- tor-dev - lists.torproject.org

Fwd: [Wikitech-l] Can we help Tor users make legitimate edits?
by Zack Weinberg 31 Dec '12

31 Dec '12

The author asked me to forward this message to tor-dev. I can vouch for their personal interest in making something happen here and their being in a position of ability to do so from Wikimedia's end. Replies should go to wikitech-l and/or the author as well as here. It looks like there was quite a bit of a thread there already: http://lists.wikimedia.org/pipermail/wikitech-l/2012-December/065345.html (note in particular that their primary concern seems to be "sockpuppets" rather than spammers). ---- Begin forwarded message: From: Sumana Harihareswara <sumanah(a)wikimedia.org> To: wikitech-l(a)lists.wikimedia.org Subject: [Wikitech-l] Can we help Tor users make legitimate edits? TL;DR: A few ideas follow on how we could possibly help legit editors contribute from behind Tor proxies. I am just conversant enough with the security problems to make unworkable suggestions ;-), so please correct me, critique & suggest solutions, and perhaps volunteer to help. The current situation: https://en.wikipedia.org/wiki/Wikipedia:Advice_to_users_using_Tor_to_bypass… We generally don't let anyone edit or upload from behind Tor; the TorBlock extension stops them. One exception: a person can create an account, accumulate lots of good edits, and then ask for an IP block exemption, and then use that account to edit from behind Tor. This is unappealing because then there's still a bunch of in-the-clear editing that has to happen first, and because then site functionaries know that the account is going to be making controversial edits (and could possibly connect it to IPs in the future, right?). And right now there's no way to truly *anonymously* contribute from behind Tor proxies; you have to log in. However, since JavaScript delivery is hard for Tor users, I'm not sure how much editing from Tor -- vandalism or legit -- is actually happening. (I hope for analytics on this and thus added it to https://www.mediawiki.org/wiki/Analytics/Dreams .) We know at least that there are legitimate editors who would prefer to use Tor and can't. People have been talking about how to improve the situation for some time -- see http://cryptome.info/wiki-no-tor.htm and https://lists.torproject.org/pipermail/tor-dev/2012-October/004116.html . It'd be nice if it could actually move forward. I've floated this problem past Tor and privacy people, and here are a few ideas: 1) Just use the existing mechanisms more leniently. Encourage the communities (Wikimedia & Tor) to use https://en.wikipedia.org/wiki/Wikipedia:Request_an_account (to get an account from behind Tor) and to let more people get IP block exemptions even before they've made any edits (< 30 people have gotten exemptions on en.wp in 2012). Add encouraging "get an exempt account" language to the "you're blocked because you're using Tor" messaging. Then if there's an uptick in vandalism from Tor then they can just tighten up again. 2) Encourage people with closed proxies to re-vitalize https://en.wikipedia.org/wiki/Wikipedia:WOCP . Problem: using closed proxies is okay for people with some threat models but not others. 3) Look at Nymble - http://freehaven.net/anonbib/#oakland11-formalizing and http://cgi.soic.indiana.edu/~kapadia/nymble/overview.php . It would allow Wikimedia to distance itself from knowing people's identities, but still allow admins to revoke permissions if people acted up. The user shows a real identity, gets a token, and exchanges that token over tor for an account. If the user abuses the site, Wikimedia site admins can blacklist the user without ever being able to learn who they were or what other edits they did. More: https://cs.uwaterloo.ca/~iang/ Ian Golberg's, Nick Hopper's, and Apu Kapadia's groups are all working on Nymble or its derivatives. It's not ready for production yet, I bet, but if someone wanted a Big Project.... 3a) A token authorization system (perhaps a MediaWiki extension) where the server blindly signs a token, and then the user can use that token to bypass the Tor blocks. (Tyler mentioned he saw this somewhere in a Bugzilla suggestion; I haven't found it.) 4) Allow more users the IP block exemption, possibly even automatically after a certain number of unreverted edits, but with some kind of FlaggedRevs integration; Tor users can edit but their changes have to be reviewed before going live. We could combine this with (3); Nymble administrators or token-issuers could pledge to review edits coming from Tor. But that latter idea sounds like a lot of social infrastructure to set up and maintain. Thoughts? Are any of you interested in working on this problem? #tor on the OFTC IRC server is full of people who'd be interested in talking about this. -- Sumana Harihareswara Engineering Community Manager Wikimedia Foundation

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Xeronet's torrc
by Nathan Freitas 31 Dec '12

