between the green onion and the address bar), and select
     "Forbid scripts globally". Note that vanilla NoScript actually whitelists
     several domains even when you try to disable scripts globally, whereas
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     Tor Browser's NoScript configuration disables all of them. </p>
 
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     <p>The more klunky way to disable JavaScript is to go to about:config,
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     find javascript.enabled, and set it to false.</p>
 
     <p>There is also a very simple addon available at addons.mozilla.org
     called QuickJS, which provides a toolbar toggle for the javascript.enabled
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     about:config control. There are no configuration options for the addon,
     it just switches the javascript.enabled entry between true and false and
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     provides a button for it. </p>
 
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     <p>If you want to be extra safe, use both the about:config setting and
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     NoScript. </p>
 
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     <p>As for whether you should disable it or leave it enabled, that's <a
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     href="#TBBJavaScriptEnabled">a tradeoff we leave to you</a>.</p>
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     <hr>
 
     <a id="VerifyDownload"></a>
     <h3><a class="anchor" href="#VerifyDownload">How do I verify the download
     (sha256sums.txt)?</a></h3>
 
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     <p>Instructions are on the <a
     href="<page docs/verifying-signatures>#BuildVerification">verifying
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     signatures</a> page.</p>
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     <hr>
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     <a id="NewIdentityClosingTabs"></a>
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     <h3><a class="anchor" href="#NewIdentityClosingTabs">Why does "New
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     Identity" close all my open tabs?</a></h3>
 
     <p>
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     That's actually a feature, since it's discarding your application-level
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     browser data too.
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     </p>
 
     <p>
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     We're working on ways to make the behavior less surprising, e.g. a popup
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     warning or auto restoring tabs. See ticket
     <a href="https://trac.torproject.org/projects/tor/ticket/9906">#9906</a>
     and ticket
     <a href="https://trac.torproject.org/projects/tor/ticket/10400">#10400</a>
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     to follow progress there.
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     </p>
 
     <hr>
 
     <a id="ConfigureRelayOrBridge"></a>
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     <h3><a class="anchor" href="#ConfigureRelayOrBridge">How do I configure Tor
     as a relay or bridge?</a></h3>
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     <p>
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     You've got three options.
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     </p>
 
     <p>
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     First (best option), if you're on Linux, you can install the
     <a href="<page download/download-unix>">system Tor package</a>
     (e.g. apt-get install tor) and then set it up to be a relay
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     (<a href="https://www.torproject.org/docs/tor-relay-debian">instructions</a>).
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     You can then use TBB independent of that.
     </p>
 
     <p>
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     Second (complex option), you can edit your <a href="#torrc">torrc file</a>
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     directly to add the following lines:
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     </p>
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     <pre>
     ORPort 443
     Exitpolicy reject *:*
     BridgeRelay 1  # only add this line if you want to be a bridge
     </pre>
 
     <hr>
 
     <a id="Timestamps"></a>
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     <h3><a class="anchor" href="#Timestamps">Why are the file timestamps
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     from 2000?</a></h3>
 
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     <p>One of the huge new features in TBB 3.x is the "deterministic build"
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     process, which allows many people to build Tor Browser and verify that they
     all make exactly the same package. See Mike's
     <a href="https://blog.torproject.org/blog/deterministic-builds-part-one-cyberwar-and-global-compromise">
     first blog</a> post for the motivation, and his
     <a href="https://blog.torproject.org/blog/deterministic-builds-part-two-technical-details">
     second blog post</a> for the technical details of how we do it.
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     </p>
 
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     <p>Part of creating identical builds is having everybody use the same
     timestamp. Mike picked the beginning of 2000 for that time. The reason
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     you might see 7pm in 1999 is because of time zones. </p>
 
     <hr>
 
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     <a id="TBBSourceCode"></a>
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     <h3><a class="anchor" href="#TBBSourceCode">Where is the source code for
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     Tor Browser? How do I verify a build?</a></h3>
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     <p>
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     Tor Browser is built from the
     <a href="https://gitweb.torproject.org/builders/tor-browser-build.git/">
     tor-browser-build.git git repository</a>. You can have a look at the
     <a href="https://gitweb.torproject.org/builders/tor-browser-build.git/tree/README">
     README file</a> for the build instructions.
     There is also some informations in the
     <a href="https://trac.torproject.org/projects/tor/wiki/doc/TorBrowser/Hacking">
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     Tor Browser Hacking Guide</a>. Also see our
     <a href="<page docs/verifying-signatures>">fingerprint verification guide</a>.
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     </p>
 
     <hr>
 
     <a id="AdvancedTorUsage"></a>
     <h2><a class="anchor" href="#AdvancedTorUsage">Advanced Tor usage:</a></h2>
 
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     <a id="torrc"></a><a id="datadir"></a>
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     <h3><a class="anchor" href="#torrc">I'm supposed to "edit my torrc".
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     What does that mean? Where's tor's data directory?</a></h3>
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     <p>
     Tor uses a text file called torrc that contains configuration
     instructions for how your Tor program should behave. The default
     configuration should work fine for most Tor users.
     </p>
 
     <p>
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     If you installed Tor Browser on Windows or Linux, torrc is in the data
     directory, which is <code>Browser/TorBrowser/Data/Tor</code> inside your
     Tor Browser directory. For the tor service on Windows see
     <a href="#NTService">Windows NT</a>.
     </p>
 
     <p>
     If you're on macOS, the torrc is in the data directory at
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     <code>~/Library/Application Support/TorBrowser-Data/Tor</code>.
     To get to it, press cmd-shift-g while in Finder and copy/paste that
     directory into the box that appears.
     </p>
 
     <p>
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     Otherwise, if you are using Tor without Tor Browser, it looks for torrc at
     differentt possible locations:
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     </p>
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     <ul>
     <li>
     <code>/usr/local/etc/tor/torrc</code> if you compiled tor from source
     </li>
     <li>
     <code>/etc/tor/torrc</code> or <code>/etc/torrc</code> if you installed a
     pre-built package. The data directory usually is
     <code>/var/lib/tor/</code>, if not defined otherwise with
     <code>DataDirectory</code> in torrc.
     </li>
     <li><code>$HOME/.torrc</code>: fallback location if above file is not found.
     </li>
     <li>
     You can define a different location for torrc with <code>-f FILE</code> and
     set another data directory with <code>--DataDirectory DIR</code> as options
     to tor.
     </li>
     </ul>
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     <p>
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     Once you've created or changed your torrc file, you will need to restart or
     reload tor for the changes to take effect. On Debian use
     <code>system tor reload</code>. (For advanced users, note that you
     actually only need to send Tor a HUP signal, not actually restart it.)
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     </p>
 
     <p>
     For other configuration options you can use, see the
     <a href="<page docs/tor-manual>">Tor manual page</a>. Have a look at
     <a href="https://gitweb.torproject.org/tor.git/tree/src/config/torrc.sample.in">
     the sample torrc file</a> for hints on common configurations. Remember, all
     lines beginning with # in torrc are treated as comments and have no effect
     on Tor's configuration.
     </p>
 
     <hr>
 
     <a id="Logs"></a>
     <h3><a class="anchor" href="#Logs">How do I set up logging, or see Tor's
     logs?</a></h3>
 
     <p>
     You'll have to go find the log files by
     hand. Here are some likely places for your logs to be:
     </p>
 
     <ul>
       <li>On OS X, Debian, Red Hat, etc, the logs are in /var/log/tor/
       </li>
       <li>On Windows, there are no default log files currently. If you enable
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       logs in your <a href="#torrc">torrc</a> file, they default to <code>\username\Application
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       Data\tor\log\</code> or <code>\Application Data\tor\log\</code>
       </li>
       <li>If you compiled Tor from source, by default your Tor logs to
       <a href="http://en.wikipedia.org/wiki/Standard_streams">"stdout"</a>
       at log-level notice. If you enable logs in your torrc file, they
       default to <code>/usr/local/var/log/tor/</code>.
       </li>
     </ul>
 
     <p>
     To change your logging setup by hand, <a href="#torrc">edit your
     torrc</a>
     and find the section (near the top of the file) which contains the
     following line:
     </p>
 
     <pre>
     \## Logs go to stdout at level "notice" unless redirected by something
     \## else, like one of the below lines.
     </pre>
 
     <p>
     For example, if you want Tor to send complete debug, info, notice, warn,
     and err level messages to a file, append the following line to the end
     of the section:
     </p>
 
     <pre>
     Log debug file c:/program files/tor/debug.log
     </pre>
 
     <p>
     Replace <code>c:/program files/tor/debug.log</code> with a directory
     and filename for your Tor log.
     </p>
 
     <hr>
 
     <a id="LogLevel"></a>
     <h3><a class="anchor" href="#LogLevel">What log level should I use?</a></h3>
 
     <p>
     There are five log levels (also called "log severities") you might see in
     Tor's logs:
     </p>
 
     <ul>
 
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     <li>"err": something bad just happened, and we can't recover. Tor will
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     exit.</li>
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     <li>"warn": something bad happened, but we're still running. The bad
     thing might be a bug in the code, some other Tor process doing something
     unexpected, etc. The operator should examine the message and try to
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     correct the problem.</li>
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     <li>"notice": something the operator will want to know about.</li>
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     <li>"info": something happened (maybe bad, maybe ok), but there's
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     nothing you need to (or can) do about it.</li>
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     <li>"debug": for everything louder than info. It is quite loud indeed.</li>
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     </ul>
 
     <p>
     Alas, some of the warn messages are hard for ordinary users to correct --
     the developers are slowly making progress at making Tor automatically react
     correctly for each situation.
     </p>
 
     <p>
     We recommend running at the default, which is "notice". You will hear about
     important things, and you won't hear about unimportant things.
     </p>
 
     <p>
     Tor relays in particular should avoid logging at info or debug in normal
     operation, since they might end up recording sensitive information in
     their logs.
     </p>
 
     <hr>
 
     <a id="DoesntWork"></a>
     <h3><a class="anchor" href="#DoesntWork">I installed Tor but it's not
     working.</a></h3>
 
     <p>
     Once you've got Tor Browser up and running, the first question to ask is
     whether your Tor client is able to establish a circuit.
     </p>
 
     <p>If Tor can establish a circuit, Tor Browser will automatically launch
     the browser for you. You can also check in the <a href="#Logs">Tor logs</a>
     for a line saying that Tor "has successfully opened a circuit. Looks like
     client functionality is working."
     </p>
 
     <p>
     If Tor can't establish a circuit, here are some hints:
     </p>
 
     <ol>
 
     <li>Check your system clock. If it's more than a few hours off, Tor will
     refuse to build circuits. For Microsoft Windows users, synchronize your
     clock under the clock -&gt; Internet time tab. In addition, correct the
     day and date under the 'Date &amp; Time' Tab. Also make sure your time
     zone is correct.</li>
     <li>Is your Internet connection <a href="#FirewallPorts">firewalled
     by port</a>, or do you normally need to use a
     <a href="<#NeedToUseAProxy">proxy</a>?
     </li>
     <li>Are you running programs like Norton Internet Security or SELinux that
     block certain connections, even though you don't realize they do? They
     could be preventing Tor from making network connections.</li>
     <li>Are you in China, or behind a restrictive corporate network firewall
     that blocks the public Tor relays? If so, you should learn about
     <a href="<page docs/bridges>">Tor bridges</a>.</li>
     <li>Check your <a href="#Logs">Tor logs</a>. Do they give you any hints
     about what's going wrong?</li>
 
     </ol>
 
     <hr />
 
     <a id="TorCrash"></a>
     <h3><a class="anchor" href="#TorCrash">My Tor keeps crashing.</a></h3>
     <p>
     We want to hear from you! There are supposed to be zero crash bugs in Tor.
     This FAQ entry describes the best way for you to be helpful to us. But even
     if you can't work out all the details, we still want to hear about it, so
     we can help you track it down.
     </p>
     <p>
     First, make sure you're using the latest version of Tor (either the latest
     stable or the latest development version).
     </p>
 
     <p>
     Second, make sure your version of libevent is new enough. We recommend at
     least libevent 1.3a.
     </p>
 
     <p>
     Third, see if there's already an entry for your bug in the
     <a href="https://bugs.torproject.org/">Tor bugtracker</a>. If so, check if
     there are any new details that you can add.
     </p>
 
     <p>
     Fourth, is the crash repeatable? Can you cause the crash? Can you isolate
     some of the circumstances or config options that make it happen? How
     quickly or often does the bug show up?
     Can you check if it happens with other versions of Tor, for example the
     latest stable release?
     </p>
 
     <p>
     Fifth, what sort of crash do you get?
     </p>
 
     <ul>
 
     <li>
     Does your Tor log include an "assert failure"? If so, please tell us that
     line, since it helps us figure out what's going on. Tell us the previous
     couple of log messages as well, especially if they seem important.
     </li>
     <li>
     If it says "Segmentation fault - core dumped" then you need to do a bit
     more to track it down. Look for a file like "core" or "tor.core" or
     "core.12345" in your current directory, or in your Data Directory.
     If it's there, run "gdb tor core" and then "bt", and include the output.
     If you can't find a core, run "ulimit -c unlimited", restart Tor, and try
     to make it crash again. (This core thing will only work on Unix -- alas,
     tracking down bugs on Windows is harder. If you're on Windows, can you get
     somebody to duplicate your bug on Unix?)
     </li>
 
     <li>
     If Tor simply vanishes mysteriously, it probably is a segmentation fault
     but you're running Tor in the background (as a daemon) so you won't notice.
     Go look at the end of your log file, and look for a core file as above.
     If you don't find any good hints, you should consider running Tor in the
     foreground (from a shell) so you can see how it dies. Warning: if you
     switch to running Tor in the foreground, you might start using a different
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     <a href="#torrc">torrc</a> file, with a different default Data Directory; see the
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     <a href="#UpgradeOrMove">relay-upgrade FAQ entry</a>
     for details.
     </li>
     <li>
     If it's still vanishing mysteriously, perhaps something else is killing it?
     Do you have resource limits (ulimits) configured that kill off processes
     sometimes? On Linux, try running "dmesg" to see if the out-of-memory killer
     removed your process. (Tor will exit cleanly if it notices that it's run
     out of memory, but in some cases it might not have time to notice.) In very
     rare circumstances, hardware problems could also be the culprit.
     </li>
     </ul>
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     <p>
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     Sixth, if the above ideas don't point out the bug, consider increasing your
     log level to "loglevel debug". You can look at the log-configuration FAQ
     entry for instructions on what to put in your torrc file. If it usually
     takes a long time for the crash to show up, you will want to reserve a
     whole lot of disk space for the debug log. Alternatively, you could just
     send debug-level logs to the screen (it's called "stdout" in the torrc),
     and then when it crashes you'll see the last couple of log lines it had
     printed.
     (Note that running with verbose logging like this will slow Tor down
     considerably, and note also that it's generally not a good idea
     security-wise to keep logs like this sitting around.)
     </p>
 
