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Versions: (draft-mdns-ice-candidates) 00 01 02 03

RTCWEB                                                         Y. Fablet
Internet-Draft                                                Apple Inc.
Intended status: Informational                               J. de Borst
Expires: September 26, 2019                                    J. Uberti
                                                                 Q. Wang
                                                                  Google
                                                          March 25, 2019


  Using Multicast DNS to protect privacy when exposing ICE candidates
                draft-ietf-rtcweb-mdns-ice-candidates-03

Abstract

   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 concealing IP addresses with
   dynamically generated Multicast DNS (mDNS) 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 26, 2019.

Copyright Notice

   Copyright (c) 2019 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Description . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  ICE Candidate Gathering . . . . . . . . . . . . . . . . .   3
       3.1.1.  Procedure . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Implementation Guidance . . . . . . . . . . . . . . .   4
     3.2.  ICE Candidate Processing  . . . . . . . . . . . . . . . .   6
       3.2.1.  Procedure . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Implementation Guidance . . . . . . . . . . . . . . .   6
     3.3.  Additional Privacy Considerations . . . . . . . . . . . .   6
       3.3.1.  Statistics  . . . . . . . . . . . . . . . . . . . . .   7
       3.3.2.  Interactions With TURN Servers  . . . . . . . . . . .   7
       3.3.3.  Generated Name Reuse  . . . . . . . . . . . . . . . .   8
       3.3.4.  Specific Browsing Contexts  . . . . . . . . . . . . .   8
       3.3.5.  Network Interface Enumeration . . . . . . . . . . . .   8
       3.3.6.  Monitoring of Sessions  . . . . . . . . . . . . . . .   8
   4.  Potential Limitations . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Reduced Connectivity  . . . . . . . . . . . . . . . . . .   9
     4.2.  Connection Setup Latency  . . . . . . . . . . . . . . . .   9
     4.3.  Backward Compatibility  . . . . . . . . . . . . . . . . .  10
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Normal Handling . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Peer-reflexive Candidate From Slow Signaling  . . . . . .  11
     5.3.  Peer-reflexive Candidate From Slow Resolution . . . . . .  11
     5.4.  IPv4, IPv6, and STUN handling . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     6.1.  mDNS Message Flooding . . . . . . . . . . . . . . . . . .  14
     6.2.  Malicious Responses to Deny Name Registration . . . . . .  15
     6.3.  Unsolicited ICE Communications  . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17








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1.  Introduction

   As detailed in [IPHandling], exposing client private IP addresses by
   default to web applications maximizes the probability of successfully
   creating direct peer-to-peer connections between 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
   providing a mechanism for WebRTC implementations to register
   ephemeral mDNS [RFC6762] names for local private IP addresses, and
   then provide those names, rather than the IP addresses, in their ICE
   candidates.  While this technique is intended to benefit WebRTC
   implementations in web browsers, by preventing collection of private
   IP addresses by arbitrary web pages, it can also be used by any
   endpoint that wants to avoid disclosing information about its local
   network to remote peers on other networks.

   WebRTC and WebRTC-compatible endpoints [Overview] that receive ICE
   candidates with mDNS names will resolve these names to IP addresses
   and perform ICE processing as usual.  In the case where the endpoint
   is a web application, the WebRTC implementation will manage this
   resolution internally and will not disclose the actual IP addresses
   to the application.

2.  Terminology

   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 [RFC2119].

3.  Description

   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
   agent.

3.1.  ICE Candidate Gathering

   This section outlines how mDNS should be used by ICE agents to
   conceal local IP addresses.




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3.1.1.  Procedure

   For each host candidate gathered by an ICE agent as part of the
   gathering process described in [RFC8445], Section 5.1.1, the
   candidate is handled as described below.

   1.  Check whether this IP address satisfies the ICE agent's policy
       regarding whether an address is safe to expose.  If so, expose
       the candidate and abort this process.

   2.  Check whether the ICE agent has previously generated, registered,
       and stored an mDNS hostname for this IP address as per Steps 3,
       4, and 6.  If it has, skip ahead to Step 7.

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

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

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

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

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

   ICE agents can implement this procedure in any way as long as it
   produces equivalent results.  An implementation may for instance pre-
   register mDNS hostnames by executing steps 3 to 6 and prepopulate an
   ICE agent accordingly.  By doing so, only step 7 of the above
   procedure will be executed at the time of gathering candidates.

