how to do client seperation using Vendor ID in ISC DHCP??????

Simon Hobson simon at
Mon Dec 26 16:16:33 UTC 2011

<ameen.shajahan at> wrote:

>Let us assume that there would be 2 servers running , so 2 clients 
>  should connect to the server.
>I am using ISC dhcp-4.2.3 code.
>  Linux server --------------->switch                        PC1
>running two process
>  i:e dhcpd 
>PC1 as client 1
>PC2 as client 2
>in switch , two ports are used to connect PC1 and PC2
>these two ports I have configured as vlans 1 and 2.  (access port)
>Vlan 1 for PC1 connection.
>Vlan 2 for PC2 connection

Taking into account some of your other requests, it might be better 
if you just described what you are trying to achieve. Each request 
brings in something different, and it's hard to work out what you 
have set up, what you are trying to do, and whether you are making 
other mistakes that are making it hard for your to achieve what you 

I'm concerned that perhaps you are missing some of the fundamental 
networking knowledge, and hence don't properly understand what you 
are trying to configure. But we can't help with that unless we can 
understand where the problem may lie.

Take this latest one, which follows on from your question about 
handling two different subnets.
You describe the PCs are being on different VLANs - and BTW, your 
attempt at a diagram is no help as it's been completely mangled by 
the time I see it.

If the PCs are on different VLANs, then there is no difficulty at all 
separating them because they are physically separated - conceptually 
it's like having them on separate switches. Thus you can trunk the 
VLANs into the server (in which case they appear as separate 
interfaces and can (for most purposes) be treated in the same way 
you'd treat two separate NICs). Or you can use a router to route one 
or both of the client subnets to the server, and thus use a relay 
agent to handle DHCP for those clients not directly attached to the 

SO, lets assume we have PCs on VLANs 10 and 20, and you want to 
service these as (say) and If you 
trunk these VLANs to the server, then (on Linux) you'd have :

eth0.10 as (say)
eth0.20 as

You could run DHCP on just one of these - ie "dhcpd <options> eth0.10"
or you could run two different instances of DHCP - ie :
   dhcpd <options> eth0.10
   dhcpd <options> eth0.20
But note that they must use different leases and config files.

Or you can run dhcpd on both of them with one instance - ie
   dhcpd <options> eth0.10 eth0.20
or if there are no interfaces you don't want to service, just :
   dhcpd <options>

Or, the server may be connected to one VLAN (it could be any, it 
doesn't have to be 10 or 20), and run a relay agent to service the 
clients. Eg, the server could be in VLAN 1 connected to the switch 
with a non trunk port (eg eth0 on There would be one or 
more relay agents connected to VLANs 10 and 20.
The server would then listen on the one NIC, and automagically handle 
the different subnets. Ie your config file just needs to be :

<global options>
subnet ... {
# Note - no range as we assume no clients here

subnet ... {
   <subnet options>
   range ;
subnet ... {
   <subnet options>
   range ;

Simon Hobson

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