[Dnsmasq-discuss] DHCP leases in multiple VLANs

Simon Kelley simon at thekelleys.org.uk
Wed Sep 8 22:08:49 BST 2010

Ferenc Wagner wrote:

>> DHCP clients should maybe do this anyway: it's dodgy to have the same
>> MAC address in more than one broadcast domain at the same time.
> Could you please explain why that is dodgy?

If there can be only a single lease for a unique MAC address then  a
single host can't create lots of leases and use up all the available
adresses. Relaxing this to allow only one lease per subnet requires that
the DHCP server have an accurate view of whether two addresses are in
the same subnet.

>> It may be possible to relax dnsmasq a bit: really only the IP address
>> needs to be unique in the lease database, but I'm not quite sure there
>> wouldn't be obscure consequences.
> Maybe if several subnets are used in one VLAN...  But as far as I can
> see, ISC DHCPD does something like this (the lease file is keyed by IP
> address only) and it seems to fare quite well.  Please consider enabling
> this in dnsmasq, too (although its lack isn't a show-stopper for me
> right now).

I think it's possible, but it's one of those changes that makes me
nervous: it feels like there's the possibility of breaking lots of
current installations.



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