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Re: HTTP Reassembly issue PAF enabled
From: Russ Combs <rcombs () sourcefire com>
Date: Fri, 5 Apr 2013 12:15:50 -0400

On Fri, Apr 5, 2013 at 11:56 AM, Parmendra Pratap
<parmendra.pratap () yahoo com> wrote:
Hi Hui

Thanks again for a quick response.

Have tried -Q flag , but since I am using Pcap DAQ it fails to start with -Q
set.
However it does start wiith --test-inline mode , which I assume works no
different from -Q except that drops are not enabled.

Actually, normalizations are not enabled in that case.  You should see
something like this in your startup output:

WARNING: tcp normalizations disabled because not inline.

Try running with:

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

My snort.conf does have preprocessor normalize_tcp: ips have set.

But even with the above set up I can still replicate the same issue related
to incorrect tcp flags.
As I said before , I think it seems like a matter of setting the tcp header
of the last packet that completes the PDU inside Packet->tcph->th_flags in
the s5 preprocessor when doing a PAF based flushing.
I can see that the direction i.e. sourceIP/port and destIP/port being
reversed for the same reason in the Packet struct when doing a reassembly
based flushing from s5.

Thanks
Parmendra

__________________________________________________________________________

Hi Parmendra,

To be clear, you must use IPS mode to get what you want, so you need to
1) use -Q  when you run snort
2) Enable Normalization for TCP:

preprocessor normalize_tcp: ips

Best,
Hui.
On 04/04/2013 08:25 AM, Parmendra Pratap wrote:
Hi Hui
Thanks for a quick reply.
I tried the use case with Snort 2.9.4.5.
Does not make a difference.
Issue is still replicable with the steps outlined in my root email.
This is what I think is going on , based on few tests and source
lookups <excuse my newbieness if it reflects anywhere below :) >-
Stream5 reassembly does not tag a packet as complete PDU until it
recieves subsequent ack gainst the packet no matter wheter or not the
packet actually holds complete PDU  (in this case HTTP) or not.
With PAF enabled this prevents the URI Bufs from being created and
inspected in HTTP inspect module until the next packet arrives (ie ack
against the original packet that contained the HTTP req).
When the HTTP inspect URI Bufs based match fires, with PAF ON,  its
always(mostly?) when the ack on reverse direction is received.
The spo_alert* modules simply uses the header data provided in the
Packet->tcph which holds the header from the current packet ie ack
packet from the server ... and hence the incorrect TCP header display
with PAF on.
There is no way curently to get the correct TCP headers unless Stream
5 is queried to give the original raw packet <spo_log_tcp_dump.c does
that>.
Realworld issue arising from the above is incorrect TCP header data in
alerts. TCP dumps are OK though for reason mentioned above.
There seems multiple ways to get around this:
Generate TCP dump on all alerts and assume the alerts will have
incorrect TCP headers
Write an alert output plugin that inspects the raw packet for correct
TCP headers etc
Add more metadata to Packet struct which can provide the correct TCP
headers at least for the last packet that completed the PDU in the
alert output plugins.
Last option looks the most organic and least sub optimal one to me.
Given my little experience with snort so far , I wont be surprised if
any of the above stated flow is incorrect.
I will more than appreciate if someone can correct me above and
enlighten me more about the internals :) .
Thanks
Parmendra



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the most talented Cisco Certified professionals. Visit the 
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