dhcpd-options(5)dhcpd-options(5)NAMEdhcp-options - Dynamic Host Configuration Protocol options
DESCRIPTION
The Dynamic Host Configuration protocol allows the client
to receive options from the DHCP server describing the
network configuration and various services that are avail-
able on the network. When configuring dhcpd(8) or
dhclient(8) , options must often be declared. The syntax
for declaring options, and the names and formats of the
options that can be declared, are documented here.
REFERENCE: OPTION STATEMENTS
DHCP option statements always start with the option key-
word, followed by an option name, followed by option data.
The option names and data formats are described below.
It is not necessary to exhaustively specify all DHCP
options - only those options which are needed by clients
must be specified.
Option data comes in a variety of formats, as defined
below:
The ip-address data type can be entered either as an
explicit IP address (e.g., 239.254.197.10) or as a domain
name (e.g., haagen.isc.org). When entering a domain name,
be sure that that domain name resolves to a single IP
address.
The int32 data type specifies a signed 32-bit integer.
The uint32 data type specifies an unsigned 32-bit integer.
The int16 and uint16 data types specify signed and
unsigned 16-bit integers. The int8 and uint8 data types
specify signed and unsigned 8-bit integers. Unsigned
8-bit integers are also sometimes referred to as octets.
The text data type specifies an NVT ASCII string, which
must be enclosed in double quotes - for example, to spec-
ify a root-path option, the syntax would be
option root-path "10.0.1.4:/var/tmp/rootfs";
The domain-name data type specifies a domain name, which
must not enclosed in double quotes. This data type is
not used for any existing DHCP options. The domain name
is stored just as if it were a text option.
The flag data type specifies a boolean value. Booleans
can be either true or false (or on or off, if that makes
more sense to you).
The string data type specifies either an NVT ASCII string
enclosed in double quotes, or a series of octets specified
in hexadecimal, seperated by colons. For example:
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option dhcp-client-identifier "CLIENT-FOO";
or
option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
SETTING OPTION VALUES USING EXPRESSIONS
Sometimes it's helpful to be able to set the value of a
DHCP option based on some value that the client has sent.
To do this, you can use expression evaluation. The dhcp-
eval(5) manual page describes how to write expressions.
To assign the result of an evaluation to an option, define
the option as follows:
option my-option = expression ;
For example:
option hostname = binary-to-ascii (16, 8, "-",
substring (hardware, 1, 6));
STANDARD DHCP OPTIONS
The documentation for the various options mentioned below
is taken from the latest IETF draft document on DHCP
options. Options not listed below may not yet be imple-
mented, but it is possible to use such options by defining
them in the configuration file. Please see the DEFINING
NEW OPTIONS heading later in this document for more infor-
mation.
The standard options are:
option all-subnets-local flag;
This option specifies whether or not the client may
assume that all subnets of the IP network to which the
client is connected use the same MTU as the subnet of
that network to which the client is directly connected.
A value of true indicates that all subnets share the
same MTU. A value of false means that the client
should assume that some subnets of the directly con-
nected network may have smaller MTUs.
option arp-cache-timeout uint32;
This option specifies the timeout in seconds for ARP
cache entries.
option bootfile-name text;
This option is used to identify a bootstrap file. If
supported by the client, it should have the same effect
as the filename declaration. BOOTP clients are
unlikely to support this option. Some DHCP clients
will support it, and others actually require it.
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option boot-size uint16;
This option specifies the length in 512-octet blocks of
the default boot image for the client.
option broadcast-address ip-address;
This option specifies the broadcast address in use on
the client's subnet. Legal values for broadcast
addresses are specified in section 3.2.1.3 of STD 3
(RFC1122).
option cookie-servers ip-address [, ip-address... ];
The cookie server option specifies a list of RFC 865
cookie servers available to the client. Servers should
be listed in order of preference.
option default-ip-ttl uint8;
This option specifies the default time-to-live that the
client should use on outgoing datagrams.
option default-tcp-ttl uint8;
This option specifies the default TTL that the client
should use when sending TCP segments. The minimum
value is 1.
option dhcp-client-identifier string;
This option can be used to specify the a DHCP client
identifier in a host declaration, so that dhcpd can
find the host record by matching against the client
identifier.
