NTP client (Posix UDP)

Minimal NTP client over UDP using only the Posix VAPI (--pkg posix): Posix.getaddrinfo for host and port lookup (IPv4 and IPv6), UDP socket / connect / read / write, byte order on the transmit timestamp, and Posix.ctime / time_t for printing.

The service name "123" is passed to getaddrinfo so the port does not need to be filled in by hand. Posix.getnameinfo prints the numeric address that was actually used. Always call Posix.freeaddrinfo on the result list.

Program

Save as ntp-client.vala:

struct NtpPacket {
    uint8 li_vn_mode;
    uint8 stratum;
    uint8 poll;
    uint8 precision;
    uint32 root_delay;
    uint32 root_dispersion;
    uint32 ref_id;
    uint32 ref_tm_s;
    uint32 ref_tm_f;
    uint32 orig_tm_s;
    uint32 orig_tm_f;
    uint32 rx_tm_s;
    uint32 rx_tm_f;
    uint32 tx_tm_s;
    uint32 tx_tm_f;
}

int main () {
    const string HOSTNAME = "pool.ntp.org";
    Posix.AddrInfo hints = Posix.AddrInfo ();
    hints.ai_family = Posix.AF_UNSPEC;
    hints.ai_socktype = Posix.SOCK_DGRAM;
    hints.ai_protocol = Posix.IPProto.UDP;

    Posix.AddrInfo* res;
    int gai = Posix.getaddrinfo (HOSTNAME, "123", hints, out res);
    if (gai != 0) {
        stderr.printf ("getaddrinfo: %s\n", Posix.gai_strerror (gai));
        return 1;
    }

    int sockfd = -1;
    unowned Posix.AddrInfo* chosen = null;
    for (unowned Posix.AddrInfo* ai = res; ai != null; ai = ai.ai_next) {
        int fd = Posix.socket (ai.ai_family, ai.ai_socktype, ai.ai_protocol);
        if (fd < 0) {
            continue;
        }
        if (Posix.connect (fd, ai.ai_addr, (Posix.socklen_t) ai.ai_addrlen) == 0) {
            sockfd = fd;
            chosen = ai;
            break;
        }
        Posix.close (fd);
    }

    if (sockfd < 0 || chosen == null) {
        stderr.printf ("Could not connect to any resolved address\n");
        Posix.freeaddrinfo (res);
        return 1;
    }

    var hostbuf = new char[256];
    var servbuf = new char[64];
    if (Posix.getnameinfo (*chosen.ai_addr, (Posix.socklen_t) chosen.ai_addrlen,
            hostbuf, servbuf, Posix.NI_NUMERICHOST | Posix.NI_NUMERICSERV) == 0) {
        stderr.printf ("Using %s port %s\n", (string) hostbuf, (string) servbuf);
    }

    var packet = NtpPacket ();
    assert (sizeof (NtpPacket) == 48);
    packet.li_vn_mode = 0x1b;

    if (Posix.write (sockfd, &packet, sizeof (NtpPacket)) < 0) {
        stderr.printf ("Can't send UDP packet: errno %d\n", Posix.errno);
        Posix.close (sockfd);
        Posix.freeaddrinfo (res);
        return 1;
    }

    if (Posix.read (sockfd, &packet, sizeof (NtpPacket)) < 0) {
        stderr.printf ("Can't read from socket: errno %d\n", Posix.errno);
        Posix.close (sockfd);
        Posix.freeaddrinfo (res);
        return 1;
    }
    Posix.close (sockfd);
    Posix.freeaddrinfo (res);

    packet.tx_tm_s = Posix.ntohl (packet.tx_tm_s);
    packet.tx_tm_f = Posix.ntohl (packet.tx_tm_f);
    const uint64 NTP_TIMESTAMP_DELTA = 2208988800ULL;
    time_t tx_tm = (time_t) ((int64) packet.tx_tm_s - (int64) NTP_TIMESTAMP_DELTA);
    unowned string? utc_str = Posix.ctime (ref tx_tm);
    stdout.printf ("Current UTC time is %s", utc_str ?? "unknown\n");
    return 0;
}

Compile and run

valac --pkg posix ntp-client.vala
./ntp-client

Posix.ctime returns a libc-formatted string (usually including a trailing newline). Outbound UDP to port 123 must be allowed. The program prints diagnostics on stderr and the time string on stdout.