Since writing Updating Namecheap DDNS from CentOS 7, I had the desire to rewrite parts of my DDNS update script, making it compatible with Synology DSM 6. Synology's DSM does have a built-in DDNS update client, but it is not currently compatible with Namecheap.

This script should still work on most versions of Linux and macOS. In fact, it might be slightly more compatible than the version in my previous post, as I am no longer using xmllint to parse the result of the DDNS update.

Installation

The installation process for this script is identical to that outlined in a previous post, Reconnecting a failed VPN connection on Synology DSM 6. If you need help with the following steps, please refer to that post, in which I describe them in more detail.

When creating a scheduled User-defined script task in Task Scheduler, name it something like Update Namecheap DDNS. The script does not need to run as root, but it won't hurt if you want to do so.

I would recommend running this script every 15 minutes, all days of the week. You can run it more frequently (e.g. every 5 minutes), but please note that due to the way DNS lookups are cached, you might receive multiple email notifications when your public IP address changes until the TTL expires.

This script is configured to work with the Send run details only when the script terminates abnormally option in DSM, so I would recommend enabling it if you choose to receive email notifications. Otherwise, you will receive a notification every time the script runs.

Finally, you will need to copy and paste the entire body of the following script into the User-defined script box. Before you click OK, make sure you modify the variables near the top of the script.

#!/bin/bash

## Namecheap DDNS update client for Synology DSM 6
## Written by Ian Harrier

###
# Variables
# These need to be customized to your environment
#
# FQDN
#   The FQDN of the (A) record to be updated
#   Supports either 'domain.tld' or 'subdomain.domain.tld'
# PASSWORD
#   The DDNS update password from Namecheap
# NAME_SERVER
#   The DNS server used to resolve the DDNS (A) record
#   Recommended to use one of the domain's name servers
###

FQDN='subdomain.domain.tld'  
PASSWORD='<secret password from Namecheap admin console>'  
NAME_SERVER='dns1.registrar-servers.com'

###
# Retrieve the current public IPv4 address
###

IPv4_CURRENT=$(nslookup myip.opendns.com resolver1.opendns.com | awk '/^Address: / { print $2 }')
if [[ $IPv4_CURRENT =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
	echo "[I] The current IPv4 address is '$IPv4_CURRENT'"
else
	echo "[E] Unable to retrieve valid IPv4 address from 'opendns.com'. Exiting..."
	exit 1
fi


###
# Retrieve the reported public IPv4 address from the DDNS entry
###

IPv4_DDNS=$(nslookup $FQDN $NAME_SERVER | awk '/^Address: / { print $2 }')
if [[ $IPv4_DDNS =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
	echo "[I] The DDNS (A) record reports '$IPv4_DDNS'"
else
	echo "[E] Unable to resolve '$FQDN' using name server '$NAME_SERVER'. Exiting..."
	exit 1
fi

###
# Compare the results
###

if [[ "$IPv4_CURRENT" == "$IPv4_DDNS" ]]; then
	echo -n "[I] IPv4 addresses match. No need to update. Exiting..."
	exit 0
else
	echo "[W] IPv4 addresses DO NOT match. Need to update."
fi

###
# Generate $HOST and $DOMAIN
###

# If $FQDN is in the format 'domain.tld' (i.e. one occurrence of '.')
if [[ $(echo $FQDN | awk 'BEGIN{FS="."} {print NF?NF-1:0}') == 1 ]]; then
	HOST='@'
	DOMAIN=$FQDN
fi

# If $FQDN is in the format 'subdomain.domain.tld' (i.e. two occurrences of '.')
if [[ $(echo $FQDN | awk 'BEGIN{FS="."} {print NF?NF-1:0}') == 2 ]]; then
	HOST=$(echo $FQDN | awk -F "." '{print $1}')
	DOMAIN=$(echo $FQDN | awk -F "." '{print $2 "." $3}')
fi

###
# Update DDNS
###

echo "[I] Updating '$FQDN' to '$IPv4_CURRENT'"
UPDATE_RESULT=$(curl -s "https://dynamicdns.park-your-domain.com/update?host=$HOST&domain=$DOMAIN&password=$PASSWORD&ip=$IPv4_CURRENT")

###
# Report error(s)
###

UPDATE_ERRORS=$(echo $UPDATE_RESULT | sed -n "s/.*<ErrCount>\(.*\)<\/ErrCount>.*/\1/p")

if [[ $UPDATE_ERRORS -gt 0 ]]; then
	echo "[E] There were $UPDATE_ERRORS error(s):"
	for (( i = 1; i <= $UPDATE_ERRORS; i++ )); do
		echo -n " $i. "
		echo $UPDATE_RESULT | sed -n "s/.*<Err$i>\(.*\)<\/Err$i>.*/\1/p"
	done
	echo "[I] Exiting..."
else
	echo "[I] Update successful. Exiting..."
fi

exit 1

Conclusion

Like the previous version of this script, when your public IP address changes, your Namecheap DDNS record will be updated accordingly. But as an added bonus, DSM is also able to send email notifications when important events occur.