Droms R. The DHCP handbook

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Much like the lease on an apartment, the lease on an address is an agreement

between the DHCP server and the DHCP client that defines the time period during

which the client will use that address. When the lease expires, the DHCP server can

reallocate the address, with a new lease, to a new computer. Therefore, when the

lease granted to

desktop1 for the use of its address expires, dhcpserve is free to real-

locate the address to a new computer. In the same way the lease on an apartment

can be renewed, the lease on an IP address can be renewed. This allows a DHCP

client to continue to use the same address for much longer than the duration of an

individual lease. This mechanism is described in more detail in Chapter 3,

“Configuring the DHCP Server.”

NOTE

At a many large conferences, terminal rooms with network connections for laptops are

provided. Addresses are assigned to the laptops using DHCP. Because there are usually more

laptops than IP addresses available on such a terminal room network, the addresses must be

reassigned to newly arriving laptops as other laptops leave the network. The addresses are

assigned with relatively short lease times (for example, 2 hours) so that soon after a laptop is

disconnected from the terminal room network, the DHCP server can reassign that address to

another laptop.

Many more issues are related to the use of address leases than are suggested by the

simple example previously described. Chapter 19, “Tuning a DHCP Service,” explains

more about allocating leases, lease extension, and explicit lease termination.

It is important that DHCP servers manage IP addresses carefully because serious prob-

lems can be caused by allocating the same address to two different computers, and

these can be difficult to debug. For example, network communication with comput-

ers assigned to the same IP address may be unreliable and IP datagrams sent to an

address used by both computers may be delivered to only one or the other computer.

A DHCP server must take care not to reallocate to one computer an address currently

in use by another computer, as this would result in both computers using the same

address.

Two Alternative Mechanisms to Leasing

A lease might seem like an overly complex mechanism for the coordination of the

use of IP addresses. This section describes two alternative mechanisms:

• Explicit notification from a computer to the server that an address can be

reassigned

• Dynamic probing of addresses

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Both of these mechanisms were considered by the IETF DHC Working Group during

the development of the DHCP specification. Although both mechanisms are useful

and are, in fact, recommended for use in the DHCP specification to help detect some

duplicate assignments, neither can guarantee that two or more clients will not use

the same IP address.

Explicit Departure Messages

One way in which an IP address allocation server might learn that a computer has

left the 192.168.13.0 subnet is for that computer to send an explicit message that

informs the server that it is leaving. After receiving such a message, the server would

be free to allocate the address of the departed computer to another computer.

Unfortunately, it is impossible to depend on a computer to send a message when it is

disconnected from the network. Computers are not always shut down gracefully, and

users may not remember to send the departure message prior to disconnecting a

computer. Also, if a computer has been disconnected from the segment due to a

change in the network architecture—perhaps the computer has been connected to a

different hub in the wiring closet—there may be no reason to send a departure

message.

Dynamic Address Probing

Another strategy that can be used to determine when an IP address is no longer in

use is to probe to see if a computer is already using that address. For example, an IP

address server might send an ICMP echo request (

ping) to determine whether a candi-

date address might already be in use. If the server receives a matching ICMP echo

reply, a computer must be using the candidate address, so the server would continue

to probe for an unused address.

This strategy is impractical because it cannot reliably determine whether a specific

address has been assigned to a computer. If the IP address server receives an echo

reply after probing an address, that address is definitely in use. However, not receiv-

ing a reply does not necessarily mean that the address is not in use. A network

problem could exist between the server and the computer using the candidate

address, or that computer might simply be turned off. In either case, the server

would not receive an echo reply, even though the address is in use.

Benefits of DHCP Leases

DHCP specifies the use of leases in address allocation so that a server can know reli-

ably when it can reallocate an address. The lease constitutes an agreement between

the client and the server; the server cannot reallocate the address until the lease has

expired, and the client cannot use the address after the lease has expired. If the

computer to which the address was allocated never contacts the server again, the

server must wait until the lease expires before reallocating the address. Even if the

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computer to which the address was allocated has active network connections in

place, it must terminate those connections and stop using the address as soon as the

lease expires.

