Droms R. The DHCP handbook

Подождите немного. Документ загружается.

NOTE

Although RFC 2132 suggests the typed format for client identifiers, this is not required, and

some DHCP clients simply send a text string with no type identifier.

server identifier

Option code: 54

Length: 4

Data: IP address

The server identifier option gives the IP address of the server involved in the

DHCP transaction. This option is used by the server to send the server’s IP address to

the client, and it is used by the client to identify the server to which the client

intends to deliver a

DHCPREQUEST message. If a server has more than one network

interface, it uses the IP address of the interface on which it received the DHCP

message to which it is responding.

The

server identifier option differs from the siaddr field in the DHCP message

header section. A client uses the server identifier to determine the source of a DHCP

message delivered to the client and to indicate for which server a broadcast DHCP

message is intended. The

siaddr field holds the IP address of the server that the

client should contact to obtain additional bootstrap services, such as additional

configuration information or an operating system kernel, through a network proto-

col such as TFTP.

requested address

Option code: 50

Length: 4

Data: IP address

The requested IP address option contains the IP address that the client requests

when it does not have explicit confirmation that its current address is valid. A client

includes its previous IP address in a

requested IP address option when sending a

DHCPREQUEST message during restart.

CHAPTER 9 DHCP Options126

012 3273 CH09 10/3/02 4:57 PM Page 126

lease time

Option code: 51

Length: 4

Data: lease time

The value of the lease time field indicates the duration of the lease for an address

assigned to a client. The

lease time value is an unsigned 32-bit number that repre-

sents the length of the lease, in seconds. The reserved value FFFFFFFF

indicates a

lease that never expires (in other words, a lease that is of infinite duration).

When a server sends a message to a client, the

lease time option represents the

length of the lease the server selected according to the network’s policies. When a

client sends a message to a server, the

lease time option represents the length of

the lease the client is requesting from the server. The lease time that the server

supplies is authoritative, and the client must honor it, regardless of the lease time it

requests.

lease renewal time

(T1)

Option code: 58

Length: 4

Data: T1

T1 represents the point in time when a client begins extending the lease on its

address. T1 and T2 are always specified as a number of seconds relative to the current

time. Beginning at time T1, the client unicasts

DHCPREQUEST messages to the server

from which the lease on the address was obtained. This option specifies the value in

T1 to the client as a 32-bit unsigned integer representing T1 in seconds. If the server

does not use this option to specify T1 to the client, the client uses half the initial

lease duration for T1.

lease renewal time

(T2)

Option code: 59

Length: 4

Data: T2

DHCP-Specific Options 127

012 3273 CH09 10/3/02 4:57 PM Page 127

T2 is the time when a client begins finding a new server through which it can

extend the lease on its address. Beginning at time T2, the client broadcasts

DHCPRE-

QUEST messages to locate a server that is willing to extend its lease. This option speci-

fies

T2 as a 32-bit unsigned integer representing T2 in seconds. If the server does not

use this option to specify T2 to the client, the client uses seven-eighths of the initial

lease duration for T2.

Some clients ignore the values of T1 and T2 that are specified by the server in these

options. Instead, these clients compute T1 to be one-half of the duration of the lease

and T2 to be seven-eighths of the duration of the lease. If you find that a client is

not attempting to extend its lease at the time specified in the

T1 option, the client

might be one that ignores values of T1 and T2 that are sent from the server.

vendor class identifier

Option code: 60

Length: n

Data: vendor class identifier

A DHCP client uses the vendor class identifier option to pass information about

the client’s vendor type and configuration. The server uses this option to interpret

the contents of the

vendor-specific options field, and (optionally) to select

specific configuration parameters for a client. The identifier is a string of opaque byte

values that is not terminated with a null character.

vendor-specific information

Option code: 43

Length: n

Data: vendor-specific information

The vendor-specific information option carries information that is interpreted

according to the client vendor type, as specified in the

vendor class identifier

option. This option enables a vendor to define new options used only by its clients,

without going through the standards process or consuming limited option code

space.

