[Snort-users] Embedded Fragment?

Glenn Forbes Fleming Larratt glratt at ...4500...
Tue Feb 19 23:08:04 EST 2002


On Tue, 19 Feb 2002, Daniel Holden wrote:

> Can someone tell me (or point me to the docs) what the following means?
>
> Feb 19 21:52:52 crow kernel: ipt_unclean: (embedded packet) Embedded
> fragment.
> Feb 19 21:52:51 crow kernel: IN=eth0 OUT= MAC=uglyMacAddressHere
> SRC=192.5.4.146 DST=209.142.39.160 LEN=56 TOS=0x00 PREC=0x00 TTL=245
> ID=22342 DF PROTO=ICMP TYPE=3 CODE=13 [SRC=209.142.39.160 DST=204.152.187.2
> LEN=64 TOS=0x00 PREC=0x00 TTL=55 ID=47316 FRAG:64 PROTO=TCP ]

	It's opaque to me whether your system "crow" is generating,
	recording, or dropping the packet in question, but:

- The packet itself is an ICMP 3/13 packet, meaning type 3, code 13
(see http://www.iana.org/assignments/icmp-parameters); type 3 is
"destination unreachable", code 13 is "Communication Administratively
Prohibited", i.e. an ACL, probably on host 192.5.4.146, dropped a packet
from host 209.142.39.160.

- A truncated portion of the packet that was dropped is included *within*
the ICMP packet; enough to discern that it was a 64-byte fragment of a TCP
packet, from host 209.142.39.160 to host 204.152.187.2 .

- What's it all mean? A 64-byte fragment either implies a final fragment
or an extraordinarily short-MTU segment in the path - or crafted fragmentation
for some probably unfriendly purpose. Need more data (like a hex dump - from
Snort ;) .

	-g


> Thank you
> Daniel Holden
> dholden at ...2819...
> http://www.idsb.net
>
>
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Glenn Forbes Fleming Larratt          glratt at ...604...
http://is.rice.edu/~glratt

There are imaginary bugs to chase in heaven.





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