31 Dec '12

I was just made aware of this "advanced" torrc configuration below. Any comments on it, from a client-only/mobile device perspective? I can understand how it might deanonymize you, but it might be a trade-off that users seeking fast circumvention only may want to make. http://xeronet.primeoptic.net/tor/torrc.php *** Advanced Torrc Settings - Did You Edit The Config ? This is a working example of an advanced configuration for use with the Tor Browser Bundle. N.B. Vidalia will show the following 'warning' message if you use this Torrc file and specifically StrictNodes 1 : "You have asked to exclude certain relays from all positions in your circuits. Expect hidden services and other Tor features to be broken in unpredictable ways." If something is broken or you cannot connect, your should set StrictNodes to 0 again. If you don't want to see warning messages the you can always switch them off in Vidalia > Messages > Options. xeronet Torrc - v1.4 - 12/12/2012 - Download ... 'Save As'. This file can be used to replace the existing Torrc file in your Tor Browser Bundle > Data > Tor The idea is to make Tor faster and safer for regular internet browsing and it does work ! How does it work ? Well, Tor works just great 'out-of-the-box', however, by tweaking settings and controlling how Tor connects to its own network we can improve on privacy and security. (1) Block 'Bad' Exit Nodes using: ExcludeNodes 'Bad' Exit Nodes are flagged in red here: http://torstatus.blutmagie.de N.B. torstatus.blutmagie.de will probably load very slowly in your web browser and might even appear to 'freeze' ! It contains a lot of 'live' data. Be patient and it will load up OK. (2) Block 'problematic' internet countries using: ExcludeNodes 'problematic' internet countries can be found here: http://map.opennet.net and here: https://wikipedia.org/wiki/Internet_censorship_by_country N.B. The 'default' list of blocked Countries has been selected by including those Countries using Pervasive and Substantial blocking of Internet Tools and Political, Social, Conflict and/or Security website filtering... You have lots of Tor servers to choose from... Why should you use a Tor server in a country that heavily filters its own citizens or perhaps even worse... (Don't worry - In doing this you will not be preventing access to the Tor network from users in these Countries.) Recommended: 'problem' internet countries Block List: Afghanistan, Algeria, Armenia, Argentina, Azerbaijan, Bangladesh, Belarus, Burma, China, Colombia, Cuba, Egypt, Eritrea, Ethiopia, Gambia, Georgia, Ghana, Guatemala, India, Indonesia, Iraq, Iran, Israel, Jordan, Kazakhstan, Kuwait, Kyrgyzstan, Laos, Lebanon, Libya, Macau, Malawi, Mali, Malaysia, Mauritania, Mexico, Moldova, Mongolia, Morocco, Nepal, Nigeria, North Korea, Oman, Pakistan, Palestinian Territories, Paraguay, Peru, Philippines, Qatar, Russia, Rwanda, Saudi Arabia, Somalia, South Africa, South Korea, Sudan, Sri Lanka, Syria, Taiwan, Tajikistan, Thailand, Tunisia, Turkey, Turkmenistan, UAE, Uganda, Uzbekistan, Venezuela, Vietnam, Yemen, Zimbabwe. See: https://wikipedia.org/wiki/List_of_Internet_top-level_domains for Country Codes. N.B. You might also consider adding / blocking your own country or location, if it is not already included in the list. This will have obvious benefits in increasing both your privacy and anonymity. Additional: 'slow' internet countries (below 1000 kbps avg.) Avoid List: Angola, Benin, Bolivia, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Comoros, Republic of the Congo, Democratic Republic of the Congo, Côte d'Ivoire, Djibouti, Equatorial