Linux virbr0 what is

CentOS

I have installed centos 7, server with gui option. I don’t remember that I turned on any kind of virtualisation.
Latter, I have noticed that virbr0 interface has showed.
What is the purpose of virbr0 interface, do I need it, and if not, how can I disable it?

avij Retired Moderator Posts: 3046 Joined: 2010/12/01 19:25:52 Location: Helsinki, Finland Contact:

Re: What is purpose of virbr0 interface

Post by avij » 2017/03/06 19:58:24

That is indeed related to virtualization. I’m not sure about this, but my best guess for disabling virbr0 is to run systemctl stop libvirtd.service , which will stop the virtualization service. If this helped, systemctl disable libvirtd.service will disable the service permanently.

TrevorH Site Admin Posts: 32853 Joined: 2009/09/24 10:40:56 Location: Brighton, UK

Re: What is purpose of virbr0 interface

Post by TrevorH » 2017/03/06 20:57:18

The default desktop install includes gnome-boxes (possibly just ‘boxes’) which is a form of virt that uses libvirt. Uninstalling it should fix it too.

The future appears to be RHEL or Debian. I think I’m going Debian.
Info for USB installs on http://wiki.centos.org/HowTos/InstallFromUSBkey
CentOS 5 and 6 are deadest, do not use them.
Use the FAQ Luke

Re: What is purpose of virbr0 interface

Post by kriticar » 2017/03/06 23:49:26

systemctl disable libvirtd.service

will I be able latter to enable the service with

systemctl disable libvirtd.service

avij Retired Moderator Posts: 3046 Joined: 2010/12/01 19:25:52 Location: Helsinki, Finland Contact:

Re: What is purpose of virbr0 interface

Post by avij » 2017/03/07 08:10:04

I suppose you meant systemctl enable libvirtd.service , but yeah, that will re-enable the service the next time you reboot. To start the service without rebooting, you can use systemctl start libvirtd.service , but that won’t be a permanent change.

Re: What is purpose of virbr0 interface

Post by kriticar » 2017/03/07 08:33:46

Re: What is purpose of virbr0 interface

Post by jlehtone » 2017/03/07 09:52:58

TrevorH wrote: The default desktop install includes gnome-boxes (possibly just ‘boxes’) which is a form of virt that uses libvirt. Uninstalling it should fix it too.

Ahh, gnome-boxes requires libvirt.

Libvirt, if installed, has libvirtd.service enabled by default.
Libvirt has a network definition «default» preconfigured and set to autostart.
The «virbr0» is a bridge interface that represents a network (a bridge is essentially a network switch).

Therefore, by installing libvirt (for any reason) by default dynamically creates the «virbr0» on every boot (and adds some netfilter rules too).

One can disable the autostart of the default network.
One can remove the definition of the default network.
One can stop and disable the libvirtd.service.
One can uninstall the libvirt packages.

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Для чего нужен virbr0-nic

При использовании KVM в списке интерфейсов основной системы появляется минимум два дополнительных интерфейса virbr0 и virbr0-nic. Назначение первого говорит само за себя — виртуальный мост, который будет связывать созданные Вами виртуальные машины с основной машиной. Назначение второго является не очевидным и в первые минуты загадочным. Если выполнить команду:

# brctl show bridge name bridge id STP enabled interfaces virbr0 8000.52540072bcb8 yes virbr0-nic

видно, что virbr0-nic является одним из сетевых интерфейсов созданного моста virbr0. Оказывается virbr0-nic создается исключительно как workaround для следующей задачи.

Когда мы создаем любой мост, его MAC адрес назначается на основании MAC адреса первого добавленного сетевого интерфейса. Если из моста удалить все привязанные интерфейсы и добавить новый, то MAC адрес существующего моста измениться на MAC адрес нового привязанного сетевого интерфейса. А так как с виртуальными машинами происходят постоянные включения/выключения, то их виртуальные интерфейсы vnetX постоянно появляются и исчезают. Чтобы сохранить MAC адрес созданного моста virbr0 постоянным разработчики пошли на такой финт ушами и создали виртуальный интерфейс virbr0-nic, значением которого инициализируется MAC адрес моста и остается постоянным вне зависимости от создаваемых и удаляемых виртуальных машин. Убедиться в этом можно выполнением следующей команды:

# ip link . 3: virbr0: mtu 1500 qdisc noqueue state UP link/ether 52:54:00:72:bc:b8 brd ff:ff:ff:ff:ff:ff 4: virbr0-nic: mtu 1500 qdisc noop master virbr0 state DOWN qlen 500 link/ether 52:54:00:72:bc:b8 brd ff:ff:ff:ff:ff:ff .

Наглядно видно, что MAC адреса virbr0 и virbr0-nic идентичные.

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How virbr0-nic is created?

How can I create a virtual network interface like virbr0-nic ? I am trying to find a way to create a NIC like virbr0-nic but everything which I can find on the Internet is how to create an interface attached to the physical interface like eth0:0 . When I write

# brctl show bridge name bridge id STP enabled interfaces virbr0 8000.525400e0af01 yes virbr0-nic virbr1 8000.525400e8a6b1 yes virbr1-nic vnet1 

usually that’s associated with something like a bridge specific to a bit of software. What’re you REALLY trying to do?

I have installed KVM and noticed that it created new NIC virbr0-nic. I am just wondering how can I create my own NIC such as this one.

5 Answers 5

These are dummy devices. You can run

To create a network interface called dummy0 .

If you want more than one device, you can create say 5 with

modprobe dummy numdummies=5 

You can then control these devices like any other network device.

