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Document: draft-cheshire-dnsext-dns-sd-04.txt Stuart Cheshire
Internet-Draft Marc Krochmal
Category: Standards Track Apple Computer, Inc.
Expires 10th February 2007 10th August 2006
DNS-Based Service Discovery
<draft-cheshire-dnsext-dns-sd-04.txt>
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.
For the purposes of this document, the term "BCP 79" refers
exclusively to RFC 3979, "Intellectual Property Rights in IETF
Technology", published March 2005.
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/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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 that desired service, using only standard DNS queries. In short,
this is referred to as DNS-based Service Discovery, or DNS-SD.
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Table of Contents
1. Introduction...................................................3
2. Conventions and Terminology Used in this Document..............4
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...................................8
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.................13
6.3 DNS-SD TXT Record Size........................................14
6.4 Rules for Names in DNS-SD Name/Value Pairs....................14
6.5 Rules for Values in DNS-SD Name/Value Pairs...................16
6.6 Example TXT Record............................................17
6.7 Version Tag...................................................17
7. Application Protocol Names....................................18
7.1 Selective Instance Enumeration................................19
7.2 Service Name Length Limits....................................20
8. Flagship Naming...............................................22
9. Service Type Enumeration......................................23
10. Populating the DNS with Information...........................24
11. Relationship to Multicast DNS.................................24
12. Discovery of Browsing and Registration Domains................25
13. DNS Additional Record Generation..............................26
14. Comparison with Alternative Service Discovery Protocols.......27
15. Real Examples.................................................29
16. User Interface Considerations.................................30
16.1 Service Advertising User-Interface Considerations.............30
16.2 Client Browsing User-Interface Considerations.................31
17. IPv6 Considerations...........................................34
18. Security Considerations.......................................34
19. IANA Considerations...........................................34
20. Acknowledgments...............................................35
21. Deployment History............................................35
22. Copyright Notice..............................................36
23. Normative References..........................................37
24. Informative References........................................37
25. Authors' Addresses............................................38
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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.
Note that the DNS-SD service does NOT have to be provided by the same
DNS server hardware that is currently providing an organization's
conventional host name lookup service (the service we traditionally
think of when we say "DNS"). By delegating the "_tcp" subdomain,
all the workload related to DNS-SD can be offloaded to a different
machine. This flexibility, to handle DNS-SD on the main DNS server,
or not, at the network administrator's discretion, is one of the
things that makes DNS-SD so compelling.
Even when the DNS-SD functions are delegated to a different machine,
the benefits of using DNS remain: It is mature technology, well
understood, with multiple independent implementations from different
vendors, a wide selection of books published on the subject, and an
established workforce experienced in its operation. In contrast,
adopting some other service discovery technology would require every
site in the world to install, learn, configure, operate and maintain
some entirely new and unfamiliar server software. Faced with these
obstacles, it seems unlikely that any other service discovery
technology could hope to compete with the ubiquitous deployment
that DNS already enjoys.
This proposal is also compatible with (but not dependent on) the
proposal outlined in "Multicast DNS" [mDNS].
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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].
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 a Protocol to Replace AppleTalk
NBP" [NBP]. That document draws upon examples from two decades 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 precomposed UTF-8-encoded text [RFC
3629]. 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], namely: the first label of the pair is the Application
Protocol Name, and the second label is either "_tcp" or "_udp",
depending on the transport protocol used by the application.
More details are given in Section 7, "Application Protocol Names".
The <Domain> portion of the Service Instance Name specifies the DNS
subdomain within which the service names are registered. It may be
"local", meaning "link-local Multicast DNS" [mDNS], or it may be
a conventional unicast DNS domain name, such as "apple.com.",
"cs.stanford.edu.", or "eng.us.ibm.com." Because service names are
not host names, they are not constrained by the usual rules for host
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names [RFC 1033][RFC 1034][RFC 1035], and rich-text service
subdomains are allowed and encouraged, for example:
Building 2, 1st Floor.apple.com.
Building 2, 2nd Floor.apple.com.
Building 2, 3rd Floor.apple.com.
Building 2, 4th Floor.apple.com.
In addition, because Service Instance Names are not constrained by
the limitations of host names, this document recommends that they
be stored in the DNS, and communicated over the wire, encoded as
straightforward canonical precomposed UTF-8, Unicode Normalization
Form C [UAX15]. In cases where the DNS server returns an NXDOMAIN
error for the name in question, client software MAY choose to retry
the query using "Punycode" [RFC 3492] encoding, if possible.
