[Snort-devel] Flowbits Set and Not Checked Against SRC/DST Networks

Eoin Miller eoin.miller at ...3055...
Fri Jun 24 16:10:08 EDT 2011

We were having some issues when setting flowbits for sessions based upon IP lists. I think I have narrowed down the problem after creating and running these rules:

alert tcp $HOME_NET any <> [,,,,,] any (msg:"TEST talking to localhost"; flags:S; flowbits:set,AOL.test; classtype:bad-unknown; sid:7000004; rev:1;)
alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"TEST http_uri to localhost"; flowbits:isset,AOL.test; content:"/"; http_uri; classtype:bad-unknown; sid:7000005; rev:1;)

The first rule never creates alerting output. But the second rule will fire on every HTTP request from our client net while these are both running. What I think is occurring is that the packet causes an alert and a flowbit to be set for the stream because it sees the initial SYN packet (which is true). But the alert output is never created because it is actually checked against the SRC/DST networks in the rule and therefor it gets suppressed. But, the flowbit must not be getting compared against the SRC/DST network ranges and ports and unset if it doesn't match. Can I bug you guys to take a look into this?

# snort --version
   ,,_     -*> Snort! <*-
  o"  )~   Version IPv6 GRE (Build 135)
   ''''    By Martin Roesch & The Snort Team: http://www.snort.org/snort/snort-team
           Copyright (C) 1998-2011 Sourcefire, Inc., et al.
           Using libpcap version 1.1.1
           Using PCRE version: 8.02 2010-03-19
           Using ZLIB version: 1.2.3

I haven't really gone through the code to much to see if this is the issue, but I think the reasoning is sound based upon output review/testing.

-- Eoin

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