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dns-lg/draft-daley-dns-schema.txt

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DNS Extensions Working Group J. Daley
Internet-Draft S. Morris
Intended status: Informational Nominet UK
Expires: July 28, 2008 January 25, 2008
Schema definition for DNS
draft-daley-dns-schema-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 28, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This memo describes a syntax for encoding DNS Resource Records in
XML, and a schema to define that syntax written in XML Schema. It
can be used to represent all DNS RDATA. This can be used by diverse
applications as a common format.
DNS Resource Records are represented as XML elements with the name of
the element taken from the mnemonic used to represent the DNS
Resource Record in presentation format. The RDATA is represented as
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XML attributes. The attribute names are taken from the RDATA field
names specified in the normative RFC.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements for the XML syntax for DNS RRs . . . . . . . 4
2.2. Requirements for the XML Schema to describe the XML
syntax for DNS RRs . . . . . . . . . . . . . . . . . . . . 5
2.3. Supported DNS RRs . . . . . . . . . . . . . . . . . . . . 5
2.4. Out of scope issues . . . . . . . . . . . . . . . . . . . 6
3. The XML syntax for DNS RRs . . . . . . . . . . . . . . . . . . 7
3.1. Design considerations . . . . . . . . . . . . . . . . . . 7
3.1.1. Unique element for each RR . . . . . . . . . . . . . . 7
3.1.2. RDATA representation . . . . . . . . . . . . . . . . . 7
3.1.3. RRSet element . . . . . . . . . . . . . . . . . . . . 7
3.1.4. Elements that represent RRs that can be used
inside an RRSet and standalone . . . . . . . . . . . . 8
3.1.5. Element for holding raw RR data . . . . . . . . . . . 8
4. The schema to define the XML syntax . . . . . . . . . . . . . 8
4.1. Design considerations . . . . . . . . . . . . . . . . . . 8
4.1.1. Use of XML Schema . . . . . . . . . . . . . . . . . . 8
4.1.2. Use of XML Namespaces . . . . . . . . . . . . . . . . 8
4.1.3. Base RR element and base attributes . . . . . . . . . 9
4.1.4. RDATA attribute types . . . . . . . . . . . . . . . . 9
4.1.5. RDATA attribute values . . . . . . . . . . . . . . . . 10
4.1.6. Required or optional attributes . . . . . . . . . . . 10
4.1.7. Global attribute declarations . . . . . . . . . . . . 10
4.1.8. Top level container element . . . . . . . . . . . . . 10
4.1.9. Specific attribute considerations . . . . . . . . . . 11
4.2. Limitations of the schema . . . . . . . . . . . . . . . . 14
4.2.1. Spurious attributes . . . . . . . . . . . . . . . . . 14
4.2.2. No restrictions on what RRs can be appended to an
RRset . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.3. Inconsistent or imprecise RFCs . . . . . . . . . . . . 15
4.2.4. Additional presentation formats . . . . . . . . . . . 15
4.2.5. Different wire formats for RDATA with semantic
equivalence . . . . . . . . . . . . . . . . . . . . . 15
4.3. Extending the schema . . . . . . . . . . . . . . . . . . . 15
4.3.1. The extension mechanism . . . . . . . . . . . . . . . 15
4.3.2. Creating an extension . . . . . . . . . . . . . . . . 17
4.3.3. Using an extension . . . . . . . . . . . . . . . . . . 17
4.4. Implementing new versions of the schema . . . . . . . . . 18
4.4.1. Use of version specific namespaces . . . . . . . . . . 18
4.4.2. Possible type changes in a new version . . . . . . . . 18
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5. Semantic Web considerations . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Full Schema . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 40
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
10.1. Normative References . . . . . . . . . . . . . . . . . . . 40
10.2. Informative References . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43
Intellectual Property and Copyright Statements . . . . . . . . . . 44
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1. Introduction
Historically, DNS Resource Records (RRs) have a presentation format
and wire format. The presentation format is typically used to
conveniently store DNS RRs in Human Readable Form. The wire format
is typically used in transport and communication between DNS protocol
elements.This memo describes a syntax for encoding DNS RRs in XML
[W3C.REC-XML] and a schema to define that syntax, written in XML
Schema [W3C.REC-XMLSchema-1]. This syntax and corresponding schema
definition are called dns-schema. The use case for dns-schema is to
have a set of rules to which a DNS RR must conform to, in order to
limit common mistakes in DNS Presentation Format, and to have a
common encoding between applications for DNS RRs.
