Changing the Default for Directed Broadcasts in Routers
Routers should ignore directed broadcasts unless an administrator turns them on.
Senie, D., "Changing the Default for Directed Broadcasts in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, August 1999, <https://www.rfc-editor.org/info/rfc2644>.
@misc{rfc2644,
series = {Request for Comments},
number = 2644,
howpublished = {RFC 2644},
publisher = {RFC Editor},
doi = {10.17487/RFC2644},
url = {https://www.rfc-editor.org/info/rfc2644},
author = {D. Senie},
title = {{Changing the Default for Directed Broadcasts in Routers}},
pagetotal = 4,
year = 1999,
month = aug,
} RFC text
Contents
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 RFC 2119.
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", RFC 1812,
June 1995.
[2] Ferguson, P. and D. Senie, "Ingress Filtering", RFC 2267, 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", RFC 2002, 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
8. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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AI ELI5
On this page
§1The Problem With the Old Default
Router Requirements (RFC 1812) said routers must receive and forward directed broadcasts. It also said routers MUST have an option to disable this, but that the option MUST default to permitting them.
The author notes that directed broadcasts have uses, yet on the Internet backbone their use appears to be comprised entirely of malicious attacks on other networks. Changing the required default would help ensure new routers connected to the Internet do not add to the problems already present.
The old rule said routers must pass these big shouts along by default. Almost the only people using them on the big Internet were attackers.
§2Smurf Attacks and Amplifiers
Damaging denial of service attacks led to the writing of RFC 2267 on Ingress Filtering. A recent trend in Smurf Attacks is to target networks that permit directed broadcasts from outside their networks. By permitting them, those systems become Smurf Amplifiers.
Ingress filtering remains the best way to limit these attacks, but 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.
The memo also mentions that Mobile IP (RFC 2002) had provisions for directed broadcasts in dynamic agent discovery. It is unclear whether that is useful, and the text suggests it may be worthwhile to consider removing that language.
Attackers send one shout to a network that echoes it to everyone, and all the echoes land on a victim. A network that allows the shout is helping the attacker without meaning to.
§3The New Router Requirements
Section 4.2.2.11 (d) of RFC 1812 is replaced. A directed broadcast of the form { <Network-prefix>, -1 } MUST NOT be used as a source address. A router MAY originate such packets. A router MAY have a configuration option to receive them, but that option MUST be disabled by default, so the router MUST NOT receive them unless the end user configures it.
The second paragraph of section 5.3.5.2 is also replaced. A router MAY have options to enable receiving and forwarding network-prefix-directed broadcasts on an interface, and these options MUST default to blocking receipt and blocking forwarding.
Routers now start with the door shut. A person has to switch the feature on on purpose before the router will pass the big shouts along.
§4Security Considerations
The whole point of the memo is security. The goal stated in the RFC is to reduce the efficacy of certain types of denial of service attacks. No other considerations are listed.
The goal is simple: make it harder for bad guys to use other people's networks to flood someone.