Please be aware that some DHCP clients, when configured
with client identifiers that are ASCII text, will
prepend a zero to the ASCII text. So you may need to
write:
option dhcp-client-identifier "\0foo";
rather than:
option dhcp-client-identifier "foo";
option dhcp-max-message-size uint16;
This option, when sent by the client, specifies the
maximum size of any response that the server sends to
the client. When specified on the server, if the
client did not send a dhcp-max-message-size option, the
size specified on the server is used. This works for
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BOOTP as well as DHCP responses.
option dhcp-parameter-request-list uint16;
This option, when sent by the client, specifies which
options the client wishes the server to return. Nor-
mally, in the ISC DHCP client, this is done using the
request statement. If this option is not specified by
the client, the DHCP server will normally return every
option that is valid in scope and that fits into the
reply. When this option is specified on the server,
the server returns the specified options. This can be
used to force a client to take options that it hasn't
requested, and it can also be used to tailor the
response of the DHCP server for clients that may need a
more limited set of options than those the server would
normally return.
option domain-name text;
This option specifies the domain name that client
should use when resolving hostnames via the Domain Name
System.
option domain-name-servers ip-address [, ip-address... ];
The domain-name-servers option specifies a list of
Domain Name System (STD 13, RFC 1035) name servers
available to the client. Servers should be listed in
order of preference.
option extensions-path-name text;
This option specifies the name of a file containing
additional options to be interpreted according to the
DHCP option format as specified in RFC2132.
option finger-server ip-address [, ip-address... ];
The Finger server option specifies a list of Finger
available to the client. Servers should be listed in
order of preference.
option font-servers ip-address [, ip-address... ];
This option specifies a list of X Window System Font
servers available to the client. Servers should be
listed in order of preference.
option host-name string;
This option specifies the name of the client. The name
may or may not be qualified with the local domain name
(it is preferable to use the domain-name option to
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specify the domain name). See RFC 1035 for character
set restrictions.
option ieee802-3-encapsulation flag;
This option specifies whether or not the client should
use Ethernet Version 2 (RFC 894) or IEEE 802.3 (RFC
1042) encapsulation if the interface is an Ethernet. A
value of false indicates that the client should use RFC
894 encapsulation. A value of true means that the
client should use RFC 1042 encapsulation.
option ien116-name-servers ip-address [, ip-address... ];
The ien116-name-servers option specifies a list of IEN
116 name servers available to the client. Servers
should be listed in order of preference.
option impress-servers ip-address [, ip-address... ];
The impress-server option specifies a list of Imagen
Impress servers available to the client. Servers
should be listed in order of preference.
option interface-mtu uint16;
This option specifies the MTU to use on this interface.
The minimum legal value for the MTU is 68.
option ip-forwarding flag;
This option specifies whether the client should config-
ure its IP layer for packet forwarding. A value of
false means disable IP forwarding, and a value of true
means enable IP forwarding.
option irc-server ip-address [, ip-address... ];
The IRC server option specifies a list of IRC available
to the client. Servers should be listed in order of
preference.
option log-servers ip-address [, ip-address... ];
The log-server option specifies a list of MIT-LCS UDP
log servers available to the client. Servers should be
listed in order of preference.
option lpr-servers ip-address [, ip-address... ];
The LPR server option specifies a list of RFC 1179 line
printer servers available to the client. Servers
should be listed in order of preference.
