While the RHEL kernel accepts a leapsecond for any month, ntpd in RHEL does not hand over the leapsecond in all cases. The ntp version ntp-4.2.4p8 sets the kernel leap bit only on end of June or December. The leap information will be forwarded to the clients on any day though. ntp Versions 4.2.6 and later set the kernel leap bit on end of any month, but require that a majority of its servers agree on the leap instead of only one as in 4.2.4 and before.

NTP servers can issue a leap second at the end of any month.

This page is a simple description of testing the leap-second implementation in NTP

I’ll list all the necessary steps of setting up a test server with fake time for the following tests, all the commands are based on Ubuntu 12.04 LTS system and may differ from other distributions and/or systems.

A test server can be simply simulated by setting up a NTP server and a file from NIST that describes all leap seconds. I use a pre-build ntpd from Ubuntu’s repo to simplify the commands, if a ntp-dev is needed, you have to compile it by own.

The test I write for leap second is very simple, a 50-times loop is executed and check current system time every 100ms (using nanosleep() to set the time step), then out put the data of all the 5 seconds to a file.