[Snort-devel] 2 questions about Stream5 handling of missing data

John Eure john.eure at ...2499...
Thu Jan 30 22:51:39 EST 2014


Hello,

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 (2.9.6.0), 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.  :)

Thanks,
John Eure
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