How to Fix "upstream sent too big header while reading response header from upstream" Error

Insufficient Buffer Size in Nginx Config

Nginx has default buffer sizes that may not be enough for all situations. The standard settings work for typical websites, but can be too small for complex applications. When the buffer size is too small, Nginx can't handle large headers, causing the "upstream sent too big header" error.

Buffer size limits can trigger this error when:

• The upstream server sends headers that exceed the buffer size.
• Multiple headers combine to surpass the total buffer capacity.
• Complex applications generate headers larger than the default Nginx settings can handle.

Large Response Headers from Upstream Servers

PHP or FastCGI applications can generate large headers, which can cause problems for Nginx. These applications might create large session data or set many cookies, resulting in headers that are too big for Nginx to process with its default settings.

Cookies and session data affect header size:

• Cookies store user preferences and login information, which can grow over time.
• Session data may contain large amounts of information about the user's current browsing session.
• Some applications use headers to pass data between different parts of the system, increasing header size.

When these factors combine, they can create headers that exceed Nginx's buffer capacity, triggering the "upstream sent too big header" error.

To fix this issue, you may need to increase the buffer size in your Nginx config. You can do this by editing the fastcgi_buffers directive or adjusting the proxy_buffer_size and proxy_buffers settings. After making changes, remember to reload Nginx for the new settings to take effect.

Increasing Nginx Buffer Sizes

To fix the "upstream sent too big header" error, you can increase the Nginx buffer sizes. This involves changing the proxy_buffer_size and related directives in your Nginx config file.

To adjust the proxy buffer size, add or change these lines in your Nginx config:

proxy_buffer_size 16k;
proxy_buffers 4 16k;
proxy_busy_buffers_size 32k;

For different cases, consider these buffer size settings:

• Small to medium websites: 16k buffer size
• Large applications: 32k or 64k buffer size
• Very large applications with many headers: 128k buffer size

Test different settings to find the best balance for your case.

Adjusting FastCGI Buffer Parameters

If you use FastCGI, you need to set the fastcgi_buffers and fastcgi_buffer_size parameters. Add or change these lines in your Nginx config:

fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;

These settings allow for bigger headers while keeping good performance. You can change these values based on your needs, but be careful of memory use. Larger buffer sizes can help applications with big headers, but they also use more server memory.

Optimizing Upstream Server Responses

To prevent the "upstream sent too big header" error, you can also improve the responses from your upstream servers:

1. Reduce header size in PHP or other backend applications:

   • Remove any unneeded header information.
   • Use compression for large data sets in headers.

2. Minimize cookies and session data:

   • Remove old or unused cookies.
   • Store large session data on the server instead of in cookies.
   • Use smaller session IDs.

Fine-tuning Proxy Pass Settings

The proxy_pass directive is important in Nginx proxy configurations. Use it to set the protocol and address of a proxied server:

proxy_pass http://backend;

This directive sends requests to the backend server. Define the backend server in an upstream block for load balancing and failover support.

Using try_files can improve request handling:

try_files $uri $uri/ @backend;

location @backend {
    proxy_pass http://backend;
}

This checks if the requested file exists locally before sending the request to the backend server, reducing load on the upstream server.

Implementing Reverse Proxy Best Practices

Set client_max_body_size to control the maximum size of client request bodies. This helps with large file uploads:

client_max_body_size 10M;

Adjust this value based on your app's needs, but don't set it too high to avoid potential issues.

The proxy_busy_buffers_size directive controls how much data can be buffered when sending to the client:

proxy_busy_buffers_size 64k;

This setting balances memory usage and performance. A larger value can improve performance for clients with slow connections but uses more memory.