   An implementation may also detect that mDNS is not supported by the
   available network interfaces.  The ICE agent may skip steps 3 and 4
   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
   address.

3.1.2.  Implementation Guidance







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3.1.2.1.  Determining Address Privacy and Server-Reflexive Candidates

   Naturally, an address that is already exposed to the Internet does
   not need to be protected by mDNS, as it can be trivially observed by
   the web server or remote endpoint.  However, determining this ahead
   of time is not straightforward; while the fact that an IPv4 address
   is private can sometimes be inferred by its value, e.g., whether it
   is an [RFC1918] address, the reverse is not necessarily true.  IPv6
   addresses present their own complications, e.g., private IPv6
   addresses as a result of NAT64 [RFC6146].

   Instead, the determination of whether an address is public can be
   reliably made as part of the ICE gathering process.  For each mDNS
   host candidate generated according the guidance above, the usual STUN
   [RFC5389] request is sent to a STUN server.  This can be done for
   both IPv4 and IPv6 local addresses, provided that the application has
   configured both IPv4 and IPv6 STUN servers.  If the STUN response
   returns the same value as the local IP address, this indicates the
   address is in fact public.

   Regardless of the result, a server-reflexive candidate will be
   generated; the transport address of this candidate is an IP address
   and therefore distinct from the hostname transport address of the
   associated mDNS candidate, and as such MUST NOT be considered
   redundant per the guidance in [RFC8445], Section 5.1.3.  To avoid
   accidental IP address, this server-reflexive candidate MUST have its
   raddr field set to 0.0.0.0 and its rport field set to 0.

   Once an address has been identified as public, the ICE agent MAY
   cache this information and omit mDNS protection for that address in
   future ICE gathering phases.

3.1.2.2.  Special Handling for IPv6 Addresses

   As noted in [IPHandling], private IPv4 addresses are especially
   problematic because of their unbounded lifetime.  However, the
   [RFC4941] IPv6 addresses recommended for WebRTC have inherent privacy
   protections, namely a short lifetime and the lack of any stateful
   information.  Accordingly, implementations MAY choose to not conceal
   [RFC4941] addresses with mDNS names as a tradeoff for improved peer-
   to-peer connectivity.

3.1.2.3.  mDNS Candidate Encoding

   The mDNS name of an mDNS candidate MUST be used in the connection-
   address field of its candidate attribute.  When an mDNS candidate is
   the default candidate, its mDNS name MUST be used in the connection-
   address field of the SDP "c=" line.  Since an mDNS candidate also



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   conceals its address family, the "c=" line SHOULD use "IP4" in the
   address-type field.

   Any candidates exposed to the 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

   This section outlines how received ICE candidates with mDNS names are
   processed by ICE agents, and is relevant to all endpoints.

3.2.1.  Procedure

   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 as defined in [RFC8445].

   4.  Otherwise, ignore the candidate.

3.2.2.  Implementation Guidance

   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 SHOULD ignore candidates where the hostname resolution
   returns more than one IP address.

   An ICE agent MAY add additional restrictions regarding the ICE
   candidates it will resolve using mDNS, as this mechanism allows
   attackers to send ICE traffic to devices with well-known mDNS names.

3.3.  Additional 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



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   IP addresses out of ICE candidates, implementations must take steps
   to prevent these IP addresses from being exposed to web applications
   through other means.

3.3.1.  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 7.3.1.3 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].

3.3.2.  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
   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.







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3.3.3.  Generated Name 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.  In the case of a web application, the use of
   registered mDNS hostnames SHOULD be scoped by the web application
   origin, and SHOULD have the lifetime of the page executing the web
   application.

3.3.4.  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.

3.3.5.  Network Interface Enumeration

   Even when local IP addresses are not exposed, the number of mDNS
   hostname candidates can still provide a fingerprinting dimension.
   This is in particular the case for network interfaces with limited
   connectivity that will not generate server-reflexive or relay
   candidates.

   The more mDNS names an endpoint exposes through mDNS hostname
   candidates, the higher the fingerprinting risk.  One countermeasure
   is to limit this number to a small value.

   Note that no additional fingerprinting risk is introduced when
   restricting mDNS hostname candidates to default route only.