TODO an example of usage.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.1. Terminology
'element that represents an RR' An XML element, defined in dns-
schema that represents an RR.
2. Requirements
2.1. Requirements for the XML syntax for DNS RRs
The first requirement is to develop an XML syntax for DNS RRs, with
the following characteristics:
1. A succinct syntax for the XML wrapper around the DNS data such
that a human reading or writing it can do so as quickly and
easily as possible.
2. This syntax should use as closely as possible the presentation
format for RDATA fields given in various RFCs, even if that
reduces overall readability.
3. This syntax should be independent of any name server
implementation.
4. This syntax should use a single element for each RR.
5. This syntax should allow the encoding of an RR of unknown type as
described in [RFC3597].
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6. This syntax should allow the encoding of any RDATA that might
appear in a datagram with the correct over structure for a DNS
datagram, even if the data contained in that datagram did not
conform to the DNS specifications. This is necessary to allow
dns-schema to be used with 'broken' DNS data.
2.2. Requirements for the XML Schema to describe the XML syntax for DNS
RRs
The second requirement, leading from the first, is to develop a
schema definition that describes the XML syntax with the following
characteristics:
1. The schema should allow conforming XML to be validated before
this XML was sent to an application, or for a server to validate
it upon receipt before any further processing.
2. This schema must not impose any restrictions on the future
definition of a new RR or a change to an existing RR.
3. This schema must allow for any new RR to be described as an
extension of it and used as easily as any RR described in it.
4. When a new version of this schema is brought out, it must be
possible for it to include new RRs or changes to existing RRs
that have been described in new RFCs, without preventing the
continuing use of any other extensions.
5. This schema should make a semantic link between RDATA fields that
represent semantically equivalent data.
2.3. Supported DNS RRs
The following RFCs and Resource Records are supported in dns-schema:
o From [RFC1035], A, CNAME, HINFO, MB, MG, MINFO, MR, MX, NS, NULL,
PTR, SOA, TXT and WKS.
o From [RFC1183], AFSDB, ISDN, RP, RT and X25.
o From [RFC1706], NSAP.
o From [RFC1712], GPOS.
o From [RFC1876], LOC.
o From [RFC2163], PX.
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o From [RFC2230], KX.
o From [RFC2538], CERT.
o From [RFC2672], DNAME.
o From [RFC2782], SRV.
o From [RFC2845], TSIG.
o From [RFC2874], A6.
o From [RFC2930], TKEY.
o From [RFC3403], NAPTR.
o From [RFC3596], AAAA.
o From [RFC4025], IPSECKEY.
o From [RFC4034], DNSKEY, DS, NSEC and RRSIG.
o From [RFC4255], SSHFP.
o From [RFC4408], SPF.
o From [RFC4431], DLV.
o From [RFC4701], DHCID.
Obsolete DNS resource records are not supported. Neither are the NB
and NBSTAT RRs defined in [RFC1002].
2.4. Out of scope issues
There are some issues that are out of scope for this memo:
o No use is made of Master File Format [RFC1035], section 5.1.
o No use is made of the DNS header section of a DNS datagram.
o This memo speculates on possible uses of dns-schema, but does not
require any such use.
