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/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.