Configuring TUN/TAP virtual network interface for use with QEMU on Xubuntu 24.04

I am planning to run qemu-system-ppc to play around QEMU and PPC with networking support. The quest and U-Boot will need to interact with external services, namely, tftp. The recommended network is to have TUN/TAP virtual network interface configured on the host. My host is Xubuntu 24.04/Ubuntu 24.04.

Let's start by creating tap0 using systemd

u1@m18:~$ sudo nvim /etc/systemd/system/tap0.service

Copy/paste content below

	[Unit]
	Description=Create TAP interface tap0
	Before=network-pre.target
	Wants=network-pre.target
	DefaultDependencies=no
	[Service]
	Type=oneshot
	ExecStart=/sbin/ip tuntap add dev tap0 mode tap user u1
	ExecStartPost=/sbin/ip link set tap0 up
	RemainAfterExit=yes
	[Install]
	WantedBy=multi-user.target

view raw tap0.service hosted with ❤ by GitHub

Change u1 to the user who needs access to tap0

Run command below to create tap0. This should also presist after reboot

u1@m18:~$ sudo systemctl daemon-reexec
u1@m18:~$ sudo systemctl enable --now tap0.service

Create a backup of existing netplan configuration

u1@m18:~$ mkdir ~/backup
u1@m18:~$ cd /etc/netplan
u1@m18:~$ sudo mv * ~/backup

Create new netplan configuration, let us call it 01-netcfg.yaml

u1@m18:~$ sudo touch /etc/netplan01-netcfg.yaml
u1@m18:~$ sudo chmod 600 /etc/netplan/01-netcfg.yaml
u1@m18:~$ sudo nvim /etc/netplan/01-netcfg.yaml

Copy/paste gist below into 01-netcfg.yaml.

	network:
	version: 2
	renderer: networkd
	ethernets:
	ens33:
	dhcp4: no
	dhcp6: no
	tap0:
	optional: true
	bridges:
	br0:
	interfaces: [ens33, tap0] # Added tap0 here
	dhcp4: yes
	dhcp6: no
	parameters:
	stp: true
	forward-delay: 4

view raw _etc_netplan_01-netcfg.yaml hosted with ❤ by GitHub

Note that ens33 is my interface name, so update that as appropriate.

Now disable Network Manager and enable networkd. The command below will reconfure your network, so you need to be on the console and not via ssh.

u1@m18:~$ sudo systemctl stop NetworkManager.service
u1@m18:~$ sudo systemctl disable NetworkManager.service
u1@m18:~$ sudo systemctl start systemd-networkd.service
u1@m18:~$ sudo systemctl enable systemd-networkd.service

Now apply netplan configuration

u1@m18:~$ sudo netplan apply

Running QEMU below and issuing dhcp at U-boot prompt should allow the guest to get IP address from DHCP server. Of course, I am assuming that there is DHCP server in the network.

u1@m18:~$ qemu-system-ppc -M mpc8544ds -cpu mpc8540 -m 512 -nographic -netdev tap,id=net0,ifname=tap0,script=no,downscript=no -device e1000,netdev=net0 -serial mon:stdio