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Understanding the DHCP Server Client conversation (very intresting subject)

 
 
Weamfox
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Posts: n/a
 
      03-01-2008
Dear Friends,

Through my researches and my studies I tried to understand the DHCP Server
client conversation in Windows Server 2003 environment and I'm little bit
lost here.

Now, According to this table that is listed on Microsoft web page
http://support.microsoft.com/kb/169289

Source Dest Source Dest
Packet
MAC addr MAC addr IP addr IP addr
Description
------------------------------------------------------------------------------------------------------
Client Broadcast 0.0.0.0 255.255.255.255 DHCP
Discover
* DHCPsrvr Broadcast DHCPsrvr 255.255.255.255 DHCP Offer
Client Broadcast 0.0.0.0 255.255.255.255 DHCP
Request
** DHCPsrvr Broadcast DHCPsrvr 255.255.255.255 DHCP ACK


I have some questions regarding the DHCP conversation manner.
The DHCP server sends a Broadcast MSG offering the IP address to the client
(*) and at the end it sends AGAIN a Broadcast MSG to acknowledge the IP
address that's been offered (**).

Now, my questions are :-

1) I understand that the Client is sending a broadcast message because it
doesn't know any concrete DHCP server in order to request an IP address, but
why the DHCP sends also a broadcast message to the client (*) (**)?

2) Let's agree that the DHCP server is broadcasting its offer, what happen
if there is more than one client (lets say 50 or more) is getting same IP
address that's suppose to be offered to a concrete client at the same time?
Doesn't this make a high traffic in the network making all the clients go
back and forth with the DHCPNACK, DHCPDECLINE and the DHCPACK?????
NOTE: I understand that the client sends an ARP request with the offered
IP address if it gets a reply that means that this IP address is in use and
the client sends DHCPDECLINE, this is from the CLIENT side but what about
the server side and the whole network?? :-s

3) Why don't the DHCP server cuts the crap and use the client MAC address to
send the DHCPOFFER and the DHCPACK instate of broadcasting to all clients?
PLEASE NOTE : That the MAC address is in the second layer (Data Link layer)
in the OSI model while the IP address is in the third layer (Network layer)
which make it more reasonable for the DHCP server to use the the MAC address
of the client that is requesting the service to send him the IP address
instate of BROADCASTING the DHCPOFFER and the DHCPACK and making high
traffic.

I'm sorry for make it long but I think that the subject is worth it....

Yours,
Weamfox








 
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Neil
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      03-01-2008
did you hear "Weamfox" <(E-Mail Removed)> say in
news:(E-Mail Removed):

>
> 1) I understand that the Client is sending a broadcast message because
> it doesn't know any concrete DHCP server in order to request an IP
> address, but why the DHCP sends also a broadcast message to the client
> (*) (**)?
>


OY! ok, who exactly would it be able to direct it to? Without the
specified DHCP server, I shout to everyone on the network (subnet) that I
can. This will go to all IPs that can acceptt it (255.255.255.255) and
all MAC adresses as well (FFFFFFFFFFFF). Think of it from human
perspectives. You walk into a pitch black room. If you aren't sure if
there is anyone in the room, You shout "ANYBODY THERE?" you don't specify
"hay Mike, are you there?". This is the DHCP DISCOVERY

So, when the DHCP server hears this it now knows a little comething. The
client (who doesn't yet have an IP address), has supplied it's MAC
address to the DHCP server. BUT, since the client doesn't have an
address, the DHCP server send the DHCP OFFER as a unicast, to the whole
network (since client is unassigned it goes to IP of 255.255.255.255) but
a specified MAC adress. From the human perspective, this is like the
other guy in the dark room responding "Dave, is that you? It's me Tom"

> 2) Let's agree that the DHCP server is broadcasting its offer, what
> happen if there is more than one client (lets say 50 or more) is
> getting same IP address that's suppose to be offered to a concrete
> client at the same time? Doesn't this make a high traffic in the
> network making all the clients go back and forth with the DHCPNACK,
> DHCPDECLINE and the DHCPACK????? NOTE: I understand that the client
> sends an ARP request with the offered IP address if it gets a reply
> that means that this IP address is in use and the client sends
> DHCPDECLINE, this is from the CLIENT side but what about the server
> side and the whole network?? :-s
>


hold up, the DHCP server is simply listening. It's not continously
offering out addresses left right and center. Each OFFER is associated
with a specified MAC adress. There is actually an outside possibility
that with 2 DHCP Servers on the same subnet, the client may be offered
more than one address, but the DHCP servers will only offer one address
per request. The client will pick which it will use. That's the part of
the DHCP REQUEST.

