现在的虚拟化架构越来越普遍了。最近摸索了下centos下如何构建KVM虚拟机NAT网络，在此记录下点滴。
1.硬件检测
KVM虚拟机需要CPU支持虚拟化技术。Intel的叫vmx,AMD的叫svm.
cat /proc/cpuinfo|grep svm
2.安装kvm内核
yum groupinstall KVM -y
安装完后载入确认
modprobe kvm-amd
lsmod|grep kvm
3.配置VNC
/etc/libvirt/qemu.conf
vnc_listen = "0.0.0.0"
/etc/init.d/libvirtd start
4.安装虚拟机：
virt-install --name os_11 --hvm --ram 512 --vcpus 2 --disk path=/opt/vms/os_11.img,size=20 --network network:default --accelerate --vnc --vncport=5911 --os-variant rhel5.4 --cdrom /root/ubuntu-11.04-server-amd64.iso -d
然后用VNC连接5911端口开始安装
5.克隆虚拟机：
virt-clone -o os_11 -n os_12 -f /data/vm/os_12.img
virt-clone -o os_11 -n os_13 -f /data/vm/os_13.img
修改虚拟机配置文件/etc/libvirt/qemu/os_12.xml的vnc端口
<domain type='kvm'>
<name>os_12</name>
<uuid>3aab5774-1810-9c67-4b97-24c9c9db071f</uuid>
<memory>524288</memory>
<currentMemory>524288</currentMemory>
<vcpu>2</vcpu>
<os>
<type arch='x86_64' machine='rhel5.4.0'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='utc'>
<timer name='pit' tickpolicy='delay'/>
</clock>
<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='/opt/vms/os_12.img'/>
<target dev='hda' bus='ide'/>
<address type='drive' controller='0' bus='0' unit='0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'/>
<interface type='network'>
<mac address='FE:16:36:5E:54:30'/>
<source network='default'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='5912' autoport='no' keymap='en-us'/>
<video>
<model type='cirrus' vram='9216' heads='1'/>
</video>
</devices>
</domain>
6.启动虚拟机（ubuntu 11.04）：
virsh start os_11
virsh start os_12
virsh start os_13
7.查看virbr0的网段，默认192.168.122.1
vnc连接虚拟机设置os_11 IP：192.168.122.11
vnc连接虚拟机设置os_12 IP：192.168.122.12
vnc连接虚拟机设置os_13 IP：192.168.122.13
网卡配置文件：
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.122.11
netmask 255.255.255.0
network 192.168.122.0
broadcast 192.168.122.255
gateway 192.168.122.1
8.此时应该可以ssh到虚拟机了，并且虚拟机上也能上网了。
9.注意问题
(1)确认安装libvirtd,virsh等相关依赖
(2)在物理机上需要配置允许IP转发：
echo 1 > /proc/sys/net/ipv4/ip_forward ,
vi /etc/sysctl.conf 修改net.ipv4.ip_forward = 1
(3)启用iptables服务，需要进行ip转发,重新设置ip转发用virsh net-start default,我的配置如下：
#iptables -L -vn
Chain INPUT (policy ACCEPT 1181K packets, 492M bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:53
0 0 ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:53
0 0 ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:67
0 0 ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:67
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
39 12560 ACCEPT all -- * virbr0 0.0.0.0/0 192.168.122.0/24 state RELATED,ESTABLISHED
56 3833 ACCEPT all -- virbr0 * 192.168.122.0/24 0.0.0.0/0
0 0 ACCEPT all -- virbr0 virbr0 0.0.0.0/0 0.0.0.0/0
0 0 REJECT all -- * virbr0 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
0 0 REJECT all -- virbr0 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
Chain OUTPUT (policy ACCEPT 953K packets, 48M bytes)
pkts bytes target prot opt in out source destination
(4)虚拟机的配置文件在/etc/libvirt/qemu/中，可以修改xml调整虚拟机的配置