Dynamic Host Configuration Working                            D. Hankins
Group                                                             Google
Internet-Draft                                           October 1, 2011
Intended status: Informational
Expires: April 3, 2012

                Guidelines for Creating New DHCP Options


   This document seeks to provide guidance to prospective DHCP Option
   authors, to help them in producing option formats that are easily

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  When to Use DHCP . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  General Principles . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Reusing Other Options  . . . . . . . . . . . . . . . . . . . .  5
   5.  Avoid Conditional Formatting . . . . . . . . . . . . . . . . .  7
   6.  Avoid Aliasing . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Considerations for Creating New Formats  . . . . . . . . . . .  8
   8.  The Dangers of Sub Options . . . . . . . . . . . . . . . . . .  8
   9.  Option Size  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. Clients Request their Options  . . . . . . . . . . . . . . . . 11
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   13. Informative References . . . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Background on ISC DHCP  . . . . . . . . . . . . . . . 15
     A.1.  Atomic DHCP  . . . . . . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   Most protocol developers ask themselves if a protocol will work, or
   work efficiently.  These are important questions, but another less
   frequently considered question is whether the proposed protocol
   presents itself needless barriers to adoption by deployed software.

   DHCPv4 [RFC2131] and DHCPv6 [RFC3315] software implementors are not
   merely faced with the task of a given option's format on the wire.
   The option must "fit" into every stage of the system's process, from
   the user interface where configuration is entered to the machine
   interfaces where configuration is consumed.  To help understand the
   potential implementation challenges of any new DHCP Option, one
   implementation's approach to tackling DHCP Option formats
   (Appendix A) has been included as an Appendix.

   Another more frequently overlooked aspect of rapid adoption is the
   question: Would the option would require operators to be intimately
   familiar with the option's internal format in order to make use of
   it?  Most DHCP software provides a facility for "unknown options" at
   the time of publication to be configured by hand by an operator.  But
   if doing so requires extensive reading (more than can be covered in a
   simple FAQ for example), it inhibits adoption.

   So although a given solution would work, and might even be space,
   time, or aesthetically optimal, a given option is presented with a
   series of ever-worsening challenges to be adopted;

   o  If it doesn't fit neatly into existing config files.

   o  If it requries new source code changes to be adopted, and hence
      upgrades of deployed software.

   o  If it does not share its deployment fate in a general manner with
      other options, standing alone in requiring code changes or
      reworking configuration file syntaxes.

   There are many things DHCP option authors can do to form DHCP Options
   to stay off this list entirely, or failing that, to make software
   implementors lives easier and improve its chances for widespread

2.  When to Use DHCP

   Principally, DHCP carries configuration parameters for its clients.
   Any knob, dial, slider, or checkbox on the client system, such as "my
   domain name servers", "my hostname", or even "my shutdown

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   temperature" are candidates for being configured by DHCP.

   The presence of such a knob isn't enough, because DHCP also presents
   the extension of an administrative domain - the operator of the
   network to which the client is currently attached.  Someone runs not
   only the local switching network infrastructure that the client is
   directly (or wirelessly) attached to, but the various methods of
   accessing the external Internet via local assist services that
   network must also provide (such as domain name servers, or routers).
   This means that in addition to the existence of a configuration
   parameter, one must also ask themselves if it is reasonable for this
   parameter to be set by the directly attached network's

   Bear in mind that the client still reserves the right to ignore
   values received via DHCP (for example, due to having a value manually
   configured by its own operator), and that at least one main use case
   for DHCP is the corporate enterprise.  So even if the local Net
   Cafe's operator is not a likely source of the candidate
   configuration, there may be other DHCP servers in a client's lifetime
   which are.

3.  General Principles

   The primary principle to follow in order to enhance an option's
   adoptability is certainly simplification.  But more specifically, to
   create the option in such a way that it should not require any new or
   special case software to support.  If old software currently deployed
   and in the field can adopt the option through supplied configuration
   conveniences then it's fairly well assured that new software can
   easily formally adopt it.

