Internet Engineering Task Force S. Cheshire
Internet-Draft Apple Inc.
Intended status: Informational T. Lemon
Expires: April 26, 2019 Nibbhaya Consulting
October 23, 2018
Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-00
Abstract
The Service Registration Protocol for DNS-Based Service Discovery
uses the standard DNS Update mechanism to enable DNS-Based Service
Discovery using only unicast packets. This eliminates the dependency
on Multicast DNS as the foundation layer, which greatly improves
scalability and improves performance on networks where multicast
service is not an optimal choice, particularly 802.11 (Wi-Fi) and
802.15.4 (IoT) networks. DNS-SD Service registration uses public
keys and SIG(0) to allow services to defend their registrations
against attack.
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 April 26, 2019.
Copyright Notice
Copyright (c) 2018 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
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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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Service Registration Protocol . . . . . . . . . . . . . . . . 4
2.1. What to publish . . . . . . . . . . . . . . . . . . . . . 5
2.2. Where to publish it . . . . . . . . . . . . . . . . . . . 6
2.3. How to publish it . . . . . . . . . . . . . . . . . . . . 6
2.3.1. How DNS-SD Service Registration differs from standard
RFC2136 DNS Update . . . . . . . . . . . . . . . . . 7
2.3.2. Testing using standard RFC2136-compliant servers . . 7
2.3.3. How to allow services to update standard
RFC2136-compliant servers . . . . . . . . . . . . . . 7
2.4. How to secure it . . . . . . . . . . . . . . . . . . . . 8
2.4.1. First-Come First-Served Naming . . . . . . . . . . . 8
2.4.2. SRP Server Behavior . . . . . . . . . . . . . . . . . 9
2.5. TTL Consistency . . . . . . . . . . . . . . . . . . . . . 12
2.6. Maintenance . . . . . . . . . . . . . . . . . . . . . . . 12
2.6.1. Cleaning up stale data . . . . . . . . . . . . . . . 12
2.6.2. Sleep Proxy . . . . . . . . . . . . . . . . . . . . . 13
3. Security Considerations . . . . . . . . . . . . . . . . . . . 14
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
5. Delegation of 'services.arpa.' . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
DNS-Based Service Discovery [RFC6763] is a component of Zero
Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].
This document describes an enhancement to DNS-Based Service Discovery
[RFC6763] that allows services to automatically register their
services using the DNS protocol rather than using Multicast DNS
[RFC6762] (mDNS). There is already a large installed base of DNS-SD
clients that can do service discovery using the DNS protocol. This
extension makes it much easier to take advantage of this existing
functionality.
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This document is intended for three audiences: implementors of
software that provides services that should be advertised using DNS-
SD, implementors of DNS servers that will be used in contexts where
DNS-SD registration is needed, and administrators of networks where
DNS-SD service is required. The document is intended to provide
sufficient information to allow interoperable implementation of the
registration protocol.
DNS-Based Service Discovery (DNS-SD) allows services to advertise the
fact that they provide service, and to provide the information
required to access that service. Clients can then discover the set
of services of a particular type that are available. They can then
select a service from among those that are available and obtain the
information required to use it.
The Service Registration Protocol for DNS-SD (SRP), described in this
document, provides a reasonably secure mechanism for publishing this
information. Once published, these services can be readily
discovered by clients using standard DNS lookups.
The DNS-SD specification [RFC6763], Section 10 ("Populating the DNS
with Information"), briefly discusses ways that services can publish
their information in the DNS namespace. In the case of mDNS, it
allows services to publish their information on the local link, using
names in the ".local" namespace, which makes their services directly
discoverable by peers attached to that same local link.
RFC6763 also allows clients to discover services using the DNS
protocol [RFC1035]. This can be done by having a system
administrator manually configure service information in the DNS, but
manually populating DNS authoritative server databases is costly and
potentially error-prone, and requires a knowledgable network
administrator. Consequently, although all DNS-SD client
implementations of which we are aware support DNS-SD using DNS
queries, in practice it is used much less frequently than mDNS.
