How to Manage KVM Host Network using Networking Scripts

CloudSpinx
September 6, 2025
5:04 pm
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Configuring networks on Linux systems is achievable with numerous tools and frameworks. Although today’s releases like RHEL, CentOS Stream, and Fedora rely on using NetworkManager with nmcli as its command-line client, traditional network scripts remain predominant — on server and virtualization installations where determinism, stability, and precise control are issues.

Networking scripts refer to system configuration files in /etc/sysconfig/network-scripts/ on RHEL-based systems and in /etc/network/interfaces on Debian-based systems. The scripts they define determine how network interfaces are brought up at bootup and how they behave at runtime.

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Scripts for network on a KVM host are highly useful since administrators would be able to have persistent configurations for:

Static IP Addresses
Linux bridges for connecting virtual machines to the externeral network
Bonded interfaces for performance and for redundancy
VLANs and high-end topologies employed for virtualization labs or production clusters

Ensute bridge utilities package are installed:

# Debian-based systems
sudo apt update && sudo apt install bridge-utils

# SLES / openSUSE
sudo zypper -n install bridge-utils

# Arch-based systems
sudo pacman -Syu bridge-utils

1. Create Linux bridge (No VLAN)

This method of Linux bridge creation involves manually editing the traditional network configuration files to define a virtual network bridge.

The resuting bridge allows multiple network interfaces to communicate as if they are on the same physical network segment.

Debian-based Systems

Ensure the bridge utilities package is installed:

sudo apt update && sudo apt install bridge-utils

List available networkn interfaces in your machine:

ip link show

Once you have the name of your physical interface, set the variables to be used in network cofigurations.

# Specofy Physical interface added to bridge (e.g ens3)
PHY_INT=ens3

# Specify the name of the bridge interface (e.g br0)
BR_NAME=br0

If using static IP address on the bridge provide extra configurations

# Static IP address of the bridge
IPADDR=192.168.1.10
# The subnet mask
NETMASK=255.255.255.0
# The default gateway
GATEWAY=192.168.1.1
# DNS server 1
DNS1=8.8.8.8
# DNS server 2
DNS2=8.8.4.4

For Debian-based systems, we edit the network interfaces file /etc/network/interfaces to configure the bridge. But first backup the current network configuration file:

sudo cp /etc/network/interfaces{,.bak}

Bridge using DHCP

sudo tee -a /etc/network/interfaces<<EOF
auto ${BR_NAME}
iface ${BR_NAME} inet dhcp
    bridge_ports ${PHY_INT}
    bridge_stp off       # disable Spanning Tree Protocol
    bridge_fd 0          # no forwarding delay
    bridge_maxwait 0     # no delay for interface to come up

auto ${PHY_INT}
iface ${PHY_INT} inet manual
EOF

Bridge with Static IP Addressing

To include IP address, subnet mask, gateway, and DNS servers in the bridge configuration, adjust like below.

sudo tee -a /etc/network/interfaces<<EOF
auto lo
iface lo inet loopback

auto ${BR_NAME}
iface ${BR_NAME} inet static
    address ${IPADDR}
    netmask ${NETMASK}
    gateway ${GATEWAY}
    dns-nameservers ${DNS1} ${DNS2}
    bridge_ports ${PHY_INT}
    bridge_stp off
    bridge_fd 0
    bridge_maxwait 0

auto ${PHY_INT}
iface ${PHY_INT} inet manual
EOF

Before we restart networking service, open the file manually to validate configurations, and delete any duplicates.

# Open and validate settings, rectify any duplicates
sudo nano /etc/network/interfaces

Go through the settings and make any changes accordingly. Once done, restart the networking service for the changes to be applied.

sudo systemctl restart networking

RHEL-based Systems

Install bridge-utils package.

# CentOS Stream / Rocky / AlmaLinux
sudo dnf -y install epel-release
sudo dnf -y install bridge-utils

# RHEL 9
sudo subscription-manager repos \
 --enable codeready-builder-for-rhel-9-$(arch)-rpms
sudo dnf install \
https://dl.fedoraproject.org/pub/epel/epel-release-latest-9.noarch.rpm
sudo dnf -y install bridge-utils

# RHEL 8
sudo subscription-manager repos \
 --enable codeready-builder-for-rhel-8-$(arch)-rpms
sudo dnf install \
https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
sudo dnf -y install bridge-utils

Check your ethernet interface name.

ip link show
# OR
ip addr

Set the variables to be used.

