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:

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:

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

To use the web console to manage VMs, install the web console VM plug-in.

Procedure

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.
Click Network Interfaces.
The Networks Interfaces pane appears with information about the virtual network interface configured for the VM.
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.
To edit the virtual network interface settings, Click Edit. The Virtual Network Interface Settings dialog opens.
Change the interface type, source, or model.
Click Save. The network interface is modified.
Note
Changes to the virtual network interface settings take effect only after restarting the VM.

Additional resources

For instructions on viewing information about all of the VMs to which the web console session is connected, see Section 6.2.1, “Viewing a virtualization overview in the web console”.
For instructions on viewing information about the storage pools to which the web console session is connected, see Section 6.2.2, “Viewing storage pool information using the web console”.
For instructions on viewing basic information about a selected VM to which the web console session is connected, see Section 6.2.3, “Viewing basic virtual machine information in the web console”.
For instructions on viewing resource usage for a selected VM to which the web console session is connected, see Section 6.2.4, “Viewing virtual machine resource usage in the web console”.
For instructions on viewing disk information about a selected VM to which the web console session is connected, see Section 6.2.5, “Viewing virtual machine disk information in the web console”.

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

To use the web console to manage VMs, install the web console VM plug-in.

Procedure

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

To use the web console to manage VMs, install the web console VM plug-in.

Procedure

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.
Click Network Interfaces.
The Network Interfaces pane appears with information about the virtual network interfaces configured for the VM.
Click Unplug in the row of the virtual network interface you want to disconnect.
The selected virtual network interface disconnects from the VM.

13.3. Recommended virtual machine networking configurations using the command-line interface

In many scenarios, the default VM networking configuration is sufficient. However, if adjusting the configuration is required, you can use the command-line interface (CLI) to do so. The following sections describe selected VM network setups for such situations.

13.3.1. Configuring externally visible virtual machines using the command-line interface

By default, a newly created VM connects to a NAT-type network that uses virbr0, the default virtual bridge on the host. This ensures that the VM can use the host’s network interface controller (NIC) for connecting to outside networks, but the VM is not reachable from external systems.

If you require a VM to appear on the same external network as the hypervisor, you must use bridged mode instead. To do so, attach the VM to a bridge device connected to the hypervisor’s physical network device. To use the command-line interface for this, follow the instructions below.

Prerequisites

A shut-down existing VM.
The IP configuration of the hypervisor. This varies depending on the network connection of the host. As an example, this procedure uses a scenario where the host is connected to the network using an ethernet cable, and the hosts' physical NIC MAC address is assigned to a static IP on a DHCP server. Therefore, the ethernet interface is treated as the hypervisor IP.
To obtain the IP configuration of the ethernet interface, use the ip addr utility:
```
# ip addr
[...]
enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 54:ee:75:49:dc:46 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.148/24 brd 10.0.0.255 scope global dynamic noprefixroute enp0s25
```

Procedure

Create and set up a bridge connection for the physical interface on the host. For instructions, see the Configuring a network bridge.
Note that in a scenario where static IP assignment is used, you must move the IPv4 setting of the physical ethernet interface to the bridge interface.
Modify the VM’s network to use the created bridged interface. For example, the following sets testguest to use bridge0.
```
# virt-xml testguest --edit --network bridge=bridge0
Domain 'testguest' defined successfully.
```
Start the VM.
```
# virsh start testguest
```
In the guest operating system, adjust the IP and DHCP settings of the system’s network interface as if the VM was another physical system in the same network as the hypervisor.
The specific steps for this will differ depending on the guest OS used by the VM. For example, if the guest OS is RHEL 8, see Configuring an Ethernet connection.

Verification

Ensure the newly created bridge is running and contains both the host’s physical interface and the interface of the VM.

# ip link show master bridge0
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master bridge0 state UP mode DEFAULT group default qlen 1000
    link/ether 54:ee:75:49:dc:46 brd ff:ff:ff:ff:ff:ff
10: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master bridge0 state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether fe:54:00:89:15:40 brd ff:ff:ff:ff:ff:ff

Ensure the VM appears on the same external network as the hypervisor:

In the guest operating system, obtain the network ID of the system. For example, if it is a Linux guest:

# ip addr
[...]
enp0s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:09:15:46 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.150/24 brd 10.0.0.255 scope global dynamic noprefixroute enp0s0

From an external system connected to the local network, connect to the VM using the obtained ID.
```
# ssh root@10.0.0.150
root@10.0.0.150's password:
Last login: Mon Sep 24 12:05:36 2019
root~#*
```
If the connection works, the network has been configured successfully.