     <hr />
a2904cae
 
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     <a id="ChooseEntryExit"></a>
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     <h3><a class="anchor" href="#ChooseEntryExit">Can I control which nodes
     (or country) are used for entry/exit?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
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     Yes. You can set preferred entry and exit nodes as well as
     inform Tor which nodes you do not want to use.
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     The following options can be added to your config file
     <a href="#torrc">torrc</a> or specified on the command line:
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     </p>
     <dl>
       <dt><tt>EntryNodes $fingerprint,$fingerprint,...</tt></dt>
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         <dd>A list of preferred nodes to use for the first hop in the
 circuit, if possible.
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         </dd>
       <dt><tt>ExitNodes $fingerprint,$fingerprint,...</tt></dt>
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         <dd>A list of preferred nodes to use for the last hop in the
 circuit, if possible.
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         </dd>
       <dt><tt>ExcludeNodes $fingerprint,$fingerprint,...</tt></dt>
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         <dd>A list of nodes to never use when building a circuit.
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         </dd>
       <dt><tt>ExcludeExitNodes $fingerprint,$fingerprint,...</tt></dt>
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         <dd>A list of nodes to never use when picking an exit.
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             Nodes listed in <tt>ExcludeNodes</tt> are automatically in
 this list.
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         </dd>
     </dl>
     <p>
0b68c29c
     <em>We recommend you do not use these</em>
37855015
     &mdash; they are intended for testing and may disappear in future
 versions.
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     You get the best security that Tor can provide when you leave the
     route selection to Tor; overriding the entry / exit nodes can mess
     up your anonymity in ways we don't understand.
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     </p>
     <p>
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     Note also that not every circuit is used to deliver traffic outside of
     the Tor network. It is normal to see non-exit circuits (such as those
32316cbf
     used to connect to onion services, those that do directory fetches,
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     those used for relay reachability self-tests, and so on) that end at
     a non-exit node. To keep a node from being used entirely, see
     <tt>ExcludeNodes</tt> and <tt>StrictNodes</tt> in the
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     <a href="<page docs/tor-manual>">manual</a>.
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     </p>
     <p>
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     Instead of <tt>$fingerprint</tt> you can also specify a
     <a href="https://secure.wikimedia.org/wikipedia/en/wiki/ISO_3166-1_alpha-2">
     2 letter ISO3166 country code</a> in curly braces (for example
     <tt>{de}</tt>), or an ip address pattern (for example 255.254.0.0/8).
     Make sure there are no spaces between the commas and the list items.
2a9aaa80
     </p>
5e8d08ab
 
2a9aaa80
     <p>
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     If you want to access a service directly through Tor's Socks interface
     (eg. using ssh via connect.c), another option is to set up an internal
     mapping in your configuration file using <tt>MapAddress</tt>.
2a9aaa80
     See the manual page for details.
     </p>
0b68c29c
 
ed5ac546
     <hr>
d97ab35a
 
5e8d08ab
     <a id="FirewallPorts"></a>
     <h3><a class="anchor" href="#FirewallPorts">My firewall only allows a
     few outgoing ports.</a></h3>
 
     <p>
     If your firewall works by blocking ports, then you can tell Tor to only
     use the ports when you start your Tor Browser. Or you can add the ports
     that your firewall permits by adding "FascistFirewall 1" to your
d7568ec6
     <a href="#torrc">torrc configuration file</a>.
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     By default, when you set this Tor assumes that your firewall allows only
     port 80 and port 443 (HTTP and HTTPS respectively). You can select a
     different set of ports with the FirewallPorts torrc option.
     </p>
 
     <p>
     If you want to be more fine-grained with your controls, you can also
     use the ReachableAddresses config options, e.g.:
     </p>
 
     <pre>
       ReachableDirAddresses *:80
       ReachableORAddresses *:443
     </pre>
 
     <hr>
d97ab35a
 
6d9ff1cf
     <a id="DefaultExitPorts"></a>
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     <h3><a class="anchor" href="#DefaultExitPorts">Is there a list of default exit
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     ports?</a></h3>
91d870fd
     <p>
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     The default open ports are listed below but keep in mind that, any port or
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     ports can be opened by the relay operator by configuring it in
     <a href="#torrc">torrc</a> or modifying the source code.
     <!-- TODO should we update this? -->
     The default according to src/or/policies.c from the source code release
     tor-0.2.4.16-rc:
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     </p>
     <pre>
   reject 0.0.0.0/8
   reject 169.254.0.0/16
   reject 127.0.0.0/8
   reject 192.168.0.0/16
   reject 10.0.0.0/8
   reject 172.16.0.0/12
   reject *:25
   reject *:119
   reject *:135-139
   reject *:445
   reject *:563
   reject *:1214
   reject *:4661-4666
   reject *:6346-6429
   reject *:6699
   reject *:6881-6999
   accept *:*
     </pre>
     <p>
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     A relay will block access to its own IP address, as well local network
     IP addresses. A relay always blocks itself by default. This prevents
     Tor users from accidentally accessing any of the exit operator's local
     services.
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     </p>
 
     <hr>
 
0e8964cc
     <a id="WarningsAboutSOCKSandDNSInformationLeaks"></a>
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     <h3><a class="anchor" href="#WarningsAboutSOCKSandDNSInformationLeaks">I
     keep seeing these warnings about SOCKS and DNS information leaks.
0e8964cc
     Should I worry?</a></h3>
     <p>
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     The warning is:
0e8964cc
     </p>
     <p>
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     Your application (using socks5 on port %d) is giving Tor only an IP
     address. Applications that do DNS resolves themselves may leak
     information. Consider using Socks4A (e.g. via Polipo or socat) instead.
0e8964cc
     </p>
     <p>
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     If you are running Tor to get anonymity, and you are worried about an
     attacker who is even slightly clever, then yes, you should worry. Here's why.
0e8964cc
     </p>
     <p>
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     <b>The Problem.</b> When your applications connect to servers on the
     Internet, they need to resolve hostnames that you can read (like
     www.torproject.org) into IP addresses that the Internet can use (like
     209.237.230.66). To do this, your application sends a request to a DNS
     server, telling it the hostname it wants to resolve. The DNS server
     replies by telling your application the IP address.
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     </p>
     <p>
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     Clearly, this is a bad idea if you plan to connect to the remote host
     anonymously: when your application sends the request to the DNS server,
     the DNS server (and anybody else who might be watching) can see what
     hostname you are asking for. Even if your application then uses Tor to
     connect to the IP anonymously, it will be pretty obvious that the user
     making the anonymous connection is probably the same person who made
     the DNS request.
0e8964cc
     </p>
     <p>
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     <b>Where SOCKS comes in.</b> Your application uses the SOCKS protocol
     to connect to your local Tor client. There are 3 versions of SOCKS you
     are likely to run into: SOCKS 4 (which only uses IP addresses), SOCKS 5
     (which usually uses IP addresses in practice), and SOCKS 4a (which uses
     hostnames).
0e8964cc
     </p>
     <p>
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     When your application uses SOCKS 4 or SOCKS 5 to give Tor an IP address,
     Tor guesses that it 'probably' got the IP address non-anonymously from a
     DNS server. That's why it gives you a warning message: you probably aren't
     as anonymous as you think.
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     </p>
     <p>
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     <b>So what can I do?</b> We describe a few solutions below.
0e8964cc
     </p>
     <ul>
     <li>If your application speaks SOCKS 4a, use it. </li>
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     <li>If you only need one or two hosts, or you are good at programming,
     you may be able to get a socks-based port-forwarder like socat to work
     for you; see <a
     href="https://trac.torproject.org/projects/tor/wiki/doc/TorifyHOWTO">the
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     Torify HOWTO</a> for examples. </li>
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     <li>Tor ships with a program called tor-resolve that can use the Tor
     network to look up hostnames remotely; if you resolve hostnames to IPs
     with tor-resolve, then pass the IPs to your applications, you'll be fine.
0e8964cc
     (Tor will still give the warning, but now you know what it means.) </li>
 
d7568ec6
     <!-- TODO I'm not sure if this project is still maintained or not
5e8d08ab
     <li>You can use TorDNS as a local DNS server to rectify the DNS leakage.
     See the Torify HOWTO for info on how to run particular applications
     anonymously.</li>
     !-->
0e8964cc
     </ul>
5e8d08ab
 
13b97478
     <p>If you think that you applied one of the solutions properly but still
     experience DNS leaks please verify there is no third-party application
     using DNS independently of Tor. Please see <a
     href="#AmITotallyAnonymous">the FAQ entry on whether you're really
     absolutely anonymous using Tor</a> for some examples.
0e8964cc
     </p>
 
     <hr>
 
f30e672d
     <a id="SocksAndDNS"></a>
5e8d08ab
     <h3><a class="anchor" href="#SocksAndDNS">How do I check if my application
     that uses SOCKS is leaking DNS requests?</a></h3>
f30e672d
 
     <p>
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     These are two steps you need to take here. The first is to make sure
     that it's using the correct variant of the SOCKS protocol, and the
     second is to make sure that there aren't other leaks.
f30e672d
     </p>
 
     <p>
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     Step one: add "TestSocks 1" to your torrc file, and then watch your
     logs as you use your application. Tor will then log, for each SOCKS
     connection, whether it was using a 'good' variant or a 'bad' one.
     (If you want to automatically disable all 'bad' variants, set
     "SafeSocks 1" in your <a href="#torrc">torrc</a> file.)
f30e672d
     </p>
 
     <p>
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     Step two: even if your application is using the correct variant of
     the SOCKS protocol, there is still a risk that it could be leaking
     DNS queries. This problem happens in Firefox extensions that resolve
     the destination hostname themselves, for example to show you its IP
     address, what country it's in, etc. These applications may use a safe
     SOCKS variant when actually making connections, but they still do DNS
     resolves locally. If you suspect your application might behave like
     this, you should use a network sniffer like <a
     href="https://www.wireshark.org/">Wireshark</a> and look for
     suspicious outbound DNS requests. I'm afraid the details of how to look
     for these problems are beyond the scope of a FAQ entry though -- find
     a friend to help if you have problems.
91d870fd
     </p>
 
     <hr>
 
6d5896ac
     <a id="TorClientOnADifferentComputerThanMyApplications"></a>
5e8d08ab
     <h3><a class="anchor" href="#TorClientOnADifferentComputerThanMyApplications">
     I want to run my Tor client on a different computer than my applications.
6d5896ac
     </a></h3>
     <p>
     By default, your Tor client only listens for applications that
     connect from localhost. Connections from other computers are
     refused. If you want to torify applications on different computers
d7568ec6
     than the Tor client, you should edit your <a href="#torrc">torrc</a> to define
6d5896ac
     SocksListenAddress 0.0.0.0 and then restart (or hup) Tor. If you
     want to get more advanced, you can configure your Tor client on a
     firewall to bind to your internal IP but not your external IP.
     </p>
 
     <hr>
 
     <a id="ServerClient"></a>
     <h3><a class="anchor" href="#ServerClient">Can I install Tor on a
     central server, and have my clients connect to it?</a></h3>
     <p>
      Yes. Tor can be configured as a client or a relay on another
      machine, and allow other machines to be able to connect to it
      for anonymity. This is most useful in an environment where many
      computers want a gateway of anonymity to the rest of the world.
      However, be forwarned that with this configuration, anyone within
      your private network (existing between you and the Tor
      client/relay) can see what traffic you are sending in clear text.
      The anonymity doesn't start until you get to the Tor relay.
      Because of this, if you are the controller of your domain and you
      know everything's locked down, you will be OK, but this configuration
      may not be suitable for large private networks where security is
      key all around.
     </p>
     <p>
d7568ec6
     Configuration is simple, editing your <a href="#torrc">torrc</a> file's SocksListenAddress
5e8d08ab
     according to the following examples:
6d5896ac
     </p>
 
5e8d08ab
     <pre>
6d5896ac
   #This provides local interface access only,
   #needs SocksPort to be greater than 0
   SocksListenAddress 127.0.0.1
 
   #This provides access to Tor on a specified interface
   SocksListenAddress 192.168.x.x:9100
 
   #Accept from all interfaces
   SocksListenAddress 0.0.0.0:9100
    </pre>
5e8d08ab
 
6d5896ac
     <p>
5e8d08ab
     You can state multiple listen addresses, in the case that you are
     part of several networks or subnets.
6d5896ac
     </p>
5e8d08ab
 
6d5896ac
     <pre>
5e8d08ab
       SocksListenAddress 192.168.x.x:9100 #eth0
       SocksListenAddress 10.x.x.x:9100 #eth1
6d5896ac
     </pre>
5e8d08ab
 
6d5896ac
     <p>
5e8d08ab
     After this, your clients on their respective networks/subnets would specify
     a socks proxy with the address and port you specified SocksListenAddress
     to be.
6d5896ac
     </p>
5e8d08ab
 
6d5896ac
     <p>
5e8d08ab
     Please note that the SocksPort configuration option gives the port ONLY for
     localhost (127.0.0.1). When setting up your SocksListenAddress(es), you need
     to give the port with the address, as shown above.
     </p>
 
6d5896ac
     <p>
5e8d08ab
     If you are interested in forcing all outgoing data through the central Tor
     client/relay, instead of the server only being an optional proxy, you may
     find the program iptables (for *nix) useful.
6d5896ac
     </p>
 
2c28979a
     <hr>
6d5896ac
 
f858ba1f
     <a id="IPv6"></a>
     <h3><a class="anchor" href="#IPv6">How do I use Tor from an IPv6 only host/computer?</a></h3>
aac5a17d
     <p>
     IPv6 is supported since Tor version 0.2.8.x or newer. To activate it add
     the following two entries into your torrc file:
     </p>
f858ba1f
     <pre>
   ClientUseIPv4 0
   ClientUseIPv6 1
     </pre>
aac5a17d
     <p>
1fee6ee4
     Note that as of 2018 there aren't many IPv6 users, or IPv6 guards, so Tor over IPv6
2c28979a
     is less anonymous than Tor over IPv4. If you are interested in developing you can review the IPv6 implemetation status at our
1fee6ee4
     <a href="https://trac.torproject.org/projects/tor/wiki/org/roadmaps/Tor/IPv6Features">IPv6Features</a>
     wiki page, known issues can be found with the
     <a href="https://trac.torproject.org/projects/tor/query?status=!closed&keywords=~ipv6">ipv6 keyword</a>.
aac5a17d
     </p>
f858ba1f
 
6d5896ac
     <hr>
 
6f107e8d
     <a id="RunningATorRelay"></a>
f87eea1a
     <h2><a class="anchor" href="#RunningATorRelay">Running a Tor relay:</a></h2>
6f107e8d
 
01f0dd22
     <a id="HowDoIDecide"></a>
13b97478
     <h3><a class="anchor" href="#HowDoIDecide">How do I decide if I should
01f0dd22
     run a relay?</a></h3>
     <p>
5e8d08ab
     We're looking for people with reasonably reliable Internet connections, 
     that have at least 1 MByte/second (that is 8 MBit/second) available
     bandwidth each way. If that's you, please consider
     <a href="<wiki>TorRelayGuide">running a Tor relay</a>.
660b94dc
     </p>
5e8d08ab
 
660b94dc
     <p>
5e8d08ab
     Even if you do not have at least 8 MBit/s of available bandwidth you can
     still help the Tor network by running a
     <a href="<page docs/pluggable-transports>#operator">Tor bridge with obfs4
     support</a>.
     In that case you should have at least 1 MBit/s of available bandwidth.
01f0dd22
     </p>
2b63e88f
 
01f0dd22
     <hr>
2b63e88f
 
fbd1ac87
     <a id="MostNeededRelayType"></a>
5e8d08ab
     <h3><a class="anchor" href="#MostNeededRelayType">What type of relays are
     most needed?</a></h3>
fbd1ac87
     <p>
     <ul>
5e8d08ab
     <li>The exit relay is the most needed relay type but it also comes with the
     highest legal exposure and risk (and you should NOT run them from your
     home).</li>
     <li>If you are looking to run a relay with minimal effort, fast guard
     relays are also very useful</li>
fbd1ac87
     <li>followed by bridges.</li>
     </ul>
     </p>
 
