KVM Bridged Network Not Working (2023)


This following worked for me for Ubuntu 12.04. You should disable your computer's firewall as you test this so it won't interfere.

The /etc/default/qemu-kvm file should be as originally installed.

You will need to have bridge-utilsKVM Bridged Network Not Working (1) qemu-kvmKVM Bridged Network Not Working (2) and libvirt-binKVM Bridged Network Not Working (3) installed. Any users using virtual machines should be added to the libvirtd group.

There no longer seems to be any need to add CAP_NET_ADMIN capability.

Network Setup

The default network mode is the User mode, also called SLIRP. It uses a predefined virbr0 bridge which is NAT routed to the guest computer. The NAT routing uses the kernel's ip_forwarding feature and iptables. Bridge mode uses a virtual bridge in the guest to which the (unnumbered) Ethernet interface connects, and on which both the host and the guest have their network interfaces.

The following diagrams may make the differences clearer:

KVM Bridged Network Not Working (4)

You can see how the default User network is defined with:

virsh net-dumpxml default

I can set up the bridged mode with the following approaches:

In /etc/network/interfaces (from the bridging part of the post you mention in your question):

auto loiface lo inet loopback#auto eth0#iface eth0 inet dhcpauto eth0iface eth0 inet manualauto br0iface br0 inet dhcp bridge_ports eth0 bridge_stp off bridge_fd 0 bridge_maxwait 0

Reboot; and make sure that wireless networking isn't active. Check the default IP route with ip route. It must be using the br0 interface.

N.B. If your Ethernet isn't hooked up when this change is made you need to have your Ethernet cable plugged in and getting a carrier or the boot will hang for two minutes and you won't have network capability That's because the eth0 interface, by being in this file, must come up before the boot can proceed normally.

N.B. Generally you can't use a wireless network instead of eth0 because of their inability to use multiple MAC addresses (I infer that they need a second one for the bridge).

As an alternative you can disable the use of Ethernet and make sure that it does not have an IP address, and that there isn't a default route set up with ip route. Then:

 sudo ifconfig eth0 up sudo brctl addbr br0 sudo brctl addif br0 eth0 sudo ifconfig br0 up sudo dhclient br0 &

You could also supply a static IP address here, as well as defining the default route and DNS address. For this example dhclient does this.

Here's my route table:

$ip route listdefault via dev br0 metric 100 dev br0 scope link metric 1000 dev br0 proto kernel scope link src dev virbr0 proto kernel scope link src

Using kvm

I can then boot a bridged kvm machine with:

 $ sudo kvm -name Quantal -m 1024 -hda foo.qcow2 --soundhw ac97 -device virtio-net,netdev=tunnel -netdev tap,id=tunnel,ifname=vnet0

The -netdev tap parameter makes sudo a requirement. As the VM is started qemu-kvm runs the following commands:

ifconfig vnet0 upbrctl addif brctl addif br0 vnet0

This is done by /etc/qemu-ifup

The VM's vnet0 interface is added to the br0 bridge because the default route above uses that bridge interface. If it weren't there the tap interface instead would be added to the virbr0 interface. Since that's not connected to the Internet, NAT would be used to connect the guest to the host and the Internet, in my experiments. You can direct the vnet0 to a particular bridge in /etc/default/qemu-kvm. Using virt-manager below you can explicitly direct which bridge to connect to.

Because of the above commands issued by qemu-kvm, and the -netdev tap,id=tunnel,ifname=vnet0 parameter, the vm virtual machine is connected to the vnet0 tunnel, and the tunnel is connected to the br0 bridge.

I can now directly ssh into this guest VM from another computer on my network.

My host ifconfig (note the vnet0 interface that appears on my network when the VM is running):

$ifconfigbr0 Link encap:Ethernet HWaddr 00:1e:33:88:07:e5 inet addr: Bcast: Mask: inet6 addr: fe80::21e:33ff:fe88:7e5/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:6526 errors:0 dropped:0 overruns:0 frame:0 TX packets:7543 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:2712940 (2.7 MB) TX bytes:1071835 (1.0 MB)eth0 Link encap:Ethernet HWaddr 00:1e:33:88:07:e5 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:7181 errors:0 dropped:0 overruns:0 frame:0 TX packets:7740 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:2974585 (2.9 MB) TX bytes:1096580 (1.0 MB) Interrupt:43 Base address:0x6000 lo Link encap:Local Loopback inet addr: Mask: inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:10 errors:0 dropped:0 overruns:0 frame:0 TX packets:10 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:664 (664.0 B) TX bytes:664 (664.0 B)vnet0 Link encap:Ethernet HWaddr ca:0c:73:c3:bc:45 inet6 addr: fe80::c80c:73ff:fec3:bc45/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:226 errors:0 dropped:0 overruns:0 frame:0 TX packets:429 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:26919 (26.9 KB) TX bytes:58929 (58.9 KB)virbr0 Link encap:Ethernet HWaddr d6:18:22:db:ff:93 inet addr: Bcast: Mask: UP BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

