How can I setup dhcpd to renew lease with a diffrent Ip each time...

Simon Hobson dhcp1 at
Thu Jul 6 15:56:34 UTC 2006

David W. Hankins wrote:
>On Thu, Jul 06, 2006 at 05:07:32PM +0200, Sébastien CRAMATTE wrote:
>>  I'm working for a cable operator and I would like to know how can I
>>  configure
>>  the dhcpd to renew lease with a new IP each time...
>There's no way to do this with the ISC DHCP software distribution as
>we release it.
>>  Currently my CPE keep always the same IP and this is a problem because
>>  they have nearly a fixed address ....
>'Nearly' is a more important distinction to me than you seem to
>Even if you gave out a random IP address, what makes you think your
>customers won't use dynamic DNS to locate their systems?
>Randomizing IP address allocations doesn't win you anything, so all
>it does is help things break more often and give your customers poor
>I strongly advise against this practice.

Always the diplomat !

If Sébastien cares to look back through the 
archives, he will find that not everyone is as 
diplomatic as that. I'd avoid shooting the 
messenger, since I'm sure it's higher management 
that have 'invented' this 'new' technique and 
demanded it's implementation.

In short, forcing address changes on customers WILL break things.

For example, the download that's been going for 
two hours and will complete soon - address 
changes, connection broken. So what will the 
customer do ? Try again ! So unless they (or the 
software they use) is capable of restarting the 
transfer, then they download all that data again 
- it could be 4GB of a 4.7GB DVD (legal) 
download. That's bad for you, it's bad for the 
server at the other end, and it's bad for the 

Of course, this will be worst for those customers 
who aren't technically literate and least likely 
to know about 'reget' and such things. They will 
simply get to know that their downloads break 
every so often while their friends on other ISPs 
have no problem.

It will not however stop people using a dynamic 
dns service to host stuff - such systems were 
specifically made to work around 
stup^H^H^H^Hmisguided ISPs who think dynamic 
addresses are a good idea.

Then of course it means you have more record 
keeping to keep track of who had what address & 
when - for dealing with complaints.

It also makes the problem worse where someone 
gets their email blocked because another customer 
used to have that address and was being used 
(without their knowledge most likely) to spew out 

If you MUST stop people running servers etc, then 
the correct way to do it is by filtering the 
connections. If your only reason is to control 
upstream bandwidth then apply traffic shaping. If 
you attempt to do either by address hopping your 
clients then it WILL fail, but in doing so you 
find yourselves derided by technical users (who 
will know what you are doing and how to work 
around it), and grumbled at by non-technical 
users (who will simply get to know your service 
as 'unreliable').

I strongly recommend going back to management and 
explaining that it won't work - it will just get 
you a bad reputation. Unless you have a monopoly 
then the result is that you'll lose customers to 
other ISPs who don't run a broken network.

It's actually in your own interests to make 
clients have as static an address as possible 
(best still is static assignments) - it's going 
to be much easier to monitor and control that way.


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