rfc3514.txt

Network Working Group                                        S. Bellovin
Internet-Draft                                        AT&T Labs Research
Expires: 3 October 2018                                     1 April 2018


                  The Security Flag in the IPv4 Header

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.

Status of This Memo

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Syntax
   4.  Normative References

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 [RFC0791] header.  Benign packets have this bit set to 0; those
   that are used for an attack will have the bit set to 1.

2.  Terminology

   The keywords <bcp14>:MUST, <bcp14>:MUST NOT, <bcp14>:REQUIRED,
   <bcp14>:SHALL, <bcp14>:SHALL NOT, <bcp14>:SHOULD, <bcp14>:SHOULD NOT,
   <bcp14>:RECOMMENDED, <bcp14>:MAY, and <bcp14>:OPTIONAL, when they
   appear in this document, are to be interpreted as described in
   [RFC2119].

3.  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.

   The bit field is laid out as follows:

4.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981, <https://www.rfc-
              editor.org/info/rfc791>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.