Possibility to configure network interface to be `admin down`

[suom1]

Feature Request

We would need a solution for putting a interface in admin down since the current ignore: true statement only ignores any configuration for a specific interface.

Description

In our infrastructure we have a dedicated network for DHCP (PXE) running IPv4 and IPv6 (and in future only IPv6, when all dependencies in the boot process are IPv6 compatible).

We however noticed a problem where we couldn't apply default gateway on IPv6 once we applied our production configuration when ignore: true was applied on the deployment interface. This is because Talos picks up default gateway from SLAAC on the deployment interface when ignore: true is applied and resulted with us not being able to install the correct default gateway in our routing table.

Because of this we would like to have something commonly known in networking as admin down for interfaces. This can be achieved in netplan for instance with activation-mode: off which completely disables the interface.

[smira]

As a workaround, just set lower metric: for your preferred default gateway.