One document matched: draft-cheshire-nat-pmp-00.txt
Document: draft-cheshire-nat-pmp-00.txt Stuart Cheshire
Internet-Draft Marc Krochmal
Expires 7th December 2005 Kiren Sekar
Apple Computer, Inc.
7th June 2005
NAT Port Mapping Protocol (NAT-PMP)
<draft-cheshire-nat-pmp-00.txt>
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
For the purposes of this document, the term "BCP 79" refers
exclusively to RFC 3979, "Intellectual Property Rights in IETF
Technology", published March 2005.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes a protocol for automating the process of
creating Network Address Translation (NAT) port mappings. Included
in the protocol is a method for retrieving the public IP address of a
NAT gateway, thus allowing a client to make this public IP address
and port number known to peers that may wish to communicate with it.
This protocol is implemented in current Apple products including
Mac OS X v10.4 Tiger and Bonjour for Windows.
Expires 7th December 2005 Cheshire, et al. [Page 1]
Internet Draft NAT Port Mapping Protocol 7th June 2005
1. Introduction
Network Address Translation (NAT) is a method of sharing one public
internet address with a number of devices. This document is focused
on what "IP Network Address Translator (NAT) Terminology and
Considerations" [RFC 2663] calls "NAPTs" (Network Address/Port
Translators). A full description of NAT is beyond the scope of this
document. The following brief overview will cover the aspects
relevant to this port mapping protocol. For more information on
NAT, see "Traditional IP Network Address Translator" [RFC 3022].
NATs have one or more public IP addresses. A private network is set
up behind the NAT. Devices behind the NAT are assigned private
addresses and the private address of the NAT device is used as the
gateway.
When a packet from any device behind the NAT is sent to an address on
the public internet, the packet first passes through the NAT box. The
NAT box looks at the source port and address. In some cases, a NAT
will also keep track of the destination port and address. The NAT
then creates a mapping from the private address and private port to a
public address and public port if a mapping does not already exist.
The NAT box replaces the private address and port number in the
packet with the public entries from the mapping and sends the packet
on to the next gateway.
When a packet from any address on the internet is received on the
NAT's public side, the NAT will look up the destination port (public
port) in the list of mappings. If an entry is found, it will contain
the private address and port that the packet should be sent to. The
NAT gateway will then rewrite the destination address and port with
those from the mapping. The packet will then be forwarded to the new
destination addresses. If the packet did not match any mapping, the
packet will most likely be dropped. Various NATs implement different
strategies to handle this. The important thing to note is that if
there is no mapping, the NAT does not know which private address the
packet should be sent to.
Mappings are usually created automatically as a result of observing
outbound traffic. There are a few exceptions. Some NATs may allow
manually-created permanent mappings that map a public port to a
specific private IP address and port. Such a mapping allows incoming
connections to the device with that private address. Some NATs also
implement a default mapping where any inbound traffic that does not
match a mapping will always be forwarded to a specific private
address. Both types of mappings are usually set up manually through
some configuration tool.
Without these manually-created inbound port mappings, clients behind
the NAT would be unable to receive inbound connections, which
represents a loss of connectivity when compared to the original
Expires 7th December 2005 Cheshire, et al. [Page 2]
Internet Draft NAT Port Mapping Protocol 7th June 2005
Internet architecture [ETEAISD]. For those who view this loss of
connectivity as a bad thing, NAT-PMP allows clients to operate much
more like a host directly connected to the unrestricted public
Internet, with an unrestricted public IP address. NAT-PMP allows
client hosts to communicate with the NAT gateway to request the
creation of inbound mappings on demand. Having created a NAT mapping
to allow inbound connections, the client can then record its public
IP address and public port number in a public registry (e.g. the
world-wide Domain Name System) or otherwise make it accessible to
peers that wish to communicate with it.
2. Conventions and Terminology Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" [RFC 2119].
3. Protocol and Packet Format
NAT Port Mapping Protocol runs over UDP. Every packet starts with an
8 bit version followed by an 8 bit operation code.
This document specifies version 0 of the protocol. Any NAT-PMP
gateway implementing this version of the protocol, receiving a
packet with a version number other than 0, MUST return result code 1
(Unsupported Version).
Opcodes between 0 and 127 are client requests. Opcodes from 128 to
255 are server responses. Responses always contain a 16 bit result
code in network byte order. A result code of zero indicates success.
