One document matched: draft-rfced-info-srisuresh-04.txt
Differences from draft-rfced-info-srisuresh-03.txt
Transport Working Group P. Srisuresh
INTERNET-DRAFT Lucent Technologies
Obsoletes: RFC 1631 K. Egevang
Category: Informational Intel Corporation
Expire in six months February 1998
The IP Network Address Translator (NAT)
<draft-rfced-info-srisuresh-04.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
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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 feature by
which IP addresses are mapped from one group to another, transparent
to users. Network Address Port Translation, or NAPT is an extension
to Basic NAT, in that many network addresses and their TCP/UDP ports
are translated to a single network address and its TCP/UDP ports.
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Together, these two operations, traditionally referred to as NAT,
provide a mechanism to connect an isolated routing realm with private
unregistered addresses to the external routing network with globally
unique registered addresses.
1. Introduction
The need for IP Address translation arises when a network's internal
IP addresses cannot be used outside the network either for security
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 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 the users within the domain
by centralizing changes to a single address translation router.
Basic Address translation feature would allow local hosts on a
private network to transparently access the external global 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 feature.
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 feature. It is
mandatory that all requests and responses pertaining to a session
be routed via the same NAT router. For this reason, we recommend
that NATs be operated 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. Address translation is predominantly
application independent, with the exception of FTP and a few
other applications. Encoded FTP sessions and any encoded sessions
in general that might include IP addresses in the encoding will
not be supported by NAT.
This solution has the disadvantage of taking away the end-to-end
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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
security assured by IPSec cannot be assumed to end hosts, so long
as there exists a NAT router along the route. The advantage of
this approach however is that it can be installed without changes
to hosts or routers.
2. Terminology and concepts used
2.1. Session flow vs. Packet flow
Connection or session flows are different from packet flows.
A session flow indicates the direction in which the session was
initiated with reference to a network port. Packet flow is the
direction in which the packet has traveled with reference to a
network port.
Take for example, an outbound telnet session. The telnet session
consists of packet flows in both inbound and outbound directions.
Outbound telnet packets carry terminal keystrokes and inbound
telnet packets carry screen displays from the telnet server.
Performing address or TCP port translation for a telnet session
would involve translation of incoming as well as outgoing packets
belonging to that session.
Packets belonging to a TCP/UDP session are uniquely identified
by the tuple of (source IP address, source TU port, target IP
address, target TU port). Packets belonging to all other sessions
are characterized simply by the tuple of (source IP address, target
IP address, IP protocol). A session is uniquely identified by the
first packet of that session.
2.2. TU ports, Server ports, Client ports
For the reminder of this document, we will refer TCP/UDP ports
associated with an IP address simply as "TU ports".
For most TCP/IP hosts, TU port range 0-1023 is used by servers
listening for incoming connections. Clients trying to initiate
a connection typically select a TU port in the range of 1024-65535.
However, this convention is not universal and not always followed.
Some client stations initiate connections using a TU port number
in the range of 0-1023, and there are servers listening on TU
port numbers in the range of 1024-65535.
A complete list of TU port services may be found in Ref[2].
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2.3. Start of session for TCP, UDP and others
The first packet of every TCP session tries to establish a session
and contains connection startup information. The first packet of a
TCP session may be recognized by the presence of SYN bit and
absence of ACK bit in the TCP flags. All TCP packets, with the
exception of the first packet must have the ACK bit set.
However, there is no deterministic way of recognizing the start of
a UDP based session or any non-TCP session.
2.4. Application Level gateway (ALG)
Not all applications lend themselves easily to translation by NATs;
especially those that include IP addresses and TCP/UDP ports in the
payload. Application Level Gateways (ALGs) are application
specific translation agents that allow hosts from one routing realm
to connect to hosts in a different realm. The ALGs may optionally
utilize address/port assignments by NAT and perform translations of
packets pertaining to the application.
3. Overview of NAT
The Address Translation operation presented in this document is
called NAT, for Network Address Translator. This is also sometimes
referred to as "Traditional NAT", as there are many variations of
address translation that lend themselves to different applications.
