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Date: Thu, 14 Jul 2005 23:01:30 -0700
To: "Giovanni P. Tirloni" <gpt@tirloni.org>
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From: stephen@dino.dnsalias.com (Stephen J. Bevan)
Cc: freebsd-net@freebsd.org, Alex Povolotsky <tarkhil@webmail.sub.ru>
Subject: Re: GRE and PF problem
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Giovanni P. Tirloni writes:
 >   I don't know how PF keeps tracks of ICMP packets but there must be a 
 > way for it to distinguish between a packet destined to 192.168.0.1 or 0.2.

An ICMP ECHO REQUEST message has a 16-bit id field which can be
altered by NAT to identify the originating machine.

There isn't really an equivalent when using a minimal GRE header.  If
GRE checksums are turned on then the 16-bit Reserved1 field could be
abused for NAT purposes.