[Snort-sigs] Not-ing out ports

Lay, James james.lay at ...3513...
Mon May 13 10:19:05 EDT 2013

Guessing you'll want the !25 on both ends since it's bidirectional:


alert tcp !25 any <> any !25




From: John Wiltberger [mailto:johwiltb at ...2420...] 
Sent: Monday, May 13, 2013 5:01 AM
To: snort-sigs at lists.sourceforge.net
Subject: [Snort-sigs] Not-ing out ports


So I have a question.  When dealing with bi-directional signatures (I
know, they aren't ideal in the least, but sometimes you can't help who
develops your signatures), if you choose to not-out a port (as in !<port
number>), does snort run a boolean OR on the traffic (as in 'if source
port != <port number OR destination port != <port number>)?

Reason is, I have a signature that's header is 'alert tcp any any <> any
!25', yet it is still alerting off of traffic over port 25.  I'm sorry
if these seems confusing, I can't think of a better way of stating this.
Any thoughts?

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