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


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