> 3) Why don't the DHCP server cuts the crap and use the client MAC
> address to send the DHCPOFFER and the DHCPACK instate of broadcasting
> to all clients? PLEASE NOTE : That the MAC address is in the second
> layer (Data Link layer) in the OSI model while the IP address is in
> the third layer (Network layer) which make it more reasonable for the
> DHCP server to use the the MAC address of the client that is
> requesting the service to send him the IP address instate of
> BROADCASTING the DHCPOFFER and the DHCPACK and making high traffic.
>


Um, dude, read this...

http://en.wikipedia.org/wiki/DHCP

too much typing...

--
The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X

- I feel much better now that I have given up Hope.
 
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Neil
Guest
Posts: n/a
 
      03-01-2008
did you hear "Weamfox" <(E-Mail Removed)> say in news:0FF41841-B797-
http://www.velocityreviews.com/forums/(E-Mail Removed):

> I'm sorry for make it long but I think that the subject is worth it....


If you REALLY want to see what's going on, get a copy of netmon etherreal
or soemthing of that ilk, drop to a command prompt as the tool is running
and do an IPCONFIG /RELEASE and an IPCONFIG /RENEW. View the results and
you will see from teh DHCP DISCOVER, OFFER,REQUEST, and ACK what is
really happening. Look at the IP addresses of each and teh MAC addresses
of each. That will help you immensely.

--
The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X

- When all else is lost, the future still remains.
 
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Weamfox
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Posts: n/a
 
      03-02-2008
Hi Neil...
I understand all what you said and I already been in wikipedia and already
have seen whats written there but whats concerning me is:

1) why the DHCP server (BROADCAST) the DHCPOFFER and the DHCPACK??? it
already knows the client that is requesting the service from its MAC
address??
why not the DHCP server just sends its offer to the client that is
requesting the service according to its MAC address??

2) you said the (DHCP server send the DHCP OFFER as a unicast, to the whole
network) but a specified MAC address.
-How come it sends a UNICAST to the whole network????
-How come it sends to the WHOLE network (based on the IP address) but a
SPECIFIED MAC address????





--------------------------------------------------------------------------------
"Neil" <guess!!!@gmail.com> wrote in message
news:Xns9A54AF046338Dneilmcsegmailcom@207.46.248.1 6...
> did you hear "Weamfox" <(E-Mail Removed)> say in
> news:(E-Mail Removed):
>
>>
>> 1) I understand that the Client is sending a broadcast message because
>> it doesn't know any concrete DHCP server in order to request an IP
>> address, but why the DHCP sends also a broadcast message to the client
>> (*) (**)?
>>

>
> OY! ok, who exactly would it be able to direct it to? Without the
> specified DHCP server, I shout to everyone on the network (subnet) that I
> can. This will go to all IPs that can acceptt it (255.255.255.255) and
> all MAC adresses as well (FFFFFFFFFFFF). Think of it from human
> perspectives. You walk into a pitch black room. If you aren't sure if
> there is anyone in the room, You shout "ANYBODY THERE?" you don't specify
> "hay Mike, are you there?". This is the DHCP DISCOVERY
>
> So, when the DHCP server hears this it now knows a little comething. The
> client (who doesn't yet have an IP address), has supplied it's MAC
> address to the DHCP server. BUT, since the client doesn't have an
> address, the DHCP server send the DHCP OFFER as a unicast, to the whole
> network (since client is unassigned it goes to IP of 255.255.255.255) but
> a specified MAC adress. From the human perspective, this is like the
> other guy in the dark room responding "Dave, is that you? It's me Tom"
>
>> 2) Let's agree that the DHCP server is broadcasting its offer, what
>> happen if there is more than one client (lets say 50 or more) is
>> getting same IP address that's suppose to be offered to a concrete
>> client at the same time? Doesn't this make a high traffic in the
>> network making all the clients go back and forth with the DHCPNACK,
>> DHCPDECLINE and the DHCPACK????? NOTE: I understand that the client
>> sends an ARP request with the offered IP address if it gets a reply
>> that means that this IP address is in use and the client sends
>> DHCPDECLINE, this is from the CLIENT side but what about the server
>> side and the whole network?? :-s
>>

>
> hold up, the DHCP server is simply listening. It's not continously
> offering out addresses left right and center. Each OFFER is associated
> with a specified MAC adress. There is actually an outside possibility
> that with 2 DHCP Servers on the same subnet, the client may be offered
> more than one address, but the DHCP servers will only offer one address
> per request. The client will pick which it will use. That's the part of
> the DHCP REQUEST.
>
>> 3) Why don't the DHCP server cuts the crap and use the client MAC
>> address to send the DHCPOFFER and the DHCPACK instate of broadcasting
>> to all clients? PLEASE NOTE : That the MAC address is in the second
>> layer (Data Link layer) in the OSI model while the IP address is in
>> the third layer (Network layer) which make it more reasonable for the
>> DHCP server to use the the MAC address of the client that is
>> requesting the service to send him the IP address instate of
>> BROADCASTING the DHCPOFFER and the DHCPACK and making high traffic.
>>

>
> Um, dude, read this...
>
> http://en.wikipedia.org/wiki/DHCP
>
> too much typing...
>
> --
> The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X
>
> - I feel much better now that I have given up Hope.
>

 
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LRM
Guest
Posts: n/a
 
      03-02-2008
"Neil" <guess!!!@gmail.com> wrote in message
news:Xns9A54AFA963F28neilmcsegmailcom@207.46.248.1 6...
> did you hear "Weamfox" <(E-Mail Removed)> say in news:0FF41841-B797-
> (E-Mail Removed):
>
>> I'm sorry for make it long but I think that the subject is worth it....

