How To Troubleshoot Common HTTP Error Codes
Introduction
When accessing a web server or application, every HTTP request that is received by a server is responded to with an HTTP status code. HTTP status codes are three-digit codes, and are grouped into five different classes. The class of a status code can be quickly identified by its first digit:
1xx: Informational
2xx: Success
3xx: Redirection
4xx: Client Error
5xx: Server Error
400 Bad Request
The 400 status code, or Bad Request error, means the HTTP request that was sent to the server has invalid syntax.
Here are a few examples of when a 400 Bad Request error might occur:
The user's cookie that is associated with the site is corrupt. Clearing the browser's cache and cookies could solve this issue
Malformed request due to a faulty browser
Malformed request due to human error when manually forming HTTP requests (e.g. using curl incorrectly)
401 Unauthorized
The 401 status code, or an Unauthorized error, means that the user trying to access the resource has not been authenticated or has not been authenticated correctly. This means that the user must provide credentials to be able to view the protected resource.
An example scenario where a 401 Unauthorized error would be returned is if a user tries to access a resource that is protected by HTTP authentication, as in this Nginx tutorial. In this case, the user will receive a 401 response code until they provide a valid username and password (one that exists in the .htpasswd file) to the web server.
403 Forbidden
The 403 status code, or a Forbidden error, means that the user made a valid request but the server is refusing to serve the request, due to a lack of permission to access the requested resource. If you are encountering a 403 error unexpectedly, there are a few typical causes that are explained here.
File Permissions
403 errors commonly occur when the user that is running the web server process does not have sufficient permissions to read the file that is being accessed.
To give an example of troubleshooting a 403 error, assume the following situation:
The user is trying to access the web server's index file, from http://example.com/index.html
The web server worker process is owned by the www-data user
On the server, the index file is located at /usr/share/nginx/html/index.html
If the user is getting a 403 Forbidden error, ensure that the www-data user has sufficient permissions to read the file. Typically, this means that the other permissions of the file should be set to read. There are several ways to ensure this, but the following command will work in this case:
sudo chmod o=r /usr/share/nginx/html/index.html
Index File Does Not Exist
If the user is trying to access a directory that does not have a default index file, and directory listings are not enabled, the web server will return a 403 Forbidden error. For example, if the user is trying to access http://example.com/emptydir/, and there is no index file in the emptydir directory on the server, a 403 status will be returned.
If you want directory listings to be enabled, you may do so in your web server configuration.
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