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Re: HTTP Reassembly issue PAF enabled
From: Hui Cao <hcao () sourcefire com>
Date: Thu, 04 Apr 2013 10:57:47 -0400

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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