``` Filename: 266-removing-current-obsolete-clients.txt Title: Removing current obsolete clients from the Tor network Author: Nick Mathewson Created: 14 Jan 2016 Status: Superseded Superseded-by: 264, 272. 1. Introduction Frequently, we find that very old versions of Tor should no longer be supported on the network. To remove relays is easy enough: we simply update the directory authorities to stop listing relays that advertise versions that are too old. But to disable clients is harder. In another proposal I describe a system for letting future clients go gracefully obsolete. This proposal explains how we can safely disable the obsolete clients we have today (and all other client versions of Tor to date, assuming that they will someday become obsolete). 1.1. Why disable clients? * Security. Anybody who hasn't updated their Tor client in 5 years is probably vulnerable to who-knows-what attacks. They aren't likely to get much anonymity either. * Withstand zombie installations. Some Tors out there were once configured to start-on-boot systems that are now unmaintained. (See 1.4 below.) They put needless load on the network, and help nobody. * Be able to remove backward-compatibility code. Currently, Tor supports some truly ancient protocols in order to avoid breaking ancient versions or Tor. This code needs to be maintained and tested. Some of it depends on undocumented or deprecated or non-portable OpenSSL features, and makes it hard to produce a conforming Tor server implementation. * Make it easier to write a conforming Tor relay. If a Tor relay needs to support every Tor client back through the beginning of time, that makes it harder to develop and test compatible implementations. 1.2. Is this dangerous? I don't think so. This proposal describes a way to make older clients gracefully disconnect from the network only when a majority of authorities agree that they should. A majority of authorities already have the ability to inflict arbitrary degrees of sabotage on the consensus document. 1.3. History The earliest versions of Tor checked the recommended-versions field in the directory to see whether they should keep running. If they saw that their version wasn't recommended, they'd shut down. There was an "IgnoreVersion" option that let you keep running anyway. Later, around 2004, the rule changed to "shut down if the version is _obsolete_", where obsolete was defined as "not recommended, and older than a version that is recommended." In 0.1.1.7-alpha, we made obsolete versions only produce a warning, and removed IgnoreVersion. (See 3ac34ae3293ceb0f2b8c49.) We have still disabled old tor versions. With Tor 0.2.0.5-alpha, we disabled Tor versions before 0.1.1.6-alpha by having the v1 authorities begin publishing empty directories only. In version 0.2.5.2-alpha, we completely removed support for the v2 directory protocol used before Tor 0.2.0; there are no longer any v2 authorities on the network. Tor versions before 0.2.1 will currently not progress past fetching an initial directory, because they believe in a number of directory authority identity keys that no longer sign the directory. Tor versions before 0.2.4 are (lightly) throttled in multihop circuit creation, because we prioritize ntor CREATE cells over TAP ones when under load. 1.4. The big problem: slow zombies and fast zombies It would be easy enough to 'disable' old clients by simply removing server support for the obsolete protocols that they use. But there's a problem with that approach: what will the clients do when they fail to make connections, or to extend circuits, or whatever else they are no longer able to do? * Ideally, I'd like such clients to stop functioning _quietly_. If they stop contacting the network, that would be best. * Next best would be if these clients contacted the network only occasionally and at different times. I'll call these clients "slow zombies". * Worse would be if the clients contact the network frequently, over and over. I'll call these clients "fast zombies". They would be at their worst when they focus on authorities, or when they act in synchrony to all strike at once. One goal of this proposal is to ensure that future clients do not become zombies at all; and that ancient clients become slow zombies at worst. 2. Some ideas that don't work. 2.1. Dropping connections based on link protocols. Tor versions before 0.2.3.6-alpha use a renegotiation-based handshake instead of our current handshake. We could detect these handshakes and close the connection at the relay side if the client attempts to renegotiate. I've tested these changes on versions maint-0.2.0 through maint-0.2.2. They result in zombies with the following behavior: The client contact each authority it knows about, attempting to make a one-hop directory connection. It fails, detects a failure, then reconnects more and more slowly ... but one hour later, it resets its connection schedule and starts again. In the steady state this appears to result in about two connections per client per authority per hour. That is probably too many. (Most authorities would be affected: of the authorities that existed in 0.2.2, gabelmoo has moved and turtles has shut down. The authorities Faravahar and longclaw are new. The authorities moria1, tor26, dizum, dannenberg, urras, maatuska and maatuska would all get hit here.) [two maatuskas? -RD] (We could simply remove the renegotiation-detection code entirely, and reply to all connections with an immediate VERSIONS cell. The behavior would probably be the same, though.) If we throttled connections rather than closing them, we'd only get one connection per authority per hour, but authorities would have to keep open a potentially huge number of sockets. 