Amazon Ec2 Elastic IP – Outbound IP

You should always have the instance applications bind to the local address (10.x.x.x) since this is what the machines see as their ethernet connection. The external IP address is an address you can reach externally, then Amazon takes care of routing that to the actual machine.

You other question answers this question more explicitly for your case though.

How strict is the Elastic IP requirement?

What you're describing is an Elastic Load Balancer (ELB). Unfortunately, these don't work with Elastic IPs.

ELBs can balance HTTP traffic and offload the SSL part of HTTPS traffic. They can also do plain TCP balancing which is what you're describing in the question. They can "stick" to an instance, even if it is rebooted and gets a new IP address.

If it must be an Elastic IP, IPTables would be the tool I'd use and the instructions for that are no different within Amazon EC2 than anywhere else. You will have to reconfigure IPTables every time any of your back-end instances reboots and gets a new IP address (unless your instances are in a Virtual Private Cloud (VPC), which I would highly recommend if you go down the IPTables route.)