24.3. Setting Up a Local Reference Clock

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/html/refclock.htm. Every driver is associated with a number. In xntp, the actual configuration takes place by means of pseudo IPs. 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 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/html/driverNN.htm (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/html. The file /usr/share/doc/packages/xntp-doc/html/refclock.htm provides links to the driver pages describing the driver parameters.