[Snort-users] Port Negation not working?

Todd Wease twease at ...1935...
Wed Jan 2 18:28:54 EST 2008


Hello Paul,

Since the rule is an 'ip' rule, the port is insignificant.


Todd

Paul Melson wrote:
> 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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