Network Working Group D. Senie
Request for Comments: 3235 Amaranth Networks Inc.
Category: Informational January 2002
Network Address Translator (NAT)-Friendly
Application Design Guidelines
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
This document discusses those things that application designers might
wish to consider when designing new protocols. While many common
Internet applications will operate cleanly in the presence of Network
Address Translators, others suffer from a variety of problems when
crossing these devices. Guidelines are presented herein to help
ensure new protocols and applications will, to the extent possible,
be compatible with NAT (Network Address Translation).
1. Introduction
Other documents that describe Network Address Translation (NAT)
discuss the Terminology and Considerations [RFC2663] and Protocol
Issues [RFC3022], [RFC3027] or discuss the implications of NAT
[RFC2993]. All of those relate to various issues with the NAT
mechanism, effects on protocols and effects upon general Internet
architecture.
It is the focus of this document to provide recommendations to
authors of new protocols about the effects to consider when designing
new protocols such that special handling is not required at NAT
gateway points.
When a protocol is unable to pass cleanly through a NAT, the use of
an Application Level Gateway (ALG) may still permit operation of the
protocol. Depending on the encoding used in a protocol, an ALG may
be difficult or easy to construct, though in some cases it may not be
possible at all. While adjunct to NAT, the formulation of protocols
that cannot directly operate through NAT should be considered such
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that the ALG design may be simple and automated. ALGs typically
operate inside small routers along with the NAT component. Ideally,
the ALG should be simple and not require excessive computation or
state storage.
Many of the same issues in application design that create issues for
NAT (and thus can require ALG support) are also issues for firewalls.
An application designer would do well to keep this in mind, as any
protocol that does require special handling by NAT or firewall
products will be more difficult to deploy than those that require no
special handling.
2. Discussion
Network Address Translation presents a challenge to some existing
applications. In many cases, it should be possible for developers of
new applications to avoid problems if they understand the issues.
This document aims to provide the application designer with
information on what things they can do and what to avoid when trying
to build applications that are able to function across NAT.
The proliferation of NAT, especially in homes and small offices
cannot be dismissed. The marketing of these technologies to homes
and small businesses is often focused on a single-computer
environment, and thus providers only give out a single IP address to
each user. NAT has become a popular choice for connecting more than
a single system per location.
Clearly the most common problem associated with NAT implementations
is the passing of addressing data between stations. Where possible,
applications should find alternatives to such schemes. Studying a
few existing protocols will serve to highlight the different
approaches possible.
Two common forms of Traditional NAT exist. With Basic NAT, only the
IP addresses of packets are altered by the NAT implementation. Many
applications will operate correctly with Basic NAT. The other common
form is Network Address Port Translation. With NAPT, both the IP
addresses and the source and destination ports (for TCP and UDP) are
potentially altered by the gateway. As such, applications passing
only port number information will work with Basic NAT, but not with
NAPT.
Application designers should strive for compatibility with NAPT, as
this form of NAT is the most widely deployed. This is also the form
of NAT that will likely see the greatest penetration in homes and
small offices. Not all applications lend themselves to the
architectural model imposed by NAPT.
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3. Recommendations and Examples
Application designers who work within the constraints of NAT, and who
do not rely on the presence of ALGs will generally find the easier
acceptance in user communities where NAT is common. When designing a
new application or service, the requirement for an ALG will limit
deployment until the required additional code is incorporated into
the many devices which implement NAT.
Each of the areas called out below are examples of issues to consider
when building an application. This list is likely not comprehensive,
but does cover a number of important issues and considerations.
3.1 Issues and Recommendations affecting all types of Network Address
Translators
3.1.1. Peer-to-Peer Applications Limitations
Peer to peer applications are problematic in a NAT world. Client-
server applications are more generally workable. Peer-to-peer
applications rely on each peer being reachable as a server (i.e.,
bound to a listening port, and able to accept connections) for the
other to connect to. With NAPT, there are likely many machines
behind one address. With other types of NAT such as Basic NAT with
Static Address Assignment (providing one-to-one mappings), there is a
greater chance of making such applications work.
Some implementations of NAT can be made to function for UDP-based
peer-to-peer applications. This capability is dependent on the
methodology used to implement the UDP sessions in the NAT device. If
the NAT device tracks the tuple (private address, private port,
public port) then it is possible for an outbound UDP packet to
establish a channel by which incoming traffic can flow from a source
other than that originally contacted by the system. The source IP
address is NOT used in this case to match incoming packets to UDP
sessions, allowing any source address using the UDP port number to be
translated.
NAT devices which track source and destination IP addresses, in
addition to port numbers, will not permit third-party packets. NAT
is often implemented in conjunction along with stateful-inspection
firewall functionality. As such the latter implementation of UDP
association tracking would be considered more secure.
