Bug#931130: Document that local configuration for dummy and bonding modules are getting overwritten by systemd

[Michael Biebl]

Am 28.06.19 um 12:58 schrieb Justin B Rye:
> Michael Biebl wrote:
>> Afaiu, the kernel default is to create one dummy device by default when
>> the module is loaded.

.. or bond device, for that matter.

>> I assume there might be cases where users rely on that default behaviour
>> without having explicitly configured anything.
> 
> Do we have any idea what their use case would be?  Apparently a
> different one from Baptiste's, if it's true that he needed to
> explicitly configure "numdummies=1"...

Dunno why Baptiste had to do that.
I just verified that by removing /lib/modprobe.d/systemd.conf, I got the
following when running "modprobe dummy" and "modprobe bonding":

6: bond0: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
group default qlen 1000
    link/ether 82:e5:b3:41:67:17 brd ff:ff:ff:ff:ff:ff
7: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group
default qlen 1000
    link/ether 5e:eb:30:64:69:ef brd ff:ff:ff:ff:ff:ff

With the modprobe.d config file installed, there obviously was neither a
dummy0 nor bond0 device.

Fwiw, I know too little about those dummy and bonding devices and how
they are usually used.
E.g. network-manager and networkd seem to create them on the fly as
needed. So I'm not sure, which users will actually be affected by that
change.
Would be good to know how Baptiste is setting up those devices. If he is
doing it manually via some scripting of low level tool or uses a higher
level network management tool
Fwiw, with "ip link add dummy0 type dummy" (or "ip link add bond0 type
bond") I was e.g. able to create such a device manually as well.
I wonder whether such an approach isn't better then statically setting
the number of devices via a kernel module option.
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