trunk/docs/en/faq.wml (c18f0dd52) - tor-webwml.git

faq.wml

## translation metadata
# Revision: $Revision: 26299 $
# Translation-Priority: 2-medium

#include "head.wmi" TITLE="Tor Project: FAQ" CHARSET="UTF-8"
<div id="content" class="clearfix">
  <div id="breadcrumbs">
    <a href="<page index>">Home &raquo; </a>
    <a href="<page docs/documentation>">Documentation &raquo; </a>
    <a href="<page docs/faq>">FAQ</a>
  </div>
  <div id="maincol">
    
    <h1>Tor FAQ</h1>
    <hr>

<p>General questions:</p>
    <ul>
    <li><a href="#WhatIsTor">What is Tor?</a></li>
    <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>
    <li><a href="#WhyCalledTor">Why is it called Tor?</a></li>
    <li><a href="#Backdoor">Is there a backdoor in Tor?</a></li>
    <li><a href="#DistributingTor">Can I distribute Tor?</a></li>
    <li><a href="#SupportMail">How can I get support?</a></li>
    <li><a href="#Forum">Is there a Tor forum?</a></li>
    <li><a href="#WhySlow">Why is Tor so slow?</a></li>
    <li><a href="#FileSharing">How can I share files anonymously through Tor?</a></li>
    <li><a href="#OutboundPorts">Do I have to open all these outbound ports on my firewall?</a></li>
    <li><a href="#Funding">What would The Tor Project do with more
    funding?</a></li>
    <li><a href="#IsItWorking">How can I tell if Tor is working, and that my connections really are anonymized?</a></li>
    <li><a href="#FTP">How do I use my browser for ftp with Tor?</a></li>
    <li><a href="#Metrics">How many people use Tor? How many relays or
    exit nodes are there?</a></li>
    <li><a href="#SSLcertfingerprint">What are your SSL certificate
    fingerprints?</a></li>
    </ul>

<p>Compilation and Installation:</p>
    <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>
    <li><a href="#GetTor">Your website is blocked in my country. How
    do I download Tor?</a></li>
    <li><a href="#CompileTorWindows">How do I compile Tor under
Windows?</a></li>
    <li><a href="#VirusFalsePositives">Why does my Tor executable appear
to
    have a virus or spyware?</a></li>
    <li><a href="#LiveCD">Is there a LiveCD or other bundle that
includes Tor?</a></li>
    </ul>

<p>Tor Browser Bundle:</p>
    <ul>

<li><a href="#TBBFlash">Why can't I view videos on YouTube and other
    Flash-based sites?</a></li>
    <li><a href="#TBBSocksPort">I want to
    run another application through the Tor launched by Tor Browser
    Bundle.</a></li>
    <li><a href="#TBBPolipo">I need an HTTP proxy. Where did Polipo
    go?</a></li>
    <li><a href="#TBBOtherExtensions">Can I install other Firefox
    extensions? Which extensions should I avoid using?</a></li>
    <li><a href="#TBBJavaScriptEnabled">Why is NoScript configured to
allow JavaScript by default in the Tor Browser Bundle?  Isn't that
unsafe?</a></li>
    <li><a href="#TBBOtherBrowser">I want to use Chrome/IE/Opera/etc
    with Tor.</a></li>
    <li><a href="#TBBCloseBrowser">I want to leave Tor Browser Bundle
    running but close the browser.</a></li>

<li><a href="#GoogleCAPTCHA">Google makes me solve a CAPTCHA or
tells
    me I have spyware installed.</a></li>
    <li><a href="#ForeignLanguages">Why does Google show up in foreign languages?</li></a>
    <li><a href="#GmailWarning">Gmail warns me that my account may have
    been compromised.</a></li>
    </ul>

<p>Advanced Tor usage:</p>
    <ul>
    <li><a href="#torrc">I'm supposed to "edit my torrc". What does
    that mean?</a></li>
    <li><a href="#Logs">How do I set up logging, or see Tor's
    logs?</a></li>
    <li><a href="#LogLevel">What log level should I use?</a></li>
    <li><a href="#DoesntWork">Tor is running, but it's not working
    correctly.</a></li>
    <li><a href="#VidaliaPassword">Tor/Vidalia prompts for a password at
    start.</a></li>
    <li><a href="#ChooseEntryExit">Can I control which nodes (or
country)
    are used for entry/exit?</a></li>
    <li><a href="#FirewallPorts">My firewall only allows a few outgoing
    ports.</a></li>
    <li><a href="#ExitPorts">Is there a list of default exit ports?</a></li>
    <li><a href="#SocksAndDNS">How do I check if my application that uses SOCKS is leaking DNS requests?</a></li>
    <li><a href="#DifferentComputer">I want to run my Tor client on a 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>
    </ul>

<p>Running a Tor relay:</p>
    <ul>
    <li><a href="#RelayFlexible">How stable does my relay need to
be?</a></li>
    <li><a href="#ExitPolicies">I'd run a relay, but I don't want to
deal
    with abuse issues.</a></li>
    <li><a href="#RelayOrBridge">Should I be a normal relay or bridge
    relay?</a></li>
    <li><a href="#MultipleRelays">I want to run more than one
relay.</a></li>
    <li><a href="#RelayMemory">Why is my Tor relay using so much
memory?</a></li>
    <li><a href="#WhyNotNamed">Why is my Tor relay not named?</a></li>
    <li><a href="#RelayDonations">Can I donate for a relay rather than
    run my own?</a></li>
    </ul>

<p>Running a Tor hidden service:</p>

<p>Anonymity and Security:</p>
    <ul>
    <li><a href="#KeyManagement">Tell me about all the keys Tor
uses.</a></li>
    <li><a href="#EntryGuards">What are Entry Guards?</a></li>
    <li><a href="#ChangePaths">How often does Tor change its paths?</a></li>
    <li><a href="#CellSize">Tor uses hundreds of bytes for every IRC line. I can't afford that!</a></li>
    <li><a href="#OutboundConnections">Why does netstat show these outbound connections?</a></li>
    </ul>

<p>Alternate designs that we don't do (yet):</p>
    <ul>
    <li><a href="#EverybodyARelay">You should make every Tor user be a
    relay.</a></li>
    <li><a href="#TransportIPnotTCP">You should transport all IP
packets,
    not just TCP packets.</a></li>
    <li><a href="#HideExits">You should hide the list of Tor relays,
    so people can't block the exits.</a></li>
    <li><a href="#ChoosePathLength">You should let people choose their path length.</a></li>
    <li><a href="#SplitEachConnection">You should split each connection over many paths.</a></li>
    <li><a href="#UnallocatedNetBlocks">Your default exit policy should block unallocated net blocks too.</a></li>
    <li><a href="#BlockWebsites">Exit policies should be able to block websites, not just IP addresses.</a></li>
    <li><a href="#BlockContent">You should change Tor to prevent users from posting certain content.</a></li>
    <li><a href="#IPv6">Tor should support IPv6.</a></li>
    </ul>