NAT/Firewall device implementations could be constructed to have a
software switch within them, permitting the consumer the ability to
select whether they want the greater security, or greater ability to
run peer-to-peer applications.
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3.1.2. Applications Requiring End-to-End IPSec Will Fail
Use of IPSec for end-to-end security will not function in the
presence of a NAT implementation. Application designers may want to
explore the use of Transport Layer Security (TLS) [RFC2246] as a
transport mode that will traverse NAT cleanly. See [RFC2709] for
additional discussion on combining NAT with Tunnel-mode IPSec
security on the same device.
3.1.3. Use DNS Names, Not IP Addresses In Payload
Applications should, where possible, use fully qualified domain names
rather than IP addresses when referring to IP endpoints. When
endpoints are across a NAT gateway, private addresses must not be
allowed to leak to the other endpoint. An example of where this can
happen today is with the HTTP and HTML protocols. It is possible for
web pages to be specified with numeric IP addresses, rather than with
names, for example http://192.168.1.10/index.html could be used as a
URL, but would likely create a problem if this address is on a server
located behind a NAT gateway. Users outside the gateway would not be
able to reach the address 192.168.1.10, and so would not see the
page.
Further exacerbating the problem is the possibility of duplicate
addresses between realms. If a server offers a link with a private
address space IP address embedded within it, such as 192.168.1.10,
the page referenced may resolve to a system on the local network the
browser is on, but would be a completely different server. The
resulting confusion to end-users would be significant. Sessions
involving multiple NAT implementations would be exceptionally
vulnerable to address reuse issues of this sort.
3.1.4. Multicast Considerations
Not all NAT devices implement multicast routing protocols.
Application designers should verify whether the devices in the
networks where their applications will be deployed are able to
process multicast traffic if their applications rely on that
capability.
3.1.5. Retention Of Address Mapping
With the exception of statically configured NAT bindings,
applications should not assume address mapping will be maintained
from one session (association between machines, for whatever protocol
for a period of time) to another. An example of this is RSVP, which
forms one connection to reserve the resources, then the actual
session for which resources were reserved is started. The sessions
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do not necessarily overlap. There is no guarantee that the NAT
implementation will keep the binding association. As such,
applications that rely on subsequent sessions being mapped to the
same host IP address may not function without an ALG.
Another consideration is the number of addressing realms. It is
entirely possible to have multiple levels of NAT implementations
between the two end points involved. As such, one must think about
the lifetime of such mappings at all such levels.
Load balancers and other devices may use a single IP address and port
to map to multiple actual end points. Many products implement
variations on this theme, sometimes using NAT, sometimes using other
technologies. The lack of guarantee of mapping is important to
understand, since the mapping to one actual system to another may not
survive across such intermediate boxes.
Don't assume systems know their own IP addresses. A system behind a
NAT may be reachable via a particular IP address, but that address
may not be recognized by the system itself. Consider the case of
Static, one-to-one mapping using Basic NAT. A server in this context
will have an IP address from the private realm, and may not know the
public address which maps to it. Similarly, some such systems may
not know their own DNS names, while others may. This is largely
dependent on the configuration of the servers and the network within
the private realm.
3.2 Recommendations for NAPT
As many of the issues specifically address NAPT issues, this section
will group these issues. NAPT is the most common form of NAT in
actual deployment in routers, especially in smaller offices and home
offices.
3.2.1 IP Addresses Specific To A Realm
Avoid the use of IP address and port number information within the
payload of packets. While in some cases ALGs will permit such
protocols to function, this presupposes every NAT device can be
updated in a timely fashion to support a new protocol. Since this is
unlikely, application writers are urged to avoid placing addressing
information in payloads all together.
In addition to avoiding addresses and port numbers within packet
payloads, it is important to avoid assumptions of (address, port)
tuples are unique beyond the scope of the present session. Load
balancing devices implementing NAT may, for example, map subsequent
sessions to other systems in the private realm.
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3.2.2 Avoid Session Bundles
Independent sessions, such as used by POP or SMTP, are preferred to
protocols that attempt to manage a bundle of related sessions, such
as FTP. The term "session" here is used to refer to any association
between end systems, and may be using any transport protocol or
combination of protocols (UDP, TCP, SCTP).
In the FTP protocol, port information is passed over one TCP
connection and is used to construct a second TCP connection for
passing the actual data. Use of a separate connection to transfer
the file data makes determination of file end quite simple, however
other schemes could be envisioned which could use a single
connection.
