005 - Host not reachable

Introduction

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

  • Obtain an IP address from the IP name of your site (your site URL without the leading 'http://'). This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
  • Open an IP socket connection to that IP address.
  • Write an HTTP data stream through that socket.
  • Receive an HTTP data stream back from your 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 005 occurs in the second step above i.e. our CheckUpDown robot obtained an IP address, but is unable to open a valid socket connection to that IP address. Consequently there is no possibility of an exchange of HTTP data streams between us and your Web server.

Reasons for 005 errors

005 errors can be difficult to analyse precisely because they occur at a relatively low level (socket creation) in the IP communications hierarchy. They can also be highly transient. For example a surge in IP traffic somewhere between our computer and yours can easily cause a time out condition.

We are unable to open a valid socket connection to your Web server (the 'Host' referred to in the message). Your Web server software may not be running or the entire computer that hosts your Web server may be down. There may also be a failure on other intervening equipment e.g. the host cannot be reached because of an intervening firewall, or an intermediate router may be down.

The IP address for your Web site may change. For example you may switch your Web site from one ISP to another or switch your Web site from one computer to another on an internal LAN. So there is now a different IP address for your IP name. This new information is propagated out over the Internet to a large number of DNSs, which takes time. It is possible for one DNS to return an obsolete IP address - one for a computer that no longer recognises your IP name. In this case no Web server is available to respond and the socket connection fails. So a recent change of IP address may cause a 005 error.

The IP address we use may be completely valid, but your Web server simply fails to respond. In other words, the DNS provides the correct IP address for the computer which hosts your Web site, but the Web server on that computer is unable to respond in time (within less than 20 seconds). It may simply be too busy dealing with other HTTP requests, or it may be down altogether, or it may be listening on a different port. Or there may be some problem with the general configuration of computers dealing with IP traffic at your computer location, so your Web server simply never receives the socket connection request at all.

Any of these conditions mean that our socket connection request goes into an IP 'black hole' - no response ever comes back to us. You see this often via a 'Read timed out' note for the 005 error.

Resolving 005 errors

If you see a lot of 005 errors, the first thing you can do is check that your Web server is indeed up and responding within an acceptable period of time. You can do this using a browser over the Internet to browse your site. Note that you should be definitely be 'out on the Internet' when you do this - simply browsing your site over a local (LAN) connection is *not* a satisfactory check. If your Web server responds promptly (in well under 20 seconds), then it is likely that the 005 error represents a 'spike' of some kind. To analyse this adequately requires good system logs - primarily on the computer which hosts your Web server and on all other computers which sit between that computer and the Internet itself. Examining these logs, and making sense of them, can be a large undertaking.

We are confident that 005 errors do indeed represent 'down' time i.e. if we get a 005 error then it is highly likely that other Internet users would have got some kind of error reported to their browser at that time, or simply given up and surfed off somewhere else.

So 005 errors can not be ignored. We suggest you contact us for further discussion (email preferred) if you see persistent 005 errors.