You can see in https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html that
this should be "HairPin" not "Hairpin". Using "Hairpin" results in
```
Oct 25 18:55:03 my-host systemd-networkd[843736]: /etc/systemd/network/10-bridge.network:11:
Unknown key name 'Hairpin' in section 'Bridge', ignoring.
```
A further bug to our strange multi-user.target depending on
network-online.target issue is that systemd recently changed the
behaviour of systemd-networkd-wait-online to no longer consider the
absence of interfaces with RequiredForOnline to be sufficient to be
online: https://github.com/systemd/systemd/pull/27825
On the advice of the systemd developers
(https://github.com/systemd/systemd/issues/29388), this commit changes
the configuration of systemd-networkd-wait-online to pass --any by
default, and lets the default DHCP interfaces be RequiredForOnline
as they would be by default if the option is omitted.
It is plausible that systemd-networkd-wait-online may still fail if
there are no interfaces at all. However, that probably cannot be
avoided.
RequiredForOnline takes a boolean or a minimum operational state and an
optional maximum operational state. In the latter case, range values are
separated with colon.
Underneath, systemd-networkd’s reload is just `networkctl reload`. Per
`man networkctl`, calling `reload` is expected to fully handle new,
modified, and removed .network files, but it only handles *new* .netdev
files. For simplicity, assume .network -> reload and .netdev -> restart.
It’s desirable to perform reload instead of restart, as restart has the
potential to bring down interfaces, resulting in a loss of network
connectivity.
Example 10. of man page of systemd.network(5) shows:
```
Example 10. MacVTap
This brings up a network interface "macvtap-test" and attaches it to "enp0s25".
# /usr/lib/systemd/network/25-macvtap.network
[Match]
Name=enp0s25
[Network]
MACVTAP=macvtap-test
```
Which is a MACVTAP example and is currently unsupported in NixOS.
This is useful for people using "modern" technologies with virtual machines.
According to networkd netdev's manpage:
```
Independent=
Takes a boolean. When true, the vxlan interface is created without any underlying network interface. Defaults to false, which means that a .network
file that requests this VXLAN interface using VXLAN= is required for the VXLAN to be created.
```
is a valid option for [VXLAN] section.
According to systemd.netdev manpage:
```
MACAddress=
Specifies the MAC address to use for the device, or takes the special value "none". When "none", systemd-networkd does not request the MAC address for
the device, and the kernel will assign a random MAC address. For "tun", "tap", or "l2tp" devices, the MACAddress= setting in the [NetDev] section is
not supported and will be ignored. Please specify it in the [Link] section of the corresponding systemd.network(5) file. If this option is not set,
"vlan" device inherits the MAC address of the master interface. For other kind of netdevs, if this option is not set, then the MAC address is
generated based on the interface name and the machine-id(5).
Note, even if "none" is specified, systemd-udevd will assign the persistent MAC address for the device, as 99-default.link has
MACAddressPolicy=persistent. So, it is also necessary to create a custom .link file for the device, if the MAC address assignment is not desired.
```
Therefore, `none` is an acceptable value.
This essentially backports
https://github.com/systemd/systemd/pull/27791. `systemd-networkd.service`
is sent the `SIGTERM` signal, but it is not required to be stopped
before `initrd-switch-root.target` is reached, despite the use of
`systemctl isolate initrd-switch-root.target`. This is because when
there is no ordering at all between two units, and a transaction stops
one and starts the other, the two operations can happen
simultaneously. This means the service could still be running when
`switch-root` actually occurs. Then, stage 2 systemd will see the
service still running and decide it doesn't need to add a start
operation for it to its initial transaction. Finally, the service
exits, but only after it's already too late. If, however, there is any
ordering at all between a stopping unit and a starting unit, then the
stop operation will be done first. This way, we ensure that the
service is properly exited before doing `switch-root`.
This is something to keep in mind going forward. There may be other
services that need this treatment. These `before` and `conflicts`
definitions are the correct way to ensure a unit is actually stopped
before you reach initrd-switch-root
The `B` in bridge should be capitalized.
It currently leads to an evuluation error:
```
error: attribute 'sectionbridgeVLAN' missing
at /nix/store/7wmrwj0sgwg1iivxk43lpkqjhji57mq7-source/nixos/modules/system/boot/networkd.nix:2386:56:
2385| example = { VLAN = "10-20"; };
2386| type = types.addCheck (types.attrsOf unitOption) check.network.sectionbridgeVLAN;
| ^
2387| description = lib.mdDoc ''
Did you mean sectionBridgeVLAN?
```
