wdiff draft-ietf-rtcweb-mdns-ice-candidates-00.txt draft-ietf-rtcweb-mdns-ice-candidates-01.txt

RTCWEB Y. Fablet
Internet-Draft Apple Inc.
Intended status: Informational J. De de Borst
Expires: March 16, April 25, 2019 J. Uberti
Q. Wang
Google
September 12,
October 22, 2018

Using Multicast DNS to protect privacy when exposing ICE candidates
draft-ietf-rtcweb-mdns-ice-candidates-00
draft-ietf-rtcweb-mdns-ice-candidates-01

Abstract

WebRTC applications rely on collect ICE candidates to enable peer-to-peer
connections between clients in as many network configurations as
possible. part of the process of
creating peer-to-peer connections. To maximize the probability to create of a
direct peer-to-
peer peer-to-peer connection, client private IP addresses are often exposed
without user consent. This is currently used as a way to track
users.
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 using with
dynamically generated names resolvable
through Multicast DNS [RFC6763]. (mDNS) [RFC6762] names.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on March 16, April 25, 2019.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Privacy Concerns Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Principle
2.1. ICE Candidate Gathering . . . . . . . . . . . . . . . . . 3
2.2. ICE Candidate Processing . . . . . . . . . . 3
3.1. ICE Candidate Gathering . . . . . . 4
2.2.1. Handling of Peer-Reflexive Remote Candidate . . . . . 4
3. Examples . . . . . . . . . . 3
3.2. ICE Candidate Processing . . . . . . . . . . . . . . . . 4
4. Privacy Guidelines . . . . . . . . . . . . . . . . . . . . . 4 6
4.1. APIs leaking Leaking IP addresses Addresses . . . . . . . . . . . . . . . . 4 6
4.2. Interactions With TURN Servers . . . . . . . . . . . . . 6
4.3. Generated names reuse Names Reuse . . . . . . . . . . . . . . . . . . 5
4.3. 7
4.4. Specific execution contexts Browsing Contexts . . . . . . . . . . . . . . . 5 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. mDNS Message Flooding . . . . . . . . . . . . . . . . . . 7
5.2. Malicious Responses to Deny Name Registration . . . . . . 8
5.3. Monitoring of Sessions . . . . . . . . . . . . . . . . . 9
6. Specification Requirements . . . . . . . . . . . . . . . . . 5
6. 9
7. Informative References . . . . . . . . . . . . . . . . . . . 5 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 10

1. Introduction

As detailed in [IPHandling], exposing client private IP addresses
allows maximizing by
default maximizes the probability to of successfully create a creating direct
peer-to-peer connection between two clients. This information is also used by many web sites
as clients, but creates 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
significant surface for user fingerprinting. [IPHandling] if recognizes
this issue, but also admits that there is an explicit
user action no current solution to trust the web site: host ICE candidates are
exposed this
problem; implementations that choose 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 Mode 3 to address the
privacy concerns often suffer from failing or suboptimal connections for
applications relying solely on data channel.
in WebRTC applications. This is particularly an issue on unmanaged
networks, typically home homes or small offices offices, where NAT loopback might may
not be supported.

To overcome the shortcomings of the above two approaches, this

This document proposes an overall solution to register dynamically generated names using
Multicast DNS when gathering ICE candidates. These dynamically
generated this problem by
registering ephemeral mDNS names are used to replace for each local private IP addresses in host address,
and then providing those names, rather than the IP addresses, to the
web application when it gathers ICE candidates. Only clients that can WebRTC
implementations 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 addresses and perform ICE
processing as usual, but 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 actual 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 addresses 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 exposed to determine that two contexts
run in the same device would defeat some of
the protections provided
by modern browsers.

3. web application.

2. Principle

This section uses the concept of ICE agent as define defined in [RFC5245]. [RFC8445].
In the remainder of the document, it is assumed that each browser
execution browsing
context (as defined in Section 7.1 of [HTMLSpec]) has its own ICE
agent.

3.1.

2.1. ICE Candidate Gathering

For any host ICE candidate gathered by a browsing context an ICE agent as part of
[RFC5245] [RFC8445]
section 4.1.1, obfuscation of 5.1.1, the candidate is done processed as follows:

1. Check whether the context ICE agent registered has a name usable registered mDNS hostname
resolving to the ICE host candidate candidate's 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 one exists, skip
ahead to
it. Expose the candidate and abort these steps.

3. Step 6.

2. Generate a random unique name, typically mDNS hostname. The unique name MUST consist of
a version 4 UUID as defined in [RFC4122].

4. [RFC4122], followed by ".local".

3. Register the unique name using Multicast DNS.

5. candidate's mDNS hostname as defined in [RFC6762].

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

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

6. Replace the IP address of the ICE host candidate with its mDNS
hostname, and expose the name
with ".local" appended candidate as usual.

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.

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 it. Expose not expose the host candidate.

3.2.

This procedure ensures that a 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.

2.2. 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", end with ".local" or if the value contains more than one ".",
then process the candidate as defined in
[RFC5245]. [RFC8445].

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

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

4. Otherwise, ignore the candidate.

Multicast DNS resolution might end up retrieving

An ICE agent may use a hostname resolver that transparently supports
both an IPv4 Multicast and
IPv6 address. Unicast DNS. In that case, this case the IPv6 address resolution of a
".local" name may be used preferably
to happen through Unicast DNS, see [RFC6762] section
3.

An ICE agent that supports mDNS candidates MUST support the IPv4 situation
where the hostname resolution results in more than one IP address.

