Droms R. The DHCP handbook

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Example 8.1

ETHER: ----- Ether Header -----

ETHER: Destination = ff:ff:ff:ff:ff:ff, (broadcast)

ETHER: Source = 8:0:20:7c:fb:89, Sun

IP: ----- IP Header -----

IP: Protocol = 17 (UDP)

IP: Source address = 0.0.0.0, OLD-BROADCAST

IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 68

UDP: Destination port = 67 (BOOTPS)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1c

DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 0.0.0.0

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: Message type = DHCPDISCOVER

As described in Chapter 7, this message is sent to the Ethernet broadcast address

FF:FF:FF:FF:FF:FF and to the IP limited broadcast address 255.255.255.255. The client

uses the IP address 0.0.0.0 as the UDP source address and in the

ciaddr address. The

DHCP message type option identifies this message as a DHCPDISCOVER message, and

the client does not include additional DHCP options.

The

DHCPOFFER Message

After the server receives the

DHCPDISCOVER message from the client, it finds an

address to assign to the client and puts it in a

DHCPOFFER message. The server also

includes in the

DHCPOFFER message other configuration parameters for the client, as

defined by the server’s configuration file. After the server has completed the

DHCPOF-

FER message, it sends the message back to the client.

Example 8.2 shows the DHCPOFFER message that is sent in response to the previous

DHCPDISCOVER message.

Example 8.2

ETHER: ----- Ether Header -----

ETHER: Destination = ff:ff:ff:ff:ff:ff, (broadcast)

ETHER: Source = 8:0:20:76:f:8, Sun

IP: ----- IP Header -----

IP: Protocol = 17 (UDP)

IP: Source address = 192.168.11.252

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IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 67

UDP: Destination port = 68 (BOOTPC)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1c

DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 192.168.11.25

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: Message type = DHCPOFFER

DHCP: DHCP Server Identifier = 192.168.11.252

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

This message is sent to Ethernet broadcast address ff:ff:ff:ff:ff:ff and to the newly

assigned IP address 192.168.11.25. The server copies the transaction identifier,

c8206f1c

, from desktop1’s DHCPDISCOVER message, so the client can identify the

DHCPOFFER response. The server gives the client a lease time of 30 days and indicates

that the client should try to extend its lease after 15 days.

The

DHCPREQUEST Message

After

desktop1 receives the DHCPOFFER message from dhcpserve, it sends a DHCPRE-

QUEST message, asking for the configuration information from dhcpserve. The

DHCPREQUEST message is shown in Example 8.3.

Example 8.3

ETHER: ----- Ether Header -----

ETHER: Destination = FF:FF:FF:FF:FF:FF, (broadcast)

ETHER: Source = 8:0:20:7c:fb:89, Sun

IP: ----- IP Header -----

IP: Protocol = 17 (UDP)

IP: Source address = 0.0.0.0, OLD-BROADCAST

IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 68

UDP: Destination port = 67 (BOOTPS)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1d

Client States 107

Example 8.2 Continued

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DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 0.0.0.0

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: DHCP Server Identifier = 192.168.11.252

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

DHCP: Message type = DHCPREQUEST

DHCP: Requested IP Address = 192.168.11.25

In this DHCPREQUEST message, desktop1 asks for the address and other configuration

parameters that

dhcpserve supplied in the DHCPOFFER message. desktop1 uses a new

transaction identifier, and broadcasts the message using the FF:FF:FF:FF:FF:FF link-

layer and 255.255.255.255 IP-layer broadcast addresses.

The

DHCPACK Message

After receiving the

DHCPREQUEST message, dhcpserve checks the requested address

and configuration parameters to ensure that the address is still available and the

parameters are correct.

dhcpserve records the assigned address and sends the DHCPACK

message shown in Example 8.4 to desktop1.

