RTCWEB                                                         Y. Fablet
Internet-Draft                                                Apple Inc.
Intended status: Informational                               J. de Borst
Expires: April 25, 2019                                        J. Uberti
                                                                 Q. Wang
                                                        October 22, 2018

  Using Multicast DNS to protect privacy when exposing ICE candidates


   WebRTC applications collect ICE candidates as part of the process of
   creating peer-to-peer connections.  To maximize the probability of a
   direct peer-to-peer connection, client private IP addresses are
   included in this candidate collection.  However, disclosure of these
   addresses has privacy implications.  This document describes a way to
   share local IP addresses with other clients while preserving client
   privacy.  This is achieved by obfuscating IP addresses with
   dynamically generated Multicast DNS (mDNS) [RFC6762] names.

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
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   This Internet-Draft will expire on April 25, 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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Principle . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  ICE Candidate Gathering . . . . . . . . . . . . . . . . .   3
     3.2.  ICE Candidate Processing  . . . . . . . . . . . . . . . .   4
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
     5.1.  Statistics  . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Interactions With TURN Servers  . . . . . . . . . . . . .   6
     5.3.  Generated Names Reuse . . . . . . . . . . . . . . . . . .   7
     5.4.  Specific Browsing Contexts  . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     6.1.  mDNS Message Flooding . . . . . . . . . . . . . . . . . .   7
     6.2.  Malicious Responses to Deny Name Registration . . . . . .   8
     6.3.  Monitoring of Sessions  . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Specification Requirements  . . . . . . . . . . . . . . . . .   9
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   As detailed in [IPHandling], exposing client private IP addresses by
   default maximizes the probability of successfully creating direct
   peer-to-peer connection between two clients, but creates a
   significant surface for user fingerprinting.  [IPHandling] recognizes
   this issue, but also admits that there is no current solution to this
   problem; implementations that choose to use Mode 3 to address the
   privacy concerns often suffer from failing or suboptimal connections
   in WebRTC applications.  This is particularly an issue on unmanaged
   networks, typically homes or small offices, where NAT loopback may
   not be supported.

   This document proposes an overall solution to this problem by
   registering ephemeral mDNS names for each local private IP address,
   and then providing those names, rather than the IP addresses, to the
   web application when it gathers ICE candidates.  WebRTC
   implementations resolve these names to IP addresses and perform ICE

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   processing as usual, but the actual IP addresses are not exposed to
   the web application.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Principle

   This section uses the concept of ICE agent as defined in [RFC8445].
   In the remainder of the document, it is assumed that each browsing
   context (as defined in Section 7.1 of [HTMLSpec]) has its own ICE

3.1.  ICE Candidate Gathering

   For any host candidate gathered by an ICE agent as part of [RFC8445],
   Section 5.1.1, the candidate is handled as follows:

   1.  Check whether the ICE agent has a usable registered mDNS hostname
       resolving to the ICE candidate's IP address.  If one exists, skip
       ahead to Step 6.

   2.  Generate a unique mDNS hostname.  The unique name MUST consist of
       a version 4 UUID as defined in [RFC4122], followed by ".local".

   3.  Register the candidate's mDNS hostname as defined in [RFC6762].

   4.  If registering of the mDNS hostname fails, abort these steps.
       The candidate is not exposed.

   5.  Store the mDNS hostname and its related IP address in the ICE
       agent for future reuse.

   6.  Replace the IP address of the ICE candidate with its mDNS
       hostname and provide the candidate to the web application.

   An ICE agent can implement this procedure in any way so long as it
   produces equivalent results to this procedure.

   An implementation may for instance pre-register mDNS hostnames by
   executing steps 3 to 5 and prepopulate an ICE agent accordingly.  By
   doing so, only step 6 of the above procedure will be executed at the
   time of gathering candidates.

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   An implementation may also detect that mDNS is not supported by the
   available network interfaces.  The ICE agent may skip steps 2 and 3
   and directly decide to not expose the host candidate.

   This procedure ensures that an mDNS name is used to replace only one
   IP address.  Specifically, an ICE agent using an interface with both
   IPv4 and IPv6 addresses MUST expose a different mDNS name for each

   Any server-reflexive candidates generated from an mDNS local
   candidate MUST have their raddr field set to and their rport
   field set to 0.

   Any candidates exposed to the web application via local descriptions
   MUST be identical to those provided during candidate gathering (i.e.,
   MUST NOT contain private host IP addresses).

3.2.  ICE Candidate Processing

   For any remote ICE candidate received by the ICE agent, the following
   procedure is used:

   1.  If the connection-address field value of the ICE candidate does
       not end with ".local" or if the value contains more than one ".",
       then process the candidate as defined in [RFC8445].

   2.  Otherwise, resolve the candidate using mDNS.

   3.  If it resolves to an IP address, replace the mDNS hostname of the
       ICE candidate with the resolved IP address and continue
       processing of the candidate.

