Strange FreeBSD behavior when trying to forward beetween ipsec crypted gif's. May be a problem with ICMP unreach packets at all

[David DeSimone]

Tom Judge <tom at tomjudge.com> wrote:
>
> > Question:  Why FW2 does not send ICMP need-fragment-but-DF-set
> > message to HostB ? 
> 
> If you take a look at icmp_error() in sys/netinet/ip_icmp.c you will
> see that icmp errors are not sent for packets that have been
> previously been decrypted by IPSec.  I have a feeling that this is
> because icmp_reflect() will not use the correct output path but I may
> be and most probably am wrong.  Bjoern (CC'd) can probably shed more
> light on it. 

It is not done because the resulting ICMP packets may very well end up
being sent to an unrelated host or router which has no SA policy
defined, meaning that the ICMP would be sent unencrypted.

This would expose some packet data in the payload of the ICMP message
that had been previously sent as securely encrypted, giving an attacker
some known-plaintext data to use in attempting to guess session keys,
among other information exposed.

-- 
David DeSimone == Network Admin == fox at verio.net
  "I don't like spinach, and I'm glad I don't, because if I
   liked it I'd eat it, and I just hate it." -- Clarence Darrow


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