The HTTP protocol, for example, uses a header and content length
approach to passing data. In this model, all data is transferred
over the single TCP connection, with the header portion indicating
the length of the data to follow. HTTP has evolved to allow multiple
objects to be passed on a single connection (thereby cutting the
connection establishment overhead). Clearly a new file transfer
function could be built that would perform most of the functions of
FTP without the need for additional TCP connections.
The goal is to keep to single connections where possible. This keeps
us from needing to pass addressing information of any sort across the
network. However, multiplexing traffic over a single connection can
create problems as well.
3.2.3. Session Bundles Originate From Same End
Origination of connections is an important consideration. Where
possible, the client should originate all connections. The FTP
protocol is the most obvious example, where by default the server
opens the data connection to a port on the client (the client having
specified the port number via a PORT command over the control TCP
session).
As pointed out in [RFC1579], the use of the passive open option in
FTP (PASV) remedies this situation as the client is responsible for
opening the connection in this case. With client-opened connections,
the standard functions of NAPT will process the request as it would
any other simple TCP connection, and so an ALG is not required.
In cases where session bundles are unavoidable, each session in the
bundle should originate from the same end station.
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3.2.4. Choice of Transport Protocol
NAPT gateways must track which sessions are alive, and flush old
sessions. TCP has clear advantages in this area, since there are
specific beginning and end of session indicators in the packets (SYN
and FIN packets). While UDP works for some types of applications
with NAT, there can be issues when that data is infrequent. Since
there is no clean way to know when an end station has finished using
a UDP session, NAT implementations use timeouts to guess when a UDP
session completes. If an application doesn't send data for a long
period of time, the NAT translation may time out.
NAT implementations also use timers to guess when TCP sessions have
disappeared. While TCP sessions should disappear only after FIN
packets are exchanged, it is possible that such packets may never
come, for example if both end stations die. As such, the NAT
implementation must use a timer for cleaning up its resources.
NAT implementers in many cases provide several timeouts, one for live
TCP sessions, one for TCP sessions on which a FIN has been seen, and
one for UDP sessions. It is best when such flexibility is provided,
but some implementations appear to apply a single timer to all
traffic.
Protocols other than TCP and UDP can work with Traditional NAT in
many cases, provided they are not carrying addressing information.
For NAPT implementations use of any protocols other than TCP and UDP
will be problematic unless or until such protocols are programmed
into the implementations.
It's important to note that NAPT deployments are based on the
assumption of a client-server application model, with the clients in
the private realm.
3.2.5. IP Fragmentation
Applications should attempt to avoid fragmentation when packets pass
over NAPT devices. While not always practical or possible, there are
failures that can occur with NAPT. Specifically, if two stations in
the private realm pick matching fragmentation identifiers, and talk
to the same remote host, it may be impossible to determine which
fragments belong to which session. A clever NAPT implementation
could track fragmentation identifiers and map those into a unique
space, though it is not clear how many do so.
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Ideally, applications should limit packet size, use Path MTU
Discovery or both. Unfortunately, at least some firewall/NAT devices
block Path MTU Discovery, apparently believing all ICMP packets are
evil.
Some implementations of NAT may implement fragment reassembly prior
to Forwarding, however many do not. Application designers are
advised to design assuming the devices do not reassemble fragments.
3.3 Issues and recommendations for Basic NAT
If only Basic NAT implementations are involved, not NAPT, then many
of the issues above do not apply. This is not to say that this form
of NAT is better or worse than NAPT. Application designers may think
they could just specify users must use Basic NAT, and many
application issues would go away. This is unrealistic, however, as
many users have no real alternative to NAPT due to the way their
providers sell service.
Many of the issues raised earlier still apply to Basic NAT, and many
protocols will not function correctly without assistance.
3.3.1. Use IP and TCP/UDP Headers Alone
Applications that use only the information in the IP and TCP or UDP
headers for communication (in other words, do not pass any additional
addressing information in the payload of the packets), are clearly
easier to support in a NAT environment. Where possible, applications
designers should try to limit themselves in this area.
This comes back to the same recommendation made for NAPT, that being
to use a single connection whenever possible.
The X windowing system, for example, uses fixed port numbers to
address X servers. With X, the server (display) is addressed via
ports 6000 through 6000 + n. These map to hostname:0 through
hostname:n server displays. Since only the address and port are
used, the NAT administrator could map these ports to one or more
private addresses, yielding a functioning solution.
The X example, in the case of NAPT, requires configuration of the NAT
implementation. This results in the ability for no more than one
station inside the NAT gateway to use such a protocol. This approach
to the problem is thus OK for NAT but not recommended for NAPT
environments.
