Changes between Version 56 and Version 57 of doc/FAQUnanswered


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Apr 23, 2010, 4:48:48 AM (9 years ago)
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trac
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  • doc/FAQUnanswered

    v56 v57  
    88= Unanswered FAQ Questions =
    99
    10 1) '''Why do I keep getting messages telling me that my clock has just jumped ahead and that my circuits will be assumed broken? ''(eg.
    11 Oct 02 10:14:53.619 [notice] Your clock just jumped 1056 seconds forward; assuming established circuits no longer work.'') I've got a cron job to sync the time every eight hours and it's never out by more than a second.'''
     10'''Why does Firefox/Privoxy/Tor return Privoxy 404 pages so frequently -- almost every time -- when properly configured, even on sites like google.com?  How to mitigate?''' (Votes: 2)
    1211
    13 -- On my system, this happens when Vidalia gets into trouble. It seems that when tor and V communicate, tor can wind up waiting for V to respond, or for the V process to be killed.
    14 
    15 
    16 2)'''If an attacker has access to past logs of ISP and any given visted site, does the prng of the tor client allow the attacker to guess which circuit it used next?'''
    17 
    18 Tor uses cryptographically strong random numbers provided by OpenSSL when choosing nodes to use in a circuit. How OpenSSL implements this is operating system specific. If there's a weakness in Tor's method of choosing nodes, it probably isn't in the random number generator.
    19 
    20 *** Hmm, if the prng is deterministic, and you can narrow one result of calling it by knowing what host was chosen, can you, knowing the algorithm, however good it is, thereby narrow the result of the next call to it?  This question might be a bit ignorant; for example the prng might use other data on the client computer instead of following an algorithm to return the next item.
    21 
    22 *** Thanks for your comments, BTW, as a lot of us are wondering the answers to these FAQU.
    23 
    24 *** It has nothing todo with determinism. It is a matter of predictability. Without using real entropy, everything done on conventional computers should be deterministic (try teling that to people who use Windows ;). However, the part that we are interested in is whether someone can predict what Tor is going to next choose.
    25 
    26   Cryptographic number generators have the property that they (shouldn't) give up their internal state by their external outputs quickly. So, unless the attacker gets to see a lot of outputs - node choices - without reseeding from real entropy they are pretty much screwed provided the cryptograpphic prng isn't broken.
    27 
    28 
    29 '''Is there a signal i can send the tor client to tell it to switch circuits immediately?'''
    30 
    31 You can connect to Tor's control port and send "authenticateCRLFsignal newnymCRLF" where CRLF is a carriage return line feed pair.
    32 
    33 '''Why does Firefox/Privoxy/Tor return Privoxy 404 pages so frequently -- almost every time -- when properly configured, even on sites like google.com?  How to mitigate?  The tor process is running fine.'''
    34 (Votes: 2)
    35 
    36 The first problem is that Privoxy doesn't retry in case of
    37 DNS errors. It shows the 404 no such domain message right away.
    38 The second problem is that some browser cache Privoxy's error
    39 messages and Firefox is one of them.
    40 
    41 The Privoxy patch described at
    42 http://www.fabiankeil.de/sourcecode/privoxy/
    43 lets Privoxy retry in case of connection problems
    44 and makes sure, the browser doesn't reuse a cached
    45 error message.
    46 
    47 '''For that matter, why is DNS the main failure mode?  Who is timing out and why?  Can Tor (1) change the timeout, (2) deprecate bad servers, or (3) cache DNS locally so it doesn't have to make a long, slow, failure-prone DNS lookup every time?'''
    48 
    49 You should be sending hostnames to Tor over SOCKS4a or SOCKS5. In that case, the Tor exit node will resolve the hostname before making a connection for you. Unless the exit node is misconfigured, there shouldn't be a problem with DNS resolves timing out.