Guinea, Gabon, Guinea, Guinea-Bissau, Guyana, Liberia, Mozambique, Namibia, Niger, Rwanda, Sao Tome and Príncipe, Senegal, Sierra Leone, Swaziland, Tanzania, Uganda, Zambia. See: http://www.akamai.com/stateoftheinternet/ for avg. internet speeds. (3) Block potentially mis-configured servers using: ExcludeNodes Mis-configured nodes might include: default or Unnamed servers etc. (4) Select fast (high bandwidth) Entry servers using: EntryNodes (5) Select fast (high bandwidth) Exit servers using: ExitNodes Fast (high bandwidth) servers can be found here: http://torstatus.blutmagie.de N.B. Servers selected for this example Torrc have been chosen because they are run by individuals or non-profit organizations with an interest or involvement in supporting internet privacy and security, freedom of speech and / or the free software movements i.e. torservers.net, globenet.org, riseup.net, privacyfoundation.ch, privacyfoundation.de, tor.noisebridge.net, fsf.org, team-cymru.org, eff.org and others. (6) Use StrictNodes 1 to enforce the server selection. N.B. "If StrictNodes is set to 1, Tor will treat the ExcludeNodes option as a requirement to follow for all the circuits you generate, even if doing so will break functionality for you. If StrictNodes is set to 0, Tor will still try to avoid nodes in the ExcludeNodes list, but it will err on the side of avoiding unexpected errors. Specifically, StrictNodes 0 tells Tor that it is okay to use an excluded node when it is necessary to perform relay reachability self-tests, connect to a hidden service, provide a hidden service to a client, fulfill a .exit request, upload directory information, or download directory information. (Default: 0)" (7) Use FascistFirewall 1 to force port 80 (http) and port 443 (https) access. N.B. "If 1, Tor will only create outgoing connections to ORs running on ports that your firewall allows (defaults to 80 and 443; see FirewallPorts). This will allow you to run Tor as a client behind a firewall with restrictive policies, but will not allow you to run as a server behind such a firewall. If you prefer more fine-grained control, use ReachableAddresses instead." If you choose to do this then make sure that your selected Nodes use port 80 and/or port 443 (8) Use UseEntryGuards 1 for increased security. N.B. "If this option is set to 1, we pick a few long-term entry servers, and try to stick with them. This is desirable because constantly changing servers increases the odds that an adversary who owns some servers will observe a fraction of your paths. (Defaults to 1.)" (9) Use ClientOnly 1 for the Tor Browser Bundle. N.B. "If set to 1, Tor will under no circumstances run as a server or serve directory requests. The default is to run as a client unless ORPort is configured. (Usually, you don’t need to set this; Tor is pretty smart at figuring out whether you are reliable and high-bandwidth enough to be a useful server.) (Default: 0)" (10) Tips: Do add Authority servers to your EntryNodes list. Do add ExitNodes as EntryNodes. Don't add EntryNodes as ExitNodes ! Do block new 'bad' Nodes in ExcludeNodes. Do check the status of the nodes that you have selected on a regular basis. Do find and add new bridge nodes as EnrtyNodes if you require them for access. If you have problems connecting to Tor then changing FascistFirewall to 0 and/or StrictNodes to 0 will probably fix the issue. Do read the Tor Manual: https://www.torproject.org/docs/tor-manual.html > CLIENT OPTIONS Remember: You can view or edit your Torrc file using Notepad.exe or another text editor. This example Torrc file will be updated when necessary, So do check back here occasionally for a new version. Thank you and safe browsing.

