RTCWEB Y. Fablet
Internet-Draft Apple Inc.
Intended status: Informational J. de Borst
Expires: April 25, 2019 J. Uberti
Q. Wang
Google
October 22, 2018
Using Multicast DNS to protect privacy when exposing ICE candidates
draft-ietf-rtcweb-mdns-ice-candidates-01
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 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
Task Force (IETF). Note that other groups may also distribute
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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 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
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. Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. ICE Candidate Gathering . . . . . . . . . . . . . . . . . 3
2.2. ICE Candidate Processing . . . . . . . . . . . . . . . . 4
2.2.1. Handling of Peer-Reflexive Remote Candidate . . . . . 4
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Privacy Guidelines . . . . . . . . . . . . . . . . . . . . . 6
4.1. APIs Leaking IP Addresses . . . . . . . . . . . . . . . . 6
4.2. Interactions With TURN Servers . . . . . . . . . . . . . 6
4.3. Generated Names Reuse . . . . . . . . . . . . . . . . . . 7
4.4. Specific Browsing Contexts . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . 9
7. 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
processing as usual, but the actual IP addresses are not exposed to
the web application.
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2. 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
agent.
2.1. ICE Candidate Gathering
For any host candidate gathered by an ICE agent as part of [RFC8445]
section 5.1.1, the candidate is processed 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 expose the 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 not expose the host candidate.
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.
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2.2. 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, see [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
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.
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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.
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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
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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;
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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 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 2.1.
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 2.1.
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.
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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.
5.3. 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
proposal.
6. 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
[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 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>.
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[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>.
[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>.
[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 Borst
Google
Email: jeroendb@google.com
Justin Uberti
Google
Email: juberti@google.com
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Qingsi Wang
Google
Email: qingsi@google.com
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