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DHCP Options
DHCP provides a framework for passing configuration information to hosts on a TCP/IP network.
Configuration parameters and other control information are carried in tagged data items that are stored
in the options field of the DHCP message. The data items themselves are also called options.
This appendix contains DHCP options and BOOTP vendor extensions from RFC 2132, and includes the
validation type for each option, as indicated in Table B-10 on page B-14.
This appendix also contains the standard Microsoft client options and several tables displaying the
options sorted by categories.
Option Descriptions
The following sections describe the DHCP options in detail:
• RFC 1497 Vendor Extensions, page B-1
• IP Layer Parameters Per Host, page B-3
APPENDIX
B
• IP Layer Parameters Per Interface, page B-4
• Link Layer Parameters Per Interface, page B-4
• TCP Parameters, page B-5
• Application and Service Parameters, page B-5
• DHCPv4 Extension Options, page B-8
• DHCPv6 Options, page B-10
• Microsoft Client Options, page B-13
• Options by Number, page B-14
• Options by Network Registrar Name, page B-18
• Option Validation Types, page B-24
RFC 1497 Vendor Extensions
Table B-1 on page B-2 lists the vendor extensions as defined in RFC 1497.
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B-1
Option Descriptions
Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
Table B-1 RFC 1497 Vendor Extension Options
Option Name No. Length Description
Pad 0 1 octet Causes the subsequent fields to align on word boundaries.
End 255 1 octet End of valid information in the vendor field. Subsequent
octets should be filled with the Pad options.
Subnet Mask 1 4 octets Client subnet mask, as per RFC 950. If both the Subnet Mask
and the Router option are specified in a DHCP reply, the
Subnet Mask option must be first.
Time Offset 2 4 octets Offset of the client subnet, in seconds, from Universal Time
(UT). The offset is expressed as a twos-complement 32-bit
integer. A positive offset indicates a location east of the zero
meridian and a negative offset indicates a location we s t of the
zero meridian.
Router 3 4 octet minimum;
multiples of 4
Time Server 4 4 octet minimum;
multiples of 4
Name Server
Option
Domain Name
Server
5 4 octet minimum;
multiples of 4
6 4 octet minimum;
multiples of 4
List of IP addresses for routers on the client subnet. Routers
should be in order of preference.
List of RFC 868 compliant time servers available to the
client. Servers should be in order of preference.
List of IEN 116 name servers available to the client. Servers
should be in order of preference.
List of Domain Name System (STD 13, RFC 1035) name
servers available to the client. Servers should be in order of
preference.
Log Server 7 4 octet minimum;
multiples of 4
Cookie Server 8 4 octet minimum;
multiples of 4
LPR Server 9 4 octet minimum;
multiples of 4
Impress
Server
Resource
Location
10 4 octet minimum;
multiples of 4
11 4 octet minimum;
multiples of 4
Server
List of MIT-LCS UDP log servers available to the client.
Servers should be in order of preference.
List of RFC 865-compliant cookie servers available to the
client. Servers should be in order of preference.
List of RFC 1179-compliant line printer servers available to
the client. Servers should be in order of preference.
List of Imagen Impress servers available to the client. Servers
should be in order of preference.
List of RFC 887-compliant resource location servers
available to the client. Servers should be in order of
preference.
Host Name 12 1 octet minimum Name of the client. The name may or may not be qualified
with the local domain name. See RFC 1035 for the character
set restrictions.
Boot File Size 13 2 octets Number of 512-octet blocks in the default boot file.
Merit Dump
File
14 1 octet minimum Path name of a file to which the client core image should be
placed in the event the client crashes. The path is formatted
as a character string consisting of characters from the NVT
ASCII character set.
Domain Name 15 1 octet minimum Domain name that the client should use when resolving
hostnames through the Domain Name System.
Swap Server 16 4 octets IP address of the client swap server.
