Chapter 13. Configuring virtual machine network connections

For your virtual machines (VMs) to connect over a network to your host, to other VMs on your host, and to locations on an external network, the VM networking must be configured accordingly. To provide VM networking, the RHEL 8 hypervisor and newly created VMs have a default network configuration, which can also be modified further. For example:

  • You can enable the VMs on your host to be discovered and connected to by locations outside the host, as if the VMs were on the same network as the host.
  • You can partially or completely isolate a VM from inbound network traffic to increase its security and minimize the risk of any problems with the VM impacting the host.

The following sections explain the various types of VM network configuration and provide instructions for setting up selected VM network configurations.

13.1. Understanding virtual networking

The connection of virtual machines (VMs) to other devices and locations on a network has to be facilitated by the host hardware. The following sections explain the mechanisms of VM network connections and describe the default VM network setting.

13.1.1. How virtual networks work

Virtual networking uses the concept of a virtual network switch. A virtual network switch is a software construct that operates on a host machine. VMs connect to the network through the virtual network switch. Based on the configuration of the virtual switch, a VM can use use an existing virtual network managed by the hypervisor, or a different network connection method.

The following figure shows a virtual network switch connecting two VMs to the network:

vn 02 switchandtwoguests

From the perspective of a guest operating system, a virtual network connection is the same as a physical network connection. Host machines view virtual network switches as network interfaces. When the libvirtd service is first installed and started, it creates virbr0, the default network interface for VMs.

To view information about this interface, use the ip utility on the host.

$ ip addr show virbr0
3: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
 UNKNOWN link/ether 1b:c4:94:cf:fd:17 brd ff:ff:ff:ff:ff:ff
 inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0

By default, all VMs on a single host are connected to the same NAT-type virtual network, named default, which uses the virbr0 interface. For details, see Section 13.1.2, “Virtual networking default configuration”.

For basic outbound-only network access from VMs, no additional network setup is usually needed, because the default network is installed along with the libvirt package, and is automatically started when the libvirtd service is started.

If a different VM network functionality is needed, you can create additional virtual networks and network interfaces and configure your VMs to use them. In addition to the default NAT, these networks and interfaces can be configured to use one of the following modes:

13.1.2. Virtual networking default configuration

When the libvirtd service is first installed on a virtualization host, it contains an initial virtual network configuration in network address translation (NAT) mode. By default, all VMs on the host are connected to the same libvirt virtual network, named default. VMs on this network can connect to locations both on the host and on the network beyond the host, but with the following limitations:

  • VMs on the network are visible to the host and other VMs on the host, but the network traffic is affected by the firewalls in the guest operating system’s network stack and by the libvirt network filtering rules attached to the guest interface.
  • VMs on the network can connect are not visible to locations outside the host. Outbound traffic is affected by the NAT rules, as well as the host system’s firewall.

The following diagram illustrates the default VM network configuration:

vn 08 network overview

13.2. Using the web console for managing virtual machine network interfaces

Using the RHEL 8 web console, you can manage the virtual network interfaces for the virtual machines to which the web console is connected. You can:

13.2.1. Viewing and editing virtual network interface information in the web console

Using the RHEL 8 web console, you can view and modify the virtual network interfaces on a selected virtual machine (VM):

Prerequisites

Procedure

  1. In the Virtual Machines interface, click the row of the VM whose information you want to see.

    The row expands to reveal the Overview pane with basic information about the selected VM and controls for shutting down and deleting the VM.

  2. Click Network Interfaces.

    The Networks Interfaces pane appears with information about the virtual network interface configured for the VM.

    cockpit vNIC info

    The information includes the following:

    • Type - The type of network interface for the VM. Types include virtual network, bridge to LAN, and direct attachment.

      Note

      Generic Ethernet connection is not supported in RHEL 8.2.

    • Model type - The model of the virtual network interface.
    • MAC Address - The MAC address of the virtual network interface.
    • IP Address - The IP address of the virtual network interface.
    • Source - The source of the network interface. This is dependent on the network type.
    • State - The state of the virtual network interface.
  3. To edit the virtual network interface settings, Click Edit. The Virtual Network Interface Settings dialog opens.

    web console virtual network if settings
  4. Change the interface type, source, or model.
  5. Click Save. The network interface is modified.

    Note

    Changes to the virtual network interface settings take effect only after restarting the VM.

Additional resources

13.2.2. Connecting virtual network interfaces in the web console

Using the RHEL 8 web console, you can reconnect disconnected virtual network interface configured for a selected virtual machine (VM).

Prerequisites

Procedure

  1. In the Virtual Machines interface, click the row of the VM whose virtual network interface you want to connect.

    The row expands to reveal the Overview pane with basic information about the selected VM and controls for shutting down and deleting the VM.

  2. Click Networks.

    The Networks pane appears with information about the virtual network interfaces configured for the VM.

    cockpit vNIC plug

  3. Click Plug in the row of the virtual network interface you want to connect.

    The selected virtual network interface connects to the VM.

13.2.3. Disconnecting virtual network interfaces in the web console

Using the RHEL 8 web console, you can disconnect the virtual network interfaces connected to a selected virtual machine (VM).

Prerequisites

Procedure

  1. In the Virtual Machines interface, click the row of the VM whose virtual network interface you want to disconnect.

    The row expands to reveal the Overview pane with basic information about the selected VM and controls for shutting down and deleting the VM.

