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Configuring the KVM Network (Bridging Mode)

It is assumed that the KVM has been installed. To access the MySQL service on the KVM from another PC in the LAN, you can connect the KVM (guest) to the host machine in bridging mode, and enable the VM to obtain an independent IP address on the LAN.

Configuring a Network Bridge

  1. Check the network.
    ip a

    This example illustrates how to bridge the KVM to the network segment (192.168.224.XXX, subnet mask: 255.255.255.0) where enp3s0 is located.

  2. Check for network bridges and ensure that the br0 network bridge does not exist.
    1
    		 
    brctl show
  3. Stop the NetworkManager service to prevent configuration modifications from taking effect unexpectedly.
    1
    		 
    systemctl stop NetworkManager
  4. Create a br0 network bridge.
    1. Open the ifcfg-br0 file. 
      1
      		 
      vim /etc/sysconfig/network-scripts/ifcfg-br0
    2. Press i to enter the insert mode and add the following content to the file: 
      DEVICE=br0
ONBOOT=yes
TYPE=Bridge
NM_CONTROLLED=yes
BOOTPROTO=static
IPADDR=192.168.224.2
NETMASK=255.255.255.0
GATEWAY=192.168.224.1
STP=on
DELAY=0.0
      • Replace the values of IPADDR and GATEWAY with actual ones.
      • If you perform the following operations to create a network bridge, the network bridge becomes invalid after the host machine is restarted. Replace the IP address in the example with the actual one. 
        brctl addbr br0
brctl addif br0 enp3s0
ifconfig br0 192.168.224.2/24
service network restart
brctl show
ip a
    3. Press Esc, type :wq!, and press Enter to save the file and exit.
  5. Modify the ifcfg-enp3s0 file to connect the enp3s0 network port to the br0 bridge.
    1. Back up the ifcfg-enp3s0 file. 
      mv /etc/sysconfig/network-scripts/ifcfg-enp3s0 /home/
    2. Open the ifcfg-enp3s0 file. 
      1
      		 
      vim /etc/sysconfig/network-scripts/ifcfg-enp3s0
    3. Press i to enter the insert mode and add the following content to the file: 
      HWADDR=a8:49:4d:f6:84:dc
DEVICE=enp3s0
TYPE=Ethernet
ONBOOT=yes
NM_CONTROLLED=yes
BRIDGE=br0

      Check the HWADDR value of enp3s0. In the command output, the MAC address following ether is the HWADDR value.

      ip a
    4. Press Esc, type :wq!, and press Enter to save the file and exit.
  6. Make the ifcfg-br0 and ifcfg-enp3s0 files take effect.
    • CentOS: 
      1
      		 
      systemctl restart network
    • openEuler: 
      1
      		 
      systemctl restart NetworkManager

      If the configuration does not take effect after the network service is restarted, check whether the previous configurations are correct. Alternatively, restart the host machine and then restart the network service.

  7. Check the network.
    1
    		 
    ip a

  8. View the network bridge.
    1
    		 
    brctl show

    If enp3s0 is displayed under interfaces of br0, the network bridge is configured successfully.

Configuring the VM

  1. Modify the VM configuration.
    1. Open the VM configuration file. 
      1
      		 
      vim /etc/libvirt/qemu/vm1.xml

      Or

      1
      		 
      virsh edit vm1
    2. Press i to enter the insert mode, find the <interface> tag under the <devices> tag, and change br1 to br0.

