RTCWEB                                                         Y. Fablet
Internet-Draft                                                Apple Inc.
Intended status: Informational                               J. De Borst
Expires: March 16, 2019                                        J. Uberti
                                                                 Q. Wang
                                                      September 12, 2018

  Using Multicast DNS to protect privacy when exposing ICE candidates


   WebRTC applications rely on ICE candidates to enable peer-to-peer
   connections between clients in as many network configurations as
   possible.  To maximize the probability to create a direct peer-to-
   peer connection, client private IP addresses are often exposed
   without user consent.  This is currently used as a way to track
   users.  This document describes a way to share IP addresses with
   other clients while preserving client privacy.  This is achieved by
   obfuscating IP addresses using dynamically generated names resolvable
   through Multicast DNS [RFC6763].

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 March 16, 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

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   (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
   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Privacy Concerns  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Principle . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  ICE Candidate Gathering . . . . . . . . . . . . . . . . .   3
     3.2.  ICE Candidate Processing  . . . . . . . . . . . . . . . .   4
   4.  Privacy Guidelines  . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  APIs leaking IP addresses . . . . . . . . . . . . . . . .   4
     4.2.  Generated names reuse . . . . . . . . . . . . . . . . . .   5
     4.3.  Specific execution contexts . . . . . . . . . . . . . . .   5
   5.  Specification Requirements  . . . . . . . . . . . . . . . . .   5
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   As detailed in [IPHandling], exposing client private IP addresses
   allows maximizing the probability to successfully create a connection
   between two clients.  This information is also used by many web sites
   as a way to fingerprint and identify users without their consent.

   The first approach exposes client private IP addresses by default, as
   can be seen from websites such as [IPLeak].  The second approach
   implemented in the WebKit engine enforces the following policy:

   1.  By default, use mode 3 as defined in [IPHandling]: any host ICE
       candidate is filtered out.

   2.  Use mode 2 as defined in [IPHandling] if there is an explicit
       user action to trust the web site: host ICE candidates are
       exposed to the web site based on the use of
       navigator.mediaDevices.getUserMedia, which typically prompts the
       user to grant or deny access to cameras/microphones.

   The second approach supports most common audio/video conference
   applications but leads to failing or suboptimal connections for
   applications relying solely on data channel.  This is particularly an
   issue on unmanaged networks, typically home or small offices where
   NAT loopback might not be supported.

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   To overcome the shortcomings of the above two approaches, this
   document proposes to register dynamically generated names using
   Multicast DNS when gathering ICE candidates.  These dynamically
   generated names are used to replace private IP addresses in host ICE
   candidates.  Only clients that can resolve these dynamically
   generated names using Multicast DNS will get access to the actual
   client IP address.

2.  Privacy Concerns

   The gathering of ICE candidates without user consent is a well-known
   fingerprinting technique to track users.  This is particularly a
   concern when users are connected through a NAT which is a usual
   configuration.  In such a case, knowing both the private IP address
   and the public IP address will usually identify uniquely the user
   device.  Additionally, Internet web sites can more easily attack
   intranet web sites when knowing the intranet IP address range.

   A successful WebRTC connection between two peers is also a potential
   thread to user privacy.  When a WebRTC connection latency is close to
   zero, the probability is high that the two peers are running on the
   same device.  Browsers often isolate contexts one from the other.
   Private browsing mode contexts usually do not share any information
   with regular browsing contexts.  The WebKit engine isolates third-
   party iframes in various ways (cookies, ITP) to prevent user
   tracking.  Enabling a web application to determine that two contexts
   run in the same device would defeat some of the protections provided
   by modern browsers.

3.  Principle

   This section uses the concept of ICE agent as define in [RFC5245].
   In the remainder of the document, it is assumed that each browser
   execution context has its own ICE agent.

3.1.  ICE Candidate Gathering

   For any host ICE candidate gathered by a browsing context as part of
   [RFC5245] section 4.1.1, obfuscation of the candidate is done as

   1.  Check whether the context ICE agent registered a name resolving
       to the ICE host candidate IP address.

   2.  If the ICE agent registered the name, replace the IP address of
       the ICE host candidate with the name with ".local" appended to
       it.  Expose the candidate and abort these steps.

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   3.  Generate a random unique name, typically a version 4 UUID as
       defined in [RFC4122].

   4.  Register the unique name using Multicast DNS.

   5.  If registering of the unique name fails, abort these steps.  The
       candidate is not exposed.

   6.  Store the name and its related IP address in the ICE agent for
       future reuse.

   7.  Replace the IP address of the ICE host candidate with the name
       with ".local" appended to it.  Expose the candidate.

3.2.  ICE Candidate Processing

   For any remote host 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 finish by ".local", process the candidate as defined in

   2.  Otherwise, remove the ".local" suffix to the value and resolve it
       using Multicast DNS.

   3.  If it resolves to an IP address, replace the value of the ICE
       host candidate by the resolved IP address and continue processing
       of the candidate.

   4.  Otherwise, ignore the candidate.

   Multicast DNS resolution might end up retrieving both an IPv4 and
   IPv6 address.  In that case, the IPv6 address may be used preferably
   to the IPv4 address.

4.  Privacy Guidelines

4.1.  APIs leaking IP addresses

   When there is no user consent, the following filtering should be done
   to prevent private IP address leakage:

   1.  host ICE candidates with an IP address are not exposed as ICE
       candidate events.

   2.  Server reflexive ICE candidate raddr field is set to and
       rport to 0.

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   3.  SDP does not expose any a=candidate line corresponding to a host
       ICE candidate which contains an IP address.

   4.  RTCIceCandidateStats dictionaries exposed to web pages do not
       contain any 'ip' member if related to a host ICE candidate.

4.2.  Generated names reuse

   Dynamically generated names can be used to track users if used too
   often.  Conversely, registering too many names will also generate
   useless processing.  The proposed rule is to create and register a
   new generated name for a given IP address on a per execution context.

4.3.  Specific execution contexts

   Privacy might also be breached if two execution contexts can identify
   whether they are run in the same device based on a successful peer-
   to-peer connection.  The proposed rule is to not register any name
   using Multicast DNS for any ICE agent belonging to:

   1.  A third-party browser execution context, i.e. a context that is
       not same origin as the top level execution context.

   2.  A private browsing execution context.

5.  Specification Requirements

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

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

   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 Multicast DNS based ICE

6.  Informative References

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

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

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

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

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

Authors' Addresses

   Youenn Fablet
   Apple Inc.

   Email: youenn@apple.com

   Jeroen De Borst

   Email: jeroendb@google.com

   Justin Uberti

   Email: juberti@google.com

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   Qingsi Wang

   Email: qingsi@google.com

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