Nginx/Apache/Load Balancers Bad Gateway

The server, acting as a gateway or proxy, received an invalid response from an upstream server.

Common Causes

Backend application server (like PHP-FPM, Node.js, or Gunicorn) is down or crashed
Timeout between proxy server and backend server
Misconfigured proxy_pass directive in Nginx/Apache
Firewall or network issues blocking communication to upstream server
Insufficient resources (CPU/memory) on backend server

Verify if your application server (PHP-FPM, Node, etc.) is running and listening on the correct port.

BASH

 $ sudo systemctl status php-fpm
# Or check specific port
sudo netstat -tlnp | grep :9000

Manually test if the proxy can reach the backend server using curl or telnet.

BASH

 $ curl -v http://backend-server:3000/
telnet backend-server 3000

Adjust Nginx proxy timeout settings if backend responses are slow.

BASH

 $ location / {
    proxy_pass http://backend;
    proxy_connect_timeout 60s;
    proxy_send_timeout 60s;
    proxy_read_timeout 60s;
}

Examine both proxy and backend server logs for detailed error messages.

BASH

 $ sudo tail -f /var/log/nginx/error.log
sudo journalctl -u php-fpm -f

Ensure firewall allows traffic between proxy and backend servers.

BASH

 $ sudo iptables -L -n | grep 3000
# If using AWS/GCP, check security group rules