Mapping port 80 to port 3000 using iptables

Port numbers less that 1024 are considered "privileged" ports, and you generally must be root to bind a listener to them.

Rather than running a network application as root, map the privileged port to a non-privileged one:

sudo iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3000

Now requests to port 80 will be forwarded on to port 3000.