[Snort-sigs] SO rules vs regular rules

Mike Cox mike.cox52 at ...2420...
Wed Feb 3 14:05:35 EST 2010

Thanks for the info.  I wasn't going to use the Snort Labs converter
(honestly I didn't know it existed) but just write them in C.  From what
Brian said, it sounds like doing so will be hit or miss unless I'm an expert
at knowing Snort's internal optimizations (which I not).

-Mike Cox

On Wed, Feb 3, 2010 at 12:50 PM, Patrick Mullen <pmullen at ...435...>wrote:

> > If you are planning on using the converter published at
> > labs.snort.org, you will not see any performance gain for straight
> > translation of rules to shared objects.  In all practical terms, the
> > detection from the shared object rules using the output the converter
> > are exactly the same as textual rules.
> Do not use the converter present on Snort Labs to convert standard
> text rules to SO rules in an effort to improve detection speed.  The
> automatically generated detection function does not implement looping
> or "back stepping" like a normal text rule does and could introduce
> false negatives.  This does *NOT* imply that the SO rules we ship do
> not have these features -- the version online has a very minor
> difference from the one we use internally so I (we) don't need to
> explain a minor code change that needs to be done to use the custom
> detection function rather than the built-in detection functions.
> To explain what I'm talking about, imagine the following rule fragment:
> content:"A"; content:"B"; distance:3; within:1;
> And the packet data:
> AA012B
> Without looping and back checking, as done in the autogenerated
> detection rule from the online snort rules to SO rules converter, the
> first content match would match on the first "A" then check the second
> content match and see "2" and return RULE_NOMATCH.  With looping
> (something you can easily add manually but is beyond the scope of the
> SO Rules generator), when the second content match failed, it would
> retry the first content match and find the second "A".  From there, it
> would check the second content match and successfully find the "B" and
> return RULE_MATCH.
> If you're interested, the change to make the converted SO rule operate
> like a text rule is to set the function pointer within the Rule struct
> to NULL.  This will tell snort to use the internal detection functions
> and will never call the ruleXXXXeval() function.
> But as Brian said, it won't improve performance anyway.  I just wanted
> to explain why you wouldn't want to do it from a purely
> detection/functional point of view.
> Thanks,
> ~Patrick
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