Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute LeavesBrandenburg InternetWorking675 Spruce Dr.SunnyvaleCA94086United States of America+1.408.246.8253dcrocker@bbiw.nethttp://bbiw.net/dnsopDNSDomain Name System> Formally, any DNS Resource Record (RR) may occur under any domain name.
However, some services use an operational convention for defining
specific interpretations of an RRset by locating the records in a
DNS branch under the parent domain to which the RRset actually
applies. The top of this subordinate branch is defined by a naming
convention that uses a reserved node name, which begins with
the underscore character (e.g., "_name"). The underscored naming construct defines a semantic
scope for DNS record types that are associated with the parent
domain above the underscored branch. This specification explores
the nature of this DNS usage and defines the "Underscored and Globally Scoped DNS Node Names" registry with IANA. The purpose of this
registry is to avoid collisions resulting from the use of the same
underscored name for different services.The core Domain Name System (DNS) technical specifications ( and ) assign no
semantics to domain names or their parts, and no constraints upon
which resource record (RR) types are permitted to be stored under
particular names .
Over time, some leaf node names, such as
www and ftp,
have come to imply support for particular services, but this is a
matter of operational convention rather than defined protocol
semantics.
This freedom in the basic technology has permitted a wide range of
administrative and semantic policies to be used -- in parallel. DNS
data semantics have been limited to the specification of particular
resource record types on the expectation that new resource record
types would be added as needed. Unfortunately, the addition of new resource record types has proven
extremely challenging, with significant adoption and use barriers
occurring over the life of the DNS.
As an alternative to defining a new RR TYPE, some DNS service
enhancements call for using an existing resource record type but
specifying a restricted scope for its occurrence. Scope is meant as
a static property, not one dependent on the nature of the query.
It is an artifact of the DNS name. That scope is a leaf node
containing the specific resource record sets that are formally
defined and constrained. Specifically:
The leaf occurs in a branch having a distinguished naming
convention: there is a parent domain name to which the
scoped data applies. The branch is under this name. The
sub-branch is indicated by a sequence of one or more
reserved DNS node names; at least the first (highest) of
these names begins with an underscore (e.g., "_name"). Because the DNS rules for a "host" (host name)
do not allow use of the underscore character,
the underscored name is distinguishable from all legal host names . Effectively, this convention for naming leaf nodes
creates a space for the listing of "attributes" -- in the
form of resource record types -- that are associated with the
parent domain above the underscored sub-branch. The scoping feature is particularly useful when generalized
resource record types are used -- notably
TXT, SRV,
and URI. It provides
efficient separation of one use of them from others. Absent this
separation, an undifferentiated mass of these RRsets is returned
to the DNS client, which then must parse through the internals of
the records in the hope of finding ones that are relevant. Worse,
in some cases, the results are ambiguous because a record type
might not adequately self-identify its specific purpose. With
underscore-based scoping, only the relevant RRsets are
returned.A simple example is DomainKeys Identified Mail (DKIM), which
uses _domainkey to define a place
to hold a TXT record containing signing information for the
parent domain. This specification formally defines how underscored names are
used as "attribute" enhancements for their parent domain names.
For example, the domain name "_domainkey.example." acts as an
attribute of the parent domain name "example.". To avoid
collisions resulting from the use of the same underscored
names for different applications using the same resource record
type, this document establishes the "Underscored and Globally Scoped DNS Node Names" registry with IANA. Use of such node names, which begin with an underscore character,
is reserved when they are the underscored name closest to the DNS
root; as in that case, they are considered "global". Underscored names that are farther down the hierarchy are
handled within the scope of the global underscored node name.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
Some resource record types are used in a fashion that can create
scaling problems if an entire RRset associated with a domain
name is aggregated in the leaf node for that name. An
increasingly popular approach, with excellent scaling properties,
places the RRset under a specially named branch, which is in turn
under the node name that would otherwise contain the RRset. The
rules for naming that branch define the context for interpreting
the RRset. That is, rather than: the arrangement is: A direct lookup to the subordinate leaf node produces
only the desired record types, at no greater cost than a typical
DNS lookup. As defined in , the DNS uses names
organized in a tree-structured or hierarchical fashion. A domain
name might have multiple node names that begin with
the underscore character (e.g., "_name"). A global underscored node name is the one that is
closest to the root of the DNS hierarchy, also called the
highest level or topmost. In the presentation convention
described in Section 3.1 of , this is the
rightmost name beginning with an underscore. In other
presentation environments, it might be positioned differently. To
avoid concern for the presentation variations, the qualifier
"global" is used here.DNS wildcards interact poorly with underscored names in two ways:
Since wildcards are only interpreted as leaf names, one cannot
create the equivalent of a wildcard name for prefixed names. A
name such as label.*.example.com is not a wildcard. Conversely, a wildcard such as *.example.com can match any name
including an underscored name. So, a wildcard might match an
underscored name, returning a record that is the type controlled
by the underscored name but is not intended to be used in the
underscored context and does not conform to its rules. Originally, different uses of underscored node names
developed largely without coordination. For TXT records, there is
no consistent, internal syntax that permits distinguishing among
the different uses. In the case of the SRV RR and URI RR,
distinguishing among different types of use was part of the
design (see and ). The
SRV and URI specifications serve as templates, defining RRs that
might only be used for specific applications when there is an
additional specification. The template definition included
reference to two levels of tables of names from which
underscored names should be drawn. The lower-level (local scope)
set of _service names is defined in
terms of other IANA tables, namely any table with symbolic names.