The Discovery Proxy [I-D.ietf-dnssd-hybrid] provides one way to
automatically populate the DNS namespace, but is only appropriate on
networks where services are easily advertised using mDNS. This
document describes a solution more suitable for networks where
multicast is inefficient, or where sleepy devices are common, by
supporting both offering of services, and discovery of services,
using unicast.
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2. Service Registration Protocol
Services that implement SRP use DNS Update [RFC2136] [RFC3007] to
publish service information in the DNS. Two variants exist, one for
full-featured hosts, and one for devices designed for "Constrained-
Node Networks" [RFC7228].
Full-featured hosts are either configured manually with a
registration domain, or use the "dr._dns-sd._udp.<domain>" query
([RFC6763] Section 11) to learn the default registration domain from
the network. RFC6763 says to discover the registration domain using
either ".local" or a network-supplied domain name for <domain>.
Services using SRP MUST use the domain name received through the
DHCPv4 Domain Name option ([RFC2132] section 3.17), if available, or
the Neighbor Discovery DNS Search List option [RFC8106]. If the DNS
Search List option contains more than one domain name, it MUST NOT be
used. If neither option is available, the Service Registration
protocol is not available on the local network.
Manual configuration of the registraton domain can be done either by
querying the list of available registration zones ("r._dns-sd._udp")
and allowing the user to select one from the UI, or by any other
means appropriate to the particular use case being addressed. Full-
featured devices construct the names of the SRV, TXT, and PTR records
describing their service(s) as subdomains of the chosen service
registration domain. For these names they then discover the zone
apex of the closest enclosing DNS zone using SOA queries
[I-D.ietf-dnssd-push]. Having discovered the enclosing DNS zone,
they query for the "_dnssd-srp._tcp<zone>" SRV record to discover the
server to which they should send DNS updates.
For devices designed for Constrained-Node Networks [RFC7228] some
simplifications are used. Instead of being configured with (or
discovering) the service registration domain, the (proposed) special-
use domain name [RFC6761] "services.arpa" is used. Instead of
learning the server to which they should send DNS updates, a fixed
IPv6 anycast address is used (value TBD). Anycasts are sent using
UDP unless TCP is required due to the size of the update. It is the
responsibility of a Constrained-Node Network supporting SRP to
provide appropriate anycast routing to deliver the DNS updates to the
appropriate server. It is the responsibility of the SRP server
supporting a Constrained-Node Network to handle the updates
appropriately. In some network environments, updates may be accepted
directly into a local "services.arpa" zone, which has only local
visibility. In other network environments, updates for names ending
in "services.arpa" may be rewritten internally to names with broader
visibility.
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The reason for these different assumptions is that Constrained-Node
Networks generally require special egress support, and Anycast
packets captured at the Constrained-Node Network egress can be
assumed to have originated locally. Low-power devices that typically
use Constrained-Node Networks may have very limited battery power.
The additional DNS lookups required to discover an SRP server and
then communicate with it will increase the power required to
advertise a service; for low-power devices, the additional
flexibility this provides does not justify the additional use of
power.
General networks have the potential to have more complicated
topologies at the Internet layer, which makes anycast routing more
difficult. Such networks may or may not have the infrastructure
required to route anycast to a server that can process it. However,
they can be assumed to be able to provide registration domain
discovery and routing. By requiring the use of TCP, the possibility
of off-network spoofing is eliminated.
We will discuss several parts to this process: how to know what to
publish, how to know where to publish it (under what name), how to
publish it, how to secure its publication, and how to maintain the
information once published.
2.1. What to publish
We refer to the DNS Update message sent by services using SRP as an
SRP update. Three types of updates appear in an SRP update: Service
Discovery records, Service Description records, and Host Description
records.
o Service Discovery records are one or more PTR RRs, mapping from
the generic service type (or subtype) to the specific Service
Instance Name.
o Service Description records are exactly one SRV RR, exactly one
KEY RR, and one or more TXT RRs, both with the same name, the
Service Instance Name ([RFC6763] section 4.1). In principle
Service Description records can include other record types, with
the same Service Instance Name, though in practice they rarely do.
The Service Instance Name MUST be referenced by one or more
Service Discovery PTR records, unless it is a placeholder service
registration for an intentionally non-discoverable service name.
o The Host Description records for a service are a KEY RR, used to
claim exclusive ownership of the service registration, and one or
more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of
the host where the service resides.
