Internet DRAFT - draft-pusateri-dnssd-update-proxy
draft-pusateri-dnssd-update-proxy
DNSSD Working Group T. Pusateri
Internet-Draft Unaffiliated
Intended status: Standards Track March 6, 2019
Expires: September 7, 2019
DNS Update Proxy for Service Discovery
draft-pusateri-dnssd-update-proxy-01
Abstract
This document describes a method to dynamically map multicast DNS
announcements into the unicast DNS namespace for use by service
discovery clients. It does not define any new protocols but uses
existing DNS protocols in new ways. This solves existing problems
with service discovery across multiple IP subnets in a simple, yet
efficient, manner.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 7, 2019.
Copyright Notice
Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. DNS Subdomain Model . . . . . . . . . . . . . . . . . . . . . 3
3.1. Subdomain Naming . . . . . . . . . . . . . . . . . . . . 4
3.2. Domain Name Discovery . . . . . . . . . . . . . . . . . . 5
3.3. Client Service Discovery . . . . . . . . . . . . . . . . 5
4. Update Proxy Behavior . . . . . . . . . . . . . . . . . . . . 6
4.1. mDNS Service Announcements . . . . . . . . . . . . . . . 6
4.2. Service Caching and Refresh . . . . . . . . . . . . . . . 6
4.3. mDNS Probing . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Link-local Addressing . . . . . . . . . . . . . . . . . . 8
4.5. IPv6 and IPv4 on Same Link . . . . . . . . . . . . . . . 8
4.6. Multiple Logical IP Subnets . . . . . . . . . . . . . . . 8
4.7. Proxy Redundancy . . . . . . . . . . . . . . . . . . . . 9
4.8. Service Filtering and Translation . . . . . . . . . . . . 9
5. DNS UPDATE . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Selection of Authoritative Unicast DNS Server . . . . . . 10
5.2. DNS UPDATE Sections . . . . . . . . . . . . . . . . . . . 10
5.2.1. Zone Section . . . . . . . . . . . . . . . . . . . . 10
5.2.2. Prerequisite Section . . . . . . . . . . . . . . . . 11
5.2.3. Update Section . . . . . . . . . . . . . . . . . . . 11
5.2.4. Additional Data Section . . . . . . . . . . . . . . . 11
6. DNS Authoritative Server Behavior . . . . . . . . . . . . . . 12
6.1. DNS Push Notifications . . . . . . . . . . . . . . . . . 12
6.2. DNSSEC Compatibility . . . . . . . . . . . . . . . . . . 12
6.3. DNS Update Record Lifetimes . . . . . . . . . . . . . . . 12
7. Transitioning to Unicast . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Comparison to Discovery Proxy . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Multicast DNS is used today for link-local service discovery. While
this has worked reasonably well on the local link, current deployment
reveals two problems. First, mDNS wasn't designed to traverse across
multi-subnet campus networks. Second, IP multicast doesn't work
across all link types and can be problematic on 802.11 Wifi networks.
Therefore, a solution is desired to contain legacy multicast DNS
service discovery and transition to a unicast DNS service discovery
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model. By mapping the current mDNS discovered services into regular
authoritative unicast DNS servers, clients from any IP subnet can
make unicast queries through normal unicast DNS resolvers.
There are many ways to map services discovered using multicast DNS
into the unicast namespace. This document describes a way to do the
mapping using a proxy that sends DNS UPDATE messages [RFC2136]
directly to an authoritative unicast DNS server. While it is
possible for each host providing a service to send it's own DNS
UPDATE, key management has prevented widespread deployment of DNS
UPDATEs across a domain. By having a limited number of proxies
sitting on one or more IP subnets, it is possible to provide secure
DNS updates at a manageable scale. Future work to automate secure
DNS UPDATEs on a larger scale is needed.
This document will explain how services on each .local domain will be
mapped into the unicast DNS namespace and how unicast clients will
discover these services. It is important to note that no changes are
required in either the clients, DNS authoritative servers, or DNS
resolver infrastructure. In addition, while the Update proxy is a
new logical concept, it requires no new protocols to be defined and
can be built using existing DNS libraries.