<p>Abuse:</p>
    <ul>
    <li><a href="#Criminals">Doesn't Tor enable criminals to do bad
things?</a></li>
    <li><a href="#RespondISP">How do I respond to my ISP about my exit
    relay?</a></li>
    </ul>

<p>For other questions not yet on this version of the FAQ, see the
<a
    href="<wikifaq>">wiki FAQ</a> for now.</p>

<hr>

<p>
    The name "Tor" can refer to several different components.
    </p>

<p>
    The Tor software is a program you can run on your computer that
helps keep
    you safe on the Internet. Tor protects you by bouncing your
communications
    around a distributed network of relays run by volunteers all around
    the world: it prevents somebody watching your Internet connection
from
    learning what sites you visit, and it prevents the sites you visit
    from learning your physical location. This set of volunteer relays
is
    called the Tor network. You can read more about how Tor works on the
<a
    href="<page about/overview>">overview page</a>.
    </p>

<p>
    The Tor Project is a non-profit (charity) organization that
maintains
    and develops the Tor software.
    </p>

<hr>

<a id="Torisdifferent"></a>
    <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 from doing
bad
things.  Some simple proxy providers use SSL to secure your connection
to them.
This may protect 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 who you are and where 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 doing any number of things, such as
watching your traffic, injecting their own advertisements into your
traffic
stream, and 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, Tor does not modify, or even know, what you
are
sending into it.  It merely relays your traffic, completely encrypted
through
the Tor network and has it pop out somewhere else in the world,
completely
intact.  The Tor client is required because we assume you trust your
local
computer.  The Tor client manages the encryption and the path chosen
through
the network.  The relays located all over the world merely pass
encrypted
packets between themselves.</p>
    <p>
    <dl>
    <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 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, such as visiting a bank or
e-commerce website, or encrypted mail connections, etc, it will only
know the
destination.  It won't be able to see the data inside the traffic
stream.  You
are still protected from this node figuring out who you are and if using
encryption, what data you're sending to the destination.</dd>
    </dl>
    </p>

<hr>

<a id="CompatibleApplications"></a>
    <h3><a class="anchor" href="#CompatibleApplications">What programs
can I use with Tor?</a></h3>

<p>
    There are two pieces to "Torifying" a program: connection-level
anonymity
    and application-level anonymity. Connection-level anonymity focuses
on
    making sure the application's Internet connections get sent through
Tor.
    This step is normally done by configuring
    the program to use your Tor client as a "socks" proxy, but there are
    other ways to do it too. For application-level anonymity, you need
to
    make sure that the information the application sends out doesn't
hurt
    your privacy. (Even if the connections are being routed through Tor,
you
    still don't want to include sensitive information like your name.)
This
    second step needs to be done on a program-by-program basis, which is
    why we don't yet recommend very many programs for safe use with Tor.
    </p>

<p>
    Most of our work so far has focused on the Firefox web browser. The
    bundles on the <a href="<page download/download>">download page</a>
automatically
    install the <a href="<page torbutton/index>">Torbutton Firefox
    extension</a> if you have Firefox installed. As of version 1.2.0,
    Torbutton now takes care of a lot of the connection-level and
    application-level worries.
    </p>

<p>
    There are plenty of other programs you can use with Tor,
    but we haven't researched the application-level anonymity
    issues on them well enough to be able to recommend a safe
    configuration. Our wiki has a list of instructions for <a
    href="<wiki>doc/TorifyHOWTO">Torifying
    specific applications</a>. There's also a <a
    href="<wiki>doc/SupportPrograms">list
    of applications that help you direct your traffic through Tor</a>.
    Please add to these lists and help us keep them accurate!
    </p>

<hr>

<a id="WhyCalledTor"></a>
    <h3><a class="anchor" href="#WhyCalledTor">Why is it called
Tor?</a></h3>

<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>

<p>
    (It's also got a fine translation from German and Turkish.)
    </p>

<p>
    Note: even though it originally came from an acronym, Tor is not
spelled
    "TOR". Only the first letter is capitalized. In fact, we can usually
    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
    spell it wrong.
    </p>

<hr>

<a id="Backdoor"></a>
    <h3><a class="anchor" href="#Backdoor">Is there a backdoor in
Tor?</a></h3>

<p>
    There is absolutely no backdoor in Tor. Nobody has asked us to put
one
    in, and 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.
    </p>

<p>
    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!
    </p>

<p>
    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.
    </p>

<p>
    Also, there might be accidental bugs in Tor that could affect your
    anonymity. We periodically find and fix anonymity-related bugs, so
make
    sure you keep your Tor versions up-to-date.
    </p>

<hr>

<a id="DistributingTor"></a>
    <h3><a class="anchor" href="#DistributingTor">Can I distribute
Tor?</a></h3>

<p>
    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
    ask us for specific permission.
    </p>

<p>
    However, if you want to redistribute the Tor software you must
follow our
    <a href="<gitblob>LICENSE">LICENSE</a>.
    Essentially this means that you need to include our LICENSE file
along
    with whatever part of the Tor software you're distributing.
    </p>

<p>
    Most people who ask us this question don't want to distribute just
the
    Tor software, though. They want to distribute the <a
    href="<page projects/torbrowser>">Tor Browser</a>. This includes <a
    href="https://www.mozilla.org/en-US/firefox/all-aurora.html">Mozilla
    Aurora</a> and <a href="<page projects/vidalia>">Vidalia</a>.
    You will need to follow the licenses for those programs
    as well. Both of them are distributed under the <a
    href="https://www.fsf.org/licensing/licenses/gpl.html">GNU General
    Public License</a>. The simplest way to obey their licenses is
    to include the source code for these programs everywhere you
    include the bundles themselves. Look for "source" packages on
    the <a href="<page projects/vidalia>">Vidalia page</a> and <a
    href="https://www.mozilla.org/en-US/firefox/all-aurora.html">Mozilla
    Aurora</a> pages.
    </p>

<p>
    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.
    </p>

<p>
    Lastly, you should realize that we release new versions of the
    Tor software frequently, and sometimes we make backward incompatible
    changes. So if you distribute a particular version of the Tor
software, it
    may not be supported &mdash; or even work &mdash; six months later.
This
    is a fact of life for all security software under heavy development.
    </p>