3.3.6.  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



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   the session are on the same network, the fact they are communicating
   can be discovered.

   Mitigation of this threat is beyond the scope of this proposal.

4.  Potential Limitations

4.1.  Reduced Connectivity

   With typical ICE, endpoints on the same network will usually be able
   to establish a direct connection between their local IP addresses.
   When using the mDNS technique, a direct connection is still possible,
   but only if at least one side can properly resolve the provided mDNS
   candidates.  This may not be possible in all scenarios.

   First, some networks may entirely disable mDNS.  Second, mDNS queries
   have limited scope.  On large networks, this may mean that an mDNS
   name cannot be resolved if the remote endpoint is too many segments
   away.

   When mDNS fails, ICE will attempt to fall back to either NAT hairpin,
   if supported, or TURN relay if not.  This may result in reduced
   connectivity, reduced throughput and increased latency, as well as
   increased cost in case of TURN relay.

   One potential mitigation, as discussed in Section 3.3, is to not
   conceal candidates created from [RFC4941] IPv6 addresses.  This
   permits connectivity even in large internal networks or where mDNS is
   disabled.

   The exact impact of the mDNS technique is being researched
   experimentally and will be provided before publication of this
   document.

4.2.  Connection Setup Latency

   As noted in Section 3, ICE agents using the mDNS technique are
   responsible for registering and resolving mDNS names as part of the
   ICE process.  These steps may delay establishment of a direct peer-
   to-peer connection, compared to when raw local IP addresses are used.

   Given that these mDNS registrations and queries are typically
   occurring on a local network, any associated delays should be small.
   Also, as noted in Section 3.1, pre-registration can be employed to
   eliminate gathering delays entirely.






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4.3.  Backward Compatibility

   For the most part, backward compatibility does not present a
   significant issue for the mDNS technique.  When an endpoint that
   supports mDNS communicates with an endpoint that does not, the legacy
   endpoint will still provide its local IP addresses, and accordingly a
   direct connection can still be attempted, even though the legacy
   endpoint cannot resolve the mDNS names provided by the new endpoint.
   In the event the legacy endpoint attempts to resolve mDNS names using
   Unicast DNS, this may cause ICE to take somewhat longer to fully
   complete, but should not have any effect on connectivity or
   connection setup time.

   However, some legacy endpoints are not fully spec-compliant and can
   behave unpredictably in the presence of ICE candidates that contain a
   hostname, potentially leading to ICE failure.  Such endpoints have
   been identified during testing of this technique, but appear to be
   rare.

5.  Examples

   The examples below show how the mDNS technique is used during ICE
   processing.  The first example shows a simple case, the next two
   examples demonstrate how peer-reflexive candidates for local IP
   addresses can be created due to timing differences, and the final
   example shows a real-world case with IPv4, IPv6, and STUN.

5.1.  Normal Handling

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

           ICE Agent 1 (192.0.2.1)           ICE Agent 2 (192.0.2.2)
       <Register mDNS |                                 |
         name N1,     |                                 |
         192.0.2.1>   |                                 |
                      |------- mDNS Candidate N1 ------>|
                      |                                 | <Register mDNS
                      |                                 |  name N2,
                      |                                 |  192.0.2.2>
                      |<------ mDNS Candidate N2 -------|
       <Resolve       |                                 | <Resolve
        mDNS name N2> |                                 |  mDNS name N1>
                      |<=== STUN check to 192.0.2.1 ====|
                      |==== STUN check to 192.0.2.2 ===>|
                      |                                 |





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   The exchange of ICE candidates relies on out-of-band signaling, for
   example, the SDP Offer/Answer procedure defined in [ICESDP].  In the
   above example, the candidate attributes in the SDP messages to
   exchange the mDNS candidates between ICE Agent 1 and 2 are as
   follows:

   ICE Agent 1:

a=candidate:1 1 udp 2122262783 1f4712db-ea17-4bcf-a596-105139dfd8bf.local
  54596 typ host

   ICE Agent 2:

a=candidate:1 1 udp 2122262783 2579ef4b-50ae-4bfe-95af-70b3376ecb9c.local
  61606 typ host

