BEHAVE WG P. Srisuresh
Internet Draft Consultant
Expires: April 4, 2007 B. Ford
M.I.T.
S. Sivakumar
Cisco Systems
S. Guha
Cornell U.
October 4, 2006
NAT Behavioral Requirements for ICMP protocol
<draft-ietf-behave-nat-icmp-01.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.
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 identifies the behavioral properties required of the
Network Address Translator (NAT) devices in conjunction with the
ICMP protocol. The objective of this memo is to make NAT devices
more predictable and compatible with diverse application protocols
that traverse the devices. Companion documents provide behavioral
recommendations specific to TCP, UDP and other protocols.
Srisuresh, et. al. [Page 1]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
Table of Contents
1. Introduction and Scope .......................................
2. Terminology ..................................................
3. ICMP Query Handling ..........................................
3.1. ICMP Query Mapping .....,,...............................
3.2. ICMP Query Session Timeouts .............................
4. ICMP Error Forwarding ........................................
4.1. ICMP Error Payload Validation .....,,....................
4.2. ICMP Error Packet Translation ...........................
4.2.1. ICMP Error Packet Received from External Realm ....
4.2.2. ICMP Error Packet Received from Private Realm .....
4.3. NAT Sessions Pertaining to ICMP Error Payload ...........
5. Hairpinning Support for ICMP packets .........................
6. Rejection of Outbound Flows Disallowed by NAT ................
7. Conformance to RFC 1812 ......................................
7.1. IP packet fragmentation .................................
7.1.1. Generating "Packet too Big" ICMP error Message ....
7.1.2. Forwarding "Packet too big" ICMP Error Message ....
7.2. Generating "Time Exceeded" Error Message ................
7.3. RFC 1812 Conformance Requirements summary ...............
8. Summary of Requirements ......................................
9. Security Considerations ......................................
10. IANA Considerations ..........................................
11. Acknowledgements .............................................
1. Introduction and Scope
As pointed out in RFC 3424 [UNSAF], NAT implementations vary
widely in terms of how they handle different traffic. The purpose
of this document is to define a specific set of requirements for NAT
behavior with regard to ICMP messages. The objective is to reduce
the unpredictability and brittleness the NAT devices (NATs)
introduce. This document is an adjunct to [BEH-UDP] and [BEH-TCP],
and other protocol specific behave document(s) in the future which
define requirements for NATs when handling protocol specific traffic.
The requirements of this specification apply to Traditional NATs as
described in [NAT-TRAD]. Traditional NAT has two variations, namely,
Basic NAT and Network Address Port Translator (NAPT). Of these, NAPT
is by far the most commonly deployed NAT device. NAPT allows
multiple private hosts to share a single public IP address
simultaneously.
This document only covers the ICMP aspects of NAT traversal.
Traditional NAT inherently mandates a certain level of firewall like
Srisuresh, et. al. [Page 2]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
functionality. However, firewall functionality in general or any
other middlebox functionality is out of the scope of this
specification.
This document focuses strictly on the behavior of the NAT device,
and not on the behavior of applications that traverse NATs.
A separate document [BEH-APP] provides recommendations for
application designers on how to make applications work robustly over
NATs that follow the behavioral requirements specified here and the
adjunct Behave documents.
Even though ICMP is a transport protocol on top of IP, ICMP message
processing is often considered an integral of IP and is independent
of other transport protocols. As such, many of the ICMP behavioral
requirements discussed in this document apply to all IP protocols.
In case a requirement in this document conflicts with protocol
specific behave requirement(s), protocol specific behave documents
will take precedence. Note, the authors are not aware of any
conflicts between this and any other IETF document at the time of
this writing.
Section 2 describes the terminology used throughout the document.
Sections 3, 4 and 5 discuss the behavioral requirements for a NAT
device when processing ICMP packets. Section 3 summarizes all the
requirements in one place.
2. Terminology
Definitions for majority of the NAT terms used throughout the
document may be found in [NAT-TERM] and [BEH-UDP]. The term
"NAT Session" is adapted from [NAT-MIB] and denotes the entity
within a NAT device that represents the translation glue for a
session traversing the NAT device.