Give it a MAC address with

ip link set dummy0 address aa:aa:aa:bb:bb:bb 

Give it an IP address with

ip addr add 10.0.0.1/24 dev dummy0 

Add it to your existing bridge with

brctl addif virbr0 dummy0 

Set it up, set it down, NAT off it, and so on.

This answers seems to be more correct than the alternative but it lacks detail about just creating an interface that’s equivalent of what KVM setups by default (virbr0-nic).

The poster of the question seems to understand what the bridge does and what the dummy device does, he just wanted to know how to create similar interfaces. You are welcome to improve the answer if you feel it could be better.

The libvirt is using TUN device for this purpose. You can manually create this device by following command:

virbr0-nic stands for Virtual bridge NIC.

It’s basically a bridge between your physical Network Card and your Virtual Machine’s virtual Network Card.

To manage bridged interface you can use the brctl command. You can list all your bridged interfaces with

and add or modify bridges accordingly to your needs. To create a new bridge use

where will be your new bridge’s name (as virbr0-nic).Then you can add interfaces to the bridge with

You can refer to the man page for additional information.

This actually doesn’t tell about creating a new «virbr1-nic», just a new «virbr1» whose interface to be set («virbr1-nic») is implied to exist previously.

I don’t think it is a bridge between physical and virtual either. I have to do masquerading between virbr0 and the physical for packets to get routed.

Sorry, but virbr0-nic is not a bridge. It is a dummy device that exists only to provide a MAC address for the bridge: unix.stackexchange.com/a/444863/8928

It is not clear exactly which types of devices virbr0-nic and vnet1 are in your setup. There are a few types of virtual devices which can be useful to include in a bridge, some of which can be created using the ip command.

One kind of virtual devices is veth which creates pairs of connected virtual Ethernet interfaces.

ip link add veth0 type veth peer name veth1 

In this example veth0 and veth1 are arbitrary interface names that I came up with for the example. This method can for example be useful if you want to move one of the two interfaces to a different networking namespace.

Another kind of virtual interface is vlan where you create a single virtual Ethernet interface attached to a specific 802.1q tag on a physical interface:

ip link add link eth0 name eth0.10 type vlan id 10 

There is a man page with information about even more types of virtual interfaces which can be created with the ip command. The name of the man page depends on which version you have installed, I have seen it named ip-link or just ip .

Additionally if you are using any virtualization there are virtual interfaces connecting host and VM. The specifics of those depend on the virtualization solution you are using.

but i don’t have this /usr/bin/tunctl -t virbr0-nic i thing that is installed from cockpit and i can’t find a delete option when i will use a virtual machine is it fine when it exist but for networking with systemd-networkd i have many problems with it. Cockpit will use Network-Manager and the configuration files from it. after the installation my network is going down all devices and applications goes up with many fails.

If you check the ‘ifconfig -a’ after a fresh install of RHEL/CentOS 6,7 system, you would find the interfaces virbr0 name. Here is an example from freshly installed CentOS 7 system.

virbr0: flags=4099[UP,BROADCAST,MULTICAST] mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:d5:f2:0c txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0-nic: flags=4098[BROADCAST,MULTICAST] mtu 1500 ether 52:54:00:d5:f2:0c txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 

What is virbr0 interface The virbr0 bridge interface is created by libvirtd’s default network configuration. libvirtd is the service which provides a basis for the host to act as a hypervisor. So in case you are not using xen virtualization, you can either prevent libvirtd’s default network from being activated on boot, or you could prevent libvirtd itself from activating on boot. The former will prevent any VM guest attached to libvirtd’s default network from having network connectivity and the latter would prevent VMs from running at all. Which is fine if you are not using it. Disable libvirtd default network

You can disable libvirtd’s default network temporarily using the virsh command. This will not persist across reboot.

virsh net-destroy default

virsh net-autostart default —disable

Remove libvirtd default network To permanently remove the libvirtd default network:

virsh net-undefine default 

To permanently disable the libvirtd service from starting at boot on RHEL5 and RHEL6:

To permanently disable the libvirtd service from starting at boot on RHEL7:

systemctl disable libvirtd.service 

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CentOS / RHEL 6,7 : How to disable or delete virbr0 interface

If you check the ‘ifconfig -a’ after a fresh install of RHEL/CentOS 6,7 system, you would find the interfaces virbr0 name. Here is an example from freshly installed CentOS 7 system.

# ifconfig -a . virbr0: flags=4099[UP,BROADCAST,MULTICAST] mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:d5:f2:0c txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0-nic: flags=4098[BROADCAST,MULTICAST] mtu 1500 ether 52:54:00:d5:f2:0c txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

What is virbr0 interface

The virbr0 bridge interface is created by libvirtd’s default network configuration. libvirtd is the service which provides a basis for the host to act as a hypervisor. So in case you are not using xen virtualization, you can either prevent libvirtd’s default network from being activated on boot, or you could prevent libvirtd itself from activating on boot. The former will prevent any VM guest attached to libvirtd’s default network from having network connectivity and the latter would prevent VMs from running at all. Which is fine if you are not using it.

Disable libvirtd default network

1. You can disable libvirtd’s default network temporarily using the virsh command. This will not persist across reboot.

2. To permanently disable the libvirtd default network from being created at boot:

# virsh net-autostart default --disable

Remove libvirtd default network

To permanently remove the libvirtd default network:

# virsh net-undefine default

To permanently disable the libvirtd service from starting at boot on RHEL5 and RHEL6:

To permanently disable the libvirtd service from starting at boot on RHEL7:

# systemctl disable libvirtd.service

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