randomizing lease renewal?

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Mar 29 19:16:07 UTC 2007

Jeff A. Earickson wrote:

>We are running ISC DHCP 3.0.5.  A question...  Is it possible
>to force a client to obtain a *different* IP number from an
>IP pool upon lease renewal, instead of getting back the same IP
>for reuse?  Example: the pool is and the client has
>IP number leased from the pool.  It goes to renew the
>lease.  I want to make sure that it gets a *new* number upon
>renewal, and does not get back.  Can this be done?

Two questions :

1) Have you looked in the archives where you'll find the same 
question flamed a few times.

2) Why ? In the years I've been on this list we've only ever been 
given one sensible reason for such a function, but normally it's a 
variation on "we want to make it hard for people to run servers".

With the ISC server it is not possible, it is specifically against 
the rfc and is deeply ingrained in the code I believe.

As to preventing people running servers, well it doesn't work - 
dynamic dns service allow people to update their dns as fast as you 
can change their IP address. Also, for reasons I can't be bothered to 
either retype or find,copy, and paste, it is really, really bad for 
the internet.

However, you aren't clear whether this is simply for one client for a 
once-only change, or a general thing. Giving you the benefit of the 
doubt, if you need to change a clients address as a once-only thing 
(as part of address reorganisation for example) then you can simply 
remove it from any pools and it will not be renewed. If you create a 
new pool with that address and explicitly "deny booting" then the 
server will NAK it as soon as a client attempts to renew it instead 
is simply ignoring the requests.

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