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 RFC 2119 [RFC2119].
ICMP messages are broadly grouped into two classes, namely "ICMP
Query" messages and "ICMP Error" messages in section 3.2.2 of
[RFC1122]. The following explanations further illustrate these
ICMP message classes.
ICMP Query Messages - All ICMP query messages are characterized
by an Identifier field in the ICMP header. The Identifier field used
by the ICMP Query messages is also referred as "Query Identifier" or
"Query Id", for short throughout the document. A Query Id is used by
Srisuresh, et. al. [Page 3]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
query senders and responders as the equivalent of a TCP/UDP port to
identify an ICMP Query session.
ICMP Error Messages - All ICMP error messages are characterized by
the fact that they embed an IP packet (which includes IP header plus
a minimum of 64 bits of transport data) that triggered the ICMP
error message. Unlike ICMP Query messages, ICMP error messages do
not have a Query Id in the ICMP header.
3. ICMP Query Handling
This section lists the behavioral requirements for a NAT device
when processing ICMP Query packets. The following sub sections
discuss requirements specific to ICMP Query handling in detail.
3.1. ICMP Query Mapping
A NAT device MUST permit ICMP query based applications to be
initiated from private hosts to the external hosts. Specifically,
a NAT device must transparently forward any ICMP Query packets
initiated from the nodes behind NAT devices and the responses to
these Query packets in the opposite direction. As specified in
[NAT-TRAD], this requires translating the IP header. A NAPT device
further translates the ICMP Query Id and the associated checksum in
the ICMP header prior to forwarding.
The mapping of ICMP Query identifier within the NAT device SHOULD
be external endpoint independent. Say, an internal host A sent an
ICMP query out to an external host B using Query Id X. And, say,
the NAT assigned this an external mapping of Query id X' on the
NAT's public address. If host A reused the Query Id X to send ICMP
queries to the same or different external host, the NAT device
SHOULD reuse the same Query Id mapping (i.e., map private host's
Query id X to Query id X' on NAT's public IP address) instead of
assigning a different mapping. This is similar to the "endpoint
independent mapping" requirement specified in the TCP and UDP
behave requirements [BEH-TCP, BEH-UDP].
Below is justification for making the endpoint independent mapping
for ICMP query IDs a SHOULD [RFC2119] requirement. ICMP Ping and
ICMP traceroute ([RFC1147]) are two most commonly known legacy
applications built on top of ICMP query messages. Neither of these
applications require the ICMP Query Id to be retained across
different sessions with external hosts. But, that may not be case
with future applications. In the future, when an endhost
application reuses the same Query identifier in sessions with
different target hosts, the endhost application might require that
Srisuresh, et. al. [Page 4]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
the endpoint identity (i.e., the tuple of IP address and Query
Identifier) appears the same across all its target hosts. Such a
requirement will be valid to make in an IP network without NAT
devices. When NAT devices enforce endpoint mapping that is external
host independent, the above assumption will be valid to make even in
the world with NAT devices. Given the dichotomy between legacy
applications not requiring endpoint independent mapping and future
applications that might require it, the requirement level is kept
at SHOULD [RFC2119].
REQ-1: A NAT device MUST permit ICMP query based applications to be
initiated from private hosts to the external hosts.
a) NAT mapping of ICMP Query identifiers SHOULD be external host
independent.
3.2. ICMP Query Session Timeouts
When an application initiates an ICMP query transiting a NAT device,
the NAT associates a timer to the ICMP query NAT session. This is
so the ICMP query NAT session is freed up if the NAT session remains
idle for longer than the timeout set by the timer. Query response
times can vary. Ideally speaking, the timeout should be set to
round trip time (RTT). RTT is 2x MSL (Maximum Segment Lifetime).
As per RFC 793, MSL is the maximum amount of time a TCP segment can
exist in a network before being delivered to the recipient. This is
the maximum duration of time you wait for a packet to reach the
target node before declaring that the packet will no longer be
delivered. The recommended value for MSL is 120 seconds, even though
several implementations set this to 60 seconds or 30 seconds. When
MSL is 120 seconds, the RTT (2x MSL) would be 240 seconds.
