Dynamic DNS client for Namecheap using bash & cron

In addition to running this website, I also run a home server. For convenience, I point a subdomain of cmetcalfe.ca at it so even though it's connected using a dynamic IP (and actually seems to change fairly frequently), I can get access to it from anywhere.

As a bit of background, the domain for this website is registered and managed through Namecheap. While they do provide a recommended DDNS client for keeping a domain's DNS updated, it only runs on Windows.

Instead, after enabling DDNS for the domain and reading Namecheap's article on using the browser to update DDNS I came up with the following dns-update script.

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#!/bin/sh

# Abort if anything goes wrong (negates the need for error-checking)
set -e

# Uses drill instead of dig
resolve() {
    #dig "$1" @resolver1.opendns.com +short 2> /dev/null
    line=$(drill "$1" @resolver1.opendns.com 2> /dev/null | sed '/;;.*$/d;/^\s*$/d' | grep "$1")
    echo "$line" | head -1 | cut -f5
}

dns=$(resolve <subdomain>.cmetcalfe.ca)
curr=$(resolve myip.opendns.com)
if [ "$dns" != "$curr" ]; then
    if curl -s "https://dynamicdns.park-your-domain.com/update?host=<subdomain>&domain=cmetcalfe.ca&password=<my passkey>" | grep -q "<ErrCount>0</ErrCount>"; then
        echo "Server DNS record updated ($dns -> $curr)"
    else
        echo "Server DNS record update FAILED (tried $dns -> $curr)"
    fi
fi

It basically checks if the IP returned by a DNS query for the subdomain matches the current IP of the server (as reported by an OpenDNS resolver) and if it doesn't, sends a request to update the DNS. The echo commands are there just to output some record of the IP changing. Maybe I'll do some analysis of it at some point.

To run the script every 30 minutes and redirect any output from it to the syslog, the following crontab entry can be used:

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*/30 * * * * /path/to/dns-update | /usr/bin/logger -t dns-update

With the script automatically running every 30 minutes I can now be confident that my subdomain will always be pointing at my home server whenever I need access to it.