"if a recent report is to be believed, Valve is looking at your browsing history. Reportedly, the company's Valve Anti Cheat system (VAC) looks at all the domains you have visited, and if it finds that you've frequented hack sites, you'll be banned. 'The new functionality has been slammed by gamers, who claim it is "more like spyware than anti-cheat". Valve has not responded to the allegations, but all Steam users have agreed to abide by specific online conduct and not to use cheats. The company's privacy policy also explains that Valve may collect "personally identifiable information", but promises not to share it with other parties.'"
From the linked article:
"The thing is, the DNS cache stores details of every site your computer has touched, whether or not you've actively visited it. If an article you are reading links to a blacklisted site, your DNS will record the address of that site, as well as the article. More subtly - if a page you are reading is using a graphic hosted on a blacklisted site, then that blacklisted site will again show up in your DNS. Even if you've never actively visited a cheat website, there may be traces of them in your DNS, and that's what VAC is reportedly now looking for.
The news was first posted to the Counter-Strike: Global Offensive Reddit, explaining that VAC now:
Goes through all your DNS Cache entries (ipconfig /displaydns)
Hashes each one with MD5
Reports back to VAC Servers"
You can flush your cache via the following procedures. I'd recommend doing so before you launch a Steam game.
Windows® 8
Press Win+X to open the WinX Menu. Right-click on Command Prompt and select Run as Administrator. (for Windows 8.1, click on "Command Prompt (Admin)" Type the following command and press Enter: ipconfig /flushdns If the command was successful, you will see the following message: Windows IP configuration successfully flushed the DNS Resolver Cache.
Windows 7
Click the Start button. Enter cmd in the Start menu search field. Right-click on Command Prompt and select Run as Administrator. Type the following command and press Enter: ipconfig /flushdns If the command was successful, you will see the following message: Windows IP configuration successfully flushed the DNS Resolver Cache.
Windows XP, 2000, or Vista®
Click the Start button. On the Start menu, click Run....
If you do not see the Run command in Vista, enter run in the Search bar.
Type the following command in the Run text box: ipconfig /flushdns
MacOS® 10.7 and 10.8
Click Applications. Click Utilities. Double-click the Terminal application. Type the following command: sudo killall -HUP mDNSResponder Warning: To run this command, you will need to know the computer's Admin account password.
MacOS 10.5 and 10.6
Click Applications. Click Utilities. Double-click the Terminal application. Type the following command: sudo dscacheutil -flushcache