One document matched: draft-cheshire-dnsext-dns-sd-00.txt
Document: draft-cheshire-dnsext-dns-sd-00.txt Stuart Cheshire
Category: Standards Track Apple Computer, Inc.
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DNS-Based Service Discovery
<draft-cheshire-dnsext-dns-sd-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. 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
Distribution of this memo is unlimited.
This document updates the document previously titled
"Discovering Named Instances of Abstract Services using DNS"
(draft-cheshire-dnsext-nias-00.txt)
Abstract
This document describes a convention for naming and structuring DNS
resource records. Given a type of service that a client is looking
for, and a domain in which the client is looking for that service,
this convention allows clients to discover a list of named instances
of a that desired service, using only standard DNS queries. In short,
this is referred to as DNS-based Service Discovery, or DNS-SD.
Acknowledgements
This concepts described in this document have been explored and
developed with help from Erik Guttman, Paul Vixie, Bill Woodcock,
and others.
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Table of Contents
1. Introduction....................................................3
2. Conventions and Terminology Used in this Document...............3
3. Design Goals....................................................4
4. Service Instance Enumeration....................................5
4.1 Structured Instance Names.......................................5
4.2 User Interface Presentation.....................................7
4.3 Internal Handling of Names......................................7
4.4 What You See Is What You Get....................................7
4.5 Ordering of Service Instance Name Components....................9
5. Service Name Resolution........................................11
6. Data Syntax for DNS-SD TXT Records.............................12
6.1 General Format Rules for DNS TXT Records.......................12
6.2 DNS TXT Record Format Rules for use in DNS-SD..................12
6.3 DNS-SD TXT Record Size.........................................13
6.4 Rules for Names in DNS-SD Name/Value Pairs.....................14
6.5 Rules for Values in DNS-SD Name/Value Pairs....................15
6.6 Example TXT Record.............................................16
6.7 Version Tag....................................................16
7. Selective Instance Enumeration.................................17
8. Flagship Naming................................................17
9. Service Type Enumeration.......................................19
10. Populating the DNS with Information............................19
11. Relationship to Multicast DNS..................................20
12. Comparison with Alternative Service Discovery Protocols........20
13. Real Example...................................................22
14. IPv6 Considerations............................................23
15. Security Considerations........................................23
16. IANA Considerations............................................23
17. Copyright......................................................23
18. Normative References...........................................24
19. Informative References.........................................25
20. Author's Address...............................................25
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1. Introduction
This document describes a convention for naming and structuring DNS
resource records. Given a type of service that a client is looking
for, and a domain in which the client is looking for that service,
this convention allows clients to discover a list of named instances
of a that desired service, using only standard DNS queries. In short,
this is referred to as DNS-based Service Discovery, or DNS-SD.
This document proposes no change to the structure of DNS messages,
and no new operation codes, response codes, resource record types, or
any other new DNS protocol values. This document simply proposes a
convention for how existing resource record types can be named and
structured to facilitate service discovery.
This proposal is entirely compatible with today's existing unicast
DNS server and client software.
This proposal is also compatible with (but not dependent on) the
proposal for Multicast DNS outlined in "Performing DNS queries via IP
Multicast" [mDNS-SC].
2. Conventions and Terminology Used in this Document
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 "Key words for use in
RFCs to Indicate Requirement Levels" [RFC 2119].
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3. Design Goals
A good service discovery protocol needs to have many properties,
three of which are mentioned below:
(i) The ability to query for services of a certain type in a certain
logical domain and receive in response a list of named instances
(network browsing, or "Service Instance Enumeration").
(ii) Given a particular named instance, the ability to efficiently
resolve that instance name to the required information a client needs
to actually use the service, i.e. IP address and port number, at the
very least (Service Name Resolution).
(iii) Instance names should be relatively persistent. If a user
selects their default printer from a list of available choices today,
then tomorrow they should still be able to print on that printer --
even if the IP address and/or port number where the service resides
have changed -- without the user (or their software) having to repeat
the network browsing step a second time.
In addition, if it is to become successful, a service discovery
protocol should be so simple to implement that virtually any
device capable of implementing IP should not have any trouble
implementing the service discovery software as well.
These goals are discussed in more detail in the remainder of this
document. A more thorough treatment of service discovery requirements
may be found in "Requirements for the Replacement of AppleTalk Name
Binding Protocol" [NBP]. That document draws upon examples from a
decade-and-a-half of operational experience with AppleTalk Name
Binding Protocol to develop a list of universal requirements which
are broadly applicable to any potential service discovery protocol.
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4. Service Instance Enumeration
DNS SRV records [RFC 2782] are useful for locating instances of a
particular type of service when all the instances are effectively
indistinguishable and provide the same service to the client.
For example, SRV records with the (hypothetical) name
"_http._tcp.example.com." would allow a client to discover a list of
all servers implementing the "_http._tcp" service (i.e. Web servers)
for the "example.com." domain. The unstated assumption is that all
these servers offer an identical set of Web pages, and it doesn't
matter to the client which of the servers it uses, as long as it
selects one at random according to the weight and priority rules laid
out in RFC 2782.
Instances of other kinds of service are less easily interchangeable.
