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DHCP client / server question

Posted on 2004-04-09
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Last Modified: 2011-10-03
Hi all,
I have two DHCP servers -  A and B , both running and serving clients. Address ratio between them is 80/20 ( as MS suggests). Both are on the same subnet and no routers /switches are between either one of them and the clients they serve (see diagram below).
I want a particular client to receive its IP configuration from server B and not server A. I even created a reservation for the client on server B to make sure that this machine always gets the IP address from this server and the address is every time the same. So far so good. But...
When I restart/renew this machine's IPCONFIG-uration,  server A is the one that responds first to the request and assigns an IP address from its scope. In order to get the address lease from server B I need to stop the DHCP server at server A and then server B without its "competitor" does the job and assigns the reserved IP address.

My question:
Is there a way to "force" a registration so a particular client to get its IP configuration from a particular DHCP server without the need to stop and start the running services?

                                                           A        B                                                    clients...........................................
                                                          ----      ----                                              ---     ---   ---   ---   ---
                                                          -  -      -   -                                               -        -     -      -      -
                                                          -  -      -   -                                              ---     ---   ---   ---   ---
                                                          ----      ----
                                                           |          |                                                 |        |     |     |     |
                                                           |          |                                                 |        |     |     |     |
                                                           \          \                                                 /        /     /      /     /
 out<---------+ router/ fwall +------------------------------------------------------------------------------------------------------------


Thanks for your time.
                           
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Question by:RICuser
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13 Comments
 
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by:parkerig
ID: 10794604
Hi,

On the below link http://www.microsoft.com/technet/prodtechnol/winntas/support/sur_dhcp.mspx
it is suggested that you can only have one scope per subnet. If this is the case then both DHCP servers theoritically need to be the same. However reading on mentions the below key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\{9F50FA55-7C4E-4DD3-B62A-A6EB1878BE08}\Parameters\Tcpip

Where the {9f5......} is the NIC. Suggest got to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ and then search for DHCP.

Inside here is the IP address of the DHCP server used to resolve address  and some other gems like time outs etc. Try changing the parameters here to DHCP server B

Ian
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by:Gareth Gudger
ID: 10795232
Create the same reservation on both machines even if the reservation does not fall within the scope on one.
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by:The--Captain
ID: 10795859
Why are you running two DHCP servers to handle such a small network?

>It would be a hard task for me to explain and justify "going back static" before the management

There's no reason it has to be all or nothing...  I assume server A and B have static addresses.  What's wrong with one more static machine?  I guess what I'm saying is, when you want a machine to have a specific IP all the time (like server A and B), you generally configure them with a static address.  This doesn't mean every other workstation has to have a static address.

May I ask why you need this particular client to always have the same IP...?

Cheers,
-Jon

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by:parkerig
ID: 10795886
We use two DHCP servers so if one is unavailable the other can offer an IP address.
I do however wonder why they are set up differently ( assuming this ) as RICuser specifically wants some machine to use DHCP from machine B

All and all I don't think you can choose as DHCP is a broadcast / receive type protocol
The microsoft link above suggests this.
Anyway from a theory point of view its an interesting question.

One way of doing it is to use Zone Alarm ( or similar software firewall ) and block broadcasts from server A

Firewall Downloads
http://downloads-zdnet.com.com/3120-20-0.html?qt=firewall&tg=dl-2001

I personally use Zone Alarm, Sygate is also very good
Cheers
Ian
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by:The--Captain
The--Captain earned 100 total points
ID: 10796277
>We use two DHCP servers so if one is unavailable the other can offer an IP address

Yeah, be he has it split 20/80...  If more than 20% of the address space is consumed, and the 80% server goes down, he's screwed.

>I do however wonder why they are set up differently

Indeed - I think we're on the same page.

I'm no MS networking guru by any means, but I always thought that this sort of failover was what a PDC/BDC setup was for...  Is DHCP not a component of PDC/BDC replication/failover...?