An Update proxy is an ideal service to run on routers and/or switches
to map local services into a larger network infrastructure.
2. Requirements Language
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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
3. DNS Subdomain Model
Each .local domain which logically maps to an IP subnet is modeled as
a separate subdomain in the unicast DNS hierarchy. Each of these
subdomains must be browsable (respond to PTR queries for "b._dns-
sd._udp.<subdomain>."). See Section 11 of [RFC6763] for more details
about browsing. In the context of the Update proxy, these subdomains
are typically special use subdomains for mDNS mappings.
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3.1. Subdomain Naming
The browseable subdomain label is prepended to the domain name and
separated by a period. See [RFC7719] for more information on
subdomains and labels. It is not important that the label be human
readable or have organizational significance. End users will not be
interacting with these labels. The main requirement is that they be
unique within the domain for each IP subnet. Subdomain labels can be
obtained by the proxy in several ways. The following methods should
be attempted in order to assure consistency among redundant proxies:
1. address-derived domain enumeration through local resolver
The proxy issues a PTR query for the registration or browse
domains based on the IP subnet. Separate queries are performed
for IPv4 and IPv6 on the same link since they are different IP
subnets. Since the Update proxy will be registering services
with DNS UPDATE, it should begin querying for registration
domains and fallback to browse domains if no registration domains
are configured.
As an example, suppose a proxy was connected to IPv4 subnet
"203.0.113.0/24". In order to determine if there was a subdomain
name for this subnet, the base domain name to query would be
derived as "0.113.0.203.in-addr.arpa." The proxy would issue a
PTR query for the following names in order to find the subdomain
for the IP subnet:
"dr._dns-sd._udp.0.113.0.203.in-addr.arpa."
"r._dns-sd._udp.0.113.0.203.in-addr.arpa."
"db._dns-sd._udp.0.113.0.203.in-addr.arpa."
"b._dns-sd._udp.0.113.0.203.in-addr.arpa."
"lb._dns-sd._udp.0.113.0.203.in-addr.arpa."
The first response with an answer should be the subdomain name
including the domain name for the network and further queries
through this list are not needed. If multiple answers are
returned in the same response, any one of the answers can be used
but the proxy should only use a single subdomain name for the IP
subnet.
The Update proxy should periodically rediscover the subdomain
name at approximately 5 minute intervals for each IP subnet
adding appropriate random jitter across IP subnets so as to
prevent synchronization.
2. proxy local configuration override
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If no answer is returned, the proxy may have local configuration
containing a subdomain name for the network. If so, this
subdomain should be used.
3. algorithmic subdomain label generation
If no local configuration is present for the IP subnet, the proxy
may generate a unique label and use that for the subdomain by
appending a common domain name. One such algorithm is to take
the network form of an IPv4 subnet without a prefix length (host
portion all zeros) and convert it to a hexadecimal string. This
will give a 8 character unique string to use as a subdomain
label. For the example above, this label would be "cb007100".
In the cases where through either local configuration or algorithmic
generation of subdomain names, a subdomain name is known by the proxy
but cannot be address derived through a PTR query, the Update proxy
SHOULD register the appropriate address-derived enumeration records
through an UPDATE to the zone master. If the IP subnet is removed or
becomes inactive on the Update proxy, the proxy MUST attempt to
remove these records.
3.2. Domain Name Discovery
The base domain name to use for each subdomain also has to be
discovered on a per IP subnet basis. In most cases, the domain name
will be the same for all IP subnets because they are all contained in
a single administrative domain. However, this is not required and a
proxy administrator may need to span multiple administrative
boundaries requiring different domain names on different IP subnets
(and therefore, subdomains).
There is not a direct query to discover a separate domain name but
the domain name is included with the subdomain in the response to the
PTR query above in Section 3.1. If the PTR query returns an empty
response, then the domain name can be obtained from local proxy
configuration and if no domain name is specified there, the default
domain for the host should be used.
