Changing mysql default port

Question

I got the following 2 commands from mysql website. I tried to change the default mysql port from 3306 to 4040 and back. But it did not work as expected.

Redirect to Proxy:

iptables -t nat -I PREROUTING -s ! 127.0.0.1 -p tcp --dport 3306 -j REDIRECT --to-ports 4040

Back to default:

iptables -t nat -D PREROUTING -s ! 127.0.0.1 -p tcp --dport 3306 -j REDIRECT --to-ports 4040

What am I missing here?

Update:

The change is being shown up in the status as follows:

# /etc/init.d/iptables status
Table: nat
Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    REDIRECT   tcp  -- !127.0.0.1            0.0.0.0/0           tcp dpt:3306 redir ports 4040 
2    REDIRECT   tcp  -- !127.0.0.1            0.0.0.0/0           tcp dpt:3306 redir ports 4040 

Chain POSTROUTING (policy ACCEPT)
num  target     prot opt source               destination         

Answer 1

For what you want to achieve, you have the ports the wrong way around. You want to redirect from 4040 to 3306. So try:

iptables -t nat -I PREROUTING -s ! 127.0.0.1 -p tcp --dport 4040 -j REDIRECT --to-ports 3306

You also need to make sure you remove the other incorrect rules as well.

P.S. If this is a production system that can't even afford downtime to change the listener port, you should not be trying iptables rules from the Internet! Test it on a test machine first.