[Snort-users] Port Negation not working?

Todd Wease twease at ...1935...
Thu Jan 3 10:09:39 EST 2008


Paul,

Thought of something you could try.  Add a pass rule for port 25 for 
each of the 'ip' rules that are firing on the MIME encoded SMTP traffic, 
e.g.

pass tcp $EXTERNAL_NET any -> $HOME_NET 25 \
(msg:"No alert port 25 - SHELLCODE base64 x86 NOOP"; \
content:"RERERERERERERERERERERERERERERERER"; sid:10000001;)


Todd

Todd Wease wrote:
> 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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