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My question is basically what the title says: does my ISP know what sites I have visited if I am using Tor?

I know that my ISP can know that I am using Tor. In addition I know that my ISP cannot read the data leaving or coming at my PC. Can it however know what sites I have visited, e.g. can it see that I am searching something on Google?

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    No. That's one of the purposes of Tor. – John Deters Dec 5 '13 at 23:31
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    Google uses https now, which is more secure against eavesdroppers and man in the middle. So even without Tor, your ISP wouldn't be able to see what you are searching on Google as everything is encrypted. But you could use a VPN service instead of Tor. This is because using Tor could raise suspicion for some companies. – jingyang Dec 6 '13 at 5:42
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    Mind you, your ISP would know what websites you were visiting even with HTTPS, as the certificate would reveal that much. They wouldn't know what you're doing on the site, though (they could make educated guesses, though). – Kitsune Dec 6 '13 at 6:27
  • Its taken me over a year, to learn how to use it correctly, just remember anything you do, does not in any way divulge any information about you, gives any clues, even in emails, names etc,,,, and away you go, its quite good fun at times, just knowing your blotting out the nosey spies! – user3552 Sep 4 '14 at 23:39
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    One of the best visual explanations I've ever seen :) eff.org/pages/tor-and-https – Storm Sep 27 '15 at 9:52
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If you're using Tor and it's all working correctly then your ISP can't see what websites you're visiting.

Tor hides the identity of who you're connecting to (the webserver), so no issues there. Additionally, since Tor encrypts your traffic your ISP can't see your HTTP requests, so they can't see what websites you're trying to download.

There's also the issue of stuff like DNS. If you try to resolve a URL using DNS in the clear (without using Tor) then that reveals what websites you intend to visit.

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    It's important to reiterate that, by default, your computer's Internet connection will try to resolve DNS using servers that are often automatically setup during IP negotiation, and belong to your ISP. You should check that DNS queries are resolved with Tor, via DNScrypt to OpenDNS, etc. – roguesys Dec 7 '13 at 1:12
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    The recommended way to use Tor is to download Torbrowser, which takes care of DNS leaks for you (as well as mitigating a whole range of other things you might not configure correctly on your own). – flamsmark Dec 10 '13 at 11:01
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Actually you need to run Tor as a router, only then you'll never leak a DNS info, but there's also a trick with persistent cookies: if you've visited some sites from your PC before going through Tor or logged into your impersonated profile - it can be used in metadata processing to deanonymize you even if you're behind a Tor router.