Network Working Group                                       T. Henderson
Internet-Draft                                        The Boeing Company
Expires: November 18, 2006                                   P. Nikander
                                            Ericsson Research NomadicLab
                                                            May 17, 2006

                   Using HIP with Legacy Applications

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The Host Identity Protocol and architecture (HIP) proposes to add a
   cryptographic name space for network stack names.  From an
   application viewpoint, HIP-enabled systems support a new address
   family (e.g., AF_HOST), but it may be a long time until such HIP-
   aware applications are widely deployed even if host systems are
   upgraded.  This informational document discusses implementation and
   API issues relating to using HIP in situations in which the system is

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   HIP-aware but the applications are not.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Approaches for supporting legacy applications  . . . . . . . .  5
     3.1.  Using IP addresses in applications . . . . . . . . . . . .  5
     3.2.  Using DNS  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Connecting directly to a HIT . . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Introduction

   The Host Identity Protocol (HIP) [1] is an experimental effort in the
   IETF and IRTF to study a new public-key-based name space for use as
   host identifiers in Internet protocols.  Fully deployed, the HIP
   architecture will permit applications to explicitly request the
   system to connect to another named host by expressing a location-
   independent name of the host when the system call to connect is
   performed.  However, there will be a transition period during which
   systems become HIP-enabled but applications are not.

   When applications and systems are both HIP-aware, the coordination
   between the application and the system can be straightforward.  For
   example, using the terminology of the widely used sockets API, the
   application can issue a system call to connect to another host by
   naming it explicitly, and the system can perform the necessary name-
   to-address mapping to assign appropriate routable addresses to the
   packets.  To enable this, a new address family (e.g., AF_HOST) could
   be defined, and additional API extensions could be defined (such as
   allowing IP addresses to be passed in the system call, along with the
   host name, as hints of where to initially try to reach the host).

   This note does not define a native HIP API such as described above.
   Rather, this note is concerned with the scenario in which the
   application is not HIP-aware and a traditional IP-address-based API
   is used by the application.  To use HIP in such a situation, there
   are a few basic possibilities: i) allow applications to use IP
   addresses as before, and provide a mapping from IP address to host
   identity (and back to IP address) within the system, ii) take
   advantage of domain name resolution to provide the application with
   either an alias for the host identifier or (in the case of IPv6) the
   host identity tag (HIT) itself, and iii) support the use of HITs
   directly (without prior DNS resolution) in place of IPv6 addresses.
   This note describes several variations of the above strategies and
   suggests some pros and cons to each approach.

   When HITs are used (rather than IP addresses) as peer names at the
   system API, they can provide a type of "channel binding" (Section
   1.1.6 of [2]) in that the ESP association formed by HIP is
   cryptographically bound to the name (HIT) invoked by the calling

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2.  Terminology

   Host Identity Tag: A 128-bit quantity formed by the hash of a Host
      Identity.  More details are available in [1].

   Local Scope Identifier: A 32- or 128-bit quantity locally
      representing the Host Identity at the IPv4 or IPv6 API.

   Referral:  An event when the application passes what it believes to
      be an IP address to another application instance on another host,
      within its application data stream.  An example is the FTP PORT

   Resolver: The system function used by applications to resolve domain
      names to IP addresses.

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3.  Approaches for supporting legacy applications

   This section provides examples of how legacy applications, using
   legacy APIs, can operate over a HIP-enabled system and use HIP.  The
   examples are organized by the name used by an application (or
   application user) to name the peer system: an IP address, a domain
   name, or a HIT.

   While the text below concentrates on the use of the connect system
   call, the same argument can also be applied to datagram-based system

   Recent work in the shim6 group has categorized the ways in which
   current applications use IP addresses [3].  These uses include short-
   lived local handles, long-lived application associations, callbacks,
   referrals, and identity comparisons.  Each of the below mechanisms
   has implications on these different uses of IP addresses by legacy

3.1.  Using IP addresses in applications

   Consider the case in which an application issues a "connect(ip)"
   system call to connect to a system named by address "ip", but for
   which we would like to enable HIP to protect the communications.
   Since the application or user does not (can not) indicate a desire to
   use HIP through the standard sockets API, the decision to invoke HIP
   must be done on the basis of host policy.  For example, if an IPsec-
   like implementation of HIP is being used, a policy may be entered
   into the security policy database that mandates to use or try HIP
   based on a match on the source or destination IP address, or other
   factors.  The mapping of IP address to host identity may be
   implemented by modifying the host operating system or by wrapping the
   existings sockets API, such as in the TESLA approach [4].

