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When I open a clear net site like torproject.org in the Tor Browser, the browser shows me this:enter image description here

How does it know there is a onion site available for the URL?

Is there a file that maps clear site URLs to their dark net counterparts that is bundled with the browser?

Or does it magically discovers the hidden URLs somehow?

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As far as I can tell this is done by:

Onion-Location: http://2gzyxa5ihm7nsggfxnu52rck2vv4rvmdlkiu3zzui5du4xyclen53wid.onion/index.html

header when accessing https:///www.torproject.org.

The header is shortly described here: https://support.torproject.org/onionservices/onion-location/ :

What ".onion available" means in my browser?

Onion-Location is a new HTTP header that web sites can use to advertise their onion counterpart. If the web site that you're visiting has an onion site available, a purple suggestion pill will prompt at the URL bar saying ".onion available". When you click on ".onion available", the web site will be reloaded and redirected to its onion counterpart. At the moment, Onion-Location is available for Tor Browser desktop (Windows, macOS and GNU/Linux). You can learn more about Onion-Location in the Tor Browser Manual. If you're an onion service operator, learn how to configure Onion-Location in your onion site.

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  • tldr: add <meta http-equiv="onion-location" content="http://youronionserviceaddress.onion"> to your html <head>. see also Metalink to declare multiple mirrors
    – milahu
    Commented Dec 6, 2022 at 12:50

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