Internet Engineering Task Force (IETF) S. Cheshire
Request for Comments: 8766 Apple Inc.
Category: Standards Track June 2020
ISSN: 2070-1721
Discovery Proxy for Multicast DNS-Based Service Discovery
Abstract
This document specifies a network proxy that uses Multicast DNS to
automatically populate the wide-area unicast Domain Name System
namespace with records describing devices and services found on the
local link.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8766.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Operational Analogy
3. Conventions and Terminology Used in This Document
4. Compatibility Considerations
5. Discovery Proxy Operation
5.1. Delegated Subdomain for DNS-based Service Discovery Records
5.2. Domain Enumeration
5.2.1. Domain Enumeration via Unicast Queries
5.2.2. Domain Enumeration via Multicast Queries
5.3. Delegated Subdomain for LDH Host Names
5.4. Delegated Subdomain for Reverse Mapping
5.5. Data Translation
5.5.1. DNS TTL Limiting
5.5.2. Suppressing Unusable Records
5.5.3. NSEC and NSEC3 Queries
5.5.4. No Text-Encoding Translation
5.5.5. Application-Specific Data Translation
5.6. Answer Aggregation
6. Administrative DNS Records
6.1. DNS SOA (Start of Authority) Record
6.2. DNS NS Records
6.3. DNS Delegation Records
6.4. DNS SRV Records
6.5. Domain Enumeration Records
7. DNSSEC Considerations
7.1. Online Signing Only
7.2. NSEC and NSEC3 Records
8. IPv6 Considerations
9. Security Considerations
9.1. Authenticity
9.2. Privacy
9.3. Denial of Service
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Implementation Status
A.1. Already Implemented and Deployed
A.2. Already Implemented
A.3. Partially Implemented
Acknowledgments
Author's Address
1. Introduction
Multicast DNS [RFC6762] and its companion technology DNS-based
Service Discovery [RFC6763] were created to provide IP networking
with the ease of use and autoconfiguration for which AppleTalk was
well known [RFC6760] [ZC] [ROADMAP].
For a small home network consisting of just a single link (or a few
physical links bridged together to appear as a single logical link
from the point of view of IP), Multicast DNS [RFC6762] is sufficient
for client devices to look up the ".local" host names of peers on the
same home network, and to use Multicast DNS-based Service Discovery
(DNS-SD) [RFC6763] to discover services offered on that home network.
For a larger network consisting of multiple links that are
interconnected using IP-layer routing instead of link-layer bridging,
link-local Multicast DNS alone is insufficient because link-local
Multicast DNS packets, by design, are not propagated onto other
links.
Using link-local multicast packets for Multicast DNS was a conscious
design choice [RFC6762]. Even when limited to a single link,
multicast traffic is still generally considered to be more expensive
than unicast, because multicast traffic impacts many devices instead
of just a single recipient. In addition, with some technologies like
Wi-Fi [IEEE-11], multicast traffic is inherently less efficient and
less reliable than unicast, because Wi-Fi multicast traffic is sent
at lower data rates, and is not acknowledged [MCAST]. Increasing the
amount of expensive multicast traffic by flooding it across multiple
links would make the traffic load even worse.
Partitioning the network into many small links curtails the spread of
expensive multicast traffic but limits the discoverability of
services. At the opposite end of the spectrum, using a very large
local link with thousands of hosts enables better service discovery
but at the cost of larger amounts of multicast traffic.
Performing DNS-based Service Discovery using purely Unicast DNS is
more efficient and doesn't require large multicast domains but does
require that the relevant data be available in the Unicast DNS
namespace. The Unicast DNS namespace in question could fall within a
traditionally assigned globally unique domain name, or it could be
within a private local unicast domain name such as ".home.arpa"
[RFC8375].
In the DNS-SD specification [RFC6763], Section 10 ("Populating the
DNS with Information") discusses various possible ways that a
service's PTR, SRV, TXT, and address records can make their way into
the Unicast DNS namespace, including manual zone file configuration
[RFC1034] [RFC1035], DNS Update [RFC2136] [RFC3007], and proxies of
various kinds.
One option is to make the relevant data available in the Unicast DNS
namespace by manual DNS configuration. This option has been used for
many years at IETF meetings to advertise the IETF terminal room
printer. Details of this example are given in Appendix A of the
Roadmap document [ROADMAP]. However, this manual DNS configuration
is labor intensive, error prone, and requires a reasonable degree of
DNS expertise.
Another option is to populate the Unicast DNS namespace by having the
devices offering the services do that themselves, using DNS Update
[REG-PROT] [DNS-UL]. However, this requires configuration of DNS
Update keys on those devices, which has proven onerous and
impractical for simple devices like printers and network cameras.
Hence, to facilitate efficient and reliable DNS-based Service
Discovery, a hybrid is needed that combines the ease of use of
Multicast DNS with the efficiency and scalability of Unicast DNS.
This document specifies a type of proxy called a "Discovery Proxy"
that uses Multicast DNS [RFC6762] to discover Multicast DNS records
on its local link on demand, and makes corresponding DNS records
visible in the Unicast DNS namespace.
In principle, similar mechanisms could be defined for other local
discovery protocols, by creating a proxy that (i) uses the protocol
in question to discover local information on demand, and then (ii)
makes corresponding DNS records visible in the Unicast DNS namespace.
Such mechanisms for other local discovery protocols could be
addressed in future documents.
The design of the Discovery Proxy is guided by the previously
published DNS-based Service Discovery requirements document
[RFC7558].
In simple terms, a descriptive DNS name is chosen for each link in an
organization. Using a DNS NS record, responsibility for that DNS
name is delegated to a Discovery Proxy physically attached to that
link. When a remote client issues a unicast query for a name falling
within the delegated subdomain, the normal DNS delegation mechanism
results in the unicast query arriving at the Discovery Proxy, since
it has been declared authoritative for those names. Now, instead of
consulting a textual zone file on disk to discover the answer to the
query as a traditional authoritative DNS server would, a Discovery
Proxy consults its local link, using Multicast DNS, to find the
answer to the question.
For fault tolerance reasons, there may be more than one Discovery
Proxy serving a given link.
Note that the Discovery Proxy uses a "pull" model. Until some remote
client has requested data, the local link is not queried using
Multicast DNS. In the idle state, in the absence of client requests,
the Discovery Proxy sends no packets and imposes no burden on the
network. It operates purely "on demand".
An alternative proposal that has been discussed is a proxy that
performs DNS updates to a remote DNS server on behalf of the
Multicast DNS devices on the local network. The difficulty with this
is that Multicast DNS devices do not routinely announce their records
on the network. Generally, they remain silent until queried. This
means that the complete set of Multicast DNS records in use on a link
can only be discovered by active querying, not by passive listening.
Because of this, a proxy can only know what names exist on a link by
issuing queries for them, and since it would be impractical to issue
queries for every possible name just to find out which names exist
and which do not, there is no reasonable way for a proxy to
programmatically learn all the answers it would need to push up to
the remote DNS server using DNS Update. Even if such a mechanism
were possible, it would risk generating high load on the network
continuously, even when there are no clients with any interest in
that data.
Hence, having a model where the query comes to the Discovery Proxy is
much more efficient than a model where the Discovery Proxy pushes the
answers out to some other remote DNS server.
A client seeking to discover services and other information performs
this by sending traditional DNS queries to the Discovery Proxy or by
sending DNS Push Notification subscription requests [RFC8765].
How a client discovers what domain name(s) to use for its DNS-based
Service Discovery queries (and, consequently, what Discovery Proxy or
Proxies to use) is described in Section 5.2.
The diagram below illustrates a network topology using a Discovery
Proxy to provide discovery service to a remote client.
+--------+ Unicast +-----------+ +---------+ +---------+
| Remote | Communication | Discovery | | Network | | Network |
| Client |---- . . . ----| Proxy | | Printer | | Camera |
+--------+ +-----------+ +---------+ +---------+
| | | |
------------ --------------------------------------------
Multicast-capable LAN segment (e.g., Ethernet)
Figure 1: Example Deployment
Note that there need not be any Discovery Proxy on the link to which
the remote client is directly attached. The remote client
communicates directly with the Discovery Proxy using normal unicast
TCP/IP communication mechanisms, potentially spanning multiple IP
hops, possibly including VPN tunnels and other similar long-distance
communication channels.
2. Operational Analogy
A Discovery Proxy does not operate as a multicast relay or multicast
forwarder. There is no danger of multicast forwarding loops that
result in traffic storms, because no multicast packets are forwarded.
A Discovery Proxy operates as a _proxy_ for remote clients,
performing queries on their behalf and reporting the results back.
A reasonable analogy is making a telephone call to a colleague at
your workplace and saying, "I'm out of the office right now. Would
you mind bringing up a printer browser window and telling me the
names of the printers you see?" That entails no risk of a forwarding
loop causing a traffic storm, because no multicast packets are sent
over the telephone call.