>
> If you REALLY want to see what's going on, get a copy of netmon etherreal
> or soemthing of that ilk, drop to a command prompt as the tool is running
> and do an IPCONFIG /RELEASE and an IPCONFIG /RENEW. View the results and
> you will see from teh DHCP DISCOVER, OFFER,REQUEST, and ACK what is
> really happening. Look at the IP addresses of each and teh MAC addresses
> of each. That will help you immensely.
>

Yeah, I couldn't find anything good on TV Saturday either....


 
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Neil
Guest
Posts: n/a
 
      03-03-2008
did you hear "LRM" <(E-Mail Removed)> say in news:e0ThQmHfIHA.5900
@TK2MSFTNGP02.phx.gbl:

> Yeah, I couldn't find anything good on TV Saturday either....


Really? I quite enjoyed SNL. Hillary was on.

--
The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X

- Zenocide: the killing of ancient philosophers.
 
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kpg*
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Posts: n/a
 
      03-03-2008
Neil <guess!!!@gmail.com> wrote in news:Xns9A565B82749B9neilmcsegmailcom@
207.46.248.16:

> did you hear "LRM" <(E-Mail Removed)> say in news:e0ThQmHfIHA.5900
> @TK2MSFTNGP02.phx.gbl:
>
>> Yeah, I couldn't find anything good on TV Saturday either....

>
> Really? I quite enjoyed SNL. Hillary was on.


Yeah, but you live in Canadia.
 
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Neil
Guest
Posts: n/a
 
      03-03-2008
did you hear "Weamfox" <(E-Mail Removed)> say in
news:(E-Mail Removed):

> Hi Neil...
> I understand all what you said and I already been in wikipedia and
> already have seen whats written there but whats concerning me is:
>
> 1) why the DHCP server (BROADCAST) the DHCPOFFER and the DHCPACK???
> it already knows the client that is requesting the service from its
> MAC address??
> why not the DHCP server just sends its offer to the client that is
> requesting the service according to its MAC address??
>


Think of it from the perspective of having multiple DHCP servers on the
same subnet. If two DHCP servers make the offer, the client needs to pick
one and then the selected server sends it's acknowledgement along with
options to the client.

> 2) you said the (DHCP server send the DHCP OFFER as a unicast, to the
> whole network) but a specified MAC address.
> -How come it sends a UNICAST to the whole network????
> -How come it sends to the WHOLE network (based on the IP address) but
> a SPECIFIED MAC address????
>
>


Because there is no assigned IP address for the client yet, there are
limited options for the server. It has to send it to all IP address
(255.255.255.255) but knows of a specific MAC address.

Seriously, grab a network sniffer, fire up a DHCP server and look at the
packets. It's incredibly helpful. When I taught TCP/IP (NT 4 days) I
would sometimes grab out the NETMON tool and go through a bit of it if
the student was having an issue. It was also well taught in the NT in the
Enterprise course (which spent WAY TOO much time in the NETMON tool).
Ethereal would be a good tool to try. Learn it and love it.

--
The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X


- 'Bother,' said Pooh, as he took aim from behind the grassy knoll.
 
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Neil
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Posts: n/a
 
      03-03-2008
did you hear "kpg*" <(E-Mail Removed)> say in
news:Xns9A5651D36A48Dipostthereforeiam@207.46.248. 16:

> Yeah, but you live in Canadia.


It's true, American politics are much funnier to Canadians. Canadian
politcs just make me sad.

--
The InterNeil "V2 w/tabbed browsing & decreased verbosity" MCNGP Triple X

- Remaining time multiplied by distress is a constant.
 
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kpg*
Guest
Posts: n/a
 
      03-03-2008
Neil <guess!!!@gmail.com> wrote in news:Xns9A5661E7AA401neilmcsegmailcom@
207.46.248.16:

> did you hear "kpg*" <(E-Mail Removed)> say in
> news:Xns9A5651D36A48Dipostthereforeiam@207.46.248. 16:
>
>> Yeah, but you live in Canadia.

>
> It's true, American politics are much funnier to Canadians. Canadian
> politcs just make me sad.


Yeah, Like all those problems you have with everyone wanting to speak
a different lanugage! A county with two languages...you Candians are
muy comico.

 
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