netplan: fix mtu if provided by network config for all rendered types

When network configuration for any interface defines maximum transmission
values (MTU) the netplan, eni and sysconfig renders will take into account
any device-level, or subnet-level mtu values.

When network configuration has conflicting device-level and ipv4 subnet
mtu values, the subnet-specific value is honored and a warning will be
logged about any ignored device-level setting.

LP: #1774666

6 files changed, 91 insertions, 12 deletions

diff --git a/cloudinit/net/eni.py b/cloudinit/net/eni.py
index c6a71d16..bd20a361 100644
--- a/cloudinit/net/eni.py
+++ b/cloudinit/net/eni.py
@@ -10,9 +10,12 @@ from . import ParserError
 from . import renderer
 from .network_state import subnet_is_ipv6
 
+from cloudinit import log as logging
 from cloudinit import util
 
 
+LOG = logging.getLogger(__name__)
+
 NET_CONFIG_COMMANDS = [
     "pre-up", "up", "post-up", "down", "pre-down", "post-down",
 ]
@@ -61,7 +64,7 @@ def _iface_add_subnet(iface, subnet):
 
 
 # TODO: switch to valid_map for attrs
-def _iface_add_attrs(iface, index):
+def _iface_add_attrs(iface, index, ipv4_subnet_mtu):
     # If the index is non-zero, this is an alias interface. Alias interfaces
     # represent additional interface addresses, and should not have additional
     # attributes. (extra attributes here are almost always either incorrect,
@@ -100,6 +103,13 @@ def _iface_add_attrs(iface, index):
             value = 'on' if iface[key] else 'off'
         if not value or key in ignore_map:
             continue
+        if key == 'mtu' and ipv4_subnet_mtu:
+            if value != ipv4_subnet_mtu:
+                LOG.warning(
+                    "Network config: ignoring %s device-level mtu:%s because"
+                    " ipv4 subnet-level mtu:%s provided.",
+                    iface['name'], value, ipv4_subnet_mtu)
+            continue
         if key in multiline_keys:
             for v in value:
                 content.append("    {0} {1}".format(renames.get(key, key), v))
@@ -377,12 +387,15 @@ class Renderer(renderer.Renderer):
         subnets = iface.get('subnets', {})
         if subnets:
             for index, subnet in enumerate(subnets):
+                ipv4_subnet_mtu = None
                 iface['index'] = index
                 iface['mode'] = subnet['type']
                 iface['control'] = subnet.get('control', 'auto')
                 subnet_inet = 'inet'
                 if subnet_is_ipv6(subnet):
                     subnet_inet += '6'
+                else:
+                    ipv4_subnet_mtu = subnet.get('mtu')
                 iface['inet'] = subnet_inet
                 if subnet['type'].startswith('dhcp'):
                     iface['mode'] = 'dhcp'
@@ -397,7 +410,7 @@ class Renderer(renderer.Renderer):
                     _iface_start_entry(
                         iface, index, render_hwaddress=render_hwaddress) +
                     _iface_add_subnet(iface, subnet) +
-                    _iface_add_attrs(iface, index)
+                    _iface_add_attrs(iface, index, ipv4_subnet_mtu)
                 )
                 for route in subnet.get('routes', []):
                     lines.extend(self._render_route(route, indent="    "))
@@ -409,7 +422,8 @@ class Renderer(renderer.Renderer):
             if 'bond-master' in iface or 'bond-slaves' in iface:
                 lines.append("auto {name}".format(**iface))
             lines.append("iface {name} {inet} {mode}".format(**iface))
-            lines.extend(_iface_add_attrs(iface, index=0))
+            lines.extend(
+                _iface_add_attrs(iface, index=0, ipv4_subnet_mtu=None))
             sections.append(lines)
         return sections
 
diff --git a/cloudinit/net/netplan.py b/cloudinit/net/netplan.py
index 63443484..40143634 100644
--- a/cloudinit/net/netplan.py
+++ b/cloudinit/net/netplan.py
@@ -34,7 +34,7 @@ def _get_params_dict_by_match(config, match):
                 if key.startswith(match))
 
 
-def _extract_addresses(config, entry):
+def _extract_addresses(config, entry, ifname):
     """This method parse a cloudinit.net.network_state dictionary (config) and
        maps netstate keys/values into a dictionary (entry) to represent
        netplan yaml.
@@ -124,6 +124,15 @@ def _extract_addresses(config, entry):
 
             addresses.append(addr)
 
