Allow

In HTTP (HyperText Transfer Protocol), the Allow header is an essential element, especially when considering the role it plays in conveying the methods supported by a specific resource. In this article, we’ll dissect this header, understand its usage, syntax, and implications on HTTP communication.

What is the ‘Allow’ Header?

The Allow header is an entity header that is used to list the set of HTTP request methods supported by a resource. The methods are sent as part of the HTTP response, indicating to the client what methods are permissible for the given URL.

Syntax and Usage

The Allow header’s value is a comma-separated list of HTTP methods that are permitted on the resource. Its basic syntax is as follows:

Allow: <method>, <method>, ...

Where <method> corresponds to any standard HTTP method, such as GET, POST, DELETE, PUT, OPTIONS, etc. Here is a sample Allow header included in an HTTP response:

HTTP/1.1 200 OK
Allow: GET, POST, HEAD
Content-Type: text/html; charset=UTF-8

In this instance, the Allow header communicates that the GET, POST, and HEAD methods are acceptable for the requested resource.

The Role in HTTP Communication

The Allow header is typically included in the response to an OPTIONS request, providing a list of methods that the client may use for subsequent requests to the resource.

Consider this HTTP request:

OPTIONS /index.html HTTP/1.1
Host: www.example.com

And the corresponding response:

HTTP/1.1 200 OK
Allow: GET, HEAD, POST
Content-Length: 0

In this case, the client has asked the server what methods are allowed for /index.html using an OPTIONS request. The server responded that GET, HEAD, and POST are all acceptable methods.

Furthermore, the Allow header is also returned with 405 (Method Not Allowed) and 501 (Not Implemented) status responses to indicate which methods are supported.

Summary

In the vast landscape of HTTP, the Allow header represents an essential part of resource manipulation and method control. By providing a list of acceptable methods for a resource, it promotes clear communication between the client and server, enabling more robust and flexible web applications.

While it might seem insignificant at first, understanding and correctly implementing the Allow header is crucial for effective client-server communication. It ensures clients interact with resources appropriately, thereby reducing errors and enhancing the overall experience on the web.

So, the next time you work on setting server responses, remember the utility of the Allow header and its role in defining resource interactions.