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). In
the common case, 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. Note: The software handling the response
should be careful not to make invalid assumptions though, since it
*is* possible, though rare, for a service enumeration in one domain
to return the names of services in a different domain. Similarly,
when using subtypes (see "Selective Instance Enumeration") the
<Service> of the discovered instance my not be exactly the same as
the <Service> that was requested.
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 preceding
them with a backslash ("." becomes "\."). Likewise, any backslashes
in the <Instance> portion should also be escaped by preceding them
with a backslash ("\" becomes "\\"). Having done this, the three
components of the name may be safely concatenated. The backslash-
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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
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
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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 with. Requiring the user to understand
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 behaviors 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 logical service -- even
though the hardware offering the service stayed the same. If the
user-visible name doesn't change, then conceptually the service being
offered is the same logical service -- even if the hardware offering
the service is new hardware brought in to replace some old equipment.
There are certainly arguments on both sides of this debate.
Nonetheless, the designers of any service discovery protocol have
to make a choice between between having the primary identifiers be
hidden, or having them be visible, and these are the reasons that
we chose to make them visible. We're not claiming that there are no
disadvantages of having primary identifiers be visible. We considered
both alternatives, and we believe that the few disadvantages
of visible identifiers are far outweighed by the many problems
caused by use of hidden identifiers.
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>
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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 next-most
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 the 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
name of the service instance, but doesn't have any idea what the
service does. We would argue that 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
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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.
5. Service Name Resolution
Given a particular Service Instance Name, when a client needs to
contact that service, it sends a DNS query for the SRV record of
that name.
The result of the DNS query 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.
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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. However,
in the overwhelmingly common case, a single advertised DNS-SD service
instance is described by exactly one SRV record, and in this common
case the priority and weight fields of the SRV record SHOULD both be
set to zero.
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. Every DNS-SD service MUST have a TXT record in
addition to its SRV record, with same name, even if the service has
no additional data to store and the TXT record contains no more than
a single zero byte.
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). (Note that when using Multicast
DNS [mDNS] the maximum packet size is 9000 bytes, which imposes an
upper limit on the size of TXT records of about 8800 bytes.)
The format of the data within a DNS TXT record is one or more
strings, packed together in memory without any intervening gaps
or padding bytes for word alignment.
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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.
An empty TXT record containing zero strings is disallowed by RFC
1035. DNS-SD implementations MUST NOT emit empty TXT records.
DNS-SD implementations receiving empty TXT records MUST treat them
as equivalent to a one-byte TXT record containing a single zero byte
(i.e. a single empty string).
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 its 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.
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 MUST 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 to 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 can 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
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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 to 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
-- 200 bytes or less.
In cases where more data is justified (e.g. LPR printing), keeping
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 the 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!
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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.
A given attribute name may appear at most once in a TXT record.
The reason for this simplifying rule is to facilitate the creation
of client libraries that parse the TXT record into an internal data
structure, such as a hash table or dictionary object that maps from
names to values, and then make that abstraction available to client
code. The rule that a given attribute name may not appear more than
once simplifies these abstractions because they aren't required to
support the case of returning more than one value for a given key.
If a client receives a TXT record containing the same attribute name
more than once, then the client MUST 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 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.
When examining a TXT record for a given named attribute, there are
therefore four broad categories of results 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")
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 true/false boolean interpretation:
* Present implies 'true'
* Absent implies 'false'
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For other keys it may be sensible to define other semantics, such as
value/no-value/unknown:
* 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 actual
name/value keys used by DNS-SD network printers.)
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.
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, Base-64 or UU
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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 actually 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 | 0x0E | DNS-SD Is Cool |
---------------------------------------------------------------
6.7 Version Tag
It is recommended that authors defining DNS-SD profiles include an
attribute of the form "txtvers=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 to help clients maintain
backwards compatibility with older implementations if it 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 txtvers number
higher (or lower) than the version(s) they know how to interpret.
Note that the version number in the txtvers tag describes the version
of the TXT record specification being used to create this TXT record,
not the version of the application protocol that will be used if the
client subsequently decides to contact that service. Ideally, every
DNS-SD TXT record specification starts at txtvers=1 and stays that
way forever. Improvements can be made by defining new keys that older
clients silently ignore. The only reason to increment the version
number is if the old specification is subsequently found to be so
horribly broken that there's no way to do a compatible forward
revision, so the txtvers number has to be incremented to tell all the
old clients they should just not even try to understand this new TXT
record.
