DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST

Sujata Verma sujataverma3 at yahoo.com
Wed Apr 18 10:46:28 UTC 2012

Thanks for reply.

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

In windows server the default gw is The DHCP pool configured gateway is ( relay agent ip) 

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. 

In linux pc i have only enabled ipv4_forward and disabled iptables completely. Is there any other configuration which can result in my observation like this ?

Please let me know your observations, after seeing the captures. I have tried many times with similar observation.

--- 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, 2:38 PM

Sujata Verma wrote:

> I am using dhcp-4.2.2rc1. I have started the dhcp relay with options
> dhcrelay -d -i eth0 -i eth1 serverip
> everything is working fine except that when client release the ip and send DHCPRELEASE packet on server side i get 3 packets for each DHCPRELEASE.
> One is directly sent to server ( which is valid packet) the other two are duplicate of this packet which is generated by relay agent with giaddr as address of eth0 and eth1 interface.
> Is this normal behavior? The same is observed with unicast DHCPREQUEST .

Are you sure about the unicast packets ? On a switched network, the packet should only ever bee seen by the server - it shouldn't reach the relay agent.

For any boradcast packets, I'd expect to see two copies - one picked up directly by the server, and one picked up by the relay agent. This is inherent in the way the relay agent needs (due to the way it's built) to listen on both interfaces. I wouldn't expect to see a packet with the relay's eth1 address in it - they aren't connected to the same network are they ?

-- Simon Hobson

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