KVM的默认网络模式为NAT，就是说借助宿主机模式上网，现在我们想改成桥接模式，这样外界就可以直接和宿主机里的虚拟机通讯了。 
1，首先看一下默认的网络设置 

# cat /etc/libvirt/qemu/networks/default.xml 
<network> 
<name>default</name> 
<uuid>1f118032-0345-4725-adcf-6b94f6aca19c</uuid> 
<bridge name="virbr0" /> 
<mac address='52:54:00:F1:6B:6E'/> 
<forward/> 
<ip address="192.168.122.1" netmask="255.255.255.0"> 
<dhcp> 
<range start="192.168.122.2" end="192.168.122.254" /> 
</dhcp> 
</ip> 
</network> 

2，再看一下虚拟机配置文件里面的网络设置 
#cat /etc/libvirt/qemu/ubuntu10.04.xml 
<!-- 
WARNING: THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY TO BE 
OVERWRITTEN AND LOST. Changes to this xml configuration should be made using: 
virsh edit ubuntu10.04 
or other application using the libvirt API. 
--> 
<domain type='kvm'> 
<name>ubuntu10.04</name> 
<uuid>2eeb1395-39a2-a169-4da3-f22b20a1d34f</uuid> 
<memory unit='KiB'>1048576</memory> 
<currentMemory unit='KiB'>1048576</currentMemory> 
<vcpu placement='static'>1</vcpu> 
<os> 
<type arch='x86_64' machine='rhel6.3.0'>hvm</type> 
<boot dev='hd'/> 
</os> 
<features> 
<acpi/> 
<apic/> 
<pae/> 
</features> 
<clock offset='utc'/> 
<on_poweroff>destroy</on_poweroff> 
<on_reboot>restart</on_reboot> 
<on_crash>restart</on_crash> 
<devices> 
<emulator>/usr/libexec/qemu-kvm</emulator> 
<disk type='file' device='disk'> 
<driver name='qemu' type='raw' cache='none'/> 
<source file='/var/lib/libvirt/images/ubuntu10.04.img'/> 
<target dev='vda' bus='virtio'/> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/> 
</disk> 
<disk type='block' device='cdrom'> 
<driver name='qemu' type='raw'/> 
<target dev='hdc' bus='ide'/> 
<readonly/> 
<address type='drive' controller='0' bus='1' target='0' unit='0'/> 
</disk> 
<controller type='usb' index='0'> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/> 
</controller> 
<controller type='ide' index='0'> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/> 
</controller> 
<interface type='bridge'> 原来是interface type='default' 
<mac address='52:54:00:50:58:7e'/> 
<source bridge='br1'/> 原来是source network='default',这里的br1要根据你的物理网卡下面有讲 
<model type='virtio'/> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> 
</interface> 
<serial type='pty'> 
<target port='0'/> 
</serial> 
<console type='pty'> 
<target type='serial' port='0'/> 
</console> 
<input type='mouse' bus='ps2'/> 
<graphics type='vnc' port='-1' autoport='yes'/> 
<sound model='ich6'> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/> 
</sound> 
<video> 
<model type='cirrus' vram='9216' heads='1'/> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/> 
</video> 
<memballoon model='virtio'> 
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/> 
</memballoon> 
</devices> 
</domain> 
[/code] 
3，改完之后要更新一下配置文件 
# virsh define ubuntu10.04.xml 
定义域 ubuntu10.04（从 ubuntu10.04.xml） 
#然后就可以virsh start ubuntu10.04了，不过现在物理网卡这块还没有改好 
4，物理网卡可以手动更改，不过现在有现成的工具包，可以用这个更改 
#wget http://www.convirture.com/downloads/convirture-tools/2.0.1/convirture-tools-2.0.1.tar.gz 
# tar zxvf convirture-tools-2.0.1.tar.gz 
# /root/convirture-tools/install/managed_server/scripts/convirt-tool --bridge setup 
我的这里出现了点小问题，发现网络没起来，所以不要远程来做这个操作，主要原因是里面的配置文件有点需要 