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RFC 6763 describes the details of what each of these types of updates
contains and is the definitive source for information about what to
publish; the reason for summarizing this here is to provide the
reader with enough information about what will be published that the
service registration process can be understood at a high level
without first learning the full details of DNS-SD. Also, the
"Service Instance Name" is an important aspect of first-come, first-
serve naming, which we describe later on in this document.
2.2. Where to publish it
Multicast DNS uses a single namespace, ".local", which is valid on
the local link. This convenience is not available for DNS-SD using
the DNS protocol: services must exist in some specific unicast
namespace.
As described above, full-featured devices are responsible for knowing
in what domain they should register their services. Devices made for
Constrained-Node Networks register in the (proposed) special use
domain name [RFC6761] "services.arpa", and let the SRP server handle
rewriting that to a different domain if necessary.
2.3. How to publish it
It is possible to issue a DNS Update that does several things at
once; this means that it's possible to do all the work of adding a
PTR resource record to the PTR RRset on the Service Name if it
already exists, or creating one if it doesn't, and creating or
updating the Service Instance Name and Host Description in a single
transaction.
An SRP update is therefore implemented as a single DNS Update message
that contains a service's Service Discovery records, Service
Description records, and Host Description records.
Updates done according to this specification are somewhat different
than regular DNS Updates as defined in RFC2136. RFC2136 uses a
fairly heavyweight process for updating: you might first attempt to
add a name if it doesn't exist; if that fails, then in a second
message you might update the name if it does exist but matches
certain preconditions. Because the registration protocol uses a
single transaction, some of this adaptability is lost.
In order to allow updates to happen in a single transaction, SRP
updates do not include update constraints. The constraints specified
in Section 2.4.2 are implicit in the processing of SRP updates, and
so there is no need for the service sending the SRP update to put in
any explicit constraints.
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2.3.1. How DNS-SD Service Registration differs from standard RFC2136
DNS Update
DNS-SD Service Registration is based on standard RFC2136 DNS Update,
with some differences:
o It implements first-come first-served name allocation, protected
using SIG(0) [RFC2931].
o It enforces policy about what updates are allowed.
o It optionally performs rewriting of "services.arpa" to some other
domain.
o It optionally performs automatic population of the address-to-name
reverse mapping domains.
o An SRP server is not required to implement general DNS Update
prerequsite processing.
o Simplified clients are allowed to send updates to an anycast
address, for names ending in "services.arpa"
2.3.2. Testing using standard RFC2136-compliant servers
It may be useful to set up a DNS server for testing that does not
implement SRP. This can be done by configuring the server to listen
on the anycast address, or advertising it in the _dnssd-srp._tcp SRV
record. It must be configured to be authoritative for
"services.arpa", and to accept updates from hosts on local networks
for names under "services.arpa" without authentication, since such
servers will not have support for FCFS authentication Section 2.4.1.
A server configured in this way will be able to successfully accept
and process SRP updates from services that send SRP updates.
However, no constraints will be applied, and this means that the test
server will accept internally inconsistent SRP updates, and will not
stop two SRP updates, sent by different services, that claim the same
name(s), from overwriting each other.
2.3.3. How to allow services to update standard RFC2136-compliant
servers
Ordinarily SRP updates will fail when sent to an RFC 2136-compliant
server that does not implement SRP because the zone being updated is
"services.arpa", and no DNS server that is not an SRP server should
normally be configured to be authoritative for "services.arpa".
Therefore, a service that sends an SRP update can tell that the
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receiving server does not support SRP, but does support RFC2136,
because the RCODE will either be NOTZONE, NOTAUTH or REFUSED, or
because there is no response to the update request (when using the
anycast address)
In this case a service MAY attempt to register itself using regular
RFC2136 DNS updates. To do so, it must discover the default
registration zone and the DNS server designated to receive updates
for that zone, as described earlier using the _dns-update._udp SRV
record. It can then make the update using the port and host pointed
to by the SRV record, and should use appropriate constraints to avoid
overwriting competing records. Such updates are out of scope for
SRP, and a service that implements SRP MUST first attempt to use SRP
to register itself, and should only attempt to use RFC2136 backwards
compatibility if that fails. Although the owner name for the SRV
record specifies the UDP protocol for updates, it is also possible to
use TCP, when the update is too large.
