Droms R. The DHCP handbook

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The user class option (as described in RFC 3004) gives textual information that a

DHCP client can use to identify the type or category of user or applications it repre-

sents. The data in the

user class option is structured as a sequence of separate user

class values. Each value consists of a single byte that gives the length of the value,

followed by the actual data values. A DHCP server uses the

user class option values

to select the configuration information to be assigned to the client.

TCP/IP Stack Configuration Parameters

The following sections describe several types of TCP/IP stack configuration parame-

ters. Most of the parameters apply to the IP layer—either to all IP traffic on the client

or to the traffic on a specific interface. Other parameters configure the link layer and

the TCP layer.

IP Layer Parameters for the Client

Each of the options in the following sections supplies a configuration parameter that

applies to all IP traffic on the client.

router

Option code: 3

Length: n

Data: list of IP addresses

The router option lists the addresses of default routers for the client to add to its

routing table. The routers are listed by their IP addresses, in order of preference. The

length field gives the total length of the list of routers and must be a multiple of 4; if

the list contains r routers, the length is

4r.

default IP time to live

Option code: 23

Length: 1

Data: 1 byte

The default IP time to live option gives the default value for the TTL (Time to

Live) field in the IP header of datagrams that the client transmits.

IP forwarding enable/disable

Option code: 19

Length: 1

Data: 0 disable forwarding

1 enable forwarding

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The IP forwarding enable/disable option controls whether a client that has more

than one interface should forward IP datagrams between its interfaces.

maximum datagram reassembly size

Option code: 22

Length: 2

Data: 16-bit integer

The maximum datagram reassembly size option specifies the largest fragmented IP

datagram the client is prepared to reassemble.

Nonlocal Source Routing Options

Two options —nonlocal source route enable/disable and policy filter—

control the forwarding of IP datagrams with nonlocal source routes (see Section 3.3.5

of RFC1122).

nonlocal source route enable/disable

Option code: 20

Length: 1

Data: 0 disable forwarding of datagrams with nonlocal source routes

1 enable forwarding of datagrams with nonlocal source routes

The nonlocal source route enable/disable option controls whether the client

forwards datagrams with nonlocal source routes.

policy filter

Option code: 21

Length: n

Data: list of filters

The policy filter option controls the filters applied to datagrams with nonlocal

source routes. Each filter includes an IP address and a subnet mask. The client

discards an IP datagram with a source route whose next hop does not match one of

the addresses (with its associated subnet mask applied) given in the

policy filter

option. The length field for the policy filter option must be a multiple of 8; if

f filters exist, the length is

8f.

Path Maximum Transmission Unit (PMTU) Options

The options described in the following sections control the PMTU mechanism in the

client.

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PMTU aging timeout

Option code: 24

Length: 4

Data: 32-bit integer

The PMTU aging timeout option specifies the timeout value to be used when PMTU

values are being aged. The option gives the timeout value in seconds.

PMTU plateau table

Option code:

Length: n

Data: list of 16-bit integers

The PMTU plateau table option specifies a table of MTU sizes for use in the PMTU

mechanism. Each MTU size is an unsigned 16-bit integer. The MTU sizes are listed in

order, from smallest to largest. The minimum MTU value must be at least 68. The

length of the PMTU plateau table list must be a multiple of 2; if p plateau values

exist, the length is

2p.

Options Defining IP Layer Parameters for a Specific Interface

The options in the following sections give configuration parameters that apply to the

IP traffic through a specific interface. Because a DHCP client with multiple network

interfaces must use DHCP separately for each interface, it can receive customized

configuration parameters for each interface.

subnet mask

Option code: 1

Length: 4

Data: subnet mask

The subnet mask option carries the subnet mask for the interface to which the

DHCP message is delivered. The subnet mask is represented as a 32-bit integer in

network byte order, with bits set to 1 corresponding to each bit in the address to be

used in a network or subnet number.

broadcast address

Option code: 28

Length: 4

Data: IP address

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The broadcast address option specifies the IP broadcast address for the network to

which the configured interface is attached. The address is represented as a 32-bit IP

address. Examples of broadcast addresses are all 1s (255.255.255.255), 0, or a network

broadcast address such as 192.1.1.255 or 192.1.1.0.

MTU for the interface

Option code:

Length: 2

Data: MTU in bytes for this interface

The MTU for the interface option gives the MTU to be used for this interface,

overriding any MTU deduced from the network interface hardware.

all subnets local

Option code: 27

Length: 1

Data: 1 all subnets have same MTU

0 some subnets may have smaller MTU

If the all subnets local option has value 1, the client can assume that all network

segments in the network to which the client is attached have the same MTU. If this

option has value

0, some network segments may have a smaller MTU.

perform mask discovery

Option code: 29

Length: 1

Data: 1 perform mask discovery

0 don’t perform mask discovery

If the perform mask discovery option has the value 1, the client should use the

ICMP

address mask request message (see RFC 950) to obtain the subnet mask for

this interface.

supply subnet mask

Option code: 30

Length: 1

Data: 1 supply subnet mask

0 don’t supply subnet mask

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If the supply subnet mask option has the value 1, the client should respond to

ICMP

address mask request messages.

static routes

Option code: 33

Length: n

Data: list of IP address pairs

The static routes option lists static routes for the client to install in its routing

table. Each static route is listed as the address of the destination and the address of

the router to use for that destination. If duplicate static routes exist for a destination,

the routes are listed in decreasing order of priority. The length of this option must be

a multiple of 8; if s static routes exist, the length is

8s.

NOTE

Since the definition of classless interdomain routing (CIDR), the static routes option is no

longer useful. The classless static routes option carries routes with subnet masks and

should be used in place of the static routes option.

