protected override async Task <IDisposable> CreateChallengeHandler(IChallengeContext ch, string hostName, IKey accountKey)
{
var cnameQuery = await lookupClient.QueryAsync($"_acme-challenge.{hostName}", QueryType.CNAME).ConfigureAwait(true);
var cnameRecord = cnameQuery.Answers.CnameRecords().Single();
var fullName = cnameRecord.CanonicalName.Value.TrimEnd('.');
Log.WriteVerboseLine("DNS CNAME target:");
Log.WriteVerboseLine(fullName);
var txt = accountKey.DnsTxt(ch.Token);
Log.WriteVerboseLine("DNS value:");
Log.WriteVerboseLine(txt);
return(new TxtRecord(this, fullName, txt));
}