Dynamic Host Configuration Working D. Hankins
Group ISC
Internet-Draft July 2007
Intended status: Informational
Expires: January 2, 2008
Guidelines for Creating New DHCP Options
draft-ietf-dhc-option-guidelines-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
94063
Abstract
This document seeks to provide guidance to prospective DHCP Option
authors, to help them in producing option formats that are easily
adoptable.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. When to Use DHCP . . . . . . . . . . . . . . . . . . . . . . . 3
3. General Principles . . . . . . . . . . . . . . . . . . . . . . 4
4. Reusing Other Options . . . . . . . . . . . . . . . . . . . . 4
5. Conditional Formatting is Hard . . . . . . . . . . . . . . . . 7
6. Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. New Formats . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Option Size . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Clients Request their Options . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Background on ISC DHCP . . . . . . . . . . . . . . . 11
Appendix A.1. Atomic DHCP . . . . . . . . . . . . . . . . . . . . 12
12. Informative References . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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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 is wether the proposed protocol presents itself
needless barriers to adoption by deployed software.
DHCPv4 [1] and DHCPv6 [2] 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, which includes
user interface considerations. As an aide to understanding the
potential implementation challenges of any new DHCP Option, one
implementation's approach to tackling DHCP Option formats
(Appendix A) has been included in an Appendix.
Another, and more frequently overlooked, aspect of rapid adoption is
wether or not 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 going to have a
rough time being adopted by deployed software if it requires code
changes. A rougher time still, if it does not share its deployment
fate in a general manner with other options of pressing need.
There are many things DHCP option authors can do to form DHCP Options
to make software implementors lives easier, and improve the chances
that the Option is formally adopted in deployed software after it has
been assigned an Option Code.
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
temperature" are candidates for being configured by DHCP.
The presence of such a knob isn't enough, however, because
secondarily, DHCP also presents the extension of an administrative
domain - that of the systems 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
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external Internet via local assist services that network must also
provide (such as domain name servers, or routers). This means taht
in addition to the existence of a configuration parameter, one must
also ask themselves if it is reasoanble for this parameter to be set
by the DHCP server operator.
Bear in mind that the client still reserves the right to over-ride or
ignore values received via DHCP (eg due to having a manually
configured value by its operator), and that at least one main use
case for DHCP is the corporate enterprise - so even if the local Net
Cafe is not a suitable source of this configuration, it is likely
that the client will at some point return to a network whose operator
is also the system's rightful master.
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 ([3], [4]), DHCPv4
message type option [5], link selection options ([6], [7]), and so
forth).
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.
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 [5] containing many option definitions.
Although some may not like the way an old option that solves a
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similar problem was approached, and it may waste space or processing
time or have ugly characteristics, it can usually be said that
duplicating that which has already been adopted has the greatest
chance of being adopted quickly and easily.
So it is preferrable to consider the bulk of DHCP options already
allocated, and consider which of those solve a similar problem. It
may even be that an option that solves the problem already exists.
But as there are so many options to choose from, this isn't entirely
practical. So, the following list of common option formats 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
options.