>One way of doing it is to use Zone Alarm ( or similar software firewall ) and block broadcasts from server A

Am I incorrect in thinking that the client originates a broadcast request, and not the server?  Either way, a decent personal firewall should provide a workaround (although I'm still wondering why this machine is not simply classified as "static" and addressed accordingly [like the servers presumably are]).

Cheers,
-Jon
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by:Gareth Gudger
Gareth Gudger earned 100 total points
ID: 10797572
>> We use two DHCP servers so if one is unavailable the other can offer an IP address

> Yeah, be he has it split 20/80...  If more than 20% of the address space is consumed, and the 80% server goes down, he's screwed.

Those are the Microsoft recommendeations Captain....it was set up by the book.

>>  do however wonder why they are set up differently

> I'm no MS networking guru by any means, but I always thought that this sort of failover was what a PDC/BDC setup was for...  Is DHCP not a component of PDC/BDC replication/failover...?

No DHCP could be on a member server. No DC functions assist with DHCP at all. The redundancy element of DHCP is to run two computer which split your addresses or cluster it. And how many people cluster a DHCP server.... P&G, General Electric, :)

>> One way of doing it is to use Zone Alarm ( or similar software firewall ) and block broadcasts from server A

> Am I incorrect in thinking that the client originates a broadcast request, and not the server?  Either way, a decent personal firewall should provide a workaround (although I'm still wondering why this machine is not simply classified as "static" and addressed accordingly [like the servers presumably are]).

Yea I would agree. I would go static any day before using that piece of crap on my network. You should be able to create the same reservation on both servers though. It doesn't matter if they are identical unlike the lease pools.
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by:parkerig
ID: 10798249
Hi again,
With the firewall you can block the broadcast from the client ( as you have mentioned ) and also block the reply from the server A.

Cheers
Ian
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parkerig earned 100 total points
ID: 10798433
RICuser,
I have spent some more time on this and just discovered that the DHCP request appears to ask the server that is first in the DNS order. Hence if you change the order for DNS you should be able to set which machine uses which DHCP server.

Ian
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Author Comment

by:RICuser
ID: 10802444
Thaks Ian, I will rty that workaround and will post with th result.
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by:RICuser
ID: 10802466
The --Captain

> Yeah, be he has it split 20/80...  If more than 20% of the address space is consumed, and the 80% server goes down, he's screwed.


That's correct Jon, I agree.But that is what MS suggests.I always wonder how server B will accomodate 80% of the clients
if server A went down? Thanks god , haven't had that  problem ( so far) ! This to me isn't real redundancy and besides the complicated DHCP clustering I can't think of a better and simpler solution.


Thanks for taking the time Jon.
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Author Comment

by:RICuser
ID: 10802577

As per why I use 2 DHCPs  with the same options , I am just folowing MS's guidelines :

From Microsoft's help:

Partitioning the Address Pool

You will probably decide to install more than one DHCP server, so that the failure of any individual server will not prevent DHCP clients from starting. However, DHCP does not provide a way for DHCP servers to cooperate in ensuring that assigned addresses are unique. Therefore, you must divide the available address pool among the DHCP servers to prevent duplicate address assignment...

and later :

Using more than one DHCP server on the same subnet provides increased fault tolerance for servicing DHCP clients located on it. With two DHCP servers, if one server is unavailable, the other server can take its place and continue to lease new addresses or renew existing clients.

A common practice when balancing a single network and scope range of addresses between two DHCP servers is to have 80 percent of the addresses distributed by one DHCP server and the remaining 20 percent provided by a second.

Don't tell me MS is wrong ;-))


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by:Pascal666
Pascal666 earned 100 total points
ID: 10804689
80/20 works because the backup server only needs to assign addresses to those boxes whose lease expires while the primary is offline.  Most of your boxes leases won't expire in the time it takes you to fix your primary server (in theory) so you don't have a problem.

-Pascal
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Expert Comment

by:sleepydeez
ID: 10810048
Statically addressing that computer sounds like the easiest way to solve that problem... unless there is a different reason for needing the address to come from a certain DHCP server....  Also you could set up a user class for the desktop in question and put that in the scope but then again, you could accomplish the same thing with a reservation (in this case)...

sleepy
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