Example 8.4

ETHER: ----- Ether Header -----

ETHER: Destination = ff:ff:ff:ff:ff:ff, (broadcast)

ETHER: Source = 8:0:20:76:f:8, Sun

IP: ----- IP Header -----

IP: Source address = 192.168.11.252

IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 67

UDP: Destination port = 68 (BOOTPC)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1d

DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 192.168.11.25

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: Message type = DHCPACK

DHCP: DHCP Server Identifier = 192.168.11.252

CHAPTER 16 Working with Graphs108

Example 8.3 Continued

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DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

Figure 8.2 shows a time line of the messages exchanged by desktop1 and dhcpserve

as the client obtains its initial address.

When desktop1 receives this DHCPACK message, it records the assignment informa-

tion and configures its TCP/IP software with the IP address and other parameters.

Then,

desktop1 can begin using TCP/IP.

Client States 109

Example 8.4 Continued

DHCPOFFERDHCPDISCOVER DHCPACKDHCPREQUEST

desktop1

Time

dhcpserve

FIGURE 8.2 A time line of messages exchanged between a client and a server to assign

an initial address.

Confirming an IP Address When Restarting

Every time desktop1 restarts—for example, after it is powered on—it checks whether

it has recorded an address with a lease that has not expired. After

desktop1 is

installed and assigned its initial address, it typically has an address it can reuse. If

desktop1 has an IP address whose lease has not expired, it goes into INIT-REBOOT

state and attempts to confirm that its address is still valid.

The

INIT-REBOOT DHCPREQUEST Message

The client sends the IP address to be confirmed in a

DHCPREQUEST message, which is

received and checked by all DHCP servers that are configured for the network

segment to which the client is attached. Example 8.5 shows a

DHCPREQUEST message

sent by a client in the

INIT-REBOOT state.

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Example 8.5

ETHER: ----- Ether Header -----

ETHER: Destination = ff:ff:ff:ff:ff:ff, (broadcast)

ETHER: Source = 8:0:20:7c:fb:89, Sun

IP: ----- IP Header -----

IP: Protocol = 17 (UDP)

IP: Source address = 0.0.0.0, OLD-BROADCAST

IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 68

UDP: Destination port = 67 (BOOTPS)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1f

DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 0.0.0.0

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

DHCP: Message type = DHCPREQUEST

DHCP: Requested IP Address = 192.168.11.25

This message is broadcast because desktop1’s address may be invalid, even if the

lease on that address has not expired. For example, if

desktop1 moves to a new

office, or if the network architect assigns a new address to the network to which

desktop1 is attached, desktop1 has an address that does not match the local IP

subnet.

When

dhcpserve receives desktop1’s DHCPREQUEST message, it extracts desktop1’s

requested address from the

options section and checks that the address is from an IP

subnet assigned to the network segment to which

desktop1 is attached. Unless

desktop1 has been moved, the address is usually correct, so desktop1 is allowed to

use it.

The

DHCPACK Message

dhcpserve replies to desktop1 with a DHCPACK message. dhcpserve puts all the

configuration parameters in the response message and returns the message to

desktop1. Example 8.6 shows the details of a DHCPACK message.

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Example 8.6

ETHER: ----- Ether Header -----

ETHER: Destination = ff:ff:ff:ff:ff:ff, (broadcast)

ETHER: Source = 8:0:20:76:f:8, Sun

IP: ----- IP Header -----

IP: Source address = 192.168.11.252

IP: Destination address = 255.255.255.255, BROADCAST

UDP: ----- UDP Header -----

UDP: Source port = 67

UDP: Destination port = 68 (BOOTPC)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1f

DHCP: Client address (ciaddr) = 0.0.0.0

DHCP: Your client address (yiaddr) = 192.168.11.25

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: Message type = DHCPACK

DHCP: DHCP Server Identifier = 192.168.11.252

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

When desktop1 receives the DHCPACK message, it uses the parameters from the

message to set its IP address and protocol software configuration. Then it is ready to

use the network. Figure 8.3 shows a time line of the messages exchanged between

desktop1 and dhcpserve as the client rechecks its IP address when restarting.

