DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST
Sujata Verma
sujataverma3 at yahoo.com
Thu Apr 19 08:05:28 UTC 2012
Thanks for your reply . We cannot run the command giving only one interface , either it has to be all or none. So it can be like
dhcrelay –d 192.168.9.2
or
dhcrelay –d –i eth0 -i eth1 192.168.9.2
If we give only one interface it will silently discard the packet. I have tested it.
Please see below bug for more information.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=648401
Regards,
Sujata
--- On Wed, 4/18/12, arunprasad <arunprasad at pointred.co> wrote:
From: arunprasad <arunprasad at pointred.co>
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, 10:47 PM
Use this command:- dhcrelay –d –i eth0 192.168.9.2 ensure that from 80.1.1.1/24 your DHCP-Server is reachable (vice versa). RegardsArun. From: arunprasad [mailto:arunprasad at pointred.co]
Sent: Wednesday, April 18, 2012 10:05 PM
To: 'Users of ISC DHCP'
Subject: RE: DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST 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.. 80.1.1.1/24 interface. PS:- IP Forwarding needs to enable. If your setup is as below. Windows 2003 server ( 192.168.9.2/23)-------------------(192.168.8.177/23) Linux PC(Relay) (80.1.1.1/24----------eth0----Windows XP Client ( DHCP client getting IP 80.1.1.2/24) RegardsArun. 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?
@Arun,
There is no switch in between, the setup to be precise is
Win Server-----------cross cable--------------Linux PC------------hub-------Win XP client
192.168.9.1/24 192.168.9.2/24 10.1.1.2/8 10.1.1.3/8 ( 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?
Regards,
Sujata
--- 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 PMSujata Verma wrote:
> My setup is like this : eth1
> Windows 2003 server ( 192.168.9.2/23)-------------------(192.168.8.177/23) Linux PC(Relay) (80.1.1.1/24----------eth0----Windows XP Client ( DHCP client getting IP 80.1.1.2/24)
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 80.1.1.1 .
>
> 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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