Network Working Group D. Senie
Request for Comments: 2644 Amaranth Networks Inc.
Updates: 1812 August 1999
BCP: 34
Category: Best Current Practice

Changing the Default for Directed Broadcasts in Routers

Status of this Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

1. IntrodUCtion

Router Requirements [1] specifies that routers must receive and
forward directed broadcasts. It also specifies that routers MUST have
an option to disable this feature, and that this option MUST default
to permit the receiving and forwarding of directed broadcasts. While
directed broadcasts have uses, their use on the Internet backbone
appears to be comprised entirely of malicious attacks on other
networks.

Changing the required default for routers would help ensure new
routers connected to the Internet do not add to the problems already
present.

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.

2. Discussion

Damaging denial of service attacks led to the writing of [2] on
Ingress Filtering. Many network providers and corporate networks have
endorsed the use of these methods to ensure their networks are not
the source of such attacks.

A recent trend in Smurf Attacks [3] is to target networks which
permit directed broadcasts from outside their networks. By permitting
directed broadcasts, these systems become "Smurf Amplifiers."

While the continued implementation of ingress filters remains the
best way to limit these attacks, restricting directed broadcasts
should also receive priority.

Network service providers and corporate network operators are urged
to ensure their networks are not susceptible to directed broadcast
packets originating outside their networks.

Mobile IP [4] had provisions for using directed broadcasts in a
mobile node's use of dynamic agent discovery. While some
implementations support this feature, it is unclear whether it is
useful. Other methods of achieving the same result are documented in
[5]. It may be worthwhile to consider removing the language on using
directed broadcasts as Mobile IP progresses on the standards track.

3. Recommendation

Router Requirements [1] is updated as follows:

Section 4.2.2.11 (d) is replaced with:

(d) { <Network-prefix>, -1 }

Directed Broadcast - a broadcast directed to the specified network
prefix. It MUST NOT be used as a source address. A router MAY
originate Network Directed Broadcast packets. A router MAY have a
configuration option to allow it to receive directed broadcast
packets, however this option MUST be disabled by default, and thus
the router MUST NOT receive Network Directed Broadcast packets
unless specifically configured by the end user.

Section 5.3.5.2, second paragraph replaced with:

A router MAY have an option to enable receiving network-prefix-
directed broadcasts on an interface and MAY have an option to
enable forwarding network-prefix-directed broadcasts. These
options MUST default to blocking receipt and blocking forwarding
of network-prefix-directed broadcasts.

4. Security Considerations

The goal of this document is to reduce the efficacy of certain types
of denial of service attacks.

5. References

[1] Baker, F., "Requirements for IP Version 4 Routers", RFC1812,
June 1995.

[2] Ferguson, P. and D. Senie, "Ingress Filtering", RFC2267, January
1998.

[3] See the pages by Craig Huegen at:
http://www.quadrunner.com/~chuegen/smurf.txt, and the CERT
advisory at: http://www.cert.org/advisories/CA-98.01.smurf.Html

[4] Perkins, C., "IP Mobility Support", RFC2002, October 1996.

[5] P. Calhoun, C. Perkins, "Mobile IP Dynamic Home Address
Allocation Extensions", Work in Progress.

6. Acknowledgments

The author would like to thank Brandon Ross of Mindspring and Gabriel
Montenegro of Sun for their input.

7. Author's Address

Daniel Senie
Amaranth Networks Inc.
324 Still River Road
Bolton, MA 01740

Phone: (978) 779-6813
EMail: dts@senie.com

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