Starting at 00:00 UTC on January 1, 2017, CNAME resolutions on some machines stopped working due to a bug triggered by the universal addition of one leap second, which affected both some authoritative DNS and origin DNS lookups, causing 5xx errors. Authoritative DNS was less affected since resolvers were served SERVFAIL and possibly retried until they hit a healthy instance. We deployed a fix at 01:30 UTC and it is rolling out to most affected sites.https://www.cloudflarestatus.com/incidents/1fczgjmknplp
Краткий перевод: Была проблема с DNS lookups из-за добавления "координационной" секунды.