Iptables Tutorial 1.2.2

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DNAT target

The DNAT target is used to do Destination Network Address Translation, which means that it is used to rewrite the Destination IP address of a packet. If a packet is matched, and this is the target of the rule, the packet, and all subsequent packets in the same stream will be translated, and then routed on to the correct device, host or network. This target can be extremely useful, for example,when you have a host running your web server inside a LAN, but no real IP to give it that will work on the Internet. You could then tell the firewall to forward all packets going to its own HTTP port, on to the real web server within the LAN. We may also specify a whole range of destination IP addresses, and the DNAT mechanism will choose the destination IP address at random for each stream. Hence, we will be able to deal with a kind of load balancing by doing this.

Note that the DNAT target is only available within the PREROUTING and OUTPUT chains in the nat table, and any of the chains called upon from any of those listed chains. Note that chains containing DNAT targets may not be used from any other chains, such as the POSTROUTING chain.

Table 11-5. DNAT target options

Option --to-destination
Example iptables -t nat -A PREROUTING -p tcp -d 15.45.23.67 --dport 80 -j DNAT --to-destination 192.168.1.1-192.168.1.10
Explanation The --to-destination option tells the DNAT mechanism which Destination IP to set in the IP header, and where to send packets that are matched. The above example would send on all packets destined for IP address 15.45.23.67 to a range of LAN IP's, namely 192.168.1.1 through 10. Note, as described previously, that a single stream will always use the same host, and that each stream will randomly be given an IP address that it will always be Destined for, within that stream. We could also have specified only one IP address, in which case we would always be connected to the same host. Also note that we may add a port or port range to which the traffic would be redirected to. This is done by adding, for example, an :80 statement to the IP addresses to which we want to DNAT the packets. A rule could then look like --to-destination 192.168.1.1:80 for example, or like --to-destination 192.168.1.1:80-100 if we wanted to specify a port range. As you can see, the syntax is pretty much the same for the DNAT target, as for the SNAT target even though they do two totally different things. Do note that port specifications are only valid for rules that specify the TCP or UDP protocols with the --protocol option.

Since DNAT requires quite a lot of work to work properly, I have decided to add a larger explanation on how to work with it. Let's take a brief example on how things would be done normally. We want to publish our website via our Internet connection. We only have one IP address, and the HTTP server is located on our internal network. Our firewall has the external IP address $INET_IP, and our HTTP server has the internal IP address $HTTP_IP and finally the firewall has the internal IP address $LAN_IP. The first thing to do is to add the following simple rule to the PREROUTING chain in the nat table:

iptables -t nat -A PREROUTING --dst $INET_IP -p tcp --dport 80 -j DNAT \

--to-destination $HTTP_IP

Now, all packets from the Internet going to port 80 on our firewall are redirected (or DNAT'ed) to our internal HTTP server. If you test this from the Internet, everything should work just perfect. So, what happens if you try connecting from a host on the same local network as the HTTP server? It will simply not work. This is a problem with routing really. We start out by dissecting what happens in a normal case. The external box has IP address $EXT_BOX, to maintain readability.

Packet leaves the connecting host going to $INET_IP and source $EXT_BOX.

Packet reaches the firewall.

Firewall DNAT's the packet and runs the packet through all different chains etcetera.

Packet leaves the firewall and travels to the $HTTP_IP.

Packet reaches the HTTP server, and the HTTP box replies back through the firewall, if that is the box that the routing database has entered as the gateway for $EXT_BOX. Normally, this would be the default gateway of the HTTP server.


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