ISC-DHCP-REFERENCES D. Hankins
ISC
May 2007
ISC DHCP References Collection
Copyright Notice
Copyright (c) 2006-2007,2009 by Internet Systems Consortium, Inc.
("ISC")
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Abstract
This document describes a collection of Reference material that ISC
DHCP has been implemented to.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition: Reference Implementation . . . . . . . . . . . . . 3
3. Low Layer References . . . . . . . . . . . . . . . . . . . . . 4
3.1. Ethernet Protocol References . . . . . . . . . . . . . . . 6
3.2. Token Ring Protocol References . . . . . . . . . . . . . . 6
3.3. FDDI Protocol References . . . . . . . . . . . . . . . . . 6
3.4. Internet Protocol Version 4 References . . . . . . . . . . 6
3.5. Unicast Datagram Protocol References . . . . . . . . . . . 6
4. BOOTP Protocol References . . . . . . . . . . . . . . . . . . 6
5. DHCP Protocol References . . . . . . . . . . . . . . . . . . . 7
5.1. DHCPv4 Protocol . . . . . . . . . . . . . . . . . . . . . 7
5.1.1. Core Protocol References . . . . . . . . . . . . . . . 7
5.2. DHCPv6 Protocol References . . . . . . . . . . . . . . . . 7
5.3. DHCP Option References . . . . . . . . . . . . . . . . . . 8
5.3.1. Relay Agent Information Option Options . . . . . . . . 10
5.3.2. Dynamic DNS Updates References . . . . . . . . . . . . 10
5.3.3. Experimental: Failover References . . . . . . . . . . 11
5.4. DHCP Procedures . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
As a little historical anecdote, ISC DHCP once packaged all the
relevant RFCs and standards documents along with the software
package. Until one day when a voice was heard from one of the many
fine institutions that build and distribute this software... they
took issue with the IETF's copyright on the RFC's. It seems the
IETF's copyrights don't allow modification of RFC's (except for
translation purposes).
Our main purpose in providing the RFCs is to aid in documentation,
but since RFCs are now available widely from many points of
distribution on the Internet, there is no real need to provide the
documents themselves. So, this document has been created in their
stead, to list the various IETF RFCs one might want to read, and to
comment on how well (or poorly) we have managed to implement them.
2. Definition: Reference Implementation
ISC DHCP, much like its other cousins in ISC software, is self-
described as a 'Reference Implementation.' There has been a great
deal of confusion about this term. Some people seem to think that
this term applies to any software that once passed a piece of
reference material on its way to market (but may do quite a lot of
things that aren't described in any reference, or may choose to
ignore the reference it saw entirely). Other folks get confused by
the word 'reference' and understand that to mean that there is some
special status applied to the software - that the software itself is
the reference by which all other software is measured. Something
along the lines of being "The DHCP Protocol's Reference Clock," it is
supposed.
The truth is actually quite a lot simpler. Reference implementations
are software packages which were written to behave precisely as
appears in reference material. They are written "to match
reference."
If the software has a behaviour that manifests itself externally
(whether it be something as simple as the 'wire format' or something
higher level, such as a complicated behaviour that arises from
multiple message exchanges), that behaviour must be found in a
reference document.
Anything else is a bug, the only question is whether the bug is in
reference or software (failing to implement the reference).
This means:
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o To produce new externally-visible behaviour, one must first
provide a reference.
o Before changing externally visible behaviour to work around simple
incompatibilities in any other implementation, one must first
provide a reference.
That is the lofty goal, at any rate. It's well understood that,
especially because the ISC DHCP Software package has not always been
held to this standard (but not entirely due to it), there are many
non-referenced behaviours within ISC DHCP.
The primary goal of reference implementation is to prove the
reference material. If the reference material is good, then you
should be able to sit down and write a program that implements the
reference, to the word, and come to an implementation that is
distinguishable from others in the details, but not in the facts of
operating the protocol. This means that there is no need for
'special knowledge' to work around arcane problems that were left
undocumented. No secret handshakes need to be learned to be imparted
with the necessary "real documentation".
Also, by accepting only reference as the guidebook for ISC DHCP's
software implementation, anyone who can make an impact on the color
texture or form of that reference has a (somewhat indirect) voice in
ISC DHCP's software design. As the IETF RFC's have been selected as
the source of reference, that means everyone on the Internet with the
will to participate has a say.
