Internet Engineering Task Force S. Cheshire
Internet-Draft Apple Inc.
Intended status: Informational T. Lemon
Expires: January 3, 2019 Nibbhaya Consulting
July 2, 2018
Service Registration Protocol for DNS-Based Service Discovery
draft-sctl-service-registration-01
Abstract
The DNS-SD Service Registration Protocol 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 (WiFi) and 802.15 (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 January 3, 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
carefully, as they describe your rights and restrictions with respect
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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.
1. Introduction
DNS-Based Service Discovery [RFC6763] is a component of Zero
Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].
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 DNS-SD Service Registration protocol, described in this document,
provides a reasonably secure mechanism for publishing this
information: what services are offered, and how to use them. Once
published, these services can be readily discovered by clients using
standard DNS lookups.
In the DNS-Based Service Discovery 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 Multicast DNS [RFC6762], allows clients to directly query
services on the local link for names in the ".local" namespace.
RFC6763 also allows clients to discover services using the DNS
protocol [RFC1035]; this is done either by having a system
administrator manually configure service information in the DNS, or
by using a Discovery Proxy [I-D.ietf-dnssd-hybrid], which performs
mDNS queries on behalf clients issuing queries using DNS. This
eliminats the "link local" limitation of mDNS, but provides no
additional security, and still relies on multicast.
Manual configuration of DNS servers is costly and failure-prone, and
requires a knowledgable network administrator. Consequently,
although all DNS-SD implementations of which we are aware support it,
it is much less frequently used than mDNS. This document describes a
solution: a way to provide DNS-SD using DNS that can be as automatic
as multicast DNS, but with better performance, scalability and
security.
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2. Service Registration Protocol
Services using the DNS-SD Service Registration Protocol use DNS
Update [RFC2136] [RFC3007] to publish service information in the DNS.
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
RFC 6763 describes the details of what is to be published. In
general terms, a service will have a name under which it offers
services ([RFC6763] section 4.1.1) and one or more service names
under which that instance name appears ([RFC6763] section 4.1.2).
The full details of how this works are described in section 4 of that
document in its entirety. A service publishes its contact
information using an SRV record on the Service Instance Name. It can
also publish TXT records with additional information about the
service; this is discussed in section 6 of RFC 6763.
RFC 6763 is the definitive source for information about what to
publish; the reason for mentioning it here is that the reader may
prefer to have an overview of the whole service registration process
before digging into the details. 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: the portion of the DNS namespace in which services
on the local network are to be published must be discovered by the
service before it can register itself.
Names published using DNS-SD service registration will be published
under some name other than .local. However, the process of
discovering what that name is is complicated, and for any given
network it should always be the case that there will be just one
namespace in which registered names will be published. Rather than
requiring the service to discover this name before issuing a
registration, the service SHOULD simply use the name ".local." The
DNS server that receives the registration request will rewrite all
instances of the terminal label ".local" to use the local
registration domain name. The response to the DNS Update being used
to register the service will contain the rewritten names, instead of
".local". Subsequent updates should still use the ".local" domain
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and not the registration domain, since the registration domain may
change over time or when the service is physically moved to a new
network.
2.3. How to publish it
DNS Updates are very flexible. As a consequence, it is possible to
do the entire registration in a single DNS message. The update
consists of two elements. The first updates the Service Name by
adding a PTR record pointing to the Service Instance Name. The
second updates the Service Instance Name. The second creates or
updates the Service Instance Name update adds an SRV record and a KEY
record, and optionally adds a TXT record with extra information about
the service. The contents of the KEY record are described in the
section on First-Come First-Served Naming (Section 2.4.1). The
update is signed using the private key that corresponds to the public
key in the KEY record, using the SIG(0) protocol [RFC2931].
The update may be rejected. If the chosen service instance name is
not permitted, or is already taken, the update will be returned with
the error code YXDOMAIN. In this case, the service will need to
choose a new instance name and try again.
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 the Service Instance Name and the key
used to register that Service Instance Name, no other service can add
or update the information associated with that Service Instance Name.
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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 that can be used.
When sending DNS updates, the service includes a KEY record
containing the public portion of the key, which is stored as an RRset
under the Service Instance Name. It is permissible for a device that
offers more than one service under more than one Service Instance
Name to use the same KEY on each such name.
The update is signed using SIG(0), using the private key that
corresponds to the public key in the KEY record.