If more than one option is carried in the

vendor-specific information option,

then the options are encoded in the same way as DHCP options and encapsulated

in the

vendor-specific information option data area. For example, if a vendor

defines two options with option codes 125 and 126, a

vendor-specific

information option that carries those vendor codes is encoded as follows:

43 9 125 4 192 168 7 4 126 1 1

CHAPTER 9 DHCP Options128

012 3273 CH09 10/3/02 4:57 PM Page 128

In this example, the option code is 43, specifying the vendor-specific information

option. The length is 9, giving the length of all the data for the option. The first

encapsulated option is the vendor’s option

125, with a length of 4 and data

192.168.7.4. The second encapsulated option is 126, with a length of 1 and data 1.

parameter request list

Option code: 55

Length: n

Data: n option codes

A DHCP client uses the parameter request list option to request specific parame-

ter values from a server. Each byte in the parameter request list is a DHCP option

code that the client wants the server to provide. The server includes in its response

to the client values for the requested option, along with other options that are

required by DHCP.

NOTE

The exact behavior of the parameter request list option is not defined. Some DHCP

servers return just the options that are listed in the parameter request list only if there is a

parameter request list option. Some DHCP servers return options that are not listed in the

parameter request list option anyway. The safest things a DHCP client can do are either

to specify in the parameter request list every option that it wants to receive or to not send a

parameter request list option at all.

message

Option code: 56

Length: n

Data: n characters

DHCP servers and clients use the message option to transmit an error message to a

DHCP message recipient. The format of the contents of the

message option is

unspecified and is typically a

character-string message that is displayed to a user

or recorded in a log file.

maximum DHCP message size

Option code: 57

Length: 2

Data: length

DHCP-Specific Options 129

012 3273 CH09 10/3/02 4:57 PM Page 129

A client or server uses the maximum DHCP message size option to advertise that it

will accept incoming messages that are larger than the default maximum size for

DHCP message (576 bytes). The length is stored as an unsigned 16-bit integer and

must not be less than 576.

option overload

Option code: 52

Length: 1

Data: 1 file field holds options

2sname field holds options

3 both fields hold options

If the option overload option is present in a DHCP message, the message recipient

concatenates the specified fields with the

options field and interprets the options in

the resulting list. See the section “Examples of Message Formats” in Chapter 6 for a

more detailed explanation.

TFTP server name

Option code: 66

Length: n

Data: n characters

The TFTP server name option identifies a TFTP server for the client to use in the

next phase of its bootstrap process, when the

sname field in the DHCP header has

been used for DHCP options. The name is a string that is not terminated with a null

character.

bootfile name

Option code: 67

Length: n

Data: n characters

The bootfile name option identifies a bootfile name for the client to use when the

file field in the DHCP header is used for DHCP options. The name is a string that is

not terminated with a null character.

CHAPTER 9 DHCP Options130

012 3273 CH09 10/3/02 4:57 PM Page 130

pad

Option code: 0

The pad option carries no information and is skipped when the options field is inter-

preted. It can be used, for example, to pad the

options section to the BOOTP stan-

dard of 64 bytes.

end

Option code: 255

The end option indicates the end of the options carried in the options field. Because

the end of the

options field in the DHCP message can be inferred from the length of

the UDP message in which the DHCP message is transmitted, some DHCP clients

and servers do not send the

end option.

NOTE

Only the pad and end options do not include length or data fields.

Host Configuration Parameters Options

These options provide configuration parameters that apply to the host.

host name

Option code: 12

Length: n

Data: n characters

The host name option gives the client’s name. This name can be only the client’s

name or the client’s name, qualified with the local domain name. The

host name

option is a string that is not terminated with a null character.

A client uses the host name option to inform the DHCP server of the name the

client is using. A server uses the host name option to inform the client of the name

it should use for itself. RFCs 2131 and 2132 are unclear as to how clients and servers

should react if they disagree about the name the client should use.

Host Configuration Parameters Options 131

012 3273 CH09 10/3/02 4:57 PM Page 131

domain name

Option code: 15

Length: n

Data: n characters

The domain name option specifies the name of the client’s domain for resolving

names in the Domain Name System (DNS). The name is a string of characters that is

not terminated with a null character.

NOTE

The domain name option specifies only a single domain for name resolution. The domain

option, which is about to be accepted as a standard DHCP option, specifies a list of

domain names used in name resolution.

client FQDN

Option code: 81

Length: n

Data: flag (1 byte)

rcode1 (1 byte)

rcode2 (1 byte)

domain name (n-3)

The client FQDN option is used by both DHCP clients and DHCP servers, in order to

negotiate whether and how the client and server will update the domain name

server with the client’s address information. A DHCP server only sends an FQDN

option to a DHCP client in response to a message from the client that contains an

FQDN option.

