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Security Concerns with IP Tunneling

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6169.
Authors James Hoagland , Suresh Krishnan , Dave Thaler
Last updated 2015-10-14 (Latest revision 2010-10-25)
Replaces draft-ietf-v6ops-teredo-security-concerns
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state RFC 6169 (Informational)
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Responsible AD Ron Bonica
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IPv6 Operations Working Group                                S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Informational                                 D. Thaler
Expires: April 28, 2011                                        Microsoft
                                                             J. Hoagland
                                                        October 25, 2010

                  Security Concerns With IP Tunneling


   A number of security concerns with IP tunnels are documented in this
   memo.  The intended audience of this document includes network
   administrators and future protocol developers.  The primary intent of
   this document is to raise the awareness level regarding the security
   issues with IP tunnels as deployed today.

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 April 28, 2011.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Tunnels May Bypass Security  . . . . . . . . . . . . . . . . .  3
     2.1.  Network Security Bypass  . . . . . . . . . . . . . . . . .  3
     2.2.  IP Ingress and Egress Filtering Bypass . . . . . . . . . .  5
     2.3.  Source Routing After the Tunnel Client . . . . . . . . . .  6
   3.  Challenges in Inspecting and Filtering Content of Tunneled
       Data Packets . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Inefficiency of Selective Network Filtering of All
           Tunneled Packets . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Problems with deep packet inspection of tunneled data
           packets  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Increased Exposure Due to Tunneling  . . . . . . . . . . . . .  9
     4.1.  NAT Holes Increase Attack Surface  . . . . . . . . . . . .  9
     4.2.  Exposure of a NAT Hole . . . . . . . . . . . . . . . . . . 11
     4.3.  Public Tunnels Widen Holes in Restricted NATs  . . . . . . 12
   5.  Tunnel Address Concerns  . . . . . . . . . . . . . . . . . . . 12
     5.1.  Feasibility of Guessing Tunnel Addresses . . . . . . . . . 12
     5.2.  Profiling Targets Based on Tunnel Address  . . . . . . . . 13
   6.  Additional Security Concerns . . . . . . . . . . . . . . . . . 14
     6.1.  Attacks Facilitated By Changing Tunnel Server Setting  . . 14
   7.  Mechanisms to secure the use of tunnels  . . . . . . . . . . . 16
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   11. Informative References . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   With NAT devices becoming increasingly more prevalent, there have
   recently been many tunneling protocols developed that go through NAT
   devices or firewalls by tunneling over UDP or TCP.  For example,
   Teredo [RFC4380], L2TPv2 [RFC2661], and L2TPv3 [RFC3931] all tunnel
   IP packets over UDP.  Similarly, many SSL VPN solutions that tunnel
   IP packets over HTTP (and hence over TCP) are deployed today.

   This document discusses security concerns with tunneling IP packets,
   and includes recommendations where relevant.

   The primary intent of this document is to help improve security
   deployments using tunnel protocols.  In addition, the document aims
   to provide information that can be used in any new or updated tunnel
   protocol specification.  The intended audience of this document
   includes network administrators and future protocol developers.

2.  Tunnels May Bypass Security

2.1.  Network Security Bypass

2.1.1.  Problem

   Tunneled IP traffic may not receive the intended level of inspection
   or policy application by network-based security devices unless such
   devices are specifically tunnel-aware.  This reduces defense in depth
   and may cause security gaps.  This applies to all network-located
   devices and to any end-host based firewalls whose existing hooking
   mechanism(s) would not show them the IP packet stream after the
   tunnel client does decapsulation or before it does encapsulation.

2.1.2.  Discussion

   Evasion by tunneling is often a problem for network-based security
   devices such as network firewalls, intrusion detection and prevention
   systems, and router controls.  To provide such functionality in the
   presence of tunnels, the developer of such devices must add support
   for parsing each new protocol.  There is typically a significant lag
   between when the security developer recognizes that a tunnel will be
   used (or will be remotely usable) to a significant degree and when
   the parsing can be implemented in a product update, the update tested
   and released, and customers begin using the update.  Late changes in
   the protocol specification or in the way it is implemented can cause
   additional delays.  This becomes a significant security concern when
   a delay in applied coverage is occurring frequently.  One way to cut
   down on this lag is for security developers to follow the progress of

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   new IETF protocols but this will still not account for any new
   proprietary protocols.

