DNS, or the Domain Name System, is the part of the internet that converts www.techsupportalert.com (which you can understand but which your browser can't), into an IP address of 220.127.116.11 (which your browser can understand, but which you probably don't want to).
When you type techsupportalert.com into your browser, the first thing your computer does is to summon the DNS system to look up the correct IP address, so that it can contact the correct server and request the page you asked for. Surfing to http://18.104.22.168 would work just as well (click the link if you don't believe me) but it's not a feasible way to use the internet.
Your PC, or rather your router, knows how to contact the DNS system because the router is programmed with the IP address of a DNS server. There are lots of DNS servers available. Your internet service provider probably runs one, and your router is probably configured to use it. There are also some additional public ones, such as the well-known one run by Google.
Because DNS is so important to the correct functioning of the internet, hackers often try to intercept it. For example, some malware or viruses will try to change the setting in your router so that you are now using the DNS server belonging to the hackers. Once they have control of your DNS server, they can control all of your internet usage. For example, they could direct all of your surfing to their own malware-ridden web site. Or they could create fake sites which look like legitimate ones, but which also contain malware. And because your internet connection is now running through a hacked DNS server, directing you to that fake site is easy.
Security company F-Secure has put together a tool called Router Checker. It simply checks the DNS settings in your router, and warns you if the settings appear to indicate that you're connecting to a known rogue DNS server. If that's the case, you can then log into your router and fix the problem.
You'll find the Router Checker at https://campaigns.f-secure.com/router-checker and it's entirely web-based, so there's nothing to install. Just click the Start button and wait for the results.
My thanks, as ever, to Lex Davidson for this handy tip.
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