In practice, ICMP Ping and ICMP traceroute ([RFC1147]), the two most
commonly known legacy applications built on top of ICMP query
messages take less than 10 seconds when run to successful
completion.
Setting the ICMP NAT session timeout to a very large duration (say,
240 seconds) could potentially tie up precious NAT resources such as
query mappings and NAT Sessions for the whole duration. On the other
hand, setting the timeout very low can result in premature freeing
of NAT resources and applications failing to complete. The ICMP Query
session timeout needs to be a balance between the two extremes.
60 seconds timemout is a balance between the two extremes. ICMP
query session timer MUST not expire in less than 60 seconds. We
RECOMMEND however that the administrator(s) be allowed to configure
the timer.
Srisuresh, et. al. [Page 5]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
REQ-2: An ICMP Query session timer MUST NOT expire in less than 60
seconds.
a) It is RECOMMENDED that the ICMP Query session timer be made
configurable.
4. ICMP Error Forwarding
Applications depend on ICMP error messages from end hosts and
intermediate devices being forwarded reliably by the NAT devices.
A NAT device MUST conform to a number of requirements to ensure
reliable forwarding. The following sub-sections discuss the
requirements in detail.
4.1. ICMP Error Payload Validation
Appendix C of [ICMP-ATK] points out that newer revision end host
TCP stacks do not accept ICMP error messages with a mismatched
IP or TCP checksum in the embedded payload. NAT devices should
ensure that the embedded payload is not corrupted. Only after the
embedded payload is validated, should the NAT proceed to consider
the error packet for forwarding.
If the IP checksum of the embedded payload does not validate, the
NAT device SHOULD simply drop the error packet. [ICMP] stipulates
that an ICMP error message should embed IP header and a minimum of
64 bits of the IP payload. Section 4.3.2.3 of [RFC1812] further
recommends that an ICMP error originator SHOULD include as much of
the original packet as possible in the payload without the length of
the ICMP datagram exceeding 576 bytes. If the embedded packet is a
complete IP packet, including the entire transport segment, and the
transport protocol of the embedded packet requires the recipient to
validate the checksum, the NAT device SHOULD validate the transport
checksum. If the transport checksum fails, the NAT device SHOULD
drop the error packet. If the transport protocol is UDP and the
checksum is set to zero, the UDP protocol does not require the
recipient to validate the UDP checksum.
When the IP packet embedded within the ICMP error message includes
IP options, the NAT device must not assume that the transport header
of the embedded packet is at a fixed offset (as would be the case
when there are no IP options associated with the packet) from the
start of the embedded packet. Specifically, the NAT device MUST
index past all IP options when locating the start of transport
header for the embedded packet.
REQ-3: When an ICMP error packet is received, the NAT device
SHOULD do the following.
a) If the IP checksum of the embedded packet fails to validate, drop
Srisuresh, et. al. [Page 6]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
the error packet; and
b) If the embedded packet includes IP options, traverse past the
IP options to locate the start of transport header for the embedded
packet.
c) If the embedded packet contains the entire transport segment,
and the transport protocol of the embedded packet requires the
recipient to validate the transport checksum, and the checksum
fails to validate, drop the error packet.
4.2. ICMP Error Packet Translation
Section 4.3 of the RFC 3022 describes the various fields within
an ICMP error message a NAT device translates. In this section,
we describe the requirements a NAT device must conform to while
doing the translations.
Consider the following scenario in figure 1. Say, NAT-xy is a NAT
device connecting hosts in private and external networks.
Router-x and Host-x are in the external network. Router-y and
Host-y are in the private network. The subnets in the external
network are routable from the private as well as the external
domains. Whereas, the subnets in the private network are only
routable within the private domain. When Host-y initiated a session
to Host-x, let us say that the NAT device assigned a mapping of
Host-y' to associate with Host-y in the external network. The
following subsections describe the processing of ICMP error
messages on the NAT device(NAT-xy), when the NAT device receives an
ICMP error message in response to a packet pertaining to this
session.
