On Sun, Aug 29, 2010 at 12:48 AM, Eric Dumazet [off-list ref] wrote:

It seems we forget to reset skb->ip_summed to CHECKSUM_NONE in various
tunnels (gre, ipip, sit, ip6_tunnel), before re-entering stack.

Add ip_summed initialization in skb_tunnel_rx(), and remove it from
ipmr / ip6mr

Signed-off-by: Eric Dumazet <redacted>

This is intentional behavior.  For CHECKSUM_COMPLETE packets we
subtract off the checksum of the encapsulating headers that we remove
so we can still get the benefit of checksum hardware for the inner
packet as well.  GRE does this using skb_postpull_rcsum(), for IPIP it
is implicit because the outer IP header was already removed, etc.