Design For A Tor DNS-based Exit List Status: This is a suggested design for a DNS Exit List (DNSEL) for Tor exit nodes. See http://exitlist.torproject.org/ for an implementation. Why? It's useful for third parties to be able to tell when a given connection is coming from a Tor exit node. Potential applications range from "anonymous user" cloaks on IRC networks like oftc, to networks like Freenode that apply special authentication rules to users from these IPs, to systems like Wikipedia that may want to make a priority of _unblocking_ shared IPs more liberally than non-shared IPs, since shared IPs presumably have non-abusive users as well as abusive ones. Since Tor provides exit policies, not every Tor server will connect to every address:port combination on the Internet. Unless you're trying to penalize hosts for supporting anonymity, it makes more sense to answer the fine-grained question "which Tor servers will connect to _me_?" than the coarse-grained question "which Tor servers exist?" The fine-grained approach also helps Tor server ops who share an IP with their Tor server: if they want to access a site that blocks Tor users, they can exclude that site from their exit policy, and the site can learn that they won't send it anonymous connections. Tor already ships with a tool (the "contrib/exitlist" script) to identify which Tor nodes might open anonymous connections to any given exit address. But this is a bit tricky to set up, so only sites like Freenode and OFTC that are dedicated to privacy use it. Conversely, providers of some DNSEL implementations are providing coarse-grained lists of Tor hosts -- sometimes even listing servers that permit no exit connections at all. This is rather a problem, since support for DNSEL is pretty ubiquitous. How? Keep a running Tor instance, and parse the cached-routers and cached-routers.new files as new routers arrive. To tell whether a given server allows connections to a certain address:port combo, look at the definitions in dir-spec.txt or follow the logic of the current exitlist script. If bug 405 is still open when you work on this (https://bugs.torproject.org/flyspray/index.php?do=details&id=405), you'll probably want to extend it to look at only the newest descriptor for each server, so you don't use obsolete exit policy data. FetchUselessDescriptors would probably be a good torrc option to enable. If you're also running a directory cache, you get extra-fresh information. The DNS interface Standard DNSEL, if I understand right, looks like this: There's some authoritative name server for foo.example.com. You want to know if 1.2.3.4 is in the list, so you query for an A record for 4.3.2.1.foo.example.com. If the record exists and has the value 127.0.0.2[DNSBL-EMAIL], 1.2.3.4 is in the list. If you get an NXDOMAIN error, 1.2.3.4 is not in the list. If you ask for a domain name outside of the foo.example.com zone, you get a Server Failure error[RFC 1035]. Assume that the DNSEL answers queries authoritatively for some zone, torhosts.example.com. Below are some queries that could be supported, though some of them are possibly a bad idea. Query type 1: "General IP:Port" Format: {IP1}.{port}.{IP2}.ip-port.torhosts.example.com Rule: Iff {IP1} is a Tor server that permits connections to {port} on {IP2}, then there should be an A record with the value 127.0.0.2. Example: "1.0.0.10.80.4.3.2.1.ip-port.torhosts.example.com" should have the value 127.0.0.2 if and only if there is a Tor server at 10.0.0.1 that allows connections to port 80 on 1.2.3.4. Example use: I'm running an IRC server at w.x.y.z:9999, and I want to tell whether an incoming connection is from a Tor server. I set up my IRC server to give a special mask to any user coming from an IP listed in 9999.z.y.x.w.ip-port.torhosts.example.com. Later, when I get a connection from a.b.c.d, my ircd looks up "d.c.b.a.9999.z.y.x.w.ip-port.torhosts.example.com" to see if it's a Tor server that allows connections to my ircd. Query type 2: "IP-port group" Format: {IP}.{listname}.list.torhosts.example.com Rule: Iff this Tor server is configured with an IP:Port list named {listname}, and {IP} is a Tor server that permits connections to any member of {listname}, then there exists an A record. Example: Suppose torhosts.example.com has a list of IP:Port called "foo". There is an A record for 4.3.2.1.foo.list.torhosts.example.com if and only if 1.2.3.4 is a Tor server that permits connections to one of the addresses in list "foo". Example use: Suppose torhosts.example.com has a list of hosts in "examplenet", a popular IRC network. Rather than having them each set up to query the appropriate "ip-port" list, they could instead all be set to query a central examplenet.list.torhosts.example.com. Problems: We'd be better off if each individual server queried about hosts that allowed connections to itself. That way, if I wanted to allow anonymous connections to foonet, but I wanted to be able to connect to foonet from my own IP without being marked, I could add just a few foonet addresses to my exit policy. Query type 3: "My IP, with port" Format: {IP}.{port}.me.torhosts.example.com Rule: An A record exists iff there is a tor server at {IP} that permits connections to {port} on the host that requested the lookup. Example: "4.3.2.1.80.me.torhosts.example.com" should have an A record if and only if there is a Tor server at 1.2.3.4 that allows connections to port 80 of the querying host. Example use: Somebody wants to set up a quick-and-dirty Tor detector for a single webserver: just point them at 80.me.torhosts.example.com. Problem: This would be easiest to use, but DNS gets in the way. If you create DNS records that give different results depending on who is asking, you mess up caching. There could be a fix here, but might not. RECOMMENDATION: Just build ip-port for now, and see what demand is like. There's no point in building mechanisms nobody wants. Web interface: Should provide the same data as the dns interface. Other issues: After a Tor server op turns off their server, it stops publishing server descriptors. We should consider that server's IP address to still represent a Tor node until 48 hours after its last descriptor was published. 30-60 minutes is not an unreasonable TTL. There could be some demand for address masks and port lists. Address masks wider than /8 make me nervous here, as do port ranges. We need an answer for what to do about hosts which exit from different IPs than their advertised IP. One approach would be for the DNSEL to launch periodic requests to itself through all exit servers whose policies allow it -- and then see where the requests actually come from. References: [DNSBL-EMAIL] Levine, J., "DNS Based Blacklists and Whitelists for E-Mail", http://tools.ietf.org/html/draft-irtf-asrg-dnsbl-02, November 2005. [RFC 1035] Mockapetris, P., "Domain Names - Implementation and Specification", RFC 1035, November 1987.