Tcl Library Source Code

Hm.

I suspect that the event-loop has to be running for this to work.

That said, the docs definitely do not mention that, and these are the examples from the docs too.

This needs a check.

Heh. While the package has a testsuite this (IMHO core) functionality of it is not covered :(

Could possibly be done using Google's nameservers.

That begs the question, do you possibly have no nameservers set up ?
(Which platform ?)

The output of the [::dns::nameservers] command could help here to see what the package thinks it has available.