我們知道：kvm虛擬化環境安裝好後，ifconfig會發現多了一個虛擬網路卡virbr0
這是由於安裝和啟用了libvirt服務後生成的，libvirt在伺服器（host）上生成一個 virtual network switch (virbr0)，host上所有的虛擬機器（guests）通過這個 virbr0 連起來。預設情況下 virbr0 使用的是 NAT 模式（採用 IP Masquerade），所以這種情況下 guest 通過 host 才能訪問外部。
但是大多數情況下：
我們虛擬機器使用的是bridge（網橋）直接連到區域網裡，所以這個 virbr0 不是必須的（注：不要搞混淆了，bridge 和這裡的 virbr0 bridge 是互不相干的）。
那麼如何關掉這個 virbr0 呢？
下面對關閉這個virbr0虛擬網路卡的操作做一記錄：
總結為以下三步：
1）首先net-destroy
2）接著net-undefine
3）最後重啟libvirtd讓設定生效
[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檢視，發現virtbr0網路卡已經消失了！
[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
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:1572393 errors:0 dropped:0 overruns:0 frame:0
TX packets:115724 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:196738740 (187.6 MiB) TX bytes:8750347 (8.3 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:350174 errors:0 dropped:0 overruns:0 frame:0
TX packets:350174 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:68170777 (65.0 MiB) TX bytes:68170777 (65.0 MiB)