feat: add a Caddy reverse-proxy option to the NixOS module

README.md

@@ -58,7 +58,7 @@ For a NixOS host, `nixosModules.default` provides `services.gsmol-bridge`:
 }
 ```
 
-The service runs under `DynamicUser`, restarts on failure, and — like the standalone script — listens on `127.0.0.1` by default. `nginx.enable` puts a TLS-terminating reverse proxy in front rather than exposing the bridge directly: it has no TLS of its own, and a browser refuses a `ws://` connection from an `https://` page, so a real deployment needs the proxy regardless of `--allow-origin`. `openFirewall = true` is for exposing the bridge's port directly instead, which is not the recommended path (see "Notes and limits").
+The service runs under `DynamicUser`, restarts on failure, and — like the standalone script — listens on `127.0.0.1` by default. `nginx.enable` (or `caddy.enable`, wiring into `services.caddy.virtualHosts` — automatic HTTPS, no `enableACME`/`forceSSL` equivalent to set) puts a TLS-terminating reverse proxy in front rather than exposing the bridge directly: it has no TLS of its own, and a browser refuses a `ws://` connection from an `https://` page, so a real deployment needs one of the two regardless of `--allow-origin`. Enable only one; a deployment that replaces `services.caddy.configFile` with a hand-written Caddyfile instead of using `virtualHosts` needs the domain added there directly, proxying to `host`:`port`. `openFirewall = true` is for exposing the bridge's port directly instead of either, which is not the recommended path (see "Notes and limits").
 
 `services.gsmol-bridge.presetServer = { host, publicKey }` bakes a server's key into the pages this deployment serves, so a first-time visitor is pre-pinned instead of asked to copy a base32 key into settings by hand (§4). The value reaches the browser over this deployment's own TLS, so it only stands in for "from the operator, through a trusted channel" when the deployment operator and the smolmaild operator are the same trusted party — set it only for a server you operate or otherwise vouch for. It seeds the first run only: the browser stores it as an ordinary pin from then on, replaceable or removable in settings like any other.
 

nix/module.nix

@@ -72,8 +72,9 @@ in
         Address the bridge listens on. The bridge speaks plain HTTP/WS with no
         TLS of its own, and browsers refuse a `ws://` connection from an
         `https://` page — so leave this at loopback and put a reverse proxy
-        (see `services.gsmol-bridge.nginx`) in front for TLS and public
+        (`services.gsmol-bridge.nginx` or `.caddy`) in front for TLS and
-        exposure, rather than binding this directly to a public interface.
+        public exposure, rather than binding this directly to a public
+        interface.
       '';
     };
 
@@ -123,10 +124,42 @@ in
         description = "Public hostname to serve gsmol from.";
       };
     };
+
+    caddy = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Put a Caddy virtual host in front of the bridge, with automatic
+          HTTPS — no `enableACME`/`forceSSL` to set, Caddy does this itself
+          for any site address that isn't `http://`-prefixed.
+
+          This wires into the standard `services.caddy.virtualHosts` option
+          and has no effect if your own configuration replaces
+          `services.caddy.configFile` outright with a hand-written Caddyfile
+          — that style of deployment needs the domain added to that file
+          instead (e.g. an imported drop-in), proxying to
+          `services.gsmol-bridge.host`:`.port`.
+        '';
+      };
+
+      domain = lib.mkOption {
+        type = lib.types.str;
+        example = "mail.example.org";
+        description = "Public hostname to serve gsmol from.";
+      };
+    };
   };
 
   config = lib.mkIf cfg.enable (lib.mkMerge [
     {
+      assertions = [
+        {
+          assertion = !(cfg.nginx.enable && cfg.caddy.enable);
+          message = "services.gsmol-bridge: enable only one of .nginx or .caddy";
+        }
+      ];
+
       systemd.services.gsmol-bridge = {
         description = "gsmol web client and Smol Mail bridge";
         wantedBy = [ "multi-user.target" ];
@@ -186,5 +219,16 @@ in
         };
       };
     })
+
+    (lib.mkIf cfg.caddy.enable {
+      services.caddy.enable = true;
+      # No proxyWebsockets-style flag needed: Caddy's reverse_proxy upgrades a
+      # WebSocket connection transparently, and it self-provisions TLS for a
+      # non-http:// site address, so neither forceSSL nor enableACME has a
+      # caddy-side equivalent to set here.
+      services.caddy.virtualHosts.${cfg.caddy.domain}.extraConfig = ''
+        reverse_proxy ${cfg.host}:${toString cfg.port}
+      '';
+    })
   ]);
 }