     <hr>
 
01f0dd22
     <a id="WhyIsntMyRelayBeingUsedMore"></a>
13b97478
     <h3><a class="anchor" href="#WhyIsntMyRelayBeingUsedMore">Why isn't my
01f0dd22
     relay being used more?</a></h3>
     <p>
13b97478
     If your relay is relatively new then give it time. Tor decides which
     relays it uses heuristically based on reports from Bandwidth
     Authorities. These authorities take measurements of your relay's
     capacity and, over time, directs more traffic there until it reaches
     an optimal load. The lifecycle of a new relay is explained in more
01f0dd22
     depth in <a href="https://blog.torproject.org/blog/lifecycle-of-a-new-relay">
     this blog post</a>.
     </p>
     <p>
13b97478
     If you've been running a relay for a while and still having issues
01f0dd22
     then try asking on the <a href=
     "https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-relays/">
13b97478
     tor-relays list</a>.
01f0dd22
     </p>
2b63e88f
 
01f0dd22
     <hr>
 
a756dc11
     <a id="IDontHaveAStaticIP"></a>
5e8d08ab
     <h3><a class="anchor" href="#IDontHaveAStaticIP">Can I run a Tor relay
     using a dynamic IP address?</a></h3>
a756dc11
 
     <p>
d7568ec6
     Tor can handle relays with dynamic IP addresses just fine. Just leave the
     "Address" line in your <a href="#torrc">torrc</a> blank, and Tor will guess.
a756dc11
     </p>
 
     <hr>
 
8ba2d867
     <a id="IPv6Relay"></a>
     <h3><a class="anchor" href="#IPv6Relay">Can I use IPv6 on my relay?</a></h3>
 
     <p>
5e8d08ab
     Tor has <a href="<wiki>org/roadmaps/Tor/IPv6Features">partial</a> support
     for IPv6 and we encourage every relay operator to
     <a href="<wiki>TorRelayGuide#IPv6">enable IPv6 functionality</a> in their
d7568ec6
     <a href="#torrc">torrc</a> configuration files when IPv6 connectivity is
     available. For the time being Tor will require IPv4 addresses on relays,
     you can not run a Tor relay on a host with IPv6 addresses only.
8ba2d867
     </p>
 
     <hr>
a756dc11
     <a id="PortscannedMore"></a>
13b97478
     <h3><a class="anchor" href="#PortscannedMore">Why do I get portscanned
a756dc11
     more often when I run a Tor relay?</a></h3>
 
     <p>
13b97478
     If you allow exit connections, some services that people connect to
     from your relay will connect back to collect more information about you.
     For example, some IRC servers connect back to your identd port to record
     which user made the connection. (This doesn't really work for them,
     because Tor doesn't know this information, but they try anyway.) Also,
     users exiting from you might attract the attention of other users on the
     IRC server, website, etc. who want to know more about the host they're
     relaying through.
a756dc11
     </p>
     <p>
13b97478
     Another reason is that groups who scan for open proxies on the Internet
     have learned that sometimes Tor relays expose their socks port to the
     world. We recommend that you bind your socksport to local networks only.
a756dc11
     </p>
     <p>
5e8d08ab
     In any case, you need to keep up to date with your security. See this
     <a href="https://trac.torproject.org/projects/tor/wiki/doc/OperationalSecurity">
     article on operational security for Tor relays</a> for more suggestions.
a756dc11
     </p>
 
     <hr>
 
2b63e88f
     <a id="HighCapacityConnection"></a>
5e8d08ab
     <h3><a class="anchor" href="#HighCapacityConnection">How can I get Tor to
     fully make use of my high capacity connection?</a></h3>
2b63e88f
 
a043d47b
     <p>
5e8d08ab
     See <a href="http://archives.seul.org/or/relays/Aug-2010/msg00034.html">
     this tor-relays thread</a>.
a043d47b
     </p>
2b63e88f
 
     <hr>
 
2a9aaa80
     <a id="RelayFlexible"></a>
5e8d08ab
     <h3><a class="anchor" href="#RelayFlexible">How stable does my relay need
     to be?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
     We aim to make setting up a Tor relay easy and convenient:
     </p>
0b68c29c
 
2a9aaa80
     <ul>
8b81e16d
     <li>It's fine if the relay goes offline sometimes. The directories
     notice this quickly and stop advertising the relay. Just try to make
     sure it's not too often, since connections using the relay when it
     disconnects will break.
     </li>
5e8d08ab
     <li>Each Tor relay has an <a href="#ExitPolicies">exit policy</a> that
     specifies what sort of outbound connections are allowed or refused from
     that relay. If you are uncomfortable allowing people to exit from your
     relay, you can set it up to only allow connections to other Tor relays.
2a9aaa80
     </li>
     <li>Your relay will passively estimate and advertise its recent
5e8d08ab
     bandwidth capacity, so high-bandwidth relays will attract more users than
     low-bandwidth ones. Therefore having low-bandwidth relays is useful too.
2a9aaa80
     </li>
     </ul>
0b68c29c
 
ed5ac546
     <hr>
2b63e88f
 
da4a6f08
     <a id="OutgoingFirewall"></a>
b75b913d
     <h3><a class="anchor" href="#OutgoingFirewall">How should I configure
3b4bfbda
     the outgoing filters on my relay?</a></h3>
da4a6f08
 
     <p>
3b4bfbda
     All <em>outgoing</em> connections must be allowed, so that each relay can 
     communicate with every other relay.
da4a6f08
     </p>
     <p>
3b4bfbda
     In many jurisdictions, Tor relay operators are legally protected by the 
     same <em>common carrier</em> regulations that prevent internet service 
     providers from being held liable for third-party content that passes 
     through their network. Exit relays that filter some traffic would 
     likely forfeit those protections. 
da4a6f08
     </p>
     <p>
3b4bfbda
     Tor promotes free network access without interference. 
     Exit relays must not filter the traffic 
     that passes through them to the internet. 
     Exit relays found to be filtering traffic will get the <a 
da4a6f08
     href="#WhatIsTheBadExitFlag">BadExit</a> flag once detected.
     </p>
 
     <hr>
 
a756dc11
     <a id="BandwidthShaping"></a>
13b97478
     <h3><a class="anchor" href="#BandwidthShaping">What bandwidth shaping
a756dc11
     options are available to Tor relays?</a></h3>
 
     <p>
d7568ec6
     There are two options you can add to your <a href="#torrc">torrc</a> file:
a756dc11
     </p>
     <ul>
     <li>
13b97478
     BandwidthRate is the maximum long-term bandwidth allowed (bytes per
4a938356
     second). For example, you might want to choose "BandwidthRate 10 MBytes"
     for 10 megabytes per second (a fast connection), or "BandwidthRate 500
35be4c37
     KBytes" for 500 kilobytes per second (a decent cable connection).
5300884f
     The minimum BandwidthRate setting is 75 kilobytes per second.
a756dc11
     </li>
     <li>
13b97478
     BandwidthBurst is a pool of bytes used to fulfill requests during
     short periods of traffic above BandwidthRate but still keeps the
     average over a long period to BandwidthRate. A low Rate but a high
     Burst enforces a long-term average while still allowing more traffic
     during peak times if the average hasn't been reached lately. For example,
4a938356
     if you choose "BandwidthBurst 500 KBytes" and also use that for your
     BandwidthRate, then you will never use more than 500 kilobytes per second;
     but if you choose a higher BandwidthBurst (like 5 MBytes), it will allow
a756dc11
     more bytes through until the pool is empty.
     </li>
     </ul>
     <p>
13b97478
     If you have an asymmetric connection (upload less than download) such
     as a cable modem, you should set BandwidthRate to less than your smaller
     bandwidth (Usually that's the upload bandwidth). (Otherwise, you could
     drop many packets during periods of maximum bandwidth usage -- you may
     need to experiment with which values make your connection comfortable.)
     Then set BandwidthBurst to the same as BandwidthRate.
a756dc11
     </p>
     <p>
13b97478
     Linux-based Tor nodes have another option at their disposal: they can
     prioritize Tor traffic below other traffic on their machine, so that
5e8d08ab
     their own personal traffic is not impacted by Tor load. A
     <a href="https://gitweb.torproject.org/tor.git/tree/contrib/operator-tools/linux-tor-prio.sh">
     script to do this</a> can be found in the Tor source distribution's contrib
13b97478
     directory.
a756dc11
     </p>
     <p>
13b97478
     Additionally, there are hibernation options where you can tell Tor to
     only serve a certain amount of bandwidth per time period (such as 100
     GB per month). These are covered in the <a
     href="#LimitTotalBandwidth">hibernation entry</a> below.
a756dc11
     </p>
     <p>
4a938356
     Note that BandwidthRate and BandwidthBurst are in <b>Bytes</b>, not Bits.
a756dc11
     </p>
 
     <hr>
 
     <a id="LimitTotalBandwidth"></a>
13b97478
     <h3><a class="anchor" href="#LimitTotalBandwidth">How can I limit the
a756dc11
     total amount of bandwidth used by my Tor relay?</a></h3>
     <p>
d7568ec6
     The accounting options in the <a href="#torrc">torrc</a> file allow you to specify the maximum
13b97478
     amount of bytes your relay uses for a time period.
a756dc11
     </p>
     <pre>
     AccountingStart day week month [day] HH:MM
     </pre>
     <p>
13b97478
     This specifies when the accounting should reset. For instance, to setup
     a total amount of bytes served for a week (that resets every Wednesday
     at 10:00am), you would use:
a756dc11
     </p>
     <pre>
     AccountingStart week 3 10:00
f97d8cfe
     AccountingMax 500 GBytes
a756dc11
     </pre>
     <p>
13b97478
     This specifies the maximum amount of data your relay will send during an
     accounting period, and the maximum amount of data your relay will receive
     during an account period. When the accounting period resets (from
a756dc11
     AccountingStart), then the counters for AccountingMax are reset to 0.
     </p>
     <p>
f97d8cfe
     Example: Let's say you want to allow 50 GB of traffic every day in each
13b97478
     direction and the accounting should reset at noon each day:
a756dc11
     </p>
     <pre>
     AccountingStart day 12:00
f97d8cfe
     AccountingMax 50 GBytes
a756dc11
     </pre>
     <p>
13b97478
     Note that your relay won't wake up exactly at the beginning of each
     accounting period. It will keep track of how quickly it used its
     quota in the last period, and choose a random point in the new interval
     to wake up. This way we avoid having hundreds of relays working at the
     beginning of each month but none still up by the end.
a756dc11
     </p>
     <p>
13b97478
     If you have only a small amount of bandwidth to donate compared to your
     connection speed, we recommend you use daily accounting, so you don't
     end up using your entire monthly quota in the first day. Just divide
     your monthly amount by 30. You might also consider rate limiting to
     spread your usefulness over more of the day: if you want to offer X GB
4a938356
     in each direction, you could set your RelayBandwidthRate to 20*X KBytes.
f97d8cfe
     For example,
5e8d08ab
     if you have 50 GB to offer each way, you might set your RelayBandwidthRate
     to 1000 KBytes: this way your relay will always be useful for at least half
     of each day.
a756dc11
     </p>
5e8d08ab
 
4a938356
     <pre>
     AccountingStart day 0:00
     AccountingMax 50 GBytes
     RelayBandwidthRate 1000 KBytes
     RelayBandwidthBurst 5000 KBytes # allow higher bursts but maintain average
     </pre>
efb55037
 
     <hr>
 
     <a id="RelayWritesMoreThanItReads"></a>
13b97478
     <h3><a class="anchor" href="#RelayWritesMoreThanItReads">Why does my relay
ac519bf8
     write more bytes onto the network than it reads?</a></h3>
efb55037
 
13b97478
     <p>You're right, for the most part a byte into your Tor relay means a
efb55037
     byte out, and vice versa. But there are a few exceptions:</p>
 
13b97478
     <p>If you open your DirPort, then Tor clients will ask you for a copy of
     the directory. The request they make (an HTTP GET) is quite small, and the
     response is sometimes quite large. This probably accounts for most of the
efb55037
     difference between your "write" byte count and your "read" byte count.</p>
 
13b97478
     <p>Another minor exception shows up when you operate as an exit node, and
     you read a few bytes from an exit connection (for example, an instant
     messaging or ssh connection) and wrap it up into an entire 512 byte cell
efb55037
     for transport through the Tor network.</p>
 
     <hr>
 
     <a id="Hibernation"></a>
13b97478
     <h3><a class="anchor" href="#Hibernation">Why can I not browse anymore
ac519bf8
     after limiting bandwidth on my Tor relay?</a></h3>
efb55037
 
13b97478
     <p>The parameters assigned in the <a
     href="#LimitTotalBandwidth">AccountingMax</a> and <a
     href="#BandwidthShaping">BandwidthRate</a> apply to both client and
     relay functions of the Tor process. Thus you may find that you are unable
     to browse as soon as your Tor goes into hibernation, signaled by this
efb55037
     entry in the log:</p>
 
13b97478
     <pre>Bandwidth soft limit reached; commencing hibernation. No new
efb55037
     connections will be accepted</pre>
 
13b97478
     <p>The solution is to run two Tor processes - one relay and one client,
     each with its own config. One way to do this (if you are starting from a
efb55037
     working relay setup) is as follows:</p>
 
     <ul>
d7568ec6
         <li>In the relay Tor <a href="#torrc">torrc</a> file, simply set the SocksPort to 0.</li>
13b97478
         <li>Create a new client torrc file from the torrc.sample and ensure
         it uses a different log file from the relay. One naming convention
efb55037
         may be torrc.client and torrc.relay.</li>
13b97478
         <li>Modify the Tor client and relay startup scripts to include
efb55037
         '-f /path/to/correct/torrc'.</li>
ecaa2485
         <li>In Linux/BSD/Mac OS X, changing the startup scripts to Tor.client
efb55037
         and Tor.relay may make separation of configs easier.</li>
     </ul>
 
a756dc11
     <hr>
0b68c29c
 
2a9aaa80
     <a id="ExitPolicies"></a>
37855015
     <h3><a class="anchor" href="#ExitPolicies">I'd run a relay, but I
 don't want to deal with abuse issues.</a></h3>
0b68c29c
 
2a9aaa80
     <p>
     Great. That's exactly why we implemented exit policies.
     </p>
0b68c29c
 
2a9aaa80
     <p>
     Each Tor relay has an exit policy that specifies what sort of
5e8d08ab
     outbound connections are allowed or refused from that relay. The exit
2a9aaa80
     policies are propagated to Tor clients via the directory, so clients
     will automatically avoid picking exit relays that would refuse to
     exit to their intended destination. This way each relay can decide
536ce692
     the services, hosts, and networks it wants to allow connections to,
5e8d08ab
     based on abuse potential and its own situation. Read the FAQ entry on
     <a href="<page docs/faq-abuse>#TypicalAbuses">issues you might encounter</a>
0b68c29c
     if you use the default exit policy, and then read Mike Perry's
e233c281
     <a href="<blog>tips-running-exit-node">tips
2a9aaa80
     for running an exit node with minimal harassment</a>.
     </p>
0b68c29c
 