A similar analogy, instead of enlisting another human being to
initiate the service discovery operation on your behalf, is to log in
to your own desktop work computer using screen sharing and then run
the printer browser yourself to see the list of printers. Or, log in
using Secure Shell (ssh) and type "dns-sd -B _ipp._tcp" and observe
the list of discovered printer names. In neither case is there any
risk of a forwarding loop causing a traffic storm, because no
multicast packets are being sent over the screen-sharing or ssh
connection.
The Discovery Proxy provides another way of performing remote
queries, which uses a different protocol instead of screen sharing or
ssh. The Discovery Proxy mechanism can be thought of as a custom
Remote Procedure Call (RPC) protocol that allows a remote client to
exercise the Multicast DNS APIs on the Discovery Proxy device, just
as a local client running on the Discovery Proxy device would use
those APIs.
When the Discovery Proxy software performs Multicast DNS operations,
the exact same Multicast DNS caching mechanisms are applied as when
any other client software on that Discovery Proxy device performs
Multicast DNS operations, regardless of whether that be running a
printer browser client locally, a remote user running the printer
browser client via a screen-sharing connection, a remote user logged
in via ssh running a command-line tool like "dns-sd", or a remote
user sending DNS requests that cause a Discovery Proxy to perform
discovery operations on its behalf.
3. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The Discovery Proxy builds on Multicast DNS, which works between
hosts on the same link. For the purposes of this document, a set of
hosts is considered to be "on the same link" if:
* when any host from that set sends a packet to any other host in
that set, using unicast, multicast, or broadcast, the entire link-
layer packet payload arrives unmodified, and
* a broadcast sent over that link, by any host from that set of
hosts, can be received by every other host in that set.
The link-layer _header_ may be modified, such as in Token Ring Source
Routing [IEEE-5], but not the link-layer _payload_. In particular,
if any device forwarding a packet modifies any part of the IP header
or IP payload, then the packet is no longer considered to be on the
same link. This means that the packet may pass through devices such
as repeaters, bridges, hubs, or switches and still be considered to
be on the same link for the purpose of this document, but not through
a device such as an IP router that decrements the IP TTL or otherwise
modifies the IP header.
4. Compatibility Considerations
No changes to existing devices are required to work with a Discovery
Proxy.
Existing devices that advertise services using Multicast DNS work
with a Discovery Proxy.
Existing clients that support DNS-based Service Discovery over
Unicast DNS work with a Discovery Proxy. DNS-based Service Discovery
over Unicast DNS was introduced in Mac OS X 10.4 Tiger in April 2005
and has been included in Apple products introduced since then,
including the iPhone and iPad. It has also been included in products
from other vendors, such as Microsoft Windows 10.
An overview of the larger collection of associated DNS-based Service
Discovery technologies, and how the Discovery Proxy technology
relates to those, is given in the Service Discovery Road Map document
[ROADMAP].
5. Discovery Proxy Operation
In a typical configuration, a Discovery Proxy is configured to be
authoritative [RFC1034] [RFC1035] for four or more DNS subdomains,
listed below. Authority for these subdomains is delegated from the
parent domain to the Discovery Proxy in the usual way for DNS
delegation, via NS records.
A DNS subdomain for DNS-based Service Discovery records.
This subdomain name may contain rich text, including spaces and
other punctuation. This is because this subdomain name is used
only in graphical user interfaces, where rich text is appropriate.
A DNS subdomain for host name records.
This subdomain name SHOULD be limited to letters, digits, and
hyphens in order to facilitate the convenient use of host names in
command-line interfaces.
One or more DNS subdomains for IPv4 Reverse Mapping records.
These subdomains will have names that end in "in-addr.arpa".
One or more DNS subdomains for IPv6 Reverse Mapping records.
These subdomains will have names that end in "ip6.arpa".
In an enterprise network, the naming and delegation of these
subdomains is typically performed by conscious action of the network
administrator. In a home network, naming and delegation would
typically be performed using some automatic configuration mechanism
such as Home Networking Control Protocol (HNCP) [RFC7788].
These three varieties of delegated subdomains (service discovery,
host names, and reverse mapping) are described below in Sections 5.1,
5.3, and 5.4.
How a client discovers where to issue its DNS-based Service Discovery
queries is described in Section 5.2.
5.1. Delegated Subdomain for DNS-based Service Discovery Records
In its simplest form, each link in an organization is assigned a
unique Unicast DNS domain name such as "Building 1.example.com" or
"2nd Floor.Building 3.example.com". Grouping multiple links under a
single Unicast DNS domain name is to be specified in a future
companion document, but for the purposes of this document, assume
that each link has its own unique Unicast DNS domain name. In a
graphical user interface these names are not displayed as strings
with dots as shown above, but something more akin to a typical file
browser graphical user interface (which is harder to illustrate in a
text-only document) showing folders, subfolders, and files in a file
system.
+---------------+--------------+-------------+-------------------+
| *example.com* | Building 1 | 1st Floor | Alice's printer |
| | Building 2 | *2nd Floor* | Bob's printer |
| | *Building 3* | 3rd Floor | Charlie's printer |
| | Building 4 | 4th Floor | |
| | Building 5 | | |
| | Building 6 | | |
+---------------+--------------+-------------+-------------------+
Figure 2: Illustrative GUI
Each named link in an organization has one or more Discovery Proxies
that serve it. This Discovery Proxy function could be performed by a
device like a router or switch that is physically attached to that
link. In the parent domain, NS records are used to delegate
ownership of each defined link name (e.g., "Building 1.example.com")
to one or more Discovery Proxies that serve the named link. In other
words, the Discovery Proxies are the authoritative name servers for
that subdomain. As in the rest of DNS-based Service Discovery, all
names are represented as-is using plain UTF-8 encoding and, as
described in Section 5.5.4, no text-encoding translations are
performed.
With appropriate VLAN configuration [IEEE-1Q], a single Discovery
Proxy device could have a logical presence on many links and serve as
the Discovery Proxy for all those links. In such a configuration,
the Discovery Proxy device would have a single physical Ethernet
[IEEE-3] port, configured as a VLAN trunk port, which would appear to
software on that device as multiple virtual Ethernet interfaces, one
connected to each of the VLAN links.
As an alternative to using VLAN technology, using a Multicast DNS
Discovery Relay [RELAY] is another way that a Discovery Proxy can
have a "virtual" presence on a remote link.
When a DNS-SD client issues a Unicast DNS query to discover services
in a particular Unicast DNS subdomain
(e.g., "_ipp._tcp.Building 1.example.com. PTR ?"), the normal DNS
delegation mechanism results in that query being forwarded until it
reaches the delegated authoritative name server for that subdomain,
namely, the Discovery Proxy on the link in question. Like a
conventional Unicast DNS server, a Discovery Proxy implements the
usual Unicast DNS protocol [RFC1034] [RFC1035] over UDP and TCP.
However, unlike a conventional Unicast DNS server that generates
answers from the data in its manually configured zone file, a
Discovery Proxy learns answers using Multicast DNS. A Discovery
Proxy does this by consulting its Multicast DNS cache and/or issuing
Multicast DNS queries, as appropriate according to the usual protocol
rules of Multicast DNS [RFC6762], for the corresponding Multicast DNS
name, type, and class, with the delegated zone part of the name
replaced with ".local" (e.g., in this case,
"_ipp._tcp.local. PTR ?"). Then, from the received Multicast DNS
data, the Discovery Proxy synthesizes the appropriate Unicast DNS
response, with the ".local" top-level label of the owner name
replaced with the name of the delegated zone. Further details of the
name translation rules are described in Section 5.5. Rules
specifying how long the Discovery Proxy should wait to accumulate
Multicast DNS responses before sending its unicast reply are
described in Section 5.6.
The existing Multicast DNS caching mechanism is used to minimize
unnecessary Multicast DNS queries on the wire. The Discovery Proxy
is acting as a client of the underlying Multicast DNS subsystem and
benefits from the same caching and efficiency measures as any other
client using that subsystem.
Note that the contents of the delegated zone, generated as it is by
performing ".local" Multicast DNS queries, mirrors the records
available on the local link via Multicast DNS very closely, but not
precisely. There is not a full bidirectional equivalence between the
two. Certain records that are available via Multicast DNS may not
have equivalents in the delegated zone possibly because they are
invalid or not relevant in the delegated zone or because they are
being suppressed because they are unusable outside the local link
(see Section 5.5.2). Conversely, certain records that appear in the
delegated zone may not have corresponding records available on the
local link via Multicast DNS. In particular, there are certain
administrative SRV records (see Section 6) that logically fall within
the delegated zone but semantically represent metadata _about_ the
zone rather than records _within_ the zone. Consequently, these
administrative records in the delegated zone do not have any
corresponding counterparts in the Multicast DNS namespace of the
local link.
