RFC7857
Internet Engineering Task Force (IETF) R. Penno Request for Comments: 7857 Cisco BCP: 127 S. Perreault Updates: 4787, 5382, 5508 Jive Communications Category: Best Current Practice M. Boucadair, Ed. ISSN: 2070-1721 Orange
S. Sivakumar
Cisco
K. Naito
NTT
April 2016
Updates to Network Address Translation (NAT) Behavioral Requirements
Abstract
This document clarifies and updates several requirements of RFCs 4787, 5382, and 5508 based on operational and development experience. The focus of this document is Network Address Translation from IPv4 to IPv4 (NAT44).
This document updates RFCs 4787, 5382, and 5508.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7857.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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5. Endpoint-Independent Mapping (EIM) Protocol Independence . . 8 6. Endpoint-Independent Filtering (EIF) Protocol Independence . 8
Contents
- 1 Introduction
- 2 TCP Session Tracking
- 3 Port Overlapping Behavior
- 4 Address Pooling Paired (APP)
- 5 Endpoint-Independent Mapping (EIM) Protocol Independence
- 6 Endpoint-Independent Filtering (EIF) Protocol Independence
- 7 Endpoint-Independent Filtering (EIF) Mapping Refresh
- 8 Port Parity
- 9 Port Randomization
Introduction
RFC4787, RFC5382, and RFC5508 contributed to enhance Network Address Translation (NAT) interoperability and conformance. Operational experience gained through widespread deployment and evolution of NAT indicates that some areas of the original documents need further clarification or updates. This document provides such clarifications and updates.
Scope
The goal of this document is to clarify and update the set of requirements listed in RFC4787, RFC5382, and RFC5508. The document focuses exclusively on NAT44.
The scope of this document has been set so that it does not create new requirements beyond those specified in the documents cited above.
Requirements related to Carrier-Grade NAT (CGN) are defined in RFC6888.
Terminology
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 RFC2119.
The reader is assumed to be familiar with the terminology defined in RFC2663, RFC4787, RFC5382, and RFC5508.
In this document, the term "NAT" refers to both "Basic NAT" and "Network Address/Port Translator (NAPT)" (see Section 3 of RFC4787). As a reminder, 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 addresses and transport identifiers (such as a TCP/UDP port or ICMP query ID); refer to Section 2 of RFC3022.
TCP Session Tracking
RFC5382 specifies TCP timers associated with various connection states but does not specify the TCP state machine a NAT44 should follow as a basis to apply such timers.
Update: The TCP state machine depicted in Figure 1, adapted from
RFC6146, SHOULD be implemented by a NAT for TCP session tracking purposes.
+----------------------------+
| |
V |
+------+ Client |
|CLOSED|-----SYN------+ |
+------+ | |
^ | |
|TCP_TRANS T.O. | |
| V |
+-------+ +-------+ |
| TRANS | | INIT | |
+-------+ +-------+ |
| ^ | |
data pkt | | |
| Server/Client RST | |
| TCP_EST T.O. | |
V | Server SYN |
+--------------+ | |
| ESTABLISHED |<---------+ |
+--------------+ |
| | |
Client FIN Server FIN |
| | |
V V |
+---------+ +----------+ |
| C FIN | | S FIN | |
| RCV | | RCV | |
+---------+ +----------+ |
| | |
Server FIN Client FIN TCP_TRANS
| | T.O.
V V |
+----------------------+ |
| C FIN + S FIN RCV |-----------------+
+----------------------+
Legend:
* Messages sent or received from the server are
prefixed with "Server".
* Messages sent or received from the client are
prefixed with "Client".
* "C" means "Client-side".
* "S" means "Server-side".
* TCP_EST T.O. refers to the established connection
idle-timeout as defined in RFC5382.
* TCP_TRANS T.O. refers to the transitory connection
idle-timeout as defined in RFC5382.
Figure 1: Simplified Version of the TCP State Machine
TCP Transitory Connection Idle-Timeout
The transitory connection idle-timeout is defined as the minimum time a TCP connection in the partially open or closing phases must remain idle before the NAT considers the associated session a candidate for removal (REQ-5 of RFC5382). However, RFC5382 does not clearly state whether these can be configured separately.
