[Bug 236888] ppp daemon limits mtu to 1492 for PPPoE

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236888

            Bug ID: 236888
           Summary: ppp daemon limits mtu to 1492 for PPPoE
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: freebsd.68fba at nospam.spacesurfer.com

The ppp daemon in FreeBSD 11.2 limits the maximum MTU to 1492 for the ppp/tun
device. Changing the MTU and MRU in the ppp.conf file to values above 1492
doesn't work. This is because the limit is hard coded in
/usr/src/usr.sbin/ppp/ether.c

My ADSL modem (vigor 130) supports an MTU above 1500 and if I change the limit
in ether.c to 1500, set my ethernet mtu to 1508 or more, then ppp will
negotiate a higher MTU/MRU for the link and it will work fine (I tried 1500
byte ping probes with the DF flag set and it works)

Unfortunately, you can't just change the ether.c value because it uses the
limit as the default and so my old modem (can only do 1492 or less) will not
work unless I set the MTU to 1492 in the ppp.conf file. It would probably be
best to leave the default at 1492 since this is the most common maximum MTU for
ADSL modems, but allow it to be overridden to whatever you want in the config
file.

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