DHCP Relay: Duplicate packets for DHCPRELEASE and DHCPREQUEST

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Apr 18 11:51:06 UTC 2012

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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