If a word processing application were to look up the (hypothetical)
SRV record "_ipp._tcp.example.com." to find the list of IPP printers
at Example Co., then picking one at random and printing on it would
probably not be what the user wanted.
The remainder of this section describes how SRV records may be used
in a slightly different way to allow a user to discover the names
of all available instances of a given type of service, in order to
select the particular instance the user desires.
4.1 Structured Instance Names
This document borrows the logical service naming syntax and semantics
from DNS SRV records, but adds one level of indirection. Instead of
requesting records of type "SRV" with name "_ipp._tcp.example.com.",
the client requests records of type "PTR" (pointer from one name to
another in the DNS namespace).
In effect, if one thinks of the domain name "_ipp._tcp.example.com."
as being analogous to an absolute path to a directory in a file
system then the PTR lookup is akin to performing a listing of that
directory to find all the files it contains. (Remember that domain
names are expressed in reverse order compared to path names: An
absolute path name is read from left to right, beginning with a
leading slash on the left, and then the top level directory, then the
next level directory, and so on. A fully-qualified domain name is
read from right to left, beginning with the dot on the right -- the
root label -- and then the top level domain to the left of that, and
the second level domain to the left of that, and so on. If the fully-
qualified domain name "_ipp._tcp.example.com." were expressed as a
file system path name, it would be "/com/example/_tcp/_ipp".)
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The result of this PTR lookup for the name "<Service>.<Domain>" is a
list of zero or more PTR records giving Service Instance Names of the
form:
Service Instance Name = <Instance> . <Service> . <Domain>
The <Instance> portion of the Service Instance Name is a single DNS
label, containing arbitrary UTF-8-encoded text [RFC 2279]. It is a
user-friendly name, meaning that it is allowed to contain any
characters, without restriction, including spaces, upper case, lower
case, punctuation -- including dots -- accented characters, non-roman
text, and anything else that may be represented using UTF-8.
DNS recommends guidelines for allowable characters for host names
[RFC 1033][RFC 1034][RFC 1035], but Service Instance Names are not
host names. Service Instance Names are not intended to ever be typed
in by a normal user; the user selects a Service Instance Name by
selecting it from a list of choices presented on the screen.
Note that just because this protocol supports arbitrary UTF-8-encoded
names doesn't mean that any particular user or administrator is
obliged to make use of that capability. Any user is free, if they
wish, to continue naming their services using only letters, digits
and hyphens, with no spaces, capital letters, or other punctuation.
DNS labels are currently limited to 63 octets in length. UTF-8
encoding can require up to four octets per Unicode character, which
means that in the worst case, the <Instance> portion of a name could
be limited to fifteen Unicode characters. However, the Unicode
characters with longer UTF-8 encodings tend to be the more obscure
ones, and tend to be the ones that convey greater meaning per
character.
Note that any character in the commonly-used 16-bit Unicode space can
be encoded with no more than three octets of UTF-8 encoding. This
means that an Instance name can contain up to 21 Kanji characters,
which is a sufficiently expressive name for most purposes.
The <Service> portion of the Service Instance Name consists of a pair
of DNS labels, following the established convention for SRV records
[RFC 2782]. The first label of the service pair is the application
protocol name, as recorded in the IANA list of assigned application
protocol names and port numbers [ports]. The second label of the
service pair is either "_tcp" or "_udp", depending on the transport
protocol used by the application.
The <Domain> portion of the Service Instance Name is a conventional
DNS domain name, consisting of as many labels as appropriate. For
example, "apple.com.", "cs.stanford.edu.", and "eng.us.ibm.com." are
all valid domain names for the <Domain> portion of the Service
Instance Name.
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4.2 User Interface Presentation
The names resulting from the PTR lookup are presented to the user in
a list for the user to select one (or more). Typically only the first
label is shown (the user-friendly <Instance> portion of the name).
The <Service> and <Domain> are already known to the user, these
having been provided by the user in the first place, by the act of
indicating the service being sought, and the domain in which to look
for it.
Having chosen the desired named instance, the Service Instance Name
may then be used immediately, or saved away in some persistent
user-preference data structure for future use, depending on what is
appropriate for the application in question.
4.3 Internal Handling of Names
If the <Instance>, <Service> and <Domain> portions are internally
concatenated together into a single string, then care must be taken
with the <Instance> portion, since it is allowed to contain any
characters, including dots.
Any dots in the <Instance> portion should be escaped by preceeding
them with a backslash ("." becomes "\."). Likewise, any backslashes
in the <Instance> portion should also be escaped by preceeding them
with a backslash ("\" becomes "\\"). Having done this, the three
components of the name may be safely concatenated. The
backslash-escaping allows literal dots in the name (escaped) to be
distinguished from label-separator dots (not escaped).
The resulting concatenated string may be safely passed to standard
DNS APIs like res_query(), which will interpret the string correctly
provided it has been escaped correctly, as described here.
4.4 What You See Is What You Get
Some service discovery protocols decouple the true service identifier
from the name presented to the user. The true service identifier used
by the protocol is an opaque unique id, often represented using a
long string of hexadecimal digits, and should never be seen by the
typical user. The name presented to the user is merely one of the
ephemeral attributes attached to this opaque identifier.