<hr>

<a id="SupportMail"></a>
    <h3><a class="anchor" href="#SupportMail">How can I get
support?</a></h3>

<p>Your best bet is to first try the following:</p>
    <ol>
    <li>Read through this <a href="<page docs/faq>">FAQ</a>.</li>
    <li>Read through the <a href="<page
docs/documentation>">documentation</a>.</li>
    <li>Read through the <a

href="https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-talk">
tor-talk
    archives</a> and see if your question is already answered.</li>
    <li>Join our <a href="ircs://irc.torproject.org#tor">irc channel</a>
and
    state the issue and wait for help.</li>
    <li>Send an email to <a

href="mailto:help@rt.torproject.org">help@rt.torproject.org</a>.</li>
    <li>If all else fails, try <a href="<page about/contact>">contacting
us</a> directly.</li>
    </ol>

<p>If you find your answer, please stick around on the IRC channel
or the
    mailing list to help others who were once in your position.</p>

<hr>

<a id="Forum"></a>
    <h3><a class="anchor" href="#Forum">Is there a Tor forum?</a></h3>

<p>Not yet, but we're working on it. Most forum software is
    a disaster to maintain and keep secure, and at the same time
    too many of the Tor developers are spread too thin to be able
    to contribute enough to a forum.
    </p>

<hr>

<p>
    There are many reasons why the Tor network is currently slow.
    </p>

<p>
    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
    will always be present. You shouldn't expect to see university-style
    bandwidth through Tor.
    </p>

<p>
    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
    handle file-sharing traffic load.
    </p>

<p>
    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.
    </p>

<ul>

<li>
    <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
    all the users who want privacy and security on the Internet.
    </li>

<li>
    <a href="<page projects/vidalia>">Help us make Tor more usable</a>.
We
    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>

<li>
    There are some bottlenecks in the current Tor network. Help us
design
    experiments to track down and demonstrate where the problems are,
and
    then we can focus better on fixing them.
    </li>

<li>
    Tor needs some architectural changes too. One important change is to
    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
    get to spend more time on it.
    </li>

<li>
    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>.
    </li>

<li>
    Help find sponsors for Tor. Do you work at a company or government
agency
    that uses Tor or has a use for Internet privacy, e.g. to browse the
    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.
    </li>

<li>
    If you can't help out with any of the above, you can still help out
    individually by <a href="<page donate/donate>">donating a bit of
money to the
    cause</a>. It adds up!
    </li>

</ul>

<hr>

<a id="FileSharing"></a>
    <h3><a class="anchor" href="#FileSharing">How can I share files anonymously through Tor?"</a></h3>

<p>
    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 excessively wastes everyone's bandwidth (slows down browsing). Also, Bittorrent over Tor <a href="https://blog.torproject.org/blog/bittorrent-over-tor-isnt-good-idea">is not anonymous</a>!
    </p>

<hr>

<a id="Funding"></a>
    <h3><a class="anchor" href="#Funding">What would The Tor Project do
with more funding?</a></h3>

<p>
    The Tor network's <a

href="https://metrics.torproject.org/network.html#networksize">several
    thousand</a> relays push <a
    href="https://metrics.torproject.org/network.html#bandwidth">over
    1GB per second on average</a>. We have <a

href="https://metrics.torproject.org/users.html#direct-users">several
    hundred thousand daily users</a>. But the Tor network is not yet
    self-sustaining.
    </p>

<p>
    There are six main development/maintenance pushes that need
attention:
    </p>

<ul>

<li>
    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.
    </li>

<li>
    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.
    </li>

<li>
    Relay support: the Tor network is run by volunteers, but they still
need
    attention with prompt bug fixes, explanations when things go wrong,
    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.,
    Tor relays have problems on Win XP currently.
    </li>

<li>
    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
    the difficult and confusing steps inside Tor. We've got a start on
this
    with the <a href="<page projects/vidalia>">Vidalia GUI</a>, but much
more work
    remains &mdash; usability for privacy software has never been easy.
    </li>

<li>
    Incentives: We need to work on ways to encourage people to configure
    their Tors as relays and exit nodes rather than just clients.
    <a href="#EverybodyARelay">We need to make it easy to become a
relay,
    and we need to give people incentives to do it.</a>
    </li>

<li>
    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
    behind these.
    </li>

</ul>

<p>
    We're continuing to move forward on all of these, but at this rate
    <a href="#WhySlow">the Tor network is growing faster than the
developers
    can keep up</a>.
    Now would be an excellent time to add a few more developers to the
effort
    so we can continue to grow the network.
    </p>

<p>
    We are also excited about tackling related problems, such as
    censorship-resistance.
    </p>

<p>
    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.
    </p>

<p>
    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
    director for information on making grants or major donations.
    </p>

<hr>

<a id="OutboundPorts"></a>
    <h3><a class="anchor" href="#OutboundPorts">Do I have to open all these outbound ports on my firewall?</a></h3>

<p>
    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 fetching updates to the directory). There are a variety of these ports, but many of them are running on 80, 443, 9001, and 9030.
    </p>
    <p>
So 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 -- and thus the most security -- as well as the most robustness in your connectivity, you'll want to let it connect to all of them.
    </p>
    <p>
If you really need to connect to only a small set of ports, see the FAQ entry on firewalled ports.
    </p>
    <p>
Note that if you're running as a Tor relay, you must allow outgoing connections to every other relay, and to anywhere your exit policy advertises that you allow. The cleanest way to do that is to simply allow all outgoing connections at your firewall. If you don't, clients will try to use these connections and things won't work. 
    </p>
    
    <hr>
    
    <a id="IsItWorking"></a>
    <h3><a class="anchor" href="#IsItWorking">How can I tell if Tor is working, and that my connections really are anonymized?</a></h3>

<p>
    There are sites you can visit that will tell you if you appear to be 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.
    </p>
    <p>
If that site is down, you can still test, but it will involve more effort: <a>http://ipid.shat.net</a> and <a>http://www.showmyip.com/</a> will tell you what your IP address appears to be, but you'll need to know your current IP address so you can compare and decide whether you're using Tor correctly.
    </p>
    <p>
To learn your IP address on OS X, Linux, BSD, etc, run "ifconfig". On Windows, go to the Start menu, click Run and enter "cmd". At the command prompt, enter "ipconfig /a".
    </p>
    <p>
If you are behind a NAT or firewall, though, your IP address will be within the range of 10.XXX.XXX.XXX, 192.168.XXX.XXX, or 172.16.XXX.XXX - 172.31.XXX.XXX, which is not your public IP address. In this case, you should check your IP address with one of the sites above without using Tor, and then check again using Tor to see whether your IP address has changed. 
    </p>
    
