Port Forwarding

Question

I have a simple requirement of a software level port forwarding/tunnelling of socket based communication.

• I have a source server and port using Sockets. This is a java program which works both in windows and linux and this is irrelevant.
• I have devices which keep sending data to this port. There may be a bi-directional communication
• I want to redirect this data to another remote server and port. So for the clients they will not have to worry about change of ip address whenever I move my app server.

Are there any tools/deamon/service programs which I can use to configure and do this?

I tried SSH, but to my understanding this needs a SSH protocol enabled server. In my case this is not applicable. I also tried using JSch but this again is an implementation of SSH in java format.

Can someone throw some pointers? Is it possible to use iptables NAT in linux?

Musket

Are you using any standard protocol or your own custom protocol ? – Suh

It is just a TCP/IP data communication. – Musket

Answer 1

You can try netcat or socat (it's more powerful than netcat)

An example for socat to forward port 80 using tcp4:

socat tcp4-listen:80,fork tcp4:{another server}:{another port}

and refer to http://en.wikipedia.org/wiki/Netcat#Port_Forwarding_or_Port_Mapping for netcat

Both are not java-related.

Plantagenet

Wow! How did I forget this!! I have a question though! Can socat or netcat automatically reconnect if a connection is broken? – Musket

I don't know if there is a built-in option to reconnect, but it can be client's job to reconnect if anything wrong – Plantagenet

This says "2011/12/04 15:52:25 socat[4328] E connect(3, AF=2 ipaddress:5000, 16): Address already in use". I tried the command with reuseaddr option also. – Musket

Just so that everybody is benefitted, the command I finally used "socat -d -d -d -d TCP4-LISTEN:9002,fork,reuseaddr TCP4:127.0.0.1:9000" which enables unlimited number of simultaneous connections. Thanks for the answer. – Musket

Answer 2

There is a TCP/IP port forwarding utility named portforward available in code.google.com. It is entirely written in Java.

Radiotelephony

Will give this a try & confirm if this would suit. – Musket

Answer 3

If you're running xinetd on your system already, it provides a simple port forwarding mechanism that might be useful if you're not running IPTables already.

If you are running IPTables, Server Fault has an excellent, short question with a very similar goal. Though I find it a bit terse more detailed documentation is available.

Excurvature