Xenserver环境：
一：环境准备
内网：192.168.2.100
外网：x.x.x.x
1.1：登陆XenCenter
1.2：进入Xenserver中的Networking选项
1.3：点选下边的Configure...按钮，进入Configure IP Addresses对话框
1.4：点选Add IP address新建虚拟交换机
1.5：Network 1 网卡连接外网
Network 2 网卡连接内网虚拟交换机
二：建立Centos7虚拟机并配置网卡
2.1：vim /etc/sysconfig/network-scripts/ifcfg-eth1
TYPE=Ethernet PROXY_METHOD=none BROWSER_ONLY=no #BOOTPROTO=dhcp BOOTPROTO=static DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=stable-privacy NAME=eth1 #UUID=dd48994a-7f5c-44c1-a8d3-107f4e4b579f DEVICE=eth1 #ONBOOT=no ONBOOT=yes IPADDR=x.x.x.x（固定IP或可联通外网的IP） NETMASK=255.255.255.x GATEWAY=x.x.x.x DNS1=8.8.8.8 DNS2=x.x.x.x
2.2：vim /etc/sysconfig/network-scripts/ifcfg-eth2
TYPE=Ethernet PROXY_METHOD=none BROWSER_ONLY=no #BOOTPROTO=dhcp BOOTPROTO=static DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=stable-privacy NAME=eth3 #UUID=34b419e0-ca01-4ca4-964b-45d2a9973002 DEVICE=eth3 #ONBOOT=no ONBOOT=yes IPADDR=192.168.2.100 NETMASK=255.255.255.0
2.3：ping baidu.com
三：配置ipv4转发
3.1：查看IPv4转发状态，默认为0即关闭状态
cat /proc/sys/net/ipv4/ip_forward
3.2：开启转发
echo 1 > /proc/sys/net/ipv4/ip_forward
四：借助iptables做地址转发：
4.1：配置iptables做SNAT，基于源的数据包转发
iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -j SNAT --to-source x.x.x.x
4.2：192.168.2.0网段的虚拟机，需要上外网，只要把网关配置成192.168.2.100即可
五：加开机运行：
5.1：为了防止重启后这些配置失效，将这两条命令加入到rc.local中，使其开机自动运行，
vim /etc/rc.d/rc.local
#!/bin/bash # THIS FILE IS ADDED FOR COMPATIBILITY PURPOSES # # It is highly advisable to create own systemd services or udev rules # to run scripts during boot instead of using this file. # # In contrast to previous versions due to parallel execution during boot # this script will NOT be run after all other services. # # Please note that you must run 'chmod +x /etc/rc.d/rc.local' to ensure # that this script will be executed during boot. touch /var/lock/subsys/local echo 1 > /proc/sys/net/ipv4/ip_forward iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -j SNAT --to-source x.x.x.x
5.2：因为CentOS7开始，rc.local默认没有执行权限，还要加一条命令
chmod +x /etc/rc.d/rc.local
重启测试：reboot