   For example, for L2TP or Teredo, an unaware network security device
   would inspect or regulate the outer IP and the IP-based UDP layer as
   normal, but it would not recognize that there is an additional IP
   layer contained inside the UDP payload to which it needs to apply the
   same controls as it would to a native packet.  (Of course, if the
   device discards the packet due to something in the IP or UDP header,
   such as referring to an unknown protocol, the embedded packet is no
   longer a concern.)  In addition, if the tunnel does encryption, the
   network-based security device may not be able to do much, just as if
   IPsec end-to-end encryption were used without tunneling.

   Network security controls being not applied must be a concern to
   those that set them up, since those controls are supposed to provide
   an additional layer of defense against external attackers.  If
   network controls are being bypassed due to the use of tunneling, the
   strength of the defense (i.e. the number of layers of defense) is
   reduced.  Since security administrators may have a significantly
   reduced level of confidence without this layer, this becomes a
   concern to them.

   One implication of the security control bypass is that defense in
   depth has been reduced, perhaps down to zero unless a local firewall
   is in use as recommended in [RFC4380].  However, even if there are
   host-based security controls that recognize tunnels, security
   administrators may not have configured them with full security
   control parity, even if all controls that were maintained by the
   network are available on the host.  Thus there may be gaps in desired

   Compounding this is that, unlike what would be the case for native
   IP, some network administrators will not even be aware that their
   hosts are globally reachable, if the tunnel provides connectivity to/
   from the Internet; for example, they may not be expecting this for
   hosts behind a stateful firewall.  In addition, Section 3.2 discusses
   how it may not be efficient to find all tunneled traffic for network
   devices to examine.

2.1.3.  Recommendations

   Security administrators who do not consider tunneling an acceptable
   risk should disable tunnel functionality either on the end-nodes
   (hosts) or on the network nodes at the perimeter of their network.
   However, there may be an awareness gap.  Thus, due to the possible
   negative security consequences, tunneling functionality should be
   easy to disable on the host and through a central management facility

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   if one is provided.

   To minimize security exposure due to tunnels, we recommend that a
   tunnel be an interface of last resort, independent of IP version.
   Specifically, we suggest that when both native and tunneled access to
   a remote host is available, that the native access be used in
   preference to tunneled access except when the tunnel endpoint is
   known to not bypass security (e.g., an IPsec tunnel to a gateway
   provided by the security administrator of the network).  This should
   also promote greater efficiency and reliability.

   Note that although Rule 7 of [RFC3484] section 6 will prefer native
   connectivity over tunnels, this rule is only a tie-breaker when a
   choice is not made by earlier rules; hence tunneling mechanisms that
   are tied to a particular range of IP address space will be decided
   based on the prefix precedence.  For example, using the prefix policy
   mechanism of [RFC3484] section 2.1, Teredo might have a precedence of
   5 so that native IPv4 is preferred over Teredo.

2.2.  IP Ingress and Egress Filtering Bypass

2.2.1.  Problem

   IP addresses inside tunnels are not subject to ingress and egress
   filtering in the network they tunnel over, unless extraordinary
   measures are taken.  Only the tunnel endpoints can do such filtering.

2.2.2.  Discussion

   Ingress filtering (sanity-checking incoming destination addresses)
   and egress filtering (sanity-checking outgoing source addresses) are
   done to mitigate attacks and to make it easier to identify the source
   of a packet and are considered to be a good practice. e.g. ingress
   filtering at the network perimeter should not allow packets with a
   source address that belongs to the network to enter the network from
   the outside the network.  This function is most naturally (and in the
   general case, by requirement) done at network boundaries.  Tunneled
   IP traffic bypassing this network control is a specific case of
   Section 2.1, but is illustrative.

2.2.3.  Recommendations

   Tunnel servers can apply ingress and egress controls to tunneled IP
   addresses passing through them to and from tunnel clients.

   Tunnel clients could make an effort to conduct ingress and egress

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   Implementations of protocols that embed an IPv4 address in a tunneled
   IPv6 address directly between peers should perform filtering based on
   checking the correspondence.

   Implementations of protocols that accept tunneled packets directly
   from a server, relay or protocol peer do filtering the same way as it
   would be done on a native link with traffic from a router.

