[DEV]

My Ubuntu VPS shows Nginx 502 Bad Gateway after a PHP update. I do not know which PHP-FPM version is installed. What safe checks should I do first? Please do not suggest restarting services before checking the cause.

[ALEX]

Diagnosis for Nginx 502 Bad Gateway

A 502 error usually means Nginx cannot communicate with the backend (PHP-FPM). Since you recently updated PHP, the socket path or service status is the likely culprit. Do not restart services yet.

Safe Diagnostic Steps

1. Check PHP-FPM Status
  Verify if the service is running.
 

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systemctl status php*-fpm

  Note the exact version (e.g., php8.2-fpm) and whether it is "active (running)".

2. Inspect Nginx Error Log
  This will show the specific connection error (e.g., "connect() failed" or "no such file or directory").
 

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tail -n 20 /var/log/nginx/error.log

3. Verify Socket Path
  Compare the socket path in your Nginx config with the actual PHP-FPM socket.
  - Nginx config:

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grep -r "fastcgi_pass" /etc/nginx/

  - PHP-FPM config:

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grep "listen" /etc/php/*/fpm/pool.d/www.conf

  Ensure they match exactly (e.g.,

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/run/php/php8.2-fpm.sock

).

4. Check Permissions
  Ensure the Nginx user (usually

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www-data

) can read/write the socket.
 

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ls -l /run/php/

Next Steps
If PHP-FPM is stopped, check its log:

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journalctl -u php*-fpm -n 20

.
If the socket path mismatch is found, update the Nginx config or PHP-FPM config to match, then reload (not restart) Nginx:

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nginx -t && systemctl reload nginx

.

Please provide the output of steps 1 and 2 for further analysis.