Websites are unavailable with 502 Bad Gateway on Plesk: upstream sent too big header while reading response header

Symptoms

• Websites are unavailable:
  

  PLESK_INFO: 502 Bad Gateway

• In the log files /var/log/nginx/error.log or /var/www/vhosts/example.com/logs/proxy_error_log the following error message can be found:
  

  CONFIG_TEXT: [error] 14790#0: *188 upstream sent too big header while reading response header from upstream, client: 203.0.113.2, server: subdomain.example.com, request: "POST /admin_catalog.php HTTP/1.1", upstream: "http://10.0.0.1:7080/admin_catalog.php", host: "subdomain.example.com", referrer: "http://subdomain.example.com/admin_catalog.php"

Cause

A request cannot be processed by the proxy server because of insufficient buffer size.

Resolution 

In Plesk interface for few domains

• Login into Plesk.
• For domains with PHP handler "FPM application served by Nginx", increase 'buffers" and 'buffer_size' in  section under Plesk > Domains > example.com > Apache & nginx Settings >Additional nginx directives.
  For example:

  CONFIG_TEXT: fastcgi_buffers 8 16k;
  fastcgi_buffer_size 32k;

  - For other PHP handlers:

  CONFIG_TEXT: proxy_buffers 8 16k;
  proxy_buffer_size 32k;

Using SSH connection for the whole server

1. Connect to the server using SSH.
2. Increase 'buffers" and 'buffer_size' in /etc/nginx/nginx.conf under http { section. For example:
   

   CONFIG_TEXT: proxy_buffers 8 16k;
   proxy_buffer_size 32k;
   fastcgi_buffers 8 16k;
   fastcgi_buffer_size 32k;

3. Restart nginx service:
   

   # service nginx restart

Increase values of these parameters if it is needed.