Ubuntu 18, VPN and DNS: curl wont resolve VPN-host, but nslookup and dig can

i found that apparently in Ubuntu 18 the whole DNS-setup is very confusing. Im connected through an pritunl VPN to my kube-cluster and im trying to use the kube-dns server. So i first tried to use https://github.com/jonathanio/update-systemd-resolved to update my DNS settings with the pushed DNS server from the VPN, but it seems that currently something is broken (https://github.com/jonathanio/update-systemd-resolved/issues/64). As im ok with hard-coding the DNS ip somewhere, i tried putting the IP in some places: installing resolvconf and putting it in /etc/resolvconf/resolv.conf.d/head, putting it in /etc/systemd/resolved.conf, of course also trying to put it directly into /etc/resolv.conf, as im a naiv person. After restarting some things a couple of times, i reached an even more confusing state:

% dig pritunl-ui.infra.svc.cluster.local                      
[...]
;; ANSWER SECTION:
pritunl-ui.infra.svc.cluster.local. 30 IN A 10.15.246.61
[...]

So this looks good, but:

% curl 'https://pritunl-ui.infra.svc.cluster.local' --insecure
curl: (6) Could not resolve host: pritunl-ui.infra.svc.cluster.local

Calling the IP does work though:

% curl 'https://10.15.246.61' --insecure
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<title>Redirecting...</title>
<h1>Redirecting...</h1>
<p>You should be redirected automatically to target URL: <a href="https://10.15.246.61/login">https://10.15.246.61/login</a>.  If not click the link.

I also tried turning-it-off-and-on-again (my host machine), but stays the same. nslookup does also work fine, tracepath doesnt.

% systemd-resolve --status
Global
         DNS Servers: 10.15.240.10
          DNS Domain: svc.cluster.local
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 3 (tun0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 10.15.240.10
          DNS Domain: default.svc.cluster.local
                      ~.

Link 2 (wlp2s0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 192.168.178.1
                      fd00::cece:1eff:feba:6468
          DNS Domain: ~.
                      fritz.box

Any ideas how to make this work?

To answer my own question: I digged a bit deeper and learned a bit about avahi, nscd, systemd-resolve and the magic of nsswitch. So apparently the problem was this line in my /etc/nsswitch.conf:

hosts:          files mdns4_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] dns myhostname

I missed that readme from update-systemd-resolved actually recommended changing this line. So i changed it to

hosts:          files dns resolve myhostname

Et Voilà, now curl can reach the host:

$ getent ahosts pritunl-ui.infra.svc.cluster.local
10.15.246.61    STREAM pritunl-ui.infra.svc.cluster.local
10.15.246.61    DGRAM  
10.15.246.61    RAW

I also tried to just remove the [NOTFOUND=return] part, and that seems to have the same effect, it also works. Maybe it says something like "if mdns fails, stop here and ignore the rest"..? Which would be a very odd strategy i think.

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