5.2. Domain Enumeration
A DNS-SD client performs Domain Enumeration [RFC6763] via certain PTR
queries, using both unicast and multicast.
If a DNS-SD client receives a Domain Name configuration via DHCP then
it issues unicast queries derived from this domain name. It also
issues unicast queries using names derived from its IPv4 subnet
address(es) and IPv6 prefix(es). These unicast Domain Enumeration
queries are described in Section 5.2.1. A DNS-SD client also issues
multicast Domain Enumeration queries in the "local" domain [RFC6762],
as described in Section 5.2.2. The results of all the Domain
Enumeration queries are combined for DNS-based Service Discovery
purposes.
5.2.1. Domain Enumeration via Unicast Queries
The (human or automated) administrator creates Unicast DNS Domain
Enumeration PTR records [RFC6763] to inform clients of available
service discovery domains. Two varieties of such Unicast DNS Domain
Enumeration PTR records exist: those with names derived from the
domain name communicated to the clients via DHCP option 15 [RFC2132],
and those with names derived from either IPv4 subnet address(es) or
IPv6 prefix(es) in use by the clients. Below is an example showing
the name-based variety, where the DHCP server configured the client
with the domain name "example.com":
b._dns-sd._udp.example.com. PTR Building 1.example.com.
PTR Building 2.example.com.
PTR Building 3.example.com.
PTR Building 4.example.com.
db._dns-sd._udp.example.com. PTR Building 1.example.com.
lb._dns-sd._udp.example.com. PTR Building 1.example.com.
The meaning of these records is defined in the DNS-based Service
Discovery specification [RFC6763] but, for convenience, is repeated
here. The "b" ("browse") records tell the client device the list of
browsing domains to display for the user to select from. The "db"
("default browse") record tells the client device which domain in
that list should be selected by default. The "db" domain MUST be one
of the domains in the "b" list; if not, then no domain is selected by
default. The "lb" ("legacy browse") record tells the client device
which domain to automatically browse on behalf of applications that
don't implement user interface for multi-domain browsing (which is
most of them at the time of writing). The "lb" domain is often the
same as the "db" domain, or sometimes the "db" domain plus one or
more others that should be included in the list of automatic browsing
domains for legacy clients.
Note that in the example above, for clarity, space characters in
names are shown as actual spaces. If this data is manually entered
into a textual zone file for authoritative server software such as
BIND, care must be taken because the space character is used as a
field separator, and other characters like dot ('.'), semicolon
(';'), dollar ('$'), backslash ('\'), etc., also have special
meaning. These characters have to be escaped when entered into a
textual zone file, following the rules in Section 5.1 of the DNS
specification [RFC1035]. For example, a literal space in a name is
represented in the textual zone file using '\032', so
"Building 1.example.com" is entered as "Building\0321.example.com".
DNS responses are limited to a maximum size of 65535 bytes. This
limits the maximum number of domains that can be returned for a
Domain Enumeration query as follows:
A DNS response header is 12 bytes. That's typically followed by a
single qname (up to 256 bytes) plus qtype (2 bytes) and qclass
(2 bytes), leaving 65275 for the Answer Section.
An Answer Section Resource Record consists of:
* Owner name, encoded as a compression pointer, 2 bytes
* RRTYPE (type PTR), 2 bytes
* RRCLASS (class IN), 2 bytes
* TTL, 4 bytes
* RDLENGTH, 2 bytes
* RDATA (domain name), up to 256 bytes
This means that each Resource Record in the Answer Section can take
up to 268 bytes total, which means that the Answer Section can
contain, in the worst case, no more than 243 domains.
In a more typical scenario, where the domain names are not all
maximum-sized names, and there is some similarity between names so
that reasonable name compression is possible, each Answer
Section Resource Record may average 140 bytes, which means that the
Answer Section can contain up to 466 domains.
It is anticipated that this should be sufficient for even a large
corporate network or university campus.
5.2.2. Domain Enumeration via Multicast Queries
In the case where Discovery Proxy functionality is widely deployed
within an enterprise (either by having a Discovery Proxy physically
on each link, or by having a Discovery Proxy with a remote "virtual"
presence on each link using VLANs or Multicast DNS Discovery Relays
[RELAY]), this offers an additional way to provide Domain Enumeration
configuration data for clients.
Note that this function of the Discovery Proxy is supplementary to
the primary purpose of the Discovery Proxy, which is to facilitate
_remote_ clients discovering services on the Discovery Proxy's local
link. This publication of Domain Enumeration configuration data via
link-local multicast on the Discovery Proxy's local link is performed
for the benefit of _local_ clients attached to that link, and
typically directs those clients to contact other distant Discovery
Proxies attached to other links. Generally, a client does not need
to use the local Discovery Proxy on its own link, because a client is
generally able to perform its own Multicast DNS queries on that link.
(The exception to this is when the local Wi-Fi access point is
blocking or filtering local multicast traffic, requiring even local
clients to use their local Discovery Proxy to perform local
discovery.)
A Discovery Proxy can be configured to generate Multicast DNS
responses for the following Multicast DNS Domain Enumeration queries
issued by clients:
b._dns-sd._udp.local. PTR ?
db._dns-sd._udp.local. PTR ?
lb._dns-sd._udp.local. PTR ?
This provides the ability for Discovery Proxies to indicate
recommended browsing domains to DNS-SD clients on a per-link
granularity. In some enterprises, it may be preferable to provide
this per-link configuration information in the form of Discovery
Proxy configuration data rather than by populating the Unicast DNS
servers with the same data (in the "ip6.arpa" or "in-addr.arpa"
domains).
Regardless of how the network operator chooses to provide this
configuration data, clients will perform Domain Enumeration via both
unicast and multicast queries and then combine the results of these
queries.
5.3. Delegated Subdomain for LDH Host Names
DNS-SD service instance names and domains are allowed to contain
arbitrary Net-Unicode text [RFC5198], encoded as precomposed UTF-8
[RFC3629].
Users typically interact with service discovery software by viewing a
list of discovered service instance names on a display and selecting
one of them by pointing, touching, or clicking. Similarly, in
software that provides a multi-domain DNS-SD user interface, users
view a list of offered domains on the display and select one of them
by pointing, touching, or clicking. To use a service, users don't
have to remember domain or instance names, or type them; users just
have to be able to recognize what they see on the display and touch
or click on the thing they want.
In contrast, host names are often remembered and typed. Also, host
names have historically been used in command-line interfaces where
spaces can be inconvenient. For this reason, host names have
traditionally been restricted to letters, digits, and hyphens (LDH)
with no spaces or other punctuation.
While we do want to allow rich text for DNS-SD service instance names
and domains, it is advisable, for maximum compatibility with existing
usage, to restrict host names to the traditional letter-digit-hyphen
rules. This means that while the service name
"My Printer._ipp._tcp.Building 1.example.com" is acceptable and
desirable (it is displayed in a graphical user interface as an
instance called "My Printer" in the domain "Building 1" at
"example.com"), the host name "My-Printer.Building 1.example.com" is
less desirable (because of the space in "Building 1").
To accommodate this difference in allowable characters, a Discovery
Proxy SHOULD support having two separate subdomains delegated to it
for each link it serves: one whose name is allowed to contain
arbitrary Net-Unicode text [RFC5198], and a second more constrained
subdomain whose name is restricted to contain only letters, digits,
and hyphens, to be used for host name records (names of 'A' and
'AAAA' address records). The restricted names may be any valid name
consisting of only letters, digits, and hyphens, including Punycode-
encoded names [RFC3492].
For example, a Discovery Proxy could have the two subdomains
"Building 1.example.com" and "bldg-1.example.com" delegated to it.
The Discovery Proxy would then translate these two Multicast DNS
records:
My Printer._ipp._tcp.local. SRV 0 0 631 prnt.local.
prnt.local. A 203.0.113.2
into Unicast DNS records as follows:
My Printer._ipp._tcp.Building 1.example.com.
SRV 0 0 631 prnt.bldg-1.example.com.
prnt.bldg-1.example.com. A 203.0.113.2
Note that the SRV record name is translated using the rich-text
domain name ("Building 1.example.com"), and the address record name
is translated using the LDH domain ("bldg-1.example.com"). Further
details of the name translation rules are described in Section 5.5.
A Discovery Proxy MAY support only a single rich-text Net-Unicode
domain and use that domain for all records, including 'A' and 'AAAA'
address records, but implementers choosing this option should be
aware that this choice may produce host names that are awkward to use
in command-line environments. Whether or not this is an issue
depends on whether users in the target environment are expected to be
using command-line interfaces.
A Discovery Proxy MUST NOT be restricted to support only a letter-
digit-hyphen subdomain, because that results in an unnecessarily poor
user experience.