If there is a need to indicate which version number(s) of the
application protocol the service implements, the recommended key
name for this is "protovers".
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7. Application Protocol Names
The <Service> portion of a Service Instance Name consists of a pair
of DNS labels, following the established convention for SRV records
[RFC 2782], namely: the first label of the pair is an underscore
character followed by the Application Protocol Name, and the second
label is either "_tcp" or "_udp".
Application Protocol Names may be no more than fourteen characters
(not counting the mandatory underscore), conforming to normal DNS
host name rules: Only lower-case letters, digits, and hyphens; must
begin and end with lower-case letter or digit.
Wise selection of an Application Protocol Name is very important,
and the choice is not always as obvious as it may appear.
In some cases, the Application Protocol Name merely names and refers
to the on-the-wire message format and semantics being used. FTP is
"ftp", IPP printing is "ipp", and so on.
However, it is common to "borrow" an existing protocol and repurpose
it for a new task. This is entirely sensible and sound engineering
practice, but that doesn't mean that the new protocol is providing
the same semantic service as the old one, even if it borrows the same
message formats. For example, the local network music playing
protocol implemented by iTunes on Macintosh and Windows is little
more than "HTTP GET" commands. However, that does *not* mean that it
is sensible or useful to try to access one of these music servers by
connecting to it with a standard web browser. Consequently, the
DNS-SD service advertised (and browsed for) by iTunes is "_daap._tcp"
(Digital Audio Access Protocol), not "_http._tcp". Advertising
"_http._tcp" service would cause iTunes servers to show up in
conventional Web browsers (Safari, Camino, OmniWeb, Opera, Netscape,
Internet Explorer, etc.) which is little use since it offers no pages
containing human-readable content. Similarly, browsing for
"_http._tcp" service would cause iTunes to find generic web servers,
such as the embedded web servers in devices like printers, which is
little use since printers generally don't have much music to offer.
Similarly, NFS is built on top of SUN RPC, but that doesn't mean it
makes sense for an NFS server to advertise that it provides "SUN RPC"
service. Likewise, Microsoft SMB file service is built on top of
Netbios running over IP, but that doesn't mean it makes sense for
an SMB file server to advertise that it provides "Netbios-over-IP"
service. The DNS-SD name of a service needs to encapsulate both the
"what" (semantics) and the "how" (protocol implementation) of the
service, since knowledge of both is necessary for a client to
usefully use the service. Merely advertising that a service was
built on top of SUN RPC is no use if the client has no idea what
the service actually does.
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Another common mistake is to assume that the service type advertised
by iTunes should be "_daap._http._tcp." This is also incorrect.
Similarly, a protocol designer implementing a network service that
happens to use Simple Object Access Protocol [SOAP] should not feel
compelled to have "_soap" appear somewhere in the Application
Protocol Name. Part of the confusion here is that the presence of
"_tcp" or "_udp" in the <Service> portion of a Service Instance Name
has led people to assume that the structure of a service name has to
reflect the internal structure of how the protocol was implemented.
This is not correct. All that is required is that the service be
identified by a unique Application Protocol Name. Making the
Application Protocol Name at least marginally descriptive of
what the service does is desirable, though not essential.
The "_tcp" or "_udp" should be regarded as little more than
boilerplate text, and care should be taken not to attach too much
importance to it. Some might argue that the "_tcp" or "_udp" should
not be there at all, but this format is defined by RFC 2782, and
that's not going to change. In addition, the presence of "_tcp" has
the useful side-effect that it provides a convenient delegation point
to hand off responsibility for service discovery to a different DNS
server, if so desired.
7.1. 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 hypothetical 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._sub._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. "_ftp._tcp")
serve to narrow the list of results, not to create more namespace.
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Subtypes are appropriate when it is desirable for different kinds
of clients to be able to browse for services at two levels of
granularity. In the example above, we hypothesize two classes of
ftp client: clients that can provide username and password when
connecting, and clients that can only do anonymous login. The set of
ftp servers on the network is the same in both cases; the difference
is that the more capable client wants to discover all of them,
whereas the more limited client only wants to find the subset of
those ftp servers that it can talk to. Subtypes are only appropriate
in two-level scenarios such as this one, where some clients want to
find the full set of services of a given type, and at the same time
other clients only want to find some subset. Generally speaking, if
there is no client that wants to find the entire set, then it's
neither necessary nor desirable to use the subtype mechanism. If all
clients are browsing for some particular subtype, and no client
exists that browses for the parent type, then an Application Protocol
Name representing the logical service should be defined, and software
should simply advertise and browse for that particular service type
directly. In particular, just because a particular network service
happens to be implemented in terms of some other underlying protocol,
like HTTP, Sun RPC, or SOAP, doesn't mean that it's sensible for that
service to be defined as a subtype of "_http", "_sunrpc", or "_soap".
