Network Working Group
Internet Engineering Task Force (IETF) C. Huitema
Internet-Draft
Request for Comments: 8882 Private Octopus Inc.
Intended status:
Category: Informational D. Kaiser
Expires: September 13, 2020
ISSN: 2070-1721 University of Luxembourg
March 12,
September 2020
DNS-SD
DNS-Based Service Discovery (DNS-SD) Privacy and Security Requirements
draft-ietf-dnssd-prireq-08
Abstract
DNS-SD (DNS (DNS-based Service Discovery) normally discloses information
about devices offering and requesting services. This information
includes
host names, hostnames, network parameters, and possibly a further
description of the corresponding service instance. Especially when
mobile devices engage in DNS DNS-based Service Discovery at a public
hotspot, serious privacy problems arise. We analyze the requirements
of a privacy-respecting discovery service.
Status of This Memo
This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2020.
https://www.rfc-editor.org/info/rfc8882.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Threat Analysis . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Service Discovery Scenarios . . . . . . . . . . . . . . . 4
3.1.1. Private Client and Public Server . . . . . . . . . . 4
3.1.2. Private Client and Private Server . . . . . . . . . . 5
3.1.3. Wearable Client and Server . . . . . . . . . . . . . 6
3.2. DNS-SD Privacy Considerations . . . . . . . . . . . . . . 8
3.2.1. Information made available via Made Available Via DNS-SD Resource Records . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Privacy Implication of Publishing Service Instance
Names . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Privacy Implication of Publishing Node Names . . . . 10
3.2.4. Privacy Implication of Publishing Service Attributes 10
3.2.5. Device Fingerprinting . . . . . . . . . . . . . . . . 11
3.2.6. Privacy Implication of Discovering Services . . . . . 12
3.3. Security Considerations . . . . . . . . . . . . . . . . . 12
3.3.1. Authenticity, Integrity & Integrity, and Freshness . . . . . . . . . 12
3.3.2. Confidentiality . . . . . . . . . . . . . . . . . . . 12
3.3.3. Resistance to Dictionary Attacks . . . . . . . . . . 13
3.3.4. Resistance to Denial-of-Service Attacks . . . . . . . 13
3.3.5. Resistance to Sender Impersonation . . . . . . . . . 13
3.3.6. Sender Deniability . . . . . . . . . . . . . . . . . 13
3.4. Operational Considerations . . . . . . . . . . . . . . . 13
3.4.1. Power Management . . . . . . . . . . . . . . . . . . 13
3.4.2. Protocol Efficiency . . . . . . . . . . . . . . . . . 14
3.4.3. Secure Initialization and Trust Models . . . . . . . 14
3.4.4. External Dependencies . . . . . . . . . . . . . . . . 15 Dependencies
4. Requirements for a DNS-SD Privacy Extension . . . . . . . . . 15
4.1. Private Client Requirements . . . . . . . . . . . . . . . 15
4.2. Private Server Requirements . . . . . . . . . . . . . . . 16
4.3. Security and Operation . . . . . . . . . . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1.
6.1. Normative References . . . . . . . . . . . . . . . . . . 17
7.2.
6.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
DNS
DNS-Based Service Discovery (DNS-SD) [RFC6763] over Multicast DNS
(mDNS) [RFC6762] enables zero-configuration service discovery in
local networks. It is very convenient for users, but it requires the
public exposure of the offering and requesting identities along with
information about the offered and requested services. Parts of the
published information can seriously breach the user's privacy. These
privacy issues and potential solutions are discussed in [KW14a],
[KW14b]
[KW14b], and [K17]. While the multicast nature of mDNS makes these
risks obvious, most risks derive from the observability of
transactions. These risks also need to be mitigated when using
server-based variants of DNS-SD.
There are cases when nodes connected to a network want to provide or
consume services without exposing their identities to the other
parties connected to the same network. Consider, for example, a
traveler wanting to upload pictures from a phone to a laptop when
both are connected to the Wi-Fi network of an Internet cafe, or two
travelers who want to share files between their laptops when waiting
for their plane in an airport lounge.
