How can I configure a DHCP server to assign addresses based on the OS that is running
Marc Chamberlin
marc at marcchamberlin.com
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 192.168.2.15 regardless of whether it is
connected wirelessly or wired. If the laptop is running Linux I want to
give the laptop the address of 192.168.2.10, 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;
fixed-address 192.168.2.15;
}
host laptop_Vista_ethernet {
hardware ethernet 00:1B:24:3C:88:3E;
fixed-address 192.168.2.15;
}
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";
fixed-address 192.168.2.10;
}
host laptop_Vista_wireless {
option dhcp-client-identifier "\001\000\032sU}\017";
fixed-address 192.168.2.15;
}
host laptop_Vista_ethernet {
option dhcp-client-identifier "\001\000\0266\302e\244";
fixed-address 192.168.2.15;
}
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..
Marc..
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