- Validate config as a task (Ansible's validate requires specifying the config file).

tasks/main.yml

@@ -35,10 +35,14 @@
       owner: root
       group: 0
       mode: 0o0644
-      validate: '{{ dhcp_conf_validate[ansible_os_family]|default(omit) }}'
   notify:
       - Restart ISC DHCP server
 
+- name: Validate config
+  when: ansible_os_family in dhcp_conf_validate
+  command: '{{ dhcp_conf_validate[ansible_os_family] }}'
+  changed_when: False
+
 - name: Enable service
   service:
       name: '{{ dhcp_service[ansible_os_family] }}'