Summary

A computer communicates with a DHCP server in four basic ways. When the

computer is first turned on, it contacts the server for a new IP address and other

configuration parameters. Each time the computer is turned on after it has obtained

its initial configuration information, it confirms its address and parameters with the

server. As long as the computer remains connected to the same network segment, it

periodically contacts the DHCP server to renew the lease on the address it is using. If

the computer is moved to a new network segment, the server notifies the computer

that its address is no longer valid; the computer then contacts the server to obtain a

new address and configuration parameters that are appropriate to the new network

segment.

A DHCP server allocates a currently unused address to a computer that is newly

attached to a network segment. As more computers are attached to a network

segment, all the available addresses might be assigned. When all the addresses

have been allocated, a DHCP server begins to reuse previously allocated

addresses that are no longer in use. DHCP uses leases on addresses to determine

when an address is no longer in use. A DHCP server does not reassign an address

before its lease expires, and a computer must stop using an address as soon as the

lease on that address expires.

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IN THIS CHAPTER

• Specifying the Basic Network

Architecture

• Required Configuration

Parameters

• Specifying Leases

• Other DHCP Options

• Extending a Lease and

Moving Between Subnets

• Other Configuration

Information

Configuring the DHCP

Server

In managing an enterprise network, the network architect

designs the network architecture and determines the

configuration parameters to be assigned to hosts through-

out the network. When the network architecture has been

determined, the network architect must indicate the struc-

ture of the network to the DHCP server. Based on that

structure, the DHCP server selects configuration parameters

and appropriate addresses for DHCP clients.

The examples in this chapter are based on the GSI network

architecture example and scenarios described in Chapter 2,

“An Example of DHCP in Operation.” The configuration

files are designed for use with the ISC DHCP server and use

the syntax of the ISC DHCP server configuration files.

Specifying the Basic Network

Architecture

The network architect describes the network architecture

to the DHCP server by identifying the IP subnets, the

addresses, and the subnet masks for each of those subnets.

Using this information, the DHCP server associates incom-

ing DHCP messages with subnets in the network. Based on

the subnet from which a DHCP message was received, the

server selects an appropriate IP address to assign to the

client or determines that a DHCP client has moved to a

new subnet.

The ISC DHCP server configuration file is an ASCII text file

that contains a series of declarations describing the

network to be managed by the server. The server reads and

parses the file when it first starts running.

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CHAPTER 3 Configuring the DHCP Server30

Subnet Declarations

The basic subnet declaration in the ISC server configuration file follows the format

in Example 3.1.

Example 3.1

subnet subnet-address netmask subnet-mask {

subnet declarations

}

In this subnet declaration,

subnet-address

is the IP address of the subnet, and

subnet-

mask

is the subnet mask to be used with this subnet. Both

subnet-address

and

subnet-

mask

are written in dotted-decimal notation.

NOTE

In the examples in this chapter, keywords are shown in bold, and arguments that must be

supplied are shown in italic.

The sample network shown in Figure 3.1 is described with the partial configuration

file shown in Example 3.2. The sample configuration file includes a subnet declara-

tion for each of the five subnets, with the IP address for each subnet and the

255.255.255.0 subnet mask. Figure 3.1 shows the IP addresses in the network.

Internet

Router

192.168.11.0

192.168.12.0

192.168.13.0

192.168.14.0

192.168.15.0

FIGURE 3.1 IP addresses in the GSI network.

You can include comments in the configuration file for the ISC server as lines that

begin with the

character. Example 3.2 includes several comments that explain

some of the details of the configuration file.

