I would like that my Dhcp server renew lease using different IP

Sébastien CRAMATTE s.cramatte at wanadoo.fr
Fri Jul 6 19:47:38 UTC 2007

Simon Hobson escribió:
> Sébastien CRAMATTE wrote:
>> I've got /22 subnet  (1024Ip's)  ...
>> What happens is that my customers always keep the same IP like if paid
>> for a static one ...
>> More over some clients change their windows property and force the IP as
>> static ...
>> I would like that my Dhcp server renew lease using different IP  and
>> prevent duplicates ... not every day  but at least every week or month ...
>> Does it exits a mean ?
> Not directly as it would be specifically in breach of the RFC.
> You could however force some changes with some 
> external scripting - I think several ideas have 
> been mooted in the past, none of them simple or 
> pretty !
> 1) Have several ranges and turn one of them off 
> for a while in a rolling fashion. Eg
> a = 50-99
> b = 100-149
> c = 150-199
> d = 200-249
> For a while you include all 4 ranges, then for a 
> period at least as long as your max lease you 
> drop range a - this will force all the clients 
> with addresses in that range to get new 
> addresses. You can go back to all four ranges for 
> a while, then drop range b, and so on.
> This does mean that some clients will have their 
> addresses changed id session which will break any 
> connections they have open.
I've got all my ranges into LDAP directory ...
I think that this method could match my need ...
> 2) Have a script 'mop up' any expired leases by 
> creating very short 'dummy' leases. You could do 
> this by running a script that periodically 
> attempts to lease lots of addresses with 
> different client-ids and a very short lease time. 
> Thus any client that allowed it's lease to expire 
> would have it's expired lease 'taken over' by one 
> of these dummy requests - next time it asks for a 
> lease there will be no record of it's previous 
> lease and so it will get a different address.
> This will result in devices getting new addresses 
> only when it allows it's lease to expire (so 
> won't break anything), but won't have any effect 
> on devices that stay permanently connected (or 
> disconnect for less than half the lease time).
We use cable modem that are connected 24Hours a day ... :(
> 3) Much more sophisticated would be to link your 
> access equipment into a script so that when a 
> client disconnects, you can trigger a process 
> that will terminate it's lease and re-allocate 
> the address (for a short term, ideally at least 
> as long as the clients remaining lease time) to a 
> dummy client (as in option 2) so that the client 
> cannot have the same address back.
Quite complicate :(
> None of these are going to be particularly pretty !

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