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Friday, 24 January 2020

404 Not Found Error: What It Is and How to Fix It

The 404 Not Found Error is an HTTP response status code, which indicates that the requested resource could not be found. Like most HTTP response codes, and particularly for those codes that indicate an error, the cause of a 404 Not Found Error can be difficult to track down and resolve. There are well over 50 potential status codes that represent the complex relationship between the client, a web application, a web server, and often multiple third-party web services, so determining the cause of a particular status code can be a challenge under the best of circumstances.
In this article, we’ll explore the 404 Not Found Error by examining what might cause this error, while providing a few tips and tricks to help you diagnose and debug your own application that is experiencing such issues. We’ll also look at a handful of the most common content management systems (CMSs) that are in use today, giving you some insight into potential problem areas within these systems that might cause an unexpected 404 Not Found Error, so let’s get started!

404 Not Found Error: What It Is and How to Fix It

Server- or Client-Side?

All HTTP response status codes that are in the 4xx category are considered client error responses. These types of messages contrast with errors in the 5xx category, such as the 502 Bad Gateway Error we looked at last week, which are considered server error responses. That said, the appearance of a 4xx error doesn’t necessarily mean the issue is something to do with the client, where the client is the web browser or device being used to access the application. Oftentimes, if you’re trying to diagnose an issue with your own application, you can immediately ignore most client-side code and components, such as HTML, cascading style sheets (CSS), client-side JavaScript, and so forth. This doesn’t apply solely to web sites, either. Many smart phone apps that have a modern looking user interface are actually powered by a normal web application behind the scenes; one that is simply hidden from the user.
That said, since a 404 Not Found Error indicates that the resource that’s trying to be accessed is unavailable, it’s entirely possible the issue steps from the client. You may be trying to access an invalid URL, the browser could be sending invalid credentials to the application, and so on. We’ll explore some of these scenarios (and potential solutions) down below, but be aware that, even though the 404 Not Found Error is considered a client error response, it doesn’t inherently mean we can rule out either the client nor the server as the root of the problem. In these scenarios, the server is still the network object that is producing the 404 Not Found Error, and returning it as the HTTP response code to the client, but it could be that the client is causing the issue in some way.

Start With a Thorough Application Backup

As with anything, it’s better to have played it safe at the start than to screw something up and come to regret it later on down the road. As such, it is critical that you perform a full backup of your application, database, and so forth, before attempting any fixes or changes to the system. Even better, if you have the capability, create a complete copy of the application onto a secondary staging server that isn’t “live,” or isn’t otherwise active and available to the public. This will give you a clean testing ground with which to test all potential fixes to resolve the issue, without threatening the security or sanctity of your live application.

Diagnosing a 404 Not Found Error

As mentioned, a 404 Not Found Error indicates that the client (web browser) is receiving a message from the server (remote computer) that the specific resource (web page/URL) is unavailable. Such an error can occur in a few scenarios:
  • The client sent a proper request to the server, and the server successfully received said request, but the server was unable to find a valid resource at that particular location. This is typically due to an invalid URL specified by the client; one which the server does not recognize as valid for any number of reasons. This represents the majority of 404 Not Found Errors, and results in an actual 404 response code being sent by the server.
  • Alternatively, some web applications “fake” 404 Not Found Errors when an invalid resource is requested. In such cases, the server returns a standard 200 OK response code, which normally indicates the resource loaded as expected. However, the server then has a custom “404 page” that it displays, indicating to the user that the requested resource was, in fact, not found. However, since the server didn’t actually return with a 404 response code, such fake errors are typically referred to as soft 404 errors. This is generally considered bad practice on the part of the server/application, since many automated softwares (such as search engine web crawlers) rely on legitimate 404 Not Found Errors to determine if resources/links are valid.
In both cases, the provided URL could have been valid in the past, but the server has failed to provide a server-side redirect, which would typically be used to redirect a request made to an invalid or outdated resource to the new, intended target resource.

Troubleshooting on the Client-Side

Since the 404 Not Found Error is a client error response code, it’s best to start by troubleshooting any potential client-side issues that could be causing this error. Here are a handful of tips to try on the browser or device that is giving you problems.

Check the Requested URL

The most common cause of a 404 Not Found Error is simply inputting an incorrect URL. Domain names (e.g. airbrake.io) are case-insensitive, meaning that this mixed case link to AirBrAKe.IO works just as well as the normal, lowercase version of airbrake.io. However, path, query, or fragment portions that appear after the domain name, are quite often case-sensitive, unless the application/server the configuration is explicitly designed to pre-process all URLs as lowercase before execution.
For example, while airbrake.io can be upper, lower, or mixed case, a link to airbrake.io/BLOG/ (with BLOG in uppercase) is invalid, resulting in our good friend the 404 Not Found Error. Of course, the lowercase version to airbrake.io/blog/ works just fine, as expected.
This is all to say that it’s quite common to have a minor typo in some part of a URL, which often results in an unexpected 404 Not Found Error.

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