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option mask-supplier flag;
This option specifies whether or not the client should
respond to subnet mask requests using ICMP. A value of
false indicates that the client should not respond. A
value of true means that the client should respond.
option max-dgram-reassembly uint16;
This option specifies the maximum size datagram that
the client should be prepared to reassemble. The mini-
mum value legal value is 576.
option merit-dump text;
This option specifies the path-name of a file to which
the client's core image should be dumped in the event
the client crashes. The path is formatted as a charac-
ter string consisting of characters from the NVT ASCII
character set.
option mobile-ip-home-agent ip-address [, ip-address... ];
This option specifies a list of IP addresses indicating
mobile IP home agents available to the client. Agents
should be listed in order of preference, although nor-
mally there will be only one such agent.
option nds-context string;
The nds-context option specifies the name of the ini-
tial Netware Directory Service for an NDS client.
option nds-servers ip-address [, ip-address... ];
The nds-servers option specifies a list of IP addresses
of NDS servers.
option nds-tree-name string;
The nds-context option specifies NDS tree name that the
NDS client should use.
option netbios-dd-server ip-address [, ip-address... ];
The NetBIOS datagram distribution server (NBDD) option
specifies a list of RFC 1001/1002 NBDD servers listed
in order of preference.
option netbios-name-servers ip-address [, ip-address...];
The NetBIOS name server (NBNS) option specifies a list
of RFC 1001/1002 NBNS name servers listed in order of
preference. NetBIOS Name Service is currently more
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commonly referred to as WINS. WINS servers can be
specified using the netbios-name-servers option.
option netbios-node-type uint8;
The NetBIOS node type option allows NetBIOS over TCP/IP
clients which are configurable to be configured as
described in RFC 1001/1002. The value is specified as
a single octet which identifies the client type.
Possible node types are:
1 B-node: Broadcast - no WINS
2 P-node: Peer - WINS only.
4 M-node: Mixed - broadcast, then WINS
8 H-node: Hybrid - WINS, then broadcast
option netbios-scope string;
The NetBIOS scope option specifies the NetBIOS over
TCP/IP scope parameter for the client as specified in
RFC 1001/1002. See RFC1001, RFC1002, and RFC1035 for
character-set restrictions.
option nwip-domain string;
The name of the NetWare/IP domain that a NetWare/IP
client should use.
option nwip-suboptions string;
A sequence of suboptions for NetWare/IP clients - see
RFC2242 for details. Normally this option is set by
specifying specific NetWare/IP suboptions - see the
NETWARE/IP SUBOPTIONS section for more information.
option nis-domain text;
This option specifies the name of the client's NIS (Sun
Network Information Services) domain. The domain is
formatted as a character string consisting of charac-
ters from the NVT ASCII character set.
option nis-servers ip-address [, ip-address... ];
This option specifies a list of IP addresses indicating
NIS servers available to the client. Servers should be
listed in order of preference.
option nisplus-domain text;
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This option specifies the name of the client's NIS+
domain. The domain is formatted as a character string
consisting of characters from the NVT ASCII character
set.
option nisplus-servers ip-address [, ip-address... ];
This option specifies a list of IP addresses indicating
NIS+ servers available to the client. Servers should
be listed in order of preference.
option nntp-server ip-address [, ip-address... ];
The NNTP server option specifies a list of NNTP avail-
able to the client. Servers should be listed in order
of preference.
option non-local-source-routing flag;
This option specifies whether the client should config-
ure its IP layer to allow forwarding of datagrams with
non-local source routes (see Section 3.3.5 of [4] for a
discussion of this topic). A value of 0 means disallow
forwarding of such datagrams, and a value of true means
allow forwarding.
option ntp-servers ip-address [, ip-address... ];
This option specifies a list of IP addresses indicating
NTP (RFC 1035) servers available to the client.
Servers should be listed in order of preference.
option path-mtu-aging-timeout uint32;
This option specifies the timeout (in seconds) to use
when aging Path MTU values discovered by the mechanism
defined in RFC 1191.
option path-mtu-plateau-table uint16 [, uint16... ];
This option specifies a table of MTU sizes to use when
performing Path MTU Discovery as defined in RFC 1191.
The table is formatted as a list of 16-bit unsigned
integers, ordered from smallest to largest. The mini-
mum MTU value cannot be smaller than 68.
option perform-mask-discovery flag;
This option specifies whether or not the client should
perform subnet mask discovery using ICMP. A value of
false indicates that the client should not perform mask
discovery. A value of true means that the client
should perform mask discovery.