As described in Section 5.2.1, for clarity, in examples here space
characters in names are shown as actual spaces. If this dynamically
discovered data were to be manually entered into a textual zone file
(which it isn't), then spaces would need to be represented using
'\032', so "My Printer._ipp._tcp.Building 1.example.com" would become
"My\032Printer._ipp._tcp.Building\0321.example.com".
Note that the '\032' representation does not appear in DNS messages
sent over the air. In the wire format of DNS messages, spaces are
sent as spaces, not as '\032', and likewise, in a graphical user
interface at the client device, spaces are shown as spaces, not as
'\032'.
5.4. Delegated Subdomain for Reverse Mapping
A Discovery Proxy can facilitate easier management of reverse mapping
domains, particularly for IPv6 addresses where manual management may
be more onerous than it is for IPv4 addresses.
To achieve this, in the parent domain, NS records are used to
delegate ownership of the appropriate reverse mapping domain to the
Discovery Proxy. In other words, the Discovery Proxy becomes the
authoritative name server for the reverse mapping domain. For fault
tolerance reasons, there may be more than one Discovery Proxy serving
a given link.
If a given link is using the IPv4 subnet 203.0.113/24, then the
domain "113.0.203.in-addr.arpa" is delegated to the Discovery Proxy
for that link.
If a given link is using the IPv6 prefix 2001:0DB8:1234:5678::/64,
then the domain "8.7.6.5.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa" is
delegated to the Discovery Proxy for that link.
When a reverse mapping query arrives at the Discovery Proxy, it
issues the identical query on its local link, as a Multicast DNS
query. The mechanism to force an apparently unicast name to be
resolved using link-local Multicast DNS varies depending on the API
set being used. For example, in the "dns_sd.h" APIs (available on
macOS, iOS, Bonjour for Windows, Linux, and Android), using
kDNSServiceFlagsForceMulticast indicates that the
DNSServiceQueryRecord() call should perform the query using Multicast
DNS. Other API sets have different ways of forcing multicast
queries. When the host owning that IPv4 or IPv6 address responds
with a name of the form "something.local", the Discovery Proxy
rewrites it to use its configured LDH host name domain instead of
".local" and returns the response to the caller.
For example, a Discovery Proxy with the two subdomains
"113.0.203.in-addr.arpa" and "bldg-1.example.com" delegated to it
would translate this Multicast DNS record:
2.113.0.203.in-addr.arpa. PTR prnt.local.
into this Unicast DNS response:
2.113.0.203.in-addr.arpa. PTR prnt.bldg-1.example.com.
In this example the "prnt.local" host name is translated using the
delegated LDH subdomain, as described in Section 5.5.
Subsequent queries for the prnt.bldg-1.example.com address record,
falling as it does within the bldg-1.example.com domain, which is
delegated to this Discovery Proxy, will arrive at this Discovery
Proxy where they are answered by issuing Multicast DNS queries and
using the received Multicast DNS answers to synthesize Unicast DNS
responses, as described above.
Note that this description assumes that all addresses on a given IPv4
subnet or IPv6 prefix are mapped to host names using the Discovery
Proxy mechanism. It would be possible to implement a Discovery Proxy
that can be configured so that some address-to-name mappings are
performed using Multicast DNS on the local link, while other address-
to-name mappings within the same IPv4 subnet or IPv6 prefix are
configured manually.
5.5. Data Translation
For the delegated rich-text and LDH subdomains, generating
appropriate Multicast DNS queries involves translating from the
configured DNS domain (e.g., "Building 1.example.com") on the Unicast
DNS side to ".local" on the Multicast DNS side.
For the delegated reverse-mapping subdomain, generating appropriate
Multicast DNS queries involves using the appropriate API mechanism to
indicate that a query should be performed using Multicast DNS, as
described in Section 5.4.
Generating appropriate Unicast DNS responses from the received
Multicast DNS answers involves translating back from ".local" to the
appropriate configured Unicast DNS domain as necessary, as described
below.
In the examples below, the delegated subdomains are as follows:
Delegated subdomain for rich-text names Building 1.example.com.
Delegated subdomain for LDH names bldg-1.example.com.
Delegated subdomain for IPv4 reverse mapping 113.0.203.in-addr.arpa.
Names in Multicast DNS answers that do not end in ".local" do not
require any translation.
Names in Multicast DNS answers that end in ".local" are only
meaningful on the local link, and require translation to make them
useable by clients outside the local link.
Names that end in ".local" may appear both as the owner names of
received Multicast DNS answer records, and in the RDATA of received
Multicast DNS answer records.
In a received Multicast DNS answer record, if the owner name ends
with ".local", then the ".local" top-level label is replaced with the
name of the delegated subdomain as was used in the originating query.
In a received Multicast DNS answer record, if a name in the RDATA
ends with ".local", then the name is translated according to the
delegated subdomain that was used in the originating query, as
explained below.
For queries in subdomains delegated for LDH host names, ".local"
names in RDATA are translated to that delegated LDH subdomain. For
example, a query for "thing.bldg-1.example.com" will be translated to
a Multicast DNS query for "thing.local". If that query returns this
CNAME record:
thing.local. CNAME prnt.local.
then both the owner name and the name in the RDATA are translated
from ".local" to the LDH subdomain "bldg-1.example.com":
thing.bldg-1.example.com. CNAME prnt.bldg-1.example.com.
For queries in subdomains delegated for reverse mapping names,
".local" names in RDATA are translated to the delegated LDH
subdomain, if one is configured, or to the delegated rich-text
subdomain otherwise. For example, consider a reverse mapping query
that returns this PTR record:
2.113.0.203.in-addr.arpa. PTR prnt.local.
The owner name is not translated because it does not end in ".local".
The name in the RDATA is translated from ".local" to the LDH
subdomain "bldg-1.example.com":
2.113.0.203.in-addr.arpa. PTR prnt.bldg-1.example.com.
For queries in subdomains delegated for rich-text names, ".local"
names in RDATA are translated according to whether or not they
represent host names (i.e., RDATA names that are the owner names of A
and AAAA DNS records). RDATA names ending in ".local" that represent
host names are translated to the delegated LDH subdomain, if one is
configured, or to the delegated rich-text subdomain otherwise. All
other RDATA names ending in ".local" are translated to the delegated
rich-text subdomain. For example, consider a DNS-SD service browsing
PTR query that returns this PTR record for IPP printing:
_ipp._tcp.local. PTR My Printer._ipp._tcp.local.
Both the owner name and the name in the RDATA are translated from
".local" to the rich-text subdomain:
_ipp._tcp.Building 1.example.com.
PTR My Printer._ipp._tcp.Building 1.example.com.
In contrast, consider a query that returns this SRV record for a
specific IPP printing instance:
My Printer._ipp._tcp.local. SRV 0 0 631 prnt.local.
As for all queries, the owner name is translated to the delegated
subdomain of the originating query, the delegated rich-text subdomain
"Building 1.example.com". However, the ".local" name in the RDATA is
the target host name field of an SRV record, a field that is used
exclusively for host names. Consequently it is translated to the LDH
subdomain "bldg-1.example.com", if configured, instead of the rich-
text subdomain:
My Printer._ipp._tcp.Building 1.example.com.
SRV 0 0 631 prnt.bldg-1.example.com.
Other beneficial translation and filtering operations are described
below.
5.5.1. DNS TTL Limiting
For efficiency, Multicast DNS typically uses moderately high DNS TTL
values. For example, the typical TTL on DNS-SD service browsing PTR
records is 75 minutes. What makes these moderately high TTLs
acceptable is the cache coherency mechanisms built in to the
Multicast DNS protocol, which protect against stale data persisting
for too long. When a service shuts down gracefully, it sends goodbye
packets to remove its service browsing PTR record(s) immediately from
neighboring caches. If a service shuts down abruptly without sending
goodbye packets, the Passive Observation Of Failures (POOF) mechanism
described in Section 10.5 of the Multicast DNS specification
[RFC6762] comes into play to purge the cache of stale data.
A traditional Unicast DNS client on a distant remote link does not
get to participate in these Multicast DNS cache coherency mechanisms
on the local link. For traditional Unicast DNS queries (those
received without using Long-Lived Queries (LLQ) [RFC8764] or DNS Push
Notification subscriptions [RFC8765]), the DNS TTLs reported in the
resulting Unicast DNS response MUST be capped to be no more than ten
seconds.
Similarly, for negative responses, the negative caching TTL indicated
in the SOA record [RFC2308] should also be ten seconds (see
Section 6.1).
This value of ten seconds is chosen based on user-experience
considerations.
For negative caching, suppose a user is attempting to access a remote
device (e.g., a printer), and they are unsuccessful because that
device is powered off. Suppose they then place a telephone call and
ask for the device to be powered on. We want the device to become
available to the user within a reasonable time period. It is
reasonable to expect it to take on the order of ten seconds for a
simple device with a simple embedded operating system to power on.
Once the device is powered on and has announced its presence on the
network via Multicast DNS, we would like it to take no more than a
further ten seconds for stale negative cache entries to expire from
Unicast DNS caches, making the device available to the user desiring
to access it.