   Some protocols such as 6to4 [RFC3056], Teredo, and ISATAP [RFC5214]
   allow both other hosts and a router over a common tunnel.  To perform
   host-based filtering with such protocols a host would need to know
   the outer IP address of each router from which it could receive
   traffic, so that packets from hosts beyond the router will be
   accepted even though the source address would not embed the router's
   IP address.  Router addresses might be learned via Secure Neighbor
   Discovery (SEND) [RFC3971] or some other mechanism (e.g., [RFC5214]
   section 8.3.2).

2.3.  Source Routing After the Tunnel Client

2.3.1.  Problem

   If the encapsulated IP packet specifies source routing beyond the
   recipient tunnel client, the host may forward the IP packet to the
   specified next hop.  This may be unexpected and contrary to
   administrator wishes and may have bypassed network-based source
   routing controls.

2.3.2.  Discussion

   A detailed discussion of issues related to source routing can be
   found in [RFC5095] and [SECA-IP].

2.3.3.  Recommendations

   Tunnel clients should by default discard tunneled IP packets that
   specify additional routing, as recommended in [RFC5095] and
   [SECA-IP], though they may also allow the user to configure what
   source routing types are allowed.  All pre-existing source routing
   controls should be upgraded to apply these controls to tunneled IP
   packets as well.

3.  Challenges in Inspecting and Filtering Content of Tunneled Data

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3.1.  Inefficiency of Selective Network Filtering of All Tunneled

3.1.1.  Problem

   There is no mechanism to both efficiently and immediately filter all
   tunneled packets (other than the obviously faulty method of filtering
   all packets).  This limits the ability to prevent tunnel use on a

3.1.2.  Discussion

   Given concerns about tunnel security or a network's lack of
   preparedness for tunnels, a network administrator may wish to simply
   block all use of tunnels that bypass security policies.  He or she
   may wish to do so using network controls; this could be either due to
   not having the capability to disable tunneling on all hosts attached
   to the network or due to wanting an extra layer of prevention.

   One simple method of doing this easily for many tunnel protocols is
   to block outbound packets to the UDP or TCP port used (e.g.,
   destination UDP port is 3544 for Teredo, UDP port 1701 for L2TP,
   etc.).  This prevents a tunnel client from establishing a new tunnel.
   However, existing tunnels will not necessarily be affected if the
   blocked port is used only for initial setup.  In addition, if the
   blocking is applied on the outside of the client's NAT device, the
   NAT device will retain the port mapping for the client.  In some
   cases, however, blocking all traffic to a given outbound port (e.g.,
   port 80) may interfere with non-tunneled traffic so this should be
   used with caution.

   Another simple alternative, if the tunnel server addresses are well-
   known, is to filter out all traffic to/from such addresses.

   The other approach is to find all packets to block in the same way as
   would be done for inspecting all packets (Section 3.2).  However;
   this faces the difficulties in terms of efficiency of filtering, as
   is discussed there.

3.1.3.  Recommendations

   Developers of protocols that tunnel over UDP or TCP (including HTTP)
   to reach the Internet should disable their protocols in networks that
   wish to enforce security policies on the user traffic.  (Windows, for
   example, disables Teredo by default if it detects that it is within
   an enterprise network that contains a Windows domain controller.)

   Administrators of such networks may wish to filter all tunneled

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   traffic at the boundaries of their networks.  It is sufficient to
   filter out the tunneled connection requests (if they can be
   identified) to stop further tunneled traffic.  The easiest mechanism
   for this would be to filter out outgoing traffic sent to the
   destination port defined by the tunneling protocol, and incoming
   traffic with that source port.  Similarly, in certain cases, it is
   also possible to use the IP protocol field to identify and filter
   tunneled packets. e.g. 6to4 [RFC3056] is a tunneling mechanism that
   uses the IPv4 packets to carry encapsulated IPv6 packets, and can be
   identified by the IPv4 protocol type 41.

3.2.  Problems with deep packet inspection of tunneled data packets

3.2.1.  Problem

   There is no efficient mechanism for network-based devices, which are
   not the tunnel endpoint, to inspect the contents of all tunneled data
   packets, the way they can for native packets.  This makes it
   difficult to apply the same controls as they do to native IP.

3.2.2.  Discussion

   Some tunnel protocols are easy to identify, such as if all data
   packets are encapsulated using a well-known UDP or TCP port that is
   unique to the protocol.

   Other protocols, however, either use dynamic ports for data traffic,
   or else share ports with other protocols (e.g., tunnels over HTTP).