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option policy-filter ip-address ip-address
[, ip-address ip-address...];
This option specifies policy filters for non-local
source routing. The filters consist of a list of IP
addresses and masks which specify destination/mask
pairs with which to filter incoming source routes.
Any source routed datagram whose next-hop address does
not match one of the filters should be discarded by the
client.
See STD 3 (RFC1122) for further information.
option pop-server ip-address [, ip-address... ];
The POP3 server option specifies a list of POP3 avail-
able to the client. Servers should be listed in order
of preference.
option resource-location-servers ip-address
[, ip-address...];
This option specifies a list of RFC 887 Resource Loca-
tion servers available to the client. Servers should
be listed in order of preference.
option root-path text;
This option specifies the path-name that contains the
client's root disk. The path is formatted as a charac-
ter string consisting of characters from the NVT ASCII
character set.
option router-discovery flag;
This option specifies whether or not the client should
solicit routers using the Router Discovery mechanism
defined in RFC 1256. A value of false indicates that
the client should not perform router discovery. A
value of true means that the client should perform
router discovery.
option router-solicitation-address ip-address;
This option specifies the address to which the client
should transmit router solicitation requests.
option routers ip-address [, ip-address... ];
The routers option specifies a list of IP addresses for
routers on the client's subnet. Routers should be
listed in order of preference.
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option slp-directory-agent boolean ip-address [, ip-
address... ];
This option specifies two things: the IP addresses of
one or more Service Location Protocol Directory Agents,
and whether the use of these addresses is mandatory.
If the initial boolean value is true, the SLP agent
should just use the IP addresses given. If the value
is false, the SLP agent may additionally do active or
passive multicast discovery of SLP agents (see RFC2165
for details).
Please note that in this option and the slp-service-
scope option, the term "SLP Agent" is being used to
refer to a Service Location Protocol agent running on a
machine that is being configured using the DHCP proto-
col.
Also, please be aware that some companies may refer to
SLP as NDS. If you have an NDS directory agent whose
address you need to configure, the slp-directory-agent
option should work.
option slp-service-scope boolean text;
The Service Location Protocol Service Scope Option
specifies two things: a list of service scopes for SLP,
and whether the use of this list is mandatory. If the
initial boolean value is true, the SLP agent should
only use the list of scopes provided in this option;
otherwise, it may use its own static configuration in
preference to the list provided in this option.
The text string should be a comma-seperated list of
scopes that the SLP agent should use. It may be omit-
ted, in which case the SLP Agent will use the aggre-
gated list of scopes of all directory agents known to
the SLP agent.
option smtp-server ip-address [, ip-address... ];
The SMTP server option specifies a list of SMTP servers
available to the client. Servers should be listed in
order of preference.
option static-routes ip-address ip-address
[, ip-address ip-address...];
This option specifies a list of static routes that the
client should install in its routing cache. If multi-
ple routes to the same destination are specified, they
are listed in descending order of priority.
The routes consist of a list of IP address pairs. The
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dhcpd-options(5)dhcpd-options(5)
first address is the destination address, and the sec-
ond address is the router for the destination.
The default route (0.0.0.0) is an illegal destination
for a static route. To specify the default route, use
the routers option. Also, please note that this
option is not intended for classless IP routing - it
does not include a subnet mask. Since classless IP
routing is now the most widely deployed routing stan-
dard, this option is virtually useless, and is not
implemented by any of the popular DHCP clients, for
example the Microsoft DHCP client.
option streettalk-directory-assistance-server ip-address
[, ip-address...];
The StreetTalk Directory Assistance (STDA) server
option specifies a list of STDA servers available to
the client. Servers should be listed in order of pref-
erence.
option streettalk-server ip-address [, ip-address... ];
The StreetTalk server option specifies a list of
StreetTalk servers available to the client. Servers
should be listed in order of preference.
option subnet-mask ip-address;
The subnet mask option specifies the client's subnet
mask as per RFC 950. If no subnet mask option is pro-
vided anywhere in scope, as a last resort dhcpd will
use the subnet mask from the subnet declaration for the
network on which an address is being assigned. How-
ever, any subnet-mask option declaration that is in
scope for the address being assigned will override the
subnet mask specified in the subnet declaration.
option swap-server ip-address;
This specifies the IP address of the client's swap
server.
option tcp-keepalive-garbage flag;
This option specifies the whether or not the client
should send TCP keepalive messages with a octet of
garbage for compatibility with older implementations.
