网络有两种方式，一种是通过bridge, 一种通过NAT

在NAT方式下，会使用virbr0进行通信，以允许虚拟机访问网络服务。
目前我们环境是桥接，所有没有作用。查看和禁用：
# virsh net-list
# virsh net-destroy default
# virsh net-undefine default
# service libvirtd restart
或者
#ifconfig virbr0 down
#brctl delbr virbr0

当然，不禁用对我们系统也没有影响。

在网桥方式下，设计的操作有

1. 工具安装：

yum install bridge-utils
yum install tunctl
# tunctl - create and manage persistent TUN/TAP interfaces

2. 脚本创建：

# tunctl - create and manage persistent TUN/TAP interfaces
#!/bin/sh
PATH=$PATH:/usr/sbin:/sbin
sudo brctl addbr br0
sudo ifconfig eth0 0.0.0.0
sudo brctl addif br0 eth0
sudo ifconfig br0 192.168.1.120 netmask 255.255.255.0 up
sudo route add -net 192.168.1.0 netmask 255.255.255.0 br0
sudo route add default gw 192.168.1.1 br0
sudo tunctl -b -u john
sudo ifconfig tap0 up
sudo brctl addif br0 tap0
#export SDL_VIDEO_X11_DGAMOUSE=0
#sudo iptables -I RH-Firewall-1-INPUT -i br0 -j ACCEPT
qemu-kvm ~/win2k.img -m 512 -net nic -net tap,ifname=tap0,script=no

    或者QEMU TAP选项中的脚本

#/etc/kvm/qemu-ifup-br1
#tunctl   -t interface         Specifies the desired interface name.
switch=br1
if [ -n "$1" ];then
        tunctl -u `whoami` -t $1
        ip link set $1 up
        sleep 0.5s
        brctl addif ${switch} $1
        exit 0
else
        echo "Error: no interface specified"
        exit 1
fi

整个环境网络设备参数如下：

$brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.128abd3da3a0       no              tap22
                                                        tap15
                                                        tap13
                                                        tap24
                                                        tap17
                                                        tap20
                                                        tap5
                                                        tap7
                                                        tap2
                                                        eth0
br1             8000.1e7c411d6778       no              tap23
                                                        tap16
                                                        tap14
                                                        tap25
                                                        tap18
                                                        tap21
                                                        tap6
                                                        tap8
                                                        tap3
                                                        eth1
                                                        tap1
virbr0          8000.000000000000       yes

$ifconfig

br0       Link encap:Ethernet  HWaddr 12:8A:BD:3D:A3:A0
          inet addr:192.168.16.226  Bcast:192.168.16.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:56149092 errors:0 dropped:0 overruns:0 frame:0
          TX packets:43895050 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:5336444585 (4.9 GiB)  TX bytes:7401435140 (6.8 GiB)

br1       Link encap:Ethernet  HWaddr 1E:7C:41:1D:67:78
          inet addr:172.31.16.226  Bcast:172.31.16.255  Mask:255.255.255.0
          inet6 addr: fe80::1c7c:41ff:fe1d:6778/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:379283800 errors:0 dropped:0 overruns:0 frame:0
          TX packets:185801315 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:265058942288 (246.8 GiB)  TX bytes:66398472590 (61.8 GiB)

tap1      Link encap:Ethernet  HWaddr 1E:7C:41:1D:67:78
          inet6 addr: fe80::1c7c:41ff:fe1d:6778/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:168 errors:0 dropped:0 overruns:0 frame:0
          TX packets:84299454 errors:0 dropped:0 overruns:9 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:27000 (26.3 KiB)  TX bytes:10826727533 (10.0 GiB)

.....

tap25     Link encap:Ethernet  HWaddr 92:6C:35:97:95:9C
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1523 errors:0 dropped:0 overruns:0 frame:0
          TX packets:33235 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:1815274 (1.7 MiB)  TX bytes:6320065 (6.0 MiB)

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
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:37 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:7248 (7.0 KiB)