    <hr>
    
    <a id="FTP"></a>
    <h3><a class="anchor" href="#FTP">How do I use my browser for ftp with Tor?
    </a></h3>

<p>Use the Tor Browser Bundle. If you want a separate application for an 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". </p>
    <hr>
    
    <a id="Metrics"></a>
    <h3><a class="anchor" href="#Metrics">How many people use Tor? How
many relays or exit nodes are there?</a></h3>

<p>
    All this and more about measuring Tor can be found at the <a
    href="https://metrics.torproject.org/">Tor Metrics Portal</a>.</p>
    <hr>

<a id="SSLcertfingerprint"></a>
    <h3><a class="anchor" href="#SSLcertfingerprint">What are the SSL
certificate fingerprints for Tor's various websites?</a></h3>
    <p>
    <pre>
    *.torproject.org SSL certificate from Digicert:
    The serial number is:
06:DE:97:E5:1D:C3:9D:C2:64:8D:AC:72:DD:41:01:FC
    The SHA-1 fingerprint is: 1f9d306e8bfccfcb03981a71a27a9f5d1e0876ce
    The SHA-256 fingerprint is:
3613d2b22a750094760c41ad19db52a4f05bdea80172e2578761ad967f7ed9aa

blog.torproject.org SSL certificate from RapidSSL:
    The serial number is: 00:EF:A3
    The SHA-1 fingerprint is: 50af43db8438e67f305a3257d8ef198e8c42f13f
    </pre>
    </p>
    <hr>

<a id="HowUninstallTor"></a>
    <h3><a class="anchor" href="#HowUninstallTor">How do I uninstall
Tor?</a></h3>

<p>
    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.
    </p>

<p>
    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. The
proper way to
    completely remove Tor, Vidalia, and Torbutton for Firefox on any
version of Windows is as follows:
    </p>

<ol>
    <li>In your taskbar, right click on Vidalia (the green onion or the
black head)
    and choose exit.</li>
    <li>Right click on the taskbar to bring up TaskManager. Look for
tor.exe in the
    Process List. If it's running, right click and choose End
Process.</li>
    <li>Click the Start button, go to Programs, go to Vidalia, choose
Uninstall.
    This will remove the Vidalia bundle, which includes Tor.</li>
    <li>Start Firefox. Go to the Tools menu, choose Add-ons. Select
Torbutton.
    Click the Uninstall button.</li>
    </ol>

<p>
    If you do not follow these steps (for example by trying to uninstall
    Vidalia and Tor while they are still running), you will need to
    reboot and manually remove the directory "Program Files\Vidalia
Bundle".
    </p>

<p>
    For Mac OS X, follow the <a
    href="<page docs/tor-doc-osx>#uninstall">uninstall directions</a>.
    </p>

<p>
    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.
    </p>

<hr>

<a id="PGPSigs"></a>
    <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; the <a href="<page projects/torbrowser>">Tor Browser
Bundle</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 via email: log in to your Gmail account and mail
'<tt>gettor@gettor.torproject.org</tt>'. If you include the word 'help'
in the body of the email, it will reply with instructions. Note that
only a few webmail providers are supported, since they need to be able
to receive very large attachments.
</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>

<hr>

<a id="CompileTorWindows"></a>
    <h3><a class="anchor" href="#CompileTorWindows">How do I compile Tor
under Windows?</a></h3>

<p>
    Try following the steps at <a
href="<gitblob>doc/tor-win32-mingw-creation.txt">
    tor-win32-mingw-creation.txt</a>.
    </p>

<p>
    (Note that you don't need to compile Tor yourself in order to use
    it. Most people just use the packages available on the <a
href="<page
    download/download>">download page</a>.)
    </p>

<hr>

<a id="VirusFalsePositives"></a>
    <h3><a class="anchor" href="#VirusFalsePositives">Why does my Tor
executable appear to have a virus or spyware?</a></h3>

<p>
    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 &mdash; 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 pick a
better vendor.
    </p>

<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
<a
    href="#CompileTorWindows">recompile it yourself</a>.
    </p>

<hr>

<a id="LiveCD"></a>
    <h3><a class="anchor" href="#LiveCD">Is there a LiveCD or other
bundle that includes Tor?</a></h3>

<p>
    Yes.  Use <a href="https://tails.boum.org/">The Amnesic Incognito
    Live System</a> or <a href="<page projects/torbrowser>">the Tor
Browser
    Bundle</a>.
    </p>

<hr>

<a id="TBBFlash"></a>
<h3><a class="anchor" href="#TBBFlash">Why can't I view videos on
YouTube
and other Flash-based sites?</a></h3>

<p>
YouTube and similar 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: <a href="http://decloak.net">completely disregarding
proxy settings</a>, 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 own
cookies</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>

<p>
<a href="https://www.youtube.com/html5">YouTube offers experimental HTML5 video
support</a> for many of their videos. You can use their Advanced Search to
find HTML5 videos.
</p>

<hr>

<a id="TBBSocksPort"></a>
<h3><a class="anchor" href="#TBBSocksPort">
I want to run another application through the Tor launched by Tor
Browser Bundle.</a></h3>

<p>
Typically Tor listens for Socks connections on port 9050. TBB listens
on port 9150.
The goal is to avoid conflicting with a "system" Tor install,
so you can run a system Tor and TBB at the same time. We're <a
href="https://trac.torproject.org/projects/tor/ticket/3948">working on
a feature</a> where Tor will try the usual ports first and then back
off to a random choice if they're already in use.
</p>

<hr>

<a id="TBBPolipo"></a>
<h3><a class="anchor" href="#TBBPolipo">I need an HTTP proxy. Where did
Polipo go?</a></h3>

<p>
In the past, Tor bundles included an HTTP proxy like Privoxy or Polipo,
solely to work around a bug in Firefox that was finally fixed in Firefox
6. Now you don't need a separate HTTP proxy to use Tor, and in fact
leaving it out makes you safer because Torbutton has better control over
Firefox's interaction with websites.
</p>

<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 &mdash; Tor runs a Socks
proxy on port 9050 on Windows, or <a href="#TBBSocksPort">see above</a>
for OSX and Linux.
</p>

<p>
If that fails, 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>

<hr>

<a id="TBBOtherExtensions"></a>
<h3><a class="anchor" href="#TBBOtherExtensions">Can I install other
Firefox extensions?</a></h3>

