Script to port-forward to a dynamic IP address. Run in CRON.

update_iptables.sh

#!/bin/bash

HOST=myhost.example.com

IP=`dig +short $HOST | sed -e '1d'`
LAST_IP_FILE=/tmp/LAST_IP
LAST_IP=`cat $LAST_IP_FILE 2> /dev/null`

tcpPorts=(8080 8880 8843 8443)
udpPorts=(3478 10001)

if [ ! "$IP" = "$LAST_IP" ] ; then
  echo "IP changed. Updating iptables..."

  # Clear previous rules
  iptables -t nat --flush POSTROUTING
  iptables -t nat --flush PREROUTING

  # Enable IP forwarding
  sysctl net.ipv4.ip_forward=1

  echo $IP > $LAST_IP_FILE
  for p in ${tcpPorts[@]}; do
    echo "TCP:$p"
    iptables -t nat -A PREROUTING -p tcp --dport $p -j DNAT --to-destination $IP:$p
  done

  for p in ${udpPorts[@]}; do
    echo "UDP:$p"
    iptables -t nat -A PREROUTING -p udp --dport $p -j DNAT --to-destination $IP:$p
  done
  
  iptables -t nat -A POSTROUTING -j MASQUERADE
  echo "Done!"
else
  echo "IP hasn't changed. Skip."
fi