Causes and countermeasures for 400 Bad Request error in Apache 2.4

Introduction

　When setting up and operating a web server, it is not uncommon for requests from clients to not be processed correctly and a “400 Bad Request” error to occur. Apache 2.4 in particular introduces stricter request handling, so this problem is increasingly encountered when migrating from previous versions.

　In this article, we will explain in detail the main causes of 400 Bad Request errors in Apache 2.4 and their solutions.

Common causes of 400 Bad Request errors

1. Bad request format: Apache will return a 400 error if the request line or headers are not compliant with RFC7230. In particular, stricter handling of whitespace characters may cause requests that were previously allowed to fail.

2. Use of illegal characters: For example, Apache 2.4 may return a 400 error if the virtual host’s `ServerName` contains disallowed characters such as an underscore (_).

3. Misconfiguration: Misconfiguration of `mod_ssl` or other modules can also cause 400 errors.

Solutions and countermeasures

1. Request validation: Capture requests on the client and server side and check whether they are compliant with RFC7230. In particular, you need to pay attention to small details such as whitespace before CRLF after HTTP/1.1.

2. Correcting illegal characters: Check if `ServerName` contains illegal characters and correct them if necessary. For example, you might want to replace underscores with hyphens or dots.

3. Review configuration: Double check the configuration of `mod_ssl` and other related modules and modify as necessary. Especially when migrating to a new version, be aware of changed default values ​​and new directives.

Additional considerations

4. Check your security settings: Apache 2.4 has improved security. Settings such as `HttpProtocolOptions` and `mod_security` have become stricter, so make sure there are no flaws in these settings. In particular, you need to be careful that settings for blocking invalid requests do not accidentally block legitimate requests as well.

5. Detailed analysis of logs: When an error occurs, analyze Apache’s `error_log` and `access_log` in detail. Logs often contain details about the request and the cause of the error, which can help you troubleshoot the issue.

6. Leverage communities and documentation: Because Apache is widely used, information is shared in many communities and forums. For specific errors, refer to Apache’s official documentation or technical sites such as Stack Overflow.

Add related links

– [Apache HTTP Server Documentation]

– [RFC7230 – Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing]

　I hope this article helps those who are facing 400 Bad Request error with Apache 2.4. There are many causes and solutions for errors, so please take the appropriate action depending on the situation.