Network Working Group P. Srisuresh
Request for Comments: 3022 Jasmine Networks
Obsoletes: 1631 K. Egevang
Category: Informational Intel Corporation
January 2001
Traditional IP Network Address Translator (Traditional NAT)
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 (2001). All Rights Reserved.
Preface
The NAT operation described in this document extends address
translation introduced in RFC 1631 and includes a new type of network
address and TCP/UDP port translation. In addition, this document
corrects the Checksum adjustment algorithm published in RFC 1631 and
attempts to discuss NAT operation and limitations in detail.
Abstract
Basic Network Address Translation or Basic NAT is a method by which
IP addresses are mapped from one group to another, transparent to end
users. Network Address Port Translation, or NAPT is a method by
which many network addresses and their TCP/UDP (Transmission Control
Protocol/User Datagram Protocol) ports are translated into a single
network address and its TCP/UDP ports. Together, these two
operations, referred to as traditional NAT, provide a mechanism to
connect a realm with private addresses to an external realm with
globally unique registered addresses.
The need for IP Address translation arises when a network's internal
IP addresses cannot be used outside the network either for privacy
reasons or because they are invalid for use outside the network.
Network topology outside a local domain can change in many ways.
Customers may change providers, company backbones may be reorganized,
or providers may merge or split. Whenever external topology changes
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with time, address assignment for nodes within the local domain must
also change to reflect the external changes. Changes of this type
can be hidden from users within the domain by centralizing changes to
a single address translation router.
Basic Address translation would (in many cases, except as noted in
[NAT-TERM] and section 6 of this document) allow hosts in a private
network to transparently access the external network and enable
access to selective local hosts from the outside. Organizations with
a network setup predominantly for internal use, with a need for
occasional external access are good candidates for this scheme.
Many Small Office, Home Office (SOHO) users and telecommuting
employees have multiple Network nodes in their office, running
TCP/UDP applications, but have a single IP address assigned to their
remote access router by their service provider to access remote
networks. This ever increasing community of remote access users
would be benefited by NAPT, which would permit multiple nodes in a
local network to simultaneously access remote networks using the
single IP address assigned to their router.
There are limitations to using the translation method. It is
mandatory that all requests and responses pertaining to a session be
routed via the same NAT router. One way to ascertain this would be
to have NAT based on a border router that is unique to a stub domain,
where all IP packets are either originated from the domain or
destined to the domain. There are other ways to ensure this with
multiple NAT devices. For example, a private domain could have two
distinct exit points to different providers and the session flow from
the hosts in a private network could traverse through whichever NAT
device has the best metric for an external host. When one of the NAT
routers fail, the other could route traffic for all the connections.
There is however a caveat with this approach, in that, rerouted flows
could fail at the time of switchover to the new NAT router. A way to
overcome this potential problem is that the routers share the same
NAT configuration and exchange state information to ensure a fail-
safe backup for each other.
Address translation is application independent and often accompanied
by application specific gateways (ALGs) to perform payload monitoring
and alterations. FTP is the most popular ALG resident on NAT
devices. Applications requiring ALG intervention must not have their
payload encoded, as doing that would effectively disables the ALG,
unless the ALG has the key to decrypt the payload.
This solution has the disadvantage of taking away the end-to-end
significance of an IP address, and making up for it with increased
state in the network. As a result, end-to-end IP network level
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security assured by IPSec cannot be assumed to end hosts, with a NAT
device enroute. The advantage of this approach however is that it
can be installed without changes to hosts or routers.
Definition of terms such as "Address Realm", "Transparent Routing",
"TU Ports", "ALG" and others, used throughout the document, may be
found in [NAT-TERM].
The Address Translation operation presented in this document is
referred to as "Traditional NAT". There are other variations of NAT
that will not be explored in this document. Traditional NAT would
allow hosts within a private network to transparently access hosts in
the external network, in most cases. In a traditional NAT, sessions
are uni-directional, outbound from the private network. Sessions in
the opposite direction may be allowed on an exceptional basis using
static address maps for pre-selected hosts. Basic NAT and NAPT are
two variations of traditional NAT, in that translation in Basic NAT
is limited to IP addresses alone, whereas translation in NAPT is
extended to include IP address and Transport identifier (such as
TCP/UDP port or ICMP query ID).
