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Can't see how they find this over the web. I'm on Windows, its in my network config. It changes when I use a VPN provider, but not when I use a local socks proxy connected to a remote server over SSH (ProxyCap). None of the publicly available sites are finding my correct IP, but leaktest finds my ISP DNS.

I imagine the protection is in using a VPN that is creating a virtual network adapter and therefore setting DNS providers there, but how is the website detecting my provider?

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up vote 9 down vote accepted

From the source of https://www.dnsleaktest.com/:

<iframe style="display:none" src="https://1segRNWUwPK0Y21Bm1M0.dnsleaktest.com/"></iframe>
<iframe style="display:none" src="https://ldJT4mFLnijeQDBhQX2D.dnsleaktest.com/"></iframe>
<iframe style="display:none" src="https://nC4B4vChnPXPshinJoyw.dnsleaktest.com/"></iframe>

That site generates a random host name and arranges for your browser to request content from that host name. Since the host name has never been served to anyone else before, your browser's request to resolve it is sure to make its way to the primary server for the dnsleaktest.com domain. Since they run their own DNS, whoever they receive the DNS requests from is your DNS provider.

Whether forcing your DNS traffic to go through a tunnel instead of via your ISP depends on who you want privacy from. If you don't want the sites you visit to know where you're from, it's a mild leak: most sites don't bother to track where your DNS requests come from, and anyway they'd have a hard time figuring out which ones are yours, and your requests are likely to be served by some cache anyway. However any site could use the same trick as dnsleaktest if they cared. If you're using a tunnel because you don't want your ISP to know which sites you're visiting, making sure that no DNS request ever reaches your ISP is crucial.

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Perfect, thanks! –  JW2014 Sep 21 '13 at 18:10
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