Fix: “Requested operation is not valid: network ‘default’ is not active”

You try to start or install a VM and libvirt refuses. In virt-manager the message looks like this:

Error starting domain: Requested operation is not valid: network 'default' is not active

During an install it can appear as Unable to complete install: 'Requested operation is not valid: network 'default' is not active', and Python tools show libvirt.libvirtError: Requested operation is not valid: network 'default' is not active.

It means exactly what it says: the VM has a network interface attached to the libvirt virtual network named default (the NAT network with bridge virbr0), and that network is not running, so the VM cannot be started.

Step 1: check the network state

Make sure you look at the same connection the VM uses. Most VMs live in qemu:///system, which needs root or libvirt group access; running virsh as a normal user may show a different, per-user list.

sudo virsh net-list --all

If default is listed as inactive, go to step 2. If it is not listed at all, go to step 4.

Step 2: start it and enable autostart (most common fix)

sudo virsh net-start default
sudo virsh net-autostart default
sudo virsh net-list --all

net-start only lasts until the next host reboot; net-autostart makes libvirt start the network every time. Now start the VM again.

Step 3: if net-start fails

• The bridge already exists. If you see internal error: Network is already in use by interface virbr0, a leftover bridge is blocking it. Find the bridge name in the network XML (sudo virsh net-dumpxml default), check nothing else uses it, then remove it, for example sudo ip link set virbr0 down and sudo ip link delete virbr0, and run net-start again.
• dnsmasq cannot start. libvirt runs its own dnsmasq for DHCP and DNS on the network. The libvirt wiki names a system-wide dnsmasq already listening on libvirt’s bridge as a common cause. Check the libvirt daemon log or journalctl for a dnsmasq error. Either disable the system dnsmasq if you do not need it, or set bind-interfaces plus interface= or listen-address= in /etc/dnsmasq.conf so it only uses your physical interface, restart it, then start the network.
• Firewall backend missing. An error like internal error: Failed to initialize a valid firewall backend means libvirt cannot set up NAT rules. Install the firewall packages your distribution’s libvirt expects (for example dnsmasq and an iptables/nftables provider), restart the libvirt daemon, and try again.
• Address conflict. If the network’s subnet (often 192.168.122.0/24) overlaps a real network on the host, change the network’s IP range with sudo virsh net-edit default and restart it.

Step 4: if the default network does not exist

The libvirt wiki documents re-creating it from the example definition that libvirt installs:

sudo virsh net-define /usr/share/libvirt/networks/default.xml
sudo virsh net-autostart default
sudo virsh net-start default

If that file is missing, your distribution may ship the default network in a separate package (for example a libvirt-daemon-config-network package on some distributions); install it or define your own network XML.

Alternative: use a different network

If you do not want the NAT network at all, attach the VM to a host bridge instead with virsh edit VM (change the interface to type='bridge' with <source bridge='br0'/>), or create new VMs with virt-install --network bridge=br0.

Official documentation

• libvirt wiki: Networking
• libvirt wiki: Virtual network ‘default’ has not been started
• virsh manual page (libvirt.org)