Linux iptables – Nat port forwarding using PREROUTING

One can use iptables to forward a specific port to another port using NAT PREROUTING chain. This can be used to make a server available on a different port for users.

1. Add NAT forwarding using PREROUTING chain

   $ sudo iptables -t nat -A PREROUTING -p tcp --dport 81 -j REDIRECT --to-port 80
   

2. Add NAT forwarding using PREROUTING chain on a specific interface

   $ sudo iptables -t nat -A PREROUTING -p tcp -d INTERFACE_IP --dport 443 -j REDIRECT --to-port 8443
   

3. List iptable nat PREROUTE rules

   ## -n (numeric)
   $ sudo iptables -t nat -L PREROUTING -n  --line-numbers
   Chain PREROUTING (policy ACCEPT)
   num  target     prot opt source               destination         
   1    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:81 redir ports 80
   2    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:82 redir ports 80
   

4. List all rules with line numbers

   $ sudo iptables -n -L -v --line-numbers
   

5. Delete a specific rule

   $ sudo iptables -t nat -D PREROUTING [NUM]