001 - Unknown Host


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 first step above i.e. the client is simply unable to determine an IP address corresponding to the IP name for the Web site.

Reasons for 001 errors

The most obvious reason for an 001 error is that the site IP name is fundamentally incorrect. It may have been mistyped or contain an invalid prefix e.g. be instead of You simply need to correct the URL.

If the IP name is indeed correct, then an 001 error indicates a failure in the DNS service. DNSs are typically highly redundant (if one is unavailable, then there are many others which can be used in their place). Given the millions of IP names used on the Internet, there are a very large number of DNSs on the Internet. These can not all be operational the whole time and the data they hold can not be kept perfectly synchronised. The role of DNSs is critical to the whole Internet, so many people work to ensure the DNS service works quickly and accurately. Despite all this effort, it is still possible for the DNS to be unable to provide an IP address given a valid IP name, which gives a 001 error.

Resolving 001 errors - general

Assuming you have started with a valid IP name, then an 001 error may still occur if the IP name is relatively new (you have a new Web site). This sort of DNS failure typically disappears within a couple of days once the new IP address/name details have propagated out to a sufficient number of global DNS systems. It is very rare for this to take more than a week. So if you see 001 errors on a brand-new Web site, simply wait a while. If you continue to get these errors a week or two later, it may be that your ISP has simply forgotten to broadcast the IP details out to the Internet.

Apart from the reasons above, 001 errors should be comparatively rare.

Resolving 001 errors - CheckUpDown

If the IP name (URL to monitor) on your CheckUpDown account is incorrect, you simply need to change it - see under the 'Manage' button on your main account page.

001 errors on your CheckUpDown accout may not always be detected by other Internet users (as 'Site not found' messages in their browsers) given the inevitable use of different DNS systems all over the Internet. Nevertheless 001 errors should never really occur once the Web site is well-established, and then only sporadically. We suggest you contact us for further discussion (email preferred) if you see persistent 001 errors.