I've been thinking of the concept of an ad blocker that runs at the OS level, rather than as a browser extension. I know that I can place x.com in Windows' %windows%\system32\drivers\etc\hosts file and point it to the IP of y.com, and on y.com I can serve up content that says, "This ad blocked by Example Ad Blocker". However, the domain list I have is quite large -- like literally a thousand domains and growing, and so this wouldn't work well in file lookups. Does Windows permit some way to programmatically, like Qt/C++, add a DNS reroute rule in a more speedy way?
There's a risk of doing domain intercepts and DLL hooks using APIs because AV products and/or Microsoft would have to whitelist you and certify you so that your activity doesn't look like a virus. And the odds of them doing that are not only low (unless you're a multimillion dollar company), but they want to protect their ad marketing too.
The best option is to make a browser extension for each of the browsers. You can even check the source code of the AdBlock Chrome extension to see how it works. The trouble with that in 2017, however, is that there's no common browser extension platform just yet. It's getting much closer, but it's still not standardized yet. The new standard uses the Chrome standard. Opera, Firefox, Edge, and of course Chrome support this new standard to some degree, but it's kind of unsmooth still. And for anyone outside of that, such as IE11 or earlier, they're not going to have your Chrome-style browser extension and you'll have to go the seriously hard route to make one just for those earlier browsers or ask the customer to upgrade when your adware product installs.
If you want something that doesn't require a browser extension, then the option you want is to add another DNS server connection in the user's DNS client settings. I don't know how to do this yet via C#, Qt/C++, or C++. However, you can shell out from those languages and use the "netsh" command to create those DNS connections. Probably a good strategy would be to find the user's default gateway IP. Then, make the DNS priority like so:
So, it would be something like these 4 netsh commands:
netsh delete dnsserver "Wireless Network Connection" all
netsh interface ip add dns name="Wireless Network Connection" addr=1.1.1.1 index=1
netsh interface ip add dns name="Wireless Network Connection" addr=192.168.254.254 index=2
netsh interface ip add dns name="Wireless Network Connection" addr=8.8.8.8 index=3
Change "Wireless Network Connection" to "Local Area Connection" if they are using a cable for their computer instead of wireless. (Few do that these days.)
Change 1.1.1.1 to the IP address of your special DNS server.
Change 192.168.254.254 to the IP address of their default gateway.
The third rule (8.8.8.8) tells the computer to use Google's DNS if all else fails. This is important because they could disconnect their laptop at home and go to a café or something, and we need their DNS stuff to still work.
Now, once you get the DNS client settings right, you need a cheap Linux cloud host to serve up the DNS server and web server. You might even need more than one in case one goes down for maintenance, and possibly on a different cloud zone or even cloud hosting provider.
For the DNS product, if you have Linux skills, you can install and configure dnsmasq pretty easily to get a cheap and easy to manage DNS server on Linux. Or, if you search your Linux repositories, you can find other DNS servers, some more robust than others, some harder to use than others.
For the web product, you can install NGINX or Apache on each of the two DNS servers. Then, you can make a configuration where any domain connection can come to it and it will load a web page for that domain. The web page can say something like, "Ad Blocked By X Ad Blocker" or whatever you want in very small font (small enough to fill the ad spot).
Once this is all in place, you'll have to reboot the Win PC client and also clear their browser cache and history so that DNS will route through the new arrangement.
The end result is that when people on that Windows PC surf the web and load an ad, their OS will make a DNS request to translate domain name to IP address. The first DNS server they'll reach will be your private DNS server. It can then say that x.com ad domain (as an example) is the IP address of your private DNS server. That private web server will then be contacted and it will display the ad block message. For all other requests not served up by your DNS servers, they'll go to their default gateway. If that's not serving up DNS as needed, then they'll failsafe to the Google DNS on 8.8.8.8. So, web browsing will work fine, minus ads.
As for a bad domain list, there's a community-maintained bad domains list here on Github.
The trouble with the private DNS server that you host is that you're now having to pay a bandwidth bill for gobs of connections to it. That's probably undesirable unless you've got a proper way to monetize that. A better strategy would be to NOT use a private DNS server on the web and use a local DNS server and a local web server. You'd have to code both of those or use some third-party product for that. The trouble there, however, is that you may have some commercial licensing problems with that, or increased costs, and it won't work for some web developers who already use a web server on their workstation.
Therefore, as you can see from the added costs, hassle, and workstation configuration nuance troubles, the best strategy would be to use the browser extension for ad blocking.
However, even at that, how are you going to differentiate your product from the free ad blockers out there that are doing a sensational job already?