   The implication of this is that network-based devices that wish to
   passively inspect (and perhaps selectively apply policy to) all
   encapsulated traffic must inspect all TCP or UDP packets (or at least
   all packets not part of a session that is known not to be a tunnel).
   This is imperfect since a heuristic must then be applied to determine
   if a packet is indeed part of a tunnel.  This may be too slow to make
   use of in practice, especially if it means that all TCP or UDP
   packets must be taken off of the device's "fast path".

   One heuristic that can be used on packets to determine if they are
   tunnel-related or not is as follows.  For each known tunnel protocol,
   attempt parsing the packet as if it were a packet of that protocol,
   destined to the local host (i.e., where the local host has the
   destination address in the inner IP header, if any).  If all syntax
   checks pass, up to and including the inner IP header (if the tunnel
   doesn't use encryption), then treat the packet as if it is a tunneled
   packet of that protocol.

   It is possible that non-tunnel packets will match as tunneled using

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   this heuristic, but tunneled packets (of the known types of tunnels)
   should not escape inspection, absent implementation bugs.

   For some protocols, it may be possible to monitor setup exchanges to
   know to expect that data will be exchanged on certain ports later.
   (Note that this does not necessarily apply to Teredo, for example,
   since communicating with another Teredo client behind a cone NAT
   [RFC5389] device does not require such signaling.  In such cases this
   control will not work.  However, deprecation of the cone bit as
   discussed in [RFC5991] means this technique may indeed work with
   updated Teredo implementations.)

3.2.3.  Recommendations

   As illustrated above, it should be clear that inspecting the contents
   of tunneled data packets is highly complex and often impractical.
   For this reason, if a network wishes to monitor IP traffic, tunneling
   across, as opposed to tunneling to, the security boundary is not
   recommended.  For example, to provide an IPv6 transition solution,
   the network should provide native IPv6 connectivity or a tunnel
   solution (e.g., ISATAP or 6over4) that encapsulates data packets
   between hosts and a router within the network.

4.  Increased Exposure Due to Tunneling

4.1.  NAT Holes Increase Attack Surface

4.1.1.  Problem

   If the tunnel allows inbound access from the public Internet, the
   opening created in a NAT device due to a tunnel client increases its
   Internet attack surface area.  If vulnerabilities are present, this
   increased exposure can be used by attackers and their programs.

   If the tunnel allows inbound access only from a private network
   (e.g., a remote network to which one has VPN'ed), the opening created
   in the NAT device still increases its attack surface area, although
   not as much as in the public Internet case.

4.1.2.  Discussion

   When a tunnel is active, a mapped port is maintained on the NAT
   device through which remote hosts can send packets and perhaps
   establish connections.  The following sequence is intended to sketch
   out the processing on the tunnel client host that can be reached
   through this mapped port; the actual processing for a given host may
   be somewhat different.

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   1.  Link-layer protocol processing

   2.  (Outer) IP host firewall processing

   3.  (Outer) IP processing by stack

   4.  UDP/TCP processing by stack

   5.  Tunnel client processing

   6.  (Inner) IP host firewall processing

   7.  (Inner) IP processing by stack

   8.  Various upper layer processing may follow

   The inner firewall (and other security) processing may or may not be
   present, but if it is, some of the inner IP processing may be
   filtered.  (For example, [RFC4380] section 7.1 recommends that an
   IPv6 host firewall be used on all Teredo clients.)

   (By the virtue of the tunnel being active, we can infer that the
   inner host firewall is unlikely to do any filtering based on the
   outer IP.)  Any of this processing may expose vulnerabilities an
   attacker can exploit; similarly these may expose information to an
   attacker.  Thus, even if firewall filtering is in place (as is
   prudent) and filters all incoming packets, the exposed area is larger
   than if a native IP Internet connection were in place, due to the
   processing that takes place before the inner IP is reached
   (specifically, the UDP/TCP processing, the tunnel client processing,
   and additional IP processing, especially if one is IPv4 and the other
   is IPv6).

   One possibility is that a layer 3 targeted worm makes use of a
   vulnerability in the exposed processing.  The main benefit tunneling
   provides to worms is enabling L3 reachability to the end host.  Even
   a thoroughly firewalled host could be subject to a worm that spreads
   with a single UDP packet if the right remote code vulnerability is

4.1.3.  Recommendations

   This problem seems inherent in tunneling being active on a host, so
   the solution seems to be to minimize tunneling use.

