Re: DHCP Relay serving clients on the same network as a DHCP server‏

[Glenn Satchell]

Miroslav Mzik wrote:
> Dear dhcp-users list members,
> 
> I am turning to you with a question regarding ISC DHCP Relay 
> functionality. The man page for dhcrelay says:
> 
> ------------
> In general, it must listen not only on those network interfaces to which 
> clients are attached, but also on those network interfaces to which the 
> server (or the router that reaches the server) is attached.
> ------------
> 
> ------------
> The relay agent should not relay packets received on a physical network 
> to DHCP servers on the same physical network - if they do, the server 
> will receive duplicate packets. In order to fix this, however, the relay 
> agent needs to be able to learn about the network topology, which 
> requires that it have a configuration file.
> ------------
> 
> I am facing the above mentioned problem - the DHCP Relay processes 
> packets coming from clients located on the same physical network as my 
> DHCP server and thus the server receives duplicate packets. Is there a 
> solution to this prob lem? I cannot find any documentation for 
> /etc/sysconfig/dhcrelay file directives which might help solving this issue.
> 
> Thank you.
> 
> Miroslav Mzik

There are no dhcrelay directives to fix this. If the clients are on the 
same physical subnet as the server then they can talk to it directly and 
do not need dhcrelay. dhcrelay is intended for remote subnets to forward 
broadcasts on behalf of clients.

So the man page is referring to a case where the router for a subnet 
might be a Linux or Unix box with more than one network interface. It is 
a common case to run dhcrelay -i eth0 to listen on eth0, but dhcrelay 
needs to listen on, say, eth1 also if this is the network connection 
that routes packets back to the dhcp server. In most other cases 
dhcrelay runs on a box with a single interface.

The only time that is dhcp and dhcrelay are run on the same subnet is 
with a switch that has a relay agent within the switch to add variables 
such as option 82 (agent.remote-id and agent.circuit-id) for the switch 
ports. But then this is using the switch's relay agent not dhcrelay.

Perhaps you could describe your network and how dhcp and dhcrelay are 
deployed and what you are trying to achieve?

regards,
-glenn