Responses also contain a 32 bit unsigned integer corresponding to the
number of seconds since the NAT gateway was rebooted or since its
port mapping state was reset.
This protocol SHOULD only be used when the client determines that its
primary source IPv4 address is in the range of private IP addresses
defined in RFC 1918. This includes the address ranges 10/8,
172.16/12, and 192.168/16.
Clients always send their Port Mapping Protocol requests to their
default gateway, as learned via DHCP [RFC 2131], or similar means.
This protocol is designed for small home networks, with a single
logical link (subnet) where the client's default gateway is also the
NAT translator for that network. For more complicated networks where
the NAT translator is some device other than the client's default
gateway, this protocol is not appropriate.
Expires 7th December 2005 Cheshire, et al. [Page 3]
Internet Draft NAT Port Mapping Protocol 7th June 2005
3.1 Determining the Public Address
To determine the public address, the client behind the NAT sends the
following UDP payload to port 5351 of the configured gateway address:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers = 0 | OP = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This packet is sent by clients to determine the public IP address and
to determine whether or not the gateway supports this protocol. After
sending the request, the client then waits for the NAT gateway to
respond. If after 250ms, the gateway doesn't respond (and doesn't
generate "ICMP Port Unreachable" messages), the client SHOULD
re-issue its request. The client SHOULD repeat this process with the
interval between attempts doubling each time. If, after two minutes,
the client has not received any response, then it SHOULD conclude
that this gateway does not support NAT Port Mapping Protocol and MAY
log an error message indicating this fact.
A compatible NAT gateway MUST generate a response with the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers = 0 | OP = 128 + 0 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds Since Start of Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public IP Address (a.b.c.d) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This response indicates that the NAT gateway implements this version
of the protocol and returns the public IP address of the NAT gateway.
If the result code is non-zero, the value of Public IP Address is
undefined (MUST be set to zero on transmission, and MUST be ignored
on reception).
The NAT gateway MUST fill in the "Seconds Since Start of Epoch" field
with the time elapsed since its port mapping table was initialized on
startup or reset for any other reason (see Section 3.7 "Seconds Since
Start of Epoch").
If the client receives an "ICMP Port Unreachable" message from the
gateway, then it SHOULD conclude that this gateway does not support
NAT Port Mapping Protocol and MAY log an error message.
Expires 7th December 2005 Cheshire, et al. [Page 4]
Internet Draft NAT Port Mapping Protocol 7th June 2005
3.1.1 Announcing Address Changes
When the public IP address of the NAT changes, the NAT gateway MUST
send a gratuitous response to the link-local multicast address
224.0.0.1, port 5351 with the packet format above to notify clients
of the new public IP address. To compensate for packet loss, the NAT
gateway SHOULD multicast 10 address change notifications. The
interval between the first two notifications SHOULD be 250ms, and the
interval between each subsequent notification SHOULD double.
3.2 Creating a Mapping
To create a mapping, the client sends a UDP packet to port 5351 of
the gateway's private IP address with the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers = 0 | OP = x | Reserved (MUST be zero) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Private Port | Requested Public Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Port Mapping Lifetime in Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opcodes supported:
1 - Map UDP
2 - Map TCP
The reserved field MUST be set to zero on transmission and MUST be
ignored on reception. The RECOMMENDED Port Mapping Lifetime for
this protocol is 3600 seconds.
After sending the port mapping request, the client then waits for the
NAT gateway to respond. If after 250ms, the gateway doesn't respond,
the client SHOULD re-issue its request. The client SHOULD repeat
this process with the interval between attempts doubling each time.
If after two minutes, the client has not received any response, it
SHOULD log an error message and give up.
Expires 7th December 2005 Cheshire, et al. [Page 5]
Internet Draft NAT Port Mapping Protocol 7th June 2005
The NAT gateway responds with the following packet format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers = 0 | OP = 128 + x | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds Since Start of Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Private Port | Mapped Public Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port Mapping Lifetime in Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'x' in the OP field MUST match what the client requested. Some
NAT gateways are incapable of creating a UDP port mapping without
also creating a corresponding TCP port mapping, and vice versa, and
these gateways MUST NOT implement NAT Port Mapping Protocol until
this deficiency is fixed. A NAT gateway which implements this
protocol MUST be able to create TCP-only and UDP-only port mappings.
If a NAT gateway silently creates a pair of mappings for a client
that only requested one mapping, then it may expose that client to
receiving inbound UDP packets or inbound TCP connection requests that
it did not ask for and does not want.
