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Synchronize your network with NTP
NTP (Network Time Protocol) allows clock synchronization between computer systems. The following HOWTO describes:
- configuring an NTP server on Slackware Linux;
- synchronizing client PCs with your local NTP server.
Synchronize a LAN server or a public root server with an NTP server on the Internet
Create an empty log file:
# touch /var/log/ntp.log
Visit http://www.pool.ntp.org and choose a list of servers according to your country.
Configure the NTP service by editing /etc/ntp.conf
. You might backup the existing ntp.conf
file and start from scratch.
In the example below, the list of four servers is chosen for my company's location (France):
# /etc/ntp.conf driftfile /etc/ntp/drift logfile /var/log/ntp.log server 0.fr.pool.ntp.org server 1.fr.pool.ntp.org server 2.fr.pool.ntp.org server 3.fr.pool.ntp.org server 127.127.1.0 fudge 127.127.1.0 stratum 10 restrict default nomodify nopeer notrap restrict 127.0.0.1 mask 255.0.0.0
Here's a little explanation for some options:
- The
fudge 127.127.1.10 stratum 10
directive is a “dummy” server acting as fallback IP in case the external time source becomes momentarily unreachable. When this happens, NTP will continue to work and base itself on this “internal” server. - NTP has its own arsenal of rules to limit access to the service, which can be used independently from a firewall. The
restrict
directives in the above configuration prevent distant computers from changing the servers' configuration (firstrestrict
statement), and the machine is configured to trust itself (secondrestrict
statement). - A
restrict
statement without any argument but followed by the hostname boils down to anallow all
.
Manage the NTP service
Before starting the service, proceed to an initial adjustment of your system clock:
# ntpdate pool.ntp.org
ntpdate
command is normally considered obsolete, but it still comes in handy when performing important time adjustments. The “orthodox” way would be to use the ntpd -g
command - the official replacement for ntpdate
- but its use will fail if your system clock is off for more than half an hour.
Activate the NTP service:
# chmod 0755 /etc/rc.d/rc.ntpd
Manage the NTP service:
# /etc/rc.d/rc.ntpd start|stop|restart|status
Now display the list of servers your machine is actually connected to:
# ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== *panopea.unstabl 213.251.128.249 2 u 30 64 377 56.136 -249.48 80.680 +88-190-17-126.r 145.238.203.14 2 u 29 64 377 77.571 -205.94 94.278 +62.210.255.117 192.93.2.20 2 u 29 64 377 77.097 -249.57 85.641 -ntp.univ-poitie 145.238.203.10 3 u 29 64 377 57.747 -191.58 107.002 LOCAL(0) .LOCL. 10 l 164 64 374 0.000 0.000 0.001
The little *
asterisk preceding one of the above lines means your machine is effectively synchronized with the respective NTP server.
Synchronizing at boot time
If instead of using a daemon, all you want to do is to synchronize time once per boot, it is possible to add the command ntpd -qg &
in the bottom of the file /etc/rc.d/rc.local
. This will synchronize the time once in the background and will exit and it needs /etc/rc.d/rc.ntpd
to not be an executable.
~# cat /etc/rc.d/rc.local #!/bin/sh # # /etc/rc.d/rc.local: Local system initialization script. # # Put any local startup commands in here. Also, if you have # anything that needs to be run at shutdown time you can # make an /etc/rc.d/rc.local_shutdown script and put those # commands in there. . . . ntpd -qg &
Synchronize your client PC(s) with your local NTP server
In a LAN, it is considered good practice to synchronize only one machine - the server - with a public NTP server, and the client PCs with the local server. This saves bandwidth and takes some load off the public NTP servers.
As above, proceed to an initial adjustment of the system clock:
# ntpdate pool.ntp.org
Create an empty logfile:
# touch /var/log/ntp.log
Now configure NTP to synchronize with the LAN server. Replace the example's IP (192.168.2.1
) with your real server's IP:
# /etc/ntp.conf driftfile /etc/ntp/drift logfile /var/log/ntp.log server 192.168.2.1 server 127.127.1.0 fudge 127.127.1.0 stratum 10 restrict default ignore restrict 127.0.0.1 mask 255.0.0.0 restrict 192.168.2.1 mask 255.255.255.255
- The three
restrict
statements mean we're blocking all NTP traffic except for the client itself and the server.
Activate and start the NTP service:
# chmod 0755 /etc/rc.d/rc.ntpd # /etc/rc.d/rc.ntpd start
As above, use the ntpq -p
command to check if the synchronization went well:
# ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== *192.168.2.1 81.19.16.225 3 u 916 1024 377 0.367 7.897 2.552 LOCAL(0) .LOCL. 10 l 10h 64 0 0.000 0.000 0.000
Firewall considerations
The NTP services uses UDP port 123. Open this port if you want to allow remote machines to connect to your NTP server.
Sources
- Originally written by Niki Kovacs