July 2022 - tor-dev - lists.torproject.org

Two features that would help load-balanced bridges
by David Fifield 01 Jun '23

01 Jun '23

After the blocking of Tor in Russia in December 2022, the number of Snowflake users rapidly increased. Eventually the tor process became the limiting factor for performance, using all of one CPU core. In a thread on tor-relays, we worked out a design where we run multiple instances of tor on the same host, all with the same identity keys, in order to effectively use all the server's CPU resources. It's running on the live bridge now, and as a result the bridge's bandwidth use has roughly doubled. Design thread https://forum.torproject.net/t/tor-relays-how-to-reduce-tor-cpu-load-on-a-s… Installation instructions https://gitlab.torproject.org/tpo/anti-censorship/team/-/wikis/Survival-Gui… Two details came up that are awkward to deal with. We have workaround for them, but they could benefit from support from core tor. They are: 1. Provide a way to disable onion key rotation, or configure a custom onion key. 2. Provide a way to set a specific authentication cookie for ExtORPort SAFE_COOKIE authentication, or a new authentication type that doesn't require credentials that change whenever tor is restarted. I should mention that, apart from the load-balancing design we settled on, we have brainstormed some other options for scaling the Snowflake bridge or bridges. At this point, none of these ideas can immediately be put into practice, because there's no way to tell tor "connect to one of these bridges at random, but only one," or "connect to this bridge, but accept any of these fingerprints." https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowfl… # Disable onion key rotation Multiple tor instances with the same identity keys will work fine for the first 5 weeks (onion-key-rotation-days + onion-key-grace-period-days), but after that time the instances will have independently rotated their onion keys, and clients will have connection failures unless the load balancer happens to connect them to the instance whose descriptor they have cached. This post investigates what the failure looks like: https://lists.torproject.org/pipermail/tor-relays/2022-January/020238.html Examples of what could work here are a torrc option to set onion-key-rotation-days to a large value, an option to disable onion key rotation, an option to set a certain named file as the onion key. What we are doing now is a bit of a nasty hack: we create a directory named secret_onion_key.old, so that a failed replace_file causes an early exit from rotate_onion_key. https://gitweb.torproject.org/tor.git/tree/src/feature/relay/router.c?h=tor… There are a few apparently benign side effects, like tor trying to rebuild its descriptor every hour, but it's effective at stopping onion key rotation. https://lists.torproject.org/pipermail/tor-relays/2022-January/020277.html # Stable ExtORPort authentication ExtORPort (extended ORPort) is a protocol that lets a pluggable transport attach transport and client IP metadata to a connection, for metrics purposes. In order to connect to the ExtORPort, the pluggable transport needs to authenticate using a scheme like ControlPort authentication. https://gitweb.torproject.org/torspec.git/tree/proposals/217-ext-orport-aut… tor generates a secret auth cookie and stores it in a file. When the pluggable transport process is managed by tor, tor tells the pluggable transport where to find the file by setting the TOR_PT_AUTH_COOKIE_FILE environment variable. In the load-balanced configuration, the pluggable transport server (snowflake-server) is not run and managed by tor. It is an independent daemon, so it doesn't have access to TOR_PT_AUTH_COOKIE_FILE (which anyway would be a different path for every tor instance). The bigger problem is that tor regenerates the auth cookie and rewrites the file on every restart. All the tor instances have different cookies, and snowflake-server does not know which it will get through the load balancer, so it doesn't know what cookie to use. Examples of what would work here are an option to use a certain file as the auth cookie, an option to leave the auth cookie file alone if it already exists, or a new ExtORPort authentication type that can use the same credentials across multiple instances. What we're doing now is using a shim program, extor-static-cookie, which presents an ExtORPort interface with a static auth cookie for snowflake-server to authenticate with, then re-authenticates to the ExtORPort of its respective instance of tor, using that instance's auth cookie. https://lists.torproject.org/pipermail/tor-relays/2022-January/020183.html

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GSoC 2022 - Tor Weather: Improving the Tor Network
by Sarthik Gupta 19 Sep '22

