DHCP "static" assignments

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Aug 9 11:58:35 UTC 2013

Peter Rathlev wrote:
>You could also rely on ISC DHCP following the RFC and (mostly) always
>assigning the same address to a device over time. On the other hand I
>have seen our servers not do this; we have had clients whose lease-time
>was exceeded doing a DISCOVER and receive a new address, even though the
>lease file contained an expired lease for the same MAC address / client
>identifier. If the lease-time is long enough this doesn't happen. We
>never found out exactly why, but this change of addresses is contrary to
>both what the ISC DHCP documentation says will happen and what the RFC
>says should happen if I read them correctly.

Once a lease is expired, it's then eligable for re-use. So if (in this case) a printer is switched off for a long time, it's address may well be allocated to another device before it gets switched back on again. Addresses are re-used on a "least recently used" basis, and whether you have a problem will depend on how long the device is off for, how many free addresses you have*, and your churn rate.

* Watch out for bootp clients and abandoned leases. Both tie up an address "forever" and can deplete your pool of free addresses. IIRC addresses leased to bootp clients are never returned to free, and abaondoned addresses are only recovered when there's nothing else left.

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