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Virtual Private Network (VPN) traffic leakages in dual-stack hosts/ networks

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7359.
Author Fernando Gont
Last updated 2014-03-03
Replaces draft-gont-opsec-vpn-leakages
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Warren "Ace" Kumari
Shepherd write-up Show Last changed 2013-10-21
IESG IESG state Became RFC 7359 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES.
Responsible AD Joel Jaeggli
Send notices to,
IANA IANA review state Version Changed - Review Needed
opsec                                                            F. Gont
Internet-Draft                                       Huawei Technologies
Intended status: Informational                             March 3, 2014
Expires: September 4, 2014

  Virtual Private Network (VPN) traffic leakages in dual-stack hosts/


   The subtle way in which the IPv6 and IPv4 protocols co-exist in
   typical networks, together with the lack of proper IPv6 support in
   popular Virtual Private Network (VPN) products, may inadvertently
   result in VPN traffic leaks.  That is, traffic meant to be
   transferred over an encrypted and integrity protected VPN connection
   may leak out of such connection and be sent in the clear on the local
   network towards the final destination.  This document discusses some
   scenarios in which such VPN leakages may occur as a result of
   employing IPv6-unaware VPN software.  Additionally, this document
   offers possible mitigations for this issue.

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

   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 4, 2014.

Copyright Notice

   Copyright (c) 2014 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
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IPv4 and IPv6 co-existence  . . . . . . . . . . . . . . . . .   4
   4.  Virtual Private Networks in IPv4/IPv6 dual-stack
       hosts/networks  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Inadvertent VPN traffic-leakages in legitimate scenarios  . .   5
   6.  VPN traffic-leakage attacks . . . . . . . . . . . . . . . . .   5
   7.  Mitigations to VPN traffic-leakage vulnerabilities  . . . . .   6
     7.1.  Fixing VPN client software  . . . . . . . . . . . . . . .   6
     7.2.  Operational Mitigations . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   It is a very common practice for users to employ VPN software when
   employing a public (and possibly-rogue) local network.  This is
   typically done not only to gain access to remote resources may not
   otherwise accessible from the public Internet, but also to secure the
   host's traffic against attackers that might be connected to the same
   local network as the victim host.  The latter case constitutes the
   problem space of this document.  Indeed, it is sometimes assumed that
   employing a VPN connection makes the use of insecure protocols (e.g.,
   that transfer sensitive information in the clear) acceptable, as a
   VPN provides security services (such as data integrity and/or
   confidentiality) for all communications made over that VPN.  However,
   this document illustrates that under certain circumstances, some
   traffic might not be mapped onto the VPN and thus be sent in the
   clear on the local network.

   Many VPN products that are typically employed for the aforementioned
   VPN connections only support the IPv4 protocol: that is, they perform
   the necessary actions such that IPv4 traffic is sent over the VPN
   connection, but they do nothing to secure IPv6 traffic originated

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   from (or being received at) the host employing the VPN client.
   However, the hosts themselves are typically dual-stacked: they
   support (and enable by default) both IPv4 and IPv6 (even if such IPv6
   connectivity is simply "dormant" when they connect to IPv4-only
   networks).  When the IPv6 connectivity of such hosts is enabled, they
   may end up employing an IPv6-unaware VPN client in a dual-stack
   network.  This may have "unexpected" consequences, as explained

   The subtle way in which the IPv4 and IPv6 protocols interact and co-
   exist in dual-stacked networks might, either inadvertently or as a
   result of a deliberate attack, result in VPN traffic leakages -- that
   is, traffic meant to be transferred over a VPN connection could leak
   out of the VPN connection and be transmitted in the clear from the
   local network to the final destination, without employing the VPN
   services at all.

   Section 3 provides some background about IPv6 and IPv4 co-existence,
   summarizing how IPv6 and IPv4 interact on a typical dual-stacked
   network.  Section 4 describes the underlying problem that leads to
   the aforementioned VPN traffic leakages.  Section 5 describes
   legitimate scenarios in which such traffic leakages might occur,
   while Section 6 describes how VPN traffic leakages can be triggered
   by deliberate attacks.

2.  Terminology

   When employing the term "Virtual Private Network" (or its acronym,
   "VPN"), this document refers to IPsec-based or TLS-based tunnels,
   where traffic is encapsulated and sent from a client to a middle-box,
   to access multiple network services (possibly employing different
   transport and/or application protocols).

   Our use of the term "Virtual Private Networks" excludes the so-called
   SSL/TLS VPN portals (a front-end provided by the middlebox to add
   security to a normally-unsecured site).  Further discussion of SSL-
   based VPNs can be found in [SSL-VPNs].