A value of false indicates that a garbage octet should
not be sent. A value of true indicates that a garbage
octet should be sent.
option tcp-keepalive-interval uint32;
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This option specifies the interval (in seconds) that
the client TCP should wait before sending a keepalive
message on a TCP connection. The time is specified as
a 32-bit unsigned integer. A value of zero indicates
that the client should not generate keepalive messages
on connections unless specifically requested by an
application.
option tftp-server-name text;
This option is used to identify a TFTP server and, if
supported by the client, should have the same effect as
the server-name declaration. BOOTP clients are
unlikely to support this option. Some DHCP clients
will support it, and others actually require it.
option time-offset int32;
The time-offset option specifies the offset of the
client's subnet in seconds from Coordinated Universal
Time (UTC).
option time-servers ip-address [, ip-address... ];
The time-server option specifies a list of RFC 868 time
servers available to the client. Servers should be
listed in order of preference.
option trailer-encapsulation flag;
This option specifies whether or not the client should
negotiate the use of trailers (RFC 893 [14]) when using
the ARP protocol. A value of 0 indicates that the
client should not attempt to use trailers. A value of
true means that the client should attempt to use trail-
ers.
option uap-servers text;
This option specifies a list of URLs, each pointing to
a user authentication service that is capable of pro-
cessing authentication requests encapsulated in the
User Authentication Protocol (UAP). UAP servers can
accept either HTTP 1.1 or SSLv3 connections. If the
list includes a URL that does not contain a port compo-
nent, the normal default port is assumed (i.e., port 80
for http and port 443 for https). If the list includes
a URL that does not contain a path component, the path
/uap is assumed. If more than one URL is specified in
this list, the URLs are seperated by spaces.
option user-class string;
This option is used by some DHCP clients as a way for
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users to specify identifying information to the client.
This can be used in a similar way to the vendor-class-
identifier option, but the value of the option is spec-
ified by the user, not the vendor. Most recent DHCP
clients have a way in the user interface to specify the
value for this identifier, usually as a text string.
option vendor-class-identifier string;
This option is used by some DHCP clients to identify
the vendor type and possibly the configuration of a
DHCP client. The information is a string of bytes
whose contents are specific to the vendor and are not
specified in a standard. To see what vendor class
identifier a clients are sending, you can write the
following in your DHCP server configuration file:
set vendor-class option vendor-class-identifier;
This will result in all entries in the DHCP server
lease database file for clients that sent vendor-
class-identifier options having a set statement that
looks something like this:
set vendor-class "SUNW.Ultra-5_10";
The vendor-class-identifier option is normally used
by the DHCP server to determine the options that are
returned in the vendor-encapsulated-options option.
Please see the VENDOR ENCAPSULATED OPTIONS section of
the dhcpd.conf manual page for further information.
option vendor-encapsulated-options string;
The vendor-encapsulated-options option can contain
either a single vendor-specific value or one or more
vendor-specific suboptions. This option is not nor-
mally specified in the DHCP server configuration file
- instead, a vendor class is defined for each vendor,
vendor class suboptions are defined, values for those
suboptions are defined, and the DHCP server makes up
a response on that basis.
Some default behaviours for well-known DHCP client
vendors (currently, the Microsoft Windows 2000 DHCP
client) are configured automatically, but otherwise
this must be configured manually - see the VENDOR
ENCAPSULATED OPTIONS section of the dhcpd.conf manual
page for details.
option x-display-manager ip-address [, ip-address...
