VPS (10.8.0.0/24) <——> (tun0) Server (ppp1) <——> iPhone (172.16.31.0/24)
iptables -A INPUT -p tcp –dport 109 -j ACCEPT
iptables -A INPUT -i ppp1 -j ACCEPT
iptables -A FORWARD -i ppp1 -j ACCEPT
iptables -A FORWARD -o ppp1 -j ACCEPT
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -I POSTROUTING -s 172.16.31.0/24 -o ppp0 -j MASQUERADE
ip route add default dev tun0 table 200
ip rule add priority 100 from 172.16.31.0/24 table 200
iptables -t nat -I POSTROUTING -o tun0 -j SNAT –to 10.8.0.6