Ever wish you could intercept packets traveling up or down the IP stack of your host? And I'm not talking about listening in, like raw sockets or libpcap (tcpdump). I mean literally stop the packet from further propagating through the IP stack and then (possibly after some changes), reinjecting it back? Well, the time to dream is over, because divert sockets for Linux are here!
Divert sockets do exactly that - they filter out certain packets based on firewall specifications and bring them to you in user space. You then have the freedom of simply reinjecting them back as if nothing happened, mangling them first and then reinjecting them, or not reinjecting them at all.
As the name suggests, this mechanism utilizes a special type of RAW socket called divert (IPPROTO_DIVERT) that allow you to receive and send on them just like regular sockets. The difference is that a divert socket is bound to a port, into which the firewall can be instructed to send certain packets. Anything that a firewall can filter out can be sent into a divert socket.
Divert sockets first appeared as part of FreeBSD. Divert sockets under Linux is a port of this mechanism that strives to be source-code compatible in terms of user-space programs that utilize it.