The DHCPv4 [RFC2131] publicly defined options range, options 1 - 127,
is nearly used up. Efforts such as [RFC3679] help extend the life of
this space, but ultimately the space will be exhausted.
This document reclassifies much of the site-specific option range,
which has not been widely used for its original intended purpose, to
extend the publicly defined options space.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The DHCP option space (0 - 255) is divided into two ranges [RFC2132]:
1. 1 - 127 are publicly defined options, now allocated in accordance
2. 128 - 254 are site-specific options.
Options 0 (pad) and 255 (end) are special and defined in [RFC2131].
3.1. Publicly Defined Options Range
The publicly defined options space (1 - 127) is nearly exhausted.
Recent work [RFC3679] will buy more time, as several allocated but
unused option codes have been reclaimed. A review could be made from
time to time to determine whether there are other option codes that
can be reclaimed.
A longer-term solution to the eventual exhaustion of the publicly
defined options space is desired. The DHC WG evaluated several
1. Using options 126 and 127 to carry 16-bit options as originally
proposed by Ralph Droms in late 1996. However, this significantly
penalizes the first option assigned to this new space, as it
requires implementing the 16-bit option support. Because of this,
options 126 and 127 have been reclaimed [RFC3679].
2. Using a new magic cookie and 16-bit option code format. However,
* penalizes the first option assigned to this new space, as it
requires significant changes to clients, servers, and relay
* could adversely impact existing clients, servers, and relay
agents that fail to properly check the magic cookie value,
* requires support of both message formats for the foreseeable
* requires clients to send multiple DHCPDISCOVER messages -- one
for each magic cookie.
3. Reclassifying a portion of the site-specific option codes as
publicly defined. The impact is minimal, as only those sites
presently using options in the reclassified range need to renumber
3.2. Site-Specific Options Range
The site-specific option range is rather large (127 options in all)
and little used. The original intent of the site-specific option
range was to support local (to a site) configuration options, and it
is difficult to believe a site would need 127 options for this
purpose. Further, many DHCP client implementations do not provide a
well documented means to request site-specific options from a server
or to allow applications to extract the returned option values.
Some vendors have made use of site-specific option codes that violate
the intent of the site-specific options, as the options are used to
configure features of their products and thus are specific to many
sites. This usage could potentially cause problems if a site that
has been using the same site-specific option codes for other purposes
deploys products from one of the vendors, or if two vendors pick the
same site-specific options.
4. Reclassifying Options
The site-specific option codes 128 to 223 are hereby reclassified as
publicly defined options. This leaves 31 site-specific options, 224
To allow vendors that have made use of site-specific options within
the reclassified range to publish their option usage and to request
an official assignment of the option number to that usage, the
following procedure will be used to reclassify these options:
1. The reclassified options (128 to 223) will be placed in the
"Unavailable" state by IANA. These options are not yet available
for assignment to publicly defined options.
2. Vendors that currently use one or more of the reclassified options
have 6 months following this RFC's publication date to notify the
DHC WG and IANA that they are using particular options numbers and
agree to document that usage in an RFC. IANA will move these
options from the "Unavailable" to "Tentatively Assigned" state.
Vendors have 18 months from this RFC's publication date to start
the documentation process by submitting an Internet-Draft.
NOTE: If multiple vendors of an option number come forward and can
demonstrate that their usage is in reasonably wide use, none of
the vendors will be allowed to keep the current option number, and
they MUST go through the normal process of getting a publicly
assigned option [RFC2939].
3. Any options still classified as "Unavailable" 6 months after the
RFC publication date will be moved to the "Unassigned" state by
IANA. These options may then be assigned to any new publicly
defined options in accordance with [RFC2939].
4. For those options in the "Tentatively Assigned" state, vendors
have 18 months following this RFC's publication date to submit an
Internet-Draft documenting the option. The documented usage MUST
be consistent with the existing usage. When the option usage is
published as an RFC, IANA will move the option to the "Assigned"
If no Internet-Draft is published within the 18 months or should
one of these Internet-Drafts expire after the 18 months, IANA will
move the option to the "Unassigned" state, and the option may then
be assigned to any new publicly defined options in accordance with
Sites presently using site-specific option codes within the
reclassified range SHOULD take steps to renumber these options to
values within the remaining range. If a site needs more than 31
site-specific options, the site must switch to using suboptions, as
has been done for other options, such as the Relay Agent Information
5. Security Considerations
This document in and by itself provides no security, nor does it
impact existing DCHP security as described in [RFC2131].
6. IANA Considerations
IANA is requested to
1. expand the publicly defined DHCPv4 options space from 1 - 127 to 1
- 223. The new options (128 - 223) are to be listed as
"Unavailable" and MUST NOT be assigned to any publicly defined
2. receive notices from vendors that have been using one or more of
the options in the 128-223 range that they are using the option
and are willing to document that usage. IANA will list these
options as "Tentatively Assigned".
3. change the listing of any options listed as "Unavailable" to
"Available" 6 months from this RFC's publication date. These
options may now be assigned in accordance with [RFC2939].
4. change the listing of any options listed as "Tentatively-Assigned"
to "Unavailable" 18 months from this RFC's publication date and
periodically thereafter as long as there is an option listed as
"Tentatively-Assigned", if no un-expired Internet-Draft exists
documenting the usage.
Many thanks to Ralph Droms and Ted Lemon for their valuable input and
earlier work on the various alternatives.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
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