   For example, it can be active only when it is really needed and only
   for as long as needed.  So, the tunnel interface can be initially not
   configured and only used when it is entirely the last resort.  The

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   interface should then be deactivated (ideally, automatically) again
   as soon as possible.  Note however that the hole will remain in the
   NAT device for some amount of time after this, so some processing of
   incoming packets is inevitable unless the client's native IP address
   behind the NAT device is changed.

4.2.  Exposure of a NAT Hole

4.2.1.  Problem

   Attackers are more likely to know about a tunnel client's NAT hole
   than a typical hole in the NAT device.  If they know about the hole,
   they could try to use it.

4.2.2.  Discussion

   There are at least three reasons why an attacker may be more likely
   to learn of the tunnel client's exposed port than a typical NAT
   exposed port:

   1.  The NAT mapping for a tunnel is typically held open for a
       significant period of time, and kept stable.  This increases the
       chance of it being discovered.

   2.  In some protocols (e.g., Teredo), the external IP address and
       port are contained in the client's address that is used end-to-
       end and possibly even advertised in a name resolution system.
       While the tunnel protocol itself might only distribute this
       address in IP headers, peers, routers, and other on-path nodes
       still see the client's IP address.  Although this point does not
       apply directly to protocols (e.g., L2TP) that do not construct
       the inner IP address based on the outer IP address, the inner IP
       address is still known to peers, routers, etc. and can still be
       reached by attackers without knowing the external IP address or

   3.  The tunnel protocol often contains more messages that are
       exchanged and with more parties (e.g., due to a longer path
       length) than without using the tunnel, offering more chance for
       visibility into the port and address in use.

4.2.3.  Recommendations

   The recommendations from Section 4.1 seem to apply here as well:
   minimize tunnel use.

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4.3.  Public Tunnels Widen Holes in Restricted NATs

4.3.1.  Problem

   Tunnels that allow inbound connectivity from the Internet (e.g.,
   Teredo, tunnel brokers, etc) essentially disable the filtering
   behavior of the NAT for all tunnel client ports.  This eliminates NAT
   devices filtering for such ports and may eliminate the need for an
   attacker to spoof an address.

4.3.2.  Discussion

   NATs that implement Address-Dependent or Address and Port-Dependent
   Filtering [RFC4787] limit the source of incoming packets to just
   those that are a previous destination.  This poses a problem for
   tunnels that intend to allow inbound connectivity from the Internet.

   Various protocols (e.g., Teredo, STUN [RFC5389], etc.) provide a
   facility for peers, upon request, to become a previous destination.
   This works by sending a "bubble" packet via a server, which is passed
   to the client, and then sent by the client (through the NAT) to the

   This removes any NAT-based barrier to attackers sending packets in
   through the client's service port.  In particular, an attacker would
   no longer need to either be an actual previous destination or to
   forge its addresses as a previous destination.  When forging, the
   attacker would have had to learn of a previous destination and then
   would face more challenges in seeing any returned traffic.

4.3.3.  Recommendations

   If the tunnel can provide connectivity to the Internet, the tunnel
   client should run a host firewall on the tunnel interface.  Also,
   minimizing public tunnel use (see Section 4.1.3) would lower the
   attack opportunity related to this exposure.

5.  Tunnel Address Concerns

5.1.  Feasibility of Guessing Tunnel Addresses

5.1.1.  Problem

   For some types of tunneling protocols, it may be feasible to guess IP
   addresses assigned to tunnels, either when looking for a specific
   client or when looking for an arbitrary client.  This is in contrast
   to native IPv6 addresses in general, but is no worse than for native

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   IPv4 addresses today.

   For example, some protocols (e.g., 6to4 and Teredo) use well-defined
   address ranges.  As another example, using well-known public servers
   for Teredo or tunnel brokers also implies using a well known address

5.2.  Profiling Targets Based on Tunnel Address

5.2.1.  Problem

   An attacker encountering an address associated with a particular
   tunneling protocol or well-known tunnel server has the opportunity to
   infer certain relevant pieces of information that can be used to
   profile the host before sending any packets.  This can reduce the
   attacker's footprint and increase the attacker's efficiency.

