How do I respond with a “syn ack” packet when receiving a “syn” packet through a raw socket?

Question

I am curious about raw sockets and and how to create them and would like to implement my own TCP mechanism. I have read some examples and have succeeded with sending both custom made TCP packets and UDP packets with my own written IP header (of course influenced by a lot of examples). I have checked with Wireshark that the packet is reaching its destination, so everything is fine so far.

But regarding TCP packets, to make the full handshake:

Client     Server
syn  ---> 
     <---  syn ack 
ack  --->

What do I need from the server´s point of view to get the syn packet so I can send the syn ack back to the client?

Answer 1

To receive packets on a raw socket, just call recv or recvfrom on it. The OS will return you a copy of the next packet addressed to the machine, with headers and all, which should include address info. Watch the destination address, port, and transport protocol, and ignore any that aren't what you were expecting. (Since the point of a raw socket is that there's no built-in notion of ports or anything the OS could use to route packets to sockets, it doesn't know what program to send it to...so every raw socket should receive every packet addressed to the machine. Meaning you might receive lots of crap you have no interest in.)

Once you see a packet addressed to "you", just build a SYN/ACK packet and send it to the address and port listed as the source in the received packet.

Note, though: the OS will often do its own processing of TCP and UDP packets (including sending ICMP "port unreachable" or other responses for ports it doesn't have listeners for)...and doing your own processing on top of that is bound to cause wackiness. If you're going to implement your own flavor of TCP, you might want to use a different protocol number. (Of course, then most clients won't be able to connect to it...you'd have to make a client as well.)