netdev - Re: Question on DHCP

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Date:	Wed, 13 Oct 2010 09:31:34 +0200
From:	Stefano Salsano <stefano.salsano@...roma2.it>
To:	Stephen Hemminger <shemminger@...tta.com>,
	Sri Ram Vemulpali <sri.ram.gmu06@...il.com>
CC:	linux-netdev <netdev@...r.kernel.org>
Subject: Re: Question on DHCP

Stephen Hemminger wrote:
> On Tue, 12 Oct 2010 21:33:25 -0400
> Sri Ram Vemulpali <sri.ram.gmu06@...il.com> wrote:
> 
>> Hi All,
>>
>>    I have very basic stupid doubt. Can anyone help to resolve this.
>>
>> DHCP, is used for dynamically configuring IP address, mask, MTU, DNS
>> domain, .... for a host. DHCP is a application layer protocol, but the
>> values it configures are related to IP layer. How come DHCP can
>> configure when its data packet in embedded in to IP datagram, which
>> are disparate layers. I am little confused. Please clarify. Thanks.
>>
> 
> DHCP client uses UDP to send broadcast packet and find the nearest
> server.  The Linux DHCP client ends up using a AF_PACKET socket because
> it wants to seem MAC address of server.

you may also want to refer to RFC 2131 (Dynamic Host Configuration 
Protocol), in section 2 it states:

In the case of a client using DHCP for initial configuration (before
    the client's TCP/IP software has been completely configured), DHCP
    requires creative use of the client's TCP/IP software and liberal
    interpretation of RFC 1122.  The TCP/IP software SHOULD accept and
    forward to the IP layer any IP packets delivered to the client's
    hardware address before the IP address is configured;

-- 
*******************************************************************
Stefano Salsano
Dipartimento Ingegneria Elettronica
Universita' di Roma "Tor Vergata"
Via del Politecnico, 1 - 00133 Roma - ITALY

http://netgroup.uniroma2.it/Stefano_Salsano/

E-mail  : stefano.salsano@...roma2.it
Cell.   : +39 320 4307310
Office  : (Tel.) +39 06 72597770  (Fax.) +39 06 72597435
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