Here are the major implementations: http://www.apple.com/library/test/success.html http://www.apple.com/library/test/success.html http://clients3.google.com/ge

Here are the major implementations: http://www.apple.com/library/test/success.html http://www.apple.com/library/test/success.html http://clients3.google.com/generate_204 http://clients3.google.com/generate_204 http://www.msftncsi.com/ncsi.txt http://www.msftncsi.com/ncsi.txt And some other implementations of dubious longevity: http://start.ubuntu.com/connectivity-check http://start.ubuntu.com/connectivity-check http://network-test.debian.org/ http://network-test.debian.org/ Now, if only NetworkManager could add support for this.