Require non-empty user agent

doc/protocol.md

@@ -119,4 +119,7 @@ Multiple set messages may be sent in one message where each message is separated
 
 4005  Closed to protect your security
   When the client does something that compromises its security and the server closes the connection to force the client to fix its code.
+
+4006  Identify yourself
+  When the client is not providing sufficient information in the User-Agent header.
 ```

src/server.js

@@ -121,6 +121,13 @@ wss.on('connection', (ws, req) => {
     return;
   }
 
+  if (!req.headers['user-agent']) {
+    logger.info('A connection was closed for lacking a valid user-agent.');
+    ws.send('Please provide a valid User-Agent header as required by https://docs.turbowarp.org/cloud-variables#advanced. If you use a cloud variable library, contact the author to find out how to do that.');
+    ws.close(4006);
+    return;
+  }
+
   const client = new Client(ws, req);
 
   let isHandshaking = false;