bug#52520: Multicast is off by default

[Mathieu Othacehe]

Hey,

> Can we instead use <network-link> for these purposes?  It is already
> there, just unused.

Mmh, indeed. Well it is already used to create vlan and veth
interfaces. What we could do then, is expose the <network-link> record
as a direct equivalent of the Guile-Netlink <link> record.

The links list of the <static-networking> record would contain all the
links that are managed by Guix. We would require all "device" fields
that appear in the <network-address> records to have their matching link
declared. If the link doesn't already exists (vlan, veth type) then
link-add is called, otherwise nothing happens. The link existence can be
tested using link-name->index. link-set would always be called
afterwards to setup link properties.

static-networking-shepherd-service would then create on service per link
present in the <static-networking> links list. The provisioned name
would be (concat 'networking- (network-link-name link)).

On tear down, which is equivalent to "herd stop networking-<link>", we
would call link-del if the link was created by Guix with link-add, or
(link-set ... #:down #t) if the link was already present. I'm not sure
how to distinguish between those two cases at this step.

This sounds like a complex plan, that will moreover require and
adaptation of existing static-networking configurations, but I cannot
think of anything easier to fix this issue and the other one I reported.

Thanks,

Mathieu