[Networkd][DHCP] Allow users to provide custom DHCP client identifier via DUIDRawData

[eduvfalc]

Component

systemd-networkd

Is your feature request related to a problem? Please describe

I recently had an assignment in which I needed to set the DHCP client identifier in order to receive a pre-allocated IP from the DHCP server. The current netword implementation allows users to modify the client identifier to some extent, but it has some limitations. In my case, I needed to set the DHCP identifier as the content provided by the DUID only, without IAID prepended to it, but currently supported configuration does not allow such behavior. For that, I had created a patch which works just fine, and I thought that it would be nice to bring it to the table in case this is a relevant feature for future releases.

Describe the solution you'd like

I would like to be able to fully customize the DHCP client identifier, without prepending the IAID or performing strict checks on payload size, as currently done for the supported DUIDTypes.

Describe alternatives you've considered

I tried setting the client identifier via DUIDRawData for different DUIDTypes, but all of the currently available types have limitations in terms of how many bytes I am allowed to write as the client identifier. For instance, DUIDType=vendor only lets me send 8 bytes of DUID and also adds an IAID to it. If I don't want a IAID and I set a ClientIdentifier=duid-only, this IAID still gets added, but with no particular meaning to it since it is 0. I think that the best option is to have a DUIDType which reflects the generic type 0, in which case one could use the DUIDRawData field to fully customize the client identifier.

The systemd version you checked that didn't have the feature you are asking for

251