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3. The XML syntax for DNS RRs
The syntax by which the DNS RRs is represented has been chosen
carefully to meet the given requirements of being succinct and easy
to read and write for a human. This is an example of a resource
record described in this syntax:
<A owner="example" class="IN" ttl="86400" address="192.0.2.1"/>
and this is an example of an RRSet:
<RRSet owner="example" class="IN" type="A" ttl="86400" >
<A address="192.0.2.1" />
<A address="192.0.2.2" />
</RRSet>
3.1. Design considerations
The design considerations behind this syntax are:
3.1.1. Unique element for each RR
Each DNS RR has a corresponding element. That ensures that any
schema can constrain the allowable attributes on a per RR basis. The
alternative would be to create a generic <RR> element, but in that
case a schema could not constrain the allowable attributes and would
place more emphasis on the application to validate this.
3.1.2. RDATA representation
All RDATA can be correctly represented in attributes, rather than
elements. This significantly reduces the verbosity of the XML.
For each element that represents an RR, the attributes specified
correspond to those specified in the normative RFC that defines the
RDATA for that RR. The names of the attributes are those given the
corresponding RR. For example, the MX element has the specific
attributes of 'preference' and 'exchange' as specified in [RFC1035].
3.1.3. RRSet element
An element that represents an RRSet is included, using the definition
from [RFC2136] that it is a set of RRs that share the same owner,
class and type. In addition a ttl attribute is specified because
[RFC2181] requires all the RRs in an RRSet to share the same ttl.
If an RR is ever defined with the mnemonic of 'RRSET' then this would
present future versions of dns-schema with a naming conflict.
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3.1.4. Elements that represent RRs that can be used inside an RRSet and
standalone
The elements that represent RRs can be used standalone or inside an
RRSet. When used inside an RRSet the owner and class attributes are
superfluous and the ttl attribute may only be used subject to the
restrictions of [RFC2181].
3.1.5. Element for holding raw RR data
An element called binaryRR is included that can hold an RR of any
type. This has an attribute 'rtype' that holds the DNS type code, an
attribute 'rdlength' that holds the length of the RDATA and another
attribute 'rdata' that holds the raw rdata. An application MUST
encode the rdata in hexadecimal data encoding as specified as part of
the the presentation format of an RR of unknown type in [RFC3597].
This element can also be used to contain 'broken' DNS data,
satisfying this requirement.
4. The schema to define the XML syntax
4.1. Design considerations
The schema appears in full in Section 8. The design considerations
behind this schema are:
4.1.1. Use of XML Schema
This schema is written using XML Schemas [W3C.REC-XMLSchema-1] and
[W3C.REC-XMLSchema-2] because this is a W3C standard and provides the
necessary level of flexibility to correctly specify the preferred
syntax. Other schema languages could have been used just as well.
4.1.2. Use of XML Namespaces
XML Namespaces [W3C.REC-xml-names] need to be used in the schema to
reference the defined types. Any document validated against dns-
schema must contain a namespace reference in order for it to validate
properly. For example
<dnsSchema xmlns="urn:ietf:params:xml:ns:dns"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:dns dns-1.0.xsd">
In that example the default namespace is set to refer to elements and
attributes from dns-schema. A third party extension could be
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included in the namespace declarations, with a specified prefix, and
so all use of the extension would be clearly identified by use of
that prefix. This is described more fully in Section 4.3
4.1.3. Base RR element and base attributes
An abstract 'RR' element is declared with a defined base type that
contains the base attributes of 'class', 'ttl' and 'owner'. All
elements that represent RRs are derived from this base element and
have a type derived from the base type. This ensures that all
elements that represent RRs have those attributes. This is achieved
using the XML Schema [W3C.REC-XMLSchema-1] feature of
substitutionGroup
This same mechanism is used by any new RRs that are created in
extensions, which ensures they are treated equally to built-in RRs
rather than needing to appear in a separate extension element. This
is covered further in Section 4.3
It should be noted that, as this is an abstract element, it cannot be
used in a conforming XML document.
4.1.4. RDATA attribute types
The type of an attribute is chosen to represent the presentation
format for the RDATA field specified in the relevant RFC. For
example a field specified as 32 bit unsigned integer is represented
using the XML Schema [W3C.REC-XMLSchema-2] type of 'unsignedInt'.