<p>
Yes. Just install them like normal. But be sure to avoid extensions like
Foxyproxy that screw up your proxy settings. Also, avoid
privacy-invasive
extensions (for example, pretty much anything with the word Toolbar in
its name).
</p>

<p>
Generally, extensions that require registration, and/or provide
additional information about websites you are visiting, should be
suspect.
</p>

<p>
Extensions you might like include
 <a href="https://addons.mozilla.org/firefox/addon/953">RefControl</a> (referer spoofing),
 <a href="https://addons.mozilla.org/firefox/addon/1474">SafeCache</a>,
 <a href="https://addons.mozilla.org/en-US/firefox/addon/6623">Better Privacy</a>,
 <a href="https://addons.mozilla.org/firefox/addon/1865">AdBlock Plus</a> (EasyPrivacy+EasyList),
 <a href="https://addons.mozilla.org/firefox/addon/82">Cookie Culler</a>,
 <a href="https://addons.mozilla.org/en-US/firefox/addon/9727/">Request Policy</a> and
 <a href="https://addons.mozilla.org/en-US/firefox/addon/certificate-patrol/">Certificate Patrol</a>.
</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 the Tor Browser Bundle?
Isn't that unsafe?</a></h3>

<p>
We configure NoScript to allow JavaScript by default in the Tor
Browser Bundle 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>
Unfortunately, Torbutton only works with Firefox right now, and without
<a href="https://www.torproject.org/torbutton/en/design/">Torbutton's
extensive privacy fixes</a> there are many ways for websites or other
attackers to recognize you, track you back to your IP address, and so
on.
In short, using any browser besides Tor Browser Bundle with Tor is a
really bad idea.
</p>

<p>
We're working with the Chrome team to <a
href="https://blog.torproject.org/blog/google-chrome-incognito-mode-tor-
and-fingerprinting">fix
some bugs and missing APIs in Chrome</a> so it will be possible to write
a
Torbutton for Chrome. No support for any other browser is on the
horizon.
</p>

<hr>

<a id="TBBCloseBrowser"></a>
<h3><a class="anchor" href="#TBBCloseBrowser">I want to leave Tor
Browser
Bundle running but close the browser.</a></h3>

<p>
We're working on a way to make this possible on all platforms. Please
be patient.
</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>

<p>
Torbutton 1.2.5 (released in mid 2010) detects Google captchas and can
automatically redirect you to a more Tor-friendly search engine such as
DuckDuckGo, ixquick, or Bing.
</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. The easy way to avoid this "feature" is to use <a>http://google.com/ncr</a>.
</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: http://google.com/search?q=...&amp;hl=en&amp;..g
</p>
<p>
In Firefox you can search for the google.src file and add the line &lt;input name="hl" value="en"&gt;g to it. Then restart Firefox and it will automatically add the "hl=en" name/value pair to all queries made from the search bar so you will get English results regardless of which Google server you have been sent to. Note that this file is actually 'hidden' as part of the application container on Macs. To get to this file on a Mac you have to right click on the Firefox application icon and select "Show Package Contents" then navigate to Contents/MacOS/searchplugins.
</p>
<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. You can also set your language by first selecting it in the Language Tools section, search for something simple. Then extract the language from the URL. In this example, we'll choose Hebrew:  <a>http://www.google.com/search?lr=lang_g'''iw</a>. Next, use that string in the url:  <a>http://google.com/intl/iw/</a>. This can obviously be set as your homepage or bookmarked if necessary. 
</pb>

<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="torrc"></a>
<h3><a class="anchor" href="#torrc">I'm supposed to "edit my torrc".
What does that mean?</a></h3>

<p>
Tor installs 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. Users of Vidalia can
make common changes through the Vidalia interface &mdash; only advanced
users should need to modify their torrc file directly.
</p>
<p>
Tor Browser Bundle users should edit your torrc through Vidalia. Open
the
Vidalia Control Panel. Choose Settings. Choose Advanced. Click the
button
labelled "Edit current torrc". Remember to make sure the checkbox for
"Save Settings." is checked. Hit the Ok button and you are done.
</p>
<p>
Otherwise, you will need to edit the file manually.
The location of your torrc file depends on the way you installed Tor:
</p>
<ul>
<li>If you installed Tor Browser Bundle, look for
<code>Data/Tor/torrc</code> inside your Tor Browser Bundle directory.
</li>
<li>On Windows, if you installed a Tor bundle with Vidalia, you can
find your torrc file in the Start menu under Programs -&gt; Vidalia
Bundle -&gt; Tor, or you can find it by hand in <code>\Documents and
Settings\<i>username</i>\Application Data\Vidalia\torrc</code>. If you
installed Tor without Vidalia, you can find your torrc in the Start
menu under Programs -&gt; Tor, or manually in either <code>\Documents
and Settings\Application Data\tor\torrc</code> or <code>\Documents and
Settings\<i>username</i>\Application Data\tor\torrc</code>.
</li>
<li>On OS X, if you use Vidalia, edit
<code>~/.vidalia/torrc</code>. Otherwise, open your favorite text editor
and load <code>/Library/Tor/torrc</code>.
</li>
<li>On Unix, if you installed a pre-built package, look for
<code>/etc/tor/torrc</code> or <code>/etc/torrc</code> or consult your
package's documentation.
</li>
<li>Finally, if you installed from source, you may not have a torrc
installed yet: look in <code>/usr/local/etc/</code> and note that you
may need to manually copy <code>torrc.sample</code> to
<code>torrc</code>.
</li>
</ul>

<p>
If you use Vidalia, be sure to exit both Tor and Vidalia before you edit
your torrc file manually. Otherwise Vidalia might overwrite your
changes.
</p>

<p>
Once you've changed your torrc, you will need to restart Tor for the
changes to take effect. (For advanced users on OS X and Unix, note that
you actually only need to send Tor a HUP signal, not actually restart
it.)
</p>

<p>
For other configuration options you can use, look at the <a href="<page
docs/tor-manual>">Tor manual page</a>. 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>
If you installed a Tor bundle that includes Vidalia, then Vidalia has a
window called "Message Log" that will show you Tor's log messages. Click
on "Advanced" to see more details. You can click on "Settings" to change
your log verbosity or save the messages to a file. You're all set.
</p>

<p>
If you're not using Vidalia, 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
logs in your torrc file, they default to <code>\username\Application
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>
    <li>"err": something bad just happened, and we can't recover. Tor will exit.</li>
    <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 correct the problem.</li>
    <li>"notice": something the operator will want to know about.</li>
    <li>"info": something happened (maybe bad, maybe ok), but there's nothing you need to (or can) do about it.</li>
    <li>"debug": for everything louder than info. It is quite loud indeed.</li> 
</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>