   There are at least two classes of DHCP options: A bulk class of
   options which are provided explicitly to carry data from one side of
   the DHCP exchange to the other (such as nameservers, domain names, or
   time servers), and a protocol class of options which require special
   processing on the part of the DHCP software or are used during
   special processing (such as the FQDN options ([RFC4702], [RFC4704]),
   DHCPv4 message type option [RFC2132], link selection options
   ([RFC3011], [RFC3527]), and so forth; these options carry data that
   is the result of a routine in some DHCP software).

   The guidelines laid out here should be understood to be relaxed for
   the protocol class of options.  Wherever special-case-code is already
   required to adopt the DHCP option, it is substantially more
   reasonable to format the option in a less generic fashion, if there
   are measurable benefits to doing so.

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4.  Reusing Other Options

   In DHCPv4, there are now nearly one hundred and thirty options, at
   least as IETF standards, which might be used as an example.  There is
   also one handy document [RFC2132] containing many option definitions.

   There is a tradeoff between the adoptability of previously defined
   option formats, and the advantages new or specialized formats can
   provide.  In the balance, it is usually preferrable to reuse
   previously used option formats.

   However, it isn't very practical to consider the bulk of DHCP options
   already allocated, and consider which of those solve a similar
   problem.  So, the following list of common option format fragments is
   provided as a shorthand.  Please note that it is not complete in
   terms of exampling every option format ever devised...it is only a
   list of option format fragments which are used in two or more

   |      Fragment |  Size | Types of Uses                             |
   |  ipv4-address |     4 | Default gateway, requested address,       |
   |               |       | subnet mask [RFC2132], addresses of       |
   |               |       | servers ([RFC2132], [RFC2241], [RFC2242], |
   |               |       | [RFC3495], [RFC3634], [RFC4174]), as a    |
   |               |       | component in a list of routes [RFC3442].  |
   |  ipv6-address |    16 | DHCPv6 server unicast address [RFC3315],  |
   |               |       | addresses of servers ([RFC3319],          |
   |               |       | [RFC3646], [RFC3898], [RFC4075],          |
   |               |       | [RFC4280]).                               |
   |        32-bit |     4 | Signed or unsigned varieties. Used for    |
   |       integer |       | timezone time offset [RFC2132]            |
   |               |       | (deprecated by [RFC4833]). Other uses for |
   |               |       | host configuration values such as path    |
   |               |       | MTU aging timeouts, ARP cache timeouts,   |
   |               |       | TCP keepalive intervals [RFC2132]. Also   |
   |               |       | used by the DHCPv4 protocol for relative  |
   |               |       | times, and times since epoch.             |
   |        16-bit |     2 | Client configuration parameters, such as  |
   |       integer |       | MTU, maximum datagram reassembly limits,  |
   |               |       | the DHCPv4 maximum message size           |
   |               |       | [RFC2132], or the elapsed time option     |
   |               |       | [RFC3315] in DHCPv6.                      |

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   | 8-bit integer |     1 | Used for host configuration parameters,   |
   |               |       | such as the default IP TTL, default TCP   |
   |               |       | TTL, NetBIOS node type [RFC2132]. Also    |
   |               |       | used for protocol features, such as the   |
   |               |       | DHCPv4 Option Overload (as flags), DHCP   |
   |               |       | Message Type (as an enumeration) or       |
   |               |       | DHCPv6 Preference [RFC3315].              |
   |     NVT-ASCII | unlim | This is the kitchen sink of common        |
   |          Text |       | fragments. Common uses are for filenames  |
   |               |       | (such as TFTP paths), host or domain      |
   |               |       | names (but this should be discouraged),   |
   |               |       | or protocol features such as textual      |
   |               |       | messages such as verbose error            |
   |               |       | indicators. Since the size of this format |
   |               |       | cannot be determined (it is not NULL      |
   |               |       | terminated), it consumes any remaining    |
   |               |       | space in the option.                      |
   |      DNS Wire | unlim | Presently used for 'domain search' lists  |
   | Format Domain |       | in both DHCPv4 [RFC3397] and DHCPv6       |
   |     Name List |       | [RFC3646], but also used in DHCPv6 for    |
   |     [RFC1035] |       | any host or domain name. A field          |
   |               |       | formatted this way may have a determinate |
   |               |       | length if the number of root labels is    |
   |               |       | limited, but use of this format as being  |
   |               |       | a determinate length should be            |
   |               |       | discouraged in DHCPv4, less so in DHCPv6. |
   |   'suboption' | unlim | The Relay Agent Information Option        |
   | encapsulation |       | [RFC3046], vendor options [RFC2132],      |
   |               |       | Vendor Identified Vendor SubOptions       |
   |               |       | ([RFC3925], [RFC3315]). Commonly used for |
   |               |       | situations where the full format cannot   |
   |               |       | be known initially, such as where there   |
   |               |       | seems to be some room for later protocol  |
   |               |       | work to expand the amount of information  |
   |               |       | carried, or where the full extent of data |
   |               |       | carried is defined in a private           |
   |               |       | specification (such as with vendor        |
   |               |       | options). Encapsulations do not use 'PAD' |
   |               |       | and 'END' options in DHCPv4, and there    |
   |               |       | are no such options in DHCPv6, so this    |
   |               |       | format also is of indeterminate length.   |

