User guide

Dynamic Host Configuration Protocol DHCP client
iMG/RG Software Reference Manual (System Administration)
5-4
Note: Note that you will still need to have a suitable subnet declaration – for example, a subnet 192.169.219.0
with netmask 255.255.255.0, as shown earlier. Any configuration options you define in this subnet will
also be offered to every fixed host you have added which is also on the given subnet.
It is also possible to assign a maximum lease duration to fixed DHCP clients as follows:
dhcpserver set fixedhost myhost maxleasetime 7200
In this context, fixed lease duration would normally be used to allow DHCP clients to see changes in offered
options quickly. The IP address itself is always guaranteed to be available for assignment to the specific host
(unless there are other DHCP servers on the same network that are deliberately configured to conflict).
You might see the following message if you have ever turned off the DHCP server:
Note: Note the DHCP server is not currently enabled.
If you see this, issue the following command:
dhcpserver enable
The final step is to tell the system to update the DHCP server software with the new IP interface and configu-
ration that has been defined. To do this, issue the following command:
dhcpserver update
Note: NO configuration changes that you have made on the DHCP server will take effect until you enter the
DHCPSERVER UPDATE command.
5.1.3 DHCP client
A DHCP client uses the facilities of the IP stack to transmit and receive DHCP packets. This information is pro-
cessed by the client and passed back to the IP stack to complete interface configuration for the lease duration.
A DHCP client is created on a given interface by using the IP SET INTERFACE command with the parameter
DHCP enabled. After this, the IP settings are discovered for the interface (It's possible define one or more
interfaceconfig rules to customize the option that must be requested).
This section describes how these settings are discovered.
Firstly, the interface is disabled for all non-DHCP traffic. This will reset the IP address and subnet mask of each
nominated interface to 0.0.0.0.
The DHCP client learns its required configuration details via a DHCPDISCOVER request.
If configuration details are not successfully obtained using DHCP, the DHCP client will retry indefinitely in
order to learn them, as described in RFC2131 (unless the interface is disabled). Retry characteristics can be
defined using DHCPCLIENT SET RETRY command.
Once the DHCP client has accepted a suitable configuration for the interface, it has to configure the IP stack
appropriately. This involves allocating the new IP address to the interface and configuring the subnet for the
interface.