<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 the Tor bundle 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, the onion icon in
Vidalia will turn green (and if you're running Tor Browser Bundle, it
will
automatically launch a browser for you). You can also check in the
Vidalia
Control Panel to make sure it says "Connected to the Tor
network!" under Status. For those not using Vidalia, check your <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>Are you sure Tor is running? If you're using Vidalia, you may have
to click on the onion and select "Start" to launch Tor.</li>
<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="<wikifaq>#MyInternetconnectionrequiresanHTTPorSOCKSproxy.">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="VidaliaPassword"></a>
<h3><a class="anchor" href="#VidaliaPassword">Tor/Vidalia prompts for a
password at start.</a></h3>

<p>
Vidalia interacts with the Tor software via Tor's "control port". The
control port lets Vidalia receive status updates from Tor, request a new
identity, configure Tor's settings, etc. Each time Vidalia starts Tor,
Vidalia sets a random password for Tor's control port to prevent other
applications from also connecting to the control port and potentially
compromising your anonymity.
</p>

<p>
Usually this process of generating and setting a random control password
happens in the background. There are three common situations, though,
where Vidalia may prompt you for a password:
</p>

<ol>
<li>You're already running Vidalia and Tor. For example, this situation
can happen if you installed the Vidalia bundle and now you're trying to
run the Tor Browser Bundle. In that case, you'll need to close the old
Vidalia and Tor before you can run this one.
</li>
<li>Vidalia crashed, but left Tor running with the last known random
password. After you restart Vidalia, it generates a new random password,
but Vidalia can't talk to Tor, because the random passwords are
different.
<br />
If the dialog that prompts you for a control password has a Reset
button,
you can click the button and Vidalia will restart Tor with a new random
control password.
<br />
If you do not see a Reset button, or if Vidalia is unable to restart
Tor for you, you can still fix the problem manually. Simply go into your
process or task manager, and terminate the Tor process. Then use Vidalia
to restart Tor and all will work again.
</li>
<li>You had previously set Tor to run as a Windows NT service. When Tor
is set to
run as a service, it starts up when the system boots. If you configured
Tor to start as a service through Vidalia, a random password was set
and saved in Tor. When you reboot, Tor starts up and uses the random
password it saved. You login and start up Vidalia. Vidalia attempts to
talk to the already running Tor. Vidalia generates a random password,
but it is different than the saved password in the Tor service.
<br />
You need to reconfigure Tor to not be a service. See the FAQ entry on
<a href="<wikifaq>#HowdoIrunmyTorrelayasanNTservice">running Tor as a
Windows NT service</a>
for more information on how to remove the Tor service.
</li>
</ol>

<hr>

<a id="ChooseEntryExit"></a>
    <h3><a class="anchor" href="#ChooseEntryExit">Can I control which
nodes (or country) are used for entry/exit?</a></h3>

<p>
    Yes. You can set preferred entry and exit nodes as well as
    inform Tor which nodes you do not want to use.
    The following options can be added to your config file <a
    href="#torrc">"torrc"</a> or specified on the command line:
    </p>
    <dl>
      <dt><tt>EntryNodes $fingerprint,$fingerprint,...</tt></dt>
        <dd>A list of preferred nodes to use for the first hop in the
circuit, if possible.
        </dd>
      <dt><tt>ExitNodes $fingerprint,$fingerprint,...</tt></dt>
        <dd>A list of preferred nodes to use for the last hop in the
circuit, if possible.
        </dd>
      <dt><tt>ExcludeNodes $fingerprint,$fingerprint,...</tt></dt>
        <dd>A list of nodes to never use when building a circuit.
        </dd>
      <dt><tt>ExcludeExitNodes $fingerprint,$fingerprint,...</tt></dt>
        <dd>A list of nodes to never use when picking an exit.
            Nodes listed in <tt>ExcludeNodes</tt> are automatically in
this list.
        </dd>
    </dl>
    <p>
    <em>We recommend you do not use these</em>
    &mdash; they are intended for testing and may disappear in future
versions.
    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.
    </p>
    <p>
    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 used to connect to hidden services, those that do directory fetches, 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 <a href="<page docs/tor-manual>">manual</a>.
    </p>
    <p>
    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), or a node
    nickname. Make sure there are no spaces between the commas and the
    list items.
    </p>
    <p>
    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>.
    See the manual page for details.
    </p>

<hr>

<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 that your firewall permits by adding "FascistFirewall 1"
to
your <a href="<page docs/faq>#torrc">torrc
configuration file</a>, or by clicking "My firewall only lets me connect
to certain ports" in Vidalia's Network Settings window.
</p>

<p>
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>

<a id="ExitPorts"></a>
    <h3><a class="anchor" href="#ExitPorts">Is there a list of default exit ports?</a></h3>
    <p>
The default open ports are listed below but keep in mind that, any port or ports can be opened by the relay operator by configuring it in torrc or modifying the source code. But the default according to src/or/policies.c from the source code release tor-0.2.4.16-rc is: 
    </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>
    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. 
    </p>

<hr>

<a id="RelayFlexible"></a>
    <h3><a class="anchor" href="#RelayFlexible">How stable does my relay
need to be?</a></h3>

<p>
    We aim to make setting up a Tor relay easy and convenient:
    </p>

<ul>
    <li>Tor has built-in support for <a
    href="<wikifaq>#WhatbandwidthshapingoptionsareavailabletoTorrelays">
    rate limiting</a>. Further, if you have a fast
    link but want to limit the number of bytes per
    day (or week or month) that you donate, check out the <a

href="<wikifaq>#HowcanIlimitthetotalamountofbandwidthusedbymyTorrelay">
hibernation
    feature</a>.
    </li>
    <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.
    </li>
    <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>
    <li>We can handle relays with dynamic IPs just fine &mdash; simply
    leave the Address config option blank, and Tor will try to guess.
    </li>
    <li>If your relay is behind a NAT and it doesn't know its public
    IP (e.g. it has an IP of 192.168.x.y), you'll need to set up port
    forwarding. Forwarding TCP connections is system dependent but
    <a href="<wikifaq>#ImbehindaNATFirewall">this FAQ entry</a>
    offers some examples on how to do this.
    </li>
    <li>Your relay will passively estimate and advertise its recent
    bandwidth capacity, so high-bandwidth relays will attract more users
than
    low-bandwidth ones. Therefore having low-bandwidth relays is useful
too.
    </li>
    </ul>

<hr>

<a id="RunARelayBut"></a>
    <a id="ExitPolicies"></a>
    <h3><a class="anchor" href="#ExitPolicies">I'd run a relay, but I
don't want to deal with abuse issues.</a></h3>

