Document how to use sops and sops-nix

README.md

@@ -18,3 +18,46 @@ infra-rebuild switch git matrix
 By default infra-rebuild tries to use the FQDN from the nixosConfiguration of the host for deployment.
 However to override individual parts of the deployment target, a [`deployment_configuration.json`](./deployment_configuration.json) can be used.
 This is exactly what we're doing to set the default deployment user to `colmena-deploy` and have custom target hostnames for Chaosknoten hosts, since they don't have an FQDN defined in their nixosConfiguration.
+
+## Setting up secrets with sops-nix for a host
+
+1. Convert the hosts SSH host public key to an age public key.
+   This can be done by connecting to the host and running:
+   ```
+   cat /etc/ssh/ssh_host_ed25519_key.pub | ssh-to-age
+   ``` 
+2. Add the resulting age public key to the `.sops.yaml` as a YAML anchor in keys.
+   It should be named something like: `host_age_hostname`
+3. Add a new creation rule for the hosts config directory.
+   It should probably have all admin keys and the hosts age key. \
+   You can use existing creation rules as a reference.
+4. Create a file containing the relevant secrets in the hosts config directory.
+   This can be accomplished with a command similar to this:
+   ```
+   sops config/hosts/hostname/secrets.yaml
+   ```
+   Note: Nested keys don't seem to be compatible with sops-nix.
+5. Add the following entry to the modules of the hosts `nixosConfiguration`:
+   ```
+   sops-nix.nixosModules.sops
+   ```
+6. Create a `sops.nix` in the hosts config directory containing the following content to include the `secrets.yaml`:
+   ```
+   { ... }:
+
+   {
+     sops = {
+       defaultSopsFile = ./secrets.yaml;
+     };
+   }
+   ```
+7. Make sure the `sops.nix` gets imported. For example in the `default.nix`.
+8. To use a secret stored under e.g. `forgejo_git_smtp_password`, you can then do something like the following:
+   ```
+   sops.secrets."forgejo_git_smtp_password" = {
+     mode = "0440";
+     owner = "forgejo";
+     group = "forgejo";
+     restartUnits = [ "forgejo.service" ];
+   };
+   ```