automatic port detection for autossh

autossh_wrapper.sh

#!/bin/sh
# little wrapper to choose a random port for autossh
# falling back to $fallback_port

set -u

autossh_bin=${AUTOSSH_BIN:-/usr/local/bin/autossh.real}
fallback_port=${AUTOSSH_FALLBACK_PORT:-21021}

# XXX what if these are not available?
egrep=/usr/bin/egrep
lsof=/usr/sbin/lsof
od=/usr/bin/od
tr=/usr/bin/tr

# backwards compatibility, skip guess if -M is passed
echo "$@" | $egrep -q -- '-f?M ?[0-9]+'
if [ $? -eq 0 ] || [ -n "${AUTOSSH_PORT:-}" ]; then
    exec $autossh_bin "$@"
fi

# take an int port and check whether it is in use (i.e. locally bound)
# unix command semantics: if in use return 0 else return 1
port_in_use() {
    [ -z "$1" ] && return 0

    $lsof -i tcp:$1 -s tcp:listen -Fp 1>/dev/null
    free_all=$?

    $lsof -i tcp@127.0.0.1:$1 -s tcp:listen -Fp 1>/dev/null
    free_localhost=$?

    [ $free_all -eq 0 ] || [ $free_localhost -eq 0 ]
    return $?
}

portguess=""
if [ -r "/dev/urandom" ]; then
    for t in $(seq 1 42); do
        # get a random int for the tcp port
        randport=$( $od -i -N2 -An /dev/urandom | $tr -d ' ' )
        randport_1=$(( $randport + 1 ))

        [ "$randport" -le 1024 ] && continue
        [ "$randport" -ge 65535 ] && continue

        # check if port is in use, race condition between here
        # and the exec
        if ! port_in_use $randport && ! port_in_use $randport_1; then
            portguess=$randport
            break
        fi
    done
fi

if [ -z "$portguess" ]; then
    fallback=$fallpack_port
    fallback_1=$(( $fallback_port + 1 ))
    if ! port_in_use $fallback && ! port_in_use $fallback_1; then
        portguess=$fallback_port
    else
        echo "unable to find a suitable tunnel port"
        exit 1
    fi
fi

export AUTOSSH_PORT="$portguess"

exec $autossh_bin "$@"