Using "set-name" with interface specific DNS with a v2 network config causes a KeyError

This bug was first reported at https://github.com/kubernetes-sigs/image-builder/issues/712 and occurs when the v2 network configuration directive "set-name" is used in conjunction with interface specific DNS settings.

Cloud-Provider: VMware, but does not matter as this bug is distro and DS agnostic
Config: The metadata was set to the following:

    instance-id: "wlan-1-md-0-775d8846bf-9bfrd"
    local-hostname: "wlan-1-md-0-775d8846bf-9bfrd"
    wait-on-network:
      ipv4: false
      ipv6: false
    network:
      version: 2
      ethernets:
        id0:
          match:
            macaddress: "00:50:56:a1:d8:a7"
          set-name: "eth0"
          wakeonlan: true
          addresses:
          - "10.196.27.122/28"
          gateway4: "10.196.27.126"
          nameservers:
            addresses:
            - "10.102.102.132"
            - "10.102.102.133"
            - "10.90.24.1"
            search:
            - "refsa1.bn.schiff.telekom.de"

Again though, the platform is likely irrelevant as this seems to be a bug introduced with https://github.com/canonical/cloud-init/commit/abd2da5777195e7e432b0d53a3f7f29d071dd50e, and can occur on any platform with and datasource as long as network v2 config is used with "set-name" and interface specific DNS.

The bug can be surfaced via unit test by patching the v21.3 version of Cloud-Init with the following:

    diff --git a/cloudinit/net/tests/test_network_state.py b/cloudinit/net/tests/test_network_state.py
    index 84e8308a..c0aa78a0 100644
    --- a/cloudinit/net/tests/test_network_state.py
    +++ b/cloudinit/net/tests/test_network_state.py
    @@ -52,6 +52,7 @@ network:
         eth1:
           match:
             macaddress: '66:77:88:99:00:11'
    + set-name: "eth2"
           nameservers:
             search: [foo.local, bar.local]
             addresses: [4.4.4.4]

Next, run the affected test from the root of the Cloud-Init source tree:

    $ make clean_pyc && \
      PYTHONPATH="$(pwd)" \
      python3 -m pytest -v cloudinit/net/tests/test_network_state.py

The output will resemble the following:

    ====================================================================== test session starts ======================================================================
    platform darwin -- Python 3.9.7, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- /usr/local/opt/python@3.9/bin/python3.9
    cachedir: .pytest_cache
    rootdir: /Users/akutz/Projects/cloud-init, configfile: tox.ini
    collected 10 items

    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_empty_v1_config_gets_network_state PASSED [ 10%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_empty_v2_config_gets_network_state PASSED [ 20%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_missing_version_returns_none PASSED [ 30%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_unknown_versions_returns_none PASSED [ 40%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_valid_config_gets_network_state PASSED [ 50%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfig::test_version_2_passes_self_as_config PASSED [ 60%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseConfigV2::test_version_2_ignores_renderer_key PASSED [ 70%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseNameservers::test_v1_nameservers_valid PASSED [ 80%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseNameservers::test_v1_nameservers_invalid PASSED [ 90%]
    cloudinit/net/tests/test_network_state.py::TestNetworkStateParseNameservers::test_v2_nameservers FAILED [100%]

    =========================================================================== FAILURES ============================================================================
    _____________________________________________________ TestNetworkStateParseNameservers.test_v2_nameservers ______________________________________________________

    self = <cloudinit.net.tests.test_network_state.TestNetworkStateParseNameservers object at 0x10a7db9d0>

        def test_v2_nameservers(self):
    > config = self._parse_network_state_from_config(V2_CONFIG_NAMESERVERS)

    cloudinit/net/tests/test_network_state.py:139:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    cloudinit/net/tests/test_network_state.py:114: in _parse_network_state_from_config
        return network_state.parse_net_config_data(yaml['network'])
    cloudinit/net/network_state.py:1074: in parse_net_config_data
        nsi.parse_config(skip_broken=skip_broken)
    cloudinit/net/network_state.py:261: in parse_config
        self.parse_config_v2(skip_broken=skip_broken)
    cloudinit/net/network_state.py:310: in parse_config_v2
        self._v2_common(command)
    cloudinit/net/network_state.py:722: in _v2_common
        self._handle_individual_nameserver(name_cmd, iface)
    cloudinit/net/network_state.py:91: in decorator
        return func(self, command, *args, **kwargs)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = <cloudinit.net.network_state.NetworkStateInterpreter object at 0x10a7e4f10>
    command = {'address': ['4.4.4.4'], 'search': ['foo.local', 'bar.local'], 'type': 'nameserver'}, iface = 'eth1'