B-2
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Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
Table B-1 RFC 1497 Vendor Extension Options (continued)
Option Name No. Length Description
Root Path 17 1 octet minimum Path name that contains the client root disk. The path is
Extensions
18 1 octet minimum Uses a string to specify a file, retrievable through TFTP. The
Path
IP Layer Parameters Per Host
Table B-2 lists the options that affect the operation of the IP layer on a per-host basis.
Table B-2 IP Layer Parameters Per Host Options
Option Descriptions
formatted as a character string consisting of characters from
the NVT ASCII character set.
file contains information that can be interpreted in the same
way as the 64-octet vendor-extension field within the
BOOTP response, with these exceptions: the length of the file
is unconstrained, and all references to instances of this option
in the file are ignored.
Option Name No. Length Description
IP Forwarding
Enable/Disable
Non-Local
Source Routing
Enable/Disable
Policy Filter 21 8 octet minimum;
19 1 octet Specifies whether the client should configure its IP layer
for packet forwarding. Values: 0=disable; 1=enable
20 1 octet Specifies whether the client should configure its IP layer to
allow forwarding of datagrams with non-local source
routes. Values: 0=disable; 1=enable
Policy filters for non-local source routing. The filters
multiples of 8
consist of a list of IP addresses and masks that 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.
Maximum
Datagram
22 2 octets Maximum size datagram that the client should be prepared
to reassemble. Value: 576 minimum
Reassembly
Size
Default IP
Time-to-Live
Path MTU
Aging Timeout
Path MTU
Plateau Table
23 1 octet Default TTL that the client should use on outgoing
datagrams. Values: 1 to 255
24 4 octets Timeout (in seconds) to use when aging Path MTU values
(defined in RFC 1191).
25 2 octets minimum;
multiples of 2
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. Value: 68 minimum
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B-3
Option Descriptions
IP Layer Parameters Per Interface
Table B-3 lists the options that affect the operation of the IP layer on a per-interface basis. A client can
issue multiple requests, one per interface, to configure interfaces with their specific parameters.
Table B-3 IP Layer Parameters Per Interface Options
Option Name No. Length Description
Interface MTU 26 2 octets Maximum time to live to use on this interface.
All Subnets
Are Local
Broadcast
Address
Perform Mask
Discovery
Mask Supplier 30 1 octet Specifies whether or not the client should respond to subnet
Perform Router
Discovery
Router
Solicitation
Address
Static Route 33 8 octet minimum;
27 1 octet Specifies whether or not the client can assume that all
28 4 octets Broadcast address in use on the client subnet.
29 1 octet Specifies whether or not the client should perform subnet
31 1 octet Specifies whether or not the client should solicit routers
32 4 octets Address to which the client should transmit router
multiples of 8
Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
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.
Values: 1=all subnets share same MTU; 0=some
directly-connected subnets can have smaller MTUs
mask discovery using ICMP. Values: 0=disable; 1=enable
mask requests using ICMP.
Values: 0=do not respond; 1=respond
using the Router Discovery mechanism defined in RFC
1256. Values: 0=disable; 1=enable
solicitation requests.
List of static routes that the client should install in its
routing cache. If multiple routes to the same destination are
specified, they are in descending order of priority. The
routes consist of a list of IP address pairs. The first address
is the destination address, and the second address is the
router for the destination. The default route (0.0.0.0) is an
illegal destination for a static route.
Link Layer Parameters Per Interface
Table B-4 lists the options that affect the operation of the data link layer on a per-interface basis.
Table B-4 Link Layer Parameters Per Interface Options
Option Name No. Length Description
Trailer
Encapsulation
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B-4
34 1 octet Specifies whether or not the client should negotiate the use of trailers
(RFC 893) when using the ARP protocol. Values: 0=do not use; 1=use
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Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
Table B-4 Link Layer Parameters Per Interface Options (continued)
Option Name No. Length Description
ARP Cache
Timeout
Ethernet
Encapsulation
35 4 octets Timeout in seconds for ARP cache entries.
