How does nextdns know my current dns resolver ?

Nextdns website is making some kind of dns queries through POST to xx.test.nextdns.io

and the response comes like

    "status": "unconfigured",
    "client": "8.17.x.x",
    "resolver": "162.158.x.x",
    "server": "anex...."


Can you please tell how is this happening and what is the technology behind this ?

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  • We use a unique generated hostname that forces your resolver to contact our auth DNS to get the IP of your resolver. Then we use the Host header of the subsequent HTTP request containing this unique hostname to return the discovered resolver IP. This is the same technique used by DNS leak test services.

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