The upper-level (global scope) SRV naming field is
_proto, although its pool of names is
not explicitly defined. The aggregate effect of these independent efforts was a long list
of underscored names that were reserved without
coordination, which invites an eventual name-assignment
collision. The remedy is this base document and a companion document (), which define a
registry for these names and attempt to register all those
already in use as well as to direct changes to the
pre-registry specifications that used global underscored
node names. A registry for global DNS node names that begin with an underscore
is defined here. The purpose of the
"Underscored and Globally Scoped DNS Node Names" registry is to
avoid collisions resulting from the use of the same underscored
name for different applications. If a public specification calls for use of an
underscored node name, the global underscored
node name -- the underscored name that is closest to the DNS root
-- MUST be entered into this registry.An underscored name defines the scope of use for specific resource
record types, which are associated with the domain name that is the
"parent" to the branch defined by the underscored name. A given name
defines a specific, constrained context for one or more RR TYPEs,
where use of such record types conforms to the defined constraints.
Within a leaf that is underscore scoped, other RRsets that are not
specified as part of the scope MAY be used.Structurally, the registry is defined as a single, flat table of RR
TYPEs, under node names beginning with underscore. In some cases,
such as for use of an SRV record, the full scoping name might be
multi-part, as a sequence of underscored names. Semantically, that
sequence represents a hierarchical model, and it is theoretically
reasonable to allow reuse of a subordinate underscored name in a
different, global underscored context; that is, a subordinate name
is meaningful only within the scope of the global underscored node name.
Therefore, they are ignored by this "Underscored and Globally Scoped DNS Node Names" registry. This registry is for the definition of highest-level
-- that is, global -- underscored node name used.NAME_service1_protoB._service2_protoB._service3_protoC._service3_useX._protoD._service4_protoE._region._authorityOnly global underscored node names are registered in the "Underscored and Globally Scoped DNS Node Names" registry. From the example above, that would mean _service1, _service2, _service3, _service 4, and _authority would be listed in the IANA registry.
The use of underscored node names is specific to each RR TYPE
that is being scoped. Each name defines a place but does not
define the rules for what appears underneath that place,
either as additional underscored naming or as a leaf node with
resource records. Details for those rules are provided by
specifications for individual RR TYPEs. The sections below
describe the way that existing underscored names are used with
the RR TYPEs that they name.Definition and registration of subordinate underscored node
names are the responsibility of the specification that creates
the global underscored node name registry entry.That is, if a scheme using a global underscored node name has one or
more subordinate levels of underscored node naming, the namespaces
from which names for those lower levels are chosen are controlled by
the parent underscored node name. Each registered global underscored
node name owns a distinct, subordinate namespace. This section provides guidance for specification writers, with a basic template they can use, to register new entries in the "Underscored and Globally Scoped DNS Node Names"
registry. The text can be added to specifications using
RR TYPE / _NODE NAME combinations that have not already been
registered:Per RFC 8552, please add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry:RR Type_NODE NAME Reference{RR TYPE}_{DNS global node name} {citation for the document making the addition.}IANA has established the "Underscored and Globally Scoped DNS Node Names" registry. This section describes the registry, the definitions, the initial entries, the use of_ta and _example, and the use of as guidance for expert review. IANA has also updated the "Enumservices Registrations" registry with a pointer to this document.The "Underscored and Globally Scoped DNS Node Names" registry includes any DNS node
name that begins with the underscore character ("_",
ASCII 0x5F) and is the underscored node name closest to the root;
that is, it defines the highest level of a DNS branch under a
"parent" domain name. This registry operates under the IANA rules for
"Expert Review" registration; see .The contents of each entry in the registry are
defined in .Each entry in the registry MUST contain values for all of
the fields specified in .Within the registry, the combination of RR Type and _NODE
NAME MUST be unique.The table is to be maintained with entries sorted by the