Most attributes are typed by a simple type built in to XML Schema
[W3C.REC-XMLSchema-2]. Some attributes are typed indirectly by the
use of a new simple type defined in dns-schema. These are in most
cases just the simplest restriction of a base type. This is done for
a number of reasons:
o Where there are multiple presentation formats for a single RDATA
field and so the defined type is a union of two built-in types.
o Where the type chosen allows for a greater range of values than is
allowed in the RDATA field wire format. Using a defined type
allows for later development where the range of values is further
constrained to match those of the wire format. Such restriction
on values is not implemented in this version of dnsSchema as
explained in Section 4.1.5
o Where there are fields that share the same semantic use (for
example an IP address or domain name). This use of types is to
prepare for later semantic inferences as explained in Section 5
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4.1.5. RDATA attribute values
The values of attributes that represent RDATA are not constrained to
match the allowable values in the relevant RFC. This is done for a
number of reasons:
o To reduce the complexity and processing requirements of the schema
validation.
o To aid extensibility. It should be noted that the only way to
restrict attributes to a list of known values is to use a feature
of XML Schema [W3C.REC-XMLSchema-1], attribute value enumerations,
that cannot be extended.
It is therefore down to the application to check the values of
attributes that represent RDATA to ensure they are meaningful.
4.1.6. Required or optional attributes
Extensive use is made of the XML Schema [W3C.REC-XMLSchema-1]
attribute 'use="required"' by which the use of an attribute in
conforming documents is mandated. This is used when the normative
RFC for that RR states that an RDATA MUST exist.
4.1.7. Global attribute declarations
Global attribute declarations are not used, instead a similar effect
is attained by the use of defined types. The reason for not using
global attribute declarations is to simplify an instance document,
which would otherwise require namespace declarations for such
attributes, complicating those documents.
4.1.8. Top level container element
There is an element in the schema called 'dnsSchema' that does not
represent any DNS data. It is provided as an optional top-level
container element, which can be used in a document as the opening
element and contain an arbitrary list of 'RRSet' elements and
elements representing RRs.
However it does not have to be used, as both the 'RRSet' element and
the elements representing RRs are declared as top level elements and
so can be used directly in a valid document.
It would be sensible for the 'dnsSchema' element to be used in
document that only references this schema (a standalone document), as
a container for a set of elements and not to be used where dnsSchema
elements would appear embedded in elements defined in another schema.
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For example, a standalone document might look like this
<dnsSchema xmlns="urn:ietf:params:xml:ns:dns"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:dns dns-1.0.xsd" >
<A owner="example1" class="IN" ttl="86400" address="192.0.2.2" />
<A owner="example2" class="IN" ttl="86400" address="192.0.2.1" />
</dnsSchema>
Whereas a fragment of a document where dnsSchema is embedded, might
look like this
:
<someApp>
<someCommand>
<dns:A owner="example2" class= IN" ttl="86400"
address="192.0.2.1" />
</someCommand>
</someApp>
:
4.1.9. Specific attribute considerations
There are some special cases that require further discussion, these
are described below:
4.1.9.1. Anonymous RDATA
The SPF, TXT and DHCID RRs have a single anonymous RDATA field just
referred to as the RDATA in the normative RFC. For each of these the
attribute that represents the RDATA is called 'rdata'.
4.1.9.2. IP address in RDATA
An IPv4 address is defined to be of type 'ipaddressType', which in
turn is defined to be a 'string' with the maximum length restricted
and is intended to hold the address in dotted-quad notation. A later
development may be to use a regular expression to ensure that the
contents of the string are indeed a valid IP address, but that is not
tackled in this memo.
IP v6 addresses are similarly defined. The maximum length is set to
allow for embedded dotted quad in the IPv6 address.
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4.1.9.3. Domain names in RDATA
Attributes for RDATA fields that are used for domain names are all of
the type 'domainType'. This is defined to be a 'string' with the
maximum length restricted. No use is made of a regular expression to
check that the contents of an attribute are a correctly formatted
domain name.
4.1.9.4. The rdata attribute of the binaryRR element
[RFC3597] specifies a presentation format for an RR of an unknown
type. In dns-schema 'rdata' is of the type hexBinary.
