nss_dns addons and thread safety

[George Beshers <gbeshers at comcast dot net>]

This concerns creating an add-on to resolv/nss_dns to support multicastdns
as used in Apple's Rendezvous, i.e.,

http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt

1)  This uses multicast on 5353 and/or a local stream created using
/var/run/mDNSresponder to a local mDNSResponder server.  Other
than patching is there a good way to add thread-specific state (e.g.,
by extending _res) so that the socket can remain open between calls?
I think that all I really need to store is the file descriptor.

2)  Ideally, I would like to re-use the domain searching code but I don't
see a hook into the send mechanism that would allow me to do that.
[I don't feel that qhook and rhook are appropriate since the user might
doing something with them.]  Suggestions welcome.

3)  Finally, IPv6 status/todo list?

Thanks in advance,

George