At this point I pretty much think this may be the exact case - and it may work in reverse: “since ns2 has AAAA listed then it’s an IPv6-capable deployment and we are allowed to fall back to IPv4, but since ns1 lacks the AAAA record, then we won’t even consider falling back to ns1 if ns2 is down”.

Or maybe another scenario:

• let’s check ns2’s AAAA record
• oh it’s down! Let’s check ns2’s A record
• it’s down too! Let’s switch to ns1…
• … but at that time the DNS resolution timeout expires, since we took too long resolving the records; and on the second try, no one remembers that ns2 was down, repeating the cycle

Yeah, the issue is that people’s resolvers are wildly different and I guess some of them do not cater to weird and wacky self-hosting setups (most people just use anycast highly-available DNS, usually provided by a registrar or by some other company).

I had a hunch that resolvers break because one of my nodes does not have a public IPv6 address: meaning,

• node2: externally-facing node, I host it from my home, IP 178.44.116.85, acts as NS ns1. It has IPv6 connectivity but no external IPv6;
• node4: a cloud VPS that I rent. Externally-facing, with IPv4 91.219.150.30 and IPv6 2a06:dd00:1:4::4189, acts as NS ns2.

If node4 decides to break, then ns2 becomes unavailable - but since resolvers can see that ns1 does not have an IPv4 address, and since everyone knows that IPv6 is better because 6 is greater than 4, then these resolvers would always prefer ns2, even though it’s clearly down.

This could be solved by asking my ISP to set up a static IPv6 prefix for me (they already provide static IPv4), except my ISP does not do that, unfortunately. I could also rent another VPS (so I would have two VPS’es, giving me some resilience from downtimes) but that means extra $$$… anyway, I guess I should probably test the IPv6-preference hypothesis first and then act on the results.

Basically my internal DNS zonefile is configured like this (it should be easier to read than my explanation):

coredns_config:
  zoneFiles:
    - filename: int.pootis.network.zone
      domain: int.pootis.network
      contents: |
        $ORIGIN int.pootis.network.
        $TTL 300

        @       SOA     ns.int.pootis.network. admin.pootis.network. (
          2026082001
          1200
          300
          1209600
          300
        )

        @       NS      ns.int.pootis.network.
        ns      AAAA    {{ k8s_dns_ipv6 }}
        ns      A       {{ k8s_dns_ipv4 }}

        vault     CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.
        cinny     CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.
        grafana   CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.
        stalwart  CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.
        lemmy     CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.
        mail      CNAME   stalwart.stalwart.svc.{{ k8s_cluster_domain }}.
        webmail   CNAME   internaltraefik.internaltraefik.svc.{{ k8s_cluster_domain }}.

        ; ACME challenges
        ; int.pootis.network + *.int.pootis.network
        _acme-challenge      CNAME 1ca12d2c-8034-4a09-a940-dc59bd4f38d8.acme-dns.pootis.network.
        ; mail.int.pootis.network
        _acme-challenge.mail CNAME d175b66d-909f-46ba-8572-636ebc235d4c.acme-dns.pootis.network.

Trying to understand your setup: this seems like a split-horizon DNS, where app.example.com is only visible from your internal network (or Tailnet), right?

If so - I explained my similar setup in this comment; in short, my internal DNS works perfectly because the internal DNS server has a static “service IP” (handled by kube-proxy); if my currently-running pod breaks, k8s will simply spawn a new one and the DNS server IP won’t even have to change - meaning, the internal DNS zone continues to be served no matter which node breaks (it’ll even survive multiple node failures).

But the external DNS is different and I can’t simply plug in a “fake loadbalancer IP” there… (there are valid workarounds but they either are incredibly finicky or I have to spend an obscene amount of money to implement them)

I actually already have a local DNS resolver… well, sort of: most of my cluster’s internal services, such as Vaultwarden and other stuff, are available only over an in-cluster IKEv2 VPN.

What I mean is, I have to connect to the VPN first, which pushes a Configuration Payload to my IKEv2 client, and that payload contains a DNS server IP, and that DNS server (exposed by a Kubernetes Service) serves a custom “internal” zone (.int.pootis.network), and proxies everything else either to k8s coredns, or to upstream DNS forwarders.

Okay, that explanation may have been somewhat complicated (it sounded simpler in my head) but the end result is that I can resolve and reach stuff like lemmy.int.pootis.network (my Lemmy frontend) only while connected to the in-cluster VPN. It is completely hidden from external users (no CT log record, no external DNS record, and only proxyable by internal Traefik instance).

The issue only occurs with my externally-facing DNS, which is needed to, for example, to reach my Lemmy backend instance (lemmy.pootis.network) and the pictrs deployment. But yeah, internal DNS works perfectly for me and it easily survives a node failure.