36 1 octet Specifies whether or not the client should use Ethernet Version 2 (RFC
TCP Parameters
Table B-5 lists the options that affect the operation of the TCP layer on a per-interface basis.
Table B-5 TCP Parameter Options
Option Name No. Length Description
TCP Default
TTL
TCP Keepalive
Interval
TCP Keepalive
Garbage
37 1 octet Default TTL that the client should use when sending TCP segments.
38 4 octets Interval (in seconds) that the client TCP should wait before sending a
39 1 octet Specifies the whether or not the client should send TCP keep-alive
Option Descriptions
894) or IEEE 802.3 (RFC 1042) encapsulation if the interface is an
Ethernet.
Value: 0=use RFC 894 encapsulation; 1=use RFC 1042 encapsulation
Value: minimum 1
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. Value: 32-bit unsigned; 0=do not generate
keepalive messages unless specifically requested.
messages with an octet of garbage for compatibility with older
implementations.
Values: 0=do not send; 1=send
Application and Service Parameters
Table B-6 lists some miscellaneous options used to configure miscellaneous applications and services.
Table B-6 Application and Service Parameter Options
Option Name No. Length Description
Network
Information
Service (NIS)
Domain
Network
Information
Service (NIS)
Servers
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40 1 octet minimum Name of the client NIS domain. The domain is formatted
41 4 octet minimum;
multiples of 4
as a character string consisting of characters from the NVT
ASCII character set.
List of IP addresses indicating NIS servers available to the
client. Servers should be in order of preference.
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Option Descriptions
Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
Table B-6 Application and Service Parameter Options (continued)
Option Name No. Length Description
Network Time
Protocol Servers
42 4 octet minimum;
multiples of 4
List of IP addresses indicating NTP servers that are
available to the client. Servers should be in order of
preference.
Vendor-Specific
Information
43 1 octet minimum This option is used by clients and servers to exchange
vendor-specific information. The information is an opaque
object of n octets, presumably interpreted by
vendor-specific code on the clients and servers. The
definition of this information is vendor specific. The
vendor is indicated in the dhcp-class-identifier option.
Servers not equipped to interpret the vendor-specific
information sent by a client must ignore it (although it can
be reported). Clients that do not receive desired
vendor-specific information should make an attempt to
operate without it, although they can do so (and announce
they are doing so) in a degraded mode.
If a vendor potentially encodes more than one item of
information in this option, then the vendor should encode
the option using encapsulated vendor-specific options as
described here.
The encapsulated vendor-specific options field should be
encoded as a sequence of code, length, and value fields of
identical syntax to the DHCP options field with these
exceptions:
• There should not be a magic cookie field in the
encapsulated vendor-specific extensions field.
NetBIOS over
TCP/IP Name
Server
NetBIOS over
TCP/IP
Datagram
Distribution
Server
44 4 octet minimum;
multiples of 4
45 4 octet minimum;
multiples of 4
• Codes other than 0 or 255 can be redefined by the
vendor within the encapsulated vendor-specific
extensions field, but should conform to the
tag-length-value syntax defined in section 2.
Code 255 (END), if present, signifies the end of the
encapsulated vendor extensions, not the end of the vendor
extensions field.
If the code 255 is not present, then the end of the enclosing
vendor-specific information field is taken as the end of the
encapsulated vendor-specific extensions field.
List of RFC 1001/1002 NBNS name servers in order of
preference.
List of RFC 1001/1002 NBDD servers in order of
preference.
