ntpdate

Set the date and time using the Network Time Protocol (NTP).

Description

The ntpdate command is used to set the local date and time. It polls the specified NTP servers to obtain a set of samples and applies the standard NTP clock filter and selection algorithms to choose the best sample.

The ntpdate command adjusts the clock using the following methods:

• If the clock offset is greater than 0.5 seconds, it sets the clock time directly by calling the settimeofday subroutine. This is the preferred method at boot time.
• If the clock offset is less than 0.5 seconds, it adjusts the clock time by calling the adjtime subroutine with the offset. This method tends to maintain the drift clock more accurately at the expense of some stability. When ntpdate is run regularly from a cron command (e.g., every one or two hours) instead of running a daemon, it can ensure sufficient accuracy and avoid abrupt clock steps.

Using multiple servers significantly improves the reliability and accuracy of the ntpdate command. Although a single server can be used, providing at least three or four servers will yield better performance.

If an NTP daemon like xntpd is already running on the same host, ntpdate will refuse to set the date.

You must have root privileges to run this command.

Syntax

ntpdate [ -b] [ -d] [ -s] [ -u] [ -aKeyid] [ -eAuthenticationDelay] [ -kKeyFile] [ -oVersion] [ -pSamples] [ -tTimeOut] Server...

Options

-aKeyid               # Use Keyid to authenticate all packets.
-b                    # Force the time to be stepped using the settimeofday subroutine.
-d                    # Enable debug mode. Determine what ntpdate would do without actually setting the clock. Results are displayed on the screen. This flag uses unprivileged ports.
-eAuthenticationDelay # Specify the processing delay for authentication in seconds.
-kKeyFile             # Specify a different name for the key file instead of the default /etc/ntp.keys.
-oVersion             # Specify the NTP version to use (1, 2, or 3). The default is 3.
-pSamples             # Specify the number of samples to acquire from each server (between 1 and 8). The default is 4.
-s                    # Log messages to the syslog facility instead of standard output. Useful when running ntpdate via cron.
-tTimeOut             # Specify the timeout for waiting for a response. The value is rounded to the nearest multiple of 0.2 seconds. The default is 1 second.
-u                    # Use an unprivileged port for outgoing packets. This is useful when behind a firewall that blocks incoming traffic to privileged ports.
-q                    # Query only - do not set the clock.