# Specify the name of the physical interface (eth0)
PHY_INT=eth0

# Specify the name of the bridge interface (br0)
BR_NAME=br0

If using static IP address on the bridge provide extra configurations.

# Static IP address of the bridge
IPADDR=192.168.1.10
# The subnet mask
NETMASK=255.255.255.0
# The default gateway
GATEWAY=192.168.1.1
# DNS server 1
DNS1=8.8.8.8
# DNS server 2
DNS2=8.8.4.4

It’s recommended to back up existing network configurations before making any changes.

sudo cp /etc/sysconfig/network-scripts/ifcfg-${PHY_INT} \
/etc/sysconfig/network-scripts/ifcfg-${PHY_INT}.bak

Create a new network configuration file for defined bridge interface:

Using DHCP – If using DHCP server to assign IP address to bridge

sudo tee /etc/sysconfig/network-scripts/ifcfg-${BR_NAME}<<EOF
DEVICE=${BR_NAME}
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes
DELAY=0
EOF

Using Static IP Address – Manually assigning IP address information to bridge.

sudo tee /etc/sysconfig/network-scripts/ifcfg-${BR_NAME}<<EOF
DEVICE=${BR_NAME}
TYPE=Bridge
BOOTPROTO=none
IPADDR=${IPADDR}
NETMASK=${NETMASK}
GATEWAY=${GATEWAY}
DNS1=${DNS2}
DNS2=${DNS2}
ONBOOT=yes
DELAY=0
EOF

Configure Physical Interface:

sudo tee /etc/sysconfig/network-scripts/ifcfg-${PHY_INT}<<EOF
DEVICE=${PHY_INT}
TYPE=Ethernet
BRIDGE=${BR_NAME}
BOOTPROTO=none
ONBOOT=yes
EOF

Apply the changes by restarting the network service.

sudo systemctl restart network

Ensure the bridge is configured correctly by cheking its status:

ip add show ${BR_NAME}

2. Create Linux bridge (With VLAN)

Debian-based systems

Let’s assume the physcial interface is eno1 and VLAN ID is 100. Here’s how you will create a sub-vlan interface.

sudo apt update
sudo apt install bridge-utils vlan

Ensure that the 8021q module required for VLAN is loaded.

sudo modprobe 8021q

To ensure it loads on boot, add it to /etc/modules.

echo 8021q | sudo tee -a /etc/modules

Add VLAN interface to the /etc/network/interfaces file.

sudo nano /etc/network/interfaces

Using DHCP

auto eno1.100
iface eno1.100 inet manual
  vlan-raw-device eno1

auto br0
iface br0 inet dhcp
  bridge_ports eno1.100
  bridge_stp off
  bridge_fd 0
  bridge_maxwait 0

Using Static IP Address

auto eno1.100
iface eno1.100 inet manual
  vlan-raw-device eno1

auto br0
iface br0 inet static
  address 192.168.1.10
  netmask 255.255.255.0
  gateway 192.168.1.1
  dns-nameservers 8.8.8.8 8.8.4.4
  bridge_ports eno1.100
  bridge_stp off
  bridge_fd 0
  bridge_maxwait 0

When done with the configurations restart networking service.

sudo systemctl restart networking

RHEL-based systems

Install necessary packages on your system.

sudo dnf install -y NetworkManager network-scripts

Confirm that kernel modules are loaded:

sudo modprobe --first-time 8021q
modinfo 8021q

Step 1: Configure physical network Interface

Edit the physical interface configuration:

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0

Add and modify the contents. Don’t forget to replace eth0 with your actual interface as seen from ip ad or ip link show commands:

DEVICE=eth0
TYPE=Ethernet
BOOTPROTO=none
ONBOOT=yes

Step 2: Create VLAN Interface

Create a configuration file for the VLAN interface. Here VLAN ID used is 100.

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0.100

Add the following content to the file. Modify to work for your network setup.

DEVICE=eth0.100
BOOTPROTO=none
ONBOOT=yes
VLAN=yes
BRIDGE=br0

Step 3: Create Bridge Interface

Create a configuration file for the bridge interface.

sudo vi /etc/sysconfig/network-scripts/ifcfg-br0

Bridge Using DHCP

DEVICE=br0
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes
STP=off

Bridge with Static IP Configurations

DEVICE=br0
TYPE=Bridge
BOOTPROTO=none
ONBOOT=yes
STP=off
IPADDR=192.168.1.10
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4

Restart network service when done.