Additional resources

For instructions on creating an externally visible VM using the web console, see Section 13.4.1, “Configuring externally visible virtual machines using the web console”.
For additional information on bridged mode, see Section 13.5.3, “Virtual networking in bridged mode”.
In certain situations, such as when a using client-to-site VPN while the VM is hosted on the client, using bridged mode for making your VMs available to external locations is not possible.
To work around this problem, you can set a destination NAT for the VM. For details, see the Configuring and managing networking document.

13.3.2. Isolating virtual machines from each other using the command-line interface

To prevent a virtual machine (VM) from communicating with other VMs on your host, for example to avoid data sharing or to increase system security, you can completely isolate the VM from host-side network traffic.

By default, a newly created VM connects to a NAT-type network that uses virbr0, the default virtual bridge on the host. This ensures that the VM can use the host’s NIC for connecting to outside networks, as well as to other VMs on the host. This is a generally secure connection, but in some cases, connectivity to the other VMs may be a security or data privacy hazard. In such situations, you can isolate the VM by using direct macvtap connection in private mode instead of the default network.

In private mode, the VM is visible to external systems and can receive a public IP on the host’s subnet, but the VM and the host cannot access each other, and the VM is also not visible to other VMs on the host.

For instructions to set up macvtap private mode on your VM using the CLI, see below.

Prerequisites

An existing VM with the default NAT setup.
The name of the host interface that you want to use for the macvtap connection. The interface you must select will vary depending on your use case and the network configuration on your host. As an example, this procedure uses the host’s physical ethernet interface.
To obtain the name of the targeted interface:
```
$ ip addr
[...]
2: enp0s31f6: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN group default qlen 1000
    link/ether 54:e1:ad:42:70:45 brd ff:ff:ff:ff:ff:ff
[...]
```

Procedure

Use the selected interface to set up private macvtap on the selected VM. The following example configures macvtap in private mode on the enp0s31f6 interface for the VM named panic-room.
```
# virt-xml panic-room --edit --network type=direct,source=enp0s31f6,source.mode=private
Domain panic-room XML defined successfully
```

Verification

Start the updated VM.

# virsh start panic-room
Domain panic-room started

List the interface statistics for the VM.

# virsh domstats panic-room --interface
Domain: 'panic-room'
  net.count=1
  net.0.name=macvtap0
  net.0.rx.bytes=0
  net.0.rx.pkts=0
  net.0.rx.errs=0
  net.0.rx.drop=0
  net.0.tx.bytes=0
  net.0.tx.pkts=0
  net.0.tx.errs=0
  net.0.tx.drop=0

If the command displays similar output, the VM has been isolated successfully.

Additional resources

For instructions on isolating a VM using the web console, see Section 13.4.2, “Isolating virtual machines from each other using the web console”.
For additional information about macvtap private mode, see Section 13.5.6, “Direct attachment of the virtual network device”.
For additional security measures that you can set on a VM, see Chapter 15, Securing virtual machines.

13.4. Recommended virtual machine networking configurations using the web console

In many scenarios, the default VM networking configuration is sufficient. However, if adjusting the configuration is required, you can use the RHEL 8 web console to do so. The following sections describe selected VM network setups for such situations.

13.4.1. Configuring externally visible virtual machines using the web console

If you require a VM to appear on the same external network as the hypervisor, you must use bridged mode instead. To do so, attach the VM to a bridge device connected to the hypervisor’s physical network device. To use the RHEL 8 web console for this, follow the instructions below.

Prerequisites

To use the web console to manage VMs, install the web console VM plug-in.
A shut-down existing VM.
The IP configuration of the hypervisor. This varies depending on the network connection of the host. As an example, this procedure uses a scenario where the host is connected to the network using an ethernet cable, and the hosts' physical NIC MAC address is assigned to a static IP on a DHCP server. Therefore, the ethernet interface is treated as the hypervisor IP.
To obtain the IP configuration of the ethernet interface, go to the Networking tab in the web console, and see the Interfaces section.