Srisuresh, et. al. [Page 7]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
Host-x
|
---------------+-------------------
|
+-------------+
| Router-x |
+-------------+
External Network |
--------------------+--------+-------------------
| ^
| | (Host-y', Host-x)
| |
+-------------+
| NAT-xy |
+-------------+
|
Private Network |
----------------+------------+----------------
|
+-------------+
| Router-y |
+-------------+
|
----------------+-------+--------
| ^
| | (Host-y, Host-x)
| |
Host-y
Figure 1. A Session from a private host traversing a NAT device.
4.2.1. ICMP Error Packet Received from External Realm
Say, a packet from Host-y to Host-x triggered an ICMP error message
from one of Router-x or Host-x (both of which are in the external
domain). Such an ICMP error packet will have one of Router-x or
Host-x as the source IP address and Host-y' as the destination IP
address as described in figure 2 below.
Srisuresh, et. al. [Page 8]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
Host-x
|
---------------+-------------------
|
+-------------+
| Router-x |
+-------------+
External Network |
--------------------+--------+-------------------
|
| | ICMP Error Packet to Host-y'
| v
+-------------+
| NAT-xy |
+-------------+
|
Private Network |
----------------+------------+----------------
|
+-------------+
| Router-y |
+-------------+
|
----------------+-------+--------
|
Host-y
Figure 2. ICMP error Packet Received from External Network
When the NAT device receives the ICMP error packet, the NAT device
must use the packet embedded within the ICMP error message (i.e.,
the IP packet from Host-y to Host-x) to look up the NAT Session the
embedded packet belongs to. If the NAT device does not have an
active mapping for the embedded payload, the NAT SHOULD silently
drop the ICMP error packet. Otherwise, the NAT device SHOULD use
the matching NAT Session to translate the embedded payload. Note,
when the payload is modified, the ICMP checksums will also need
updating.
The NAT device SHOULD also use the matching NAT Session to translate
the destination IP address in the outer IP header. In the outer
header, the source IP address will remain unchanged because the
originator of the ICMP error message (Host-x or Router-x) is in
external domain and routable from the private domain.
REQ-4: If a NAT device receives an ICMP error packet from external
realm, and the NAT does not have an active mapping for the embedded
Srisuresh, et. al. [Page 9]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
payload, the NAT SHOULD silently drop the ICMP error packet. If the
the NAT has active mapping for the embedded payload, then the NAT
MUST do the following prior to forwarding the packet.
a) Revert the IP and transport headers of the embedded IP packet to
their original form, using the matching mapping; and
b) Leave the ICMP error type and code unchanged; and
c) Modify the destination IP address of the outer IP header to be
same as the source IP address of the embedded packet after
translation.
4.2.2. ICMP Error Packet Received from Private Realm
Now, say, a packet from Host-x to Host-y triggered an ICMP error
message from one of Router-y or Host-y (both of which are in the
private domain). Such an ICMP error packet will have one of
Router-y or Host-y as the source IP address and Host-x as the
destination IP address as specified in figure 3 below.
Host-x
|
---------------+-------------------
|
+-------------+
| Router-x |
+-------------+
External Network |
--------------------+--------+-------------------
|
|
|
+-------------+
| NAT-xy |
+-------------+
| ^
| | ICMP Error Packet to Host-x
Private Network |
----------------+------------+----------------
|
+-------------+
| Router-y |
+-------------+
|
----------------+-------+--------
|
Host-y
Figure 3. ICMP Error Packet Received from Private Network
Srisuresh, et. al. [Page 10]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
When the NAT device receives the ICMP error packet, the NAT device
must use the packet embedded within the ICMP error message (i.e.,
the IP packet from Host-x to Host-y) to look up the NAT Session the
embedded packet belongs to. If the NAT device does not have an
active mapping for the embedded payload, the NAT SHOULD silently
drop the ICMP error packet. Otherwise, the NAT device MUST use
the matching NAT Session to translate the embedded payload. Note,
when the payload is modified, the ICMP checksums will also need
updating.
In the outer header, the destination IP address will remain
unchanged, as the IP addresses for Host-x is already in the external
domain. If the ICMP error message is generated by Host-y, the NAT
a NAPT device must simply use the NAT Session to translate the
source IP address Host-y to Host-y'. However, if the ICMP error
message is originated by the intermediate node Router-y, and the
NAT device is a Basic NAT ([NAT-TRAD]), and it has active mapping
for the IP address that sent the ICMP error, the NAT device MUST
translate the source IP address of the ICMP error packet with the
public IP address in the mapping. Otherwise, the NAT device MUST
simply use its own IP address in the external domain to translate
the source IP address.