DHCP clients use the

client FQDN option to tell DHCP servers what hostname or

fully qualified domain name (FQDN) the client wants to use. When a DHCP server

receives a

client FQDN option from a client, it can respond to the client with a

client FQDN option that either confirms the client’s choice or provides the client

with the name the server wants the client to use.

The first byte of the

client FQDN option is a flag byte. Four bits in the flag byte

have been assigned codes:

CHAPTER 9 DHCP Options132

012 3273 CH09 10/3/02 4:57 PM Page 132

Bit Code

1 S

2 O

4 E

8 N

If the client wants the DHCP server to update its A record, the client sets the S bit

in the

flag byte. If the client intends to update its own A record, it clears the S bit. If

the client does not want the DHCP server to do any DNS update at all for the client,

the client sets the

N bit; otherwise, it leaves the N bit clear. DHCP clients never set the

O bit.

If the DHCP server receives a client FQDN option from the client and it supports the

client FQDN option, it always sends a client FQDN option back to the client. If the

server is configured to update the client’s A record, it sets both the

O and the S bits to

inform the client of its policy. DHCP servers never set the

N bit.

DHCP clients and servers can both set the E bit. The DHCP server always sets the E

bit the same way that it is set in the client’s message. All new DHCP clients must set

the

E bit, but older DHCP clients might not set it. If it is set, the E bit indicates

that the domain name is encoded in DNS wire format. If it is clear, the

E bit indi-

cates that the domain name is encoded as in the DNS presentation format (for

example, the ASCII string

foo.example.com is in presentation format).

The rcode1 and rcode2 bytes are present for backward compatibility with old clients,

but they are not used.

When the client sends an FQDN option, it puts the unqualified hostname or fully

qualified domain name that it wants to use in the domain name field.

When the server responds to the client, it either sends the client the same name that

the client sent or, if it wants to provide the client with a different name, it sends that

name instead.

Chapter 23, ”Configuring DHCP–DDNS Interactions,” describes how DHCP clients

and servers do DNS updates and how the

client FQDN option is used. The client

FQDN option has only recently begun to be implemented, and many clients do not

support it. Clients that do support it usually also send a

hostname option. No stan-

dard says which option the server should use if both are present, although common

sense suggests that the

client FQDN option should be used.

Host Configuration Parameters Options 133

012 3273 CH09 10/3/02 4:57 PM Page 133

time offset

Option code: 2

Length: 4

Data: 32-bit signed integer

The time offset option specifies which time zone offset from coordinated universal

time the client should use. (Coordinated universal time is often abbreviated UTC,

which is the abbreviation for the French name, Universal Temp Coordiné.) The offset

is a signed 32-bit integer that expresses the offset in seconds. A positive offset indi-

cates a location east of the zero meridian, and a negative offset indicates a location

west of the zero meridian.

bootfile size

Option code: 13

Length: 2

Data: 16-bit unsigned integer

The bootfile size option gives the size of the client’s bootfile. The size is an

unsigned 16-bit integer that specifies the number of 512-byte blocks in the bootfile.

root path

Option code: 17

Length: n

Data: n characters

The root path option gives the filename (that is, the full pathname) of the directory

that is being used as the client’s root disk partition. Diskless clients use it to mount a

network disk from the server that is identified in the

siaddr or sname fields.

swap server

Option code: 16

Length: 4

Data: IP address

CHAPTER 9 DHCP Options134

012 3273 CH09 10/3/02 4:57 PM Page 134

The swap server option gives the IP address of a server that provides a swap space

service (that is, a service such as providing swap storage through the network for a

diskless workstation) for the client.

extensions path

Option code: 18

Length: n

Data: n characters

The extensions path option gives the filename (that is, the full pathname) of a file

that contains additional options for the client. The name is a string that is not termi-

nated with a null character. The client obtains a copy of the file identified in the

extensions path option by using TFTP from the server that is identified in the

siaddr or sname fields. After retrieving the file, the client interprets the contents of

the file by using the same syntax used in the contents of the

options field.

merit dump file

Option code: 14

Length: n

Data: n characters

The merit dump file option gives the name of a file where the client should dump

a core image if the client crashes. The name is a string that is not terminated with a

null character.

NOTE

DHCP options that specify filenames do not specify a particular format for the filenames. The

format of the filenames is dictated by the needs of the client and of the servers that the client

will be contacting.

user class

Option code: 77

Length: n

Data: n characters

Host Configuration Parameters Options 135

012 3273 CH09 10/3/02 4:57 PM Page 135