Unless mentioned otherwise, Address Translation or NAT throughout
this document will pertain to traditional NAT, namely Basic NAT as
well as NAPT. Only the stub border routers as described in figure 1
below may be configured to perform address translation.
\ | / . /
+---------------+ WAN . +-----------------+/
|Regional Router|----------------------|Stub Router w/NAT|---
+---------------+ . +-----------------+\
. | \
. | LAN
. ---------------
Stub border
Figure 1: Traditional NAT Configuration
Basic NAT operation is as follows. A stub domain with a set of
private network addresses could be enabled to communicate with
external network by dynamically mapping the set of private addresses
to a set of globally valid network addresses. If the number of local
nodes are less than or equal to addresses in the global set, each
local address is guaranteed a global address to map to. Otherwise,
nodes allowed to have simultaneous access to external network are
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limited by the number of addresses in global set. Individual local
addresses may be statically mapped to specific global addresses to
ensure guaranteed access to the outside or to allow access to the
local host from external hosts via a fixed public address. Multiple
simultaneous sessions may be initiated from a local node, using the
same address mapping.
Addresses inside a stub domain are local to that domain and not valid
outside the domain. Thus, addresses inside a stub domain can be
reused by any other stub domain. For instance, a single Class A
address could be used by many stub domains. At each exit point
between a stub domain and backbone, NAT is installed. If there is
more than one exit point it is of great importance that each NAT has
the same translation table.
For instance, in the example of figure 2, both stubs A and B
internally use class A private address block 10.0.0.0/8 [RFC 1918].
Stub A's NAT is assigned the class C address block 198.76.29.0/24,
and Stub B's NAT is assigned the class C address block
198.76.28.0/24. The class C addresses are globally unique no other
NAT boxes can use them.
\ | /
+---------------+
|Regional Router|
+---------------+
WAN | | WAN
| |
Stub A .............|.... ....|............ Stub B
| |
{s=198.76.29.7,^ | | v{s=198.76.29.7,
d=198.76.28.4}^ | | v d=198.76.28.4}
+-----------------+ +-----------------+
|Stub Router w/NAT| |Stub Router w/NAT|
+-----------------+ +-----------------+
| |
| LAN LAN |
------------- -------------
| |
{s=10.33.96.5, ^ | | v{s=198.76.29.7,
d=198.76.28.4}^ +--+ +--+ v d=10.81.13.22}
|--| |--|
/____\ /____\
10.33.96.5 10.81.13.22
Figure 2: Basic NAT Operation
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When stub A host 10.33.96.5 wishes to send a packet to stub B host
10.81.13.22, it uses the globally unique address 198.76.28.4 as
destination, and sends the packet to its primary router. The stub
router has a static route for net 198.76.0.0 so the packet is
forwarded to the WAN-link. However, NAT translates the source
address 10.33.96.5 of the IP header to the globally unique
198.76.29.7 before the packet is forwarded. Likewise, IP packets on
the return path go through similar address translations.
Notice that this requires no changes to hosts or routers. For
instance, as far as the stub A host is concerned, 198.76.28.4 is the
address used by the host in stub B. The address translations are
transparent to end hosts in most cases. Of course, this is just a
simple example. There are numerous issues to be explored.
Say, an organization has a private IP network and a WAN link to a
service provider. The private network's stub router is assigned a
globally valid address on the WAN link and the remaining nodes in the
organization have IP addresses that have only local significance. In
such a case, nodes on the private network could be allowed
simultaneous access to the external network, using the single
registered IP address with the aid of NAPT. NAPT would allow mapping
of tuples of the type (local IP addresses, local TU port number) to
tuples of the type (registered IP address, assigned TU port number).