4.1.9.5. XML in RDATA
Any data in attributes that represent an RDATA field that can contain
XML MUST be escaped using the rules given in [W3C.REC-XML]
Because escaping is a standard part of XML, no specific type is
defined to use for those fields where escaping may be required.
4.1.9.6. Unparsed data in RDATA
A number of RDATA fields are defined in RFCs as containing any text
data. Again any data in the attributes that represent these RDATA
fields MUST be escaped following the rules given in [W3C.REC-XML]
4.1.9.7. Variable length binary data in RDATA
There are a number of examples where RDATA contains a binary field
such as set of flags or a bit map field. For example WKS has a
variable length bit map field, with no defined presentation format.
These fields are represented either by the XML Schema
[W3C.REC-XMLSchema-2] types of 'hexBinary' or 'base64Binary'
depending on context. XML Schema [W3C.REC-XMLSchema-2] in turn
references [RFC2045] for the definition of base64.
4.1.9.8. Preferences in RDATA
A number of RRs have a preference RDATA field, namely KX, MX, PX, RT,
NAPTR. The attributes that represent the preference field for these
RRs are all defined to be of the type 'preferenceType' on the
potentially contentious grounds that they are semantically
equivalent.
Additionally the IPSECKEY RR has a precedence RDATA field, which is
defined as being equivalent to the preference RDATA field of the MX
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RR. The attribute representing this field is therefore also defined
as being of type 'preferenceType'.
4.1.9.9. Seconds (units of time) in RDATA
Many RDATA fields are defined as unsigned 32 bit integers that record
a number of seconds. This is used in three separate ways:
o Fields such as the 'refresh' field of the SOA RR, contain seconds
as a relative value.
o Fields such as the 'signature expiration' field of the RRSIG RR,
contain the number of seconds since the epoch.
o Fields such as the 'ttl' field of all RRs, contain a relative
number of seconds but with specific semantic usage of Time To
Live.
These three separate ways are represented by attributes of three
different defined types:
o Attributes that represent RDATA fields of relative seconds are
defined to be of type 'secondsType'. This is an unsigned int.
o Attributes that represent RDATA fields of seconds since the epoch
are defined to be of type 'secondsSinceEpochType'. Different RRs
define different presentation formats for these fields, some using
straight integers such as the 'inception' field of the TKEY RR,
but the 'signature inception' field of the RRSIG RR additionally
defines the 14 character 'YYYYMMDDHHmmSS' string format.
Accordingly 'secondsSinceEpochType' is a union of two simple
types, one that allows the integer representation and one that
allows a string representation of 14 characters, sufficient for
the 'YYYYMMDDHHmmSS' representation.
o Attributes that represent TTL are defined to be of type 'ttlType',
which is derived from 'secondsType'. This enables the semantic
equivalence of fields that represent TTL.
Unfortunately there are anomalies with seconds (units of time) in
RDATA. These are explained further in section Section 4.2
4.1.9.10. [RFC4034] algorithms in RDATA
Three RRs defined in [RFC4034], namely DNSKEY, RRSIG and DS all have
an algorithm field in their RDATA. This is specified as having two
presentation formats, an unsigned integer and a string mnemonic. The
attributes that represent these fields have the type 'algorithmType'.
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This is defined as a union of two simple types, one that allows
integer representation and one that allows a string representation.
The string type is not restricted to the mnemonic values given in
[RFC4034].
4.1.9.11. DNSKEY flags and protocol in RDATA
The flags and protocol RDATA fields of DNSKEY (defined in [RFC4034])
are represented in integer format and so the attributes that
represent these fields are only defined to be the appropriate integer
type.
