Snort mailing list archives
XFF/ExtraData not always logged for drop rules
From: Mike Cox <mike.cox52 () gmail com>
Date: Wed, 24 Jun 2015 09:49:08 -0400
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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Current thread:
- XFF/ExtraData not always logged for drop rules Mike Cox (Jun 24)
- Re: XFF/ExtraData not always logged for drop rules Mike Cox (Jun 25)
- Re: XFF/ExtraData not always logged for drop rules Carter Waxman (cwaxman) (Jun 25)
- Re: XFF/ExtraData not always logged for drop rules Mike Cox (Jun 25)
- Re: XFF/ExtraData not always logged for drop rules Mike Cox (Jun 26)
- Re: XFF/ExtraData not always logged for drop rules Carter Waxman (cwaxman) (Jun 25)
- Re: XFF/ExtraData not always logged for drop rules Mike Cox (Jun 25)