RFC 3514 (rfc3514) - Page 1 of 6


The Security Flag in the IPv4 Header



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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
   [RFC 791] 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 [RFC 2119].

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


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