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

[Justin B Rye]

Baptiste BEAUPLAT wrote:
>> 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"...
> 
> In my case, I didn't need to specify the "numdummies=1" as it is the
> default (when systemd override is not installed).
> 
> Someone that needed only one interface with bonding or dummy would have
>  to just load the module while someone that required more interfaces
> would have to explicitly set numdummies.

Okay, back to the drawing board:

 _Module configuration for bonding and dummy interfaces_

 Systems using channel bonding and/or dummy interfaces (for instance to
 configure a machine as a router) may encounter problems upgrading to buster
 due to the file /lib/modprobe.d/systemd.conf now shipped by udev (intended
 to simplify configuration via systemd-networkd). This file contains the lines

	options bonding max_bonds=0
	options dummy numdummies=0

 Admins who were depending on different values will need to ensure they are
 set in the correct way to take precedence. A file in /lib/modprobe.d can be
 overridden by one with the same name under /etc/modprobe.d, but the names
 are processed in alphabetical order, so /lib/modprobe.d/systemd.conf follows
 and overrides (for instance) /etc/modprobe.d/dummy.conf. Make sure that any
 local configuration file has a name that sorts after "systemd.conf", such
 as "/etc/modprobe.d/zz-local.conf".

It sounds as if we should also be adding something to the effect that

 With systemd it may be simpler to configure virtual network devices
 using a .netdev file - see
 "https://www.freedesktop.org/software/systemd/man/systemd.netdev.html#[Bond] Section Options".

but (a) is that odd #fragment-id going to choke docbook? and (b) does
it need any hedging ("using an appropriately de-frobularised version
of systemd-networkd and a .netdev file")?
-- 
JBR	with qualifications in linguistics, experience as a Debian
	sysadmin, and probably no clue about this particular package