The problem with this approach is that it decouples user perception
from reality:
* What happens if there are two service instances, with different
unique ids, but they have inadvertently been given the same
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user-visible name? If two instances appear in an on-screen list
with the same name, how does the user know which is which?
* Suppose a printer breaks down, and the user replaces it with
another printer of the same make and model, and configures the new
printer with the exact same name as the one being replaced:
"Stuart's Printer". Now, when the user tries to print, the
on-screen print dialog tells them that their selected default
printer is "Stuart's Printer". When they browse the network to see
what is there, they see a printer called "Stuart's Printer", yet
when the user tries to print, they are told that the printer
"Stuart's Printer" can't be found. The hidden internal unique id
that the software is trying to find on the network doesn't match
the hidden internal unique id of the new printer, even though its
apparent "name" and its logical purpose for being there are the
same. To remedy this, the user typically has to delete the print
queue they have created, and then create a new (apparently
identical) queue for the new printer, so that the new queue will
contain the right hidden internal unique id. Having all this hidden
information that the user can't see makes for a confusing and
frustrating user experience, and exposing long ugly hexadecimal
strings to the user and forcing them to understand what they mean
is even worse.
* Suppose an existing printer is moved to a new department, and given
a new name and a new function. Changing the user-visible name of
that piece of hardware doesn't change its hidden internal unique
id. Users who had previously created print queues for that printer
will still be accessing the same hardware by its unique id, even
though the logical service that used to be offered by that hardware
has ceased to exist.
To solve these problems requires the user or administrator to be
aware of the supposedly hidden unique id, and to set its value
correctly as hardware is moved around, repurposed, or replaced,
thereby contradicting the notion that it is a hidden identifier that
human users never need to deal. Requiring the user to this expert
behind-the-scenes knowledge of what is *really* going on is just
one more burden placed on the user when they are trying to diagnose
why their computers and network devices are not working as expected.
These anomalies and counter-intuitive behaviours can be eliminated by
maintaining a tight bidirectional one-to-one mapping between what the
user sees on the screen and what is really happening "behind the
curtain". If something is configured incorrectly, then that is
apparent in the familiar day-to-day user interface that everyone
understands, not in some little-known rarely-used "expert" interface.
In summary: The user-visible name is the primary identifier for a
service. If the user-visible name is changed, then conceptually the
service being offered is a different service -- even if the hardware
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in question has been recycled from some other part of the
organization where it was previously in use providing the same type
of service to a different group of people, or for a different
purpose. If the user-visible name stays the same, then conceptually
the service being offered is the same service -- even if the hardware
in question is new hardware brought in to replace some old equipment
that was broken, worn out, or out-of-date.
4.5 Ordering of Service Instance Name Components
There have been questions about why services are named using DNS
Service Instance Names of the form:
Service Instance Name = <Instance> . <Service> . <Domain>
instead of:
Service Instance Name = <Service> . <Instance> . <Domain>
There are three reasons why it is beneficial to name service
instances with the parent domain as the most-significant (rightmost)
part of the name, then the abstract service type as the nextmost
significant, and then the specific instance name as the
least-significant (leftmost) part of the name:
4.5.1. Semantic Structure
The facility being provided by browsing ("Service Instance
Enumeration") is effectively enumerating the leaves of a tree
structure. A given domain offers zero or more services. For each
of those service types, there may be zero or more instances of
that service.
The user knows what type of service they are seeking. (If they are
running an FTP client, they are looking for FTP servers. If they
have a document to print, they are looking for entities that speak
some known printing protocol.) The user knows in which
organizational or geographical domain they wish to search. (The
user does not want a single flat list of every single printer on
the planet, even if such a thing were possible.) What the user
does not know in advance is whether they service they seek is
offered in the given domain, or if so, how many instances are
offered, and the names of those instances. Hence having the
instance names be the leaves of the tree is consistent with this
semantic model.
Having the service types be the terminal leaves of the tree would
imply that the user knows the domain name, and already knows the
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name of the service instance, but doesn't have any idea what the
service does. Clearly this is a less useful model.
4.5.2. Network Efficiency
When a DNS response contains multiple answers, name compression
works more effectively if all the names contain a common suffix.
If many answers in the packet have the same <Service> and
<Domain>, then each occurrence of a Service Instance Name can be
expressed using only the <Instance> part followed by a two-byte
compression pointer referencing a previous appearance of
"<Service>.<Domain>". This efficiency would not be possible
if the <Service> component appeared first in each name.
4.5.3. Operational Flexibility
This name structure allows subdomains to be delegated along
logical service boundaries. For example, the network administrator
at Example Co. could choose to delegate the "_tcp.example.com."
subdomain to a different machine, so that the machine handling
service discovery doesn't have to be the same as the machine
handling other day-to-day DNS operations. (It *can* be the same
machine if the administrator so chooses, but the point is that the
administrator is free to make that choice.) Furthermore, if the
network administrator wishes to delegate all information related
to IPP printers to a machine dedicated to that specific task, this
is easily done by delegating the "_ipp._tcp.example.com."
subdomain to the desired machine. It is also convenient to set
security policies on a per-zone/per-subdomain basis. For example,
the administrator may choose to enable DNS Dynamic Update [RFC
2136] [RFC 3007] for printers registering in the
"_ipp._tcp.example.com." subdomain, but not for other
zones/subdomains. This easy flexibility would not exist if the
<Service> component appeared first in each name.
