[ale] DHCP and M$

James Kinney jkinney at localnetsolutions.com
Wed Feb 25 13:09:36 EST 2004

> I went and plumbed the MS knowledgebase and reread the RFC (2131). Its
> been a *LONG* time since I had to admin a DHCP server. Here's what I
> found:
>  From MS: If the client has previously had a DHCP assigned IP address
> and it is restarted, the client will specifically request the previously
> leased IP address in a special DHCPREQUEST packet. The Source address is
> and the Destination is the broadcast address
> Microsoft clients will populate the DHCP Option Field DHCP: Requested
> Address with the previously assigned address. Strictly RFC compliant
> clients will populate the CIADDR Field with the address requested. The
> Microsoft DHCP server will accept either.
> So, the client should be asking again for a new address at boot time,
> and the DHCP server on the other end should be looking at it and saying
> "well, that address isn't valid now, so please ask for a new one." If
> the server decides that that address is ok, it can either acknowledge
> the request, or just clam up.
> So, even if the client had a lease that wasn't due to expire, the act of
> rebooting should cause a new request. I suspect that it is, but the DHCP
> server on the other end is either acknowledging the reqest, or not
> saying anything, so the windows client is going ahead and using the old
> one. This would indicate a misconfiguration of the DHCP server (not
> necessarily the one giving out the original request with the very long
> leases, but the one not telling the client when it asks that the address
> it has isn't a good one anymore). Maybe the DHCP server has some bad LAN
> definitions or subnet masks, etc. somewhere?
> of course, if you don't reboot, all bets are off.
> One suggestion, if the DHCP server can't be fixed, is to put the
> ipconfig /releasee and ipconfig /renew commands in a .bat file that the
> users can run. Label it "double click me to connect to network" or
> something.
> Mike
The .bat file was a solution until I ran into people that could not be
given super-user status on the laptop.  The secondary DHCP server is a
Linux box not under my jurisdiction. And both networks are
which doesn't help. The big problem is that the gateway is different for
each network.

More digging...

> James Kinney wrote:
>> Sorry to ask this here as it is truly a M$ problem.
>> WinXP seems to aggressively cache DHCP client settings. So much so that
>> plugging in a laptop into another network and rebooting will not reset
>> the
>> ip address. It will keep the original one unless ipconfig /release,
>> ipconfig /renew are run manually.
>> The problem: User must be a power user or higher to run those commands.
>> _THESE_ users should never have power user status
>> Some setup details: The DHCP server that passes the address that won't
>> go
>> away is set to have lease times of days. For the desktop machines, this
>> is
>> OK. For the in-and-out laptops, this is a mess. Could the extra long
>> lease
>> time be adding to the misery?
>> The more I use M$ servers the more I like pencil and paper.
> --
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> Mike Murphy
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James P. Kinney III
CEO & Director of Engineering
Local Net Solutions,LLC


GPG ID: 829C6CA7 James P. Kinney III (M.S. Physics)
<jkinney at localnetsolutions.com>
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