TLS T. Fossati
Internet-Draft H. Tschofenig, Ed.
Updates: 6347 (if approved) Arm Limited
Intended status: Standards Track July 08, 2019
Expires: January 9, 2020
Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-tschofenig-tls-dtls-rrc-00
Abstract
This document specifies a return routability check for use in context
of the Connection ID (CID) construct for the Datagram Transport Layer
Security (DTLS) protocol versions 1.2 and 1.3.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Application Layer Return Routability Check . . . . . . . . . 3
4. The Return Routability Check Message . . . . . . . . . . . . 4
5. RRC Example . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security and Privacy Considerations . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Working Group Information . . . . . . . . . . . . . 9
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
In "classical" DTLS, selecting a security context of an incoming DTLS
record is accomplished with the help of the 5-tuple, i.e. source IP
address, source port, transport protocol, destination IP address, and
destination port. Changes to this 5 tuple can happen for a variety
reasons over the lifetime of the DTLS session. In the IoT content
NAT rebinding is a common reason with sleepy devices. Other examples
include end host mobility and multi-homing. Without CID, if the
source IP address and/or source port changes during the lifetime of
an ongoing DTLS session then the receiver will be unable to locate
the correct security context. As a result, the DTLS handshake has to
be re-run.
A CID is an identifier carried in the record layer header of a DTLS
datagram that gives the receiver additional information for selecting
the appropriate security context. The CID mechanism has been
specified in [I-D.ietf-tls-dtls-connection-id] for DTLS 1.2 and in
[I-D.ietf-tls-dtls13] for DTLS 1.3.
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An on-path adversary could intercept and modify the source IP address
(and the source port). Even if receiver checks the authenticity and
freshness of the packet, the recipient is fooled into changing the
CID-to-IP/port association. This attack is possible because the
network and transport layer identifiers, such as source IP address
and source port numbers, are not integrity protected and
authenticated by the DTLS record layer.
This attack makes strong assumptions on the attacker's abilities, and
moreover it only misleads the peer until the next message gets
through un-intercepted.
A return routability check (RRC) is performed by the receiving peer
before the CID-to-IP address/port binding is updated in that peer's
session state database. This is done in order to provide a certain
degree of confidence to the receiving peer that the sending peer is
reachable at the indicated address and port.
Without such a return routability check, an adversary can redirect
traffic towards a third party or a black hole.
While an equivalent check can be performed at the application layer
(modulo the DTLS API exposing the address update event to the calling
application), it is advantageous to offer this functionality at the
DTLS layer. Section 3 describes the application layer procedure and
Section 4 specifies a new message to perform this return routability
check.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document assumes familiarity with the CID solutions defined for
DTLS 1.2 [I-D.ietf-tls-dtls-connection-id] and for DTLS 1.3
[I-D.ietf-tls-dtls13].
3. Application Layer Return Routability Check
When a record with CID is received that has the source address of the
enclosing UDP datagram different from the one previously associated
with that CID, the receiver MUST NOT update its view of the peer's IP
address and port number with the source specified in the UDP datagram
before cryptographically validating the enclosed record(s). This is
to ensure that a man-on-the-middle attacker that sends a datagram
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with a different source address/port on an existing CID session does
not successfully manage to re-route any return traffic.
Furthermore, when using CID, anti-replay protection MUST be enabled.
This is to ensure that a man-on-the-middle attacker sending a
previously captured record with a modified source IP address and port
will not be able to successfully pass the above check (since the
datagram is very likely discarded on receipt - if it falls outside
the replay window).
The two countermeasures cannot complete stop a man-in-the-middle
attacker who performs a DoS on the sender or uses the receiver as as
backscatter source for a DDoS attack. For a more generic protection,
a return routability check is needed.
It is RECOMMENDED that implementations of the CID functionaliy
described in [I-D.ietf-tls-dtls-connection-id] and in
[I-D.ietf-tls-dtls13] added peer address update events to their APIs.
Applications can then use these events as triggers to perform an
application layer return routability check, for example one that is
based on successful exchange of minimal amount of ping-pong traffic
with the peer.