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5. Service Name Resolution
Given a particular Service Instance Name, when a client needs to
contact that service, it sends a DNS request for the SRV record of
that name.
The result of the DNS request is a SRV record giving the port number
and target host where the service may be found.
The use of SRV records is very important. There are only 65535 TCP
port numbers available. These port numbers are being allocated
one-per-application-protocol at an alarming rate. Some protocols like
the X Window System have a block of 64 TCP ports allocated
(6000-6063). If we start allocating blocks of 64 TCP ports at a time,
we will run out even faster. Using a different TCP port for each
different instance of a given service on a given machine is entirely
sensible, but allocating large static ranges, as was done for X, is a
very inefficient way to manage a limited resource. On any given host,
most TCP ports are reserved for services that will never run on that
particular host. This is very poor utilization of the limited port
space. Using SRV records allows each host to allocate its available
port numbers dynamically to those services running on that host that
need them, and then advertise the allocated port numbers via SRV
records. Allocating the available listening port numbers locally
on a per-host basis as needed allows much better utilization of the
available port space than today's centralized global allocation.
In some environments there may be no compelling reason to assign
managed names to every host, since every available service is
accessible by name anyway, as a first-class entity in its own right.
However, the DNS packet format and record format still require a host
name to link the target host referenced in the SRV record to the
address records giving the IPv4 and/or IPv6 addresses for that
hardware. In the case where no natural host name is available, the
SRV record may give its own name as the name of the target host, and
then the requisite address records may be attached to that same name.
It is perfectly permissible for a single name in the DNS hierarchy to
have multiple records of different type attached. (The only
restriction being that a given name may not have both a CNAME record
and other records at the same time.)
In the event that more than one SRV is returned, clients MUST
correctly interpret the priority and weight fields -- i.e. Lower
numbered priority servers should be used in preference to higher
numbered priority servers, and servers with equal priority should be
selected randomly in proportion to their relative weights.
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6. Data Syntax for DNS-SD TXT Records
Some services discovered via Service Instance Enumeration may need
more than just an IP address and port number to properly identify the
service. For example, printing via the LPR protocol often specifies a
queue name. This queue name is typically short and cryptic, and need
not be shown to the user. It should be regarded the same way as the
IP address and port number -- it is one component of the addressing
information required to identify a specific instance of a service
being offered by some piece of hardware. Similarly, a file server may
have multiple volumes, each identified by its own volume name. A Web
server typically has multiple pages, each identified by its own URL.
In these cases, the necessary additional data is stored in a TXT
record with the same name as the SRV record. The specific nature of
that additional data, and how it is to be used, is service-dependent,
but the overall syntax of the data in the TXT record is standardized,
as described below.
6.1 General Format Rules for DNS TXT Records
A DNS TXT record can be up to 65535 (0xFFFF) bytes long. The total
length is indicated by the length given in the resource record header
in the DNS message. There is no way to tell directly from the data
alone how long it is (e.g. there is no length count at the start, or
terminating NULL byte at the end).
The format of the data within a DNS TXT record is zero or more
strings, packed together in memory without any intervening gaps or
padding bytes for word alignment.
The format of each constituent string within the DNS TXT record is a
single length byte, followed by 0-255 bytes of text data.
These format rules are defined in Section 3.3.14 of RFC 1035, and are
not specific to DNS-SD. DNS-SD simply specifies a usage convention
for what data should be stored in those constituent strings.
6.2 DNS TXT Record Format Rules for use in DNS-SD
DNS-SD uses DNS TXT records to store arbitrary name/value pairs
conveying additional information about the named service. Each
name/value pair is encoded as it's own constituent string within the
DNS TXT record, in the form "name=value". Everything up to the first
'=' character is the name. Everything after the first '=' character
to the end of the string (including subsequent '=' characters, if
any) is the value. Specific rules governing names and values are
given below. Each author defining a DNS-SD profile for discovering
instances of a particular type of service should define the base set
of name/value attributes that are valid for that type of service.
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Using this standardized name/value syntax within the TXT record makes
it easier for these base definitions to be expanded later by defining
additional named attributes. If an implementation sees unknown
attribute names in a service TXT record, it SHOULD silently ignore
them.
The TCP (or UDP) port number of the service, and target host name,
are given in the SRV record. This information -- target host name and
port number -- MUST NOT be duplicated using name/value attributes in
the TXT record.
The intention of DNS-SD TXT records is convey a small amount of
useful additional information about a service. Ideally it SHOULD NOT
be necessary for a client to retrieve this additional information
before it an usefully establish a connection to the service. For a
well-designed TCP-based application protocol, it should be possible,
knowing only the host name and port number, to open a connection to
that listening process, and then perform version- or feature-
negotiation to determine the capabilities of the service instance.
