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70-291 DHCP Question

 
 
Mr Roflcopter
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      06-12-2005

Hello All,

My second first (see post '70-291 - Didn't fail!' on the 31st of May for
an explanation of that) attempt at 291 is on Tuesday.

I have a question regarding DHCP and DHCP relays - how does the DHCP
server know what set of configuration to serve to a client?

What I mean is, let's say we have a DHCP server that will serve two
subnets, with a DHCP relay on the remote subnet or a 1549 compliant
router in between. There are two scopes on the DHCP server, one for the
local subnet and one for the remote subnet.

Now, what confuses me is this - when the DHCP sever receives a
DHCPDISCOVER packet, it will just pick the next address from the free
pool of addresses and send it as a DHCPOFFER, yes?

So, how does the DHCP server know which scope to offer an address from?
Obviously if a DHCPDISCOVER packet is relayed from the remote subnet,
that client will need a different default gateway address. OK, we could
use client reservations, but the books I have make no mention of using
that in this situation.

Yours confusedly,

--
Chris M.
 
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TSH
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Posts: n/a
 
      06-12-2005
On Sun, 12 Jun 2005 15:56:19 +0100, Mr Roflcopter wrote:

> Hello All,
>
> My second first (see post '70-291 - Didn't fail!' on the 31st of May for
> an explanation of that) attempt at 291 is on Tuesday.
>
> I have a question regarding DHCP and DHCP relays - how does the DHCP
> server know what set of configuration to serve to a client?
>
> What I mean is, let's say we have a DHCP server that will serve two
> subnets, with a DHCP relay on the remote subnet or a 1549 compliant
> router in between. There are two scopes on the DHCP server, one for the
> local subnet and one for the remote subnet.
>
> Now, what confuses me is this - when the DHCP sever receives a
> DHCPDISCOVER packet, it will just pick the next address from the free
> pool of addresses and send it as a DHCPOFFER, yes?
>
> So, how does the DHCP server know which scope to offer an address from?
> Obviously if a DHCPDISCOVER packet is relayed from the remote subnet,
> that client will need a different default gateway address. OK, we could
> use client reservations, but the books I have make no mention of using
> that in this situation.
>
> Yours confusedly,



Router has e.g. two defined addresses, one for each port ... when it
receives DHCP discover from client it replaces Client IP address in
datagram header with its own IP and forwards it to the defined DHCP server
.... DHCP server than compares that IP address with all subnet ranges
defined on server and supplies IP address from suitable range.
 
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Mr Roflcopter
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Posts: n/a
 
      06-12-2005
TSH wrote:
>>I have a question regarding DHCP and DHCP relays - how does the DHCP
>>server know what set of configuration to serve to a client?

>
> Router has e.g. two defined addresses, one for each port ... when it
> receives DHCP discover from client it replaces Client IP address in
> datagram header with its own IP and forwards it to the defined DHCP server
> ... DHCP server than compares that IP address with all subnet ranges
> defined on server and supplies IP address from suitable range.



Cheers TSH,

That makes sense to me - I don't understand why the books don't make any
mention of this. Perhaps they expect the readers to think that this will
'just work' without questioning how.

Thanks again,

--
Chris M.
 
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TSH
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      06-12-2005
On Sun, 12 Jun 2005 17:36:13 +0100, Mr Roflcopter wrote:

> TSH wrote:
>> Router has e.g. two defined addresses, one for each port ... when it
>> receives DHCP discover from client it replaces Client IP address in
>> datagram header with its own IP and forwards it to the defined DHCP server
>> ... DHCP server than compares that IP address with all subnet ranges
>> defined on server and supplies IP address from suitable range.

>
>
> Cheers TSH,
>
> That makes sense to me - I don't understand why the books don't make any
> mention of this. Perhaps they expect the readers to think that this will
> 'just work' without questioning how.
>
> Thanks again,



I agree with you, I had a same problem with this one ... you should do a
google search on "DHCP Discover" and check it out a little bit ... there is
more to it than my simplified explanation ...
 
