Standard practices say no non-root process gets to talk to the Internet on a port less than 1024. How, then, could I get Node talking on port 80 on EC2? (I wanted it to go as fast as possible and use the smallest possible share of my teeny tiny little micro-instance's resources, so proxying through nginx or Apache seemed suboptimal.)
Add a port forwarding rule via iptables.
First, I listed the rules currently running on the NAT (Network Address Translation) table:
[ec2-user@ip-XX-XXX-XX-X ~]$ sudo iptables -t nat -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
I saw nothing, so I felt free to add a rule forwarding packets sent to external port 80 to internal port 8000:
[ec2-user@ip-XX-XXX-XX-X ~]$ sudo iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-ports 8000
When I listed again, I saw a new PREROUTING chain:
[ec2-user@ip-XX-XXX-XX-X ~]$ sudo iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
REDIRECT tcp -- anywhere anywhere tcp dpt:http redir ports 8000
I checked my Node script, which was running on port 8000, and (yes!) it was responding on port 80.
During my early attempts I screwed up a bunch of times. I removed busted rules by specifying the right table, the right chain, and the right line number, like so:
[ec2-user@ip-XX-XXX-XX-X ~]$ sudo iptables -t nat -D PREROUTING 1
This removed the first line from the PREROUTING chain in my nat table.