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<rfc ipr="none">
  <front>
    <title>ISC DHCP References Collection</title>

    <author initials="D.H." surname="Hankins" fullname="David W. Hankins">
      <organization abbrev="ISC">Internet Systems Consortium,
				 Inc.
      </organization>

      <address>
	<postal>
	  <street>950 Charter Street</street>
	  <city>Redwood City</city>
	  <region>CA</region>
	  <code>94063</code>
	</postal>

	<phone>+1 650 423 1300</phone>
	<email>David_Hankins@isc.org</email>
      </address>
    </author>

    <date month="May" year="2007"/>

    <note title="Copyright Notice">
	<t>Copyright (c) 2006-2007,2009 by Internet Systems Consortium, Inc.
	("ISC")</t>

	<t>Permission to use, copy, modify, and distribute this software for
	any purpose with or without fee is hereby granted, provided that the
	above copyright notice and this permission notice appear in all
	copies.</t>

	<t>THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
	WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
	MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
	ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
	WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
	ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
	OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.</t>
    </note>

    <keyword>ISC</keyword>
    <keyword>DHCP</keyword>
    <keyword>Reference Implementation</keyword>

    <abstract>
	<t>This document describes a collection of Reference material that
	ISC DHCP has been implemented to.</t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction">
	<t>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).</t>

	<t>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.</t>
    </section>

    <section title="Definition: Reference Implementation">
	<t>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.</t>

	<t>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."</t>

	<t>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.</t>

	<t>Anything else is a bug, the only question is whether the
	bug is in reference or software (failing to implement the
	reference).</t>

	<t>This means:</t>

      <list style="symbols">
	<t>To produce new externally-visible behaviour, one must first
	provide a reference.</t>

	<t>Before changing externally visible behaviour to work around
	simple incompatibilities in any other implementation, one must
	first provide a reference.</t>
      </list>

	<t>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.</t>

	<t>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".</t>

	<t>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.</t>
    </section>

    <section title="Low Layer References">
	<t>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.</t>

	<t>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.</t>

	<t>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 <xref target="RFC2131">RFC2131</xref>.</t>

      <list style="numbers">
	<t>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
	address yet) interface.</t>

	<t>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.</t>

	<t>Transmit a UDP packet from IP:Self, Ethernet:Seelf, destined to
	IP:remote-system LinkLayer:remote-system, without transmitting a
	single ARP.</t>

	<t>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.</t>
      </list>

	<t>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, <xref target="RFC2131">
	RFC2131</xref> very explicitly demands that these packets' IP
	destination addresses be set to 255.255.255.255.</t>

	<t>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.</t>

	<t>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.</t>

	<t>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.</t>

	<t>Modern unixes have opened up some facilities that diminish how
	much of this sort of nefarious kludgery is necessary, but have not
	found 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.</t>

      <section title="Ethernet Protocol References">
	<t>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 <xref target="RFC0894">
	RFC894</xref> (see the section titled "Packet format"), and
	the following URL is also thought to be useful.</t>

	<t>http://en.wikipedia.org/wiki/DIX</t>
      </section>

      <section title="Token Ring Protocol References">
	<t>IEEE 802.5 defines the Token Ring framing format used by ISC
	DHCP.</t>
      </section>

      <section title="FDDI Protocol References">
	<t><xref target="RFC1188">RFC1188</xref> is the most helpful
	reference ISC DHCP has used to form FDDI packets.</t>
      </section>

      <section title="Internet Protocol Version 4 References">
	<t><xref target="RFC0760">RFC760</xref> fundamentally defines the
	bare IPv4 protocol which ISC DHCP implements.</t>
      </section>

      <section title="Unicast Datagram Protocol References">
	<t><xref target="RFC0768">RFC768</xref> 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.</t>
      </section>
    </section>

    <section title="BOOTP Protocol References">
	<t>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.</t>