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Opt-in model for flash proxy
by David Fifield 28 Dec '12

28 Dec '12

Alexandre recently finished up work adding an opt-in option for flash proxies. Now, clicking on the badge will bring up this page with yes/no buttons: http://crypto.stanford.edu/flashproxy/options.html The default model is still opt-out, though this may change in the future. If you want to make the badge on your site opt-in-only, add the cookierequired query parameter to your iframe, and encourage your visitors to visit the options page. The options work by setting a cookie. <iframe src="//crypto.stanford.edu/flashproxy/embed.html?cookierequired" width="80" height="15" frameborder="0" scrolling="no"></iframe> To summarize, here is what happens if you use cookierequired or not. The only difference is when the user hasn't set a cookie at all. Without cookierequired: no cookie cookie=no cookie=yes Will I be a proxy? yes no yes With cookierequired: no cookie cookie=no cookie=yes Will I be a proxy? no no yes If we switch to opt-in by default in the future, we'll ignore the cookierequired parameter and always use the no/no/yes part of the table. It will also help us switch to opt-in-only if we can get lots of people to opt in in advance. David Fifield

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Brainstorming a Tor censorship analysis tool
by Philipp Winter 26 Dec '12

26 Dec '12

Hi there, Deliverable 6 for sponsor Z says: > 6. Start a tool that a censored developer can run to discover why their Tor is > failing to connect: brainstorm a list of "things to check", and sort them by > how useful they'd be to check / how hard they'd be to build. (#7137) The deliverable is due on Feb. 28, 2013 so we should get started. Some background about the deliverable: The reason for this project is that debugging possible censorship events is tedious right now. We often have no access to machines in censoring countries and we are dependent on users creating packet dumps for us. This tool should speed up and automate this process to some extent. Censored users should run it and the tool should then collect data which should then somehow reach us. I created the following wiki page which should contain all the necessary information: https://censorshipwiki.torproject.org/TorCensorshipAnalyzer Please add/modify stuff and share your opinion. Since there is quite some overlap with OONI, it would be great if the OONI people could give feedback. Cheers, Philipp

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Testing in Tor [was Re: Brainstorming a Tor censorship analysis tool]
by Nick Mathewson 23 Dec '12

