[Snort-users] http_inspect missing requests

Russ rucombs at ...589...
Fri Feb 3 17:06:21 EST 2017

The final 3 GET requests were not acknowledged by the TCP server and so 
weren't processed.  If you run in IPS mode you will see them get them 
processed.  To enable IPS mode, make sure you have

     preprocessor normalize_tcp: ips

in your conf and add these args to your command line:

     --daq dump --daq-var load-mode=read-file -Q

The dump DAQ allows you to test inline mode with pcaps (it will create a 
new pcap with only the packets allowed to pass); -Q enables inline mode; 
and normalize_tcp: ips enables stream normalization.

On 2/3/17 1:27 PM, Felix Erlacher wrote:
> Hi all,
> I have a pcap trace containing HTTP traffic. I began to wonder because
> Snort did not trigger all alerts I was expecting. So I extracted the TCP
> stream in question and looked at it more closely. My impression is that
> for some reason the HTTP preprocessor is not parsing all GET requests.
> If I load this trace in Wireshark, than "follow TCP stream", it shows me
> 10 GET requests.
> If I use ngrep to manually inspect the trace, I count 10 GET requests as
> well.
> But the HTTP Inspect preprocessor of Snort tells me it found only 7 GET
> requests?!
> What could possibly be the problem?
> Some peculiarities of the trace:
> Heavy usage of HTTP/1.1 pipelining
> While Wireshark and the Snort DAQ tell me they processed 13 packets,
> HTTP inspect tells me it processed 17 packets.
> This trace contains checksum errors and a tcp RST in the last packet.
> I am using Snort with snort.conf from tarball and "-k none" switch.
> I would be happy to share the trace, but for privacy reasons I don't
> want to do that on the list. In case someone wants to take a look, just
> drop me a mail.
> thanks and greetings
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