Clarification: This document clarifies that a NAT SHOULD provide
different configurable parameters for configuring the open and closing idle timeouts.
To accommodate deployments that consider a partially open timeout of 4 minutes as being excessive from a security standpoint, a NAT MAY allow the configured timeout to be less than 4 minutes. However, a minimum default transitory connection idle-timeout of 4 minutes is RECOMMENDED.
TCP RST
RFC5382 leaves the handling of TCP RST packets unspecified.
Update: This document adopts a similar default behavior as in
RFC6146. Concretely, when the NAT receives a TCP RST matching an existing mapping, it MUST translate the packet according to the NAT mapping entry. Moreover, the NAT SHOULD wait for 4 minutes before deleting the session and removing any state associated with it if no packets are received during that 4-minute timeout.
Notes:
* Admittedly, the NAT has to verify whether received TCP RST
packets belong to a connection. This verification check is
required to avoid off-path attacks.
* If the NAT immediately removes the NAT mapping upon receipt of
a TCP RST message, stale connections may be maintained by
endpoints if the first RST message is lost between the NAT and
the recipient.
Port Overlapping Behavior
REQ-1 from RFC4787 and REQ-1 from RFC5382 specify a specific port overlapping behavior; that is, the external IP address and port can be reused for connections originating from the same internal source IP address and port irrespective of the destination. This is known as Endpoint-Independent Mapping (EIM).
Update: This document clarifies that this port overlapping behavior
may be extended to connections originating from different internal source IP addresses and ports as long as their destinations are different.
The following mechanism MAY be implemented by a NAT:
If destination addresses and ports are different for outgoing
connections started by local clients, a NAT MAY assign the same
external port as the source ports for the connections. The
port overlapping mechanism manages mappings between external
packets and internal packets by looking at and storing their
5-tuple (protocol, source address, source port, destination
address, and destination port).
This enables concurrent use of a single NAT external port for multiple transport sessions, which allows a NAT to successfully process packets in a network that has a limited number of IP addresses (e.g., deployment with a high address space multiplicative factor (refer to Appendix B of RFC6269)).
Address Pooling Paired (APP)
The "IP address pooling" behavior of "Paired" (APP) was recommended in REQ-2 from RFC4787, but the behavior when an external IPv4 runs out of ports was left undefined.
Clarification: This document clarifies that if APP is enabled, new
sessions from a host that already has a mapping associated with an external IP that ran out of ports SHOULD be dropped. A configuration parameter MAY be provided to allow a NAT to start using ports from another external IP address when the one that anchored the APP mapping ran out of ports. Tweaking this configuration parameter is a trade-off between service continuity and APP strict enforcement. Note, this behavior is sometimes referred to as "soft-APP".
As a reminder, the recommendation for the particular case of a CGN is that an implementation must use the same external IP address mapping for all sessions associated with the same internal IP address, be they TCP, UDP, ICMP, something else, or a mix of different protocols RFC6888.
Update: This behavior SHOULD apply also for TCP.
Endpoint-Independent Mapping (EIM) Protocol Independence
REQ-1 from RFC4787 and REQ-1 from RFC5382 do not specify whether EIM are protocol dependent or protocol independent. For example, if an outbound TCP SYN creates a mapping, it is left undefined whether outbound UDP packets can reuse such mapping.
Update: EIM mappings SHOULD be protocol dependent. A configuration
parameter MAY be provided to allow protocols that multiplex TCP and UDP over the same source IP address and port number to use a single mapping. The default value of this configuration parameter MUST be protocol-dependent EIM.
This update is consistent with the stateful Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers (NAT64) RFC6146 that clearly specifies three binding information bases (TCP, UDP, and ICMP).
Endpoint-Independent Filtering (EIF) Protocol Independence
REQ-8 from RFC4787 and REQ-3 from RFC5382 do not specify whether mappings with Endpoint-Independent Filtering (EIF) are protocol independent or protocol dependent. For example, if an outbound TCP SYN creates a mapping, it is left undefined whether inbound UDP packets matching that mapping should be accepted or rejected.