3. Low Layer References
It may surprise you to realize that ISC DHCP implements 802.1
'Ethernet' framing, Token Ring, and FDDI. In order to bridge the gap
there between these physical and DHCP layers, it must also implement
IP and UDP framing.
The reason for this stems from Unix systems' handling of BSD sockets
(the general way one might engage in transmission of UDP packets) on
unconfigured interfaces, or even the handling of broadcast addressing
on configured interfaces.
There are a few things that DHCP servers, relays, and clients all
need to do in order to speak the DHCP protocol in strict compliance
with RFC2131 [RFC2131].
1. Transmit a UDP packet from IP:0.0.0.0 Ethernet:Self, destined to
IP:255.255.255.255 LinkLayer:Broadcast on an unconfigured (no IP
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address yet) interface.
2. Receive a UDP packet from IP:remote-system LinkLayer:remote-
system, destined to IP:255.255.255.255 LinkLayer:Broadcast, again
on an unconfigured interface.
3. Transmit a UDP packet from IP:Self, Ethernet:Seelf, destined to
IP:remote-system LinkLayer:remote-system, without transmitting a
single ARP.
4. And of course the simple case, a regular IP unicast that is
routed via the usual means (so it may be direct to a local
system, with ARP providing the glue, or it may be to a remote
system via one or more routers as normal). In this case, the
interfaces are always configured.
The above isn't as simple as it sounds on a regular BSD socket. Many
unix implementations will transmit broadcasts not to 255.255.255.255,
but to x.y.z.255 (where x.y.z is the system's local subnet). Such
packets are not received by several known DHCP client implementations
- and it's not their fault, RFC2131 [RFC2131] very explicitly demands
that these packets' IP destination addresses be set to
255.255.255.255.
Receiving packets sent to 255.255.255.255 isn't a problem on most
modern unixes...so long as the interface is configured. When there
is no IPv4 address on the interface, things become much more murky.
So, for this convoluted and unfortunate state of affairs in the unix
systems of the day ISC DHCP was manufactured, in order to do what it
needs not only to implement the reference but to interoperate with
other implementations, the software must create some form of raw
socket to operate on.
What it actually does is create, for each interface detected on the
system, a Berkeley Packet Filter socket (or equivalent), and program
it with a filter that brings in only DHCP packets. A "fallback" UDP
Berkeley socket is generally also created, a single one no matter how
many interfaces. Should the software need to transmit a contrived
packet to the local network the packet is formed piece by piece and
transmitted via the BPF socket. Hence the need to implement many
forms of Link Layer framing and above. The software gets away with
not having to implement IP routing tables as well by simply utilizing
the aforementioned 'fallback' UDP socket when unicasting between two
configured systems is the need.
Modern unixes have opened up some facilities that diminish how much
of this sort of nefarious kludgery is necessary, but have not found
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the state of affairs absolutely absolved. In particular, one might
now unicast without ARP by inserting an entry into the ARP cache
prior to transmitting. Unconfigured interfaces remain the sticking
point, however...on virtually no modern unixes is it possible to
receive broadcast packets unless a local IPv4 address has been
configured, unless it is done with raw sockets.
3.1. Ethernet Protocol References
ISC DHCP Implements Ethernet Version 2 ("DIX"), which is a variant of
IEEE 802.2. No good reference of this framing is known to exist at
this time, but it is vaguely described in RFC894 [RFC0894] (see the
section titled "Packet format"), and the following URL is also
thought to be useful.
http://en.wikipedia.org/wiki/DIX
3.2. Token Ring Protocol References
IEEE 802.5 defines the Token Ring framing format used by ISC DHCP.
3.3. FDDI Protocol References
RFC1188 [RFC1188] is the most helpful reference ISC DHCP has used to
form FDDI packets.
3.4. Internet Protocol Version 4 References
RFC760 [RFC0760] fundamentally defines the bare IPv4 protocol which
ISC DHCP implements.
3.5. Unicast Datagram Protocol References
RFC768 [RFC0768] defines the User Datagram Protocol that ultimately
carries the DHCP or BOOTP protocol. The destination DHCP server port
is 67, the client port is 68. Source ports are irrelevant.
