The Web server (running the Web site) thinks that the HTTP data stream sent by the client (e.g. your Web browser or our CheckUpDown robot) should include a 'Content-Length' specification. This is typically used only for HTTP methods that result in the placement of data on the Web server, not the retrieval of data from it.
This error seldom occurs in most Web traffic, particularly when the client system is a Web browser. The problem can only be resolved by examining what your client system is trying to do then discussing with your ISP why the Web server expects a 'Content-Length' specification.
Our service monitors your site for HTTP errors like 411. This error should simply never occur on your CheckUpDown account. If it does, it typically indicates defective programming of our systems or of the Web server which manages the site. The 'Content-Length' specification is irrelevant to the type of access we use when we check your URL.
Please contact us (email preferred) whenever you encounter 411 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 '411'.