      If the <interface> tag does not exist, add the following content under the <devices> tag:

      <interface type='bridge'>
      <mac address='52:54:00:7a:95:48'/>
      <source bridge='br1'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</interface>
      • <interface type='bridge'> indicates that the VM uses the bridge mode.
      • The MAC address in <mac address='52:54:00:7a:95:48'/> is automatically generated after the VM is installed and is used as the MAC address of the VM NIC eth0. Generally, you do not need to change the MAC address. If you need to change the MAC address, ensure that it is different from the MAC address of the br0 network bridge.
      • <source bridge='br0'/> indicates the connection to br0 network bridge on the host machine.
    3. Press Esc, type :wq!, and press Enter to save the file and exit.
  2. Make the VM configuration take effect.
    1
    		 
    systemctl restart libvirtd
  3. Stop the VM.
    1
2
3
    		 
    virsh list --all
virsh shutdown vm1
virsh list --all

  4. Start the VM.
    1
2
    		 
    virsh start vm1
virsh list --all

  5. Check the network bridge.
    1
    		 
    brctl show

    If vnet0 is displayed under interfaces of br0, vm1 is successfully connected to the network bridge. (vnet0 exists only after the vm1.xml configuration takes effect and vm1 is successfully started.)

  6. Log in to the VM.
    virsh list --all
virsh console vm1 --force
  7. Check the network.
    ip a
  8. Modify the NIC configuration.
    • CentOS:
      1. Open the ifcfg-eth0 file. 
        1
        		 
        vim /etc/sysconfig/network-scripts/ifcfg-eth0
      2. Press i to enter the insert mode and modify the following content.

        Set IPADDR to an unused IP address that is on the same network segment as the host machine. Set this parameter based on your requirements.

        TYPE=Ethernet
PROXY_METHOD=none
BROWSER_ONLY=no
BOOTPROTO=static
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_FAILURE_FATAL=no
IPV6_ADDR_GEN_MODE=stable-privacy
NAME=eth0
UUID=aca69508-f4c7-4ff2-9cc5-668ac45e429e
DEVICE=eth0
ONBOOT=yes
IPADDR=192.168.224.3
NETMASK=255.255.255.0
GATEWAY=192.168.224.1
      3. Press Esc, type :wq!, and press Enter to save the file and exit.
    • openEuler:
      1. Open the ifcfg-enp1s0 file. 
        1
        		 
        vim /etc/sysconfig/network-scripts/ifcfg-enp1s0
      2. Press i to enter the insert mode and modify the following content. Generally, you only need to modify the information in bold. Set IPADDR to an unused IP address that is on the same network segment as the host machine. Set this parameter based on your requirements. 
        TYPE=Ethernet
PROXY_METHOD=none
BROWSER_ONLY=no
BOOTPROTO=static
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_FAILURE_FATAL=no
IPV6_ADDR_GEN_MODE=stable-privacy
NAME=enp1s0
UUID=dedf1652-5eff-3fb4-be23-c658b834774f
DEVICE=enp1s0
ONBOOT=yes
AUTOCONNECT_PRIORITY=-999
IPADDR=192.168.224.13
NETMASK=255.255.255.0
GATEWAY=192.168.224.1
      3. Press Esc, type :wq!, and press Enter to save the file and exit.
  9. Make the NIC take effect.
    • CentOS: 
      systemctl restart network
ip a
    • openEuler: 
      systemctl restart NetworkManager
ifdown enp1s0
ifup enp1s0
ip a

Verifying the Network

  1. Ping the VM and the host machine from each other.
  2. Ping the VM and other hosts on the LAN from each other.
  3. Ping the host machine and other hosts on the LAN from each other.
    • If the ping fails, on the host machine, capture packets of br0 and analyze the problem. 
      yum -y install tcpdump
tcpdump -i br0
    • If all of the host machines, VMs, and other hosts are successfully pinged, the bridge configuration is complete.
    • If the VM can communicate with only the host machine but not with other hosts on the LAN, you may need to disable the network filter on the host machine.
      1. Disable the network filter.
      2. Open the sysctl.conf file: 
        1
        		 
        vim /etc/sysctl.conf
      3. Press i to enter the insert mode and add the following content to the file: 
        net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
      4. Press Esc, type :wq!, and press Enter to save the file and exit.
      5. Reload the kernel parameters for the configuration to take effect. 
        1
        		 
        sysctl -p