Common Option Fragments
+---------------+-------+-------------------------------------------+
| Fragment | Size | Types of Uses |
+---------------+-------+-------------------------------------------+
| ipv4-address | 4 | Default gateway, requested address, |
| | | subnet mask [5], addresses of servers |
| | | ([5], [8], [9], [10], [11], [12]), as a |
| | | component in a list of routes [13]. |
| ipv6-address | 16 | DHCPv6 server unicast address [2], |
| | | addresses of servers ([14], [15], [16], |
| | | [17], [18]). |
| 32-bit | 4 | Signed or unsigned varieties. Deprecated |
| integer | | [19] use for timezone time offset [5]. |
| | | Other uses for host configuration values |
| | | such as path mtu aging timeouts, arp |
| | | cache timeouts, tcp keepalive intervals |
| | | [5]. 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 [5], or |
| | | the elapsed time option [2] in DHCPv6. |
| 8-bit integer | 1 | Used for host configuration parameters, |
| | | such as the default IP TTL, default TCP |
| | | TTL, NetBIOS node type [5]. Also used |
| | | for protocol features, such as the DHCPv4 |
| | | Option Overload (as flags), DHCP Message |
| | | Type (as an enumeration) or DHCPv6 |
| | | Preference [2]. |
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| 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 [21] and DHCPv6 [15], but |
| Name List | | also used in DHCPv6 for any host or |
| [20] | | 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 [22], |
| encapsulation | | vendor options [5], Vendor Identified |
| | | Vendor SubOptions ([23], [2]). 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
One approach to manufacturing simple DHCP Options is to assemble the
option out of whatever common fragments fit - possibly allowing one
or more fragments to repeat to fill the 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 values at
the tail end of the option. If there are more than one variable/
indeterminate length values, consider the use of multiple options or
suboptions.
This estimates that implementations will be able to reuse code paths
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designed to support the other options.
5. Conditional Formatting is Hard
Placing a byte at the start of the option which informs the software
how to process the remaining bytes of the option may appear simple to
the casual observer. But there are no such conditional formatting
methods in existence today, so it must therefore require new, special
case code, be written for this purpose.
Wherever possible, used fixed length buffers to carry the information
desired. Obviously, this becomes less possible as the fixed length
buffer approaches large sizes, at least in DHCPv4, where DHCP packet
space is limited.
6. Aliasing
Providing multiple different formats of the same configuration
information, such as an IP address, name, or URL, which are all
intended to provide the same location information, is undesirable.
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 parameter should have only one option to
configure it. 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), late binding (such as with DNS), or completeness
(such as with a URL).
7. New Formats
If the Option simply will not fit into any existing work, the last
recourse is to contrive a new format to fit.
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When doing so, it is not enough to gauge wether 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
might reasonably have any other uses, and if so, to create the option
with the foreknowledge that it may later become a common fragment.
One specific consideration to evaluate, is wether 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. Option Size
DHCPv4 [1] 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 bytes. This means that the
options payload space will be 312 bytes, which you will have to share
with other options. This space can be extended by making use of
Option Overloading [5], which allows the use of the BOOTP FILE and
SNAME header fields for carrying DHCPv4 options (adding 192 bytes),
but these header fields will not be available for overloading if they
have been configured to carry a value.
DHCPv6 [2] carries a much more relaxed restriction, as it appears
ready and able to accept packet sizes up to 64KB, putting options
payload space at very nearly the same number (there are very few, and
small, header fields). But it is still undesirable to produce
fragments, and it's still very possible that the client's MTU is not
very large (or that client software is not prepared to retain a 64KB
buffer). So it is still best advice to design options to a ~500 byte
payload limitation.
This is easily accomplished by preferring option formats which
contain the desired information in the smallest form factor, in the
absence of other motivations. One example is to use a 4-octet IPv4
address rather than a fully qualified domain name. There may be
motivations to use the fully qualified domain name anyway, such as
externally supplied load balancers, or other protocol features.
When it is not possible to compress the configuration contents either
because of the number of optional parameters that must be identified,
or because it is expected that very large configurations are valid,
it may be preferrable 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.
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In the case where a DHCPv4 option may, or will, exceed 255 bytes in
length (and thus exceed the 'length' field's ability to contain it),
a DHCPv4 option 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 [1],
prior to evaluating their contents. This is an important distinction
that is sometimes overlooked by authors - these multiple options do
not represent multiple options formatted precisely as you have
defined, but rather one option that has been split along any
arbitrary point into multiple containers. 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, unless of course the option is very likely to exceed 255
bytes, or the documented example(s) are this big.
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
bytes, 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.
9. Clients Request their Options
In both DHCP protocols, there exists as part of the protocol
definition an option whose purpose is twofold - to inform the server
what option(s) 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 are formed by elements of the protocol's
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
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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.