2.4. How to secure it
Traditional DNS update is secured using the TSIG protocol, which uses
a secret key shared between the client (which issues the update) and
the server (which authenticates it). This model does not work for
automatic service registration.
The goal of securing the DNS-SD Registration Protocol is to provide
the best possible security given the constraint that service
registration has to be automatic. It is possible to layer more
operational security on top of what we describe here, but what we
describe here improves upon the security of mDNS. The goal is not to
provide the level of security of a network managed by a skilled
operator.
2.4.1. First-Come First-Served Naming
First-Come First-Serve naming provides a limited degree of security:
a service that registers its service using DNS-SD Registration
protocol is given ownership of a name for an extended period of time
based on the key used to authenticate the DNS Update. As long as the
registration service remembers thename and the key used to register
that name, no other service can add or update the information
associated with that. FCFS naming is used to protect both the
Service Description and the Host Description.
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2.4.1.1. Service Behavior
The service generates a public/private key pair. This key pair MUST
be stored in stable storage; if there is no writable stable storage
on the client, the client MUST be pre-configured with a public/
private key pair in read-only storage that can be used. This key
pair MUST be unique to the device.
When sending DNS updates, the service includes a KEY record
containing the public portion of the key in each Host Description
update and each Service Description update. Each KEY record MUST
contain the same public key. The update is signed using SIG(0),
using the private key that corresponds to the public key in the KEY
record. The lifetimes of the records in the update is set using the
EDNS(0) Update Lease option [I-D.sekar-dns-ul].
The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records
[RFC6763] uses the LEASE field of the Update Lease option, and is
typically set to two hours. This means that if a device is
disconnected from the network, it does not appear in the user
interfaces of devices looking for services of that type for too long.
The lifetime of the KEY records is set using the KEY-LEASE field of
the Update Lease Option, and should be set to a much longer time,
typically 14 days. The result of this is that even though a device
may be temporarily unplugged, disappearing from the network for a few
days, it makes a claim on its name that lasts much longer.
This means that even if a device is unplugged from the network for a
few days, and its services are not available for that time, no other
rogue device can come along and immediately claim its name the moment
it disappears from the network. In the event that a device is
unplugged from the network and permanently discarded, then its name
is eventually cleaned up and made available for re-use.
2.4.2. SRP Server Behavior
The SRP server first validates that the SRP update is a syntactically
and semantically valid DNS Update according to the rules specified in
RFC2136.
The SRP server checks each update in the SRP update to see that it
contains a Service Discovery update, a Service Description update,
and a Host Description update. Order matters in DNS updates.
Specifically, deletes must precede adds for records that the deletes
would affect; otherwise the add will have no effect. This is the
only ordering constraint; aside from this constraint, updates may
appear in whatever order is convenient when constructing the update.
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An update is a Service Discovery update if it contains
o exactly one RRset update,
o which is for a PTR RR,
o which points to a Service Instance Name
o for which an update is present in the SRP update.
o Service Discovery updates do not contain any deletes, and do not
contain any other updates.
An update is a Service Description update if, for the appropriate
Service Instance Name, it contains
o exactly one "Delete all RRsets from a name" update,
o exactly one SRV RRset update,
o exactly one KEY RR update that adds a KEY RR that contains the
public key corresponding to the private key that was used to sign
the message,
o one or more TXT RRset updates,
o and the target of the SRV record update references a hostname for
which there is a Host Description update in the SRP update.
o Service Descriptions do not update any other records.
An update is a Host Description update if, for the appropriate
hostname, it contains
o exactly one "Delete all RRsets from a name" update,
o one or more A or AAAA RR update(s)
o exactly one KEY RR update that adds a KEY RR that contains the
public key corresponding to the private key that was used to sign
the message,
o there is a Service Instance Name update in the SRP update that
updates an SRV RR so that it points to the hostname being updated
by this update.
o Host Description updates do not update any other records.