# If using network service
sudo systemctl restart network.service

# If using NetworkManager
sudo systemctl restart NetworkManager

Validate your network configurations:

# List bridges
brctl show

# IP configurations
ip addr

# Interfaces
ip link show

3. Create Linux bridge on Bond / Team interface

In this example we assume:

eth0 is your management interface
eth1 and eth2 are the interfaces bonded together
Bond name is bond0

Available bonding modes

Bonding Mode	Description	Switch Configuration
`0 (balance-rr)`	Round-robin distribution of packets across interfaces.	Static Etherchannel (not LACP-negotiated)
`1 (active-backup)`	One interface is active, others are in backup mode.	Autonomous ports
`2 (balance-xor)`	XOR hashing of MAC addresses to distribute packets across interfaces.	Requires specific switch configuration
`3 (broadcast)`	Broadcast packets to all interfaces.	Not recommended for most use cases
`4 (802.3ad)`	LACP (Link Aggregation Control Protocol) for dynamic link aggregation.	LACP-capable switch required
`5 (balance-tlb)`	Transmit load balancing based on destination MAC address hash.	No specific switch configuration required
`6 (balance-alb)`	Adaptive load balancing based on traffic patterns.	No specific switch configuration required

BONDING_OPTS Parameters

The BONDING_OPTS parameter in a bonding interface configuration allows you to specify additional options for the bonding driver. Here’s a table of commonly used options:

Option	Description
`mode`	Specifies the bonding mode (e.g., `0`, `1`, `4`).
`miimon`	ARP/ICMP link monitoring interval in milliseconds.
`downdelay`	Delay before marking a slave down (milliseconds).
`updelay`	Delay before marking a slave up (milliseconds).
`lacp_rate`	LACP rate (slow/fast) for mode 4 (802.3ad).
`xmit_hash_policy`	Transmit hash policy (e.g., `layer2`, `layer3+4`).
`arp_ip`	IP address for ARP requests.
`fail_over_mac`	MAC address to use for the bond interface.

On Debian-Based Systems

Follow these steps on a Debian based Linux machine.

Step 1: Install Packages and Load modules

sudo apt update
sudo apt install vlan ifenslave bridge-utils

Load necessary modules

sudo modprobe bonding vlan
echo bonding | sudo tee -a /etc/modules

Step 2: Configure Bonding Interface

Edit the /etc/network/interfaces file to create a bonding interface

sudo nano /etc/network/interfaces

Define interfaces used in bond (bond0)

# Define interfaces used in bonded interface
auto eth1
iface eth1 inet manual
bond-master bond0

auto eth2
iface eth2 inet manual
bond-master bond0

Add bond network configurations

Using mode 0 (balance-rr) example:

auto bond0
iface bond0 inet manual
  bond-slaves eth1 eth2
  bond-miimon 100
  bond-mode balance-rr

Using mode 4 (802.3ad) example:

auto bond0
iface bond0 inet manual
  bond-slaves eth1 eth2
  bond-miimon 100
  bond-mode 802.3ad
  bond-lacp-rate 1

Using mode 1 (active-backup) example:

auto bond0
iface bond0 inet manual
  bond-slaves eth1 eth2
  bond-miimon 100
  bond-mode active-backup

Step 3: Add bridge configurations

We can complete the setup by creating a linux bridge with static IP address.

Bridge with DHCP

# Bridge setup
auto br0
iface br0 inet dhcp
  bridge_ports bond0
  bridge_stp off       # Disable Spanning Tree Protocol
  bridge_fd 0          # Forwarding delay 0 for fast forwarding
  bridge_maxwait 0     # No delay during bridge creation

Bridge with static IP address

# Bridge setup
auto br0
iface br0 inet static
  address 192.168.1.10
  netmask 255.255.255.0
  gateway 192.168.1.1
  dns-nameservers 8.8.8.8 8.8.4.4
  bridge_ports bond0
  bridge_stp off       # Disable Spanning Tree Protocol
  bridge_fd 0          # Forwarding delay 0 for fast forwarding
  bridge_maxwait 0     # No delay during bridge creation

Restart networking service to changes to take effect:

sudo systemctl restart networking

Creating a bond on VLAN interface

By combining bonding (link aggregation) and VLAN (virtual LAN) tagging features, you can optimize network connectivity and segmentation effectively.

Let’s consider the following environment:

Physical interfaces used in bonding eth1 and eth2
VLAN ID: 100
Bridge name: br0
Bond name: br0

Here are complete network configurations that can be customized to your needs.

