Setting up a $connect route that requires a WebSocket subprotocol

Clients can use the Sec-WebSocket-Protocol field to request a WebSocket subprotocol during the connection to your WebSocket API. You can set up an integration for the $connect route to allow connections only if a client requests a subprotocol that your API supports.

The following example Lambda function returns the Sec-WebSocket-Protocol header to clients. The function establishes a connection to your API only if the client specifies the myprotocol subprotocol.

For an AWS CloudFormation template that creates this example API and Lambda proxy integration, see ws-subprotocol.yaml.

export const handler = async (event) => {
    if (event.headers != undefined) {
        const headers = toLowerCaseProperties(event.headers);
        
        if (headers['sec-websocket-protocol'] != undefined) {
            const subprotocolHeader = headers['sec-websocket-protocol'];
            const subprotocols = subprotocolHeader.split(',');
            
            if (subprotocols.indexOf('myprotocol') >= 0) {
                const response = {
                    statusCode: 200,
                    headers: {
                        "Sec-WebSocket-Protocol" : "myprotocol"
                    }
                };
                return response;
            }
        }
    }
    
    const response = {
        statusCode: 400
    };
        
    return response;
};

function toLowerCaseProperties(obj) {
    var wrapper = {};
    for (var key in obj) {
        wrapper[key.toLowerCase()] = obj[key];
    }
    return wrapper;
}    
        

You can use wscat to test that your API allows connections only if a client requests a subprotocol that your API supports. The following commands use the -s flag to specify subprotocols during the connection.

The following command attempts a connection with an unsupported subprotocol. Because the client specified the chat1 subprotocol, the Lambda integration returns a 400 error, and the connection is unsuccessful.

wscat -c wss://api-id.execute-api.region.amazonaws.com/beta -s chat1
error: Unexpected server response: 400

The following command includes a supported subprotocol in the connection request. The Lambda integration allows the connection.

wscat -c wss://api-id.execute-api.region.amazonaws.com/beta -s chat1,myprotocol
connected (press CTRL+C to quit)

To learn more about invoking WebSocket APIs, see Invoking a WebSocket API.