Google.com and clients1.google.com/generate_204

Question

I was looking into google.com\'s Net activity in firebug just because I was curious and noticed a request was returning \"204 No Content.\"

It turns out that a 204 N

Answer 1

Many applications access this URL to determine if they have a connection that only leads to a captive portal.

The idea is that any captive portal thinks this is a "normal" website, and then redirects you to its portal site, which is returned with a status 200. If an application tries to access any normal website, it is confronted with a totally unexpected response and may have problems figuring out what's wrong. However, with this URL it's easy: If you get status 200, you are inside a captive portal, and you can tell your user to do something about it (usually either log in to the portal using a browser, or turn WiFi off and rely on 3G, if they are using a phone). If you get status 204, you got connected to Google, so your application is actually connected to the internet.

Microsoft and Apple use a slightly different approach; they both have some URLs that return a very specific short text message with a status 200, so instead of accessing the Google url you can for example go to "captive.apple.com" and check for status 200 with data = "Success" and nothing else. If you get status 200 and not exactly that data then you are again in a captive portal.