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DHCP Message Relaying and BOOTP Relay Agents
(Page 2 of 2)
DHCP Relaying Process
Since DHCP was designed specifically
to support BOOTP relay agents, the agents behave in DHCP much as they
do in BOOTP. Of course DHCP has much more complex message exchanges,
but as we've already seen, they are all still designed around the notion
of a client request and server response. There are just more requests
and responses. The BOOTP agent looks for broadcasts sent by the client
and then forwards them to the server just
as described in the topic on BOOTP relay agent behavior,
and then returns replies from the server. The additional information
in the DHCP protocol is implemented using additions to the BOOTP message
format in the form of DHCP options, which the relay agent doesn't look
at. It just treats them as it does BOOTP requests and replies.
So, in summary, when a relay agent
is used, here's what the various client requests and server replies
in the DHCP operation section become:
- Client Request: When a client broadcasts
a request, the relay agent intercepts it on UDP port 67. It checks the
Hops field, and discards the request if the value is greater
than 16; otherwise it increments the field. The agent puts its own address
into the GIAddr field unless another relay agent has already
put its address in the field. It then forwards the client request to
a DHCP server, either unicast or broadcast on another network.
- Server Reply: The server sees a non-zero
value in GIAddr and sends the reply to the relay agent whose
IP address is in that field. The relay agent then sends the reply back
to the client, using either unicast or broadcast as discussed in the
topic on DHCP addressing.
One difference between BOOTP and
DHCP is that certain communications from the client to the server are
unicast. The most noticeable instance of this is when a client tries
to renew its lease with a specific DHCP server. Since it sends this
request unicast, it can go to a DHCP server on a different network using
conventional IP routing, and the relay agent does not need to be involved.
Key Concept: To permit DHCP clients and DHCP servers to reside on different physical networks, an intermediary device is required to facilitate message exchange between networks. DHCP uses the same mechanism for this as BOOTP: the deployment of BOOTP relay agents. The relay agent captures client requests, forwards them to the server, and then returns the servers responses back to the client.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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