This page provides an introduction to the common networking configurations used by libvirt based applications. This information applies to all hypervisors, whether Xen, KVM or another.

The two common setups are "virtual network" or "shared physical device". The former is identical across all distributions and available out-of-the-box. The latter needs distribution specific manual configuration.

NAT forwarding (aka "virtual networks")

Host configuration (NAT)

Every standard libvirt installation provides NAT based connectivity to virtual machines out of the box. This is the so called 'default virtual network'. You can verify that it is available with

# virsh net-list --all
Name                 State      Autostart
default              active     yes

If it is missing, then the example XML config can be reloaded & activated

# virsh net-define /usr/share/libvirt/networks/default.xml
Network default defined from /usr/share/libvirt/networks/default.xml
# virsh net-autostart default
Network default marked as autostarted
# virsh net-start default
Network default started

When the libvirt default network is running, you will see an isolated bridge device. This device explicitly does *NOT* have any physical interfaces added, since it uses NAT + forwarding to connect to outside world. Do not add interfaces

# brctl show
bridge name bridge id       STP enabled interfaces
virbr0      8000.000000000000   yes

Libvirt will add iptables rules to allow traffic to/from guests attached to the virbr0 device in the INPUT, FORWARD, OUTPUT and POSTROUTING chains. It will also attempt to enable ip_forward. Some other applications may disable it, so the best option is to add the following to /etc/sysctl.conf

net.ipv4.ip_forward = 1

If you are already running dnsmasq on your machine, please see libvirtd and dnsmasq.

Guest configuration (NAT)

Once the host configuration is complete, a guest can be connected to the virtual network based on the network name. E.g. to connect a guest to the 'default' virtual network, you need to edit the domain configuration file for this guest:

virsh edit <guest>

where <guest> is the name or uuid of the guest. Add the following snippet of XML to the config file:

<interface type='network'>
   <source network='default'/>
   <mac address='00:16:3e:1a:b3:4a'/>

N.B. the MAC address is optional and will be automatically generated if omitted.

Applying modifications to the network

Sometimes, one needs to edit the network definition and apply the changes on the fly. The most common scenario for this is adding new static MAC+IP mappings for the network's DHCP server. If you edit the network with "virsh net-edit", any changes you make won't take effect until the network is destroyed and re-started, which unfortunately will cause a all guests to lose network connectivity with the host until their network interfaces are explicitly re-attached (which is automatically done as a side effect of restarting the libvirtd service).

virsh net-update

Fortunately, many changes to the network configuration (including the aforementioned addition of a static MAC+IP mapping for DHCP) can be done with "virsh net-update", which can be told to enact the changes immediately. For example, to add a DHCP static host entry to the network named "default" mapping MAC address 53:54:00:00:01 to IP address and hostname "bob", you could use this command:

virsh net-update default add ip-dhcp-host \
      "<host mac='52:54:00:00:00:01' \
       name='bob' ip='' />" \
       --live --config

Along with the "add" subcommand, virsh net-update also has a "delete" sub-command as well as "modify" (for some items), "add-first", and "add-last".

The config items in a network that can be changed with virsh net-update are:

ip-dhcp-range (add/delete only, no modify)
forward-interface (add/delete only)

In each case, the final argument on the commandline (aside from "--live --config") should be the XML section that you want to add/modify or delete. For example, the proper XML for "virsh net-update default add forward-interface" would be something like "<interface dev='eth20'/>" (note the careful use of quotes - due to the XML containing spaces and shell redirection characters, you must put quotes around the entire XML snippet, but this means that any quotes within the XML must either be single quotes, or be escaped with a backslash.)

Arbitrary changes to the network

Although the most common cases of changing network config can be handled with "virsh net-update", there are some parts of the config that can't be modified in this way, and in those cases you will be left with all running guests detached from the network after it is restarted. This can be remedied by restarting the libvirtd service, which checks that all guest tap devices are connected to their proper bridges during initialization.

Forwarding Incoming Connections

By default, guests that are connected via a virtual network with <forward mode='nat'/> can make any outgoing network connection they like. Incoming connections are allowed from the host, and from other guests connected to the same libvirt network, but all other incoming connections are blocked by iptables rules.

If you would like to make a service that is on a guest behind a NATed virtual network publicly available, you can setup libvirt's "hook" script for qemu to install the necessary iptables rules to forward incoming connections to the host on any given port HP to port GP on the guest GNAME:

  1. Determine a) the name of the guest "G" (as defined in the libvirt domain XML), b) the IP address of the guest "I", c) the port on the guest that will receive the connections "GP", and d) the port on the host that will be forwarded to the guest "HP".

    (To assure that the guest's IP address remains unchanged, you can either configure the guest OS with static ip information, or add a <host> element inside the <dhcp> element of the network that is used by your guest. See the libvirt network XML documentation address section for defails and an example.)

