Understanding and Fixing "Upstream Prematurely Closed Connection" in Nginx

Seeing "upstream prematurely closed connection" in Nginx? This error means the upstream server closed the connection too soon. Here’s how to fix it.

Common causes of "upstream prematurely closed connection" in Nginx

• Upstream server errors

  • Server crashes
  • Misconfigurations

• Timeouts

  • FastCGI, Proxy, or Upstream timeouts

• Buffer and size limitations

  • Inadequate buffer sizes
  • Header size limits

• Application issues

  • Bugs or errors in the application on upstream server

How to fix the "upstrem prematurely closed connection" error

• Adjust Nginx timeouts

  • Increase proxy_read_timeout
  • Increase fastcgi_read_timeout

  proxy_read_timeout 90s;
  fastcgi_read_timeout 90s;
  

• Increase Buffers

  • Adjust proxy_buffer_size
  • Modify proxy_buffers

  proxy_buffer_size 16k;
  proxy_buffers 4 16k;
  

Use this example to adjust your Nginx settings:

http {
    proxy_read_timeout 90s;
    proxy_connect_timeout 90s;
    proxy_send_timeout 90s;

    server {
        location / {
            proxy_pass http://upstream_server;
            proxy_buffer_size 16k;
            proxy_buffers 4 16k;
            proxy_busy_buffers_size 32k;
        }
    }
}

Upstream server errors

Upstream server crashes can cause this error. Check the server logs for clues. Fix any found issues and restart the server. Alse check CPU and memory usage on upstream server.

• Check logs on upstream servers

  • Look at Application logs
  • Check upstream system server logs

  dmesg -T
  journalctl -xe --since '3 hours ago'
  

• Check server resources on upstream servers

  • Check CPU usage

    top
    

  • Monitor memory usage

    free -h
    

Conclusion

Fixing the "upstream prematurely closed connection" error is crucial for smooth operation. By following these steps, you can troubleshoot and resolve the issue.

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