	<t>The ISC DHCP server supports BOOTP clients conforming to
	<xref target="RFC0951">RFC951</xref> and <xref target="RFC1542">
	RFC1542</xref>.</t>
    </section>

    <section title="DHCP Protocol References">
      <section title="DHCPv4 Protocol">
	<t>"The DHCP[v4] Protocol" is not defined in a single document.  The
	following collection of references of what ISC DHCP terms "The
	DHCPv4 Protocol".</t>

        <section title="Core Protocol References">
	  <t><xref target="RFC2131">RFC2131</xref> 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.</t>

	  <t><xref target="RFC2132">RFC2132</xref> 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.</t>
        </section>
      </section>

      <section title="DHCPv6 Protocol References">
	<t>For now there is only one document that specifies the DHCPv6
	protocol (there have been no updates yet), <xref target="RFC3315">
	RFC3315</xref>.</t>

	<t>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
	IA_NAs within one packet from the client, our client only supports
	sending one.  There is no relay support.</t>

	<t>DHCPv6 introduces some new and uncomfortable ideas to the common
	software library.</t>

	<list style="numbers">
	  <t>Options of zero length are normal in DHCPv6.  Currently, all
	  ISC DHCP software treats zero-length options as errors.</t>

	  <t>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.</t>

	  <t>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.</t>
	</list>

	<t>Precisely how to correctly support the above conundrums has not
	quite yet been settled, so support is incomplete.</t>
      </section>

      <section title="DHCP Option References">
	<t><xref target="RFC2241">RFC2241</xref> defines options for
	Novell Directory Services.</t>

	<t><xref target="RFC2242">RFC2242</xref> defines an encapsulated
	option space for NWIP configuration.</t>

	<t><xref target="RFC2485">RFC2485</xref> defines the Open Group's
	UAP option.</t>

	<t><xref target="RFC2610">RFC2610</xref> defines options for
	the Service Location Protocol (SLP).</t>

	<t><xref target="RFC2937">RFC2937</xref> 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.</t>

	<t><xref target="RFC3004">RFC3004</xref> defines the User-Class
	option.  Note carefully that ISC DHCP currently does not implement
	to this reference, but has (inexplicably) selected an incompatible
	format: a plain text string.</t>

	<t><xref target="RFC3011">RFC3011</xref> 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.</t>

	<t><xref target="RFC3319">RFC3319</xref> defines the SIP server
	options for DHCPv6.</t>

	<t><xref target="RFC3396">RFC3396</xref> 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.</t>

	<t><xref target="RFC3397">RFC3397</xref> documents the Domain-Search
	Option, which allows the configuration of the /etc/resolv.conf
	'search' parameter in a way that is <xref target="RFC1035">RFC1035
	</xref> wire format compatible (in fact, it uses the RFC1035 wire
	format).  ISC DHCP has both client and server support, and supports
	RFC1035 name compression.</t>

	<t><xref target="RFC3646">RFC3646</xref> documents the DHCPv6
	name-servers and domain-search options.</t>

	<t><xref target="RFC3633">RFC3633</xref> documents the Identity
	Association Prefix Delegation, which is included here for protocol
	wire reference, but which is not supported by ISC DHCP.</t>

	<t><xref target="RFC3679">RFC3679</xref> documents a number of
	options that were documented earlier in history, but were not
	made use of.</t>

	<t><xref target="RFC3898">RFC3898</xref> documents four NIS options
	for delivering NIS servers and domain information in DHCPv6.</t>

	<t><xref target="RFC3925">RFC3925</xref> 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.</t>

	<t><xref target="RFC3942">RFC3942</xref> redefined the 'site local'
	option space.</t>

	<t><xref target="RFC4075">RFC4075</xref> defines the DHCPv6 SNTP
	Servers option.</t>

	<t><xref target="RFC4242">RFC4242</xref> defines the Information
	Refresh Time option, which advises DHCPv6 Information-Request
	clients to return for updated information.</t>

