Network Working Group L. Daigle, Ed.
Request for Comments: 3424 Internet Architecture Board
Category: Informational IAB
November 2002
IAB Considerations for UNilateral Self-Address Fixing (UNSAF)
Across Network Address Translation
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
As a result of the nature of Network Address Translation (NAT)
Middleboxes, communicating endpoints that are separated by one or
more NATs do not know how to refer to themselves using addresses that
are valid in the addressing realms of their (current and future)
peers. Various proposals have been made for "UNilateral Self-Address
Fixing (UNSAF)" processes. These are processes whereby some
originating endpoint attempts to determine or fix the address (and
port) by which it is known to another endpoint - e.g. to be able to
use address data in the protocol exchange, or to advertise a public
address from which it will receive connections.
This document outlines the reasons for which these proposals can be
considered at best as short term fixes to specific problems and the
specific issues to be carefully evaluated before creating an UNSAF
proposal.
1. Introduction
As a result of the nature of Network Address (and port) Translation
(NAT) Middleboxes, communicating endpoints that are separated by one
or more NATs do not know how to refer to themselves using addresses
that are valid in the addressing realms of their (current and future)
peers - the address translation is locked within the NAT box. For
some purposes, endpoints need to know the addresses (and/or ports) by
which they are known to their peers. There are two cases: 1) when
the client initiates communication, starting the communication has
the side effect of creating an address binding in the NAT device and
allocating an address in the realm that is external to the NAT box;
and 2) a server will be accepting connections from outside, but
because it does not initiate communication, no NAT binding is
created. In such cases, a mechanism is needed to fix such a binding
before communication can take place.
"UNilateral Self-Address Fixing (UNSAF)" is a process whereby some
originating process attempts to determine or fix the address (and
port) by which it is known - e.g. to be able to use address data in
the protocol exchange, or to advertise a public address from which it
will receive connections.
There are only heuristics and workarounds to attempt to achieve this
effect; there is no 100% solution. Since NATs may also dynamically
reclaim or readjust translations, "keep-alive" and periodic re-
polling may be required. Use of these workarounds MUST be considered
transitional in IETF protocols, and a better architectural solution
is being sought. The explicit intention is to deprecate any such
workarounds when sound technical approaches are available.
2. Architectural issues affecting UNSAF Systems
Generally speaking, the proposed workarounds are for cases where a
standard protocol communication is to take place between two
endpoints, but in order for this to occur, a separate step of
determining (or fixing) the perceived address of an endpoint in the
other endpoint's addressing realm is required. Proposals require
that an endpoint seeking to "fix" its address contact a participating
service (in a different address realm) to determine (reflect) its
address. Thus, there is an "UNSAF client" partnering with some form
of "UNSAF service" that may or may not be associated with the target
endpoint of the actual desired communication session. Throughout
this memo, the terms "UNSAF server" and "UNSAF service" should be
understood to generically refer to whatever process is participating
in the UNSAF address determination for the originating process (the
UNSAF client).
Any users of these workarounds should be aware that specific
technical issues that impede the creation of a general solution
include:
o there *is* no unique "outside" to a NAT - it may be impossible to
tell where the target endpoint is with respect to the initiator;
how does an UNSAF client find an appropriate UNSAF server to
reflect its address? (See Appendix C).
o specifically because it is impossible to tell where address realms
are bounded ("inside" or "outside", "private" or "public", or
several "private" realms routing traffic), an address can only be
determined relative to one specific point in the network. If the
UNSAF service that reflected an UNSAF client's address is in a
different NAT-masqueraded subnet from some other service X that
the client wishes to use, there is _no_ guarantee that the
client's "perceived" address from the UNSAF partner would be the
same as the address viewed from the perspective of X. (See
Appendix C).
o absent "middlebox communication (midcom)", there is no usable way
to let incoming communications make their way through a middlebox
(NAT, firewall) under proper supervision. By circumventing the
NAT, UNSAF mechanisms may also (inadvertently) circumvent security
mechanisms. The particular danger is that internal machines are
unwittingly exposed to all the malicious communications from the
external side that the firewall is intended to block. This is
particularly unacceptable if the UNSAF process is running on one
machine which is acting on behalf of several.
o proposed workarounds include the use of "ping"-like address
discovery requests sent from the UNSAF client (initiator) to the
UNSAF server (listener), to which the listener responds with the
transport address of the initiator - in the address realm of the
listener. However, with connection-less transports, e.g. UDP,
IPsec ESP, etc., an UNSAF process must take care to react to
changes in NAT bindings for a given application flow, since it may
change unpredictably.
o if the UNSAF client uses periodic retries to refresh/reevaluate
the address translation state, both the UNSAF client and the UNSAF
server are required to maintain information about the presumed
state of the communication in order to manage the address
illusion.
o since the UNSAF server is not integrated with the middlebox, it
can only operate on the assumption that past behavior is a
predictor of future behavior. It has no special knowledge of the
address translation heuristic or affecting factors.
o the communication exchange is made more "brittle" by the
introduction of other servers (UNSAF servers) that need to be
reachable in order for the communication to succeed - more boxes
that are "fate sharing" in the communication.
Workarounds may mitigate some of these problems through tight scoping
of applicability and specific fixes. For example:
o rather than finding the address from "the" outside of the NAT, the
applicability of the approach may be limited to finding the
"self-address" from a specific service, for use exclusively with
that service.
o limiting the scope to outbound requests for service (or service
initiation) in order to prevent unacceptable security exposures.
