[Bug 220351] Path MTU discovery works only after resetting an internal interface MTU (mpd5, ng_nat, ipfw)

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=220351

Eugene Grosbein <eugen at freebsd.org> changed:

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

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

Lowering interface MTU of internal machine also lowers its default route 'mtu'
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=1500 for internal hosts and have no MTU-related network
problems.

Please test and report back.

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