4. BOOTP Protocol References
The DHCP Protocol is strange among protocols in that it is grafted
over the top of another protocol - BOOTP (but we don't call it "DHCP
over BOOTP" like we do, say "TCP over IP"). BOOTP and DHCP share UDP
packet formats - DHCP is merely a conventional use of both BOOTP
header fields and the trailing 'options' space.
The ISC DHCP server supports BOOTP clients conforming to RFC951
[RFC0951] and RFC1542 [RFC1542].
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5. DHCP Protocol References
5.1. DHCPv4 Protocol
"The DHCP[v4] Protocol" is not defined in a single document. The
following collection of references of what ISC DHCP terms "The DHCPv4
Protocol".
5.1.1. Core Protocol References
RFC2131 [RFC2131] defines the protocol format and procedures. ISC
DHCP is not known to diverge from this document in any way. There
are, however, a few points on which different implementations have
arisen out of vagueries in the document. DHCP Clients exist which,
at one time, present themselves as using a Client Identifier Option
which is equal to the client's hardware address. Later, the client
transmits DHCP packets with no Client Identifier Option present -
essentially identifying themselves using the hardware address. Some
DHCP Servers have been developed which identify this client as a
single client. ISC has interpreted RFC2131 to indicate that these
clients must be treated as two separate entities (and hence two,
separate addresses). Client behaviour (Embedded Windows products)
has developed that relies on the former implementation, and hence is
incompatible with the latter. Also, RFC2131 demands explicitly that
some header fields be zeroed upon certain message types. The ISC
DHCP Server instead copies many of these fields from the packet
received from the client or relay, which may not be zero. It is not
known if there is a good reason for this that has not been
documented.
RFC2132 [RFC2132] defines the initial set of DHCP Options and
provides a great deal of guidance on how to go about formatting and
processing options. The document unfortunately waffles to a great
extent about the NULL termination of DHCP Options, and some DHCP
Clients (Windows 95) have been implemented that rely upon DHCP
Options containing text strings to be NULL-terminated (or else they
crash). So, ISC DHCP detects if clients null-terminate the host-name
option and, if so, null terminates any text options it transmits to
the client. It also removes NULL termination from any known text
option it receives prior to any other processing.
5.2. DHCPv6 Protocol References
For now there is only one document that specifies the DHCPv6 protocol
(there have been no updates yet), RFC3315 [RFC3315].
Support for DHCPv6 was added first in version 4.0.0. The server and
client support only IA_NA. While the server does support multiple
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IA_NAs within one packet from the client, our client only supports
sending one. There is no relay support.
DHCPv6 introduces some new and uncomfortable ideas to the common
software library.
1. Options of zero length are normal in DHCPv6. Currently, all ISC
DHCP software treats zero-length options as errors.
2. Options sometimes may appear multiple times. The common library
used to treat all appearance of multiple options as specified in
RFC2131 - to be concatenated. DHCPv6 options may sometimes
appear multiple times (such as with IA_NA or IAADDR), but often
must not.
3. The same option space appears in DHCPv6 packets multiple times.
If the packet was got via a relay, then the client's packet is
stored to an option within the relay's packet...if there were two
relays, this recurses. At each of these steps, the root "DHCPv6
option space" is used. Further, a client packet may contain an
IA_NA, which may contain an IAADDR - but really, in an abstract
sense, this is again re-encapsulation of the DHCPv6 option space
beneath options it also contains.
Precisely how to correctly support the above conundrums has not quite
yet been settled, so support is incomplete.
5.3. DHCP Option References
RFC2241 [RFC2241] defines options for Novell Directory Services.
RFC2242 [RFC2242] defines an encapsulated option space for NWIP
configuration.
RFC2485 [RFC2485] defines the Open Group's UAP option.
RFC2610 [RFC2610] defines options for the Service Location Protocol
(SLP).
RFC2937 [RFC2937] defines the Name Service Search Option (not to be
confused with the domain-search option). The Name Service Search
Option allows eg nsswitch.conf to be reconfigured via dhcp. The ISC
DHCP server implements this option, and the ISC DHCP client is
compatible...but does not by default install this option's value.
One would need to make their relevant dhclient-script process this
option in a way that is suitable for the system.
RFC3004 [RFC3004] defines the User-Class option. Note carefully that
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ISC DHCP currently does not implement to this reference, but has
(inexplicably) selected an incompatible format: a plain text string.