小的更改，我的是UUID注释了。 
# ls ifcfg-* 
ifcfg-br1 ifcfg-br2 ifcfg-br3 ifcfg-br4 ifcfg-em1 ifcfg-em2 ifcfg-em3 ifcfg-em4 ifcfg-lo 
我这里有四个物理网卡，所以生成了四个bridge,不过真实用的只有br1 
这个工具好有一个好处就是保存了原配置文件，如果你不想用时还可以还原 
# ls saved.ifcfg-em* 
saved.ifcfg-em1.20121128.101114 saved.ifcfg-em3.20121128.101114 
saved.ifcfg-em2.20121128.101114 saved.ifcfg-em4.20121128.101114 
看下ifceg-em1 
# cat ifcfg-em1 
DEVICE="em1" 
#BOOTPROTO=none 
NM_CONTROLLED="yes" 
ONBOOT=yes 
TYPE="Ethernet" 
#UUID="3bc7fdcc-42b1-4223-b202-e697305897c8" 
#IPADDR172.16.10.81 
PREFIX=24 
GATEWAY=172.16.10.254 
DNS1=172.16.10.5 
DEFROUTE=yes 
IPV4_FAILURE_FATAL=yes 
IPV6INIT=no 
NAME="System em1" 
HWADDR=90:B1:1C:20:4D:A5 
BRIDGE=br1 
看下ifcfg-br1 
# cat ifcfg-br1 
DEVICE="br1" 
BOOTPROTO=none 
NM_CONTROLLED="yes" 
ONBOOT=yes 
#UUID="3bc7fdcc-42b1-4223-b202-e697305897c8" 
IPADDR=172.16.10.81 
PREFIX=24 
GATEWAY=172.16.10.254 
DNS1=172.16.10.5 
DEFROUTE=yes 
IPV4_FAILURE_FATAL=yes 
IPV6INIT=no 
NAME="System br1" 
TYPE=Bridge 
最后看下网卡 
# ifconfig 
br1 Link encap:Ethernet HWaddr 90:B1:1C:20:4D:A5 
inet addr:172.16.10.81 Bcast:172.16.10.255 Mask:255.255.255.0 
inet6 addr: fe80::92b1:1cff:fe20:4da5/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 
RX packets:10219 errors:0 dropped:0 overruns:0 frame:0 
TX packets:3776 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:0 
RX bytes:576031 (562.5 KiB) TX bytes:15928008 (15.1 MiB) 
em1 Link encap:Ethernet HWaddr 90:B1:1C:20:4D:A5 
inet6 addr: fe80::92b1:1cff:fe20:4da5/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 
RX packets:10362 errors:0 dropped:0 overruns:0 frame:0 
TX packets:13131 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:1000 
RX bytes:766684 (748.7 KiB) TX bytes:16489098 (15.7 MiB) 
Interrupt:35 
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:144411 errors:0 dropped:0 overruns:0 frame:0 
TX packets:144411 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:0 
RX bytes:147381248 (140.5 MiB) TX bytes:147381248 (140.5 MiB) 
virbr0 Link encap:Ethernet HWaddr 52:54:00:F1:6B:6E 
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:2028 errors:0 dropped:0 overruns:0 frame:0 
TX packets:1135 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:0 
RX bytes:233311 (227.8 KiB) TX bytes:89372 (87.2 KiB) 
vnet0 Link encap:Ethernet HWaddr FE:54:00:50:58:7E 
inet6 addr: fe80::fc54:ff:fe50:587e/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 
RX packets:161 errors:0 dropped:0 overruns:0 frame:0 
TX packets:3117 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:500 
RX bytes:15533 (15.1 KiB) TX bytes:263947 (257.7 KiB) 
vnet1 Link encap:Ethernet HWaddr FE:54:00:B5:1A:19 
inet6 addr: fe80::fc54:ff:feb5:1a19/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 
RX packets:56 errors:0 dropped:0 overruns:0 frame:0 
TX packets:794 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:500 
RX bytes:8754 (8.5 KiB) TX bytes:42245 (41.2 KiB) 
5,启动虚拟机ubuntu10.04后配置网卡 
#cat /etc/network/interface 
auto lo 
iface lo inet loopback 
auto eth0 
iface eth0 inet static 
address 172.16.10.100 
netmask 255.255.255.0 
gateway 172.16.10.254 
#/etc/init.d/networking restart 
启动即可