Listing active Tor servers...

Want to know the list of active Tor servers at any given time? It turns out that this is fairly easy to do.

When a Tor client starts up, and from time-to-time during normal operation, it needs to refresh it's list of active servers. The Tor network maintains several "directory servers" which keep track of which nodes are willing to perform onion routing functions. Because the Tor client may not have yet started (it can't join the Tor network before it knows which Tor servers to use), anyone can fetch the directory from the public Internet using HTTP.

The basic idea is pretty simple. Just visit one of the directory servers with a URL like the following:

http://belegost.mit.edu/tor/

The belegost.mit.edu system is one of the five or so authoritative Tor directories on the Internet, so it should be fairly stable. The file it returns contains all the information that a Tor client needs to know about each of the servers, including IP addresses and port numbers. The Tor directory protocol document can help you interpret the details fairly easily.

Of course, if you just want to know the list of active servers for monitoring or blocking purposes, you can just run the following perl script, which will dump out the server names, IP addresses and onion routing ports for you.

#!/usr/bin/perl
#
# Fetch the list of known Tor servers (from an existing Tor server) and
# display some of the basic info for each router.

use LWP::Simple;

# Hostname of an existing Tor router.  We use one of the directory authorities
# since that's pretty much what they're for.
$INITIAL_TOR_SERVER = "18.244.0.114";   # moria2 / belegost.mit.edu
$INITIAL_TOR_PORT = 80;

# Fetch the list of servers
$content = get("http://$INITIAL_TOR_SERVER:$INITIAL_TOR_PORT/tor/");
@lines = split /\n/,$content;

foreach $router (@lines) {
    if($router =~ m/^router\s+(\S+)\s+(\S+)\s+(\S+)\s+(\S+)\s+(\S+)\s*$/m) {
        ($name, $address, $or_port, $socks_port, $directory_port) =
            ($1, $2, $3, $4, $5, $6);
        print "$name $address $or_port\n";
    }
}

Update 2008-06-09 11:38: In the nearly two years since I wrote this original post, the Tor folks have updated their directory protocol, and this script no longer works. Please see my newer post for an update and some working code.