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Tamusthefirst
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Posts: n/a
 
      06-13-2005


Mr Roflcopter wrote:
> Hello All,
>
> My second first (see post '70-291 - Didn't fail!' on the 31st of May for
> an explanation of that) attempt at 291 is on Tuesday.
>
> I have a question regarding DHCP and DHCP relays - how does the DHCP
> server know what set of configuration to serve to a client?
>
> What I mean is, let's say we have a DHCP server that will serve two
> subnets, with a DHCP relay on the remote subnet or a 1549 compliant
> router in between. There are two scopes on the DHCP server, one for the
> local subnet and one for the remote subnet.
>
> Now, what confuses me is this - when the DHCP sever receives a
> DHCPDISCOVER packet, it will just pick the next address from the free
> pool of addresses and send it as a DHCPOFFER, yes?
>
> So, how does the DHCP server know which scope to offer an address from?
> Obviously if a DHCPDISCOVER packet is relayed from the remote subnet,
> that client will need a different default gateway address. OK, we could
> use client reservations, but the books I have make no mention of using
> that in this situation.
>
> Yours confusedly,
>
> --
> Chris M.


http://www.warriorsofthe.net/movie.html
download the movie
hth.
Tam

 
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doctordns@gmail.com
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      06-13-2005
The router acting as a DHCP Relay agent is one scenairo, but you can
have another system on the subnet acting as the relay agent. The relay
agent has to listen for the DHCP Discover, and then wait for a period
for a DHCP Server to NOT answer the request. I configure my relay
agents to wait for 3 seconds before relaying.

Secondlhy, TSH's explanation has a tiny error. The DHCP relay agent
places the address that the original DHCP Discover was received from
into the GIADDR feild of the DHCP request which the relay agent
unicast-sends to the (configured) DHCP server. This address is in the
DHCP request, and not in the packets UDP datagram header. A small
point...

Finally, some books do indeed make mention of this - look for the MS
Press TCP/IP Protocols and Services book! <grin>

 
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TSH
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Posts: n/a
 
      06-13-2005
On 13 Jun 2005 09:06:17 -0700, http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:

> Secondlhy, TSH's explanation has a tiny error. The DHCP relay agent
> places the address that the original DHCP Discover was received from
> into the GIADDR feild of the DHCP request which the relay agent
> unicast-sends to the (configured) DHCP server. This address is in the
> DHCP request, and not in the packets UDP datagram header. A small
> point...


That's correct, but as i said, mine explanation was simplified and i
pointed him to get more information
 
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Wayne
Guest
Posts: n/a
 
      06-14-2005
Yes, it's a fantastic movie, but unfortunately shows absolutely *nothing*
about how DHCP works.

--
Wayne McGlinn
Brisbane, Oz
http://spaces.msn.com/members/wmcglinn
"help, help I'm being repressed!"
Dennis.



"Tamusthefirst" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
>
> Mr Roflcopter wrote:
>> Hello All,
>>
>> My second first (see post '70-291 - Didn't fail!' on the 31st of May for
>> an explanation of that) attempt at 291 is on Tuesday.
>>
>> I have a question regarding DHCP and DHCP relays - how does the DHCP
>> server know what set of configuration to serve to a client?
>>
>> What I mean is, let's say we have a DHCP server that will serve two
>> subnets, with a DHCP relay on the remote subnet or a 1549 compliant
>> router in between. There are two scopes on the DHCP server, one for the
>> local subnet and one for the remote subnet.
>>
>> Now, what confuses me is this - when the DHCP sever receives a
>> DHCPDISCOVER packet, it will just pick the next address from the free
>> pool of addresses and send it as a DHCPOFFER, yes?
>>
>> So, how does the DHCP server know which scope to offer an address from?
>> Obviously if a DHCPDISCOVER packet is relayed from the remote subnet,
>> that client will need a different default gateway address. OK, we could
>> use client reservations, but the books I have make no mention of using
>> that in this situation.
>>
>> Yours confusedly,
>>
>> --
>> Chris M.

>
> http://www.warriorsofthe.net/movie.html
> download the movie
> hth.
> Tam
>



 
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doctordns@gmail.com
Guest
Posts: n/a
 
      06-14-2005
Having spent many, many hours tracing DHCP, it's the little things that
count!

Maybe I should get a life, but DHCP is rather near and dear to my
heart. 6 years ago, I wrote my 1st magazine article on the subject
(reprinted in http://www.kapoho.com/articles/Art01.htm) and to this
day, I get 1-2 mails a month from folks asking questions.



 
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Guest
Posts: n/a
 
      06-14-2005
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Having spent many, many hours tracing DHCP, it's the little things that
> count!
>
> Maybe I should get a life, but DHCP is rather near and dear to my
> heart. 6 years ago, I wrote my 1st magazine article on the subject
> (reprinted in http://www.kapoho.com/articles/Art01.htm) and to this
> day, I get 1-2 mails a month from folks asking questions.
>
>
>


I read the article. Looks decent enough... Paragraphs and complete
sentences.

When do you think they'll be able to get this stuff to 'just work' instead
of 'making you work'?

Microcephalic S. Bob


 
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