<p>
    Great. That's exactly why we implemented exit policies.
    </p>

<p>
    Each Tor relay has an exit policy that specifies what sort of
    outbound connections are allowed or refused from that relay. The
exit
    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
    the services, hosts, and networks he wants to allow connections to,
    based on abuse potential and his own situation. Read the FAQ entry
on
    <a href="<page docs/faq-abuse>#TypicalAbuses">issues you might
encounter</a>
    if you use the default exit policy, and then read Mike Perry's
    <a href="<blog>tips-running-exit-node-minimal-harassment">tips
    for running an exit node with minimal harassment</a>.
    </p>

<p>
    The default exit policy allows access to many popular services
    (e.g. web browsing), but <a
href="<wikifaq>#Istherealistofdefaultexitports">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
    using Vidalia's "Sharing" tab, or by manually 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
    "reject *:*" (or un-check all the boxes in Vidalia). 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
    (that is, your computer can resolve Internet addresses correctly).
    If there are any resources that your computer can't reach (for
example,
    you are behind a restrictive firewall or content filter), please
    explicitly reject them in your exit policy &mdash; otherwise Tor
users
    will be impacted too.
    </p>

<hr>

<a id="DifferentComputer"></a>
    <h3><a class="anchor" href="#DifferentComputer">I want to run my Tor client on a different computer than my applications.</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 than the Tor client, you should edit your torrc to define SocksListenAddress 0.0.0.0 g 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>
Configuration is simple, editing your torrc file's SocksListenAddress according to the following examples:
    </p>
    <pre>
  SocksListenAddress 127.0.0.1 #This provides local interface access only, needs SocksPort to be greater than 0
  SocksListenAddress 192.168.x.x:9100 #This provides access to Tor on a specified interface
  SocksListenAddress 0.0.0.0:9100 #Accept from all interfaces
    </pre>
    <p>
You can state multiple listen addresses, in the case that you are part of several networks or subnets.
    </p>
    <pre>
  SocksListenAddress 192.168.x.x:9100 #eth0
  SocksListenAddress 10.x.x.x:9100 #eth1
    </pre>
    <p>
After this, your clients on their respective networks/subnets would specify a socks proxy with the address and port you specified SocksListenAddress to be. 
    </p>
    <p>
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>
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. 
    </p>

<hr>

<a id="RelayOrBridge"></a>
    <h3><a class="anchor" href="#RelayOrBridge">Should I be a normal
relay or bridge relay?</a></h3>

<p><a href="<page docs/bridges>">Bridge relays</a> (or "bridges" for
short)
    are <a href="<page docs/tor-doc-relay>">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.
    </p>

<p>Being a normal relay vs being a bridge relay is almost the same
    configuration: it's just a matter of whether your relay is listed
    publicly or not.
    </p>

<p>
    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.
    </p>

<p>
    Several countries, including China and Iran, have found ways to
    detect and block connections to Tor bridges.
    <a href="<page projects/obfsproxy>">Obfsproxy</a> bridges address
    this by adding another layer of obfuscation.
    </p>

<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 a normal relay, since we need more exits. If you can't be an
    exit and only have a little bit of bandwidth, be a bridge. Thanks
    for volunteering!
    </p>

<hr>

<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>
    MyFamily $fingerprint1,$fingerprint2,$fingerprint3
</pre>

<p>
where each fingerprint is the 40 character identity fingerprint (without
spaces). You can also list them by nickname, but fingerprint is safer.
Be
sure to prefix the digest strings with a dollar sign ('$') so that the
digest is not confused with a nickname in the config file.
</p>

<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>

<hr>

<a id="RelayMemory"></a>
    <h3><a class="anchor" href="#RelayMemory">Why is my Tor relay using
so much memory?</a></h3>

<p>If your Tor relay is using more memory than you'd like, here are
some
    tips for reducing its footprint:
    </p>

<ol>
    <li>If you're on Linux, you may be encountering memory fragmentation
    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
    open, you are probably losing a lot of memory to OpenSSL's internal
    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>

<li>If you're running on Solaris, OpenBSD, NetBSD, or
    old FreeBSD, Tor is probably forking separate processes
    rather than using threads. Consider switching to a <a
    href="<wikifaq>#WhydoesntmyWindowsorotherOSTorrelayrunwell">better
    operating system</a>.</li>

<li>If you still can't handle the memory load, consider reducing the
    amount of bandwidth your relay advertises. Advertising less
bandwidth
    means you will attract fewer users, so your relay shouldn't grow
    as large. See the <tt>MaxAdvertisedBandwidth</tt> option in the man
    page.</li>

</ol>

<p>
    All of this said, fast Tor relays do use a lot of ram. It is not
unusual
    for a fast exit relay to use 500-1000 MB of memory.
    </p>

<hr>

<a id="WhyNotNamed"></a>
    <h3><a class="anchor" href="#WhyNotNamed">Why is my Tor relay not
named?</a></h3>

<p>
    We currently use these metrics to determine if your relay should be
named:<br>
    </p>
    <ul>
    <li>The name is not currently mapped to a different key. Existing
mappings
    are removed after 6 months of inactivity from a relay.</li>
    <li>The relay must have been around for at least two weeks.</li>
    <li>No other router may have wanted the same name in the past
month.</li>
    </ul>

<hr>

<a id="RelayDonations"></a>
    <h3><a class="anchor" href="#RelayDonations">Can I donate for a
    relay rather than run my own?</a></h3>

<p>
    Sure! We recommend these non-profit charities that are happy to turn
    your donations into better speed and anonymity for the Tor network:
    </p>
    <ul>
    <li><a href="https://www.torservers.net/">torservers.net</a>
    is a German charitable non-profit that runs a wide variety of
    exit relays worldwide. They also like donations of bandwidth from
    ISPs.</li>
    <li><a
href="https://www.noisebridge.net/wiki/Noisebridge_Tor">Noisebridge</a>
    is a US-based 501(c)(3) non-profit that collects donations and turns
    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>
    </ul>

<p>
    These organizations are not the same as <a href="<page
    donate/donate>">The Tor Project, Inc</a>, but we consider that a
    good thing. They're both run by nice people who are part of the
    Tor community.
    </p>

<p>
    Note that there can be a tradeoff here between anonymity and
    performance. The Tor network's anonymity comes in part from
diversity,
    so if you are in a position to run your own relay, you will be
    improving Tor's anonymity more than by donating. At the same time
    though, economies
    of scale for bandwidth mean that combining many small donations into
    several larger relays is more efficient at improving network
    performance. Improving anonymity and improving performance are both
    worthwhile goals, so however you can help is great!
    </p>

