Dynamic Host Configuration Working D. Hankins
Group ISC
Internet-Draft February 17, 2009
Intended status: Informational
Expires: August 21, 2009
Guidelines for Creating New DHCP Options
draft-ietf-dhc-option-guidelines-04
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Abstract
This document seeks to provide guidance to prospective DHCP Option
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authors, to help them in producing option formats that are easily
adoptable.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. When to Use DHCP . . . . . . . . . . . . . . . . . . . . . . . 3
3. General Principles . . . . . . . . . . . . . . . . . . . . . . 4
4. Reusing Other Options . . . . . . . . . . . . . . . . . . . . 5
5. Conditional Formatting is Hard . . . . . . . . . . . . . . . . 7
6. Avoid Aliasing . . . . . . . . . . . . . . . . . . . . . . . . 7
7. New Formats . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. The Dangers of Sub Options . . . . . . . . . . . . . . . . . . 8
9. Option Size . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Clients Request their Options . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
13. Appendix A: Background on ISC DHCP . . . . . . . . . . . . . . 12
13.1. Atomic DHCP . . . . . . . . . . . . . . . . . . . . . . . 14
14. Informative References . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
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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, which
includes user interface considerations. To help understand the
potential implementation challenges of any new DHCP Option, one
implementation's approach to tackling DHCP Option formats
(Section 13) 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, creating a pressing need for 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
adoption.
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.
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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 some DHCP server operators, which roughly
translates to the local network administrator.
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).
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
options.
+---------------+-------+-------------------------------------------+
| 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 values 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. 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 the only conditional formatting methods
that are in widepsread 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, 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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conversions.
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).
7. New Formats
If the Option simply will not fit into any existing work, 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
Options.
Sub options seem very attractive, because they allow the encoding of
multiple variable length fields within the single "parent" option.
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", and the software will not include the
entire option if there is not sufficient space for only the last sub-
option to fit in the DHCP packet.
Consequently, it is not advisable to group options that may not be
requested at the same time by the same client under an encapsulated
space.
Another attraction sub options present is ease of extending the
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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 most options will be later
extended.
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.
Additionaly, DHCPv4 option concatenation (described in detail below)
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 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 [RFC2132], 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 [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, RFC3315
explicitly refers readers to RFC2460 Section 5, which describes an
MTU of 1280 octets and a minimum fragment reassembly of 1500 octets.
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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
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.
The DHCPv4 code and length tags are each a single byte. As the
length field describes the length of the DHCP option's contents (not
including the code and length bytes), any option whose contents'
length exceeds 255 bytes 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
[RFC2131], prior to evaluating their contents. This is an important
distinction that is sometimes overlooked by authors - these multiple
options are not individually 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, 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.
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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.
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 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 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
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
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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
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. 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
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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' [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
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.
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 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
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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
2.
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.
13.1. 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.
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
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
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.
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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.
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.
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;
14. 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.
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[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
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.
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[RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration
Protocol version 4 (DHCPv4) Options", RFC 3942,
November 2004.
[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.
Author's Address
David W. Hankins
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
US
Phone: +1 650 423 1307
Email: David_Hankins@isc.org
Hankins Expires August 21, 2009 [Page 17]
