Nginx 502 Error: Backend Problem
Diagnosing communication between Nginx and PHP-FPM.
The 502 Bad Gateway error with Nginx means the reverse proxy cannot get a valid response from the backend, usually PHP-FPM. It's a communication problem between the two services.
Causes vary: PHP-FPM crash, timeout, misconfigured socket, or exhausted resources. This guide helps you identify the exact source of the problem.
Main Causes
- PHP-FPM down: PHP-FPM service is stopped or crashed. No worker available to process requests.
- Socket doesn't exist: Nginx tries to connect to a Unix socket that doesn't exist or has wrong permissions.
- Upstream timeout: PHP-FPM takes too long to respond and Nginx gives up.
- Exhausted workers: All PHP-FPM workers are busy, new requests are rejected.
Diagnosis
- PHP-FPM status: Check that PHP-FPM is active: systemctl status php-fpm.
- Nginx logs: Check /var/log/nginx/error.log for exact message (upstream, timeout, connection refused).
- Socket/Port: Verify socket or port configured in Nginx matches PHP-FPM.
Solutions
- Restart PHP-FPM: systemctl restart php-fpm to relaunch the service.
- Increase workers: Adjust pm.max_children in PHP-FPM config based on available memory.
- Increase timeouts: Configure fastcgi_read_timeout in Nginx if legitimate scripts are slow.
Frequently Asked Questions
How to see if PHP-FPM is overloaded?
Enable pm.status_path in PHP-FPM to see active/idle workers. If all are "active", you're short on workers.
Socket or TCP, which is better?
Unix Socket is faster on the same server. TCP is needed if PHP-FPM is on a different server.
Why does PHP-FPM crash silently?
Often due to memory overflow. Check PHP memory_limit and system memory. Check PHP-FPM logs.
Does MoniTao detect 502 errors?
Yes, MoniTao monitors HTTP code and alerts you immediately on 5xx errors including 502.
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