Upstream Sent Too Big Header While Reading Response Header From Upstream

The error upstream sent too big header while reading response header from upstream in Nginx indicates that the upstream server (such as a backend application server or FastCGI server) is sending response headers that exceed the maximum allowed size configured in Nginx. This can occur if the upstream server is including large headers, such as cookies or custom headers.

To resolve this issue, you need to adjust the Nginx configuration to increase the maximum allowed size for headers. Here’s a step-by-step guide on how to address this issue:

1. Increase the proxy_buffers and proxy_buffer_size Settings

If you are using Nginx as a reverse proxy, you can increase the buffer sizes in the Nginx configuration.

Configuration Steps:

1. Open the Nginx Configuration File:

   This could be in /etc/nginx/nginx.conf, or in a site-specific configuration file under /etc/nginx/sites-available/ or /etc/nginx/conf.d/.

    

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   sudo nano /etc/nginx/nginx.conf
   

2. Add or Update the http Block:

   Increase the proxy_buffer_size and proxy_buffers values. These directives control the size of buffers used to read the response from the upstream server.

    

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   http {
       # Increase buffer size
       proxy_buffer_size 128k;
       proxy_buffers 8 128k;
       proxy_busy_buffers_size 256k;
   
       # Other settings...
   }
   
   

   Adjust the buffer sizes according to your needs. The proxy_buffer_size sets the size of the buffer for reading the response header, while proxy_buffers sets the number and size of the buffers used for reading the response body.

3. Save and Close the File.

4. Test the Configuration:

    

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   sudo nginx -t
   

   Ensure there are no syntax errors in the configuration.

5. Reload Nginx:

    

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   sudo systemctl reload nginx
   

   This applies the new configuration.

2. Increase the fastcgi_buffers and fastcgi_buffer_size Settings

If you are using FastCGI with Nginx, you can increase the buffer sizes related to FastCGI.

Configuration Steps:

1. Open the Nginx Configuration File:

    

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   sudo nano /etc/nginx/nginx.conf
   

2. Add or Update the http Block:

   Increase the fastcgi_buffer_size and fastcgi_buffers values.

    

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   http {
       # Increase FastCGI buffer size
       fastcgi_buffer_size 128k;
       fastcgi_buffers 8 128k;
   
       # Other settings...
   }
   
   

3. Save and Close the File.

4. Test and Reload Nginx:

    

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   sudo nginx -t
   sudo systemctl reload nginx
   

3. Adjust client_header_buffer_size

If the issue is related to large client headers, you may need to adjust the client_header_buffer_size directive.

Configuration Steps:

1. Open the Nginx Configuration File:

    

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   sudo nano /etc/nginx/nginx.conf
   

2. Add or Update the http Block:

    

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   http {
       # Increase client header buffer size
       client_header_buffer_size 1k;
       large_client_header_buffers 4 4k;
   
       # Other settings...
   }
   
   

   Adjust client_header_buffer_size and large_client_header_buffers based on the header sizes you expect.

3. Save and Close the File.

4. Test and Reload Nginx:

    

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   sudo nginx -t
   sudo systemctl reload nginx
   

4. Check Upstream Server Configuration

Sometimes the upstream server configuration itself may be the cause of oversized headers. Check the configuration of your upstream application or server to ensure it is not sending unnecessarily large headers.

Summary

• For Reverse Proxy Configurations:
  • Increase proxy_buffer_size, proxy_buffers, and proxy_busy_buffers_size.
• For FastCGI Configurations:
  • Increase fastcgi_buffer_size and fastcgi_buffers.
• For Large Client Headers:
  • Adjust client_header_buffer_size and large_client_header_buffers.
• Review and Adjust Upstream Server:
  • Ensure the upstream server is configured correctly and is not sending excessively large headers.

By adjusting these settings, you should be able to resolve the upstream sent too big header while reading response header from upstream error and handle large headers from your upstream server.

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