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    <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 on my magazine's
    CD?</a></li>
    <li><a href="#SupportMail">How can I get an answer to my
    Tor support mail?</a></li>
    <li><a href="#WhySlow">Why is Tor so slow?</a></li>
    <li><a href="#Funding">What would The Tor Project do with more
    funding?</a></li>
    <li><a href="#Metrics">How many people use Tor? How many relays or
    exit nodes are there?</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>Running Tor:</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>
    </ul>

<p>Running a Tor client:</p>
    <ul>
    <li><a href="#DoesntWork">I installed Tor and Polipo but it's not
    working.</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="#GoogleCaptcha">Google makes me solve a Captcha or tells
    me I have spyware installed.</a></li>
    <li><a href="#GmailWarning">Gmail warns me that my account may have
    been compromised.</a></li>
    <li><a href="#FirewallPorts">My firewall only allows a few outgoing
    ports.</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>
    </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>
    </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>
    </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 see 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 isn't recording your personal details.
    </p>
    <p>
    Tor passes your traffic through at least 3 different servers before sending
it on to the destination.  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>TheOnionRouter/TorifyHOWTO">Torifying
    specific applications</a>. There's also a <a
    href="<wiki>TheOnionRouter/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 on my magazine's CD?</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 Tor bundles, which
    typically include <a href="http://www.pps.jussieu.fr/~jch/software/polipo/">Polipo</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 the <a
    href="http://www.pps.jussieu.fr/~jch/software/polipo/">Polipo
    download page</a>.
    </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 an answer to my Tor support mail?</a></h3>

<p>There is no official support for Tor. Your best bet is to 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="http://archives.seul.org/or/talk">OR-TALK Archives</a> and
    see if your question is already answered.</li>
    <li>Join our <a href="irc://irc.oftc.net#tor">irc channel</a> and
    state the issue and wait for help.</li>
    <li>Send an email to tor-assistants at torproject.org. These are
    volunteers who may be able to help you but you may not get a response
    for days.</li>
    </ol>

<p>If you find your answer, please stick around on the IRC channel or the
    mailing list and answer questions from others.</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>
    There are some steps that individuals
    can take to improve their Tor performance. <a
    href="<wiki>TheOnionRouter/FireFoxTorPerf">You
    can configure your Firefox to handle Tor better</a>, <a
    href="http://www.pps.jussieu.fr/~jch/software/polipo/tor.html">you can use
    Polipo with Tor</a>, or you can try <a href="<page download/download>">upgrading
    to the latest version of Tor</a>.  If this works well, please help by
    documenting what you did, and letting us know about it.
    </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="Funding"></a>
    <h3><a class="anchor" href="#Funding">What would The Tor Project do with more funding?</a></h3>

<p>
    We have about 1800 relays right now, pushing over 150 MB/s average
    traffic. We have several hundred thousand active users. 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 <a href="#RelayOS">stability</a> 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>

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

<p>
    This 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, Torbutton for Firefox, and Polipo 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 and Polipo.</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, Tor, and Polipo 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 google cache
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 AT 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://amnesia.boum.org/">The (Amnesic) Incognito
    Live System</a> or <a href="<page projects/torbrowser>">the Tor Browser
    Bundle</a>.
    </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>
The location of your torrc file depends on the way you installed Tor:
</p>

<ul>
<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. 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. You
can click on "Settings" to see more details, or to 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="DoesntWork"></a>
<h3><a class="anchor" href="#DoesntWork">I installed Tor and Polipo but it's not working.</a></h3>

<p>
Once you've installed the Tor bundle, there are two questions to ask:
first, is your Tor able to establish a circuit? Second, is your
Firefox correctly configured to send its traffic through Tor?
</p>

<p>If Tor can establish a circuit, the onion icon in
Vidalia will turn green. 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 XP users, synchronize your clock under
the clock -&gt; Internet time tab. In addition, correct the day and date
under the 'Date &amp; Time' Tab.</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>

<p>
Step two is to confirm that Firefox is correctly configured to send its
traffic through Tor. Try the <a href="https://check.torproject.org/">Tor
Check</a> site and see whether it thinks you are using Tor. See <a
href="<wikifaq>#HowcanItellifTorisworkingandthatmyconnectionsreallyareanonymizedArethereexternalserversthatwilltestmyconnection">the
Tor Check FAQ entry</a> for details.
</p>

