HTTP Error 501 Not implemented

Introduction

The Web server (running the Web site) does not understand or does not support the HTTP method it finds in the HTTP data stream sent to it by the client (e.g. a Web browser or our CheckUpDown robot). The methods defined by the HTTP protocol are as follows:

  • OPTIONS: Find out the communication options available for a particular URL resource. Allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without a specific action involving transfer of data.
  • GET: Retrieve the information identified by the URL resource e.g. GET a particular Web page or image. The most common method by far.
  • HEAD: Identical to GET except that the server returns header information only, not the actual information identified by the URL resource. Useful to obtain metainformation about the entity implied by the request without transferring the entity-body itself. Often used to test hypertext links for validity, accessibility, and recent modification.
  • POST: Submit data to the Web server such as 1) post a message to a bulletin board, newsgroup or mailing list, 2) provide input data - typically from a CGI form - to a data-handling process, 3) add a record directly to a database.
  • PUT: Set (place/replace) the data for a particular URL to the new data submitted by the client. For example, upload a new Web page to a server.
  • DELETE: Remove the data associated with the URL resource. For example, delete a Web page.
  • TRACE: Run a remote, application-layer loop-back of the request message. Effectively a 'ping' which tests what data the Web server is receiving from the client.
  • CONNECT: Reserved for use with tunnelling (e.g. SSL) via a proxy server. This method is defined only for HTTP version 1.1, not the earlier version 1.0.

If the method in the request HTTP data stream is not one of the above, then a 501 error will result. Or the method may be valid but not actually supported by the Web server. This typically only happens for newer methods such as CONNECT when received by older Web servers. (Last updated: March 2012).

Fixing 501 errors - general

This error should be very rare in any Web browser. It is more likely if the client is not a Web browser - particularly if the Web server is old. In either case if the client has specified a valid request type, then the Web server is either responding incorrectly or simply needs to be upgraded.

Fixing 501 errors - CheckUpDown

Our service monitors your site for errors like 501. CheckUpDown only ever uses the GET request type, which should be supported by all Web servers including the very oldest. So you should not see this error very often on your CheckUpDown account, if at all.

Please contact us (email preferred) whenever you encounter 501 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 so we can agree whether the particular HTTP request types should be supported, and whether there is some defect in the Web server program code handling those request types. Fixing the problem may require Web server program code changes, which could take some time.


501 errors in the HTTP cycle

Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:

(1) Obtain an IP address from the IP name of the site (the site URL without the leading 'http://'). This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
(2) Open an IP socket connection to that IP address.
(3) Write an HTTP data stream through that socket.
(4) Receive an HTTP data stream back from the Web server in response. This data stream contains status codes whose values are determined by the HTTP protocol. Parse this data stream for status codes and other useful information.

This error occurs in the final step above when the client receives an HTTP status code that it recognises as '501'.


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