DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST

arunprasad arunprasad at pointred.co
Wed Apr 18 16:35:26 UTC 2012

Hi Sujata,


dhcrelay -d -i eth0 -i eth1 serverip 


-i ifname 

Listen for DHCPv4/BOOTP queries on interface ifname. Multiple interfaces may
be specified by using more than one -i option. If no interfaces are
specified on the command line, dhcrelay will identify all network
interfaces, eliminating non-broadcast interfaces if possible, and attempt to
listen on all of them. 


So try running dhrelay on your downstream interface only i.e..


PS:- IP Forwarding needs to enable. If your setup is as below.


Windows 2003 server (
Linux PC(Relay) ( XP Client ( DHCP
client getting IP 








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 6:36 PM
To: Users of ISC DHCP
Subject: Re: DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST


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 :
>   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

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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