70-291 DHCP Question

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.

TSH
Sun Jun 12 10:50:17 CDT 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.

Mr
Sun Jun 12 11:36:13 CDT 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.

TSH
Sun Jun 12 13:32:50 CDT 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 :) ...

Tamusthefirst
Mon Jun 13 04:49:49 CDT 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

doctordns
Mon Jun 13 11:06:17 CDT 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>

TSH
Mon Jun 13 12:54:54 CDT 2005

On 13 Jun 2005 09:06:17 -0700, doctordns@gmail.com 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 :)

Wayne
Mon Jun 13 20:19:13 CDT 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" <tom_smith60@hotmail.com> wrote in message
news:1118656189.924428.220870@g44g2000cwa.googlegroups.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
>

doctordns
Tue Jun 14 01:40:29 CDT 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.

:-)

The
Tue Jun 14 02:04:58 CDT 2005

<doctordns@gmail.com> wrote in message
news:1118731229.507707.279720@g14g2000cwa.googlegroups.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

doctordns
Tue Jun 14 17:19:15 CDT 2005

Well - for the most part, this stuff does just work:

1. In XP, a client will use APIPA as a network address if no DHCP is
found, and will re-attempt to find a DHCP server regularly. Thus
transient netowork issues are over come. Plus the 169.254.0.0/16
address is a giveaway for troubleshooting.

2. When using most home and small business routers/gateways/nat
products, the client and the server pretty well just work too. Setting
this up, for example Connection Sharing in XP is very simple.

3. For more complex stuff, it's not easy to define what 'just working
means' in all scenairos. But for the most part, DHCP/BOOTP relay agents
are pretty easy to setup.

4. DHCP was not designed to be a robust technology. It's simple,
basically link local broadcast based technology. It was also based on
IPv4 which was never designed to 'just work'!!

5. IPv6 provides a better solution.

So all in all, it does just work.

Thomas

Mike
Wed Jun 15 02:33:24 CDT 2005

Thanks for the thought-provoking question.

Even though you passed your 70-291 exam, I'm taking mine this Friday.

I found the answer to your question here:
http://support.microsoft.com/?kbid=120932

It explains that the replay ip is known to the dhcp server and it tries to
match the scope to it.

Mike

"Mr Roflcopter" <nobody@nowhere.special> wrote in message
news:42ac4cfc$0$2032$ed2e19e4@ptn-nntp-reader04.plus.net...
>
> 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.