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- 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.
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