draft-huitema-dnssd-privreq.xml

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<rfc category="info" 
     docName="draft-huitema-dnssd-prireq-00"
     ipr="trust200902">

<front>
    <title abbrev="DNS-SD Privacy Requirements">
      DNS-SD Privacy and Security Requirements
    </title>

   <author fullname="Christian Huitema" initials="C." surname="Huitema">
      <organization>Private Octopus Inc.</organization>
      <address>
        <postal>
          <street></street>
          <city>Friday Harbor</city>
          <code>98250</code>
          <region>WA</region>
          <country>U.S.A.</country>
        </postal>
        <email>huitema@huitema.net</email>
        <uri>http://privateoctopus.com/</uri>
      </address>
    </author>

    <date year="2018" />

    <abstract>
        <t>
DNS-SD (DNS Service Discovery) normally discloses information about devices offering and 
requesting services. This information includes host names, network parameters, and possibly 
a further description of the corresponding service instance. Especially when mobile devices 
engage in DNS Service Discovery over Multicast DNS at a public hotspot, serious privacy 
problems arise. We analyze the requirements of a privacy respecting discovery service.
</t>
    </abstract>
</front>

<middle>
<section title="Introduction">
<t>
DNS-SD <xref target="RFC6763" /> over mDNS <xref target="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 <xref target="KW14a" />, <xref target="KW14b" /> and <xref target="K17" />.
</t>
<t>
There are cases when nodes connected to a network want to provide
or consume services without exposing their identity 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 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.
</t>
<t>
We expect that these exchanges will start with a discovery 
procedure using DNS-SD <xref target="RFC6763" /> over mDNS <xref target="RFC6762" />.
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, and then connect to it.
</t>
<t>
When analyzing these scenarios in <xref target="analysis"/>, we find that
the DNS-SD messages leak identifying information such as the instance name,
the host name or service properties.
</t>
<section title="Requirements">
<t>
  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in <xref target="RFC2119" />.
</t>
</section>
</section>

<section title="DNS-SD Discovery Scenarios" anchor="scenarios">
<t>
DNS-Based Service Discovery (DNS-SD) is defined in <xref target="RFC6763" />.
It allows nodes to publish the availability of an instance of a service by
inserting specific records in the DNS (<xref target="RFC1033"/>,
<xref target="RFC1034"/>, <xref target="RFC1035"/>) or by publishing
these records locally using
multicast DNS (mDNS) <xref target="RFC6762"/>.
Available services are described using three types of records:
</t>
<t>
<list style="hanging">
<t hangText="PTR Record:">Associates a service type in the domain with
an "instance" name of this service type.
</t>
<t hangText="SRV Record:">Provides the node name, port number, priority and
weight associated with the service instance, in conformance with <xref target="RFC2782" />.
</t>
<t hangText="TXT Record:">Provides a set of attribute-value pairs describing
specific properties of the service instance.
</t>
</list>
</t>

<t>In the remaining sections, we review common discovery scenarios provided
by DNS-SD and discuss their privacy requirements.</t>

<section title="Private client and public server" anchor="privclipubserv" >

<t>
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 hotel or at an airport.
</t>
<t>
<figure>
<artwork>
                                     ( Taking notes:
                                     ( David is printing
                                     ( a document
                                      ~~~~~~~~~~~
                                                  o
         ___                                        o   ___
        /   \                                         _|___|_
        |   |                                          |* *|
         \_/      __                                    \_/
          |      / /   Discovery   +----------+          |
         /|\    /_/  &lt;-----------&gt; |  +----+  |         /|\
        / | \__/                   +--|    |--+        / | \
       /  |                           |____/          /  |  \
      /   |                                          /   |   \
         / \                                            / \
        /   \                                          /   \
       /     \                                        /     \
      /       \                                      /       \
     /         \                                    /         \

</artwork>
</figure>
</t>
<t>
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 just for
tracking people or as a preliminary to targeted attacks.
</t>
<t>
The requirement in that scenario is that the discovery activity
should not disclose the identity of the client.
</t>
</section>

<section title="Private client and private server" anchor="privcliprivserv" >
<t>
The second private discovery scenario involves 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 for 
example an airport's lounge.
</t>
<t>
<figure>
<artwork>
                                        ( Taking notes:
                                        ( David is meeting
                                        ( with Stuart
                                          ~~~~~~~~~~~
                                                     o
         ___                               ___         o   ___
        /   \                             /   \          _|___|_
        |   |                             |   |           |* *|
         \_/      __               __      \_/             \_/
          |      / /   Discovery   \ \      |               |
         /|\    /_/  &lt;-----------&gt;  \_\    /|\             /|\
        / | \__/                       \__/ | \           / | \
       /  |                                 |  \         /  |  \
      /   |                                 |   \       /   |   \
         / \                               / \             / \
        /   \                             /   \           /   \
       /     \                           /     \         /     \
      /       \                         /       \       /       \
     /         \                       /         \     /         \

