70-291 DHCP Question

Discussion in 'MCSE' started by Mr Roflcopter, Jun 12, 2005.

  1. 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.
     
    Mr Roflcopter, Jun 12, 2005
    #1
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  2. Mr Roflcopter

    TSH Guest

    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.
     
    TSH, Jun 12, 2005
    #2
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  3. 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.
     
    Mr Roflcopter, Jun 12, 2005
    #3
  4. Mr Roflcopter

    TSH Guest

    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 :) ...
     
    TSH, Jun 12, 2005
    #4
  5. 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
     
    Tamusthefirst, Jun 13, 2005
    #5
  6. Mr Roflcopter

    Guest

    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>
     
    , Jun 13, 2005
    #6
  7. Mr Roflcopter

    TSH Guest

    On 13 Jun 2005 09:06:17 -0700, 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 :)
     
    TSH, Jun 13, 2005
    #7
  8. Mr Roflcopter

    Wayne Guest

    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" <> wrote in message
    news:...
    >
    >
    > 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
    >
     
    Wayne, Jun 14, 2005
    #8
  9. Mr Roflcopter

    Guest

    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.

    :)
     
    , Jun 14, 2005
    #9
  10. Mr Roflcopter

    Guest Guest

    <> wrote in message
    news:...
    > 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
     
    Guest, Jun 14, 2005
    #10
  11. Mr Roflcopter

    Guest

    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
     
    , Jun 14, 2005
    #11
  12. Mr Roflcopter

    Mike Cano Guest

    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" <> wrote in message
    news:42ac4cfc$0$2032$...
    >
    > 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.
     
    Mike Cano, Jun 15, 2005
    #12
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