Thank you everyone for your answers. At first I was wondering if some
special config had to be done on the DHCP server itself, but that isn't
the case. Gaz, you're saying that DHCP takes the source of the request
into account when it dishes out addresses, and it looks like you're
right. The DHCP server will respond to a DHCPDISCOVER request from a
bootp relay agent (ie a router with an ip helper address set) with an
address that's on the same subnet as the agent.
This is from RFC 2131, it's a listing of the order of preference when a
DHCP server assigns an address. Note: "giaddr" is the address of the
bootp relay agent. Check out the 4th bullet:
When a server receives a DHCPDISCOVER message from a client, the
server chooses a network address for the requesting client. If no
address is available, the server may choose to report the problem to
the system administrator. If an address is available, the new address
SHOULD be chosen as follows:
o The client's current address as recorded in the client's current
binding, ELSE
o The client's previous address as recorded in the client's (now
expired or released) binding, if that address is in the server's
pool of available addresses and not already allocated, ELSE
o The address requested in the 'Requested IP Address' option, if
that
address is valid and not already allocated, ELSE
o A new address allocated from the server's pool of available
addresses; the address is selected based on the subnet from
which
the message was received (if 'giaddr' is 0) or on the address of
the relay agent that forwarded the message ('giaddr' when not
0).
Again, thanks for all the help.
Hal Logan
Network Specialist / Adjunct Faculty
Computing and Engineering Technology
Manatee Community College
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On
> Behalf Of
> Gaz
> Sent: Tuesday, November 27, 2001 5:34 AM
> To: [EMAIL PROTECTED]
> Subject: Re: DHCP Question [7:27380]
>
>
> Dave said:
>
> "The only reason for this I can fathom is because the DHCP
> request is no
> longer a broadcast it now has the source address of the
> router interface
> that the helper address is setup on and it seems DHCP will take this
> into account when dishing out addresses."
>
> That's exactly the assumption I've always made, but I'm not totally
> convinced. I'd like to sniff the packet and actually see whether it's
> dependant upon the source address or something contained within the
> packet. As per usual IF I get chance I'll put a sniffer on.
>
> Anybody know for sure?
>
> Gaz
>
> wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > As mentioned in an earier reply..you must have your DHCP Scopes
> > correctly set up
> >
> > I've found that with NT4/W2k DHCP servers
> > that, If for example your DHCP server is set up to dish out
> addresses
> > in
> the
> > range of 192.168.1.1 to 100 mask 255.255.255.0 and it receives a
> > request
> for
> > an address directed from the helper routers interface which has an
> > address of 10.1.1.1 (which means your DHCP clients will be
> on the same
>
> > network) it will ignore that request. The only reason for
> this I can
> > fathom is because the DHCP request is no longer a broadcast
> it now has
>
> > the source address of the router interface that the helper
> address is
> > setup on and it seems DHCP will take this into account when dishing
> > out addresses.
> >
> > I've found that if a scope in the 10.1.1.0 range is setup on the
> > server my DHCP clients will recieve an IP address in the
> correct 10.
> > range with no problem.
> >
> > I have two scopes on my DHCP server
> > 172.16.60.1 - 172.16.61.254 Subnet 255.255.254.0 and 10.222.36.1
> > -10.222.37.254 Subnet 255.255.254.0
> >
> > my router interface configured to forward DHCP reqests is set up as
> follows
> > ..
> > ip address 10.222.36.2 255.255.254.0
> > ip helper-address 155.131.60.40 (MY DHCP SERVER address )
> > my DHCP clients never get an address from the wrong range
> if i disable
>
> > the 10.222.36.0 range my dhcp clients behind the router
> don't get an
> > address at all ...
> >
> > Hope this make sense and helps and if i'm talking pants
> please someone
>
> > put me straight ...
> >
> > Regards Dave
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=27460&t=27460
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