After implementation of nginx reverse-proxy Js Client connects with Flask socketio but doesn't receive any messages

Question

For some reason my Angular client connects with the backend server (apparently successfully) but it doens't receive any message. Neither direct messages nor broadcasted.

This problem was introduced after using Nginx configured as reverse-proxy for the backend. I followed the latest official documentation of flask socketio module but still didn't found any clue of what's going on.

On the client I connect and prepare to receive messages with:

const API = "http://146.250.180.213/api/"
::
socket: io;
::
    this.socket = io(API);
    ::
    this.socket.on('connect', function() {
                console.log('Conection with server estblished - socketio');
            });
    ::
    this.socket.on('update_monitor', (data: any) => {
        console.log('Broadcasted message received! Data:', data);
    });

On Flask I start the server and define endpoints with:

app = Flask(__name__)
socketio = SocketIO(app)

if __name__ == '__main__':
    socketio.run(app, port=5000, debug=True)

===

@app.route('/test_endpoint', methods=['GET'])
def test_endpoint():
    socketio.emit('update_monitor', {"mrtp": app.config['MOST_RECENT_TIMESTAMP_PROCESSED'], 'updated_elements': ['ESICAS23C_ESICAS23']})
    return jsonify({'message': 'ok'}), 200

@socketio.on('connect')
def connect():
    print('Client connected')

I use the 'test_endpoint' to test the socketio mechanism by requesting it with Postman.

On Nginx, I followed the configuration provided by the documentation:

server {
    listen 0.0.0.0:80;

    root   /usr/share/nginx/html;
    index  index.html index.htm;
    include /etc/nginx/mime.types;

    location / {
        try_files $uri /index.html;
    }

    location /socket.io {
        # include proxy_params; dont know what this stands for
        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_pass http://127.0.0.1:5000/socket.io;
    }

    location /grafana/ {
            proxy_pass http://localhost:3000/;
            proxy_hide_header X-Frame-Options;
    }

    location /api {
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_hide_header X-Frame-Options;
            proxy_connect_timeout       9999;
            proxy_send_timeout          9999;
            proxy_read_timeout          9999;
            send_timeout                9999;
            rewrite  ^/api/(.*)  /$1 break;
            proxy_pass http://localhost:5000;
    }
}

Both on client and backend I see that they connects, but client never receives any message.

Already checked nginx (docker logs) logs output and nothing abnormal happens. There isn't any error message, anywhere. Any clue of what's happening? Suggestions?

Answer 1

The solution is to use http://146.250.180.213/ as the API url i.e. without the /api/ namespace.