Hello Jiri:

This issue popped up on my radar while looking at a dhclient ticket. After digging into it I was able to readily reproduce it which seemed to be the a sticking point prior to this.  All one need to do is configure large lease times and then run clients which do not issue DHCPDISCOVERs but rather go straight to DHCPREQUESTs.

You'll be pleased to learn that this issue will be fixed in 4.4.0 and 4.3.6, the latter should be available late June '17.  Our fix is slightly different the yours but it works well enough.

As always, thanks for your continued efforts and patience.

Sincerely,

Thomas Markwalder
ISC Software Engineering