  2. Stop the guest if it's running.

  3. Create the file /etc/libvirt/hooks/qemu (or add the following to an already existing hook script), with contents similar to the following (replace GNAME, IP, GP, and HP appropriately for your setup):

    Use the basic script below or see an "advanced" version, which can handle several different machines and port mappings here (improvements are welcome) or here's a python script which does a similar thing and is easy to understand and configure (improvements are welcome):

    # IMPORTANT: Change the "VM NAME" string to match your actual VM Name.
    # In order to create rules to other VMs, just duplicate the below block and configure
    # it accordingly.
    if [ "${1}" = "VM NAME" ]; then
       # Update the following variables to fit your setup
       if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -D FORWARD -o virbr0 -p tcp -d $GUEST_IP --dport $GUEST_PORT -j ACCEPT
        /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
       if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -I FORWARD -o virbr0 -p tcp -d $GUEST_IP --dport $GUEST_PORT -j ACCEPT
        /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
  4. chmod +x /etc/libvirt/hooks/qemu

  5. Restart the libvirtd service.

  6. Start the guest.

    (NB: This method is a hack, and has one annoying flaw in versions of libvirt prior to 0.9.13 - if libvirtd is restarted while the guest is running, all of the standard iptables rules to support virtual networks that were added by libvirtd will be reloaded, thus changing the order of the above FORWARD rule relative to a reject rule for the network, hence rendering this setup non-working until the guest is stopped and restarted. Thanks to the new "reconnect" hook in libvirt-0.9.13 and newer (which is used by the above script if available), this flaw is not present in newer versions of libvirt (however, this hook script should still be considered a hack).

Bridged networking (aka "shared physical device")

Host configuration (bridged)

The NAT based connectivity is useful for quick & easy deployments, or on machines with dynamic/sporadic networking connectivity. More advanced users will want to use full bridging, where the guest is connected directly to the LAN. The instructions for setting this up vary by distribution, and even by release.

Important Note: Unfortunately, wireless interfaces cannot be attached to a Linux host bridge, so if your connection to the external network is via a wireless interface ("wlanX"), you will not be able to use this mode of networking for your guests.

Important Note: If, after trying to use the bridge interface, you find your network link becomes dead and refuses to work again, it might be that the router/switch upstream is blocking "unauthorized switches" in the network (for example, by detecting BPDU packets). You'll have to change its configuration to explicitly allow the host machine/network port as a "switch".

Fedora/RHEL Bridging

This outlines how to setup briding using standard network initscripts and systemctl.

Using NetworkManager directly

If your distro was released some time after 2015 and uses NetworkManager, it likely supports bridging natively. See these instructions for creating a bridge directly with NetworkManager:

Disabling NetworkManager (for older distros)

If your distro was released before 2015, the NetworkManager version likely does not handle bridging, so it is necessary to use "classic" network initscripts for the bridge, and to explicitly mark them as independent from NetworkManager (the "NM_CONTROLLED=no" lines in the scripts below).

If desired, you can also completely disable NetworkManager:

# chkconfig NetworkManager off
# chkconfig network on
# service NetworkManager stop
# service network start

Creating network initscripts

In the /etc/sysconfig/network-scripts directory it is neccessary to create 2 config files. The first (ifcfg-eth0) defines your physical network interface, and says that it will be part of a bridge:

# cat > ifcfg-eth0 <<EOF

Obviously change the HWADDR to match your actual NIC's address. You may also wish to configure the device's MTU here using e.g. MTU=9000.

The second config file (ifcfg-br0) defines the bridge device:

# cat > ifcfg-br0 <<EOF

WARNING: The line TYPE=Bridge is case-sensitive - it must have uppercase 'B' and lower case 'ridge'

After changing this restart networking (or simply reboot)

# service network restart

The final step is to disable netfilter on the bridge:

# cat >> /etc/sysctl.conf <<EOF
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
# sysctl -p /etc/sysctl.conf

It is recommended to do this for performance and security reasons. See Fedora bug #512206. Alternatively you can configure iptables to allow all traffic to be forwarded across the bridge:

# echo "-I FORWARD -m physdev --physdev-is-bridged -j ACCEPT" > /etc/sysconfig/iptables-forward-bridged
# lokkit --custom-rules=ipv4:filter:/etc/sysconfig/iptables-forward-bridged
# service libvirtd reload

You should now have a "shared physical device", to which guests can be attached and have full LAN access

# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.000000000000       yes
br0             8000.000e0cb30550       yes             eth0

Note how this bridge is completely independant of the virbr0. Do *NOT* attempt to attach a physical device to 'virbr0' - this is only for NAT connectivity

Debian/Ubuntu Bridging

See the debian wiki for up to date instructions of bridging:

Guest configuration (Bridge)

In order to let your virtual machines use this bridge, their configuration should include the interface definition as described in Bridge to LAN. In essence you are specifying the bridge name to connect to. Assuming a shared physical device where the bridge is called "br0", the following guest XML would be used:

<interface type='bridge'>
   <source bridge='br0'/>
   <mac address='00:16:3e:1a:b3:4a'/>
   <model type='virtio'/>   # try this if you experience problems with VLANs

NB, the mac address is optional and will be automatically generated if omitted.