We expect that these exchanges will start with a discovery procedure
using DNS-SD over mDNS. One of the devices will publish the
availability of a service, such as a picture library or a file store
in our examples. The user of the other device will discover this
service,
service and then connect to it.
When analyzing these scenarios in Section 3.1, we find that the DNS-
SD messages leak identifying information information, such as the service instance
name, Service
Instance Name, the host name, hostname, or service properties. We use the
following definitions:
Identity
In this document, the term "identity" refers to the identity of
the entity (legal person) operating a device.
Disclosing an Identity
In this document document, "disclosing an identity" means showing the
identity of operating entities to devices external to the
discovery process; process, e.g., devices on the same network link that are
listening to the network traffic but are not actually involved in
the discovery process. This document focuses on identity
disclosure by data conveyed via messages on the service discovery
protocol layer. Still, identity leaks on deeper layers, e.g., the
IP layer, are mentioned.
Disclosing Information
In this document document, "disclosing information" is also focused on
disclosure of data conveyed via messages on the service discovery
protocol layer, such as generic non-identity but including both identity-revealing information and
other still potentially sensitive data.
2. Threat Model
This document considers the following attacker types sorted by
increasing power. All these attackers can either be passive (they
just listen to network traffic they have access to) or active (they
additionally can craft and send malicious packets).
external
An external attacker is not on the same network link as victim
devices engaging in service discovery; thus, the external attacker
is in a different multicast domain.
on-link
An on-link attacker is on the same network link as victim devices
engaging in service discovery; thus, the on-link attacker is in
the same multicast domain. This attacker can also mount all
attacks an external attacker can mount.
MITM
A Man in the Middle Man-in-the-Middle (MITM) attacker either controls (parts of) a
network link or can trick two parties to send traffic via the
attacker; thus, the MITM attacker has access to unicast traffic
between devices engaging in service discovery. This attacker can
also mount all attacks an on-link attacker can mount.
3. Threat Analysis
In this section section, we analyse analyze how the attackers described in the
previous section might threaten the privacy of entities operating
devices engaging in service discovery. We focus on attacks
leveraging data transmitted in service discovery protocol messages.
3.1. Service Discovery Scenarios
In this section, we review common service discovery scenarios and
discuss privacy threats and their privacy requirements. In all three
of these common scenarios scenarios, the attacker is of the type passive on-
link.
3.1.1. Private Client and Public Server
Perhaps the simplest private discovery scenario involves a single
client connecting to a public server through a public network. A
common example would be a traveler using a publicly available printer
in a business center, in an a hotel, or at an airport.
( Taking notes:
( David is printing
( a document document.
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | client server |* *|
\_/ __ \_/
| / / Discovery +----------+ |
/|\ /_/ <-----------> | +----+ | /|\
/ | \__/ +--| |--+ / | \
/ | |____/ / | \
/ | / | \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
David adversary Adversary
In that scenario, the server is public and wants to be discovered,
but the client is private. The adversary will be listening to the
network traffic, trying to identify the visitors' devices and their
activity. Identifying devices leads to identifying people, either
for surveillance of these individuals in the physical world or as a
preliminary step for a targeted cyber attack.
The requirement in that scenario is that the discovery activity
should not disclose the identity of the client.
3.1.2. Private Client and Private Server
The second private discovery scenario involves a private client
connecting to a private server. A common example would be two people
engaging in a collaborative application in a public place, such as an
airport's lounge.
( Taking notes:
( David is meeting
( with Stuart Stuart.
