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

3 files changed, 38 insertions, 11 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