libvirt and bridged networking

libvirt and virt-manager are a blessing. They bring powerful, free, open source management to Xen- and KVM-based virtualization environments.

I’ve been using both for quite a while. Also, I’ve always prefered bridged networking support for my virtual machines over NAT. While NAT is non-disruptive and allows for isolation, I typically like to easily access services provided by my virtual machines, like SSH or NFSv4. Turns out that setting bridged networking support in libvirt is very easy, as long as bridged interface is detected by libvirt.

The simplest solution consists of creating a bridge interface that enslaves all the physical networks interfaces used to connect to the LAN or the Internet. For example, in Ubuntu, in order to enslave eth0 to a br0 bridge interface, while using DHCP for IPv4 address configuration, /etc/network/interfaces needs to look like this:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet manual

# The bridge
auto br0
iface br0 inet dhcp
        bridge_ports eth0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0

Next time, when creating a new virtual machine, it will be possible to use bridged networking in addition to NAT-based networking. There is one caveat, at least in Ubuntu: libvirt and virt-manager by default connect to qemu:///user instead of qemu:///system. This is neither good nor bad by itself. qemu:///user allows a non-privileged user to create and use virtual machines and the process of creating and destroying the virtual network interfaces used by the virtual machines is done within the context of the user running virt-manager. Due to lack of root privileges, virtual machines are limited to QEMU’s usermode networking support. In order to use advanced networking feautures like bridged networking, make sure you connect to qemu:///system instead. That is typically achieved by running virt-manager as root (which is not necessarily nice). I tried playing with udev and device ownership and permission masks but it all boils down to the inability of a non-privileged user to use brcrl to enslave network interfaces to a bridge.

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One thought on “libvirt and bridged networking

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