Update: EIF filtering SHOULD be protocol dependent. A configuration
parameter MAY be provided to make it protocol independent. The default value of this configuration parameter MUST be protocol- dependent EIF.
This behavior is aligned with the update in Section 5.
Applications that can be transported over a variety of transport protocols and/or support transport fallback schemes won't experience connectivity failures if the NAT is configured with protocol-independent EIM and protocol-independent EIF.
Endpoint-Independent Filtering (EIF) Mapping Refresh
The NAT mapping Refresh direction may have a "NAT Inbound refresh behavior" of "True" according to REQ-6 from RFC4787, but RFC4787 does not clarify how this behavior applies to EIF mappings. The issue in question is whether inbound packets that match an EIF mapping but do not create a new session due to a security policy should refresh the mapping timer.
Clarification: This document clarifies that even when a NAT has an
inbound refresh behavior set to "TRUE", such packets SHOULD NOT refresh the mapping. Otherwise, a simple attack of a packet every two minutes can keep the mapping indefinitely.
Update: This behavior SHOULD apply also for TCP.
Outbound Mapping Refresh and Error Packets
Update: In the case of NAT outbound refresh behavior, ICMP Errors or
TCP RST outbound packets sent as a response to inbound packets SHOULD NOT refresh the mapping. Other packets that indicate the host is not interested in receiving packets MAY be configurable to also not refresh state, such as a Session Traversal Utilities for NAT (STUN) error response RFC5389 or IKE INVALID_SYNTAX RFC7296.
Port Parity
Update: A NAT MAY disable port parity preservation for all dynamic
mappings. Nevertheless, A NAT SHOULD support means to explicitly request to preserve port parity (e.g., RFC7753).
Note: According to RFC6887, dynamic mappings are said to be dynamic in the sense that they are created on demand, either implicitly or explicitly:
1. Implicit dynamic mappings refer to mappings that are created
as a side effect of traffic such as an outgoing TCP SYN or
outgoing UDP packet. Implicit dynamic mappings usually have a
finite lifetime, though this lifetime is generally not known
to the client using them.
2. Explicit dynamic mappings refer to mappings that are created
as a result, for example, of explicit Port Control Protocol
(PCP) MAP and PEER requests. Explicit dynamic mappings have a
finite lifetime, and this lifetime is communicated to the
client.
Port Randomization
Update: A NAT SHOULD follow the recommendations specified in
Section 4 of RFC6056, especially:
A NAPT that does not implement port preservation RFC4787 RFC5382 SHOULD obfuscate selection of the ephemeral port of a packet when it is changed during translation of that packet.
A NAPT that does implement port preservation SHOULD obfuscate
the ephemeral port of a packet only if the port must be changed
as a result of the port being already in use for some other
session.
A NAPT that performs parity preservation and that must change
the ephemeral port during translation of a packet SHOULD
obfuscate the ephemeral ports. The algorithms described in
this document could be easily adapted such that the parity is
preserved (i.e., force the lowest order bit of the resulting
port number to 0 or 1 according to whether even or odd parity
is desired).
10. IP Identification (IP ID)
Update: A NAT SHOULD handle the Identification field of translated
IPv4 packets as specified in Section 5.3.1 of RFC6864.
11. ICMP Query Mappings Timeout
Section 3.1 of RFC5508 specifies that ICMP Query mappings are to be maintained by a NAT. However, the specification doesn't discuss Query mapping timeout values. Section 3.2 of RFC5508 only discusses ICMP Query session timeouts.
Update: ICMP Query mappings MAY be deleted once the last session
using the mapping is deleted.
12. Hairpinning Support for ICMP Packets
REQ-7 from RFC5508 specifies that a NAT enforcing Basic NAT must support traversal of hairpinned ICMP Query sessions.
Clarification: This implicitly means that address mappings from
external address to internal address (similar to Endpoint- Independent Filters) must be maintained to allow inbound ICMP Query sessions. If an ICMP Query is received on an external address, a NAT can then translate to an internal IP.
REQ-7 from RFC5508 specifies that all NATs must support the traversal of hairpinned ICMP Error messages.
Clarification: This behavior requires a NAT to maintain address
mappings from external IP address to internal IP address in addition to the ICMP Query mappings described in Section 3.1 of RFC5508.
