Technical data

Configuring the DHCP Client
8.1 Key Concepts
8.1.2 Requesting a Lease
A DHCP server allocates IP addresses to clients on a temporary or permanent
basis. This time period is called a lease. A client can request a lease for some
period of time, which the DHCP server can either honor or assign a different
time period depending on the policy in force. A client may request a lease for an
infinite period of time, but the server may choose to give out a lengthy but not
infinite lease. For whatever time period the DHCP server assigns, the DHCP
server guarantees not to reassign the IP address to any other system until the
lease expires.
Lease times are represented in DHCP dialogs as relative time to be interpreted
with respect to the client’s clock. If there is drift between the client’s clock and
the server’s clock, the server may consider the lease expired before the client
does. To compensate, the server may return a shorter lease duration to the client
than the server commits to its local database of client information.
8.1.3 Requesting Parameters
The first service provided by a DHCP server is to provide storage of network
parameters for network clients. DHCP clients can query the DHCP server to
retrieve the configuration parameters. In its initial discover or request message,
a client can supply a list of parameters for which it needs information. If the
server does not return any or all of the values for the requested parameters, the
client uses default values for any missing values.
8.1.4 Understanding How the DHCP Client Operates
When your system has an interface configured under DHCP control, the following
sequence of steps occur at TCP/IP Services startup time:
1. The TCPIP$STARTUP procedure installs the DHCP client image,
TCPIP$DHCP_CLIENT.EXE, with the appropriate OpenVMS system
privileges.
2. Then it issues the following command to start the interface:
$ TCPIP START COMMUNICATION/INITIALIZE
This command creates a subprocess and runs the DHCPCONF utility as
follows to set up the interface:
DHCPCONF -W 30 ifname START
Alternatively, the command procedure issues the following command if the
interface is the primary interface:
DHCPCONF -P -W 30 ifname START
The -w 30 option on the DHCPCONF command line tells DHCPCONF to
wait for a maximum of 30 seconds before returning. This wait prevents
the TCPIP$STARTUP procedure from hanging indefinitely when there are
problems reaching a DHCP server. If the 30-second timer expires, the DHCP
client process will, by default, continue to complete the DHCP dialog until it
is successful or it is shut down.
Configuring the DHCP Client 8–3