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Mon, 20 Nov 2006:

I've been playing around with twisted for a while. It is an excellent framework to write protocol servers in python. I was mostly interested in writing a homebrew DNS server with the framework, something which could run plugin modules to add features like statistical analysis of common typos in domain names and eventually writing up something which would fix typos, like what opendns does.

To my surprise, twisted already came with a DNS server - twisted.names. And apparently, this was feature compatible with what I wanted to do - except that there was a distinct lack of documentation to go with it.

7 hours and a few coffees later, I had myself a decent solution. Shouldn't have taken that long, really - but I was lost in all that dynamically typed polymorphism.

from twisted.internet.protocol import Factory, Protocol
from twisted.internet import reactor
from twisted.protocols import dns
from twisted.names import client, server

class SpelDnsReolver(client.Resolver):
    def filterAnswers(self, message):
        if message.trunc:
            return self.queryTCP(message.queries).addCallback(self.filterAnswers)
            if(len(message.answers) == 0):
                query = message.queries[0]
                # code to do a dns rewrite
                return self.queryUDP(<alternative>).addCallback(self.filterAnswers)
        return (message.answers, message.authority, message.additional)

verbosity = 0
resolver = SpelDnsReolver(servers=[('', 53)])
f = server.DNSServerFactory(clients=[resolver], verbose=verbosity)
p = dns.DNSDatagramProtocol(f)
f.noisy = p.noisy = verbosity

reactor.listenUDP(53, p)
reactor.listenTCP(53, f)

That's the entire code (well, excluding the rewrite sections). Should I even bother to explain how the code works ? It turned out to be so childishly simple, that I feel beaten to the punch by the twisted framework. To actually run it in server mode, you can start it with twistd -y speldns.py and you have your own DNS server !

In conclusion, I hope I have grossed a few of you out by trying to do soundex checks on dns sub-domains.

DNS is not a directory service.
         -- Paul Vixie

posted at: 04:45 | path: /hacks | permalink | Tags: , ,