My bridge configuration while running the VM:

$brctl showbridge name bridge id STP enabled interfacesbr0 8000.001e338807e5 no eth0 vnet0virbr0 8000.000000000000 yes

Note that both the virtual machine's vnet0 interface and the eth0 interface are connected to the br0 bridge.

And the MAC's on the br0 interface:

$brctl showmacs br0port no mac addr is local? ageing timer 1 00:05:5d:cf:64:61 no 2.54 1 00:19:d2:42:5d:3f no 36.76 1 00:19:df:da:af:7c no 2.86 1 00:1e:33:88:07:e5 yes 0.00 1 00:60:0f:e4:17:d6 no 0.79 2 52:54:00:12:34:56 no 0.80 1 58:6d:8f:17:5b:c0 no 5.91 1 c8:aa:21:be:8d:16 no 167.69 2 ca:0c:73:c3:bc:45 yes 0.00

Note that the br0 interface connects my host computer to the same bridge being used by the guest.

You can check that you are bridged rather than NAT routed to your own network by using traceroute If the first node is your network's router rather than the guest's ip address your network should be working correctly.

See this documentation.


Be sure that you have installed virt-manager and hal. The hal package is a suggested dependency for virt-manager and is used to determine the network configuration of your system when creating or editing guests.

While having the br0 bridge defined as above I created a virtual machine with virt-manager as follows:

KVM Bridged Network Not Working (5)KVM Bridged Network Not Working (6)KVM Bridged Network Not Working (7)KVM Bridged Network Not Working (8)KVM Bridged Network Not Working (9)

I was able to go directly to the rest of my home network and to the Internet from this guest. I was also able to ssh into it from the other (non-host, non-guest) Ubuntu computer on my home network.

Here's the very long kvm command run by virt-manager (for comparison with EApubs or anyone else having trouble with this):

/usr/bin/kvm -S -M pc-1.0 -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name precise -uuid f057a729-eda6-4b85-84dc-f100c9ae3789 -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/precise.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -drive file=/media/natty/home/gruber/ubuntu-kvm/tmpW8gSGB.qcow2,if=none,id=drive-ide0-0-0,format=qcow2 -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 -netdev tap,fd=18,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=52:54:00:0e:da:9b,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -vnc -vga cirrus -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5

Here's the network portion of the virtual machine description in /etc/libvirt/qemu/quantal.xml

 <interface type='bridge'> <mac address='52:54:00:b0:8e:aa'/> <source bridge='br0'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface>

According to this link, for performance and reliability, it may be best to set the network device model to virtio, you can do this by in the virt-viewer by pressing the i button, going to the NIC setting, and setting the "Device model" to virtio. You could also add this to the XML above by adding the line:

 <model type='virtio'/>

In Summary

All this took on 12.04 was:

  1. Installing virt-manager, bridge-utils, qemu-kvm, and related packages
  2. Make sure each user wishing to use kvm are in the libvirtd group.
  3. Defining /etc/network/interfaces as above (which match the quoted article)
  4. Reboot, making sure Ethernet is plugged in and wireless (if any) is off.
  5. Either run kvm against an image directly with, e.g. -device e1000,netdev=tunnel -netdev tap,id=tunnel,ifname=vnet0, or create a virtual machine with virt-manager, specifying network Bridge br0 under the Step 4->Advanced Options panel.

No further changes were needed to networking, capabilities, templates, or configurations.

To expose a service in your new guest to the Internet you should:

  1. Prepare and configure any firewall service you will need.
  2. Either assign a static address in your guest configuration or in your DHCP service.
  3. If you are using a NAT router open a port for the service you are implementing directing it to the guest's IP address.

Remember to test and re-enable the firewall service for your host computer. It may need any entry to forward traffic to the guest.

See https://help.ubuntu.com/community/KVM/Installation, https://help.ubuntu.com/community/KVM/Networking, and https://help.ubuntu.com/12.04/serverguide/libvirt.html.

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