acme4j/src/doc/docs/challenge/dns-01.md

dns-01 Challenge

With the dns-01 challenge, you prove to the CA that you are able to control the DNS records of the domain to be authorized, by creating a TXT record with a signed content.

Dns01Challenge provides a digest string:

Dns01Challenge challenge = auth.findChallenge(Dns01Challenge.class);
String domain = auth.getIdentifier().getDomain();

String digest = challenge.getDigest();

The CA expects a TXT record at _acme-challenge.${domain} with the digest string as value.

The validation was successful if the CA was able to fetch the TXT record and got the correct digest returned.