NAT operation described here is a router function that involves
a) dynamic address assignment and address translation or
b) dynamic TCP/UDP port assignment and translation of network
address and TCP/UDP port. We will call the former Basic NAT and
the latter NAPT. Together they are referred to as NAT. Unless
mentioned otherwise, Address Translation or NAT throughout this
document will pertain to 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: NAT Configuration
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3.1 Overview of Basic NAT
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 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 to be mapped to an address from global set. Otherwise,
local nodes allowed to have simultaneous access to external network
are limited by the number of addresses in global set. In addition,
individual local addresses may be statically mapped to specific
global addresses to ensure guaranteed access to the outside or to
expose a local node for access from the outside. Multiple 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.
\ | /
+---------------+
|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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For instance, in the example of figure 2, both stubs A and B
internally use class A address 10.0.0.0. Stub A's NAT is
assigned the class C address 198.76.29.0, and Stub B's NAT is
assigned the class C address 198.76.28.0. The class C addresses
are globally unique no other NAT boxes can use them.
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 it's 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 completely transparent. Of course, this is just a simple
example. There are numerous issues to be explored.
3.2. Overview of NAPT
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 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 10.0.0.0. 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 net 138.76.0.0 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 TU port service 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.
4.0. Translation phases of a session.
There are three phases to Address translation, as follows.
4.1. Address binding:
Address binding is the phase in which a local node IP address is
associated with a global address for purposes of translation. For
addresses that have static mapping, the binding happens at startup
time. Otherwise, a local address is bound to a global address
dynamically at the time of session initiation from the local node.
Once a local address is bound to a global address, all subsequent
sessions originating from the same local address will use the same
binding for session based packet translation.
In the case of NAPT, where many local addresses are mapped to a
single globally unique address, the binding would be from (local
IP addr, TU port#) to a TU port of Registered IP address. As
with Basic NAT, this binding is determined at the time of session
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initiation.
4.2. Address lookup and translation:
Once address binding is established for a connection setup
through a NAT port, all subsequent packets belonging to the same
connection will be subject to address lookup (and TU port lookup,
in the case of NAPT) for translation purposes.
For outbound packets of a session, the source IP address (and
source TU port, in case of NAPT) and related fields (such as
IP, TCP, UDP and ICMP header checksums) will undergo translation.
For inbound packets of a session, the destination IP address
(and destination TU port, in case of NAPT) and related fields
such as IP, TCP, UDP and ICMP header checksums) will undergo
translation.
4.3. Address unbinding:
Address unbinding is the phase in which a local node IP address is
no longer associated with a global address for purposes of
translation. When the last session based on an address binding is
terminated, it is safe to do the address unbinding after session
termination.
The end of a TCP session is detected when FIN is acknowledged by
both halves of the session or when either half sets RST bit in
TCP flags field. Within a couple seconds after this, the session
can be safely assumed to have been terminated. Dynamically bound
TCP entries that have not been used for say, 24 hours, should
also be safe to delete from the bound list. Dynamically bound
non-TCP entries that have not been used for say, 1 minute, should
also be safe to delete from the bound list. Session timeouts for
TCP and non-TCP sessions could optionally be made user
configurable. Another good way to handle session terminations is
to timestamp entries and keep them as long as possible and retire
the longest idle session when it becomes necessary.
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5.0. Packet Translations
NATs are, generally speaking, application independent in that
the translations are limited to IP/TCP/UDP/ICMP headers and
ICMP error messages only. NATs also do not change the payload
of any the packets, as payloads tend to be application specific.
However, there are exceptions to this rule. One of the most
popular internet applications FTP would not work by this purist
approach of NATs. FTP control session carries in its payload the
IP address and TCP port information pertaining to the data
session it supports. So, NATs are extended to support FTP
application as an exception. Some vendors may choose to expand
the function of NAT routers to include other applications
requiring modifications in payload.
Keeping NATs application independent implies having to work
some of the commonly used utilities (which use IP addresses in
payload) around NAT. DNS service is one of them. It is
recommended that internal DNS servers maintain mapping of names
to IP addresses for internal hosts as well as some external
hosts. External DNS servers maintain name mapping for external
hosts alone and not for any of the internal hosts. If the local
network does not have an internal DNS server, all DNS requests
will be directed to external DNS server to find address mapping
for the external hosts.
