Site breakage from DNS ad-blockers

So this past week I had an unpleasant experience with my adblocker interfering with a paid service I use. The adblocker I use is NextDNS.io, a DNS-based service sort of like a "Pi Hole in the Cloud". I have a fairly standard, but strict, configuration, and so it's subscribed to several blocklists, and it works its miracles on every device I have, even when I'm away from home.

Now I've had issues before, such as when the taxi company sent me a tracking link and I couldn't open it, and I found out much later that NextDNS was blocking it as a brand-new domain name. Once whitelisted, everything worked fine.

But this week's issues were much more subtle and sneaky. I use GrubHub a lot. Every day. And I always pay with PayPal; I don't even have credit cards saved in my account. But a few days ago, that PayPal option disappeared. I couldn't even change the options in my account preferences.

GrubHub Support does not know how to support their apps or website. They have no technical assistance available at Tier 1. They asked me if I was using the app, and they repeatedly invited me to install the app, as if that would change anything (I tried; it didn't.) And so at this point, I was blaming the bug on their devs, that some UI feature had changed and I was getting degraded service. But their Support reps weren't interested in troubleshooting.

So I hobbled by for a few days and ordered from other services. I could've added a card to GH, but I didn't feel like it, and they insisted that they still accepted PayPal as payment and so I felt like I should wait until it was available again.

Yesterday I contacted support again and pursued it a little more. Eventually, I was connected to a manager. I went back-and-forth with the manager a while without resolving it. (Their chat does not permit any uploads or screenshots, so it's sort of Stone Age.) And then I received email from Support! Natasha said I could send her screenshots and explain what's up. So I did this, and then Natasha asked me if I could see a "Venmo" option and she said that perhaps an adblocker is getting in the way.

So I replied and showed her no Venmo, and then I checked out NextDNS logs. Sure enough, there were a ton of domains being blocked when I hit the GrubHub site. I carefully whitelisted three that looked likely, and all my payment options were restored, even Venmo (I'd never seen Venmo before!)

In fact, my mobile PWA is acting completely differently now, as if I had an instant upgrade with new UI features. So I emailed Natasha to thank her.

I also mentioned to Natasha that I was deeply disturbed by the breadth and depth of telemetry that is being sent to platforms such as Pinterest, TikTok, Facebook, Twitter, and all sorts of janky ad-tech sites, just because I am ordering a meal. I told her that is invasive and unwelcome, and I plan to continue blocking everything besides what is necessary to place an order.

So I suppose that the moral of the story is this: if you're running an adblocker, and some site malfunctions mysteriously, check your adblocker before hitting up Support, because things can fail sneakily when you're tampering with traffic in unexpected ways.