Client States 111

DHCPACKDHCPREQUEST

desktop1

Time

dhcpserve

FIGURE 8.3 A time line of messages exchanged between a client and a server when the

client restarts.

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If desktop1 receives no response to its broadcast message when it first starts up, it

may be that DHCP servers are inaccessible due to a power outage or a temporary

network problem. Rather than keep DHCP clients from using the network if the

servers do not respond, a client can use its previous address if the lease on that

address is still valid.

Extending a Lease

Suppose desktop1 continues running without being turned off or restarting.

Eventually, the lease on the address assigned to

desktop1 will run out. The leasing

mechanism is designed to give DHCP servers a reliable way to reclaim unused

addresses, not to take away addresses from computers that are still in use. DHCP

provides a way for a computer to extend the lease on its address without interrupt-

ing network use.

A DHCP client extends its lease by sending a message to a server, requesting more

time on the lease. The request for an extension is sent in a

DHCPREQUEST message,

and the client can ask for a lease of whatever length it chooses. At this point, the

DHCP server decides how long an extension to grant and returns the new lease dura-

tion to the client in a

DHCPACK message.

The choice of lease length is up to the server. In fact, the server can choose not to

extend the lease or it can ignore lease extension requests altogether.

The Lease Extension Request

A DHCP client is said to be in the

RENEWING state when it begins asking for a lease

extension. Example 8.7 shows an example of a

DHCPREQUEST message from desktop1,

asking for an extension of its current lease on 192.168.11.0. Note that the message is

sent directly to

dhcpserve and includes the length of the extension desktop1 wants

as well as the other protocol parameters that

desktop1 is using.

Example 8.7

ETHER: ----- Ether Header -----

ETHER: Destination = 8:0:20:76:f:8, Sun

ETHER: Source = 8:0:20:7c:fb:89, Sun

IP: ----- IP Header -----

IP: Protocol = 17 (UDP)

IP: Source address = 192.168.11.25

IP: Destination address = 192.168.11.252

UDP: ----- UDP Header -----

UDP: Source port = 68

UDP: Destination port = 67 (BOOTPS)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1c

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DHCP: Client address (ciaddr) = 192.168.11.25

DHCP: Your client address (yiaddr) = 192.168.11.25

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: DHCP Server Identifier = 192.168.11.252

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

DHCP: Message type = DHCPREQUEST

The Lease Extension Response

After receiving the

DHCPREQUEST message from desktop1, the server confirms that its

configured lease policy allows it to extend the lease on the address assigned to

desktop1. If the lease can be extended, the server determines the appropriate config-

uration parameters for

desktop1 and sends a DHCPACK message with options for those

parameters. Example 8.8 shows

dhcpserve’s response to desktop1’s request to extend

the lease.

Example 8.8

ETHER: ----- Ether Header -----

ETHER: Destination = 8:0:20:7c:fb:89, Sun

ETHER: Source = 8:0:20:76:f:8, Sun

IP: Protocol = 17 (UDP)

IP: Source address = 192.168.11.252

IP: Destination address = 192.168.11.25

UDP: ----- UDP Header -----

UDP: Source port = 67

UDP: Destination port = 68 (BOOTPC)

DHCP: ----- Dynamic Host Configuration Protocol -----

DHCP: Transaction ID = 0xc8206f1c

DHCP: Client address (ciaddr) = 192.168.11.25

DHCP: Your client address (yiaddr) = 192.168.11.25

DHCP: Client hardware address (chaddr) = 08:00:20:7C:FB:89

DHCP: ----- (Options) field options -----

DHCP: Message type = DHCPACK

DHCP: DHCP Server Identifier = 192.168.11.252

DHCP: IP Address Lease Time = 2592000 seconds

DHCP: Renewal (T1) Time Value = 1296000 seconds

DHCP: Rebinding (T2) Time Value = 2268000 seconds

DHCP: Subnet Mask = 255.255.255.0

Client States 113

Example 8.7 Continued

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In this response, dhcpserve agrees to desktop1’s request for an extension of 2592000

seconds. When

desktop1 receives the DHCPACK message from dhcpserve, it records

the lease duration and other parameters from the message. Figure 8.4 shows the time

line of these messages.

