User`s guide

XSR User’s Guide 297
Chapter 12 How DHCP Works
Configuring DHCP
How DHCP Works
DHCP’s client-server model defines a set of messages exchanged between
two systems. A simplified description client-server communications follows:
1
A client issues a broadcast message (DISCOVER) to locate available
DHCP Servers on its local subnet. This message may include
suggested values for the
network address and duration of a lease.
Also, BOOTP relay agents may pass the message on to DHCP
Servers not on the same physical subnet.
2 A response (OFFER) is sent from a DHCP Server to the client with an
offer of configuration parameters including an available network IP
address, among others. Before the server actually allocates the new
address, it will check that the address is free by pinging it.
3 A client sends a message (REQUEST) to servers for one of the
following purposes:
– Requesting offered parameters from one server and implicitly
declining offers from all others,
– Confirming the correctness of a previously allocated address
after, for example, a system reboot,
– Extending the lease on a particular network address.
4
The selected server sends a message (ACK) to a client with
configuration parameters - a
binding - including a committed network
address,
client-identifier or hardware-address and commits its lease
to the binding database. Or, the server sends a message (NACK)
indicating the client’s idea of a network address is incorrect or the
client’s lease has expired.
5 The client performs a final check (ARPs the allocated network
address) on the parameters and at this point is configured.
6 The client may relinquish its lease on a network address by sending a
message (RELEASE) to the server identifying the lease with its client
identifier (hardware/network addresses). If the client used a client ID
when it got the lease, it will use the same identifier in the message.
Alternately, when a lease is near expiration, the client tries to renew it. If
unsuccessful in renewing by a certain period, the client enters a rebinding
state and sends a DISCOVER message to restart the process.
DHCP also sets various options/extensions to clients which are outlined in
“Assigned Network Configuration Values to Clients: Options” on page 299.