[systemd-devel] networkd: Support setting mtu / mac address by interface name

[Marcel Holtmann]

Hi Lennart,

>>>> Moreover, if we
>>>> give people this feature I'm pretty sure we'll get lots of people
>>>> expecting it to work also for any other sort of name and getting
>>>> confused when it doesn't.
>>> 
>>> Well, this is something we can fix by documentation, no?
>>> 
>>> Or maybe name the match option differently, maybe OriginalName= or
>>> KernelName= or so, and then only matching interfaces where you know
>>> that the name was selected by userspace in the first place?
>> 
>> I like the idea of OriginalName, much less likely to get people
>> confused. I now implemented that, with the restriction that we cannot
>> match on renamed names. For now I left it open to match on ethX style
>> names, as people in principle could do sensible things like
>> "OriginalName=eth*" or even "OriginalName=eth0" when we know there is
>> only one interface.
>> 
>> One thing to consider would be to disallow renaming from a .link file
>> where the OriginalName was used to match. That way we don't have the
>> somewhat odd situation that a .link file can only be applied once (we
>> do not remember the original name, so cannot match on that the second
>> time around, as that would be a mess)...
> 
> Maybe we should even store the original name in a udev property, so
> that we can make this fully idempotent simply because we can always
> check this new property for the original name passed down from the
> kernel?

you do realize that once the kernel renamed the name, it is free to reuse that name again. So eth0 as OriginalName can then be present multiple times.

I think that you can only have OriginalName matching if a renaming has never occurred. Otherwise this is really not deterministic. Since foo0 (renamed from eth0) and and bar0 (renamed from eth0) will both match OriginalName=eth0. The difference is only timing and that is inhering racy of course.

Regards

Marcel