DNS
Test whether a DNS server is an open resolver.
An open resolver answers recursive queries for anyone. That is useful for amplification attacks. Authoritative servers should keep recursion off.
The test sends example.com A with RD=1 to UDP/53. RA=1 means recursion is on.
If recursion is on at your authoritative NS, cache-poisoning surface grows and UDP/53 can be used for amplification. Limit allow-recursion to your own network.
It is designed as a public resolver. Your own authoritative server should not be.
Timeout or a network error. A filter is not the same as “recursion on”.
An internal resolver should not be open to the internet. Only probe IPs you operate.
The server is not answering recursively — expected for an authoritative-only server.
No. It is one example.com A query. Still do not scan hosts without permission.