An example I remember is when Virgin Media replaced any DNS queries returning NXDOMAIN with their own "search page" plastered with ads.

https://shkspr.mobi/blog/2021/03/advanced-network-error-sear...

Some mobile phone network carriers would inject certain PII (such as your mobile number) into HTTP request headers sent to ad networks. I'm sure they still do it, but by requiring the site to have some extra request to be made instea.

Some carriers also passed info in these headers to be used to make billing calls to the Telco, such as subscribing to premium services with minimal (if any) consent from the user.

I got stung by this years ago. An ad for mobile ringtones showed up on some news site I visited on my phone, I scrolled past without clicking. Next minute I get a text telling me I've subscribed signed up to a (iirc) $25/month ringtone plan and that it'd show up on my phone bill. It took an hour on the phone to my carrier to block the charges and get a refund. The ringtone company kept messaging me for a year that I needed to contact them and "fix" the issue.

As someone who was sometimes using http as a back end transport for "edi" data (hey at the time it was usually ftp in that industry so no worse, but at least http got around nat problems than even pasv didn't), one time in the mid 00's I was in a coffee shop and just speechless flabbergasted outraged to discover the contents of a random web page I happened to know the contents of get some extra html and js injected into it from the isp (the isp not the coffee shop).

"WHO THE F gave them the right to modify the data in my tcp session!!!!".

Spying on it I assumed but modifying one byte of it? A naively written edi that did not expect that would be completely destroyed.

Probably shouldn't have been so stupid for 15 years already by that point. There is no such thing as the right, there is only the ability.

If someone has the ability to do something, they will. Reputation, right & wrong, none of that actually exists or matters.

I didn't track down if the mechanism was based on dns. Could have been but I don't see why it would have had to be. But I was also not attempting to use any non default dns on that device at that time anyway. Obviously since they are trasporting unencrypted data they could modify it any time simply because it's in their hands.

dns would just be one handy low-hanging-fruit way to detour the data to a convenient dedicated purpose server to do the work. But not special or necessary at all. If they want to do the same thing without hijacking dns, they can with almost not even any difference in total effort.

Last I checked Verizon FIOS still has the opt-out. You have to go into your account preferences and manually disable it because spying is on by default.

This is a good reminder to login and take a look, I wouldn't put it past them to change the setting name and reset the default.

edit: apparently Verizon has zero record of the username I've used for my FIOS account for years (and have saved in my password manager). Their account reset also fails with an error. What a dumpster fire of a company.