diff --git a/masterserver/README.md b/masterserver/README.md
index 91a90383..7d306e09 100644
--- a/masterserver/README.md
+++ b/masterserver/README.md
@@ -6,10 +6,10 @@ Servers need to send frequent updates to let the master server know they are sti
 
 ## Prerequisites
 
--   MySQL server (see `config.inc.php` for default configuration)
--   PHP 5.?+
-    -   Socket PHP extension
-    -   pdo_mysql extension
+- MySQL server (see `config.inc.php` for default configuration)
+- PHP 5.?+
+  - Socket PHP extension
+  - pdo_mysql extension
 
 ## /register.php (POST)
 
@@ -17,13 +17,25 @@ Register a game server by writing its details to the database.
 
 ### Parameters
 
-All three parameters are required.
+`port`, `version`, and `name` are required.  `ip` is optional.
 
 - **port**: The game server's port, such as `35666`.
-- **version**: The game server's version, such as `20191101`.
+- **version**: The game server's version, such as `20241208`.
 - **name**: A name to identify the server to clients.
+- **ip** _(optional)_: The IP address or hostname to advertise to clients. When omitted, the TCP connection's source address is used.
 
-The game server's IP address is also recorded.
+  If advertising with a reverse proxy (running with the `server=listen` option on the proxy server, and with `serverproxy=<IP ADDRESS>` on the game server), supply the reverse proxy's address here so that clients connect to the proxy. The IP must contain only valid IPv4/IPv6/hostname characters and be at most 253 characters long.
+
+On first registration, the master server opens a TCP connection to the advertised address and port to verify the server is publicly reachable.  Subsequent registrations from the same address and port act as a heartbeat and skip this check.
+
+### Responses
+
+| Body | Meaning |
+| ---- | ------- |
+| `OK` | Registered successfully |
+| `CONNECT FAILED: ip:port` | Master server could not TCP-connect to the advertised address — port forwarding or the reverse proxy is not reachable |
+| `'port' not set` (HTTP 400) | Missing required parameter |
+| `Invalid 'ip'` (HTTP 400) | `ip` override contains disallowed characters or exceeds 253 characters |
 
 ### Example
 
@@ -34,7 +46,14 @@ $ curl -d "port=35666&name=Test&version=20190111" -X POST http://127.0.0.1:8000/
 OK
 ```
 
-If the registry server fails to connect to the game server, it responds:
+Registering the server at 127.0.0.1 via a reverse proxy at 203.0.113.1:
+
+```
+$ curl -d "port=35666&name=Test&version=20190111&ip=203.0.113.1" -X POST http://127.0.0.1:8000/register.php
+OK
+```
+
+If the master server fails to connect to the advertised address:
 
 ```
 CONNECT FAILED: 127.0.0.1:35666
@@ -42,7 +61,7 @@ CONNECT FAILED: 127.0.0.1:35666
 
 ## /list.php (GET)
 
-Retrieve a list of registered servers, each in a colon-delimted line of (in order) its IP address, port, version, and name.
+Retrieve a list of registered servers, each as a colon-delimited line of (in order) its IP address, port, version, and name.
 
 ### Example
 
diff --git a/masterserver/register.php b/masterserver/register.php
index e8084a56..90e47415 100644
--- a/masterserver/register.php
+++ b/masterserver/register.php
@@ -21,11 +21,26 @@
 $name = $_POST['name'];
 $version = (int)($_POST['version']);
 
+// Optional IP override: used when the game server tunnels through a reverse
+// proxy and wants to advertise the proxy's address instead of its own NAT'd IP.
+$ip = $_SERVER['REMOTE_ADDR'];
+if (isset($_POST['ip']))
+{
+    $candidate = trim($_POST['ip']);
+    if (strlen($candidate) > 0 && strlen($candidate) <= 253 && preg_match('/^[0-9a-zA-Z:.\-]+$/', $candidate))
+        $ip = $candidate;
+    else
+    {
+        http_response_code(400);
+        die("Invalid 'ip'");
+    }
+}
+
 $db = new PDO($database['dsn'], $database['user'], $database['password']);
 $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
 
 $db->exec("CREATE TABLE IF NOT EXISTS `servers` (
-  `ip` varchar(32) NOT NULL,
+  `ip` varchar(253) NOT NULL,
   `port` int(11) NOT NULL,
   `version` int(11) NOT NULL,
   `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
@@ -34,22 +49,22 @@
 );");
 
 $q = $db->prepare("SELECT * FROM `servers` WHERE `ip` = :ip AND `port` = :port AND `version` = :version");
-$result = $q->execute(array("ip" => $_SERVER['REMOTE_ADDR'], "port" => $port, "version" => $version));
+$result = $q->execute(array("ip" => $ip, "port" => $port, "version" => $version));
 if($q->fetch() === false)
 {
     //First time server is added. Try if we can connect to the server. If not, then port forwarding is most likely not setup.
     $socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
-    $result = @socket_connect($socket, $_SERVER['REMOTE_ADDR'], $port);
+    $result = @socket_connect($socket, $ip, $port);
     if ($result === false)
     {
-        die("CONNECT FAILED: " . $_SERVER['REMOTE_ADDR'] . ":" . $port);
+        die("CONNECT FAILED: " . $ip . ":" . $port);
     }else{
         socket_close($socket);
     }
 }
 
 $q = $db->prepare("INSERT INTO `servers`(`ip`, `port`, `version`, `name`) VALUES (:ip, :port, :version, :name) ON DUPLICATE KEY UPDATE `name` = :name, `version` = :version, `last_update` = NOW()");
-$q->execute(array("ip" => $_SERVER['REMOTE_ADDR'], "port" => $port, "version" => $version, "name" => $name));
+$q->execute(array("ip" => $ip, "port" => $port, "version" => $version, "name" => $name));
 
 echo "OK";
 ?>
diff --git a/src/multiplayer_server.cpp b/src/multiplayer_server.cpp
index 18dac4b8..4b06bff4 100644
--- a/src/multiplayer_server.cpp
+++ b/src/multiplayer_server.cpp
@@ -84,6 +84,7 @@ void GameServer::connectToProxy(sp::io::network::Address address, int port)
         LOG(ERROR) << "Failed to connect to proxy";
         return;
     }
+    proxy_address = address.getHumanReadable()[0];
 
     ClientInfo info;
     socket->setBlocking(false);
@@ -718,7 +719,10 @@ void GameServer::runMasterServerUpdateThread()
     sp::io::http::Request http(hostname, port);
     while(!isDestroyed() && master_server_url != "")
     {
-        auto response = http.post(uri, "port=" + string(listen_port) + "&name=" + server_name + "&version=" + string(version_number));
+        string body = "port=" + string(listen_port) + "&name=" + server_name + "&version=" + string(version_number);
+        if (proxy_address != "")
+            body += "&ip=" + proxy_address;
+        auto response = http.post(uri, body);
         if (response.status != 200)
         {
             LOG(WARNING) << "Failed to register at master server " << master_server_url << " (status " << response.status << ")";
diff --git a/src/multiplayer_server.h b/src/multiplayer_server.h
index 10a9e027..4f6cf5ed 100644
--- a/src/multiplayer_server.h
+++ b/src/multiplayer_server.h
@@ -92,6 +92,7 @@ class GameServer : public Updatable
     std::vector<uint32_t> ecs_entity_version;
 
     string master_server_url;
+    string proxy_address;
     std::thread master_server_update_thread;
     MasterServerState master_server_state = MasterServerState::Disabled;
 public: