One-to-one NAT

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One-to-one NAT (aka Static NAT) is a way to make systems behind a firewall and configured with private IP addresses (those addresses reserved for private use in RFC 1918) appear to have public IP addresses.¹

These examples have been tested and work with multiple public static IPs. Tested with:

  • WRT54GL v.1.1 with DD-WRT RC6 and it works just fine.²
  • DD-WRT v24-sp2 (01/01/09) mega on Linksys WRT 600N V1

Contents

Setup

Startup

Set up new public static IP on dd-wrt WAN interface vlan1. This must be done for each public static IP and should be save to the Startup script using the Save Statup button.

ifconfig vlan1:1 PUBLIC_IP1 netmask NETMASK broadcast BROADCAST
ifconfig vlan1:2 PUBLIC_IP2 netmask NETMASK broadcast BROADCAST

Examples:

ifconfig vlan1:1 173.xxx.xxx.249 netmask 255.255.255.240 broadcast 173.xxx.xxx.255
ifconfig vlan1:2 173.xxx.xxx.250 netmask 255.255.255.240 broadcast 173.xxx.xxx.255

Firewall

SNAT/DNAT

route all packets for the new public ip, to a certain local ip

iptables -t nat -I PREROUTING -i vlan1 -d PUBLIC_IP -j DNAT --to-destination LAN_IP

masquerade returned packets from the local ip to the public ip

iptables -t nat -I POSTROUTING -o vlan1 -s LAN_IP -j SNAT --to-source PUBLIC_IP

This may be a better method to masquerade returned packets from local IP to the public IP. (tested on DD-WRT v24-sp2 (01/01/09) mega)

iptables -t nat -I POSTROUTING 1 -p all -s LAN_IP -j SNAT --to-source PUBLIC_IP

Example:

iptables -t nat -I POSTROUTING 1 -p all -s 192.168.0.20 -j SNAT --to-source 173.xxx.xxx.250

translate anything else from the lan to the "main" router ip

iptables -t nat -I POSTROUTING -o vlan1 -s LAN_SUBNET (ex. 192.168.1.0/24) -j SNAT --to-source MAIN_IP
PORT FORWARD

Forward port X to above local IP

iptables -I FORWARD -p tcp -i vlan1 -d LAN_IP --dport X -j ACCEPT

Examples:

iptables -I FORWARD -p tcp -d 192.168.0.20 --dport 21 -j ACCEPT
iptables -I FORWARD -p tcp -d 192.168.0.20 --dport 80 -j ACCEPT
iptables -I FORWARD -p tcp -d 192.168.0.20 --dport 5900 -j ACCEPT

You could also replace above rule(s) with the following:

iptables -I FORWARD -p all -i vlan1 -d LAN_IP -j ACCEPT 

Which instead of forwarding just a single port, will let through all tcp/udp connections on all ports to this public ip-->lan ip.

In other words forwarding all connections would be no firewalling what so ever.

Resources

¹ http://www.shorewall.net/NAT.htm
² http://www.dd-wrt.com/phpBB2/viewtopic.php?t=24555