Automate the Docker DNS Pain Away

6 minute read

Like many other nerds, I have somewhat of a homelab at home. These days it’s not as complicated as it used to be, consisting largely of a big “NAS”1, a Home Assistant box, and a couple other small things.

The NAS, being a beefy server machine, runs a bunch of Docker containers for various things — Octoprint, Docspell (which runs 2 of its own containers + Solr), etc.. It also runs NixOS2, and all of these containers are fronted by the Let’s Encrypt and Nginx infrastructure that it provides. To avoid exposing ports on the NAS’s “host” network, I point Nginx virtual hosts directly at container IPs, like so:

services.nginx.virtualHosts."" = {
  enableACME = true;
  acmeRoot = null;
  forceSSL = true;
  locations."/" = {
	proxyPass = "";
	proxyWebsockets = true;
	extraConfig = "client_max_body_size 0;";

This generally works, except…one of the things that NixOS does as part of a nixos-rebuild --switch when using the declarative Nginx configuration is an Nginx configuration check. Normally, this is great: if I screw up the configuration somehow (e.g. injecting some bad configuration), it won’t take down Nginx. However, it has a big downside: if containers are restarted/container configuration changes, assigned IPs are not stable3, and Nginx configuration will fail to validate.

Previously, I’d tried a number of things that purported to provide a Docker <-> DNS translation, subscribing to Docker daemon events and running a DNS server that I could point other things at. In practice, this never worked quite right: despite telling systemd-resolved that the dns-proxy-server container should be used for DNS, rebuilds (and thus Nginx config checks) would frequently fail because the upstreams would fail to respond on the proxyPass ports.

I was about to embark on a “stupid scratch a homelab itch” project and write something that connects to Docker, listens for events, and updates Route534, when Pete Keen suggested that I check out the docker-gen project, and then pointed me at dnscontrol as well. Sensing an opportunity to hit a Pareto optimal5, I set about hacking up some systemd services and a dnsconfig.js.tmpl file, and an hour or so later, had something extremely feasible.

For the purposes of this writeup, I’m going to assume that you already have the dnscontrol and docker-gen binaries somewhere on your system. In my case, they’re in /nas/homes/brajkovic/bin. I also assume that you’re using Nix/NixOS, because I didn’t write the units manually, but hopefully these declarations for the systemd units are simple enough to manually write the full unit.

The systemd units


First, the docker-gen unit — docker-gen knows how to run as a daemon and listen to events, so we can run it as a normal systemd service:"docker-gen-dns" = {
  path = [

  script = ''
    docker-gen -config docker-gen.cfg

  serviceConfig.WorkingDirectory = "/nas/homes/brajkovic/.config/dns";
  wantedBy = [ "" ];

The working directory is where the docker-gen.cfg file lives, it’ll be in the next section.


Next, the dnscontrol unit — in this case, we register it as a oneshot unit, because docker-gen will run systemd to start it."" = {
  path = [

  script = ''
    dnscontrol version
    dnscontrol preview
    dnscontrol push

  serviceConfig.Type = "oneshot";
  serviceConfig.WorkingDirectory = "/nas/homes/brajkovic/.config/dns";
  after = [ "" ];

The config files


This file configures docker-gen’s behavior, and is super simple:

dest = "dnsconfig.js"
notifycmd = "systemctl start"
template = "dnsconfig.js.tmpl"
watch = true
wait = "500ms:2s"

It tells docker-gen to source the template from dnsconfig.js.tmpl, write it to dnsconfig.js, and then run our dnscontrol unit as the “notify” command after it’s done updating the template. Setting watch to true puts docker-gen in daemon mode, and wait configures the hysteresis: it will wait at least 500ms, at most 2 seconds, to debounce changes.


The dnscontrol template, also deceptively simple:

var REG_NONE = NewRegistrar("none");
var DSP_R53 = NewDnsProvider("r53_main");

D("", REG_NONE, DnsProvider(DSP_R53),
{{range $key, $value := .}}
    {{if $value.IP}}
    // {{ $value.Name }} ({{$value.ID}} from {{$value.Image.Repository}})
    A("{{ $value.Name }}", "{{$value.IP}}"),
    // Allow letsencrypt to issue certificate for this domain
    CAA("@", "issue", ""),
    // Allow ACM to issue certificates for this domain
    CAA("@", "issue", ""),
    // Allow no CA to issue wildcard certificate for this domain
    CAA("@", "issuewild", ";"),
    // Report all violation to If CA does not support
    // this record then refuse to issue any certificate
    CAA("@", "iodef", "", CAA_CRITICAL)

This is mostly basic JavaScript, plus some Go template language — we emit all the A records for the Docker images, and some really basic CAA records so that we can issue certs if we need to for those DNS names6.


The basic “credentials” file for dnscontrol:

  "r53_main": {
    "TYPE": "ROUTE53"

This doesn’t actually have any credentials, because those are provided by the standard AWS SDK credentials mechanism — I should probably do something better with those secrets, but if you can either log into or physically steal my NAS, you’ve earned my AWS creds.

Wrapping It All Up

Putting all that together, we’re done. The docker-gen daemon runs, supervised by its systemd unit. When it needs to, it spawns dnscontrol, but it mostly just sits there idly — I had to restart it to get any recent output, and it said:

Jul 31 22:32:58 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:32:58 Watching docker events
Jul 31 22:32:58 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:32:58 Contents of dnsconfig.js did not change. Skipping notification 'systemctl start'

When I manually killed a container7, you can see the expected output when things do happen:

Jul 31 22:35:09 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:35:09 Received event die for container 236c84adea38
Jul 31 22:35:10 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:35:10 Debounce minTimer fired
Jul 31 22:35:10 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:35:10 Received event stop for container 236c84adea38
Jul 31 22:35:10 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:35:10 Generated 'dnsconfig.js' from 7 containers
Jul 31 22:35:10 hagal docker-gen-dns-start[3691698]: 2023/07/31 22:35:10 Running 'systemctl start'

The unit was indeed started, and you can see dnscontrol applies the changes:

Jul 31 22:35:11 hagal[3712349]: [INFO: Diff2 algorithm in use. Welcome to the future!]
Jul 31 22:35:11 hagal[3712349]: ******************** Domain:
Jul 31 22:35:12 hagal[3712349]: 1 correction (r53_main)
Jul 31 22:35:12 hagal[3712349]: #1: - DELETE A ttl=300
Jul 31 22:35:12 hagal[3712349]: SUCCESS!
Jul 31 22:35:12 hagal[3712349]: Done. 1 corrections.

Overall, really simple, and like I said, hits a strong Pareto optimal: an all-in-one solution would be cool, but bodging together some existing tools and a few systemd services provided satisfying short-term relief.

  1. A Supermicro 6028U-TR4T+, with dual Xeon E5-2650L v3 processors, 128 GB of RAM, and all the disks I could cram in. I expect it to last me…a damn long time. 

  2. Which was in some ways a mistake, and in some ways really speeds things along. 

  3. Manually managing IPAM for Docker containers is not my idea of a good time: see the aforementioned mild regret of using NixOS — I don’t actually want to spend that much sysadmin time. 

  4. Where my domain is hosted, but likely it would have ended up supporting pluggable providers, because I can’t build anything without overbuilding it. 

  5. 80% of the desired outcome, 20% of the work. 

  6. Not that we need to — the day-to-day records that I use live on the root domain. 

  7. Just for fun, but I do need to cut this container out of the configuration for good. 

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