#!/bin/bash 

# This script is used to automatically generate the GRE tunnel between two hosts based on FQDN

# CONSTANTS
# If not defined in the environment, the following variables will be used

if [[ -z $GRE_TUNNEL_NAME ]]; then
    GRE_TUNNEL_NAME="gre1"
    GRE_LOCAL_IP="45.13.XX.XX"
    GRE_LAN_IP_4="192.168.100.1/24"
    GRE_LAN_IP_6="2a0e:fd45:2a0c::1/64"
    FQDN="remote.example.com"
fi

# FUNCTIONS
function usage() {
    echo "Usage: $0 <remote_host_fqdn>"
    exit 1
}

function check_root() {
    if [[ $EUID -ne 0 ]]; then
        echo "This script must be run as root"
        exit 1
    fi
}

function get_current_ip() {
    # Get the dst IP on the GRE tunnel
    CURRENT_IP=$(ip addr show $GRE_TUNNEL_NAME | grep -Po 'peer \K[\d.]+')
    if [[ -z $CURRENT_IP ]]; then
        echo "GRE tunnel is down or not configured"
        # Create the GRE tunnel
        CURRENT_IP=""
    fi
}

function get_remote_ip() {
    # Get the remote IP on the GRE tunnel
    REMOTE_IP=$(dig +short $FQDN | tail -n1)
    if [[ -z $REMOTE_IP ]]; then
        echo "Failed to resolve the remote host FQDN ($FQDN)"
        exit 1 
    fi
}

function check_gre_tunnel() {
    # Check if the GRE tunnel is up
    if [[ -z $CURRENT_IP ]]; then
        echo "GRE tunnel is down or not configured"
        # Return 1 to indicate that the GRE tunnel is down
        return 1
    fi
}

function configure_gre_tunnel() {
    # Configure the GRE tunnel
    # Delete the GRE tunnel if it exists
    echo "LOG - $(date) - REMOTE IP: $REMOTE_IP"
    echo "ip link del $GRE_TUNNEL_NAME"
    ip link del $GRE_TUNNEL_NAME
    # Create the GRE tunnel
    echo "ip link add $GRE_TUNNEL_NAME type gre remote $REMOTE_IP local $GRE_LOCAL_IP ttl 255"
    ip link add $GRE_TUNNEL_NAME type gre remote $REMOTE_IP local $GRE_LOCAL_IP ttl 255
    echo "ip link set $GRE_TUNNEL_NAME up"
    ip link set $GRE_TUNNEL_NAME up
    echo "ip addr add  $GRE_LAN_IP_4 dev $GRE_TUNNEL_NAME"
    ip addr add  $GRE_LAN_IP_4 dev $GRE_TUNNEL_NAME
    echo "ip addr add  $GRE_LAN_IP_6 dev $GRE_TUNNEL_NAME"
    ip addr add  $GRE_LAN_IP_6 dev $GRE_TUNNEL_NAME
}

function check_remote_ip() {
    # Check if the remote IP is the same as the one configured on the GRE tunnel
    if [[ $REMOTE_IP == $CURRENT_IP ]]; then
        echo "GRE tunnel is already configured with the right remote IP"
        # Return 0 to indicate that the GRE tunnel is already configured
        exit 0
    else
        echo "GRE tunnel is configured with the wrong remote IP"
        configure_gre_tunnel
        echo "Updated GRE tunnel with the right remote IP"
        exit 0        
    fi
}

# MAIN
function main() {
    check_root

    echo "LOG - $(date) - Starting autogre.sh"
    echo "LOG - $(date) - FQDN: $FQDN"
    echo "LOG - $(date) - GRE_TUNNEL_NAME: $GRE_TUNNEL_NAME"
    echo "LOG - $(date) - GRE_LOCAL_IP: $GRE_LOCAL_IP"
    echo "LOG - $(date) - GRE_LAN_IP_4: $GRE_LAN_IP_4"
    echo "LOG - $(date) - GRE_LAN_IP_6: $GRE_LAN_IP_6"

    echo "LOG - $(date) - Checking if the GRE tunnel is configured"
    get_current_ip

    echo "LOG - $(date) - Checking if the remote IP is the same as the one configured on the GRE tunnel"
    get_remote_ip

    echo "LOG - $(date) - Checking if the GRE tunnel is up"
    if check_gre_tunnel ; then
        echo "LOG - $(date) - GRE tunnel is up"
        check_remote_ip
        echo "LOG - $(date) - GRE tunnel is configured with the right remote IP"
    else
        echo "LOG - $(date) - GRE tunnel is down"
        configure_gre_tunnel
        echo "LOG - $(date) - Updated GRE tunnel with the right remote IP"
    fi
}

main