Create temporary hostkeys for test if there are none

and if we are not writing the main configuration file

tasks/install.yml

@@ -22,22 +22,24 @@
   notify: reload_sshd
 
 - name: Make sure hostkeys are available and have expected permissions
-  vars:
+  vars: &share_vars
     # This mimics the macro body_option() in sshd_config.j2
     # The explicit to_json filter is needed for Python 2 compatibility
+    __sshd_hostkeys_from_config: >-
+      {% if sshd_HostKey is defined %}
+        {{ sshd_HostKey | to_json }}
+      {% elif sshd['HostKey'] is defined %}
+        {{ sshd['HostKey'] | to_json }}
+      {% elif __sshd_defaults['HostKey'] is defined and not sshd_skip_defaults %}
+        {{ __sshd_defaults['HostKey'] | to_json }}
+      {% else %}
+        []
+      {% endif %}
     __sshd_verify_hostkeys: >-
       {% if not sshd_verify_hostkeys %}
         []
       {% elif sshd_verify_hostkeys == 'auto' %}
-        {% if sshd_HostKey is defined %}
-          {{ sshd_HostKey | to_json}}
-        {% elif sshd['HostKey'] is defined %}
-          {{ sshd['HostKey'] | to_json }}
-        {% elif __sshd_defaults['HostKey'] is defined and not sshd_skip_defaults %}
-          {{ __sshd_defaults['HostKey'] | to_json }}
-        {% else %}
-          []
-        {% endif %}
+        {{ __sshd_hostkeys_from_config }}
       {% else %}
         {{ sshd_verify_hostkeys | to_json }}
       {% endif %}
@@ -61,15 +63,46 @@
       loop: "{{ __sshd_verify_hostkeys | from_json | list }}"
 
 - name: Apply configuration
-  template:
-    src: sshd_config.j2
-    dest: "{{ sshd_config_file }}"
-    owner: "{{ sshd_config_owner }}"
-    group: "{{ sshd_config_group }}"
-    mode: "{{ sshd_config_mode }}"
-    validate: "{{ sshd_binary }} -t -f %s"
-    backup: "{{ sshd_backup }}"
-  notify: reload_sshd
+  vars:
+    <<: *share_vars
+  block:
+    - name: Create a temporary hostkey for syntax verification if needed
+      tempfile:
+        state: directory
+      register: sshd_test_hostkey
+      when:
+        - __sshd_hostkeys_from_config | from_json == []
+        - sshd_config_file != "/etc/ssh/sshd_config"
+
+    - name: Generate temporary hostkey
+      shell: "ssh-keygen -q -t rsa -f {{ sshd_test_hostkey.path }}/rsa_key -C '' -N ''"
+      when: sshd_test_hostkey.path is defined
+
+    - name: Create the configuration file
+      template:
+        src: sshd_config.j2
+        dest: "{{ sshd_config_file }}"
+        owner: "{{ sshd_config_owner }}"
+        group: "{{ sshd_config_group }}"
+        mode: "{{ sshd_config_mode }}"
+        validate: >-
+          {% if sshd_test_hostkey is defined and sshd_test_hostkey.path is defined %}
+            {{ sshd_binary }} -t -f %s -h {{ sshd_test_hostkey.path }}/rsa_key
+          {% else %}
+            {{ sshd_binary }} -t -f %s
+          {% endif %}
+        backup: "{{ sshd_backup }}"
+      notify: reload_sshd
+  rescue:
+    - name: re-raise the error
+      fail:
+        msg: "{{ ansible_failed_result }}"
+  always:
+    - name: Remove temporary host keys
+      file:
+        path: "{{ sshd_test_hostkey.path }}"
+        state: absent
+      when: sshd_test_hostkey.path is defined
 
 - name: Install systemd service files
   block: