IPv6 tunnel MTU problems: it seems to be gif problem
man gif says:
If the outer protocol is IPv4, gif does not try to perform path MTU dis-
covery for the encapsulated packet (DF bit is set to 0).
If the outer protocol is IPv6, path MTU discovery for encapsulated pack-
ets 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, it seems, when "outer" interface has MTU less than 1500 (1460,
for example, as in my case), MTU on gif interface should be set less
(even if outer protocol is IPv4, and inner one is IPv6). But gif
doesn't allow MTU less than default 1280 (if_gif.h contains this
Could it be source with my problems with IPv6-over-IPv4-over-PPPoE