                     Table 1: Common Option Fragments

   The easiest approach to manufacturing trivially deployable DHCP
   Options is to assemble the option out of whatever common fragments
   fit - possibly allowing a group of fragments to repeat to fill the

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   remaining space (if present) and so provide multiple values.  Place
   all fixed size values at the start of the option, and any variable/
   indeterminate sized value at the tail end of the option.

   This estimates that implementations will be able to reuse code paths
   designed to support the other options.

5.  Avoid Conditional Formatting

   Placing a octet at the start of the option which informs the software
   how to process the remaining octets of the option may appear simple
   to the casual observer.  But the only conditional formatting methods
   that are in widespread use today are 'protocol' class options.  So
   conditional formatting requires new code to be written, as well as
   introduces an implementation problem; as it requires that all
   speakers implement all current and future conditional formats.

   Conditional formatting is absolutely not recommended, except in cases
   where the DHCP option has already been deployed experimentally, and
   all but one conditional format is deprecated.

6.  Avoid Aliasing

   Options are said to be aliases of each other if they provide input to
   the same configuration parameter.  A commonly proposed example is to
   configure the location of some new service ("my foo server") using a
   binary IP address, a domain name field, and a URL.  This kind of
   aliasing is undesirable, and is best avoided.

   In this case, where three different formats are supposed, it triples
   the work of the software involved, requiring support for not merely
   one format, but support to produce and digest all three.  Since
   clients cannot predict what values the server will provide, they must
   request all formats...so in the case where the server is configured
   with all formats, DHCP option space is wasted on option contents that
   are redundant.

   It also becomes unclear which types of values are mandatory, and how
   configuring some of the options may influence the others.  For
   example, if an operator configures the URL only, should the server
   synthesize a domain name and IP address?

   A single configuration value on a host is probably presented to the
   operator (or other software on the machine) in a single field or
   channel.  If that channel has a natural format, then any alternative
   formats merely make more work for intervening software in providing

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   So the best advice is to choose the one method that best fulfills the
   requirements, be that for simplicity (such as with an IP address and
   port pair), late binding (such as with DNS), or completeness (such as
   with a URL).

   On the specific subject of desiring to configure a value using a
   Fully Qualified Domain Name instead of a binary IP address, note that
   most DHCP server implementations will happily accept a Domain Name
   entered by the administrator, and use DNS resolution to render binary
   IP addresses in DHCP replies to clients.  Consequently, consider the
   extra packet overhead incurred on the client's end to perform DNS
   resolution itself.  The client may be operating on a battery and
   packet transmission is a non-trivial use of power, and the extra RTT
   delays the client must endure before the service is configured are at
   least two factors to consider in making a decision on format.

7.  Considerations for Creating New Formats

   If the option simply will not fit into any existing work by using
   fragments, the last recourse is to create a new format to fit.

   When doing so, it is not enough to gauge whether or not the option
   format will work in the context of the option presently being
   considered.  It is equally important to consider if the new format's
   fragments might reasonably have any other uses, and if so, to create
   the option with the foreknowledge that its parts may later become a
   common fragment.

   One specific consideration to evaluate is whether or not options of a
   similar format would need to have multiple or single values encoded
   (whatever differs from the current option), and how that might be
   accomplished in a similar format.