While a NAT gateway MUST NOT automatically create mappings for TCP
when the client requests UDP, and vice versa, the NAT gateway SHOULD
reserve the companion port so the same client can choose to map it in
the future. For example, if a client requests to map TCP port 80, as
long as the client maintains the lease for that TCP port mapping,
another client with a different IP address SHOULD NOT be able to
successfully acquire the mapping for UDP port 80.
The client normally requests the public port matching the private
port. If that public port is not available, the NAT gateway MUST
return a public port that is available or return an error code if no
ports are available. If the client has no preference about what
public port is assigned, then it should set the requested public port
to zero.
The source address of the packet MUST be used for the private address
in the mapping. This protocol is not intended to facilitate one
device behind a NAT creating mappings for other devices. If there
are legacy devices that require inbound mappings, permanent mappings
can be created manually by the administrator, just as they are today.
Expires 7th December 2005 Cheshire, et al. [Page 6]
Internet Draft NAT Port Mapping Protocol 7th June 2005
If a mapping already exists for a given private port on a given local
client, and that client requests another mapping for the same private
port, possibly requesting a different public port, then the mapping
request should succeed, returning the already-assigned public port.
This is necessary to handle the case where a client requests a
mapping with requested public port X, and is granted a mapping with
actual public port Y, but the acknowledgement packet gets lost.
When the client retransmits its mapping request, it should get back
the same positive acknowledgement as was sent the first time.
The NAT gateway SHOULD NOT accept mapping requests destined to the
NAT gateway's public IP address or received on its public network
interface. Only packets received on the private interface(s) with a
destination address matching the private address(es) of the NAT
gateway should be allowed.
The NAT gateway MUST fill in the "Seconds Since Start of Epoch" field
with the time elapsed since its port mapping table was initialized on
startup or reset for any other reason (see Section 3.7 "Seconds Since
Start of Epoch").
The Port Mapping Lifetime is an unsigned integer in seconds. The NAT
gateway MAY reduce the lifetime from what the client requested. The
NAT gateway SHOULD NOT offer a lease lifetime greater than that
requested by the client.
The client SHOULD begin trying to renew the mapping halfway to expiry
time, like DHCP. The renewal packet should look exactly the same as
a request packet, except that the client SHOULD set the requested
public port to what the router previously mapped. If the router
crashes or is rebooted, this helps the router recover its mapping
state.
3.3 Destroying a Mapping
A mapping may be destroyed in a variety of ways. If a client fails
to renew a mapping, then when its lifetime expires the mapping MUST
be automatically deleted. In the common case where the gateway
device is a combined DHCP server and NAT gateway, when a client's
DHCP address lease expires, the gateway device MAY automatically
delete any mappings belonging to that client. Otherwise a new client
being assigned the same IP address could receive unexpected inbound
UDP packets or inbound TCP connection requests that it did not ask
for and does not want.
Expires 7th December 2005 Cheshire, et al. [Page 7]
Internet Draft NAT Port Mapping Protocol 7th June 2005
A client MAY also send an explicit packet to request deletion of a
mapping that is no longer needed. A client requests explicit
deletion of a mapping by sending a message to the NAT gateway
requesting the mapping, with the Requested Lifetime in Seconds set
to 0. The requested public port MUST be set to zero by the client
on sending, and MUST be ignored by the gateway on reception.
When a mapping is destroyed successfully as a result of the client
explicitly requesting the deletion, the NAT gateway MUST send a
response packet which is formatted as defined in section 3.2. The
response MUST contain a result code of 0, the private port as
indicated in the deletion request, a public port of 0, and a lifetime
of 0. The NAT gateway MUST respond to a request to destroy a mapping
that does not exist as if the request were successful. This is
because of the case where the acknowledgement is lost, and the client
retransmits its request to delete the mapping. In this case the
second request to delete the mapping MUST return the same response
packet as the first request.
If the deletion request was unsuccessful, the response MUST contain a
non-zero result code and the requested mapping; the lifetime is
undefined (MUST be set to zero on transmission, and MUST be ignored
on reception). If the client attempts to delete a port mapping which
was manually assigned by some kind of configuration tool, the NAT
gateway MUST respond with a 'Not Authorized' error, result code 2.
When a mapping is destroyed as a result of its lifetime expiring or
for any other reason, if the NAT gateway's internal state indicates
that there are still active TCP connections traversing that now-
defunct mapping, then the NAT gateway SHOULD send appropriately-
constructed TCP RST (reset) packets both to the local client and to
the remote peer on the Internet to terminate that TCP connection.