That would only be useful if there were some class of client for
which it is sensible to say, "I want to discover a service on the
network, and I don't care what it does, as long as it does it using
the SOAP XML RPC mechanism."
As with the TXT record name/value pairs, the list of possible
subtypes, if any, are defined and specified separately for each basic
service type. Note that the example given here using "_ftp" is a
hypothetical one. The "_ftp" service type does not (currently) have
any subtypes defined. Subtypes are currently a little-used feature
of DNS-SD, and experience will show whether or not they ultimately
prove to have broad applicability.
7.2 Service Name Length Limits
As described above, application protocol names are allowed to be up
to fourteen characters long. The reason for this limit is to leave
as many bytes of the domain name as possible available for use
by both the network administrator (choosing service domain names)
and the end user (choosing instance names).
A domain name may be up to 255 bytes long, including the final
terminating root label at the end. Domain names used by DNS-SD
take the following forms:
<Instance>.<app>._tcp.<servicedomain>.<parentdomain>.
<sub>._sub.<app>._tcp.<servicedomain>.<parentdomain>.
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The first example shows a service instance name, i.e. the name of the
service's SRV and TXT records. The second shows a subtype browsing
name, i.e. the name of a PTR record pointing to service instance
names (see "Selective Instance Enumeration").
The instance name <Instance> may be up to 63 bytes. Including the
length byte used by the DNS format when the name is stored in a
packet, that makes 64 bytes.
When using subtypes, the subtype identifier is allowed to be up to
63 bytes, plus the length byte, making 64. Including the "_sub"
and its length byte, this makes 69 bytes.
The application protocol name <app> may be up to 14 bytes, plus the
underscore and length byte, making 16. Including the "_udp" or "_tcp"
and its length byte, this makes 21 bytes.
Typically, DNS-SD service records are placed into subdomains of their
own beneath a company's existing domain name. Since these subdomains
are intended to be accessed through graphical user interfaces, not
typed on a command-line they are frequently long and descriptive.
Including the length byte, the user-visible service domain may be up
to 64 bytes.
The terminating root label at the end counts as one byte.
Of our available 255 bytes, we have now accounted for 69+21+64+1 =
155 bytes. This leaves 100 bytes to accommodate the organization's
existing domain name <parentdomain>. When used with Multicast DNS,
<parentdomain> is "local", which easily fits. When used with parent
domains of 100 bytes or less, the full functionality of DNS-SD is
available without restriction. When used with parent domains longer
than 100 bytes, the protocol risks exceeding the maximum possible
length of domain names, causing failures. In this case, careful
choice of short <servicedomain> names can help avoid overflows.
If the <servicedomain> and <parentdomain> are too long, then service
instances with long instance names will not be discoverable or
resolvable, and applications making use of long subtype names
may fail.
Because of this constraint, we choose to limit Application Protocol
Names to 14 characters or less. Allowing more characters would not
add to the expressive power of the protocol, and would needlessly
lower the limit on the maximum <parentdomain> length that may be
safely used.
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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
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", just because 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 a placeholder SRV record (priority=0, weight=0, port=0,
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target host = hostname of device) with that name. If, when it
attempts to create this SRV record, it finds that a record with the
same name already exists, then it knows that this name is already
taken by some entity implementing at least 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 detect a conflict when
they try to use it. The SRV record needs to contain the target host
name in order for the conflict detection rules to operate. If two
different devices were to create placeholder SRV records both using a
null target host name (just the root label), then the two SRV records
would be seen to be in agreement so no conflict would be registered.
By defining a common well-known flagship protocol for the class,
future devices that may not even know about each other's protocols
establish 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.
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._dns-sd._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.
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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 queries 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 queries on the local link
for appropriate PTR and SRV names ending with ".local." [mDNS]
11. Relationship to Multicast DNS
DNS-Based Service Discovery is only peripherally related to Multicast
DNS, in that the standard unicast DNS queries used by DNS-SD may also
be performed using multicast when appropriate, which is particularly
beneficial in Zeroconf environments [ZC].