5.2.2.  Discussion

   The tunnel address reveals some information about the nature of the

   o  That a host has a tunnel address associated with a given protocol
      means that the client is running on some platform for which there
      exists a tunnel client implementation of that protocol.  In
      addition, if some platforms have that protocol installed by
      default and where the host's default rules for using it make it
      susceptible to being in use, then it is more likely to be running
      on such a platform than on one where it is not used by default.
      For example, as of this writing, seeing a Teredo address suggests
      that the host it is on is probably running Windows.

   o  Similarly, the use of an address associated with a particular
      tunnel server also suggests some information.  Tunnel client
      software is often deployed, installed, and/or configured using
      some degree of automation.  It seems likely that the majority of
      the time the tunnel server that results from the initial
      configuration will go unchanged from the initial setting.
      Moreover, the server that is configured for use may be associated
      with a particular means of installation, which often suggests the
      platform.  For example, if the server field in a Teredo address is
      one of the IPv4 addressees to which
      resolves, it suggests that the host is running Windows.

   o  The external IPv4 address of a NAT device can of course be readily
      associated with a particular organization or at least an ISP, and
      hence putting this address in an IPv6 address reveals this
      information.  However, this is no different than using a native IP

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      address, and hence is not new with tunneling.

   o  It is also possible that external client port numbers may be more
      often associated with particular client software or the platform
      on which it is running.  The usefulness of this for platform
      determination is, however, reduced by the different NAT port
      number assignment behaviors.  In addition, the same observations
      would apply to use of UDP or TCP over native IP as well, and hence
      this is not new with tunneling.

   The platform, tunnel client software, or organization information can
   be used by an attacker to target attacks more carefully.  For
   example, an attacker may decide to attack an address only if it is
   likely to be associated with a particular platform or tunnel client
   software with a known vulnerability.  (This is similar to the ability
   to guess some platforms based on the OUI in the EUI-64 portion of an
   IPv6 address generated from a MAC address, since some platforms are
   commonly used with interface cards from particular vendors.)

5.2.3.  Recommendations

   If installation programs randomized the server setting, that would
   reduce the extent to which they can be profiled.  Similarly,
   administrators can choose to change the default setting to reduce the
   degree to which they can be profiled ahead of time.

   Randomizing the tunnel client port in use would mitigate any
   profiling that can be done based on the external port, especially if
   multiple different tunnel clients did this.  Further discussion on
   randomizing ports can be found at [TSV-PORT].

   It is recommended that tunnel protocols minimize the propagation of
   knowledge about whether the NAT is a cone NAT.

6.  Additional Security Concerns

6.1.  Attacks Facilitated By Changing Tunnel Server Setting

6.1.1.  Problem

   If an attacker could either change a tunnel client's server setting
   or change the IP addresses to which a configured host name resolves
   (e.g., by intercepting DNS queries) AND the tunnel is not
   authenticated, it would let the attacker become a man in the middle.
   This would allow them to at least monitor peer communication and at
   worst to impersonate the remote peer.

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6.1.2.  Discussion

   A client's server has good visibility into the client's communication
   with IP peers.  If the server were switched to one that records this
   information and makes it available to third parties (e.g.,
   advertisers, competitors, spouses, etc.) then sensitive information
   would be disclosed, especially if the client's host prefers the
   tunnel over native IP.  Assuming the server provides good service,
   the user would not have reason to suspect the change.

   Full interception of IP traffic could also be arranged (including
   pharming) which would allow any number of deception or monitoring
   attacks including phishing.  We illustrate this with an example
   phishing attack scenario.

   It is often assumed that the tunnel server is a trusted entity.  It
   may be possible for malware or a malicious user to quietly change the
   client's tunnel server setting and have the user be unaware their
   trust has been misplaced for an indefinite period of time.  However,
   malware or a malicious user can do much worse than this, so this is
   not a significant concern.  Hence it is only important that an
   attacker on the network cannot change the client's server setting.

   1.  A phisher sets up a malicious tunnel server (or tampers with a
       legitimate one).  This server, for the most part, provides
       correct service.

   2.  An attacker, by some means, switches the host's tunnel server
       setting, or spoofs a DNS reply, to point to the above server.  If
       neither DNS nor the tunnel setup is secured (i.e., if the client
       does not authenticate the information), then the attacker's
       tunnel server is seen as legitimate.

   3.  A user on the victim host types their bank's URL into his/her

   4.  The bank's hostname resolves to one or more IP addresses and the
       tunnel is selected for socket connection for whatever reason
       (e.g., the tunnel provides IPv6 connectivity and the bank has an
       IPv6 address).