2a9aaa80
     <p>
0b68c29c
     The default exit policy allows access to many popular services
5e8d08ab
     (e.g. web browsing), but <a href="#DefaultExitPorts">restricts</a> some due
     to abuse potential (e.g. mail) and some since the Tor network can't handle
     the load (e.g. default file-sharing ports). You can change your exit policy
     by editing your <a href="<page docs/faq>#torrc">torrc</a> file. If you want
     to avoid most if not all abuse potential, set it to <var>"reject *:*"</var>.
     This setting means that your relay will be used for relaying traffic inside
     the Tor network, but not for connections to external websites or other
     services.
     </p>
 
     <p>
     If you do allow any exit connections, make sure name resolution works
2a9aaa80
     (that is, your computer can resolve Internet addresses correctly).
5e8d08ab
     If there are any resources that your computer can't reach (for example,
2a9aaa80
     you are behind a restrictive firewall or content filter), please
5e8d08ab
     explicitly reject them in your exit policy &mdash; otherwise Tor users
2a9aaa80
     will be impacted too.
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
a10d8980
     <a id="PackagedTor"></a>
2b63e88f
     <h3><a class="anchor" href="#PackagedTor">Should I install Tor from my
a10d8980
     package manager, or build from source?</a></h3>
     <p>
2b63e88f
     If you're using Debian or Ubuntu especially, there are a number of benefits
     to installing Tor from the <a
f26db68a
     href="<page docs/debian>">Tor Project's repository</a>.
a10d8980
     </p>
     <ul>
       <li>
f26db68a
       Your ulimit -n gets set to 32768 &mdash; high enough for Tor to
       keep open all the connections it needs.
a10d8980
       </li>
       <li>
2b63e88f
       A user profile is created just for Tor, so Tor doesn't need to run as
a10d8980
       root.
       </li>
       <li>
       An init script is included so that Tor runs at boot.
       </li>
       <li>
2b63e88f
       Tor runs with --verify-config, so that most problems with your
       config file get caught.
a10d8980
       </li>
       <li>
       Tor can bind to low level ports, then drop privileges.
       </li>
     </ul>
 
     <hr>
 
5bdf9aee
     <a id="WhatIsTheBadExitFlag"></a>
13b97478
     <h3><a class="anchor" href="#WhatIsTheBadExitFlag">What is the
5bdf9aee
     BadExit flag?</a></h3>
 
13b97478
     <p>When an exit is misconfigured or malicious it's assigned the BadExit
     flag. This tells Tor to avoid exiting through that relay. In effect,
5bdf9aee
     relays with this flag become non-exits.</p>
 
     <hr>
 
     <a id="IGotTheBadExitFlagWhyDidThatHappen"></a>
     <h3><a class="anchor" href="#IGotTheBadExitFlagWhyDidThatHappen">I got
     the BadExit flag why did that happen?</a></h3>
 
13b97478
     <p>If you got this flag then we either discovered a problem or suspicious
5e8d08ab
     activity when routing traffic through your exit and weren't able to contact
     you. Please reach out to the
     <a href="mailto:bad-relays@lists.torproject.org">bad-relays team</a>
4913c11c
     so we can sort out the issue.
     </p>
5bdf9aee
 
     <hr>
 
     <a id="MyRelayRecentlyGotTheGuardFlagAndTrafficDroppedByHalf"></a>
5e8d08ab
     <h3><a class="anchor" href="#MyRelayRecentlyGotTheGuardFlagAndTrafficDroppedByHalf">
     My relay recently got the Guard flag and traffic dropped by half.</a></h3>
5bdf9aee
     <p>
13b97478
     Since it's now a guard, clients are using it less in other positions, but
     not many clients have rotated their existing guards out to use it as a
     guard yet. Read more details in this <a
     href="https://blog.torproject.org/blog/lifecycle-of-a-new-relay">blog
     post</a> or in <a href="http://freehaven.net/anonbib/#wpes12-cogs">Changing
     of the Guards: A Framework for Understanding and Improving Entry Guard
5bdf9aee
     Selection in Tor</a>.
     </p>
2b63e88f
 
5bdf9aee
     <hr>
 
2a9aaa80
     <a id="RelayOrBridge"></a>
5e8d08ab
     <h3><a class="anchor" href="#RelayOrBridge">Should I be a normal relay or
     bridge relay?</a></h3>
0b68c29c
 
5e8d08ab
     <p><a href="<page docs/bridges>">Bridge relays</a> (or "bridges" for short)
     are <a href="<wiki>TorRelayGuide">Tor relays</a> that aren't listed in the
     public Tor directory. That means that ISPs or governments trying to block
     access to the Tor network can't simply block all bridges.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>Being a normal relay vs being a bridge relay is almost the same
5e8d08ab
     configuration: it's just a matter of whether your relay is listed publicly
     or not.
2a9aaa80
     </p>
0b68c29c
 
6c03dfec
     <p>
5e8d08ab
     So bridges are useful a) for Tor users in oppressive regimes, and b) for
     people who want an extra layer of security because they're worried somebody
     will recognize that it's a public Tor relay IP address they're contacting.
2a9aaa80
     </p>
0b68c29c
 
6c03dfec
     <p>
5e8d08ab
     Several countries, including China and Iran, have found ways to detect and
     block connections to Tor bridges.
c21cddf4
     <a href="<page docs/pluggable-transports>">Obfsproxy</a> bridges address
55d1a604
     this by adding another layer of obfuscation.
6c03dfec
     </p>
 
5e8d08ab
     <p>So should you run a normal relay or bridge relay? If you have lots
     of bandwidth, you should definitely run a normal relay. If you're willing
     to <a href="#ExitPolicies">be an exit</a>, you should definitely run an
     exit relay, since we need more exits. If you can't be an exit and only have
     a little bit of bandwidth, setup an
b42016e8
     <a href="<page docs/pluggable-transports>#operator">obfs4 bridge</a>.
     Thanks for volunteering!
2a9aaa80
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
5e8d08ab
     <a id="UpgradeOrMove"></a>
     <h3><a class="anchor" href="#UpgradeOrMove">I want to upgrade/move my relay.
     How do I keep the same key?</a></h3>
 
     <p>
     When upgrading your Tor relay, or moving it on a different computer, the
     important part is to keep the same identity keys (stored in
     "keys/ed25519_master_id_secret_key" and "keys/secret_id_key" in your
     DataDirectory). Keeping backups of the identity keys so you can restore
     a relay in the future is the recommended way to ensure the reputation of
     the relay won't be wasted.
     </p>
 
     <p>
     This means that if you're upgrading your Tor relay and you keep the same
d7568ec6
     <a href="#torrc">torrc and the same DataDirectory</a>, then the upgrade should just work and
5e8d08ab
     your relay will keep using the same key. If you need to pick a new
     DataDirectory, be sure to copy your old
     keys/ed25519_master_id_secret_key and keys/secret_id_key over.
     </p>
 
     <p>
     Note: As of Tor 0.2.7 we are using new generation identities for relays
     based on ed25519 elliptic curve cryptography. Eventually they will
     replace the old RSA identities, but that will happen in time, to ensure
     compatibility with older versions. Until then, each relay will have both
     an ed25519 identity (identity key file:
     keys/ed25519_master_id_secret_key) and a RSA identity (identity key
     file: keys/secret_id_key). You need to copy / backup both of them in
     order to restore your relay, change your DataDirectory or migrate the
     relay on a new computer.
     </p>
 
     <hr>
 
     <a id="OfflineED25519"></a>
     <h3><a class="anchor" href="#OfflineED25519">How do offline ed25519
     identity keys work? What do I need to know?</a></h3>
 
     <p>
     As of Tor 0.2.7 offline ed25519 identity keys are supported. In simple
     words, it works like this:
     </p>
 
     <ul>
     <li>there is a master ed25519 identity secret key file named
     "ed25519_master_id_secret_key". This is the most important one, so make
     sure you keep a backup in a secure place - the file is sensitive and
     should be protected. Tor could encrypt it for you if you generate it
     manually and enter a password when asked.</li>
     <li>a medium term signing key named "ed25519_signing_secret_key" is
     generated for Tor to use. Also, a certificate is generated named
     "ed25519_signing_cert" which is signed by the master identity secret key
     and confirms that the medium term signing key is valid for a certain
     period of time. The default validity is 30 days, but this can be
     customized by setting "SigningKeyLifetime N days|weeks|months" in
d7568ec6
     <a href="#torrc">torrc</a>.</li>
5e8d08ab
     <li>there is also a master public key named
     "ed25519_master_id_public_key, which is the actual identity of the relay
     advertised in the network. This one is not sensitive and can be easily
     computed from "ed5519_master_id_secret_key".</li>
     </ul>
 
     <p>
     Tor will only need access to the medium term signing key and certificate
     as long as they are valid, so the master identity secret key can be kept
     outside DataDirectory/keys, on a storage media or a different computer.
     You'll have to manually renew the medium term signing key and
     certificate before they expire otherwise the Tor process on the relay
     will exit upon expiration.
     </p>
 
     <p>
     This feature is optional, you don't need to use it unless you want to.
     If you want your relay to run unattended for longer time without having
     to manually do the medium term signing key renewal on regular basis,
     best to leave the master identity secret key in DataDirectory/keys, just
     make a backup in case you'll need to reinstall it.  If you want to use
     this feature, you can consult our
     <a href="https://trac.torproject.org/projects/tor/wiki/doc/TorRelaySecurity/OfflineKeys">
     more detailed guide</a> on the topic.
     </p>
 
     <hr>
 
     <a id="NTService"></a>
     <h3><a class="anchor" href="#NTService">How do I run my Tor relay as an NT
     service?</a></h3>
 
     <p>
     You can run Tor as a service on all versions of Windows except Windows
     95/98/ME.
     </p>
 
     <p>
     If you've already configured your Tor to be a relay, please note that when
     you enable Tor as a service, it will use a different DatagDirectory, and
     thus will generate a different key. If you want to keep using the old key,
     see the Upgrading your Tor relay FAQ entry for how to restore the old
     identity key.
     </p>
 
     <p>
     To install Tor as a service, you can simply run:
     </p>
 
     <pre>
     tor --service install
     </pre>
 
     <p>
     A service called Tor Win32 Service will be installed and started. This
     service will also automatically start every time Windows boots, unless
     you change the Start-up type. An easy way to check the status of Tor,
     start or stop the service, and change the start-up type is by running
     services.msc and finding the Tor service in the list of currently
     installed services.
     </p>
 
     <p>
     Optionally, you can specify additional options for the Tor service using
     the -options argument. For example, if you want Tor to use C:\tor\torrc,
d7568ec6
     instead of the default <a href="#torrc">torrc</a>, and open a control port on port 9151, you
5e8d08ab
     would run:
     </p>
 
     <pre>
     tor --service install -options -f C:\tor\torrc ControlPort 9151
     </pre>
 
     <p>
     You can also start or stop the Tor service from the command line by typing:
     </p>
 
     <pre>
     tor --service start
     </pre>
 
     <p>
     or
     </p>
 
     <pre>
     tor --service stop
     </pre>
 
     <p>
     To remove the Tor service, you can run the following command:
     </p>
 
     <pre>
     tor --service remove
     </pre>
 
     <p>
     If you are running Tor as a service and you want to uninstall Tor entirely,
     be sure to run the service removal command (shown above) first before
     running the uninstaller from "Add/Remove Programs". The uninstaller is
     currently not capable of removing the active service.
     </p>
 
     <hr>
 
     <a id="VirtualServer"></a>
     <h3><a class="anchor" href="#VirtualServer">Can I run a Tor relay from my
     virtual server account?</a></h3>
 
     <p>
     Some ISPs are selling "vserver" accounts that provide what they call a
     virtual server -- you can't actually interact with the hardware, and
     they can artificially limit certain resources such as the number of file
     descriptors you can open at once. Competent vserver admins are able to
     configure your server to not hit these limits. For example, in SWSoft's
     Virtuozzo, investigate /proc/user_beancounters. Look for "failcnt" in
     tcpsndbuf, tcprecvbuf, numothersock, and othersockbuf. Ask for these to
     be increased accordingly. Xen, Virtual Box and VMware virtual servers have
     no such limits normally.
     </p>
 
     <p>
     If the vserver admin will not increase system limits another option is
     to reduce the memory allocated to the send and receive buffers on TCP
     connections Tor uses. An experimental feature to constrain socket buffers
     has recently been added. If your version of Tor supports it, set
     "ConstrainedSockets 1" in your configuration. See the tor man page for
     additional details about this option.
     </p>
 
     <p>
     Unfortunately, since Tor currently requires you to be able to connect to
     all the other Tor relays, we need you to be able to use at least 1024 file
     descriptors. This means we can't make use of Tor relays that are crippled
     in this way.
     </p>
f549ee31
 
5e8d08ab
     <p>
     We hope to fix this in the future, once we know how to build a Tor network
     with restricted topologies -- that is, where each node connects to only a
     few other nodes. But this is still a long way off.
     </p>
9b92b660
 
f549ee31
     <hr>
 
5e8d08ab
     <a id="MultipleRelays"></a>
     <h3><a class="anchor" href="#MultipleRelays">I want to run more than one
     relay.</a></h3>
 
     <p>
     Great. If you want to run several relays to donate more to the network,
     we're happy with that. But please don't run more than a few dozen on
     the same network, since part of the goal of the Tor network is dispersal
     and diversity.
     </p>
 
     <p>
     If you do decide to run more than one relay, please set the "MyFamily"
     config option in the <a href="#torrc">torrc</a> of each relay, listing
     all the relays (comma-separated) that are under your control:
     </p>
 
     <pre>
21be8103
     MyFamily $fingerprint1,$fingerprint2,$fingerprint3
5e8d08ab
     </pre>
21be8103
 
5e8d08ab
     <p>
     where each fingerprint is the 40 character identity fingerprint (without
     spaces).
     </p>
21be8103
 
5e8d08ab
     <p>
     That way clients will know to avoid using more than one of your relays
     in a single circuit. You should set MyFamily if you have administrative
     control of the computers or of their network, even if they're not all in
     the same geographic location.
     </p>
21be8103
 
     <hr>
 
f549ee31
     <a id="WrongIP"></a>
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     <h3><a class="anchor" href="#WrongIP">My relay is picking the wrong
f30e672d
     IP address.</a></h3>
f549ee31
     <p>
5e8d08ab
     Tor guesses its IP address by asking the computer for its hostname, and
     then resolving that hostname. Often people have old entries in their
     <var>/etc/hosts</var> file that point to old IP addresses.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     If that doesn't fix it, you should use the "Address" config option to
     specify the IP you want it to pick. If your computer is behind a NAT and
     it only has an internal IP address, see the following FAQ entry on
     <a href="#RelayFlexible">dynamic IP addresses</a>.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     Also, if you have many addresses, you might also want to set
     "OutboundBindAddress" so external connections come from the IP you intend
     to present to the world.
f549ee31
     </p>
 
     <hr>
 
     <a id="BehindANAT"></a>
     <h3><a class="anchor" href="#BehindANAT">I'm behind a NAT/Firewall.</a></h3>
 
     <p>
5e8d08ab
     See <a href="http://portforward.com/">portforward.com</a> for directions on
     how to port forward with your NAT/router device.
     </p>
 
     <p>
     If your relay is running on a internal net you need to setup port
     forwarding. Forwarding TCP connections is system dependent but the
     firewalled-clients FAQ entry offers some examples on how to do this.
     </p>
 
     <p>
     Also, here's an example of how you would do this on GNU/Linux if you're
     using iptables:
     </p>
 