Similar reasoning applies to capping positive TTLs at ten seconds.
In the event of a device moving location, getting a new DHCP address,
or other renumbering events, we would like the updated information to
be available to remote clients in a relatively timely fashion.
However, network administrators should be aware that many recursive
resolvers by default are configured to impose a minimum TTL of 30
seconds. If stale data appears to be persisting in the network to
the extent that it adversely impacts user experience, network
administrators are advised to check the configuration of their
recursive resolvers.
For received Unicast DNS queries that use LLQ [RFC8764] or DNS Push
Notifications [RFC8765], the Multicast DNS record's TTL SHOULD be
returned unmodified, because the notification channel exists to
inform the remote client as records come and go. For further details
about Long-Lived Queries and its newer replacement, DNS Push
Notifications, see Section 5.6.
5.5.2. Suppressing Unusable Records
A Discovery Proxy SHOULD offer a configurable option, enabled by
default, to suppress Unicast DNS answers for records that are not
useful outside the local link. When the option to suppress unusable
records is enabled:
* For a Discovery Proxy that is serving only clients outside the
local link, DNS A and AAAA records for IPv4 link-local addresses
[RFC3927] and IPv6 link-local addresses [RFC4862] SHOULD be
suppressed.
* Similarly, for sites that have multiple private address realms
[RFC1918], in cases where the Discovery Proxy can determine that
the querying client is in a different address realm, private
addresses SHOULD NOT be communicated to that client.
* IPv6 Unique Local Addresses [RFC4193] SHOULD be suppressed in
cases where the Discovery Proxy can determine that the querying
client is in a different IPv6 address realm.
* By the same logic, DNS SRV records that reference target host
names that have no addresses usable by the requester should be
suppressed, and likewise, DNS-SD service browsing PTR records that
point to unusable SRV records should similarly be suppressed.
5.5.3. NSEC and NSEC3 Queries
Multicast DNS devices do not routinely announce their records on the
network. Generally, they remain silent until queried. This means
that the complete set of Multicast DNS records in use on a link can
only be discovered by active querying, not by passive listening.
Because of this, a Discovery Proxy can only know what names exist on
a link by issuing queries for them, and since it would be impractical
to issue queries for every possible name just to find out which names
exist and which do not, a Discovery Proxy cannot programmatically
generate the traditional Unicast DNS NSEC [RFC4034] and NSEC3
[RFC5155] records that assert the nonexistence of a large range of
names.
When queried for an NSEC or NSEC3 record type, the Discovery Proxy
issues a qtype "ANY" query using Multicast DNS on the local link and
then generates an NSEC or NSEC3 response with a Type Bit Map
signifying which record types do and do not exist for just the
specific name queried, and no other names.
Multicast DNS NSEC records received on the local link MUST NOT be
forwarded unmodified to a unicast querier, because there are slight
differences in the NSEC record data. In particular, Multicast DNS
NSEC records do not have the NSEC bit set in the Type Bit Map,
whereas conventional Unicast DNS NSEC records do have the NSEC bit
set.
5.5.4. No Text-Encoding Translation
A Discovery Proxy does no translation between text encodings.
Specifically, a Discovery Proxy does no translation between Punycode
encoding [RFC3492] and UTF-8 encoding [RFC3629], either in the owner
name of DNS records or anywhere in the RDATA of DNS records (such as
the RDATA of PTR records, SRV records, NS records, or other record
types like TXT, where it is ambiguous whether the RDATA may contain
DNS names). All bytes are treated as-is with no attempt at text-
encoding translation. A client implementing DNS-based Service
Discovery [RFC6763] will use UTF-8 encoding for its unicast DNS-based
Service Discovery queries, which the Discovery Proxy passes through
without any text-encoding translation to the Multicast DNS subsystem.
Responses from the Multicast DNS subsystem are similarly returned,
without any text-encoding translation, back to the requesting unicast
client.
5.5.5. Application-Specific Data Translation
There may be cases where Application-Specific Data Translation is
appropriate.
For example, AirPrint printers tend to advertise fairly verbose
information about their capabilities in their DNS-SD TXT record. TXT
record sizes in the range of 500-1000 bytes are not uncommon. This
information is a legacy from lineprinter (LPR) printing, because LPR
does not have in-band capability negotiation, so all of this
information is conveyed using the DNS-SD TXT record instead.
Internet Printing Protocol (IPP) printing does have in-band
capability negotiation, but for convenience, printers tend to include
the same capability information in their IPP DNS-SD TXT records as
well. For local Multicast DNS (mDNS) use, this extra TXT record
information is wasteful but not fatal. However, when a Discovery
Proxy aggregates data from multiple printers on a link, and sends it
via unicast (via UDP or TCP), this amount of unnecessary TXT record
information can result in large responses. A DNS reply over TCP
carrying information about 70 printers with an average of 700 bytes
per printer adds up to about 50 kilobytes of data. Therefore, a
Discovery Proxy that is aware of the specifics of an application-
layer protocol such as AirPrint (which uses IPP) can elide
unnecessary key/value pairs from the DNS-SD TXT record for better
network efficiency.
Also, the DNS-SD TXT record for many printers contains an "adminurl"
key (e.g., "adminurl=
http://printername.local/status.html"). For
this URL to be useful outside the local link, the embedded ".local"
host name needs to be translated to an appropriate name with larger
scope. It is easy to translate ".local" names when they appear in
well-defined places: as a record's owner name, or in domain name
fields in the RDATA of record types like PTR and SRV. In the
printing case, some application-specific knowledge about the
semantics of the "adminurl" key is needed for the Discovery Proxy to
know that it contains a name that needs to be translated. This is
somewhat analogous to the need for NAT gateways to contain ALGs
(Application-Level Gateways) to facilitate the correct translation of
protocols that embed addresses in unexpected places.
To avoid the need for application-specific knowledge about the
semantics of particular TXT record keys, protocol designers are
advised to avoid placing link-local names or link-local IP addresses
in TXT record keys if translation of those names or addresses would
be required for off-link operation. In the printing case, the
consequence of failing to translate the "adminurl" key correctly
would be that, when accessed from a different link, printing will
still work, but clicking the "Admin" user interface button will fail
to open the printer's administration page. Rather than duplicating
the host name from the service's SRV record in its "adminurl" key,
thereby having the same host name appear in two places, a better
design might have been to omit the host name from the "adminurl" key
and instead have the client implicitly substitute the target host
name from the service's SRV record in place of a missing host name in
the "adminurl" key. That way, the desired host name only appears
once and is in a well-defined place where software like the Discovery
Proxy is expecting to find it.
Note that this kind of Application-Specific Data Translation is
expected to be very rare; it is the exception rather than the rule.
This is an example of a common theme in computing. It is frequently
the case that it is wise to start with a clean, layered design with
clear boundaries. Then, in certain special cases, those layer
boundaries may be violated where the performance and efficiency
benefits outweigh the inelegance of the layer violation.
These layer violations are optional. They are done primarily for
efficiency reasons and generally should not be required for correct
operation. A Discovery Proxy MAY operate solely at the mDNS layer
without any knowledge of semantics at the DNS-SD layer or above.
5.6. Answer Aggregation
In a simple analysis, simply gathering multicast answers and
forwarding them in a unicast response seems adequate, but it raises
the question of how long the Discovery Proxy should wait to be sure
that it has received all the Multicast DNS answers it needs to form a
complete Unicast DNS response. If it waits too little time, then it
risks its Unicast DNS response being incomplete. If it waits too
long, then it creates a poor user experience at the client end. In
fact, there may be no time that is both short enough to produce a
good user experience and at the same time long enough to reliably
produce complete results.
Similarly, the Discovery Proxy (the authoritative name server for the
subdomain in question) needs to decide what DNS TTL to report for
these records. If the TTL is too long, then the recursive resolvers
issuing queries on behalf of their clients risk caching stale data
for too long. If the TTL is too short, then the amount of network
traffic will be more than necessary. In fact, there may be no TTL
that is both short enough to avoid undesirable stale data and, at the
same time, long enough to be efficient on the network.
Both these dilemmas are solved by the use of DNS Long-Lived Queries
(LLQ) [RFC8764] or its newer replacement, DNS Push Notifications
[RFC8765].
Clients supporting unicast DNS-based Service Discovery SHOULD
implement DNS Push Notifications [RFC8765] for improved user
experience.
Clients and Discovery Proxies MAY support both LLQ and DNS Push
Notifications, and when talking to a Discovery Proxy that supports
both, the client may use either protocol, as it chooses, though it is
expected that only DNS Push Notifications will continue to be
supported in the long run.
When a Discovery Proxy receives a query using LLQ or DNS Push
Notifications, it responds immediately using the Multicast DNS
records it already has in its cache (if any). This provides a good
client user experience by providing a near-instantaneous response.
