ICMP Security Failures Messages
ICMP messages for reporting authentication, encryption, and authorization failures in IPsec
Karn, P. and W. Simpson, "ICMP Security Failures Messages", RFC 2521, DOI 10.17487/RFC2521, March 1999, <https://www.rfc-editor.org/info/rfc2521>.
@misc{rfc2521,
series = {Request for Comments},
number = 2521,
howpublished = {RFC 2521},
publisher = {RFC Editor},
doi = {10.17487/RFC2521},
url = {https://www.rfc-editor.org/info/rfc2521},
author = {P. Karn and W. Simpson},
title = {{ICMP Security Failures Messages}},
pagetotal = 9,
year = 1999,
month = mar,
} RFC text
Contents
Network Working Group P. Karn
Request for Comments: 2521 Qualcomm
Category: Experimental W. Simpson
DayDreamer
March 1999
ICMP Security Failures Messages
Status of this Memo
This document defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). Copyright (C) Philip Karn
and William Allen Simpson (1994-1999). All Rights Reserved.
Abstract
This document specifies ICMP messages for indicating failures when
using IP Security Protocols (AH and ESP).
Table of Contents
1. Introduction .......................................... 1
2. Message Formats ....................................... 1
2.1 Bad SPI ......................................... 2
2.2 Authentication Failed ........................... 2
2.3 Decompression Failed ............................ 2
2.4 Decryption Failed ............................... 2
2.5 Need Authentication ............................. 3
2.6 Need Authorization .............................. 3
3. Error Procedures ...................................... 3
SECURITY CONSIDERATIONS ...................................... 4
HISTORY ...................................................... 5
ACKNOWLEDGEMENTS ............................................. 5
REFERENCES ................................................... 5
CONTACTS ..................................................... 6
COPYRIGHT .................................................... 7
1. Introduction
This mechanism is intended for use with the Internet Security
Protocols [RFC-1825 et sequitur] for authentication and privacy. For
statically configured Security Associations, these messages indicate
that the operator needs to manually reconfigure, or is attempting an
unauthorized operation. These messages may also be used to trigger
automated session-key management.
The datagram format and basic facilities are already defined for ICMP
[RFC-792].
Up-to-date values of the ICMP Type field are specified in the most
recent "Assigned Numbers" [RFC-1700]. This document concerns the
following values:
40 Security Failures
2. Message Formats
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Original Internet Headers + 64 bits of Payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 40
Code Indicates the kind of failure:
0 = Bad SPI
1 = Authentication Failed
2 = Decompression Failed
3 = Decryption Failed
4 = Need Authentication
5 = Need Authorization
Checksum Two octets. The ICMP Checksum.
Reserved Two octets. For future use; MUST be set to zero
when transmitted, and MUST be ignored when received.
Pointer Two octets. An offset into the Original Internet
Headers that locates the most significant octet of
the offending SPI. Will be zero when no SPI is
present.
Original Internet Headers ...
The original Internet Protocol header, any
intervening headers up to and including the
offending SPI (if any), plus the first 64 bits (8
octets) of the remaining payload data.
This data is used by the host to match the message
to the appropriate process. If a payload protocol
uses port numbers, they are assumed to be in the
first 64-bits of the original datagram's payload.
Usage of this message is elaborated in the following sections.
2.1. Bad SPI
Indicates that a received datagram includes a Security Parameters
Index (SPI) that is invalid or has expired.
2.2. Authentication Failed
Indicates that a received datagram failed the authenticity or
integrity check for a given SPI.
Note that the SPI may indicate an outer Encapsulating Security
Protocol when a separate Authentication Header SPI is hidden inside.
2.3. Decompression Failed
Indicates that a received datagram failed a decompression check for a
given SPI.
2.4. Decryption Failed
Indicates that a received datagram failed a decryption check for a
given SPI.
2.5. Need Authentication
Indicates that a received datagram will not be accepted without
additional authentication.
In this case, either no SPI is present, or an unsuitable SPI is
present. For example, an encryption SPI without integrity arrives
from a secure operating system with mutually suspicious users.
