Problem: HTTP Headers with Underscores Dropped by Nginx
Nginx, a web server and reverse proxy, drops HTTP headers with underscores by default. This can cause issues when using custom headers or third-party services that use headers with underscores, leading to data loss or problems in web applications.
Nginx's Default Handling of Underscore Headers
The Silent Drop: Nginx's Default Behavior
Nginx drops headers with underscores by default. This happens without any warning or error message. When Nginx gets a request with headers containing underscores, it removes these headers before sending the request to the backend server or application. This removal can cause issues in web applications that use these custom headers for functionality or data transfer.
The hidden nature of this process makes it hard for developers and system administrators to find the cause of problems related to missing header data. Users might spend time debugging their applications, not knowing that Nginx is removing their custom headers. This default behavior can be a problem when working with third-party services or APIs that use headers with underscores, as it can disrupt the flow of information between systems.
Tip: Check Nginx Configuration
To verify if Nginx is dropping headers with underscores, you can add a custom header with an underscore in your client request and then use a backend logging mechanism or debugging tool to check if the header reaches your application. If the header is missing, it's likely being dropped by Nginx.
The Root Cause: CGI Legacy and Header Mapping
Historical Context: CGI Variables and Header Conversion
Nginx's behavior of dropping headers with underscores stems from Common Gateway Interface (CGI) legacy. CGI is an old standard for web servers to run external programs and return their output to web clients. It influenced how web servers handle HTTP headers.
In CGI, HTTP headers are converted to environment variables for scripts to use. During this conversion, dashes and underscores in header names are changed to underscores. For example, a header like "Custom-Header" becomes "HTTP_CUSTOM_HEADER" as a CGI environment variable. This process can create confusion when headers already have underscores.
To avoid these ambiguities, Nginx drops headers with underscores by default. This decision prevents issues where different headers might map to the same CGI variable name. For example, "Custom_Header" and "Custom-Header" would both become "HTTP_CUSTOM_HEADER" in CGI, making it unclear which original header the variable represents.
While this approach solves the ambiguity problem, it can cause unexpected behavior in modern web applications that don't use CGI and rely on custom headers with underscores. Understanding this historical context helps explain why Nginx handles underscore-containing headers this way, even though it may not always be the desired behavior in current web development practices.
Tip: Handling Underscore Headers in Nginx
To allow headers with underscores in Nginx, you can use the underscores_in_headers on; directive in your Nginx configuration. Add this line to your server or location block to enable the processing of headers containing underscores.
HTTP Standards and Header Naming Conventions
Clarifying the HTTP RFC
The HTTP RFC (Request for Comments) allows the use of underscores in header names. This is often misunderstood by developers and system administrators. The HTTP specification permits many characters in header names, including underscores.
The confusion often comes from misinterpreting the ASCII character rules in the RFC. The HTTP/1.1 specification (RFC 2616) refers to RFC 822 for header field formats. RFC 822 states that field names must use printable ASCII characters, which include characters with decimal values between 33 and 126, except for the colon. The underscore character (decimal value 95) is within this allowed range.
Some developers think underscores are not allowed because certain web servers, like Apache or Nginx, treat them as invalid by default. However, this is a server-specific choice, not a requirement of the HTTP standard.
While underscores are allowed in HTTP header names according to the RFC, using them may cause issues with some servers or proxies that aren't set up to handle them. For this reason, it's often better to use hyphens instead of underscores in custom header names to avoid potential problems.
Tip: Header Naming Best Practices
When creating custom HTTP headers, use hyphens instead of underscores to improve compatibility across different servers and systems. For example, use "Custom-Header" rather than "Custom_Header".
Example: HTTP Header Compatibility
Consider a scenario where you're developing an API that uses custom headers to transmit additional information. You might be tempted to use a header like "API_Version" to indicate the version of your API. However, to maximize compatibility, it's better to use "API-Version" instead. This ensures your header will be correctly processed by a wider range of servers and intermediaries.
Solving the Underscore Header Issue in Nginx
Enabling Underscore Headers: The Configuration Solution
To solve the issue of Nginx dropping headers with underscores, you can use the 'underscores_in_headers on;' directive in your Nginx configuration. This directive tells Nginx to accept and pass through headers that contain underscores.
To implement this solution, add the following line to your Nginx configuration:
underscores_in_headers on;You can place this directive in different locations within your Nginx configuration file, depending on how broadly you want to apply it:
- Server block: If you want to enable underscore headers for a specific server, add the directive inside the server block. For example:
server {
listen 80;
server_name example.com;
underscores_in_headers on;
# Other server configurations...
}- HTTP block: To enable underscore headers for all servers, place the directive in the http block of your Nginx configuration:
http {
underscores_in_headers on;
# Other http configurations...
}- Location block: If you only need to allow underscore headers for specific locations, add the directive to the relevant location block:
location /api {
underscores_in_headers on;
# Other location configurations...
}After adding the directive, restart Nginx to apply the changes. This will allow Nginx to process and pass through headers containing underscores, solving the issue of missing custom headers in your applications.
Remember to test your configuration after making these changes to make sure everything works as expected.
Tip: Verify Underscore Headers
After enabling underscore headers in Nginx, you can verify if they're being passed through correctly by using a tool like cURL. Here's an example command:
curl -I -H "X_Custom_Header: TestValue" http://your-domain.comThis command sends a request with a custom header containing an underscore. Check the response headers to confirm that your custom header is present and has not been dropped by Nginx.