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12. Discovery of Browsing and Registration Domains (Domain Enumeration)
One of the main reasons for DNS-Based Service Discovery is so that
when a visiting client (e.g. a laptop computer) arrives at a new
network, it can discover what services are available on that network
without manual configuration. This logic that applies to discovering
services without manual configuration also applies to discovering the
domains in which services are registered without requiring manual
configuration.
This discovery is performed recursively, using Unicast or Multicast
DNS. Five special RR names are reserved for this purpose:
b._dns-sd._udp.<domain>.
db._dns-sd._udp.<domain>.
r._dns-sd._udp.<domain>.
dr._dns-sd._udp.<domain>.
lb._dns-sd._udp.<domain>.
By performing PTR queries for these names, a client can learn,
respectively:
o A list of domains recommended for browsing
o A single recommended default domain for browsing
o A list of domains recommended for registering services using
Dynamic Update
o A single recommended default domain for registering services.
o The final query shown yields the "legacy browsing" domain.
Sophisticated client applications that care to present choices
of domain to the user, use the answers learned from the previous
four queries to discover those domains to present. In contrast,
many current applications browse without specifying an explicit
domain, allowing the operating system to automatically select an
appropriate domain on their behalf. It is for this class of
application that the "legacy browsing" query is provided, to allow
the network administrator to communicate to the client operating
systems which domain should be used for these applications.
These domains are purely advisory. The client or user is free to
browse and/or register services in any domains. The purpose of these
special queries is to allow software to create a user-interface that
displays a useful list of suggested choices to the user, from which
they may make a suitable selection, or ignore the offered suggestions
and manually enter their own choice.
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The <domain> part of the name may be "local" (meaning "perform the
query using link-local multicast) or it may be learned through some
other mechanism, such as the DHCP "Domain" option (option code 15)
[RFC 2132] or the DHCP "Domain Search" option (option code 119)
[RFC 3397].
The <domain> part of the name may also be derived from the host's IP
address. The host takes its IP address, and calculates the logical
AND of that address and its subnet mask, to derive the 'base' address
of the subnet. It then constructs the conventional DNS "reverse
mapping" name corresponding to that base address, and uses that
as the <domain> part of the name for the queries described above.
For example, if a host has address 192.168.12.34, with subnet mask
255.255.0.0, then the 'base' address of the subnet is 192.168.0.0,
and to discover the recommended legacy browsing domain for devices
on this subnet, the host issues a DNS PTR query for the name
"lb._dns-sd._udp.0.0.168.192.in-addr.arpa."
Sophisticated clients may perform domain enumeration queries both in
"local" and in one or more unicast domains, and then present the user
with an aggregate result, combining the information received from all
sources.
13. DNS Additional Record Generation
DNS has an efficiency feature whereby a DNS server may place
additional records in the Additional Section of the DNS Message.
These additional records are typically records that the client did
not explicitly request, but the server has reasonable grounds to
expect that the client might request them shortly.
This section recommends which additional records should be generated
to improve network efficiency for both unicast and multicast DNS-SD
responses.
13.1 PTR Records
When including a PTR record in a response packet, the
server/responder SHOULD include the following additional records:
o The SRV record(s) named in the PTR rdata.
o The TXT record(s) named in the PTR rdata.
o All address records (type "A" and "AAAA") named in the SRV rdata.
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13.2 SRV Records
When including an SVR record in a response packet, the
server/responder SHOULD include the following additional records:
o All address records (type "A" and "AAAA") named in the SRV rdata.
13.3 TXT Records
When including a TXT record in a response packet, no additional
records are required.
13.4 Other Record Types
In response to address queries, or other record types, no additional
records are required by this document.
14. 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 analyzer 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. The Zeroconf host profile [ZCHP]
requires 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
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clients to register with the central aggregation server. Virtually
every company with an IP network already runs a 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
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.
Zeroconf environments already require a multicast-based DNS-like
name lookup protocol for mapping host names to addresses, so it
makes sense to let one multicast-based protocol do both jobs.
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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15. Real Examples
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.
15.1 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._ftp._tcp.dns-sd.org
_ftp._tcp.dns-sd.org
name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org
_ftp._tcp.dns-sd.org
name = Registered\032Users'\032Only._ftp._tcp.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.
15.2 Question: What FTP servers allow anonymous access?
nslookup -q=ptr _anon._sub._ftp._tcp.dns-sd.org
_anon._sub._ftp._tcp.dns-sd.org
name = Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
_anon._sub._ftp._tcp.dns-sd.org
name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org
Answer: Only "Apple QuickTime Files" and "Microsoft Developer Files"
allow anonymous access.