To edit the virtual machine's configuration, use:

virsh edit <VM name>

For more information, see this FAQ entry

PCI Passthrough of host network devices

It is possible to directly assign a host's PCI network device to a guest. One pre-requisite for doing this assignment is that the host must support either the Intel VT-d or AMD IOMMU extensions. There are two methods of setting up assignment of a PCI device to a guest:

Assignment with <hostdev>

This is the traditional method of assigning any generic PCI device to a guest. It's covered well in the following guide:

libvirt PCI Device Assignment

Assignment with <interface type='hostdev'> (SRIOV devices only)

SRIOV network cards provide multiple "Virtual Functions" (VF) that can each be individually assigned to a guest using PCI device assignment, and each will behave as a full physical network device. This permits many guests to gain the performance advantage of direct PCI device assignment, while only using a single slot on the physical machine.

These VFs can be assigned to guests in the traditional manner using <hostdev>, however that method ends up being problematic because (unlike regular network devices) SRIOV VF network devices do not have permanent unique MAC addresses, but are instead given a new and different random MAC address each time the host OS is rebooted. The result will be that even if the guest is assigned the same VF each time, any time the host is rebooted the guest will see that its network adapter has a new MAC address, which will lead to the guest believing there is new hardware connected, requiring re-configuration of the guest's network settings.

It is possible for the host to set the MAC address prior to assigning the VF to the guest, but there is no provision for this in the <hostdev> settings (since <hostdev> is for a generic PCI device, it knows nothing of function-specific items like MAC address). In order to solve this problem, libvirt-0.9.10 added a new <interface type='hostdev'> (documented here). This new type of interface device behaves as a hybrid of an <interface> and a <hostdev> - libvirt will first do any network-specific hardware/switch initialization indicated (such as setting the MAC address, and/or associating with an 802.1Qbh switch), then perform the PCI device assignment to the guest.

In order to use <interface type='hostdev'>, you must have an SRIOV-capable network card, host hardware that supports either the Intel VT-d or AMD IOMMU extensions, and you must learn the PCI address of the VF that you wish to assign (see this document for instructions on how to do that).

Once you have verified/learned the above information, you can edit your guest's domain configuration to have a device entry like the following:

  <interface type='hostdev' managed='yes'>
      <address type='pci' domain='0x0' bus='0x00' slot='0x07' function='0x0'/>
    <mac address='52:54:00:6d:90:02'>
    <virtualport type='802.1Qbh'>
      <parameters profileid='finance'/>

(Note that if you do not provide a mac address, one will be automatically generated, just as with any other type of interface device. Also, the <virtualport> element is only used if you are connecting to an 802.11Qgh hardware switch (802.11Qbg (a.k.a. "VEPA") switches are currently not supported in this mode)).

When the guest starts, it should see a network device of the type provided by the physical adapter, with the configured MAC address. This MAC address will remain unchanged across guest and host reboots.

Assignment from a pool of SRIOV VFs in a libvirt <network> definition

Hard coding the PCI address of a particular VF into a guest's configuration has two serious limitations:

  1. The specified VF must be available any time the guest is started, implying that the administrator must permanently assign each VF to a single guest (or modify the configuration of a guest to specify a currently unused VF's PCI address each time the guest is started).

  2. If the guest is moved to another host, that host must have exactly the same hardware in the same location on the PCI bus (or, again, the guest configuration must be modified prior to start).

    Starting with libvirt 0.10.0, it is possible to avoid both of these problems by creating a libvirt network with a device pool containing all the VFs of an SR-IOV device, then configuring the guest to reference this network; each time the guest is started, a single VF will be allocated from the pool and assigned to the guest; when the guest is stopped, the VF will be returned to the pool for use by another guest.

    The following is an example network definition that will make available a pool of all VFs for the SR-IOV adapter with its PF (Physical Function) at "eth3' on the host:

      <forward mode='hostdev' managed='yes'>
        <pf dev='eth3'/>

    To use this network, place the above text in, e.g., /tmp/passthrough.xml (replaceing "eth3" with the netdev name of your own SR-IOV device's PF), then execute the following commands:

    virsh net-define /tmp/passthrough.xml
    virsh net-autostart passthrough
    virsh net-start passthrough.

    Although only a single device is shown, libvirt will automatically derive the list of all VFs associated with that PF the first time a guest is started with an interface definition like the following:

    <interface type='network'>
      <source network='passthrough'>

    You can verify this by running "virsh net-dumpxml passthrough" after starting the first guest that uses the network; you will get output similar to the following:

    <network connections='1'>
      <forward mode='hostdev' managed='yes'>
        <pf dev='eth3'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x1'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x3'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x5'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x7'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x1'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x3'/>
        <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x5'/>