The Network Time Protocol daemon (ntpd) is an operating system program that maintains the

system time In computer science and computer programming, system time represents a computer system's notion of the passage of time. In this sense, ''time'' also includes the passing of days on the calendar. System time is measured by a ''system clock'', w ...

in synchronization with time servers using the

Network Time Protocol The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable- latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in c ...

(NTP).

Description

The ntpd program is an operating-system

daemon Daimon or Daemon (Ancient Greek: , "god", "godlike", "power", "fate") originally referred to a lesser deity or guiding spirit such as the daimons of ancient Greek religion and mythology and of later Hellenistic religion and philosophy. The word ...

that sets and maintains a computer system's system time in synchronization with Internet-standard

time server Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to co ...

s. It is a complete implementation of the

(NTP) version 4, but retains compatibility with versions 1, 2, and 3 as defined by RFC 1059, RFC 1119, and RFC 1305, respectively. ntpd performs most computations in 64-bit

floating point In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be ...

arithmetic and uses 64-bit fixed point operations only when necessary to preserve the ultimate precision, about 232

picosecond A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 ...

s. While ordinary workstations and networks cannot achieve the ultimate

precision Precision, precise or precisely may refer to: Science, and technology, and mathematics Mathematics and computing (general) * Accuracy and precision, measurement deviation from true value and its scatter * Significant figures, the number of digit ...

, future processors and networks may require it. xntpd is the Network Time Protocol version three (1992) daemon software. The "x" was added to the name because the branch of code that eventually became NTPv3 was "experimental". The name of the software changed back to "ntpd" for version four because the inventor of NTP, Dave Mills, decided that something probably should not remain "experimental" for about twenty years in the absence of dramatic change. ntpd uses a single configuration-file to run the daemon in server and/or client modes. The configuration file, usually named ntp.conf, is located in the /etc directory. Other important files include the drift file, which ntpd uses to correct for hardware-

clock skew Clock skew (sometimes called timing skew) is a phenomenon in synchronous digital circuit systems (such as computer systems) in which the same sourced clock signal arrives at different components at different times due to gate or, in more advanced ...

in the absence of a connection to a more accurate upstream time-server.

Implementations

{{Further, Network Time Protocol#Implementations

NTP implementations

NTP (RFC 5905):
The NTP Reference Implementation
from The NTP Project at the University of Delaware.

– Free Windows port of The NTP Reference Implementation from http://www.ntp.org with an easy-to-use installer
NTPsec
a hardened implementation derived from NTP Classic, Dave Mills's original. *

chrony chrony is an implementation of the Network Time Protocol (NTP). It's an alternative to ntpd, which is a reference implementation of NTP. It runs on Unix-like operating systems (including Linux and macOS) and is released under the GNU GPL v2. It' ...

– chronyd implements the NTP protocol and can act as either a client or a server. *

OpenNTPD OpenNTPD (also known as OpenBSD NTP Daemon) is a Unix daemon implementing the Network Time Protocol to synchronize the local clock of a computer system with remote NTP servers. It is also able to act as an NTP server to NTP-compatible clients. ...

– A portable Simple NTPD implementation by the

OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project em ...

group

Simple NTP (SNTP) implementations

SNTP (RFC 4330):
clockspeed
– A simplest available and secure suite of NTP/SNTP client, clock skew eliminator, and precise time synchronization server and client
dntpd
– A simpl
client ntpd
in

DragonFly BSD DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD in Ju ...

ConnMan
– ConnMan contains an NTP implementation. *

BusyBox BusyBox is a software suite that provides several Unix utilities in a single executable file. It runs in a variety of POSIX environments such as Linux, Android, and FreeBSD, although many of the tools it provides are designed to work with in ...

, since version 1.16.2, has included an SNTP client and server based on OpenNTP.
systemd-timesyncd
– A Linux and systemd specific client implementation of SNTP.

References

- Network Time Protocol (NTP) daemon''
Mills, D. L. The University of Delaware, USA. 2005. (Date Accessed: 19 August 2005)

External links