For example, when connecting to an AppleShare server over TCP, the
client enters into a protocol exchange with the server to determine
which version of the AppleShare protocol the server implements, and
which optional features or capabilities (if any) are available. For a
well-designed application protocol, clients should be able to connect
and use the service even if there is no information at all in the TXT
record. In this case, the information in the TXT record should be
viewed as a performance optimization -- when a client discovers many
instances of a service, the TXT record allows the client to know some
rudimentary information about each instance without having open a TCP
connection to each one and interrogate every service instance
separately. Extreme care should be taken when doing this to ensure
that the information in the TXT record is in agreement with the
information retrieved by a client connecting over TCP.
There are legacy protocols which provide no feature negotiation
capability, and in these cases it may be useful to convey necessary
information in the TXT record. For example, when printing using the
old Unix LPR (port 515) protocol, the LPR service provides no way for
the client to determine whether a particular printer accepts
PostScript, or what version of PostScript, etc. In this case it is
appropriate to embed this information in the TXT record, because the
alternative is worse -- passing around written instructions to the
users, arcane manual configuration of "/etc/printcap" files, etc.
6.3 DNS-SD TXT Record Size
The total size of a typical DNS-SD TXT record is intended to be small
-- 100 bytes or less.
In cases where more data is justified (e.g. LPR printing), keeping
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the total size under 400 bytes should allow it to fit in a single
standard 512-byte DNS message. (This standard DNS message size is
defined in RFC 1035.)
In extreme cases where even this is not enough, keeping size of the
TXT record under 1300 bytes should allow it to fit in a single
1500-byte Ethernet packet.
Using TXT records larger than 1300 bytes is NOT RECOMMENDED at this
time.
6.4 Rules for Names in DNS-SD Name/Value Pairs
The "Name" MUST be at least one character. Strings beginning with an
'=' character (i.e. the name is missing) SHOULD be silently ignored.
The characters of "Name" MUST be printable US-ASCII values
(0x20-0x7E), excluding '=' (0x3D).
Spaces in the name are significant, whether leading, trailing, or in
the middle -- so don't include any spaces unless you really intend
that!
Case is ignored when interpreting a name, so "papersize=A4",
"PAPERSIZE=A4" and "Papersize=A4" are all identical.
If there is no '=', then it is a boolean attribute, and is simply
identified as being present, with no value.
When examining a TXT record for a given named attribute, there are
therefore four broad categories of result which may be returned:
* Attribute not present (Absent)
* Attribute present, with no value
(e.g. "Anon Allowed" -- server allows anonymous connections)
* Attribute present, with empty value (e.g. "Installed PlugIns=" --
server supports plugins, but none are presently installed)
* Attribute present, with non-empty value
(e.g. "Installed PlugIns=JPEG,MPEG2,MPEG4")
Unless specified otherwise by a particular DNS-SD profile, a given
attribute name may appear at most once in a TXT record. If a client
receives a TXT record containing the same attribute name more than
once, then the client SHOULD silently ignore all but the first
occurrence of that attribute. For client implementations that process
a DNS-SD TXT record from start to end, placing name/value pairs into
a hash table, using the name as the hash table key, this means that
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if the implementation attempts to add a new name/value pair into the
table and finds an entry with the same name already present, then the
new entry being added should be silently discarded instead. For
client implementations that retrieve name/value pairs by searching
the TXT record for the requested name, they should search the TXT
record from the start, and simply return the first matching name they
find.
Each author defining a DNS-SD profile for discovering instances of a
particular type of service should define the interpretation of these
different kinds of result. For example, for some keys, there may be a
natural boolean interpretation:
* Absent implies 'false'
* Present with no value implies 'true'
For other keys it may be sensible to define other semantics, such as:
* Present with value implies that value.
E.g. "Color=4" for a four-color ink-jet printer,
or "Color=6" for a six-color ink-jet printer.
* Present with empty value implies 'false'. E.g. Not a color printer.
* Absent implies 'Unknown'. E.g. A print server connected to some
unknown printer where the print server doesn't actually know if the
printer does color or not -- which gives a very bad user experience
and should be avoided wherever possible.
(Note that this is a hypothetical example, not an example of real
name/value keys for printing.)
As a general rule, attribute names that contain no dots are defined
as part of the open-standard definition written by the person or
group defining the DNS-SD profile for discovering that particular
service type. Vendor-specific extensions should be given names of the
form "keyname.company.com=value", using a domain name legitimately
registered to the person or organization creating the vendor-specific
key. This reduces the risk of accidental conflict if different
organizations each define their own vendor-specific keys.
6.5 Rules for Values in DNS-SD Name/Value Pairs
If there is an '=', then everything after the first '=' to the end of
the string is the value. The value can contain any eight-bit values
including '='. Leading or trailing spaces are part of the value, so
don't put them there unless you intend them to be there. Any
quotation marks around the value are part of the value, so don't put
them there unless you intend them to be part of the value.
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The value is opaque binary data. Often the value for a particular
attribute will be US-ASCII (or UTF-8) text, but it is legal for a
value to be any binary data. For example, if the value of a key is an
IPv4 address, that address should simply be stored as four bytes of
binary data, not as a variable-length 7-15 byte ASCII string giving
the address represented in textual dotted decimal notation.
