Dynamic DNS update wrapper for nsupdate

ddclient.service

# /etc/systemd/system/ddclient.service
[Unit]
Description=Dynamic DNS update wrapper for nsupdate

[Service]
Type=simple
Restart=no
ExecStart=/usr/local/sbin/my-ddclient

ddclient.timer

# /etc/systemd/system/ddclient.timer
[Unit]
Description=Dynamic DNS update wrapper for nsupdate
Wants=network.target

[Timer]
OnCalendar=*:0/5
Persistent=1

[Install]
WantedBy=timers.target

my-ddclient.sh

#!/bin/bash
# /usr/local/sbin/my-ddclient
# The actual ddclient keeps breaking things so we replaced it with a shell script.
# SPDX-License-Identifier: GPL-3.0-or-later

echo "Updating IPv4/IPv6 host records for $HOSTNAME."

valid_fqdn='^[-a-zA-Z0-9]{1,63}\.([-a-zA-Z0-9]{1,63}\.?)+\.?$'
valid_length='^.{4,254}$'
valid_ipv4='^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$'
valid_ipv6='^([0-9a-fA-F]{1,4}::?){1,7}[0-9a-fA-F]{1,4}$'

if [[ ! $HOSTNAME =~ $valid_fqdn || ! $HOSTNAME =~ $valid_length ]]; then
    echo "Invalid FQDN for host: $HOSTNAME"
    exit 1
fi

nameserver=$(dig +short soa "${HOSTNAME#*.}" | awk '{print $1}')
current_ipv4=$(curl api.ipify.org 2>/dev/null) # Alternate: ipv4.icanhazip.com
current_ipv6=$(curl api6.ipify.org 2>/dev/null) # Alternate: ipv6.icanhazip.com
published_ipv4=$(dig +short a "$HOSTNAME" @"$nameserver")
published_ipv6=$(dig +short aaaa "$HOSTNAME" @"$nameserver")

if [[ ! $current_ipv4 =~ $valid_ipv4 ]]; then
    echo "No valid IPv4 address found. $current_ipv4"
elif [[ $current_ipv4 == "$published_ipv4" ]]; then
    echo "IPv4 address matches published record. $current_ipv4"
else
    update_cmd+="update delete $HOSTNAME 3600 in a\n"
    update_cmd+="update add $HOSTNAME 3600 in a $current_ipv4\n"
fi

if [[ ! $current_ipv6 =~ $valid_ipv6 ]]; then
    echo "No valid IPv6 address found. $current_ipv6"
elif [[ $current_ipv6 == "$published_ipv6" ]]; then
    echo "IPv6 address matches published record. $current_ipv6"
else
    update_cmd+="update delete $HOSTNAME 3600 in aaaa\n"
    update_cmd+="update add $HOSTNAME 3600 in aaaa $current_ipv6\n"
fi

if [[ -n $update_cmd ]]; then
    update_cmd+="show\nsend\n"
else
    echo 'No addresses to update, exiting.'
    exit 0
fi

nsupdate -k /etc/"${HOSTNAME%%.*}".tsig <<<"$(echo -e "$update_cmd")"

echo "Record update complete."
exit 0

Need to account for possible nsupdate failures on this one.