fix(ssh): use openssh.authorizedPrincipals for cloudflare certificates

docs/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## 2026-09-06
 
+- **Fixed OpenSSH authorized principals certificate authentication for Cloudflare Tunnel**:
+  - Replaced manual `environment.etc."ssh/authorized_principals/${username}"` symlink and `Match User` config with NixOS native `users.users.<name>.openssh.authorizedPrincipals`.
+  - Root cause: `environment.etc` without an explicit `mode` creates symlinks pointing into `/nix/store`, which has group-writable mode `0775` (`nixbld` group). Under `StrictModes yes`, sshd refused authentication with `bad ownership or modes for directory /nix/store`, causing certificate principal matching to fail with `Certificate does not contain an authorized principal`.
+  - Setting `users.users.<name>.openssh.authorizedPrincipals` causes NixOS to generate `/etc/ssh/authorized_principals.d/<name>` with `mode = "0444"`, copying the file instead of symlinking into the store, and automatically configuring `services.openssh.settings.AuthorizedPrincipalsFile = "/etc/ssh/authorized_principals.d/%u"`.
+  - Also added `mode = "0444"` to `/etc/ssh/ca.pub` and moved `TrustedUserCAKeys` into `services.openssh.settings`.
+
 - **Configured OpenID Connect (OIDC) authentication for Mealie**:
   - Configured `services.mealie.settings` with OIDC settings pointing to Pocket ID (`auth.masu.rs`), using client ID `040925ed-b39e-4442-b8e8-369c948c0cd2`.
   - Added secret management for `mealie-oidc-secret.age` via `secrets.mealie-oidc-secret`, using `prefix = "OIDC_CLIENT_SECRET="` to generate an environment file.

platforms/nixos/modules/nmasur/presets/services/cloudflared.nix

@@ -25,7 +25,7 @@
 # Set ca = "<public key>"
 
 let
-  inherit (config.nmasur.settings) username;
+  inherit (config.nmasur.settings) username hostnames;
   cfg = config.nmasur.presets.services.cloudflared;
 in
 
@@ -68,27 +68,24 @@ in
 
     # Grant Cloudflare access to SSH into this server
     environment.etc = {
-      "ssh/ca.pub".text = ''
-        ${cfg.tunnel.ca}
-      '';
-
-      # Must match the username portion of the email address in Cloudflare
-      # Access
-      "ssh/authorized_principals".text = ''
-        ${username}
-      '';
+      "ssh/ca.pub" = {
+        text = ''
+          ${cfg.tunnel.ca}
+        '';
+        mode = "0444";
+      };
     };
 
+    users.users.${username}.openssh.authorizedPrincipals = [
+      username
+      "${username}@${hostnames.mail}"
+    ];
+
     # Adjust SSH config to allow access from Cloudflare's certificate
-    services.openssh.extraConfig = ''
-      PubkeyAuthentication yes
-      TrustedUserCAKeys /etc/ssh/ca.pub
-      Match User '${username}'
-        AuthorizedPrincipalsFile /etc/ssh/authorized_principals
-        # if there is no existing AuthenticationMethods
-        AuthenticationMethods publickey
-    '';
-    services.openssh.settings.Macs = [ "hmac-sha2-512" ]; # Fix for failure to find matching mac
+    services.openssh.settings = {
+      TrustedUserCAKeys = "/etc/ssh/ca.pub";
+      Macs = [ "hmac-sha2-512" ]; # Fix for failure to find matching mac
+    };
 
     # Create credentials file for Cloudflare
     secrets.cloudflared = {