REQ-5: If a NAT device receives an ICMP error packet from private
realm, and the NAT does not have an active mapping for the embedded
payload, the NAT SHOULD silently drop the ICMP error packet. If the
the NAT has active mapping for the embedded payload, then the NAT
MUST do the following prior to forwarding the packet.
a) Revert the IP and transport headers of the embedded
IP packet to their original form, using the matching mapping; and
b) Leave the ICMP error type and code unchanged; and
c) If the NAT is a Basic NAT ([NAT-TRAD]), and the NAT has active
mapping for the IP address that sent the ICMP error, translate the
source IP address of the ICMP error packet with the public IP address
in the mapping. In all other cases, translate the source IP address
of the ICMP error packet with its own public IP address.
4.3. NAT Sessions Pertaining to ICMP Error Payload
While processing an ICMP error packet, a NAT device MUST NOT
refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet. This is in spite of the
fact that the NAT device uses the NAT Session to translate the
embedded payload. By not effecting the NAT Sessions, the NAT
device is able to retain them, even as someone spoofs ICMP error
Srisuresh, et. al. [Page 11]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
messages pertaining to the NAT Sessions.
REQ-6: While processing an ICMP error packet, a NAT device MUST NOT
refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet.
5. Hairpinning Support for ICMP packets
[BEH-UDP] and [BEH-TCP] mandate support for hairpinning for UDP and
TCP sessions respectively on NAT devices. A NAT device also needs to
support hairpinning for ICMP Query sessions as well. Specifically,
ICMP query hairpinning MUST be supported on Basic NATs. Say, for
example, individual private hosts register their NAT assigned
external IP address with a rendezvous server. Other hosts that wish
to initiate ICMP Query sessions to the registered hosts might do so
using the public address registered with the Rendezvous server. For
this reason, Basic NAT devices MUST support the traversal of
hairpinned ICMP query sessions.
Packets belonging to any of the hairpinned sessions could in turn
trigger ICMP error messages directed to the source of hairpinned
IP packets. Such hairpinned ICMP error messages will traverse the
NAT devices enroute. All NAT devices (i.e, Basic NAT as well as
NAPT devices) MUST support the traversal of hairpinned ICMP error
messages. Specifically, the NAT device must translate not only the
embedded hairpinned packet, but also the outer IP header that is
hairpinned.
A hairpinned ICMP error message is received from a node in private
network. As such, the ICMP error processing requirement specified
in Req-5 is applicable in its entirety in processing the ICMP
error message. In addition, the NAT device MUST translate the
destination IP address of the outer IP header to be same as the
source IP address of the embedded IP packet after the translation.
REQ-7: NAT devices enforcing Basic NAT ([NAT-TRAD]) MUST support the
traversal of hairpinned ICMP query sessions. All NAT devices (i.e,
Basic NAT as well as NAPT devices) MUST support the traversal of
hairpinned ICMP error messages.
a) When forwarding a hairpinned ICMP error message, the NAT device
MUST translate the destination IP address of the outer IP header to
be same as the source IP address of the embedded IP packet after
the translation.
6. Rejection of Outbound Flows Disallowed by NAT
Srisuresh, et. al. [Page 12]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
A NAT device typically permits all outbound sessions. However,
a NAT device may disallow some outbound sessions due to resource
constraints or administration considerations. For example, a NAT
device may not permit the first packet of a new outbound session,
if the NAT device is out of resources (out of addresses or TCP/UDP
ports or a NAT Session resource) to set up a state for the session,
or, the specific session is administratively restricted by the NAT
device.
When the first packet of an outbound flow is prohibited by a NAT
device due to resource constraints or administration considerations,
the NAT device SHOULD send ICMP destination unreachable message.
Section 5.2.7.1 of [RFC1812] recommends routers to use ICMP code 13
(Communication administratively prohibited) when they
administratively filter packets. As such, a NAT device MUST use
ICMP code 13 when generating an ICMP error message.
