apt install bridge-utils

/etc/network/interfaces

auto lo br0
iface lo inet loopback

iface br0 inet static
    bridge_ports eth0
        address 192.168.12.2/24
        gateway 192.168.12.1
#        dns-nameservers 192.168.12.1
        bridge_waitport 0
        bridge_maxwait 0
        bridge_df 0
        bridge_stp off

Bridged-Netzwerk manuell einrichten

ip addr flush dev eth0
ip link add name br0 type bridge
ip link set eth0 master br0
ip addr add 192.168.12.2/24 dev br0
ip link set br0 up
ip route add default via 192.168.12.1

Kernel Parameter

echo 0 > /proc/sys/net/bridge/bridge-nf-call-arptables
echo 0 > /proc/sys/net/bridge/bridge-nf-call-ip6tables
echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables

Für QEMU VM

qemu-system-x86_64 -m 4069 -cpu host -smp 2 -machine type=pc,accel=kvm \
-hda /media/ramdisk/vm21.img -display none -k de \
-netdev tap,id=net21,ifname=tap21,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown \
-device e1000,netdev=net21,mac=52:54:00:12:34:21 \
-boot c &

Alle Config-Files unter /etc/systemd/network/

ls -l 
-rw-r--r-- 1 root root 30 Nov  2 22:19 br0.netdev
-rw-r--r-- 1 root root 98 Nov  2 22:20 br0.network
-rw-r--r-- 1 root root 45 Nov  2 22:21 lan-eth0.network

br0.netdev

[NetDev]
Name=br0
Kind=bridge

br0.network

[Match]
Name=br0

[Network]
Address=192.168.1.5/24
Gateway=192.168.1.1
DNS=8.8.8.8 192.168.1.1

lan-eth0.network

[Match]
Name=eth0

[Network]
Bridge=br0