What Is Bridging

Bridging highly simulates a network card, making the router believe that the virtual machine’s network card truly exists. Personally, I feel it’s similar to resistors connected in parallel, whereas NAT (another common virtual machine network connection method) is more like parasitizing on the host’s network card.

Why Use Bridging

It allows you to treat the virtual machine as a completely independent machine, enabling mutual access with the external network (which is not possible with NAT).

How to Configure Bridging

In CentOS 6, refer to the command-line method in this article.

We don’t use the GUI method because:

  • We’re unsure which options to fill in on the last screen.
  • We don’t know how to reset if we make a wrong selection.

Command-line steps:

  1. Check if bridge-utils is installed:

    rpm -q bridge-utils

    Usually, it’s already installed. If not, install it:

    su -
yum install bridge-utils

  2. Verify your network interfaces:

    Run ifconfig to ensure you have at least three network interfaces:

    eth0      Link encap:Ethernet  HWaddr 00:18:E7:16:DA:65
          inet addr:192.168.0.117  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::218:e7ff:fe16:da65/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:556 errors:0 dropped:0 overruns:0 frame:0
          TX packets:414 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:222834 (217.6 KiB)  TX bytes:48430 (47.2 KiB)
          Interrupt:16 Base address:0x4f00

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:8 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:480 (480.0 b)  TX bytes:480 (480.0 b)

virbr0    Link encap:Ethernet  HWaddr 52:54:00:2A:C1:7E
          inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:13 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:2793 (2.7 KiB)

  3. Navigate to the network scripts directory:

    su –
cd /etc/sysconfig/network-scripts

  4. Bring down the eth0 interface:

    ifdown eth0

    This step is crucial and must be performed locally. When I first configured this, I didn’t shut down the network (since I was working remotely). I didn’t realize that updating the ifcfg-eth0 configuration without restarting the network would immediately apply changes, resulting in loss of network connectivity.

  5. Edit ifcfg-eth0:

    In the ifcfg-eth0 file, include:

    DEVICE=eth0
ONBOOT=yes
BRIDGE=br0

    Keep only these three lines in the file. There’s no need to configure an IP address here. Bridging seems to replace the original network card with the bridge, so you can delegate the configuration to the bridge.

  6. Create a new file ifcfg-br0:

    DEVICE=br0
ONBOOT=yes
TYPE=Bridge
BOOTPROTO=static
IPADDR=xxx.xxx.xxx.xxx   # Use the IP you originally had in ifcfg-eth0
GATEWAY=xxx.xxx.xxx.xxx  # Your gateway address
NETMASK=255.255.255.0    # Your netmask
DNS1=xxx.xxx.xxx.xxx     # Your primary DNS server
DNS2=xxx.xxx.xxx.xxx     # Your secondary DNS server (if any)
STP=on
DELAY=0

    Note: Replace xxx.xxx.xxx.xxx with your actual network settings.

  7. Bring up the interfaces:

    ifup br0
ifup eth0

  8. Verify the bridge interface:

    Check ifconfig to ensure that br0 is now present.

  9. Update firewall rules:

    Edit /etc/sysconfig/iptables and add:

    -A INPUT -i br0 -j ACCEPT

    (This is a general example; you may need to adjust it based on your specific firewall configuration.)

  10. Restart the firewall:

    service iptables restart

  11. Configure bridging in virt-manager:

    When creating a new virtual machine using virt-manager, you can now select br0 for the network interface. Without this bridge, the bridging option would not be available.

Note: When configuring the IP inside the virtual machine, be sure to specify the GATEWAY. Otherwise, the virtual machine will only be able to access the internal network and not the external network. At this point, the virtual machine won’t automatically discover the gateway.