[Snort-devel] Option for one-line "raw" packet dump (ascii and hex) in alert_fast output module

Roberto Moreda moreda at ...3615...
Wed Oct 7 19:29:22 EDT 2015

Hi, all.

I hope I’m not bringing up an old or closed subject. I made some searches and I couldn’t find anything clear about my problem :-) 

For some time I have been using Snort as a source of “security events” for several log consolidation or SIEM systems. Most of those systems assume one-line alerts as input, with the minimal info (i.e. name/id of the signature, severity, category, source and destination). The problem is that analysts usually would like to have the payload of the package to assess false positives at once.

In order to not interfere with the usual “field recognition patterns” of such systems, I opted to extend the alert_fast output module this way:

output alert_fast: [<filename> ["packet"|"packetraw"] [<limit>]]

* packetraw: this option will cause brief single-line entries
                 to be logged with the content of the packet in raw format (ascii and hexadecimal
                 dumps) appended.

This is absolutely backwards compatible, not affecting current Snort configurations.

The result with the “packetraw” option in the alert_fast output module configuration, should be one line per alert as:

10/08/15-01:03:16.909442  [**] [3:21355:4]  <eth1> PROTOCOL-DNS potential dns ca
che poisoning attempt - mismatched txid [**] [Classification: Attempted Informat
ion Leak] [Priority: 2] {UDP} XX.XX.XX.XX:53 -> YY.YY.YY.YY:12563 ...z[…]  0001D77A[…]

Note that the ...z[…]  0001D77A[…] is shortened on purpose, but the idea is basically what’s shown. Once again, this should be backwards compatible with sane parsers in most of log consolidation or SIEM systems *and* appends the ascii and hexadecimal dump of the raw packet to each event, offering a great way to assess false positives and make accurate general searches.

I wrote a patch against to enable this behaviour, that you can see here <https://github.com/moreda/snort/compare/> in a fancy format or download here <https://github.com/moreda/snort/compare/> ready to apply. 

I know that the general idea is to avoid extra logic in the output modules, letting other processes to cope with unified2 to convert data to whatever format… but I’m pretty sure that this tiny addition could lower complexity in many deployments allowing to have payload info in a very simple way.

Please, feel free to criticise, correct or comment about my proposal.
Thank you very much!


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