8.  The Dangers of Sub Options

   Some DHCP options, such as the DHCPv4 Relay Agent Information Option
   [RFC3046] are defined to contain a series of DHCP options, possibly
   using code tags specific to that option (but not in some limited
   "protocol feature" cases in DHCPv6 [RFC3315]).  These are commonly
   referred to as Encapsulated Option Spaces or more simply, Sub

   Sub options seem very attractive, because they allow the encoding of
   multiple variable length fields within the single "parent" option.

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   However, DHCP software will only include these options on an "all or
   nothing" basis, there is no well deployed mechanism for "Sub Option
   Parameter Request Lists" (although some defined sub-option spaces,
   such as for DOCSIS, do describe sub-option scoped PRL analogues), and
   the software will not include the entire option if there is not
   sufficient space.

   Consequently, it is not advisable to group options that may not be
   requested at the same time by the same client under an encapsulated

   Another attraction sub options present is ease of extending the
   configuration value for later, related configuration.  This must be
   weighed against the cost associated with asking IANA to maintain the
   space's internally assigned option codes.  Generally, the cost to
   IANA is greater, as it is unlikely that options will be later

   The use of sub-options is not a solution to aliasing problems.  Sub-
   options that contain multiple configuration values that alias the
   same configuration element actually makes matters worse.  The only
   solution to aliasing problems is to select a single option format, or
   where that is literally impossible, to use multiple DHCP options.  In
   this way, clients may place only the options they support on their
   parameter request list, in the order they support them.  Later
   protocol maintenance may incorporate a means to select a single DHCP
   option code out of a list of aliased options, so do not concern
   yourself with packet space issues arising from receiving all the

   Additionally, DHCPv4 option concatenation (Section 9) has not been
   defined in any DHCPv4 sub-options space.  Currently there is some
   DHCP software which does concatenate multiple DHCP options present in
   a sub-option space.  There is also software that treats multiple DHCP
   option codes present in a sub-option as individual single options.
   So there is no reliably predictable default behaviour.

   Because no sub-options space has yet been defined that includes the
   possibility of having more than one instance of an option of the same
   code, any attempt to do so is discouraged.

9.  Option Size

   DHCPv4 [RFC2131] options payload space is limited, as there are a
   number of unaddressed deployment problems with DHCPv4 packet sizes.
   The end result is that you should build your option to the assumption
   that the packet will be no larger than 576 octets.  This means that

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   the options payload space will be 312 octets, which you will have to
   share with other options.  This space can be extended by making use
   of Option Overloading [RFC2132], which allows the use of the BOOTP
   FILE and SNAME header fields for carrying DHCPv4 options (adding 192
   octets), but these header fields will not be available for
   overloading if they have been configured to carry a value.

   DHCPv6 [RFC3315] is much better off.  First, through its use of link-
   local addresses, it steps aside many of the deployment problems that
   plague DHCPv4, and looks a great deal more like any other UDP based
   application; oblivious to packet sizes up to 64KB.  Second, RFC 3315
   explicitly refers readers to RFC 2460 Section 5, which describes an
   MTU of 1280 octets and a minimum fragment reassembly of 1500 octets.
   It's much more feasible to suggest that DHCPv6 is capable of having
   larger options deployed over it, and at least no common upper limit
   is yet known to have been encoded by its implementors.  It is
   impossible to describe any fixed limit that cleanly divides those too
   big from the workable.

   So in either protocol, it is advantageous to prefer option formats
   which contain the desired information in the smallest form factor
   that solves the requirements.  One example is to use a 4-octet IPv4
   address rather than a fully qualified domain name, because many DHCP
   servers will perform DNS resolution on configured FQDN's (so the DNS
   recursive lookup is performed anyway).  There may be motivations to
   use the fully qualified domain name anyway, such as if the intended
   RRSET is not an address, or if the client must refresh the name more
   frequently than common lease renewal periods.

   When it is not possible to compress the configuration contents either
   because of the simple size of the parameters, or because it is
   expected that very large configurations are valid, it may be
   preferable to use a second stage configuration.  Some examples of
   this are to provide TFTP server and pathnames, or a URL, which the
   client will load and process externally to the DHCP protocol.

