Configuring underscores_in_headers in NGINX

NGINX is a high-performance web server and reverse proxy server widely used for hosting websites, web applications, and services. One of the configuration options in NGINX is underscores_in_headers, which controls how underscores in HTTP header names are treated.

This guide explains the purpose, use cases, and configuration of the underscores_in_headers directive in the NGINX configuration file.

Overview of underscores_in_headers

The underscores_in_headers directive in NGINX is used to allow or disallow underscores (_) in HTTP header names. By default, NGINX does not allow headers with underscores, as they are considered non-standard and may lead to security vulnerabilities or compatibility issues with certain HTTP standards and applications.

Directive:

underscores_in_headers on;

• on: Allows underscores in HTTP header names.
• off: (Default) Disallows underscores in HTTP header names.

Why Use underscores_in_headers?

1. Compatibility with Custom Headers:

   • Some applications or third-party services use custom HTTP headers with underscores. Enabling underscores_in_headers allows NGINX to accept and correctly handle these custom headers.

2. Avoid Header Rejection:

   • If your application or service relies on headers with underscores (e.g., X_Custom_Header), setting this directive to on prevents NGINX from rejecting such headers, ensuring smooth operation and avoiding potential errors.

3. API Development:

   • During API development, you may encounter cases where headers with underscores are used. Enabling underscores helps in testing and development scenarios where specific headers are critical.

Potential Risks

While enabling underscores in headers can provide flexibility, it's important to note some potential risks:

• Security Concerns:

  • HTTP headers with underscores may be less secure and could potentially introduce vulnerabilities, especially if headers are dynamically generated or manipulated by attackers.

• Standard Compliance:

  • HTTP headers with underscores are not strictly compliant with HTTP standards (RFC 7230), which specify the use of hyphens (-) instead. Enabling underscores may cause interoperability issues with clients or upstream servers that enforce strict standards.

Configuration Example

To enable underscores in HTTP headers, add or modify the underscores_in_headers directive in your NGINX configuration file (nginx.conf). Typically, this directive is placed within the http block or a specific server block if you want to apply it to a particular server context.

http {
    ...
 
    # Allow underscores in HTTP header names
    underscores_in_headers on;
 
    ...
}

Alternatively, if you want to restrict this setting to a particular server context, you can place it within the server block:

server {
    listen 80;
    server_name example.com;
 
    # Allow underscores in HTTP header names for this server only
    underscores_in_headers on;
 
    location / {
        ...
    }
}

Reloading NGINX

After making the configuration changes, reload NGINX to apply the new settings:

sudo systemctl reload nginx

Or, if not using systemctl:

sudo nginx -s reload

Use Case

Enabling underscores_in_headers is useful in scenarios where:

• You are integrating with external services or APIs that use headers with underscores.
• Your application or client expects or relies on headers with underscores.
• You are in a development or testing environment where non-standard headers are being used.

However, it is recommended to review your use case carefully and assess whether allowing underscores is necessary or if alternative solutions (e.g., using hyphens in headers) could be implemented to maintain compliance with HTTP standards.

Conclusion

The underscores_in_headers directive provides flexibility for handling non-standard HTTP headers in NGINX. While it can be useful for specific scenarios, consider the potential security implications and compatibility issues before enabling this setting in production environments.

This documentation should help you understand and configure the underscores_in_headers directive in NGINX, providing better control over HTTP header handling based on your specific requirements.