This is perhaps the most important flag. By default, ntpdate uses UDP port 123. If that port is already in use by a running ntpd daemon or blocked by a firewall, the command will fail. The -u flag tells ntpdate to use an unprivileged port for outgoing packets, allowing it to work behind most firewalls.
Virtual machines (VMs) are notoriously prone to clock drift when they are paused or migrated between hosts. Adminstrators often use ntpdate to snap the time back into place after a VM resumes operation. 3. The "Midnight Fireball" Problem Avoiding clock drift on VMs - Red Hat
Forces the clock to "step" (jump) to the correct time immediately, rather than gradually adjusting it.
Understanding ntpdate : The Legacy Command for Network Time Synchronization
ntpdate is a command-line utility used to set the local date and time by polling a Network Time Protocol (NTP) server. Unlike the ntpd daemon, which runs continuously and gradually slews the clock to keep it in sync, ntpdate is a "one-shot" tool. When executed, it determines the time offset from the specified server and immediately adjusts the local system clock. How to Use the ntpdate Command The basic syntax for using the command is straightforward: sudo ntpdate [options] [time_server]
Redirects the output to the system log (syslog) rather than the standard output, which is useful for cron jobs . Common Use Cases and Limitations 1. Initial Time Setup
Ntpdate !!top!! May 2026
This is perhaps the most important flag. By default, ntpdate uses UDP port 123. If that port is already in use by a running ntpd daemon or blocked by a firewall, the command will fail. The -u flag tells ntpdate to use an unprivileged port for outgoing packets, allowing it to work behind most firewalls.
Virtual machines (VMs) are notoriously prone to clock drift when they are paused or migrated between hosts. Adminstrators often use ntpdate to snap the time back into place after a VM resumes operation. 3. The "Midnight Fireball" Problem Avoiding clock drift on VMs - Red Hat ntpdate
Forces the clock to "step" (jump) to the correct time immediately, rather than gradually adjusting it. This is perhaps the most important flag
Understanding ntpdate : The Legacy Command for Network Time Synchronization The -u flag tells ntpdate to use an
ntpdate is a command-line utility used to set the local date and time by polling a Network Time Protocol (NTP) server. Unlike the ntpd daemon, which runs continuously and gradually slews the clock to keep it in sync, ntpdate is a "one-shot" tool. When executed, it determines the time offset from the specified server and immediately adjusts the local system clock. How to Use the ntpdate Command The basic syntax for using the command is straightforward: sudo ntpdate [options] [time_server]
Redirects the output to the system log (syslog) rather than the standard output, which is useful for cron jobs . Common Use Cases and Limitations 1. Initial Time Setup