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Example 3.2

# Server subnet

subnet 192.168.11.0 netmask 255.255.255.0 {

}

# Staff subnet 1

subnet 192.168.12.0 netmask 255.255.255.0 {

}

# Staff subnet 2

subnet 192.168.13.0 netmask 255.255.255.0 {

}

# Staff subnet 3

subnet 192.168.14.0 netmask 255.255.255.0 {

}

# Staff subnet 4

subnet 192.168.15.0 netmask 255.255.255.0 {

}

Subnet Address Allocation

In addition to defining the subnets, the network architect must define the range of

addresses within each subnet, or scope, that is available for allocation by the server.

Any addresses assigned to hosts or devices through some other mechanism must be

excluded from the range of available addresses for each subnet. For example, in the

GSI network, the router interface on each subnet is assigned the host address 254.

Thus, on the 192.168.11.0 subnet, the router uses address 192.168.11.254.

The network architect manually configures the router interfaces, rather than using

DHCP to assign the addresses. The server is configured so that the range of available

addresses on each subnet does not include the router’s address.

In the ISC server configuration file, the syntax for specifying the range of available

addresses in a subnet is shown in Example 3.3.

Example 3.3

range first-available-address last-available-address;

Example 3.4 gives the configuration file for the GSI network, specifying that IP

addresses 1 through 251 are available on the server subnet and IP addresses 1

through 253 are available on the other subnets for assignment to DHCP clients in

each subnet. This configuration file reserves host address 254 on each subnet for the

router interface on that subnet. The server subnet declaration also reserves addresses

for a DHCP server and a DNS server.

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Example 3.4

# Server subnet

subnet 192.168.11.0 netmask 255.255.255.0 {

range 192.168.11.1 192.168.11.251;

# 192.168.11.252 reserved for DHCP server

# 192.168.11.253 reserved for DNS server

# 192.168.11.254 reserved for router interface

}

# Staff subnet 1

subnet 192.168.12.0 netmask 255.255.255.0 {

range 192.168.12.1 192.168.12.253;

# 192.168.12.254 reserved for router interface

}

# Staff subnet 2

subnet 192.168.13.0 netmask 255.255.255.0 {

range 192.168.13.1 192.168.13.253;

# 192.168.13.254 reserved for router interface

}

# Staff subnet 3

subnet 192.168.14.0 netmask 255.255.255.0 {

range 192.168.14.1 192.168.14.253;

# 192.168.14.254 reserved for router interface

}

# Staff subnet 4

subnet 192.168.15.0 netmask 255.255.255.0 {

range 192.168.15.1 192.168.15.253;

# 192.168.15.254 reserved for router interface

}

Required Configuration Parameters

DHCP provides other configuration parameters in addition to an IP address. In fact,

several additional parameters must be provided to a TCP/IP host before that host can

communicate with other hosts. A host must be configured with the following:

• Its local subnet mask

• The address of at least one router on its subnet

• The address of a DNS server

These parameters are provided to a DHCP client through options in the DHCP

message. Chapter 9, “DHCP Options,” describes all the DHCP options in detail. A

few of the most commonly used options are discussed in the following sections.

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Configuration Options

The syntax for specifying an option is shown in Example 3.5.

Example 3.5

option option-name option-value;

If an

option

statement appears within a subnet declaration, it is applied to any DHCP

client in that subnet. In Example 3.6, adding a routers

option

statement to the decla-

ration for the 192.168.11.0 subnet causes the DHCP server to send 192.168.11.254 as

the default router address to any DHCP client on the 192.168.11.0 subnet.

Example 3.6

# Server subnet

subnet 192.168.11.0 netmask 255.255.255.0 {

range 192.168.11.1 192.168.11.251;

# 192.168.11.252 reserved for DHCP server

# 192.168.11.253 reserved for DNS server

# 192.168.11.254 reserved for router interface

option routers 192.168.11.254;

}

In addition to the routers option, the DHCP client also needs a subnet-mask and a

domain-name-servers option. GSI maintains a DNS server at 192.168.11.253, and the

subnet mask for every GSI network is the same: 255.255.255.0. The configuration file

for GSI, including default routers, subnet masks, and DNS servers, is shown in

Example 3.7.