~~~~~~~~~~~
o
___ ___ o ___
/ \ / \ _|___|_
| | server client | | |* *|
\_/ __ __ \_/ \_/
| / / Discovery \ \ | |
/|\ /_/ <-----------> \_\ /|\ /|\
/ | \__/ \__/ | \ / | \
/ | | \ / | \
/ | | \ / | \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
David Stuart Adversary
In that scenario, the collaborative application on one of the devices
will act as a server, and the application on the other device will
act as a client. The server wants to be discovered by the client, client but
has no desire to be discovered by anyone else. The adversary will be
listening to network traffic, attempting to discover the identity of
devices as in the first scenario, scenario and also attempting to discover the
patterns of traffic, as these patterns reveal the business and social
interactions between the owners of the devices.
The requirement in that scenario is that the discovery activity
should not disclose the identity of either the client or the server, server
nor reveal the business and social interactions between the owners of
the devices.
3.1.3. Wearable Client and Server
The third private discovery scenario involves wearable devices. A
typical example would be the watch on someone's wrist connecting to
the phone in their pocket.
( Taking notes:
( David is here. His watch is
( talking to his phone phone.
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | client |* *|
\_/ \_/
| _/ |
/|\ // /|\
/ | \__/ ^ / | \
/ |__ | Discovery / | \
/ |\ \ v / | \
/ \\_\ / \
/ \ server / \
/ \ / \
/ \ / \
/ \ / \
David Adversary
This third scenario is in many ways similar to the second scenario.
It involves two devices, one acting as server and the other acting as
client, and it leads to the same requirement of the discovery traffic
not disclosing the identity of either the client or the server. The
main difference is that the devices are managed by a single owner,
which can lead to different methods for establishing secure relations
between the devices. There is also an added emphasis on hiding the
type of devices that the person wears.
In addition to tracking the identity of the owner of the devices, the
adversary is interested in the characteristics of the devices, such
as type, brand, and model. Identifying the type of device can lead
to further attacks, from theft to device-specific hacking. The
combination of devices worn by the same person will also provide a
"fingerprint" of the person, risking identification.
This scenario also represents the general case of bringing private
IoT
Internet-of-Things (IoT) devices into public places. A wearable IoT
device might act as a DNS-SD/mDNS client client, which allows attackers to
infer information about devices' owners. While the attacker might be
a person person, as in the example figure, this could also be abused for large scale
large-scale data collection installing stationary IoT-device-tracking
servers in frequented public places.
The issues described in Section 3.1.1 3.1.1, such as identifying people or
using the information for targeted attacks attacks, apply here too.
3.2. DNS-SD Privacy Considerations
While the discovery process illustrated in the scenarios in
Section 3.1 most likely would be based on [RFC6762] as a means for
making service information available, this document considers all
kinds of means for making DNS-SD resource records available. These
means comprise of but are not limited to mDNS [RFC6762], DNS servers
([RFC1033]
([RFC1033], [RFC1034], and [RFC1035]), using SRP [I-D.ietf-dnssd-srp], the use of Service
Registration Protocol (SRP) [SRP], and multi-link [RFC7558] networks.
The discovery scenarios in Section 3.1 illustrate three separate
abstract privacy requirements that vary based on the use case. These
are not limited to mDNS.
1. Client identity privacy: Client identities are not leaked during
service discovery or use.
2. Multi-entity, mutual client and server identity privacy: Neither
client nor server identities are leaked during service discovery
or use.
3. Single-entity, mutual client and server identity privacy:
Identities of clients and servers owned and managed by the same
legal person are not leaked during service discovery or use.
In this section, we describe aspects of DNS-SD that make these
requirements difficult to achieve in practice. While it is intended
to be thorough, it is not possible to be exhaustive.
Client identity privacy, if not addressed properly, can be thwarted
by a passive attacker (see Section 2). The type of passive attacker
necessary depends on the means of making service information
available. Information conveyed via multicast messages can be
obtained by an on-link attacker. Unicast messages are easy harder to access
access, but if the transmission is not encrypted, but encrypted they could still be
accessed by an attacker with access to network routers or bridges.
Using multi-
link multi-link service discovery solutions [RFC7558], external
attackers have to be taken into consideration as well, e.g., when
relaying multicast messages to other links.