];
This option specifies a list of systems that are
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running the X Window System Display Manager and are
available to the client. Addresses should be listed
in order of preference.
option www-server ip-address [, ip-address... ];
The WWW server option specifies a list of WWW avail-
able to the client. Servers should be listed in
order of preference.
RELAY AGENT INFORMATION OPTION
An IETF draft, draft-ietf-dhc-agent-options-11.txt,
defines a series of encapsulated options that a relay
agent can add to a DHCP packet when relaying it to the
DHCP server. The server can then make address allocation
decisions (or whatever other decisions it wants) based on
these options. The server also returns these options in
any replies it sends through the relay agent, so that the
relay agent can use the information in these options for
delivery or accounting purposes.
The current draft defines two options. To reference
these options in the dhcp server, specify the option space
name, "agent", followed by a period, followed by the
option name. It is not normally useful to define values
for these options in the server, although it is permissi-
ble. These options are not supported in the client.
option agent.circuit-id string;
The circuit-id suboption encodes an agent-local identi-
fier of the circuit from which a DHCP client-to-server
packet was received. It is intended for use by agents
in relaying DHCP responses back to the proper circuit.
The format of this option is currently defined to be
vendor-dependent, and will probably remain that way,
although the current draft allows for for the possibil-
ity of standardizing the format in the future.
option agent.remote-id string;
The remote-id suboption encodes information about the
remote host end of a circuit. Examples of what it
might contain include caller ID information, username
information, remote ATM address, cable modem ID, and
similar things. In principal, the meaning is not
well-specified, and it should generally be assumed to
be an opaque object that is administratively guaranteed
to be unique to a particular remote end of a circuit.
THE CLIENT FQDN SUBOPTIONS
The Client FQDN option, currently defined in the Internet
Draft draft-ietf-dhc-fqdn-option-00.txt is not a standard
yet, but is in sufficiently wide use already that we have
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dhcpd-options(5)dhcpd-options(5)
implemented it. Due to the complexity of the option for-
mat, we have implemented it as a suboption space rather
than a single option. In general this option should not
be configured by the user - instead it should be used as
part of an automatic DNS update system.
option fqdn.no-client-update flag;
When the client sends this, if it is true, it means the
client will not attempt to update its A record. When
sent by the server to the client, it means that the
client should not update its own A record.
option fqdn.server-update flag;
When the client sends this to the server, it is
requesting that the server update its A record. When
sent by the server, it means that the server has
updated (or is about to update) the client's A record.
option fqdn.encoded flag;
If true, this indicates that the domain name included
in the option is encoded in DNS wire format, rather
than as plain ASCII text. The client normally sets
this to false if it doesn't support DNS wire format in
the FQDN option. The server should always send back
the same value that the client sent. When this value
is set on the configuration side, it controls the for-
mat in which the fqdn.name suboption is encoded.
option fqdn.rcode1 flag;
option fqdn.rcode1 flag;
These options specify the result of the updates of the
A and PTR records, respectively, and are only sent by
the DHCP server to the DHCP client. The values of
these fields are those defined in the DNS protocol
specification.
option fqdn.name text;
Specifies the domain name that the client wishes to
use. This can be a fully-qualified domain name, or a
single label. If there is no trailing generally
update that name in some locally-defined domain.
If you wish to use any of these suboptions, we strongly
recommend that you refer to the Client FQDN option draft
(or standard, when it becomes a standard) - the documenta-
tion here is sketchy and incomplete in comparison, and is
just intended for reference by people who already under-
stand the Client FQDN option specification.
15
dhcpd-options(5)dhcpd-options(5)THE NETWARE/IP SUBOPTIONS
RFC2242 defines a set of encapsulated options for Novell
NetWare/IP clients. To use these options in the dhcp
server, specify the option space name, "nwip", followed by
a period, followed by the option name. The following
options can be specified:
option nwip.nsq-broadcast flag;
If true, the client should use the NetWare Nearest
Server Query to locate a NetWare/IP server. The
behaviour of the Novell client if this suboption is
false, or is not present, is not specified.
option nwip.preferred-dss ip-address [, ip-address... ];
This suboption specifies a list of up to five IP
addresses, each of which should be the IP address of a
NetWare Domain SAP/RIP server (DSS).
