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Weird DHCP/DNS issue - systemd-networkd

Usually, the [Match] section of the .network file is configured with either the MACAddress or the interface Name keys:

[Match]
MACAddress=0a:bb:f2:cf:bb:01
[Match]
Name=ens5

So, when the uplink interface gets replaced with the tap interface and since the tap interface has the same attributes as the uplink interface, systemd-networkd is happy and things are ok.

But when the [Match] section is configured with something like PermanentMACAddress key

[Match]
PermanentMACAddress=0a:bb:f2:cf:bb:01

which is not supported with virtual interfaces like tap/tun

$ ethtool -P ens5
Permanent address: not set
$

then the match will fail and systemd-networkd will not manage the tap interface,

$ networkctl
IDX LINK    TYPE     OPERATIONAL SETUP
  1 lo      loopback carrier     unmanaged
  3 docker0 bridge   no-carrier  unmanaged
  4 ens5    ether    routable    unmanaged

3 links listed.
$

This can lead to two very undesirable consequences:

  1. DNS config is wiped off leading to DNS failures
  2. If uplink is configured using dhcp then upon lease expiry, since systemd-networkd is not managing the interface, it will not do the dhcp lease renewal thus ultimately bricking the node.

In order to prevent the above from happening, create a .network file for the tap interface under /etc/systemd/network and configure the [Match] section with either the interface Name or MACAddress key. For example, say, the uplink interface is ens5 and its .network file is /run/systemd/network/10-netplan-ens5.network then first copy this file to /etc/systemd/network and rename it:

sudo cp /run/systemd/network/10-netplan-ens5.network /etc/systemd/network/ens5.network

Secondly, in the file /etc/systemd/network/ens5.network, remove the PermanentMACAddress key in the [Match] section and add the Name key if not present already:

$ sdiff /run/systemd/network/10-netplan-ens5.network    /etc/systemd/network/ens5.network

[Match]                            [Match]
PermanentMACAddress=0a:bb:f2:cf:bb:01         <
Name=ens5                           Name=ens5

[Network]                           [Network]
DHCP=ipv4                           DHCP=ipv4
LinkLocalAddressing=ipv6                 LinkLocalAddressing=ipv6

[DHCP]                            [DHCP]
RouteMetric=100                       RouteMetric=100
UseMTU=true                           UseMTU=true
$

And then

sudo systemctl daemon-reload
sudo systemctl restart systemd-networkd