When it comes to troubleshooting web server issues, one of the most common errors encountered is the dreaded "404 not found" error. This occurs when a user attempts to access a resource from a web server, such as a web page or image, but the server is unable to locate the requested item. In this post, I will discuss how to troubleshoot and resolve such errors on Apache2 webservers installed on Debian systems.
When encountering a "404 not found" error, the first step to troubleshooting the issue is to check the Apache2 error log. This log contains detailed information regarding any errors that occurred during the request process. On a Debian system, the Apache2 error log can typically be found at "/var/log/apache2/error.log". Upon opening the log, you should search for entries related to the request that caused the 404 error. Such entries will include the requested resource name, along with the corresponding HTTP status code (in this case, 404) and details of why the server was unable to locate the file.
Once you have identified the exact cause of the "404 not found" error, it is then a matter of correcting the issue. Here, the cause could be almost anything, from targeting a file or directory that does not exist, to an incorrect configuration within the server itself. Common examples of such issues include problems with the file permissions, incorrectly configured Rewrite rules, or missing/invalid symbolic links.
For instance, if the 404 error occurs when attempting to request a file, you can begin by ensuring that the file exists in the correct location, and that the correct permissions are applied to it. Then, you can verify that the file is valid by using the "file" command. This will provide details such as the content type of the file, along with the size and checksum. If any of these details are incorrect, then the file may be corrupted or otherwise invalid.
In the case of a Rewrite rule issue, you can review the server configuration for any errors or typos. On a Debian system, such rules can typically be found in the "/etc/apache2/sites-enabled/" directory. The content of the configuration files should be validated to ensure that the Rewrite rules are correctly written for each request.
Lastly, if the 404 errors are occurring due to a missing/invalid symbolic link, you should check the link path to verify that it is pointing to an existing resource. If necessary, you can also check to make sure that the link is correctly configured within the server configuration file located in the "/etc/apache2/sites-enabled/" directory.
In conclusion, the "404 not found" error on Apache2 webservers in Debian systems can be caused by numerous issues, ranging from file permission errors to invalid/corrupted files, or incorrect configuration settings. By following the troubleshooting steps outlined above, you can discover the exact cause of the issue, and then take the required steps to resolve it.