This model fits the requirements of most Small Office Home Office
(SOHO) groups to access external network using a single service
provider assigned IP address. This model could be extended to allow
inbound access by statically mapping a local node per each service TU
port of the registered IP address.
In the example of figure 3 below, stub A internally uses class A
address block 10.0.0.0/8. The stub router's WAN interface is
assigned an IP address 138.76.28.4 by the service provider.
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\ | /
+-----------------------+
|Service Provider Router|
+-----------------------+
WAN |
|
Stub A .............|....
|
^{s=138.76.28.4,sport=1024, | v{s=138.76.29.7, sport = 23,
^ d=138.76.29.7,dport=23} | v d=138.76.28.4, dport = 1024}
+------------------+
|Stub Router w/NAPT|
+------------------+
|
| LAN
--------------------------------------------
| ^{s=10.0.0.10,sport=3017, | v{s=138.76.29.7, sport=23,
| ^ d=138.76.29.7,dport=23} | v d=10.0.0.10, dport=3017}
| |
+--+ +--+ +--+
|--| |--| |--|
/____\ /____\ /____\
10.0.0.1 10.0.0.2 ..... 10.0.0.10
Figure 3: Network Address Port Translation (NAPT) Operation
When stub A host 10.0.0.10 sends a telnet packet to host 138.76.29.7,
it uses the globally unique address 138.76.29.7 as destination, and
sends the packet to it's primary router. The stub router has a
static route for the subnet 138.76.0.0/16 so the packet is forwarded
to the WAN-link. However, NAPT translates the tuple of source
address 10.0.0.10 and source TCP port 3017 in the IP and TCP headers
into the globally unique 138.76.28.4 and a uniquely assigned TCP
port, say 1024, before the packet is forwarded. Packets on the
return path go through similar address and TCP port translations for
the target IP address and target TCP port. Once again, notice that
this requires no changes to hosts or routers. The translation is
completely transparent.
In this setup, only TCP/UDP sessions are allowed and must originate
from the local network. However, there are services such as DNS that
demand inbound access. There may be other services for which an
organization wishes to allow inbound session access. It is possible
to statically configure a well known TU port service [RFC 1700] on
the stub router to be directed to a specific node in the private
network.
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In addition to TCP/UDP sessions, ICMP messages, with the exception of
REDIRECT message type may also be monitored by NAPT router. ICMP
query type packets are translated similar to that of TCP/UDP packets,
in that the identifier field in ICMP message header will be uniquely
mapped to a query identifier of the registered IP address. The
identifier field in ICMP query messages is set by Query sender and
returned unchanged in response message from the Query responder. So,
the tuple of (Local IP address, local ICMP query identifier) is
mapped to a tuple of (registered IP address, assigned ICMP query
Identifier) by the NAPT router to uniquely identify ICMP queries of
all types from any of the local hosts. Modifications to ICMP error
messages are discussed in a later section, as that involves
modifications to ICMP payload as well as the IP and ICMP headers.
In NAPT setup, where the registered IP address is the same as the IP
address of the stub router WAN interface, the router has to be sure
to make distinction between TCP, UDP or ICMP query sessions
originated from itself versus those originated from the nodes on
local network. All inbound sessions (including TCP, UDP and ICMP
query sessions) are assumed to be directed to the NAT router as the
end node, unless the target service port is statically mapped to a
different node in the local network.
Sessions other than TCP, UDP and ICMP query type are simply not
permitted from local nodes, serviced by a NAPT router.
The translation phases with traditional NAT are same as described in
[NAT-TERM]. The following sub-sections identify items that are
specific to traditional NAT.
With Basic NAT, a private address is bound to an external address,
when the first outgoing session is initiated from the private host.
Subsequent to that, all other outgoing sessions originating from the
same private address will use the same address binding for packet
translation.
In the case of NAPT, where many private addresses are mapped to a
single globally unique address, the binding would be from the tuple
of (private address, private TU port) to the tuple of (assigned
address, assigned TU port). As with Basic NAT, this binding is
determined when the first outgoing session is initiated by the tuple
of (private address, private TU port) on the private host. While not
a common practice, it is possible to have an application on private
host establish multiple simultaneous sessions originating from the
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same tuple of (private address, private TU port). In such a case, a
single binding for the tuple of (private address, private TU port)
may be used for translation of packets pertaining to all sessions
originating from the same tuple on a host.