RFC3011 [RFC3011] defines the Subnet-Selection plain DHCPv4 option.
Do not confuse this option with the relay agent "link selection" sub-
option, although their behaviour is similar.
RFC3319 [RFC3319] defines the SIP server options for DHCPv6.
RFC3396 [RFC3396] documents both how long options may be encoded in
DHCPv4 packets, and also how multiple instances of the same option
code within a DHCPv4 packet will be decoded by receivers.
RFC3397 [RFC3397] documents the Domain-Search Option, which allows
the configuration of the /etc/resolv.conf 'search' parameter in a way
that is RFC1035 [RFC1035] wire format compatible (in fact, it uses
the RFC1035 wire format). ISC DHCP has both client and server
support, and supports RFC1035 name compression.
RFC3646 [RFC3646] documents the DHCPv6 name-servers and domain-search
options.
RFC3633 [RFC3633] documents the Identity Association Prefix
Delegation, which is included here for protocol wire reference, but
which is not supported by ISC DHCP.
RFC3679 [RFC3679] documents a number of options that were documented
earlier in history, but were not made use of.
RFC3898 [RFC3898] documents four NIS options for delivering NIS
servers and domain information in DHCPv6.
RFC3925 [RFC3925] documents a pair of Enterprise-ID delimited option
spaces for vendors to use in order to inform servers of their "vendor
class" (sort of like 'uname' or 'who and what am I'), and a means to
deliver vendor-specific and vendor-documented option codes and
values.
RFC3942 [RFC3942] redefined the 'site local' option space.
RFC4075 [RFC4075] defines the DHCPv6 SNTP Servers option.
RFC4242 [RFC4242] defines the Information Refresh Time option, which
advises DHCPv6 Information-Request clients to return for updated
information.
RFC4280 [RFC4280] defines two BCMS server options.
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RFC4388 [RFC4388] defined the DHCPv4 LEASEQUERY message type and a
number of suitable response messages, for the purpose of sharing
information about DHCP served addresses and clients.
RFC4580 [RFC4580] defines a DHCPv6 subscriber-id option, which is
similar in principle to the DHCPv4 relay agent option of the same
name.
RFC4649 [RFC4649] defines a DHCPv6 remote-id option, which is similar
in principle to the DHCPv4 relay agent remote-id.
5.3.1. Relay Agent Information Option Options
RFC3046 [RFC3046] defines the Relay Agent Information Option and
provides a number of sub-option definitions.
RFC3256 [RFC3256] defines the DOCSIS Device Class sub-option.
RFC3527 [RFC3527] defines the Link Selection sub-option.
5.3.2. Dynamic DNS Updates References
The collection of documents that describe the standards-based method
to update dns names of DHCP clients starts most easily with RFC4703
[RFC4703] to define the overall architecture, travels through RFCs
4702 [RFC4702] and 4704 [RFC4704] to describe the DHCPv4 and DHCPv6
FQDN options (to carry the client name), and ends up at RFC4701
[RFC4701] which describes the DHCID RR used in DNS to perform a kind
of atomic locking.
ISC DHCP adoped early versions of these documents, and has not yet
synched up with the final standards versions.
For RFCs 4702 and 4704, the 'N' bit is not yet supported. The result
is that it is always set zero, and is ignored if set.
For RFC4701, which is used to match client identities with names in
the DNS as part of name conflict resolution. Note that ISC DHCP's
implementation of DHCIDs vary wildly from this specification. First,
ISC DHCP uses a TXT record in which the contents are stored in
hexadecimal. Second, there is a flaw in the selection of the
'Identifier Type', which results in a completely different value
being selected than was defined in an older revision of this
document...also this field is one byte prior to hexadecimal encoding
rather than two. Third, ISC DHCP does not use a digest type code.
Rather, all values for such TXT records are reached via an MD5 sum.
In short, nothing is compatible, but the principle of the TXT record
is the same as the standard DHCID record. However, for DHCPv6 FQDN,
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we do use DHCID type code '2', as no other value really makes sense
in our context.
5.3.3. Experimental: Failover References
The Failover Protocol defines a means by which two DHCP Servers can
share all the relevant information about leases granted to DHCP
clients on given networks, so that one of the two servers may fail
and be survived by a server that can act responsibly.
Unfortunately it has been quite some years since the last time this
document was edited, and the authors no longer show any interest in
fielding comments or improving the document.