Server identity privacy can be thwarted by a passive attacker in the
same way as client identity privacy. Additionally, active attackers
querying for information have to be taken into consideration as well.
This is mainly relevant for unicast-based discovery, where listening
to discovery traffic requires a MITM attacker; however, an external
active attacker might be able to learn the server identity by just
querying for service information, e.g. e.g., via DNS.
3.2.1. Information made available via Made Available Via DNS-SD Resource Records
DNS-Based Service Discovery (DNS-SD) is defined in [RFC6763]. It
allows nodes to publish the availability of an instance of a service
by inserting specific records in the DNS ([RFC1033], [RFC1034], and
[RFC1035]) or by publishing these records locally using multicast DNS
(mDNS) [RFC6762]. Available services are described using three types
of records:
PTR Record: Record
Associates a service type in the domain with an "instance" name of
this service type.
SRV Record: Record
Provides the node name, port number, priority and weight
associated with the service instance, in conformance with
[RFC2782].
TXT Record: Record
Provides a set of attribute-value pairs describing specific
properties of the service instance.
3.2.2. Privacy Implication of Publishing Service Instance Names
In the first phase of discovery, clients obtain all PTR records
associated with a service type in a given naming domain. Each PTR
record contains a Service Instance Name defined in Section 4 of
[RFC6763]:
Service Instance Name = <Instance> . <Service> . <Domain>
The <Instance> portion of the Service Instance Name is meant to
convey enough information for users of discovery clients to easily
select the desired service instance. Nodes that use DNS-SD over mDNS
[RFC6762] in a mobile environment will rely on the specificity of the
instance name to identify the desired service instance. In our
example of users wanting to upload pictures to a laptop in an
Internet Cafe, cafe, the list of available service instances may look like:
Alice's Images . _imageStore._tcp . local
Alice's Mobile Phone . _presence._tcp . local
Alice's Notebook . _presence._tcp . local
Bob's Notebook . _presence._tcp . local
Carol's Notebook . _presence._tcp . local
Alice will see the list on her phone and understand intuitively that
she should pick the first item. The discovery will "just work".
(Note that our examples of service names conform to the specification
in section Section 4.1 of [RFC6763], [RFC6763] but may require some character escaping
when entered in conventional DNS software.)
However, DNS-SD/mDNS will reveal to anybody that Alice is currently
visiting the Internet Cafe. cafe. It further discloses the fact that she
uses two devices, shares an image store, and uses a chat application
supporting the _presence protocol on both of her devices. She might
currently chat with Bob or Carol, as they are also using a _presence
supporting chat application. This information is not just available
to devices actively browsing for and offering services, services but to anybody
passively listening to the network traffic, i.e. i.e., a passive on-link
attacker.
There is, of course, also no authentication requirement to claim a
particular instance name, so an active attacker can provide resources
that claim to be Alice's but are not.
3.2.3. Privacy Implication of Publishing Node Names
The SRV records contain the DNS name of the node publishing the
service. Typical implementations construct this DNS name by
concatenating the "host name" "hostname" of the node with the name of the local
domain. The privacy implications of this practice are reviewed in
[RFC8117]. Depending on naming practices, the host name hostname is either a
strong identifier of the device, or device or, at a minimum minimum, a partial
identifier. It enables tracking of both the device, device and, by
extension, the device's owner.
3.2.4. Privacy Implication of Publishing Service Attributes
The TXT record's attribute-value pairs contain information on the
characteristics of the corresponding service instance. This in turn
reveals information about the devices that publish services. The
amount of information varies widely with the particular service and
its implementation:
o
* Some attributes, such as the paper size available in a printer,
are the same on many devices, devices and thus only provide limited
information to a tracker.
o
* Attributes that have freeform free-form values, such as the name of a
directory, may reveal much more information.
Combinations of individual attributes have more information power
than specific attributes, attributes and can potentially be used for
"fingerprinting" a specific device.
Information contained in TXT records not only breaches privacy by
making devices trackable, trackable but might directly contain private
information about the user. For instance instance, the _presence service
reveals the "chat status" to everyone in the same network. Users
might not be aware of that.