19 Sep '22

Hello everyone, I’m Sarthik Gupta (irc: sarthikg), a recent grad from Punjab Engineering College, Chandigarh & currently working as a Software Engineer at Soroco. This summer, I’ll be working on GSoC Project: “Tor Weather: Improving the Tor Network” with my mentors GeKo & Silvia. I’m extremely excited to be a part of Tor and am interested in working on this project! Briefing about the project: As of now, if any relay disappears from the tor-network, no one will know. This causes the network to lose out on bandwidth from nodes which have been down for hours because no-one knew they were down. Tor-Weather aims at solving this by creating a notification service which relay operators can subscribe to in order to get various types of updates for their relays. The tor-weather service will offer a plethora of notifications options for the relays. These include, the node being down, running on EOL/Outdated version, losing a flag, ranking in top 20/50/100, etc. These notifications can be subscribed & customised by the relay operators to fit their needs using a web-frontend. A detailed proposal for this project is available at: <https://gitlab.torproject.org/sarthikg/tor-weather/-/wikis/Proposal> https://gitlab.torproject.org/sarthikg/tor-weather/-/wikis/Proposal. I plan to keep this wiki updated with the progress & design decisions taken throughout the development of the project. Suggestions are always welcomed! Please reach out to us in irc (#tor-dev) for any ideas, questions, or suggestions you might have. Thanks, Sarthik Gupta

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Counting HS descriptor uploads
by Michael Rogers 17 Aug '22

17 Aug '22

Hi, The Briar team is working on a new app that uses Tor hidden services, and we're trying to work out when the server publishing a hidden service should consider the service to be reachable by clients. We've tried counting HS descriptor uploads via control port events, but we found that when republishing a hidden service that has been published before, the number of descriptor uploads seems to vary. When republishing a hidden service, is it guaranteed that at least one copy of the descriptor will be uploaded? Or are there circumstances where Tor might decide that enough copies were previously uploaded, and still contain up-to-date information about introduction points etc, so no new copies need to be uploaded? Thanks, Michael

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bridge:// URI and QR codes
by meskio 10 Aug '22

10 Aug '22

Hello, We have a messy situation to share bridges between devices. BridgeDB and Tor Browser have QR codes but they use different format for them. There has being some attempts to standarize a bridge URI[0][1], but it looks like we never finished that work. Pier has produced an RFC of a bridge URI proposal: https://gitlab.torproject.org/tpo/applications/team/-/wikis/Design-Document… If you have an application that uses (or plan to use) bridge URIs or QR codes I'm interested on your take on that proposal. Do your application already have any kind of support for bridge URIs or QR codes? If so how do they look like? Will it break things if we change them? What do you think of the proposal? How can we improve it? Thank you. [0] https://gitlab.torproject.org/legacy/trac/-/issues/15035 [1] https://gitlab.torproject.org/tpo/applications/fenix/-/issues/40189 -- meskio | https://meskio.net/ -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- My contact info: https://meskio.net/crypto.txt -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Nos vamos a Croatan.

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Proposal 341: A better algorithm for out-of-sockets eviction
by Nick Mathewson 27 Jul '22