Packets pertaining to NAT managed sessions undergo translation
in either direction. Individual packet translation issues are
covered in detail in the following sub-sections.
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.
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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;
}
5.1. Header Manipulations
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/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.
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.
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Modifications to ICMP and FTP packets are considered separately
in the following subsections. ICMP packet modifications section
covers modifications to ICMP headers as well.
5.2. FTP sessions
The arguments to the File Transfer Protocol (FTP) PORT command and
PASV response include an IP address and a TCP port (in ASCII!). If
the IP address in PORT command or PASV response is local to the
stub domain, then NAT must substitute this. Because the address
and TCP port are encoded in ASCII, this may result in a change in
the size of packet. For instance, 10,18,177,42,64,87 is 18 ASCII
characters, whereas 193,45,228,137,64,87 is 20 ASCII characters.
If the new size is same as the previous, only the TCP checksum
needs adjustment as a result of change of data. If the new size
is less than or greater than the previous, TCP sequence numbers
must also be changed to reflect the change in length of FTP control
data portion.
A special table is used to correct the TCP sequence and acknowledge
numbers with source port FTP or destination port FTP. The table
entries should have source, destination, 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
and acknowledge numbers are decremented by this delta.
The sequence number adjustment must be coded carefully, not to harm
performance for TCP in general. Of course, if the FTP session is
encrypted, PORT command and/or PASV response will fail.
5.3. ICMP packet modifications
All ICMP error messages (with the exception of Redirect message type)
will need to be modified, when passed through NAT. The ICMP error
message types needing NAT modification would include
Destination-Unreachable, Source-Quench, Time-Exceeded and
Parameter-Problem. NAT should not attempt to modify a Redirect
message type.
Changes to ICMP error message will include a minimum of two address
modifications and three checksum modifications. This is because these
ICMP messages contain part of the original IP packet in the payload.
In order for NAT to be completely transparent to the host, the IP
address of the IP header embedded in the payload of the ICMP packet
must be modified, the checksum field of the same IP header must
correspondingly be modified, and the ICMP header checksum must also
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be modified to reflect changes made to the IP header and checksum in
the payload. Furthermore, the normal IP header must also be
modified.
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.
5.4. IP option handling
An IP datagram with any of the IP options Record Route, Strict
Source Route or Loose Source Route would involve IP addresses of the
intermediate routers. A NAT intermediate router would simply leave
the addresses untranslated and not participate in the processing of
these options.
5.5. Applications with IP-address Content
Not All applications lend themselves easily to address translation
by NATs. Especially, the applications that carry IP address
(and TU port, in case of NAPT) inside the payload. Application Level
Gateways, or ALGs must be used to perform translations on packets
pertaining to such applications. ALGs may optionally utilize address
(and TU port) assignments made by NAT and perform translations
specific to the application. Some not so transparent ALGs may choose
to perform application specific authentication, logging, filtering
and other enhanced functions, not often found with application
independent NATs. Often, one or more ALGs are used in a NAT router
to complement NAT functionality for a private network.
For example, NAT routers would not translate IP addresses
within SNMP payloads. It is not uncommon for an SNMP specific
ALG to reside on a NAT router to perform SNMP MIB translations
proprietary to the private network.
And, if the payload is encrypted, then it is impossible for NATs
or even the ALGs to make the translation.
6. Miscellaneous issues
6.1. Partitioning of Local and Global Addresses
For NAT to operate as described in this draft, it is necessary
to partition the IP address space into two parts - the local
addresses used internal to stub domain, and the globally
unique addresses. Any given address must either be a local
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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 local 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 local addresses.
6.2. Private address space recommendation
The RFC listed in ref[1] 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 many independent organizations at the same time,
with NAT operation enabled on their border routers.
6.3. Routing Across NAT
The router running NAT should not advertise the local 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.
6.4. Private Networks that Span Backbones
In many cases, a private network (such as a corporate network) will
be spread over different locations and will use a public backbone
for communications between those locations. In this case, it is not
desirable to do address translation, both because large numbers of
hosts may want to communicate across the backbone, thus requiring
large address tables, and because there will be more applications
that depend on configured addresses, as opposed to going to a name
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server. We call such a private network a backbone-partitioned stub.
