how to create ippacket and send it out?

[linwenyuan]

i am doing a firewall based on netfilter ,and i want to make it could defend synflood attack,so during the three-way handshaking ,when the receiver sends the ack+syn packet ,my wathdog (a program i write in the module which is registed on the netfilter,and it is in receiver) sends the ack packet for sender to get the tcp connection completed,and begin to time, if it have not received the real ack packet from sender ,which means that the sender is invalid or nonexisted .
the sender(request for service) and receiver(supply service) are the two endpoints of tcp connection.
so the problem is that how could i create a packet and send it to the sender. Actually ,i have no idea about this.
so is there anyone can help me ?
any answer would be appreciated very much.

[cyberinstru]

Please refer syncookies and tcp_ipv4 implentation in Linux TCP stack:

Syncookies: http://lxr.linux.no/source/net/ipv4/syncookies.c

TCP_ipv4: http://lxr.linux.no/source/net/ipv4/tcp_ipv4.c

[linwenyuan]

ok ,thank you very much .