The Web server (running the Web site) does not support, or refuses to support, the HTTP protocol version specified by the client (e.g. your Web browser or our CheckUpDown robot) in the HTTP request data stream sent to the server.
The HTTP protocol has various versions identified as major.minor e.g. version 0.9, 1.0 or 1.1. The server is indicating that it is unable or unwilling to complete the request using the major version provided by the client - other than with this error message.
Assuming that your request identifies a valid major.minor version number (the request is not fundamentally corrupt), then this error should mostly only occur if you are trying to use version 1.0 or 1.1, but the Web server only supports the older 0.9 version.
Most Web browsers assume that Web servers support 1.x versions of the HTTP protocol. In practice very old versions such as 0.9 are little used nowadays, not least because they provide poorer security and performance than newer versions of the protocol. So if you see this error in your Web browser, the only option is to upgrade the Web server software. If version 1.x requests fail, it may well be because the Web server is supporting the 1.x protocol versions badly, rather than not supporting them at all.
The same arguments apply if your client is not a Web browser.
Our service monitors your site for HTTP errors like 505. It is our responsibility to support whatever versions of the HTTP protocol all Web servers understand. Because very old versions such as 0.9 are little used nowadays, our CheckUpDown robot uses 1.x versions of HTTP when we try to access the Web site. If these fail, it may well be because the Web server is supporting the 1.x protocol versions badly, rather than not supporting them at all.
Please contact us (email preferred) whenever you encounter 505 errors - there is nothing you can do to sort them out. We then have to analyse the underlying HTTP data streams and may have to liaise with your ISP and the vendor of the Web server software to agree how different HTTP protocol versions should be supported. In the worst case we may have to revise our program code supporting particular HTTP versions, which may take some time.
Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:
This error occurs in the final step above when the client receives an HTTP status code that it recognises as '505'.