To be able to use TUN/TAP network, instead of directly configuring the physical Ethernet network device — my Realtek 8169 Gigabit Ethernet, which in my computer it’s named eth1 –, we need to reconfigure the network in order to get a bridge device, named br0, with the physical Ethernet eth1 device attached to it. We can achieve this by editing /etc/network/interfaces to look like this:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
#auto eth1
#iface eth1 inet static

auto br0
iface br0 inet static
        address a.b.c.d
        netmask 255.255.255.128
        gateway v.w.x.y
        bridge_ports eth1
        bridge_fd 1
        bridge_hello 1
        bridge_stp off

The helper script /etc/qemu-ifup, which QEMU uses to bring up a TAP network interface used by the guest and bind it to the existing bridge br0, should look like this:

#!/bin/sh
echo "Executing /etc/qemu-ifup"
echo "Bringing up $1 for bridged mode..."
sudo /sbin/ifconfig $1 0.0.0.0 promisc up
echo "Adding $1 to br0..."
sudo /usr/sbin/brctl addif br0 $1
sleep 2

To allow running QEMU as an unprivileged user, we need to edit /etc/sudoers in order to grant access for running /sbin/ifconfig and /usr/sbin/brctl. It’s recommended to edit that file using visudo. The file should could look like this:

# Cmnd alias specification
Cmnd_Alias      QEMU=/sbin/ifconfig, \\
                     /sbin/modprobe, \\
                     /usr/sbin/brctl

# Defaults
Defaults        !lecture,tty_tickets,!fqdn,rootpw

# User privilege specification
root    ALL=(ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Grant access to Cmnd_Alias to user jdoe
jdoe ALL=NOPASSWD:QEMU

Now, we can launch qemu from the command line:

XLIB_SKIP_ARGB_VISUALS=1 \\
qemu -hda hda -cdrom *iso -boot d \\
-m 512 -localtime -net nic,vlan=0 \\
-net tap,ifname=tap0,script=/etc/qemu-ifup \\
-kernel-kqemu

Searching on the Internet, I found a workaround which consists on launching QEMU with the XLIB_SKIP_ARGB_VISUALS variable set to 1. For example:

XLIB_SKIP_ARGB_VISUALS=1 qemu -hda ...

I hope this will be fixed in a later release of either QEMU or Novell’s Xgl Xserver.