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For a while I've noticed that Google logins (or, at least, GMail logins) always redirect through to "youtube.com" Is Google handling all logins from youtube.com? If so, why?

(wasn't sure if I should post this on webapps or superuser, so feel free to move this or tell me to move this to the appropriate site if necessary)

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up vote 5 down vote accepted

When you log into a Google site like GMail, Google also logs you into all your various accounts Google accounts - such as YouTube. In order to be logged into a site, Google sets a cookie in your browser.

This is fine for most of their properties which are on the *.google.com domain (mail.google.com, docs.google.com, etc). However a website can only set and read a cookie for it's own domain - google.com can't set a cookie for youtube.com. To get around this, as part of the login process Google will redirect you through accounts.youtube.com to log you into YouTube - and typically your country specific domain if you are outside the US (e.g. I get logged into google.com.au as well).

The full login process is described in this SO answer, it dates from 2009 but most of it should still hold true.

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ah, that makes sense, I had completely forgotten that cookies were handled on a per-domain basis :) thanks! – MatthewSot Aug 22 '12 at 1:29

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