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List:       dhcp-server
Subject:    Re: how to configure a dhcp server to listens on a subnet that is
From:       Simon Hobson <dhcp () thehobsons ! co ! uk>
Date:       2006-02-22 9:03:17
Message-ID: a06230931c021d67c4e61 () [192 ! 168 ! 0 ! 140]
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Robin Xiao (QD/LJC) wrote:

>   I start a DHCP server with %subnet that is not in the same subnet as
>all of my network interfaces.
>But I find that this DHCP server can not listens on the specified
>%subnet.
>Can anybody tell me how can let DHCP server listens on this %subnet.

Somewhere on the remote subnet you must have a dhcp relay agent (aka 
bootp helper, aka dhcp helper). This is NOT the same as just 
forwarding the broadcast packets, the relay agent captures the 
broadcast packets from the client and inserts it's own address before 
sending it to the dhcp server - the dhcp server uses this address to 
work out the subnet the client is attached to.

The relay agent is usually run on the routers (though it can be in 
any device), how you turn it on is vendor specific. For Cisco routers 
you would give this incantation for the network to which the clients 
are connected :

   ip helper address

eg

   ip helper 10.1.2.3

You should also turn off a whole list of other protocols that the 
router will then forward :

no ip forward-protocol udp tftp
no ip forward-protocol udp nameserver
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs


So the router config would contain something like :


no ip forward-protocol udp tftp
no ip forward-protocol udp nameserver
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs

interface FastEthernet 0
   ip address 192.168.1.1 255.255.255.0
   ip helper 10.1.2.3

interface FastEthernet 1
   ip address 10.1.2.1 255.255.255.0


The relaying process is well described in the book "The DHCP 
Handbook" by Ralph Droms and Ted lemon.

Simon

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