option nwip.nearest-nwip-server ip-address
[, ip-address...];
This suboption specifies a list of up to five IP
addresses, each of which should be the IP address of a
Nearest NetWare IP server.
option nwip.autoretries uint8;
Specifies the number of times that a NetWare/IP client
should attempt to communicate with a given DSS server
at startup.
option nwip.autoretry-secs uint8;
Specifies the number of seconds that a Netware/IP
client should wait between retries when attempting to
establish communications with a DSS server at startup.
option nwip.nwip-1-1 uint8;
If true, the NetWare/IP client should support Net-
Ware/IP version 1.1 compatibility. This is only
needed if the client will be contacting Netware/IP ver-
sion 1.1 servers.
option nwip.primary-dss ip-address;
Specifies the IP address of the Primary Domain SAP/RIP
Service server (DSS) for this NetWare/IP domain. The
NetWare/IP administration utility uses this value as
Primary DSS server when configuring a secondary DSS
server.
16
dhcpd-options(5)dhcpd-options(5)DEFINING NEW OPTIONS
The Internet Software Consortium DHCP client and server
provide the capability to define new options. Each DHCP
option has a name, a code, and a structure. The name is
used by you to refer to the option. The code is a num-
ber, used by the DHCP server and client to refer to an
option. The structure describes what the contents of an
option looks like.
To define a new option, you need to choose a name for it
that is not in use for some other option - for example,
you can't use "host-name" because the DHCP protocol
already defines a host-name option, which is documented
earlier in this manual page. If an option name doesn't
appear in this manual page, you can use it, but it's prob-
ably a good idea to put some kind of unique string at the
beginning so you can be sure that future options don't
take your name. For example, you might define an option,
"local-host-name", feeling some confidence that no offi-
cial DHCP option name will ever start with "local".
Once you have chosen a name, you must choose a code. For
site-local options, all codes between 128 and 254 are
reserved for DHCP options, so you can pick any one of
these. In practice, some vendors have interpreted the
protocol rather loosely and have used option code values
greater than 128 themselves. There's no real way to
avoid this problem, but it's not likely to cause too much
trouble in practice.
The structure of an option is simply the format in which
the option data appears. The ISC DHCP server currently
supports a few simple types, like integers, booleans,
strings and IP addresses, and it also supports the ability
to define arrays of single types or arrays of fixed
sequences of types.
New options are declared as follows:
option new-name code new-code = definition ;
The values of new-name and new-code should be the name you
have chosen for the new option and the code you have cho-
sen. The definition should be the definition of the
structure of the option.
The following simple option type definitions are sup-
ported:
BOOLEAN
option new-name code new-code = boolean ;
An option of type boolean is a flag with a value of either
17
dhcpd-options(5)dhcpd-options(5)
on or off (or true or false). So an example use of the
boolean type would be:
option use-zephyr code 180 = boolean;
option use-zephyr on;
INTEGER
option new-name code new-code = sign integer width ;
The sign token should either be blank, unsigned or signed.
The width can be either 8, 16 or 32, and refers to the
number of bits in the integer. So for example, the fol-
lowing two lines show a definition of the sql-connection-
max option and its use:
option sql-connection-max code 192 = unsigned integer 16;
option sql-connection-max 1536;
IP-ADDRESS
option new-name code new-code = ip-address ;
An option whose structure is an IP address can be
expressed either as a domain name or as a dotted quad. So
the following is an example use of the ip-address type:
option sql-server-address code 193 = ip-address;
option sql-server-address sql.example.com;
TEXT
option new-name code new-code = text ;
An option whose type is text will encode an ASCII text
string. For example:
option sql-default-connection-name code 194 = text;
option sql-default-connection-name "PRODZA";
DATA STRING
option new-name code new-code = string ;
An option whose type is a data string is essentially just
a collection of bytes, and can be specified either as
quoted text, like the text type, or as a list of hexadeci-
mal contents seperated by colons whose values must be
between 0 and FF. For 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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dhcpd-options(5)dhcpd-options(5)
ENCAPSULATION
option new-name code new-code = encapsulate identifier ;
An option whose type is encapsulate will encapsulate the
contents of the option space specified in identifier.