     <pre>
     /sbin/iptables -A INPUT -i eth0 -p tcp --destination-port 9001 -j ACCEPT
     </pre>
 
     <p>
     You may have to change "eth0" if you have a different external interface
     (the one connected to the Internet). Chances are you have only one (except
     the loopback) so it shouldn't be too hard to figure out.
f549ee31
     </p>
     <hr>
 
2a9aaa80
     <a id="RelayMemory"></a>
5e8d08ab
     <h3><a class="anchor" href="#RelayMemory">Why is my Tor relay using so much
     memory?</a></h3>
0b68c29c
 
5e8d08ab
     <p>If your Tor relay is using more memory than you'd like, here are some
2a9aaa80
     tips for reducing its footprint:
     </p>
0b68c29c
 
2a9aaa80
     <ol>
     <li>If you're on Linux, you may be encountering memory fragmentation
5e8d08ab
     bugs in glibc's malloc implementation. That is, when Tor releases memory
     back to the system, the pieces of memory are fragmented so they're hard
     to reuse. The Tor tarball ships with OpenBSD's malloc implementation,
     which doesn't have as many fragmentation bugs (but the tradeoff is higher
     CPU load). You can tell Tor to use this malloc implementation instead:
     <tt>./configure --enable-openbsd-malloc</tt>
     </li>
     <li>If you're running a fast relay, meaning you have many TLS connections
2a9aaa80
     open, you are probably losing a lot of memory to OpenSSL's internal
5e8d08ab
     buffers (38KB+ per socket). We've patched OpenSSL to
     <a href="https://lists.torproject.org/pipermail/tor-dev/2008-June/001519.html">
     release unused buffer memory more aggressively</a>. If you update to
     OpenSSL 1.0.0 or newer, Tor's build process will automatically recognize
     and use this feature.
     </li>
2a9aaa80
     <li>If you still can't handle the memory load, consider reducing the
5e8d08ab
     amount of bandwidth your relay advertises. Advertising less bandwidth
2a9aaa80
     means you will attract fewer users, so your relay shouldn't grow
     as large. See the <tt>MaxAdvertisedBandwidth</tt> option in the man
5e8d08ab
     page.
     </li>
2a9aaa80
     </ol>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     All of this said, fast Tor relays do use a lot of ram. It is not unusual
2a9aaa80
     for a fast exit relay to use 500-1000 MB of memory.
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
f549ee31
     <a id="BetterAnonymity"></a>
13b97478
     <h3><a class="anchor" href="#BetterAnonymity">Do I get better anonymity
f30e672d
     if I run a relay?</a></h3>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     Yes, you do get better anonymity against some attacks.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     The simplest example is an attacker who owns a small number of Tor relays.
     They will see a connection from you, but they won't be able to know whether
     the connection originated at your computer or was relayed from somebody
     else.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     There are some cases where it doesn't seem to help: if an attacker can
     watch all of your incoming and outgoing traffic, then it's easy for them
     to learn which connections were relayed and which started at you. (In
     this case they still don't know your destinations unless they are watching
     them too, but you're no better off than if you were an ordinary client.)
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     There are also some downsides to running a Tor relay. First, while we
     only have a few hundred relays, the fact that you're running one might
     signal to an attacker that you place a high value on your anonymity.
     Second, there are some more esoteric attacks that are not as
     well-understood or well-tested that involve making use of the knowledge
     that you're running a relay -- for example, an attacker may be able to
     "observe" whether you're sending traffic even if they can't actually watch
     your network, by relaying traffic through your Tor relay and noticing
     changes in traffic timing.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     It is an open research question whether the benefits outweigh the risks.
     A lot of that depends on the attacks you are most worried about. For
     most users, we think it's a smart move.
2a9aaa80
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
5bdf9aee
     <a id="FacingLegalTrouble"></a>
13b97478
     <h3><a class="anchor" href="#FacingLegalTrouble">I'm facing legal
     trouble. How do I prove that my server was a Tor relay at a given
5bdf9aee
     time?</a></h3>
 
     <p><a href="https://exonerator.torproject.org/">
13b97478
     Exonerator</a> is a web service that can check if an IP address was a
5e8d08ab
     relay at a given time. We can also <a href="<page about/contact>">provide a
     signed letter</a> if needed.</p>
5bdf9aee
 
     <hr>
 
e71b1ebb
     <a id="RelayDonations"></a>
2ee01d9b
     <h3><a class="anchor" href="#RelayDonations">Can I donate for a
     relay rather than run my own?</a></h3>
e71b1ebb
 
     <p>
11ab6cce
     Sure! We recommend these non-profit charities that are happy to turn
e71b1ebb
     your donations into better speed and anonymity for the Tor network:
     </p>
5e8d08ab
 
e71b1ebb
     <ul>
     <li><a href="https://www.torservers.net/">torservers.net</a>
     is a German charitable non-profit that runs a wide variety of
11ab6cce
     exit relays worldwide. They also like donations of bandwidth from
     ISPs.</li>
e71b1ebb
     <li><a
37855015
 href="https://www.noisebridge.net/wiki/Noisebridge_Tor">Noisebridge</a>
e71b1ebb
     is a US-based 501(c)(3) non-profit that collects donations and turns
11ab6cce
     them into more US-based exit relay capacity.</li>
     <li><a href="https://nos-oignons.net/">Nos Oignons</a> is a French
     charitable non-profit that runs fast exit relays in France.</li>
6f107e8d
     <li><a href="https://www.dfri.se/donera/?lang=en">DFRI</a> is a
     Swedish non-profit running exit relays.</li>
e71b1ebb
     </ul>
 
     <p>
     These organizations are not the same as <a href="<page
5e8d08ab
     donate/donate>">The Tor Project, Inc</a>, but we consider that a good thing.
     They're run by nice people who are part of the Tor community.
e71b1ebb
     </p>
 
     <p>
     Note that there can be a tradeoff here between anonymity and
5e8d08ab
     performance. The Tor network's anonymity comes in part from diversity,
e71b1ebb
     so if you are in a position to run your own relay, you will be
e53eda17
     improving Tor's anonymity more than by donating. At the same time
     though, economies
e71b1ebb
     of scale for bandwidth mean that combining many small donations into
f0ebea19
     several larger relays is more efficient at improving network
e71b1ebb
     performance. Improving anonymity and improving performance are both
     worthwhile goals, so however you can help is great!
     </p>
 
     <hr>
 
5e8d08ab
     <!-- Leaving in old id to accomodate incoming links. -->
     <a id="TorOnionServices"></a><a id="TorHiddenServices"></a>
     <h2><a class="anchor" href="#TorOnionServices">Tor onion services:</a></h2>
6f107e8d
 
32316cbf
     <a id="AccessOnionServices"></a><a id="AccessHiddenServices"></a>
     <h3><a class="anchor" href="#AccessOnionServices">How do I access
     onion services?</a></h3>
2b63e88f
 
f30e672d
     <p>
32316cbf
     Tor onion services are named with a special top-level domain (TLD)
13b97478
     name in DNS: .onion. Since the .onion TLD is not recognized by the
     official root DNS servers on the Internet, your application will not
     get the response it needs to locate the service. Currently, the Tor
     directory server provides this look-up service; and thus the look-up
     request must get to the Tor network.
f30e672d
     </p>
 
5e8d08ab
     <p>
     Therefore, your application <b>needs</b> to pass the .onion hostname to
     Tor directly. You can't try to resolve it to an IP address, since there
     <i>is</i> no corresponding IP address.
     </p>
2b63e88f
 
13b97478
     <p>
     So, how do you make your application pass the hostname directly to Tor?
     You can't use SOCKS 4, since SOCKS 4 proxies require an IP from the
     client (a web browser is an example of a SOCKS client). Even though
     SOCKS 5 can accept either an IP or a hostname, most applications
     supporting SOCKS 5 try to resolve the name before passing it to the
     SOCKS proxy. SOCKS 4a, however, always accepts a hostname: You'll need
     to use SOCKS 4a.
     </p>
2b63e88f
 
13b97478
     <p>
     Some applications, such as the browsers Mozilla Firefox and Apple's
     Safari, support sending DNS queries to Tor's SOCKS 5 proxy. Most web
     browsers don't support SOCKS 4a very well, though. The workaround is
     to point your web browser at an HTTP proxy, and tell the HTTP proxy
f30e672d
     to speak to Tor with SOCKS 4a. We recommend Polipo as your HTTP proxy.
     </p>
2b63e88f
 
f30e672d
     <p>
13b97478
     For applications that do not support HTTP proxy, and so cannot use
     Polipo, <a href="http://www.freecap.ru/eng/">FreeCap</a> is an
32316cbf
     alternative. When using FreeCap set proxy protocol to SOCKS 5 and under
13b97478
     settings set DNS name resolving to remote. This
     will allow you to use almost any program with Tor without leaking DNS
32316cbf
     lookups and allow those same programs to access onion services.
f30e672d
     </p>
2b63e88f
 
f30e672d
     <p>
13b97478
     See also the <a href="#SocksAndDNS">question on DNS</a>.
2b63e88f
     </p>
 
f30e672d
     <hr>
 
32316cbf
     <a id="ProvideAnOnionService"></a><a id="ProvideAHiddenService"></a>
     <h3><a class="anchor" href="#ProvideAnOnionService">How do I provide an
     onion service?</a></h3>
2b63e88f
 
f30e672d
     <p>
c9be5827
     See the <a href="<page docs/tor-onion-service>">
32316cbf
     official onion service configuration instructions</a>.
f30e672d
     </p>
 
     <hr>
2b63e88f
 
6f107e8d
     <a id="Development"></a>
f87eea1a
     <h2><a class="anchor" href="#Development">Development:</a></h2>
6f107e8d
 
2fd90486
     <a id="VersionNumbers"></a>
13b97478
     <h3><a class="anchor" href="#VersionNumbers">What do these weird
2fd90486
     version numbers mean?</a></h3>
 
     <p>
13b97478
     Versions of Tor before 0.1.0 used a strange and hard-to-explain
991262e9
     version scheme. Let's forget about those.
2fd90486
     </p>
     <p>
13b97478
     Starting with 0.1.0, versions all look like this:
     MAJOR.MINOR.MICRO(.PATCHLEVEL)(-TAG). The stuff in parenthesis is
     optional. MAJOR, MINOR, MICRO, and PATCHLEVEL are all numbers. Only one
     release is ever made with any given set of these version numbers. The
     TAG lets you know how stable we think the release is: "alpha" is pretty
     unstable; "rc" is a release candidate; and no tag at all means that we
     have a final release. If the tag ends with "-cvs", you're looking at
     a development snapshot that came after a given release.
2fd90486
     </p>
     <p>
13b97478
     So for example, we might start a development branch with (say)
     0.1.1.1-alpha. The patchlevel increments consistently as the status
     tag changes, for example, as in: 0.1.1.2-alpha, 0.1.1.3-alpha,
     0.1.1.4-rc, 0.1.1.5-rc, etc. Eventually, we would release 0.1.1.6.
2fd90486
     The next stable release would be 0.1.1.7.
     </p>
     <p>
13b97478
     Why do we do it like this? Because every release has a unique
     version number, it is easy for tools like package manager to tell
     which release is newer than another. The tag makes it easy for users
     to tell how stable the release is likely to be.
2fd90486
     </p>
 
     <hr>
 
     <a id="PrivateTorNetwork"></a>
13b97478
     <h3><a class="anchor" href="#PrivateTorNetwork">How do I set up my
2fd90486
     own private Tor network?</a></h3>
2b63e88f
 
2fd90486
     <p>
13b97478
     If you want to experiment locally with your own network, or you're
     cut off from the Internet and want to be able to mess with Tor still,
     then you may want to set up your own separate Tor network.
2fd90486
     </p>
     <p>
13b97478
     To set up your own Tor network, you need to run your own authoritative
     directory servers, and your clients and relays must be configured so
     they know about your directory servers rather than the default public
     ones.
2fd90486
     </p>
     <p>
13b97478
     Apart from the somewhat tedious method of manually configuring a couple
     of directory authorities, relays and clients there are two separate
     tools that could help. One is Chutney, the other is Shadow.
2fd90486
     </p>
     <p>
13b97478
     <a href="https://gitweb.torproject.org/chutney.git">Chutney</a> is a
     tool for configuring, controlling and running tests on a
     testing Tor network. It requires that you have Tor and Python (2.5 or
2fd90486
     later) installed on your system. You can use Chutney to create a testing
d7568ec6
     network by generating Tor configuration files (<a href="#torrc">torrc</a>)
     and necessary keys (for the directory authorities). Then you can let
     Chutney start your Tor authorities, relays and clients and wait for the
     network to bootstrap.
13b97478
     Finally, you can have Chutney run tests on your network to see which
     things work and which do not. Chutney is typically used for running a
     testing network with about 10 instances of Tor. Every instance of Tor
     binds to one or two ports on localhost (127.0.0.1) and all Tor
     communication is done over the loopback interface. The <a
1700644e
     href="https://gitweb.torproject.org/chutney.git/tree/README">Chutney
13b97478
     README</a> is a good starting point for getting it up and running.
     </p>
     <p>
     <a href="https://github.com/shadow/shadow">Shadow</a> is a network
     simulator that can run Tor through its Scallion plug-in. Although
     it's typically used for running load and performance tests on
     substantially larger Tor test networks than what's feasible with
     Chutney, it also makes for an excellent debugging tool since you can
     run completely deterministic experiments. A large Shadow network is on
     the size of thousands of instances of Tor, and you can run experiments
     out of the box using one of Shadow's several included scallion experiment
     configurations. Shadow can be run on any linux machine without root,
     and can also run on EC2 using a pre-configured image. Also, Shadow
     controls the time of the simulation with the effect that
     time-consuming tests can be done more efficiently than in an
     ordinary testing network. The <a
     href="https://github.com/shadow/shadow/wiki">Shadow wiki</a> and
     <a href="http://shadow.github.io/">Shadow website</a> are
     good places to get started.
2fd90486
     </p>
 
     <hr>
 
e105e056
     <a id="UseTorWithJava"></a>
13b97478
     <h3><a class="anchor" href="#UseTorWithJava">How can I make my Java
2fd90486
     program use the Tor Network?</a></h3>
 
     <p>
13b97478
     The newest versions of Java now have SOCKS4/5 support built in.
     Unfortunately, the SOCKS interface is not very well documented and
     may still leak your DNS lookups. The safest way to use Tor is to
     interface the SOCKS protocol directly or go through an application-level
     proxy that speaks SOCKS4a. For an example and libraries that implement
     the SOCKS4a connection, go to Joe Foley's TorLib in the <a
     href="http://web.mit.edu/foley/www/TinFoil/">TinFoil Project</a>.
2fd90486
     </p>
 
     <p>
13b97478
     A fully Java implementation of the Tor client is now available as <a
     href="http://www.subgraph.com/orchid.html">Orchid</a>. We still consider
     Orchid to be experimental, so use with care.
2fd90486
     </p>
 
     <hr>
 
 
     <a id="WhatIsLibevent"></a>
     <h3><a class="anchor" href="#WhatIsLibevent">What is Libevent?</a></h3>
2b63e88f
 
