cert-request: allow ipa-ca.$DOMAIN dNSName for IPA servers

For detailed discussion on the purpose of this change and the design
decisions made, see `git log -1 $THIS_COMMIT~1`.

ACME support requires TLS and we want ACME clients to access the
service via the ipa-ca.$DOMAIN DNS name.  So we need to add the
ipa-ca.$DOMAIN dNSName to IPA servers' HTTP certificates.  To
facilitiate this, add a special case to the cert-request command
processing.  The rule is:

- if the dnsName being validated is "ipa-ca.$DOMAIN"
- and the subject principal is an "HTTP/..." service
- and the subject principal's hostname is an IPA server

Then that name (i.e. "ipa-ca.$DOMAIN") is immediately allowed.
Otherwise continue with the usual dnsName validation.

Part of: https://pagure.io/freeipa/issue/8186

Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipaserver/plugins/cert.py

@@ -36,7 +36,7 @@ from ipalib import api
 from ipalib import errors, messages
 from ipalib import x509
 from ipalib import ngettext
-from ipalib.constants import IPA_CA_CN
+from ipalib.constants import IPA_CA_CN, IPA_CA_RECORD
 from ipalib.crud import Create, PKQuery, Retrieve, Search
 from ipalib.frontend import Method, Object
 from ipalib.parameters import (
@@ -799,6 +799,21 @@ class cert_request(Create, BaseCertMethod, VirtualCommand):
 
                 name = gn.value
 
+                # Special case: if the DNS name is ipa-ca.$DOMAIN and if the
+                # subject principal is the HTTP service for an IPA server
+                # then allow the name.
+                if name == f'{IPA_CA_RECORD}.{self.api.env.domain}' \
+                        and principal.is_service \
+                        and principal.service_name == 'HTTP':
+                    try:
+                        self.api.Command.server_show(principal.hostname)
+                    except errors.NotFound:
+                        pass  # not an IPA server; proceed as usual
+                    else:
+                        # subject principal is an IPA server, so the
+                        # ipa-ca.$DOMAIN name is allowed
+                        continue
+
                 if _dns_name_matches_principal(name, principal, principal_obj):
                     san_dnsnames.add(name)
                     continue  # nothing more to check for this alt name