diff --git a/utils.cpp b/utils.cpp
index 87de974..1cf4428 100644
--- a/utils.cpp
+++ b/utils.cpp
@@ -356,6 +356,7 @@ bool parseHttpHeader(CirBuf &buf, std::string &websocket_key, int &websocket_ver
     bool upgradeHeaderSeen = false;
     bool connectionHeaderSeen = false;
     bool firstLine = true;
+    bool subprotocol_seen = false;
 
     std::string line;
     while (std::getline(is, line))
@@ -396,12 +397,16 @@ bool parseHttpHeader(CirBuf &buf, std::string &websocket_key, int &websocket_ver
             websocket_key = value;
         else if (name == "sec-websocket-version")
             websocket_version = stoi(value);
+        else if (name == "sec-websocket-protocol" && value_lower == "mqtt")
+            subprotocol_seen = true;
     }
 
     if (doubleEmptyLine)
     {
         if (!connectionHeaderSeen || !upgradeHeaderSeen)
             throw BadHttpRequest("HTTP request is not a websocket upgrade request.");
+        if (!subprotocol_seen)
+            throw BadHttpRequest("HTTP header Sec-WebSocket-Protocol with value 'mqtt' must be present.");
     }
 
     return doubleEmptyLine;
@@ -491,6 +496,7 @@ std::string generateWebsocketAnswer(const std::string &acceptString)
     oss << "Upgrade: websocket\r\n";
     oss << "Connection: Upgrade\r\n";
     oss << "Sec-WebSocket-Accept: " << acceptString << "\r\n";
+    oss << "Sec-WebSocket-Protocol: mqtt\r\n";
     oss << "\r\n";
     oss.flush();
     return oss.str();