Problem with multiple NICs with FAI

[Christian Meyer]

Hello there,

I'm trying to set up FAI-clients with more than only one NIC with
systemd-networkd instead of ifup.
Some clients use both WLAN (e.g. wlp3s0) and LAN (e.g. enp4s0) in the
same subnet, a different (proxy-)machine is a virtual box (XEN) with
eth0 and eth1 in different networks.

However, I don't know wheather this is a FAI bug or a config problem. I
see the following output during testing softupdates:
...
Setting SERVER=fai-server. Value extracted from FAI_CONFIG_SRC.
/usr/lib/fai/subroutines: Zeile 822: /sys/class/net/$NIC1/address:
Mehrdeutige Umlenkung.
Monitoring to server fai-server enabled.
...

The problem is clear: NIC1 are two NIC's: variables.log says:
NIC1=$'enp4s0\nwlp3s0'

Possibly this happens AFTER switching a machine to systemd-networkd, so
managing one NIC works fine, but it could be a routing problem.
According to /usr/lib/fai/subroutines it seems that there is no default
route and that "NIC1" is "everything" that is a network device:

   # determine the name of the default network interface
    # try several times if dhclient needs some more time
    local i=0
    NIC1=$(ip route | awk '/^default/ {print $5}')
    while [ ! "$NIC1" -a $i -lt 8 ]; do
        sleep 2
        NIC1=$(ip route | awk '/^default/ {print $5}')
        (( i += 1 ))
    done
    if [ -z "$NIC1" ]; then
        NIC1=$(ip route | awk '/ dev / {print $3}')
    fi
    export NIC1

On the other hand it seems that FAI doesn't really know how to deal
with more than one NIC and I can't find anything about other NIC's.
FYI: I'm using Debian stable with FAI 5.3.6 and a very own configspace.

Is there a way to proper deal with multiple network cards in FAI or is
my routing just broken? In this cases, can anyone please give me a hint
how to configure multiple NICs / networks with systemd-networkd and
systemd-resolved?

Thanks a lot,
Christian