What does the 505 error mean?

The 505 error, often displayed as Error 505 HTTP Version Not Supported, is an HTTP response status code that indicates the server does not support, or refuses to support, the HTTP protocol version used in the request.

This means that while the server understands the request itself, it cannot process it because the client is using an unsupported HTTP version (for example, HTTP/1.0 or HTTP/2). This can occur if the server is configured to only accept certain protocol versions or if it is outdated and does not understand newer versions.

The 505 error is part of the 5xx class of HTTP status codes, which signify server-side issues. Unlike many 5xx errors that may be temporary (e.g., due to overload), the 505 error usually reflects a permanent incompatibility between the client’s HTTP version and the server’s supported versions.

How to resolve the 505 error?

As a website user:

If you see a 505 HTTP Version Not Supported error while browsing a site or using a web application, there is usually little you can change directly. The issue lies with the server’s configuration or software. However, you can try the following:

• Update your browser or client software to ensure it uses a supported HTTP version.
• If the problem persists, contact the website’s support team to inform them of the error.
  

As a server administrator or developer:

To fix a 505 error, you should identify why the server is rejecting the HTTP version:

• Check supported HTTP versions: Verify the versions your server software can handle (e.g., HTTP/1.1, HTTP/2). Ensure the server is configured to support the versions your clients are using.
• Update server software: If the server is outdated, it may not understand newer HTTP versions. Upgrading your web server (NGINX, Apache, IIS, etc.) may be necessary.
• Proxy or load balancer compatibility: If you are using a reverse proxy or load balancer, ensure it supports the same HTTP versions as your backend server. Incompatibilities between components can trigger 505 errors.
• Client misconfiguration: If you control the client (such as an API integration), check that it uses a compatible HTTP version in requests. Some HTTP libraries allow explicitly setting the protocol version.
  

Recommendation:

When setting up or maintaining an HTTP server, make sure it supports the necessary HTTP versions that your audience or applications require. Regularly test your services with different clients and tools (e.g., curl, Postman) to ensure compatibility. If possible, maintain support for widely used versions like HTTP/1.1 and HTTP/2, unless you have a specific reason to limit them.