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Date:      Thu, 29 Jun 2017 19:53:14 +0000
From:      bugzilla-noreply@freebsd.org
To:        freebsd-net@FreeBSD.org
Subject:   [Bug 220351] Path MTU discovery works only after resetting an internal interface MTU (mpd5, ng_nat, ipfw)
Message-ID:  <bug-220351-2472-lCDWr8PCgb@https.bugs.freebsd.org/bugzilla/>
In-Reply-To: <bug-220351-2472@https.bugs.freebsd.org/bugzilla/>
References:  <bug-220351-2472@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D220351

Eugene Grosbein <eugen@freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|freebsd-net@FreeBSD.org     |eugen@freebsd.org
             Status|New                         |Open
                 CC|                            |eugen@freebsd.org

--- Comment #2 from Eugene Grosbein <eugen@freebsd.org> ---
(In reply to Martin Birgmeier from comment #1)

Lowering interface MTU of internal machine also lowers its default route 'm=
tu'
attribute, see output of the command 'route -n get default'.

Raising interface MTU back does NOT restore 'mtu' attribute of the default
route, so the kernel still uses 1492 as maximum IP packet size for packets
going via this route.

Correct solution for your problem is 'set iface enable tcpmssfix' command in
the mpd.conf, see /usr/local/etc/mpd5/mpd.conf.sample for example. This way=
 you
can keep default MTU=3D1500 for internal hosts and have no MTU-related netw=
ork
problems.

Please test and report back.

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