GRE（Generic Routing Encapsulation），通用路由封装协议，是思科开发的隧道协议。
需要加载ip_gre内核模块，并设置开机自动加载
modprobe ip_gre
设置开机自动加载
echo "/sbin/modprobe ip_gre > /dev/null 2>&1" > /etc/sysconfig/modules/ip_gre.modules && chmod 755 /etc/sysconfig/modules/ip_gre.modules
Host A：192.168.0.10
vi /etc/sysconfig/network-scripts/ifcfg-tun0
DEVICE=tun0 BOOTPROTO=none ONBOOT=no TYPE=GRE PEER_OUTER_IPADDR=192.168.1.100 PEER_INNER_IPADDR=192.168.168.2 MY_INNER_IPADDR=192.168.168.1
Host B：192.168.1.100
vi /etc/sysconfig/network-scripts/ifcfg-tun0
DEVICE=tun0 BOOTPROTO=none ONBOOT=no TYPE=GRE PEER_OUTER_IPADDR=192.168.0.10 PEER_INNER_IPADDR=192.168.168.1 MY_INNER_IPADDR=192.168.168.2
Host A、Host B：
ifup tun0
Host A：
ping 192.168.168.2
Host B：
ping 192.168.168.1
抓包结果如下：
15:59:15.444042 IP 192.168.0.10 > 192.168.1.100: GREv0, length 88: IP 192.168.168.1 > 192.168.168.2: ICMP echo request, id 15083, seq 1379, length 64 15:59:15.444111 IP 192.168.1.100 > 192.168.0.10: GREv0, length 88: IP 192.168.168.2 > 192.168.168.1: ICMP echo reply, id 15083, seq 1379, length 64 15:59:16.446077 IP 192.168.0.10 > 192.168.1.100: GREv0, length 88: IP 192.168.168.1 > 192.168.168.2: ICMP echo request, id 15083, seq 1380, length 64 15:59:16.446156 IP 192.168.1.100 > 192.168.0.10: GREv0, length 88: IP 192.168.168.2 > 192.168.168.1: ICMP echo reply, id 15083, seq 1380, length 64
参考资料：
1、https://www.centoshowtos.org/network-and-security/gre-tunnel/
2、https://www.linickx.com/gre-example-for-centosrhel
原文：http://www.cnblogs.com/fwonfo/p/7819289.html