23 Dec '12

On Wed, Dec 19, 2012 at 2:29 PM, Simon <simonhf(a)gmail.com> wrote: [...] > Maybe there is no automated testing for any Tor projects? At least a > quick search on the wiki only found [1] which lists possible ways to > test (but was created 7 months ago and apparently not updated since > and collecting dust) and [2] discussing a manual test procedure for > TBB. However, tor-0.2.3.25.tar.gz does reveal some test files but the > source code ratio of production code to test code is not inspiring at > first glance: [...] Be aware that we've also been using 'chutney' and 'experimentor' for integration testing. They supplement coverage a bit, though they need more tests, and each tends to hide certain classes of error. > Tor seems to have good planning compared to most open source projects. > So I would be interested in hearing why testing is apparently 'falling > between the cracks'. Why isn't there just 10 times more test LOC? Not because of any hatred or disapproval of tests--just because nobody's written that 100 kloc of testing code yet. I think that's for two main reasons: * We were in a hurry when we wrote lots of the stuff we wrote. * Large parts of the codebase have been written in a tightly coupled style that needs refactoring before it can be tested without a live Tor network at hand. * Until Tor 0.2.2, our testing framework didn't let us have tests that touched global state, which made our tests in general pretty fragile. > What > about implementing a new policy immediately: Any new production LOC > committed must be covered by tests, or peer reviewed and > democratically excluded? Goodness knows we need more review and testing. It doesn't seem fair to reject patches for lacking test coverage when they are patches to code that itself lacks coverage, though. If you write a 5-line patch to connection_ap_rewrite_and_attach, for instance, you probably shouldn't be on the hook for refactoring the whole function to make it testable, though you will be hard-pressed to write any tests for that monster without a complete refactor. It might be a reasonable goal to try to set a plan for increasing test coverage by a certain percentage with each release. If you like and you have time, it would be cool to stop by the tickets on trac.torproject.org for milestone "Tor: 0.2.4.x-final" in state "needs_review" and look to see whether you think any of them have code that would be amenable to new tests, or to look through currently untested functions and try to figure out how to make more of them tested and testable. yrs, -- Nick

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Stem code review 2012-12-21
by Sean Robinson 22 Dec '12

22 Dec '12

Stem devs, This is a review of Stem commits from 2012-12-11 through 2012-12-21. Damian, you are very good at documentation, concise and informative. I tend to hit one or the other, but not both. I recognize when someone gets the right mix. Good work. I, too, have found logging.basicConfig[0] hides too much about how it works and, now, I roll my own logging handler early. Would GETINFO/GETCONF cache hit logging[1] benefit from log_once? Or even something between log and log_once (e.g. rate_limited_log)? Is there any particular reason for moving away from readthedocs.org? I'm curious if there was more than a desire to self-host as much as possible. Good work on the Controller.get_conf() clean-up[2]. This is easier to code against with the predictable return types. I have already written a new method with this and always getting a list (event empty) reduces the amount of error checking I had to do. The heartbeat time tracking is a good idea[3]. But, I wonder, should this bother with an accessor method? Could this work as just well as an (non-callable) attribute? [0]: https://gitweb.torproject.org/stem.git/commit/a7b275a3d64301da4d23137dbadd4… [1]: https://gitweb.torproject.org/stem.git/commit/9b5ff19be35ebbf8c75772a63406b… [2]: https://gitweb.torproject.org/stem.git/commit/5b45d3125c811067d7c3ba0a11e32… [3]: https://gitweb.torproject.org/stem.git/commit/6bc92816dc4356a204fdde0f160ee… -- Sean Robinson

2 1

Idle connections in netstat
by Angus Gardner 20 Dec '12

20 Dec '12

Hi, I am fairly new to Tor and have been running a relay for a couple of weeks. I have configured a RelayBandwithRate of only 100 KB and so only see a few active connections at any one time. Recently I have been noticing, in netstat output, a high level of established but idle TCP connections to other tor-hosts. (Out of 96 ESTABLISHED connections there are 90 with Recv-Q & Send-Q of 0.) Is this part of the rolling connection set of this host or something else? Eg: #netstat -e Active Internet connections (w/o servers) Proto Recv-Q Send-Q Local Address Foreign Address State User Inode ... tcp 0 0 my.ip:55528 digi00299.digicube:9001 ESTABLISHED tor 449100 tcp 0 0 my.ip:56308 tor01.spacedump.ne:9001 ESTABLISHED tor 440912 tcp 0 0 my.ip:57934 torsrvo.snydernet:https ESTABLISHED tor 443309 tcp 0 0 my.ip:9001 tor18.anonymizer.:57588 TIME_WAIT root 0 tcp 0 0 my.ip:46943 tor.parad0x.org:https ESTABLISHED tor 449313 tcp 0 0 my.ip:9001 vodka.projekt16.b:51520 ESTABLISHED tor 449337 tcp 0 0 my.ip:9001 gamma.rueckgr.at:39215 ESTABLISHED tor 449218 tcp 0 0 my.ip:44622 dannenberg.ccc.de:https ESTABLISHED tor 431598 tcp 0 0 my.ip:39656 torsrvn.snydernet:https ESTABLISHED tor 446435 tcp 0 0 my.ip:9001 76.72.169.90:40606 TIME_WAIT root 0 tcp 0 93 my.ip:9001 torproxy10.teamcy:58154 ESTABLISHED tor 449452 tcp 0 0 my.ip:60081 v-603-kalfa-23-10:https ESTABLISHED tor 448047 tcp 0 1846 my.ip:35776 tor5.anonymizer.c:https ESTABLISHED tor 440943 . Cheers, Gus