5.2.  Peer-reflexive Candidate From Slow Signaling

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

           ICE Agent 1 (192.0.2.1)           ICE Agent 2 (192.0.2.2)
       <Register mDNS |                                 |
         name N1,     |                                 |
         192.0.2.1>   |                                 |
                      |------- mDNS Candidate N1 ------>|
                      |                                 | <Resolve
                      |                                 |  mDNS name N1>
                      |<=== STUN check to 192.0.2.1 ====|
      prflx candidate |                                 | <Register mDNS
    192.0.2.2 created |                                 |  name N2,
                      |                                 |  192.0.2.2>
                      |<------ mDNS Candidate N2 -------|
                      |                                 |

5.3.  Peer-reflexive Candidate From Slow Resolution

   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 (192.0.2.1)           ICE Agent 2 (192.0.2.2)
       <Register mDNS |                                 | <Register mDNS
         name N1,     |                                 |  name N2,
         192.0.2.1>   |                                 |  192.0.2.2>
                      |------- mDNS Candidate N1 ------>|
                      |<------ mDNS Candidate N2 -------|
<Resolve              |                                 | <Resolve
 mDNS                 |                                 |  mDNS name N1>
  .                   |<=== STUN check to 192.0.2.1 ====|
  .   prflx candidate |                                 |
  . 192.0.2.2 created |                                 |
 name                 |                                 |
 N2>                  |                                 |

5.4.  IPv4, IPv6, and STUN handling

   This last example demonstrates the overall ICE gathering process for
   two endpoints, each with a private IPv4 address and a public IPv6
   address.  They preregister their mDNS names to speed up ICE
   gathering.

               ICE Agent 1                        ICE Agent 2
               192.168.1.1         STUN           192.168.1.2
               2001:db8::1        Server          2001:db8::2
  ----------------------------------------------------------------------
                      Pre-registration of mDNS names
                   |                |                 |
    <Register mDNS |                |                 | <Register mDNS
      name N1.1,   |                |                 |  name N2.1,
      192.168.1.1> |                |                 |  192.168.1.2>
    <Register mDNS |                |                 | <Register mDNS
      name N1.2,   |                |                 |  name N2.2,
      2001:db8::1> |                |                 |  2001:db8::2>
                   |                |                 |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                    ICE Agent 1 sends mDNS candidates
                   |                |                 |
            <N1.1> |------- mDNS Candidate C1.1 ----->|
            <N1.2> |------- mDNS Candidate C1.2 ----->|
                   |                |                 | <Resolve mDNS
                   |                |                 |  name N1.1 to
                   |                |                 |  192.168.1.1>
                   |                |                 | <Resolve mDNS
                   |                |                 |  name N1.2 to
                   |                |                 |  2001:db8::1>
                   |                |                 |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
              ICE Agent 1 sends server-reflexive candidates



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                   |                |                 |
    <192.168.1.1   |--Binding Req-->|                 |
     is 192.0.2.1> |<-Binding Resp--|                 |
       <192.0.2.1> |------ srflx Candidate C1.3 ----->|
  <2001:db8::1     |--Binding Req-->|                 |
   is 2001:db8::1> |<-Binding Resp--|                 |
     <2001:db8::1> |------ srflx Candidate C1.4 ----->| <Discard C1.4
                   |                |                 |  as redundant>
                   |                |                 |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
          ICE Agent 2 sends mDNS candidates, resolution is slow
                   |                |                 |
                   |<------ mDNS Candidate C2.1 ------| <N2.1>
                   |<------ mDNS Candidate C2.2 ------| <N2.2>
   <Resolve mDNS   |                |                 |
    name N2.1 ...> |                |                 |
   <Resolve mDNS   |                |                 |
    name N2.2 ...> |                |                 |
                   |                |                 |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    ICE Agent 2 sends server-reflexive candidates, resolution completes
                   |                |                 |
                   |                |<--Binding Req---| <192.168.1.2
                   |                |---Binding Resp->|  is 192.0.2.2>
                   |<----- srflx Candidate C2.3 ------| <192.0.2.2>
                   |                |<--Binding Req---| <2001:db8::2
                   |                |---Binding Resp->|  is 2001:db8::2>
                   |<----- srflx Candidate C2.4 ------| <2001:db8::2>
                   |                |                 |
    <... N2.1 is   |                |                 |
     192.168.1.2>  |                |                 |
    <... N2.2 is   |                |                 |
     2001:db8::2,  |                |                 |
     discard C2.4> |                |                 |
                   |                |                 |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                          ICE connectivity checks
                   |                |                 |
       2001:db8::1 |<============= STUN ==============| 2001:db8::2
       2001:db8::1 |============== STUN =============>| 2001:db8::2
       192.168.1.1 |<============= STUN ==============| 192.168.1.2
       192.168.1.1 |============== STUN =============>| 192.168.1.2
         192.0.2.1 |    Failed <-- STUN --------------| 192.168.1.2
       192.168.1.1 |---------------STUN --> Failed    | 192.0.2.2
       2001:db8::1 |====== STUN(USE-CANDIDATE) ======>| 2001:db8::2

