DHCP Parameter Configuration Process For Clients With Non-DHCP Addresses
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The majority of DHCP clients make use of the protocol to obtain both an IP address and other configuration parameters. This is the reason why so much of DHCP is oriented around address assignment and leasing. A conventional DHCP client obtains all its configuration parameters at the same time it gets an IP address, using the message exchanges and processes we have seen in the preceding topics of this section.
There are cases, however, where a device with an IP address assigned using a method other than DHCP still wants to use DHCP servers to obtain other configuration parameters. The main advantage of this is administrative convenience; it allows a device with a static IP address to still be able to automatically get other parameters the same way that regular DHCP clients do.
Ironically, one common case where this capability can be used is configuring DHCP servers themselves! Administrators normally do not use DHCP to provide an IP address to a DHCP server, but they may want to use it to tell the server other parameters. In this case, the server requesting the parameters actually acts as a client for the purpose of the exchange with another server.
The original DHCP standard did not provide any mechanism for this sort of non-IP configuration to take place. RFC 2131 revised the protocol, adding a new message type (DHCPINFORM) that allows a device to request configuration parameters without going through the full leasing process. This message is used as part of a simple bidirectional communication that is separate from the leasing communications we have looked at so far. Since it doesn't involve IP address assignment, it is not part of the lease life cycle, nor is it part of the DHCP client finite state machine.
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