Droms R. The DHCP handbook

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The option statement can be in one of two forms. In the first, recommended form,

the value or values for an option are specified in the format specified by the option’s

declaration. The second form allows the entire contents of the option to be specified

as the result of a data expression. The second form is not recommended because the

configuration file parser can’t verify that the values are correct, but it is useful for

returning an option to the client whose value is derived from what the client sent.

option-values should include values that conform to the declaration of the option.

If the

option definition (see the section “The option Definition”) specifies a record

value—that is, a series of types enclosed in braces—those values should be separated

by spaces. If the

option definition specifies an array of values, each element in the

array should be separated by commas. An option whose value is an array of records

should be specified with the individual values in the record, separated by spaces, and

the records themselves should be separated by commas. Example B.3 shows

option

statements that illustrate each of these three possibilities.

Example B.3

option policy-filter 10.0.0.0 10.0.255.255;

option routers 10.0.0.1, 10.0.0.2;

option static-routes 10.0.0.0 10.0.0.18, 192.168.0.0 10.0.0.17;

The option Definition

option option-name code code = definition;

In the option definition, option-name should be the name of the option being

declared. If the option is not being defined in the

dhcp option space, option-name

should be the name of the option space in which it is being defined, followed by a

period (

.), followed by the actual name of the option. code should be the option

code—which for current DHCP options should be a number between 0 and 255.

definition should be the definition of the structure of the option—what data type

or types it is made up of and whether it is an array. The ISC DHCP server currently

supports a few simple types, such as integers, Booleans, strings, and IP addresses, and

it supports the ability to define arrays of single types or arrays of fixed sequences of

types (records).

The simple option types that are supported are described in the following sections.

The boolean Type

option name code code = boolean;

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An option of type boolean is a flag with a value of either on or off (true or false).

Here is an example of the use of the

boolean type:

option use-zephyr code 180 = boolean;

option use-zephyr on;

The integer Type

option name code code = [ signed | unsigned ] integer width;

Integers can be either signed or unsigned. If neither is specified, signed is assumed.

width can be 8, 16, or 32, and refers to the number of bits in the integer. The follow-

ing two lines show the definition of the

sql-connection-max option and its use:

option sql-connection-max code 192 = unsigned integer 16;

option sql-connection-max 1536;

The ip-address Type

option name code code = ip-address;

You can express an option whose structure is an IP address either as a domain name

or as a dotted quad. The following is an example of the use of the

ip-address type:

option sql-server-address code 193 = ip-address;

option sql-server-address sql.example.com;

The text Type

option name code code = text;

An option whose type is text encodes an ASCII text string, as in the following

example:

option sql-default-connection-name code 194 = text;

option sql-default-connection-name “PRODZA”;

The string Type

option name code code = string;

An option whose type is string is essentially a collection of bytes and can be speci-

fied either as quoted text, like the

text type, or as a list of hexadecimal byte values

separated by colons. Here is an example:

option sql-identification-token code 195 = string;

option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;

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Arrays

option name code code = array of type;

Options can contain arrays of any of the types shown in the preceding syntax

diagram, except for the text and data string types, which aren’t currently supported

in arrays. The following is an example of an array definition:

option kerberos-servers code 200 = array of ip-address;

option kerberos-servers 10.20.10.1, 10.20.11.1;

Records

option name code code = { type-1 [ , ... type-n ] };

Options can contain data structures that consist of a sequence of data types, which is

called a record type. Here is an example:

option contrived-001 code 201 = { boolean, integer 32, text };

option contrived-001 on 1772 “contrivance”;

It is also possible to have options that are arrays of records. Here is an example:

option new-static-routes code 201 = array of {

ip-address, ip-address, ip-address, integer 8 };

option new-static-routes

10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,

10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,

10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;

The Standard DHCP Options

The following are the option definitions for all the standard DHCP options:

option subnet-mask code 1 = ip-address;

option time-offset code 2 = signed integer 32;

option routers code 3 = array of ip-address;

option time-servers code 4 = array of ip-address;

option ien116-name-servers code 5 = array of ip-address;

option domain-name-servers code 6 = array of ip-address;

option log-servers code 7 = array of ip-address;

option cookie-servers code 8 = array of ip-address;

option lpr-servers code 9 = array of ip-address;

option impress-servers code 10 = array of ip-address;