    50 
    51 *** Actually, for me this happens most of the time.  I am sending through Privoxy, and tried both sockses.  So why would I be hitting so many exit nodes that FREQUENTLY time out on DNS?  Firefox 1.5, most recent stable Tor.
     12Maybe it's just my Mac but I find that Tor has a habit of dying and staying dead which leaves privoxy and firefox non-functional. The work around I've found is that you can open Terminal directly and just start Tor by typing "Tor". I also find that if I start getting a lot of 403's or 504's then I can just kill the Tor instance running in Terminal and re-start it. More often than not, that fixes the problem.
    5213
    5314'''Is the reason that gmail rarely works: gmail, tor, privoxy, firefox, your own bandwidth/latency, tor's bw/latency, or some combination?  Is it fixable?'''
    5415
    55 *** For Gmail for me, it's even worse; I have to try 5 or 6 times before I get a page.
    56 
    57 -- I know that improvements to DNS handling are due in 1.2.x-final (see bug #364).
    58 
    5916'''Why is the argument against more than 3 hops that both-ends attacks are the enemy?  Wouldn't it be better to have more than 3 if the enemy cannot mount a both-ends attack?'''
    60 
    61 This is two questions, really.
    62 
    63 To understand it better, think from the perspective of an attacker. They have a choice: they can make a both-ends attack, an attack comprising the hops and both.
    64 
    65 Now, at this point you need to think what attacker you are trying to defend against. The NSA? Sorry, your screwed - find something else. A big company? Some rogue ISP's? Ok, now that's something that Tor may be able to defend against...
    66 
    67 Now, what information are you - the attacker - trying to find? Who is talking to who? Content of some transmissions? Both?
    68 
    69 Well, for finding content of transmissions your best way is to just listen in on some exit nodes.. or create some malicious ones. That's not the attack threat we're trying to defend against, then.
    70 For finding who is talking to who? Ok, let's say that's what we're trying to find out.
    71 
    72 From this perspective, what is needed to mount a both-ends attack? Listening on both ends. If there is some mitigation technique used - like random timing - compromising those both ends could come in handy (at this point, more hops would really be useful). But there isn't - AFAIK - and so we shall suppose that listening on both ends is enough.
    73 
    74 Now, what is the alternative option? Listening to all the hops AS WELL? Oh dear, that's a little harder.. and what does it bring us? Well, we can get slightly more accurate results - maybe - as we can more closely correlate. This, of course, gets yet better if we compromise some of those hops.
    75 
    76 The key word here is slightly. Provided the number of hops is a constant - so you can get a reasonably accurate latency estimate - then, over a longer enough period of time, you will be able to correlate and get your results.
    77 
    78 The key word here is slightly. Provided the number of hops is a constant - so you can get a reasonably accurate latency estimate - then, over a longer enough period of time, you will be able to correlate and get your results.
    79 
    80 So, yes, the hops will make it slightly more secure. But only slightly. And yes, in some cases, it may be really useful.. but in those cases, you probably should be using something with much stronger anonymity than tor (if someone knowing what your saying is unacceptable, don't use tor.. not, at any rate, on it's own).
    81 
    82 Now, then, what are the disadvantages to more hops? More bandwidth is used. Latency is increased.
    83 
    84 Despite this, it's possible that having number of hops as an easily configurable option is not a bad idea... I would guess that there is an excess of middle-man nodes with the recent draconian laws in some parts of the Western oh so free world that make people interested in privacy yet at the same time too scared to actually stick their neck out. In this case, Tor as a network would likely not lose much by doing that.
    85 
    86 *** I ommitted elaboration of the cases where more hops would be really useful (I think I forgot about it..). These cases are generally when traffic originating from an exit node is wanted to be tracked down and a response can be made quickly, but does not have global observer capabilities. In that case, in order to find the entry node, going through each hop is the only sensible solution in a network with > 50 servers. How much you would gain from extra hops is difficult to answer.. it would depend, I think, on how often circuits rotate and the probability of a hop being out of the grasp of the attackers. Hops are, I think, probably most useful when you - or your data - specifically are being targeted..
