tor-dev January 2024

tor-dev@lists.torproject.org

5 participants
4 discussions

When RFC 7686 and transparent proxies collide
by Matt Jolly 28 Sep '24

28 Sep '24

Hello Everyone, I recently inadvertently opened a much larger can of worms than I'd intended when fixing a bug reported downstream where cURL would, when configured with certain DNS backends, fail to resolve .onion addresses. https://bugs.gentoo.org/887287 After doing some digging I discovered that the c-ares library was updated in 2018 to intentionally fail to resolve .onion addresses in line with RFC 7686, and another reported 'Bug' in cURL for leaking .onion DNS requests: https://github.com/c-ares/c-ares/issues/196 https://github.com/curl/curl/issues/543 I took the obviously sane and uncontroversial approach of making sure that cURL would always behave the same way regardless of the DNS backend in use, and that it would output a useful error message when it failed to resolve a .onion address. Unfortunately, this has made a lot of people very angry and been ~~widely regarded as a bad move~~panned by a small subset of downstream cURL users: https://github.com/curl/curl/discussions/11125 https://infosec.exchange/@harrysintonen/110977751446379372 https://gitlab.torproject.org/tpo/core/torspec/-/issues/202 I accept that, in particular, transproxy users are being inconvenienced, but I also don't want to go back to 'cURL leaks .onion DNS requests _sometimes_'. As a career sysadmin and downstream bug triager: this is the stuff that keeps me up late at night. Quite literally, far too often. I have found, however that the downstreams that I expected to be inconvenienced most (Whonix and Tails) simply use socks: https://github.com/Kicksecure/sdwdate/commit/5724d83b258a469b7a9a7bbc651539… https://github.com/Kicksecure/tb-updater/commit/d040c12085a527f4d39cb1751f2… https://github.com/Kicksecure/usability-misc/blob/8f722bbbc7b7f2f3a35619a5a… https://gitlab.tails.boum.org/tails/tails/-/issues/19488 https://gitlab.tails.boum.org/tails/tails/-/merge_requests/1123 I've asked in the Tor Specifications issue (inspired by Silvio's suggestions), and in the cURL issue, but I seem to be getting nowhere and the impacted users are clamouring for a quick band-aid solution, which I feel will work out worse for everyone in the long run: >How can client applications (safely): > >1.discover that they're in a Tor-enabled environment >2.resolve onion services only via Tor in that circumstance >3.not leak .onion resolution attempts at all > >Right now, not making these requests in the first place is the >safest (and correct) thing to do, however inconvenient it may be. >Rather than immediately trying to come up with a band-aid approach >to this problem, a sane mechanism needs to be implemented to: > >1.prevent each application from coming up with their own solution >2.prevent inconsistency in .onion resolution (i.e. no "oh it only >leaks if DO_ONION_RESOLUTION is set") >3.provide a standardised mechanism for applications that want to be Tor >aware to discover that they're in a Tor-enabled environment. I'm not particularly attached to that last point, but it's worth discussing. On a related note: -is the use of a transparent proxy recommended? -is there a sane alternative that involves as minimal configuration as possible for these users? I'm not sure what the best way forward is here, but I'm hoping that actual Tor developers might have a useful opinion on the matter, or at least be able to point me in the right direction. Thanks for your time, Cheers, Matt

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Fact-checking a claim about relay/bridge fingerprint authentication
by David Fifield 25 Feb '24

