tunneling smtp connections from localhost to another port

Question

Ok, so Many ISPs are now blocking port 25 on outgoing connections, so I set up an smtp server on port 1025. Now this works great and configured clients send mail properly. But for most of the code already on the server, the smtp server should be on 25, now rather than reconfigure all the code to use the new port I was thinking of establishing a tunnel or something to force all from localhost to 25 to go to localhost on 1025.

I tried this and it did not work

iptables -t nat -A PREROUTING -p tcp -i lo --dport 25 -j DNAT --to 127.0.0.1:1025

what can I do to accomplish this?

thank you

Answer 1

To forwarding from localhost use xinetd:

service smtp
{
        socket_type             = stream
        wait                    = no
        user                    = root
        redirect                = 127.0.0.1 1025
        bind                    = 127.0.0.1
}

Answer 2

You are doing it wrong, the concept is almost the contrary, try this instead:

iptables -t nat -I PREROUTING -p tcp --dport 1025 -j REDIRECT --to-ports 25

Answer 3

You should use REDIRECT not DNAT for locally forwarded ports. Something like:

iptables -t nat -A PREROUTING -i lo -p tcp --dport 25 -j REDIRECT --to-ports 1025

Answer 4

iptables -t nat -I OUTPUT 1 -o lo -d 127.0.0.1 -p tcp --dport 25 -j REDIRECT --to-ports 1025