#!/bin/bash

UNIT=socat-dns
default_dest=149.56.44.185
default_port=53

verify_service_availability()
{
    dig . @127.0.0.1 >/dev/null 2>&1
}

service_launch_command()
{
    local dest port local_port
    dest=${1:-$default_dest}
    case "$dest" in
        '') return 1 ;;
        *:*)
            port=${dest#*:}
            dest=${dest%%:*}
        ;;
        *)
            port=${2:-$default_port}
        ;;
    esac
    local_port=${3:-$port}

    printf "socat UDP-RECVFROM:%s,fork UDP-SENDTO:%s\n" "$local_port" "$dest:$port"
}

####

watchdog()
{
    PID=$1
    VERIFY=$2

    TWO_MILLION=2000000
    SLEEP_TIME=$(( ${WATCHDOG_USEC:-$TWO_MILLION} / $TWO_MILLION ))

    enable -f /usr/lib/bash/sleep sleep || true
    while $VERIFY
    do
        systemd-notify --pid="$PID" WATCHDOG=1
        sleep "$SLEEP_TIME"
    done
}

main()
{
    case "$1" in
        install)
            shift
            COMMAND=$(service_launch_command "$@")
            if systemctl is-active "$UNIT" >/dev/null
            then
                systemctl stop "$UNIT"
            fi
            systemctl reset-failed "$UNIT" 2>/dev/null

            systemd-run \
                --unit "$UNIT" \
                --property Restart="always" \
                --property WatchdogSec="5" \
                -- \
                "$0" watchdog "$COMMAND" verify_service_availability
            ;;
        watchdog)
            shift
            watchdog $$ "$2" &
            exec $1
            ;;
        status)
            systemctl status "$UNIT"
            ;;
        monitor)
            journalctl -u "$UNIT" -f
            ;;
        *)
            echo "Usage: $0 install [destination [destination port [local port]]]" >&2
            echo "       $0 status" >&2
            exit 1
            ;;
    esac
}

main "$@"