netfilter: nf_nat: support user-specified SNAT rules in LOCAL_IN

netfilter: nf_nat: support user-specified SNAT rules in LOCAL_IN

2.6.34 introduced 'conntrack zones' to deal with cases where packets
from multiple identical networks are handled by conntrack/NAT. Packets
are looped through veth devices, during which they are NATed to private
addresses, after which they can continue normally through the stack
and possibly have NAT rules applied a second time.

This works well, but is needlessly complicated for cases where only
a single SNAT/DNAT mapping needs to be applied to these packets. In that
case, all that needs to be done is to assign each network to a seperate
zone and perform NAT as usual. However this doesn't work for packets
destined for the machine performing NAT itself since its corrently not
possible to configure SNAT mappings for the LOCAL_IN chain.

This patch adds a new INPUT chain to the NAT table and changes the
targets performing SNAT to be usable in that chain.

Example usage with two identical networks (192.168.0.0/24) on eth0/eth1:

iptables -t raw -A PREROUTING -i eth0 -j CT --zone 1
iptables -t raw -A PREROUTING -i eth0 -j MARK --set-mark 1
iptables -t raw -A PREROUTING -i eth1 -j CT --zone 2
iptabels -t raw -A PREROUTING -i eth1 -j MARK --set-mark 2

iptables -t nat -A INPUT       -m mark --mark 1 -j NETMAP --to 10.0.0.0/24
iptables -t nat -A POSTROUTING -m mark --mark 1 -j NETMAP --to 10.0.0.0/24
iptables -t nat -A INPUT       -m mark --mark 2 -j NETMAP --to 10.0.1.0/24
iptables -t nat -A POSTROUTING -m mark --mark 2 -j NETMAP --to 10.0.1.0/24

iptables -t raw -A PREROUTING -d 10.0.0.0/24 -j CT --zone 1
iptables -t raw -A OUTPUT     -d 10.0.0.0/24 -j CT --zone 1
iptables -t raw -A PREROUTING -d 10.0.1.0/24 -j CT --zone 2
iptables -t raw -A OUTPUT     -d 10.0.1.0/24 -j CT --zone 2

iptables -t nat -A PREROUTING -d 10.0.0.0/24 -j NETMAP --to 192.168.0.0/24
iptables -t nat -A OUTPUT     -d 10.0.0.0/24 -j NETMAP --to 192.168.0.0/24
iptables -t nat -A PREROUTING -d 10.0.1.0/24 -j NETMAP --to 192.168.0.0/24
iptables -t nat -A OUTPUT     -d 10.0.1.0/24 -j NETMAP --to 192.168.0.0/24

Signed-off-by: Patrick McHardy <kaber@trash.net>