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: [email protected]
Reporter: [email protected]
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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