[Snort-users] Dont discard truncated packets

Felix Erlacher felix.erlacher at ...17726...
Fri Jan 27 11:26:10 EST 2017

I didnt find out how to tell Snort NOT to discard truncated packets and
very likely there is no such option. So I found another solution that
worked for me:
As Snort does not like truncated packets I had to fix the size info in
my pcap file so that packets do not appear truncated anymore.
I replayed the trace with tcpreplay and the --pktlen option on one VM
and captured the traffic with tcpdump on another VM.
Off course, the result is not the same pcap trace at all:
-packets are still truncated, just the size information is fixed.
-it is nearly impossible to have the same timing as before, this might
influence the outcome of a Snort Analysis.
-other effects that I didnt consider yet.



On 26/01/17 20:04, Felix Erlacher wrote:
> Hi all,
> I have a pcap trace with one packet containing payload for a rule I want
> to test. The packet is truncated. The rule does not trigger an alert.
> I can see in the protocol statistics that one IPv4 packet is discarded.
> As I only have one packet in the trace I assume it is discarded because
> it is truncated.
> Can I tell Snort to not discard truncated packets?
> Or better, not to discard packets with "basic encoding integrity flaws"
> as the manual calls it.
> I tried various preproc options from the manual, always with the result
> of truncated packets being discarded.
> While I am aware that having Snort analyze truncated packets might not
> be the best of ideas, it would be helpful in various test scenarios.
> BTW: I am using the "-k none" switch, so this problem shouldn't be
> caused by checksum errors.
> greets
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Felix Erlacher

Institute of Computer Science
University of Innsbruck


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