3. Practical Issues
From observations of deployed networks, it is clear that different
NAT box implementations vary widely in terms of how they handle
different traffic and addressing cases.
Some of the specific types of observed behaviors have included:
o NATs may drop fragments in either direction: without complete
TCP/UDP headers, the NAT may not make the address translation
mapping, simply dropping the packet.
o Shipping NATs often contain Application Layer Gateways (ALGs)
which attempt to be context-sensitive, depending on the source or
destination port number. The behavior of the ALGs can be hard to
anticipate and these behaviors have not always been documented.
o Most NAT implementations with ALGs that attempt to translate TCP
application protocols do not perform their functions correctly
when the substrings they must translate span across multiple TCP
segments; some of them are also known to fail on flows that use
TCP option headers, e.g. timestamps.
o NAT implementations differ markedly in their handling of packets.
Quite a few only really work reliably with TCP packets, not UDP.
Of the ones that do make any attempt to handle UDP packets, the
timers aging out flows can vary widely making it challenging to
predict behavior.
o Variation in address and port assignments can be quite frequent -
on NATs, port numbers always change, and change unpredictably;
there may be multiple NATs in parallel for load-sharing, making IP
address variations quite likely as well.
4. Architectural Considerations
By distinguishing these approaches as short term fixes, the IAB
believes the following considerations must be explicitly addressed in
any proposal:
1. Precise definition of a specific, limited-scope problem that is
to be solved with the UNSAF proposal. A short term fix should
not be generalized to solve other problems. Such generalizations
lead to the the prolonged dependence on and usage of the supposed
short term fix -- meaning that it is no longer accurate to call
it "short term".
2. Description of an exit strategy/transition plan. The better
short term fixes are the ones that will naturally see less and
less use as the appropriate technology is deployed.
3. Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
4. Identify requirements for longer term, sound technical solutions;
contribute to the process of finding the right longer term
solution.
5. Discussion of the impact of the noted practical issues with
existing deployed NATs and experience reports.
5. Security Considerations
As a general class of workarounds, UNSAF proposals may introduce
security holes because, in the absence of "middlebox communication
(midcom)", there is no feasible way to let incoming communications
make their way through a firewall under proper supervision:
respecting the firewall policies as opposed to circumventing security
mechanisms.
Appendix A. IAB Members at the time of this writing:
Harald Alvestrand
Ran Atkinson
Rob Austein
Fred Baker
Leslie Daigle
Steve Deering
Sally Floyd
Ted Hardie
Geoff Huston
Charlie Kaufman
James Kempf
Eric Rescorla
Mike St. Johns
Appendix B. Acknowledgements
This document has benefited greatly from detailed comments and
suggestions from Thomas Narten, Bernard Aboba, Keith Moore, and James
Woodyatt.
This document was originally drafted when the following people were
part of the IAB: Steve Bellovin, Brian Carpenter, Jon Crowcroft, John
Klensin and Henning Schulzrinne; it has benefited considerably from
their contributions and review.
Appendix C. Example NAT Configuration Scenario
C.1 Generic NATed Network Configuration
Here is one sample scenario wherein it is difficult to describe a
single "outside" to a given address realm (bridged by NAPTs). This
sort of configuration might arise in an enterprise environment where
different divisions have their own subnets (each using the same
private address space); the divisions are connected so that they can
pass traffic on each others' networks, but to access the global
Internet, each uses a different NAPT/firewall:
+---------+
| Box C | (192.168.4.5)
+---+-----+
|
---------------------------------+-------
|
| 192.168.3.0/24
+----+----+
| NAT 2 |
+----+----+
| 10.1.0.0/32
|
-----+-------------------------+------------+----
| |
| +----+----+
| | Box B | (10.1.1.100)
| +---------+
|
+----+----+
| NAPT 1 | (10.1.2.27)
+----+----+
| 10.1.0.0/32
|
----+-----+--
|
|
+----+----+
| Box A | (10.1.1.100)
+---------+
From the perspective of Box B, Box A's address is (some port on)
10.1.2.27. From the perspective of Box C, however, Box A's address
is some address in the space 192.168.3.0/24.
C.2 Real World Home Network Example
James Woodyatt provided the following scenario, based on current
examples of home networking products:
o the customer has existing Internet service from some broadband
service provider, using e.g. a DSL line connected to an appliance
that integrates a DSL modem with a NAT router/firewall.
o these devices are sometimes packaged with automated provisioning
firmware, so the customer may view them as part of what their ISP
provides them.
o later, the customer wants to use a host with only a wireless LAN
interface, so they install a wireless access point that ships in
its default configuration with NAT and a DHCP server enabled.
o after this, the customer has a wired LAN in one private address
realm and a wireless LAN in another private address realm.
Furthermore, most customers probably have no idea what the phrase
"address realm" means and shouldn't have to learn it. All they often
know is that the printer server is inaccessible to the wireless
laptop computer. (Why? Because the discovery protocol uses UDP
multicast with TTL=1, but that's okay because any response would just
be dropped by the NAT anyway, because there's no ALG.)
Authors' Addresses
Leslie Daigle
Editor
Internet Architecture Board
IAB
EMail: iab@iab.org
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