Linode dynamic DNS updating script

update-dns.sh

#!/bin/bash
# Modified by Chris Richardson (https://github.com/christr and https://twitter.com/christr77) on 09/20/2020
# Previous versions of this script don't work because they hadn't been updated since 2012. There are now more steps involved to set this up.

# This script update is based on information found here: https://developers.linode.com/api/v4/domains-domain-id-records-record-id/#put

# You first must find out the domain ID and resource ID numbers. In order to do this follow the steps below.
# 1. Create a Linode API Key through your account profile at https://cloud.linode.com/dashboard. Give it rights to read/write to domains only.
# 2. From a shell run the following command: LINODE_API_KEY=[insert API key from step 1 here]
# 3. Run the following command to get the domain ID number for the domain you want to manage: curl -H "Authorization: Bearer $LINODE_API_KEY" https://api.linode.com/v4/domains/
# 4. From a shell run the following command: DOMAIN_ID=[insert domain ID number from step 3 here]
# 5. Run the following command to get the resource ID number for the subdomain you want to manage: curl -H "Authorization: Bearer $LINODE_API_KEY" https://api.linode.com/v4/domains/$DOMAIN_ID/records/
# 6. From a shell run the following command: RESOURCE_ID=[insert resource ID number from step 5 here]
# 7. Run the following command to verify the current settings for this resource: curl -H "Authorization: Bearer $LINODE_API_KEY" https://api.linode.com/v4/domains/$DOMAIN_ID/records/$RESOURCE_ID
# 8. Use the information collected from these commands to complete the variables below in this script.

LINODE_API_KEY=
DOMAIN_ID=
RESOURCE_ID=
NAME=

function resource_update {
curl -H "Content-Type: application/json" \
	-H "Authorization: Bearer $LINODE_API_KEY" \
	-X PUT -d '{
		"type": "A",
		"name": "'$NAME'",
		"target": "'$WAN_IP'",
		"priority": 0,
		"weight": 0,
		"port": 0,
		"service": null,
		"protocol": null,
		"ttl_sec": 120,
		"tag": null
	}' \
	https://api.linode.com/v4/domains/$DOMAIN_ID/records/$RESOURCE_ID
}

WAN_IP=`curl -s ifconfig.me/ip`
if [ -f $HOME/.wan_ip.txt ]; then
	OLD_WAN_IP=`cat $HOME/.wan_ip.txt`
else
	echo "No file, need IP"
	OLD_WAN_IP=""
fi

if [ "$WAN_IP" = "$OLD_WAN_IP" ]; then
	echo "IP Unchanged"
else
	echo $WAN_IP > $HOME/.wan_ip.txt
	echo "Updating DNS to $WAN_IP. Results from Linode are displayed below."
	resource_update
fi

I actually prefer the current method myself because I like that no update is even initiated to Linode if the IP hasn't changed.

Back in the day, my DDNS provider (not Linode) would penalize too many API calls where nothing changed. Interesting that Chris chimed in in that thread about simply using remote_addr. I guess Linode didn't care about API calls that didn't update anything (and I wonder if Akamai will feel the same way).

The other benefit to the bit of logic there is that we get an entry in syslog when the IP changes, which can come in handy for troubleshooting ISP issues like modem/ONT resets, firmware updates, etc.

The TTL point is a good one; I can confirm this behavior (TTL of 0 sets it to "default" in DNS manager rather than literally zero seconds). According to Linode the default ttl is 86400 seconds, which as @demaniak noted is probably not what we want. I've just set mine to 120s and it updates the TTL from "Default" to 2mins.

That's a good point I didn't consider on the TTL. I modified the script to do 120. Thank you both.