After an address binding or (address, TU port) tuple binding in case
of NAPT is established, a soft state may be maintained for each of
the connections using the binding. Packets belonging to the same
session will be subject to session lookup for translation purposes.
The exact nature of translation is discussed in the follow-on
section.
Packets pertaining to NAT managed sessions undergo translation in
either direction. Individual packet translation issues are covered
in detail in the following sub-sections.
In Basic NAT model, the IP header of every packet must be modified.
This modification includes IP address (source IP address for outbound
packets and destination IP address for inbound packets) and the IP
checksum.
For TCP ([TCP]) and UDP ([UDP]) sessions, modifications must include
update of checksum in the TCP/UDP headers. This is because TCP/UDP
checksum also covers a pseudo header which contains the source and
destination IP addresses. As an exception, UDP headers with 0
checksum should not be modified. As for ICMP Query packets ([ICMP]),
no further changes in ICMP header are required as the checksum in
ICMP header does not cover IP addresses.
In NAPT model, modifications to IP header are similar to that of
Basic NAT. For TCP/UDP sessions, modifications must be extended to
include translation of TU port (source TU port for outbound packets
and destination TU port for inbound packets) in the TCP/UDP header.
ICMP header in ICMP Query packets must also be modified to replace
the query ID and ICMP header checksum. Private host query ID must be
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translated into assigned ID on the outbound and the exact reverse on
the inbound. ICMP header checksum must be corrected to account for
Query ID translation.
NAT modifications are per packet based and can be very compute
intensive, as they involve one or more checksum modifications in
addition to simple field translations. Luckily, we have an algorithm
below, which makes checksum adjustment to IP, TCP, UDP and ICMP
headers very simple and efficient. Since all these headers use a
one's complement sum, it is sufficient to calculate the arithmetic
difference between the before-translation and after-translation
addresses and add this to the checksum. The algorithm below is
applicable only for even offsets (i.e., optr below must be at an even
offset from start of header) and even lengths (i.e., olen and nlen
below must be even). Sample code (in C) for this is as follows.
void checksumadjust(unsigned char *chksum, unsigned char *optr,
int olen, unsigned char *nptr, int nlen)
/* assuming: unsigned char is 8 bits, long is 32 bits.
- chksum points to the chksum in the packet
- optr points to the old data in the packet
- nptr points to the new data in the packet
*/
{
long x, old, new;
x=chksum[0]*256+chksum[1];
x=~x & 0xFFFF;
while (olen)
{
old=optr[0]*256+optr[1]; optr+=2;
x-=old & 0xffff;
if (x<=0) { x--; x&=0xffff; }
olen-=2;
}
while (nlen)
{
new=nptr[0]*256+nptr[1]; nptr+=2;
x+=new & 0xffff;
if (x & 0x10000) { x++; x&=0xffff; }
nlen-=2;
}
x=~x & 0xFFFF;
chksum[0]=x/256; chksum[1]=x & 0xff;
}
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Changes to ICMP error message ([ICMP]) will include changes to IP and
ICMP headers on the outer layer as well as changes to headers of the
packet embedded within the ICMP-error message payload.
In order for NAT to be transparent to end-host, the IP address of the
IP header embedded within the payload of ICMP-Error message must be
modified, the checksum field of the embedded IP header must be
modified, and lastly, the ICMP header checksum must also be modified
to reflect changes to payload.
In a NAPT setup, if the IP message embedded within ICMP happens to be
a TCP, UDP or ICMP Query packet, you will also need to modify the
appropriate TU port number within the TCP/UDP header or the Query
Identifier field in the ICMP Query header.
Lastly, the IP header of the ICMP packet must also be modified.
One of the most popular applications, "FTP" ([FTP]) would require an
ALG to monitor the control session payload to determine the ensuing
data session parameters. FTP ALG is an integral part of most NAT
implementations.
