Dynamic DNS using Cloudflare Workers

In this post I’ll try to describe a simple solution, that I came up with, to solve the issue of dynamically updating DNS records when the IP addresses of your machines/instances changes frequently.

While Dynamic DNS isn’t a new thing and many services/tools around the internet already provide solutions to this problem (for more than 2 decades), I had a few requirements that ruled out most of them:

• I didn’t want to sign up to a new account in one of these external services.
• I would prefer to use a domain name under my control.
• I don’t trust the machine/instance that executes the update agent, so according to the principle of the least privilege, the client should only able to update one DNS record.

The first and second points rule out the usual DDNS service providers and the third point forbids me from using the Cloudflare API as is (like it is donein other blog posts), since the permissions we are allowed to setup for a new API token aren’t granular enough to only allow access to a single DNS record, at best I would’ve to give access to all records under that domain.

My solution to the problem at hand was to put a worker is front of the API, basically delegating half of the work to this “serverless function”. The flow is the following;

• agent gets IP address and timestamp
• agent signs the data using a previously known key
• agent contacts the worker
• worker verifies signature, IP address and timestamp
• worker fetches DNS record info of a predefined subdomain
• If the IP address is the same, nothing needs to be done
• If the IP address is different, worker updates DNS record
• worker notifies the agent of the outcome

Nothing too fancy or clever, right? But is works like a charm.

I’ve published my implementation on GitHub with a FOSS license, so anyone can modify and reuse. It doesn’t require any extra dependencies, it consists of only two files and you just need to drop them at the right locations and you’re ready to go. The repository can be found here and the README.md contains the detailed steps to deploy it.

There are other small features that could be implemented, such as using the same worker with several agents that need to update different records, so only one of these “serverless functions” would be required. But these improvements will have wait for another time, for now I just needed something that worked well for this particular case and that could be easily deployed in a short time.