[Snort-devel] XFF/ExtraData not always logged for drop rules

Mike Cox mike.cox52 at ...2499...
Wed Jun 24 09:49:08 EDT 2015


VRT and Snort Devs,


The X-Forwared-For ("XFF") and/or True-Client-IP data is stored in
what is called "ExtraData" and written to the unified2 file.
ExtraData is often not written to the unified2 file at the same time
as other alert data and can depend on stream flushing as well as alert
flushing.

Looking at Stream5, there are a number of cases where the XFF
ExtraData is not logged on 'drop' rules even though it is available in
the data stream.  From what I can tell, the ExtraData gets written by
purge_alerts() which gets called by purge_to_seq() which gets called
by purge_flushed_ackd() (sometimes these functions get called when
dealing with TIME_WAIT timer stuff but let's ignore those cases for
now).  However, purge_flushed_ackd() only purges flushed and acked
bytes (as I understand it, bytes that are flushed *and* ackd).

So in these situations, when 'drop' rules trigger, the ExtraData is not written:

1) A single packet triggers the drop rule and it is inspected as a
packet and not part of a (reassembled) stream.
  - The Stream5 preprocessor doesn't get involved enough to do the
appropriate flushing/purging required to write the ExtraData.

2) A reassembled stream "packet" triggers the drop rule *and* the
normalize_tcp preprocessor is enabled (i.e. 'normalize_tcp: ips' which
is going to be the Protocol-IPS or Footprint-IPS flush policy
depending on PAF).
  - Snort is in pre-ACK mode and so it doesn't wait on the ACK to
flush the data to detection.  Since the flushing happens before the
ACK is received (and the ACK isn't processed anyway since the stream
is blocked by the block rule), the ExtraData never gets written.

I can understand and somewhat accept why the ExtraData isn't written
for scenario 1 although this happens when the HTTP Inspect
preprocessor is already engaged so it seems feasible to log the
ExtraData/XFF.  Can this be done?

For scenario 2, can I make a feature request that the ExtraData gets
logged appropriately in this case?  I'm guessing that people who run
Snort inline also have normalize and PAF enabled and it makes sense to
me that 'drop' rules would still write ExtraData, especially since
Stream5 is fully involved.
Once drop rules fire, the stream gets blocked (assuming the DAQ
supports this) so it makes sense to go ahead and compile/write out the
ExtraData since nothing else on that stream is going to get fully
processed.

I haven't looked much at Stream6 although it looks like most of the
code from Stream5 so I'm not sure why the version number change.

Thanks!

-Mike Cox

P.S. setting 'flush_on_alert' for Stream5 doesn't seem to have any
affect on these scenarios.
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