        @ensure_command_keys(['address'])
        def _handle_individual_nameserver(self, command, iface):
            _iface = self._network_state.get('interfaces')
            nameservers, search = self._parse_dns(command)
    > _iface[iface]['dns'] = {'nameservers': nameservers, 'search': search}
    E KeyError: 'eth1'

    cloudinit/net/network_state.py:546: KeyError
    ----------------------------------------------------------------------- Captured log call -----------------------------------------------------------------------
    2021-10-08 00:57:41 DEBUG cloudinit.net.network_state:network_state.py:670 v2(ethernets) -> v1(physical):
    {'type': 'physical', 'name': 'eth0', 'mac_address': '00:11:22:33:44:55', 'match': {'macaddress': '00:11:22:33:44:55'}}
    2021-10-08 00:57:41 DEBUG cloudinit.net.network_state:network_state.py:670 v2(ethernets) -> v1(physical):
    {'type': 'physical', 'name': 'eth2', 'mac_address': '66:77:88:99:00:11', 'match': {'macaddress': '66:77:88:99:00:11'}}
    2021-10-08 00:57:41 DEBUG cloudinit.net.network_state:network_state.py:711 v2_common: handling config:
    {'eth0': {'match': {'macaddress': '00:11:22:33:44:55'}, 'nameservers': {'search': ['spam.local', 'eggs.local'], 'addresses': ['8.8.8.8']}}, 'eth1': {'match': {'macaddress': '66:77:88:99:00:11'}, 'set-name': 'eth2', 'nameservers': {'search': ['foo.local', 'bar.local'], 'addresses': ['4.4.4.4']}}}
    ======================================================================= warnings summary ========================================================================
    ../../../../usr/local/lib/python3.9/site-packages/_pytest/config/__init__.py:1183
      /usr/local/lib/python3.9/site-packages/_pytest/config/__init__.py:1183: PytestDeprecationWarning: The --strict option is deprecated, use --strict-markers instead.
        self.issue_config_time_warning(

    conftest.py:68
      /Users/akutz/Projects/cloud-init/conftest.py:68: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
      Use @pytest.fixture instead; they are the same.
        @pytest.yield_fixture(autouse=True)

    conftest.py:169
      /Users/akutz/Projects/cloud-init/conftest.py:169: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
      Use @pytest.fixture instead; they are the same.
        def httpretty():

    -- Docs: https://docs.pytest.org/en/stable/warnings.html
    ==================================================================== short test summary info ====================================================================
    FAILED cloudinit/net/tests/test_network_state.py::TestNetworkStateParseNameservers::test_v2_nameservers - KeyError: 'eth1'
    ============================================================ 1 failed, 9 passed, 3 warnings in 0.60s ============================================================

This is is occurring because the code to iterate over the interfaces when configuring interface-specific DNS is using the original interface name, not the one from the "set-name" directive. The following patch corrects the issue:

    diff --git a/cloudinit/net/network_state.py b/cloudinit/net/network_state.py
    index 95b064f0..06ff8e96 100644
    --- a/cloudinit/net/network_state.py
    +++ b/cloudinit/net/network_state.py
    @@ -543,7 +543,11 @@ class NetworkStateInterpreter(metaclass=CommandHandlerMeta):
         def _handle_individual_nameserver(self, command, iface):
             _iface = self._network_state.get('interfaces')
             nameservers, search = self._parse_dns(command)
    - _iface[iface]['dns'] = {'nameservers': nameservers, 'search': search}
    + try:
    + _iface[iface]['dns'] = {'nameservers': nameservers, 'search': search}
    + except:
    + print("original iface name: %s\niface dict: %s\n" % (iface, _iface))
    + raise

         @ensure_command_keys(['destination'])
         def handle_route(self, command):
    diff --git a/cloudinit/net/tests/test_network_state.py b/cloudinit/net/tests/test_network_state.py
    index 84e8308a..45e99171 100644
    --- a/cloudinit/net/tests/test_network_state.py
    +++ b/cloudinit/net/tests/test_network_state.py
    @@ -52,6 +52,7 @@ network:
         eth1:
           match:
             macaddress: '66:77:88:99:00:11'
    + set-name: "ens92"
           nameservers:
             search: [foo.local, bar.local]
             addresses: [4.4.4.4]

Now the above test passes. A PR will be opened on Cloud-Init's GitHub repository with the above patch.