Further, TXT records often contain version information about
services, allowing potential attackers to identify devices running
exploit-prone versions of a certain service.
3.2.5. Device Fingerprinting
The combination of information published in DNS-SD has the potential
to provide a "fingerprint" of a specific device. Such information
includes:
o List
* A list of services published by the device, which can be retrieved
because the SRV records will point to the same host name.
o hostname.
* Specific attributes describing these services.
o
* Port numbers used by the services.
o
* Priority and weight attributes in the SRV records.
This combination of services and attributes will often be sufficient
to identify the version of the software running on a device. If a
device publishes many services with rich sets of attributes, the
combination may be sufficient to identify the specific device and
track its owner.
An argument is sometimes made that devices providing services can be
identified by observing the local traffic, traffic and that trying to hide the
presence of the service is futile. However, there are good reasons
for the discovery service layer to avoid unnecessary exposure:
1. Providing privacy at the discovery layer is of the essence for
enabling automatically configured privacy-preserving network
applications. Application layer protocols are not forced to
leverage the offered privacy, but if device tracking is not
prevented at the deeper layers, including the service discovery
layer, obfuscating a certain service's protocol at the
application layer is futile.
2. Further, in the case of mDNS based mDNS-based discovery, even if the
application layer does not protect privacy, typically services are
typically provided via unicast unicast, which requires a MITM attacker, while
whereas identifying services based on multicast discovery
messages just requires an on-link attacker.
The same argument can be extended to say that the pattern of services
offered by a device allows for fingerprinting the device. This may
or may not be true, since we can expect that services will be
designed or updated to avoid leaking fingerprints. In any case, the
design of the discovery service should avoid making a bad situation
worse,
worse and should should, as much as possible possible, avoid providing new
fingerprinting information.
3.2.6. Privacy Implication of Discovering Services
The consumers of services engage in discovery, discovery and in doing so reveal
some information information, such as the list of services they are interested in
and the domains in which they are looking for the services. When the
clients select specific instances of services, they reveal their
preference for these instances. This can be benign if the service
type is very common, but it could be more problematic for sensitive
services, such as some private messaging services.
One way to protect clients would be to somehow encrypt the requested
service types. Of course, just as we noted in Section 3.2.5, traffic
analysis can often reveal the service.
3.3. Security Considerations
For each of the operations described above, we must also consider
security threats we are concerned about.
3.3.1. Authenticity, Integrity & Integrity, and Freshness
Can devices (both servers and clients) trust the information they
receive? Has it been modified in flight by an adversary? Can
devices trust the source of the information? Is the source of
information fresh, i.e., not replayed? Freshness may or may not be
required depending on whether the discovery process is meant to be
online. In some cases, publishing discovery information to a shared
directory or registry, rather than to each online recipient through a
broadcast channel, may suffice.
3.3.2. Confidentiality
Confidentiality is about restricting information access to only
authorized individuals. Ideally Ideally, this should only be the appropriate
trusted parties, though it can be challenging to define who are "the
appropriate trusted parties." In some use cases, this may mean that
only mutually authenticated and trusting clients and servers can read
messages sent for one another. The process of service discovery in
particular is often used to discover new entities that the device did
not previously know about. It may be tricky to work out how a device
can have an established trust relationship with a new entity it has
never previously communicated with.
3.3.3. Resistance to Dictionary Attacks
It can be tempting to use (publicly computable) hash functions to
obscure sensitive identifiers. This transforms a sensitive unique
identifier
identifier, such as an email address address, into a "scrambled" but still
unique identifier. Unfortunately Unfortunately, simple solutions may be vulnerable
to offline dictionary attacks.
3.3.4. Resistance to Denial-of-Service Attacks
In any protocol where the receiver of messages has to perform
cryptographic operations on those messages, there is a risk of a
brute-force flooding attack causing the receiver to expend excessive
amounts of CPU time and, where appliciable, applicable, battery power just
processing and discarding those messages.
