DHCP relay IP and gateway IP, possible bad logic?

Alan DeKok aland at deployingradius.com
Mon Mar 4 21:59:51 CET 2013

Phil Mayers wrote:
> Second, "reply to giaddr" is mandated in the DHCP spec; are you *sure*
> you have "other DHCP servers" which "reply to source ip"? Which servers?

  The issue is that giaddr serves two purposes.  In the request, it
indicates that the server MUST send the reply to that IP.

  In the reply, it means that the client sends the NEXT request to the

  ASCII art helps:

	client --> 1  NAS  2 ---> server

  The client sends broadcast packets to the NAS, using a private network
 The NAS unicasts them FROM NAS address "2" to the server, using giaddr
= 2.  NAS address "2" and the server are on a public network.

  The server knows that the NAS has a private address.  So it sends the
unicast answer back to NAS address 2", with giaddr = NAS address "1".

  The NAS broadcasts (or unicasts) this response back to the client.

  On a renew, the client unicasts the packet to NAS address "1", which
forwards it to the server using address "2", and giaddr ==2.

  And the whole process starts again.

  I think I know have a handle on DHCP and RADIUS.  My head is getting

  Alan DeKok.

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