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Proposal 205: Remove global client-side DNS caching
by Nick Mathewson 19 Dec '12

19 Dec '12

Filename: 205-local-dnscache.txt Title: Remove global client-side DNS caching Author: Nick Mathewson Created: 20 July 2012 Status: Open 0. Overview This proposal suggests that, for reasons of security, we move client-side DNS caching from a global cache to a set of per-circuit caches. This will break some things that used to work. I'll explain how to fix them. 1. Background and Motivation Since the earliest Tor releases, we've kept a client-side DNS cache. This lets us implement exit policies and exit enclaves -- if we remember that www.mit.edu is 18.9.22.169 the first time we see it, then we can avoid making future requests for www.mit.edu via any node that blocks net 18. Also, if there happened to be a Tor node at 18.9.22.169, we could use that node as an exit enclave. But there are security issues with DNS caches. A malicious exit node or DNS server can lie. And unlike other traffic, where the effect of a lie is confined to the request in question, a malicious exit node can affect the behavior of future circuits when it gives a false DNS reply. This false reply could be used to keep a client connecting to an MITM'd target, or to make a client use a chosen node as an exit enclave for that node, or so on. With IPv6, tracking attacks will become even more possible: A hostile exit node can give every client a different IPv6 address for every hostname they want to resolve, such that every one of those addresses is under the attacker's control. And even if the exit node is honest, having a cached DNS result can cause Tor clients to build their future circuits distinguishably: the exit on any subsequent circuit can tell whether the client knew the IP for the address yet or not. Further, if the site's DNS provides different answers to clients from different parts of the world, then the client's cached choice of IP will reveal where it first learned about the website. So client-side DNS caching needs to go away. 2. Design 2.1. The basic idea I propose that clients should cache DNS results in per-circuit DNS caches, not in the global address map. 2.2. What about exit policies? Microdescriptor-based clients have already dropped the ability to track which nodes declare which exit policies, without much ill effect. As we go forward, I think that remembering the IP address of each request so that we can match it to exit policies will be even less effective, especially if proposals to allow AS-based exit policies can succeed. 2.3. What about exit enclaves? Exit enclaves are already borken. They need to move towards a cross-certification solution where a node advertises that it can exit to a hostname or domain X.Y.Z, and a signed record at X.Y.Z advertises that the node is an enclave exit for X.Y.Z. That's out-of-scope for this proposal, except to note that nothing proposed here keeps that design from working. 2.4. What about address mapping? Our current address map algorithm is, more or less: N = 0 while N < MAX_MAPPING && exists map[address]: address = map[address] N = N + 1 if N == MAX_MAPPING: Give up, it's a loop. Where 'map' is the union of all mapping entries derived from the controller, the configuration file, trackhostexits maps, virtual-address maps, DNS replies, and so on. With this design, the DNS cache will not be part of the address map. That means that entries in the address map which relied on happening after the DNS cache entries can no longer work so well. These would include: A) Mappings from an IP address to a particular exit, either manually declared or inserted by TrackHostExits. B) Mappings from IP addresses to other IP addresses. C) Mappings from IP addresses to hostnames. We can try to solve these by introducing an extra step of address mapping after the DNS cache is applied. In other words, we should apply the address map, then see if we can attach to a circuit. If we can, we try to apply that circuit's dns cache, then apply the address map again. 2.5. What about the performance impact? That all depends on application behavior. If the application continues to make all of its requests with the hostname, there shouldn't be much trouble. Exit-side DNS caches and exit-side DNS will avoid any additional round trips across the Tor network; compared to that, the time to do a DNS resolution at the exit node *should* be small. That said, this will hurt performance a little in the case where the exit node and its resolver don't have the answer cached, and it takes a long time to resolve the hostname. If the application is doing "resolve, then connect to an IP", see 2.6 below. 2.6. What about DNSPort? If the application is doing its own DNS caching, they won't get much security benefit from here. If the application is doing a resolve before each connect, there will be a performance hit when the resolver is using a circuit that hadn't previously resolved the address. Also, DNSPort users: AutomapHostsOnResolve is your friend. 3. Alternate designs and future directions 3.1. Why keep client-side DNS caching at all? A fine question! I am not sure it actually buys us anything any longer, since exits also have DNS caching. Shall we discuss that? It would sure simplify matters. 3.2. The impact of DNSSec Once we get DNSSec support, clients will be able to verify whether an exit's answers are correctly signed or not. When that happens, we could get most of the benefits of global DNS caching back, without most of the security issues, if we restrict it to DNSSec-signed answers.