    8717
    8818'''How can I be sure that sending DNS through tor doesn't get spoofed sites?'''
    8919
    90 Use a method for authenticating the site you want to connect to, such as ensuring an SSL certificate is associated with the domain name and signed by a trusted authority, or verifying an SSH fingerprint out-of-band. Hidden services on Tor are not subject to this problem because their .onion names are self-authenticating as hashes of their public keys.
    91 
    9220'''How does tor relate to ipv6 and how should typical applications handle ipv6 if they use tor (or tor via Privoxy)?'''
    9321
    94 
    95 ---- /!\ '''Edit conflict - other version:''' ----
    96 Like a dog talking to a quasar... I never was good with similes.
    97 
    98 Tor carries TCP data (does it? Maybe it just carries some data that is then transported over TCP... I know, for instance, that it also contains IP and port... TCP doesn't) over another layer - currently IPv4. There is work to make Tor work with IPV6, but I am not aware of it being completed.
    99 
    100 As for how an application would interface with it -- depends. It could use SOCKS; in that case, I think it would have to use SOCKS5 in the event of addressing IPv6 (rather than domains). This could be hacked around by adding a .ipv6 domain - nasty.
    101 
    102 Privoxy? Same as currently, if it handles it..
     22'''What does *this* message mean?'''  (should this q be removed?)
    10323
    10424'''What version of libevent should I be using?'''
     
    10626the latest.  at least 1.1
    10727
    108 '''How to use Tor in squid? For using Tor on a network using Squid as proxy, for example...'''
     28'''How to use Tor in squid? For using Tor on a network using Squid as proxy, for example...
    10929
    110 '''How to use Tor with PF (Packet Filter, found in OpenBSD, NetBSD, DragonFlyBSD and FreeBSD)?'''
     30'''How to use Tor with PF (Packet Filter, found in OpenBSD, NetBSD, DragonFlyBSD and FreeBSD)?
    11131
    112 
    113 
    114 '''How does Tor work with tabbed browsing, say with Firefox?  Do these requests all follow the same circuit through the Tor network? Can an eavesdropper link a user across all sites opened simultaneously in tabs?'''
    115 
    116 ver 1.5 Works fine for me, I use No-Script Plugin to help be even safer. Anyone else have a problem with Firefox. Weither the request follow the same curcuit is out of my realm. My surfing experience is good to just fine. ProBastion
     32'''How does Tor work with tabbed browsing, say with Firefox? Do these requests all follow the same circuit through the Tor network? Can an eavesdropper link a user across all sites opened simultaneously in tabs?'''
    11733
    11834They will most likely all use the same circuit.  http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#ChangePaths
     
    12440If you wish to do this, you will need to make a copy of your Privoxy config file, and comment out the line that causes Privoxy to use the Tor service.  Once you have done this, to switch over, just stop Privoxy, swap the config files, and restart it. You can also automate the process with a very simple shell script - an OSX version including sample config files and a shell script can be found [http://idlecircuits.com/privoxyswitcher.zip here], and the script can be used as an example for other *nix systems.
    12541
    126 -- In fact, there is no need to stop/restart privoxy. On my system, I have the privoxy config file owned by me, so I can edit it directly. Changing between tor and no-tor is as simple as editing one line.
    127 
    128 Here's the relevant lines from my privoxy config file:
    129 {{{
    130 # Tor:
    131 #
    132 ## forward-socks4a / localhost:9050 . 
    133 forward-socks4a .onion localhost:9050 .
    134 
    135 # Do not torrify these (high volume/speed concerns, as well as PhP BBS
    136 # systems that consider a changed IP to be a new login.):
    137 forward .blood-bowl.net .
    138 forward .qemu-forum.ipi.fi .
