DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST

Sujata Verma sujataverma3 at yahoo.com
Wed Apr 18 13:05:48 UTC 2012

Thanks for the reply. 

Yes, it is duplicating the unicast DHCPREQUEST packets as well. 

So is it a bug, is there anything i can do to stop doing it ( in code ) ,  any pointers? 


There is no switch in between, the setup to be precise is 

Win Server-----------cross cable--------------Linux PC------------hub-------Win XP client             ( from server)

I changed the subnet as  above but same behavior.

One more observation : 

if i disable ip_forward flag , the DHCP relay agent is sending only one DHCPRELEASE packet with correct giaddr ( incoming eth0 interface ). But as forwarding is disabled it is understood that the client DHCPRELEASE  message is not forwarded to the server but why the other "eth1" interface DHCPRELEASE message is not being generated from relay agent? 

Any pointers? 


--- On Wed, 4/18/12, Simon Hobson <dhcp1 at thehobsons.co.uk> wrote:

From: Simon Hobson <dhcp1 at thehobsons.co.uk>
Subject: Re: DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST
To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
Date: Wednesday, April 18, 2012, 5:21 PM

Sujata Verma wrote:

> My setup is like this :                                                                                                              eth1
>   Windows 2003 server ( Linux PC(Relay) ( XP Client ( DHCP client getting IP

That's better

> I am attaching the wireshark capture for both client and server. In this we can see that the DHCPDISCOVER is only being sent once to the server which relay is forwarding while adding giaddr field as .
> When DHCPRELEASE packet is being sent, it is sent directly to the server and with other two packets as duplicates with giaddr as of two interfaces.

My guess (I've not actually used the ISC relay) is that it's sniffing the packets and picking up the unicast packet from the client to the server both when it enters the relay on eth0 and when it exits via eth1. That, to my understanding, is wrong.

It would be interesting to see what happens to a unicast DHCP-Request packet ? The Request packet in your captures is a broadcast packet which is being relayed by the Relay Agent (correctly). You will need to (temporarily) tweak your server config down to a short lease time so that the client will renew in a timescale that's workable.
Based on the above, I suspect you'll see those packets duplicated as well.

-- Simon Hobson

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