Why KVM failed to find a suitable default network? Let’s see the troubleshooting details in our latest blog. Bobcares, as a part of our Server Management Service offers solutions to every query that comes our way.

Overview

1. Troubleshooting “KVM Failed to Find a Suitable Default Network”
2. Error Causes
3. How to Fix the Error?
4. Conclusion

Troubleshooting “KVM Failed to Find a Suitable Default Network”

The Kernel-based Virtual Machine (KVM) or libvirt (the utility that manages KVM) is unable to locate or launch a default virtual network, which is the cause of the issue. For virtual machines operating under KVM to have networking features (such as NAT or bridged networking), this network is essential.

Error Causes

There are several reasons why this error may occur, including:

1. Absence of Default Network Configuration: It’s possible that the default network configuration has been deleted or is not configured.

2. libvirt Not Running: It’s possible that the libvirtd service, which controls networking and virtual machines, isn’t launched or operating correctly.

3. Corrupted Network Configuration: It’s possible that the default network definition is incorrectly defined or corrupted.

4. Network Bridge Issues: The virtual network may not start due to network bridging issues, such as misconfigured DNSSQ or missing kernel components.

5. Permissions or SELinux Problems: On systems with AppArmor or SELinux installed, libvirt may not be able to manage the network due to specific permissions problems.

Error Causes

1. Check Installation

i. Verify libvirt: Make sure libvirt is installed:

dpkg -l | grep libvirt

ii. Install if missing:

sudo apt install libvirt-daemon-system libvirt-clients bridge-utils

iii. Check default network:

virsh net-list --all

If “default” is missing, it needs to be created or started.

2. Start or Define the Default Network

i. Start the default network (if inactive):

sudo virsh net-start default

ii. If missing, create the default network:

Generate the XML:

<network>
<name>default</name>
<uuid>YOUR-UUID-HERE</uuid>
<forward mode='nat'/>
<bridge name='virbr0' stp='on' delay='0'/>
<mac address='YOUR-MAC-HERE'/>
<ip address='192.168.122.1' netmask='255.255.255.0'>
<dhcp>
<range start='192.168.122.2' end='192.168.122.254'/>
</dhcp>
</ip>
</network>

Generate unique values for UUID and MAC:

uuidgen

Define and start:

sudo virsh net-define default.xml
sudo virsh net-autostart default
sudo virsh net-start default

3. Check libvirtd Service

i. Check if libvirtd is running:

sudo systemctl status libvirtd

ii. Start if inactive:

sudo systemctl start libvirtd
sudo systemctl enable libvirtd

4. Restart Networking Services

i. Restart services if issues persist:

sudo systemctl restart libvirtd
sudo systemctl restart network-manager
sudo systemctl restart dnsmasq # If dnsmasq is used

5. Check Kernel Modules & Bridging

i. Check the tun module:

lsmod | grep tun

ii. If missing, load it:

sudo modprobe tun

iii. Check network bridge:

brctl show

iv. Ensure virbr0 is listed.

6. SELinux or AppArmor Issues

i. For SELinux (CentOS/Fedora):

Check for SELinux blocks:

sudo ausearch -m avc -ts recent

Fix issues with:

sudo setsebool -P virt_use_nfs 1

ii. For AppArmor (Ubuntu):

Check blocks:

sudo dmesg | grep apparmor

7. Verify dnsmasq

i. Check dnsmasq logs for errors:

journalctl -u libvirtd | grep dnsmasq

8. Check Logs for More Details

i. Inspect logs if none of the steps work:

sudo journalctl -xe
sudo journalctl -u libvirtd

This should help us troubleshoot and fix the KVM default network issue step-by-step.

[Want to learn more? Reach out to us if you have any further questions.]

Conclusion

In summary, fixing the “KVM Failed to Find a Suitable Default Network” error involves checking that libvirt is installed, ensuring the default network is active or created, verifying services like libvirtd are running, and inspecting kernel modules or logs for any issues. Following these steps should help resolve most KVM networking problems efficiently.