Setting up dynamic DNS for Namecheap using Dockerized Caddy

2024-06-24

The internet is unusable without ad blocking, so I route all of our network traffic through Pi-hole running on my home server to block ads. I want that same experience when I'm away from home, so I set up a VPN with WireGuard to allow me to use my home Pi-hole as the DNS server for my phone and laptop when on the go.

This works great except for when my home network falls over, or I lose power, or when the unexpected happens. We recently went on vacation and, of course, the day we got there, some "unexpected" happened and I was suddenly without an ad blocker on my phone. Before figuring out what went wrong, I quickly spun up a similar WireGuard + Pi-hole setup on a Droplet so I could use the internet without losing my mind.

Thanks to a friend who stopped by our apartment for an unrelated reason, I learned that the issue was that our ISP had changed our public IP on us. This is the first time this has happened to me in the.... five? six? years I've been running a homelab, over four different ISPs.

I thought dynamic DNS would save me, but it seemed clear that I'd configured ddclient incorrectly. Since I run Caddy, I wondered if I could shove dynamic DNS into that. It turns out that's possible with this random "Caddy app"I found, as long as I install the right Caddy DNS provider. I use namecheap, so I grabbed that one.

Using this Caddy app and a Caddy DNS provider involes compiling Caddy with these modules. My home server uses Docker Compose (on Arch, btw) so I need to make an image with the DNS provider built in. The documentation has an example for this (under adding custom Caddy modules), which is great!

My Caddy block previously looked like this:

x-common-variables: &common-variables
  PGID: 1000
  PUID: 1000
  TZ: America/New_York

services:
  caddy:
    container_name: caddy
    image: caddy:2.8.4
    restart: unless-stopped
    environment:
      <<: *common-variables
      HOST: "my.host" # an actual domain name here
      LOCAL_IP: 192.168.1.2
    ports:
      - "80:80"
      - "443:443"
      - "443:443/udp"
    volumes:
      - ./appdata/caddy/Caddyfile:/etc/caddy/Caddyfile
      - ./appdata/caddy/site:/srv
      - ./appdata/caddy/data:/data
      - ./appdata/caddy/config:/config

Looking at the documentation for Docker Compose, I want something like this:

x-common-variables: &common-variables
  PGID: 1000
  PUID: 1000
  TZ: America/New_York

services:
  caddy:
    container_name: caddy
    restart: unless-stopped
    build: ./custom-caddy-build
    environment:
      <<: *common-variables
      HOST: "my.host" # an actual domain name here
      LOCAL_IP: 192.168.1.2
      NAMECHEAP_SECRET_KEY: "definitely-my-secret-key"
    ports:
      - "80:80"
      - "443:443"
      - "443:443/udp"
    volumes:
      - ./appdata/caddy/Caddyfile:/etc/caddy/Caddyfile
      - ./appdata/caddy/site:/srv
      - ./appdata/caddy/data:/data
      - ./appdata/caddy/config:/config

Then, in custom-caddy-build/Dockerfile:

FROM caddy:2.8.4-builder AS builder

RUN xcaddy build\
    --with github.com/caddy-dns/namecheap\
    --with github.com/mholt/caddy-dynamicdns

FROM caddy:2.8.4

COPY --from=builder /usr/bin/caddy /usr/bin/caddy

Then I ran sudo docker-compose up -d as normal and... it just worked. Neat!

Now I configure the dynamic DNS plugin, in my Caddyfile:

{
	dynamic_dns {
		provider namecheap {
			api_key {$NAMECHEAP_SECRET_KEY}
			user my-namecheap-user
		}
		domains {
			my.host @ www
			my.host my-subdomain
			my.host my-other-subdomain
		}
	}
}

I had to enable access to the namecheap API for my account before this would work. user is just my username for namecheap. To use the API, I had to add an IP address to their allowlist. This is concerning because if my IP address changes (the exact problem I'm trying to solve), I'm not sure that I'll be able to... actually hit the namecheap API to update my dynamic DNS? We'll cross that bridge when we inevitably get to it.

Anyways, how do I check that everything worked? I set up an A+ Dynamic DNS record pointing to 127.0.0.1. I figured if that value gets updated when I restart Caddy, that means I've set things up correctly. That... did not work. The A+ record would (inexplicably) get changed to an A record with the same placeholder IP address.

My next validation attempt was to change the IP address of a subdomain. I observed a change here! The list of subdomains got a new entry for the subdomain I changed with the correct IP address, with the old placeholder one still present as well. This is apparently a known problem that is namecheap's fault so I won't sweat it too much, because the DNS does still work.

The dynamic DNS thing (a "Caddy app") has a dynamic_domains feature that I used originally, but it spammed a bunch of messages in my logs when I tried to use it:

caddy  | {"level":"info","ts":1719245958.4446826,"logger":"dynamic_dns","msg":"domain not found in DNS","domain":"atuin.my.host"}

I ended up specifying my domains manually, as above. This does turn into One More Thing I Have To Remember To Do Whenever I Add A New Service To My Home Lab but that's okay.

Last thing is to format my Caddyfile:

sudo docker exec -it caddy caddy fmt --overwrite /etc/caddy/Caddyfile

Neat!