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rfc:rfc3514

Network Working Group S. Bellovin Request for Comments: 3514 AT&T Labs Research Category: Informational 1 April 2003

                The Security Flag in the IPv4 Header

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 Firewalls, packet filters, intrusion detection systems, and the like
 often have difficulty distinguishing between packets that have
 malicious intent and those that are merely unusual.  We define a
 security flag in the IPv4 header as a means of distinguishing the two
 cases.

1. Introduction

 Firewalls [CBR03], packet filters, intrusion detection systems, and
 the like often have difficulty distinguishing between packets that
 have malicious intent and those that are merely unusual.  The problem
 is that making such determinations is hard.  To solve this problem,
 we define a security flag, known as the "evil" bit, in the IPv4
 [RFC791] header.  Benign packets have this bit set to 0; those that
 are used for an attack will have the bit set to 1.

1.1. Terminology

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [RFC2119].

2. Syntax

 The high-order bit of the IP fragment offset field is the only unused
 bit in the IP header.  Accordingly, the selection of the bit position
 is not left to IANA.

Bellovin Informational [Page 1] RFC 3514 The Security Flag in the IPv4 Header 1 April 2003

 The bit field is laid out as follows:
           0
          +-+
          |E|
          +-+
 Currently-assigned values are defined as follows:
 0x0  If the bit is set to 0, the packet has no evil intent.  Hosts,
      network elements, etc., SHOULD assume that the packet is
      harmless, and SHOULD NOT take any defensive measures.  (We note
      that this part of the spec is already implemented by many common
      desktop operating systems.)
 0x1  If the bit is set to 1, the packet has evil intent.  Secure
      systems SHOULD try to defend themselves against such packets.
      Insecure systems MAY chose to crash, be penetrated, etc.

3. Setting the Evil Bit

 There are a number of ways in which the evil bit may be set.  Attack
 applications may use a suitable API to request that it be set.
 Systems that do not have other mechanisms MUST provide such an API;
 attack programs MUST use it.
 Multi-level insecure operating systems may have special levels for
 attack programs; the evil bit MUST be set by default on packets
 emanating from programs running at such levels.  However, the system
 MAY provide an API to allow it to be cleared for non-malicious
 activity by users who normally engage in attack behavior.
 Fragments that by themselves are dangerous MUST have the evil bit
 set.  If a packet with the evil bit set is fragmented by an
 intermediate router and the fragments themselves are not dangerous,
 the evil bit MUST be cleared in the fragments, and MUST be turned
 back on in the reassembled packet.
 Intermediate systems are sometimes used to launder attack
 connections.  Packets to such systems that are intended to be relayed
 to a target SHOULD have the evil bit set.
 Some applications hand-craft their own packets.  If these packets are
 part of an attack, the application MUST set the evil bit by itself.
 In networks protected by firewalls, it is axiomatic that all
 attackers are on the outside of the firewall.  Therefore, hosts
 inside the firewall MUST NOT set the evil bit on any packets.

Bellovin Informational [Page 2] RFC 3514 The Security Flag in the IPv4 Header 1 April 2003

 Because NAT [RFC3022] boxes modify packets, they SHOULD set the evil
 bit on such packets.  "Transparent" http and email proxies SHOULD set
 the evil bit on their reply packets to the innocent client host.
 Some hosts scan other hosts in a fashion that can alert intrusion
 detection systems.  If the scanning is part of a benign research
 project, the evil bit MUST NOT be set.  If the scanning per se is
 innocent, but the ultimate intent is evil and the destination site
 has such an intrusion detection system, the evil bit SHOULD be set.

4. Processing of the Evil Bit

 Devices such as firewalls MUST drop all inbound packets that have the
 evil bit set.  Packets with the evil bit off MUST NOT be dropped.
 Dropped packets SHOULD be noted in the appropriate MIB variable.
 Intrusion detection systems (IDSs) have a harder problem.  Because of
 their known propensity for false negatives and false positives, IDSs
 MUST apply a probabilistic correction factor when evaluating the evil
 bit.  If the evil bit is set, a suitable random number generator
 [RFC1750] must be consulted to determine if the attempt should be
 logged.  Similarly, if the bit is off, another random number
 generator must be consulted to determine if it should be logged
 despite the setting.
 The default probabilities for these tests depends on the type of IDS.
 Thus, a signature-based IDS would have a low false positive value but
 a high false negative value.  A suitable administrative interface
 MUST be provided to permit operators to reset these values.
 Routers that are not intended as as security devices SHOULD NOT
 examine this bit.  This will allow them to pass packets at higher
 speeds.
 As outlined earlier, host processing of evil packets is operating-
 system dependent; however, all hosts MUST react appropriately
 according to their nature.

5. Related Work

 Although this document only defines the IPv4 evil bit, there are
 complementary mechanisms for other forms of evil.  We sketch some of
 those here.
 For IPv6 [RFC2460], evilness is conveyed by two options.  The first,
 a hop-by-hop option, is used for packets that damage the network,
 such as DDoS packets.  The second, an end-to-end option, is for
 packets intended to damage destination hosts.  In either case, the

Bellovin Informational [Page 3] RFC 3514 The Security Flag in the IPv4 Header 1 April 2003

 option contains a 128-bit strength indicator, which says how evil the
 packet is, and a 128-bit type code that describes the particular type
 of attack intended.
 Some link layers, notably those based on optical switching, may
 bypass routers (and hence firewalls) entirely.  Accordingly, some
 link-layer scheme MUST be used to denote evil.  This may involve evil
 lambdas, evil polarizations, etc.
 DDoS attack packets are denoted by a special diffserv code point.
 An application/evil MIME type is defined for Web- or email-carried
 mischief.  Other MIME types can be embedded inside of evil sections;
 this permit easy encoding of word processing documents with macro
 viruses, etc.

6. IANA Considerations

 This document defines the behavior of security elements for the 0x0
 and 0x1 values of this bit.  Behavior for other values of the bit may
 be defined only by IETF consensus [RFC2434].

7. Security Considerations

 Correct functioning of security mechanisms depend critically on the
 evil bit being set properly.  If faulty components do not set the
 evil bit to 1 when appropriate, firewalls will not be able to do
 their jobs properly.  Similarly, if the bit is set to 1 when it
 shouldn't be, a denial of service condition may occur.

8. References

 [CBR03]   W.R. Cheswick, S.M. Bellovin, and A.D. Rubin, "Firewalls
           and Internet Security: Repelling the Wily Hacker", Second
           Edition, Addison-Wesley, 2003.
 [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
           1981.
 [RFC1750] Eastlake, D., 3rd, Crocker, S. and J. Schiller, "Randomness
           Recommendations for Security", RFC 1750, December 1994.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
           IANA Considerations Section in RFCs", BCP 26, RFC 2434,
           October 1998.

Bellovin Informational [Page 4] RFC 3514 The Security Flag in the IPv4 Header 1 April 2003

 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.
 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
           Address Translator (Traditional NAT)", RFC 3022, January
           2001.

9. Author's Address

 Steven M. Bellovin
 AT&T Labs Research
 Shannon Laboratory
 180 Park Avenue
 Florham Park, NJ 07932
 Phone: +1 973-360-8656
 EMail: bellovin@acm.org

Bellovin Informational [Page 5] RFC 3514 The Security Flag in the IPv4 Header 1 April 2003

10. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bellovin Informational [Page 6]

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