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505 Error and Other 5xx Server Errors: What They Mean and How to Fix Them

#webdev #tutorial #server #wordpress

TL;DR: 505 HTTP Version Not Supported is an error that happens when a server either doesn't support the HTTP protocol version contained in the client's request or refuses to apply it. Sometimes it can be resolved on the user's side by clearing the cache. Other times, the server's software needs to be either updated or reconfigured.

Introduction

During web resource administration, server configuration, or search engine optimization, technical specialists frequently encounter 5xx group errors. For website owners, especially those running on the WordPress platform, a sudden loss of access to a resource can lead to lower search engine visibility and a loss of user traffic.

The 505 error code is relatively rare compared to others, but it's not that hard to fix. The IT specialist does, however, need a strong grasp on the processes that provoke this issue. It's also important to be able to conduct diagnostics on both the server's side and the client's.

The Meaning Behind the 505 Error

Web browsers interact with the wider web with the help of the Hypertext Transfer Protocol, better known as HTTP. The servers respond in the same "language," assigning a numeric HTTP status code to each of their responses. Typically, codes beginning with the number 5 mark a server-side issue. The 505 error code is somewhat unique among them, because it indicates the client's request has been correctly received, but it isn't getting processed. This can happen for several reasons, such as the request getting sent via the HTTP/3 protocol while the server's configuration allows for the interpretation of HTTP/1 only.

Sometimes this problem can be referred to as "505 internal server error" in conversation or search queries. Technically, this is inaccurate — a standard internal server error is represented by code 500 and points to a general internal error in the code or settings, whereas a 505 error is strictly related to the network protocol version.

How the 505 Error Code Differs from Others in the 5xx Group

Here are the related errors you can use as comparison points:

  • Error 500 is the real Internal Server Error. You can run into it when a script fails or when a mistake crawls into the .htaccess configuration file.
  • Error 502, known also as Bad Gateway, occurs as a result of an invalid response coming from an upstream node to an intermediate gateway or proxy.
  • Error 503 (Service Unavailable) is what users get when servers are taken down for maintenance, although it can also signify server overload.
  • Error 504 (Gateway Timeout) indicates the main web server hasn't responded for longer than the connection settings allow.

What Is Required for Diagnostics and Error Fixing

To perform troubleshooting procedures, a webmaster requires certain tools:

  • A terminal with the installed curl command-line utility to send test requests
  • Access to the server's configuration files or the hosting control panel
  • A web browser with developer tools enabled

Diagnostic Algorithm

Step 1. Determining the Source of the Problem

At the initial stage, it is necessary to determine whether the error is caused by the server configuration or the local parameters of the client.

Testing via curl

Using the curl utility allows sending a request with a forced specification of a particular HTTP protocol version, excluding the influence of the local cache.

To check HTTP/1.0 support, the following command is executed:

curl -I --http1.0 https://your-site.com
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A similar check is performed for HTTP/1.1:

curl -I --http1.1 https://your-site.com
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To test the HTTP/2 protocol, the following command is applied:

curl -I --http2 https://your-site.com
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If any request returns the header HTTP/1.1 505 HTTP Version Not Supported, this indicates incorrect web server settings.

An example of terminal output in the presence of an error:

HTTP/1.1 505 HTTP Version Not Supported
Date: Thu, 04 Jun 2026 12:00:00 GMT
Server: Apache
Content-Length: 226
Content-Type: text/html; charset=iso-8859-1
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Analysis via Web Browser

For a detailed study of the request parameters, the built-in browser developer tools are used (activated with the F12 key). In the Network tab, the target request is selected after reloading the page. On the Headers tab, the version of the protocol used is verified, and the response body is examined in the Response tab for additional technical details.

Methods for Fixing the Error on the Client Side

If the server processes external requests correctly, the cause of the problem may lie in the user's software. In such cases, the following measures are applied:

  • Cleaning local data, as a corrupted browser cache can force the transmission of outdated or incorrectly formed headers
  • Performing a cache-clearing procedure in the browser settings to reset saved network parameters
  • Updating the web browser, as an outdated version may not support modern markup and data transfer standards, creating compatibility issues
  • Checking intermediate node parameters, as an incorrectly configured proxy server can distort transmitted data packets

When conducting automated testing, parsing, or web analytics, it is critical to use high-quality proxy solutions that do not distort request headers. The Proxy-Seller service offers stable proxy servers that ensure correct data transmission without the risk of causing network protocol version conflicts.

Methods for Fixing the Error on the Server Side

If diagnostics indicate a failure on the hosting side, the website administrator or developer needs to make changes to the server infrastructure.

Updating Server Software

Using outdated versions of Apache or Nginx often leads to incompatibility with modern protocols. Timely software updates minimize the risk of such errors. To update server components in Linux-family operating systems, standard commands are used:

sudo apt update && sudo apt install --only-upgrade nginx
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Adjusting Web Server Configuration

The administrator must ensure that the current server configuration supports relevant standards. Below is an example of an Nginx configuration file with HTTP/2 support activated:

server {
    listen 443 ssl http2;
    server_name your-site.com;

    ssl_certificate /etc/letsencrypt/live/your-site.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/your-site.com/privkey.pem;

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_prefer_server_ciphers on;

    location / {
        try_files $uri $uri/ /index.php?$args;
    }
}
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After making changes, the configuration is checked for syntax errors:

sudo nginx -t
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If no errors are found, the service is restarted:

sudo systemctl restart nginx
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Troubleshooting in WordPress CMS

On sites running on WordPress, the problem may be caused by optimization or security plugins that override HTTP headers. As a diagnostic measure, it is recommended to temporarily disable these extensions by renaming their directories on the server, as well as to check the .htaccess file structure for non-standard redirection rules.

Conclusion

Although the 505 HTTP Version Not Supported status belongs to the category of rare network failures, understanding the mechanisms of data transmission protocols allows for a rapid localization of the source of the crash. Proper web server setup, regular updates of the server software, and clearing the local cache help maintain stable resource availability for all categories of users.

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