15.3 Question: How do I access "Apple QuickTime Files"?
nslookup -q=any "Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org."
Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
text = "path=/quicktime"
Apple\032QuickTime\032Files._ftp._tcp.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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16. User Interface Considerations
DNS-Based Service Discovery was designed by first giving careful
consideration to what constitutes a good user experience for service
discovery, and then designing a protocol with the features necessary
to enable that good user experience. This section covers two issues
in particular: Choice of factory-default names (and automatic
renaming behavior) for devices advertising services, and the
"continuous live update" user-experience model for clients
browsing to discover services.
16.1 Service Advertising User-Interface Considerations
When a DNS-SD service is advertised using Multicast DNS [mDNS],
automatic name conflict and resolution will occur if there is already
another service of the same type advertising with the same name.
As described in the Multicast DNS specification [mDNS], upon a
conflict, the service should:
1. Automatically select a new name (typically by appending
or incrementing a digit at the end of the name),
2. try advertising with the new name, and
3. upon success, record the new name in persistent storage.
This renaming behavior is very important, because it is the key
to providing user-friendly service names in the out-of-the-box
factory-default configuration. Some product developers may not
have realized this, because there are some products today where
the factory-default name is distinctly unfriendly, containing
random-looking strings of characters, like the device's Ethernet
address in hexadecimal. This is unnecessary, and undesirable, because
the point of the user-visible name is that it should be friendly and
useful to human users. If the name is not unique on the local network
the protocol will rememdy this as necessary. It is ironic that many
of the devices with this mistake are network printers, given that
these same printers also simultaneously support AppleTalk-over-
Ethernet, with nice user-friendly default names (and automatic
conflict detection and renaming). Examples of good factory-default
names are as follows:
Brother 5070N
Canon W2200 [ Apologies to makers of ]
HP LaserJet 4600 [ DNS-SD/mDNS printers ]
Lexmark W840 [ not listed. Email ]
Okidata C5300 [ the authors and we'll ]
Ricoh Aficio CL7100 [ add you to the list. ]
Xerox Phaser 6200DX
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To complete the case for why adding long ugly serial numbers to
the end of names is neither necessary nor desirable, consider
the cases where the user has (a) only one network printer,
(b) two network printers, and (c) many network printers.
(a) In the case where the user has only one network printer, a simple
name like (to use a vendor-neutral example) "Printer" is more
user-friendly than an ugly name like "Printer 0001E68C74FB".
Appending ugly hexadecimal goop to the end of the name to make
sure the name is unique is irrelevant to a user who only has one
printer anyway.
(b) In the case where the user gets a second network printer,
having it detect that the name "Printer" is already in use
and automatically instead name itself "Printer (2)" provides a
good user experience. For the users, remembering that the old
printer is "Printer" and the new one is "Printer (2)" is easy
and intuitive. Seeing two printers called "Printer 0001E68C74FB"
and "Printer 00306EC3FD1C" is a lot less helpful.
(c) In the case of a network with ten network printers, seeing a
list of ten names all of the form "Printer xxxxxxxxxxxx" has
effectively taken what was supposed to be a list of user-friendly
rich-text names (supporting mixed case, spaces, punctuation,
non-Roman characters and other symbols) and turned it into
just about the worst user-interface imaginable: a list of
incomprehensible random-looking strings of letters and digits.
In a network with a lot of printers, it would be desirable for
the people setting up the printers to take a moment to give each
one a descriptive name, but in the event they don't, presenting
the users with a list of sequentially-numbered printers is a much
more desirable default user experience than showing a list of raw
Ethernet addresses.
16.2 Client Browsing User-Interface Considerations
Of particular concern in the design of DNS-SD was the dynamic nature
of service discovery in a changing network environment. Other service
discovery protocols have been designed with an implicit unstated
assumption that the usage model is:
(a) client calls the service discovery code
(b) client gets list of discovered services
as of a particular instant in time, and then
(c) client displays list for user to select from
Superficially this usage model seems reasonable, but the problem is
that it's too optimistic. It only considers the success case, where
the user successfully finds the service they're looking for. In the
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case where the user is looking for (say) a particular printer, and
that printer's not turned on or not connected, the user first has
to attempt to remedy the problem, and then has to click a "refresh"
button to retry the service discovery (or, worse, dismiss the
browsing window entirely, and open a new one to initiate a new
network search attempt) to find out whether they were successful.