A client can request the explicit deletion of all its UDP or TCP
mappings by sending the same deletion request to the NAT gateway with
public port, private port, and lifetime set to 0. A client MAY
choose to do this when it first acquires a new IP address in order to
protect itself from port mappings that were performed by a previous
owner of the IP address. After receiving such a deletion request,
the gateway MUST delete all its UDP or TCP port mappings (depending
on the opcode). The gateway responds to such a deletion request with
a response as described above, with the private port set to zero. If
the gateway is unable to delete a port mapping, for example, because
the mapping was manually configured by the administrator, the gateway
MUST still delete as many port mappings as possible, but respond with
a non-zero result code. The exact result code to return depends on
the cause of the failure. If the gateway is able to successfully
delete all port mappings as requested, it MUST respond with a result
code of 0.
Expires 7th December 2005 Cheshire, et al. [Page 8]
Internet Draft NAT Port Mapping Protocol 7th June 2005
3.4 Result Codes
Currently defined result codes:
0 - Success
1 - Unsupported Version
2 - Not Authorized/Refused
(e.g. box supports mapping, but user has turned feature off)
3 - Network Failure
(e.g. NAT box itself has not obtained a DHCP lease)
4 - Out of resources
(NAT box cannot create any more mappings at this time)
5 - Unsupported opcode
If the result code is non-zero, the format of the packet following
the result code may be truncated. For example, if the client sends a
request to the server with an opcode of 17 and the server does not
recognize that opcode, the server SHOULD respond with a message where
the opcode is 17 + 128 and the result code is 5 (opcode not
supported). Since the server does not understand the format of
opcode 17, it may not know what to place after the result code. In
some cases, relevant data may follow the opcode to identify the
operation that failed. For example, a client may request a mapping
but that mapping may fail due to resource exhaustion. The server
SHOULD respond with the result code to indicate resource exhaustion
(4) followed by the requested port mapping so the client may identify
which operation failed.
Clients MUST be able to properly handle result codes not defined in
this document. Undefined results codes MUST be treated as fatal
errors of the request.
3.5 Recreating Mappings On Reboot
The NAT gateway MAY store mappings in persistent storage so when it
is powered off or rebooted, it remembers the port mapping state of
the network. However, maintaining this state is not necessary. When
the NAT gateway powers on or clears its port mapping state as the
result of a configuration change, it MUST reset the epoch time and
re-announce its IP address as described in Section 3.2.1 "Announcing
Address Changes". This will signal to clients on the network that
they need to redo their mappings. When a client renews its port
mappings as the result of receiving such a packet, it MUST delay each
port mapping request by a random amount of time selected with uniform
random distribution in the range 0 to 3 seconds.
Expires 7th December 2005 Cheshire, et al. [Page 9]
Internet Draft NAT Port Mapping Protocol 7th June 2005
3.6 Seconds Since Start of Epoch
Every packet sent by the NAT gateway includes a "Seconds since start
of epoch" field (SSSOE). If the NAT gateway resets or loses the
state of its port mapping table, due to reboot, power failure, or any
other reason, it MUST reset its epoch time and begin counting SSSOE
from 0 again. Whenever a client receives any packet from the NAT
gateway, either gratuitously or in response to a client request, the
client computes its own conservative estimate of the expected SSSOE
value by taking the SSSOE value in the last packet it received from
the gateway and adding 7/8 (87.5%) of the time elapsed since that
packet was received. If the SSSOE in the newly received packet is
less than the client's conservative estimate, then the client
concludes that the NAT gateway has undergone a reboot or other loss
of port mapping state, and the client MUST immediately renew all its
active port mapping leases as described in Section 3.6 "Recreating
Mappings On Reboot".
4. UNSAF Considerations
The document "IAB Considerations for UNSAF Across NAT" [RFC 3424]
covers a number of issues when working with NATs. RFC 3424 outlines
some requirements for any document that attempts to work around
problems associated with NATs. This section addresses those
requirements.
4.1 Scope
This protocol addresses the needs of TCP and UDP transport peers that
are separated from the public internet by exactly one NAT. Such
peers must have access to some form of directory server for
registering the public IP address and port at which they can be
reached.
4.2 Transition Plan
Any client making use of this protocol SHOULD implement IPv6 support.
