NTPd

ntpd stands for Network Time Protocol daemon. It is a background service that implements the Network Time Protocol (NTP) to synchronize the local system clock with one or more remote time servers. ntpd is widely used on Unix-like systems and is available as part of the NTP project; there are also ports and variants for other platforms. The goal is to maintain accurate time across machines for logging, scheduling, and time-stamped data.

ntpd operates by forming associations with NTP servers or peers and continuously exchanging timing information. It runs clock discipline algorithms to minimize time offset and frequency error, adjusting the system clock gradually (slew) or, when necessary, stepping the clock to correct large errors. It can use precision sources such as kernel PPS (pulse-per-second) input to improve accuracy and may discipline the local oscillator based on network measurements. ntpd can also provide time services to other machines, functioning as an NTP server or a symmetric peer in a cluster.

Configuration is done through an ntp.conf file, where administrators specify time sources (server, peer, or broadcast addresses), access controls (restrict lines), driftfile storage for long-term frequency drift, and optional authentication keys for server and peer authentication. ntpd supports authenticating time sources and can employ various security mechanisms defined by the NTP standards, helping prevent spoofing and abuse.

In practice, ntpd is one of the most common NTP implementations on servers and workstations. Alternatives such as Chrony, OpenNTPD, and NTPsec exist, offering different performance profiles and use cases. ntpd remains a foundational tool for maintaining synchronized time in many networks and systems.