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option resource-location-servers code 11 = array of ip-address;

option host-name code 12 = string;

option boot-size code 13 = unsigned integer 16;

option merit-dump code 14 = text;

option domain-name code 15 = text;

option swap-server code 16 = ip-address;

option root-path code 17 = text;

option extensions-path code 18 = text;

option ip-forwarding code 19 = boolean;

option non-local-source-routing code 20 = boolean;

option policy-filter code 21 = array of { ip-address, ip-address };

option max-dgram-reassembly code 22 = unsigned integer 16;

option default-ip-ttl code 23 = unsigned integer 8;

option path-mtu-aging-timeout code 24 = unsigned integer 32;

option path-mtu-plateau-table code 25 = array of unsigned integer 16;

option interface-mtu code 26 = unsigned integer 16;

option all-subnets-local code 27 = boolean;

option broadcast-address code 28 = ip-address;

option perform-mask-discovery code 29 = boolean;

option mask-supplier code 30 = boolean;

option router-discovery code 31 = boolean;

option router-solicitation-address code 32 = ip-address;

option static-routes code 33 = array of { ip-address, ip-address };

option trailer-encapsulation code 34 = boolean;

option arp-cache-timeout code 35 = unsigned integer 32;

option ieee802-3-encapsulation code 36 = boolean;

option default-tcp-ttl code 37 = unsigned integer 8;

option tcp-keepalive-interval code 38 = unsigned integer 32;

option tcp-keepalive-garbage code 39 = boolean;

option nis-domain code 40 = text;

option nis-servers code 41 = array of ip-address;

option ntp-servers code 42 = array of ip-address;

option vendor-encapsulated-options code 43 = string;

option netbios-name-servers code 44 = array of ip-address;

option netbios-dd-server code 45 = array of ip-address;

option netbios-node-type code 46 = unsigned integer 8;

option netbios-scope code 47 = text;

option font-servers code 48 = array of ip-address;

option x-display-manager code 49 = array of ip-address;

option dhcp-requested-address code 50 = ip-address;

option dhcp-lease-time code 51 = unsigned integer 32;

option dhcp-option-overload code 52 = unsigned integer 8;

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option dhcp-message-type code 53 = unsigned integer 8;

option dhcp-server-identifier code 54 = ip-address; the

option dhcp-parameter-request-list code 55 = array of unsigned integer 8;

option dhcp-message code 56 = text;

option dhcp-max-message-size code 57 = unsigned integer 16;

option dhcp-renewal-time code 58 = unsigned integer 32;

option dhcp-rebinding-time code 59 = unsigned integer 32;

option vendor-class-identifier code 60 = string;

option dhcp-client-identifier code 61 = string;

option nwip-domain code 62 = string;

option nisplus-domain code 64 = text;

option nisplus-servers code 65 = array of ip-address;

option tftp-server-name code 66 = text;

option bootfile-name code 67 = text;

option mobile-ip-home-agent code 68 = array of ip-address;

option smtp-server code 69 = array of ip-address;

option pop-server code 70 = array of ip-address;

option nntp-server code 71 = array of ip-address;

option www-server code 72 = array of ip-address;

option finger-server code 73 = array of ip-address;

option irc-server code 74 = array of ip-address;

option streettalk-server code 75 = array of ip-address;

option streettalk-directory-assistance-server code 76 = array of ip-address;

option slp-directory-agent code 78 = array of { boolean, ip-address };

option nds-servers code 85 = array of ip-address;

option nds-tree-name code 86 = string;

option nds-context code 87 = string;

option uap-servers code 98 = text;

option subnet-selection code 118 = ip-address;

option nwip.nsq-broadcast code 5 = boolean;

option nwip.preferred-dss code 6 = array of ip-address;

option nwip.nearest-nwip-server code 7 = array of ip-address;

option nwip.autoretries = unsigned integer 8;

option nwip.autoretry-secs = unsigned integer 8;

option nwip.nwip-1-1 = boolean;

option nwip.primary-dss = ip-address;

option fqdn.no-client-update = boolean;

option fqdn.server-update = boolean;

option fqdn.encoded = boolean;

option fqdn.rcode1 = unsigned integer 8;

option fqdn.rcode2 = unsigned integer 8;

option fqdn.hostname = text;

