Network Working Group P Karn Internet Draft Qualcomm W A Simpson DayDreamer expires in six months April 1996 ICMP Security Failures Messages draft-simpson-icmp-ipsec-fail-02.txt Status of this Memo Distribution of this memo is unlimited. This document is an Internet-Draft. Internet Drafts are working doc- uments of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute work- ing 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 not appropriate to use Internet Drafts as refer- ence material, or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow Directories on: ftp.is.co.za (Africa) nic.nordu.net (Europe) ds.internic.net (US East Coast) ftp.isi.edu (US West Coast) munnari.oz.au (Pacific Rim) Abstract This document specifies ICMP messages for indicating failures when using IP Security Protocols (AH and ESP). Karn & Simpson expires in six months [Page i] DRAFT ICMP Security Failures April 1996 1. Introduction This mechanism is intended for use with the Internet Security Proto- cols [RFC-1825] for authentication and privacy. For statically con- figured Security Associations, these messages indicate that the oper- ator needs to manually reconfigure, or is attempting an unauthorized operation. These messages may also be used to trigger automated ses- sion-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. Karn & Simpson expires in six months [Page 1] DRAFT ICMP Security Failures April 1996 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 pre- sent. Original Internet Headers ... The original Internet Protocol header, any interven- ing 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 Proto- col 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. Karn & Simpson expires in six months [Page 2] DRAFT ICMP Security Failures April 1996 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 addi- tional authentication. In this case, either no SPI is present, or an unsuitable SPI is pre- sent. For example, an encryption SPI without integrity arrives from a secure operating system with mutually hostile 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. Karn & Simpson expires in six months [Page 3] DRAFT ICMP Security Failures April 1996 Security Considerations When a prior Security Association between the parties has not expired, these messages SHOULD be sent with authenticatation. However, the node MUST NOT dynamically establish a new Security Asso- ciation for the sole purpose of authenticating these messages. Auto- mated 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 authen- tically 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 manage- ment 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 com- putational resources. In addition, these messages inform the recipient when the ICMP sender is under attack. Unlike other ICMP error messages, the messages pro- vide sufficient data to determine that these messages are in response to previously sent messages. Therefore, it is imperative that the recipient accept both authenti- cated and unathenticated 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. Karn & Simpson expires in six months [Page 4] DRAFT ICMP Security Failures April 1996 Acknowledgements Some of the text of this specification was derived from "Requirements for Internet Hosts -- Communication Layers" [RFC-1122] and "Require- ments 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. References [RFC-792] Postel, J., "Internet Control Message Protocol", STD 5, September 1981. [RFC-1122] Braden, R., Editor, "Requirements for Internet Hosts -- Com- munication Layers", 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 Proto- col", Naval Research Laboratory, July 1995. Karn & Simpson expires in six months [Page 5] DRAFT ICMP Security Failures April 1996 Contacts Comments about this document should be discussed on the ipsec- dev@terisa.com 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) bsimpson@MorningStar.com Karn & Simpson expires in six months [Page 6]