   We note that, in addition to the general case of "send all traffic
   through the VPN", this document additionally considers the so-called
   "split-tunnel" case, where some subset of the traffic is sent through
   the VPN, while other traffic is send to its intended destination with
   a direct routing path (i.e., without employing the VPN tunnel).  We
   note that many organizations will prevent split-tunneling in their
   VPN configurations if they would like to make sure the users data
   goes through a gateway with protections (malware detection, URL
   filtering, etc.), but others are more interested in performance of

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   the user's access or the ability for researchers to have options to
   access sites they may not be able to through the gateway.

3.  IPv4 and IPv6 co-existence

   The co-existence of the IPv4 and IPv6 protocols has a number of
   interesting and subtle aspects that may have "surprising"
   consequences.  While IPv6 is not backwards-compatible with IPv4, the
   two protocols are "glued" together by the Domain Name System (DNS).

   For example, consider a site (say, that has both
   IPv4 and IPv6 support.  The corresponding domain name
   (, in our case) will contain both A and AAAA DNS
   resource records (RRs).  Each A record will contain one IPv4 address,
   while each AAAA record will contain one IPv6 address -- and there
   might be more than one instance of each of these record types.  Thus,
   when a dual-stacked client application means to communicate with, it can request both A and AAAA records, and use any
   of the available addresses.  The preferred address family (IPv4 or
   IPv6) and the specific address that will be used (assuming more than
   one address of each family is available) varies from one protocol
   implementation to another, with many host implementations preferring
   IPv6 addresses over IPv4 addresses.

      NOTE: [RFC6724] specifies an algorithm for selecting a destination
      address from a list of IPv6 and IPv4 addresses.  [RFC6555]
      discusses the challenge of selecting the most appropriate
      destination address, along with a proposed implementation approach
      that mitigates connection-establishment delays.

   As a result of this "co-existence" between IPv6 and IPv4, when a
   dual-stacked client means to communicate with some other system, the
   availability of A and AAAA DNS resource records will typically affect
   which protocol is employed to communicate with that system.

4.  Virtual Private Networks in IPv4/IPv6 dual-stack hosts/networks

   Many Virtual Private Network (VPN) implementations do not support the
   IPv6 protocol -- or, what is worse, they completely ignore IPv6.
   This typically means that, when establishing a VPN connection, the
   VPN software takes care of the IPv4 connectivity by, e.g. inserting
   an IPv4 default route that causes all IPv4 traffic to be sent over
   the VPN connection (as opposed to sending the traffic in the clear,
   employing the local router).  However, if IPv6 is not supported (or
   completely ignored), any packets destined to an IPv6 address will be
   sent in the clear using the local IPv6 router.  That is, the VPN
   software will do nothing about the IPv6 traffic.

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   The underlying reason for which these VPN leakages may occur is that,
   while IPv4 and IPv6 are two different protocols incompatible with
   each other, the two protocols are "glued" together by the Domain Name
   System.  Therefore, for dual-stacked systems, it is not possible to
   secure the communication with another system without securing both
   protocols (IPv6 and IPv4).

5.  Inadvertent VPN traffic-leakages in legitimate scenarios

   Consider a dual-stacked host that employs IPv4-only VPN software to
   establish a VPN connection with a VPN server, and that such host now
   connects to a dual-stacked network (that provides both IPv6 and IPv4
   connectivity).  If some application on the client means to
   communicate with a dual-stacked destination, the client will
   typically query both A and AAAA DNS resource records.  Since the host
   will have both IPv4 and IPv6 connectivity, and the intended
   destination will have both A and AAAA DNS resource records, one of
   the possible outcomes is that the host will employ IPv6 to
   communicate with the intended destination.  Since the VPN software
   does not support IPv6, the IPv6 traffic will not employ the VPN
   connection, and hence will have neither integrity nor confidentiality
   protection from the source host to the final destination.

   This could inadvertently expose sensitive traffic that was assumed to
   be secured by the VPN software.  In this particular scenario, the
   resulting VPN traffic leakage is a side-effect of employing
   IPv6-unaware VPN software in a dual-stacked host/network.

6.  VPN traffic-leakage attacks

   A local attacker could deliberately trigger IPv6 connectivity on the
   victim host by sending forged ICMPv6 Router Advertisement messages
   [RFC4861].  Such packets could be sent by employing standard software
   such as rtadvd [RTADVD], or by employing packet-crafting tools such
   as [SI6-Toolkit] or THC-IPv6 [THC-IPv6].  Once IPv6 connectivity has
   been enabled, communications with dual-stacked systems could result
   in VPN traffic leakages, as previously described.

   While this attack may be useful enough (due to the increasing number
   of IPv6-enabled sites), it will only lead to traffic leakages when
   the destination system is dual-stacked.  However, it is usually
   trivial for an attacker to trigger such VPN leakages for any
   destination systems: an attacker could simply advertise himself as
   the local recursive DNS server by sending forged Router Advertisement
   messages [RFC4861] that include the corresponding RDNSS option
   [RFC6106], and then perform a DNS spoofing attack such that he can
   become a "Man in the Middle" and intercept the corresponding traffic.