The FTP ALG would require a special table to correct the TCP sequence
and acknowledge numbers with source port FTP or destination port FTP.
The table entries should have source address, destination address,
source port, destination port, delta for sequence numbers and a
timestamp. New entries are created only when FTP PORT commands or
PASV responses are seen. The sequence number delta may be increased
or decreased for every FTP PORT command or PASV response. Sequence
numbers are incremented on the outbound and acknowledge numbers are
decremented on the inbound by this delta.
FTP payload translations are limited to private addresses and their
assigned external addresses (encoded as individual octets in ASCII)
for Basic NAT. For NAPT setup, however, the translations must be
extended to include the TCP port octets (in ASCII) following the
address octets.
Considering that sessions in a traditional NAT are predominantly
outbound from a private domain, DNS ALG may be obviated from use in
conjunction with traditional NAT as follows. DNS server(s) internal
to the private domain maintain mapping of names to IP addresses for
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internal hosts and possibly some external hosts. External DNS
servers maintain name mapping for external hosts alone and not for
any of the internal hosts. If the private network does not have an
internal DNS server, all DNS requests may be directed to external DNS
server to find address mapping for the external hosts.
An IP datagram with any of the IP options Record Route, Strict Source
Route or Loose Source Route would involve recording or using IP
addresses of intermediate routers. A NAT intermediate router may
choose not to support these options or leave the addresses
untranslated while processing the options. The result of leaving the
addresses untranslated would be that private addresses along the
source route are exposed end to end. This should not jeopardize the
traversal path of the packet, per se, as each router is supposed to
look at the next hop router only.
For NAT to operate as described in this document, it is necessary to
partition the IP address space into two parts - the private addresses
used internal to stub domain, and the globally unique addresses. Any
given address must either be a private address or a global address.
There is no overlap.
The problem with overlap is the following. Say a host in stub A
wished to send packets to a host in stub B, but the global addresses
of stub B overlapped the private addressees of stub A. In this case,
the routers in stub A would not be able to distinguish the global
address of stub B from its own private addresses.
[RFC 1918] has recommendations on address space allocation for
private networks. Internet Assigned Numbers Authority (IANA) has
three blocks of IP address space, namely 10.0.0.0/8, 172.16.0.0/12,
and 192.168.0.0/16 for private internets. In pre-CIDR notation, the
first block is nothing but a single class A network number, while the
second block is a set of 16 contiguous class B networks, and the
third block is a set of 256 contiguous class C networks.
An organization that decides to use IP addresses in the address space
defined above can do so without any coordination with IANA or an
Internet registry. The address space can thus be used privately by
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many independent organizations at the same time, with NAT operation
enabled on their border routers.
The router running NAT should not advertise the private networks to
the backbone. Only the networks with global addresses may be known
outside the stub. However, global information that NAT receives from
the stub border router can be advertised in the stub the usual way.
Typically, the NAT stub router will have a static route configured to
forward all external traffic to service provider router over WAN
link, and the service provider router will have a static route
configured to forward NAT packets (i.e., those whose destination IP
address fall within the range of NAT managed global address list) to
NAT router over WAN link.
In Basic NAT setup, when private network nodes outnumber global
addresses available for mapping (say, a class B private network
mapped to a class C global address block), external network access to
some of the local nodes is abruptly cut off after the last global
address from the address list is used up. This is very inconvenient
and constraining. Such an incident can be safely avoided by
optionally allowing the Basic NAT router to switch over to NAPT setup
for the last global address in the address list. Doing this will
ensure that hosts on private network will have continued,
uninterrupted access to the external nodes and services for most
applications. Note, however, it could be confusing if some of the
applications that used to work with Basic NAT suddenly break due to
the switch-over to NAPT.
[NAT-TERM] covers the limitations of all flavors of NAT, broadly
speaking. The following sub-sections identify limitations specific
to traditional NAT.
Traditional NAT can be viewed as providing a privacy mechanism as
sessions are uni-directional from private hosts and the actual
addresses of the private hosts are not visible to external hosts.
