identity: Autodetect FreeIPA server with DNS (#50988)

* identity: Autodetect FreeIPA server with DNS

This adds the ability for the freeIPA related modules to be able to
auto-detect the IPA server through DNS.

This takes advantage of the fact that a lot of FreeIPA deployments
configure their hosts to use IPA as the nameserver.

This check is only used if we didn't set neither the ipa_host parameter,
nor the environment variable IPA_HOST.

* identity: Specify docs for DNS discovery of ipa_host

These docs specify that it can now default to DNS if the 'ipa-ca' entry
is available.

lib/ansible/module_utils/ipa.py

@@ -28,13 +28,28 @@
 # USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import json
+import socket
 
 import re
 from ansible.module_utils._text import to_bytes, to_native, to_text
 from ansible.module_utils.six import PY3
 from ansible.module_utils.six.moves.urllib.parse import quote
 from ansible.module_utils.urls import fetch_url
-from ansible.module_utils.basic import env_fallback
+from ansible.module_utils.basic import env_fallback, AnsibleFallbackNotFound
+
+
+def _env_then_dns_fallback(*args, **kwargs):
+    ''' Load value from environment or DNS in that order'''
+    try:
+        return env_fallback(*args, **kwargs)
+    except AnsibleFallbackNotFound:
+        # If no host was given, we try to guess it from IPA.
+        # The ipa-ca entry is a standard entry that IPA will have set for
+        # the CA.
+        try:
+            return socket.gethostbyaddr(socket.gethostbyname('ipa-ca'))[0]
+        except Exception:
+            raise AnsibleFallbackNotFound
 
 
 class IPAClient(object):
@@ -181,7 +196,7 @@ class IPAClient(object):
 def ipa_argument_spec():
     return dict(
         ipa_prot=dict(type='str', default='https', choices=['http', 'https'], fallback=(env_fallback, ['IPA_PROT'])),
-        ipa_host=dict(type='str', default='ipa.example.com', fallback=(env_fallback, ['IPA_HOST'])),
+        ipa_host=dict(type='str', default='ipa.example.com', fallback=(_env_then_dns_fallback, ['IPA_HOST'])),
         ipa_port=dict(type='int', default=443, fallback=(env_fallback, ['IPA_PORT'])),
         ipa_user=dict(type='str', default='admin', fallback=(env_fallback, ['IPA_USER'])),
         ipa_pass=dict(type='str', required=True, no_log=True, fallback=(env_fallback, ['IPA_PASS'])),

lib/ansible/plugins/doc_fragments/ipa.py

@@ -18,7 +18,9 @@ options:
     description:
     - IP or hostname of IPA server.
     - If the value is not specified in the task, the value of environment variable C(IPA_HOST) will be used instead.
-    - If both the environment variable C(IPA_HOST) and the value are not specified in the task, then default value is set.
+    - If both the environment variable C(IPA_HOST) and the value are not specified in the task, then DNS will be used to try to discover the FreeIPA server.
+    - The relevant entry needed in FreeIPA is the 'ipa-ca' entry.
+    - If neither the DNS entry, nor the environment C(IPA_HOST), nor the value are available in the task, then the default value will be used.
     - 'Environment variable fallback mechanism is added in version 2.5.'
     default: ipa.example.com
   ipa_user: