[BUG] [CoreDNI] Domain Names Starting with "." cause crash loop

[chasewilson]

Describe the bug
clusters with DNS pods using domain names starting with "." can cause crash loops within the DNS pods. This is in Release 2023-05-14 due to Core DNS upgrade to 1.94 which includes an update from an upstream library to prohibit domain names starting with ".".

The breaking change was introduced in this commit in last July before releasing CoreDNS 1.9.4. It's not a recent change. The impacted CoreDNS version is 1.9.4 and above.

The culprit change is in miekg/dns: miekg/dns#1376

In CoreDNS, the IsDomainName function is used to validate the zone in the server block. Previously, it allows leading "." in the zone, e.g. ".domain.com". After the new change, coredns will crash with error "zone is not a valid domain name: .domain.com"

There is a second issue in 1.9.4 (no breaking but warning in the message) caused by another change: coredns/coredns#5414

Prior to CoreDNS 1.9.4, server block supports wildcard in the zone, i.e. ".domain.com", in server block. In CoreDNS 1.9.4, a warning will be logged "Warning: Domain ".domain.com" does not follow RFC1035 preferred syntax".

With above 2 changes in CoreDNS, we want to understand the exact standard for the zone in server block. It's not documented clearly.