[ale] DHCP and M$

Mike Murphy mike at tyderia.net
Wed Feb 25 13:52:58 EST 2004

ah ha! there is a scenario I didn't really foresee. If both DHCP servers 
are managing the "same" range of addresses (for instance, most broadband 
routers by default are configured to give out and up to 
DHCP clients), the second dhcp server is probably seeing the renewal 
request with the IP address from the first, seeing that that particular 
address is still valid for it, and not requiring the client to get a new 
one, so the client is holding on to old default gateway, DNS, etc. data. 
That's a bummer.

You could modify the DHCP server you have control over to use a 
different range for DHCP address allocations than the other one (one way 
or another--I like using using 2 private netblocks for this: one for 
static addresses, one for DHCP, with a compatible subnet mask so the 2 
blocks are all one subnet). This would have the effect of forcing 
clients to get a new address when they change networks.

As long as the address is valid, and the lease hasn't run out, the new 
DHCP server is right not to force a new address on a client. Its not 
going to know about all the other stuff the client is holding on to, 
like default gateway.

The other alternative that would take the user out of the loop entirely 
might be to have a startup or login BAT that does the release and renew 
for them. That might run into some of the same permissions issues.


James Kinney wrote:
>>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
>> 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
> 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
>>>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
>>>away is set to have lease times of days. For the desktop machines, this
>>>OK. For the in-and-out laptops, this is a mess. Could the extra long
>>>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
>>Mobile: 404-545-6234
>>Email: mike at tyderia.net
>>Email Pager: pagemike at tyderia.net
>>AIM: mmichael453
>>JDAM: 33:45:14.0584N  84:21:43.038W
>>Ale mailing list
>>Ale at ale.org


Mike Murphy
781 Inman Mews Drive Atlanta GA 30307
Landline: 404-653-1070
Mobile: 404-545-6234
Email: mike at tyderia.net
Email Pager: pagemike at tyderia.net
AIM: mmichael453
JDAM: 33:45:14.0584N  84:21:43.038W

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