dissecting DHCP - Q1

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Mar 16 07:55:33 UTC 2009

Defryn, Guy wrote:

>We are reviewing all our infrastructure as part of  DR exercise.

That'll be fun ! Hope your management have a more realistic attitude 
than some of my previous management had.

>DHCP is high on the list. I want to understand how DHCP really works so
>I can advice appropriately. I have set up a lab environment so I can 
>figure out
>how DHCP really works. We have no DHCP gurus here as most of us are 
>so please forgive the stupid questions I might ask.

Then I recommend "The DHCP Handbook" by Ralph Droms and Ted Lemon - 
now in 2nd edition I believe. I think you'll find this answers all 
the questions you might not have thought of yet !

>My first question is regarding the use of the release/renew command
>This is how the lease looks like currently
>    starts 1 2009/0316 01:28:40;
>    ends 1 2009/0316 01:58:40;
>    tstp 1 2009/03/23 01:43:40;
>    binding state active;
>    next binding state expired;
>    hardware Ethernet 00:50:56:97:68:46;
>    uid "\001\000PV\227hF";
>    set ddns-rev-name ="";
>    set ddns-txt ="317dsfre7j19d04808e3cb85cdb99";
>    set ddns-fwd-name = "xp1.mydomain.co.nz";
>    client-hostname "xp1"
>When I do a release/renew my IP changes to the next available one, 
>in this case
>The binding state of previous release gets set to release and next 
>binding state is set to free.
>My question here is whether it is normal that it uses the next 
>available in pool.

That's odd, unless it's Client-ID (uid in the lease database) has 
changed, it should be given the same address. That's standard 
operation for the ISC server, and required by the RFCs.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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