If a client supports IPv6 and is running on a device with a global
IPv6 address, that IPv6 address SHOULD be preferred to the IPv4
public address using this NAT mapping protocol. In case other
clients do not have IPv6 connectivity, both the IPv4 and IPv6
addresses SHOULD be registered with whatever form of directory server
is used. Preference SHOULD be given to IPv6 addresses when
available. By implementing support for IPv6 and using this protocol
for IPv4, vendors can ship products today that will work under both
scenarios. As IPv6 is more widely deployed, clients of this protocol
following these recommendations will transparently make use of IPv6.
Expires 7th December 2005 Cheshire, et al. [Page 10]
Internet Draft NAT Port Mapping Protocol 7th June 2005
4.3 Failure Cases
Aside from NATs that do not implement this protocol, there are a
number of situations where this protocol may not work.
4.3.1 NAT Behind NAT
Some people's primary IP address, assigned by their ISP, may itself
be a NAT address. In addition, some people may have a public IP
address, but may then double NAT themselves, perhaps by choice or
perhaps by accident. Although it might be possible in principle for
one NAT gateway to recursively request a mapping from the next one,
this document does not advocate that and does not try to prescribe
how it would be done.
It would be a lot of work to implement nested NAT port mapping
correctly, and there are a number of reasons why the end result might
not be as useful as we might hope. Consider the case of an ISP that
offers each of its customers only a single NAT address. This ISP
could instead have chosen to provide each customer with a single
public IP address, or, if the ISP insists on running NAT, it could
have chosen to allow each customer a reasonable number of addresses,
enough for each customer device to have its own NAT address directly
from the ISP. If instead this ISP chooses to allow each customer
just one and only one NAT address, forcing said customer to run
nested NAT in order to use more than one device, it seems unlikely
that such an ISP would be so obliging as to provide a NAT service
that supports NAT Port Mapping Protocol. Supposing that such an ISP
did wish to offer its customers NAT service with NAT-PMP so as to
give them the ability to receive inbound connections, this ISP could
easily choose to allow each client to request a reasonable number of
DHCP addresses from that gateway. Remember that Net 10 [RFC 1918]
allows for over 16 million addresses, so NAT addresses are not in any
way in short supply. A single NAT gateway with 16 million available
addresses is likely to run out of packet forwarding capacity before
it runs out of private addresses to hand out. In this way the ISP
could offer single-level NAT with NAT-PMP, obviating the need to
support nested NAT-PMP. In addition, an ISP that is motivated to
provide their customers with unhindered access to the Internet by
allowing incoming as well as outgoing connections has better ways to
offer this service. Such an ISP could offer its customers real
public IP addresses instead of NAT addresses, or could even choose to
offer its customers full IPv6 connectivity, where no mapping or
translation is required at all.
Expires 7th December 2005 Cheshire, et al. [Page 11]
Internet Draft NAT Port Mapping Protocol 7th June 2005
4.3.2 NATs with Multiple Public IP Addresses
If a NAT maps private addresses to multiple public addresses,
then it SHOULD pick one of those addresses as the one it will
support for inbound connections, and for the purposes of this
protocol it SHOULD act as if that address were its only address.
4.3.3 NATs and Routed Private Networks
In some cases, a large network may be subnetted. Some sites may
install a NAT gateway and subnet the private network. Such
subnetting breaks this protocol because the router address is not
necessarily the address of the device performing NAT.
Addressing this problem is not a high priority. Any site with the
resources to set up such a configuration should have the resources to
add manual mappings or attain a range of globally unique addresses.
Not all NATs will support this protocol. In the case where a client
is run behind a NAT that does not support this protocol, the software
relying on the functionality of this protocol may be unusable.
4.3.4 Communication Between Hosts Behind the Same NAT
NAT gateways supporting NAT-PMP should also implement "hairpin
translation". Hairpin translation means supporting communication
between two local clients being served by the same NAT gateway.
Suppose device A is listening on private address and port 10.0.0.2:80
for incoming connections. Using NAT-PMP, device A has obtained a
mapping to public address and port x.x.x.x:80, and has recorded this
public address and port in a public directory of some kind. For
example, it could have created a DNS SRV record containing this
information, and recorded it, using DNS Dynamic Update [RFC 3007], in
a publicly accessible DNS server. Suppose then that device B, behind
the same NAT gateway as device A, but unknowing or uncaring of this
fact, retrieves device A's DNS SRV record and attempts to open a TCP
connection to x.x.x.x:80. The outgoing packets addressed to this
public Internet address will be sent to the NAT gateway for
translation and forwarding. Having translated the source address and
port number on the outgoing packet, the NAT gateway needs to be smart
enough to recognize that the destination address is in fact itself,
and then feed this packet back into its packet reception engine, to
perform the destination port mapping lookup to translate and forward
this packet to device A at address and port 10.0.0.2:80.