   The DHCPv4 code and length tags are each a single octet.  As the
   length field describes the length of the DHCP option's contents (not
   including the code and length octets), any option whose contents'
   length exceeds 255 octets can not be contained in a single option.
   These 'long options' will simply be fragmented into multiple options
   within the packet.  DHCP software processing these fragments will
   concatenate them, in the order they appear as defined by [RFC2131],
   prior to evaluating their contents.  This is an important distinction
   that is sometimes overlooked by authors - these multiple options are
   not individually formatted to convey one unit of information
   precisely as you have defined, but rather one option that has been
   split along any arbitrary octet boundary into multiple containers.

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   When documenting an example, then, try to make sure that the division
   point you select as an example does not lie on a clean division of
   your option contents - place it at an offset so as to reinforce that
   these values must be concatenated rather than processed individually.

   DHCPv4 option fragments are a basic protocol feature, so there
   usually is no reason to mention this feature in new option
   definitions, and no requirement for every option definition to be
   presented as a series of fragments.  It is only recommended to
   reinforce the existence of DHCP option fragmentation when the
   potential for large options is likely.  In this case, try to choose a
   large example data value.

   Note that option fragmentation is also a very common side-effect of
   running out of options space, and moving to overloaded FILE or SNAME
   fields.  Although the option may be considerably shorter than 255
   octets, if it does not fit in the remaining space then software may
   consume all remaining options space with one option fragment, and
   place the remainder in an overloaded field.

   Primarily it is important to remember that DHCPv4 differs from DHCPv6
   on this point: DHCPv4 can only convey one option of a given option
   code at any time - additional options (or possibly sub options, which
   do not have concisely defined semantics) of the same code will be
   concatenated together and processed at once.  DHCPv6 does allow
   multiple instances of a given option, and they are treated as
   distinct values following the defined format, however this feature is
   generally preferred to be restricted to protocol class features (such
   as the IA_* series of options); it is better to define your option as
   an array if it is possible.

   So remember that it is out of the question to define a case for
   multiple instances of your option in DHCPv4, and it is recommended to
   clarify (with normative language) if any DHCPv6 option may appear
   once or multiple times.

10.  Clients Request their Options

   The DHCPv4 Parameter Request List [RFC2132], and the DHCPv6 Option
   Request Option (OPTION_ORO) [RFC3315], are both options that serve
   two purposes - to inform the server what options the client supports
   and is willing to digest, and in what order of priority the client
   places those option contents (in the event that they will not fit in
   the packet, later options are to be dropped).

   It doesn't make sense for some options to appear on this Parameter
   Request List, such as those formed by elements of the protocol's

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   internal workings, or are formed on either end by DHCP-level software
   engaged in some exchange of information.  When in any form of doubt,
   assume that any new option must be present on the relevant option
   request list if the client desires it.

   It is a frequent mistake of option draft authors, then, to create
   text that implies that a server will simply provide the new option,
   and clients will digest it.  Generally, it's best to also specify
   that clients MUST place the new option code on the relevant list
   option, clients MAY include the new option in their packets to
   servers with hints as to values they desire, and servers MAY respond
   with the option contents (if they have been so configured).

   Under only the most dire of circumstances should a new option
   definition require special ordering of options either in the relevant
   request option, or in the order of options within the packet.
   Although the request option does imply a priority, which might be
   processed in order, a server may shuffle options around in a DHCPv4
   packet in order to make them fit, and server software may sort DHCPv6
   options into strange orders.  There is not one universal approach.

11.  Security Considerations

   DHCP does have an Authentication mechanism ([RFC3118], [RFC3315],
   [RFC4030]), where it is possible for DHCP software to discriminate
   between authentic endpoints and men in the middle.

   However, at this date the mechanism is poorly deployed.  It also does
   not provide end-to-end encryption.

   So, while creating a new option, bear in mind that DHCP packet
   contents are always transmitted in the clear, and actual production
   use of the software will probably be vulnerable at least to man-in-
   the-middle attacks from within the network, even where the network
   itself is protected from external attacks by firewalls.  In
   particular, some DHCP message exchanges are transmitted to broadcast
   or multicast addresses that are likely broadcast anyway.

