dhcpd and dhcrelay

[Glenn Satchell]

>X-Authentication-Warning: angus.ind.WPI.EDU: cra set sender to cra at WPI.EDU 
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>Date: Wed, 1 Oct 2008 10:31:33 -0400
>From: Chuck Anderson <cra at WPI.EDU>
>To: dhcp-users at isc.org
>Subject: Re: dhcpd and dhcrelay
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>On Wed, Oct 01, 2008 at 10:23:11AM +0400, Elena Timoshkina wrote:
>> > Why?  Where will you be relaying packets from/to if the packets are 
>> > arriving at the dhcpd server already?
>> Dhcrelay will work with other dhcpd server (
>> dhcrelay_servers="10.97.5.2" ).
>
>What VLAN or interface is 10.97.5.2 attached to?
>
>> > It makes no sense to run dhcrelay with a single interface.  You need 
>> > two interfaces that dhcrelay will relay the packets between.
>> How can I determine two interfaces for dhcrelay? It will work in
>> vlan667 (requests) and vlan667 (relay to server  10.97.5.2).
>> It is possible?
>
>So the server is on vlan667 and the clients are also on the same 
>vlan667?  You don't need or want a relay at all in that case.
>
>> >> But dhcrelay not binded in interface vlan667. How can I do it - to
>> >> bind dhcrelay in address 10.97.7.18 only?
>> 
>> > dhcrelay doesn't bind to IP addresses, but rather physical interfaces.
>> ok. how to bind it in vlan667 and not bind in vlan39 ?
>> > This is because the DHCP protocol needs to handle broadcasts from 
>> > unaddressed hosts (0.0.0.0 source address) and send to all-ones 
>> > broadcast 255.255.255.255.
>> yes, can dhcrelay listen only broadcasts in vlan667 (not vlan39 and
>> others)?
>
>dhcrelay -i <client-iface> -i <server-iface> 10.97.7.18
>
>If client-iface is the same as server-iface, don't run dhcrelay at 
>all.
>
Actually that last sentence may not always be correct. dhcrelay does
not have to run on a router - it could run on a single homed server on
a subnet/vlan where it listens for broadcasts and relays them to the
server. More correctly if server-iface is the same subnet as 10.97.7.18
then you don't need to run dhcrelay,

man dhcrelay has all the options.

regards,
-glenn