   There are a number of ways that HIP could be used in such a scenario.

   Manual configuration:

      Pre-existing SAs may be available due to previous administrative


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      The system could send an I1 to the Responder with an empty value
      for Responder HIT.

   Using DNS:

      If the responder has host identities registered in the forward DNS
      zone and has a PTR record in the reverse zone, the initiating
      system could perform a reverse+forward lookup to learn the HIT
      associated with the address.  Alternatively, the HIT could be
      stored in some type of HIP name service such as a DHT, keyed by IP
      address.  Unless secured with DNSSEC, the use of the reverse DNS
      map is subject to well-known security limitations (an attacker may
      cause an incorrect IP address to FQDN binding to occur).

   These types of solutions have the benefit of better supporting
   applications that use IP addresses for long-lived application
   associations, callbacks, and referrals.  They have weaker security
   properties than the approaches outlined in Section 3.2 and
   Section 3.3, however, because the binding between host identity and
   address is weak and not visible to the application or user.  In fact,
   the semantics of the application's "connect(ip)" call may be
   interpreted as "connect me to the system reachable at IP address ip"
   but perhaps no stronger semantics than that.  HIP can be used in this
   case to provide perfect forward secrecy and authentication, but not
   to strongly authenticate the peer at the onset of communications.
   DNS with DNSSEC, if trusted, may be able to provide some additional
   initial authentication, but at a cost of initial resolution latency.

   Using IP addresses at the application layer may not provide the full
   potential benefits of HIP mobility support.  It allows for mobility
   if one is able to readdress the existing sockets upon a HIP readdress
   event.  However, mobility will break in the connectionless case when
   an application caches the IP address and repeatedly calls sendto().

3.2.  Using DNS

   In the previous section, it was pointed out that a HIP-enabled system
   might make use of DNS to transparently fetch host identifiers prior
   to the onset of communication.  For applications that make use of
   DNS, the name resolution process is another opportunity to use HIP.
   If host identities are bound to domain names (with a trusted DNS) the
   following are possible:

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   Return HIP LSIs and HITs instead of IP addresses:

      The system resolver could be configured to return a Local Scope
      Identifier (LSI) or Host Identity Tag (HIT) rather than an IP
      address, if HIP information is available in the DNS that binds a
      particular domain name to a host identity, and otherwise to return
      an IP address as usual.  The system can then maintain a mapping
      between LSI and host identity and perform the appropriate
      conversion at the system call interface or below.  The application
      uses the LSI or HIT as it would an IP address.

   Locally use a HIP-specific domain name suffix:

      One drawback to spoofing the DNS resolution is that some
      applications actually may want to fetch IP addresses (e.g.,
      diagnostic applications such as ping).  One way to provide finer
      granularity on whether the resolver returns an IP address or an
      LSI is to distinguish by the presence of a domain name suffix.
      Specifically, if the application requests to resolve
      "" (or some similar suffix), then the system
      returns an LSI, while if the application requests to resolve
      "", IP address(es) are returned as usual.  Caution
      against the use of FQDN suffixes is discussed in [5].

   Since the LSI or HIT is non-routable, a couple of potential hazards
   arise, in the case of referrals, callbacks, and long-lived
   application associations.  First, applications that perform referrals
   may pass the LSI to another system that has no system context to
   resolve the LSI back to a host identity or an IP address.  Note that
   these are the same type of applications that will likely break if
   used over certain types of NATs.  Second, applications may cache the
   results of DNS queries for a long time, and it may be hard for a HIP
   system to determine when to perform garbage collection on the LSI
   bindings.  However, when using HITs, the security of using the HITs
   for identity comparison may be stronger than in the case of using IP

   It may be possible for an LSI or HIT to be routable or resolvable,
   but such a case may not have the level of security in the binding to
   host identity that a HIT has with the host identity.  For example, a
   special IP address that has some location invariance is the
   identifier-address discussed in [6].  In general, LSIs and HITs
   considered to date for HIP have been non-routable.

