DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST

arunprasad arunprasad at pointred.co
Wed Apr 18 11:05:05 UTC 2012

Hi Sujitha,


Try keeping DHCP-Server & DHCP-relay in different subnet.





From: dhcp-users-bounces+arunprasad=pointred.co at lists.isc.org
[mailto:dhcp-users-bounces+arunprasad=pointred.co at lists.isc.org] On Behalf
Of Sujata Verma
Sent: Wednesday, April 18, 2012 4:16 PM
To: Users of ISC DHCP
Subject: Re: DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST


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

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

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.
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