2.6. Need Authorization
Indicates that a received datagram will not be accepted because it
has insufficient authorization.
In this case, an authentication SPI is present that is inappropriate
for the target transport or application. The principle party denoted
by the SPI does not have proper authorization for the facilities used
by the datagram. For example, the party is authorized for Telnet
access, but not for FTP access.
3. Error Procedures
As is usual with ICMP messages, upon receipt of one of these error
messages that is uninterpretable or otherwise contains an error, no
ICMP error message is sent in response. Instead, the message is
silently discarded. However, for diagnosis of problems, a node
SHOULD provide the capability of logging the error, including the
contents of the silently discarded datagram, and SHOULD record the
event in a statistics counter.
On receipt, special care MUST be taken that the ICMP message actually
includes information that matches a previously sent IP datagram.
Otherwise, this might provide an opportunity for a denial of service
attack.
The sending implementation MUST be able to limit the rate at which
these messages are generated. The rate limit parameters SHOULD be
configurable. How the limits are applied (such as, by destination or
per interface) is left to the implementor's discretion.
Security Considerations
When a prior Security Association between the parties has not
expired, these messages SHOULD be sent with authentication.
However, the node MUST NOT dynamically establish a new Security
Association for the sole purpose of authenticating these messages.
Automated key management is computationally intensive. This could be
used for a very serious denial of service attack. It would be very
easy to swamp a target with bogus SPIs from random IP Sources, and
have it start up numerous useless key management sessions to
authentically inform the putative sender.
In the event of loss of state (such as a system crash), the node will
need to send failure messages to all parties that attempt subsequent
communication. In this case, the node may have lost the key
management technique that was used to establish the Security
Association.
Much better to simply let the peers know that there was a failure,
and let them request key management as needed (at their staggered
timeouts). They'll remember the previous key management technique,
and restart gracefully. This distributes the restart burden among
systems, and helps allow the recently failed node to manage its
computational resources.
In addition, these messages inform the recipient when the ICMP sender
is under attack. Unlike other ICMP error messages, the messages
provide sufficient data to determine that these messages are in
response to previously sent messages.
Therefore, it is imperative that the recipient accept both
authenticated and unauthenticated failure messages. The recipient's
log SHOULD indicate when the ICMP messages are not validated, and
when the ICMP messages are not in response to a valid previous
message.
There is some concern that sending these messages may result in the
leak of security information. For example, an attacker might use
these messages to test or verify potential forged keys. However,
this information is already available through the simple expedient of
using Echo facilities, or waiting for a TCP 3-way handshake.
The rate limiting mechanism also limits this form of leak, as many
messages will not result in an error indication. At the very least,
this will lengthen the time factor for verifying such information.
History
The text has been extensively reviewed on the IP Security mailing
list, in January and February of 1995 and again in December 1995.
The specification is stable, and was forwarded to the IESG by the
authors for IETF Last Call as a Proposed Standard in March 1996.
There have been several implementations.
Acknowledgements
Some of the text of this specification was derived from "Requirements
for Internet Hosts -- Communication Layers" [RFC-1122] and
"Requirements for IP Version 4 Routers" [RFC-1812].
Naganand Doraswamy and Hilarie Orman provided useful critiques of
earlier versions of this document.
Stimulating comments were also received from Jeffrey Schiller.
Special thanks to the Center for Information Technology Integration
(CITI) for providing computing resources.
References
[RFC-792] Postel, J., "Internet Control Message Protocol", STD 5,
September 1981.
[RFC-1122] Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, USC/Information Sciences
Institute, October 1989.
[RFC-1700] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
USC/Information Sciences Institute, October 1994.
[RFC-1812] Baker, F., Editor, "Requirements for IP Version 4
Routers", Cisco Systems, June 1995.
[RFC-1825] Atkinson, R., "Security Architecture for the Internet
Protocol", Naval Research Laboratory, July 1995.
Contacts
Comments about this document should be discussed on the
photuris@adk.gr mailing list.
