help understanding alarm for IP flags are invalid (1225-0)

Unanswered Question
Sep 20th, 2007

The alarm is attached. Based on my feeble attempts to read the IP header included in the alarm, the Don't fragment bit is set and the frag offset is zero. Why is this signature firing?

I have this problem too.
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Anonymous (not verified) Wed, 09/26/2007 - 10:13

It's not necessarily to do with fragmented packets, it depends on what signature actually fired. It will be one of the normalizer engine sigs, and the action on that signature will be "deny-connection-inline" meaning once the sig fires all subsequent packets on that connection are denied/dropped

mhellman Wed, 09/26/2007 - 10:31

not sure I follow you. we're not inline, we're in promiscuous mode. The signature that actually fired is in the heading of the posting...1225-0. Unless I'm reading it wrong, the trigger packet has the "do not fragment" bit set and has no fragment offset value. To the untrained eye, this appears to be a normal unfragmented packet.

mhellman Wed, 09/26/2007 - 12:34

This appears to be is a big scary bug. I didn't initially bother to valid the trigger packet source and destination IP address. I should have. The trigger packet has a destination address of and NOT The firewall logs show that there WAS a session between and during this time period. I don't know what to make of this other than at least once my IDS fired a completely and utterly mashed up mess of an alarm


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