dhcpinform or dhcprequest

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Sep 2 12:39:44 UTC 2009

Tom Schmitt wrote:

>offer, sends a request and finally gets an acknowledge from the DHCP server.
>And, at least with my configuration, the DHCP server is sending an 
>update to the DNS with the name and IP of the client.
>So far no problem.
>But the first lease the client is getting from the DHCP server has 
>only a short leasetime and the client is requiered to renew the 

The initial lease does not have to be short, and that is not the 
default setup for the ISC server.

>Now to the question:
>In my understanding the client should at some point before the 
>leasetime is ended (normally at half the leasetime) send a 
>DHCPREQUEST to renew its lease. Then the DHCP server would send a 
>DHCPACK with a longer leastime.


>But what I see in the logfile of the DHCP server is something else:
>At half of the leasetime the client is sending a DHCPINFORM instead 
>The DHCP server is answering with a DHCPACK but the leasetime stay 
>the same, the lease is not renewed.

DHCP Inform is not for lease renewal - it is separate and is designed 
to allow applications to query the server for options. Internet 
Explorer is a well known example of this, it will do a DHCP Inform to 
find out if it should be using a proxy (amongst other stuff).

>A few minutes later the leasetime is ended and the DHCP server sends 
>an update to the DNS, deleting the entry of the client.
>But the client still uses this IP, only now having trouble because 
>of the missing DNS entry.
>My question:
>Is this really a problem on clientside? (If yes, know anyone a 
>solution for this?)

Yes, it's a broken client - very broken.

Hard to see how much more broken it could be.
  - Attempts to renew using DHCP Inform
  - Fails to renew
  - Continues using address

Solution, ditch the client ! Of course, if the vendor can fix it then 
demand that they do.
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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