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Date: Fri, 07 Oct 2011 18:42:34 +0200
From: Andre Oppermann <andre@freebsd.org>
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To: Ryan Stone <rysto32@gmail.com>
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Cc: freebsd-net <freebsd-net@freebsd.org>
Subject: Re: [PATCH] Don't have ICMP Echo Replies copy fragmentation flags
 from Echo Request
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On 07.10.2011 17:54, Ryan Stone wrote:
> Currently when FreeBSD responds to a ICMP Echo Request, it takes the
> original mbuf, rewrites a couple of fields (like the src/dst IP and
> the ICMP type), and then sends that mbuf back.  As things are
> currently implemented, the Don't Fragment bit is kept in the ICMP
> replay.  This can cause problems for large ICMP Echo Requests if the
> MTU on the return route is less than the MTU on the incoming route and
> the DF bit is set(Linux's ping command sets it by default).  Is it
> intended that the DF bit from the Request be copied into the Reply?

Yes, this is intended.  It allows you to test asymmetric paths.

-- 
Andre