Fixing 'Upstream Sent Too Big Header' Error in Magento 2

The error message “upstream sent too big header while reading response header from upstream” typically occurs in a Nginx web server when Magento 2 generates a large response header. This issue can be caused by several factors, and it usually indicates that the headers being sent back from Magento 2 are larger than the default limits set in Nginx.

To fix this issue, you can try the following steps:

1. Increase Buffer and Header Size Limits:

   In your Nginx configuration file, you can increase the buffer and header size limits to accommodate larger headers. This can be done by adding or modifying the following lines in your Nginx configuration file (nginx.conf or the server block configuration):

   http {
       ...
       fastcgi_buffers 16 256k;
       fastcgi_buffer_size 256k;
       ...
       proxy_buffer_size          128k;
    	proxy_buffers              4 256k;
       proxy_busy_buffers_size    256k;
   }
   

   Adjust the buffer and buffer size values as needed based on your server’s requirements.
   The proxy parameters are useful when proxy_pass being used, eg: Varnish with ssl termination

2. Check Your Magento 2 Configuration:

   Review your Magento 2 store’s configuration settings to ensure that there are no unnecessary or excessively long headers being generated. For example, consider optimizing your theme, extensions, or any custom code that might be adding additional headers.

3. Reduce the Number of Enabled Modules and Extensions:

   Disable any unnecessary or unused modules and extensions in Magento 2. Some third-party extensions may add extra data to response headers, contributing to the problem.

4. Check for Looping or Redirect Issues:

   Ensure that your site is not stuck in a redirect loop, as this can cause excessive headers to be generated. Review your server and application code to check for any unintended redirects.

5. Monitor and Optimize Database Queries:

   Slow database queries can contribute to larger response headers. Monitor your database performance and optimize any slow queries if necessary.

6. Caching:

   Implement full-page caching using solutions like Varnish or built-in Magento 2 caching to reduce the load on your server.

7. Use a Content Delivery Network (CDN):

   Offloading some of the static content to a CDN can help reduce the size of headers and improve performance.

8. Consider Load Balancing:

   If your site experiences heavy traffic, consider using load balancing to distribute the load across multiple servers.

9. Update Software:

   Ensure that your server software (Nginx, PHP, Magento 2) and any relevant extensions are up to date.

10. Logging and Monitoring:

    Implement logging and monitoring to identify the root cause of the issue and track any unusual behavior.

After making these changes, restart Nginx to apply the configuration, and then test your site to see if the issue is resolved. Keep in mind that you should make changes to your server’s configuration carefully and consider the potential impact on your site’s performance and security. It’s a good practice to back up your server configuration before making any changes.