multi-threaded udp resolver

[Julian Assange <proff@iq.org>]

I've previously written a multi-threaded udp dns resolver in c (not
threads so much as a fsm emulating threads). I'd like to, if possible
write one in ocaml directly, rather than simply hooking into the C
code (which wouldn't be that simple anyway, due to the various timers,
management of fd's etc it needs).

While ocaml provides appropriate udp send/receive functions, the best
mechanism for understanding the structure of dns packets is unknown to
me. DNS packets are `loosely' structured. That is, there are many
different structural elements (including arrays of those elements),
and exactly how they are crammed into a packet can only be determined
by reading the structure. i.e the first part of the structure
describes the type (but not structure) of the next strucural element
and so on.

Vixie's named/bind daemon doesn't even attempt to describe the
structure in any sort of data form, but rather uses the code flow
itself to describe the structure (e.g pulling 16 bits, assigning it to
a variable, advancing the interpretation pointer by 16 bits, testing
the variable, pulling 32 bits etc). This method is incredibly
error-prone, and it's hard to see a good way of fitting it in with
ocaml's type system. Any ideas on the best way to approach this problem?

Cheers,
Julian.

-- 
Stefan Kahrs in [Kah96] discusses the
   notion of completeness--programs which never go wrong can be
   type-checked--which complements Milner's notion of
   soundness--type-checked programs never go wrong [Mil78].