Backbone-partitioned stubs should behave as though they were a non-
partitioned stub. That is, the routers in all partitions should
maintain routes to the local address spaces of all partitions. Of
course, the (public) backbones do not maintain routes to any local
addresses. Therefore, the border routers must tunnel through the
backbones using encapsulation. To do this, each NAT box will set
aside one global address for tunneling. When a NAT box x in stub
partition X wishes to deliver a packet to stub partition Y, it will
encapsulate the packet in an IP header with destination address set
to the global address of NAT box y that has been reserved for
encapsulation. When NAT box y receives a packet with that destination
address, it decapsulates the IP header and routes the packet
internally.
6.5. Switch-over from Basic NAT to NAPT
In Basic NAT setup, when local network nodes outnumber global
addresses available for mapping (say, a class B local 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 guarantee that hosts on local
network will have continued, uninterrupted access to the external
nodes and services.
7.0. NAT limitations
7.1. Privacy, Security, and Debugging Considerations
Unfortunately, NAT reduces the number of options for providing
security. With NAT, nothing that carries an IP address or TU port or
information derived from an IP address or TU port (such as the
IP/TCP/UDP/ICMP header checksum) can be encrypted. While most
application-level encryption should be ok, this prevents encryption
of TCP/UDP headers.
NAT takes away the end-to-end significance of IP addresses of the
end nodes. As a result, end-to-end IP network level security assured
by IPSec will not work for end hosts, so long as there exists a NAT
router along the route. IPSec is workable with NAT only so long as
IPSec and NAT are implemented on the same router (ex: Gateway to
Gateway security or Gateway to end node security based on VPNs).
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On the other hand, NAT itself can be seen as providing a kind of
privacy mechanism. This comes from the fact that machines on the
backbone cannot monitor which hosts are sending and receiving traffic
(assuming of course that the application data is encrypted).
The same characteristic that enhances privacy potentially makes
debugging problems (including security violations) more difficult.
If a host is abusing the Internet in some way (such as trying to
attack another machine or even sending large amounts of junk mail
or something) it is more difficult to pinpoint the source of the
trouble because the IP address of the host is hidden in a NAT router.
7.2. ARP responses to NAT mapped global addresses on a LAN interface
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 6.5 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.
7.3. Translation of fragmented FTP control packets
Translation of fragmented FTP control packets is tricky when the
packets contain "PORT" command or response to "PASV" command.
Clearly, this is a pathological case. It may be fine to simply
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discard the fragments. Alternately, NAT router could attempt
to assemble fragments first and then translate prior to
forwarding.
Yet another pathological case would be when each character of
packets containing "PORT" command or response to "PASV" is sent
in a separate datagram, unfragmented. In this case, NAT would
simply have to let the packets through, untranslated.
7.4. Translation of outbound TCP/UDP fragmented packets in NAPT setup
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.
7.5. Negative characteristics:
1. NAT is compute intensive even with the help of a clever
checksum adjust algorithm, as each data packet is subject to
NAT lookup and modifications. As a result, router forwarding
throughput will be slowed considerably.
2. NAT increases the probability of mis-addressing. For example,
same local address may be bound to different global address at
different times and vice versa. As a result, any traffic flow
study based purely on global addresses and TU ports could be
confused and might misinterpret the results.
3. NAT breaks certain applications or at least makes them more
difficult to run.
DNS is one of the most commonly used utilities that need to be
worked around the limitation of NAT as described in section 5.0.
Doing this would ensure that local addresses in private network
do not appear in the payload of DNS request and response messages.
Likewise, SNMP based management applications often require an
ALG to translate private addresses to distinguish the various
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independent nodes within private network.
4. NAT hides the identity of hosts. This is not to be confused with
security however. Security on a router must be relegated to
firewall functionality, independent of or in conjunction with
NAT operation.
8.0. Current Implementations
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.
9.0. Acknowledgements
The first author Srisuresh would like to express his thanks
and sincere gratitude to Der-hwa Gan for the knowledge and
insight gained during the many probing discussions they had
held. Der-hwa has a wide spread knowledge of routers and
applications alike and was instrumental in making the author
appreciate the many uses of NATs.