<hr>

<a id="KeyManagement"></a>
    <h3><a class="anchor" href="#KeyManagement">Tell me about all the
keys Tor uses.</a></h3>

<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
    talking to the relays they meant to talk to, and 3) signatures to
make
    sure all clients know the same set of relays.
    </p>

<p>
    <b>Encryption</b>: first, all connections in Tor use TLS link
encryption,
    so observers can't look inside to see which circuit a given cell is
    intended for. Further, the Tor client establishes an ephemeral
encryption
    key with each relay in the circuit; these extra layers of encryption
    mean that only the exit relay can read
    the cells. Both sides discard the circuit key when the circuit ends,
    so logging traffic and then breaking into the relay to discover the
key
    won't work.
    </p>

<p>
    <b>Authentication</b>:
    Every Tor relay has a public decryption key called the "onion key".
    Each relay rotates its onion key once a week.
    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.
    </p>

<p>
    <b>Coordination</b>:
    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), he 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
    is to give them a specially modified version of the software.
    </p>

<p>
    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>.
    </p>

<p>
    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
    and start meeting people.
    </p>

<hr>

<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
<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>, <a
href="http://freehaven.net/anonbib/#wright03">Defending Anonymous
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>

<hr>

<a id="ChangePaths"></a>
    <h3><a class="anchor" href="#ChangePaths">How often does Tor change its paths?</a></h3>
    <p>
     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 will switch to a new circuit immediately.)
    </p>
    <p>
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.
    </p>

<a id="OutboundConnections"></a>
    <h3><a class="anchor" href="#OutboundConnections">Why does netstat show these outbound connections?</a></h3>
    <p>
    Because that's how Tor works. It holds open a handful of connections so there will be one available when you need one. 
    </p>

<hr>

<a id="CellSize"></a>
    <h3><a class="anchor" href="#CellSize">Tor uses hundreds of bytes for every IRC line. I can't afford that!</a></h3>
    <p>
     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 afraid Tor is not useful for you right now.
    </p>
    <p>
The actual content of these fixed size cells is <a href="https://gitweb.torproject.org/torspec.git/blob/HEAD:/tor-spec.txt">documented in the main Tor spec</a>, section 3.
    </p>
    <p>
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="https://www.torproject.org/getinvolved/volunteer.html.en#Research">research ideas</a> that may involve changing the cell size. 
    </p>

<hr>

<a id="EverybodyARelay"></a>
    <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="<wikifaq>#DoIgetbetteranonymityifIrunarelay">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
    increases the size of the anonymity set.
    </p>

<p>
    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: Vidalia has an easy relay configuration interface, and
supports
    uPnP too. Tor is good at automatically detecting whether it's
reachable and
    how much bandwidth it can offer.
    </p>

<p>
    There are five steps we need to address before we can do this
though:
    </p>

<p>
    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 <a href="https://www.torproject.org/press/2008-12-19-roadmap-press-release"
>our
    development roadmap</a>.
    </p>

<p>
    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.
    </p>

<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>

<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
    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.
    </p>

<p>
    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>.
    </p>

<p>
    Please help on all of these!
    </p>

<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="<wikifaq>#DoesTorresistremotephysicaldevicefingerprinting">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
hidden 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>

<hr>

<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 a hidden service or a ".exit" address it could be 4.
</p>
<p>
 We don't want to encourage people to use paths longer than this -- 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/tor/trunk/doc/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>.
</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. We're not sure of the right trade-offs here. Please write a research paper that tells us what to do. 
</p>

<hr>

<a id="SplitEachConnection"></a>
    <h3><a class="anchor" href="#SplitEachConnection">You should split each connection over many paths.</a></h3>

<p>
 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.
    </p>
    <p>
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.
    </p>
    <p>
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>.
    </p>
    <p>
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.
    </p>

<hr>

<a id="UnallocatedNetBlocks"></a>
    <h3><a class="anchor" href="#UnallocatedNetBlocks">Your default exit policy should block unallocated net blocks too.</a></h3>

<p>
 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?
    </p>
    <p>
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. 
    </p>

<hr>

<a id="BlockWebsites"></a>
    <h3><a class="anchor" href="#BlockWebsites">Exit policies should be able to block websites, not just IP addresses.</a></h3>

<p>
 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).
    </p>
    <p>
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.
    </p>
    <p>
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. 
    </p>

<hr>

<a id="BlockContent"></a>
    <h3><a class="anchor" href="#BlockContent">You should change Tor to prevent users from posting certain content.</a></h3>

<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 this problem.
    </p>
    <p>
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. 
    </p>

<hr>

<a id="IPv6"></a>
    <h3><a class="anchor" href="#IPv6">Tor should support IPv6.</a></h3>

<p>
     That's a great idea! There are two aspects for IPv6 support that Tor needs. First, Tor needs to support exit to hosts that only have IPv6 addresses. Second, Tor needs to support Tor relays that only have IPv6 addresses.
    </p>
    <p>
The first is far easier: the protocol changes are relatively simple and isolated. It would be like another kind of exit policy.
    </p>
    <p>
The second is a little harder: right now, we assume that (mostly) every Tor relay can connect to every other. This has problems of its own, and adding IPv6-address-only relays adds problems too: it means that only relays with IPv6 abilities can connect to IPv6-address-only relays. This makes it possible for the attacker to make some inferences about client paths that it would not be able to make otherwise.
    </p>
    <p>
There is an  IPv6 exit proposal to address the first step for anonymous access to IPv6 resources on the Internet.
    </p>
    <p>
Full IPv6 support is definitely on our "someday" list; it will come along faster if somebody who wants it does some of the work.
    </p>

<hr>

<a id="Criminals"></a>
    <h3><a class="anchor" href="#Criminals">Doesn't Tor enable criminals
to do bad things?</a></h3>

<p>
    For the answer to this question and others, please see our <a
    href="<page docs/faq-abuse>">Tor Abuse FAQ</a>.
    </p>

<hr>

<a id="RespondISP"></a>
    <h3><a class="anchor" href="#RespondISP">How do I respond to my ISP
about my exit relay?</a></h3>

<p>
    A collection of templates for successfully responding to ISPs is <a
    href="<wiki>doc/TorAbuseTemplates">collected
    here</a>.
    </p>

<hr>

</div>
  
  <div id = "sidecol">
#include "side.wmi"
#include "info.wmi"
  </div>
  
</div>

#include <foot.wmi>

tor-webwml.git

Added 19 FAQ entries