One-to-one NAT

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# WAN .251 -> LAN .20 # WAN .251 -> LAN .20
-# The original post has 192.168.0.15 and it should be 192.168.0.20. I fixed it.+ # The original post has 192.168.0.15 and it should be 192.168.0.20. I fixed it.
iptables -t nat -I PREROUTING -p all -d 173.xxx.xxx.251 -j DNAT --to-destination 192.168.0.20 iptables -t nat -I PREROUTING -p all -d 173.xxx.xxx.251 -j DNAT --to-destination 192.168.0.20
iptables -t nat -I POSTROUTING -p all -s 192.168.0.15 -j SNAT --to-source 173.xxx.xxx.251 iptables -t nat -I POSTROUTING -p all -s 192.168.0.15 -j SNAT --to-source 173.xxx.xxx.251

Revision as of 22:21, 13 August 2009

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
  • Tomato 1.23 (which sounds odd, but this is just iptables; it should work in any Linux-based router, except for the interface names)

Contents

Setup

Everything in square brackets needs to be replaced by your values. Examples are at the bottom.

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]

Firewall

SNAT/DNAT

Route all packets for the new public ip, to a certain local IP.

iptables -t nat -I [PREROUTING] -p all -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] 1 -p all -s [LAN_IP] -j SNAT --to-source [PUBLIC_IP]

Translate anything else from the lan to the "main" router IP.

iptables -t nat -I [POSTROUTING] -o br0 -s [LAN_SUBNET] -j SNAT --to-source [MAIN_IP]

In that last line, br0 may not always be correct; it's br0 on Tomato at least. You can telnet to the router and use the ifconfig command to see the correct value; it's the one with the router's internal IP associated with it.

PORT FORWARD

Forward port X to above local IP

iptables -I FORWARD -p tcp -i vlan1 -d [LAN_IP] --dport X -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 for that IP address.

Copy/Paste Examples

Startup Script

# Save Startup
ifconfig vlan1:1 173.xxx.xxx.250 netmask 255.255.255.240 broadcast 173.xxx.xxx.255
ifconfig vlan1:2 173.xxx.xxx.251 netmask 255.255.255.240 broadcast 173.xxx.xxx.255

Firewall Script

# Save Firewall
iptables -t nat -I POSTROUTING -o br0 -s 192.168.0.0/24 -j SNAT --to-source 192.168.0.1

# WAN .250 -> LAN .15
iptables -t nat -I PREROUTING -p all -d 173.xxx.xxx.250 -j DNAT --to-destination 192.168.0.15
iptables -t nat -I POSTROUTING -p all -s 192.168.0.15 -j SNAT --to-source 173.xxx.xxx.250
iptables -I FORWARD -p tcp -d 192.168.0.15 --dport 21 -j ACCEPT
iptables -I FORWARD -p tcp -d 192.168.0.15 --dport 80 -j ACCEPT
iptables -I FORWARD -p tcp -d 192.168.0.15 --dport 5900 -j ACCEPT

# WAN .251 -> LAN .20
# The original post has 192.168.0.15 and it should be 192.168.0.20.  I fixed it.
iptables -t nat -I PREROUTING -p all -d 173.xxx.xxx.251 -j DNAT --to-destination 192.168.0.20
iptables -t nat -I POSTROUTING -p all -s 192.168.0.15 -j SNAT --to-source 173.xxx.xxx.251
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

Resources

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