[Snort-users] Rule works in replay file mode, but not when sniffing

Russ Combs rcombs at ...1935...
Fri Jul 12 12:55:19 EDT 2013


On Fri, Jul 12, 2013 at 12:22 PM, Pavel Rantorski <fhjull01 at ...15979...>wrote:

>  Thank you for your answers.
>
> When I use rule:
>
>
> alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:”Test file
> download”; content:”filename=“; fast_pattern:only; http_header;)
>
> It behaves in a same way - won't fire when sniffing, fires when replayed.
>
>
> You are correct that with --dirty-pig, no alert is generated even on
> replay with the sample I sent. I trimmed the sample I sent before, perhaps
> a bit too much. Larger pcap (hopefully with full session) can be found at:
>
> http://img.dreamchaser.cz/testdata7.zip (8MB)
>

Where was this pcap captured?

>
> This sample fires both rules (mine + the one suggested by Joel Esler) even
> with --dirty-pig, but no results when sniffed directly.
>

Are you sure you don't have this (
https://www.myricom.com/software/myri10ge/349-when-i-view-traffic-with-tcpdump-why-do-i-see-packets-larger-than-the-mtu.html)
going on?  I definitely saw evidence of that in your earlier Snort pcap.
If you use the same options to run Snort both in readback and replay modes
and you get different results, it is very likely due to how the packets are
presented to Snort.

>
> ------------------------------
> Date: Fri, 12 Jul 2013 11:12:05 -0400
> Subject: Re: [Snort-users] Rule works in replay file mode, but not when
> sniffing
> From: rcombs at ...1935...
> To: jesler at ...1935...
> CC: fhjull01 at ...15979...; snort-users at lists.sourceforge.net
>
>
> I think you have something else going on.  Can you send a full session
> capture?
>
> The capture you have is alerting in readback at shutdown.  To confirm,
> --dirty-pig to your command line and you won't get the alert.
>
> The reason is Content-Length: 9156548 but you only have 4101 bytes in the
> capture.  Snort is trying to reassemble more data which never shows up.
> You can get the alert to fire even with --dirty-pig if you change paf_max
> to something like 3072.
>
> If you want it fire in replay you need at least paf_max worth of the
> response body.
>
> On Fri, Jul 12, 2013 at 10:10 AM, Joel Esler <jesler at ...1935...>wrote:
>
> What happens when you do a:
>
> alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:”Test file
> download”; content:”filename=“; fast_pattern:only; http_header;)
>
>
> On Jul 12, 2013, at 9:05 AM, Pavel Rantorski <fhjull01 at ...15979...> wrote:
>
>  Hello,
> I'm testing a rule that should (eventually) detect download/upload of
> specific file types from public HTTP servers. I could not get the rule to
> trigger, so I simplified it to:
>
> alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"Test file
> download"; content:"Content-Disposition|3a|"; nocase; http_header;
> pcre:"/filename=/simH"; classtype:policy-violation; sid:1000004; rev:7;)
>
> (the rule is nowhere near complete, it is simplified to be less prone to
> mistakes)
>
> Unfortunatelly, the rule still does not work. I captured the traffic (on
> the same machine/interface that Snort was running) and verified that such
> packet is indeed there. When I let Snort analyze the traffic from this pcap
> file ('snort -A console -c /etc/snort/snort.conf -r /tmp/testdata5.pcap -l
> . -u snort'), the rule is fired on console correctly.
>
> The rule is (in standard, sniffing mode) sometimes triggered as well
> (although never from this particular server I am testing).
>
> What could be the cause of this? Snort is running in IDS mode (not inline)
> and is not dropping packets. LRO and GRO are disabled on network adapter. I
> have tried running Snort with '-k none' without any results.
>
> I have attached small pcap sample of the traffic I'm trying to catch -
> this is enough to trigger the rule in replay mode, but didn't trigger when
> sniffing.
>
> Thank you,
> Pavel
> <testdata5.pcap>
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