10. Security Considerations
DHCP does have an Authentication mechanism ([24], [2], [25]), 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 men in
the middle attacks from within the network, even where the network
itself is protected from external attacks by firewalls.
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
to explicitly remind the implementor to be aware of the potential for
long options. Either by defining a reasonable upper limit, or
explicitly reminding the implementor that an option may be
exceptionally long.
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.
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11. IANA Considerations
This document has no actions for IANA.
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
distributions.
The ISC DHCP Software Suite has to allow:
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' [26] options), or finally were of Vendor design
(Vendor Encapsulated Options [5] 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
functions:
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.
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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
applications.
If every option were formatted differently and uniquely, then we
would have to write 6 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 4.
This is not always workable - sometimes an intermediary 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 4 to
3.
This is still an intractable number of functions per each DHCP
option. So, we need a way to reduce this to small orders.
Appendix A.1. Atomic DHCP
To accomplish these goals, a common "Format String" is used to
describe, in abstract, all of the above. Each byte in this format
string represents a "DHCP Atom". We chain these 'atoms' together,
forming a sort of molecular structure for a particular DHCP Option.
Configuration Syntax language 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
optional.
As the software encounters a format string, it processes each Atom
individually to read, formulate in memory, or write to output the
various option contents.
The format strings themselves are either hard coded by the software
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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. For example, here are two predefined options,
as they might have been configured for use by an operator if the
software did not already support them.
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 };
option dhcp.path-mtu-plataeu-table 4352, 1500, 576;
option dhcp.static-routes 10.10.10.10 10.10.10.9,
10.10.10.11 10.10.10.9;
12. Informative References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[2] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[3] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified Domain
Name (FQDN) Option", RFC 4702, October 2006.
[4] Volz, B., "The Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Client Fully Qualified Domain Name (FQDN) Option",
RFC 4704, October 2006.
[5] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
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[6] Waters, G., "The IPv4 Subnet Selection Option for DHCP",
RFC 3011, November 2000.
[7] 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.
[8] Provan, D., "DHCP Options for Novell Directory Services",
RFC 2241, November 1997.
[9] Droms, R. and K. Fong, "NetWare/IP Domain Name and
Information", RFC 2242, November 1997.
[10] Beser, B. and P. Duffy, "Dynamic Host Configuration Protocol
(DHCP) Option for CableLabs Client Configuration", RFC 3495,
March 2003.
[11] 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.
[12] 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.
[13] Lemon, T., Cheshire, S., and B. Volz, "The Classless Static
Route Option for Dynamic Host Configuration Protocol (DHCP)
version 4", RFC 3442, December 2002.
[14] Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
Protocol (DHCPv6) Options for Session Initiation Protocol (SIP)
Servers", RFC 3319, July 2003.
[15] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[16] Kalusivalingam, V., "Network Information Service (NIS)
Configuration Options for Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3898, October 2004.
[17] Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
Configuration Option for DHCPv6", RFC 4075, May 2005.
[18] Chowdhury, K., Yegani, P., and L. Madour, "Dynamic Host
Configuration Protocol (DHCP) Options for Broadcast and
Multicast Control Servers", RFC 4280, November 2005.
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[19] Lear, E. and P. Eggert, "Timezone Options for DHCP", RFC 4833,
April 2007.
[20] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[21] Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol
(DHCP) Domain Search Option", RFC 3397, November 2002.
[22] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[23] Littlefield, J., "Vendor-Identifying Vendor Options for Dynamic
Host Configuration Protocol version 4 (DHCPv4)", RFC 3925,
October 2004.
[24] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[25] Stapp, M. and T. Lemon, "The Authentication Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option",
RFC 4030, March 2005.
[26] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
Author's Address
David W. Hankins
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA
US
Phone: +1 650 423 1307
Email: David_Hankins@isc.org
Hankins Expires January 2, 2008 [Page 15]
Internet-Draft DHCP Guidelines July 2007
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Hankins Expires January 2, 2008 [Page 16]
