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 (C) The Internet Society (2003). All Rights Reserved.
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
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.
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].
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:
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.