DHCP Configuration Parameters, Storage and Communication
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One of the more important oversights in DHCP's predecessor, BOOTP, is that the original protocol allowed a server to tell a client only three pieces of information: its IP address, the name of the server it could use to download a boot file, and the name of the boot file to use. This was a result of BOOTP's legacy as a protocol created primarily to let diskless workstations be bootstrapped.
Obviously, the IP address is a very important parameter, but in modern networks it isn't the only one that a client needs to be given for it to function properly. A typical host needs to be given other essential information to allow it to know how it should operate on its local network and interact with other devices. For example, it needs to know the address of a default local router, the subnet mask for the subnet it is on, parameters for creating outgoing IP datagrams, and much more.
The inability to specify additional configuration parameters in BOOTP was resolved by using the special BOOTP Vendor-Specific Area for vendor-independent vendor information fields, as first defined in RFC 1048. In DHCP, this idea has been extended further, and more important, formalized, as part of the effort to make DHCP a more general-purpose configuration tool. Configuration parameter storage, maintenance and communication is no longer an optional feature, but an essential part of the host configuration process.
Just as DHCP servers are the bosses that own and manage IP addresses, they also act as the repository for other configuration parameters that belong to DHCP clients. This centralization of parameter storage provides many of the same benefits that centralizing IP addresses in DHCP does: parameters can be checked and adjusted in a single place rather than having to go to each client machine.
Each DHCP server is programmed with parameters that are to be communicated to clients in addition to an IP address when an address is assigned. Alternately, a client that has already been assigned an address using some other mechanism may still query the DHCP server to get parameter information, using the DHCPINFORM message type. (This was actually added to the protocol in RFC 2131; it was not in the original DHCP standard.)
The exact method of storage of client parameters is to some extent implementation-dependent. Typically, there will be some parameters that apply to all clients. For example, on a small network with only one router, that router will probably be the default router for every DHCP client regardless of address.
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