</artwork>
</figure>
</t>
<t>
In that scenario, the collaborative application on one of the device will
act as server, and the application on the other device will act as client.
The server wants to be discovered by the 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, and also attempting to discover the patterns of traffic,
as these patterns reveal the business and social interactions between
the owners of the devices.
</t>
<t>
The requirement in that scenario is that the discovery activity
should not disclose the identity of either the client or the server.
</t>
</section>

<section title="Wearable client and server" anchor="wearcliserv" >
<t>
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.
</t>
<t>
<figure>
<artwork>

                                     ( Taking notes:
                                     ( David' is here. His watch is 
                                     ( talking to his phone
                                       ~~~~~~~~~~~
                                                   o
         ___                                         o  ___
        /   \                                         _|___|_
        |   |                                          |* *|
         \_/                                            \_/
          |     _/                                       |
         /|\   //                                       /|\
        / | \__/  ^                                    / | \
       /  |__     | Discovery                         /  |  \
      /   |\ \    v                                  /   |   \
         / \\_\                                         / \
        /   \                                          /   \
       /     \                                        /     \
      /       \                                      /       \
     /         \                                    /         \

</artwork>
</figure>
</t>
<t>
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 that the discovery traffic not disclose 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 device. There is
also an added emphasis in hiding the type of devices that the person
wears.
</t>
<t>
In addition to tracking the identity of the owner of the devices, the adversary
is interested by 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, allowing identification.
</t>
</section>
</section>

<section title="Privacy Considerations" anchor="analysis">

<t>The discovery scenarios in Section <xref target="scenarios"/> illustrate three
separate privacy requirements that vary based on use case:
<list style="numbers">
  <t>Client identity privacy: Client identities are not leaked during service discovery
    or use.</t>
  <t>Multi-owner, mutual client and server identity privacy: Neither client nor server identities are
    leaked during service discovery or use.</t>
  <t>Single-owner, mutual client and server identity privacy: Identities of clients and servers
    owned and managed by the same application, device, or user are not leaked during
    service discovery or use.</t>
</list>
</t>

<t>In the remaining subsections, we describe aspects of DNS-SD that make these requirements
difficult to achieve in practice.</t>

<section title="Privacy Implication of Publishing Service Instance Names" anchor="instanceLeak" >
<t>
In the first phase of discovery, client 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 <xref target="RFC6763" />:
</t>

<t>
<figure>
<artwork>
  Service Instance Name = &lt;Instance&gt; . &lt;Service&gt; . &lt;Domain&gt;
</artwork>
</figure>
</t>

<t>
The &lt;Instance&gt; 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 <xref target="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, the list of 
available service instances may look like:
</t>
<t>
<figure>
<artwork>
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
</artwork>
</figure>
</t>
<t>
Alice will see the list on her phone and understand intuitively that she should
pick the first item. The discovery will "just work".
</t>
<t>
However, DNS-SD/mDNS will reveal to anybody that Alice is currently visiting the Internet 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, but to anybody passively listening to the network traffic.
</t>
</section>

<section title="Privacy Implication of Publishing Node Names">
<t>
The SRV records contain the DNS name of the node publishing the
service. Typical implementations construct this DNS name by
concatenating the "host name" of the node with the name of the 
local domain. The privacy implications of this
practice are reviewed in <xref target="RFC8117" />.
Depending on naming practices, the host name is either a strong 
identifier of the device, or at a minimum a partial identifier.
It enables tracking of both the device, and, by extension, the device's owner.
</t>
</section>

<section title="Privacy Implication of Publishing Service Attributes">
<t>
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:
</t>
<t>
<list style="symbols">
<t>
Some attributes like the paper size available in a printer, are the
same on many devices, and thus only provide limited information
to a tracker.
</t>
<t>
Attributes that have freeform values, such as the name of a directory,
may reveal much more information.
</t>
</list>
</t>
<t>
Combinations of attributes have more information power than specific attributes,
and can potentially be used for "fingerprinting" a specific device.
</t>

