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Sincronice su red con NTP

NTP ( Network Time Protocol ) permite la sincronización de reloj entre sistemas informáticos. El siguiente HOWTO describe:

  • configurando un servidor NTP en Slackware Linux;
  • Sincronizar PCs cliente con su servidor NTP local.

Introducción

Cuando varios usuarios manipulan datos compartidos en diferentes PC clientes en una red, es importante que todas estas máquinas estén sincronizadas. Esto es especialmente cierto si comparte archivos a través de NFS, o si usa NIS para la autenticación centralizada. Obtendrás todo tipo de errores extraños si tus relojes no están sincronizados. Desafortunadamente, los relojes a bordo de los clientes no son lo suficientemente precisos.

Ahí es donde NTP ( Network Time Protocol ) es útil. Permite que las máquinas en red ajusten sus relojes para estar perfectamente sincronizados. Una serie de servidores de hora públicos en Internet permiten la recepción de la hora exacta. Desde este punto, podemos usar NTP de varias maneras.

  • El comando ntpdate realiza una corrección inicial del reloj del BIOS.
  • Este ajuste de una sola vez no es suficiente para un servidor que se supone que funciona 24/7, ya que su reloj se alejará gradualmente de la hora exacta. En ese caso, tenemos que configurar el daemon ntpd (envío con el paquete ntp ). Este demonio se pone en contacto con los servidores de hora públicos a intervalos regulares y procede con correcciones incrementales del reloj local.
  • El daemon ntpd puede a su vez configurarse como un servidor de tiempo para las máquinas cliente locales.

Se considera una buena práctica usar ntpdate para el ajuste inicial y ntpd para la sincronización de tiempo regular.

Consideraciones de Firewall

Los servicios NTP utilizan el puerto UDP 123. Abra este puerto si desea permitir que las máquinas remotas se conecten a su servidor NTP.

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 (first restrict statement), and the machine is configured to trust itself (second restrict statement).
  • A restrict statement without any argument but followed by the hostname boils down to an allow all.

Manage the NTP service

Before starting the service, proceed to an initial adjustment of your system clock:

# ntpdate pool.ntp.org
The 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 +x /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.

The first synchronization can take a few minutes, sometimes up to half an hour.

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 +x /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
Again, you might have to wait a few minutes before the first synchronization takes place.

Monitor the performance of ntpd

You will notice that the logfile /var/log/ntp.log does not contain any information about the actual accuracy of your system clock. If it's important to you, you can log the statistics of time corrections applied by NTP daemon to the system clock. To do this, add the following lines to /etc/ntp.conf:

statsdir /var/log/ntp/
statistics loopstats
filegen loopstats file loops type day link enable

You have to create the statsdir manually. Once the configuration changes are in effect, ntpd will create files named loops.YYYYMMDD in that directory. Below is an example line from one of these files:

56690 3950.569 0.001199636 2.297 0.001830770 0.571576 10

The first and second number are the UTC time (expressed as Modified Julian Date and seconds elapsed since midnight). The third and fourth number are the offsets of time (in seconds) and of frequency (in parts per million). The fifth and sixth number are their respective uncertainties.

To monitor the performance of ntpd, you can examine the plot of clock offset or frequency offset vs. time:

$ awk '{printf("%f %f %f\n", $1+$2/86400, $3, $5)}' /var/log/ntp/loops.* > time
$ awk '{printf("%f %f %f\n", $1+$2/86400, $4, $6)}' /var/log/ntp/loops.* > freq
$ gnuplot
gnuplot> set xzeroaxis
gnuplot> plot 'time' with yerror
gnuplot> plot 'freq' with yerror

Given enough data, visual examination of the plots will allow you to see peculiarities in ntpd performance, should they arise. For example, in the case illustrated by the figure below, the rapid decrease of the frequency offset was caused by replacing the power supply unit of the machine.

Sources

  • Originally written by Niki Kovacs
  • Performance monitoring section contributed by Dominik Drobek

 es:howtos:network_services:ntp ()
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