FreeBSD & Stack TCP/IP

Hi,

I have a router (FreeBSD 5.2) connected to the Internet (PPPoE) and a local
network with few computers (Win & xBSD).

If I tried to send a TCP packet with the FIN flag to my FreeBSD on the local
interface ... I got no anwser which is normal (I guess).

If I tried to send the same packet (with FIN flag) to my FreeBSD but from
the Internet, I got an answer with the RST, PUSH and SYN flags.

I do not understant why I have TWO DIFFERENT behavior.

I can give few information here :
- I use ipfw (without check state rules) and natd.
- On my local network, the fw lets every packets go through.
- My kernel is NOT compiled with TCP_DROP_SYNFIN option and sysctl gives me
''net.inet.tcp.drop_synfin=0'.
- My router is pingable on both interfaces.
- I tried to send a FIN packet on another FreeBSD on my local network, I got
no answer.

If anyone knows. Thanx

Julien