North American Network Operators Group
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Re: RFC3514

  • From: Jack Bates
  • Date: Tue Apr 01 10:31:53 2003 
Scott Francis wrote:

Comments?

(Nice to see Mr. Bellovin keeping up the holiday tradition ... :))
Yep.

" Fragments that by themselves are dangerous MUST have the evil bit
set. If a packet with the evil bit set is fragmented by an
intermediate router and the fragments themselves are not dangerous,
the evil bit MUST be cleared in the fragments, and MUST be turned
back on in the reassembled packet."

There is no guidelines for specifying how the router will determine if the fragments themselves are dangerous. An attacker may carefully design the evil packet with the expectation of fragmentation, allowing the fragments themselves to be the tool of the attack. It is therefore recommended that all fragment of a packet with the evil bit set should also have the evil bit set when fragmentation is performed by an intermediate router incapable of determining the dangerous nature of the packets.


:)

-Jack