CHAPTER 16 Working with Graphs114

DHCPACKDHCPREQUEST

desktop1

Time

dhcpserve

FIGURE 8.4 A time line of messages exchanged by a client and a server to extend a

lease.

The lease is extended without interrupting other applications. This process does not

affect the user; as long as a DHCP server that can extend the lease is available, a

DHCP client can run indefinitely without any impact from the DHCP leasing mech-

anism.

The time at which

desktop1 begins asking to extend its lease is called T1. The DHCP

server can explicitly tell a client when to extend its lease by configuring T1, which is

also known as the renewal time. If the server does not give the client a value for T1, it

defaults to one-half the original lease.

In Example 8.8, if

desktop1 does not receive a response to its lease extension

request, it retransmits the request according to the rules described in Chapter 7. As

long as the lease extension is received before the lease expires,

desktop1 can

continue using the network.

Extending a Lease from a Different Server

If a client in the RENEWING state fails to contact its original DHCP server, the client

enters the

REBINDING state, called T2, at a later time.

In the REBINDING state, the client broadcasts its DHCPREQUEST messages to all DHCP

servers, rather than unicasting them only to the server that gave it its current lease.

At this point, if another server receives the

DHCPREQUEST message and can extend the

lease, it does so, and the client can continue operating with its lease.

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NOTE

This mechanism for finding alternate servers for extending a lease was included in DHCP to

enhance reliability. If the server from which a DHCP client received its lease is unavailable for

an extended period, the client automatically locates another server. If the DHCP server that

granted a client its lease is moved to a new computer system, with a new IP address, the

client automatically finds the server at its new address when it cannot contact it at the old

address.

Using alternate DHCP servers implies that the servers can coordinate the information about

clients and leases. If dhcpserve gives an address to desktop1, other servers must learn about

that address and its lease before they can extend the lease. The DHCP specification in RFC

2131 does not define a standard way for servers to exchange lease information. Chapter 10,

“Failover Protocol Operation.” And Chapter 17, “Setting Up a Reliable DHCP Service,” discuss

some alternatives for using multiple DHCP servers.

In the REBINDING state, after a server has received the broadcast DHCPREQUEST message

from the client, it unicasts the

DHCPACK message to the client. When the client

receives a

DHCPACK message, it records the new lease and the responding server’s

address. The next time the client extends its lease, it uses this new server.

When a Lease Expires

If a client is unable to contact a server to renew its lease before the lease expires, the

client must stop using its IP address and go back to the

INIT state. When the client’s

lease on an address expires while the client is using the network, all active TCP

connections are dropped, and the user might have to reconnect to network applica-

tions that were in use. Data (and tempers!) can be lost when the client’s lease

expires, so it is important for the client to extend the lease before it expires. Ways of

preventing lease expiration are discussed in Chapter 17.

Moving to a New Network

If desktop1 moves to a new network segment while it is powered off, DHCP allows it

to quickly discover that it has moved. When it starts up after the move, it broadcasts

DHCPREQUEST message for its old IP address. When dhcpserve receives the DHCPRE-

QUEST message, it compares the new network segment to the requested address and

determines that the address will not work on that network segment. For example, if

desktop1 moves from the 192.168.11.0 subnet to the 192.168.12.0 subnet, it broad-

casts its previous address—say, 192.168.11.25—in a

DHCPREQUEST message. The relay

agent on the 192.168.12.0 subnet forwards the message to the server. The server

checks its configuration information and finds that the client’s requested address,

192.168.11.25, is not on the same network segment as the 192.168.12.0 subnet,

which means that

desktop1’s address is invalid.

Client States 115

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