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Re: 2 questions about Stream5 handling of missing data
From: "Russ Combs (rucombs)" <rucombs () cisco com>
Date: Fri, 31 Jan 2014 16:04:16 +0000
Thanks for the detailed info. I'll have a look and get back to you.
From: John Eure [john.eure () gmail com]
Sent: Thursday, January 30, 2014 10:51 PM
To: snort-devel () lists sourceforge net
Subject: [Snort-devel] 2 questions about Stream5 handling of missing data
I've been using snort to do some custom traffic inspection for a client, and have written a few plugins, including a
preprocessor plugin that uses Stream5's Protocol Aware Flushing (PAF). I've been testing out the new release
(18.104.22.168), and encountered a behavior that I hadn't seen before, and I'd like to find out whether it's a bug, or
whether it's something I should be expecting.
Normally, every time my preprocessor plugin sees a packet, the Packet struct has been zeroed out (up to the ip_options
field) and then filled with new data, so I get a clean struct each time. In this release, Stream5 got some
improvements in how it handles missing data. (Thank you, Sourcefire!) But when that new handling is triggered, I'm
seeing packets that haven't been completely zeroed out. Specifically, it's the Stream5 rebuilt pseudo-packet that is
generated after the gap in the data, which hasn't been zeroed out before the new data was added.
I've been using a bit field (the preproc_reassembly_pkt_bits field) in the Packet struct to mark packets as having been
accepted or rejected by my preprocessor, and so I was surprised to find that the bit field wasn't reset in between
packets. Is this normal behavior that I should be expecting?
Also, I've got a second, more general question, for Sourcefire. After Stream5 detects missing data on a stream, PAF
gets "reset", and the flush policy gets set to STREAM_FLPOLICY_FOOTPRINT, and never goes back to
STREAM_FLPOLICY_PROTOCOL again. So far, I've been able to work around this, but I'd much rather have a solid fix in
place. So I was wondering, is this on your roadmap for future development? At the very least, now you know there's at
least one person interested in that feature. :)
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