Mar 19, 2011

Selective IPv6 (AAAA) DNS resolution

Had IPv6 tunnel from HE for a few years now, but since I've changed ISP about a year ago, I've been unable to use it because ISP dropped sit tunnel packets for some weird reason.
A quick check yesterday revealed that this limitation seem to have been lifted, so I've re-enabled the tunnel at once.
All the IPv6-enabled stuff started using AAAA-provided IPs at once, and that resulted in some problems.
Particulary annoying thing is that ZNC IRC bouncer managed to loose connection to freenode about five times in two days, interrupting conversations and missing some channel history.
Of course, problem can be easily solved by making znc connect to IPv4 addresses, as it was doing before, but since there's no option like "connect to IPv4" and "irc.freenode.net" doesn't seem to have some alias like "ipv4.irc.freenode.net", that'd mean either specifying single IP in znc.conf (instead on DNS-provided list of servers) or filtering AAAA results, while leaving A records intact.
Latter solution seem to be better in many ways, so I decided to look for something that can override AAAA RR's for a single domain (irc.freenode.net in my case) or a configurable list of them.
I use dead-simple dnscache resolver from djbdns bundle, which doesn't seem to be capable of such filtering by itself.
ISC BIND seem to have "filter-aaaa" global option to provide A-only results to a list of clients/networks, but that's also not what I need, since it will make IPv6-only mirrors (upon which I seem to stumble more and more lately) inaccessible.
Rest of the recursive DNS resolvers doesn't seem to have even that capability, so some hack was needed here.
Useful feature that most resolvers have though is the ability to query specific DNS servers for a specific domains. Even dnscache is capable of doing that, so putting BIND with AAAA resolution disabled behind dnscache and forwarding freenode.net domain to it should do the trick.
But installing and running BIND just to resolve one (or maybe a few more, in the future) domain looks like an overkill to me, so I thought of twisted and it's names component, implementing DNS protocols.

And all it took with twisted to implement such no-AAAA DNS proxy, as it turns out, was these five lines of code:

class IPv4OnlyResolver(client.Resolver):
    def lookupIPV6Address(self, name, timeout = None):
        return self._lookup('nx.fraggod.net', dns.IN, dns.AAAA, timeout)

protocol = dns.DNSDatagramProtocol(
    server.DNSServerFactory(clients=[IPv4OnlyResolver()]) )

Meh, should've skipped the search for existing implementation altogether.

That script plus "echo IP > /etc/djbdns/cache/servers/freenode.net" solved the problem, although dnscache doesn't seem to be capable of forwarding queries to non-standard port, so proxy has to be bound to specific localhost interface, not just some wildcard:port socket.

Code, with trivial CLI, logging, dnscache forwarders-file support and redirected AAAA-answer caching, is here.

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