REQ-8: When a NAT device is unable to establish a NAT Session for a
new flow due to resource constraints or administrative restrictions,
the NAT device SHOULD send an ICMP destination unreachable message,
with a code of 13 (Communication administratively prohibited) to the
sender, and drop the original packet.
7. Conformance to RFC 1812
A NAT device is inherently an intermediate router that forwards
IP packets between private and public realms. As such, the NAT
device MUST conform to all the requirements of a router, as
specified in [RFC1812]. Section 5.2.7.1 of [RFC1812] states that
a router MUST also be able to generate ICMP Destination Unreachable
messages and SHOULD choose a response code that most closely matches
the reason the message is being generated.
Note, however, NAT devices also function as hosts on the Internet
and are bound by the conformance requirements in [RFC1122]. Protocol
specific Behave documents ([BEH-UDP], [BEH-TCP]) identify instances
where a NAT device should deviate from RFC 1122. As such, the host
behavior requirements of NAT devices specified in the protocol
specific behave drafts take precedence over RFC 1122.
The focus of this section is on conformance to router requirements.
The following sub sections identify specific instances where a NAT
device would be expected to conform to RFC 1812.
7.1. IP packet fragmentation
Many networking applications (which include TCP as well as UDP based
applications) depend on ICMP error messages from the network to
Srisuresh, et. al. [Page 13]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
perform end-to-end path MTU discovery [PMTU]. Once path MTU is
discovered, an application that chooses to avoid fragmentation may
do so by originating IP packets that fit within the maximum Path MTU
enroute and setting the DF (Don't Fragment) bit in the IP header, so
the intermediate nodes enroute do not fragment the IP packets. The
following sub-sections discuss the need for NAT devices to honor the
DF bit in the IP header and be able to generate "Packet too big"
ICMP error message when they cannot forward the IP packet without
fragmentation. Also discussed is the need to seamlessly forward
ICMP error messages generated by other intermediate devices.
7.1.1. Generating "Packet too Big" ICMP error Message
When a router is unable to forward a datagram because it exceeds
the MTU of the nexthop network and its Don't Fragment (DF) bit is
set, the router is required to return an ICMP Destination
Unreachable message to the source of the datagram, with the Code
indicating "fragmentation needed and DF set". Further, the router
MUST include the MTU of that nexthop network in the low order
16 bits of the ICMP header, as specified in [PMTU].
A NAT device MUST honor the DF bit in the IP header of the
packets transiting the device. If the DF bit is set and the
MTU on the forwarding interface of the NAT device is such that
the IP datagram cannot be forwarded without fragmentation, the
NAT device MUST issue a "packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet. The sender will resend after
taking the appropriate corrective action.
If the DF bit is not set and the MTU on the forwarding interface
of the NAT device mandates fragmentation, the NAT device must
simply send this fragmented, just as any router does [RFC1812].
7.1.2. Forwarding "Packet too big" ICMP Error Message
This is flip side of the argument for the above section. By
virtue of the address translation NAT performs, NAT may end
up being the recipient of "Packet too big" message.
When NAT device is the recipient of "Packet too big"
ICMP message from the network, the NAT device MUST forward the
ICMP message back to the intended recipient, pursuant to the
previously stated requirements REQ-3, REQ-4, REQ-5 and REQ-6.
7.2. Generating "Time Exceeded" Error Message
Srisuresh, et. al. [Page 14]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
Section 5.2.7.3 of RFC 1812 says that a router MUST generate
"Time Exceeded" ICMP error message when it discards a packet due
to an expired TTL field. A router MAY have a per interface option
to disable origination of these messages on that interface, but that
option MUST default to allowing the messages to be originated.
7.3. RFC 1812 Conformance Requirements summary
REQ-9: A NAT device MUST conform to RFC 1812 in IP packet handling.
Below are specific instances where a NAT device MUST conform to
RFC 1812.
a) If DF bit is set on a transit IP packet and the NAT
device cannot forward the packet without fragmentation, the
NAT device MUST send a "Packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet.
b) A NAT device MUST, by default, generate "Time Exceeded" ICMP
error message when it discards a packet due to an expired TTL field,
unless explicitly configured otherwise.