2.2. Blocking circuit creation under certain circumstances In tor 0.2.5.1-alpha, we began ignoring the UseNTorHandshake option, and always preferring the ntor handshake where available. Unfortunately, we can't simply drop all TAP handshakes, since clients and relays can still use them in the hidden service protocol. But we could detect these versions by: Looking for use of a TAP handshake from an IP not associated with any known relay, or on a connection where the client did not authenticate. (This could be from a bridge, but clients don't build circuits that go to an IntroPoint or RendPoint directly after a bridge.) This would still result in clients not having directories, however, and retrying once an hour. 3. Ideas that might work 3.1. Move all authorities to new ports We could have each authority known to older clients start listening for connections at a new port P. We'd forward the old port to the new port. Once sufficiently many clients were using the new ports, we could disable the forwarding. This would result in the old clients turning into zombies as above, but they would only be scrabbling at nonexistent ports, causing less load on the authorities. [This proposal would probably be easiest to implement.] 3.2. Start disabling old link protocols on relays We could have new relays start dropping support for the old link protocols, while maintaining support on the authorities and older relays. The result here would be a degradation of older client performance over time. They'd still behave zombieishly if the authorities dropped support, however. 3.3. Changing the consensus format. We could allow 'f' (short for "flag") as a synonym for 's' in consensus documents. Later, if we want to disable all Tor versions before today, we can change the consensus algorithm so that the consensus (or perhaps only the microdesc consensus) is spelled with 'f' lines instead of 's' lines. This will create a consensus which older clients and relays parse as having all nodes down, which will make them not connect to the network at all. We could similarly replace "r" with "n", or replace Running with Online, or so on. In doing this, we could also rename fresh-until and valid-until, so that new clients would have the real expiration date, and old clients would see "this consensus never expires". This would prevent them from downloading new consensuses. [This proposal would result in the quietest shutdown.] A. How to "pull the switch." This is an example timeline of how we could implement 3.3 above, along with proposal 264. TIME 0: Implement the client/relay side of proposal 264, backported to every currently extant Tor version that we still support. At the same time, add support for the new consensus type to all the same Tor versions. Don't disable anything yet. TIME 1....N: Encourage all distributions shipping packages for those old tor versions to upgrade to ones released at Time 0 or later. Keep informed of the upgrade status of the clients and relays on the Tor network. LATER: At some point after nearly all clients and relays have upgraded to the versions released at Time 0 or later, we could make the switchover to publishing the new consensus type. B. Next steps. We should verify what happens when currently extant client versions get an empty consensus. This will determine whether 3.3 will not work. Will they try to fetch a new one from the authorities at the end of the validity period. Another option is from Roger: we could add a flag meaning "ignore this consensus; it is a poison consensus to kill old Tor versions." And maybe we could have it signed only by keys that the current clients won't accept. And we could serve it to old clients rather than serving them the real consensus. And we could give it a really high expiration time. New clients wouldn't believe it. We'd need to flesh this out. Another option is also from Roger: Tell new clients about new locations to fetch directories from. Keep the old locations working for as long as we want to support them. We'd need to flesh this out too. The timeline above requires us to keep informed of the status of the different clients and relays attempting to connect to the tor network. We should make sure we'll actually able to do so. http://meetbot.debian.net/tor-dev/2016/tor-dev.2016-02-12-15.01.log.html has a more full discussion of the above ideas. ```