[Snort-sigs] rule exclusion by content

Al Lewis (allewi) allewi at cisco.com
Thu Jul 13 11:58:00 EDT 2017

You can negate content:


Taken from the “note” section


A ! modifier negates the results of the entire content search, modifiers included. For example, if using content:!"A"; within:50; and there are only 5 bytes of payload and there is no "A" in those 5 bytes, the result will return a match. If there must be 50 bytes for a valid match, use isdataat as a pre-cursor to the content.

Albert Lewis
SOURCEfire, Inc. now part of Cisco
Email: allewi at cisco.com<mailto:allewi at cisco.com>

From: Snort-sigs <snort-sigs-bounces at lists.snort.org<mailto:snort-sigs-bounces at lists.snort.org>> on behalf of "lravelo at us.hellmann.net<mailto:lravelo at us.hellmann.net>" <lravelo at us.hellmann.net<mailto:lravelo at us.hellmann.net>>
Date: Thursday, July 13, 2017 at 11:52 AM
To: "snort-sigs at lists.snort.org<mailto:snort-sigs at lists.snort.org>" <snort-sigs at lists.snort.org<mailto:snort-sigs at lists.snort.org>>
Subject: [Snort-sigs] rule exclusion by content

Good morning,

I have this rule which generates way too many alerts in squert:

alert udp $EXTERNAL_NET 53 -> $HOME_NET any (msg:"PROTOCOL-DNS TMG Firewall Client long host entry exploit attempt"; sid:19187; gid:3; rev:7; classtype:attempted-user; reference:cve,2011-1889; reference:url,technet.microsoft.com/en-us/security/bulletin/MS11-040; metadata: engine shared, soid 3|19187, policy max-detect-ips drop;)

we use OpenDNS in our environment and it seems like every single alert contains "opendns" somewhere in the content.  I'm sure there's a way to adjust or create another rule which negates the alert if the payload contains the word "opendns" but I've not seen any examples of this online.  Any help is appreciated :-)


Lazaro Ravelo
ISS Systems Engineer II

Hellmann Worldwide Logistics Inc.
10450 Doral Blvd
Doral, FL  33178
Phone:  +1 305 406 4500
Fax:  +1 305 418 4992
Direct:  +1 305 406 4574
Mobile:  +1 305 927 1386
Email:  Lazaro.Ravelo at us.hellmann.net<mailto:Lazaro.Ravelo at us.hellmann.net>
Web:  www.hellmann.com<http://www.hellmann.net/>

07/13/2017----11:43:39 AM

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