A client (e.g. your Web browser or our CheckUpDown robot) can indicate to the Web server (running the Web site) the characteristics of the data it will accept back from the Web server. This is done using 'accept headers' of the following types:
If the Web server detects that the data it wants to return is not acceptable to the client, it returns a header containing the 406 error code. (Last updated: March 2012).
This error occurs very infrequently in Web browsers, because most browsers will accept any data returned from the Web server.
If the client is not a Web browser, then anyone can only investigate the problem by looking at the Accept headers generated by the client system and the data stream returned by the Web server. If you do not have access to the source code for these systems, the only thing you can do is refer the problem to technical support people at the companies that developed the systems.
Our service monitors your site for HTTP errors like 406. This error should simply never occur in your CheckUpDown account. If it does, it typically indicates defective programming of our systems or of the Web server which manages the site. We do not use accept headers at all, so there is no reason for the Web server to generate a 406 error.
Please contact us (email preferred) whenever you encounter 406 errors - there is nothing you can do to sort them out. We then have to liaise with your ISP and the vendor of the Web server software to agree the exact reason for the error.
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 '406'.