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3.3.2. Avoid Addressing In Payload
As with NAPT, transporting IP address and/or port number information
in the payload is likely to cause trouble. As stated earlier, load
balancers and similar platforms may well map the same IP address and
port number to a completely different system. Thus it is problematic
to assume an address or port number which is valid in the realm on
one side of a NAT is valid on the other side.
3.4 Bi-directional NAT
Bi-directional NAT makes use of DNS mapping of names to point
sessions originating outside the private realm to servers in the
private realm. Through use of a DNS-ALG [RFC2694], lookups are
performed to find the proper host and packets are sent to that host.
Requirements for applications are the same as for Basic NAT.
Addresses are mapped one-to-one to servers. Unlike Traditional NAT
devices, Bi-directional NAT devices (in conjunction with DNS-ALG) are
amenable to peer-to-peer applications.
3.5 Twice NAT
Twice NAT is address translation where both source and destination IP
addresses are modified due to addressing conflicts between two
private realms. Two bi-directional NAT boxes connected together
would essentially perform the same task, though a common address
space that is not otherwise used by either private realm would be
required.
Requirements for applications to work in the Twice NAT environment
are the same as for Basic NAT. Addresses are mapped one to one.
3.6 Multi-homed NAT
Multi-homed NAT is the use of multiple NAT implementations to provide
redundancy. The multiple implementations share configuration
information so that sessions might continue in the event of a fail-
over. Unless the multiple implementations share the same external
addresses, sessions will have to restart regardless.
Requirements for multi-homed NAT are the same as for Basic NAT or
NAPT, depending on how the multi-homed NAT is implemented and
configured.
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3.7 Realm Specific IP (RSIP)
Realm Specific IP is described in [RFC2663] and defined in [RSIP] and
related documents. Clients within a private realm using RSIP are
aware of the delineation between private and public, and access a
server to allocate address (and optionally port) information for use
in conversing with hosts in the public realm. By doing this, clients
create packets that need not be altered by the RSIP server on their
way to the remote host. This technique can permit IPSec to function,
and potentially makes any application function as if there were no
special processing involved at all.
RSIP uses a view of the world in which there are only two realms, the
private and public. This isn't always the case. Situations with
multiple levels of NAT implementations are growing. For example,
some ISPs are handing out [RFC1918] addresses to their dialup users,
rather than obtaining real addresses. Any user of such an ISP who
also uses a NAT implementation will see two levels of NAT, and the
advantages of RSIP will have been wasted.
3.8 Performance Implications of Address Translation Implementations
Resource utilization on the NAT gateway should be considered. An
application that opens and closes many TCP connections, for example,
will use up more resources on the NAT router than an application
performing all transfers over a single TCP connection. HTTP 1.0
opened a connection for each object on a web page, whereas HTTP 1.1
permits the TCP session to be held open for additional objects that
may need to be transferred. Clearly the latter imposes a lower
overhead on the NAT gateway, as it is only maintaining state on a
single connection instead of multiple connections.
New session establishment will typically remain a software function
even in implementations where the packet-by-packet translation work
is handled by hardware forwarding engines. While high-performance
NAT boxes may be built, protocols that open many sessions instead of
multiplexing will be slower than those that do not.
Applications with different types of data, such as interactive
conferencing, require separate streams for the different types of
data. In such cases the protocol needs of each stream must be
optimized. While the goal of multiplexing over a single session is
preferred, clearly there are cases where this is impractical.
The latency of NAT translation overhead is implementation dependent.
On a per-packet basis, for established sessions only the source or
destination IP address is replaced, the source or destination port
(for NAPT) and the checksums for IP, and TCP or UDP are recalculated.
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The functionality can be efficiently implemented in hardware or
software.
4. Security Considerations
Network Address Translators have implications for IPSec, as noted
above. When application developers are considering whether their
applications function with NAT implementations, care should be given
to selection of security methodology. Transport Layer Security (TLS)
[RFC2246] operates across translation boundaries. End-to-end IPSec
will prove problematic in many cases.
5. References
[RFC1579] Bellovin, S., "Firewall Friendly FTP", RFC 1579, February
1994.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator (NAT)", RFC 3027,
January 2001.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RFC2709] Srisuresh, P., "Security Model with Tunnel-mode IPsec for
NAT Domains", RFC 2709, October 1999.
[RFC3102] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102, October 2001.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A.
Heffernan, "DNS extensions to Network Address Translators
(DNS_ALG)", RFC 2694, September 1999.
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6. Acknowledgements
I'd like to thank Pyda Srisuresh for his invaluable input and
feedback, and Keith Moore for his extensive comments.
7. Author's Address
Daniel Senie
Amaranth Networks Inc.
324 Still River Road
Bolton, MA 01740
Phone: (978) 779-6813
EMail: dts@senie.com
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8. Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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