2fd90486
     <p>
13b97478
     When you want to deal with a bunch of net connections at once, you
     have a few options:
2fd90486
     </p>
     <p>
13b97478
     One is multithreading: you have a separate micro-program inside the
     main program for each net connection that reads and writes to the
     connection as needed.This, performance-wise, sucks.
2fd90486
     </p>
     <p>
13b97478
     Another is asynchronous network programming: you have a single main
     program that finds out when various net connections are ready to
2fd90486
     read/write, and acts accordingly.
     </p>
     <p>
13b97478
     The problem is that the oldest ways to find out when net connections
     are ready to read/write, suck. And the newest ways are finally fast,
     but are not available on all platforms.
2fd90486
     </p>
     <p>
13b97478
     This is where Libevent comes in and wraps all these ways to find
     out whether net connections are ready to read/write, so that Tor
     (and other programs) can use the fastest one that your platform
     supports, but can still work on older platforms (these methods are
     all different depending on the platorm) So Libevent presents a
     consistent and fast interface to select, poll, kqueue, epoll,
     /dev/poll, and windows select.
2fd90486
     </p>
     <p>
13b97478
     However, On the the Win32 platform (by Microsoft) the only good
     way to do fast IO on windows with hundreds of sockets is using
     overlapped IO, which is grossly unlike every other BSD sockets
     interface.
2fd90486
     </p>
13b97478
     <p>Libevent has <a href="http://www.monkey.org/~provos/libevent/">its
2fd90486
     own website</a>.
     </p>
     <hr>
 
     <a id="MyNewFeature"></a>
13b97478
     <h3><a class="anchor" href="#MyNewFeature">What do I need to do to get
2fd90486
     a new feature into Tor?</a></h3>
2b63e88f
 
2fd90486
     <p>
13b97478
     For a new feature to go into Tor, it needs to be designed (explain what
     you think Tor should do), argued to be secure (explain why it's better
     or at least as good as what Tor does now), specified (explained at the
     byte level at approximately the level of detail in tor-spec.txt), and
     implemented (done in software).
2fd90486
     </p>
 
     <p>
13b97478
     You probably shouldn't count on other people doing all of these steps
     for you: people who are skilled enough to do this stuff generally
2fd90486
     have their own favorite feature requests.
     </p>
 
     <hr>
 
6f107e8d
     <a id="AnonymityAndSecurity"></a>
5e8d08ab
     <h2><a class="anchor" href="#AnonymityAndSecurity">Anonymity And Security:
     </a></h2>
6f107e8d
 
a2904cae
     <a id="WhatProtectionsDoesTorProvide"></a>
13b97478
     <h3><a class="anchor" href="#WhatProtectionsDoesTorProvide">What
a2904cae
     protections does Tor provide?</a></h3>
2b63e88f
 
13b97478
     <p>
     Internet communication is based on a store-and-forward model that
     can be understood in analogy to postal mail: Data is transmitted in
     blocks called IP datagrams or packets. Every packet includes a source
     IP address (of the sender) and a destination IP address (of the
     receiver), just as ordinary letters contain postal addresses of sender
     and receiver. The way from sender to receiver involves multiple hops of
     routers, where each router inspects the destination IP address and
     forwards the packet closer to its destination. Thus, every router
     between sender and receiver learns that the sender is communicating
     with the receiver. In particular, your local ISP is in the position to
     build a complete profile of your Internet usage. In addition, every
     server in the Internet that can see any of the packets can profile your
     behaviour.
     </p>
2b63e88f
 
13b97478
     <p>
     The aim of Tor is to improve your privacy by sending your traffic through
     a series of proxies. Your communication is encrypted in multiple layers
     and routed via multiple hops through the Tor network to the final
     receiver. More details on this process can be found in the <a
d7568ec6
     href="<page about/overview>">Tor overview</a>.
13b97478
     Note that all your local ISP can observe now is that you are
     communicating with Tor nodes. Similarly, servers in the Internet just
a2904cae
     see that they are being contacted by Tor nodes.
     </p>
2b63e88f
 
a2904cae
     <p>
13b97478
     Generally speaking, Tor aims to solve three privacy problems:
a2904cae
     </p>
2b63e88f
 
a2904cae
     <p>
13b97478
     First, Tor prevents websites and other services from learning
     your location, which they can use to build databases about your
     habits and interests. With Tor, your Internet connections don't
     give you away by default -- now you can have the ability to choose,
     for each connection, how much information to reveal.
a2904cae
     </p>
2b63e88f
 
a2904cae
     <p>
13b97478
     Second, Tor prevents people watching your traffic locally (such as
728b231c
     your ISP or someone with access to your home wifi or router) from
     learning what information you're fetching and where you're fetching
     it from. It also stops them from deciding what you're
13b97478
     allowed to learn and publish -- if you can get to any part of the Tor
2b63e88f
     network, you can reach any site on the Internet.
a2904cae
     </p>
 
     <p>
13b97478
     Third, Tor routes your connection through more than one Tor relay
     so no single relay can learn what you're up to. Because these relays
     are run by different individuals or organizations, distributing trust
a2904cae
     provides more security than the old <a href="#Torisdifferent">one hop proxy
13b97478
     </a> approach.
a2904cae
     </p>
2b63e88f
 
a2904cae
     <p>
13b97478
     Note, however, that there are situations where Tor fails to solve these
     privacy problems entirely: see the entry below on <a
2b63e88f
     href="#AttacksOnOnionRouting">remaining attacks</a>.
a2904cae
     </p>
2b63e88f
 
a2904cae
     <hr>
2b63e88f
 
a2904cae
     <a id="CanExitNodesEavesdrop"></a>
5e8d08ab
     <h3><a class="anchor" href="#CanExitNodesEavesdrop">Can exit nodes
     eavesdrop on communications? Isn't that bad?</a></h3>
2b63e88f
 
a2904cae
     <p>
13b97478
     Yes, the guy running the exit node can read the bytes that come in and
     out there. Tor anonymizes the origin of your traffic, and it makes sure
     to encrypt everything inside the Tor network, but it does not magically
     encrypt all traffic throughout the Internet.
a2904cae
     </p>
2b63e88f
 
a2904cae
     <p>
13b97478
     This is why you should always use end-to-end encryption such as SSL for
     sensitive Internet connections. (The corollary to this answer is that if
     you are worried about somebody intercepting your traffic and you're
5e8d08ab
     *not* using end-to-end encryption at the application layer, then
     something has already gone wrong and you shouldn't be thinking that Tor is
     the problem.)
a2904cae
     </p>
2b63e88f
 
a2904cae
     <hr>
2b63e88f
 
9fb81a6f
     <a id="AmITotallyAnonymous"></a>
13b97478
     <h3><a class="anchor" href="#AmITotallyAnonymous">So I'm totally anonymous
9fb81a6f
     if I use Tor?</a></h3>
 
     <p>
     <b>No.</b>
     </p>
     <p>
13b97478
     First, Tor protects the network communications. It separates where you
     are from where you are going on the Internet. What content and data you
     transmit over Tor is controlled by you. If you login to Google or
     Facebook via Tor, the local ISP or network provider doesn't know you
     are visiting Google or Facebook. Google and Facebook don't know where
     you are in the world. However, since you have logged into their sites,
     they know who you are. If you don't want to share information, you are
     in control.
     </p>
 
     <p>
     Second, active content, such as Java, Javascript, Adobe Flash, Adobe
     Shockwave, QuickTime, RealAudio, ActiveX controls, and VBScript, are
     binary applications. These binary applications run as your user account
     with your permissions in your operating system. This means these
     applications can access anything that your user account can access. Some
     of these technologies, such as Java and Adobe Flash for instance, run in
     what is known as a virtual machine. This virtual machine may have the
     ability to ignore your configured proxy settings, and therefore bypass
     Tor and share information directly to other sites on the Internet. The
     virtual machine may be able to store data, such as cookies, completely
     separate from your browser or operating system data stores. Therefore,
9fb81a6f
     these technologies must be disabled in your browser to use Tor safely.
     </p>
     <p>
32700e09
     That's where <a
f65daf2d
     href="<page projects/torbrowser>">Tor Browser</a> comes in. We produce
32700e09
     a web browser that is preconfigured to
13b97478
     help you control the risks to your privacy and anonymity while browsing
     the Internet. Not only are the above technologies disabled to prevent
5bd3add6
     identity leaks, Tor Browser also includes browser extensions like
9fb81a6f
     NoScript and Torbutton, as well as patches to the Firefox source
5bd3add6
     code. The full design of Tor Browser can be read <a
d7568ec6
     href="/projects/torbrowser/design/index.html.en">here</a>.
13b97478
     In designing a safe, secure solution for browsing the web with Tor,
     we've discovered that configuring <a href="#TBBOtherBrowser">other
7740f878
     browsers</a> to use Tor is unsafe.
9fb81a6f
     </p>
 
     <p>
13b97478
     Alternatively, you may find a Live CD or USB operating system more to
     your liking. The Tails team has created an <a
     href="https://tails.boum.org/">entire bootable operating system</a>
     configured for anonymity and privacy on the Internet.
9fb81a6f
     </p>
 
     <p>
13b97478
     Tor is a work in progress. There is still <a
d7568ec6
     href="<page getinvolved/volunteer>">plenty of work
13b97478
     left to do</a> for a strong, secure, and complete solution.
9fb81a6f
     </p>
 
     <hr>
 
2a9aaa80
     <a id="KeyManagement"></a>
5e8d08ab
     <h3><a class="anchor" href="#KeyManagement">Tell me about all the keys Tor
     uses.</a></h3>
0b68c29c
 
2a9aaa80
     <p>
     Tor uses a variety of different keys, with three goals in mind: 1)
     encryption to ensure privacy of data within the Tor network, 2)
     authentication so clients know they're
5e8d08ab
     talking to the relays they meant to talk to, and 3) signatures to make
2a9aaa80
     sure all clients know the same set of relays.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     <b>Encryption</b>: first, all connections in Tor use TLS link encryption,
2a9aaa80
     so observers can't look inside to see which circuit a given cell is
5e8d08ab
     intended for. Further, the Tor client establishes an ephemeral encryption
8d180ff7
     key with each relay in the circuit; these extra layers of encryption
     mean that only the exit relay can read
2a9aaa80
     the cells. Both sides discard the circuit key when the circuit ends,
37855015
     so logging traffic and then breaking into the relay to discover the
f30e672d
     key won't work.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     <b>Authentication</b>:
     Every Tor relay has a public decryption key called the "onion key".
8d180ff7
     Each relay rotates its onion key once a week.
5e8d08ab
     When the Tor client establishes circuits, at each step it
     <a href="<svnprojects>design-paper/tor-design.html#subsec:circuits">demands
     that the Tor relay prove knowledge of its onion key</a>. That way the first
     node in the path can't just spoof the rest of the path.
     Because the Tor client chooses the path, it can make sure to get Tor's
     "distributed trust" property: no single relay in the path can know about
     both the client and what the client is doing.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     <b>Coordination</b>:
5e8d08ab
     How do clients know what the relays are, and how do they know that they
     have the right keys for them? Each relay has a long-term public signing
     key called the "identity key". Each directory authority additionally has a
     "directory signing key". The directory authorities
     <a href="<specblob>dir-spec.txt">provide a signed list</a>
     of all the known relays, and in that list are a set of certificates from
     each relay (self-signed by their identity key) specifying their keys,
     locations, exit policies, and so on. So unless the adversary can control
     a majority of the directory authorities (as of 2012 there are 8 directory
     authorities), they can't trick the Tor client into using other Tor relays.
     </p>
 
     <p>
     How do clients know what the directory authorities are? The Tor software
     comes with a built-in list of location and public key for each directory
     authority. So the only way to trick users into using a fake Tor network
2a9aaa80
     is to give them a specially modified version of the software.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     How do users know they've got the right software? When we distribute the
     source code or a package, we digitally sign it with
     <a href="http://www.gnupg.org/">GNU Privacy Guard</a>. See the
     <a href="<page docs/verifying-signatures>">instructions on how to check
     Tor's signatures</a>.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     In order to be certain that it's really signed by us, you need to have
     met us in person and gotten a copy of our GPG key fingerprint, or you
     need to know somebody who has. If you're concerned about an attack on
     this level, we recommend you get involved with the security community
2a9aaa80
     and start meeting people.
     </p>
0b68c29c
 
ed5ac546
     <hr>
0b68c29c
 
5e8d08ab
     <a id="EntryGuards"></a>
     <h3><a class="anchor" href="#EntryGuards">What are Entry Guards?</a></h3>
 
     <p>
     Tor (like all current practical low-latency anonymity designs) fails
     when the attacker can see both ends of the communications channel. For
     example, suppose the attacker controls or watches the Tor relay you
     choose to enter the network, and also controls or watches the website you
     visit. In this case, the research community knows no practical low-latency
     design that can reliably stop the attacker from correlating volume and
     timing information on the two sides.
     </p>
 
     <p>
     So, what should we do? Suppose the attacker controls, or can observe,
     <i>C</i> relays. Suppose there are <i>N</i> relays total. If you select new
     entry and exit relays each time you use the network, the attacker will be
     able to correlate all traffic you send with probability around
     <i>(c/n)<sup>2</sup></i>. But profiling is, for most users, as bad as being
     traced all the time: they want to do something often without an attacker
     noticing, and the attacker noticing once is as bad as the attacker noticing
     more often. Thus, choosing many random entries and exits gives the user no
     chance of escaping profiling by this kind of attacker.
     </p>
 
     <p>
     The solution is "entry guards": each Tor client selects a few relays at
     random to use as entry points, and uses only those relays for her first hop.
     If those relays are not controlled or observed, the attacker can't win,
     ever, and the user is secure. If those relays <i>are</i> observed or
     controlled by the attacker, the attacker sees a larger <i>fraction</i>
     of the user's traffic &mdash; but still the user is no more profiled than
     before. Thus, the user has some chance (on the order of <i>(n-c)/n</i>)
     of avoiding profiling, whereas she had none before.
     </p>
 
     <p>
     You can read more at <a href="http://freehaven.net/anonbib/#wright02">
     An Analysis of the Degradation of Anonymous Protocols</a>,
efffbdc9
     <a href="http://freehaven.net/anonbib/#wright03">Defending Anonymous
5e8d08ab
     Communication Against Passive Logging Attacks</a>, and especially
     <a href="http://freehaven.net/anonbib/#hs-attack06">
     Locating Hidden Servers</a>.
     </p>
 
     <p>
     Restricting your entry nodes may also help against attackers who want
     to run a few Tor nodes and easily enumerate all of the Tor user IP
     addresses. (Even though they can't learn what destinations the users
     are talking to, they still might be able to do bad things with just a
     list of users.) However, that feature won't really become useful until
     we move to a "directory guard" design as well.
     </p>
75abc428
 
     <hr>
 
91d870fd
     <a id="ChangePaths"></a>
5e8d08ab
     <h3><a class="anchor" href="#ChangePaths">How often does Tor change its
     paths?</a></h3>
91d870fd
     <p>
13b97478
      Tor will reuse the same circuit for new TCP streams for 10 minutes,
      as long as the circuit is working fine. (If the circuit fails, Tor
f30e672d
      will switch to a new circuit immediately.)
     </p>
5e8d08ab
 
f30e672d
     <p>
5e8d08ab
     But note that a single TCP stream (e.g. a long IRC connection) will stay on
     the same circuit forever -- we don't rotate individual streams from one
     circuit to the next. Otherwise an adversary with a partial view of the
     network would be given many chances over time to link you to your
     destination, rather than just one chance.
f30e672d
     </p>
 
