Network Working Group T. Henderson
Internet-Draft The Boeing Company
Intended status: Informational P. Nikander
Expires: October 11, 2007 Ericsson Research NomadicLab
April 9, 2007
Using the Host Identity Protocol with Legacy Applications
draft-ietf-hip-applications-01
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Abstract
The Host Identity Protocol (HIP) and architecture proposes to add a
cryptographic name space for network stack names. From an
application viewpoint, HIP-enabled systems support a new address
family of host identifiers, 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
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 to map domain names to HIs . . . . . . . . . . . 6
3.3. Connecting directly to a HIT . . . . . . . . . . . . . . . 8
3.4. Local address management . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
7. Informative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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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 send packets to another named host by expressing a
location-independent name of the host when the system call to send
packets 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 Application
Programming Interface (API), the application can issue a system call
to send packets 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 for hosts 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
identifier (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 level, 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
application.
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2. Terminology
Host Identity: An abstract concept applied to a computing platform.
Host Identifier (HI): A public key of an asymmetric key pair used as
a name for a Host Identity. More details are available in [1].
Host Identity Tag (HIT): A 128-bit quantity composed with the hash
of a Host Identity. More details are available in [3] and [1].
Local Scope Identifier (LSI): 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
command.
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. Finally, some local address management issues are
discussed.
While the text below concentrates on the use of the sockets connect
system call, the same argument is also valid for other system calls
using socket addresses.
Recent work in the shim6 group has categorized the ways in which
current applications use IP addresses [4]. 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
applications.
3.1. Using IP addresses in applications
Consider the case in which an application issues a "connect(ip)"
system call to set the default destination 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
identifier may be implemented by modifying the host operating system
or by wrapping the existings sockets API, such as in the TESLA
approach [5].
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
action.
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Opportunistically:
The system could send an I1 to the Responder with an empty value
for Responder HIT.
Using DNS to map IP addresses to HIs:
If the responder has host identifiers 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 distributed hash
table (DHT), keyed by IP address. Unless secured with DNS
security extensions, the use of the reverse DNS map is subject to
well-known security limitations (an attacker may cause an
incorrect IP address to domain name 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 identifier 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 security extensions (DNSSEC) [6], if trusted, may be able to
provide some additional initial authentication, but at a cost of
initial resolution latency. Note that this usage does not
necessarily reveal to the user of the legacy application that HIP is
being used.
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 to map domain names to HIs
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 identifiers are bound to domain names (with a trusted DNS)
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the following are possible:
Return HIP LSIs and HITs instead of IP addresses:
The system resolver could be configured to return a Local Scope
Identifier (LSI) or HIT rather than an IP address, if HIP
information is available in the DNS that binds a particular domain
name to a host identifier, and otherwise to return an IP address
as usual. The system can then maintain a mapping between LSI and
host identifier 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
"www.example.com.hip" (or some similar suffix), then the system
returns an LSI, while if the application requests to resolve
"www.example.com", IP address(es) are returned as usual. Caution
against the use of domain name suffixes is discussed in [7].
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 identifier or an IP address. Note
that these are the same type of applications that will likely break
if used over certain types of network address translators (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 addresses.
It may be possible for an LSI or HIT to be routable or resolvable,
either directly or on an overlay. For example, a special IP address
that has some location invariance is the identifier-address discussed
in [8]. A term other than LSI may be needed for these routable
identifiers, since they would no longer be locally scoped. When
using DNS, returning a routable identifier would avoid the
aforementioned problems with referrals. However, the cost of
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routability may be that the hash binding between the routable
identifier and the host identifier would be weakened, since more bits
may be allocated to the hierarchical part.
3.3. Connecting directly to a HIT
The previous two sections describe the use of IP addresses and LSIs
as local handles to a host identifier. 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 send packets specifically to the named peer system. HITs
have been defined as Overlay Routable Cryptographic Hash Identifiers
(ORCHIDs) such that they cannot be confused with routable IP
addresses; see [3].
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 HIP associations would fail to establish), the
problem may otherwise be that the peer host supports HIP but is not
able to perform HIT resolution for some reason.
3.4. Local address management
The previous sections focused mainly on client behavior (HIP
initiator). We must also consider the behavior for servers.
Typically, a server may bind to a wildcard IP address and well-known
port. In the case of HIP use with legacy server implementations,
there are again a few options. As in Section 3.1 above, the system
may be configured manually to always, optionally (depending on the
client behavior), or never use HIP with a particular service, as a
matter of policy, when the server specifies a wildcard (IP) address.
When a system API call such as getaddrinfo [9] is used for resolving
local addresses, it may also return HITs or LSIs, if the system has
assigned HITs or LSIs to internal virtual interfaces (common in many
HIP implementations). The application may use such identifiers as
addresses in subsequent socket calls.
Some applications may try to bind a socket to a specific local
address. If the local address specified is an IP address, again, the
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underlying system may be configured to still use HIP. If the local
address specified is a HIT (Section 3.3), the system should enforce
that connections can only come to the specified HIT. If a system has
many HITs, an application that binds to a single HIT cannot accept
connections to the other HITs in the system. It may be possible for
a system to specify a special ORCHID value as a local HIT wildcard
value, if such wildcarding among local HIs is desired.
When a host has multiple HIs and the socket behavior does not
prescribe the use of any particular HI as a source identifier, it is
a matter of local policy as to how to select a HI to serve as a
source identifier.
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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 security of the system 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 DNS record) 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 domain 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 packets will either be sent to a node possessing the
corresponding private key or the security association will fail to be
established.
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5. IANA Considerations
This document has no actions for IANA.
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6. Acknowledgments
Jeff Ahrenholz, Gonzalo Camarillo, Alberto Garcia, Miika Komu, Teemu
Koponen, Julien Laganier, and Jukka Ylitalo have provided comments on
different versions of this draft.
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7. Informative References
[1] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-07
(work in progress), February 2007.
[2] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[3] Nikander, P., "An IPv6 Prefix for Overlay Routable Cryptographic
Hash Identifiers (ORCHID)", draft-laganier-ipv6-khi-07 (work in
progress), February 2007.
[4] Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress), July 2005.
[5] 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), pages 211-224,
March 2003.
[6] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[7] Faltstrom, P., "Design Choices When Expanding DNS",
draft-iab-dns-choices-04 (work in progress), October 2006.
[8] Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-07 (work in progress),
December 2006.
[9] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493,
February 2003.
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Authors' Addresses
Tom Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: thomas.r.henderson@boeing.com
Pekka Nikander
Ericsson Research NomadicLab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
Email: pekka.nikander@nomadiclab.com
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