3.3. Client Service Discovery
Fortunately, clients performing service discovery require no changes
in order to work with the Update proxy. Existing clients already
support wide-area bonjour which specifies how to query search domains
and subdomains for services. See section 11 of [RFC6763].
However, in order for clients to discover the subdomain for each IP
subnet, the subdomain MUST be browseable and a browse record for the
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domain must enumerate all of the subdomains. If the domain records
do not exist, the Update proxy MUST create them in the domain and
MUST ensure each subdomain is browseable.
In the future, authoritative unicast DNS servers may add support for
DNS Push Notifications [I-D.ietf-dnssd-push] which would allow
clients to maintain long lived subscriptions to services. Clients
may also wish to add support for this feature to provide an efficient
alternative to polling.
4. Update Proxy Behavior
Since no new protocols are defined, this document mostly describes
the expected behavior of the Update proxy and how it uses existing
protocols to achieve multi IP subnet service discovery. The behavior
is mostly intuitive but is described to ensure compatibility and
completeness.
4.1. mDNS Service Announcements
The Update proxy should listen to mDNS service announcements
(responses) on all interfaces it is proxying for. Multiple Update
proxies can be active on the same IP subnet at the same time. See
[RFC6762] for more information on multicast DNS.
4.2. Service Caching and Refresh
As specified in Section 8.3 of [RFC6762], service announcements are
sent multiple times for redundancy. However, there is no need to
send duplicate UPDATE messages to the authoritative unicast DNS
server. Therefore, the Update proxy should cache service
announcements and only send DNS UPDATE messages when needed.
As described in Section 8.4 of [RFC6762], a host may send "goodbye"
announcements by setting the TTL to 0. In this case, the record MUST
be removed from the cache or otherwise marked as expired and a DNS
UPDATE should be sent to the authoritative unicast DNS server
removing the record.
The Update proxy MUST also remove/expire old cache entries and remove
the records from the authoritative unicast DNS server when the cache-
flush bit is set on new announcements as described in Section 10.2 of
[RFC6762].
A host providing a service may automatically refresh the TTL in the
announcement from time to time keeping the service valid based on
subsequent multicast queries it receives. However, if no mDNS
clients are requesting the particular service for the length of the
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TTL value, the service announcement could timeout naturally. In
order to keep accurate information regarding all of the services on
the IP subnet, the Update proxy SHOULD send a unicast PTR query for
the service name directly to the host announcing the service. This
query should be sent at a random time between 5 and 10 seconds before
the TTL value indicates the announcement will expire.
As described in Section 11 of [RFC6762], the Update proxy should use
an IP source address of the IP subnet of the interface it is
transmitting over and that is on the same IP subnet as the service
provider. It is also permissible to use a link-local IP address in
the IPv6 case as long as the service itself is available on an IPv6
address that is reachable from outside the local link.
In order for the Update proxy to discover as many services available
on each IP subnet as possible, it should periodically send a PTR
multicast query for "_services._dns-sd._udp.local." on each subnet.
The unicast response bit SHOULD be set in the query in order to force
unicast responses to the Update proxy. As PTR responses are
received, The Update proxy can then send Service Instance Enumeration
PTR queries (also with the unicast response bit set) for each
service.
This was not the intended behavior of mDNS since local clients would
just ask dynamically when they needed to know all of the providers of
a service name but keeping this information up to date in the
authoritative server provides benefits to remote clients such as
faster response times and ability to use DNSSEC validation that were
not previously possible with multicast DNS. These benefits are
provided at the additional cost of a slight increase in network
activity and processing time by the hosts announcing services.
However, if the Update proxy uses unicast to query the service
providers directly, other clients are not affected by these refresh
queries and do not have to turn their radios on for queries/responses
that they have no interest in.
4.3. mDNS Probing
While Section 8.2 of [RFC6762] recommends all potential answers be
included in mDNS probe queries, because these records haven't gone
through conflict resolution, they should not be regarded as
announcements of services. Therefore, an Update proxy MUST NOT rely
on information in any section of DNS query messages.