   If an option is of a specific fixed length, it is useful to remind
   the implementer of the option data's full length.  This is easily
   done by declaring the specific value of the 'length' tag of the
   option.  This helps to gently remind implementers to validate option
   length before digesting them into likewise fixed length regions of
   memory or stack.

   If an option may be of variable size (such as having indeterminate
   length fields, such as domain names or text strings), it is advisable

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   to explicitly remind the implementor to be aware of the potential for
   long options.  Either define a reasonable upper limit (and suggest
   validating it), or explicitly remind the implementor that an option
   may be exceptionally long (to be prepared to handle errors rather
   than truncate values).

   For some option contents, "insane values" may be used to breach
   security.  An IP address field might be made to carry a loopback
   address, or local broadcast address, and depending on the protocol
   this may lead to undesirable results.  A domain name field may be
   filled with contrived contents that exceed the limitations placed
   upon domain name formatting...as this value is possibly delivered to
   "internal configuration" records of the system, it may be trusted,
   rather than validated.

   So it behooves an option's definition to contain any validation
   measures as can reasonably be made.

12.  IANA Considerations

   This document has no actions for IANA.

13.  Informative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [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.

   [RFC2241]  Provan, D., "DHCP Options for Novell Directory Services",
              RFC 2241, November 1997.

   [RFC2242]  Droms, R. and K. Fong, "NetWare/IP Domain Name and
              Information", RFC 2242, November 1997.

   [RFC3011]  Waters, G., "The IPv4 Subnet Selection Option for DHCP",
              RFC 3011, November 2000.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP

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              Messages", RFC 3118, June 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3319]  Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
              Protocol (DHCPv6) Options for Session Initiation Protocol
              (SIP) Servers", RFC 3319, July 2003.

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              November 2002.

   [RFC3442]  Lemon, T., Cheshire, S., and B. Volz, "The Classless
              Static Route Option for Dynamic Host Configuration
              Protocol (DHCP) version 4", RFC 3442, December 2002.

   [RFC3495]  Beser, B. and P. Duffy, "Dynamic Host Configuration
              Protocol (DHCP) Option for CableLabs Client
              Configuration", RFC 3495, March 2003.

   [RFC3527]  Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
              "Link Selection sub-option for the Relay Agent Information
              Option for DHCPv4", RFC 3527, April 2003.

   [RFC3634]  Luehrs, K., Woundy, R., Bevilacqua, J., and N. Davoust,
              "Key Distribution Center (KDC) Server Address Sub-option
              for the Dynamic Host Configuration Protocol (DHCP)
              CableLabs Client Configuration (CCC) Option", RFC 3634,
              December 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC3898]  Kalusivalingam, V., "Network Information Service (NIS)
              Configuration Options for Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6)", RFC 3898, October 2004.

   [RFC3925]  Littlefield, J., "Vendor-Identifying Vendor Options for
              Dynamic Host Configuration Protocol version 4 (DHCPv4)",
              RFC 3925, October 2004.

   [RFC3942]  Volz, B., "Reclassifying Dynamic Host Configuration
              Protocol version 4 (DHCPv4) Options", RFC 3942,
              November 2004.

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   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075, May 2005.

   [RFC4174]  Monia, C., Tseng, J., and K. Gibbons, "The IPv4 Dynamic
              Host Configuration Protocol (DHCP) Option for the Internet
              Storage Name Service", RFC 4174, September 2005.

   [RFC4280]  Chowdhury, K., Yegani, P., and L. Madour, "Dynamic Host
              Configuration Protocol (DHCP) Options for Broadcast and
              Multicast Control Servers", RFC 4280, November 2005.

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702, October 2006.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, October 2006.

   [RFC4833]  Lear, E. and P. Eggert, "Timezone Options for DHCP",
              RFC 4833, April 2007.

Appendix A.  Background on ISC DHCP

   The ISC DHCP software package was mostly written by Ted Lemon in
   cooperation with Nominum, Inc. Since then, it has been given to
   Internet Systems Consortium, Inc. ("ISC") where it has been
   maintained in the public interest by contributors and ISC employees.

   It includes a DHCP Server, Relay, and Client implementation, with a
   common library of DHCP protocol handling procedures.

   The DHCP Client may be found on some Linux distributions, and FreeBSD
   5 and earlier.  Variations ("Forks") of older versions of the client
   may be found on several BSDs, including FreeBSD 6 and later.

