#!/bin/bash
# This script configures running chronyd to use NTP servers obtained from
# DHCP and _ntp._udp DNS SRV records. Files with servers from DHCP are managed
# externally (e.g. by a dhclient script). Files with servers from DNS SRV
# records are updated here using the dig utility.

chronyc=/usr/bin/chronyc
helper_dir=/var/run/chrony-helper
added_servers_file=$helper_dir/added_servers

network_sysconfig_file=/etc/sysconfig/network
dhclient_servers_files=/var/lib/dhclient/chrony.servers.*
dnssrv_servers_files=$helper_dir/dnssrv@*
dnssrv_timer_prefix=chrony-dnssrv@

chrony_command() {
    $chronyc -a -n -m "$1"
}

is_running() {
    chrony_command "tracking" &> /dev/null
}

is_update_needed() {
    for file in $dhclient_servers_files $dnssrv_servers_files \
            $added_servers_file; do
        [ -e "$file" ] && return 0
    done
    return 1
}

update_daemon() {
    local all_servers_with_args all_servers added_servers

    if ! is_running; then
        rm -f $added_servers_file
        return 0
    fi

    all_servers_with_args=$(
        cat $dhclient_servers_files $dnssrv_servers_files 2> /dev/null)

    all_servers=$(
        echo "$all_servers_with_args" |
            while read server serverargs; do
                echo "$server"
            done | sort -u)
    added_servers=$( (
        cat $added_servers_file 2> /dev/null
        echo "$all_servers_with_args" |
            while read server serverargs; do
                [ -z "$server" ] && continue
                chrony_command "add server $server $serverargs" &> /dev/null &&
                    echo "$server"
            done) | sort -u)

    comm -23 <(echo -n "$added_servers") <(echo -n "$all_servers") |
        while read server; do
            chrony_command "delete $server" &> /dev/null
        done

    added_servers=$(comm -12 <(echo -n "$added_servers") <(echo -n "$all_servers"))

    [ -n "$added_servers" ] && echo "$added_servers" > $added_servers_file ||
        rm -f $added_servers_file
}

get_dnssrv_servers() {
    local name=$1

    if ! command -v dig &> /dev/null; then
        echo "Missing dig (DNS lookup utility)" >&2
        return 1
    fi

    (
        . $network_sysconfig_file &> /dev/null

        output=$(dig "$name" srv +short +ndots=2 +search 2> /dev/null)
        [ $? -ne 0 ] && return 0

        echo "$output" | while read prio weight port target; do
            server=${target%.}
            [ -z "$server" ] && continue
            echo "$server port $port ${NTPSERVERARGS:-iburst}"
        done
    )
}

check_dnssrv_name() {
    local name=$1

    if [ -z "$name" ]; then
        echo "No DNS SRV name specified" >&2
        return 1
    fi

    if [ "${name:0:9}" != _ntp._udp ]; then
        echo "DNS SRV name $name doesn't start with _ntp._udp" >&2
        return 1
    fi
}

update_dnssrv_servers() {
    local name=$1
    local srv_file=$helper_dir/dnssrv@$name servers

    check_dnssrv_name "$name" || return 1

    servers=$(get_dnssrv_servers "$name")
    [ -n "$servers" ] && echo "$servers" > "$srv_file" || rm -f "$srv_file"
}

set_dnssrv_timer() {
    local state=$1 name=$2
    local srv_file=$helper_dir/dnssrv@$name servers
    local timer=$dnssrv_timer_prefix$name.timer

    check_dnssrv_name "$name" || return 1

    if [ "$state" = enable ]; then
        systemctl enable "$timer"
        systemctl start "$timer"
    elif [ "$state" = disable ]; then
        systemctl stop "$timer"
        systemctl disable "$timer"
        rm -f "$srv_file"
    fi
}

list_dnssrv_timers() {
    systemctl --all --full -t timer list-units | grep "^$dnssrv_timer_prefix" | \
        sed "s|^$dnssrv_timer_prefix\(.*\)\.timer.*|\1|"
}

prepare_helper_dir() {
    mkdir -p $helper_dir
    exec 100> $helper_dir/lock
    if ! flock -w 20 100; then
        echo "Failed to lock $helper_dir" >&2
        return 1
    fi
}

print_help() {
    echo "Usage: $0 COMMAND"
    echo
    echo "Commands:"
    echo "	update-daemon"
    echo "	update-dnssrv-servers NAME"
    echo "	enable-dnssrv NAME"
    echo "	disable-dnssrv NAME"
    echo "	list-dnssrv"
    echo "	is-running"
    echo "	command CHRONYC-COMMAND"
}

case "$1" in
    update-daemon|add-dhclient-servers|remove-dhclient-servers)
        is_update_needed || exit 0
        prepare_helper_dir && update_daemon
        ;;
    update-dnssrv-servers)
        prepare_helper_dir && update_dnssrv_servers "$2" && update_daemon
        ;;
    enable-dnssrv)
        set_dnssrv_timer enable "$2"
        ;;
    disable-dnssrv)
        set_dnssrv_timer disable "$2" && prepare_helper_dir && update_daemon
        ;;
    list-dnssrv)
        list_dnssrv_timers
        ;;
    is-running)
        is_running
        ;;
    command|forced-command)
        chrony_command "$2"
        ;;
    *)
        print_help
        exit 2
esac

exit $?