Introduction to ntp

The ntp package contains a client and server to keep the time synchronized between various computers over a network. This package is the official reference implementation of the NTP protocol.



Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

ntp Dependencies




libcap-2.69 with PAM, libevent-2.1.12, libedit, and libopts from AutoGen

Editor Notes: https://wiki.linuxfromscratch.org/blfs/wiki/ntp

Installation of ntp

There should be a dedicated user and group to take control of the ntpd daemon after it is started. Issue the following commands as the root user:

groupadd -g 87 ntp &&
useradd -c "Network Time Protocol" -d /var/lib/ntp -u 87 \
        -g ntp -s /bin/false ntp

The update-leap command needs to be fixed in order to run properly:

sed -e 's/"(\\S+)"/"?([^\\s"]+)"?/' \
    -i scripts/update-leap/update-leap.in

Now fix an issue introduced with glibc-2.34:

sed -e 's/#ifndef __sun/#if !defined(__sun) \&\& !defined(__GLIBC__)/' \
    -i libntp/work_thread.c

Install ntp by running the following commands:

./configure --prefix=/usr         \
            --bindir=/usr/sbin    \
            --sysconfdir=/etc     \
            --enable-linuxcaps    \
            --with-lineeditlibs=readline \
            --docdir=/usr/share/doc/ntp-4.2.8p17 &&

To test the results, issue: make check.

Now, as the root user:

make install &&
install -v -o ntp -g ntp -d /var/lib/ntp

Command Explanations

CFLAGS="-O2 -g -fPIC": This environment variable is necessary to generate Position Independent Code needed for use in the package libraries.

--bindir=/usr/sbin: This parameter places the administrative programs in /usr/sbin.

--enable-linuxcaps: ntpd is run as user ntp, so use Linux capabilities for non-root clock control.

--with-lineeditlibs=readline: This switch enables Readline support for ntpdc and ntpq programs. If omitted, libedit will be used if installed, otherwise no readline capabilities will be compiled.

Configuring ntp

Config Files


Configuration Information

The following configuration file first defines various ntp servers with open access from different continents. Second, it creates a drift file where ntpd stores the frequency offset and a pid file to store the ntpd process ID. Third, it defines the location for the leap-second definition file /etc/ntp.leapseconds, that the update-leap script checks and updates, when necessary. This script can be run as a cron job and the ntp developers recommend a frequency of about three weeks for the updates. Since the documentation included with the package is sparse, visit the ntp website at https://www.ntp.org/ and https://www.ntppool.org/ for more information.

cat > /etc/ntp.conf << "EOF"
# Asia
server 0.asia.pool.ntp.org

# Australia
server 0.oceania.pool.ntp.org

# Europe
server 0.europe.pool.ntp.org

# North America
server 0.north-america.pool.ntp.org

# South America
server 2.south-america.pool.ntp.org

driftfile /var/lib/ntp/ntp.drift
pidfile   /run/ntpd.pid

leapfile  /var/lib/ntp/ntp.leapseconds

You may wish to add a Security session. For explanations, see https://www.eecis.udel.edu/~mills/ntp/html/accopt.html#restrict.

cat >> /etc/ntp.conf << "EOF"
# Security session
restrict    default limited kod nomodify notrap nopeer noquery
restrict -6 default limited kod nomodify notrap nopeer noquery

restrict ::1

Synchronizing the Time

There are two options. Option one is to run ntpd continuously and allow it to synchronize the time in a gradual manner. The other option is to run ntpd periodically (using cron) and update the time each time ntpd is scheduled.

If you choose Option one, then install the /etc/rc.d/init.d/ntp init script included in the blfs-bootscripts-20230917 package.

make install-ntpd

If you prefer to run ntpd periodically, add the following command to root's crontab:

ntpd -q

Execute the following command if you would like to set the hardware clock to the current system time at shutdown and reboot:

ln -v -sf ../init.d/setclock /etc/rc.d/rc0.d/K46setclock &&
ln -v -sf ../init.d/setclock /etc/rc.d/rc6.d/K46setclock

The other way around is already set up by LFS.


Installed Programs: calc_tickadj, ntp-keygen, ntp-wait, ntpd, ntpdate, ntpdc, ntpq, ntptime, ntptrace, sntp, tickadj and update-leap
Installed Libraries: None
Installed Directories: /usr/share/ntp, /usr/share/doc/ntp-4.2.8 and /var/lib/ntp

Short Descriptions


calculates optimal value for tick given ntp drift file


generates cryptographic data files used by the NTPv4 authentication and identification schemes


is useful at boot time, to delay the boot sequence until ntpd has set the time


is a ntp daemon that runs in the background and keeps the date and time synchronized based on response from configured ntp servers. It also functions as a ntp server


is a client program that sets the date and time based on the response from an ntp server. This command is deprecated


is used to query the ntp daemon about its current state and to request changes in that state


is a utility program used to monitor ntpd operations and determine performance


reads and displays time-related kernel variables


traces a chain of ntp servers back to the primary source


is a Simple Network Time Protocol (SNTP) client


reads, and optionally modifies, several timekeeping-related variables in older kernels that do not have support for precision timekeeping


is a script to verify and, if necessary, update the leap-second definition file