<t>
Information contained in TXT records does not only breach privacy by making devices
trackable, but might directly contain private information about the user.
For instance the _presence service reveals the "chat status" to everyone in the same network.
Users might not be aware of that.
</t>

<t>
  Further, TXT records often contain version information about services allowing potential attackers
  to identify devices running exploit-prone versions of a certain service.
</t>

</section>

<section title="Device Fingerprinting" anchor="serverFingerprint">
<t>
The combination of information published in DNS-SD has the potential to
provide a "fingerprint" of a specific device. Such information includes: 
</t>
<t>
<list style="symbols">
<t>
List of services published by the device, which can be retrieved because the
SRV records will point to the same host name.
</t>
<t>
Specific attributes describing these services.
</t>
<t>
Port numbers used by the services.
</t>
<t>
Priority and weight attributes in the SRV records.
</t>
</list>
</t>
<t>
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.
</t>

<t>
A sometimes heard argument is that devices providing services can be identified
by observing the local traffic, and that trying to hide the presence of the service
is futile.
This argument, however, does not carry much weight because
<list style="numbers">
<t>
Proving 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.
</t>
<t>
Further, even if the application layer does not protect privacy,
it is hard to record and analyse the unicast traffic (which most applications will generate)
compared to just listening to the multicast messages sent by DNS-SD/mDNS.
</t>
</list>
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, and should as much as
possible avoid providing new fingerprinting information.
</t>
</section>

<section title="Privacy Implication of Discovering Services" anchor="clientPrivacy" >
<t>
The consumers of services engage in discovery, and in doing so
reveal some 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 for example some private messaging services.
</t>
<t>
One way to protect clients would be to somehow encrypt the requested service types.
Of course, just as we noted in <xref target="serverFingerprint"/>, traffic
analysis can often reveal the service. 
</t>
</section>
</section>

<section title="Security Considerations">
  <t>For each of the operations described above, we must also consider security
  threats we are concerned about.</t>
  <section title="Authenticity, Integrity &amp; Freshness">
    <t>Can we trust the information we receive?
    Has it been modified in flight by an adversary?
    Do we 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.</t>
  </section>

  <section title="Confidentiality">
    <t>Confidentiality is about restricting information access to only
      authorized individuals. Ideally this should only be the appropriate 
      trusted parties, though it can be challenging to define who are "the 
      appropriate trusted parties." In some uses cases, this may mean that only 
      mutually authenticated and trusting clients and servers can read messages sent for one another. 
      The "Discover" operation 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.
    </t>
  </section>

  <section title="Resistance to Dictionary Attacks">
    <t>It can be tempting to use (publicly computable) hash functions to obscure sensitive identifiers.
    This transforms a sensitive unique identifier such as an email address
    into a "scrambled" (but still unique) identifier. Unfortunately simple solutions 
    may be vulnerable to offline dictionary attacks.</t>
  </section>

  <section title="Resistance to Denial-of-Service Attack">
    <t>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 battery power) just processing and discarding those messages.</t>
  </section>

  <section title="Resistance to Sender Impersonation">
    <t>Sender impersonation is an attack wherein messages such as service 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.</t>
  </section>

  <section title="Sender Deniability">
    <t>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. </t>
  </section>
</section>

<section title="Operational Considerations">
  <section title="Power Management">

    <t>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,
    while the device itself goes to sleep to reduce power consumption.
    When the proxy determines that some action is required which only
    the device itself can perform, the proxy may have some way
    (such as Ethernet "Magic Packet") to wake the device.</t>

    <t>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,
    with energy-saving techniques that rely on having (somewhat untrusted)
    network proxies answer queries on behalf of sleeping devices.</t>

  </section>

  <section title="Protocol Efficiency">

    <t>Creating a discovery protocol that has the desired security
    properties may result in a design that is not efficient.
    To perform the necessary operations the protocol may
    need to send and receive a large number of network packets.
    This may consume an unreasonable amount of network capacity
    (particularly problematic when it's shared wireless spectrum),
    cause an unnecessary level of power consumption
    (particularly problematic on battery devices)
    and may result in the discovery process being slow.</t>

    <t>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, while also managing to do that quickly and efficiently.</t>

  </section>

  <section title="Secure Initialization and Trust Models">

    <t>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,
    to convert an "untrusted entity" into a "trusted entity".</t>

