add freebsd ntrack backend

[Alexander Sack]

freebsd support would be nice.

from what I understood, the freebsd route API is more or less similar to what we have with rtnetlink in linux

So it also uses messages over a PF_ROUTE socket to signal changes to the routing table. I am not exactly sure how to get updates on the IF and IFA parts; however, we don't use that for libnl backend either atm and i am not so sure if anything but the IF link state is of any relevance for the use-case implemented.

With some luck changes to the IF and IFA will also trigger route messages, so we don't need to explicitly poll through sysctl in order to support the "temporary link down" use-case.

The manpage of the freebsd route(4) api is here:

http://www.freebsd.org/cgi/man.cgi?query=route&apropos=0&sektion=4&manpath=FreeBSD+7.2-RELEASE&format=html

one point different to the linux rtnetlink approach seems to be that you do not need to explicitly subscribe to events in order to get them. Rather you would need to opt-out to not get all system wide route changes pushed.

[Alexander Sack (asac)]

i committed the bsd stub to a topic branch ... lp:~asac/ntrack/main.lp448398.bsd-backend linking that to the bug accordingly.