GRE VPN Tunnel - Linux (Ubuntu)

interfaces

# Configurare GRE tunel (VPN) on Ubuntu Linux
# There are two ubuntu servers acting as routers on their own LAN networks 172.18.17.0/24 , 172.18.18.0/24 respectively
# Each of the servers also have public IP address
# configure /etc/network/interfaces on the servers adding *gre1* interface

#Ubuntu1 - 192.168.168.1/30 is the local IP address on the gre1 interface , this IP will comunicate with the remote gre1 tunnel interface for routing
#        - <post-up> command: 172.18.18.0/24 which is the remote LAN is routed through the tunnel to 192.168.168.2
auto gre1
iface gre1 inet tunnel
  address 192.168.168.1
  netmask 255.255.255.252
  mode gre
  endpoint <REMOTE PUBLIC IP>
  dstaddr 192.168.168.2
  local <LOCAL PUBLIC IP>
  post-up route add -net 172.18.18.0 netmask 255.255.255.0 gw 192.168.168.2

#Ubuntu2 - 192.168.168.2/30 is the local IP address on the gre1 interface , this IP will comunicate with the remote gre1 tunnel interface for routing
#        - <post-up> command: 172.18.17.0/24 which is the remote LAN is routed through the tunnel to 192.168.168.1
auto gre1
iface gre1 inet tunnel
  address 192.168.168.2 
  netmask 255.255.255.252
  mode gre
  endpoint <REMOTE PUBLIC IP>
  dstaddr 192.168.168.1
  local <LOCAL PUBLIC IP>
  post-up route add -net 172.18.17.0 netmask 255.255.255.0 gw 192.168.168.1