certbot-dns-google: enable automatic credential lookup on google cloud (#5117)

- when no credentials are passed it will try to get valid credentials
using the google metadata service
- this is a feature of the google SDK, so we don't need to handle that
explicitly
- previous behaviour with a credentials file is retained

certbot-dns-google/certbot_dns_google/__init__.py

@@ -10,7 +10,7 @@ Named Arguments
 ========================================  =====================================
 ``--dns-google-credentials``              Google Cloud Platform credentials_
                                           JSON file.
-                                          (Required)
+                                          (Required - Optional on Google Compute Engine)
 ``--dns-google-propagation-seconds``      The number of seconds to wait for DNS
                                           to propagate before asking the ACME
                                           server to verify the DNS record.
@@ -21,8 +21,8 @@ Named Arguments
 Credentials
 -----------
 
-Use of this plugin requires a configuration file containing Google Cloud
+Use of this plugin requires Google Cloud Platform API credentials
-Platform API credentials for an account with the following permissions:
+for an account with the following permissions:
 
 * ``dns.changes.create``
 * ``dns.changes.get``
@@ -33,7 +33,12 @@ Platform API credentials for an account with the following permissions:
 Google provides instructions for `creating a service account <https://developers
 .google.com/identity/protocols/OAuth2ServiceAccount#creatinganaccount>`_ and
 `information about the required permissions <https://cloud.google.com/dns/access
--control#permissions_and_roles>`_.
+-control#permissions_and_roles>`_. If you're running on Google Compute Engine,
+you can `assign the service account to the instance <https://cloud.google.com/
+compute/docs/access/create-enable-service-accounts-for-instances>`_ which
+is running certbot. A credentials file is not required in this case, as they
+are automatically obtained by certbot through the `metadata service
+<https://cloud.google.com/compute/docs/storing-retrieving-metadata>`_ .
 
 .. code-block:: json
    :name: credentials.json

certbot-dns-google/certbot_dns_google/dns_google.py

@@ -2,6 +2,7 @@
 import json
 import logging
 
+import httplib2
 import zope.interface
 from googleapiclient import discovery
 from googleapiclient import errors as googleapiclient_errors
@@ -15,6 +16,8 @@ logger = logging.getLogger(__name__)
 
 ACCT_URL = 'https://developers.google.com/identity/protocols/OAuth2ServiceAccount#creatinganaccount'
 PERMISSIONS_URL = 'https://cloud.google.com/dns/access-control#permissions_and_roles'
+METADATA_URL = 'http://metadata.google.internal/computeMetadata/v1/'
+METADATA_HEADERS = {'Metadata-Flavor': 'Google'}
 
 
 @zope.interface.implementer(interfaces.IAuthenticator)
@@ -39,14 +42,27 @@ class Authenticator(dns_common.DNSAuthenticator):
         add('credentials',
             help=('Path to Google Cloud DNS service account JSON file. (See {0} for' +
                   'information about creating a service account and {1} for information about the' +
-                  'required permissions.)').format(ACCT_URL, PERMISSIONS_URL))
+                  'required permissions.)').format(ACCT_URL, PERMISSIONS_URL),
+            default=None)
 
     def more_info(self): # pylint: disable=missing-docstring,no-self-use
         return 'This plugin configures a DNS TXT record to respond to a dns-01 challenge using ' + \
                'the Google Cloud DNS API.'
 
     def _setup_credentials(self):
-        self._configure_file('credentials', 'path to Google Cloud DNS service account JSON file')
+        if self.conf('credentials') is None:
+            try:
+                # use project_id query to check for availability of google metadata server
+                # we won't use the result but know we're not on GCP when an exception is thrown
+                _GoogleClient.get_project_id()
+            except (ValueError, httplib2.ServerNotFoundError):
+                raise errors.PluginError('Unable to get Google Cloud Metadata and no credentials'
+                                         ' specified. Automatic credential lookup is only '
+                                         'available on Google Cloud Platform. Please configure'
+                                         ' credentials using --dns-google-credentials <file>')
+        else:
+            self._configure_file('credentials',
+                                 'path to Google Cloud DNS service account JSON file')
 
             dns_common.validate_file_permissions(self.conf('credentials'))
 
@@ -65,13 +81,18 @@ class _GoogleClient(object):
     Encapsulates all communication with the Google Cloud DNS API.
     """
 