Questions about this document can also be directed to:
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com
karn@unix.ka9q.ampr.org (preferred)
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
Full Copyright Statement
Copyright (C) The Internet Society (1999). Copyright (C) Philip
Karn and William Allen Simpson (1994-1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
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AI ELI5
On this page
§1Reporting IPsec Errors
The Internet Protocol Security architecture defines how to apply encryption and authentication to IP traffic. When a system receives an IPsec-protected packet, it must verify authentication, decrypt if needed, and check if the sender is authorized. If any of these steps fail, the receiver needs a way to report the problem.
This document specifies ICMP messages for this purpose. These messages are useful for both statically configured security associations (where an operator needs to manually fix the configuration) and for automated key management protocols that detect failures and attempt to renegotiate.
When a protected message fails checks, the receiver sends back an ICMP error message saying what went wrong so the sender can fix it.
§2The Message Structure
All these messages use ICMP Type 40. The message includes a checksum, a reserved field (for future use), and a Pointer field that indicates the byte offset in the original headers where the problematic Security Parameters Index (SPI) is located. Zero is used when no SPI is present.
The message also includes the original IP header and any intermediate headers up to the SPI, plus the first 64 bits (8 octets) of the payload data. This is enough information for the receiving host to match the error to the original packet (most protocols with port numbers put those ports in the first 64 bits).
The error message is a short note that copies the original message headers and the first part of the data so you can tell which packet failed.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 40 (Security Failures) | |
CODE | Code | 1 byte | 0=Bad SPI, 1=Auth Failed, 2=Decomp Failed, 3=Decrypt Failed, 4=Need Auth, 5=Need Authz | |
CHECKSUM | Checksum | 2 bytes | ICMP checksum | |
RESERVED | Reserved | 2 bytes | For future use; must be zero | |
POINTER | Pointer | 2 bytes | Byte offset to problematic SPI (or 0) |
§3Six Types of Failures
Bad SPI (code 0): The Security Parameters Index in the packet is invalid or has expired. Authentication Failed (code 1): The packet failed the authenticity or integrity check for a given SPI; the SPI may be an outer ESP header hiding an inner Authentication Header. Decompression Failed (code 2): The packet failed a decompression check for its SPI, which can happen when IPsec includes compression.
Decryption Failed (code 3): The packet failed a decryption check for its SPI, meaning the encrypted data could not be properly decrypted. Need Authentication (code 4): The receiver will not accept the packet without additional authentication, either because no SPI is present or because an unsuitable SPI is present (for example, encryption without integrity). Need Authorization (code 5): The packet includes an authentication SPI that is inappropriate for the target transport or application; the party denoted by the SPI does not have proper authorization.
The error code says which check failed: bad SPI, authentication failed, decompression failed, decryption failed, need more authentication, or need authorization.
- 0Bad SPI — Security Parameters Index is invalid or expired
- 1Authentication Failed — Packet failed authenticity or integrity check
- 2Decompression Failed — Packet failed decompression check
- 3Decryption Failed — Packet failed decryption check
- 4Need Authentication — Receiver requires additional authentication
- 5Need Authorization — Sender lacks authorization for the target
§4Handling and Rate Limiting
Upon receipt of an uninterpretable security failure message or one that contains an error, no ICMP error message is sent in response—it is silently discarded. However, nodes SHOULD log the error for diagnosis and maintain statistics counters.
Systems MUST take special care to verify that the ICMP message actually corresponds to a previously sent IP datagram, preventing denial-of-service attacks where an attacker floods targets with bogus error messages. Implementations MUST support rate limiting of these messages; limits may be applied per destination, per interface, or by some other method. For authenticated responses (when a prior Security Association exists), these messages SHOULD be sent with authentication. However, implementations MUST NOT dynamically establish a new Security Association solely to authenticate these messages, as that would be computationally expensive and vulnerable to denial-of-service attacks.
Broken or forged error messages are ignored silently. Valid senders should check that errors match something they sent. Messages are rate-limited to prevent flooding attacks.