Port Negation not working?

["Paul Melson" <pmelson () gmail com>]

Because of a high number of false positives caused by MIME-encoded file
attachments, I have modified most of the shellcode/nop rules on one of our
sensors so that they should ignore SMTP traffic.  However, I have a handful
of rules that still fire on SMTP traffic.  Here's one example.

Original rule:
alert ip $EXTERNAL_NET any -> $HOME_NET any (msg:"SHELLCODE base64 x86
NOOP"; \
content:"RERERERERERERERERERERERERERERERER"; metadata:policy security-ips
drop; \
classtype:shellcode-detect; sid:12801; rev:1;)

Modified rule:
alert ip $EXTERNAL_NET any -> $HOME_NET !25 (msg:"SHELLCODE base64 x86
NOOP"; \
content:"RERERERERERERERERERERERERERERERER"; metadata:policy security-ips
drop; \
classtype:shellcode-detect; sid:12801; rev:1;)

I still get alerts from the modified rule for SMTP traffic containing the
"RERERE..." string.  I know that the rule/port syntax has been modified in
2.8 to allow port lists, but according to the manual[1], this syntax should
still work for port negation.  Am I missing something?

Thanks,
PaulM

[1] http://www.snort.org/docs/snort_htmanuals/htmanual_280/node169.html




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