The same characteristic that enhances privacy potentially makes
debugging problems (including security violations) more difficult. If
a host in private network is abusing the Internet in some way (such
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as trying to attack another machine or even sending large amounts of
spam) it is more difficult to track the actual source of trouble
because the IP address of the host is hidden in a NAT router.
NAT must be enabled only on border routers of a stub domain. The
examples provided in the document to illustrate Basic NAT and NAPT
have maintained a WAN link for connection to external router (i.e.,
service provider router) from NAT router. However, if the WAN link
were to be replaced by a LAN connection and if part or all of the
global address space used for NAT mapping belongs to the same IP
subnet as the LAN segment, the NAT router would be expected to
provide ARP support for the address range that belongs to the same
subnet. Responding to ARP requests for the NAT mapped global
addresses with its own MAC address is a must in such a situation with
Basic NAT setup. If the NAT router did not respond to these
requests, there is no other node in the network that has ownership to
these addresses and hence will go unresponded.
This scenario is unlikely with NAPT setup except when the single
address used in NAPT mapping is not the interface address of the NAT
router (as in the case of a switch-over from Basic NAT to NAPT
explained in 5.4 above, for example).
Using an address range from a directly connected subnet for NAT
address mapping would obviate static route configuration on the
service provider router.
It is the opinion of the authors that a LAN link to a service
provider router is not very common. However, vendors may be
interested to optionally support proxy ARP just in case.
Translation of outbound TCP/UDP fragments (i.e., those originating
from private hosts) in NAPT setup are doomed to fail. The reason is
as follows. Only the first fragment contains the TCP/UDP header that
would be necessary to associate the packet to a session for
translation purposes. Subsequent fragments do not contain TCP/UDP
port information, but simply carry the same fragmentation identifier
specified in the first fragment. Say, two private hosts originated
fragmented TCP/UDP packets to the same destination host. And, they
happened to use the same fragmentation identifier. When the target
host receives the two unrelated datagrams, carrying same
fragmentation id, and from the same assigned host address, it is
unable to determine which of the two sessions the datagrams belong
to. Consequently, both sessions will be corrupted.
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Many commercial implementations are available in the industry that
adhere to the NAT description provided in this document. Linux
public domain software contains NAT under the name of "IP
masquerade". FreeBSD public domain software has NAPT implementation
running as a daemon. Note however that Linux source is covered under
the GNU license and FreeBSD software is covered under the UC
Berkeley license.
Both Linux and FreeBSD software are free, so you can buy CD-ROMs for
these for little more than the cost of distribution. They are also
available on-line from a lot of FTP sites with the latest patches.
The security considerations described in [NAT-TERM] for all
variations of NATs are applicable to traditional NAT.
References
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC 1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
[RFC 1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 1123] Braden, R., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, October 1989.
[RFC 1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[FTP] Postel, J. and J. Reynolds, "FILE TRANSFER PROTOCOL
(FTP)", STD 9, RFC 959, October 1985.
[TCP] Defense Advanced Research Projects Agency Information
Processing Techniques Office, "TRANSMISSION CONTROL
PROTOCOL (TCP) SPECIFICATION", STD 7, RFC 793, September
1981.
Srisuresh & Egevang Informational [Page 14]
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[ICMP] Postel, J., "INTERNET CONTROL MESSAGE (ICMP)
SPECIFICATION", STD 5, RFC 792, September 1981.
[UDP] Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC
768, August 1980.
[RFC 2101] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
Behaviour Today", RFC 2101, February 1997.
Authors' Addresses
Pyda Srisuresh
Jasmine Networks, Inc.
3061 Zanker Road, Suite B
San Jose, CA 95134
U.S.A.
Phone: (408) 895-5032
EMail: srisuresh@yahoo.com
Kjeld Borch Egevang
Intel Denmark ApS
Phone: +45 44886556
Fax: +45 44886051
EMail: kjeld.egevang@intel.com
http: //www.freeyellow.com/members/kbe
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Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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