Linux KVM: Disable virbr0 NAT Interface

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The virtual network (virbr0) used for Network address translation (NAT) which allows guests to access to network services. However, NAT slows down things and only recommended for desktop installations. To disable Network address translation (NAT) forwarding type the following commands:


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Type the following command:
# ifconfig
Sample outputs:

virbr0    Link encap:Ethernet  HWaddr 00:00:00:00:00:00  
          inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
          inet6 addr: fe80::200:ff:fe00:0/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:39 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 b)  TX bytes:7921 (7.7 KiB)

Or use the following command:
# virsh net-list
Sample outputs:

Name                 State      Autostart
-----------------------------------------
default              active     yes

To disable virbr0, enter:
# virsh net-destroy default
# virsh net-undefine default
# service libvirtd restart
# ifconfig

This entry is 9 of 14 in the CentOS / Redhat (RHEL) KVM Virtulization series. Keep reading the rest of the series:
  1. CentOS / Redhat: Install KVM Virtualization Software
  2. CentOS / Redhat: KVM Bridged Network Configuration
  3. KVM virt-manager: Install CentOS As Guest Operating System
  4. KVM virt-install: Install FreeBSD / CentOS As Guest Operating System
  5. KVM: Install CentOS / RHEL Using Kickstart File (Automated Installation)
  6. Troubleshooting KVM Virtualization Problem With Log Files
  7. KVM Virsh: Redirect FreeBSD Console To A Serial Port
  8. KVM: Starting / Stopping Guest Operating Systems With virsh Command
  9. Linux KVM: Disable virbr0 NAT Interface
  10. FreeBSD / OpeBSD Running in KVM Does Not Accept FTP Traffic
  11. KVM: Start a Virtual Machine / Guest At Boot Time
  12. KVM virt-install: Install OpenBSD As Guest Operating System
  13. Linux KVM: OpenBSD Guest Hangs At Starting tty Flags
  14. KVM Virtualization: Start VNC Remote Access For Guest Operating Systems

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Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

22 comment

  1. Hi Vivek,

    Whether disabling virbr0 i mean the steps you have given above needs a restart of the machine.

      1. If we shutdown the libvirtd service as we are not using it, whether virbr0 interface will come down and not show up in ifconfig.

  6. You can use “net-autostart default –disable” instead of “net-undefine default”. This keeps the config on disk so that you may reenable it later if you need it (with “net-autostart default”).

  8. Thanks !

    Worked very well.

    Don’t know whether we can disable it at OS installation itself, if not required.

    Found this on a machine installed by 3rd party and was taking lot of load.

    regards

  10. Thanks – worked just fine on a CentOS 5.8/64 environment on a big server that I want to set up with Xen or KVM – but no NAT is needed/wanted. Next step is to set up a bridge… but not virbr0.

  14. to reverse this you need to recreate default virbr0

    # brctl addbr virbr0
# ip addr add 192.168.122.1/24 broadcast 192.168.122.255 dev virbr0
# ip link set dev virbr0 up

# uuidgen virbr0 < gen uuid for virbr0
# ifconfig | grep virbr0 < get the mac address

# vim /root/default.xml


  default
  6087308e-2b93-4d4d-aa24-dabd1668b11a
  

# virsh net-list
 Name                 State      Autostart     Persistent
----------------------------------------------------------

# cp default.xml /var/lib/libvirt/network/

# virsh net-define default.xml
# virsh net-autostart default
# service libvirtd restart

# virsh net-list
 Name                 State      Autostart     Persistent
----------------------------------------------------------
 default              active     yes           yes

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