Procedure

Create and set up a bridge connection for the physical interface on the host. For instructions, see Configuring network bridges in the web console.
Note that in a scenario where static IP assignment is used, you must move the IPv4 setting of the physical ethernet interface to the bridge interface.
Modify the VM’s network to use the bridged interface. In the Network Interfaces tab of the VM:
1. Click Add Network Interface
2. In the Add Virtual Network Interface dialog, set:
  - Interface Type to Bridge to LAN
  - Source to the newly created bridge, for example bridge0
3. Click Add
4. Optional: Click Unplug for all the other interfaces connected to the VM.
Click Run to start the VM.
In the guest operating system, adjust the IP and DHCP settings of the system’s network interface as if the VM was another physical system in the same network as the hypervisor.
The specific steps for this will differ depending on the guest OS used by the VM. For example, if the guest OS is RHEL 8, see Configuring an Ethernet connection.

Verification

In the Networking tab of the host’s web console, click the row with the newly created bridge to ensure it is running and contains both the host’s physical interface and the interface of the VM.

Ensure the VM appears on the same external network as the hypervisor.

In the guest operating system, obtain the network ID of the system. For example, if it is a Linux guest:

# ip addr
[...]
enp0s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:09:15:46 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.150/24 brd 10.0.0.255 scope global dynamic noprefixroute enp0s0

From an external system connected to the local network, connect to the VM using the obtained ID.
```
# ssh root@10.0.0.150
root@110.34.5.18's password:
Last login: Mon Sep 24 12:05:36 2019
root~#*
```
If the connection works, the network has been configured successfully.

Additional resources

For instructions on creating an externally visible VM using the CLI, see Section 13.3.1, “Configuring externally visible virtual machines using the command-line interface”.
For additional information on bridged mode, see Section 13.5.3, “Virtual networking in bridged mode”.
In certain situations, such as when a using client-to-site VPN while the VM is hosted on the client, using bridged mode for making your VMs available to external locations is not possible.
To work around this problem, you can set a destination NAT for the VM. For details, see the Configuring and managing networking document.

13.4.2. Isolating virtual machines from each other using the web console

For instructions to set up macvtap private mode on your VM using the web console, see below.

Prerequisites

To use the web console to manage VMs, install the web console VM plug-in.
An existing VM with the default NAT setup.

Procedure

In the Virtual Machines pane, click the row with the virtual machine you want to isolate.
A pane with the basic information about the VM opens.
Click the Network Interfaces tab.
Click Edit.
The Virtual Machine Interface Settings dialog opens.
Set Interface Type to Direct Attachment
Set Source to the host interface of your choice.
Note that the interface you select will vary depending on your use case and the network configuration on your host.

Verification

Start the VM by clicking Run.

In the Terminal pane of the web console, list the interface statistics for the VM. For example, to view the network interface traffic for the panic-room VM:

# virsh domstats panic-room --interface
Domain: 'panic-room'
  net.count=1
  net.0.name=macvtap0
  net.0.rx.bytes=0
  net.0.rx.pkts=0
  net.0.rx.errs=0
  net.0.rx.drop=0
  net.0.tx.bytes=0
  net.0.tx.pkts=0
  net.0.tx.errs=0
  net.0.tx.drop=0

If the command displays similar output, the VM has been isolated successfully.

Additional resources

For instructions on isolating a VM using the command-line, see Section 13.3.2, “Isolating virtual machines from each other using the command-line interface”.
For additional information about macvtap private mode, see Section 13.5.6, “Direct attachment of the virtual network device”.
For additional security measures that you can set on a VM, see Chapter 15, Securing virtual machines.

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.

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:

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).

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.

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:

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

For instructions on configuring your VMs to use bridged mode, see Section 13.3.1, “Configuring externally visible virtual machines using the command-line interface” or Section 13.4.1, “Configuring externally visible virtual machines using the web console”.
For a detailed explanation of bridge_opts parameters, used to configure bridged networking mode, see the Red Hat Virtualization Administration Guide.

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.

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.

Additional resources

For instructions on configuring your VMs to use macvtap in private mode, see Section 13.3.2, “Isolating virtual machines from each other using the command-line interface” or Section 13.4.2, “Isolating virtual machines from each other using the web console”.

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 host	Connection to other VMs on the host	Connection to outside locations	Visible 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

For additional information on networking configurations in RHEL 8, see the Configuring and managing networking document.
Specific network interface cards can be attached to VMs as SR-IOV devices, which increases their performance. For details, see Section 10.8, “Managing SR-IOV devices”.