WAN: 172.16.5.0/24
LAN: 192.168.1.0/24
router0: 192.168.1.1, [Internet], default gw [Internet]
router1: 192.168.1.3(eth2), 172.16.5.6(eth0), default gw 172.16.5.254

iptables -t nat -A PREROUTING -p tcp --dport 999 -i eth2 -j DNAT --to-destination 10.1.2.3:22:22
iptables -t nat -A POSTROUTING -p tcp -s 10.1.2.3:22 -j SNAT --to-source 192.168.1.3:999