domain-alias.sh

#!/bin/bash

domain=$1
port=$2

if [ -z "$domain" ] || [ -z "$port" ]; then
  echo "Usage: domain-alias <domain> <port>"
  exit 1
fi

existing=$(cat /etc/hosts | grep "$domain$" | cut -d' ' -f1)
if [ -n "$existing" ]; then
  ip=$existing
else
  last=$(cat /etc/hosts | grep -oP "127.0.0.\d" | cut -d'.' -f4 | sort -n | tail -1)
  next=$(expr $last + 1)
  ip="127.0.0.$next"
fi
ipfwCmd="fwd $ip,$port tcp from any to $ip dst-port 80"

ifconfig lo0 alias $ip

# Add to ipfw rules.
ipfw list | grep -q "$ipfwCmd"
if [ "$?" -eq 1 ]; then
  ipfw add $ipfwCmd > /dev/null
fi

# Append to /etc/ipfw.conf for persisting past restarts.
[[ ! -e /etc/ipfw.conf ]] && touch /etc/ipfw.conf
cat /etc/ipfw.conf | grep -q "$ipfwCmd"
if [ "$?" -eq 1 ]; then
  echo "add $ipfwCmd # $domain" >> /etc/ipfw.conf
fi

# Append alias to local IP to /etc/hosts.
if [ -z "$existing" ]; then
  echo "$ip $domain" >> /etc/hosts
fi

echo "Run apps at $ip:$port and view them at http://$domain/."

simple-version.sh

ifconfig lo0 alias $ip
ipfw add fwd $ip,$port tcp from any to $ip dst-port 80 > /dev/null
echo "$ip $domain" >> /etc/hosts

For newer versions of macOS, see https://apple.stackexchange.com/questions/254654/port-forwarding-on-macos-sierra