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4.4. Link-local Addressing
In the IPv6 case, the source address of the announcements is a link-
local IPv6 address that will probably be different than the IP subnet
that the service is being provided on. However, it is certainly
possible that link-local addressing is used with IPv4 as well. This
is not as common but exists in a zero-conf environment where no IPv4
addresses are assigned via DHCP or statically and the hosts revert to
link-local IPv4 addresses ("169.254/16"), see [RFC3927].
If the service SRV target resolves to only a link-local address, then
the service is not eligible to be advertised outside of the link and
shouldn't be sent to the authoritative unicast DNS server by the
Update proxy.
In general, the Update proxy needs to ensure that the service is
reachable outside of the link it is announced on before sending an
UPDATE to the authoritative server for the subdomain.
4.5. IPv6 and IPv4 on Same Link
Announced services may be available on IPv4, IPv6, or both on the
same link. If both IPv4 A records [RFC1035] and IPv6 AAAA records
[RFC3596] are published for an SRV target [RFC2782] name, the
administrator should provide the service over both protocols.
In some cases, this won't be possible. This will not incur any extra
delays if clients attempt connections over both IPv4 and IPv6
protocols simultaneously but if one protocol is preferred over
another, delays may occur.
4.6. Multiple Logical IP Subnets
Multiple IP subnets on the same link is just a more general case of
IPv4 and IPv6 on the same link. When multiple IP subnets exist for
the same protocol on the same link, they appear as separate
interfaces to the Update proxy and require a separate subdomain name
just as IPv4 and IPv6 do.
This is required for a client on one logical IP subnet of an
interface to communicate with a service provided by a host on a
different IP subnet of the same link.
If a SRV target resolves to addresses on multiple logical IP subnets
of the same interface, the service can be included in multiple
subdomains on the appropriate server(s) for those subdomains.
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4.7. Proxy Redundancy
Providing redundant Update proxies for the same IP subnet can be
easily achieved by virtue of the DNS UPDATE protocol. None of the
redundant proxies needs to be aware of any of the other redundant
proxies on an IP subnet.
Alternatives for ways to format DNS UPDATE messages are defined below
in Section 5.2.2 as to possible uses of the Prerequisite section for
use with redundant Update proxies.
Alternatively, a proxy MAY choose to hibernate when it discovers
another active Update proxy as described below in Section 7.
4.8. Service Filtering and Translation
In the process of registering services with an authoritative unicast
DNS server, the proxy can perform filtering and translation on the
dynamically discovered services.
As an example, suppose legacy printers are discovered that do not
support the current AirPrint feature set. The proxy can alter the
TXT record associated with the printer to add the necessary keys as
well as any additional service records to allow AirPrint clients to
discover and use the legacy printer.
As another example, suppose there is a printer that is behind a
locked door where students do not have access. In this case, the
printer's resource records MAY be filtered by the proxy so it does
not show up during a browse operation on the subnet.
An Update proxy could have rulesets that define the translations it
performs on the fly as is learns about matching services.
5. DNS UPDATE
While DNS UPDATE is well supported in authoritative DNS servers, it
typically requires some form of authentication for the server to
accept the update. The most common form is TSIG [RFC2845],[RFC4635]
which is based on a shared secret and a one way hash over the
contents of the record.
The Update proxy doesn't dictate a method of privacy or
authentication for communication to an authoritative DNS UPDATE
server. However, implementations SHOULD ensure some form of
authentication exists and even refuse to operate in an environment
without authentication.
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5.1. Selection of Authoritative Unicast DNS Server
The Update proxy should attempt to locate the authoritative DNS
UPDATE server for each subdomain in the following manner:
1. An Update proxy should first send an SRV query for "_dns-update-
tls._tcp.<subdomain>." If an answer is received, the target and
port number will provide the parameters needed for where to send
updates. The proxy can also try the TCP and UDP variants of this
service name "_dns-update._tcp" and "_dns-update._udp" if the TLS
variant does not exist. If no TLS variant is found, the proxy
can still attempt a TLS connection on the SRV port of the TCP or
UDP variant. The proxy can also attempt to connect to the target
on the reserved port (853) for DNS over TLS as defined in
Section 3.1 of [RFC7858].
