PIX Denying inbound UDP packets from DNS Server

[jimb]

Can anyone explan this message from the PIX syslog? The 208.*.*.* address is my DNS server on the outside network. The 63.*.*.* address is a client address on the inside network. The client seems to resolve DNS with no problem. What does this message indicate? Why would DNS initiate a connection to the clients?

2002-06-28 09:36:42 Syslog.Warning lan-226.dnetrouter1-1.dnet.net Jun 28 2002 09:21:12: %PIX-4-106023: Deny udp src outside:208.*.*.*/53 dst inside:63.*.*.*/1192 by access-group "outside"

[paqiu]

The message means that UDP seesion has been denied by your access-group "outside".

PIX can inspect TCP sessions with "syn" "syn-ack" "sequence number" because TCP is statefull transport protocol.

For UDP sessions, it is stateless, PIX can only set timer for each UDP session. When the timer timeout, PIX will think that session is finished.

Sometimes if your DNS request send to the DNS server, for some reason, the DNS server take quite a long to reponse, the PIX think that UDP session has timeout. PIX has cleared that DNS request session because of the timer timeout.

When the DNS response, PIX will think this is a new UDP session, it will be denied by your access-group "outside".

This situation normally will not happend only when DNS server really takes ages to response to a DNS enquiry.

Best Regards,