Simultaneously, the Discovery Proxy issues a Multicast DNS query on
the local link to discover if there are any additional Multicast DNS
records it did not already know about. Should additional Multicast
DNS responses be received, these are then delivered to the client
using additional LLQ or DNS Push Notification update messages. The
timeliness of such update messages is limited only by the timeliness
of the device responding to the Multicast DNS query. If the
Multicast DNS device responds quickly, then the update message is
delivered quickly. If the Multicast DNS device responds slowly, then
the update message is delivered slowly. The benefit of using
multiple update messages to deliver results as they become available
is that the Discovery Proxy can respond promptly because it doesn't
have to deliver all the results in a single response that needs to be
delayed to allow for the expected worst-case delay for receiving all
the Multicast DNS responses.
With a proxy that supported only standard DNS queries, even if it
were to try to provide reliability by assuming an excessively
pessimistic worst-case time (thereby giving a very poor user
experience), there would still be the risk of a slow Multicast DNS
device taking even longer than that worst-case time (e.g., a device
that is not even powered on until ten seconds after the initial query
is received), resulting in incomplete responses. Using update
messages to deliver subsequent asynchronous replies solves this
dilemma: even very late responses are not lost; they are delivered in
subsequent update messages.
Note that while normal DNS queries are generally received via the
client's configured recursive resolver, LLQ and DNS Push Notification
subscriptions may be received directly from the client.
There are two factors that determine how unicast responses are
generated:
The first factor is whether or not the Discovery Proxy already has at
least one record in its cache that answers the question.
The second factor is whether the client used a normal DNS query, or
established a subscription using LLQ or DNS Push Notifications.
Normal DNS queries are typically used for one-shot operations like
SRV or address record queries. LLQ and DNS Push Notification
subscriptions are typically used for long-lived service browsing PTR
queries. Normal DNS queries and LLQ each have different response
timing depending on the cache state, yielding the first four cases
listed below. DNS Push Notifications, the newer protocol, has
uniform behavior regardless of cache state, yielding the fifth case
listed below.
* Standard DNS query; no answer in cache:
Issue an mDNS query on the local link, exactly as a local client
would issue an mDNS query, for the desired record name, type, and
class, including retransmissions, as appropriate, according to the
established mDNS retransmission schedule [RFC6762]. The Discovery
Proxy awaits Multicast DNS responses.
As soon as any Multicast DNS response packet is received that
contains one or more positive answers to that question (with or
without the Cache Flush bit [RFC6762] set) or a negative answer
(signified via a Multicast DNS NSEC record [RFC6762]), the
Discovery Proxy generates a Unicast DNS response message
containing the corresponding (filtered and translated) answers and
sends it to the remote client.
If after six seconds no relevant Multicast DNS answers have been
received, cancel the mDNS query and return a negative response to
the remote client. Six seconds is enough time for the underlying
Multicast DNS subsystem to transmit three mDNS queries and allow
some time for responses to arrive.
(Reasoning: Queries not using LLQ or Push Notifications are
generally queries that expect an answer from only one device, so
the first response is also the only response.)
DNS TTLs in responses MUST be capped to at most ten seconds.
* Standard DNS query; at least one answer in cache:
No local mDNS queries are performed.
The Discovery Proxy generates a Unicast DNS response message
containing the answer(s) from the cache right away, to minimize
delay.
(Reasoning: Queries not using LLQ or Push Notifications are
generally queries that expect an answer from only one device.
Given RRSet TTL harmonization, if the proxy has one Multicast DNS
answer in its cache, it can reasonably assume that it has the
whole set.)
DNS TTLs in responses MUST be capped to at most ten seconds.
* Long-Lived Query (LLQ); no answer in cache:
As in the case above with no answer in the cache, plan to perform
mDNS querying for six seconds, returning an LLQ response message
to the remote client as soon as any relevant mDNS response is
received.
If after six seconds no relevant mDNS answers have been received,
and the client has not cancelled its Long-Lived Query, return a
negative LLQ response message to the remote client.
(Reasoning: We don't need to rush to send an empty answer.)
Regardless of whether or not a relevant mDNS response is received
within six seconds, the Long-Lived Query remains active for as
long as the client maintains the LLQ state, and results in the
ongoing transmission of mDNS queries until the Long-Lived Query is
cancelled. If the set of mDNS answers changes, LLQ Event Response
messages are sent.
DNS TTLs in responses are returned unmodified.
* Long-Lived Query (LLQ); at least one answer in cache:
As in the case above with at least one answer in the cache, the
Discovery Proxy generates a unicast LLQ response message
containing the answer(s) from the cache right away, to minimize
delay.
The Long-Lived Query remains active for as long as the client
maintains the LLQ state, and results in the transmission of mDNS
queries (with appropriate Known Answer lists) to determine if
further answers are available. If the set of mDNS answers
changes, LLQ Event Response messages are sent.
(Reasoning: We want a user interface that is displayed very
rapidly yet continues to remain accurate even as the network
environment changes.)
DNS TTLs in responses are returned unmodified.
* Push Notification Subscription
The Discovery Proxy acknowledges the subscription request
immediately.
If one or more answers are already available in the cache, those
answers are then sent in an immediately following DNS PUSH
message.
The Push Notification subscription remains active until the client
cancels the subscription, and results in the transmission of mDNS
queries (with appropriate Known Answer lists) to determine if
further answers are available. If the set of mDNS answers
changes, further DNS PUSH messages are sent.
(Reasoning: We want a user interface that is displayed very
rapidly yet continues to remain accurate even as the network
environment changes.)
DNS TTLs in responses are returned unmodified.
Where the text above refers to returning "a negative response to the
remote client", it is describing returning a "no error no answer"
negative response, not NXDOMAIN. This is because the Discovery Proxy
cannot know all the Multicast DNS domain names that may exist on a
link at any given time, so any name with no answers may have child
names that do exist, making it an "empty non-terminal" name.
Note that certain aspects of the behavior described here do not have
to be implemented overtly by the Discovery Proxy; they occur
naturally as a result of using existing Multicast DNS APIs.
For example, in the first case above (standard DNS query and no
answers in the cache), if a new Multicast DNS query is requested
(either by a local client on the Discovery Proxy device, or by the
Discovery Proxy software on that device on behalf of a remote
client), and there is not already an identical Multicast DNS query
active and there are no matching answers already in the Multicast DNS
cache on the Discovery Proxy device, then this will cause a series of
Multicast DNS query packets to be issued with exponential backoff.
The exponential backoff sequence in some implementations starts at
one second and then doubles for each retransmission (0, 1, 3, 7
seconds, etc.), and in others, it starts at one second and then
triples for each retransmission (0, 1, 4, 13 seconds, etc.). In
either case, if no response has been received after six seconds, that
is long enough that the underlying Multicast DNS implementation will
have sent three query packets without receiving any response. At
that point, the Discovery Proxy cancels its Multicast DNS query (so
no further Multicast DNS query packets will be sent for this query)
and returns a negative response to the remote client via unicast.
The six-second delay is chosen to be long enough to give enough time
for devices to respond, yet short enough not to be too onerous for a
human user waiting for a response. For example, using the "dig" DNS
debugging tool, the current default settings result in it waiting a
total of 15 seconds for a reply (three transmissions of the DNS UDP
query packet, with a wait of 5 seconds after each packet), which is
ample time for it to have received a negative reply from a Discovery
Proxy after six seconds.
The text above states that for a standard DNS query, if at least one
answer is already available in the cache, then a Discovery Proxy
should not issue additional mDNS query packets. This also occurs
naturally as a result of using existing Multicast DNS APIs. If a new
Multicast DNS query is requested (either locally, or by the Discovery
Proxy on behalf of a remote client) for which there are relevant
answers already in the Multicast DNS cache on the Discovery Proxy
device, and after the answers are delivered the Multicast DNS query
is immediately cancelled, then no Multicast DNS query packets will be
generated for this query.
6. Administrative DNS Records
6.1. DNS SOA (Start of Authority) Record
The MNAME field SHOULD contain the host name of the Discovery Proxy
device (i.e., the same domain name as the RDATA of the NS record
delegating the relevant zone(s) to this Discovery Proxy device).
The RNAME field SHOULD contain the mailbox of the person responsible
for administering this Discovery Proxy device.
The SERIAL field MUST be zero.
Zone transfers are undefined for Discovery Proxy zones, and
consequently, the REFRESH, RETRY, and EXPIRE fields have no useful
meaning for Discovery Proxy zones. These fields SHOULD contain
reasonable default values. The RECOMMENDED values are: REFRESH 7200,
RETRY 3600, and EXPIRE 86400.
The MINIMUM field (used to control the lifetime of negative cache
entries) SHOULD contain the value 10. This value is chosen based on
user-experience considerations (see Section 5.5.1).