   The DHCP Server implementation is known to be in wide use by many
   Unix-based servers, and comes pre-installed on most Linux

   The ISC DHCP Software Suite has to allow:

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   o  Administrators to configure arbitrary DHCP Option Wire Formats for
      options that either were not published at the time the software
      released, or are of the System Administrator's invention (such as
      'Site-Local' [RFC3942] options), or finally were of Vendor design
      (Vendor Encapsulated Options [RFC2132] or similar).

   o  Pre-defined names and formats of options allocated by IANA and
      defined by the IETF Standards body.

   o  Applications deriving their configuration parameters from values
      provided by these options to receive and understand their content.
      Often, the binary format on the wire is not helpful or digestable
      by, for example, 'ifconfig' or '/etc/resolv.conf'.

   So, one can imagine that this would require a number of software

   1.  To read operator-written configuration value into memory.

   2.  To write the in-memory representation into protocol wire format.

   3.  To read the protocol wire format into memory.

   4.  To write the in-memory format to persistent storage (to cache
       across reboots for example).

   5.  To write the in-memory format to a format that can be consumed by

   If every option were formatted differently and uniquely, then we
   would have to write 5 functions for every option.  As there is the
   potential for as many as 254 DHCPv4 options, or 65536 DHCPv6 options,
   not to mention the various encapsulated spaces ("suboptions"), this
   is not scalable.

   One simple trick is to make the in-memory format the same as the wire
   format.  This reduces the number of functions required from 5 to 3.
   This is not always workable - sometimes an intermediate format is
   required, but it is a good general case.

   Another simple trick is to use the same (or very nearly the same)
   format for persistent storage as is used to convey parameters to
   applications.  This reduces the number of functions again from 3 to

   This is still an intractable number of functions per each DHCP
   option, even without the entire DHCP option space populated.  So, we
   need a way to reduce this to small orders.

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A.1.  Atomic DHCP

   To accomplish these goals, a common "Format String" is used to
   describe, in abstract, all of the above.  Each octet in this format
   string represents a "DHCP Atom".  We chain these 'atoms' together,
   forming a sort of molecular structure for a particular DHCP option's
   defined format.

   The Configuration Syntax allows the user to construct such a format
   string without having to understand how the Atom is encoded on the
   wire, and how it is configured or presented.

   You can reasonably imagine that the various common formats of DHCP
   options described above (Table 1) each have an Atom associated with
   it.  There are special use Atoms, such as one to repeat the previous
   Atoms indefinitely (for example, for options with multiple IPv4
   addresses one after the other), and one which makes the previous Atom

   As the software encounters a format string, it processes each Atom
   individually to read from configuration into wire format, and also to
   validate and convert wire format into output format (which with some
   small exclusions is identical to the configuration format).

   The format strings themselves are either hard coded by the software
   in a table of option definitions, or are compiled at runtime through
   configuration syntax generated by the operator.

           option <space>.<option> code <number> = <definition>;

   The <space> refers to the option space, which may be the DHCPv4
   option space, the DHCPv6 option space, or any suboption space such as
   the DHCPv4 Relay Agent Information suboptions or similar.

   The <option> refers to the option's symbolic name within that space.

   The code <number> refers to the binary code assigned to this option.

   The <definition> is a complex statement that brings together DHCP
   Atoms, many of which are the aforementioned common formats, that
   compose this option.

   Below is a sample configuration for two options, whose wire formats
   are defined in [RFC2132].  The Path MTU Plateau Table option, and the
   Static Routes option.

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        option dhcp.path-mtu-plateau-table code 25 =
                                           array of unsigned integer 16;
        option dhcp.static-routes code 33 = array of { ip-address,
                                                       ip-address };

   Once the options' syntax configuration is out of the way, values to
   be carried in the options may be configured.  These acts are
   distinct; the previous configuration only prepares the parser system
   to accept the configuration below.  The below configuration actually
   supplies a value to be transmitted on the wire, relying on the above
   format definition.

        option dhcp.path-mtu-plataeu-table 4352, 1500, 576;
        option dhcp.static-routes,

Author's Address

   David W. Hankins
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: dhankins@google.com

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