13. Security Considerations
NAT behavioral considerations are discussed in RFC4787, RFC5382, and RFC5508.
Because some of the clarifications and updates (e.g., Section 2) are inspired from NAT64, the security considerations discussed in Section 5 of RFC6146 apply also for this specification.
The update in Section 3 allows for an optimized NAT resource usage. In order to avoid service disruption, the NAT must not invoke this functionality unless the packets are to be sent to distinct destination addresses.
Some of the updates (e.g., Sections 7, 9, and 11) allow for increased security compared to RFC4787, RFC5382, and RFC5508. Particularly,
o the updates in Sections 7 and 11 prevent an illegitimate node to
maintain mappings activated in the NAT while these mappings should be cleared, and
o port randomization (Section 9) complicates tracking hosts located
behind a NAT.
Sections 4 and 12 propose updates that increase the serviceability of a host located behind a NAT. These updates do not introduce any additional security concerns to RFC4787, RFC5382, and RFC5508.
The updates in Sections 5 and 6 allow for a better NAT transparency from an application standpoint. Hosts that require a restricted filtering behavior should enable specific policies (e.g., Access Control List (ACL)) either locally or by soliciting a dedicated security device (e.g., firewall). How a host updates its filtering policies is out of scope of this document.
The update in Section 8 induces security concerns that are specific to the protocol used to interact with the NAT. For example, if PCP is used to explicitly request parity preservation for a given mapping, the security considerations discussed in RFC6887 should be taken into account.
The update in Section 10 may have undesired effects on the performance of the NAT in environments in which fragmentation is massively experienced. Such an issue may be used as an attack vector against NATs.
14. References
14.1. Normative References
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
RFC4787 Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.
RFC5382 Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, DOI 10.17487/RFC5382, October 2008, <http://www.rfc-editor.org/info/rfc5382>.
RFC5508 Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508, DOI 10.17487/RFC5508, April 2009, <http://www.rfc-editor.org/info/rfc5508>.
RFC6056 Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011, <http://www.rfc-editor.org/info/rfc6056>.
RFC6146 Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <http://www.rfc-editor.org/info/rfc6146>.
RFC6864 Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013, <http://www.rfc-editor.org/info/rfc6864>.
14.2. Informative References
RFC2663 Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, DOI 10.17487/RFC2663, August 1999,
<http://www.rfc-editor.org/info/rfc2663>.
RFC3022 Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, DOI 10.17487/RFC3022, January 2001, <http://www.rfc-editor.org/info/rfc3022>.
RFC5389 Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389, DOI 10.17487/RFC5389, October 2008, <http://www.rfc-editor.org/info/rfc5389>.
RFC6269 Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269, DOI 10.17487/RFC6269, June 2011, <http://www.rfc-editor.org/info/rfc6269>.
RFC6887 Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, DOI 10.17487/RFC6887, April 2013, <http://www.rfc-editor.org/info/rfc6887>.
RFC6888 Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
April 2013, <http://www.rfc-editor.org/info/rfc6888>.
RFC7296 Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <http://www.rfc-editor.org/info/rfc7296>.
RFC7753 Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
and S. Perreault, "Port Control Protocol (PCP) Extension
for Port-Set Allocation", RFC 7753, DOI 10.17487/RFC7753,
February 2016, <http://www.rfc-editor.org/info/rfc7753>.
Acknowledgements
Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan, Lars Eggert, Gorry Fairhurst, Brandon Williams, and David Black for their review and discussion.
Many thanks to Ben Laurie for the SecDir review and Dan Romascanu for the Gen-ART review.
Dan Wing proposed some text for the configurable errors in Section 7.1.
Contributors
The following individual contributed text to the document:
Sarat Kamiset Insieme Networks United States
Authors' Addresses
Reinaldo Penno Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 United States
Email: [email protected]
Simon Perreault Jive Communications Canada
Email: [email protected]
Mohamed Boucadair (editor) Orange Rennes 35000 France
Email: [email protected]
Senthil Sivakumar Cisco Systems, Inc. United States
Email: [email protected]
Kengo Naito NTT Tokyo Japan
Email: [email protected]
