HPlogo Installing and Administering Internet Services: HP 9000 Networking > Chapter 6 Dynamic Host Configuration Protocol (DHCP)

DHCP Components and Concepts
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The primary components of DHCP discussed in this section include the DHCP server, DHCP client, and 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 addresses.

You can have multiple DHCP servers on your network as long as 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 a pool of IP addresses. It 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 asks for confirmation of its existing configuration, the server confirms the configuration.

DHCP is a superset of the older BOOTP bootstrap protocol. The HP-UX DHCP server will service older 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 needed by system- and application-level software.

HP-UX workstations can run DHCP clients automatically. Autoparms script enables HP-UX to run DHCP clients automatically. If you edit the configuration, the autoparms script will not run.

DHCP clients can include TCP/IP network printers, X terminals, and Microsoft Windows machines. In addition to supporting new DHCP clients, the HP-UX 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 has control of the IP address block. It grants DHCP clients permission 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, DHCP guarantees that the IP address assigned to the client will not be re-assigned 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 will renew 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 off-line, client A will be assigned a different IP address.

DHCP Transactions: Basic Operation

This section covers the basic interaction between a client and DHCP server to assign an IP address to a client on the network.

  1. A DHCP transaction begins when a client sends out a DHCP DISCOVER packet. This is usually a broadcast packet. At minimum, the packet will contain only the client's hardware address.

  2. 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:

    • Proposed IP address for client

    • Server's name

    • Server's IP address

    • Other configuration information

  3. The client receives the DHCP OFFER packet. The client might receive more than one offer if you have more than one DHCP server on the network. HP strongly recommends that you have only one DHCP server on the network. If the client likes 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 HP-UX client rejects offers for IP addresses with very short lease times. For example, the client will reject an offer with a lease time of 10 seconds.

  4. The DHCP server receives the DHCPREQUEST packet and grants the client its request to lease the IP address. The server sends a DHCPACK to the client. This is the official notification that the address has been granted.

  5. 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 to the client to notify it 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 not able to request an extension on the lease, the IP address is recycled.

  6. The DHCP server sends DHCPACK to extend the lease on the IP address.

Figure 6-1 “DHCP Client and Server Transaction” illustrates what takes place between the DHCP client and server.

Figure 6-1 DHCP Client and Server Transaction

DHCP Client and Server Transaction


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