-    def __init__(self, account_json):
+    def __init__(self, account_json=None):
 
         scopes = ['https://www.googleapis.com/auth/ndev.clouddns.readwrite']
+        if account_json is not None:
             credentials = ServiceAccountCredentials.from_json_keyfile_name(account_json, scopes)
-        self.dns = discovery.build('dns', 'v1', credentials=credentials, cache_discovery=False)
             with open(account_json) as account:
                 self.project_id = json.load(account)['project_id']
+        else:
+            credentials = None
+            self.project_id = self.get_project_id()
+
+        self.dns = discovery.build('dns', 'v1', credentials=credentials, cache_discovery=False)
 
     def add_txt_record(self, domain, record_name, record_content, record_ttl):
         """
@@ -183,3 +204,24 @@ class _GoogleClient(object):
 
         raise errors.PluginError('Unable to determine managed zone for {0} using zone names: {1}.'
                                  .format(domain, zone_dns_name_guesses))
+
+    @staticmethod
+    def get_project_id():
+        """
+        Query the google metadata service for the current project ID
+
+        This only works on Google Cloud Platform
+
+        :raises ServerNotFoundError: Not running on Google Compute or DNS not available
+        :raises ValueError: Server is found, but response code is not 200
+        :returns: project id
+        """
+        url = '{0}project/project-id'.format(METADATA_URL)
+
+        # Request an access token from the metadata server.
+        http = httplib2.Http()
+        r, content = http.request(url, headers=METADATA_HEADERS)
+        if r.status != 200:
+            raise ValueError("Invalid status code: {0}".format(r))
+
+        return content

certbot-dns-google/certbot_dns_google/dns_google_test.py

@@ -5,8 +5,10 @@ import unittest
 
 import mock
 from googleapiclient.errors import Error
+from httplib2 import ServerNotFoundError
 
 from certbot import errors
+from certbot.errors import PluginError
 from certbot.plugins import dns_test_common
 from certbot.plugins.dns_test_common import DOMAIN
 from certbot.tests import util as test_util
@@ -50,6 +52,11 @@ class AuthenticatorTest(test_util.TempDirTestCase, dns_test_common.BaseAuthentic
         expected = [mock.call.del_txt_record(DOMAIN, '_acme-challenge.'+DOMAIN, mock.ANY, mock.ANY)]
         self.assertEqual(expected, self.mock_client.mock_calls)
 
+    @mock.patch('httplib2.Http.request', side_effect=ServerNotFoundError)
+    def test_without_auth(self, unused_mock):
+        self.config.google_credentials = None
+        self.assertRaises(PluginError, self.auth.perform, [self.achall])
+
 
 class GoogleClientTest(unittest.TestCase):
     record_name = "foo"
@@ -74,11 +81,24 @@ class GoogleClientTest(unittest.TestCase):
 
         return client, mock_changes
 
+    @mock.patch('googleapiclient.discovery.build')
+    @mock.patch('oauth2client.service_account.ServiceAccountCredentials.from_json_keyfile_name')
+    @mock.patch('certbot_dns_google.dns_google._GoogleClient.get_project_id')
+    def test_client_without_credentials(self, get_project_id_mock, credential_mock,
+                                        unused_discovery_mock):
+        from certbot_dns_google.dns_google import _GoogleClient
+        _GoogleClient(None)
+        self.assertFalse(credential_mock.called)
+        self.assertTrue(get_project_id_mock.called)
+
     @mock.patch('oauth2client.service_account.ServiceAccountCredentials.from_json_keyfile_name')
     @mock.patch('certbot_dns_google.dns_google.open',
                 mock.mock_open(read_data='{"project_id": "' + PROJECT_ID + '"}'), create=True)
-    def test_add_txt_record(self, unused_credential_mock):
+    @mock.patch('certbot_dns_google.dns_google._GoogleClient.get_project_id')
+    def test_add_txt_record(self, get_project_id_mock, credential_mock):
         client, changes = self._setUp_client_with_mock([{'managedZones': [{'id': self.zone}]}])
+        credential_mock.assert_called_once_with('/not/a/real/path.json', mock.ANY)
+        self.assertFalse(get_project_id_mock.called)
 
         client.add_txt_record(DOMAIN, self.record_name, self.record_content, self.record_ttl)
 
@@ -197,6 +217,34 @@ class GoogleClientTest(unittest.TestCase):
 
         client.del_txt_record(DOMAIN, self.record_name, self.record_content, self.record_ttl)
 
+    def test_get_project_id(self):
+        from certbot_dns_google.dns_google import _GoogleClient
+
+        response = DummyResponse()
+        response.status = 200
+
+        with mock.patch('httplib2.Http.request', return_value=(response, 1234)):
+            project_id = _GoogleClient.get_project_id()
+            self.assertEqual(project_id, 1234)
+
+        failed_response = DummyResponse()
+        failed_response.status = 404
+
+        with mock.patch('httplib2.Http.request',
+                        return_value=(failed_response, "some detailed http error response")):
+            self.assertRaises(ValueError, _GoogleClient.get_project_id)
+
+        with mock.patch('httplib2.Http.request', side_effect=ServerNotFoundError):
+            self.assertRaises(ServerNotFoundError, _GoogleClient.get_project_id)
+
+
+class DummyResponse(object):
+    """
+    Dummy object to create a fake HTTPResponse (the actual one requires a socket and we only
+     need the status attribute)
+    """
+    def __init__(self):
+        self.status = 200
 
 if __name__ == "__main__":
     unittest.main()  # pragma: no cover

certbot-dns-google/setup.py

@@ -19,6 +19,8 @@ install_requires = [
     # will tolerate; see #2599:
     'setuptools>=1.0',
     'zope.interface',
+    # already a dependency of google-api-python-client, but added for consistency
+    'httplib2'
 ]
 
 docs_extras = [