How can I configure a DHCP server to assign addresses based on the OS that is running

Marc Chamberlin marc at
Wed May 19 21:49:19 UTC 2010

I have a need to be able to assign fixed IP addresses to various 
computers based on the operating system that is running, not based on 
MAC addresses... The purpose of this is so that our backup server, which 
automatically backs up files from each system, and can only work with 
static IP addresses, has the ability to change it's behavior based on 
which IP address/OS is used/running, when it connects to a particular 
system. Our systems, are all dual boot,  Linux and Windows (Vista or 
XP). Laptops in particular are a problem because they can be connected 
wirelessly or wired to our network, and it is unpredictable as to which 
way they  will be connected, or what operating system is in use, when 
the backup occurs (usually late at night)....

I am running a dhcp server on a Linux (SuSE11.2) system,  - Internet 
Systems Consortium DHCP Server V3.1.2p1

So here is what I am after -  For a given OS (Windows or Linux) running 
on a client, I want the DHCP server to assign a particular IP address to 
that client, regardless of which interface that client is using - wired 
or wireless. But I want a different IP address assigned to that client 
for each OS. For example, if the laptop is running Windows I want to 
give the laptop the address of regardless of whether it is 
connected wirelessly or wired. If the laptop is running Linux I want to 
give the laptop the address of, again regardless of how it 
is connected to the network.

I understand that I can configure the dhcpd.conf file so as to assign 
the same IP address to a client, for each MAC address that it has, i.e. 
as follows -

   host laptop_Vista_wireless {
     hardware ethernet 00:1A:73:55:7D:0F;

   host laptop_Vista_ethernet {
     hardware ethernet 00:1B:24:3C:88:3E;

But that won't work for Linux since it will be using the same interfaces 
with the same MAC addresses, and I need a different IP addressed assign 
to that system when it is running Linux.

With Linux I can set the dhcp client identifier, but I have never been 
able to figure out how to do so in Windows XP or Vista. I have 
discovered, by monitoring the dhcpd.leases file however, that there is 
indeed a client identifier for each of the network interface cards when 
running under Windows. Unfortunately it is not the same identifier for 
both interfaces, nor is it human readable. But I can cut and paste it so 
I have tried to configure the dhcpd.conf file as follows -

   host laptop_Linux {
     option dhcp-client-identifier "\000Linux-laptop";

   host laptop_Vista_wireless {
     option dhcp-client-identifier "\001\000\032sU}\017";

   host laptop_Vista_ethernet {
     option dhcp-client-identifier "\001\000\0266\302e\244";

And that gets me close but there is still a problem. If the user of the 
laptop switches from using one of the interfaces to the other, the dhcp 
server recognizes that there is still a lease on the IP address that it 
assigned to the initial interface, used by the laptop, and it fails to 
reassign that IP address to the new interface that the user switched to. 
So some default IP address is assigned instead, which results in my 
backup server failing to back up that system.

I realize that there is an interesting question about what happens if a 
user has both interfaces active when he/she connects to a network. I see 
from log files that Linux will try and have the dhcp server assign an IP 
address to each interface when it is activated/initialized. Since 
Windows hides everything, I am not sure what it does or how it 
prioritizes the usage of multiple interfaces. So I would appreciate some 
insight here as well, if anyone really understands the behavior of these 
operating systems.. But what I really want is that I have the ability to 
assign a fixed IP address to whichever interface the OS will decide to 
use.... And if I can assign the same IP address to each interface, then 
it should not matter in the final analysis, since I would guess the OS 
is only going to use one of the interfaces. (At least that is what it 
appears Windows does, not so sure about Linux)

I have tried to read the documentation and man pages, and fooled around 
a bit with the conditional expressions but could never get that to work. 
The documentation is pretty difficult and obtuse to understand so I give 
up and decided to simply ask... So can some kind guru help me out and 
show me how to configure dhcpd.conf so I can achieve what I am after? 
Much appreciate it and many thanks in advance..


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