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HTTP Status Code
401 - Unauthorized

4xx Client errors

The HTTP Status Code 401 is similar to 400 except that it is specifically used when required authentication has failed or has not yet been provided by the client.

What Is HTTP Status Code 401?

HTTP Status Code 401, also known as an 'unauthorized error', is a common response code that tells the client it needs to authenticate itself to gain access to the requested resource. This client can range from a typical web browser to an API making a request to a web server.

Whenever a web server gets a request for a resource, it checks for proper authentication credentials within the request header. If these credentials are missing or insufficient, the server responds with a status code of 401. This code essentially instructs the client to provide valid authentication credentials.

This HTTP error is strongly related to web security. It employs the "WWW-Authenticate" header field, or the authenticate header field, as a way to tell the client that it needs to authenticate.

Keep in mind, HTTP status code 401 is unique from other HTTP status codes such as 400 (Bad Request), 403 (Forbidden), and 500 (Internal Server Error). Each of these status codes represents a separate kind of error or issue with the request or server.

When and How Is HTTP Status Code 401 Used?

There are a number of scenarios where HTTP Status Code 401 comes into play. One common instance is when a client, like a web browser or an API, sends a request to a web server but neglects to provide valid authentication credentials. This scenario might arise when you try to access a webpage or an API key without supplying the right credentials.

Server Response to HTTP Status Code 401

When the server gets a request without valid credentials, it replies with an unauthorized error message, urging you to provide the correct credentials. But how does the server verify whether the authentication credentials are valid? It checks the 'Authorization' header field in the request header sent by the client. If this field lacks valid credentials, the server responds with HTTP status code 401.

Client Reaction to HTTP Status Code 401

If a client gets a 401 status code, it should reply by sending a new request with an 'Authorization' header field that includes valid authentication credentials. The specifics included in the 'WWW-Authenticate' header field in the 401 response can be used to figure out what type of authentication is required.

Dealing with HTTP Status Code 401 Issues

Occasionally, you might run into this error status for reasons not directly related to authentication credentials. Hitting the concurrent request rate limit, triggering a service unavailable response, or having problems with the browser cache can all lead to a 401 error.

The network tab in the browser's developer tools can be invaluable when dealing with HTTP error codes like 401. This tab can provide important diagnostic details like the exact request and response headers, the raw response data from the server, and the timeline of the request. This can shed light on why the server responded with a 401 status code and how to fix the issue.

Example Usage of HTTP Status Code 401

A Simplified Explanation

The internet allows us to interact with websites through requests. These requests are similar to knocking on a door and asking for information. In some cases, the website will require you to prove your identity before it divulges any data. This identification process is known as "authentication".

Consider trying to gain access to a members-only club. The security guard at the entrance verifies if you're a member by requesting your membership card. If you can't provide a valid card, you're denied entry. In the same way, when your browser sends a request to a website requiring authentication and you haven't offered valid credentials, the website responds with a 401 Unauthorized status code. It's as if the website is saying, "I'm sorry, but I can't let you in without proper identification."

A Technical Perspective

The HTTP 401 status code is typically returned in an HTTP transaction when the client hasn't provided valid authentication credentials for the resource requested. Here's a simple example using Python and the requests library:

import requests
response = requests.get('https://example.com/secure', auth=('user', 'wrong_password'))

In this script, we're trying to access a secure resource at `https://example.com/secure` with incorrect authentication details. Consequently, the server returns a 401 status code, indicating that the request is unauthorized.

The History of HTTP Status Code 401

The Birth of HTTP and the 401 Status Code

HTTP 401 status code has been a part of the HTTP standard since its inception. HTTP, or Hypertext Transfer Protocol, was first proposed by Tim Berners-Lee, the inventor of the World Wide Web, in 1989. It was officially adopted as a standard in 1991. The 401 Unauthorized status code is part of the HTTP/1.0 specification, which was released in 1996.

The Objective of the 401 Status Code

The goal of this status code has consistently been to indicate that the client needs to authenticate itself to receive the requested response. While the technology and protocols for authentication have progressed over time, the basic function of the 401 status code has stayed consistent.

How Does HTTP Status Code 401 Relate to Other Status Codes?

Its Relation with Other 4xx Status Codes

The 401 status code falls into the category of HTTP status codes that signal a client-side error. It shares similarities with other 4xx status codes like the 403 Forbidden, 404 Not Found, 405 Method Not Allowed, and 407 Proxy-Authentication Required status codes.

Distinguishing the 401 Status Code

The 401 Unauthorized status code is returned when the server requires the client to authenticate, whereas the 403 Forbidden status code is returned when the server declines to fulfill the request, despite knowing the identity of the client. This typically means the client lacks the necessary permissions to access the resource.

The 404 Not Found status code is returned when the server can't find the requested resource. It's equivalent to telling the client, "I can't locate what you're searching for."

The 405 Method Not Allowed status code is returned when the server understands the request method but doesn't support it for the targeted resource.

Lastly, the 407 Proxy-Authentication Required status code resembles 401 Unauthorized, but it's returned when the client must authenticate itself with a proxy.

Other Interesting Things About HTTP Status Code 401

The HTTP 401 status code signifies an unauthorized request. This occurs when a client fails to provide valid authentication credentials for the requested resource. This code is generally used with the POST and GET HTTP methods, where the POST method can change the server's state, and the GET method is used to retrieve data from a resource.

In the context of HTTP response headers, the HTTP 401 status code is closely associated with the HTTP Authentication and WWW-Authenticate HTTP Header. These headers handle and authenticate access, ensuring only authorized clients can access the requested resources.

The HTTP 401 status code also has interesting connections with other status codes. It shares similarities with codes like 403, 404, 405, and 407, all of which indicate different types of client error responses.

When you face HTTP 401 status code errors, you have several possible solutions. You might check the URL for errors, clear your browser cookies, or refresh the website and attempt to log in again. If these attempts fail, you might consider contacting the website's webmaster.

To truly comprehend the complexities of the HTTP 401 status code and its application, it's recommended to further explore HTTP methods, HTTP response headers, and similar HTTP status codes. This will give you a more detailed understanding of the code and its role within the broader HTTP protocol.

Additional Links and Codes Related to Status Code 401

Written by Benjamin Bouchet.
Last updated May 31, 2023.