Intercepting a packet, changing it and re-injecting into the network

[Eygene Ryabinkin]

Fabricio, good day!
> I'm developing a network application which needs *to intercept* a packet
> (not just *copy* a packet, like libpcap does), move this packet into my
> application (userland), do some checking in the packet and according
> with some heuristics, the application may change the payload and
> re-inject the modified packet into the network. Note that sometimes,
> I'll change the payload, drop the packet or just let it go.
> 
> So, how can a I do that in FreeBSD? I can use 6.1, 7.1, any version.

Probably you should read the divert(4) manual page and the ipfw(8)
manual, looking for the 'divert' directive. The good example of
your application is /sbin/natd.

The aforementioned facilities are present in FreeBSD at least since 4.x.
-- 
Eygene