Expires 7th December 2005 Cheshire, et al. [Page 12]
Internet Draft NAT Port Mapping Protocol 7th June 2005
4.3.5 Non UDP/TCP Transport Traffic
Any communication over transport protocols other than TCP and UDP
will not be served by this protocol. Examples are Generic Routing
Encapsulation (GRE), Authentication Header (AH) and Encapsulating
Security Payload (ESP).
4.4 Long Term Solution
As IPv6 is deployed, clients of this protocol supporting IPv6 will be
able to bypass this protocol and the NAT when communicating with
other IPv6 devices. In order to ensure this transition, any client
implementing this protocol SHOULD also implement IPv6 and use this
solution only when IPv6 is not available to both peers.
4.5 Existing Deployed NATs
Existing deployed NATs will not support this protocol. This protocol
will only work with NATs that are upgraded to support it.
5. Security Considerations
As discussed in section 3.2, only clients on the private side of the
NAT may create port mappings, and only on behalf of themselves. By
using IP address spoofing, it's possible for one client to delete the
port mappings of another client. It's also possible for one client
to create port mappings on behalf of another client. The best way to
deal with this vulnerability is to use IPSec [RFC 2401].
Since allowing incoming connections is often a policy decision, any
NAT gateway implementing this protocol SHOULD have an administration
mechanism to disable it.
Some people view the property that NATs block inbound connections as
a security benefit which is undermined by this protocol. The authors
of this document have a different point of view. In the days before
NAT, all hosts had unique public IP addresses, and had unhindered
ability to communicate with any other host on the Internet. When NAT
came along it broke this unhindered connectivity, relegating many
hosts to second-class status, unable to receive inbound connections.
This protocol goes some way to undo some of that damage. The purpose
of a NAT gateway should be to allow several hosts to share a single
address, not to simultaneously impede those host's ability to
communicate freely. Security is most properly provided by end-to-end
cryptographic security, and/or by explicit firewall functionality, as
appropriate. Blocking of certain connections should occur only as a
result of explicit and intentional firewall policy, not as an
accidental side-effect of some other technology.
Expires 7th December 2005 Cheshire, et al. [Page 13]
Internet Draft NAT Port Mapping Protocol 7th June 2005
6. Copyright Notice
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights. For the purposes of this document,
the term "BCP 78" refers exclusively to RFC 3978, "IETF Rights
in Contributions", published March 2005.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
7. IANA Considerations
No IANA services are required by this document.
8. Acknowledgments
The concepts described in this document have been explored, developed
and implemented with help from Bob Bradley, Josh Graessley, Rob
Newberry, Roger Pantos, John Saxton, and James Woodyatt.
9. References
[RFC 1918] Y. Rekhter et.al., "Address Allocation for Private
Internets", RFC 1918, February 1996.
[RFC 2119] RFC 2119 - Key words for use in RFCs to Indicate
Requirement Levels
[RFC 2131] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC 2401] Atkinson, R. and S. Kent, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RFC 3007] Wellington, B., "Simple Secure Domain Name System
(DNS) Dynamic Update", RFC 3007, November 2000.
Expires 7th December 2005 Cheshire, et al. [Page 14]
Internet Draft NAT Port Mapping Protocol 7th June 2005
[RFC 3022] RFC 3022 - Network Address Translator
[RFC 3424] RFC 3424 - IAB Considerations for UNilateral Self-Address
Fixing (UNSAF) Across Network Address Translation
[ETEAISD] J. Saltzer, D. Reed and D. Clark: "End-to-end arguments in
system design", ACM Trans. Comp. Sys., 2(4):277-88, Nov.
1984
10. Authors' Addresses
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc [at] stuartcheshire [dot] org
Marc Krochmal
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 4368
EMail: marc [at] apple [dot] com
Kiren Sekar
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 8051
EMail: kiren [at] apple [dot] com
Expires 7th December 2005 Cheshire, et al. [Page 15]
| PAFTECH AB 2003-2026 | 2026-04-23 01:10:55 |