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   As with the previous attack scenario, packet-crafting tools such as
   [SI6-Toolkit] and [THC-IPv6] can readily perform this attack.

      NOTE: Some systems are known to prefer IPv6-based recursive DNS
      servers over IPv4-based ones, and hence the "malicious" recursive
      DNS servers would be preferred over the legitimate ones advertised
      by the VPN server.

7.  Mitigations to VPN traffic-leakage vulnerabilities

7.1.  Fixing VPN client software

   There are a number of possible mitigations for the VPN traffic-
   leakage vulnerability discussed in this document.

   If the VPN client is configured by administrative decision to
   redirect all IPv4 traffic to the VPN, it should:

   1.  If IPv6 is not supported in the VPN software, disable IPv6
       support in all network interfaces.

          NOTE: For IPv6-unaware VPN clients, the most simple mitigation
          (although not necessarily the most desirable one) would be to
          disable IPv6 support in all network interface cards when a VPN
          connection is meant to be employed.  Thus, applications on the
          host running the VPN client software will have no other option
          than to employ IPv4, and hence they will simply not even try
          to send/process IPv6 traffic.

   2.  If IPv6 is supported in the VPN software, ensure that all IPv6
       traffic is also sent via the VPN.

   If the VPN client is configured to only send a subset of IPv4 traffic
   to the VPN tunnel (split-tunnel mode), then:

   1.  If the VPN client does not support IPv6, it should disable IPv6
       support in all network interfaces.

   2.  If the VPN client supports IPv6, it is the administrators
       responsibility to ensure that the correct corresponding sets of
       IPv4 and IPv6 networks get routed into the VPN tunnel.

   Additionally, VPN clients that support IPv6 should mitigate all
   Neighbor Discovery (ND) attacks that may introduce new entries in the
   routing table, such as attacks based on forged Router Advertisement
   messages containing more specific routes [RFC4191], forged ICMPv6
   Redirect messages, etc.

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   A network may prevent local attackers from successfully performing
   the aforementioned attacks against other local hosts by implementing
   First-Hop Security solutions such as Router Advertisement Guard (RA-
   Guard) [RFC6105] and DHCPv6-Shield [I-D.ietf-opsec-dhcpv6-shield].
   However, for obvious reasons, a host cannot and should not rely on
   this type of mitigations when connecting to an open network
   (cybercafe, etc.).

      NOTE: Besides, popular implementations of RA-Guard are known to be
      vulnerable to evasion attacks [RFC7113].

   Finally, we note that if (eventually) IPv6-only VPN implementations
   become available, similar issues to the ones discussed in this
   document could arise if these IPv6-only VPN implementations do
   nothing about the IPv4 traffic.

7.2.  Operational Mitigations

   While the desired mitigation for the issues discussed in this
   document is for VPN clients to be IPv6-aware, we note that in
   scenarios where this would be unfeasible, and administrator may want
   to disable IPv6 connectivity on all network interfaces of the node
   employing the IPv6-unaware VPN client.

8.  IANA Considerations

   This document has no actions for IANA.

9.  Security Considerations

   This document discusses how traffic meant to be transferred over a
   VPN connection can leak out of the VPN, and hence appear in the clear
   on the local network.  This is the result of employing IPv6-unaware
   VPN client software on dual-stacked hosts.

   Possible ways to mitigate this problem include fixing the VPN client
   software, or disabling IPv6 connectivity on all network interfaces
   when the previous option is not feasible.

10.  Acknowledgements

   The author of this document would like to thank (in alphabetical
   order) Cameron Byrne, Spencer Dawkins, Gert Doering, Stephen Farrell,
   Seth Hall, Paul Hoffman, Tor Houghton, Russ Housley, Joel Jaeggli,
   Alastair Johnson, Merike Kaeo, Panos Kampanakis, Warren Kumari,
   Henrik Lund Kramshoj, Barry Leiba, Kathleen Moriarty, Thomas
   Osterried, Jim Small, Martin Vigoureux, and Andrew Yourtchenko for
   providing valuable comments on earlier versions of this document.

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11.  References

11.1.  Normative References

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, November 2010.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

11.2.  Informative References

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113, February 2014.

              Gont, F., Will, W., and G. Velde, "DHCPv6-Shield:
              Protecting Against Rogue DHCPv6 Servers", draft-ietf-
              opsec-dhcpv6-shield-02 (work in progress), February 2014.

              "SI6 Networks' IPv6 toolkit",

              "The Hacker's Choice IPv6 Attack Toolkit",

   [RTADVD]   "rtadvd(8) manual page", <

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              Hoffman, P., "SSL VPNs: An IETF Perspective", IETF 72,
              SAAG Meeting., 2008,

Author's Address

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472

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