#!/bin/bash
UNIT=socat-dns

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

watchdog()
{
    PID=$1

    enable -f /usr/lib/bash/sleep sleep || true
    while verify_service_availability
    do
        systemd-notify --pid="$PID" WATCHDOG=1
        sleep $((${WATCHDOG_USEC:-2000000} / 2000000))
    done
}

main()
{
    case "$1" in
        install)
            shift
            DEST=${1:-149.56.44.185}
            PORT=${2:-53}
            LOCAL_PORT=${3:-$PORT}
            COMMAND="socat UDP-RECVFROM:$LOCAL_PORT,fork UDP-SENDTO:$DEST:$PORT"
            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"
            ;;
        watchdog)
	    shift
	    COMMAND="$1"
            watchdog $$ &
            exec $COMMAND
            ;;
        *)
            echo "Usage: $0 install [destination] [destination port] [local port]" >&2
            exit 1
            ;;
    esac
}

main "$@"