An SRP update MUST include at least one Service Discovery update, at
least one Service Description update, and exactly one Host
Description update. An update message that does not is not an SRP
update. An update message that contains any other updates, or any
update constraints, is not an SRP update. Such messages should
either be processed as regular RFC2136 updates, including access
control checks and constraint checks, if supported, or else rejected
with RCODE=REFUSED.
Note that if the definitions of each of these update types are
followed carefully, this means that many things that look very much
like SRP updates nevertheless are not. For example, a DNS update
that contains an update to a Service Name and an update to a Service
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Instance Name, where the Service Name does not reference the Service
Instance Name, is not a valid SRP update message, but may be a valid
RFC2136 update.
Assuming that an update message has been validated with these
conditions and is a valid SRP update, the server checks that the name
in the Host Description update exists. If so, then the server checks
to see if the KEY record on the name is the same as the KEY record in
the update. The server performs the same check for the KEY records
in any Service Description update. If any existing KEY record
corresponding to a KEY record in the SRP update does not match the
KEY record in the SRP update, then the server MUST reject the SRP
update with the YXDOMAIN RCODE.
Otherwise, the server validates the SRP update using SIG(0) on the
public key in the KEY record of the Host Description update. If the
validation fails, the server MUST reject the SRP Update with the
REFUSED RCODE. Otherwise, the SRP update is considered valid and
authentic, and is processed according to the method described in
RFC2136. The status that is returned depends on the result of
processing the update.
The server MAY add a Reverse Mapping that corresponds to the Host
Description. This is not required because the Reverse Mapping serves
no protocol function, but it may be useful for debugging, e.g. in
annotating network packet traces or logs.
The server MAY apply additional criteria when accepting updates. In
some networks, it may be possible to do out-of-band registration of
keys, and only accept updates from pre-registered keys. In this
case, an update for a key that has not been registered should be
rejected with the REFUSED RCODE.
There are at least two benefits to doing this rather than simply
using normal SIG(0) DNS updates. First, the same registration
protocol can be used in both cases, so both use cases can be
addressed by the same service implementation. Second, the
registration protocol includes maintenance functionality not present
with normal DNS updates.
Note that the semantics of using SRP in this way are different than
for typical RFC2136 implementations: the KEY used to sign the SRP
update only allows the client to update records that refer to its
Host Description. RFC2136 implementations do not normally provide a
way to enforce a constraint of this type.
The server may also have a dictionary of names or name patterns that
are not permitted. If such a list is used, updates for Service
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Instance Names that match entries in the dictionary are rejected with
YXDOMAIN.
2.5. TTL Consistency
All RRs within an RRset are required to have the same TTL
(Clarifications to the DNS Specification [RFC2181], Section 5.2). In
order to avoid inconsistencies, SRP places restrictions on TTLs sent
by services and requires that SRP Servers enforce consistency.
Services sending SRP updates MUST use consistent TTLs in all RRs
within the SRP update.
SRP update servers MUST check that the TTLs for all RRs within the
SRP update are the same. If they are not, the SRP update MUST be
rejected with a REFUSED RCODE.
Additionally, when adding RRs to an RRset, for example when
processing Service Discovery records, the server MUST use the same
TTL on all RRs in the RRset. How this consistency is enforced is up
to the implementation.
TTLs sent in SRP updates are advisory: they indicate the client's
guess as to what a good TTL would be. SRP servers may override these
TTLs. SRP servers SHOULD ensure that TTLs are reasonable: neither
too long nor too short. The TTL should never be longer than the
lease time Section 2.6.1. Shorter TTLs will result in more frequent
data refreshes; this increases latency on the client side, and
increases load on any caching resolvers and on the authoritative
server. Longer TTLs will increase the likelihood that data in caches
will be stale. TTL minimums and maximums SHOULD be configurable by
the operator of the SRP server.
2.6. Maintenance
2.6.1. Cleaning up stale data
Because the DNS-SD registration protocol is automatic, and not
managed by humans, some additional bookkeeping is required. When an
update is constructed by the client, it MUST include include an
EDNS(0) Update Lease Option [I-D.sekar-dns-ul]. The Update Lease
Option contains two lease times: the Lease Time and the Key Lease
Time.