   5.  The tunnel client uses the server for help in connecting to the
       bank's IP address.  Some tunneling protocols use a separate
       channel for signaling vs data, but this still allows the server
       to place itself in the data path by an appropriate signal to the
       client.  For example, in Teredo, the client sends a ping request
       through a server which is expected to come back through a data
       relay, and a malicious server can simply send it back itself to

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       indicate that is a data relay for the communication.

   6.  The rest works pretty much like any normal phishing transaction,
       except that the attacker acts as a tunnel server (or data relay,
       for protocols such as Teredo) and a host with the bank's IP

   This pharming type attack is not unique to tunneling.  Switching DNS
   server settings to a malicious DNS server or DNS cache poisoning in a
   recursive DNS resolver could have a similar effect.

6.1.3.  Recommendations

   In general, anti-phishing and anti-fraud provisions should help with
   aspects of this, as well as software that specifically monitors for
   tunnel server changes.

   Perhaps the best way to mitigate tunnel-specific attacks is to have
   the client either authenticate the tunnel server, or at least the
   means by which the tunnel server's IP address is determined.  For
   example, SSL VPNs use https URLs and hence authenticate the server as
   being the expected one.  Another mechanism, when IPv6 Router
   Advertisements are sent over the tunnel is to use SEcure Neighbor
   Discovery (SEND) [RFC3971] to verify that the client trusts the

   On the host, it should require an appropriate level of privilege in
   order to change the tunnel server setting (as well as other non-
   tunnel-specific settings such as the DNS server setting, etc.).
   Making it easy to see the current tunnel server setting (e.g., not
   requiring privilege for this) should help detection of changes.

   The scope of the attack can also be reduced by limiting tunneling use
   in general but especially in preferring native IPv4 to tunneled IPv6;
   this is because it is reasonable to expect that banks and similar web
   sites will continue to be accessible over IPv4 for as long as a
   significant fraction of their customers are still IPv4-only.  Please
   refer to Section 3 of [TUNNEL-LOOPS] for a detailed description and
   mitigation measures for a class of attacks based on IPv6 automatic

7.  Mechanisms to secure the use of tunnels

   This document described several security issues with tunnels.  This
   does not mean that tunnels need to be avoided at any cost.  On the
   contrary, tunnels can be very useful if deployed, operated and used
   properly.  The threats against IP tunnels are documented here.  If

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   the threats can be mitigated, network administrators can efficiently
   and securely use tunnels in their network.  Several measures can be
   taken in order to secure the operation of IPv6 tunnels:

   o  Operating on-premise tunnel servers/relays so that the tunneled
      traffic does not cross border routers.

   o  Setting up internal routing to steer traffic to these servers/

   o  Setting up of firewalls [RFC2979] to allow known and controllable
      tunneling mechanisms and disallow unknown tunnels.

8.  Acknowledgments

   The authors would like to thank Remi Denis-Courmont, Fred Templin,
   Jordi Palet Martinez, James Woodyatt, Christian Huitema, Brian
   Carpenter, Nathan Ward, Kurt Zeilenga, Joel Halpern, Erik Kline,
   Alfred Hoenes and Fernando Gont for reviewing earlier versions of the
   document and providing comments to make this document better.

9.  Security Considerations

   This entire document discusses security concerns with tunnels.

10.  IANA Considerations

   This document does not require any IANA action.

11.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

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   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              December 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5991]  Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
              Security Updates", RFC 5991, September 2010.

   [SECA-IP]  Gont, F., "Security Assessment of the Internet Protocol
              version 4", draft-ietf-opsec-ip-security-03 (work in
              progress), April 2010.

              Larsen, M. and F. Gont, "Transport Protocol Port
              Randomization Recommendations",
              draft-ietf-tsvwg-port-randomization-09 (work in progress),
              August 2010.

              Nakibly, G. and F. Templin, "Routing Loop Attack using
              IPv6 Automatic Tunnels: Problem Statement and Proposed

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              Mitigations", draft-ietf-v6ops-tunnel-loops-00 (work in
              progress), September 2010.

Authors' Addresses

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Phone: +1 514 345 7900 x42871

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Phone: +1 425 703 8835

   James Hoagland
   Symantec Corporation
   350 Ellis St.
   Mountain View, CA  94043


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