Linux iptables: Port Redirection Example

How do I redirect 80 port to 8123 using iptables?

You can easily redirect incoming traffic by inserting rules into PREROUTING chain of the nat table. You can set destination port using the REDIRECT target.

Syntax

The syntax is as follows to redirect tcp $srcPortNumber port to $dstPortNumber:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport $srcPortNumber -j REDIRECT --to-port $dstPortNumber

The syntax is as follows to redirect udp $srcPortNumber port to $dstPortNumber:

iptables -t nat -A PREROUTING -i eth0 -p udp --dport $srcPortNumber -j REDIRECT --to-port $dstPortNumber

Replace eth0 with your actual interface name. The following syntax match for source and destination ips:

iptables -t nat -I PREROUTING --src $SRC_IP_MASK --dst $DST_IP -p tcp --dport $portNumber -j REDIRECT --to-ports $rediectPort

Examples:

The following example redirects TCP port 25 to port 2525:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 25 -j REDIRECT --to-port 2525

In this example all incoming traffic on port 80 redirect to port 8123

iptables -t nat -I PREROUTING --src 0/0 --dst 192.168.1.5 -p tcp --dport 80 -j REDIRECT --to-ports 8123

Quoting from the iptables man page:

 This  target is only valid in the nat table, in the PREROUTING and OUTPUT 
       chains, and user-defined chains which are only  called  from  those
       chains.   It redirects the packet to the machine itself by changing the
       destination IP  to  the  primary  address  of  the  incoming  interface
       (locally-generated  packets  are  mapped to the 127.0.0.1 address).  It
       takes one option:

       --to-ports port[-port]
              This specifies a destination port or  range  of  ports  to  use:
              without  this,  the  destination port is never altered.  This is
              only valid if the rule also specifies -p tcp or -p udp.

The OUTPUT chain example:

iptables -t nat -I OUTPUT --src 0/0 --dst 192.168.1.5 -p tcp --dport 80 -j REDIRECT --to-ports 8123

How Do I View NAT Rules?

Type the following command:

iptables -t nat -L -n -v

How Do I Save NAT Redirect Rules?

Type the following command:

iptables-save

References:

• man page – iptables