     <hr>
 
     <a id="CellSize"></a>
13b97478
     <h3><a class="anchor" href="#CellSize">Tor uses hundreds of bytes for
f30e672d
     every IRC line. I can't afford that!</a></h3>
     <p>
13b97478
      Tor sends data in chunks of 512 bytes (called "cells"), to make it
      harder for intermediaries to guess exactly how many bytes you're
      communicating at each step. This is unlikely to change in the near
      future -- if this increased bandwidth use is prohibitive for you, I'm
f30e672d
      afraid Tor is not useful for you right now.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     The actual content of these fixed size cells is
     <a href="https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt">
     documented in the main Tor spec</a>, section 3.
f30e672d
     </p>
5e8d08ab
 
f30e672d
     <p>
5e8d08ab
     We have been considering one day adding two classes of cells -- maybe a 64
     byte cell and a 1024 byte cell. This would allow less overhead for
     interactive streams while still allowing good throughput for bulk streams.
     But since we want to do a lot of work on quality-of-service and better
     queuing approaches first, you shouldn't expect this change anytime soon
     (if ever). However if you are keen, there are a couple of
     <a href="<page getinvolved/volunteer>#Research"> research ideas</a>
     that may involve changing the cell size.
91d870fd
     </p>
 
f30e672d
     <hr>
 
91d870fd
     <a id="OutboundConnections"></a>
13b97478
     <h3><a class="anchor" href="#OutboundConnections">Why does netstat show
f30e672d
     these outbound connections?</a></h3>
91d870fd
     <p>
13b97478
     Because that's how Tor works. It holds open a handful of connections
     so there will be one available when you need one.
91d870fd
     </p>
 
     <hr>
 
f549ee31
     <a id="PowerfulBlockers"></a>
13b97478
     <h3><a class="anchor" href="#PowerfulBlockers">What about powerful blocking
f30e672d
     mechanisms?</a></h3>
f549ee31
     <p>
5e8d08ab
     An adversary with a great deal of manpower and money, and severe
     real-world penalties to discourage people from trying to evade detection,
     is a difficult test for an anonymity and anti-censorship system.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     The original Tor design was easy to block if the attacker controls Alice's
     connection to the Tor network --- by blocking the directory authorities, by
     blocking all the relay IP addresses in the directory, or by filtering based
     on the fingerprint of the Tor TLS handshake. After seeing these attacks and
     others first-hand, more effort was put into researching new circumvention
     techniques. Pluggable transports are protocols designed to allow users
     behind government firewalls to access the Tor network.
f549ee31
     </p>
5e8d08ab
 
f549ee31
     <p>
5e8d08ab
     We've made quite a bit of progress on this problem lately. You can read
     more details on the <a href="<page docs/pluggable-transports>">
     pluggable transports page</a>. You may also be interested in
     <a href="https://www.youtube.com/watch?v=GwMr8Xl7JMQ">Roger and Jake's talk
     at 28C3</a>, or <a href="https://www.youtube.com/watch?v=JZg1nqs793M">
     Runa's talk at 44con</a>.
f549ee31
     </p>
 
     <hr>
13b97478
 
f549ee31
     <a id="RemotePhysicalDeviceFingerprinting"></a>
13b97478
     <h3><a class="anchor" href="#RemotePhysicalDeviceFingerprinting">Does Tor
f30e672d
     resist "remote physical device fingerprinting"?</a></h3>
5e8d08ab
 
     <p>
     Yes, we resist all of these attacks as far as we know.
     </p>
 
f549ee31
     <p>
5e8d08ab
     These attacks come from examining characteristics of the IP headers or TCP
     headers and looking for information leaks based on individual hardware
     signatures. One example is the
     <a href="http://www.caida.org/outreach/papers/2005/fingerprinting/">
     Oakland 2005 paper</a> that lets you learn if two packet streams originated
     from the same hardware, but only if you can see the original TCP timestamps.
     </p>
 
f549ee31
     <p>
5e8d08ab
     Tor transports TCP streams, not IP packets, so we end up automatically
     scrubbing a lot of the potential information leaks. Because Tor relays use
     their own (new) IP and TCP headers at each hop, this information isn't
     relayed from hop to hop. Of course, this also means that we're limited in
     the protocols we can transport (only correctly-formed TCP, not all IP like
     ZKS's Freedom network could) -- but maybe that's a good thing at this stage.
     </p>
f549ee31
 
     <hr>
 
5bdf9aee
     <a id="IsTorLikeAVPN"></a>
     <h3><a class="anchor" href="#IsTorLikeAVPN">Is Tor like a VPN?</a></h3>
2b63e88f
 
13b97478
     <p>
5e8d08ab
     <b>Do not use a VPN as an
     <a href="http://www.nbcnews.com/news/investigations/war-anonymous-british-spies-attacked-hackers-snowden-docs-show-n21361">
     anonymity solution</a>.</b>
2b63e88f
     If you're looking for a trusted entry into the Tor network, or if you want
5e8d08ab
     to obscure the fact that you're using Tor,
d7568ec6
     <a href="<page docs/bridges>#RunningABridge">setting up
f947dc67
     a private server as a bridge</a> works quite well.
     </p>
 
     <p>
13b97478
     VPNs encrypt the traffic between the user and the VPN provider,
     and they can act as a proxy between a user and an online destination.
     However, VPNs have a single point of failure: the VPN provider.
     A technically proficient attacker or a number of employees could
     retrieve the full identity information associated with a VPN user.
     It is also possible to use coercion or other means to convince a
     VPN provider to reveal their users' identities. Identities can be
     discovered by following a money trail (using Bitcoin does not solve
     this problem because Bitcoin is not anonymous), or by persuading the
     VPN provider to hand over logs. Even
     if a VPN provider says they don't keep logs, users have to take their
     word for it---and trust that the VPN provider won't buckle to outside
     pressures that might want them to start keeping logs.
     </p>
 
     <p>
     When you use a VPN, websites can still build up a persistent profile of
     your usage over time. Even though sites you visit won't automatically
     get your originating IP address, they still know how to profile you
     based on your browsing history.
     </p>
 
     <p>
     When you use Tor the IP address you connect to changes at most every 10
     minutes, and often more frequently than that. This makes it extremely
     dificult for websites to create any sort of persistent profile of Tor
5e8d08ab
     users (assuming you did not
     <a href="<page download/download>#warning">identify yourself in other
     ways</a>). No one Tor relay can know enough information to compromise any
     Tor user because of Tor's
     <a href="<page about/overview>#thesolution">encrypted three-hop circuit</a>
     design.
08e6e1c9
     </p>
5bdf9aee
 
     <hr>
 
ff62bd3e
     <a id="Proxychains"></a>
13b97478
     <h3><a class="anchor" href="#Proxychains">Aren't 10 proxies
ff62bd3e
     (proxychains) better than Tor with only 3 hops?</a></h3>
2b63e88f
 
ff62bd3e
     <p>
13b97478
     Proxychains is a program that sends your traffic through a series of
     open web proxies that you supply before sending it on to your final
     destination. <a href="#KeyManagement">Unlike Tor</a>, proxychains
     does not encrypt the connections between each proxy server. An open proxy
     that wanted to monitor your connection could see all the other proxy
     servers you wanted to use between itself and your final destination,
     as well as the IP address that proxy hop received traffic from.
ff62bd3e
     </p>
     <p>
5e8d08ab
     Because the
     <a href="https://gitweb.torproject.org/torspec.git?a=blob_plain;hb=HEAD;f=tor-spec.txt">
13b97478
     Tor protocol</a> requires encrypted relay-to-relay connections, not
     even a misbehaving relay can see the entire path of any Tor user.
ff62bd3e
     </p>
5e8d08ab
 
ff62bd3e
     <p>
13b97478
     While Tor relays are run by volunteers and checked periodically for
     suspicious behavior, many open proxies that can be found with a search
     engine are compromised machines, misconfigured private proxies
     not intended for public use, or honeypots set up to exploit users.
ff62bd3e
     </p>
2b63e88f
 
ff62bd3e
     <hr>
2b63e88f
 
5e8d08ab
     <a id="AttacksOnOnionRouting"></a>
13b97478
     <h3><a class="anchor" href="#AttacksOnOnionRouting">What attacks remain
f30e672d
     against onion routing?</a></h3>
91d870fd
     <p>
5e8d08ab
     As mentioned above, it is possible for an observer who can view both you and
     either the destination website or your Tor exit node to correlate timings of
     your traffic as it enters the Tor network and also as it exits. Tor does not
     defend against such a threat model.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     In a more limited sense, note that if a censor or law enforcement agency has
     the ability to obtain specific observation of parts of the network, it is
     possible for them to verify a suspicion that you talk regularly to your
     friend by observing traffic at both ends and correlating the timing of only
     that traffic. Again, this is only useful to verify that parties already
     suspected of communicating with one another are doing so. In most countries,
     the suspicion required to obtain a warrant already carries more weight than
     timing correlation would provide.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
13b97478
 Furthermore, since Tor reuses circuits for multiple TCP connections, it is
 possible to associate non anonymous and anonymous traffic at a given exit
 node, so be careful about what applications you run concurrently over Tor.
 Perhaps even run separate Tor clients for these applications.
91d870fd
     </p>
 
     <hr>
 
5bdf9aee
     <a id="LearnMoreAboutAnonymity"></a>
5e8d08ab
     <h3><a class="anchor" href="#LearnMoreAboutAnonymity">Where can I learn
     more about anonymity?</a></h3>
5bdf9aee
 
     <p>
5e8d08ab
     <a href="http://freehaven.net/anonbib/topic.html#Anonymous_20communication">
     Read these papers</a> (especially the ones in boxes) to get up to speed on
     anonymous communication systems.
5bdf9aee
     </p>
 
     <hr>
6f107e8d
 
     <a id="AlternateDesigns"></a>
f87eea1a
     <h2><a class="anchor" href="#AlternateDesigns">Alternate designs:</a></h2>
6f107e8d
 
2a9aaa80
     <a id="EverybodyARelay"></a>
5e8d08ab
     <h3><a class="anchor" href="#EverybodyARelay">You should make every Tor
     user be a relay.</a></h3>
 
     <p>
     Requiring every Tor user to be a relay would help with scaling the network
     to handle all our users, and <a href="#BetterAnonymity">running a Tor relay
     may help your anonymity</a>. However, many Tor users cannot be good relays
     &mdash; for example, some Tor clients operate from behind restrictive
     firewalls, connect via modem, or otherwise aren't in a position where they
     can relay traffic. Providing service to these clients is a critical part of
     providing effective anonymity for everyone, since many Tor users are
     subject to these or similar constraints and including these clients
2a9aaa80
     increases the size of the anonymity set.
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     That said, we do want to encourage Tor users to run relays, so what we
     really want to do is simplify the process of setting up and maintaining
     a relay. We've made a lot of progress with easy configuration in the past
     few years: Tor is good at automatically detecting whether it's reachable
     and how much bandwidth it can offer.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     There are five steps we need to address before we can do this though:
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     First, we need to make Tor stable as a relay on all common operating
     systems. The main remaining platform is Windows, and we're mostly there.
     See Section 4.1 of
d7568ec6
     <a href="<page press/2008-12-19-roadmap-press-release>">
5e8d08ab
     our development roadmap</a>.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
5e8d08ab
     Second, we still need to get better at automatically estimating the right
     amount of bandwidth to allow. See item #7 on the
     <a href="<page getinvolved/volunteer>#Research">research section of the
     volunteer page</a>: "Tor doesn't work very well when relays have asymmetric
     bandwidth (e.g. cable or DSL)". It might be that
     <a href="<page docs/faq>#TransportIPnotTCP">switching to UDP transport</a>
     is the simplest answer here &mdash; which alas is not a very simple answer
     at all.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     Third, we need to work on scalability, both of the network (how to
     stop requiring that all Tor relays be able to connect to all Tor
     relays) and of the directory (how to stop requiring that all Tor
     users know about all Tor relays). Changes like this can have large
     impact on potential and actual anonymity. See Section 5 of the <a
     href="<svnprojects>design-paper/challenges.pdf">Challenges</a> paper
     for details. Again, UDP transport would help here.
     </p>
0b68c29c
 
2a9aaa80
     <p>
     Fourth, we need to better understand the risks from
     letting the attacker send traffic through your relay while
     you're also initiating your own anonymized traffic. <a
     href="http://freehaven.net/anonbib/#back01">Three</a> <a
     href="http://freehaven.net/anonbib/#clog-the-queue">different</a>
     <a href="http://freehaven.net/anonbib/#torta05">research</a> papers
     describe ways to identify the relays in a circuit by running traffic
5e8d08ab
     through candidate relays and looking for dips in the traffic while the
     circuit is active. These clogging attacks are not that scary in the Tor
     context so long as relays are never clients too. But if we're trying to
     encourage more clients to turn on relay functionality too (whether as
     <a href="<page docs/bridges>">bridge relays</a> or as normal relays), then
     we need to understand this threat better and learn how to mitigate it.
37855015
     </p>
 
     <p>
5e8d08ab
     Fifth, we might need some sort of incentive scheme to encourage people
     to relay traffic for others, and/or to become exit nodes. Here are our
     <a href="<blog>two-incentive-designs-tor">current thoughts on Tor
     incentives</a>.
2a9aaa80
     </p>
0b68c29c
 
2a9aaa80
     <p>
     Please help on all of these!
     </p>
0b68c29c
 
5e8d08ab
     <hr>
 
     <a id="TransportIPnotTCP"></a>
     <h3><a class="anchor" href="#TransportIPnotTCP">You should transport all
     IP packets, not just TCP packets.</a></h3>
 
     <p>
     This would be handy, because it would make Tor better able to handle
     new protocols like VoIP, it could solve the whole need to socksify
     applications, and it would solve the fact that exit relays need to
     allocate a lot of file descriptors to hold open all the exit
     connections.
     </p>
 
     <p>
     We're heading in this direction: see
     <a href="https://trac.torproject.org/projects/tor/ticket/1855">this trac
     ticket</a> for directions we should investigate. Some of the hard problems
     are:
     </p>
 
     <ol>
     <li>IP packets reveal OS characteristics. We would still need to do
     IP-level packet normalization, to stop things like TCP fingerprinting
     attacks. Given the diversity and complexity of TCP stacks, along with
     <a href="#RemotePhysicalDeviceFingerprinting">device fingerprinting
     attacks</a>, it looks like our best bet is shipping our own user-space TCP
     stack.
     </li>
     <li>Application-level streams still need scrubbing. We will still need
     user-side applications like Torbutton. So it won't become just a matter
     of capturing packets and anonymizing them at the IP layer.
     </li>
     <li>Certain protocols will still leak information. For example, we must
     rewrite DNS requests so they are delivered to an unlinkable DNS server
     rather than the DNS server at a user's ISP; thus, we must understand
     the protocols we are transporting.
     </li>
     <li><a href="http://crypto.stanford.edu/~nagendra/projects/dtls/dtls.html">
     DTLS</a> (datagram TLS) basically has no users, and IPsec sure is big. Once
     we've picked a transport mechanism, we need to design a new end-to-end Tor
     protocol for avoiding tagging attacks and other potential anonymity and
     integrity issues now that we allow drops, resends, et cetera.
     </li>
     <li>Exit policies for arbitrary IP packets mean building a secure IDS. Our
     node operators tell us that exit policies are one of the main reasons
     they're willing to run Tor. Adding an Intrusion Detection System to handle
     exit policies would increase the security complexity of Tor, and would
     likely not work anyway, as evidenced by the entire field of IDS and
     counter-IDS papers. Many potential abuse issues are resolved by the fact
     that Tor only transports valid TCP streams (as opposed to arbitrary IP
     including malformed packets and IP floods), so exit policies become even
     <i>more</i> important as we become able to transport IP packets. We also
     need to compactly describe exit policies in the Tor directory, so clients
     can predict which nodes will allow their packets to exit &mdash; and
     clients need to predict all the packets they will want to send in a session
     before picking their exit node!
     </li>
     <li>The Tor-internal name spaces would need to be redesigned. We support
     onion service ".onion" addresses by intercepting the addresses when they
     are passed to the Tor client. Doing so at the IP level will require a more
     complex interface between Tor and the local DNS resolver.
     </li>
     </ol>
 
