Simon Hobson dhcp at
Mon May 1 17:05:43 UTC 2006

Mike Diggins wrote:

>I am considering enabling the 'one-lease-per-client' option in DHCPD for
>my student network. They are mostly dedicated to a single network jack and
>subnet, but laptop users may roam around. I read the warnings from the man
>pages. I assume the concern is that when a client "roams" to another
>subnet, the DHCP server will automatically release his old address, even
>though the client does not. When he returns to his room, he might continue
>using the old IP address which could very well be leased to someone else.
>How much of a concern is this?
>Would you recommend this option for a Student Residence with multiple
>subnets? I want to be sure we're not wasting addresses.

Having a client return and continue to use an address that has been 
'revoked' should not be a problem - when it re-attaches to the 
network it must broadcast for a request so as to be sure that it is 
still connected to the same network. When it does that, the server 
will either NAK the request or renew the lease, either way, the 
client should not be using an address that's since been leased to 
another device.

What should be more of a concern would be devices (such as laptops) 
that have more than one interface - in which case, it could get an 
address on the wired network, sign in on the wireless, and the server 
will silently cancel the lease on the wired network. In this case, 
the client can continue using an address that the server now thinks 
is free - and if it tried to lease it to another client, it will get 
a reply to a ping and mark the address as abandoned.

For something like student halls etc, I would be inclined to simply 
use shortish leases (say a few hours) if you are short of addresses. 
But how big a problem is it ? Are you using public or private 
addressing ? If private, then you should have plenty of addresses to 
go around (there's over 16 million in the range).


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