400 Bad Request - request header or cookie too large

Learn 400 bad request - request header or cookie too large with practical examples, diagrams, and best practices. Covers ruby-on-rails, nginx development techniques with visual explanations.
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Resolving '400 Bad Request: Request Header or Cookie Too Large' in Rails with Nginx

Resolving '400 Bad Request: Request Header or Cookie Too Large' in Rails with Nginx

Learn how to diagnose and fix the '400 Bad Request' error caused by oversized request headers or cookies when running Ruby on Rails applications behind Nginx.

The '400 Bad Request - Request header or cookie too large' error is a common issue encountered by web applications, particularly those with complex authentication mechanisms or extensive session management. This error indicates that the HTTP request sent by the client includes headers or cookies exceeding the server's configured limits. In a typical Ruby on Rails application deployed with Nginx as a reverse proxy, this problem often stems from Nginx's default buffer sizes being too small to accommodate large cookies or numerous headers generated by the Rails application.

Understanding the Cause: Nginx Buffer Limits

Nginx, by default, imposes limits on the size of request headers and buffers to prevent denial-of-service attacks and conserve memory. When a client sends a request, Nginx reads the request header into a set of buffers. If the combined size of the headers (including cookies) exceeds these configured limits, Nginx responds with a '400 Bad Request' error before the request even reaches the Rails application. Common culprits for large headers include:

  • Large Session Cookies: Rails applications, especially those using cookie-based sessions, can generate large session cookies, particularly if a lot of data is stored in the session.
  • Numerous Cookies: A high number of individual cookies, even if each is small, can collectively exceed the header limit.
  • OAuth/SSO Tokens: Authentication tokens from Single Sign-On (SSO) or OAuth providers can sometimes be quite large.
  • Custom Headers: Applications or proxies adding many custom headers.

A diagram illustrating the request flow from client to Rails application via Nginx. Client sends Request (large headers) -> Nginx (Buffer Limits) -> Nginx returns '400 Bad Request' before reaching -> Rails Application. Use blue boxes for Client, Nginx, and Rails App, a red X icon over the Nginx to Rails arrow to signify failure, and a red box for '400 Bad Request'. Arrows show the flow.

Request flow with Nginx buffer limits

Identifying the Problematic Headers/Cookies

Before making changes, it's helpful to identify which specific headers or cookies are contributing to the problem. You can do this by inspecting the network requests in your browser's developer tools. Look at the 'Request Headers' section for the failing request. Pay close attention to the Cookie header and any other custom headers your application might be sending. If you have access to Nginx logs, you might also find clues there, though the '400 Bad Request' usually prevents the request from being fully logged by the application itself.

Solution: Adjusting Nginx Configuration

The most direct solution is to increase the Nginx buffer sizes. This involves modifying your Nginx configuration file, typically located at /etc/nginx/nginx.conf or within your site's specific configuration in /etc/nginx/sites-available/. You'll need to add or adjust the large_client_header_buffers directive.

This directive takes two arguments: the number of buffers and the size of each buffer. For example, 4 16k means Nginx will allocate 4 buffers, each 16 kilobytes in size, for reading large client headers. The total buffer size would be 4 * 16k = 64k.

http {
    # ... other http directives ...

    client_header_buffer_size 8k;
    large_client_header_buffers 4 32k;

    # ... other http directives ...
}

Example Nginx configuration for increased header buffers

In the example above:

  • client_header_buffer_size 8k; sets the buffer size for smaller client request headers. This is often sufficient for most requests.
  • large_client_header_buffers 4 32k; is the key directive. It tells Nginx to allocate four buffers, each 32 kilobytes in size, for large client request headers. This provides a total of 128 KB for headers. You may need to experiment with these values based on the size of your headers.

These directives should be placed within the http {} block of your Nginx configuration. After making changes, always test the configuration for syntax errors and restart Nginx.

1. Step 1

Open your Nginx configuration file. This is typically /etc/nginx/nginx.conf or a site-specific file in /etc/nginx/sites-available/your_site.

2. Step 2

Locate the http {} block. If it doesn't exist, create it.

3. Step 3

Add or modify the client_header_buffer_size and large_client_header_buffers directives within the http {} block. Start with values like client_header_buffer_size 8k; and large_client_header_buffers 4 32k;.

4. Step 4

Save the configuration file.

5. Step 5

Test the Nginx configuration for syntax errors using the command: sudo nginx -t.

6. Step 6

If the test is successful, reload or restart Nginx to apply the changes: sudo systemctl reload nginx or sudo systemctl restart nginx.

7. Step 7

Clear your browser's cookies and cache for the affected site, then try accessing the application again.

While increasing buffer sizes is a quick fix, it's also worth considering whether your application truly needs such large headers or cookies. Reviewing your application's session management, cookie usage, and authentication mechanisms for efficiency can prevent similar issues in the future and improve overall performance.