[ale] DHCP and M$

Mike Murphy mike at tyderia.net
Wed Feb 25 12:46:03 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 


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.


Mike Murphy
781 Inman Mews Drive Atlanta GA 30307
Landline: 404-653-1070
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Email: mike at tyderia.net
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JDAM: 33:45:14.0584N  84:21:43.038W

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