Example 3.7

# Server subnet

subnet 192.168.11.0 netmask 255.255.255.0 {

range 192.168.11.1 192.168.11.251;

# 192.168.11.252 reserved for DHCP server

# 192.168.11.253 reserved for DNS server

# 192.168.11.254 reserved for router interface

option routers 192.168.11.254;

option subnet-mask 255.255.255.0;

option domain-name-servers 192.168.11.253;

}

# Staff subnet 1

subnet 192.168.12.0 netmask 255.255.255.0 {

range 192.168.12.1 192.168.12.253;

Required Configuration Parameters 33

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# 192.168.12.254 reserved for router interface

option routers 192.168.12.254;

option subnet-mask 255.255.255.0;

option domain-name-servers 192.168.11.253;

}

# Staff subnet 2

subnet 192.168.13.0 netmask 255.255.255.0 {

range 192.168.13.1 192.168.13.253;

# 192.168.13.254 reserved for router interface

option routers 192.168.13.254;

option subnet-mask 255.255.255.0;

option domain-name-servers 192.168.11.253;

}

# Staff subnet 3

subnet 192.168.14.0 netmask 255.255.255.0 {

range 192.168.14.1 192.168.14.253;

# 192.168.14.254 reserved for router interface

option routers 192.168.14.254;

option subnet-mask 255.255.255.0;

option domain-name-servers 192.168.11.253;

}

# Staff subnet 4

subnet 192.168.15.0 netmask 255.255.255.0 {

range 192.168.15.1 192.168.15.253;

# 192.168.15.254 reserved for router interface

option routers 192.168.15.254;

option subnet-mask 255.255.255.0;

option domain-name-servers 192.168.11.253;

}

Specifying Leases

Chapter 2 describes the use of leases as a mechanism through which a DHCP server

knows when a host will stop using an IP address. The DHCP specification allows a

lease to be up to 2

–2 seconds (49,710 days, or about 135 years).

NOTE

–2 is expressed as FFFFFFFE

. This is the largest number that can be stored in the 32-bit

lease field in a DHCP message. DHCP uses FFFFFFFF

to represent an infinite lease.

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Lease Durations

The DHCP specification does not include rules or requirements for lease allocation or

duration; those policies are defined by the network architect and are implemented

by the DHCP server. A DHCP client might request a particular lease duration, but the

DHCP server will always choose a lease duration based on the policies for lease

assignment defined by the network architect.

Default, Minimum, and Maximum Lease Lengths

The ISC server allows the network architect to specify a default lease length, a

minimum lease length, and a maximum lease length. The default lease length is used if

the client does not request a specific lease. The minimum lease length is used to

force the client to take a longer lease than it has requested. The maximum lease

length defines the longest lease that the server can allocate. If a client requests a

lease longer than the maximum lease length, then the server simply issues a lease

equal to the maximum lease length.

The syntax for defining the default, minimum and maximum lease lengths is shown

in Example 3.8, and

time is expressed in seconds.

Example 3.8

default-lease-time time;

max-lease-time time;

min-lease-time time;

Subnet Lease Lengths

Choosing appropriate lease times for a subnet depends on the types of hosts that will

connect to that subnet. Table 3.1 lists some types of subnets that might be a part of

the GSI network and examples of lease times.

TABLE 3.1 Examples of Lease Times

Type of Subnet Primary Use Default Lease

GSI training lab Students with laptops One class period plus 10 minutes

Conference room Visitors with laptops 2 hours

“Hotel” office Staff members who use it daily 12 hours

Telecommuters DSL/cable service 7 days

Staff offices Permanent staff members 30 days

Central servers Organization servers 3 months

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