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Setting Up a Reliable DHCP Service

   

Contents Next >

Setting Up a Reliable DHCP Service

  

15.1. -

Determining Your Level of DHCP Service Reliability

  

15.2. -

Specific Failures in DHCP Service

  

15.3. -

Improving Reliability Through Long Leases

  

15.4. -

Setting Up a Secondary DHCP Server

  

15.5. -

Problems with Setting Up Redundant Servers

  

 

Summary

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The DHCP Handbook

From: The DHCP Handbook
Author: Ralph Droms; Ted Lemon
Publisher: MTP
More Information

15. Setting Up a Reliable DHCP Service

As discussed in Chapter 1, “An Introduction to DHCP,” loss of DHCP service can be a major problem if you depend on such service for automated management of IP addresses and computer configurations. If a client can't access DHCP service, it can't obtain a new address or extend the lease on an address it already has. Thus, when DHCP service is unavailable, new computers and computers that move to new network segments may be unable to use network services, although computers with existing leases on IP addresses will be unable to use the network when those leases expire. Applications running on computers that depend on network service will also be disrupted.

This chapter discusses specific ways in which the DHCP service might fail and presents solutions for those failure modes. It also describes some more general DHCP service implementations that provide additional reliability through redundant DHCP servers.

15.1. Determining Your Level of DHCP Service Reliability

Before deciding on an implementation strategy for providing reliable DHCP service, it is appropriate to review how a loss of DHCP service will affect you and to determine the appropriate level of reliability for your organization and network infrastructure. The loss of DHCP service has two major effects. DHCP clients are unable to obtain new addresses, and they are unable to extend leases on addresses previously assigned through DHCP.

15.1.1. Effects of Loss of Service

In many circumstances, the loss of DHCP service does not immediately affect most DHCP clients and network users and, in fact, may be less disruptive in the short run than the loss of DNS or network file services. DNS service is used with every new connection that requires resolution of a DNS name, so loss of DNS service is immediately obvious to users. On the other hand, a DHCP client that was assigned an address will continue to function normally, and won't attempt to contact the DHCP server until half the duration of its lease expires. Even after it begins to request an extension on its lease, the DHCP client will continue to use its address. Only if the DHCP service is still unavailable when the lease actually expires must the DHCP client stop using its address and terminate network connections.

DHCP clients that restart while DHCP service is unavailable simply continue to use their previously assigned addresses until their leases expire. As long as a client's IP address is still appropriate for the network segment to which it is connected—that is, the client hasn't moved to a new network segment, or the network segment hasn't been assigned a new network number—the client can use its old address. The client notices the loss of DHCP service only when its lease actually expires.

Of course, clients that require DHCP service cannot access the network until such service is restored. If you administer a network to which laptop computers are frequently connected and disconnected, or if clients that you support request a short lease duration, many of your clients may be quickly affected by the loss of DHCP service. And, unfortunately, this loss of service may cause DHCP clients to fail in ways that your end users may not understand (and may not be patient about!). Different DHCP clients react in different ways when they cannot contact a DHCP server. In many cases, the user experiences long startup times while the DHCP client attempts to contact the DHCP server, along with unexplained loss of network access.

NOTE

Recent DHCP clients from Microsoft and Apple have an additional feature that may cause confusion when DHCP service is unavailable. If these clients cannot contact a DHCP server, they pick an address from a range of addresses that are reserved for auto-configuration. Unfortunately, a computer that performs this auto-configuration appears to be operating normally, but in effect it is using a IP address that cannot be used to reach destination computers not connected to its local network segment. The user has no indication of network initialization failure, but cannot access network services. A DHCP client that performs auto-configuration assigns itself an IP address on the 169.254.0.0/16 network. This mechanism for auto-configuration is currently documented as an Internet Draft, “Automatically Choosing an IP Address in an Ad-Hoc IPv4 Network,” and is available at http://www.ietf.cnri.reston.va.us/internet-drafts/draft-ietf-dhc-ipv4-autoconfig-05.txt

Because of the potential problems this auto-configuration mechanism may cause, the DHC working group developed a DHCP option that controls the use of auto-configuration in DHCP clients. This option, described in RFC2563, “DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients,” enables a network administrator to turn off the use of auto-configuration in DHCP clients through DHCPOFFER messages.

You must determine your own requirements for the reliability of your DHCP service based on the ways in which your clients access your network, the length of the leases you choose, and your tolerance for calls to your help desk. Although no single solution exists that fits the needs of every network, the next section covers some specific failure modes and suggests some solutions for those scenarios.

   

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Breaking News

One of the primary architects of OpenCable, Michael Adams, explains the key concepts of this initiative in his book OpenCable Architecture.

Expert Advice

Ralph Droms, Ph.D., author of The DHCP Handbook and chair of the IETF Dynamic Host Configuration Working Group, guides you to his top picks for reliable DHCP-related information.

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