Is T-Mobile bypassing local ad blockers when using your hotspot?

I hate online ads.

That could be the end of the post, but sadly it’s not.

I understand that sites need ad revenue to function. But the online advertising ecosystem is so fundamentally broken that I refuse to participate in it, even to the detriment of the sites whose content I value. It’s possible to run a sustainable business through ads without ruining your site visitors’ experience.

There are various tools you can use to block the most obnoxious ads, but I’ve taken a very direct, hands-on approach. I’ve actually taken the time to view source on sites I visit that go overboard with ads, identify the domain names of the ad servers, and add them to my Mac’s /etc/hosts file so they resolve to localhost (127.0.0.1), effectively killing any ads from those sources.

And it works. I often see “broken image” icons, but all of those hideous animated ads screaming at my eyeballs are gone.

Except when I’m tethering to my phone’s hotspot.

Somehow, when I use my T-Mobile hotspot instead of my home wifi, all of those ads come flooding back. What. Is. Happening.

The only explanation I can think of is that T-Mobile is somehow using a proxy to bypass my local hosts file, but I though the local file always trumped anything else.

I don’t have an answer. But I don’t like it. And perhaps more importantly, if T-Mobile is doing this, what else is it doing with the data I’m sending and receiving over its network?