Date:   Thu, 6 Jan 2022 18:31:45 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc:     Andrew Lunn <andrew@lunn.ch>, Oliver Neukum <oneukum@suse.com>,
        Aaron Ma <aaron.ma@canonical.com>, henning.schild@siemens.com,
        linux-usb@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, davem@davemloft.net,
        hayeswang@realtek.com, tiwai@suse.de
Subject: Re: [PATCH 1/3 v3] net: usb: r8152: Check used MAC passthrough
 address
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On Fri, 7 Jan 2022 10:01:33 +0800 Kai-Heng Feng wrote:
> > On device creation, udev can check if it now has both interfaces? If
> > the internal interface is up, it is probably in use. Otherwise, take
> > its MAC address and assign it to the dock interface, and give the
> > internal interface a random MAC address, just in case.
> >
> > You probably need to delay NetworkManager, systemd-networkkd,
> > /etc/network/interfaces etc, so that they don't do anything until
> > after udevd has settled, indicating all devices have probably been
> > found.  
> 
> I don't think it's a good idea. On my laptop,
> systemd-udev-settle.service can add extra 5~10 seconds boot time
> delay.
> Furthermore, the external NIC in question is in a USB/Thunderbolt
> dock, it can present pre-boot, or it can be hotplugged at any time.

IIUC our guess is that this feature used for NAC and IEEE 802.1X.
In that case someone is already provisioning certificates to all 
the machines, and must provide a config for all its interfaces.
It should be pretty simple to also put the right MAC address override
in the NetworkManager/systemd-networkd/whatever config, no?