qemu-kvm 命令启动时，网络的设置

cat /etc/qemu-ifup 
#!/bin/bash
switch=virbr0
/sbin/ifconfig  $1 192.168.100.100 up
brctl addif ${switch} $1
 
改变权限： chomd 777 /etc/qemu-ifup 

 /usr/libexec/qemu-kvm  -M rhel6.2.0 -enable-kvm -m 2048 -smp 8,sockets=8,cores=1,threads=1 -name fedora -uuid f0f0f4be-f0c2-abf9-dcbc-ca5127659810
-nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/fedora.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control
-rtc base=utc -no-shutdown -drive file=/home/fedora.img,if=none,id=drive-ide0-0-0,format=raw,cache=none
-device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1
-drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0
 -net nic,model=rtl8139 -net tap,ifname=tap0,script=/etc/qemu-ifup -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb
 -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4  --vnc :10

host 的配置
4: virbr0: mtu 1500 qdisc noqueue state UNKNOWN 
    link/ether 3a:04:13:35:d0:37 brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
5: virbr0-nic: mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500
    link/ether 52:54:00:61:9a:e1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.100.100/24 brd 192.168.100.255 scope global virbr0-nic
    inet6 fe80::5054:ff:fe61:9ae1/64 scope link 
       valid_lft forever preferred_lft forever
46: tap0: mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500
    link/ether 3a:04:13:35:d0:37 brd ff:ff:ff:ff:ff:ff
    inet 192.168.100.100/24 brd 192.168.100.255 scope global tap0
    inet6 fe80::3804:13ff:fe35:d037/64 scope link 
       valid_lft forever preferred_lft forever

其中tap0是刚生成的，
[root@localhost qemu]# brctl show
bridge name      bridge id                STP   enabled    interfaces
virbr0        8000.3a041335d037       yes                  tap0
                                                                          virbr0-nic
也就是说，virbr0-nic是给宿主机的，tap0是给guest的。tap0对应于guest的eth0