Router Discovery Options

Two options—

perform router discovery and router solicitation address—

control the use of router discovery (see RFC 1256).

perform router discovery

Option code: 31

Length: 1

Data: 0 do not perform router discovery

1 perform router discovery

The perform router discovery option specifies whether the client should initiate

the router discovery protocol described in RFC 1256.

router solicitation address

Option code: 32

Length: 4

Data: IP address

The router solicitation address option gives the address the client should use to

transmit router discovery protocol messages.

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Link Layer Parameters Options

A few options affect the configuration of link-layer parameters. These options apply

only to the link-layer software for the interface over which the DHCP message is

received.

ARP cache timeout

Option code: 35

Length: 4

Data: 32-bit integer

The ARP cache timeout option specifies the lifetime for entries in the ARP cache.

The timeout value is represented in seconds.

Ethernet encapsulation

Option code: 36

Length: 1

Data: 0 Ethernet version 2 encapsulation

1 IEEE 802.3 encapsulation

The Ethernet encapsulation option selects either Ethernet version 2 (defined in

RFC 894) or IEEE 802.3 (defined in RFC 1042) encapsulation for IP datagrams.

trailer encapsulation

Option code: 34

Length: 1

Data: 0 do not use trailers

1 attempt to use trailers

The trailer encapsulation option controls whether the client should negotiate the

use of trailers (see RFC 893) by using ARP.

TCP Parameters Options

The options described in the following sections give values for TCP software configu-

ration parameters. These options affect the parameters for all TCP traffic on the

client.

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TCP default TTL

Option code: 37

Length: 1

Data: 8-bit integer

The TCP default TTL option sets the TTL that the TCP layer should use when

sending TCP segments.

TCP Keepalive Parameters Options

Two options—

TCP keepalive interval and TCP keepalive garbage—control the

operation of the TCP keepalive mechanism.

TCP keepalive interval

Option code:

Length:

Data:

do not use keepalive

nonzero integer

use keepalive interval

The TCP keepalive interval option specifies the time the client should wait before

a keepalive segment. The interval is given in seconds. If the data value is zero,

the client should not use keepalive segments unless an application specifically

requests it.

TCP keepalive garbage

Option code: 39

Length: 1

Data: 0 do not send garbage byte

1 send garbage byte

The TCP keepalive garbage option specifies whether the client should send a

garbage byte in keepalive segments (see section 4.2.3.6 of RFC 1122).

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Service Parameter Options

The options described in the following sections provide parameters for a variety of

services. Most of these options provide the address of a server or servers for a specific

service. Some options give additional information about a service, such as the

Network Information Service (NIS) or Network Information Service+ (NIS+).

time server

Option code: 4

Length: n

Data: list of IP addresses

The time server option lists addresses of RFC 868 time servers that are available to

the client. The length must be a multiple of 4; if s time servers are in the option, the

length is

4s.

name server

Option code: 5

Length: n

Data: list of IP addresses

The name server option lists addresses of Internet Name Servers (see IEN-116,

www.isi.edu/in-notes/ien/ien116.txt) that are available to the client. The length

must be a multiple of 4; if s name servers are in the option, the length is

4s.

domain server

Option code: 6

Length: n

Data: list of IP addresses

The domain server option lists addresses of Domain Name Service (DNS) servers (see

RFC 1035) that are available to the client. The length must be a multiple of 4; if s

DNS servers are in the option, the length is

4s.

log server

Option code: 7

Length: n

Data: list of IP addresses

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The Log server option lists addresses of logging servers that are available to the

client. The length must be a multiple of 4; if s logging servers are in the option, the

length is

4s.

quotes server

Option code: 8

Length: n

Data: list of IP addresses

The quotes server option lists addresses of quote of the day servers (see RFC 865)

that are available to the client. The length must be a multiple of 4; if s quote servers

are in the option, the length is

4s.

LPR server

Option code: 9

Length: n

Data: list of IP addresses

The LPR server option lists addresses of Line Printer Protocol (LPR) servers (see

RFC 1179) that are available to the client. The length must be a multiple of 4; if

s LPR servers are in the option, the length is

4s.

Impress server

Option code: 10

Length: n

Data: list of IP addresses

The Impress server option lists addresses of Impress print servers that are available

to the client. The length must be a multiple of 4; if s impress servers are in the

option, the length is

4s.

RLP server

Option code: 11

Length: n

Data: list of IP addresses

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The RLP server option lists addresses of Resource Location Protocol (RLP) servers

(see RFC 887) that are available to the client. The length must be a multiple of 4; if

s RLP servers are in the option, the length is

4s.

SMTP server

Option code: 69

Length: n

Data: list of IP addresses

The SMTP server option lists addresses of Simple Mail Transport Protocol (SMTP; see

RFC 821) servers that are available to the client. The length must be a multiple of 4;

if s SMTP servers are in the option, the length is

4s.

POP server

Option code: 70

Length: n

Data: list of IP addresses

The POP server option lists addresses of Post Office Protocol version 3 (POP3; see

RFC 1939) servers that are available to the client. If more than one address is in the

list, the POP3 servers are listed in order of preference. The length must be a multiple

of 4; if s POP3 servers are in the option, the length is

4s.

NTP server

Option code: 42

Length: n

Data: list of IP addresses

The NTP server option lists addresses of Network Time Protocol (NTP; see RFC 1305)

servers that are available to the client. If more than one address is in the list, the

NTP servers are listed in order of preference. The length must be a multiple of 4; if

s NTP servers are in the option, the length is

4s.

finger server

Option code: 73

Length: n

Data: list of IP addresses

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