Attaching a Guest Directly to a Virtualization Host Network Interface with a macvtap Driver

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External

Internal

Overview

The procedure consists in leaving the network interface unconfigured on the virtualization host and declaring a network interface of type "direct" in the guest configuration. After assignment, and after the guest boots, the corresponding interface shows up on host as follows: 

3: em2: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN qlen 1000
    link/ether 18:66:da:9f:96:d7 brd ff:ff:ff:ff:ff:ff
...
11: macvtap0@em2: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state LOWERLAYERDOWN qlen 500
    link/ether 52:54:00:02:72:ed brd ff:ff:ff:ff:ff:ff

Procedure

Configure the Virtual Device on Guest

Leave the network interface unconfigured on the virtualization host.

Assign it to the guest in configuration with virsh edit or in the corresponding XML configuration file as follows: 

...
<interface type='direct'>
   <source dev='em2' mode='private'/>
</interface>
...

where "em2" is the virtualization host network interface to be directly exposed to the guest.

Note that after creating the domain based on the XML configuration, libvirt will update the stored configuration as follows: 

...
<interface type='direct'>
   <mac address='52:54:00:02:72:ed'/>
   <source dev='em2' mode='private'/>
   <model type='rtl8139'/>
   <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
</interface>
...

Note that the MAC address declared in the configuration is different from the MAC address declared by ip addr on the virtualization host.

The new network interface shows up on the guest, alongside the default network interface eth1: 

ip addr
...
2: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 52:54:00:02:72:ed brd ff:ff:ff:ff:ff:ff
...
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 52:54:00:e4:e6:8a brd ff:ff:ff:ff:ff:ff

Primary Interface Name Instability


A certain primary interface name instability has been observed after adding the second virtual network interface in the domain definition file. The name of the primary network interface would be assigned non-deterministically to "eth0" and "eth1", respectively, upon successive reboots. An attempt to configure "device" in the domain name interface definition was ineffective. The issue seemed to go away - the network device would be always assigned to eth0 - without a good explanation after adding both /etc/sysconfig/network-scripts/ifcfg-eth0 and /etc/sysconfig/network-scripts/ifcfg-eth1.\\ \\

Configure the Network Interface on Guest

Manual Configuration

After the first boot, figure out the name of the new network interface and add a /etc/sysconfig/network-scripts/ifcfg-<interface-name> configuration file, similar to: 

NAME="ens8"
DEVICE="ens8"
TYPE="Ethernet"
ONBOOT="yes"
BOOTPROTO="none"
IPADDR="172.68.30.1"
PREFIX="24"
DEFROUTE="no"
PEERDNS="no"
PEERROUTES="no"
IPV4_FAILURE_FATAL="yes"
IPV6INIT="no"
IPV6_AUTOCONF="no"
IPV6_DEFROUTE="no"
IPV6_PEERDNS="no"
IPV6_PEERROUTES="no"
IPV6_FAILURE_FATAL="no"
UUID="8e9cbe3a-3582-4213-8a6d-73af4d93e3d0"

Make sure UUID is unique, you can generate a new UUID with uuidgen.

For more details on how to configure network interfaces, see

Configuring a Network Interface

Reboot the second time, both interfaces should be operational.

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