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Caddy Cloudflare DNS on NixOS

If you serve Caddy from behind Cloudflare and enforce, you may run into an issue with auto-provisioning ACME SSL certs.

By default, Caddy uses the HTTP validation method which requires no setup but does involve serving traffic on HTTP / port 80, including through Cloudflare. I prefer Cloudflare to enforce using HTTPS / port 443 everywhere, which means that every time I want to validate a new cert I have to temporarily disable the HTTPS enforcement and/or set the SSL settings to “flexible” instead of “strict”.

For hands-free automation, this is a non-starter!

The solution is to switch from HTTP validation to TXT validation, which doesn’t require serving HTTP but does require updating your DNS records to receive an SSL cert.

Updating DNS with Caddy

Caddy has a plugin to connect to Cloudflare’s DNS service and automatically manage the records for cert validation, but it’s not packaged in NixOS.

So the next step is to recompile Caddy with the Cloudflare DNS plugin using an overlay. Here’s what my overlay looks like:

_final: prev:


  plugins = [ "github.com/caddy-dns/cloudflare" ];
  goImports =
    prev.lib.flip prev.lib.concatMapStrings plugins (pkg: "   _ \"${pkg}\"\n");
  goGets = prev.lib.flip prev.lib.concatMapStrings plugins
    (pkg: "go get ${pkg}\n      ");
  main = ''
    package main
    import (
    	caddycmd "github.com/caddyserver/caddy/v2/cmd"
    	_ "github.com/caddyserver/caddy/v2/modules/standard"
    func main() {

in {
  caddy-cloudflare = prev.buildGo120Module {
    pname = "caddy-cloudflare";
    version = prev.caddy.version;
    runVend = true;

    subPackages = [ "cmd/caddy" ];

    src = prev.caddy.src;

    vendorSha256 = "sha256:mwIsWJYKuEZpOU38qZOG1LEh4QpK4EO0/8l4UGsroU8=";

    overrideModAttrs = (_: {
      preBuild = ''
        echo '${main}' > cmd/caddy/main.go
      postInstall = "cp go.sum go.mod $out/ && ls $out/";

    postPatch = ''
      echo '${main}' > cmd/caddy/main.go
      cat cmd/caddy/main.go

    postConfigure = ''
      cp vendor/go.sum ./
      cp vendor/go.mod ./

    meta = with prev.lib; {
      homepage = "https://caddyserver.com";
      description =
        "Fast, cross-platform HTTP/2 web server with automatic HTTPS";
      license = licenses.asl20;
      maintainers = with maintainers; [ Br1ght0ne techknowlogick ];

The primary change we’re making here is pulling in the Go module for the plugin, and updating the resulting vendorSha256.

Once we have made our overlay (I gave it a separate pname above called caddy-cloudflare) we can use it in our Caddy settings.

services.caddy.package = pkgs.caddy-cloudflare;

You could also just override the caddy package and not bother to update services.caddy.package, but I like that 1) I have to choose to use it explicitly, and 2) I can conditionally enable this version of the package only for machines that use Cloudflare, in case I

In my previous post, I explained that I use the JSON config format instead of a traditional Caddyfile, which gives me more control over options as well as the ability to merge together routes from different services.

services.caddy = {
  adapter = "''"; # Required to enable JSON
  configFile = pkgs.writeText "Caddyfile" (builtins.toJSON {
    apps.http.servers.main = {
      listen = [ ":443" ];
      routes = config.caddy.routes;

Adding the following into the JSON configuration will enable the new Cloudflare DNS plugin as an ACME DNS provider, which will automatically create the necessary records to solve the LetsEncrypt verification challenges.

apps.tls.automation.policies = [{
  issuers = [{
    module = "acme";
    challenges = {
      dns = {
        provider = {
          name = "cloudflare";
          api_token = "{env.CF_API_TOKEN}";
        resolvers = [ "" ];

However, we’ll need an API token for our Cloudflare account in order to make those changes to our DNS as securely as possible.

Setting the Cloudflare API Key

Check the Cloudflare docs for how to generate an API token from your profile. For our purposes, the token should have the following permissions:

You should take this API token and write it out into a secret text file with the following format:


This is because we will load it as an environment file, so it must be a file that a shell can evaluate.

Next, you’ll need to securely deploy your new secret file to a location on your disk. You could use one of the following methods:

In any case, make sure the file permissions are set to be read by caddy:caddy (or whatever user is running the caddy service) with 0400 access.

Then you’ll need to set the following to load the secret as an environment file for the systemd service:

systemd.services.caddy.serviceConfig.EnvironmentFile = /path/to/your/secret

I’ve also found that I needed to enable CAP_NET_BIND_SERVICE to grant the systemd service the ability to bind to lower ports (such as 443):

systemd.services.caddy.serviceConfig.AmbientCapabilities = "CAP_NET_BIND_SERVICE";

At this point, your Caddy service should be able to modify DNS records and receive LetsEncrypt certificates automatically! Take a look at my Cloudflare configuration settings and my Caddy overlay as examples.