Nginx Bad Gateway

Nginx received an invalid response from an upstream server (like PHP-FPM, Node.js, or another backend) while acting as a reverse proxy or load balancer.

Common Causes

Upstream server (application backend) is down or not responding
Upstream server process crashed or is stuck
Network connectivity issues between Nginx and upstream server
Upstream server timeout settings are too low
Resource exhaustion (memory, CPU) on the upstream server

How to Fix

1 Check Upstream Server Status

Verify if your application backend (PHP-FPM, Gunicorn, etc.) is running and accessible.

BASH

 $ systemctl status php8.1-fpm  # or your specific service
curl -I http://localhost:3000  # test backend directly

2 Increase Proxy Timeouts

Adjust Nginx proxy timeout directives if upstream is slow to respond.

BASH

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

3 Examine Nginx Error Logs

Check Nginx error logs for specific upstream failure messages.

BASH

 $ tail -f /var/log/nginx/error.log
grep "502" /var/log/nginx/error.log

4 Test Upstream Connectivity

Verify network connectivity and port accessibility to the upstream server.

BASH

 $ telnet backend-server-ip 9000  # test PHP-FPM port
nc -zv localhost 3000           # test Node.js backend

5 Restart Upstream Services

Restart the application backend service and Nginx.

BASH

 $ systemctl restart php8.1-fpm
systemctl restart nginx