25 Feb '24

Cecylia, Arlo, Serene, Shelikhoo, and I are writing a research paper about Snowflake. Here is a draft: https://www.bamsoftware.com/papers/snowflake/snowflake.20231003.e6e1c30d.pdf We're writing to check a factual claim in the section about having multiple backend bridges. Basically, we wanted it to be possible for there to be multiple Snowflake bridge sites run by different groups of people, and we did not want to share the same relay identity keys across all bridge sites, because of the increased risk of the keys being exposed. Therefore every bridge site has its own relay identity, which requires the client to know the relay fingerprints in advance and that it be the client (and not, e.g., the broker) that decides which bridge to use. 1. Is our general description (quoted below) of the design constraints as they bear on Tor correct? 2. Is §4.2 "CERTS cells" the right part of tor-spec to cite to make our point? https://gitlab.torproject.org/tpo/core/torspec/-/blob/b345ca044131b2eb18e6a… https://github.com/turfed/snowflake-paper/blob/e6e1c30dde6716dc5e54a32f2134… A Tor bridge is identified by a long-term identity public key. If, on connecting to a bridge, the client finds that the bridge's identity is not the expected one, the client will terminate the connection \cite[\S 4.2]{tor-spec}. The Tor client can configure at most one identity per bridge; there is no way to indicate (with a certificate, for example) that multiple identities should be considered equivalent. This constraint leaves two options: either all Snowflake bridges must share the same cryptographic identity, or else it must be the client that makes the choice of what bridge to use. While the former option is possible to do (by synchronizing identity keys across servers), every added bridge would increase the risk of compromising the all-important identity keys. Our vision was that different bridge sites would run in different locations with their own management teams, and that any compromise of a bridge site should affect that site only. In my own experiments, providing an incorrect relay fingerprint leads to errors in connection_or_client_learned_peer_id: https://gitlab.torproject.org/tpo/core/tor/-/blob/tor-0.4.7.13/src/core/or/… [warn] Tried connecting to router at 192.0.2.3:80 ID=<none> RSA_ID=2B280B23E1107BB62ABFC40DDCC8824814F80A71, but RSA + ed25519 identity keys were not as expected: wanted 1111111111111111111111111111111111111111 + no ed25519 key but got 2B280B23E1107BB62ABFC40DDCC8824814F80A72 + 1zOHpg+FxqQfi/6jDLtCpHHqBTH8gjYmCKXkus1D5Ko. [warn] Problem bootstrapping. Stuck at 14% (handshake): Handshaking with a relay. (Unexpected identity in router certificate; IDENTITY; count 1; recommendation warn; host 1111111111111111111111111111111111111111 at 192.0.2.3:80)

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New Proposal - UDP Application Support in Tor
by Micah Elizabeth Scott 25 Jan '24

25 Jan '24

Hello tor-dev folks! Late last year I started taking a close look at what it would take to support applications on Tor which rely on UDP networking. This was originally to be based on Nick's proposal, 339-udp-over-tor. The scope of this work so far has been specifically focused on end-user application compatibility, and excludes fundamental changes to Tor's network structure or protocols for now. This combination of approach and scope left me with more questions than answers, so I started looking deeper into the available solutions along with their expected benefits and risks. This proposal is the result of that investigation. Please find the text attached, or in the torspec repo as proposal #348: https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/348-ud… Unlike a typical proposal, this does not recommend any specific change to the Tor implementation. Several possible changes are presented, but ultimately the recommended approach is to use application-specific UDP relays to achieve compatibility. Integrated approaches are also presented, where Tor does involve itself in the transit of individual datagrams. These approaches offer advantages only when they are also part of a long-term plan to offer transport features beyond those offered by TCP. Until such a plan is in-scope, specific UDP extensions cannot be offered with confidence. I would appreciate any feedback on this proposal, whether it's about this particular shorter-term context or about longer-term plans to achieve some kind of optional unreliable transport. Thanks for your time! --beth

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Timers in Arti?
by Michael Rogers 15 Jan '24

15 Jan '24

Hi, If I understand right, the C implementation of Tor has various state machines that are driven by local libevent timers as well as events from the network. For example, when building a circuit, if there hasn't been any progress for a certain amount of time then a timer fires to handle the timeout. When running Tor on Android, we need to prevent the OS from suspending so that these timers fire when they're supposed to. This uses a lot of battery, because normally the OS would spend most of its time suspended. Unlike a laptop, an Android device with its screen turned off is constantly dipping in an out of suspension, and a lot of the platform's power optimisations are aimed at batching whatever work needs to be done so that the periods of suspension can be longer. If we allowed the OS to suspend then the timers would fire with arbitrary delays, and I don't really know what impact that would have: I'd speculate that there might be connectivity problems (eg dead circuits not being detected and replaced) and/or network-visible changes in the behaviour of circuits that could affect anonymity. So I've had a longstanding but not-very-hopeful request that maybe Tor's reliance on timers could be reduced, or at least clarified, so that we could either allow the OS to suspend without breaking anything, or at least know what was likely to break. And basically I'd just like to renew that request for Arti. Could anyone give me an overview of how these local timers are handled in Arti, or any information about what's likely to happen if the timers are arbitrarily delayed? Thanks, Michael

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