4.2. Limitations of the schema
4.2.1. Spurious attributes
Each element that represents an RR has the base attributes of
'class', 'owner' and 'ttl', which also appear as attributes of an
RRSet. Because of the syntax requirement Section 2.1 to have a
single element for each RR, it is possible for the elements within an
RRSet to have 'class', 'owner' and 'ttl' attributes that contradict
those of the RRSet. For example
<RRSet owner="example" class="IN" type="A" ttl="86400" >
<A class="CH" address="192.0.2.1" />
<A owner="different" address="192.0.2.2" />
<A ttl="400" address="192.0.2.3" />
</RRSet>
To correct this anomaly would require such significant alterations to
the schema that a number of important characteristics, including
simplicity and ease of use, would be lost.
As it is the intent of this memo to mirror the presentation format of
DNS RRs in XML, not to change them, then applications that are
presented with such inconsistent data SHOULD ignore any 'class',
'ttl' and 'owner' attributes given in the elements within an RRSet,
whether or not they contradict those of the RRSet.
Note that the use of different ttl values for each RR within an RRSet
was deprecated in section 5.2 of [RFC2181].
4.2.2. No restrictions on what RRs can be appended to an RRset
[RFC2136] lists a number of RRs that cannot be appended to an RRset.
This restriction is not enforced in this schema.
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4.2.3. Inconsistent or imprecise RFCs
Some RFCs are not clear on whether a specified RDATA field is a
signed or unsigned integer. This syntax has made a reasoned choice.
For example the 'refresh' field within the SOA RR is not explicitly
defined as signed or unsigned, but it would not make sense if a
signed integer was used here.
4.2.4. Additional presentation formats
In order to preserve semantic equivalence some attributes that
represent RDATA are of the same type and so allow the same
presentation formats when the normative RFCs do not specify those
presentation formats for that RDATA.
Examples of this include the 'inception' field of the TKEY RR. The
attribute that represents this is of the type
'secondsSinceEpochType', which allows the string representation of
'YYYYMMDDHHmmSS' even though this is not a defined presentation
format for this field.
4.2.5. Different wire formats for RDATA with semantic equivalence
There are some RDATA fields in different RRs that have the semantic
equivalence but are represented in different wire formats, making it
impossible to use common attribute types.
The one example of this is the 'timesigned' field of the TSIG RR,
which is defined as representing seconds since the epoch but has the
wire format of a 48 bit unsigned integer, when all other semantically
equivalent fields use a 32 bit unsigned integer. In this instance
this field uses its own unique type.
4.3. Extending the schema
4.3.1. The extension mechanism
All elements that represent RRs are specified using the same
mechanism that is made available for the development of third-party
extensions.
The schema defines an abstract element called 'RR', which is of an
abstract type 'RRType' that defines the three base attributes of
'class', 'ttl' and 'owner'. This is done in this code:
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<element name="RR" type="dns:RRType" />
<complexType name="RRType" abstract="true">
<attribute name="class" type="dns:classType" />
<attribute name="ttl" type="dns:ttlType" />
<attribute name="owner" type="dns:hostType" />
</complexType>
Being abstract, the element 'RR' cannot be instantiated; it is just a
placeholder that is designed to be replaced by elements that
represent DNS RRs.
All elements that represent RRs are then defined using the
substitutionGroup syntax of XML Schema [W3C.REC-XMLSchema-1]. In
order to do this, two requirements must be met
o The new element has the substitutionGroup attribute, specifying
the 'RR' element as the target.
o The new element must be of a type that is defined as an extension
of the type 'RRType'.
For example, the A element is defined in exactly this manner, as
shown by this code:
<element name="A" substitutionGroup="dns:RR" type="dns:AType" />
<complexType name="AType">
<complexContent>
<extension base="dns:RRType">
<attribute name="address" type="dns:ipaddressType" />
</extension>
</complexContent>
</complexType>
A new RR defined in a different schema follows exactly the same
requirements. For example, the 'EXAMPLE' RR could be defined as:
<element name="EXAMPLE" substitutionGroup="dns:RR"
type="dns:EXAMPLEType" />
<complexType name="EXAMPLEType">
<complexContent>
<extension base="dns:RRType">
<attribute name="exampleRDATA" type="dns:hostType" />
</extension>
</complexContent>
</complexType>
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4.3.2. Creating an extension
The purpose of an extension is to provide syntax for a DNS RR that is
not included in dns-schema. Extensions are specified in a new XML
Schema instance document, which has the following characteristics:
o declares its own XML Namespace [W3C.REC-xml-names];
o references dns-schema both as a namespace and importing that
schema;
o uses the extension mechanism to create a new element to represent
an RR as described in Section 4.3.1.