10.0. Security Considerations
Below are some of the security considerations associated with
NAT routers.
1. UDP sessions are inherently unsafe. Responses to a datagram
could come from an address different from the target address
used by sender. Below is a quote from RFC 1123, section 2.3
that confirms this.
When the local host is multihomed, a UDP-based request/
response application SHOULD send the response with
an IP source address that is the same as the specific
destination address of the UDP request datagram. The
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"specific destination address" is defined in the
"IP Addressing" section of the companion RFC [INTRO:1].
NAT implementations that do not track datagrams on a
per-session basis but lump states of multiple UDP sessions
into a single state could compromise the security even further.
2. Multicast sessions (UDP based) are another source for security
weaknesses.
Say, a host on private network initiated a multicast session.
Datagram sent by the the private host could trigger responses
in the reverse direction from multiple external hosts. NAT
implementations that use a single state to track the multicast
responses in a multicast session could potentially be the
target of security attacks. This multicast specific security
concern, however, is not unique to NAT implementations, and
exists across all hosts supporting multicast applications.
3. NAT takes away end-to-end significance of IP addresses, TU
ports, etc. and makes up for their loss by maintaining a
state for each of the sessions it supports. This type of
state management for sessions makes NAT a target for security
break-ins that hosts have had to deal with. E.g., SYN attacks.
In a SYN flood attack, an attacker host sends many SYN packets
and does not respond with an ACK to the (SYN | ACK)s sent by
the receiving host. As the receiving host is waiting for more
and more ACKs, the buffer queue will fill up and the receiving
host can no longer accept legitimate connections. This means
that attackers can block e-mail, web or any other services that
may have been provided by the receiving host.
When a NAT router is in between the attacker and the target
host, NAT is maintaining a state for each new session that
attacker is initiating. Each new SYN packet sent by the
attacker causes a new buffer to be allocated within NAT for
management of that new session. Soon, the buffer queue will
fill up and the NAT router can no longer support any
legitimate connections. This means that attacker is now able
to block all services that may have been provided by any of
the private hosts, not just the host that is the target of
attack.
One solution may be for NAT implementations to monitor
half-open sessions, and set a ceiling on the maximum number
of half-open sessions and free up buffers that were allocated
for connections that have been half-open for longer than a
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certain time period.
4. End-to-end IP network level security assured by IPSec will not
work for end hosts, so long as there exists a NAT router along
the route. IPSec is workable with NAT only so long as IPSec and
NAT are implemented on the same router (ex: Gateway to Gateway
security or Gateway to end node security based on VPNs).
REFERENCES
[1] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and,
Lear, E. "Address Allocation for Private Internets", RFC 1918
or its successor.
[2] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700 or
its successor.
[3] R. Braden, "Requirements for Internet Hosts -- Communication
Layers", RFC 1122 or its successor.
[4] R. Braden, "Requirements for Internet Hosts -- Application
and Support", RFC 1123 or its successor.
[5] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812
or its successor.
[6] J. Postel, J. Reynolds, "FILE TRANSFER PROTOCOL (FTP)",
RFC 959 or its successor.
[7] "TRANSMISSION CONTROL PROTOCOL (TCP) SPECIFICATION", RFC 793
or its successor.
[8] J. Postel, "INTERNET CONTROl MESSAGE (ICMP) SPECIFICATION",
RFC 793 or its successor.
[9] J. Postel, "User Datagram Protocol (UDP)", RFC 768 or its
successor.
[10] J. Mogul, J. Postel, "Internet Standard Subnetting Procedure",
RFC 950 or its successor.
[11] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address
Behaviour Today", RFC 2101 or its successor.
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Authors' Addresses
Pyda Srisuresh
Lucent technologies
Pleasanton, CA 94588-8519
U.S.A.
Voice: (510) 737-2153
Fax: (510) 737-2110
EMail: suresh@livingston.com
Kjeld Borch Egevang
Intel Denmark ApS
Voice: +45 44530100
Fax: +45 44531415
EMail: kbe@casetech.dk
http: //www.freeyellow.com/members/kbe
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