Enable dhclient to poll the interface state and send only

Enable dhclient to poll the interface state and send only
requests if the interface has an active link. This is a
great benefit if you often change networks with your laptop
and you do not like to kill/restart dhclient all the time.
Changes are automatically detected and the link is refreshed.

The change allows us to start dhclient in background mode
Enable dhclient to poll the interface state and send only
requests if the interface has an active link. This is a
great benefit if you often change networks with your laptop
and you do not like to kill/restart dhclient all the time.
Changes are automatically detected and the link is refreshed.

The change allows us to start dhclient in background mode
while the network cable is not plugged in.

To control the polling interval, the option -i has been
introduced. It takes seconds as parameter, the minimum is
one second, the default is five seconds.

Polling is done in seconds, not microseconds, because dhclient
does internally work with timeouts in seconds.

This change will be part of the next major ISC-dhcpd release.

Tested by:	bms, imp, and many many others.
Reviewed by:	murray, eivind, dhclient folks