Tag Archives: staticip

a smart[ish] dhcpd

After running into some wacky networking issues at a recent customer engagement, I had a brainstorm about a smart[ish] DHCPd server that could work in conjunction with DNS and static IP assignment to more intelligently fill subnet space.

Here’s the scenario we had:

Lab network space is fairly-heavily populated with static assigned addresses – in a /23 network, ie ~500 available address on the subnet, about 420 addresses were in use.

Not all statically-assigned IPs were registered in DNS.

The in-use addresses were did not leave much contiguous, unused space (little groups of 2 or4 addresses open – not ~80, or even a couple small batches of 20-30 in a row).

DNS was running on a Windows 2012 host.

DHCPd (ISC’s) was setup on an RHEL 5×64 Linux machine.

The problem with using the ISC DHCPd server, as supplied by HPSA, is that while you can configure multiple subnets to hand-out addresses on, you cannot configure multiple ranges on a single subnet. So we were unable to effectively utilize all the little gaps in assigned addresses.

Maybe this is something DNS/DHCP can do from a Windows DC, but I have an idea for how DHCPd could work a little smarter:

  • give a very large range on a given subnet (perhaps all but the gateway and broadcast addresses)
  • before handing an address out, in addition to checking the leases file for if it is free, check against DNS to see if it is in use
  • if an address is in use because it is static, update the leases file with the statically-assigned information as if it were assigned dynamically – but give it an unusually-long lease time (eg 1 month instead of 4 hours)
  • on a periodic basis (perhaps once an hour, day, week – it should be configurable), scan the whole subnet for in-use addresses (via something like nmap and checking against DNS)
    • remove all lease file entries for unused/available IPs
    • update lease file entries for used/unavailable IPs, if not already recorded

This would have the advantage of intelligently filling address gaps on a given subnet, and require less interaction between teams that want/need to be able to use DHCP and those that need/want static addresses.

Or maybe what I’m describing has already been solved, and I just don’t know how to find it.