The options at `systemd.network` (`links`, `netdevs` and `networks`) are
directly mapped to the three different unit types of `systemd-networkd(8)`.
However there's also the option `systemd.network.units` which is
basically used as a container for generated unit-configs that are linked
to `/etc/systemd/networkd`[1].
This should not be exposed to the user as it's unclear whether or not it
should be used directly which can be pretty confusing which is why I decided to
declare this option as internal (including all sub-options as `internal`
doesn't seem to be propagated to submodules).
[1] 9db75ed88f/nixos/modules/system/boot/networkd.nix (L933-L937)
This has been there since v209 [1]
```
The interface name to use. This option has lower precedence than NamePolicy=, so for this setting to take effect, NamePolicy= must either be unset, empty, disabled, or all policies configured there must fail. Also see the example below with "Name=dmz0".
Note that specifying a name that the kernel might use for another interface (for example "eth0") is dangerous because the name assignment done by udev will race with the assignment done by the kernel, and only one interface may use the name. Depending on the order of operations, either udev or the kernel will win, making the naming unpredictable. It is best to use some different prefix, for example "internal0"/"external0" or "lan0"/"lan1"/"lan3".
```
[1] 43b3a5ef61
Both options were introduced in systemd v243[1]. Those options can be
used to ensure that LinkLocalAddressing is only configured for a given
interface if DHCPv4 fails. To quote `systemd.network(5)`:
```
If "fallback" or "ipv4-fallback" is specified, then an IPv4
link-local address is configured only when DHCPv4 fails. If "fallback", an IPv6 link-local
address is always configured, and if "ipv4-fallback", the address is not configured. Note
that, the fallback mechanism works only when DHCPv4 client is enabled, that is, it requires
"DHCP=yes" or "DHCP=ipv4".
```
[1] 8bc17bb3f7
Since the bonds interface changed to a lot more possible values we create a
mapping of kernel bond attribute names and values to networkd attributes.
Those match for the most part, but have to transformed slightly.
There is also an assert that unknown options won’t slip through silently.
networkd options are always correct or up to date. This option allows to by
pass type checking. It is also easier to write because examples can be just copy
and paste from manpages.
Networkd units can contain secrets. In future also wireguard vpn will be supported by
networkd. To avoid leakage of private keys, those could be then also put outside
of the /nix/store
Having a writeable /etc/systemd/network also allows to quick fix network issues,
when upgrading `nixos-rebuild switch` would require network on its own (due
updates).
- most nixos user only require time synchronisation,
while ntpd implements a battery-included ntp server (1,215 LOCs of C-Code vs 64,302)
- timesyncd support ntp server per interface (if configured through dhcp for instance)
- timesyncd is already included in the systemd package, switching to it would
save a little disk space (1,5M)
Systemd upstream provides targets for networking. This also includes a target network-online.target.
In this PR I remove / replace most occurrences since some of them were even wrong and could delay startup.