The status of this protocol is very unsure, but ISC's implementation
of it has proven stable and suitable for use in sizable production
environments.
draft-ietf-dhc-failover-12.txt [draft-failover] describes the
Failover Protocol. In addition to what is described in this
document, ISC DHCP has elected to make some experimental changes that
may be revoked in a future version of ISC DHCP (if the draft authors
do not adopt the new behaviour). Specifically, ISC DHCP's POOLREQ
behaviour differs substantially from what is documented in the draft,
and the server also implements a form of 'MAC Address Affinity' which
is not described in the failover document. The full nature of these
changes have been described on the IETF DHC WG mailing list (which
has archives), and also in ISC DHCP's manual pages. Also note that
although this document references a RECOVER-WAIT state, it does not
document a protocol number assignment for this state. As a
consequence, ISC DHCP has elected to use the value 254.
RFC3074 [RFC3074] describes the Load Balancing Algorithm (LBA) that
ISC DHCP uses in concert with the Failover protocol. Note that
versions 3.0.* are known to misimplement the hash algorithm (it will
only use the low 4 bits of every byte of the hash bucket array).
5.4. DHCP Procedures
RFC2939 [RFC2939] explains how to go about obtaining a new DHCP
Option code assignment.
6. References
[RFC0760] Postel, J., "DoD standard Internet Protocol", RFC 760,
January 1980.
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[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0894] Hornig, C., "Standard for the transmission of IP datagrams
over Ethernet networks", STD 41, RFC 894, April 1984.
[RFC0951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1188] Katz, D., "Proposed Standard for the Transmission of IP
Datagrams over FDDI Networks", RFC 1188, October 1990.
[RFC1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[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.
[RFC2485] Drach, S., "DHCP Option for The Open Group's User
Authentication Protocol", RFC 2485, January 1999.
[RFC2610] Perkins, C. and E. Guttman, "DHCP Options for Service
Location Protocol", RFC 2610, June 1999.
[RFC2937] Smith, C., "The Name Service Search Option for DHCP",
RFC 2937, September 2000.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
[RFC3004] Stump, G., Droms, R., Gu, Y., Vyaghrapuri, R., Demirtjis,
A., Beser, B., and J. Privat, "The User Class Option for
DHCP", RFC 3004, November 2000.
[RFC3011] Waters, G., "The IPv4 Subnet Selection Option for DHCP",
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RFC 3011, November 2000.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[RFC3074] Volz, B., Gonczi, S., Lemon, T., and R. Stevens, "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
[RFC3256] Jones, D. and R. Woundy, "The DOCSIS (Data-Over-Cable
Service Interface Specifications) Device Class DHCP
(Dynamic Host Configuration Protocol) Relay Agent
Information Sub-option", RFC 3256, April 2002.
[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.
[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
November 2002.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[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.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC3679] Droms, R., "Unused Dynamic Host Configuration Protocol
(DHCP) Option Codes", RFC 3679, January 2004.
[RFC3898] Kalusivalingam, V., "Network Information Service (NIS)
Configuration Options for Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3898, October 2004.
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[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.
[RFC4075] Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
Configuration Option for DHCPv6", RFC 4075, May 2005.
[RFC4242] Venaas, S., Chown, T., and B. Volz, "Information Refresh
Time Option for Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 4242, November 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.
[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC4580] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Subscriber-ID Option", RFC 4580,
June 2006.
[RFC4649] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Remote-ID Option", RFC 4649,
August 2006.
[RFC4701] Stapp, M., Lemon, T., and A. Gustafsson, "A DNS Resource
Record (RR) for Encoding Dynamic Host Configuration
Protocol (DHCP) Information (DHCID RR)", RFC 4701,
October 2006.
[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.
[RFC4703] Stapp, M. and B. Volz, "Resolution of Fully Qualified
Domain Name (FQDN) Conflicts among Dynamic Host
Configuration Protocol (DHCP) Clients", RFC 4703,
October 2006.
[RFC4704] Volz, B., "The Dynamic Host Configuration Protocol for
IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
Option", RFC 4704, October 2006.
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[draft-failover]
Droms, R., "DHCP Failover Protocol", March 2003.
Author's Address
David W. Hankins
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
Phone: +1 650 423 1300
Email: David_Hankins@isc.org
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