These leases are promises, similar to DHCP leases [RFC2131], from the
client that it will send a new update for the service registration
before the lease time expires. The Lease time is chosen to represent
the time after the update during which the registered records other
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than the KEY record should be assumed to be valid. The Key Lease
time represents the time after the update during which the KEY record
should be assumed to be valid.
The reasoning behind the different lease times is discussed in the
section on first-come, first-served naming Section 2.4.1. SRP
servers may be configured with limits for these values. A default
limit of two hours for the Lease and 14 days for the SIG(0) KEY are
currently thought to be good choices. Clients that are going to
continue to use names on which they hold leases should update well
before the lease ends, in case the registration service is
unavailable or under heavy load.
The SRP server MUST include an EDNS(0) Update Lease option in the
response if the lease time proposed by the service has been shortened
or lengthened. The service MUST check for the EDNS(0) Update Lease
option in the response and MUST use the lease times from that option
in place of the options that it sent to the server when deciding when
to update its registration. The times may be shorter or longer than
those specified in the SRP update; the client must honor them in
either case.
Clients should assume that each lease ends N seconds after the update
was first transmitted, where N is the lease duration. Servers should
assume that each lease ends N seconds after the update that was
successfully processed was received. Because the server will always
receive the update after the client sent it, this avoids the
possibility of misunderstandings.
SRP servers MUST reject updates that do not include an EDNS(0) Update
Lease option. Dual-use servers MAY accept updates that don't include
leases, but SHOULD differentiate between SRP updates and other
updates, and MUST reject updates that would otherwise be SRP updates
updates if they do not include leases.
Lease times have a completely different function than TTLs. On an
authoritative DNS server, the TTL on a resource record is a constant:
whenever that RR is served in a DNS response, the TTL value sent in
the answer is the same. The lease time is never sent as a TTL; its
sole purpose is to determine when the authoritative DNS server will
delete stale records. It is not an error to send a DNS response with
a TTL of 'n' when the remaining time on the lease is less than 'n'.
2.6.2. Sleep Proxy
Another use of SRP is for devices that sleep to reduce power
consumption.
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In this case, in addition to the DNS Update Lease option
[I-D.sekar-dns-ul] described above, the device includes an EDNS(0)
OWNER Option [I-D.cheshire-edns0-owner-option].
The EDNS(0) Update Lease option constitutes a promise by the device
that it will wake up before this time elapses, to renew its
registration and thereby demonstrate that it is still attached to the
network. If it fails to renew the registration by this time, that
indicates that it is no longer attached to the network, and its
registration (except for the KEY in the Host Description) should be
deleted.
The EDNS(0) OWNER Option indicates that the device will be asleep,
and will not be receptive to normal network traffic. When a DNS
server receives a DNS Update with an EDNS(0) OWNER Option, that
signifies that the SRP server should set up a proxy for any IPv4 or
IPv6 address records in the DNS Update message. This proxy should
send ARP or ND messages claiming ownership of the IPv4 and/or IPv6
addresses in the records in question. In addition, proxy should
answer future ARP or ND requests for those IPv4 and/or IPv6
addresses, claiming ownership of them. When the DNS server receives
a TCP SYN or UDP packet addressed to one of the IPv4 or IPv6
addresses for which it proxying, it should then wake up the sleeping
device using the information in the EDNS(0) OWNER Option. At present
version 0 of the OWNER Option specifies the "Wake-on-LAN Magic
Packet" that needs to be sent; future versions could be extended to
specify other wakeup mechanisms.
Note that although the authoritative DNS server that implements the
SRP function need not be on the same link as the sleeping host, the
Sleep Proxy must be on the same link.
It is not required that sleepy nodes on a Constrained-Node Network
support sleep proxy. Such devices may have different mechanisms for
dealing with sleep and wakeup. An SRP registration for such a device
will be useful regardless of the mechanism whereby messages are
delivered to the sleepy end device. For example, the message might
be held in a buffer for an extended period of time by an intermediate
device on a mesh network, and then delivered to the device when it
wakes up. The exact details of such behaviors are out of scope for
this document.
3. Security Considerations
SRP updates have no authorization semantics other than first-come,
first-served. This means that if an attacker from outside of the
administrative domain of the server knows the server's IP address, it
can in principle send updates to the server that will be processed
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successfully. Servers should therefore be configured to reject
updates from source addresses outside of the administrative domain of
the server.