   4.  Otherwise, ignore the candidate.

   An ICE agent may use a hostname resolver that transparently supports
   both Multicast and Unicast DNS.  In this case the resolution of a
   ".local" name may happen through Unicast DNS as noted in [RFC6762],
   Section 3.

   An ICE agent that supports mDNS candidates MUST support the situation
   where the hostname resolution results in more than one IP address.
   In this case, the ICE agent MUST take exactly one of the resolved IP
   addresses and ignore the others.  The ICE agent SHOULD, if available,
   use the first IPv6 address resolved, otherwise the first IPv4

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4.  Examples

   In this example, mDNS candidates are exchanged between peers and
   resolved to obtain the corresponding IP addresses.

              ICE Agent 1 (             ICE Agent 2 (
        <Register     |                                 |
         mDNS name N1 |                                 |
         for> |                                 |
                      |------- mDNS Candidate N1 ------>|
                      |                                 | <Register
                      |                                 |  mDNS name N2
                      |                                 |  for>
                      |<------ mDNS Candidate N2 -------|
       <Resolve       |                                 | <Resolve
        mDNS name N2> |                                 |  mDNS name N1>
                      |<==== STUN check to =====|
                      |===== STUN check to ====>|
                      |                                 |

   The following two examples indicate how peer-reflexive candidates for
   host IP addresses can be created due to timing differences.

   In this example, a peer-reflexive candidate is generated because the
   mDNS candidate is signaled after the STUN checks begin.

              ICE Agent 1 (             ICE Agent 2 (
        <Register     |                                 |
         mDNS name N1 |                                 |
         for> |                                 |
                      |------- mDNS Candidate N1 ------>|
                      |                                 | <Resolve
                      |                                 |  mDNS name N1>
                      |<==== STUN check to =====|
      prflx candidate |                                 | <Register created |                                 |  mDNS name N2
                      |                                 |  for>
                      |<------ mDNS Candidate N2 -------|
                      |                                 |

   In this example, a peer-reflexive candidate is generated because the
   mDNS resolution for name N2 does not complete until after the STUN
   checks are received.

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              ICE Agent 1 (             ICE Agent 2 (
        <Register     |                                 | <Register
         mDNS name N1 |                                 |  mDNS name N2
         for> |                                 |  for>
                      |------- mDNS Candidate N1 ------>|
                      |<------ mDNS Candidate N2 -------|
<Resolve              |                                 | <Resolve
 ...                  |                                 |  mDNS name N1>
 mDNS                 |<==== STUN check to =====|
 ...  prflx candidate |                                 |
 name created |                                 |
 ...                  |                                 |
 N2>                  |                                 |

5.  Privacy Considerations

   The goal of this mechanism is to keep knowledge of private host IP
   addresses within the ICE agent while continuing to allow the
   application to transmit ICE candidates.  Besides keeping private host
   IP addresses out of ICE candidates, implementations must take steps
   to prevent these IP addresses from being exposed to web applications
   through other means.

5.1.  Statistics

   Statistics related to ICE candidates that are accessible to the web
   application MUST NOT contain the IP address of a local or remote mDNS
   candidate; the mDNS name SHOULD be used instead.

   In addition, a peer-reflexive remote candidate may be constructed
   from a remote host IP address as a result of an ICE connectivity
   check, as described in Section of [RFC8445].  This check may
   arrive before the candidate due to signaling or mDNS resolution
   delays, as shown in the examples above.

   To prevent disclosure of the host IP address to the application in
   this scenario, statistics related to ICE candidates MUST NOT contain
   the IP address of any peer-reflexive candidate, unless that IP has
   already been learned through signaling of a candidate with the same
   address and either the same or a different port; this includes cases
   where the signaled candidate is discarded as redundant according to
   Section 5.1.3 of [RFC8445].

5.2.  Interactions With TURN Servers

   When sending data to a TURN [RFC5766] server, the sending client
   tells the server the destination IP and port for the data.  This
   means that if the client uses TURN to send to an IP that was obtained

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   by mDNS resolution, the TURN server will learn the underlying host IP
   and port, and this information can then be relayed to the web
   application, defeating the value of the mDNS wrapping.

   To prevent disclosure of the host IP address to a TURN server, the
   ICE agent MUST NOT form candidate pairs between its own relay
   candidates and remote mDNS candidates.  Note that the converse is not
   an issue; the ICE agent MAY form candidate pairs between its own mDNS
   candidates and remote relay candidates, as in this situation host IPs
   will not be sent directly to the TURN server.

   This restriction has no effect on connectivity; in the cases where
   host IP addresses are private and need to be wrapped with mDNS names,
   they will be unreachable from the TURN server, and as noted above,
   the reverse path will continue to work normally.

5.3.  Generated Names Reuse

   It is important that use of registered mDNS hostnames is limited in
   time and/or scope.  Indefinitely reusing the same mDNS hostname
   candidate would provide applications an even more reliable tracking
   mechanism than the private IP addresses that this specification is
   designed to hide.  The use of registered mDNS hostnames SHOULD be
   scoped by origin, and SHOULD have the lifetime of the page.

