ssh-tunnel

#!/bin/bash

usage() {
	bin=$(basename $0)
	echo >&2 "Usage:"
	echo >&2 "  $bin [opts] <ssh -L forwarding spec> <ssh destination spec> [<ssh opts>]"
	echo >&2 "  $bin <ssh -L forwarding spec> <ssh destination spec> { kill | -k }"
	echo >&2
	echo >&2 "Starts non-interactive ssh -L tunnel through specified destination host,"
	echo >&2 " saving pid file in $XDG_RUNTIME_DIR,"
	echo >&2 " which can be used to stop tunnel via kill/-k option."
	echo >&2 "Example: $bin 8080:127.0.0.1:80 root@my.example.host"
	echo >&2
	echo >&2 "Options:"
	echo >&2 " -1 - oneshot - start tunnel in the foreground."
	echo >&2 "    Can be useful when setting up tunnel for the first time."
	echo >&2 " -t - tping - use 'tping' tool before running ssh, to avoid restarting ssh"
	echo >&2 "    every interval when remote host is more likely to be down than otherwise."
	echo >&2 " -r - reverse-ssh - run reverse ssh tunnel (using -R option instead of -L)."
	echo >&2 " -i <interval> - interval between connection attempts"
	echo >&2 "    in seconds (default=5), applied to both ssh/tping."
	echo >&2 " -x - debug - Use 'set -x' at the start of the script."
	exit ${1:-0}
}
[[ "$#" -ge 2 ]] || usage 1
[[ "$1" = -h || "$1" = --help ]] && usage

oneshot= tping= interval=5 tun=-L

while getopts ":h1txi:r" opt
do case "$opt" in
	h) usage 0 ;;
	i) [[ $OPTARG =~ ^[0-9]+$ ]] && interval=$OPTARG \
		|| { echo >&2 -e "ERROR: Invalid -i interval value: $OPTARG\n"; usage 1; } ;;
	1) oneshot=t ;;
	t) tping=t ;;
	r) tun=-R ;;
	x) set -x ;;
	*) { echo >&2 -e "ERROR: Invalid option: -$OPTARG\n"; usage 1; } ;;
esac; done
shift $((OPTIND-1))

fwd=$1 dst=$2
shift; shift

pid_file="$XDG_RUNTIME_DIR/.$(basename $0)"`
	`".$(echo "$fwd.$dst" | md5sum | cut -b-5)"

[[ "$1" = kill || "$1" = -k ]] && {
	pid=$(cat "$pid_file" 2>/dev/null)
	[[ -n "$pid" ]] || exit 1
	flock -n 3 3<"$pid_file" && exit 1
	pgrp=$(ps -o pgrp= "$pid" | tr -d '\040\011\012\015')
	[[ -n "$pgrp" && "$pgrp" -gt 1 ]] || exit 1
	exec kill -- -"$pgrp"
}

OPTIND=0 ssh_port= ssh_opts_ext=( "$@" )
while getopts ":p:" opt
	do case "$opt" in p) ssh_port=$OPTARG;; esac; done

ssh_opts=(
	-oControlPath=none
	-oControlMaster=no
	-oServerAliveInterval=3
	-oServerAliveCountMax=5
	-oConnectTimeout=5
	-oBatchMode=yes
	-oPasswordAuthentication=no
	-oNumberOfPasswordPrompts=0
	-oExitOnForwardFailure=yes
	-TnN
)
[[ -n "$oneshot" ]] || ssh_opts+=( -qy )
ssh_opts+=( "${ssh_opts_ext[@]}" )

tping_ct=$(( $interval * 2 ))
tping_ct=$(( $tping_ct > 5 ? $tping_ct : 5 ))
tping_opts=( -p"${ssh_port:-22}" -r"$interval" --conn-timeout="$tping_ct" )

[[ -z "$oneshot" ]] || {
	[[ -z "$tping" ]] || tping "${tping_opts[@]}" "$dst"
	exec ssh "${ssh_opts[@]}" "$tun" "$fwd" "$dst"
}

touch "$pid_file"
flock -n 3 3<"$pid_file" || exit 0
exec 3>"$pid_file"
( flock -n 3 || exit 0
	trap "rm -f '$pid_file'" EXIT
	interval_base=$interval
	while :; do
		[[ -z "$tping" ]] || {
			ts0=$(printf '%(%s)T' -1)
			tping "${tping_opts[@]}" "$dst"; }
		touch "$pid_file"
		ssh "${ssh_opts[@]}" "$tun" "$fwd" "$dst"
		[[ -z "$tping" ]] || interval=$((
			$ts0 + $interval_base - $(printf '%(%s)T' -1) ))
		[[ "$interval" -le 0 ]] || sleep "$interval"
	done ) &

echo $! >&3
exit 0