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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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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 inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 October 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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>.
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