Fixing ‘Upstream Sent Too Big Header’ Error in Nginx

If you’ve encountered the error message “upstream sent too big header while reading response header from upstream” in your Nginx server logs, you know how frustrating it can be. This error occurs when the response headers from an upstream server are too large for Nginx to handle. In this blog post, we’ll explore why this happens and how you can resolve it to keep your server running smoothly.

Understanding the Error – “Upstream Sent Too Big Header’ Error in Nginx”

Nginx often acts as a reverse proxy, meaning it forwards client requests to another server (the upstream server) and then sends the response back to the client. This error indicates that the headers sent back by the upstream server are too large for Nginx to process within its default buffer limits.

Common causes for this issue include:

• Large Cookies: Cookies set by the upstream server can sometimes grow excessively large.
• Numerous Headers: Having too many headers in the response can lead to this problem.
• Excessively Large Headers: Individual headers that are too large can also trigger this error.

Solution 1: Increase Header Buffer Size in Nginx

To handle larger headers, you can adjust the buffer size settings in your Nginx configuration.

Step-by-Step Guide:

1. Locate Your Nginx Configuration File:

The file is usually found at /etc/nginx/nginx.conf or /etc/nginx/conf.d/default.conf.

2. Modify the Configuration:

Add or update the following directives to increase buffer sizes:

http {
    # Other settings...
 
    server {
        # Other settings...
 
        # Increase buffer sizes
        proxy_buffer_size          128k;
        proxy_buffers       4         256k;
        proxy_busy_buffers_size    256k;
        large_client_header_buffers 4       16k;
    }
}

3. Reload or Restart Nginx:

Apply the changes by reloading or restarting Nginx:

sudo systemctl reload nginx 
      or
sudo systemctl restart nginx

Solution 2: Optimize Upstream Server Response Headers

Ensure that the upstream server is not sending excessively large headers. Here are some tips:

1. Reduce Cookie Sizes:

Check if cookies set by the server are unnecessarily large. Optimize or remove them if possible.

2. Minimize Headers:

Remove any unnecessary headers from the response

3. Keep Custom Headers Concise:

If you’re adding custom headers, ensure they are as small as possible.

Example Scenario:

If your application is setting large cookies for tracking purposes, consider reducing the amount of data stored in each cookie or using alternative storage methods.

Solution 3: Debugging and Detailed Logging

Detailed logging can help you identify which headers are causing the issue.

Enable Detailed Logging in Nginx:

1. Modify the Log Format and Enable Debug Logging:

Update your Nginx configuration to include detailed logging:

http {
    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for"';
 
    access_log /var/log/nginx/access.log main;
    error_log /var/log/nginx/error.log debug;
}

2. Reload or Restart Nginx:

Apply the logging changes:

sudo systemctl reload nginx
      or
sudo systemctl restart nginx

3. Analyze Logs:

Use the logs to identify large headers and adjust the upstream server configuration accordingly.

Conclusion

By increasing the buffer sizes in Nginx and optimizing the headers sent by your upstream server, you can resolve the “upstream sent too big header” error effectively. Additionally, enabling detailed logging helps in diagnosing and fixing the specific causes of large headers. Proactively monitoring and optimizing your server configurations ensures smooth operation and better performance.

Feel free to share your experiences or ask questions in the comments section below. Let’s keep the conversation going and help each other troubleshoot these common issues!