GRE Tunnel for anybody

Requirements:
- nmcli (Network Manager)
- iptables

Server 1 external ip: 1.1.1.1
Server 2 external ip: 2.2.2.2

GRE IFNAME: gre1

Server 1 gre:
 ip 10.10.1.1
 net 10.10.1.0/24
 
Server 2 gre:
 ip 10.10.2.1
 net 10.10.2.0/24

Way 1: NMCLI  (RHEL, Fedora)
===============================

Server 1:
nmcli conn add type ip-tunnel ifname gre1 mode gre remote 2.2.2.2 local 1.1.1.1 -- ip-tunnel.mtu 1500 ip-tunnel.ttl 255 ipv4.method manual ipv4.addresses 10.10.1.1 ipv4.routes "10.10.2.0/24"

Server 2:
nmcli conn add type ip-tunnel ifname gre1 mode gre remote 1.1.1.1 local 2.2.2.2 -- ip-tunnel.mtu 1500 ip-tunnel.ttl 255 ipv4.method manual ipv4.addresses 10.10.2.1 ipv4.routes "10.10.1.0/24"

Firewall (iptables):
*filter
-A INPUT -p gre -s 2.2.2.2|1.1.1.1 -j ACCEPT
-A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu


Way 2. For Ubuntu losers (except 16.04+):
=======================================
Server 1:

#!/bin/bash

DEV=gre1
LOCAL=1.1.1.1
REMOTE=2.2.2.2
IP=10.10.1.1
NET=10.10.2.0


up(){
 ip tunnel add $DEV mode gre remote $REMOTE local $LOCAL ttl 255
 ip link set $DEV up
 ip addr add $IP dev $DEV
 ip route add $NET/24 dev $DEV
}


down(){
 ip link set $DEV down
 ip tunnel del $DEV
}




case "$1" in
up)
  up
  ;;
down)
  down
  ;;
*)
  echo "gre [up|down]"
  ;;
esac


Server 2:

#!/bin/bash

DEV=gre1
LOCAL=2.2.2.2
REMOTE=1.1.1.1
IP=10.10.2.1
NET=10.10.1.0


up(){
 ip tunnel add $DEV mode gre remote $REMOTE local $LOCAL ttl 255
 ip link set $DEV up
 ip addr add $IP dev $DEV
 ip route add $NET/24 dev $DEV
}


down(){
 ip link set $DEV down
 ip tunnel del $DEV
}




case "$1" in
up)
  up
  ;;
down)
  down
  ;;
*)
  echo "gre [up|down]"
  ;;
esac