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Proposal: Tuning the Parameters for the Path Bias Defense
by Mike Perry 19 Dec '12

19 Dec '12

Also exists at https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/path-bias-tun… -------------------------------------------------------------------- Title: Tuning the Parameters for the Path Bias Defense Author: Mike Perry Created: 01-10-2012 Status: Open Target: 0.2.4.x+ Overview This proposal describes how we can use the results of simulations in combination with network scans to set reasonable limits for the Path Bias defense, which causes clients to be informed about and ideally rotate away from Guards that provide extremely low circuit success rates. Motivation The Path Bias defense is designed to defend against a type of route capture where malicious Guard nodes deliberately fail circuits that extend to non-colluding Exit nodes to maximize their network utilization in favor of carrying only compromised traffic. This attack was explored in the academic literature in [1], and a variant involving cryptographic tagging was posted to tor-dev[2] in March. In the extreme, the attack allows an adversary that carries c/n of the network capacity to deanonymize c/n of the network connections, breaking the O((c/n)^2) property of Tor's original threat model. Design Description The Path Bias defense is a client-side accounting mechanism in Tor that tracks the circuit failure rate for each of the client's guards. Clients maintain two integers for each of their guards: a count of the number of times a circuit was extended at least one hop through that guard, and a count of the number of circuits that successfully complete through that guard. The ratio of these two numbers is used to determine a circuit success rate for that Guard. The system should issue a notice log message when Guard success rate falls below 70%, a warn when Guard success rate falls below 50%, and should drop the Guard when the success rate falls below 30%. To ensure correctness, checks are performed to ensure that we do not count successes without also counting the first hop. Similarly, to provide a moving average of recent Guard activity while still preserving the ability to ensure correctness, we "scale" the success counts by an integer divisor (currently 2) when the counts exceed the moving average window (300) and when the division does not produce integer truncation. No log messages should be displayed, nor should any Guard be dropped until it has completed at least 150 first hops (inclusive). Analysis: Simulation To test the defense in the face of various types of malicious and non-malicious Guard behavior, I wrote a simulation program in Python[3]. The simulation confirmed that without any defense, an adversary that provides c/n of the network capacity is able to observe c/n of the network flows using circuit failure attacks. It also showed that with the defense, an adversary that wishes to evade detection has compromise rates bounded by: P(compromise) <= (c/n)^2 * (100/CUTOFF_PERCENT) circs_per_client <= circuit_attempts*(c/n) In this way, the defense restores the O((c/n)^2) compromise property, but unfortunately only over long periods of time (see Security Considerations below). The spread between the cutoff values and the normal rate of circuit success has a substantial effect on false positives. From the simulation's results, the sweet spot for the size of this spread appears to be 10%. In other words, we want to set the cutoffs such that they are 10% below the success rate we expect to see in normal usage. The simulation also demonstrates that larger "scaling window" sizes reduce false positives for instances where non-malicious guards experience some ambient rate of circuit failure. Analysis: Live Scan Preliminary Guard node scanning using the txtorcon circuit scanner[4] shows normal circuit completion rates between 80-90% for most Guard nodes. However, it also showed that CPU overload conditions can easily push success rates as low as 45%. Even more concerning is that for a brief period during the live scan, success rates dropped to 50-60% network-wide (regardless of Guard node choice). Based on these results, the notice condition should be 70%, the warn condition should be 50%, and the drop condition should be 30%. Future Analysis: Deployed Clients It's my belief that further analysis should be done by deploying loglines for all three thresholds in clients in the live network to utilize user reports on how often high rates of circuit failure are seen before we deploy changes to rotate away from failing Guards. I believe these log lines should be deployed in 0.2.3.x clients, to maximize the exposure of the code to varying network conditions, so that we have enough data to consider deploying the Guard-dropping cutoff in 0.2.4.x. Security Considerations While the scaling window does provide freshness and can help mitigate "bait-and-switch" attacks, it also creates the possibility of conditions where clients can be forced off their Guards due to temporary network-wide CPU DoS. This provides another reason beyond false positive concerns to set the scaling window as large as is reasonable. A DoS directed at specific Guard nodes is unlikely to allow an adversary to cause clients to rotate away from that Guard, because it is unlikely that the DoS can be precise enough to allow first hops to that Guard to succeed, but also cause extends to fail. This leaves network-wide DoS as the primary vector for influencing clients. Simulation results show that in order to cause clients to rotate away from a Guard node that previously succeeded 80% of its circuits, an adversary would need to induce a 25% success rate for around 350 circuit attempts before the client would reject it, or a 5% success rate for around 215 attempts, both using a scaling window of 300 circuits. Assuming one circuit per Guard per 10 minutes of active client activity, this is a sustained network-wide DoS attack of 60 hours for the 25% case, or 38 hours for the 5% case. Presumably this is enough time for the directory authorities to respond by altering the pb_disablepct consensus parameter before clients rotate, especially given that most clients are not active for even 38 hours on end, and will tend to stop building circuits while idle. If we raised the scaling window to 500 circuits, it would require 1050 circuits if the DoS brought circuit success down to 25% (175 hours), and 415 circuits if the DoS brought the circuit success down to 5% (69 hours). The tradeoff, though, is that larger scaling window values allow Guard nodes to compromise clients for duty cycles of around the size of this window (up to the (c/n)^2 * 100/CUTOFF_PERCENT limit in aggregate), so we do have to find balance between these concerns. Implementation Notes: Log Messages Log messages need to be chosen with care to avoid alarming users. I suggest: Notice: "Your Guard %s is failing more circuits than usual. Most likely this means the Tor network is overloaded. Success counts are %d/%d." Warn: "Your Guard %s is failing a very large amount of circuits. Most likely this means the Tor network is overloaded, but it could also mean an attack against you or potentially the Guard itself. Success counts are %d/%d." Drop: "Your Guard %s is failing an extremely large amount of circuits. [Tor has disabled use of this Guard.] Success counts are %d/%d." The second piece of the Drop message would not be present in 0.2.3.x, since the Guard won't actually be dropped. Implementation Notes: Consensus Parameters The following consensus parameters reflect the constants listed in the proposal. These parameters should also be available for override in torrc. pb_mincircs=150 The minimum number of first hops before we log or drop Guards. pb_noticepct=70 The threshold of circuit success below which we display a notice. pb_warnpct=50 The threshold of circuit success below which we display a warn. pb_disablepct=30 The threshold of circuit success below which we disable the guard. pb_scalecircs=300 The number of first hops at which we scale the counts down. pb_scalefactor=2 The integer divisor by which we scale. 1. http://freehaven.net/anonbib/cache/ccs07-doa.pdf 2. https://lists.torproject.org/pipermail/tor-dev/2012-March/003347.html 3. https://gitweb.torproject.org/torflow.git/tree/HEAD:/CircuitAnalysis/PathBi… 4. https://github.com/meejah/txtorcon/blob/exit_scanner/apps/exit_scanner/fail… -- Mike Perry