The lifetime of the DNS-SD PTR, SRV and TXT records [RFC6763] 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.
However, the lifetime of its DNS SIG(0) public key 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 way, 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, and yet the name is eventually
cleaned up and made available for re-use.
2.4.1.2. Registration Server Behavior
The Registration server checks that the DNS update contains a Service
Instance Name. In principle, each name in the update should be
evaluated as a candidate Service Instance Name. However, some names
will obviously be Service Names, and these can be skipped when
evaluating candidates. In order for a candidate to actually be a
service instance name, the following conditions must be true:
o There is at least one name that turns out NOT to be a Service
Instance Name for which there is a PTR RRset update that includes
a record pointing to the candidate.
o The candidate includes an SRV record
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o The candidate includes a KEY record
If an update does not contain a valid Service Instance Name, or if it
contains an update to a PTR RRset that references a name that is not
the Service Instance Name being updated, the update is rejected with
the NOTAUTH RCODE.
If an update contains an SRV record that contains an IP address other
than the IP address from which the update was recieved, the update is
rejected with the NOTAUTH RCODE.
Once each name for which there are updates in the DNS Update has been
considered as a candidate, it should be the case that only one name
is actually a possible Service Instance Name. If more than one name
is still a possible candidate, then the DNS Update is rejected with
the FORMERR RCODE.
If there is only one candidate, then the server checks to see if that
name already exists. If it does already exist, then the server
checks to see if the KEY record on the name is the same as the one in
the update for that name. If it is not, then the DNS Update is
rejected with the YXDOMAIN RCODE.
Otherwise, the server validates the update using SIG(0). If the
validation fails, the update is rejected with NOTAUTH. Otherwise,
the update is evaluated according to the rules described in RFC2136
for processing DNS updates, and whatever the correct result is is
returned.
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 using NOTAUTH.
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 implementation. Second, the registration
protocol includes maintenance functionality not present with normal
DNS updates.
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
Instance Names that match entries in the dictionary are rejected with
YXDOMAIN.
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2.5. Maintenance
2.5.1. 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 and an EDNS(0) Instance Lease option.
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 Update Lease time is chosen to
represent the time after the update during which the registered
records other than the KEY record should be assumed to be valid. The
Instance 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. DNS-SD
Registration Protocol servers may be configured with limits for these
values. A default limit of two hours for the Update Lease and 30
days for the Instance Lease 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.
Clients should assume that each lease ends N seconds after the update
was first transmitted, where N is the number included in the option.
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.
DNS-SD Registration Protocol servers SHOULD reject updates that do
not include a DNS update lease time. Dual-use servers may accept
updates that don't include leases, but SHOULD differentiate between
DNS-SD registration protocol updates and other updates, and SHOULD
reject updates that are known to be DNS-SD registration protocol
updates if they do not include leases.
2.5.2. Sleep Proxy
Another use of Service Registration Protocol is for devices that
sleep to reduce power consumption.
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].
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The DNS Update Lease option constitutes a promise by the device that
it will wake up before this time elapses, to renew its records and
thereby demonstrate that it is still attached to the network. If it
fails to renew the records by this time, that indicates that it is no
longer attached to the network, and its records 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 DNS server should act as a proxy for any IPv4 or
IPv6 address records in the DNS Update message. This means that the
DNS server should send ARP or ND messages claiming ownership of the
IPv4 and/or IPv6 addresses in the records in question. In addition,
the DNS server 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.
3. Security Considerations
DNS-SD Service Registration Protocol 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 successfully. Servers should
therefore be configured to reject updates from source addresses
outside of the administrative domain of the server.
Note that these rules only apply to the validation of DNS-SD
registration protocol 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 automatic
updates, the intersection of the DNS-SD service registration update
rules and whatever other update rules are present must be considered
very carefully.
4. Privacy Considerations
5. References
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5.1. 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>.
5.2. Informative 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>.
[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>.
[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>.
[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>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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.sekar-dns-ul]
Sekar, K., "Dynamic DNS Update Leases", draft-sekar-dns-
ul-01 (work in progress), August 2006.
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[I-D.cheshire-dnssd-roadmap]
Cheshire, S., "Service Discovery Road Map", draft-
cheshire-dnssd-roadmap-01 (work in progress), March 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
Stuart Cheshire
Apple Inc.
1 Infinite Loop
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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