Following my previous thread about pap2t problem, it is clear now that the problem was related to the wrong
NAT entry :
udp 17 1 src=192.168.1.108 dst=188.8.131.52 sport=5071 dport=5060 packets=106 bytes=40803 [UNREPLIED] src=184.108.40.206 dst=192.168.1.108 sport=5060 dport=5071 packets=0 bytes=0 mark=257 use=1
So what I did, I disconnected the PAP2T box from the network, and waited to see that this connection will be removed from the nat table after UDP Unreplied Timeout reached.
However, even after 5 minutes (UDP Unreplied Timeout is 30 sec) it is still there. The NAT entry looked exactly the same - no single byte was added, no packets count increase. Only after I click on Drop Idle connection button, it is removed from the table.
My understanding was that UDB Unreplied Timeout will drop any UDP Unreplied connection, if there is no activity - i.e. no more packets sent on the same port/ip. However, it is turned out to work differently.
Am I missing something ?