+    if 'mtu' in config:
+        entry_mtu = entry.get('mtu')
+        if entry_mtu and config['mtu'] != entry_mtu:
+            LOG.warning(
+                "Network config: ignoring %s device-level mtu:%s because"
+                " ipv4 subnet-level mtu:%s provided.",
+                ifname, config['mtu'], entry_mtu)
+        else:
+            entry['mtu'] = config['mtu']
     if len(addresses) > 0:
         entry.update({'addresses': addresses})
     if len(routes) > 0:
@@ -262,10 +271,7 @@ class Renderer(renderer.Renderer):
                     else:
                         del eth['match']
                         del eth['set-name']
-                if 'mtu' in ifcfg:
-                    eth['mtu'] = ifcfg.get('mtu')
-
-                _extract_addresses(ifcfg, eth)
+                _extract_addresses(ifcfg, eth, ifname)
                 ethernets.update({ifname: eth})
 
             elif if_type == 'bond':
@@ -288,7 +294,7 @@ class Renderer(renderer.Renderer):
                 slave_interfaces = ifcfg.get('bond-slaves')
                 if slave_interfaces == 'none':
                     _extract_bond_slaves_by_name(interfaces, bond, ifname)
-                _extract_addresses(ifcfg, bond)
+                _extract_addresses(ifcfg, bond, ifname)
                 bonds.update({ifname: bond})
 
             elif if_type == 'bridge':
@@ -321,7 +327,7 @@ class Renderer(renderer.Renderer):
 
                 if len(br_config) > 0:
                     bridge.update({'parameters': br_config})
-                _extract_addresses(ifcfg, bridge)
+                _extract_addresses(ifcfg, bridge, ifname)
                 bridges.update({ifname: bridge})
 
             elif if_type == 'vlan':
@@ -333,7 +339,7 @@ class Renderer(renderer.Renderer):
                 macaddr = ifcfg.get('mac_address', None)
                 if macaddr is not None:
                     vlan['macaddress'] = macaddr.lower()
-                _extract_addresses(ifcfg, vlan)
+                _extract_addresses(ifcfg, vlan, ifname)
                 vlans.update({ifname: vlan})
 
         # inject global nameserver values under each all interface which
diff --git a/cloudinit/net/sysconfig.py b/cloudinit/net/sysconfig.py
index e53b9f1b..3d719238 100644
--- a/cloudinit/net/sysconfig.py
+++ b/cloudinit/net/sysconfig.py
@@ -304,6 +304,13 @@ class Renderer(renderer.Renderer):
                     mtu_key = 'IPV6_MTU'
                     iface_cfg['IPV6INIT'] = True
                 if 'mtu' in subnet:
+                    mtu_mismatch = bool(mtu_key in iface_cfg and
+                                        subnet['mtu'] != iface_cfg[mtu_key])
+                    if mtu_mismatch:
+                        LOG.warning(
+                            'Network config: ignoring %s device-level mtu:%s'
+                            ' because ipv4 subnet-level mtu:%s provided.',
+                            iface_cfg.name, iface_cfg[mtu_key], subnet['mtu'])
                     iface_cfg[mtu_key] = subnet['mtu']
             elif subnet_type == 'manual':
                 # If the subnet has an MTU setting, then ONBOOT=True
diff --git a/doc/rtd/topics/network-config-format-v1.rst b/doc/rtd/topics/network-config-format-v1.rst
index 2f8ab54c..3b0148ca 100644
--- a/doc/rtd/topics/network-config-format-v1.rst
+++ b/doc/rtd/topics/network-config-format-v1.rst
@@ -130,6 +130,18 @@ the bond interfaces.
 The ``bond_interfaces`` key accepts a list of network device ``name`` values
 from the configuration.  This list may be empty.
 
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
+.. note::
+
+  The possible supported values of a device's MTU is not available at
+  configuration time.  It's possible to specify a value too large or to
+  small for a device and may be ignored by the device.
+
 **params**:  *<Dictionary of key: value bonding parameter pairs>*
 
 The ``params`` key in a bond holds a dictionary of bonding parameters.
@@ -268,6 +280,21 @@ Type ``vlan`` requires the following keys:
 - ``vlan_link``: Specify the underlying link via its ``name``.
 - ``vlan_id``: Specify the VLAN numeric id.
 
+The following optional keys are supported:
+
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
+.. note::
+
+  The possible supported values of a device's MTU is not available at
+  configuration time.  It's possible to specify a value too large or to
+  small for a device and may be ignored by the device.
+
+
 **VLAN Example**::
 
    network:
diff --git a/doc/rtd/topics/network-config-format-v2.rst b/doc/rtd/topics/network-config-format-v2.rst
index 335d236a..ea370ef5 100644
--- a/doc/rtd/topics/network-config-format-v2.rst
+++ b/doc/rtd/topics/network-config-format-v2.rst
@@ -174,6 +174,12 @@ recognized by ``inet_pton(3)``
 Example for IPv4: ``gateway4: 172.16.0.1``
 Example for IPv6: ``gateway6: 2001:4::1``
 
