It’s always MTU, unless it’s DNS

One of the most common questions I hear from small and even medium sized ISPs is “why should I run my own DNS resolver(s)?” The perception that DNS is hard, complicated, or even unnecessary is often cited as a reason to just farm it out to one of the “free” anycast resolver services available across the internet. Now, there are many reasons to be wary of DNS, both from the professional and the consumer side - it is a huge treasure trove of personal information about behavior, and is easily monitized by entities large enough to consume and process it.