Bash Script for free reliable Dynamic DNS (DynDNS) using the Digital Ocean API

dodyndns.sh

#!/bin/bash

##
# This script check whether the server's public IP has changed and updates an
# A DNS record with the Digital Ocean API.
# /etc/dodyndns configuration file:
# SERVER_IP=127.0.0.1
# RECORD_ID=
#
# DYNDNS_DOMAIN_PREFIX=dyndns
# DIGITAL_OCEAN_DOMAIN=example.com
# DIGITAL_OCEAN_API_TOKEN=asdfghjkl1234567890
# DNS_TTL=60
# ADMIN_EMAIL="admin@example.org"
# ADMIN_NAME="John Doe"
#
# Copyright: Itay Grudev (c) 2018
# License: MIT
#

source /etc/dodyndns

# Acquire the computer's public address
IP_DIG=$(dig +short myip.opendns.com @resolver1.opendns.com 2>/dev/null)

if [[ $IP_DIG =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
	IP=$IP_DIG
else
	IP_CURL=$(curl -s https://checkip.amazonaws.com 2>/dev/null)

	if [[ $IP_CURL =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
		IP=$IP_CURL
	fi
fi

# If the script is unable to retrieve the server's external ip address it
# should simply exit silently assuming this is normal downtime
if [ -z "${IP// }" ]; then
	exit 0
fi

# If the reported IP addresses has changed to the one on record
if [ "$SERVER_IP" != "$IP" ]; then
	function inform_admin_of_error() {
		/usr/sbin/sendmail $ADMIN_EMAIL <<-EOF
			From: Server: $(hostname) <$LOGNAME@$(hostname)>
			To: $ADMIN_NAME <$ADMIN_EMAIL>
			Content-Type: text/plain; charset=UTF-8
			X-Priority: 1 (Highest)
			X-MSMail-Priority: High
			Importance: High
			Subject: Urgent! Unable to update the server's A record.
			Dear $ADMIN_NAME,

			The public IP address of $(hostname) has changed from "$SERVER_IP" to "$IP".
			But due to a problem with the Digital Ocean API updating the A record of the server was not successful.

			Services are most likely unavailable and urgent changes to the DNS A records are probably required.

			Kind Regards,
			$LOGNAME@$(hostname) via crontab
		EOF
	}

	# Verify the authenticity of the new IP address by comparing SSH fingerprints
	TMP_KNOWN_HOSTS=$(mktemp)
	ssh-keyscan 127.0.0.1 > $TMP_KNOWN_HOSTS
	sed -i "s/127\.0\.0\.1/$IP/" $TMP_KNOWN_HOSTS
	RESPONSE=$(ssh -n -o "UserKnownHostsFile $TMP_KNOWN_HOSTS" -o "StrictHostKeyChecking yes" $IP true 2>&1)
	rm TMP_KNOWN_HOSTS

	if ! [[ $RESPONSE = *"Host key verification failed."* ]]; then
		# If everything checks out and there is no MITM update the Digital Ocean DNS
		# record. Though we first need to delete the old record.
		if [ ! -z "$RECORD_ID" ] && [ "$RECORD_ID" != " " ] && [ "$RECORD_ID" != "null" ]; then
			curl -s -X DELETE \
			-H "Content-Type: application/json" \
			-H "Authorization: Bearer ${DIGITAL_OCEAN_API_TOKEN}" \
			"https://api.digitalocean.com/v2/domains/${DIGITAL_OCEAN_DOMAIN}/records/${RECORD_ID}" 2>&1 >/dev/null
		fi

		NEW_RECORD_ID="$(curl -s -X POST \
		-H "Content-Type: application/json" \
		-H "Authorization: Bearer ${DIGITAL_OCEAN_API_TOKEN}" \
		-d "
			{
				\"type\": \"A\",
				\"name\": \"$DYNDNS_DOMAIN_PREFIX\",
				\"data\": \"$IP\",
				\"ttl\": $DNS_TTL
			}" \
		"https://api.digitalocean.com/v2/domains/${DIGITAL_OCEAN_DOMAIN}/records" | \
		jq '.domain_record.id')"

		# Verify the record id is valid
		NUMERICAL_REGEX='^[0-9]+$'
		if ! [[ $NEW_RECORD_ID =~ $NUMERICAL_REGEX ]]; then
			inform_admin_of_error
			exit 1
		fi

		# Verify the new domain name was saved
		SAVED_RECORD_ID="$(curl -s -X GET \
		-H "Content-Type: application/json" \
		-H "Authorization: Bearer ${DIGITAL_OCEAN_API_TOKEN}" \
		"https://api.digitalocean.com/v2/domains/${DIGITAL_OCEAN_DOMAIN}/records/${NEW_RECORD_ID}" | \
		jq '.domain_record.id')"

		# Verify the record id is numeric
		if [[ "$NEW_RECORD_ID" == "$SAVED_RECORD_ID" ]]; then
			# Update the configuration settings
			sed -i "s/SERVER_IP=.\+/SERVER_IP=$IP/" /etc/dodyndns
			sed -i "s/RECORD_ID=.\+/RECORD_ID=$NEW_RECORD_ID/" /etc/dodyndns
		else
			inform_admin_of_error
			exit 1
		fi
	fi
fi