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Old 05-17-2005, 11:56 AM   #1
Registered: Jan 2005
Location: Cork Ireland
Distribution: Debian
Posts: 384

Rep: Reputation: 32
Forwarding dhcp requests through a Linux router

Hi !

I've seen something when working for a customer and I've never figured out if it was doable with Linux boxes... that's not really because I need it, but I'm interested in how this could/would work.

So, my customer has got a huge LAN made of Cisco boxes. He has many VLANs, each in a different subnet. They are connected through layer 3 switches (4 catalysts 6000 routing through OSPF as a matter of fact).

It's time to use sample config.... let's say that :
VLAN1 is, that's for the PC
VLAN2 is, that's for the servers

There's only one dhcp server :

When a PC requests for an IP, the layer 3 switch that receives the request forwards it to the dhcp server (because the dhcp server's IP is configured in the switch) and uses the IP of the interface that received the request as a source IP for the forwarded dhcp request.

The dhcp server receives the request, sees the source IP and thus knows in what subnet it must select the IP to send back. So he replies to the layer 3 switch which in turn forwards the dhcp response to the PC.

I know how to set this up on Cisco routers, that's a single command line :
ip helper-address

What I wonder is :
1/ How could we do this with a Linux router ?
2/ How could we configure a Linux dhcp server to work like this ? Do you think that configuring all subnets on the dhcpd.conf file would be enough (maybe... as a matter of fact, there's no specification in the Cisco configuraiton guides on what dhcp server must be used, so it may be a standard) ?

Any comment or idea will be appreciated...

Last edited by fr_laz; 05-17-2005 at 11:58 AM.
Old 05-19-2005, 09:14 AM   #2
Registered: Jan 2005
Location: Cork Ireland
Distribution: Debian
Posts: 384

Original Poster
Rep: Reputation: 32

ok, I found... that's the dhcrelay command...


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