Kea: Support DHCP lease times per subnet

[bagobones]

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Is your feature request related to a problem? Please describe.

Kea currently only has a global lease time set however it is common to want different lease times by subnet / vlan.

• Servers: long lease times
• workstations medium lease times
• WiFi / WiFi guest networks VERY short lease times to avoid pool starvation.

Describe the solution you like

Kea should already support per subnet lease times that override the global lease time, add an option for lease time to the subnets.

Describe alternatives you considered

Set a middle of the road lease time as a workaround. Don't deploy Kea in serious business environments, stick with the old DHCP client.

Additional context

In larger networks having different lease times is important.. WiFi guest networks is the most common case where you will set a drastically shorter lease time as the pool will run out over the course of a day otherwise.

[CA-DigitalAnvil]

I would like to add my upvote for the original poster's request. Per-pool lease times in a scenario such as described would be incredibly valuable, particularly since that feature was available (and very much used) in the old ISC dhcpd server. Additionally, setting default lease times to something ridiculously small (as needed by the afore mentioned WiFi) and then using 'option-data' to crank the lease time around to something useful for those leases that are not of that flavour does not appear to be supported. An example of an elegant use of classing would be really helpful, if such a critter indeed exists.