random.dnsp.co
A different answer every single time.
What it does
random.dnsp.co returns an address with no meaning at all. The value is irrelevant: only whether it changes tells you anything. Two identical answers back to back mean a cache is between you and us.
Example
$ dig @dnsp.co random.dnsp.co +short
88.14.203.6
$ dig @dnsp.co random.dnsp.co +short
12.240.9.171
Reading the answer
a different address each time
No cache in the path.
the same address twice
Something cached the answer despite its lifetime.
Notes
- Use increment instead if you want a result you can reason about rather than just compare.