Generic debugging tools should generally display all attribute values
as a hex dump, with accompanying text alongside displaying the UTF-8
interpretation of those bytes, except for attributes where the
debugging tool has embedded knowledge that the value is some other
kind of data.
Authors defining DNS-SD profiles SHOULD NOT convert binary attribute
data types into printable text (e.g. using hexadecimal, Base64 or UU
encoding) merely for the sake of making the data be printable text
when seen in a generic debugging tool. Doing this simply bloats the
size of the TXT record, without truly making the data any more
understandable to someone looking at it in a generic debugging tool.
6.6 Example TXT Record
The TXT record below contains three syntactically valid name/value
pairs. (The meaning of these name/value pairs, if any, would depend
on the definitions pertaining to the service in question that is
using them.)
-----------------------------------------------------------------
| 0x0A | name=value | 0x08 | paper=A4 | 0x10 | Zeroconf Is Cool |
-----------------------------------------------------------------
6.7 Version Tag
It is recommended that authors defining DNS-SD profiles include an
attribute of the form "version=xxx" in their definition, and require
it to be the first name/value pair in the TXT record. This
information in the TXT record can be useful help clients maintain
backwards compatibility with older implementations if becomes
necessary to change or update the specification over time. Even if
the profile author doesn't anticipate the need for any future
incompatible changes, having a version number in the specification
provides useful insurance should incompatible changes become
unavoidable. Clients should ignore TXT records with a version number
higher (or lower) than the version(s) they know how to interpret.
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7. Selective Instance Enumeration
This document does not attempt to define an arbitrary query language
for service discovery, nor do we believe one is necessary.
However, there are some circumstances where narrowing the list of
results may be useful. A Web browser client that is able to retrieve
HTML documents via HTTP and display them may also be able to retrieve
HTML documents via FTP and display them, but only in the case of FTP
servers that allow anonymous login. For that Web browser, discovering
all FTP servers on the network is not useful. The Web browser only
wants to discover FTP servers that it is able to talk to. In this
case, a subtype of "_ftp._tcp" could be defined. Instead of issuing a
query for "_ftp._tcp.<Domain>", the Web browser issues a query for
"_anon._ftp._tcp.<Domain>", where "_anon" is a defined subtype of
"_ftp._tcp". The response to this query only includes the names of
SRV records for FTP servers that are willing to allow anonymous
login.
Note that the FTP server's Service Instance Name is unchanged -- it
is still something of the form "The Server._ftp._tcp.example.com."
The subdomain in which FTP server SRV records are registered defines
the namespace within which FTP server names are unique. Additional
subtypes (e.g. "_anon") of the basic service type (e.g. "_tcp._tcp")
serve to narrow the list of results, not to create more namespace.
As for the TXT record name/value pairs, the list of possible
subtypes, if any, are defined and specified separately for each basic
service type.
8. Flagship Naming
In some cases, there may be several network protocols available which
all perform roughly the same logical function. For example, the
printing world has the LPR protocol, and the Internet Printing
Protocol (IPP), both of which cause printed sheets to be emitted from
printers in much the same way. In addition, many printer vendors send
their own proprietary page description language (PDL) data over a TCP
connection to TCP port 9100, herein referred to as the
"pdl-datastream" protocol. In an ideal world we would have only one
network printing protocol, and it would be sufficiently good that no
one felt a compelling need to invent a different one. However, in
practice, multiple legacy protocols do exist, and a service discovery
protocol has to accommodate that.
Many printers implement all three printing protocols: LPR, IPP, and
pdl-datastream. For the benefit of clients that may speak only one of
those protocols, all three are advertised.
However, some clients may implement two, or all three of those
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printing protocols. When a client looks for all three service types
on the network, it will find three distinct services -- an LPR
service, an IPP service, and a pdl-datastream service -- all of which
cause printed sheets to be emitted from the same physical printer.
In the case of multiple protocols like this that all perform
effectively the same function, the client should suppress duplicate
names and display each name only once. When the user prints to a
given named printer, the printing client is responsible for choosing
the protocol which will best achieve the desired effect, without, for
example, requiring the user to make a manual choice between LPR and
IPP.
As described so far, this all works very well. However, consider some
future printer that only supports IPP printing, and some other future
printer that only supports pdl-datastream printing. The name spaces
for different service types are intentionally disjoint -- it is
acceptable and desirable to be able to have both a file server called
"Sales Department" and a printer called "Sales Department". However,
it is not desirable, in the common case, to have two different
printers both called "Sales Department", even if those printers are
implementing different protocols.
To help guard against this, when there are two or more network
protocols which perform roughly the same logical function, one of the
protocols is declared the "flagship" of the fleet of related
protocols. Typically the flagship protocol is the oldest and/or
best-known protocol of the set.
If a device does not implement the flagship protocol, then it instead
creates an empty SRV record (priority=0, weight=0, port=0, target
host = null) with that name. If, when it attempts to create this SRV
record, it finds that one with that name already exists, then it
knows that this name is already taken by some entity implementing
one of the protocols from the class, and it must choose another. If
no SRV record already exists, then the act of creating it stakes a
claim to that name so that future devices in the same class will not
try to use it.
