In Linux, blocking access may be done easily with ipchains, e.g.
ipchains -A input -s vancouver-webpages.com -j DENYto block access and
ipchains -D input -s vancouver-webpages.com -j DENYto allow it. On other systems, a firewall command may be used, or a static route created to a nonexistent interface or gateway, or a temporary entry made in a hosts file to an address such as 127.0.0.2.
Here, the Apache ASIS feature is used to serve a JPEG image with defined HTTP headers, specifically
Content-type: image/jpeg Pragma: no-cache Expires: 0 Cache-Control: noneWhen the page is visited, it is retrieved from cache and displayed, The check image should not be in cache and must be retrieved from the master server. If there is an error loading the image, or a timeout, as may result from the master server being down or unreachable, then a script is called to jump to the slave server.
A similar script on the slave page checks the availability of the master server and jumps back if it is up, since it is generally not desired that people remember or bookmark the slave pages. Search engines may be deterred from indexing the slave pages by using e.g. the ROBOTS meta tag <meta name="robots" content="none">