27 Jul '22

``` Filename: 341-better-oos.md Title: A better algorithm for out-of-sockets eviction Author: Nick Mathewson Created: 25 July 2022 Status: Open ``` # Introduction Our existing algorithm for handling an out-of-sockets condition needs improvement. It only handles sockets used for OR connections, and prioritizes those with more circuits. Because of these weaknesses, the algorithm is trivial to circumvent, and it's disabled by default with `DisableOOSCheck`. Here we propose a new algorithm for choosing which connections to close when we're out of sockets. In summary, the new algorithm works by deciding which kinds of connections we have "too many" of, and then by closing excess connections of each kind. The algorithm for selecting connections of each kind is different. # Intuitions behind the algorithm below We want to keep a healthy mix of connections running; favoring one kind of connection over another gives the attacker a fine way to starve the disfavored connections by making a bunch of the favored kind. The correct mix of connections depends on the type of service we are providing. Everywhere _except_ authorities, for example, inbound directory connections are perfectly fine to close, since nothing in our protocol actually generates them. In general, we would prefer to close DirPort connections, then Exit connections, then OR connections. The priority with which to close connections is different depending on the connection type. "Age of connection" or "number of circuits" may be a fine metric for how truly used an OR connection is, but for a DirPort connection, high age is suspicious. # The algorithm Define a "candidate" connection as one that has a socket, and is either an exit stream, an _inbound_ directory stream, or an OR connection. (Note that OR connections can be from clients, relays, or bridges. Note that ordinary relays should not get directory streams that use sockets, since clients always use `BEGIN_DIR` to create tunneled directory streams.) In all of the following, treat subtraction as saturating at zero. In other words, when you see "A - B" below, read it as "MAX(A-B, 0)". ## Phase 1: Deciding how many connections to close When we are find that we are low on sockets, we pick a number of sockets that we want to close according to our existing algorithm. (That is, we try to close 1/4 of our maximum sockets if we have reached our upper limit, or 1/10 of our maximum sockets if we have encountered a failure from socket(2).) Call this `N_CLOSE`. Then we decide which sockets to target based on this algorithm. 1. Consider the total number of sockets used for exit streams (`N_EXIT`), the total number used for _inbound_ directory streams (`N_DIR`), and the total number used for OR connections (`N_OR`). (In these calculations, we exclude connections that are already marked to be closed.) Call the total `N_CONN = N_DIR + N_OR + N_EXIT`. Define `N_RETAIN = N_CONN - N_CLOSE`. 2. Compute how many connections of each type are "in excess". First, calculate our target proportions: * If we are an authority, let `T_DIR` = 1. Otherwise set `T_DIR = 0.1`. * If we are an exit or we are running an onion service, let `T_EXIT = 2`. Otherwise let `T_EXIT = 0.1`. * Let `T_OR` = 1. > TODO: Should those numbers be consensus parameters? These numbers define the relative proportions of connections that we would be willing to retain retain in our final mix. Compute a number of _excess_ connections of each type by calculating. ``` T_TOTAL = T_OR + T_DIR + T_EXIT. EXCESS_DIR = N_DIR - N_RETAIN * (T_DIR / T_TOTAL) EXCESS_EXIT = N_EXIT - N_RETAIN * (T_EXIT / T_TOTAL) EXCESS_OR = N_OR - N_RETAIN * (T_OR / T_TOTAL) ``` 3. Finally, divide N_CLOSE among the different types of excess connections, assigning first to excess directory connections, then excess exit connections, and finally to excess OR connections. ``` CLOSE_DIR = MIN(EXCESS_DIR, N_CLOSE) N_CLOSE := N_CLOSE - CLOSE_DIR CLOSE_EXIT = MIN(EXCESS_EXIT, N_CLOSE) N_CLOSE := N_CLOSE - CLOSE_EXIT CLOSE_OR = MIN(EXCESS_OR, N_CLOSE) ``` We will try to close `CLOSE_DIR` directory connections, `CLOSE_EXIT` exit connections, and `CLOSE_OR` OR connections. ## Phase 2: Closing directory connections We want to close a certain number of directory connections. To select our targets, we sort first by the number of directory connections from a similar address (see "similar address" below) and then by their age, preferring to close the oldest ones first. > This approach defeats "many requests from the same address" and "Open > a connection and hold it open, and do so from many addresses". It > doesn't do such a great job with defeating "open and close frequently > and do so on many addresses." > Note that fallback directories do not typically use sockets for > handling directory connections: theirs are usually created with > BEGIN_DIR. ## Phase 3: Closing exit connections. We want to close a certain number of exit connections. To do this, we pick an exit connection at random, then close its circuit _along with all the other exit connections on the same circuit_. Then we repeat until we have closed at least our target number of exit connections. > This approach probabilistically favors closing circuits with a large > number of sockets open, regardless of how long those sockets have been > open. This defeats the easiest way of opening a large number of exit > streams ("open them all on once circuit") without making the > counter-approach ("open each exit stream on a its own circuit") much > more attractive. ## Phase 3: Closing OR connections. We want to close a certain number of OR connections, to clients, bridges, or relays. To do this, we first close OR connections with zero circuits. Then we close all OR connections but the most recent 2 from each "similar address". Then we close OR connections at random from among those _not_ to a recognized relay in the latest directory. Finally, we close OR connections at random. > We used to unconditionally prefer to close connections with fewer > circuits. That's trivial for an adversary to circumvent, though: they > can just open a bunch of circuits on their bogus OR connections, and > force us to preferentially close circuits from real clients, bridges, > and relays. > Note that some connections that seem like client connections ("not > from relays in the latest directory") are actually those created by > bridges. ## What is "A similar address"? We define two connections as having a similar address if they are in the same IPv4 /30, or if they are in the same IPv6 /90. # Acknowledgments This proposal was inspired by a set of [OOS improvements](https://gitlab.torproject.org/tpo/core/tor/-/issues/32794 ) from `starlight`.