Because nothing happens instantaneously in networking, and packets
can be lost, necessitating some number of retransmissions, a service
discovery search typically takes a few seconds. A fairly typical user
experience model is:
(a) display an empty window,
(b) display some animation like a searchlight
sweeping back and forth for ten seconds, and then
(c) at the end of the ten-second search, display
a static list showing what was discovered.
Every time the user clicks the "refresh" button they have to endure
another ten-second wait, and every time the discovered list is
finally shown at the end of the ten-second wait, the moment it's
displayed on the screen it's already beginning to get stale and
out-of-date.
The service discovery user experience that the DNS-SD designers had
in mind has some rather different properties:
1. Displaying a list of discovered services should be effectively
instantaneous -- i.e. typically 1/10 second, not 10 seconds.
2. The list of discovered services should not be getting stale
and out-of-date from the moment it's displayed. The list
should be 'live' and should continue to update as new services
are discovered. Because of the delays, packet losses, and
retransmissions inherent in networking, it is to be expected
that sometimes, after the initial list is displayed showing
the majority of discovered services, a few remaining stragglers
may continue to trickle in during the subsequent few seconds.
Even after this initial stable list has been built and displayed,
the list should remain 'live' and should continue to update.
At any future time, be it minutes, hours, or even days later,
if a new service of the desired type is discovered, it should be
displayed in the list automatically, without the user having to
click a "refresh" button or take any other explicit action to
update the display.
3. With users getting to be in the habit of leaving service discovery
windows open, and coming to expect to be able to rely on them
to show a continuous 'live' view of current network reality,
this creates a new requirement for us: deletion of stale services.
When a service discovery list shows just a static snapshot at a
moment in time, then the situation is simple: either a service was
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discovered and appears in the list, or it was not, and does not.
However, when our list is live and updates continuously with the
discovery of new services, then this implies the corollary: when
a service goes away, it needs to *disappear* from the service
discovery list. Otherwise, the result would be unacceptable: the
service discovery list would simply grow monotonically over time,
and would require a periodic "refresh" (or complete dismissal and
recreation) to clear out old stale data.
4. With users getting to be in the habit of leaving service discovery
windows open, these windows need to update not only in response
to services coming and going, but also in response to changes
in configuration and connectivity of the client machine itself.
For example, if a user opens a service discovery window when no
Ethernet cable is connected to the client machine, and the window
appears empty with no discovered services, then when the user
connects the cable the window should automatically populate with
discovered services without requiring any explicit user action.
If the user disconnects the Ethernet cable, all the services
discovered via that network interface should automatically
disappear. If the user switches from one 802.11 wireless base
station to another, the service discovery window should
automatically update to remove all the services discovered
via the old wireless base station, and add all the services
discovered via the new one.
If these requirements seem to be setting an arbitrary and
unreasonably high standard for service discovery, bear in mind that
while it may have seemed that way to some, back in the 1990s when
these ideas were first proposed, in the years since then Apple and
other companies have shipped multiple implementations of DNS-SD/mDNS
that meet and exceed these requirements. In the years since Apple
shipped Mac OS X 10.2 Jaguar with the Open Source mDNSResponder
daemon, this service discovery "live browsing" paradigm has been
adopted and implemented in a wide range of Apple and third-party
applications, including printer discovery, Safari discovery of
devices with embedded web servers (for status and configuration),
iTunes music sharing, iPhoto photo sharing, the iChat Bonjour buddy
list, SubEthaEdit multi-user document editing, etc.
With so many different applications demonstrating that the "live
browsing" paradigm is clearly achievable, these four requirements
should not be regarded as idealistic unattainable goals, but
instead as the bare minimum baseline functionality that any
credible service discovery protocol needs to achieve.
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17. 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.
18. 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.
19. IANA Considerations
This protocol builds on DNS SRV records [RFC 2782], and similarly
requires IANA to assign unique application protocol names.
Unfortunately, the "IANA Considerations" section of RFC 2782 says
simply, "The IANA has assigned RR type value 33 to the SRV RR.
No other IANA services are required by this document."
Due to this oversight, IANA is currently prevented from carrying
out the necessary function of assigning these unique identifiers.
This document proposes the following IANA allocation policy for
unique application protocol names:
Allowable names:
* Must be no more than fourteen characters long
* Must consist only of:
- lower-case letters 'a' - 'z'
- digits '0' - '9'
- the hyphen character '-'
* Must begin and end with a lower-case letter or digit.
