Hi,

I was following this post [https://www.digitalocean.com/community/tutorials/how-to-forward-ports-through-a-linux-gateway-with-iptables](http://) to set up port forwarding.

Under Firewall network details:

In the Tutorial, Private IP Address: 192.0.2.15

For my setup, Private IP Address: DYNAMIC

The Private IP address of the firewall doesnt come anywhere untill the last command,

sudo iptables -t nat -A POSTROUTING -o eth1 -p tcp –dport 80 -d 192.0.2.2 -j SNAT –to-source 192.0.2.15

How do I replace this everytime my firewall’s private ip address changes?

Thanks,

These answers are provided by our Community. If you find them useful, show some love by clicking the heart. If you run into issues leave a comment, or add your own answer to help others.

×
1 answer

Hi,

iptables does not support dynamic IP addresses/hostnames, but you can have it update the rule to the correct IP address whenever it changes.

Instead of redirecting the packets to the SNAT chain, create a new chain that sits in the middle which then redirects the rules to the SNAT chain. So, instead of POSTROUTING -> SNAT, it goes POSTROUTING -> DYNAMIC -> SNAT.

Create the new chain:

sudo iptables -t nat -N DYNAMIC

Add the rule, replacing -j SNAT [...] with -j DYNAMIC:

sudo iptables -t nat -A POSTROUTING -o eth1 -p tcp --dport 80 -d 192.0.2.2 -j DYNAMIC

Add the firewall’s IP address to the DYNAMIC chain (the rule applies to all packets in the DYNAMIC chain, because the filtering is done in the previous rule):

sudo iptables -t nat -A DYNAMIC -j SNAT --to-source 192.0.2.15

Now, test the rule and make sure it works as expected. Whenever there is a change to the firewall’s IP address, you can simply flush the DYNAMIC chain and add the rule again:

sudo iptables -t nat -F DYNAMIC
sudo iptables -t nat -A DYNAMIC -j SNAT --to-source 192.0.2.58

You can definitely automate the last two commands if you have a way of knowing what the new IP address is and when it changes. Obviously, if it’s possible to give the firewall a static IP address, that is definitely the preferred option.

Submit an Answer