    <t>One way to establish trust between two entities is
    to trust a third party to make that determination for us.
    For example, the X.509 certificates used by TLS and HTTPS web browsing
    are based on the model of trusting a third party to tell us who to trust.
    There are some difficulties in using this model for establishing
    trust for service discovery uses.
    If we want to print our tax returns or medical documents on "our"
    printer, then we need to know which printer on the network we can
    trust be be "our" printer.
    All of the printers we discover on the network may be legitimate
    printers made by legitimate printer manufacturers, but not all of
    them are "our" printer. A third-party certificate authority cannot
    tell us which one of the printers is ours.</t>

    <t>Another common way to establish a trust relationship is
    Trust On First Use (TOFU), as used by ssh.
    The first usage is a Leap Of Faith, but after that public keys
    are exchanged and at least we can confirm that subsequent
    communications are with the same entity.
    In today's world, where there may be attackers present even at that
    first use, it would be preferable to be able to establish a trust relationship
    without requiring an initial Leap Of Faith.</t>

    <t>Techniques now exist for securely establishing a trust relationship
    without requiring an initial Leap Of Faith.
    Trust can be established securely using a short passphrase or PIN with
    cryptographic algorithms such as
    <xref target="RFC5054">Secure Remote Password (SRP)</xref>
    or a Password Authenticated Key Exchange like
    <xref target="RFC8236">J-PAKE</xref>
    using a
    <xref target="RFC8235">Schnorr Non-interactive Zero-Knowledge Proof</xref>.
    </t>

    <t>Such techniques require a user to enter the correct passphrase or PIN
    in order for the cryptographic algorithms to establish working communication.
    This avoids the human tendency to simply press the "OK" button when asked
    if they want to do something on their electronic device.
    It removes the human fallibility element from the equation,
    and avoids the human users inadvertently sabotaging their own security.</t>

    <t>Using these techniques, if a user tries to print their tax return
    on a printer they’ve never used before (even though the name looks right)
    they’ll be prompted to enter a pairing PIN, and the user
    *cannot* ignore that warning. They can’t just press an “OK” button.
    They have to walk to the printer and read the displayed PIN and enter it.
    And if the intended printer is not displaying a pairing PIN, or is displaying
    a different pairing PIN, that means the user may be being spoofed,
    and the connection will not succeed, and the failure will not reveal
    any secret information to the attacker.
    As much as the human desires to "just give me an OK button to make it print"
    (and the attacker desires them to click that OK button too)
    the cryptographic algorithms do not give the user the ability to opt
    out of the security, and consequently do not give the attacker
    any way to persuade the user to opt out of the security protections.</t>

  </section>

  <section title="External Dependencies">
    <t>Trust establishment may depend on external, and optionally online, parties. 
    Systems which 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.</t>
  </section>
</section>

<section title="IANA Considerations" anchor="iana">
<t> 
This draft does not require any IANA action.
</t> 
</section>

<section title="Acknowledgments">
    <t>This draft incorporates many contributions from Stuart Cheshire and Chris Wood.
    </t>
</section>

</middle>

<back>
  
<references title="Informative References">

  &rfc2119;
  &rfc6763;
  &rfc1033;
  &rfc1034;
  &rfc1035;
  &rfc2782;
  &rfc5054;
  &rfc6762;
  &rfc8117;
  &rfc8235;
  &rfc8236;
<reference anchor="KW14a" target="http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7011331">
  <front>
    <title>Adding Privacy to Multicast DNS Service Discovery</title>
    <author initials="D." surname="Kaiser" fullname="Daniel Kaiser">
      <organization/>
    </author>
    <author initials="M." surname="Waldvogel" fullname="Marcel Waldvogel">
      <organization/>
    </author>
    <date year="2014"/>
  </front>
  <seriesInfo name="DOI" value="10.1109/TrustCom.2014.107"/>
</reference>

<reference anchor="KW14b" target="http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7056899">
  <front>
    <title>Efficient Privacy Preserving Multicast DNS Service Discovery</title>
    <author initials="D." surname="Kaiser" fullname="Daniel Kaiser">
      <organization/>
    </author>
    <author initials="M." surname="Waldvogel" fullname="Marcel Waldvogel">
      <organization/>
    </author>
    <date year="2014"/>
  </front>
  <seriesInfo name="DOI" value="10.1109/HPCC.2014.141"/>
</reference>

<reference anchor="K17" target="http://nbn-resolving.de/urn:nbn:de:bsz:352-0-422757">
  <front>
    <title>Efficient Privacy-Preserving Configurationless Service Discovery Supporting Multi-Link Networks</title>
    <author initials="D." surname="Kaiser" fullname="Daniel Kaiser">
      <organization/>
    </author>
    <date year="2017"/>
  </front>
</reference>

</references>  

</back>
</rfc>