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Re: HTTP Reassembly issue PAF enabled
From: Parmendra Pratap <parmendra.pratap () yahoo com>
Date: Thu, 4 Apr 2013 05:25:39 -0700 (PDT)
Thanks for a quick reply.
I tried the use case with Snort 126.96.36.199.
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
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
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 :) .
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