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Find the cause of a 502 Bad Gateway in the nginx log

harmless rootnginxdebian, ubuntu
Adjust the values, the command updates live
root@server
grep -E "upstream|connect\(\) failed" /var/log/nginx/error.log | tail -20

Shows the last 20 messages where nginx could not reach the backend (PHP-FPM, Apache or a Node application) or did not get a valid response. Typical messages are connect() failed (111: Connection refused) when the backend is stopped, or upstream prematurely closed connection when PHP processes crashed. The upstream: part of the message reveals which target is meant.

Note: On Plesk, use /var/www/vhosts/system/<domain>/logs/proxy_error_log as the log.

Also searched as

  • nginx 502 bad gateway cause
  • nginx upstream error log
  • connect() failed upstream
  • php-fpm not responding nginx

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