    139 }}}
    140 The line with "##" on it is the line to toggle. Remove those to enable tor, add them to disable tor.
    141 
    142 NB: Every PHP BBS site I've seen will consider you to have logged out and relogged in if your IP address -- as seen by the PHP site -- changes. This means that if tor ever switches circuits and changes exit node, those sites will reset your "unread messages". I have not been able to find a decent way to solve this with TrackHostExits, given that vidalia will overwrite my tor config occasionally (and has no support for adding these internally, so I have two editors trying to change the tor config), the length of time needed to track varies from 30 minutes at some (forced logout after thirty minutes of idle time) to 24 hours at others, dealing with the occasional dead exit node (and then you need to use a new exit node earlier), etc. And, my list of exception sites is currently 26 lines long.
    143 
    14442'''Tor works fine for POP3 email. But, Whistle-blowers and others who need anonymous
    14543political free speech must have reliable SSL smtp email services. How can Tor be used
     
    14745servers? Blocking port 25 helps to defeat spammers, but the smtp SSL/TLS ports are not
    14846generally not used by them. Is there any solution with Tor to help the free speech folk
    149 and others who need to use secure, reliable smtp services? (votes: 6)'''
     47and others who need to use secure, reliable smtp services? (votes: 2)'''
    15048
    15149
     
    15755'''Can Tor be used in a network that has NO DEFAULT ROUTE?  The only access method from this network is to use a traditional proxy.  Is there a way to chain proxies so that TOR requests are sent outbound via the standard proxy? (votes: 1)'''
    15856
    159 Maybe. If you can get some routes for the Tor servers, then that of course is great.... assuming it must go through the proxy, however, it will need to support sending Tor requests.
    160 
    161 Assuming it does.. AFAIK, Tor doesn't have built-in support directly for this. So, you'll have to make it look to Tor like it really can connect .. err.. normally.
    162 
    163 I can see several ways of doing this.
    164 
    165 You could overload the network commands and have them actually connect to the proxy.. that's easy but a hack.
    166 
    167 You could try doing something with OpenVPN; personally, I don't have any experience with it... but I'm guessing you could do something neat with it (I remember someone setting up an OpenVPN with Tor being used to route things.. so it supports socks, I think, in some way. In that case, assuming the proxy is socks... all done :) Oterwise, you could write an interface to make it socks).
    16857
    16958
     
    18271'''How can I uninstall tor? (votes: 1)'''
    18372
    184 
    185 
    186 
    187 '''I have legal questions about running Tor. Is there anybody I can contact? ( votes: 1)'''
    188 
    189 Added 2.1.06- The Developers do not provide Legal advice. period! Over at the Tor Legal FAQ there is a written section by EFF lawyers. It aims to give you an overview of some of the legal issues that arise from the Tor project. Read the Disclaimer. The FAQ does provide a dialougue on the legalality & posssible scenarios of operating a Tor Server. They also provide you with contact information to a EFF Lawyer. The Tor FAQ also provides a links to an Abuse FAQ, & Tor Technical FAQ Wiki.  See this address for more information along these lines. http://tor.eff.org/faq.html
    190 
     73'''I have legal questions about running Tor. Is there anybody I can contact? (votes: 1)'''
    19174
    19275'''If I set up Tor to only act as a router node (reject *:* in torrc) can I still be a contact point for hidden services?'''
     
    19578
    19679'''Can anonymity be broken if all Tor servers in the chain are compromised/malicious and so are keeping logs to trace the chain?'''
    197 
    198 Yes. Indeed, if all the servers in a circuit are compromised then they need not even be communicated with.. the entry node can decipher for all the (possibly even non-existent) nodes. In order to maintain a superficial view of anonymity, it would probably be good to forward it to the exit server however.
