Query

[Simon]

Soporte VT <soporte at vallecastelecom.com> wrote:

> I m working for a small company that sells the Internet as an ISP.  One of our clients calls every day about its IP address that is always the same. Even though the client is on a dynamic pool, I think, when the router automatically attempts to renew as soon as 50 percent of the lease duration has expired, the DHCP server allocated the same IP.
> 
> Is there a command configuration in which I say change the IP address no matter what? I mean, when the DHCP client tries to renew the IP address.

As others have already said, what you see happening is correct behaviour, the requested behaviour would explicitly break compliance with the RFCs for DHCP behaviour and would be quite hard to do with the ISC server. It would annoy many users when they realise why their connections drop at “random” times, and eventually push users to move to an ISP that doesn’t wilfully break things.

Where ISPs do genuinely provide a dynamic (and changing) IP address, it’s typically done when the router connects to the ISP - at least over here in the UK where PPP over <something> connections are the most common. So as long as the router maintains the DSL signal and PPP session, the IP remains stable - if the DSL connection drops, then the user gets a different IP when a new PPP session is established. That’s handled by the backend serving addresses to the PPP service - typically not DHCP.
The key thing is that the IP only changes when the PPP session is broken (and hence, any IP connections in place would also get broken), it doesn’t change otherwise.

Simon