  2. Click Network Interfaces.

    The Network Interfaces pane appears with information about the virtual network interfaces configured for the VM.

    cockpit vNIC disconnect
  3. Click Unplug in the row of the virtual network interface you want to disconnect.

    The selected virtual network interface disconnects from the VM.

13.5. Types of virtual machine network connections

To modify the networking properties and behavior of your VMs, change the type of virtual network or interface the VMs use. The following sections describe the connection types available to VMs in RHEL 8.

13.5.1. Virtual networking with network address translation

By default, virtual network switches operate in network address translation (NAT) mode. They use IP masquerading rather than Source-NAT (SNAT) or Destination-NAT (DNAT). IP masquerading enables connected VMs to use the host machine’s IP address for communication with any external network. When the virtual network switch is operating in NAT mode, computers external to the host cannot communicate with the VMs inside the host.

vn 04 hostwithnatswitch
Warning

Virtual network switches use NAT configured by iptables rules. Editing these rules while the switch is running is not recommended, because incorrect rules may result in the switch being unable to communicate.

13.5.2. Virtual networking in routed mode

When using Routed mode, the virtual switch connects to the physical LAN connected to the host machine, passing traffic back and forth without the use of NAT. The virtual switch can examine all traffic and use the information contained within the network packets to make routing decisions. When using this mode, the virtual machines (VMs) are all in a single subnet, separate from the host machine. The VM subnet is routed through a virtual switch, which exists on the host machine. This enables incoming connections, but requires extra routing-table entries for systems on the external network.

Routed mode uses routing based on the IP address:

vn 06 routed switch

Common topologies that use routed mode include DMZs and virtual server hosting.

DMZ

You can create a network where one or more nodes are placed in a controlled sub-network for security reasons. Such a sub-network is known as a demilitarized zone (DMZ).

vn 09 routed mode DMZ

Host machines in a DMZ typically provide services to WAN (external) host machines as well as LAN (internal) host machines. Since this requires them to be accessible from multiple locations, and considering that these locations are controlled and operated in different ways based on their security and trust level, routed mode is the best configuration for this environment.

Virtual server hosting

A virtual server hosting provider may have several host machines, each with two physical network connections. One interface is used for management and accounting, the other for the VMs to connect through. Each VM has its own public IP address, but the host machines use private IP addresses so that only internal administrators can manage the VMs.

vn 10 routed mode datacenter

13.5.3. Virtual networking in bridged mode

In most VM networking modes, VMs automatically create and connect to the virbr0 virtual bridge. In contrast, in bridged mode, the VM connects to an existing Linux bridge on the host. As a result, the VM is directly visible on the physical network. This enables incoming connections, but does not require any extra routing-table entries.

Bridged mode uses connection switching based on the MAC address:

vn Bridged Mode Diagram

In bridged mode, the VM appear within the same subnet as the host machine. All other physical machines on the same physical network can detect the VM and access it.

Bridged network bonding

It is possible to use multiple physical bridge interfaces on the hypervisor by joining them together with a bond. The bond can then be added to a bridge, after which the VMs can be added to the bridge as well. However, the bonding driver has several modes of operation, and not all of these modes work with a bridge where VMs are in use.

The following bonding modes are usable:

  • mode 1
  • mode 2
  • mode 4

In contrast, using modes 0, 3, 5, or 6 is likely to cause the connection to fail. Also note that media-independent interface (MII) monitoring should be used to monitor bonding modes, as Address Resolution Protocol (ARP) monitoring does not work correctly.

For more information on bonding modes, refer to the Red Hat Knowledgebase.

Common scenarios

The most common use cases for bridged mode include:

  • Deploying VMs in an existing network alongside host machines, making the difference between virtual and physical machines invisible to the end user.
  • Deploying VMs without making any changes to existing physical network configuration settings.
  • Deploying VMs that must be easily accessible to an existing physical network. Placing VMs on a physical network where they must access DHCP services.
  • Connecting VMs to an existing network where virtual LANs (VLANs) are used.

Additional resources

13.5.4. Virtual networking in isolated mode

When using isolated mode, virtual machines connected to the virtual switch can communicate with each other and with the host machine, but their traffic will not pass outside of the host machine, and they cannot receive traffic from outside the host machine. Using dnsmasq in this mode is required for basic functionality such as DHCP.

vn 07 isolated switch

13.5.5. Virtual networking in open mode

When using open mode for networking, libvirt does not generate any iptables rules for the network. As a result, libvirt does not overwrite iptables rules provided by the host, and the user can therefore manually manage the VM’s iptables rules.

13.5.6. Direct attachment of the virtual network device

You can use the macvtap driver to attach a virtual machine’s NIC directly to a specified physical interface of the host machine. The macvtap connection has a number of modes, including private mode.

In this mode, all packets are sent to the external switch and will only be delivered to a target VM on the same host machine if they are sent through an external router or gateway and these send them back to the host. Private mode can be used to prevent the individual VMs on a single host from communicating with each other.

virt macvtap modes private

13.5.7. Comparison of virtual machine connection types

The following table provides information about the locations to which selected types of virtual machine (VM) network configurations can connect, and to which they are visible.

Table 13.1. Virtual machine connection types

 Connection to the hostConnection to other VMs on the hostConnection to outside locationsVisible to outside locations

Bridged mode

YES

YES

YES

YES

NAT

YES

YES

YES

no

Routed mode

YES

YES

YES

YES

Isolated mode

YES

YES

no

no

Private mode

no

no

YES

YES

Open mode

Depends on the host’s iptables rules

13.6. Additional resources