4. Privacy Guidelines

4.1. APIs leaking
In this case, the ICE agent MUST take exactly one of the resolved IP
addresses

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

1. host others. The ICE candidates with an IP agent SHOULD, if available,
use the first IPv6 address are resolved, otherwise the first IPv4
address.

2.2.1. Handling of Peer-Reflexive Remote Candidate

A peer-reflexive remote candidate could be learned and constructed
from the source transport address of the STUN Binding request as an
ICE connectivity check. The peer-reflexive candidate could share the
same address as a remote mDNS candidate that is in the process of
being signaled or name resolution.

In addition to the elimination procedure of redundant candidates
defined in Section 5.1.3 of [RFC8445], which could remove constructed
peer-reflexive remote candidates, the address of any existing peer-
reflexive remote candidate should not be exposed to Web applications
by ICE agents that implement this proposal, as detailed in Section 4.

3. Examples

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

ICE Agent 1 (1.1.1.1) ICE Agent 2 (2.2.2.2)
<Register | |
mDNS name N1 | |
for 1.1.1.1> | |
|----------- mDNS Candidate N1 ---------->|
| | <Register
| | mDNS name N2
| | for 2.2.2.2>
|<---------- mDNS Candidate N2 -----------|
<Resolve | | <Resolve
mDNS name N2> | | mDNS name N1>
|<======== STUN check to 1.1.1.1 =========|
|========= STUN check to 2.2.2.2 ========>|
| |

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 (1.1.1.1) ICE Agent 2 (2.2.2.2)
<Register | |
mDNS name N1 | |
for 1.1.1.1> | |
|----------- mDNS Candidate N1 ---------->|
| | <Resolve
| | mDNS name N1>
|<======== STUN check to 1.1.1.1 =========|
prflx candidate | | <Register
2.2.2.2 created | | mDNS name N2
| | for 2.2.2.2>
|<---------- 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.

ICE Agent 1 (1.1.1.1) ICE Agent 2 (2.2.2.2)
<Register | | <Register
mDNS name N1 | | mDNS name N2
for 1.1.1.1> | | for 2.2.2.2>
|----------- mDNS Candidate N1 ---------->|
|<---------- mDNS Candidate N2 -----------|
<Resolve | | <Resolve
... | | mDNS name N1>
mDNS |<======== STUN check to 1.1.1.1 =========|
... prflx candidate | |
name 2.2.2.2 created | |
... | |
N2> | |

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

2. Server reflexive ICE candidate raddr field is set to 0.0.0.0 and
rport to 0.

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

4. Statistics related to ICE candidates MUST NOT contain the
resolved IP address of a remote mDNS candidate or the IP address
of a peer-reflexive candidate, unless that IP address has already
been learned through other means, e.g., receiving it in a
separate server-reflexive remote candidate.

4.2. 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.

4.3. 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.

4.4. 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.

5. Security Considerations

5.1. 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;

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.

5.2. Malicious Responses to Deny Name Registration

If the optional probing queries are implemented for the name
registration, a host
ICE candidate malicious endpoint in the local network, which contains an IP address.

4. RTCIceCandidateStats dictionaries exposed is
capable of responding mDNS queries, could send responses to web pages do not
contain any 'ip' member if related block the
use of the generated names. This would lead to a host the discarding of
this ICE candidate.

4.2. Generated names reuse

Dynamically generated names host candidate as in Step 5 in Section 2.1.

The above attack can be used to track users if used too
often. Conversely, mitigated by skipping the probing when
registering too many names will a name, which also generate
useless processing. The proposed rule is conforms to create and register a
new generated Section 8 in [RFC6762],
given that the name is randomly generated for the probabilistic
uniqueness (e.g. a given IP address on version 4 UUID) in Step 3 in Section 2.1.
However, a per execution context.

4.3. Specific execution contexts

Privacy might also be breached if two execution contexts similar attack can identify
whether they be performed by exploiting the negative
responses (defined in [RFC6762], Section 8.1), in which NSEC resource
records are run sent to claim the nonexistence of records related to the
gathered ICE host candidates.

The existence of malicious endpoints in the same device based on local network poses a successful peer-
to-peer connection. The proposed rule is
generic threat, and requires dedicated protocol suites to not register any name
using Multicast DNS for any ICE agent belonging to:

1. mitigate,
which is beyond the scope of this proposal.

5.3. Monitoring of Sessions

A third-party browser execution context, i.e. 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 context session log that is
not shows which
endpoints are communicating, and for how long. If both endpoints in
the session are on the same origin as network, the top level execution context.

2. A private browsing execution context.

5. fact they are communicating
can be discovered.

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

6. Specification Requirements

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

The proposal relies on adding the ability to register Multicast DNS mDNS 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 mDNS-based ICE
candidates.

7. 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>.

[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,
<https://www.rfc-editor.org/info/rfc4122>.

[RFC5245]

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for Offer/Answer Protocols", NAT (STUN)", RFC 5245, 5766,
DOI 10.17487/RFC5245, 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5245>.

[RFC6763]
<https://www.rfc-editor.org/info/rfc5766>.

[RFC6762] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", "Multicast DNS", RFC 6763, 6762,
DOI 10.17487/RFC6763, 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
<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>.

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

Authors' Addresses

Youenn Fablet
Apple Inc.

Email: youenn@apple.com

Jeroen De de Borst
Google

Email: jeroendb@google.com

Justin Uberti
Google

Email: juberti@google.com
Qingsi Wang
Google

Email: qingsi@google.com