EC2: Rewrite network config on AWS Classic instances every boot

AWS EC2 instances' network come in 2 basic flavors: Classic and VPC
(Virtual Private Cloud). The former has an interesting behavior of having
its MAC address changed whenever the instance is stopped/restarted. This
behavior is not observed in VPC instances.

In Ubuntu 18.04 (Bionic) the network "management" changed from ENI-style
(etc/network/interfaces) to netplan, and when using netplan we observe
the following block present in /etc/netplan/50-cloud-init.yaml:

match:
  macaddress: aa:bb:cc:dd:ee:ff

Jani Ollikainen noticed in Launchpad bug #1802073 that the EC2 Classic
instances were booting without network access in Bionic after stop/restart
procedure, due to their MAC address change behavior. It was narrowed down
to the netplan MAC match block, that kept the old MAC address after
stopping and restarting an instance, since the network configuration
writing happens by default only once in EC2 instances, in the first boot.

This patch changes the network configuration write to every boot in EC2
Classic instances, by checking against the "vpc-id" metadata information
provided only in the VPC instances - if we don't have this metadata value,
cloud-init will rewrite the network configuration file in every boot.

This was tested in an EC2 Classic instance and proved to fix the issue;
unit tests were also added for the new method is_classic_instance().

LP: #1802073

Reported-by: Jani Ollikainen <jani.ollikainen@ik.fi>
Suggested-by: Ryan Harper <ryan.harper@canonical.com>
Co-developed-by: Chad Smith <chad.smith@canonical.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/unittests/test_datasource/test_ec2.py b/tests/unittests/test_datasource/test_ec2.py
index 1a5956d9..20d59bfd 100644
--- a/tests/unittests/test_datasource/test_ec2.py
+++ b/tests/unittests/test_datasource/test_ec2.py
@@ -401,6 +401,30 @@ class TestEc2(test_helpers.HttprettyTestCase):
         ds.metadata = DEFAULT_METADATA
         self.assertEqual('my-identity-id', ds.get_instance_id())
 
+    def test_classic_instance_true(self):
+        """If no vpc-id in metadata, is_classic_instance must return true."""
+        md_copy = copy.deepcopy(DEFAULT_METADATA)
+        ifaces_md = md_copy.get('network', {}).get('interfaces', {})
+        for _mac, mac_data in ifaces_md.get('macs', {}).items():
+            if 'vpc-id' in mac_data:
+                del mac_data['vpc-id']
+
+        ds = self._setup_ds(
+            platform_data=self.valid_platform_data,
+            sys_cfg={'datasource': {'Ec2': {'strict_id': False}}},
+            md={'md': md_copy})
+        self.assertTrue(ds.get_data())
+        self.assertTrue(ds.is_classic_instance())
+
+    def test_classic_instance_false(self):
+        """If vpc-id in metadata, is_classic_instance must return false."""
+        ds = self._setup_ds(
+            platform_data=self.valid_platform_data,
+            sys_cfg={'datasource': {'Ec2': {'strict_id': False}}},
+            md={'md': DEFAULT_METADATA})
+        self.assertTrue(ds.get_data())
+        self.assertFalse(ds.is_classic_instance())
+
     @mock.patch('cloudinit.net.dhcp.maybe_perform_dhcp_discovery')
     def test_valid_platform_with_strict_true(self, m_dhcp):
         """Valid platform data should return true with strict_id true."""