     <hr>
 
     <a id="HideExits"></a>
     <h3><a class="anchor" href="#HideExits">You should hide the list of Tor
     relays, so people can't block the exits.</a></h3>
 
     <p>
     There are a few reasons we don't:
     </p>
 
     <ol>
     <li>We can't help but make the information available, since Tor clients
     need to use it to pick their paths. So if the "blockers" want it, they
     can get it anyway. Further, even if we didn't tell clients about the
     list of relays directly, somebody could still make a lot of connections
     through Tor to a test site and build a list of the addresses they see.
     </li>
 
     <li>If people want to block us, we believe that they should be allowed to
     do so.  Obviously, we would prefer for everybody to allow Tor users to
     connect to them, but people have the right to decide who their services
     should allow connections from, and if they want to block anonymous users,
     they can.
     </li>
 
     <li>Being blockable also has tactical advantages: it may be a persuasive
     response to website maintainers who feel threatened by Tor. Giving them
     the option may inspire them to <a href="<page docs/faq-abuse>#Bans">stop
     and think</a> about whether they really want to eliminate private access
     to their system, and if not, what other options they might have. The
     time they might otherwise have spent blocking Tor, they may instead
     spend rethinking their overall approach to privacy and anonymity.
     </li>
     </ol>
bfbe8320
 
ed5ac546
     <hr>
0b68c29c
 
5e8d08ab
     <a id="ChoosePathLength"></a>
     <h3><a class="anchor" href="#ChoosePathLength">You should let people choose
     their path length.</a></h3>
     <p>
     Right now the path length is hard-coded at 3 plus the number of nodes in
     your path that are sensitive. That is, in normal cases it's 3, but for
     example if you're accessing an onion service or a ".exit" address it could
     be 4.
     </p>
 
     <p>
     We don't want to encourage people to use paths longer than this &mdash; it
     increases load on the network without (as far as we can tell) providing
     any more security. Remember that
     <a href="https://svn.torproject.org/svn/projects/design-paper/tor-design.html#subsec:threat-model">
     the best way to attack Tor is to attack the endpoints and ignore the middle
     of the path</a>.
     Also, using paths longer than 3 could harm anonymity, first because it makes
     <a href="http://freehaven.net/anonbib/#ccs07-doa">"denial of security"</a>
     attacks easier, and second because it could act as an  identifier if only a
     few people do it ("Oh, there's that person who changed her path length
     again").
     </p>
 
     <p>
     And we don't want to encourage people to use paths of length 1 either.
     Currently there is no reason to suspect that investigating a single relay
     will yield user-destination pairs, but if many people are using only a
     single hop, we make it more likely that attackers will seize or break into
     relays in hopes of tracing users.
     </p>
 
     <p>
     Now, there is a good argument for making the number of hops in a path
     unpredictable. For example, somebody who happens to control the last
     two hops in your path still doesn't know who you are, but they know
     for sure which entry node you used. Choosing path length from, say,
     a geometric distribution will turn this into a statistical attack,
     which seems to be an improvement. On the other hand, a longer path
     length is bad for usability, and without further protections it seems
     likely that an adversary can estimate your path length anyway. We're
     not sure of the right trade-offs here. Please write a research paper
     that tells us what to do.
     </p>
bfbe8320
 
ed5ac546
     <hr>
0b68c29c
 
c10a4de4
     <a id="ChoosePathCountries"></a>
     <h3><a class="anchor" href="#ChoosePathCountries">You should change path
     selection to avoid entering and exiting from the same country.</a></h3>
 
     <p>
     It is better to not manually change the path. This could have unforeseen
     consquences and you'll probably screw it up, we don't understand it very
     well either.
     </p>
 
     <p>
     There are many attacks and adversaries that Tor is trying to defend against
     at once, and constraining paths has surprising trickle-down effects on the
     other attacks (e.g. if I see where you exit then I know where you *didn't*
     enter, thus reducing your entropy, sometimes by a surprising amount
     depending on what path constraints are choosen).
     </p>
 
     <p>
     In general, changing Tor's path selection makes your client look different
     from other clients. Picking your entry and exit in different countries is
     not a good defence, because it only defends against adversaries that are
     unable to rent servers in other countries.
     </p>
91d870fd
 
     <hr>
 
c10a4de4
     <a id="SplitEachConnection"></a>
13b97478
     <h3><a class="anchor" href="#SplitEachConnection">You should split
f30e672d
     each connection over many paths.</a></h3>
91d870fd
 
     <p>
5e8d08ab
     We don't currently think this is a good idea. You see, the attacks we're
     worried about are at the endpoints: the adversary watches Alice (or the
     first hop in the path) and Bob (or the last hop in the path) and learns
     that they are communicating.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     If we make the assumption that timing attacks work well on even a few
     packets end-to-end, then having *more* possible ways for the adversary to
     observe the connection seems to hurt anonymity, not help it.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
ac08645c
 
5e8d08ab
     Now, it's possible that we could make ourselves more resistant to
     end-to-end attacks with a little bit of padding and by making each circuit
     send and receive a fixed number of cells. This approach is more
     well-understood in the context of high-latency systems. See e.g.
     <a href="http://freehaven.net/anonbib/#pet05-serjantov">
     Message Splitting Against the Partial Adversary by Andrei Serjantov and
     Steven J. Murdoch</a>.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     But since we don't currently understand what network and padding
     parameters, if any, could provide increased end-to-end security, our
     current strategy is to minimize the number of places that the adversary
     could possibly see.
91d870fd
     </p>
 
     <hr>
 
34bb6a9b
     <a id="MigrateApplicationStreamsAcrossCircuits"></a>
13b97478
     <h3><a class="anchor" href="#MigrateApplicationStreamsAcrossCircuits">You
34bb6a9b
     should migrate application streams across circuits.</a></h3>
13b97478
     <p>This would be great for two reasons. First, if a circuit breaks, we
     would be able to shift its active streams onto a new circuit, so they
     don't have to break. Second, it is conceivable that we could get
     increased security against certain attacks by migrating streams
     periodically, since leaving a stream on a given circuit for many hours
34bb6a9b
     might make it more vulnerable to certain adversaries.</p>
 
13b97478
     <p>There are two problems though. First, Tor would need a much more
     bulky protocol. Right now each end of the Tor circuit just sends the
     cells, and lets TCP provide the in-order guaranteed delivery. If we
     can move streams across circuits, though, we would need to add queues
     at each end of the circuit, add sequence numbers so we can send and
     receive acknowledgements for cells, and so forth. These changes would
     increase the complexity of the Tor protocol considerably. Which leads
     to the second problem: if the exit node goes away, there's nothing we
     can do to save the TCP connection. Circuits are typically three hops
34bb6a9b
     long, so in about a third of the cases we just lose.</p>
 
13b97478
     <p>Thus our current answer is that since we can only improve things by
     at best 2/3, it's not worth the added code and complexity. If somebody
     writes a protocol specification for it and it turns out to be pretty
34bb6a9b
     simple, we'd love to add it.</p>
 
13b97478
     <p>But there are still some approaches we can take to improve the
     reliability of streams. The main approach we have now is to specify
     that streams using certain application ports prefer circuits to be
     made up of stable nodes. These ports are specified in the "LongLivedPorts"
efb55037
     <a href="#torrc">torrc</a> option, and they default to</p>
     <pre>21,22,706,1863,5050,5190,5222,5223,6667,6697,8300</pre>
13b97478
     <p>The definition of "stable" is an open research question, since we
     can only guess future stability based on past performance. Right now
     we judge that a node is stable if it advertises that it has been up
     for more than a day. Down the road we plan to refine this so it takes into
34bb6a9b
     account the average stability of the other nodes in the Tor network.</p>
 
     <hr>
 
     <a id="LetTheNetworkPickThePath"></a>
13b97478
     <h3><a class="anchor" href="#LetTheNetworkPickThePath">You should
34bb6a9b
     let the network pick the path, not the client</a></h3>
 
13b97478
     <p>No. You cannot trust the network to pick the path for relays could
     collude and route you through their colluding friends. This would give
34bb6a9b
     an adversary the ability to watch all of your traffic end to end.</p>
 
     <hr>
 
91d870fd
     <a id="UnallocatedNetBlocks"></a>
13b97478
     <h3><a class="anchor" href="#UnallocatedNetBlocks">Your default exit
f30e672d
     policy should block unallocated net blocks too.</a></h3>
91d870fd
 
     <p>
5e8d08ab
     No, it shouldn't. The default exit policy blocks certain private net blocks,
     like 10.0.0.0/8, because they might actively be in use by Tor relays and we
     don't want to cause any surprises by bridging to internal networks. Some
     overzealous firewall configs suggest that you also block all the parts of
     the Internet that IANA has not currently allocated. First, this turns into
     a problem for them when those addresses *are* allocated. Second, why should
     we default-reject something that might one day be useful?
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     Tor's default exit policy is chosen to be flexible and useful in the future:
     we allow everything except the specific addresses and ports that we
     anticipate will lead to problems.
91d870fd
     </p>
 
     <hr>
 
     <a id="BlockWebsites"></a>
13b97478
     <h3><a class="anchor" href="#BlockWebsites">Exit policies should be
f30e672d
     able to block websites, not just IP addresses.</a></h3>
91d870fd
 
     <p>
5e8d08ab
     It would be nice to let relay operators say things like "reject
     www.slashdot.org" in their exit policies, rather than requiring
     them to learn all the IP address space that could be covered by the site
     (and then also blocking other sites at those IP addresses).
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     There are two problems, though. First, users could still get around these
     blocks. For example, they could request the IP address rather than the
     hostname when they exit from the Tor network. This means operators would
     still need to learn all the IP addresses for the destinations in question.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     The second problem is that it would allow remote attackers to censor
     arbitrary sites. For example, if a Tor operator blocks www1.slashdot.org,
     and then some attacker poisons the Tor relay's DNS or otherwise changes
     that hostname to resolve to the IP address for a major news site, then
     suddenly that Tor relay is blocking the news site.
91d870fd
     </p>
 
     <hr>
 
     <a id="BlockContent"></a>
13b97478
     <h3><a class="anchor" href="#BlockContent">You should change Tor to
f30e672d
     prevent users from posting certain content.</a></h3>
91d870fd
 
13b97478
     <p> Tor only transports data, it does not inspect the contents of the
     connections which are sent over it. In general it's a very hard problem
     for a computer to determine what is objectionable content with good true
     positive/false positive rates and we are not interested in addressing
f30e672d
     this problem.
91d870fd
     </p>
5e8d08ab
 
91d870fd
     <p>
5e8d08ab
     Further, and more importantly, which definition of "certain content" could
     we use? Every choice would lead to a quagmire of conflicting personal
     morals. The only solution is to have no opinion.
91d870fd
     </p>
 
     <hr>
 
ae26aad0
     <a id="SendPadding"></a>
13b97478
     <h3><a class="anchor" href="#SendPadding">You should send padding so it's
ae26aad0
     more secure.</a></h3>
2b63e88f
 
ae26aad0
     <p>
13b97478
     Like all anonymous communication networks that are fast enough for web
     browsing, Tor is vulnerable to statistical "traffic confirmation"
     attacks, where the adversary watches traffic at both ends of a circuit
5e8d08ab
     and confirms their guess that those endpoints are communicating. It would
     be really nice if we could use cover traffic to confuse this attack. But
     there are three problems here:
ae26aad0
     </p>
2b63e88f
 
ae26aad0
     <ul>
     <li>
13b97478
     Cover traffic is really expensive. And *every* user needs to be doing
     it. This adds up to a lot of extra bandwidth cost for our volunteer
ae26aad0
     operators, and they're already pushed to the limit.
     </li>
     <li>
13b97478
     You'd need to always be sending traffic, meaning you'd need to always
     be online. Otherwise, you'd need to be sending end-to-end cover
     traffic -- not just to the first hop, but all the way to your final
     destination -- to prevent the adversary from correlating presence of
     traffic at the destination to times when you're online. What does it
     mean to send cover traffic to -- and from -- a web server? That is not
     supported in most protocols.
ae26aad0
     </li>
     <li>
13b97478
     Even if you *could* send full end-to-end padding between all users and
     all destinations all the time, you're *still* vulnerable to active
     attacks that block the padding for a short time at one end and look for
     patterns later in the path.
ae26aad0
     </li>
     </ul>
2b63e88f
 
ae26aad0
     <p>
13b97478
     In short, for a system like Tor that aims to be fast, we don't see any
     use for padding, and it would definitely be a serious usability problem.
     We hope that one day somebody will prove us wrong, but we are not
     optimistic.
ae26aad0
     </p>
2b63e88f
 
74b08c7a
     <p>
     We did however since
     <a href="https://gitweb.torproject.org/torspec.git/tree/proposals/251-netflow-padding.txt">
     implement netflow padding</a> to collapse netflow records for improved
     security. Now padding is sent between a client's Tor connection and its guard
     bidirectionally at a random interval that we control from the consensus,
     with a default of 4 to 14 seconds if the connection is idle. This has the
     goal of stymying some of the potential traffic analysis attacks out there --
     website fingerprinting, end-to-end correlation, and the things in between.
     </p>
 
     <p>For details see the
     <a href="https://blog.torproject.org/blog/network-team-hackfest-wilmington-watch">
     blog post</a> by the Tor network team, the
     <a href="https://lists.torproject.org/pipermail/tor-dev/2015-August/009326.html">
     announcement</a> on the tor-dev mailinglist or read further
     <a href="https://www.freehaven.net/anonbib/bibtex.html">publications</a> on
     padding.
     </p>
 
ae26aad0
     <hr>
 
     <a id="Steganography"></a>
5e8d08ab
     <h3><a class="anchor" href="#Steganography">You should use steganography to
     hide Tor traffic.</a></h3>
2b63e88f
 
ae26aad0
     <p>
13b97478
     Many people suggest that we should use steganography to make it hard
     to notice Tor connections on the Internet. There are a few problems
     with this idea though:
ae26aad0
     </p>
2b63e88f
 
ae26aad0
     <p>
13b97478
     First, in the current network topology, the Tor relays list <a
     href="#HideExits">is public</a> and can be accessed by attackers.
     An attacker who wants to detect or block anonymous users could
     always just notice <b>any connection</b> to or from a Tor relay's
     IP address.
ae26aad0
     </p>
2b63e88f
 
ff62bd3e
    <hr>
13b97478
 
2a9aaa80
   </div>
   <!-- END MAINCOL -->
   <div id = "sidecol">
 #include "side.wmi"
 #include "info.wmi"
   </div>
   <!-- END SIDECOL -->
 </div>
 <!-- END CONTENT -->
0b68c29c
 #include <foot.wmi>
11ab6cce