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
 **nameservers**: *<(mapping)>*
 
 Set DNS servers and search domains, for manual address configuration. There
diff --git a/tests/unittests/test_net.py b/tests/unittests/test_net.py
index e13ca3ce..5ab61cf2 100644
--- a/tests/unittests/test_net.py
+++ b/tests/unittests/test_net.py
@@ -525,6 +525,7 @@ NETWORK_CONFIGS = {
             config:
               - type: 'physical'
                 name: 'iface0'
+                mtu: 8999
                 subnets:
                   - type: static
                     address: 192.168.14.2/24
@@ -660,8 +661,8 @@ iface eth0.101 inet static
     dns-nameservers 192.168.0.10 10.23.23.134
     dns-search barley.maas sacchromyces.maas brettanomyces.maas
     gateway 192.168.0.1
-    hwaddress aa:bb:cc:dd:ee:11
     mtu 1500
+    hwaddress aa:bb:cc:dd:ee:11
     vlan-raw-device eth0
     vlan_id 101
 
@@ -757,6 +758,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
                         id: 101
                         link: eth0
                         macaddress: aa:bb:cc:dd:ee:11
+                        mtu: 1500
                         nameservers:
                             addresses:
                             - 192.168.0.10
@@ -920,6 +922,8 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
                   mtu: 1500
                   subnets:
                     - type: static
+                      # When 'mtu' matches device-level mtu, no warnings
+                      mtu: 1500
                       address: 192.168.0.2/24
                       gateway: 192.168.0.1
                       dns_nameservers:
@@ -1028,6 +1032,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
               - type: bond
                 name: bond0
                 mac_address: "aa:bb:cc:dd:e8:ff"
+                mtu: 9000
                 bond_interfaces:
                   - bond0s0
                   - bond0s1
@@ -1070,6 +1075,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
                      interfaces:
                      - bond0s0
                      - bond0s1
+                     mtu: 9000
                      parameters:
                          mii-monitor-interval: 100
                          mode: active-backup
@@ -1157,6 +1163,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
         IPADDR1=192.168.1.2
         IPV6ADDR=2001:1::1/92
         IPV6INIT=yes
+        MTU=9000
         NETMASK=255.255.255.0
         NETMASK1=255.255.255.0
         NM_CONTROLLED=no
@@ -1203,6 +1210,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
                 name: en0
                 mac_address: "aa:bb:cc:dd:e8:00"
               - type: vlan
+                mtu: 2222
                 name: en0.99
                 vlan_link: en0
                 vlan_id: 99
@@ -1238,6 +1246,7 @@ pre-down route del -net 10.0.0.0 netmask 255.0.0.0 gw 11.0.0.1 metric 3 || true
                 IPV6ADDR=2001:1::bbbb/96
                 IPV6INIT=yes
                 IPV6_DEFAULTGW=2001:1::1
+                MTU=2222
                 NETMASK=255.255.255.0
                 NETMASK1=255.255.255.0
                 NM_CONTROLLED=no
@@ -1669,6 +1678,8 @@ iface eth1 inet dhcp
 
 class TestSysConfigRendering(CiTestCase):
 
+    with_logs = True
+
     scripts_dir = '/etc/sysconfig/network-scripts'
     header = ('# Created by cloud-init on instance boot automatically, '
               'do not edit.\n#\n')
@@ -1917,6 +1928,9 @@ USERCTL=no
         found = self._render_and_read(network_config=yaml.load(entry['yaml']))
         self._compare_files_to_expected(entry['expected_sysconfig'], found)
         self._assert_headers(found)
+        self.assertNotIn(
+            'WARNING: Network config: ignoring eth0.101 device-level mtu',
+            self.logs.getvalue())
 
     def test_small_config(self):
         entry = NETWORK_CONFIGS['small']
@@ -1929,6 +1943,10 @@ USERCTL=no
         found = self._render_and_read(network_config=yaml.load(entry['yaml']))
         self._compare_files_to_expected(entry['expected_sysconfig'], found)
         self._assert_headers(found)
+        expected_msg = (
+            'WARNING: Network config: ignoring iface0 device-level mtu:8999'
+            ' because ipv4 subnet-level mtu:9000 provided.')
+        self.assertIn(expected_msg, self.logs.getvalue())
 
     def test_dhcpv6_only_config(self):
         entry = NETWORK_CONFIGS['dhcpv6_only']
@@ -2410,6 +2428,7 @@ class TestNetplanRoundTrip(CiTestCase):
 
 
 class TestEniRoundTrip(CiTestCase):
+
     def _render_and_read(self, network_config=None, state=None, eni_path=None,
                          netrules_path=None, dir=None):
         if dir is None: