Local DDNS for gateway IPs on OpenWrt

dyngwdomain.sh

#!/bin/sh
# dyngwdomain.sh - Local DDNS for gateway IPs on OpenWrt
# Licensed under MIT. (c) 2025 Aldo Adirajasa Fathoni

domain="$1"
iface="$2"

msg() {
  echo $@
  logger -t dyngwdomain.sh[$$] "$@"
}

die() {
  msg "${@}, domain=${domain} iface=${iface}" >&2
  exit 1
}

idx=$(uci show dhcp | sed -nr "s/dhcp\.@domain\[(-?[0-9]+)\]\.name='${domain}'/\1/p")
if ! [ "$idx" -ge 0 ] 2> /dev/null; then
  die "Error checking DNS record"
fi

oldip=$(uci get dhcp.@domain[${idx}].ip)
if [ -z "$oldip" ]; then                                    
  die "Error checking old gateway"                     
fi

newip=$(ip route show 0.0.0.0/0 dev "$iface" | cut -d\  -f3)
if [ -z "$newip" ]; then
  die "Error checking new gateway"
fi

if [ "$oldip" != "$newip" ]; then
  uci set dhcp.@domain[${idx}].ip="$newip"
  uci commit dhcp.@domain[${idx}].ip
  /etc/init.d/dnsmasq reload
  msg "${domain}: ${oldip} -> ${newip}"
else
  msg "${domain} still points to ${oldip}"
fi