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Lock to a DHCP Server

By noelamo ·
I need some help in locking a workstation to a specific DHCP server througgh a registry hack or any software.

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by p.j.hutchison In reply to Lock to a DHCP Server

A subnet should only be allowed to a specific DHCP by settings on the Router to allow DHCP traffic over port 67/68. Two DHCPs should NOT server the same subnet. Looks like network need configuring properly.

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by noelamo In reply to

There are 20 Vlans where the DHCP server is located. What I am trying to achive is to restrict laptop users from pluging in any other network, so that the Windows will always look for the same DHCP Server that I want it locked into

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by noelamo In reply to Lock to a DHCP Server

Let's say
VLAN1 has DHCP_A Server
VLAN2 has DHCP_B Server

Laptop1 on VLAN1 will get an IP Address from DHCP_A Server, but when Laptop1 goes to VLAN2, I want Laptop1 to try to get it's address from DHCP_A instead of DHCP_B, I know LAptop1 can't go anywhere or get any IP Adress.

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by noelamo In reply to Lock to a DHCP Server

Basically I only want Laptop1's network to work only in VLAN1

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by naterice In reply to Lock to a DHCP Server

The long and short of it is that this is not going to be possible. DHCP was simply not designed to operate in this way. I?ll start by explaining a little bit about how DHCP works.

When you connect your client to the network and request a DHCP assigned address you broadcast your desire to all computers on the network. This is a network broadcast not simply talking to one specific server (hence no setting for specific server). When a DHCP server receives your request it responds according to it?s configuration with the information that the client needs to connect to the network.

If there were more than one DHCP server on the network it would cause erratic behavior on the client side. You would never know which DHCP information you would end up with because the client would receive the information from the server that responded first.

To avoid broadcasting to all computers on the network you would have to filter the broadcasts at the router, or simply subnet your VLANs so that none of them could talk to each other.

I am pretty sure that you want is to configure your clients statically. That way when connecting to a foreign network, the configuration wouldn?t change and the clients would not request DHCP addresses in the first place (and probably not be able to communicate on the foreign network).

Hope this helps.

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by noelamo In reply to

There is a registry setting in XP for DHCP Server, I need to know if there is a way to lock it.

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by razz2 In reply to Lock to a DHCP Server

Lets clarify one thing here. The others are correct that 2 DHCP
servers on the same subnet COULD cause issues, but only if they
share or overlap the same scope address'.

DHCP is a network service and as such requires redundancy in
any LAN. Without that redundacy, if the only DHCP server died,
the clients needing an address might auto assign a 169.254.y.z
address. Older clients might get nothing. network access would
stop.

While Microsoft has been contridictory on their examples, the
standard is to have two DHCP servers, each with unique scopes
that come from the same subnet. An example would be that:

DHCP A: 192.168.1.100 --> 192.168.1.150
DHCP B: 192.168.1.151 --> 192.168.1.200

With that said, the answer is still that DHCP requests are
answered by any available DHCP server that receives the
broadcasted request. If a client gets more than 1 offer it chooses
the one it feels is best. if more than 1 seem best, it chooses the
first response it got from those.

You could use some form of sub-domains. A client will always
try the Authorized DHCP in it's logon domain first as I recall.

Good Luck,

razz

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by noelamo In reply to

These are 2 DHCP servers that they are not physically connected when I mentioned VLAN1 and VLAN2 I wanted to make the distinction but those two vlans are not physically connected. It is like you want the laptop to grab IP Address only when the guy is in the office and he cannot grab an address if plugs the Laptop to an internet cafe.

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by george.kennedy In reply to

Guys - now're gettin somewhere...
Quote "A client will always
try the Authorized DHCP in it's logon domain first as I recall."
Brief explanation - I work in the IT dept of an organisation with 3,300 PCs over 40 locations. Someone brought their D-Link router in to work & plugged it in, to try to connect to broadband via the phone line at lunch times.
The result was that every time a client needed a new / or renew an IP address, it got the address from the D-Link router & then fell off the network. Gradually PCs disappeared..... It took ages to find out why - ( the fact that it was hidden in the cieling didn't help !!).
We need to stop this happening again & an IP address fron the DHCP server in the logon domain would suit us well.
Anyone know any more about this ?
How to get a client to only accept IP addresses from a DHCP server in the logon domain?
Rgds.
Geo...

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by george.kennedy In reply to

Guys - now're gettin somewhere...
Quote "A client will always
try the Authorized DHCP in it's logon domain first as I recall."
Brief explanation - I work in the IT dept of an organisation with 3,300 PCs over 40 locations. Someone brought their D-Link router in to work & plugged it in, to try to connect to broadband via the phone line at lunch times.
The result was that every time a client needed a new / or renew an IP address, it got the address from the D-Link router & then fell off the network. Gradually PCs disappeared..... It took ages to find out why - ( the fact that it was hidden in the cieling didn't help !!).
We need to stop this happening again & an IP address fron the DHCP server in the logon domain would suit us well.
Anyone know any more about this ?
How to get a client to only accept IP addresses from a DHCP server in the logon domain?
Rgds.
Geo...

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