The kernel has taken the decision to always enforce DF bit on IPv4
tunnels, which have fixed (not inherited) TTL (e.g.
net/ipv4/ipip.c:ipip_tunnel_ioctl()). Commment by Alexey Kuznetsov in
the head of ip_gre.c explains that the reason is attempting to avoid
network loops.
But the commit c54419321455631 removed this enforcing from GRE tunnels,
not changing this behavior for IP-IP (net/ipv4/ipip.c) and SIT
(net/ipv6/sit.c).
It can be discussed whether such enforcing of DF bit is exactly the
desired behavior, but shouldn't it at least be identical across IPv4
tunnels?
Konstantin Shemyak
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