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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 Client/Server Implementation, Features and Issues 1 2 3 DHCP Client General Implementation and Management Issues

DHCP is a client/server protocol, relying on both server and client to fulfill certain responsibilities. Of the two device roles, the DHCP server is arguably the more important, because it is in the server that most of the functionality of DHCP is actually implemented. The server maintains the configuration database, keeps track of address ranges and manages leases. For this reason, DHCP servers are also typically much more complex than DHCP clients.

In essence, without a DHCP server, there really is no DHCP. Thus, deciding how to implement DHCP servers is a large part of implementing the protocol. This overall chapter is about describing the function of protocols like DHCP and not getting into details of how to implement them. However, I feel it is useful to look at some of the general issues related to how DHCP servers are set up and used, to help put into perspective how the protocol really works.

A “classical” DHCP server consists of DHCP server software running on a server hardware platform of one sort or another. A DHCP server usually will not be a dedicated computer except on very large networks. It is more common for a hardware server to provide DHCP services along with performing other functions, such as acting as an application server, general database server, providing DNS services and so forth. So, a DHCP server need not be a special computer; any device that can run a DHCP server implementation can act as a server.

In fact, the DHCP server may not even need to be a host computer at all. Today, many routers include DHCP functionality. Programming a router to act as a DHCP server allows clients that connect to the router to be automatically assigned IP addresses. This provides numerous potential advantages in an environment where a limited number of public IP addresses is shared amongst multiple clients, or where IP Network Address Translation (NAT) is used to dynamically share a small number of addresses. Since DHCP requires a database, a router that acts as a DHCP server requires some form of permanent storage. This is often implemented using flash memory on routers, while “true” servers of course use hard disk storage.

Virtually all modern operating systems include support for DHCP, including most variants of UNIX, Linux, newer versions of Microsoft Windows, Novell NetWare and others. In some cases, you may need to run the “server version” of the operating system to have a host act as a DHCP server. For example, while Microsoft Windows XP supports DHCP, I don't believe that a DHCP server comes in “Windows XP Home”, the “home user” version. (Of course, you could install one yourself!)

DHCP Client/Server Implementation, Features and Issues 1 2 3 DHCP Client General Implementation and Management Issues