<anything>.log.dnsp.co

Lists every address that looked up the same label before you.

What it does

Every lookup under log.dnsp.co is recorded. Query the same label again over TCP and you get back the addresses that have asked for it. Pick an unguessable label, use it somewhere, then see which machines followed it.

Example

$ dig +tcp @dnsp.co mytest123.log.dnsp.co +short

203.0.113.7

198.51.100.24

Reading the answer

only addresses you expect

Nothing else resolved your label.

addresses you do not recognise

Your lookups are being mirrored, replayed, or prefetched by a third party.

no results

Logging still happened. Results are only returned over TCP.

Notes

Choose a label nobody could guess, otherwise the log is meaningless.
A common use: put the label in a link and see who resolves it before you click.
Lookups are always recorded, whatever transport you used. Only the results need TCP.
No TCP available? Read the log over HTTPS on the lookup page instead.