2. The Update proxy can make a similar query for the same service in
the domain if a subdomain specific answer isn't returned: "_dns-
update-tls._tcp.<domain>." as well as the TCP and UDP variants.
3. If no matching SRV records are returned, the Update proxy SHOULD
consult local configuration policy to see if an DNS UPDATE server
has been configured.
4. If no local configuration exists for a DNS UPDATE server, the
Update proxy can query the SOA records for the subdomain and try
sending updates to the MNAME master server configured for the
subdomain. Again, using TLS/TCP is encouraged if available.
5. If DNS UPDATEs are not accepted by the server(s) represented by
the SOA MNAME master server, then the Update proxy can assume
that DNS UPDATEs are not available for the subdomain and
listening to mDNS announcements on the IP subnet would be
unproductive.
5.2. DNS UPDATE Sections
A DNS UPDATE message contains four sections as specified in
[RFC2136].
5.2.1. Zone Section
When an Update proxy is adding or removing services to/from a
subdomain, the zone section MUST contain a single zone (ZOCOUNT = 1)
and the ZNAME MUST be the subdomain being updated. ZTYPE MUST be SOA
and ZCLASS MUST be the same class as the records being added/removed.
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DNS UPDATEs to multiple subdomains MUST be performed in separate DNS
UPDATE messages with one subdomain per message.
If a new subdomain is being created for a domain by the Update proxy,
the subdomain's parent zone should be used for the ZNAME. ZTYPE MUST
be SOA and ZCLASS MUST be the same class as the subdomain's NS record
CLASS that is going to be added. Similarly for removing a subdomain.
5.2.2. Prerequisite Section
It is not necessary for the Update proxy to include any prerequisites
when adding/removing records. However, if the Update proxy wants to
have better error handling, it can add prerequisites to ensure the
state of the authoritative server is consistent.
Given that multiple Update proxies may exist for the same IP subnet
(and subdomain), it is possible that similar records may be added or
deleted to/from the authoritative server before the Update proxy's
own messages are processed. This is not to be considered a fatal
error and may happen during normal operation of redundant proxies.
The use of prerequisite can be used to identify these cases if
desired.
5.2.3. Update Section
The Update section contains all of the records that the proxy wants
to be added/removed in a single subdomain. If TIMEOUT resource
records are being manually added to the authoritative server, they
MUST be included as regular resource records in the Update section.
See Section 6.3 below for more information.
5.2.4. Additional Data Section
The Update proxy may include additional data as needed. Instances
where additional data might be included are:
1. When creating a subdomain by adding new NS records to a domain, A
or AAAA glue records MAY be needed. Though, in most cases, the
same authoritative server name / IP addresses should be used as
in the parent domain.
2. If including a lease lifetime as discussed below in Section 6.3,
the OPT recording containing the Update lease will be sent in the
additional data section.
3. The TSIG cryptographic signature of the DNS UPDATE message should
be the last resource record in the additional data section.
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6. DNS Authoritative Server Behavior
The Update proxy will rely on the authoritative server to update the
SERIAL number for the zone after each update is completed.
6.1. DNS Push Notifications
An authoritative unicast DNS server MAY support DNS Push
notifications [I-D.ietf-dnssd-push] for client queries in order to
provide more timely and more efficient responses. While this is
outside of the scope of the Update proxy, it is mentioned here for
completeness.
6.2. DNSSEC Compatibility
With mDNS, the next domain name field in an NSEC record could not
reference the next record in the zone because it was not possible to
know all of the records in the zone a priori. By mapping all known
records into a unicast subdomain, the NSEC next domain name field can
contain the next known record as defined. As new services are
discovered and UPDATEd in the authoritative unicast DNS server, the
NSEC records can be kept up to date by the authoritative server.