3.3.  Connecting directly to a HIT

   The previous two sections describe the use of IP addresses and and

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   LSIs as local handles to a host identity.  A third approach, for IPv6
   applications, is to configure the application to connect directly to
   a HIT (e.g., "connect(HIT)" as a socket call).  Although more
   cumbersome for human users (due to the flat HIT name space) than
   using either IPv6 addresses or domain names, this scenario has
   stronger security semantics, because the application is asking the
   system to connect specifically to the named peer system.

   Depending on how HITs are ultimately defined, it may be hard for a
   system to distinguish between a HIT and a routable IPv6 address.
   Elsewhere it has been proposed [7] that HITs be precluded from using
   highest-ordered bits that correspond to IPv6 addresses, so that at
   least in the near term, a system could differentiate between a HIT
   and an IPv6 address by inspection.

   Another challenge with this approach is in actually finding the IP
   addresses to use, based on the HIT.  Some type of HIT resolution
   service would be needed in this case.

   A third challenge of this approach is in supporting callbacks and
   referrals to possibly non-HIP-aware hosts.  However, since most
   communications in this case would likely be to other HIP-aware hosts
   (else the initial connect() would fail), the problem may be instead
   if the peer host supports HIP but is not able to perform HIT
   resolution for some reason.

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4.  Security Considerations

   In this section we discuss the security of the system in general
   terms, outlining some of the security properties.  However, this
   section is not intended to provide a complete risk analysis.  Such an
   analysis would, in any case, be dependent on the actual application
   using HIP, and is therefore considered out of scope.

   The three outlined scenarios differ considerably in their security
   properties.  There are further differences related to whether DNSSEC
   is used or not, and whether the DNSSEC zones are considered
   trustworthy enough from an application point of view.

   When IP addresses are used to represent the peer system, the security
   properties depend on the the configuration method.  With manual
   configuration, the system's security is comparable to a non-HIP
   system with similar IPsec policies.  The security semantics of an
   opportunistic key exchange are roughly equal to current non-secured
   IP; the exchange is vulnerable to man-in-the-middle attacks.
   However, the system is less vulnerable to connection hijacking
   attacks.  If the DNS is used, if both maps are secured (or the HITs
   stored in the reverse MAP) and the client trusts the DNSSEC
   signatures, the system may provide a fairly high security level.
   However, much depends on the details of the implementation, the
   security and administrative practises used when signing the DNS
   zones, and other factors.

   Using the forward DNS to map a DNS name into an LSI is a case that is
   closest to the most typical use scenarios today.  If DNSSEC is used,
   the result is fairly similar to the current use of certificates with
   TLS.  If DNSSEC is not used, the result is fairly similar to the
   current use of plain IP, with the exception that HIP provides
   protection against connection hijacking attacks.

   If the application is basing its operations on HITs, the connections
   become automatically secured due to the implicit channel bindings in
   HIP.  That is, when the application makes a connect(HIT) system call,
   the resulting connection will either be connected to a node
   possessing the corresponding private key or the connection attempt
   will fail.

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5.  Acknowledgments

   Jeff Ahrenholz, Miika Komu, Teemu Koponen, and Jukka Ylitalo have
   provided comments on different versions of this draft.

6.  References

   [1]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-04
        (work in progress), October 2005.

   [2]  Linn, J., "Generic Security Service Application Program
        Interface Version 2, Update 1", RFC 2743, January 2000.

   [3]  Nordmark, E., "Shim6 Application Referral Issues",
        draft-ietf-shim6-app-refer-00 (work in progress), July 2005.

   [4]  Salz, J., Balakrishnan, H., and A. Snoeren, "TESLA:  A
        Transparent, Extensible Session-Layer Architecture for End-to-
        end Network Services",  Proceedings of USENIX Symposium on
        Internet Technologies and Systems (USITS), December 2003.

   [5]  Faltstrom, P., "Design Choices When Expanding DNS",
        draft-iab-dns-choices-03 (work in progress), February 2006.

   [6]  Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
        protocol", draft-ietf-shim6-proto-03 (work in progress),
        December 2005.

   [7]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix for
        Overlay Routable Keyed Hash Identifiers (KHI)",
        draft-laganier-ipv6-khi-01 (work in progress), February 2006.

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Authors' Addresses

   Tom Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA


   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420

   Phone: +358 9 299 1

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