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Network Working Group                                        S. BellovinRequest for Comments: 3514                            AT&T Labs ResearchCategory: Informational                                     1 April 2003The Security Flag in the IPv4 HeaderStatus 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, theBellovin                     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.orgBellovin                     Informational                      [Page 5]

RFC 3514          The Security Flag in the IPv4 Header      1 April 200310. 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]