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 identified by its first digit:
This guide focuses on identifying and troubleshooting the most commonly encountered HTTP error codes, i.e. 4xx and 5xx status codes, from a system administrator’s perspective. There are many situations that could cause a web server to respond to a request with a particular error code – we will cover common potential causes and solutions.
Client errors, or HTTP status codes from 400 to 499, are the result of HTTP requests sent by a user client (i.e. a web browser or other HTTP client). Even though these types of errors are client-related, it is often useful to know which error code a user is encountering to determine if the potential issue can be fixed by server configuration.
Server errors, or HTTP status codes from 500 to 599, are returned by a web server when it is aware that an error has occurred or is otherwise not able to process the request.
error.logthat can be scanned for relevant information
Now that you have a high-level understanding of HTTP status codes, we will look at the commonly encountered errors.
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 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.
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.
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:
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
Another potential cause of 403 errors, often intentionally, is the use of an
.htaccess file. The
.htaccess file can be used to deny access of certain resources to specific IP addresses or ranges, for example.
If the user is unexpectedly getting a 403 Forbidden error, ensure that it is not being caused by your
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.
The 404 status code, or a Not Found error, means that the user is able to communicate with the server but it is unable to locate the requested file or resource.
404 errors can occur in a large variety of situations. If the user is unexpectedly receiving a 404 Not Found error, here are some questions to ask while troubleshooting:
The 500 status code, or Internal Server Error, means that server cannot process the request for an unknown reason. Sometimes this code will appear when more specific 5xx errors are more appropriate.
This most common cause for this error is server misconfiguration (e.g. a malformed
.htaccess file) or missing packages (e.g. trying to execute a PHP file without PHP installed properly).
The 502 status code, or Bad Gateway error, means that the server is a gateway or proxy server, and it is not receiving a valid response from the backend servers that should actually fulfill the request.
If the server in question is a reverse proxy server, such as a load balancer, here are a few things to check:
The 503 status code, or Service Unavailable error, means that the server is overloaded or under maintenance. This error implies that the service should become available at some point.
If the server is not under maintenance, this can indicate that the server does not have enough CPU or memory resources to handle all of the incoming requests, or that the web server needs to be configured to allow more users, threads, or processes.
The 504 status code, or Gateway Timeout error, means that the server is a gateway or proxy server, and it is not receiving a response from the backend servers within the allowed time period.
This typically occurs in the following situations:
Now that you are familiar with the most common HTTP error codes, and common solutions to those codes, you should have a good basis for troubleshooting issues with your web servers or applications.
If you encounter any error codes that were not mentioned in this guide, or if you know of other likely solutions to the ones that were described, feel free to discuss them in the comments.
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