ipsec6_common_input_cb() uses partial copy of ip6_input() to parse
headers. But this isn't correct, when we use tunnel mode IPSec.
When we stripped outer IPv6 header from the decrypted packet, it
can become IPv4 packet and should be handled by ip_input. Also when
we use tunnel mode IPSec with IPv6 traffic, we should pass decrypted
packet with inner IPv6 header to ip6_input, it will correctly handle
it and also can decide to forward it.
The "skip" variable points to offset where payload starts. In tunnel
mode we reset it to zero after stripping the outer header. So, when
it is zero, we should requeue mbuf via netisr.
Differential Revision: https://reviews.freebsd.org/D2306
Reviewed by: adrian, gnn
Sponsored by: Yandex LLC
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