Comment 6 for bug 1863873

Here is a full procedure to reproduce the issue.

Set-up
======

1. Install an amd64 Debian Buster (default network install),

2. install lxc and create a bionic amd64 container,
```bash
apt install lxc
lxc-create -t download -n bionic
lxc-start -n bionic
```

3. inside the container, deactivate dhcp (dhcp4: false )in `/etc/netplan/10-lxc.yaml`, and install the systemd packages without the bug,
```bash
lxc-attach -n bionic
sed -i 's/true/false/' /etc/netplan/10-lxc.yaml
apt install systemd=237-3ubuntu10.38 libsystemd0=237-3ubuntu10.38 libnss-systemd=237-3ubuntu10.38 libpam-systemd=237-3ubuntu10.38
exit
```

4. create a bridge on the host with a static IP and deactivate dhcp, in `/etc/network/interfaces`,
```
# 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
#allow-hotplug ens18
#iface ens18 inet dhcp
## This is an autoconfigured IPv6 interface
#iface ens18 inet6 auto

iface ens18 inet manual

auto br0
iface br0 inet static
    address 192.168.1.168
    netmask 255.255.255.0
    gateway 192.168.1.220
    bridge_ports ens18
    bridge_stp off
    bridge_waitport 0
    bridge_fd 0
```

5. on the host, modify the network configuration of the container to use the bridge with a static IP in `/var/lib/lxc/bionic/config`,
```
# Template used to create this container: /usr/share/lxc/templates/lxc-download
# Parameters passed to the template:
# Template script checksum (SHA-1): 273c51343604eb85f7e294c8da0a5eb769d648f3
# For additional config options, please look at lxc.container.conf(5)

# Uncomment the following line to support nesting containers:
#lxc.include = /usr/share/lxc/config/nesting.conf
# (Be aware this has security implications)

# Distribution configuration
lxc.include = /usr/share/lxc/config/common.conf

# For Ubuntu 14.04
lxc.mount.entry = /sys/kernel/debug sys/kernel/debug none bind,optional 0 0
lxc.mount.entry = /sys/kernel/security sys/kernel/security none bind,optional 0 0
lxc.mount.entry = /sys/fs/pstore sys/fs/pstore none bind,optional 0 0
lxc.mount.entry = mqueue dev/mqueue mqueue rw,relatime,create=dir,optional 0 0
lxc.arch = linux64

# Container specific configuration
lxc.apparmor.profile = generated
lxc.apparmor.allow_nesting = 1
lxc.rootfs.path = dir:/var/lib/lxc/bionic/rootfs
lxc.uts.name = bionic

## Network configuration
#lxc.net.0.type = empty

# Network configuration
lxc.net.0.type = veth
lxc.net.0.flags = up
lxc.net.0.link = br0
lxc.net.0.name = eth0
lxc.net.0.ipv4.gateway = 192.168.1.220
lxc.net.0.ipv4.address = 192.168.1.169/32
```

6. reboot the host.
```bash
reboot
```

Let’s do it
===========

1. Start the container and check the IP config, which should be ok,
```bash
lxc-start -n bionic
lxc-attach -n bionic
ip a
```

2. upgrade the system and check the IP config, the static is gone.
```bash
apt upgrade
ip a
exit
```

If systemd is downgraded again to 237-3ubuntu10.38, the IP is back at the next reboot of the container.