An extension schema to add an element representing a new RR called
EXAMPLE would look as follows:
<?xml version="1.0" encoding="UTF-8"?>
<schema targetNamespace="http://www.example.org/example-schema"
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ex="http://www.example.org/example-schema"
xmlns:dns="urn:ietf:params:xml:ns:dns">
<annotation>
<documentation>Example extension to dns-schema</documentation>
</annotation>
<import namespace="urn:ietf:params:xml:ns:dns"
schemaLocation="dns-1.0.xsd"/>
<element name="EXAMPLE" substitutionGroup="dns:RR"
type="ex:EXAMPLEType" />
<complexType name="EXAMPLEType">
<complexContent>
<extension base="dns:RRType">
<attribute name="exampleRDATA" type="dns:hostType" />
</extension>
</complexContent>
</complexType>
</schema>
4.3.3. Using an extension
With an extension declared as per Section 4.3.2 it can then be
referenced in a XML document that also references dns-schema. The
use of namespaces will keep the references separate.
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<dnsSchema xmlns="urn:ietf:params:xml:ns:dns"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:dns dns-1.0.xsd"
xmlns:ex="http://www.example.org/example-schema"
xsi:schemaLocation=
"http://www.example.org/example-schema example.xsd">
<ex:EXAMPLE owner="example" class="IN" ttl="86400"
exampleRDATA="whatever" />
<A owner="example" class="IN" ttl="86400" address="192.0.2.1" />
</dnsSchema>
4.4. Implementing new versions of the schema
If a new version of the schema is developed that includes within it
new RRs already described in third party extensions, the use of XML
Namespaces [W3C.REC-xml-names] will ensure that the third party
extension can continue to be used.
If dns-1.1.xsd were now available and the document updated to use
that, then the document would still validate correctly. If the
author then wanted to use the 'example' RR from the new version of
dns-schema as well as the version from the extension then they could
do so as it sits in a different namespace.
4.4.1. Use of version specific namespaces
This memo specifies two URNs that can be used to refer to dns-schema.
The first of these is a version independent reference
'urn:ietf:params:xml:ns:dns', the second is a version specific
reference 'urn:ietf:params:xml:ns:dns-1.0'. A document can use
either reference, depending on need.
4.4.2. Possible type changes in a new version
A new version may introduce a change to some attributes types in
order to maintain the semantic linkage described in Section 5, though
this would not affect any documents that use dns-schema.
This change would occur where an RDATA field existed that was
represented by an attribute defined as a base type and a new RR saw a
new RDATA field introduced that had the same semantic usage. This
would best be handled by the creation of a new defined type that both
the attributes referenced as their type.
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5. Semantic Web considerations
This schema contains no Resource Description Framework (RDF)
[W3C.PR-rdf-concepts-20031215] or variant to provide semantic
connections between the data. It does however use XML Schema
[W3C.REC-XMLSchema-2] data types in a consistent form that can be
used to derive semantic information at a later date. It should be
noted that there are two types of semantic relationships that exist
between RDATA fields but this schema makes no attempt to
differentiate them.
The first is the semantic relationship between the type of RDATA
field. For example, the MX element has an attribute 'exchange' and
the NS element has an attribute 'nsdname'; both of these are declared
to be of type domainType, since they are both domain names and so a
semantic inference as to their equivalence can be drawn.
The second is the semantic relationship between the usage of the
RDATA field. For example, the SIG and DS elements both have and
RDATA field called 'keytag', which represents the same data, not just
of the same type.
6. Security Considerations
This memo raised no security issues.
7. IANA Considerations
This memo uses URNs to describe XML namespaces [W3C.REC-xml-names]
and XML schemas [W3C.REC-XMLSchema-1] conforming to a registry
mechanism described in [RFC3688]. Two URI assignments need to be
registered by the IANA.
