xntp is preset to use the local computer clock as a time reference. Using the (BIOS) clock, however, only serves as a fallback for the case that no time source of greater precision is available. YaST facilitates the configuration of an NTP client. For a system that is not running a firewall, use either the quick or advanced configuration. For a firewall-protected system, the advanced configuration can open the required ports in SuSEfirewall2.

32.1.1. Quick NTP Client Configuration¶

The quick NTP client configuration (Network Services+NTP Configuration) consists of two dialogs. Set the start mode of xntpd and the server to query in the first dialog. To start xntpd automatically when the system is booted, click During Boot. Then specify the NTP Server Configuration. Either click Use Random Servers from pool.ntp.org if you cannot use a local time server or click Select to access a second dialog in which to select a suitable time server for your network.

Figure 32.1. YaST: Configuring an NTP Client¶

In the detailed server selection dialog, determine whether to implement time synchronization using a time server from your local network (Local NTP Server) or an Internet-based time server that takes care of your time zone (Public NTP Server). For a local time server, click Lookup to start an SLP query for available time servers in your network. Select the most suitable time server from the list of search results and exit the dialog with OK. For a public time server, select your country (time zone) and a suitable server from the list under Public NTP Server then exit the dialog with OK. In the main dialog, test the availability of the selected server with Test and quit the dialog with Finish.

32.1.2. Advanced NTP Client Configuration¶

The advanced configuration of an NTP client can be accessed under Advanced Configuration from the main dialog of the NTP Configuration module, shown in Figure 32.1, “YaST: Configuring an NTP Client”, after selecting the start-up mode as described in the quick configuration.

Figure 32.2. YaST: Complex NTP Configuration¶

In Advanced NTP Configuration, determine whether xntpd should be started in a chroot jail. By default, Run NTP Daemon in Chroot Jail is activated. This increases the security in the event of an attack over xntpd, because it prevents the attacker from compromising the entire system. Configure NTP Daemon via DHCP sets up the NTP client to get a list of the NTP servers available in your network via DHCP.

Enable Open Port in Firewall if SuSEfirewall is active, which it is by default. If you leave the port closed, it is not possible to establish a connection to the time server.

The servers and other time sources for the client to query are listed in the lower part. Modify this list as needed with Add, Edit, and Delete. Display Log provides the possibility to view the log files of your client.

Click Add to add a new source of time information. In the following dialog, select the type of source with which the time synchronization should be made. The following options are available:

Server

Another dialog enables you to select an NTP server (as described in Section 32.1.1, “Quick NTP Client Configuration”). Activate Use for Initial Synchronization to trigger the synchronization of the time information between the server and the client when the system is booted. Options allows you to specify additional options for xntpd. Refer to /usr/share/doc/packages/xntp-doc (part of the xntp-doc package) for detailed information.

Peer

A peer is a machine to which a symmetric relationship is established: it acts both as a time server and as a client. To use a peer in the same network instead of a server, enter the address of the system. The rest of the dialog is identical to the Server dialog.

Radio Clock

To use a radio clock in your system for the time synchronization, enter the clock type, unit number, device name, and other options in this dialog. Click Driver Calibration to fine-tune the driver. Detailed information about the operation of a local radio clock is available in /usr/share/doc/packages/xntp-doc/refclock.html.

Outgoing Broadcast

Time information and queries can also be transmitted by broadcast in the network. In this dialog, enter the address to which such broadcasts should be sent. Do not activate broadcasting unless you have a reliable time source like a radio controlled clock.

Incoming Broadcast

If you want your client to receive its information via broadcast, enter the address from which the respective packets should be accepted in this fields.

The easiest way to use a time server in the network is to set server parameters. For example, if a time server called ntp.example.com is reachable from the network, add its name to the file /etc/ntp.conf by adding the following line:

server ntp.example.com

To add more time servers, insert additional lines with the keyword server. After initializing xntpd with the command rcntpd start, it takes about one hour until the time is stabilized and the drift file for correcting the local computer clock is created. With the drift file, the systematic error of the hardware clock can be computed as soon as the computer is powered on. The correction is used immediately, resulting in a higher stability of the system time.

There are two possible ways to use the NTP mechanism as a client: First, the client can query the time from a known server in regular intervals. With many clients, this approach can cause a high load on the server. Second, the client can wait for NTP broadcasts sent out by broadcast time servers in the network. This approach has the disadvantage that the quality of the server is unknown and a server sending out wrong information can cause severe problems.

If the time is obtained via broadcast, you do not need the server name. In this case, enter the line broadcastclient in the configuration file /etc/ntp.conf. To use one or more known time servers exclusively, enter their names in the line starting with servers.

The software package xntp contains drivers for connecting local reference clocks. A list of supported clocks is available in the xntp-doc package in the file /usr/share/doc/packages/xntp-doc/refclock.html. Every driver is associated with a number. In xntp, the actual configuration takes place by means of pseudo IP addresses. The clocks are entered in the file /etc/ntp.conf as though they existed in the network. For this purpose, they are assigned special IP addresses in the form 127.127.t.u. Here, t stands for the type of the clock and determines which driver is used and u for the unit, which determines the interface used.

Normally, the individual drivers have special parameters that describe configuration details. The file /usr/share/doc/packages/xntp-doc/drivers/driverNN.html (where NN is the number of the driver) provides information about the particular type of clock. For example, the “type 8” clock (radio clock over serial interface) requires an additional mode that specifies the clock more precisely. The Conrad DCF77 receiver module, for example, has mode 5. To use this clock as a preferred reference, specify the keyword prefer. The complete server line for a Conrad DCF77 receiver module would be:

server 127.127.8.0 mode 5 prefer

Other clocks follow the same pattern. Following the installation of the xntp-doc package, the documentation for xntp is available in the directory /usr/share/doc/packages/xntp-doc. The file /usr/share/doc/packages/xntp-doc/refclock.html provides links to the driver pages describing the driver parameters.