DHCP Overview

DHCP (Dynamic Host Configuration Protocol) is an extension of the BOOTP protocol that defines a protocol for passing configuration information to hosts on a TCP/IP network. The DHCP and BOOTP daemons are a subsidiary of inetd, and are started or restarted automatically (that is, as requests are passed to it).

The main advantage of DHCP is its capability to automatically allocate IP addresses to clients booting on the TCP/IP network for the first time.

The DHCP server passes the IP information and other start-up information to clients, including the name of the Domain Name Service (DNS) server. Additionally, DHCP passes the following startup information:

IP Subnet Mask
IP Routes
Broadcast IP Address
DNS Server
NIS Server
NTP Server

Benefits of Using DHCP

Using DHCP reduces the labor involved in managing the network. Before DHCP, the network administrator had to manually connect and configure every computer in a network.

Because the DHCP server automatically dispenses IP addresses and other configuration information, the process of connecting a new computer to the network is much simpler. DHCP is very flexible and allows the network administrator to set up the server one time to serve many thousands of clients.

DHCP Components and Concepts

The primary components of DHCP are as follows:

DHCP server
DHCP client
DHCP leases

DHCP Servers

The DHCP server dispenses and manages network IP addresses. It assigns IP addresses to clients that are connecting to the network for the first time. When a client connects to the network, the server automatically assigns it an IP address from an appropriate pool of IP addresses.

You can have multiple DHCP servers on your network provided their subnet pools do not overlap. However, it is recommended that you have only one DHCP server for your entire network. The server is responsible for the pool of IP addresses and can give out an IP address to a client requesting a new configuration from the pool of IP addresses for which it is responsible. When a client requests for confirmation of its existing configuration, the server confirms the configuration.

DHCP is a superset of the BOOTP bootstrap protocol. The DHCP server services the BOOTP clients. And DHCP servers and clients from different vendors interoperate very well with one another.

DHCP Clients

DHCP server supplies DHCP clients with sufficient information to establish an endpoint for network communications. It also supplies other parameters required by the system- and application-level software.

HP-UX workstations can run DHCP clients automatically. The autoparms script, /sin/auto_parms, enables HP-UX systems to run DHCP clients automatically. The autoparms script does not run, if you edit the configuration file /etc/rc.config.d/netconf.

DHCP clients can include TCP/IP network printers, X terminals, and Microsoft® Windows® machines. In addition to supporting new DHCP clients, the DHCP server supports new and existing BOOTP clients.

DHCP clients are currently supported on 10 BaseT and 100 BaseT ethernets. It is not supported on FDDI and Token Ring networks.

DHCP Leases

The DHCP server controls the IP address block. It grants permission to DHCP clients to use IP addresses on a lease basis. The IP address is leased to the client for a fixed amount of time. The administrator sets the lease time, which can last from 120 seconds to infinity. During the lease period, DHCP guarantees that the IP address assigned to the client is not reassigned to another client.

Before the lease time expires, the DHCP client automatically requests an extension on its lease. As long as the client can contact the DHCP server, the server renews the lease.

For example, when client A reboots, it attempts to renew the lease it had before being powered off. If client A was powered off when the renewal time elapsed, it can be assigned a different IP address. If the IP address is still unassigned when client A comes back online, the server can assign the same IP address to client A. But if the server assigned the IP address to client B while client A was offline, client A will be assigned a different IP address.

DHCP Transactions: Basic Operation

Figure 1-3 illustrates the interaction between the DHCP client and server.

Figure 1-3 DHCP Client and Server Transaction

The following describes the interaction between the client and the DHCP server in the Figure 1-3 and the steps involved in assigning an IP address to a client on the network:

A DHCP transaction begins when a client sends out a DHCP DISCOVER packet which is usually a broadcast packet. The packet contains only the client’s hardware address.
The server receives the DHCP DISCOVER packet. If an IP address on the client subnet is available and the server is willing to grant a lease, it makes an offer by sending a DHCP OFFER packet to the client. The offer packet contains the following information:
- Proposed IP address for the client
- Server name
- Server IP address
- Other configuration information
The client receives the DHCP OFFER packet. The client may receive more than one offer if more than one DHCP server exists on the network. HP recommends that you have only one DHCP server on the network. If the client is interested in the offer, it sends a DHCPREQUEST packet to the server. This indicates a formal request to lease the IP address offered by the server.
NOTE: The client rejects offers for IP addresses with short lease times. For example, the client rejects an offer with a lease time of 10 seconds.
The DHCP server receives the DHCPREQUEST packet and leases an IP address to the client. The server sends a DHCPACK packet to the client. This is an official notification that the address has been granted.
Before the lease time expires, the DHCP client requests to extend the lease by sending a DHCPREQUEST packet to the server. The server then updates and extends the lease time. It sends a DHCPACK packet to the client to notify that the lease has been extended. These updates and lease extensions continue as long as the client is powered on. If the lease expires and the client is not powered on, and is not able to request an extension on the lease, the IP address is recycled.
The DHCP server sends a DHCPACK packet to extend the lease on the IP address.