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Shrink Tor binary size
by Sergey Ponomarev 17 Jul '22

17 Jul '22

I’m working on a firmware for routers based on OpenWrt and it needs Tor out of the box for NAT punching i.e. SSH and Web admin access. It will expose a Single Onion Service i.e. not "hidden" with just 3 hop for a better performance. In fact it just needs to connect to some random relay to have a tunnel and encrypt/decrypt traffic. The full hidden service functionality with IP/RP dances is not needed: I'm not trying to hide the location of the router. In fact most users even don't need anonymity with the 3 hop to access their router but Tor requires 3. So this is probably a kind of abuse of the Tor network but this is the only free network of proxies. At the same time a user will have an encryption while accessing the router's admin panel. I may enable HTTPS with a self signed cert but users must accept it manually which is hard for inexperienced users. The Tor is already ported to OpenWrt but its binary is more than 2mb. For 16mb routers this is not critical but a lot and I need to keep space. So is it possible to compile Tor without some features? As far as I understood from autoconf I can disable Relay functionality. But maybe I can also disable some admin api parts and SOCKS proxy. Also OpenWrt out of the box has WolfSSL so is it possible to compile tor with it instead of OpenSSL? Another problem is files sizes inside of /var/lib/tor/: 20,442 cached-certs 2,303,443 cached-microdesc-consensus 8,725,598 cached-microdescs.new 3,531 state I think that all these files may be minimized. E.g. cached-certs contains keys in PEM format which is just Base64 form of DER. So switching to plain DER certs can significantly reduce size. As a side question: is it possible to make a small relay proxy that can work part time when I sleep? Imagine that each router already has a Tor. This is potentially thousands of relay nodes and all of them have a motivation to support the network which they are using themselves. But users don’t want to lose bandwidth. I see that I can set some traffic or bandwidth limits. But maybe I can write some script that will enable or disable the relay by schedule in the evening. As far I know relays must be always online so this will make the relay unstable and it won’t be used by Tor. Is it technically possible? Regards, Sergey Ponomarev, stokito.com

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Want to build circuit via unpublished relay, can't add microdesc using control port
by ValdikSS 11 Jul '22

11 Jul '22

Hello everyone. I'm experimenting with Tor network and wanted to build a circuit via unpublished relay (PublishServerDescriptor 0). To do so, I set up a relay, got its authority descriptor, imported it with +POSTDESCRIPTOR in the client using control port and tried to build a circuit using its fingerprint with EXTENDCIRCUIT. Everything works fine with UseMicrodescriptors 0, however with MicroDescriptors activated, the client does not detect imported (full) descriptor, returning: 552 No descriptor for "fingerprint here" I found no way to import microdescriptor using control port and no other means to use unpublished nodes. How can I add microdesc which is not in consensus into router? Or how can I build circuit with a relay not in consensus otherwise?

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Tor can't read HiddenServicePort unix socket through group permissions when starting as root and using setgid?
by keyandthegate 06 Jul '22

06 Jul '22

Permissions are set so tor should be able to access through the `postfix-test-queue` user: > $ sudo ls -l /var/spool/postfix-test/public/smtpd > srw-rw-rw- 1 postfix-test postfix-test 0 █████ /var/spool/postfix-test/public/smtpd > $ sudo ls -l /var/spool/postfix-test > # ... > drwx--x--- 2 postfix-test postfix-test-queue 4096 █████ public > $ sudo -u _tor-test id > uid=130(_tor-test) gid=141(_tor-test) groups=141(_tor-test),1006(postfix-test-queue) > $ cat /etc/tor/instances/test/torrc | grep HiddenServicePort > HiddenServicePort 25 unix:/var/spool/postfix-test/public/smtpd > $ cat /run/tor-instances/test.defaults | grep User > User _tor-test Running `tor@test` via the default systemctl config shows: > $ ps -ax -o uid,gid,supgid,command | grep /usr/bin/tor > 130 141 141 /usr/bin/tor --defaults-torrc /run/tor-instances/test.defaults -f /etc/tor/instances/test/torrc Which is missing the `postfix-test-queue` `1006` user which, for example shows up if I do: > $ sudo -u _tor-test sleep 1000 & ps -ax -o uid,gid,supgid,command | grep sleep > [1] 132314 > 0 141 141,1006 sudo -u _tor-test sleep 1000 Connecting using `sudo -u` works (the message indicates successful connection): > $ sudo -u _tor-test curl --unix-socket /var/spool/postfix-test/public/smtpd http://localhost > curl: (1) Received HTTP/0.9 when not allowed But connecting via tor does not: > $ torsocks --ipv6 curl http://█████.onion:25 > █████ ERROR torsocks[134873]: Host unreachable (in socks5_recv_connect_reply() at socks5.c:539) > curl: (7) Couldn't connect to server But does if I allow access to the socket to everyone: > $ sudo chmod "o+x" /var/spool/postfix-test/public/ > $ torsocks --ipv6 curl http://█████.onion:25 > curl: (1) Received HTTP/0.9 when not allowed Tor's relevant source code: [Tor: lib/process/setuid.c Source File](https://tpo.pages.torproject.net/core/doc/tor/setuid_8c_source.html)

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