B.9. Guest Can Reach Outside Network, but Cannot Reach Host when Using macvtap Interface
- A guest virtual machine can communicate with other guests, but cannot connect to the host machine after being configured to use a macvtap (also known as
type='direct') network interface.
- Even when not connecting to a Virtual Ethernet Port Aggregator (VEPA) or VN-Link capable switch, macvtap interfaces can be useful. Setting the mode of such an interface to
bridgeallows the guest to be directly connected to the physical network in a very simple manner without the setup issues (or NetworkManager incompatibility) that can accompany the use of a traditional host bridge device.However, when a guest virtual machine is configured to use a
type='direct'network interface such as macvtap, despite having the ability to communicate with other guests and other external hosts on the network, the guest cannot communicate with its own host.This situation is actually not an error — it is the defined behavior of macvtap. Due to the way in which the host's physical Ethernet is attached to the macvtap bridge, traffic into that bridge from the guests that is forwarded to the physical interface cannot be bounced back up to the host's IP stack. Additionally, traffic from the host's IP stack that is sent to the physical interface cannot be bounced back up to the macvtap bridge for forwarding to the guests.
- Use libvirt to create an isolated network, and create a second interface for each guest virtual machine that is connected to this network. The host and guests can then directly communicate over this isolated network, while also maintaining compatibility with NetworkManager.
Procedure B.3. Creating an isolated network with libvirt
The guests are now able to reach the host at the address 192.168.254.1, and the host will be able to reach the guests at the IP address they acquired from DHCP (alternatively, you can manually configure the IP addresses for the guests). Since this new network is isolated to only the host and guests, all other communication from the guests will use the macvtap interface.
- Add and save the following XML in the
/tmp/isolated.xmlfile. If the 192.168.254.0/24 network is already in use elsewhere on your network, you can choose a different network.
<network> <name>isolated</name> <ip address='192.168.254.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.254.2' end='192.168.254.254' /> </dhcp> </ip> </network>
- Create the network with this command:
virsh net-define /tmp/isolated.xml
- Set the network to autostart with the
virsh net-autostart isolatedcommand.
- Start the network with the
virsh net-start isolatedcommand.
virsh edit name_of_guest, edit the configuration of each guest that uses macvtap for its network connection and add a new
<devices>section similar to the following (note the
<model type='virtio'/>line is optional to include):
<interface type='network'> <source network='isolated'/> <model type='virtio'/> </interface>
- Shut down, then restart each of these guests.