The Update proxy will assume that DNS updates sent to zones with
DNSSEC enabled will be updated as needed as specified in [RFC3007].
6.3. DNS Update Record Lifetimes
When the Update proxy sends an DNS UPDATE message to an authoritative
unicast DNS server, it MAY include a lease lifetime to indicate how
long the UPDATE server should keep the resource records active in the
zone. This is different from the TTL which tells resolvers how long
to keep the records in their cache. Lease lifetimes may be based on
different origin data. For example, when an IP address is assigned
to a host via DHCP, the DHCP server will provide a time period for
which the address is assigned to the host.
There are several possibilities for how a DNS UPDATE server may limit
the lifetime of records added via an update message.
1. The DNS update server MAY be configured to automatically delete
the records after a certain fixed time period (such as 24 hours).
This is a failsafe mechanism in case the origin of the record
data goes offline and does not ever try to remove the records.
2. A lease lifetime can be communicated via an OPT record as defined
in Dynamic DNS Update Leases [I-D.sekar-dns-ul]. This provides a
timeout period for all of the records added in the update message
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and is controlled by the sender of the update. This is a work in
progress and does not yet have widespread adoption among
authoritative unicast DNS server software.
3. Individual DNS TIMEOUT resource records
[I-D.pusateri-dnsop-update-timeout] can be added to the update
message to indicate the timeout value for one or any number of
the resource records contained in the update message. This is
the most flexible but also does not have any adoption among
authoritative unicast DNS server software. One advantage of the
TIMEOUT resource records is that they are stored in the
authoritative server like any other record and synchronized to
secondary servers as well. Therefore, if the primary server were
to restart or experience an extended failure, the lease lifetime
would not be lost.
Note that it is possible to use both the Dynamic DNS Update leases to
communicate the lease lifetime and for the authoritative unicast DNS
server to create TIMEOUT resource records on demand to achieve the
same result if the Update proxy does not include TIMEOUT resource
records natively.
7. Transitioning to Unicast
The design of the Update proxy is intended to work within the
existing mDNS model and allow it to scale to multiple subnets using
existing unicast DNS infrastructure. It is careful not to increase
the amount of multicast traffic used for service discovery but it is
also capable of providing a transition path to actually reduce
multicast usage incrementally. Hosts have generally not been able to
directly register their own services through DNS UPDATE because of
the security scaling problem of shared passwords needed by TSIG.
By creating trusted infrastructure in the form of an Update proxy,
services can now be registered through the Update proxy directly via
unicast. This alleviates the need to advertise over mDNS at all.
Therefore, if we allow service providers to discover the Update proxy
using either unicast or multicast queries, we can then perform all
subsequent communication over unicast. This greatly reduces the
multicast usage and the need for every host on the network to wake up
and listen to multicast responses and periodic announcements.
In order to accomodate this transition, the Update proxy MAY announce
and respond to PTR queries for the update proxy service using service
name "_dns-updateproxy-tls._tcp.local." pointing to a local instance
name. As host implementations are updated to locate an Update proxy,
they can switch entirely to unicast for all of the services they
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provide. The SRV record for the instance name can provide a target
name and port for which the service provider can connect directly
using TLS and send unicast registrations for all of its services.
The TXT record for the instance name can contain vendor specific
information associated with the Update proxy if desired. There are
no required keys in the TXT record and while the record MUST exist
(see Section 6 of [RFC6763]), it can consist of a single zero length
byte. Multiple announcements can be sent across a single TLS
connection. There are no responses expected from the Update proxy
after sending the announcements. However, Update proxies MUST only
announce this service if they intend to provide a unicast interface
for service providers on the local subnet.
Discovery of the Update proxy can also be entirely using unicast for
service providers that prefer or are not multicast capable. The
service provider would use address-derived domain enumeration as
described in Section 3.1 to determine the subdomain name of the local
link and then perform a normal unicast PTR query through the local
resolver for "_dns-updateproxy-tls._tcp.<subdomain>." to find the
Update proxy instance. In the case of redundant proxies, multiple
PTR records with different instance names may exist. The SRV records
priority and weight fields can be used to determine the preferred
Update proxy.