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option fqdn.domainname = text;

option fqdn.fqdn = text;

option agent.circuit-id code 1 = string;

option agent.remote-id code 2 = string; the

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The DHCP Message

Format

Every DHCP message includes a

fixed-format

section and

variable-format

section. The

fixed-format

section is

divided into several fields that carry the client’s hardware

and IP addresses, the server’s IP address, and other control

information. The

variable-format

section holds options

that carry additional control information about the

message and configuration parameters.

The fixed-format Section

Figure C.1 illustrates the format of the

fixed-format

section

of a DHCP message, and Table C.1 explains the use of each

of the fields identified in Figure C.1.

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APPENDIX C The DHCP Message Format504

FIGURE C.1 The fields in the fixed-format section of a DHCP message.

TABLE C.1 The Fields in the fixed-format Section of a DHCP Message

Field Length Description

op 1Message operation code; set to 1 in messages sent by a client,

and set to 2 in messages sent by a server. The two possible

values for op are carried forward from BOOTP for backward

compatibility and are sometimes called BOOTREQUEST and

BOOTREPLY, respectively.

htype 1Hardware address type; see Table C.2.

hlen 1 Link-layer address length (in bytes); defines the length of the

link-layer address in the chaddr field.

hops 1Number of relay agents that forwarded this message.

xid 4Transaction identifier; used by clients to match responses from

servers with previously transmitted requests.

secs 2 Elapsed time (in seconds) since the client began the DHCP

process.

flags 2 The leftmost bit of this field, called the broadcast bit, can be set

to 1 to indicate that messages to the client must be broadcast

(see the section “Using the Broadcast Flag” in Chapter 7,

“Transmitting DHCP Messages,” for details).

ciaddr 4Client’s IP address; set by the client when the client confirms

that its IP address is valid.

07815162324 31

htype

xid

secs flags

ciaddr

yiaddr

siaddr

giaddr

hlen hops

chaddr

(16 bytes)

sname

(64 bytes)

file

(128 bytes)

options

(variable)

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yiaddr 4Client’s IP address; set by the server to inform the client of the

client’s IP address—that is, “your” IP address.

siaddr 4 IP address of the next server for the client to use in the configu-

ration process; for example, the server to contact for TFTP

download of an operating system kernel.

giaddr 4 Relay agent (“gateway”) IP address; filled in by the relay agent

with the address of the interface through which the DHCP

message was received.

chaddr 16 Client’s link-layer address.

sname 64 DNS name of the next server for the client to use in the config-

uration process.

file 128 Name of the file for the client to request from the next server;

for example, the name of the file that contains the operating

system for this client.

The htype Field

Table C.2 lists the possible values for the htype field. RFC 2131 states that the htype

field contains the hardware type, as defined in RFC 1700, “Assigned Numbers.” This

list has been updated since RFC 1700 was issued, and the latest version is available at

www.iana.org, the Web site for the Internet Assigned Numbers Authority.

TABLE C.2 Values for the htype Field in the DHCP Message

htype Value Network Hardware Type

1 Ethernet (10Mbps–1000Mbps). In practice, this is used for all Ethernet

networks, even when IEEE 802.3 encapsulation is used.

2 Experimental Ethernet (3MB).

3 Amateur Radio AX.25.

4 Proteon ProNET Token Ring.

5 Chaos.

6 IEEE 802 networks. In practice, this is used for IBM Token Ring networks.

7 ARCNET.

8 Hyperchannel (FDDI).

9 Lanstar.

10 Autonet Short Address.

11 LocalTalk.

12 LocalNet (IBM PCNet or SYTEK LocalNET).

13 Ultra link.

14 SMDS.

The

fixed-format Section 505

TABLE C.1 Continued

Field Length Description

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