# Ethernet interfaces
auto eth1
iface eth1 inet manual
  bond-master bond0

auto eth2
iface eth2 inet manual
  bond-master bond0

# Bonding setup on VLAN 100
auto bond0.100
iface bond0.100 inet manual
  bond-slaves eth1 eth2
  bond-miimon 100
  bond-mode active-backup
  vlan-raw-device bond0

# Bridge setup
auto br0
iface br0 inet static
  address 192.168.1.10
  netmask 255.255.255.0
  gateway 192.168.1.1
  dns-nameservers 8.8.8.8 8.8.4.4
  bridge_ports bond0.100
  bridge_stp off       # Disable Spanning Tree Protocol
  bridge_fd 0          # Forwarding delay 0 for fast forwarding
  bridge_maxwait 0     # No delay during bridge creation

On RHEL-based Systems

Step 1: Install necessary packages / load module

Install all the necessary packages:

sudo dnf install -y NetworkManager teamd

Load kernel module using modprobe command:

sudo modprobe bonding && modinfo bonding

Step 2: Create Bond interface

Next create a bond interface configuration file

sudo vi /etc/sysconfig/network-scripts/ifcfg-bond0

Add and customize the contents below.

Example on using mode 1 (active-backup):

NAME=bond0
DEVICE=bond0
TYPE=Bond
ONBOOT=yes
BONDING_MASTER=yes
BOOTPROTO=none
BONDING_OPTS="mode=active-backup miimon=100"
BRIDGE=br0
NM_CONTROLLED=no

BONDING_OPTS configurations when using mode 4 (802.3ad):

BONDING_OPTS="mode=802.3ad lacp_rate=fast miimon=100"
#OR
BONDING_OPTS="mode=4 lacp_rate=fast miimon=100"

BONDING_OPTS configurations when using mode 0 (balance-rr):

BONDING_OPTS="mode=balance-rr miimon=100"
#OR
BONDING_OPTS="mode=0 miimon=100"

Step 3: Configure Bond Slaves

Create configuration files for the interfaces to be used in the bond (e.g., eth1 and eth2).

For eth1

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth1

Add configuration for eth1 interface.

NAME=bond0-slave0
DEVICE=eth1
ONBOOT=yes
BOOTPROTO=none
MASTER=bond0
SLAVE=yes
NM_CONTROLLED=no

For eth2

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth2

Add configuration for eth2 interface.

NAME=bond0-slave1
DEVICE=eth2
ONBOOT=yes
BOOTPROTO=none
MASTER=bond0
SLAVE=yes
NM_CONTROLLED=no

Step 4: Create Bridge Interface

Create a configuration file for the bridge interface.

sudo vi /etc/sysconfig/network-scripts/ifcfg-br0

With DHCP

DEVICE=br0
TYPE=Bridge
BRIDGE_PORTS=bond0
BOOTPROTO=dhcp
ONBOOT=yes
NM_CONTROLLED=no
DELAY=0

With Static IP address

DEVICE=br0
TYPE=Bridge
BRIDGE_PORTS=bond0
BOOTPROTO=static
ONBOOT=yes
IPADDR=192.168.1.10
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4
NM_CONTROLLED=no
DELAY=0

Creating a VLAN interface on a bond

Let’s consider an example:

Bond name: bond0
VLAN ID: 100

Step 1: Create bond interface configuration:

Create /etc/sysconfig/network-scripts/ifcfg-bond0

NAME=bond0
DEVICE=bond0
TYPE=Bond
ONBOOT=yes
BONDING_MASTER=yes
BOOTPROTO=none
BONDING_OPTS="mode=active-backup miimon=100"
NM_CONTROLLED=no

Step 2: Create VLAN interface on bond

Create /etc/sysconfig/network-scripts/ifcfg-bond0.100

DEVICE=bond0.100
BOOTPROTO=none
ONBOOT=yes
TYPE=VLAN
PHYSDEV=bond0
VLAN=yes
BRIDGE=br0
NM_CONTROLLED=no

Step 3: Create Linux Bridge on Bond VLAN Interface

Create /etc/sysconfig/network-scripts/ifcfg-br0

DEVICE=br0
TYPE=Bridge
BOOTPROTO=static
ONBOOT=yes
IPADDR=192.168.1.10
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4
NM_CONTROLLED=no

Restart network after making all the configurations

sudo systemctl restart network

🌍 Whether at work or on the go, access Mastering KVM Virtualization anytime, in the book you will learn:

Host Network Management using Netplan: bridges, VLANs, Bonded interfaces
Host Network Management using Network Manager (NMCLI) – bridged, VLANs, Bonds/Teamed
Host Network Management using Networking Scripts
Host Network Management using Open vSwitch

Watch out for other KVM guides from our team:

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