Once these services are initially registered with the Update proxy,
the proxy may occasionally sent unicast queries to confirm these
services are still active. A 60 second interval with appropriate
jitter is recommended for confirmation. If, in the meantime, the
service is discontinued, the service provider SHOULD reconnect to the
Update proxy and send "goodbye" announcements with TTL 0 as normally
would be done with mDNS to expire the services as described in
Section 8.4 of [RFC6762]. The Update proxy will not send any
response to these "goodbye" announcements, therefore, the connection
MUST be closed gracefully to ensure proper delivery.
8. Security Considerations
When a secure DNS UPDATE is sent to an authoritative server, it
should not be construed that this information is any more reliable
than the original mDNS announcement was for which it was based. Care
should always be taken when receiving mDNS announcements to ensure
they are source IP address is one that belongs to an IP subnet on the
received interface of the Update proxy. In addition, the TTL of the
received link local announcement MUST be 1 to ensure it was not
forwarded from a remote network.
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Each Update proxy requires configuration of a shared secret for
creation of the TSIG signature resource record contained as the last
record in the UPDATE message.
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9. References
9.1. Normative References
[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>.
[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>.
[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>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<https://www.rfc-editor.org/info/rfc2845>.
[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>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.
[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>.
[RFC4635] Eastlake 3rd, D., "HMAC SHA (Hashed Message Authentication
Code, Secure Hash Algorithm) TSIG Algorithm Identifiers",
RFC 4635, DOI 10.17487/RFC4635, August 2006,
<https://www.rfc-editor.org/info/rfc4635>.
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[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>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[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>.
9.2. Informative References
[I-D.ietf-dnssd-hybrid]
Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
progress), March 2018.
[I-D.ietf-dnssd-mdns-relay]
Lemon, T. and S. Cheshire, "Multicast DNS Discovery
Relay", draft-ietf-dnssd-mdns-relay-01 (work in progress),
July 2018.
[I-D.ietf-dnssd-push]
Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-16 (work in progress), November
2018.
[I-D.pusateri-dnsop-update-timeout]
Pusateri, T. and T. Wattenberg, "DNS TIMEOUT Resource
Record", draft-pusateri-dnsop-update-timeout-02 (work in
progress), March 2019.
[I-D.sekar-dns-ul]
Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
draft-sekar-dns-ul-02 (work in progress), August 2018.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.
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Appendix A. Comparison to Discovery Proxy
The Update Proxy defined in this document is an alternative to the
Discovery Proxy [I-D.ietf-dnssd-hybrid] and the Discovery Relay
[I-D.ietf-dnssd-mdns-relay]. This solution makes different trade-
offs than the ones made by the Discovery Proxy which offer some
advantages at a cost of increased state.
The main difference is that the Discovery Proxy builds the list of
matching services on demand by querying over mDNS and collecting the
announcements in response to client queries. Whereas the Update
proxy tries to build a complete list of services by listening for all
announcements, discovering and refreshing them, and then inserting
them into subdomains using DNS UPDATE.
The main advantages of the Update proxy include limiting further
propagation of IP multicast across the campus, providing a pathway to
eliminate multicast entirely, faster response time to client queries,
and the ability to provide DNSSEC signed security responses for
client queries.
While the Discovery Proxy increases multicast queries and responses
based on received unicast queries "O(n^2)", the Update proxy can
reduce or eliminate mDNS traffic on the local links "O(1)".
Another key difference is that the Update proxy never becomes an
authoritative unicast DNS server for the attached subdomain. It
simply updates the existing authoritative server for the domain.
Therefore, the administrator is free to use existing authoritative
DNS server infrastructure.
Author's Address
Tom Pusateri
Unaffiliated
Raleigh NC 27608
USA
Phone: +1 (919) 867-1330
Email: pusateri@bangj.com
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