first column (RR Type) and, within that, the second column
(_NODE NAME).The required Reference for an entry MUST have a stable
resolution to the organization controlling that registry
entry.A registry entry contains: Lists an RR TYPE that is
defined for use within this scope.Specifies a single,
underscored name that defines a reserved name; this
name is the global entry name for the scoped
resource record types that are associated with that
name. For characters in the name that have an
uppercase form and a lowercase form, the character
MUST be recorded as lowercase to simplify name
comparisons. Lists the specification that
defines a record type and its use under this _Node
Name. The organization producing the specification
retains control over the registry entry for the _Node
Name. Each RR TYPE that is to be used with a _Node Name MUST
have a separate registry entry. The initial entries in the registry are as follows:RR Type_NODE NAME Reference*_exampleNULL_ta-* {}OPENPGPKEY_openpgpkeySMIMEA _smimecertSRV_dccpSRV_httpSRV_ipv6SRV_ldapSRV_ocspSRV_sctpSRV_sipSRV_tcpSRV_udpSRV_xmppTLSA_daneTLSA_sctpTLSA_tcpTLSA_udpTXT_acme-challengeTXT_dmarcTXT_domainkeyTXT_mta-stsTXT_spfTXT_sztpTXT_tcpTXT_udpTXT_vouchURI_acctURI_dccpURI_emailURI_emsURI_faxURI_ftURI_h323URI_iaxURI_ical-accessURI_ical-schedURI_ifaxURI_imURI_mmsURI_presURI_pstnURI_sctpURI_sipURI_smsURI_tcpURI_udpURI_unifmsgURI_vcardURI_videomsgURI_voiceURI_voicemsgURI_vpimURI_webURI_xmppUnder the NULL RR Type, the entry _ta-*
denotes all node names beginning with the string
_ta-*. It does NOT refer to a DNS
wildcard specification.The node name _example is reserved
across all RRsets.This section provides guidance for expert review of registration
requests in the "Underscored and Globally Scoped DNS Node Names" registry.This review is solely to determine adequacy of a requested
entry in this registry, and it does not include review of other
aspects of the document specifying that entry. For example,
such a document might also contain a definition of the
resource record type that is referenced by the requested
entry. Any required review of that definition is separate from
the expert review required here. The review is for the purposes of ensuring that:The details for creating the registry entry are sufficiently
clear, precise, and completeThe combination of the underscored name, under which the
listed resource record type is used, and the resource record
type is unique in the tableFor the purposes of this expert review, other matters of the
specification's technical quality, adequacy, or the like are outside
of scope. The following note has been added to the "Enumservice Registrations" registry:When adding an entry to this registry, strong
consideration should be given to also adding an entry to
the "Underscored and Globally Scoped DNS Node Names" registry.This memo raises no security issues.DNS Attrleaf Changes: Fixing Specifications That Use Underscored Node NamesOriginal uses of an underscore character as a domain node name prefix, which creates a space for constrained interpretation of resource records, were specified without the benefit of an IANA registry. This produced an entirely uncoordinated set of name- creation activities, all drawing from the same namespace. A registry now has been defined. However the existing specifications that use underscore naming need to be modified, to be in line with the new registry. This document specifies those changes. The changes preserve existing software and operational practice, while adapting the specifications for those practices to the newer underscore registry model.Secure Zero Touch Provisioning (SZTP)This draft presents a technique to securely provision a networking device when it is booting in a factory-default state. Variations in the solution enables it to be used on both public and private networks. The provisioning steps are able to update the boot image, commit an initial configuration, and execute arbitrary scripts to address auxiliary needs. The updated device is subsequently able to establish secure connections with other systems. For instance, a device may establish NETCONF (RFC 6241) and/or RESTCONF (RFC 8040) connections with deployment-specific network management systems.Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Martin Hoffmann,
Paul Hoffman, Peter Koch, Olaf Kolkman, Murray Kucherawy, John
Levine, Benno Overeinder, and Andrew Sullivan for diligent review of
the (much) earlier draft versions. For the later enhancements, thanks to
Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Joel Jaeggli, Benjamin Kaduk,
Mirja Kuehlewind, Warren Kumari, John Levine, Benno
Overeinder, Eric Rescorla, Adam Roach, Petr Spacek,
Ondrej Sury, Paul Vixie, Tim Wicinski, and Paul Wouters. Special thanks to Ray Bellis for his persistent encouragement to
continue this effort, as well as the suggestion for an essential
simplification to the registration model.