Ntp

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XNTPD is an operating system daemon which sets and maintains the system time-of-day in synchronism with Internet standard time servers. xntpd is a complete implementation of the Network Time Protocol (NTP) version 3, as defined by RFC-1305, but also retains compatibility with version 1 and 2 servers as defined by RFC- 1059 and RFC-1119, respectively. xntpd does all computations in 64-bit fixed point arithmetic and requires no floating point support. While the ultimate precision of this design, about 232 picoseconds, is not achievable with ordinary workstations and networks of today, it may be required with future nanosecond CPU clocks and gigabit LANs.

The daemon can operate in any of several modes, including symmetric active/passive, client/server and broadcast/multicast, as described in RFC-1305. A broadcast/multicast client can discover remote servers, compute server-client propagation delay correction factors and configure itself automatically. This makes it possible to deploy a fleet of workstations without specifying configuration details specific to the local environment.

Ordinarily, xntpd reads the ntp.conf configuration file at startup time in order to determine the synchronization sources and operating modes. It is also possible to specify a working, although limited, configuration entirely on the command line, obviating the need for a configuration file. This may be particularly appropriate when the local host is to be configured as a broadcast or multicast client, with all peers being determined by listening to broadcasts at run time.

Various internal xntpd variables can be displayed and configuration options altered while the daemon is running using the ntpq and xntpdc utility programs.

  • When xntpd starts it looks at the value of umask, and if it's zero xntpd will set the umask to 022.

Commands

  • To Update system time
rdate -s time.nist.gov

<--- OR --->

/usr/sbin/ntpdate -u 0.pool.ntp.org

Then

/sbin/hwclock --systohc

Command Line Options

-a
    Enable authentication mode. The default is enabled, so this option is obsolete now. 
-A
    Disable authentication mode.

-b
    Synchronize using NTP broadcast messages.

-c conffile
    Specify the name and path of the configuration file.

-d
     Specify debugging mode. This flag may occur multiple times, with each occurrence indicating greater detail of display.

-f driftfile
    Specify the name and path of the drift file.

-k keyfile
    Specify the name and path of the file containing the NTP authentication keys.

-l logfile
    Specify the name and path of the log file. The default is the system log facility.

-m
    Synchronize using NTP multicast messages on the IP multicast group address 224.0.1.1 (requires multicast kernel).

-p pidfile
    Specify the name and path to record the daemon's process ID.

-r broadcastdelay
    Specify the default propagation delay from the broadcast/multicast server and this computer. This is used  only if the delay cannot be computed automatically by the protocol.

-s statsdir
    Specify the directory path for files created by the statistics facility.

-t key
    Add a key number to the trusted key list.

-v variable
    Add a system variable.

-V variable
    Add a system variable listed by default.

Installing NTP from Source

yum -y install gcc gawk openssl-devel
mkdir -p /var/src
cd /var/src
wget http://www.eecis.udel.edu/~ntp/ntp_spool/ntp4/ntp-dev/ntp-dev-4.2.7p335.tar.gz
tar -xzf ntp-dev-4.2.7p335.tar.gz
cd ntp-dev-4.2.7p335
./configure --enable-clockctl && make && echo $?
make install

The Configuration File

The xntpd configuration file is read at initial startup in order to specify the synchronization sources, modes and other related information. Usually, it is installed in the /etc directory, but could be installed elsewhere (see the -c conffile command line option). The file format is similar to other Unix configuration files - comments begin with a # character and extend to the end of the line; blank lines are ignored. Configuration commands consist of an initial keyword followed by a list of arguments, some of which may be optional, separated by whitespace. Commands may not be continued over multiple lines. Arguments may be host names, host addresses written in numeric, dotted-quad form, integers, floating point numbers (when specifying times in seconds) and text strings. Optional arguments are delimited by [ ] in the following descriptions, while alternatives are separated by |. The notation [ ... ] means an optional, indefinite repetition of the last item before the [ ... ].

See the following pages for configuration and control options. While there is a rich set of options available, the only required option is one or more server, peer or broadcast commands described in the Configuration Options page. The Notes on Configuring NTP and Setting up a NTP Subnet page contains an extended discussion of these options.


Files

/etc/ntp.conf - the default name of the configuration file
/etc/ntp.drift - the default name of the drift file
/etc/ntp.keys - the default name of the key file