By defining a common well-known flagship protocol for the class,
future devices that may not even know about each other's protocols
have a common ground where they can coordinate to verify uniqueness
of names.
No PTR record is created advertising the presence of empty flagship
SRV records, since they do not represent a real service being
advertised.
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9. Service Type Enumeration
In general, clients are not interested in finding *every* service on
the network, just the services that the client knows how to talk to.
(Software designers may *think* there's some value to finding *every*
service on the network, but that's just wooly thinking.)
However, for problem diagnosis and network management tools, it may
be useful for network administrators to find the list of advertised
service types on the network, even if those service names are just
opaque identifiers and not particularly informative in isolation.
For this reason, a special meta-query is defined. A DNS query for
PTR records with the name "_services._mdns._udp.<Domain>" yields
a list of PTR records, where the rdata of each PTR record is the
name of a service type. A subsequent query for PTR records with
one of those names yields a list of instances of that service type.
10. Populating the DNS with Information
How the SRV and PTR records that describe services and allow them to
be enumerated make their way into the DNS is outside the scope of
this document. However, it can happen easily in any of a number of
ways, for example:
On some networks, the administrator might manually enter the records
into the name server's configuration file.
A network monitoring tool could output a standard zone file to be
read into a conventional DNS server. For example, a tool that can
find Apple LaserWriters using AppleTalk NBP could find the list of
printers, communicate with each one to find its IP address,
PostScript version, installed options, etc., and then write out a DNS
zone file describing those printers and their capabilities using DNS
resource records. That information would then be available to DNS-SD
clients that don't implement AppleTalk NBP, and don't want to.
Future IP printers could use Dynamic DNS Update [RFC 2136] to
automatically register their own SRV and PTR records with the DNS
server.
A printer manager device which has knowledge of printers on the
network through some other management protocol could also use Dynamic
DNS Update [RFC 2136].
Alternatively, a printer manager device could implement enough of the
DNS protocol that it is able to answer DNS requests directly, and
Example Co.'s main DNS server could delegate the
_ipp._tcp.example.com subdomain to the printer manager device.
Zeroconf printers answer Multicast DNS requests on the local link
for appropriate PTR and SRV names ending with ".local." [mDNS-SC].
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11. Relationship to Multicast DNS
DNS-Based Service Discovery is not strictly related to Multicast DNS,
but the two are highly complementary, particularly in Zeroconf
environments [ZC].
Lookups for PTR records of the form "<Service>.local." are defined to
use multicast, and return a list of named instances of the form
"<Instance>.<Service>.local."
12. Comparison with Alternative Service Discovery Protocols
Over the years there have been many proposed ways to do network
service discovery with IP, but none achieved ubiquity in the
marketplace. Certainly none has achieved anything close to the
ubiquity of today's deployment of DNS servers, clients, and other
infrastructure.
The advantage of using DNS as the basis for service discovery is that
it makes use of those existing servers, clients, protocols,
infrastructure, and expertise. Existing network analyser tools
already know how to decode and display DNS packets for network
debugging.
For ad-hoc networks such as Zeroconf environments, peer-to-peer
multicast protocols are appropriate. It is almost certain that the
Zeroconf host profile [ZCHP] will specify the use of a DNS-like
protocol over IP Multicast for host name resolution in the absence of
DNS servers. Given that Zeroconf hosts will have to implement this
Multicast-based DNS-like protocol anyway, it makes sense for them to
also perform service discovery using that same Multicast-based
DNS-like software, instead of also having to implement an entirely
different service discovery protocol.
In larger networks, a high volume of enterprise-wide IP multicast
traffic may not be desirable, so any credible service discovery
protocol intended for larger networks has to provide some facility to
aggregate registrations and lookups at a central server (or servers)
instead of working exclusively using multicast. This requires some
service discovery aggregation server software to be written,
debugged, deployed, and maintained. This also requires some service
discovery registration protocol to be implemented and deployed for
clients to register with the central aggregation server. Virtually
every company with an IP network already runs DNS server, and DNS
already has a dynamic registration protocol [RFC 2136]. Given that
virtually every company already has to operate and maintain a DNS
server anyway, it makes sense to take advantage of this instead of
also having to learn, operate and maintain a different service
registration server. It should be stressed again that using the same
software and protocols doesn't necessarily mean using the same
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physical piece of hardware. The DNS-SD service discovery functions
do not have to be provided by the same piece of hardware that
is currently providing the company's DNS name service. The
"_tcp.<Domain>" subdomain may be delegated to a different piece of
hardware. However, even when the DNS-SD service is being provided by
a different piece of hardware, it is still the same familiar DNS
server software that is running, with the same configuration file
syntax, the same log file format, and so forth.
Service discovery needs to be able to provide appropriate security.
DNS already has existing mechanisms for security [RFC 2535].
In summary:
Service discovery requires a central aggregation server.
DNS already has one: It's called a DNS server.
Service discovery requires a service registration protocol.
DNS already has one: It's called DNS Dynamic Update.
Service discovery requires a query protocol
DNS already has one: It's called DNS.
Service discovery requires security mechanisms.
DNS already has security mechanisms: DNSSEC.
Service discovery requires a multicast mode for ad-hoc networks.
