Is there an AWS service (rather than creating an instance, and doing something via iptables?) that will allow me to use the API to set up port forwarding from a single Elastic IP address, onto multiple EC2 instances.
Say I have the Elastic IP 54.345.67.89. When I create an EC2 instance, rather than assign it it's own Elastic IP, I'd like to create a new port for the Elastic IP address and forward on traffic from that port to a port on the new instance.
So, 54.345.67.89:25600 would forward onto -> 10.123.456.78:25600
Then if I create another instance, I'd be able to use create another port that the Elastic IP would use to forward onto that new instance:
54.345.67.78:25601 would forward onto -> 10.123.456.79:25600
Best Answer
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.)