Keep Reverse Tunnel Open

keep_tunnel_open.sh

#!/usr/bin/env bash
#
# MIT License
# 
# Copyright (c) 2021 Vallum Sofware, LLC
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
#
# Inspired by https://stackoverflow.com/questions/1998297/ssh-check-if-a-tunnel-is-alive
#
# Tested on Ubuntu 18.04 LTS and 20.04 LTS
#

#
# See https://stackoverflow.com/a/45122025/4329358
#
function print_style {
    if [ "$2" = "info" ] ; then
        COLOR="96m";
    elif [ "$2" = "success" ] ; then
        COLOR="92m";
    elif [ "$2" = "warning" ] ; then
        COLOR="93m";
    elif [ "$2" = "danger" ] ; then
        COLOR="91m";
    else #default color
        COLOR="0m";
    fi

    STARTCOLOR="\e[$COLOR";
    ENDCOLOR="\e[0m";

    printf "${STARTCOLOR}%b${ENDCOLOR}\n" "$1";
}

usage() {
        print_style "$0 --hostname <ip-address> --port port_num [-u <username> | --user <username> Supply username (default is ${USER})] [-k private-key | --private-key private-key Supply a private key, otherwise rely in ~/.ssh/config]"
        exit 1
}

user_name="${USER}"
private_key=
while [ $# -gt 0 ]
do
    case
        "-u" | "--user")
            shift
            user_name="$1"
            shift
        -k | --private-key)
            shift
            private_key="-i $1"
            shift
        --port)
            shift
            port="$1"
            shift
        *)
            usage
        -h | --help
            usage
    esac
done

if [ ! -v host_name ]
then
    print_style "$0: --hostname is required"
    usage
elif [ ! -v port ]
    print_style "$0: --port is required"
    usage
fi

while [ true ]
do
        echo "start tunnel"
        ssh ${private_key} -N -R ${port}:localhost:22 ${user_name}@${host_name} sleep 10
        echo "ssh exits with $?"
        sleep 300
        echo "restart tunnel"
done