Registration request for the dns-schema namespace:
URI: urn:ietf:params:xml:ns:dns
Registrant Contact: See the "Author's Address" section of this memo.
XML: None. Namespace URIs do not represent an XML specification.
Registration request for the dns-schema version specific namespace:
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URI: urn:ietf:params:xml:ns:dns-1.0
Registrant Contact: See the "Author's Address" section of this memo.
XML: None. Namespace URIs do not represent an XML specification.
Registration request for the dns-schema XML schema:
URI: urn:ietf:params:xml:schema:dns-1.0
Registrant Contact: See the "Author's Address" section of this memo.
XML: See Section 8 of this memo.
8. Full Schema
<?xml version="1.0"?>
<schema targetNamespace="urn:ietf:params:xml:ns:dns"
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:dns="urn:ietf:params:xml:ns:dns">
<annotation>
<documentation> dns-schema v1.0 </documentation>
</annotation>
<!-- - - - - - - - - - - - - - - - - - -->
<!-- dnsSchema element -->
<!-- - - - - - - - - - - - - - - - - - -->
<element name="dnsSchema">
<complexType>
<choice maxOccurs="unbounded">
<element ref="dns:RRset"/>
<element ref="dns:RR"/>
</choice>
</complexType>
</element>
<!-- - - - - - - - - - - - - - - - - - -->
<!-- RRset element -->
<!-- - - - - - - - - - - - - - - - - - -->
<element name="RRset" type="dns:RRsetType"/>
<complexType name="RRsetType">
<choice>
<!-- has to be a <choice> to prevent mixed RRs in an RRset -->
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<element ref="dns:RR" maxOccurs="unbounded"/>
</choice>
<attribute name="class" type="string"/>
<attribute name="type" type="string"/>
<attribute name="owner" type="string"/>
<attribute name="ttl" type="dns:ttlType"/>
</complexType>
<!-- - - - - - - - - - - - - - - - - - -->
<!-- RR abstract element -->
<!-- - - - - - - - - - - - - - - - - - -->
<element name="RR" type="dns:RRType" abstract="true"/>
<complexType name="RRType" abstract="true">
<attribute name="owner" type="dns:domainType"/>
<attribute name="class" type="dns:classType"/>
<attribute name="ttl" type="dns:ttlType"/>
</complexType>
<!-- - - - - - - - - - - - - - - - - - -->
<!-- Attribute types -->
<!-- - - - - - - - - - - - - - - - - - -->
<simpleType name="domainType">
<restriction base="string">
<maxLength value="256"/>
</restriction>
</simpleType>
<simpleType name="classType">
<restriction base="string">
<maxLength value="2"/>
</restriction>
</simpleType>
<simpleType name="secondsType">
<restriction base="unsignedInt"/>
</simpleType>
<simpleType name="secondsSinceEpochType">
<union>
<simpleType>
<restriction base="string">
<maxLength value="14"/>
</restriction>
</simpleType>
<simpleType>
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<restriction base="unsignedInt"/>
</simpleType>
</union>
</simpleType>
<simpleType name="secondsSinceEpoch48Type">
<restriction base="unsignedLong">
<maxInclusive value="140737488355329"/>
<!-- 2**47+1 -->
</restriction>
</simpleType>
<simpleType name="ttlType">
<restriction base="dns:secondsType"/>
</simpleType>
<simpleType name="ipaddressType">
<restriction base="string">
<maxLength value="15"/>
</restriction>
</simpleType>
<simpleType name="ip6addressType">
<restriction base="string">
<maxLength value="45"/>
<!-- 45 allows for embedded dotted quad -->
</restriction>
</simpleType>
<simpleType name="algorithmType">
<union>
<simpleType>
<restriction base="string"/>
</simpleType>
<simpleType>
<restriction base="unsignedByte"/>
</simpleType>
</union>
</simpleType>
<simpleType name="certificateTypeType">
<union>
<simpleType>
<restriction base="string"/>