[systemd-devel] systemd.link MACAddress=<whatever> matches OpenVPN tun device

[gima+ml.systemd-devel at iki.fi]

> Name            : systemd (commit 
> c38499d476026d999558a7eee9c95ca2fa41e115)
> Version         : 239.2-1
I have a systemd.link file that gives my usb modem a more recognizable 
name. I saw some renaming errors in the journal and noticed that systemd 
also tried to rename my VPN device. This shouldn't happen and I 
investigated. Here's the result:

It appears that the `50-usbmodem.link` file is being applied to the 
`tunvpn` device, even though the file has a MACAddress filter to match 
only the usbmodem.


I have the following file:

> /etc/systemd/network/50-usbmodem.link
>
> [Match]
> MACAddress=aa:bb:cc:dd:ee:ff
>
> [Link]
> Name=usbmodem


And by running the following command, it can be seen that the problem 
really occurs.

> $ udevadm test-builtin net_setup_link /sys/class/net/tunvpn/
>
> calling: test-builtin
> Load module index
> Parsed configuration file /etc/systemd/network/50-usbmodem.link
> Created link configuration context.
> ID_NET_DRIVER=tun
> Config file /etc/systemd/network/50-usbmodem.link applies to device tunvpn
> link_config: autonegotiation is unset or enabled, the speed and duplex 
> are not writable.
> ID_NET_LINK_FILE=/etc/systemd/network/50-usbmodem.link
> ID_NET_NAME=usbmodem
> Unload module index
> Unloaded link configuration context.

The tun device has no ethernet address, as it's a L3 interface, so the 
MACAddress really really shouldn't match.
> $ ip link show tunvpn
> xx: tunvpn: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc 
> fq_codel state UNKNOWN mode DEFAULT group default qlen 100
>     link/none


I fixed this temporarily by adding the following line to the [Match] 
section:

> Driver=huawei_cdc_ncm

I'm not entirely sure, but this appears to be a bug.

Maybe relevant section: 
https://github.com/systemd/systemd/blob/c38499d476026d999558a7eee9c95ca2fa41e115/src/udev/net/link-config.c#L218


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