   Ice Agent 1 candidates:




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     C1.1: candidate:1 1 udp 2122262783 9b36eaac-bb2e-49bb-bb78-
                     21c41c499900.local 10004 typ host
     C1.2: candidate:2 1 udp 2122262527 76c82649-02d6-4030-8aef-
                     a2ba3a9019d5.local 10006 typ host
     C1.3: candidate:1 1 udp 1686055167 192.0.2.1
                     30004 typ srflx raddr 0.0.0.0 rport 0
     C1.4: candidate:2 1 udp 1686054911 2001:db8::1
                     10006 typ srflx raddr 0.0.0.0 rport 0

   Ice Agent 2 candidates:

     C2.1: candidate:1 1 udp 2122262783 b977f597-260c-4f70-9ac4-
                     26e69b55f966.local 20004 typ host
     C2.2: candidate:2 1 udp 2122262527 ac4595a7-7e42-4e85-85e6-
                     c292abe0e681.local 20006 typ host
     C2.3: candidate:1 1 udp 1686055167 192.0.2.2
                     40004 typ srflx raddr 0.0.0.0 rport 0
     C2.4: candidate:2 1 udp 1686054911 2001:db8::2
                     20006 typ srflx raddr 0.0.0.0 rport 0

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

   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;



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   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 general per-question and per-record multicast
   rate limiting rule, in which a given question or 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 questions or records,
   are constantly created and sent.  Therefore, a browser-wide mDNS
   message rate limit MUST be provided for all mDNS queries and
   responses that are dispatched during the ICE candidate gathering and
   processing described in Section 3.  A browser MAY implement more
   specific rate limits, e.g., to ensure a single origin does not
   prevent other origins from registering, unregistering, or resolving
   mDNS names.

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
   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.  Unsolicited ICE Communications

   As noted in Section 4.2 of [RTCWebSecurity], an attacker may use ICE
   as a way to send unsolicited network traffic to specific endpoints.
   While this is not specific to mDNS hostname candidates, this
   technique makes it easier to target devices with well-known mDNS
   names.





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   As noted in Section 3.2, ICE agents may decide to not resolve mDNS
   names, for example, if these names are not in the format defined by
   Section 3.1.

7.  IANA Considerations

   This document requires no actions from IANA.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [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,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [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,
              <https://www.rfc-editor.org/info/rfc8445>.




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8.2.  Informative References

   [HTMLSpec]
              "HTML Living Standard", n.d.,
              <https://html.spec.whatwg.org>.

   [ICESDP]   Keranen, A., "Session Description Protocol (SDP) Offer/
              Answer procedures for Interactive Connectivity
              Establishment (ICE)", April 2018,
              <https://tools.ietf.org/html/
              draft-ietf-mmusic-ice-sip-sdp>.

   [IPHandling]
              Shieh, G., "WebRTC IP Address Handling Requirements",
              April 2018, <https://tools.ietf.org/html/
              draft-ietf-rtcweb-ip-handling>.

   [Overview]
              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", November 2017,
              <https://tools.ietf.org/html/draft-ietf-rtcweb-overview>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RTCWebSecurity]
              Rescorla, E., "Security Considerations for WebRTC",
              January 2018,
              <https://tools.ietf.org/html/draft-ietf-rtcweb-security>.

   [WebRTCSpec]
              Bruaroey, J., "The WebRTC specification", n.d.,
              <https://w3c.github.io/webrtc-pc/>.

Authors' Addresses

   Youenn Fablet
   Apple Inc.

   Email: youenn@apple.com




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   Jeroen de Borst
   Google

   Email: jeroendb@google.com


   Justin Uberti
   Google

   Email: juberti@google.com


   Qingsi Wang
   Google

   Email: qingsi@google.com



































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