    199 
    200 '''What system resources does a TOR server use?  The FAQ already dicusses memory a bit.  What about CPU?  Encryption is CPU-intensive.  Specific question I'd like answered: I'll be setting up a TOR node bandwidth-limited to about 256kbps (half my upstream bandwidth).  Will an old 300MHz G3 Mac easily handle this, or will a faster processor be needed? How 'bout a P90?  Presumably, TOR's disk usage and I/O is minimal.''' (Votes: 1)
    201 
    20280
    20381== Cannot resolve Foo.onion/Resolve requests to hidden services not allowed ==
     
    261139'''Answer''': No, Tor itself is all or nothing, a request either goes through it or it does not.
    262140
    263 For privoxy, however, you can use forward lines to make some hosts use tor, some use the normal system, and others use whatever other proxy you want.
    264 {{{
    265 forward-socks4a / localhost:9050 .
    266 forward-socks4a .onion localhost:9050 .
    267 
    268 # Do not torrify these (high volume/speed concerns, as well as PhP BBS
    269 # systems that consider a changed IP to be a new login.):
    270 forward .blood-bowl.net .
    271 forward .youtube.com .
    272 forward .qemu-forum.ipi.fi .
    273 forward .vidalia-project.net .
    274 }}}
    275 
    276 Here is an example. This uses privoxy for all sites (ad filtering, etc), and then specifies that some sites go through tor, and some do not.
    277 
    278 Privoxy uses the LAST match. So, the first line says "Use Tor by default". It can be turned off. The second line says "Always use Tor for .onion". After that are lines for "Never use Tor for these".
    279 
    280 Older, wrong information:
    281141Privoxy is also all or nothing in the sense that if a request has made it to Privoxy then either Privoxy is set up to go through Tor or it's not, there does not appear to be a way to program Privoxy so it will use Tor for some requests but not others.
    282142
     
    300160
    301161The pac file solution is far from ideal. It won't apply to non-web access and it runs into problems such as the bad pac file support in Safari. It also is not secure. A malicious website can trivially bypass this mechanism by placing pictures on its website from domains that it controls but are unlikely to be on a 'black list'. Therefore this mechanism is only useful with Websites that are not in and of themselves malicious but rather, due to their nature, can collect substantial amounts of personal information that one would rather not release. A search engine is a classic example. If and when privacy is a critical concern then the only proper course of action is to get rid of the pac file and instead configure all connections to go through privoxy/tor.
    302 
    303 
    304 
    305 '''I've been banned as an contributor at Slashdot! I run a Win 2003 server, with a decent pipe. They said that if I blocked them they would let me contribute again. I did an edit on my torrc file by adding a line:
    306 
    307 reject *:66.35.250.150 (which is Slashdot.com by using an online DNS 'dig' page
    308 
    309 I add the above right after my default exit. Which was just this:
    310 #ExitPolicy accept *:6660-6667,reject *:* # allow irc ports but no more
    311 ExitPolicy accept *:119 # accept nntp as well as default exit policy
    312 #ExitPolicy reject *:* # middleman only -- no exits allowed
    313 reject *:66.35.250.15
    314 
    315 Is this the way to do this, and just block Slashdot? Any help would be helpful, I've googled, did the tor.eff site, etc. But I'm not real UNIX centric (though thats changing) so just wanted to run it by some community persons. '''
    316 
    317 
    318 Well, you don't appear to allow exits on HTTP ports so I don't know why they blocked you. Fascists? [Ed. Fascists put their belief into the state... I doubt Slashdot does. Really, it's authoritarian]
    319 
    320 Anyway, you should probably block their IP rather than ports that happen to be theirs ;) (which don't exist - surprised tor let you do that..)
    321 
    322 
    323 '''What is the significance of the changes in the Bittorrent Torify HOWTO?'''
    324 
    325 I noticed I can't connect with btdownloadcurses through proxychains any more. Looking for answers, I went back to the Torify HOWTO and noticed that it had been altered. Where it used to explain about using proxychains to run bittorrent through TOR, which I used successfully for over a year, it now says that Bittorrent "uses a mechanism similar to TOR." That was certainly news to me. How is the generic Bittorrent client technically similar to TOR in any way? I have always heard that the generic Bittorrent client offers almost no anonimity at all. Now I'm reading that Bittorrent and TOR are practically the same thing and it would be redundant to use them together. Seems a bit curious.