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Opening new SOCKS to client plugin for new circuit?
by David Fifield 15 Dec '12

15 Dec '12

I noticed a change in behavior in cb62a0b69a7d67b427224ca4c3075b49853a3a1f or thereabouts. tor opens a new SOCKS connection to a client transport plugin while bootstrapping at 50% (if descriptors are not cached) or at 85% (if descriptors are not cached). The upshot is that the flash proxy transport, for which new connections are not free, needs two connected browser proxies in order to bootstrap. Earlier revisions permit bootstrapping and circuit creation with just one connection to the proxy. I start an external proxy thus: $ ./flashproxy-client --external --unsafe-logging 2012-12-03 09:02:05 Listening remote on 0.0.0.0:9000. 2012-12-03 09:02:05 Listening remote on [::]:9000. 2012-12-03 09:02:05 Listening local on 127.0.0.1:9001. 2012-12-03 09:02:05 Listening local on [::1]:9001. The "remote" listener is waiting for WebSocket connections from a browser. The "local" listener is waiting for SOCKS connections from Tor. Then I start Tor to use the proxy: $ ./src/or/tor ClientTransportPlugin "websocket socks4 127.0.0.1:9001" UseBridges 1 Bridge "websocket 0.0.1.0:1" LearnCircuitBuildTimeout 0 CircuitBuildTimeout 60 Log "info stderr" ... Dec 03 09:02:13.000 [notice] Bootstrapped 10%: Finishing handshake with directory server. flashproxy-client notices Tor's pending SOCKS connection: 2012-12-03 09:02:13 Local connection from 127.0.0.1:55421. 2012-12-03 09:02:13 SOCKS request from 127.0.0.1:55421. 2012-12-03 09:02:13 Got SOCKS request for 0.0.1.0:1. 2012-12-03 09:02:13 locals (1): [u'127.0.0.1:55421'] 2012-12-03 09:02:13 remotes (0): [] 2012-12-03 09:02:13 Data from unlinked local 127.0.0.1:55421 (217 bytes). 2012-12-03 09:02:13 locals (1): [u'127.0.0.1:55421'] 2012-12-03 09:02:13 remotes (0): [] Then I open a browser to make a single WebSocket connection which will appear as one of the pluggable transport's "remotes". http://crypto.stanford.edu/flashproxy/embed.html?debug=1&client=127.0.0.1:9… flashproxy-client sees the new remote and starts proxying data. 2012-12-03 09:02:17 Remote connection from 127.0.0.1:51321. 2012-12-03 09:02:17 Data from WebSocket-pending 127.0.0.1:51321. 2012-12-03 09:02:17 locals (1): [u'127.0.0.1:55421'] 2012-12-03 09:02:17 remotes (1): [u'127.0.0.1:51321'] 2012-12-03 09:02:17 Linking 127.0.0.1:55421 and 127.0.0.1:51321. Now bootstrapping continues (over the WebSocket channel) until reaching 85%, and then it says "connections all too old, or too non-canonical" and makes a new SOCKS request: Dec 03 09:02:18.000 [notice] new bridge descriptor '3VXRyxz67OeRoqHn' (fresh): $7C03D29CA58BE8EED5F483F31A2DEDF6FD7CC444~3VXRyxz67OeRoqHn at 0.0.1.0 Dec 03 09:02:18.000 [notice] We now have enough directory information to build circuits. Dec 03 09:02:18.000 [notice] Bootstrapped 80%: Connecting to the Tor network. ... Dec 03 09:02:18.000 [info] circuit_handle_first_hop(): Next router is [scrubbed]: Connections all too old, or too non-canonical. Launching a new one. Dec 03 09:02:18.000 [notice] Bootstrapped 85%: Finishing handshake with first hop. Dec 03 09:02:18.000 [info] connection_read_proxy_handshake(): Proxy Client: connection to 0.0.1.0:1 successful flashproxy-client sees the SOCKS request, but because there are no more browser connections forthcoming, everything stalls at this point. 2012-12-03 09:02:18 Local connection from 127.0.0.1:55427. 2012-12-03 09:02:18 SOCKS request from 127.0.0.1:55427. 2012-12-03 09:02:18 Got SOCKS request for 0.0.1.0:1. 2012-12-03 09:02:18 locals (2): [u'127.0.0.1:55421', u'127.0.0.1:55427'] 2012-12-03 09:02:18 remotes (1): [u'127.0.0.1:51321'] 2012-12-03 09:02:18 Data from unlinked local 127.0.0.1:55427 (231 bytes). I've verified this failure to bootstrap with recent 190c1d4981e5751aabd3d894095851c830f1d570. After bisecting, I think the last commit with the old behavior (boostrapped 100%) was 751b3aabb5ab88fca16834e559a8d9835831b05f. There were some compile errors during bisecting so I couldn't narrow it to a specific revision; git reports There are only 'skip'ped commits left to test. The first bad commit could be any of: 35924435d22c2469ecbe06156c8069a928859d63 e136f7ccb4e671e33b6c92a48df819082291f5c1 4768c0efe3e9471cc367c3740d1a4ba0ab79626c 6cce6241dd4405f6cf21057f9913e07633cf18bb 519c971f6a3b89f1e81cda3c0290d4d943ec0d78 77dac97354974e8a819d8e35ad4e7a76199999b4 32337502f11e6c84e4db8591f5f81c4fc6d1da58 8b14db9628f0e8982e894034e86c8efdd78cff32 15303c32ec9d84aff8de5ed9df28e779c36c6e5c 28f108bcceab59fcf9f27e33065f64bfdb0f159a 7f952da55334d3a3693d1c6e8531fd96730265db f0f87cb68a22feaf552a18b521d3313d843f8793 838743654c1bed2bfe22789ff53a1993c005f176 9ad7ba9f2267a9ee34fafda9356f1fa86633f00f cb62a0b69a7d67b427224ca4c3075b49853a3a1f We cannot bisect more! Based on the log, cb62a0b69a7d67b427224ca4c3075b49853a3a1f seems a likely cause of the change: "Use channel_is_bad_for_new_circs(), connection_or_get_num_circs() in main.c". I thought I would be able to reproduce this with another transport or with a simple SOCKS proxy, showing two connections where there used to be one, but I can't. I see two connections even with the old code. For example with an ssh SOCKS proxy (ssh -v -D 9001 -N localhost): debug1: Connection to port 9001 forwarding to socks port 0 requested. debug1: channel 2: new [dynamic-tcpip] debug1: Connection to port 9001 forwarding to socks port 0 requested. debug1: channel 3: new [dynamic-tcpip] I guess that the difference is that previously, the second connection happens after bootstrapping is complete, while now it happens at 85%. (That is only a guess, I haven't verified it.) David Fifield

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