Question on DHCP

Question on DHCP

[Sri Ram Vemulpali]

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.

-- 
Regards,
Sri.

Re: Question on DHCP

[Stephen Hemminger]

On Tue, 12 Oct 2010 21:33:25 -0400
Sri Ram Vemulpali <sri.ram.gmu06@gmail.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.

-- 

Re: Question on DHCP

[Stefano Salsano]

Stephen Hemminger wrote:
> On Tue, 12 Oct 2010 21:33:25 -0400
> Sri Ram Vemulpali <sri.ram.gmu06@gmail.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;

-- 
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Dipartimento Ingegneria Elettronica
Universita' di Roma "Tor Vergata"
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