B-6
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Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
Table B-6 Application and Service Parameter Options (continued)
Option Name No. Length Description
NetBIOS over
46 1 octet Allows NetBIOS over TCP/IP client, which are configured
TCP/IP Node
Type
NetBIOS over
47 1 octet minimum NetBIOS over TCP/IP scope parameter for the client as
TCP/IP Scope
X Window
System Font
48 4 octet minimum;
multiples of 4
Server
X Window
System Display
49 4 octet minimum;
multiples of 4
Manager
Network
64 1 octet minimum Name of the client NIS+ domain. The domain is formatted
Information
Service (NIS+)
Domain
Network
Information
65 4 octet minimum;
multiples of 4
Service (NIS+)
Servers
Mobile IP Home
Agent
68 0 octets
minimum;
multiples of 4;
expected, 4 octets
(single home
agent address)
Simple Mail
Transport
69 4 octet minimum;
multiples of 4
Protocol
(SMTP) Server
Post Office
Protocol (POP3)
70 4 octet minimum;
multiples of 4
Server
Network News
Transport
71 4 octet minimum;
multiples of 4
Protocol
(NNTP) Server
Wor ld Wid e We b
(WWW) Server
72 4 octet minimum;
multiples of 4
Option Descriptions
as described in RFC 1001/1002.
Values: Single hexadecimal octet that identifies the client
type:
• 0x1=B-node (broadcast node)
• 0x2=P-node (point-to-point node)
• 0x4=M-node (mixed node)
• 0x8=H-node
specified in RFC 1001/1002.
List of X Window System Font servers available to the
client. Servers should be in order of preference.
List of IP addresses of systems that are running the X
Window System Display Manager and are available to the
client. Addresses should be in order of preference.
as a character string consisting of characters from the NVT
ASCII character set.
List of IP addresses indicating NIS+ servers available to
the client. Servers should be in order of preference.
List of IP addresses indicating mobile IP home agents
available to the client. Agents should be in order of
preference.
Value: 32-bit address; 0=no home agents available
List of SMTP servers available to the client. Servers
should be in order of preference.
List of POP3 servers available to the client. Servers should
be in order of preference.
List of NNTP servers available to the client. Servers should
be in order of preference.
List of World Wide Web (WWW) servers available to the
client. Servers should be in order of preference.
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Option Descriptions
Table B-6 Application and Service Parameter Options (continued)
Option Name No. Length Description
Finger Server 73 4 octet minimum;
Internet Relay
Chat Server
StreetTalk
Server
StreetTalk
Directory
Assistance
(STDA) Server
DHCPv4 Extension Options
Table B-7 lists the DHCPv4 extension options.
multiples of 4
74 4 octet minimum;
multiples of 4
75 4 octet minimum;
multiples of 4
76 4 octet minimum;
multiples of 4
Part 6: Appendices, Glossary, and Index Appendix B DHCP Options
List of Finger servers available to the client. Servers
should be in order of preference.
List of IRC servers available to the client. Servers should
be in order of preference.
List of StreetTalk servers available to the client. Servers
should be in order of preference.
List of STDA servers available to the client. Servers should
be in order of preference.
Table B-7 DHCPv4 Extensions
Option Name No. Length Description
Requested IP
Address
IP Address
Lease Time
50 4 octets Used in a client request (DHCPDISCOVER) to allow the client to
request that a particular IP address be assigned.
51 4 octets Used in a client request (DHCPDISCOVER or DHCPREQUEST)
to allow the client to request a lease time for the IP address. In a
server reply (DHCPOFFER), a DHCP server uses this option to
specify the lease time it is willing to offer.
Value: seconds, as 32-bit unsigned integer
Option Overload 52 1 octet Indicates that the DHCP sname or file fields are being overloaded
by using them to carry DHCP options. A DHCP server inserts this
option if the returned parameters will exceed the usual space
allotted for options. If this option is present, the client interprets
the specified additional fields after it concludes interpretation of
the standard option fields.
Values: 1=file field is used to hold options; 2=sname field is used
to hold options; 3=both fields are used to hold options
DHCP Message
Type
53 1 octet Used to convey the type of DHCP message. The preset value is 1
(DHCPDISCOVER). Values:
1=DHCPDISCOVER; 2=DHCPOFFER; 3=DHCPREQUEST;
4=DHCPDECLINE; 5=DHCPACK; 6=DHCPNAK;
7=DHCPRELEASE; 8=DHCPINFORM; 13=LEASEQUERY
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