[Snort-devel] Checksums

Martin Roesch roesch at ...48...
Wed Oct 25 16:19:00 EDT 2000

Checksumming should be done in the decoder, and we should probably think hard
about what to do with packets with known bad checksums.  I advocate dropping
them, since any OS worth anything (even crap from MS) is going to drop those
packets.  OTOH, there might be some analysis we'd like to do on them (covert
channel detection, etc) so we might just want to have a flag in the Packet
struct.  Either way, with the defrag and stream reassembly, we need these
functions implemented ASAP (hopefully before 1.7 ships).

One request: make it fast, make it portable! (ok, two requests...)  ;)


Christopher Cramer wrote:
> Guys,
> I've been thinking about insertion attacks recently, and it seems to me
> that a lot of snort is, or through defrag and tcp_stream will be, subject
> to a variety of insertion attacks.  Many of these are do to a lack of
> checksum testing in the decode engine.
> For the tcp stream reassembly code, I was about to add in some checksum
> tests (both the IP and the TCP checksum tests) before I incorporated the
> packet data in the tcp stream.  It occured to me that it might be better
> if we handled checksums in the decode engine.
> While I don't think we should necessarily drop packets with bad checksums,
> I think a set of flags that say which checksums suceeded and which
> failed would be a helpful addition.  This does mean the Dragos's defrag
> and my tcp_stream preprocessors should probably compute the correct
> checksum before sending our faked packets, but that's not too hard.
> If this sounds reasonable, I can go ahead and begin the implementation.
> -Chris
> ----------------------------------------------------------------------
> Dr. Christopher E. Cramer
> Assistant Research Professor
> Duke University, Department of Electrical and Computer Engineering
> 114 Hudson Hall, Box 90291, Durham, NC  27708-0291
> PH:  919-660-5248     FAX:  919-660-5293     email:  cec at ...56...
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Martin Roesch
roesch at ...48...

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