Not a newb here. Been using Kali since it was Backtrack-3 at least.

I can setup a temporary bridge interface via the `ip` command stack.

#!/usr/bin/env bash

ip link add name br0 type bridge
ip link set dev br0 up
ip addr add 192.168.xx.xx/xx dev br0
ip route append default via 192.168.xx.xx dev br0
ip link set eth0 master br0
ip addr del 192.168.xx.xx/xx dev eth0
echo "DONE"
This works fine, but it is not persistent. In order to get persistence, I have to modify `/etc/network/interfaces`. Which is done as described in the debian documentation.

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
iface eth0 inet manual

# Bridge network interface
auto br0
iface br0 inet static
        address 192.168.xx.xx
        network 192.168.x.xx
        broadcast 192.168.xx.xx
        gateway 192.168.xx.xx
        dns-nameservers xxx.xx.xx.xx xx.xx.xx.xx
        bridge_ports eth0
        bridge_fd 5
        bridge_stp off
        bridge_vlan_aware yes
        bridge_hw xx:xx:xx:xx:xx:xx
        pre-up ip addr flush dev eth0
Yet, systemctl fails when asking it to restart the networking.service. The log states:

ifup[96060]: RTNETLINK answers: File exists
ifup[96008]: ifup: failed to bring up br0
This occurs whether the device is actually up or not. That is, if I set the br0 device to the down state, I will still receive the error
RTNETLINK answers: File exists

What gives?