我们知道：kvm虚拟化环境安装好后，ifconfig会发现多了一个虚拟网卡virbr0
这是由于安装和启用了libvirt服务后生成的，libvirt在服务器（host）上生成一个 virtual network switch (virbr0)，host上所有的虚拟机（guests）通过这个 virbr0 连起来。默认情况下 virbr0 使用的是 NAT 模式（采用 IP Masquerade），所以这种情况下 guest 通过 host 才能访问外部。
但是大多数情况下：
我们虚拟机使用的是bridge（网桥）直接连到局域网里，所以这个 virbr0 不是必须的（注：不要搞混淆了，bridge 和这里的 virbr0 bridge 是互不相干的）。
那么如何关掉这个 virbr0 呢？
下面对关闭这个virbr0虚拟网卡的操作做一记录：
总结为以下三步：
（1）首先net-destroy
（2）接着net-undefine
（3）最后重启libvirtd让设置生效
[root@kvmserver ~]# ifconfig
br0 Link encap:Ethernet HWaddr C8:1F:66:F4:FC:4F
inet addr:192.168.1.8 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::ca1f:66ff:fef4:fc4f/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:135245 errors:0 dropped:0 overruns:0 frame:0
TX packets:101444 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:13942261 (13.2 MiB) TX bytes:7634359 (7.2 MiB)
em1 Link encap:Ethernet HWaddr C8:1F:66:F4:FC:4E
inet addr:103.10.86.8 Bcast:103.10.86.63 Mask:255.255.255.192
inet6 addr: fe80::ca1f:66ff:fef4:fc4e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1565647 errors:0 dropped:0 overruns:0 frame:0
TX packets:433759 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:371869769 (354.6 MiB) TX bytes:332186615 (316.7 MiB)
Interrupt:35
em2 Link encap:Ethernet HWaddr C8:1F:66:F4:FC:4F
inet6 addr: fe80::ca1f:66ff:fef4:fc4f/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1306151 errors:0 dropped:0 overruns:0 frame:0
TX packets:95447 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:163400847 (155.8 MiB) TX bytes:7174405 (6.8 MiB)
Interrupt:38
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:128908 errors:0 dropped:0 overruns:0 frame:0
TX packets:128908 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:25188477 (24.0 MiB) TX bytes:25188477 (24.0 MiB)
virbr0 Link encap:Ethernet HWaddr 52:54:00:E7:2D:22
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:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
[root@kvmserver ~]# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.c81f66f4fc4f no em2
virbr0 8000.525400e72d22 yes virbr0-nic
[root@kvmserver ~]# virsh net-list
Name State Autostart Persistent
--------------------------------------------------
br0 active yes yes
default active yes yes
[root@kvmserver ~]# virsh net-destroy default
Network default destroyed
[root@kvmserver ~]# virsh net-undefine default
Network default has been undefined
[root@kvmserver ~]# service libvirtd restart
Stopping libvirtd daemon: [ OK ]
Starting libvirtd daemon: [ OK ]
[root@kvmserver ~]# virsh net-list
Name State Autostart Persistent
--------------------------------------------------
br0 active yes yes
[root@kvmserver ~]# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.c81f66f4fc4f no em2
再次ifconfig查看，发现virtbr0网卡已经消失了！
[root@kvmserver ~]# ifconfig
br0 Link encap:Ethernet HWaddr C8:1F:66:F4:FC:4F
inet addr:192.168.1.8 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::ca1f:66ff:fef4:fc4f/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:161858 errors:0 dropped:0 overruns:0 frame:0
TX packets:123581 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:15735341 (15.0 MiB) TX bytes:9481865 (9.0 MiB)
em1 Link encap:Ethernet HWaddr C8:1F:66:F4:FC:4E
inet addr:103.10.86.8 Bcast:103.10.86.63 Mask:255.255.255.192
inet6 addr: fe80::ca1f:66ff:fef4:fc4e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1850577 errors:0 dropped:0 overruns:0 frame:0
TX packets:696879 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:407720396 (388.8 MiB) TX bytes:592419489 (564.9 MiB)
Interrupt:35