Point IP to an application instance

Question

I was just wondering how I go about pointing a static IP address which I have purchased to an application instance on my server, using a different IP from the one I currently have pointing to my main server?

This baffles me completely, I know of two commands that are related but I don't know how to force them to point the IP to a certain port on my localhost. I think they are ARP and route. I could be completely wrong.

All I want to do is point different IP addresses to different instances of Java.

Also I'm using CentOS 5.9

Answer 1

IP addresses do not point to ports or applications. They point to (virtual) machines, or containers, or contexts.

An application listens on a port, which can be (depending on what it passes to bind) on either all IP addresses on the machine, or on one IP address. So the best thing to do would be to get your application to call bind with the correct IP. You can have two different programs listening on port 80 (for example) as long as they're listening on a different IP address. Many programs allow you to specify the IP to listen on, for sometimes using the syntax 10.0.0.1:1234 where 10.0.0.1 is the IP and 1234 the port. When you leave out the IP, they default to 0.0.0.0 which is all addresses on the machine.

If your application can't do that, you can—as goldilocks hinted—work around it using NAT. Basically, you have each app listen on a different port, say 8080 and 8081. Then you set up NAT rules to redirect port 80:

iptables -t nat -A PREROUTING -d 10.0.0.1 -p tcp --dport 80 -j DNAT --to-destination 8080
iptables -t nat -A PREROUTING -d 10.0.0.2 -p tcp --dport 80 -j DNAT --to-destination 8081

You can add additional firewall rules block direct access to ports 8080 and 8081, of course.

You could also use ipvs to do this. Or a proxy server. There are probably several more ways.