In the event that there are multiple Discovery Proxy devices on a
link for fault tolerance reasons, this will result in clients
receiving inconsistent SOA records (different MNAME and possibly
RNAME) depending on which Discovery Proxy answers their SOA query.
However, since clients generally have no reason to use the MNAME or
RNAME data, this is unlikely to cause any problems.
6.2. DNS NS Records
In the event that there are multiple Discovery Proxy devices on a
link for fault tolerance reasons, the parent zone MUST be configured
with NS records giving the names of all the Discovery Proxy devices
on the link.
Each Discovery Proxy device MUST be configured to answer NS queries
for the zone apex name by giving its own NS record, and the NS
records of its fellow Discovery Proxy devices on the same link, so
that it can return the correct answers for NS queries.
The target host name in the RDATA of an NS record MUST NOT reference
a name that falls within any zone delegated to a Discovery Proxy.
Apart from the zone apex name, all other host names (names of A and
AAAA DNS records) that fall within a zone delegated to a Discovery
Proxy correspond to local Multicast DNS host names, which logically
belong to the respective Multicast DNS hosts defending those names,
not the Discovery Proxy. Generally speaking, the Discovery Proxy
does not own or control the delegated zone; it is merely a conduit to
the corresponding ".local" namespace, which is controlled by the
Multicast DNS hosts on that link. If an NS record were to reference
a manually determined host name that falls within a delegated zone,
that manually determined host name may inadvertently conflict with a
corresponding ".local" host name that is owned and controlled by some
device on that link.
6.3. DNS Delegation Records
Since the Multicast DNS specification [RFC6762] states that there can
be no delegation (subdomains) within a ".local" namespace, this
implies that any name within a zone delegated to a Discovery Proxy
(except for the zone apex name itself) cannot have any answers for
any DNS queries for RRTYPEs SOA, NS, or DS. Consequently:
* for any query for the zone apex name of a zone delegated to a
Discovery Proxy, the Discovery Proxy MUST generate the appropriate
immediate answers as described above, and
* for any query for any name below the zone apex, for RRTYPEs SOA,
NS, or DS, the Discovery Proxy MUST generate an immediate negative
answer.
6.4. DNS SRV Records
There are certain special DNS records that logically fall within the
delegated Unicast DNS subdomain, but rather than mapping to their
corresponding ".local" namesakes, they actually contain metadata
pertaining to the operation of the delegated Unicast DNS subdomain
itself. They do not exist in the corresponding ".local" namespace of
the local link. For these queries, a Discovery Proxy MUST generate
immediate answers, whether positive or negative, to avoid delays
while clients wait for their query to be answered.
For example, if a Discovery Proxy implements Long-Lived Queries
[RFC8764], then it MUST positively respond to
"_dns-llq._udp.<zone> SRV" queries, "_dns-llq._tcp.<zone> SRV"
queries, and "_dns-llq-tls._tcp.<zone> SRV" queries as appropriate.
If it does not implement Long-Lived Queries, it MUST return an
immediate negative answer for those queries, instead of passing those
queries through to the local network as Multicast DNS queries and
then waiting unsuccessfully for answers that will not be forthcoming.
If a Discovery Proxy implements DNS Push Notifications [RFC8765],
then it MUST positively respond to "_dns-push-tls._tcp.<zone>"
queries. Otherwise, it MUST return an immediate negative answer for
those queries.
A Discovery Proxy MUST return an immediate negative answer for
"_dns-update._udp.<zone> SRV" queries, "_dns-update._tcp.<zone> SRV"
queries, and "_dns-update-tls._tcp.<zone> SRV" queries, since using
DNS Update [RFC2136] to change zones generated dynamically from local
Multicast DNS data is not possible.
6.5. Domain Enumeration Records
If the network operator chooses to use address-based unicast Domain
Enumeration queries for client configuration (see Section 5.2.1), and
the network operator also chooses to delegate the enclosing reverse
mapping subdomain to a Discovery Proxy, then that Discovery Proxy
becomes responsible for serving the answers to those address-based
unicast Domain Enumeration queries.
As with the SRV metadata records described above, a Discovery Proxy
configured with delegated reverse mapping subdomains is responsible
for generating immediate (positive or negative) answers for address-
based unicast Domain Enumeration queries, rather than passing them
though to the underlying Multicast DNS subsystem and then waiting
unsuccessfully for answers that will not be forthcoming.
7. DNSSEC Considerations
7.1. Online Signing Only
The Discovery Proxy acts as the authoritative name server for
designated subdomains, and if DNSSEC is to be used, the Discovery
Proxy needs to possess a copy of the signing keys in order to
generate authoritative signed data from the local Multicast DNS
responses it receives. Offline signing is not applicable to
Discovery Proxy.
7.2. NSEC and NSEC3 Records
In DNSSEC, NSEC and NSEC3 records are used to assert the nonexistence
of certain names, also described as "authenticated denial of
existence" [RFC4034] [RFC5155].
Since a Discovery Proxy only knows what names exist on the local link
by issuing queries for them, and since it would be impractical to
issue queries for every possible name just to find out which names
exist and which do not, a Discovery Proxy cannot programmatically
synthesize the traditional NSEC and NSEC3 records that assert the
nonexistence of a large range of names. Instead, when generating a
negative response, a Discovery Proxy programmatically synthesizes a
single NSEC record asserting the nonexistence of just the specific
name queried and no others. Since the Discovery Proxy has the zone
signing key, it can do this on demand. Since the NSEC record asserts
the nonexistence of only a single name, zone walking is not a
concern, and NSEC3 is therefore not necessary.
Note that this applies only to traditional immediate DNS queries,
which may return immediate negative answers when no immediate
positive answer is available. When used with a DNS Push Notification
subscription [RFC8765], there are no negative answers, merely the
absence of answers so far, which may change in the future if answers
become available.
8. IPv6 Considerations
An IPv4-only host and an IPv6-only host behave as "ships that pass in
the night". Even if they are on the same Ethernet [IEEE-3], neither
is aware of the other's traffic. For this reason, each link may have
_two_ unrelated ".local." zones: one for IPv4 and one for IPv6.
Since, for practical purposes, a group of IPv4-only hosts and a group
of IPv6-only hosts on the same Ethernet act as if they were on two
entirely separate Ethernet segments, it is unsurprising that their
use of the ".local." zone should occur exactly as it would if they
really were on two entirely separate Ethernet segments.
It will be desirable to have a mechanism to "stitch" together these
two unrelated ".local." zones so that they appear as one. Such a
mechanism will need to be able to differentiate between a dual-stack
(v4/v6) host participating in both ".local." zones, and two different
hosts: one IPv4-only and the other IPv6-only, which are both trying
to use the same name(s). Such a mechanism will be specified in a
future companion document.
At present, it is RECOMMENDED that a Discovery Proxy be configured
with a single domain name for both the IPv4 and IPv6 ".local." zones
on the local link, and when a unicast query is received, it should
issue Multicast DNS queries using both IPv4 and IPv6 on the local
link and then combine the results.
9. Security Considerations
9.1. Authenticity
A service proves its presence on a link by its ability to answer
link-local multicast queries on that link. If greater security is
desired, then the Discovery Proxy mechanism should not be used, and
something with stronger security should be used instead such as
authenticated secure DNS Update [RFC2136] [RFC3007].
9.2. Privacy
The Domain Name System is, generally speaking, a global public
database. Records that exist in the Domain Name System name
hierarchy can be queried by name from, in principle, anywhere in the
world. If services on a mobile device (like a laptop computer) are
made visible via the Discovery Proxy mechanism, then when those
services become visible in a domain such as "My House.example.com",
it might indicate to (potentially hostile) observers that the mobile
device is in the owner's home. When those services disappear from
"My House.example.com", that change could be used by observers to
infer when the mobile device (and possibly its owner) may have left
the house. The privacy of this information may be protected using
techniques like firewalls, split-view DNS, and Virtual Private
Networks (VPNs), as are customarily used today to protect the privacy
of corporate DNS information.
The privacy issue is particularly serious for the IPv4 and IPv6
reverse zones. If the public delegation of the reverse zones points
to the Discovery Proxy, and the Discovery Proxy is reachable
globally, then it could leak a significant amount of information.
Attackers could discover hosts that otherwise might not be easy to
identify, and learn their host names. Attackers could also discover
the existence of links where hosts frequently come and go.
The Discovery Proxy could provide sensitive records only to
authenticated users. This is a general DNS problem, not specific to
the Discovery Proxy. Work is underway in the IETF to tackle this
problem [RFC7626].
