Every time you open a website, your computer quietly asks a directory service for the address. That service is the DNS, and by default it answers every question without judgment — including the ones that lead somewhere bad.
A filtering DNS keeps a list of sites known to be fraudulent or to carry malware, and refuses to hand over those addresses. Your browser gets nothing back, so the page never loads. It is a different phone book, not a program.
Where to put it
What it stops
The link in the email that looks almost right
The fake bank page behind a text message
Sites that quietly install malware
What it will not stop
A phone call from someone claiming to be Microsoft
Someone already inside the machine
Money already sent
Confirm it worked
Open malware.testcategory.com. If the page refuses to load, it is working. That is a harmless test page Cloudflare publishes for exactly this.
Undo it
Set the DNS assignment back to Automatic or DHCP.
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