8. Summary of Requirements
This section summarizes the requirements discussed in the preceding
sections.
REQ-1: A NAT device MUST permit ICMP query based applications to be
initiated from private hosts to the external hosts.
a) NAT mapping of ICMP Query identifiers SHOULD be external host
independent.
REQ-2: An ICMP Query session timer MUST NOT expire in less than 60
seconds.
a) It is RECOMMENDED that the ICMP Query session timer be made
configurable.
REQ-3: When an ICMP error packet is received, the NAT device
SHOULD do the following.
a) If the IP checksum of the embedded packet fails to validate, drop
the error packet; and
b) If the embedded packet includes IP options, traverse past the
IP options to locate the start of transport header for the embedded
packet.
c) If the embedded packet contains the entire transport segment,
and the transport protocol of the embedded packet requires the
recipient to validate the transport checksum, and the checksum
fails to validate, drop the error packet.
Srisuresh, et. al. [Page 15]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
REQ-4: If a NAT device receives an ICMP error packet from external
realm, and the NAT does not have an active mapping for the embedded
payload, the NAT SHOULD silently drop the ICMP error packet. If the
the NAT has active mapping for the embedded payload, then the NAT
MUST do the following prior to forwarding the packet.
a) Revert the IP and transport headers of the embedded IP packet to
their original form, using the matching mapping; and
b) Leave the ICMP error type and code unchanged; and
c) Modify the destination IP address of the outer IP header to be
same as the source IP address of the embedded packet after
translation.
REQ-5: If a NAT device receives an ICMP error packet from private
realm, and the NAT does not have an active mapping for the embedded
payload, the NAT SHOULD silently drop the ICMP error packet. If the
the NAT has active mapping for the embedded payload, then the NAT
MUST do the following prior to forwarding the packet.
a) Revert the IP and transport headers of the embedded
IP packet to their original form, using the matching mapping; and
b) Leave the ICMP error type and code unchanged; and
c) If the NAT is a Basic NAT ([NAT-TRAD]), and the NAT has active
mapping for the IP address that sent the ICMP error, translate the
source IP address of the ICMP error packet with the public IP address
in the mapping. In all other cases, translate the source IP address
of the ICMP error packet with its own public IP address.
REQ-6: While processing an ICMP error packet, a NAT device MUST NOT
refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet.
REQ-7: NAT devices enforcing Basic NAT ([NAT-TRAD]) MUST support the
traversal of hairpinned ICMP query sessions. All NAT devices (i.e,
Basic NAT as well as NAPT devices) MUST support the traversal of
hairpinned ICMP error messages.
a) When forwarding a hairpinned ICMP error message, the NAT device
MUST translate the destination IP address of the outer IP header to
be same as the source IP address of the embedded IP packet after
the translation.
REQ-8: When a NAT device is unable to establish a NAT Session for a
new flow due to resource constraints or administrative restrictions,
the NAT device SHOULD send an ICMP destination unreachable message,
with a code of 13 (Communication administratively prohibited) to the
sender, and drop the original packet.
REQ-9: A NAT device MUST conform to RFC 1812 in IP packet handling.
Below are specific instances where a NAT device MUST conform to
RFC 1812.
Srisuresh, et. al. [Page 16]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
a) If DF bit is set on a transit IP packet and the NAT
device cannot forward the packet without fragmentation, the
NAT device MUST send a "Packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet.
b) A NAT device MUST, by default, generate "Time Exceeded" ICMP
error message when it discards a packet due to an expired TTL field,
unless explicitly configured otherwise.
9. Security Considerations
This document does not introduce any new security concerns related
to ICMP error message handling in the NAT devices. However, the
document does propose counter measures to mitigate security concerns
that already exist with ICMP error messages.
[ICMP-ATK] lists a number of ICMP attacks that can be directed
against end host TCP stacks and suggests remedies to counter the
attacks. [TCP-SOFT] describes improvements to the handling of
ICMP error messages in many of the existing TCP/IP stacks, including
Linux. Section 4 of this document describes a number of measures by
which NAT devices should validate and update the embedded payload
in ICMP error messages prior to forwarding. These measure ensure
that NATs forward the ICMP error messages reliably, as stipulated
in [ICMP-ATK].
