默认外网eth1, 默认路由不用改, 为了正确路由到内网eth0:

新增外网 eth2, NewIP 配置路由:

ip route add default via NewGW dev eth2 src NewIP table 200
ip rule add from NewIP table 200
ip rule add fwmark 0x200 table 200

配置iptables mangle 表: ( eth0 : LAN )

-A PREROUTING -i eth0 -m conntrack --ctstate RELATED,ESTABLISHED -j CONNMARK --restore-mark
-A PREROUTING -i eth2 -m conntrack --ctstate NEW -j CONNMARK --set-mark 0x200

配置rp_filter:

net.ipv4.conf.all.rp_filter = 2
net.ipv4.conf.eth2.rp_filter = 2