<p>
If it thinks you're not using Tor, here are some hints:
</p>

<ol>
<li>Did you install the Torbutton extension for Firefox? The installation
bundles include it, but sometimes people forget to install it. Make sure
it says "Tor enabled" at the bottom right of your Firefox window. (For
expert users, make sure your http proxy is set to localhost port
8118.)</li>
<li>Do you have incompatible Firefox extensions like FoxyProxy
installed? If so, uninstall them. (Note that using FoxyProxy is NOT
a sufficient substitute for Torbutton. There are many known attacks
against a browser setup that does not include Torbutton. Read more
in the <a href="<page torbutton/torbutton-faq>">Torbutton FAQ</a> and the <a
href="https://www.torproject.org/torbutton/design/">Torbutton design</a>
specification.)</li>
<li>If your browser says "The proxy server is refusing connections.",
check that Polipo (the http proxy that passes traffic between Firefox
and Tor) is running. On Windows, look in the task manager and check for
a polipo.exe. On OS X, open the utilities folder in your applications
folder, and open Terminal.app. Then run "ps aux|grep polipo".</li>
<li>If you're upgrading from OS X, some of the earlier OS X installers
were broken in really unfortunate ways. You may find that <a href="<page
docs/tor-doc-osx>#uninstall">uninstalling everything</a> and then
installing a fresh bundle helps. Alas, the current uninstall instructions
may not apply anymore to your old bundle. Sorry.</li>
<li>If you're on Linux, make sure Privoxy isn't running, since it will
conflict with the port that our Polipo configuration file picks.</li>
<li>If you installed Polipo yourself (not from a bundle), did you edit the
config file as described? Did you restart Polipo after this change? Are
you sure?</li>
<li>For Red Hat Linux and related systems, do you have SELinux enabled? If
so, it might be preventing Polipo from talking to Tor. We also run across
BSD users periodically who have local firewall rules that prevent some
connections to localhost.</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>
    The <tt>EntryNodes</tt> and <tt>ExitNodes</tt> config options are
    treated as a request, meaning if the nodes are down or seem slow,
    Tor will still avoid them. You can make the option mandatory by
    setting <tt>StrictExitNodes 1</tt> or <tt>StrictEntryNodes 1</tt>
    &mdash; but if you do, your Tor connections will stop working
    if all of the nodes you have specified become unreachable.
    See the <a href="<page docs/documentation>#NeatLinks">Tor status pages</a>
    for some nodes you might pick.
    </p>
    <p>
    Instead of <tt>$fingerprint</tt> you can also specify a 2 letter
    ISO3166 country code in curly braces (for example {de}), 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="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
Ixquick or Bing.
</p>

<hr />

<a id="GmailWarning"></a>
<h3><a class="anchor" href="#GmailWarning">Gmail warns me that my account may have been compromised.</a></h3>

<p>
Sometimes, after you've used Gmail over Tor, Google presents a
pop-up notification that your account may have been compromised.
The notification window lists a series of IP addresses and locations
throughout the world recently used to access your account.
</p>

<p>
In general this is a false alarm: Google saw a bunch of logins from
different places, as a result of running the service via Tor, and decided
it was a good idea to confirm the account was being accessed by it's
rightful owner.
</p>

<p>
Even though this may be a biproduct of using the service via tor,
that doesn't mean you can entirely ignore the warning. It is
<i>probably</i> a false positive, but it might not be since it is
possible for someone to hijack your Google cookie.
</p>

<p>
Cookie hijacking is possible by either physical access to your computer
or by watching your network traffic.  In theory only physical access
should compromise your system because Gmail and similar services
should only send the cookie over an SSL link. In practice, alas, it's <a
href="http://fscked.org/blog/fully-automated-active-https-cookie-hijacking">
way more complex than that</a>.
</p>

<p>
And if somebody <i>did</i> steal your google cookie, they might end
up logging in from unusual places (though of course they also might
not). So the summary is that since you're using Tor, this security
measure that Google uses isn't so useful for you, because it's full of
false positives. You'll have to use other approaches, like seeing if
anything looks weird on the account, or looking at the timestamps for
recent logins and wondering if you actually logged in at those times.
</p>

<hr>

<a id="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="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="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 main Tor directory. That means
    that even an ISP or government trying to filter connections to the Tor
    network probably won't be able to block all the 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
    publically or not.
    </p>

<p>Right now, there are a small number of places in the world that filter
    connections to the Tor network. So getting a lot of bridges running
    right now is mostly a backup measure, a) in case the Tor network does
    get blocked somewhere, 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>So should you run a normal relay or bridge relay? If you have
    lots of bandwidth, you should definitely run a normal relay &mdash;
    bridge relays see very little use these days. 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, then flip a coin. 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="http://archives.seul.org/or/dev/Jun-2008/msg00001.html">release
    unused buffer memory more aggressively</a>. If you update to OpenSSL
    1.0.0-beta5, 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="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, so 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".
    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.
    Each relay rotates its onion key once a week.
    </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 threshold of the 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>

[https://www.torproject.org/docs/faq#EntryGuards Answer moved to our new FAQ page]

<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 user 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="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 plan to finally address that in 2009. 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="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>TheOnionRouter/TorAbuseTemplates">collected
    here</a>.
    </p>

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