For example, a rogue entity could bombard the NAT device with a
large number of ICMP errors. If the NAT device did not
validate the legitimacy of the ICMP error packets, the ICMP errors
would be forwarded directly to the end nodes. End hosts not capable
of defending themselves against such bogus ICMP error attacks could
be adversely impacted by such attacks. Req-3 recommends validating
embedded payload prior to forwarding. Checksum validation by itself
does not protect end hosts from attacks. However, checksum
validation mitigates endhosts from malformed ICMP error attacks.
Req-4 and Req-5 further mandate that when a NAT device does not find
a mapping selection for the embedded payload, the NAT should drop
the ICMP error packets, without forwarding.
A rogue source could also try and send bogus ICMP error messages for
the active NAT sessions, with an intent to destroy the sessions.
Req-6 averts such an attack by ensuring that an ICMP error message
does not effect the state of a session on the NAT device.
10. IANA Considerations
Srisuresh, et. al. [Page 17]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
There are no IANA considerations.
11. Acknowledgements
The authors wish to thank Fernando Gont and Dan Wing for providing
valuable input and offering generous amount of their time in shaping
the ICMP requirements. Their valuable feedback makes this document a
better read. The authors highly appreciate that.
Normative References
[BEH-UDP] Audet, F. and Jennings, C., "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-04.txt (Work In
Progress), September 2005.
[ICMP] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[NAT-MIB] Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N.,
and Wang, C., "Definitions of Managed Objects for Network
Address Translators (NAT)", RFC 4008, March 2005.
[NAT-TERM] Srisuresh, P. and Holdrege, M., "IP Network Address
Translator (NAT) Terminology and Considerations", RFC 2663,
August 1999.
[NAT-TRAD] Srisuresh, P., and Egevang, K., "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC1147] Stine, R., "FYI on a Network Management Tool Catalog: Tools
for Monitoring and Debugging TCP/IP Internets and
Interconnected Devices", RFC 1147, April 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative References
[BEH-APP] Ford, B., Srisuresh, P., and Kegel, D., "Application Design
Guidelines for Traversal through Network Address
Translators", draft-ford-behave-app-02.txt (Work In
Srisuresh, et. al. [Page 18]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
Progress), March 2006.
[BEH-TCP] Guha, S., Biswas, K., Ford, B., Francis, P., Sivakumar, S.,
and Srisuresh, P., "NAT Behavioral Requirements for
Unicast TCP", draft-ietf-behave-tcp-00.txt (Work In
Progress), February 2006.
[ICMP-ATK] Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-00.txt (Work In Progress),
February 2006.
[PMTU] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[TCP-SOFT] Gont, F., "TCP's Reaction to Soft Errors",
draft-ietf-tcpm-tcp-soft-errors-00.txt (Work In Progress),
February 2006.
[UNSAF] Daigle, L., and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
Author's Addresses:
Pyda Srisuresh
Consultant
20072 Pacifica Dr.
Cupertino, CA 95014
U.S.A.
Phone: (408)836-4773
E-mail: srisuresh@yahoo.com
Bryan Ford
Computer Science and Artificial Intelligence Laboratory
Massachusetts Institute of Technology
77 Massachusetts Ave.
Cambridge, MA 02139
U.S.A.
Phone: (617) 253-5261
E-mail: baford@mit.edu
Web: http://www.brynosaurus.com/
Senthil Sivakumar
Cisco Systems, Inc.
Srisuresh, et. al. [Page 19]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
7100-8 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709-4987
U.S.A.
Phone: +1 919 392 5158
Email: ssenthil@cisco.com
Saikat Guha
Cornell University
331 Upson Hall
Ithaca, NY 14853
U.S.A.
Email: saikat@cs.cornell.edu
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Copyright Statement
Copyright (C) The Internet Society (2006).
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.
This document and the information contained herein are provided on an
Srisuresh, et. al. [Page 20]
Internet-Draft NAT Behavioral Requirements for ICMP October 2006
"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.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Srisuresh, et. al. [Page 21]