ubuntu
写入网卡/etc/network/interfaces

ubuntu通用

auto tun0
iface tun0 inet static
address <local tunnel IP>
netmask <tunnel subnet mask>
pre-up iptunnel add tun1 mode gre local <local IP> remote <remote IP> ttl 255
up ifconfig tun1 multicast
pointopoint <remote tunnel IP>
post-down iptunnel del tun1

Centos类方法
约定：
hostA IP：公网 100.xx.xx.101 内网10.0.0.1
hostB IP：公网192.xx.xx.101 内网10.0.0.2

HOST A配置

cat /etc/sysconfig/network-scripts/ifcfg-tun0
DEVICE=tun0
BOOTPROTO=none
ONBOOT=yes
TYPE=GRE
PEER_OUTER_IPADDR=192.xx.xx.102 #hostB IPv4
PEER_INNER_IPADDR=10.0.0.2
MY_INNER_IPADDR=10.0.0.1

host B配置

cat /etc/sysconfig/network-scripts/ifcfg-tun0
DEVICE=tun0
BOOTPROTO=none
ONBOOT=yes
TYPE=GRE
PEER_OUTER_IPADDR=100.xx.xx.101 #hostA IPv4
PEER_INNER_IPADDR=10.0.0.1
MY_INNER_IPADDR=10.0.0.2

设定好配置文件用ifup上线隧道做测试，ifdown下线隧道。

 ifup tun0

最后用ping和tcpdump抓包分析

tcpdump -n -i eth1 proto 47

上述内容采摘整理自：    
https://www.linickx.com/gre-example-for-centosrhel    
https://onvox.net/articles/2009-01-19-how-to-create-ipv4-gre-tunnels-in-ubuntu