[Oisf-users] Understanding Suricata's "xbit" implementation.

Champ Clark III cclark at quadrantsec.com
Fri Feb 24 20:16:45 UTC 2017


Over the past year I have become quite the Suricata fan.  So much so 
that I ended up in Washington DC to attend Suricon 2016.  It was a great 
experience and I plan on attending Suricon 2017!

My question has to do with how Suricata handles "xbits".  I work on a 
project that uses "Snort like" rules for log analysis called "Sagan" 
(http://sagan.io). It is multi-threaded and also supports the concept of 
"xbits".  My goal is to align how Sagan works with "xbits" with how 
Suricata works with "xbits".  After a little bit of research,  I've 
found some Suricata examples of "xbits" usage. For example:

xbits:set,EXAMPLEBIT, track ip_dst, expire 60;
xbits:isset, EXAMPLEBIT, track ip_dst;

1) My question is, when the xbit is "set",  is the "track ip_dst" 
needed?  In Sagan,  this is what I would do:

xbits: set,EXAMPLEBIT,60;
xbits: isset,by_src,EXAMPLEBIT;

When we "set" an xbit,  both the source and destination are stored in 
memory.   This way when we use a "isset", "isnotset" or "unset",  we can 
do so by "by_src", "by_dst", "both" or "reverse" (Suricata's 
"toggle"?).  This means that Sagan really only uses the src/dst on 
"isset", "isnotset", "unset" xbits and not with "set".

2) Is it possible to use multiple "xbits" with isset/isnotset/etc.  In 
Sagan,  this is what I would do:


This would mean, "if the source (by_src) has EXAMPLEBIT1 or EXAMPLEBIT2 
or EXAMPLEBIT3 isset".   You could also do:


This would mean, "if the source (by_src) has EXAMPLEBIT1 and EXAMPLEBIT2 
and EXAMPLEBIT3 isset".

Hopefully my questions are clear.

We've found the "xbit" concept to be insanely powerful.  One of my goals 
this year is to store "xbit" data from both Sagan and Suricata into a 
Redis database.   This way,  xbit data could be shared globally from a 
log and packet analysis level.

Thank you!

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