RFC3514 - The Security Flag in the IPv4 Header

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.

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.

The bit field is laid out as follows:

+-+

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.

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

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, RFC791, September

1981.

[RFC1750] Eastlake, D., 3rd, Crocker, S. and J. Schiller, "Randomness

Recommendations for Security", RFC1750, December 1994.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an

IANA Considerations Section in RFCs", BCP 26, RFC2434,

October 1998.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6

(IPv6) Specification", RFC2460, December 1998.

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network

Address Translator (Traditional NAT)", RFC3022, 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

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