Add systemd_resolved_disable_stub_listener (#9875)

docs/dns-stack.md

@@ -143,6 +143,11 @@ coredns_default_zone_cache_block: |
   }
 ```
 
+### systemd_resolved_disable_stub_listener
+
+Whether or not to set `DNSStubListener=no` when using systemd-resolved. Defaults to `true` on Flatcar.
+You might need to set it to `true` if CoreDNS fails to start with `address already in use` errors.
+
 ## DNS modes supported by Kubespray
 
 You can modify how Kubespray sets up DNS for your cluster with the variables ``dns_mode`` and ``resolvconf_mode``.

roles/kubernetes/preinstall/defaults/main.yml

@@ -140,3 +140,6 @@ redhat_os_family_extensions:
 # Extending some distributions into the debian os family
 debian_os_family_extensions:
   - "UnionTech OS Server 20"
+
+# Sets DNSStubListener=no, useful if you get "0.0.0.0:53: bind: address already in use"
+systemd_resolved_disable_stub_listener: "{{ ansible_os_family in ['Flatcar', 'Flatcar Container Linux by Kinvolk'] }}"

roles/kubernetes/preinstall/templates/resolved.conf.j2

@@ -14,7 +14,7 @@ Domains={{ searchdomains|default([]) | join(' ') }}
 #MulticastDNS=no
 DNSSEC=no
 Cache=no-negative
-{% if ansible_os_family in ["Flatcar", "Flatcar Container Linux by Kinvolk"] %}
+{% if systemd_resolved_disable_stub_listener | bool %}
 DNSStubListener=no
 {% else %}
 #DNSStubListener=yes