TCP/IP address and parameter assignment - Dynamic Host Configuration Protocol

Transmission Control Protocol/Internet Protocol (TCP/IP) enables communication between machines with configured addresses. Part of the burden a network administrator must face is address assignment and parameter distribution for all machines on the network. Commonly, this is a process in which the administrator dictates the configuration to each user, allowing the user to configure his own machine. However, misconfigurations and misunderstandings can generate service calls that the administrator must deal with individually. The Dynamic Host Configuration Protocol (DHCP) gives the network administrator a method to remove the end user from this configuration problem and maintain the network configuration in a centralized location.

DHCP is an application-layer protocol that allows a client machine on the network, to get an IP address and other configuration parameters from the server. It gets information by exchanging packets between a daemon on the client and another on the server. Most operating systems now provide a DHCP client in their base package.

To obtain an address, the DHCP client daemon (dhcpcd) broadcasts a DHCP discover message, which is received by the server and processed. (Multiple servers can be configured on the network for redundancy.) If a free address is available for that client, a DHCP offer message is created, This message contains an IP address and other options that are appropriate for that client. The client receives the server DHCP offer and stores it while waiting for other offers. When the client chooses the best offer, it broadcasts a DHCP request that specifies which server offer it wants.

All configured DHCP servers receive the request. Each checks to see if it is the requested server. If not, the server frees the address assigned to that client. The requested server marks the address as assigned and returns a DHCP acknowledgment, at which time, the transaction is complete. The client has an address for the period of time (lease) designated by the server.

When half of the lease time is used, the client sends the server a renew packet to extend the lease time. If the server is willing to renew, it sends a DHCP acknowledgment. If the client does not get a response from the server that owns its current address, it broadcasts a DHCP rebind packet to reach the server if, for example, the server has been moved from one network to another. If the client has not renewed its address after the full lease time, the interface is brought down and the process starts over. This cycle prevents multiple clients on a network from being assigned the same address.

The DHCP server assigns addresses based on keys. Four common keys are network, class, vendor, and client ID. The server uses these keys to get an address and a set of configuration options to return to the client.

network

Identifies which network segment the packet came from. The network key allows the server to check its address database and assign an address by network segment.

class

Is completely client configurable. It can specify an address and options. This key can be used to denote machine function in the network or to describe how machines are grouped for administrative purposes. For example, the network administrator might want to create a netbios class that contains options for NetBIOS clients or an accounting class that represents Accounting department machines that need access to a specific printer.

vendor

Helps identify the client by its hardware/software platform (for example, a Microsoft Windows 95 client or an OS/2 Warp client).

client ID

Identifies the client either through the machine host name or its medium access control (MAC) layer address. The client ID is specified in the configuration file of the dhcpcd daemon. Also, the client ID can be used by the server to pass options to a specific client or prohibit a particular client from receiving any parameters.

These keys can be used by the configuration either singularly or in combinations. If multiple keys are provided by the client and multiple addresses can be assigned, only one is chosen, and the option set is derived from the chosen key first. For more detailed information about the selection of keys and addresses, see DHCP configuration.

A relay agent is needed so initial broadcasts from the client can leave the local network. This agent is called the BOOTP relay agent. The relay agents act as forwarding agents for DHCP and BOOTP packets.