Also, amplification attacks have to be taken into consideration.
Messages with larger payloads should only be sent as an answer to a
query sent by a verified client.
3.3.5. Resistance to Sender Impersonation
Sender impersonation is an attack wherein messages messages, such as service
offers
offers, are forged by entities who do not possess the corresponding
secret key material. These attacks may be used to learn the identity
of a communicating party, actively or passively.
3.3.6. Sender Deniability
Deniability of sender activity, e.g., of broadcasting a discovery
request, may be desirable or necessary in some use cases. This
property ensures that eavesdroppers cannot prove senders issued a
specific message destined for one or more peers.
3.4. Operational Considerations
3.4.1. Power Management
Many modern devices, especially battery-powered devices, use power
management techniques to conserve energy. One such technique is for
a device to transfer information about itself to a proxy, which will
act on behalf of the device for some functions, functions while the device
itself goes to sleep to reduce power consumption. When the proxy
determines that some action is required required, which only the device itself
can perform, the proxy may have some way to wake the device, as
described for example in [SLEEP-PROXY].
In many cases, the device may not trust the network proxy
sufficiently to share all its confidential key material with the
proxy. This poses challenges for combining private discovery that
relies on per-query cryptographic operations, operations with energy-saving
techniques that rely on having (somewhat untrusted) network proxies
answer queries on behalf of sleeping devices.
3.4.2. Protocol Efficiency
Creating a discovery protocol that has the desired security
properties may result in a design that is not efficient. To perform
the necessary operations operations, the protocol may need to send and receive a
large number of network packets, packets or require an inordinate amount of
multicast transmissions. This may consume an unreasonable amount of
network capacity, particularly problematic when it is a shared
wireless spectrum. Further, it may cause an unnecessary level of
power consumption consumption, which is particularly problematic on battery
devices,
devices and may result in the discovery process being slow.
It is a difficult challenge to design a discovery protocol that has
the property of obscuring the details of what it is doing from
unauthorized observers, observers while also managing to perform efficiently.
3.4.3. Secure Initialization and Trust Models
One of the challenges implicit in the preceding discussions is that
whenever we discuss "trusted entities" versus "untrusted entities",
there needs to be some way that trust is initially established, established to
convert an "untrusted entity" into a "trusted entity".
The purpose of this document is not to define the specific way in
which trust can be established. Protocol designers may rely on a
number of existing technologies, including PKI, Trust On First Use
(TOFU), or using the use of a short passphrase or PIN with cryptographic
algorithms
algorithms, such as Secure Remote Password (SRP) [RFC5054] or a
Password Authenticated
Password-Authenticated Key Exchange like J-PAKE [RFC8236] using a
Schnorr Non-interactive Zero-Knowledge Proof [RFC8235].
Protocol designers should consider a specific usability pitfall when
trust is established immediately prior to performing discovery.
Users will have a tendency to "click OK" in order to achieve their
task. This implicit vulnerability is avoided if the trust
establishment requires more significant participation of the user,
such as entering a password or PIN.
3.4.4. External Dependencies
Trust establishment may depend on external parties. Optionally, this
might involve synchronous communication. Systems which that have such a
dependency may be attacked by interfering with communication to
external dependencies. Where possible, such dependencies should be
minimized. Local trust models are best for secure initialization in
the presence of active attackers.
4. Requirements for a DNS-SD Privacy Extension
Given the considerations discussed in the previous sections, we state
requirements for privacy preserving DNS-SD in the following
subsections.
Defining a solution according to these requirements is intended to
lead to a solution that does not transmit privacy-violating DNS-SD
messages and further does not open pathways to new attacks against
the operation of DNS-SD.
However, while this document gives advice on which privacy protecting
mechanisms should be used on deeper-layer network protocols and on
how to actually connect to services in a privacy-preserving way,
stating corresponding requirements is out of the scope of this
document. To mitigate attacks against privacy on lower layers, both
servers and clients must use privacy options available at lower
layers, and
layers and, for example example, avoid publishing static IPv4 or IPv6
addresses,
addresses or static IEEE 802 MAC Media Access Control (MAC) addresses.