    326 
    327 As a sub-question, let me just ask directly: Is it true that Bittorrent through TOR via proxychains no longer works?
    328 
    329 Also, the same page now mentions a technique of using Tor to connect to the tracker only, as opposed to the peers, by including the line --tracker-proxy 127.0.0.1:8118: on the command line. However, I see no documentation of this option in the btdownloadcurses client and I find it a bit suspicious that the format of this option uses a hyphen rather than an underscore as all the other command line options that are listed as being compatible with btdownloadcurses use underscores to separate options with two words such as --check_hashes <arg> or --report_hash_failures <arg>. Is that a typo or an undocumented option that just happens to deviate from the naming convetion of all the other options?
    330 
    331 '''How do you start and stop Tor and Privoxy in OS X (Panther) if you did not install the startup script? (needs to be added to installation instructions)'''
    332 
    333 
    334 On my system, the privoxy startup file ultimately runs
    335 {{{sudo $INSTALLDIR/privoxy --pidfile $pidfile }}}
    336 
    337 with INSTALLDIR being where you installed it, and pidfile being a filename that will hold the process ID.
    338 
    339 Tor can be started as a normal user -- just run the tor program. On my system, it runs as
    340 {{{/usr/bin/tor ControlPort 9051}}}
    341 
    342 Note that Vidalia is responsible for starting tor, normally.
    343 
    344 '''How do you configure the proxy if you are using Tor and Privoxy in OS X (Panther) with a router's firewall and the built-in OS X firewall, e.g. when using Wi-fi to connect to wireless router?  (needs to be added to installation instructions)'''
    345 
    346 I'm not sure that this is OS X specific. For any firewall, you need to add two incoming ports, 9001 and 9030 by default, to the list of approved ports.
    347 
    348 For Mac OS X, go to control panel, sharing, firewall, and then click "new" twice. The first one is port 9001, label "Tor Server", the second is port 9030, label "Tor Directory Mirror". Both of these are TCP ports, and the "port name" field should be "other".
    349 
    350 If you have BOTH the Os X firewall, and the router firewall, then you also need to open those ports on the router. Details are router specific.
    351 
    352 '''What to do (troubleshooting) if browsing slows to a crawl with Tor and Privoxy running in OS X?'''
    353 
    354 My best solution so far is to put these lines in the tor torrc file:
    355 {{{CircuitBuildTimeout 6
    356 NewCircuitPeriod 3
    357 ExcludeNodes charlesbabbage,tutzing,TFTor,freetux4ever
    358 LongLivedPorts 80,23,21,22,706,1863,5050,5190,5222,5223,6667,8300,8888}}}
    359 
    360 I'm sure some people will cringe at "6" for the build timeout. It normally is 60 -- which means that a circuit that is slow enough to need a full minute for creation will be considered fast enough for use. Sadly, a timeout of 5 causes far too many failures -- with a timeout of 6, browsing is "mostly fast". This ensures that the entry and middle nodes will be fast -- it does NOT protect against bad exit nodes. The four nodes that seem to consistently give me trouble are listed as "do not use".
    361 
    362 Finally, port 80 is added to the "long lived ports" list. The idea here is that if a site isn't reliable, web browsing will be likely to have interruptions.
    363 
    364 
    365 '''I am running a Tor server on one computer on a network. Can I stop the other PCs on the same network from being k-lined on QuakeNet?'''
    366 
    367 '''Would it make sense to support binding to multiple ports in Tor server (e.g. to bind to ports 443, 22, 5190 etc.) for clients behind _really_ restrictive firewalls? If this was implemented one day, maybe you could also support binding to multiple specific IP addresses on multihomed servers?'''
    368 
    369 
    370 ----
    371 CategoryHomepage