	<t><xref target="RFC4280">RFC4280</xref> defines two BCMS server
	options.</t>

	<t><xref target="RFC4388">RFC4388</xref> 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.</t>

	<t><xref target="RFC4580">RFC4580</xref> defines a DHCPv6
	subscriber-id option, which is similar in principle to the DHCPv4
	relay agent option of the same name.</t>

	<t><xref target="RFC4649">RFC4649</xref> defines a DHCPv6 remote-id
	option, which is similar in principle to the DHCPv4 relay agent
	remote-id.</t>

        <section title="Relay Agent Information Option Options">
	  <t><xref target="RFC3046">RFC3046</xref> defines the Relay Agent
	  Information Option and provides a number of sub-option
	  definitions.</t>

	  <t><xref target="RFC3256">RFC3256</xref> defines the DOCSIS Device
	  Class sub-option.</t>

	  <t><xref target="RFC3527">RFC3527</xref> defines the Link Selection
  	  sub-option.</t>
        </section>

	<section title="Dynamic DNS Updates References">
	  <t>The collection of documents that describe the standards-based
	  method to update dns names of DHCP clients starts most easily
	  with <xref target="RFC4703">RFC4703</xref> to define the overall
	  architecture, travels through RFCs <xref target="RFC4702">4702</xref>
	  and <xref target="RFC4704">4704</xref> to describe the DHCPv4 and
	  DHCPv6 FQDN options (to carry the client name), and ends up at
	  <xref target="RFC4701">RFC4701</xref> which describes the DHCID
	  RR used in DNS to perform a kind of atomic locking.</t>

	  <t>ISC DHCP adoped early versions of these documents, and has not
	  yet synched up with the final standards versions.</t>

	  <t>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.</t>

	  <t>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, we do use DHCID type code '2',
	  as no other value really makes sense in our context.</t>
	</section>

	<section title="Experimental: Failover References">
	  <t>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.</t>

	  <t>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.</t>

	  <t>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.</t>

	  <t><xref target="draft-failover">draft-ietf-dhc-failover-12.txt</xref>
	  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.</t>

	  <t><xref target="RFC3074">RFC3074</xref> 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).</t>
	</section>
      </section>

      <section title="DHCP Procedures">
	<t><xref target="RFC2939">RFC2939</xref> explains how to go about
	obtaining a new DHCP Option code assignment.</t>
      </section>
    </section>
  </middle>

  <back>
    <references>
	&rfc760;
	&rfc768;
	&rfc894;
	&rfc951;
	&rfc1035;
	&rfc1188;
	&rfc1542;
	&rfc2131;
	&rfc2132;
	&rfc2241;
	&rfc2242;
	&rfc2485;
	&rfc2610;
	&rfc2937;
	&rfc2939;
	&rfc3004;
	&rfc3011;
	&rfc3046;
	&rfc3074;
	&rfc3256;
	&rfc3315;
	&rfc3319;
	&rfc3396;
	&rfc3397;
	&rfc3527;
	&rfc3633;
	&rfc3646;
	&rfc3679;
	&rfc3898;
	&rfc3925;
	&rfc3942;
	&rfc4075;
	&rfc4242;
	&rfc4280;
	&rfc4388;
	&rfc4580;
	&rfc4649;
	&rfc4701;
	&rfc4702;
	&rfc4703;
	&rfc4704;

	<reference anchor='draft-failover'>
	  <front>
	    <title>DHCP Failover Protocol</title>

	    <author initials='R.' surname='Droms' fullname='Ralph Droms'>
		<organization abbrev='Cisco'>Cisco Systems</organization>
	    </author>

	    <date month='March' year='2003'/>
	  </front>
	  <format type="TXT" octets="312151" target="https://www.isc.org/sw/dhcp/drafts/draft-ietf-dhc-failover-12.txt"/>
	</reference>
    </references>
  </back>
</rfc>

