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

[Herbert J. Skuhra]

Herbert J Skuhra skrev:
> 
> bugzilla-noreply skrev:
>> 
>> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=220351
>> Eugene Grosbein <eugen at freebsd.org> changed:
>> 
>> What    |Removed                     |Added
>> ----------------------------------------------------------------------------
>> 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.
> 
> I think I have a similar problem, but with IPv6 and pf! :-(
> 
> - Problem to load e.g. https://www.heise.de on the first try
> - Can not connect to Skype for Business over IPv6 (unfortunately I
> sometimes need it for job)

Skype for Business (S4B) on Mac OS X 10.12.6 (Beta5) is actually
trying to connect over IPv4. The connection fails until I lower MTU to
1440. After resetting MTU to 1500 and restarting S4B it connects fine
for a while ('route -n get' default shows 1500). I use 'set iface
enable tcpmssfix' in mpd5.conf and I've tried:

- set iface mtu and set link mtu/mru/mrru in mpd5.conf
- set scrub ... max-mss in pf.conf

--
Herbert