Examples of encapsulated options in the DHCP protocol as
it currently exists include the vendor-encapsulated-
options option, the netware-suboptions option and the
relay-agent-information option.
option space local;
option local.demo code 1 = text;
option local-encapsulation code 197 = encapsulate local;
option local.demo "demo";
ARRAYS
Options can contain arrays of any of the above types
except for the text and data string types, which aren't
currently supported in arrays. An example of an array
definition is as follows:
option kerberos-servers code 200 = array of ip-address;
option kerberos-servers 10.20.10.1, 10.20.11.1;
RECORDS
Options can also contain data structures consisting of a
sequence of data types, which is sometimes called a record
type. For example:
option contrived-001 code 201 = { boolean, integer 32, text };
option contrived-001 on 1772 "contrivance";
It's also possible to have options that are arrays of
records, for example:
option new-static-routes code 201 = array of {
ip-address, ip-address, ip-address, integer 8 };
option 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;
VENDOR ENCAPSULATED OPTIONS
The DHCP protocol defines the vendor-encapsulated-options
option, which allows vendors to define their own options
that will be sent encapsulated in a standard DHCP option.
The format of the vendor-encapsulated-options option is
either a series of bytes whose format is not specified, or
a sequence of options, each of which consists of a single-
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dhcpd-options(5)dhcpd-options(5)
byte vendor-specific option code, followed by a single-
byte length, followed by as many bytes of data as are
specified in the length (the length does not include
itself or the option code).
The value of this option can be set in one of two ways.
The first way is to simply specify the data directly,
using a text string or a colon-seperated list of hexadeci-
mal values. For example:
option vendor-encapsulated-options
2:4:AC:11:41:1:
3:12:73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
4:12:2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
The second way of setting the value of this option is to
have the DHCP server generate a vendor-specific option
buffer. To do this, you must do four things: define an
option space, define some options in that option space,
provide values for them, and specify that that option
space should be used to generate the vendor-encapsulated-
options option.
To define a new option space in which vendor options can
be stored, use the option space statement:
option space name ;
The name can then be used in option definitions, as
described earlier in this document. For example:
option space SUNW;
option SUNW.server-address code 2 = ip-address;
option SUNW.server-name code 3 = text;
option SUNW.root-path code 4 = text;
Once you have defined an option space and the format of
some options, you can set up scopes that define values for
those options, and you can say when to use them. For
example, suppose you want to handle two different classes
of clients. Using the option space definition shown in
the previous example, you can send different option values
to different clients based on the vendor-class-identifier
option that the clients send, as follows:
class "vendor-classes" {
match option vendor-class-identifier;
}
option SUNW.server-address 172.17.65.1;
option SUNW.server-name "sundhcp-server17-1";
subclass "vendor-classes" "SUNW.Ultra-5_10" {
vendor-option-space SUNW;
20
dhcpd-options(5)dhcpd-options(5)
option SUNW.root-path "/export/root/sparc";
}
subclass "vendor-classes" "SUNW.i86pc" {
vendor-option-space SUNW;
option SUNW.root-path "/export/root/i86pc";
}
As you can see in the preceding example, regular scoping
rules apply, so you can define values that are global in
the global scope, and only define values that are specific
to a particular class in the local scope. The vendor-
option-space declaration tells the DHCP server to use
options in the SUNW option space to construct the vendor-
encapsulated-options option.
SEE ALSOdhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-
eval(5), dhcpd(8), dhclient(8), RFC2132, RFC2131, draft-
ietf-dhc-agent-options-??.txt.
AUTHOR
The Internet Software Consortium DHCP Distribution was
written by Ted Lemon <mellon@isc.org> under a contract
with Vixie Labs. Funding for this project was provided
through the Internet Software Consortium. Information
about the Internet Software Consortium can be found at
http://www.isc.org/isc.
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