4. The Return Routability Check Message
enum {
invalid(0),
change_cipher_spec(20),
alert(21),
handshake(22),
application_data(23),
heartbeat(24), /* RFC 6520 */
return_routability_check(TBD), /* NEW */
(255)
} ContentType;
The newly introduced return_routability_check message contains a
cookie. The semantic of the cookie is similar to the cookie used in
the HelloRetryRequest message defined in [RFC8446].
The return_routability_check message MUST be authenticated and
encrypted using the currently active security context.
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The endpoint that observes the peer's address update MUST stop
sending any buffered application data (or limit the sending rate to a
TBD threshold) and initiate the return routability check that
proceeds as follows:
1. A cookie is placed in the return_routability_check message;
2. The message is sent to the observed new address and a timeout T
is started;
3. The peer endpoint, after successfully verifying the received
return_routability_check message echoes it back;
4. When the initiator receives and verifies the
return_routability_check message, it updates the peer address
binding;
5. If T expires, or the address confirmation fails, the peer address
binding is not updated.
After this point, any pending send operation is resumed to the bound
peer address.
struct {
opaque cookie<1..2^16-1>;
} Cookie;
struct {
Cookie cookie;
} return_routability_check;
5. RRC Example
The example shown in Figure 1 illustrates a client and a server
exchanging application payloads protected by DTLS with an
unilaterally used CIDs. At some point in the communication
interaction the IP address used by the client changes and, thanks to
the CID usage, the security context to interpret the record is
successfully located by the server. However, the server wants to
test the reachability of the client at his new IP address, to avoid
being abused (e.g., as an amplifier) by an attacker impersonating the
client.
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Client Server
------ ------
Application Data ========>
<CID=100>
Src-IP=A
Dst-IP=Z
<======== Application Data
Src-IP=Z
Dst-IP=A
<<------------->>
<< Some >>
<< Time >>
<< Later >>
<<------------->>
Application Data ========>
<CID=100>
Src-IP=B
Dst-IP=Z
<<< Unverified IP
Address B >>
<-------- Return Routability Check
(cookie)
Src-IP=Z
Dst-IP=B
Return Routability Check -------->
(cookie)
Src-IP=B
Dst-IP=Z
<<< IP Address B
Verified >>
<======== Application Data
Src-IP=Z
Dst-IP=B
Figure 1: Return Routability Example
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6. Security and Privacy Considerations
As all the datagrams in DTLS are authenticated, integrity and
confidentiality protected there is no risk that an attacker
undetectably modifies the contents of those packets. The IP
addresses in the IP header and the port numbers of the transport
layer are, however, not authenticated. With the introduction of the
CID, care must be taken to test reachability of a peer at a given IP
address and port.
Note that the return routability checks do not protect against third-
party flooding if the attacker is along the path, as the attacker can
forward the return routability checks to the real peer (even if those
datagrams are cryptographically authenticated).
7. IANA Considerations
IANA is requested to allocate an entry to the existing TLS
"ContentType" registry, for the return_routability_check(TBD) defined
in this document.
8. Open Issues
- Should the return routability check use separate sequence numbers
and replay windows?
- Should the heartbeat message be re-used instead of the proposed
new message exchange?
9. References
9.1. Normative References
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
id-05 (work in progress), May 2019.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-31 (work in progress), March
2019.
[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>.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[1] mailto:tls@ietf.org
[2] https://www1.ietf.org/mailman/listinfo/tls
[3] https://www.ietf.org/mail-archive/web/tls/current/index.html
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RFC EDITOR: PLEASE REMOVE THE THIS SECTION
- Initial version
Appendix B. Working Group Information
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [1]. Information on the group and
information on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/tls [2]
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html [3]
We would like to thank Achim Kraus, Hanno Becker and Manuel Pegourie-
Gonnard for their input to this document.
Authors' Addresses
Thomas Fossati
Arm Limited
EMail: thomas.fossati@arm.com
Hannes Tschofenig (editor)
Arm Limited
EMail: hannes.tschofenig@arm.com
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