[Bug 240059] Min MTU for a interface GIF interface is to high

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240059

            Bug ID: 240059
           Summary: Min MTU for a interface GIF interface is to high
           Product: Base System
           Version: 12.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: sirl33tname at gmail.com

The Min MTU for a GIF interface is 1280.
See: 
https://github.com/freebsd/freebsd/blob/c4ed3acb9f59d3498905e95d5d445c5046fdfdb6/sys/net/if_gif.h#L80-L82
#define GIF_MTU_MIN     (1280) /* Minimum MTU */

There are some cases where this is still to high. 
And to quote the man page:

If the outer protocol is IPv6, path MTU discovery for encapsulated
packets may affect communication over the interface.  The first bigger-
than-pmtu packet may be lost.  To avoid the problem, you may want to set
the interface MTU for gif to 1240 or smaller, when the outer header is
IPv6 and the inner header is IPv4.

So I would expect that it is possible to set the MTU for a GIF interface lower
than 1280.

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