A resolver decides more than a server does
Most people think of DNS as the thing that turns a name into a number, and that is what it is, but it is also the place where a decision gets made about whether the connection happens at all.
That is why it is worth knowing what is happening there, even if all you wanted was to know why the page is slow.
What the lookup actually returns
A resolver answers a query with a record, and the record is one of a handful of types. The address record is the one everybody knows. The others are the ones that decide whether you reach the thing at all.
There is the record that points at a name instead of an address, which is the normal way a CDN works: the same host resolves to different addresses depending on where you are asking from, and the answer changes over the day.
There is the record that says "this name does not exist". That is the one a blocklist returns, and it is the one your device sees as a failure.
The difference between blocking and failing
A blocked name and a broken name look identical from the client. Both fail to resolve. The difference is visible only from the server side, in the query log.
That log is the interesting part. A resolver that keeps a report of what was asked and what was answered turns the network into something you can audit, instead of something you have to take on faith.
LockFilter publishes its reports alongside the list itself, and it is at https://lockfilter.com.
The same idea, one layer down, is a resolver that resolves normally but explains what it found. disable.page is at https://disable.page.
Why this matters when something breaks
Here is the practical case. Something on your network stops loading. The reflex is to blame the site. Half the time it is a name that resolved to nothing because of a rule somebody added last week.
If the resolver keeps the query log, the answer is a lookup instead of an archaeology project. You find out that the name was answered with "does not exist", you find the rule that did it, and you are done.
The alternative is guessing, and guessing is expensive.
