Port or'ing

["Craig Mueller" <cmueller () alebra com>]

I'd like to write a rule that detects any inbound TCP connection,  that 
alerts when the dest port != a defined set of ports.
Example: Allowed inbound TCP ports is 25, 80, 443. If a connection 
attempt to any port not in the list, then alert. Resulting in a 
connection attempt to 22 triggers alert..

Currently I use a rule in this form, to look for TCP port 25

alert tcp $EXTERNAL_NET any -> $HOME_NET !25  (msg:" TCP port TCP !25"; 
classtype:Critical;)

I know this does not work, but I think something this could work:
alert tcp $EXTERNAL_NET any -> $HOME_NET ![25,80,443]  (msg:" TCP port 
TCP !25,80,443"; classtype:Critical;)



Thanks

--
Craig Mueller CISSP
Senior Consultant
Alebra Technologies
www.alebra.com
612-436-8204




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