* Must not already be assigned to some other protocol in the
existing IANA "list of assigned application protocol names
and port numbers" [ports].
These identifiers are allocated on a First Come First Served basis.
In the event of abuse (e.g. automated mass registrations, etc.),
the policy may be changed without notice to Expert Review [RFC 2434].
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
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advocates that on or before the date a product ships, developers
should properly register their service names.
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 competitors.
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.
This proposed IANA allocation policy is not in force until this
document is published as an RFC. In the meantime, unique application
protocol names may be registered according to the instructions at
<http://www.dns-sd.org/ServiceTypes.html>. As of August 2006, there
are roughly 300 application protocols in currently shipping products
that have been so registered as using DNS-SD for service discovery.
20. Acknowledgments
The concepts described in this document have been explored, developed
and implemented with help from Richard Brown, Erik Guttman, Paul
Vixie, and Bill Woodcock.
Special thanks go to Bob Bradley, Josh Graessley, Scott Herscher,
Roger Pantos and Kiren Sekar for their significant contributions.
21. Deployment History
The first implementations of DNS-Based Service Discovery and
Multicast DNS were initially developed during the late 1990s,
but the event that put them into the media spotlight was Steve Jobs
demonstrating it live on stage in his keynote presentation opening
Apple's annual Worldwide Developers Conference in May 2002, and
announcing Apple's adoption of the technology throughout its hardware
and software product line. Three months later, in August 2002, Apple
shipped Mac OS X 10.2 Jaguar, and millions of end-users got their
first exposure to Zero Configuration Networking with DNS-SD/mDNS
in applications like Safari, iChat, and printer setup. A month later,
in September 2002, Apple released the entire source code for the
mDNS Responder daemon under its Darwin Open Source project, with
code not just for Mac OS X, but also for a range of other platforms
including Windows, VxWorks, Linux, Solaris, FreeBSD, etc.
Many hardware makers were quick to see the benefits of Zero
Configuration Networking. Printer makers especially were enthusiastic
early adopters, and within a year every major printer manufacturer
was shipping DNS-SD/mDNS-enabled network printers. If you've bought
any network printer at all in the last few years, it was probably one
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that supports DNS-SD/mDNS, even if you didn't know that at the time.
For Mac OS X users, telling if you have DNS-SD/mDNS printers on your
network is easy because they automatically appear in the "Bonjour"
submenu in the "Print" dialog of every Mac application. Microsoft
Windows users can get a similar experience by installing Bonjour for
Windows (takes about 90 seconds, no restart required) and running the
Bonjour for Windows Printer Setup Wizard [B4W].
The Open Source community has produced several independent
implementations of DNS-Based Service Discovery and Multicast DNS,
some in C like Apple's mDNSResponder daemon, and others in a variety
of different languages including Java, Python, Perl, and C#/Mono.
22. Copyright Notice
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights. For the purposes of this document,
the term "BCP 78" refers exclusively to RFC 3978, "IETF Rights
in Contributions", published March 2005.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
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23. 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 2782] Gulbrandsen, A., et al., "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2782, February 2000.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[UAX15] "Unicode Normalization Forms"
http://www.unicode.org/reports/tr15/
24. Informative References
[B4W] Bonjour for Windows <http://www.apple.com/bonjour/>
[mDNS] Cheshire, S., and M. Krochmal, "Multicast DNS",
Internet-Draft (work in progress),
draft-cheshire-dnsext-multicastdns-06.txt, August 2006.
[NBP] Cheshire, S., and M. Krochmal,
"Requirements for a Protocol to Replace AppleTalk NBP",
Internet-Draft (work in progress),
draft-cheshire-dnsext-nbp-05.txt, August 2006.
[RFC 2132] Alexander, S., and Droms, R., "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC 2136] Vixie, P., et al., "Dynamic Updates in the Domain Name
System (DNS UPDATE)", RFC 2136, April 1997.
[RFC 2434] Narten, T., and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", RFC 2434,
October 1998.
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[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.
[RFC 3397] Aboba, B., and Cheshire, S., "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397, November
2002.
[SOAP] Nilo Mitra, "SOAP Version 1.2 Part 0: Primer",
W3C Proposed Recommendation, 24 June 2003
http://www.w3.org/TR/2003/REC-soap12-part0-20030624
[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.
25. Authors' Addresses
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc [at] stuartcheshire [dot] org
Marc Krochmal
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 4368
EMail: marc [at] apple [dot] com
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