For services advertised on a single network link, link local link-local IP
addresses should be used; see [RFC3927] and [RFC4291] for IPv4 and
IPv6, respectively. Static servers advertising services globally via
DNS can hide their IP addresses from unauthorized clients using the
split mode topology shown in [I-D.ietf-tls-esni]. Encrypted Server Name Indication [ESNI].
Hiding static MAC addresses can be achieved via MAC address
randomization (see [RFC7844]).
4.1. Private Client Requirements
For all three scenarios described in Section 3.1, client privacy
requires DNS-SD messages to:
1. Avoid disclosure of the client's identity, either directly or via
inference, to nodes other than select servers.
2. Avoid exposure of linkable identifiers that allow tracing client
devices.
3. Avoid disclosure of the client's interest in specific service
instances or service types to nodes other than select servers.
When listing and resolving services via current DNS-SD deployments,
clients typically disclose their interest in specific services types
and specific instances of these types, respectively.
In addition to the exposure and disclosure risks noted above,
protocols and implementations will have to consider fingerprinting
attacks (see Section 3.2.5) that could retrieve similar information.
4.2. Private Server Requirements
Servers like the "printer" discussed in scenario 1 Section 3.1.1 are public, but
the servers discussed in scenarios 2 Sections 3.1.2 and 3 are 3.1.3 are, by essence essence,
private. Server privacy requires DNS-SD messages to:
1. Avoid disclosure of the server's identity, either directly or via
inference, to nodes other than authorized clients. In
particular, Servers servers must avoid publishing static identifiers identifiers,
such as host names hostnames or service names. When those fields are
required by the protocol, servers should publish randomized
values. (See [RFC8117] for a discussion of host names.) hostnames.)
2. Avoid exposure of linkable identifiers that allow tracing
servers.
3. Avoid disclosure to unauthorized clients of service instance
names Service Instance
Names or service types of offered services.
4. Avoid disclosure to unauthorized clients of information about the
services they offer.
5. Avoid disclosure of static IPv4 or IPv6 addresses.
When offering services via current DNS-SD deployments, servers
typically disclose their hostnames (SRV, A/AAAA), instance names of
offered services (PRT, (PTR, SRV), and information about services (TXT).
Heeding these requirements protects a server's privacy on the DNS-SD
level.
The current DNS-SD user interfaces present the list of discovered
service names to the users, users and let them pick a service from the list.
Using random identifiers for service names renders that UI flow
unusable. Privacy-respecting discovery protocols will have to solve
this issue, for example example, by presenting authenticated or decrypted
service names instead of the randomized values.
4.3. Security and Operation
In order to be secure and feasible, a DNS-SD privacy extension needs
to consider security and operational requirements including:
1. Avoiding significant CPU overhead on nodes or significantly
higher network load. Such overhead or load would make nodes
vulnerable to denial of service denial-of-service attacks. Further, it would
increase power consumption consumption, which is damaging for IoT devices.
2. Avoiding designs in which a small message can trigger a large
amount of traffic towards an unverified address, as this could be
exploited in amplification attacks.
5. IANA Considerations
This draft does not require any document has no IANA action. actions.
6. Acknowledgments
This draft incorporates many contributions from Stuart Cheshire and
Chris Wood. Thanks to Florian Adamsky for extensive review and
suggestions on the organization of the threat model. Thanks to Barry
Leiba for an extensive review. Thanks to Roman Danyliw, Ben Kaduk,
Adam Roach and Alissa Cooper for their comments during IESG review.
7. References
7.1.
6.1. Normative References
[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>.
7.2.
6.2. Informative References
[I-D.ietf-dnssd-srp]
Cheshire, S. and T. Lemon, "Service Registration Protocol
for DNS-Based Service Discovery", draft-ietf-dnssd-srp-02
(work in progress), July 2019.
