dhcpinform or dhcprequest

Glenn Satchell Glenn.Satchell at uniq.com.au
Wed Sep 2 23:10:10 UTC 2009

>Date: Wed, 02 Sep 2009 13:25:15 -0300
>From: Shawn Holland <sholland at sandara.ca>
>Subject: Re: dhcpinform or dhcprequest
>To: Users of ISC DHCP <dhcp-users at lists.isc.org>
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>On Wed, 2009-09-02 at 17:22 +0200, Tom Schmitt wrote:
>> -------- Original-Nachricht --------
>> > Datum: Wed, 2 Sep 2009 13:39:44 +0100
>> > Von: Simon Hobson <dhcp1 at thehobsons.co.uk>
>> > An: Users of ISC DHCP <dhcp-users at lists.isc.org>
>> > Betreff: Re: dhcpinform or dhcprequest
>> > >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.
>> > 
>> > Correct
>> > 
>> > >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 
>> > >of a DHCPREQUEST.
>> > >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.
>> Thank you for the good explanation. This is very helpfull.
>> Sadly I have not much hope that the vendor will change anything: The Client 
is a HP workstation with Windows XP and Microsoft is not well known for changing 
their software based on a request from a single person.
>To me it sounds like a broken client as its up to the client to renew
>its lease. The DHCP server can only tell it when the lease expires but
>not when to renew.
>As for troubleshooting the client, for what its worth, you could try
>resetting the IP stack from a command line with:
>netsh int ip reset reset.txt
>This helps a lot of weird IP issues.

Also try this to see what the PC thinks is happenning:

ipconfig /all

Release the current lease and then get a new one:

ipconfig /release

ipconfig /renew

As others have said, XP doesn't normally behave this way, so probably
something is not right.


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