9.3. Denial of Service
A remote attacker could use a rapid series of unique Unicast DNS
queries to induce a Discovery Proxy to generate a rapid series of
corresponding Multicast DNS queries on one or more of its local
links. Multicast traffic is generally more expensive than unicast
traffic, especially on Wi-Fi links [MCAST], which makes this attack
particularly serious. To limit the damage that can be caused by such
attacks, a Discovery Proxy (or the underlying Multicast DNS subsystem
that it utilizes) MUST implement Multicast DNS query rate limiting
appropriate to the link technology in question. For today's
802.11b/g/n/ac Wi-Fi links (for which approximately 200 multicast
packets per second is sufficient to consume approximately 100% of the
wireless spectrum), a limit of 20 Multicast DNS query packets per
second is RECOMMENDED. On other link technologies like Gigabit
Ethernet, higher limits may be appropriate. A consequence of this
rate limiting is that a rogue remote client could issue an excessive
number of queries resulting in denial of service to other legitimate
remote clients attempting to use that Discovery Proxy. However, this
is preferable to a rogue remote client being able to inflict even
greater harm on the local network, which could impact the correct
operation of all local clients on that network.
10. IANA Considerations
This document has no IANA actions.
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<
https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <
https://www.rfc-editor.org/info/rfc1035>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <
https://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<
https://www.rfc-editor.org/info/rfc2119>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<
https://www.rfc-editor.org/info/rfc2308>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <
https://www.rfc-editor.org/info/rfc3629>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<
https://www.rfc-editor.org/info/rfc3927>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<
https://www.rfc-editor.org/info/rfc4034>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<
https://www.rfc-editor.org/info/rfc4862>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<
https://www.rfc-editor.org/info/rfc5155>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<
https://www.rfc-editor.org/info/rfc5198>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<
https://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<
https://www.rfc-editor.org/info/rfc6763>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <
https://www.rfc-editor.org/info/rfc8174>.
[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<
https://www.rfc-editor.org/info/rfc8490>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<
https://www.rfc-editor.org/info/rfc8765>.
11.2. Informative References
[DNS-UL] Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
Work in Progress, Internet-Draft, draft-sekar-dns-ul-02, 2
August 2018,
<
https://tools.ietf.org/html/draft-sekar-dns-ul-02>.
[IEEE-1Q] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Bridges and Bridged Networks", IEEE Std
802.1Q-2014, DOI 10.1109/IEEESTD.2014.6991462, 2014,
<
https://ieeexplore.ieee.org/document/6991462>.
[IEEE-3] IEEE, "IEEE Standard for Ethernet",
DOI 10.1109/IEEESTD.2018.8457469, IEEE Std 802.3-2018,
December 2008,
<
https://ieeexplore.ieee.org/document/8457469>.
[IEEE-5] IEEE, "Telecommunications and information exchange between
systems - Local and metropolitan area networks - Part 5:
Token ring access method and physical layer
specifications", IEEE Std 802.5-1998, 1998,
<
https://standards.ieee.org/standard/802_5-1998.html>.
[IEEE-11] IEEE, "Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", IEEE Std 802.11-2016,
December 2016,
<
https://standards.ieee.org/standard/802_11-2016.html>.
[MCAST] Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
Zuniga, "Multicast Considerations over IEEE 802 Wireless
Media", Work in Progress, Internet-Draft, draft-ietf-
mboned-ieee802-mcast-problems-11, 11 December 2019,
<
https://tools.ietf.org/html/draft-ietf-mboned-ieee802-
mcast-problems-11>.
[OHP] "ohybridproxy - an mDNS/DNS hybrid-proxy based on
mDNSResponder", commit 464d6c9, June 2017,
<
https://github.com/sbyx/ohybridproxy/>.
[REG-PROT] Cheshire, S. and T. Lemon, "Service Registration Protocol
for DNS-Based Service Discovery", Work in Progress,
Internet-Draft, draft-sctl-service-registration-02, 15
July 2018, <
https://tools.ietf.org/html/draft-sctl-
service-registration-02>.
[RELAY] Cheshire, S. and T. Lemon, "Multicast DNS Discovery
Relay", Work in Progress, Internet-Draft, draft-sctl-
dnssd-mdns-relay-04, 21 March 2018,
<
https://tools.ietf.org/html/draft-sctl-dnssd-mdns-relay-
04>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<
https://www.rfc-editor.org/info/rfc2132>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<
https://www.rfc-editor.org/info/rfc2136>.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<
https://www.rfc-editor.org/info/rfc3007>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<
https://www.rfc-editor.org/info/rfc3492>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<
https://www.rfc-editor.org/info/rfc4193>.
[RFC6760] Cheshire, S. and M. Krochmal, "Requirements for a Protocol
to Replace the AppleTalk Name Binding Protocol (NBP)",
RFC 6760, DOI 10.17487/RFC6760, February 2013,
<
https://www.rfc-editor.org/info/rfc6760>.
[RFC7558] Lynn, K., Cheshire, S., Blanchet, M., and D. Migault,
"Requirements for Scalable DNS-Based Service Discovery
(DNS-SD) / Multicast DNS (mDNS) Extensions", RFC 7558,
DOI 10.17487/RFC7558, July 2015,
<
https://www.rfc-editor.org/info/rfc7558>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<
https://www.rfc-editor.org/info/rfc7626>.
[RFC7788] Stenberg, M., Barth, S., and P. Pfister, "Home Networking
Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
2016, <
https://www.rfc-editor.org/info/rfc7788>.
[RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<
https://www.rfc-editor.org/info/rfc8375>.
[RFC8764] Cheshire, S. and M. Krochmal, "Apple's DNS Long-Lived
Queries Protocol", RFC 8764, DOI 10.17487/RFC8764, June
2020, <
https://www.rfc-editor.org/info/rfc8764>.
[ROADMAP] Cheshire, S., "Service Discovery Road Map", Work in
Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
23 October 2018, <
https://tools.ietf.org/html/draft-
cheshire-dnssd-roadmap-03>.
[ZC] Cheshire, S. and D.H. Steinberg, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc.,
ISBN 0-596-10100-7, December 2005.
Appendix A. Implementation Status
Some aspects of the mechanism specified in this document already
exist in deployed software. Some aspects are new. This section
outlines which aspects already exist and which are new.
A.1. Already Implemented and Deployed
Domain enumeration by the client ("b._dns-sd._udp.<zone>" queries) is
already implemented and deployed.
Performing unicast queries to the indicated discovery domain is
already implemented and deployed.
These are implemented and deployed in Mac OS X 10.4 Tiger and later
(including all versions of Apple iOS, on all models of iPhones,
iPads, Apple TVs and HomePods), in Bonjour for Windows, and in
Android 4.1 "Jelly Bean" (API Level 16) and later.
Domain enumeration and unicast querying have been used for several
years at IETF meetings to make terminal room printers discoverable
from outside the terminal room. When an IETF attendee presses
"Cmd-P" on a Mac, or selects AirPrint on an iPad or iPhone, and the
terminal room printers appear, it is because the client is sending
Unicast DNS queries to the IETF DNS servers. A walk-through giving
the details of this particular specific example is given in
Appendix A of the Roadmap document [ROADMAP].
The Long-Lived Query mechanism [RFC8764] referred to in this
specification exists and is deployed but was not standardized by the
IETF. The IETF has developed a superior Long-Lived Query mechanism
called DNS Push Notifications [RFC8765], which is built on DNS
Stateful Operations [RFC8490]. DNS Push Notifications is implemented
and deployed in Mac OS X 10.15 and later, and iOS 13 and later.
A.2. Already Implemented
A minimal portable Discovery Proxy implementation has been produced
by Markus Stenberg and Steven Barth, which runs on OS X and several
Linux variants including OpenWrt [OHP]. It was demonstrated at the
Berlin IETF in July 2013.
Tom Pusateri has an implementation that runs on any Unix/Linux
system. It has a RESTful interface for management and an
experimental demo command-line interface (CLI) and web interface.
Ted Lemon also has produced a portable implementation of Discovery
Proxy, which is available in the mDNSResponder open source code.
A.3. Partially Implemented
At the time of writing, existing APIs make multiple domains visible
to client software, but most client user interfaces lump all
discovered services into a single flat list. This is largely a
chicken-and-egg problem. Application writers were naturally
reluctant to spend time writing domain-aware user interface code when
few customers would benefit from it. If Discovery Proxy deployment
becomes common, then application writers will have a reason to
provide a better user experience. Existing applications will work
with the Discovery Proxy but will show all services in a single flat
list. Applications with improved user interfaces will show services
grouped by domain.
Acknowledgments
Thanks to Markus Stenberg for helping develop the policy regarding
the four styles of unicast response according to what data is
immediately available in the cache. Thanks to Anders Brandt, Ben
Campbell, Tim Chown, Alissa Cooper, Spencer Dawkins, Ralph Droms,
Joel Halpern, Ray Hunter, Joel Jaeggli, Warren Kumari, Ted Lemon,
Alexey Melnikov, Kathleen Moriarty, Tom Pusateri, Eric Rescorla, Adam
Roach, David Schinazi, Markus Stenberg, Dave Thaler, and Andrew
Yourtchenko for their comments.
Author's Address
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email:
[email protected]