DNS doesn't have one right now, but it will soon, to meet Zeroconf
requirements.
It makes more sense to use the existing software that every network
needs already, instead of deploying an entire parallel system just
for service discovery.
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13. Real Example
The following examples were prepared using standard unmodified
nslookup and standard unmodified BIND running on GNU/Linux.
Note: In real products, this information is obtained and presented to
the user using graphical network browser software, not command-line
tools, but if you wish you can try these examples for yourself as you
read along, using the command-line tools already available on your
own Unix machine.
13.1 Question: What FTP servers are being advertised from dns-sd.org?
nslookup -q=ptr _ftp._tcp.dns-sd.org.
_ftp._tcp.dns-sd.org name=Apple\032QuickTime\032Files.dns-sd.org
_ftp._tcp.dns-sd.org name=Microsoft\032Developer\032Files.dns-sd.org
_ftp._tcp.dns-sd.org name=Registered\032Users'\032Only.dns-sd.org
Answer: There are three, called "Apple QuickTime Files",
"Microsoft Developer Files" and "Registered Users' Only".
Note that nslookup escapes spaces as "\032" for display purposes,
but a graphical DNS-SD browser does not.
13.2 Question: What FTP servers allow anonymous access?
nslookup -q=ptr _anon._ftp._tcp.dns-sd.org
_anon._ftp._tcp.dns-sd.org
name=Apple\032QuickTime\032Files.dns-sd.org
_anon._ftp._tcp.dns-sd.org
name=Microsoft\032Developer\032Files.dns-sd.org
Answer: Only "Apple QuickTime Files" and "Microsoft Developer Files"
allow anonymous access.
13.3 Question: How do I access "Apple QuickTime Files"?
nslookup -q=any "Apple\032QuickTime\032Files.dns-sd.org."
Apple\032QuickTime\032Files.dns-sd.org text = "path=/quicktime"
Apple\032QuickTime\032Files.dns-sd.org
priority = 0, weight = 0, port= 21 host = ftp.apple.com
ftp.apple.com internet address = 17.254.0.27
ftp.apple.com internet address = 17.254.0.31
ftp.apple.com internet address = 17.254.0.26
Answer: You need to connect to ftp.apple.com, port 21, path
"/quicktime". The addresses for ftp.apple.com are also given.
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14. IPv6 Considerations
IPv6 has no significant differences, except that the address of the
SRV record's target host is given by the appropriate IPv6 address
records instead of the IPv4 "A" record.
15. Security Considerations
DNSSEC [RFC 2535] should be used where the authenticity of
information is important. Since DNS-SD is just a naming and usage
convention for records in the existing DNS system, it has no specific
additional security requirements over and above those that already
apply to DNS queries and DNS updates.
16. IANA Considerations
The IANA will continue having to allocating symbolic service/protocol
names, just as they do today every time someone requests a TCP or UDP
port number. In future, as more applications start using DNS SRV
records, it may make sense for IANA to start allocating symbolic
service/protocol names without an associated hard-coded port number.
The textual nature of service/protocol names means that there are
almost infinitely many more of them available than the finite set of
65535 possible port numbers. This means that developers can produce
experimental implementations using unregistered service names with
little chance of accidental collision, providing service names are
chosen with appropriate care. However, this document strongly
advocates that on or before the date a product ships, developers
should register their service names with IANA.
Some developers have expressed concern that publicly registering
their service names (and port numbers today) with IANA before a
product ships may give away clues about that product to their
competition. For this reason, IANA should consider allowing service
name applications to remain secret for some period of time, much as
US patent applications remain secret for two years after the date of
filing.
17. Copyright
Copyright (C) The Internet Society 20th December 2002.
All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
18. Normative References
[ports] IANA list of assigned application protocol names and port
numbers <http://www.iana.org/assignments/port-numbers>
[RFC 1033] Lottor, M., "Domain Administrators Operations Guide",
RFC 1033, November 1987.
[RFC 1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC 1035, November 1987.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC 2782] Gulbrandsen, A., et al., "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2782, February 2000.
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19. Informative References
[mDNS-SC] Cheshire, S., "Performing DNS queries via IP Multicast",
Internet-Draft (work in progress),
draft-cheshire-dnsext-multicastdns-01.txt, December 2002.
[NBP] Cheshire, S., "Requirements for the Replacement of
AppleTalk Name Binding Protocol", Internet-Draft (work
in progress), draft-cheshire-dnsext-nbp-01.txt, December
2002.
[RFC 2136] Vixie, P., et al., "Dynamic Updates in the Domain Name
System (DNS UPDATE)", RFC 2136, April 1997.
[RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC 3007] Wellington, B., et al., "Secure Domain Name System (DNS)
Dynamic Update", RFC 3007, November 2000.
[ZC] Williams, A., "Requirements for Automatic Configuration
of IP Hosts", Internet-Draft (work in progress),
draft-ietf-zeroconf-reqts-12.txt, September 2002.
[ZCHP] Guttman, E., "Zeroconf Host Profile Applicability
Statement", Internet-Draft (work in progress),
draft-ietf-zeroconf-host-prof-01.txt, July 2001.
20. Author's Address
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org
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