[I-D.ietf-tls-esni]
[ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood,
"Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-05 (work "TLS
Encrypted Client Hello", Work in progress), November 2019. Progress, Internet-Draft,
draft-ietf-tls-esni-07, June 1, 2020,
<https://tools.ietf.org/html/draft-ietf-tls-esni-07>.
[K17] Kaiser, D., "Efficient Privacy-Preserving
Configurationless Service Discovery Supporting Multi-Link
Networks", August 2017,
<http://nbn-resolving.de/urn:nbn:de:bsz:352-0-422757>.
<https://nbn-resolving.de/urn:nbn:de:bsz:352-0-422757>.
[KW14a] Kaiser, D. and M. Waldvogel, "Adding Privacy to Multicast
DNS Service Discovery", DOI 10.1109/TrustCom.2014.107,
September 2014, <http://ieeexplore.ieee.org/xpl/ <https://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=7011331>.
[KW14b] Kaiser, D. and M. Waldvogel, "Efficient Privacy Preserving
Multicast DNS Service Discovery",
DOI 10.1109/HPCC.2014.141, August 2014,
<http://ieeexplore.ieee.org/xpl/
<https://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=7056899>.
[RFC1033] Lottor, M., "Domain Administrators Operations Guide",
RFC 1033, DOI 10.17487/RFC1033, November 1987,
<https://www.rfc-editor.org/info/rfc1033>.
[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>.
[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>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, DOI 10.17487/RFC5054, November
2007, <https://www.rfc-editor.org/info/rfc5054>.
[RFC7558] Lynn, K., Cheshire, S., Blanchet, M., and D. Migault,
"Requirements for Scalable DNS-Based Service Discovery
(DNS-SD) / Multicast DNS (mDNS) Extensions", RFC 7558,
DOI 10.17487/RFC7558, July 2015,
<https://www.rfc-editor.org/info/rfc7558>.
[RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC7844, May 2016,
<https://www.rfc-editor.org/info/rfc7844>.
[RFC8117] Huitema, C., Thaler, D., and R. Winter, "Current Hostname
Practice Considered Harmful", RFC 8117,
DOI 10.17487/RFC8117, March 2017,
<https://www.rfc-editor.org/info/rfc8117>.
[RFC8235] Hao, F., Ed., "Schnorr Non-interactive Zero-Knowledge
Proof", RFC 8235, DOI 10.17487/RFC8235, September 2017,
<https://www.rfc-editor.org/info/rfc8235>.
[RFC8236] Hao, F., Ed., "J-PAKE: Password-Authenticated Key Exchange
by Juggling", RFC 8236, DOI 10.17487/RFC8236, September
2017, <https://www.rfc-editor.org/info/rfc8236>.
[SLEEP-PROXY]
Cheshire, S., "Understanding Sleep Proxy Service",
December 2009,
<http://stuartcheshire.org/SleepProxy/index.html>.
[SRP] Lemon, T. and S. Cheshire, "Service Registration Protocol
for DNS-Based Service Discovery", Work in Progress,
Internet-Draft, draft-ietf-dnssd-srp-04, July 13, 2020,
<https://tools.ietf.org/html/draft-ietf-dnssd-srp-04>.
Acknowledgments
This document incorporates many contributions from Stuart Cheshire
and Chris Wood. Thanks to Florian Adamsky for extensive review and
suggestions on the organization of the threat model. Thanks to Barry
Leiba for an extensive review. Thanks to Roman Danyliw, Ben Kaduk,
Adam Roach, and Alissa Cooper for their comments during IESG review.
Authors' Addresses
Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
U.S.A.
United States of America
Email: huitema@huitema.net
URI: http://privateoctopus.com/
Daniel Kaiser
University of Luxembourg
6, avenue de la Fonte
L-4364 Esch-sur-Alzette 4364
Luxembourg
Email: daniel.kaiser@uni.lu
URI: https://secan-lab.uni.lu/