5.4.  Specific Browsing Contexts

   As noted in [IPHandling], privacy may be breached if a web
   application running in two browsing contexts can determine whether it
   is running on the same device.  While the approach in this document
   prevents the application from directly comparing local private IP
   addresses, a successful local WebRTC connection can also present a
   threat to user privacy.  Specifically, when the latency of a WebRTC
   connection latency is close to zero, the probability is high that the
   two peers are running on the same device.

   To avoid this issue, browsers SHOULD NOT register mDNS names for
   WebRTC applications running in a third-party browsing context (i.e.,
   a context that has a different origin than the top-level browsing
   context), or a private browsing context.

6.  Security Considerations

6.1.  mDNS Message Flooding

   The implementation of this proposal requires the mDNS querying
   capability of the browser for registering mDNS names or adding remote
   ICE host candidates with such names.  It also requires the mDNS

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   responding capability of either the browser or the operating platform
   of the browser for registering, removing or resolving mDNS names.  In

   o  the registration of name requires optional probing queries and
      mandatory announcing responses ([RFC6762], Section 8), and this is
      performed at the beginning of ICE gathering;

   o  the addition of remote ICE host candidates with mDNS names
      generates mDNS queries for names of each candidate;

   o  the removal of names could happen when the browsing context of the
      ICE agent is destroyed in an implementation, and goodbye responses
      should be sent to invalidate records generated by the ICE agent in
      the local network ([RFC6762], Section 10.1).

   A malicious Web application could flood the local network with mDNS
   messages by:

   o  creating browsing contexts that create ICE agents and start
      gathering of local ICE host candidates;

   o  destroying these local candidates soon after the name registration
      is done;

   o  adding fictitious remote ICE host candidates with mDNS names.

   [RFC6762] defines a per-record multicast rate limiting rule, in which
   a given record on a given interface cannot be sent less than one
   second since its last transmission.  This rate limiting rule however
   does not mitigate the above attacks, in which new names, hence new
   records, are constantly created and sent.  A browser-wide mDNS
   message rate limit MUST be provided for all messages that can be
   indirectly dispatched by a web application, namely the probing
   queries, announcement responses, resolution queries, and goodbye
   responses associated with mDNS.

6.2.  Malicious Responses to Deny Name Registration

   If the optional probing queries are implemented for the name
   registration, a malicious endpoint in the local network, which is
   capable of responding mDNS queries, could send responses to block the
   use of the generated names.  This would lead to the discarding of
   this ICE host candidate as in Step 5 in Section 3.1.

   The above attack can be mitigated by skipping the probing when
   registering a name, which also conforms to Section 8 in [RFC6762],
   given that the name is randomly generated for the probabilistic

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   uniqueness (e.g. a version 4 UUID) in Step 3 in Section 3.1.
   However, a similar attack can be performed by exploiting the negative
   responses (defined in [RFC6762], Section 8.1), in which NSEC resource
   records are sent to claim the nonexistence of records related to the
   gathered ICE host candidates.

   The existence of malicious endpoints in the local network poses a
   generic threat, and requires dedicated protocol suites to mitigate,
   which is beyond the scope of this proposal.

6.3.  Monitoring of Sessions

   A malicious endpoint in the local network may also record other
   endpoints who are registering, unregistering, and resolving mDNS
   names.  By doing so, they can create a session log that shows which
   endpoints are communicating, and for how long.  If both endpoints in
   the session are on the same network, the fact they are communicating
   can be discovered.

   As above, mitigation of this threat is beyond the scope of this

7.  IANA Considerations

   This document requires no actions from IANA.

8.  Specification Requirements

   The proposal relies on identifying and resolving any mDNS-based ICE
   candidates as part of adding/processing a remote candidate.  [ICESDP]
   section 4.1 could be updated to explicitly allow mDNS names in the
   connection-address field.

   The proposal relies on adding the ability to register mDNS names at
   ICE gathering time.  This could be described in [ICESDP] and/or

   The proposal allows updating [IPHandling] so that mode 2 is not the
   mode used by default when user consent is not required.  Instead, the
   default mode could be defined as mode 3 with mDNS-based ICE

9.  Informative References

              "HTML Living Standard", n.d.,

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   [ICESDP]   Keranen, A., "Session Description Protocol (SDP) Offer/
              Answer procedures for Interactive Connectivity
              Establishment (ICE)", April 2018,

              Shieh, G., "WebRTC IP Address Handling Requirements",
              April 2018, <https://tools.ietf.org/html/

   [IPLeak]   "IP/DNS Detect", n.d., <https://ipleak.net>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,

              Bruaroey, J., "The WebRTC specification", n.d.,

Authors' Addresses

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   Youenn Fablet
   Apple Inc.

   Email: youenn@apple.com

   Jeroen de Borst

   Email: jeroendb@google.com

   Justin Uberti

   Email: juberti@google.com

   Qingsi Wang

   Email: qingsi@google.com

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