For Anycast updates, this validation must be enforced by every router
that connects the Constrained-Device Network to the unconstrained
portion of the network. For TCP updates, the initial SYN-SYN+ACK
handshake prevents updates being forged by an off-network attacker.
In order to ensure that this handshake happens, Service Discovery
Protocol servers MUST NOT accept TCP Fast Open payloads.
Note that these rules only apply to the validation of SRP updates. A
server that accepts updates from DNS-SD registration protocol clients
may also accept other DNS updates, and those DNS updates may be
validated using different rules. However, in the case of a DNS
service that accepts SRP updates, the intersection of the SRP update
rules and whatever other update rules are present must be considered
very carefully.
For example, a normal, authenticated RFC2136 update to any RR that
was added using SRP, but that is authenticated using a different key,
could be used to override a promise made by the registration
protocol, by replacing all or part of the service registration
information with information provided by a different client. An
implementation that allows both kinds of updates should not allow
updates to records added by SRP updates using different
authentication and authorization credentials.
4. Privacy Considerations
5. Delegation of 'services.arpa.'
In order to be fully functional, there must be a delegation of
'services.arpa.' in the '.arpa.' zone [RFC3172]. This delegation
should be set up as was done for 'home.arpa', as a result of the
specification in [RFC8375]Section 7.
6. IANA Considerations
IANA is requested to record the domain name 'services.arpa.' in the
Special-Use Domain Names registry [SUDN]. IANA is requested, with
the approval of IAB, to implement the delegation requested in
Section 5.
IANA is further requested to add a new entry to the "Transport-
Independent Locally-Served Zones" subregistry of the the "Locally-
Served DNS Zones" registry[LSDZ]. The entry will of for
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'services.arpa.' with the description "DNS-SD Registration Protocol
Special-Use Domain", listing this document as the reference.
IANA is also requested to add a new entry to the Service Names and
Port Numbers registry for dnssd-srp with a transport type of tcp. No
port number is to be assigned. The reference should be to this
document, and the Assignee and Contact information should reference
the authors of this document. The Description should be as follows:
Availability of DNS Service Discovery Service Registration Protocol
Service for a given domain is advertised using the "_dnssd-
srp._tcp.<domain>." SRV record gives the target host and port where
DNSSD Service Registration Service is provided for the named domain.
7. Acknowledgments
Thanks to Toke Hoeiland-Joergensen for a thorough technical review,
to Tamara Kemper for doing a nice developmental edit, Tim Wattenberg
for doing a service implementation at the Montreal Hackathon at IETF
102, Tom Pusateri for reviewing during the hackathon and afterwards,
and [...] more reviewers to come, hopefully.
8. References
8.1. Normative References
[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>.
[I-D.sekar-dns-ul]
Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
draft-sekar-dns-ul-02 (work in progress), August 2018.
[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>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
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[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>.
[SUDN] "Special-Use Domain Names Registry", July 2012,
<https://www.iana.org/assignments/special-use-domain-
names/special-use-domain-names.xhtml>.
[LSDZ] "Locally-Served DNS Zones Registry", July 2011,
<https://www.iana.org/assignments/locally-served-dns-
zones/locally-served-dns-zones.xhtml>.
8.2. Informative 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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[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>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
2000, <https://www.rfc-editor.org/info/rfc2931>.
[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>.
[RFC3152] Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
DOI 10.17487/RFC3152, August 2001,
<https://www.rfc-editor.org/info/rfc3152>.
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[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>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[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-push]
Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-15 (work in progress), September
2018.
[I-D.cheshire-dnssd-roadmap]
Cheshire, S., "Service Discovery Road Map", draft-
cheshire-dnssd-roadmap-02 (work in progress), October
2018.
[I-D.cheshire-edns0-owner-option]
Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", draft-
cheshire-edns0-owner-option-01 (work in progress), July
2017.
[ZC] Cheshire, S. and D. Steinberg, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc. ,
ISBN 0-596-10100-7, December 2005.
Authors' Addresses
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Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
Ted Lemon
Nibbhaya Consulting
P.O. Box 958
Brattleboro, Vermont 05302
United States of America
Email: mellon@fugue.com
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