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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Client/Server Implementation, Features and Issues

DHCP Server General Implementation and Management Issues 1 DHCP Message Relaying and BOOTP Relay Agents

I said in the previous topic that without DHCP servers, there would be no DHCP, which is true. DHCP servers are where most of the protocol is implemented, so they are the heart of the protocol. With servers you have DHCP, but without clients, there's nothing to actually use DHCP, so nobody would even care. Thus, even though they are less critical than servers when viewed strictly from the point of view of the protocol, DHCP clients are still quite important.

Just as a DHCP server consists of server software running on a server platform or hardware acting as a server, a DHCP client is simply DHCP client software running on a client device. Most often, a client device is a host computer connected to a TCP/IP internetwork. DHCP is today so widely accepted that virtually all hosts include DHCP client software. The DHCP client is usually integrated into graphical operating systems like Windows, or is implemented using a specific client daemon like dhclient or dhcpcd on UNIX/Linux.

Since the entire idea behind DHCP is to put the server in charge of parameter storage, configuration and address management, DHCP clients are relatively simple. The client implements the messaging protocol and communicates parameters received from the DHCP server to the rest of the TCP/IP software components as needed. It doesn't do a whole lot else.

In fact, there's not really much for an administrator to do to set up a client to use DHCP. In some operating systems, it's as simple as “throwing a switch”, by enabling DHCP support within the client itself. This prompts the client to then stop using any manually-configured parameters and start searching for a DHCP server instead. The server then becomes responsible for the client's configuration and address assignment.

Since the client doesn't do a great deal in DHCP other than communicate with the server, not much is required in the way of user software for a DHCP client. In most cases, control over the DHCP client software is accomplished using a TCP/IP configuration utility. Windows clients use the programs ipconfig or winipcfg to display the status of their current DHCP leases. These programs also allow the client to manually release the current lease or renew it.

Releasing the lease means early lease termination using the DHCPRELEASE message. This is usually the only way that a lease is terminated. Renewing the lease is a manual version of the automated renewal process. Releasing and renewing the lease may be done in sequence to reset a client that is in a confused state or is having some other type of DHCP or connectivity problem.

DHCP Server General Implementation and Management Issues 1 DHCP Message Relaying and BOOTP Relay Agents