draft-ietf-dnsop-reflectors-are-evil-03.txt   draft-ietf-dnsop-reflectors-are-evil-04.txt 
Network Working Group J. Damas Network Working Group J. Damas
Internet-Draft ISC Internet-Draft ISC
Intended status: Informational F. Neves Intended status: Best Current F. Neves
Expires: August 17, 2007 Registro.br Practice Registro.br
February 13, 2007 Expires: January 10, 2008 July 9, 2007
Preventing Use of Recursive Nameservers in Reflector Attacks Preventing Use of Recursive Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-03.txt draft-ietf-dnsop-reflectors-are-evil-04.txt
Status of this Memo Status of this Memo
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This Internet-Draft will expire on August 17, 2007. This Internet-Draft will expire on January 10, 2008.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document describes ways to prevent the use of default configured This document describes ways to prevent the use of default configured
recursive nameservers as reflectors on DOS attacks. Recommended recursive nameservers as reflectors in Denial of Service (DoS)
configuration as measures to mitigate the attack are given. attacks. Recommended configuration as measures to mitigate the
attack are given.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3 2. Document Terminology . . . . . . . . . . . . . . . . . . . . . 3
3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4 3. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . . . 8
1. Introduction 1. Introduction
Recently, DNS [RFC1034] has been named as a major factor in the Recently, DNS [RFC1034] has been named as a major factor in the
generation of massive amounts of network traffic used in Denial of generation of massive amounts of network traffic used in Denial of
Service (DoS) attacks. These attacks, called reflector attacks, are Service (DoS) attacks. These attacks, called reflector attacks, are
not due to any particular flaw in the design of the DNS or its not due to any particular flaw in the design of the DNS or its
implementations, aside perhaps the fact that DNS relies heavily on implementations, aside perhaps the fact that DNS relies heavily on
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mitigate the scale of an attack by modifying authoritative server mitigate the scale of an attack by modifying authoritative server
configurations are limited. This document's recommendations are configurations are limited. This document's recommendations are
concerned with recursive nameservers only. concerned with recursive nameservers only.
In this document we describe the characteristics of the attack and In this document we describe the characteristics of the attack and
recommend DNS server configurations that specifically alleviate the recommend DNS server configurations that specifically alleviate the
problem described, while pointing to the only truly real solution: problem described, while pointing to the only truly real solution:
the wide-scale deployment of ingress filtering to prevent use of the wide-scale deployment of ingress filtering to prevent use of
spoofed IP addresses [BCP38]. spoofed IP addresses [BCP38].
2. Problem Description 2. Document Terminology
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].
3. Problem Description
Because most DNS traffic is stateless by design, an attacker could Because most DNS traffic is stateless by design, an attacker could
start a DoS attack in the following way: start a DoS attack in the following way:
1. The attacker starts by configuring a record (LRECORD) on any zone 1. The attacker starts by configuring a record (LRECORD) on any zone
he has access to (AZONE), normally with large RDATA and TTL. he has access to (AZONE), normally with large RDATA and TTL.
2. Taking advantage of clients (ZCLIENTS) on non-BCP38 networks, the 2. Taking advantage of clients (ZCLIENTS) on non-BCP38 networks, the
attacker then crafts a query for LRECORD using the source address attacker then crafts a query for LRECORD using the source address
of their target victim and sends it to an open recursive of their target victim and sends it to an open recursive
nameserver (ORNS). nameserver (ORNS).
3. Each ORNS proceeds with the resolution, caches the LRECORD and 3. Each ORNS proceeds with the resolution, caches the LRECORD and
finally sends it to the target. After this first lookup, access finally sends it to the target. After this first lookup, access
to the authoritative nameservers for AZONE is normally no longer to the authoritative nameservers for AZONE is normally no longer
necessary. The LRECORD will remain cached for the duration of necessary. The LRECORD will remain cached for the duration of
the TTL at the ORNS even if the AZONE is corrected. the TTL at the ORNS even if the AZONE is corrected.
4. Cleanup of the AZONE might, depending on the implementation used 4. Cleanup of the AZONE might, depending on the implementation used
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historical reasons, from times when the Internet was a much closer- historical reasons, from times when the Internet was a much closer-
knit community, some nameserver implementations have been made knit community, some nameserver implementations have been made
available with default configurations that when used for recursive available with default configurations that when used for recursive
nameservers made the server accessible to all hosts on the Internet. nameservers made the server accessible to all hosts on the Internet.
For years this was a convenient and helpful configuration, enabling For years this was a convenient and helpful configuration, enabling
wider availability of services. As this document aims to make wider availability of services. As this document aims to make
apparent, it is now much better to be conscious of ones own apparent, it is now much better to be conscious of ones own
nameserver services and focus the delivery of services on the nameserver services and focus the delivery of services on the
intended audience of those services, be they a university campus, an intended audience of those services, be they a university campus, an
enterprise or an ISP's customers. The authors also want to draw the enterprise or an ISP's customers. The target audience also includes
attention of small network operators and private server managers who operators of small network operators and private server managers who
decide to operate nameservers with the aim of optimising their DNS decide to operate nameservers with the aim of optimising their DNS
service, as these are more likely to use default configurations as service, as these are more likely to use default configurations as
shipped by implementors. shipped by implementors.
3. Recommended Configuration 4. Recommended Configuration
From the description of the problem in the previous section it From the description of the problem in the previous section it
follows that the solution to these sort of attacks is the wide follows that the solution to these sort of attacks is the wide
deployment of ingress filtering [BCP38] in routers to prevent use of deployment of ingress filtering [BCP38] in routers to prevent use of
address spoofing as a viable course of action to prevent the attacks. address spoofing as a viable course of action to prevent the attacks.
In situations were more complex network setups are in place, "Ingress In situations were more complex network setups are in place, "Ingress
Filtering for Multihomed Network" [BCP84] maybe a useful additional Filtering for Multihomed Network" [BCP84] maybe a useful additional
reference. reference.
Nonetheless, the fact remains that DNS servers acting as open Nonetheless, the fact remains that DNS servers acting as open
recursive servers provide an easy means to obtain great rates of recursive servers provide an easy means to obtain great rates of
amplification for attack traffic, requiring only a small amount of amplification for attack traffic, requiring only a small amount of
traffic from the attack sources to generate a vast amount of traffic traffic from the attack sources to generate a vast amount of traffic
towards the victim. towards the victim.
The authors also want to note that with the increasing length of With the increasing length of authoritative DNS responses derived
authoritative DNS responses derived from deployment of DNSSEC and from deployment of DNSSEC and NAPTR as used in ENUM services,
NAPTR as used in ENUM services, authoritative servers will eventually authoritative servers will eventually be more useful as actors in
be more useful as actors in this sort of amplification attack, this sort of amplification attack, stressing even more the need for
stressing even more the need for deployment of BCP 38. deployment of BCP 38.
In this section we describe the Current Best Practice for operating In this section we describe the Best Current Practice for operating
recursive nameservers. Following these recommendations would reduce recursive nameservers. Following these recommendations would reduce
the chances of having a given recursive nameserver be used for the the chances of having a given recursive nameserver be used for the
generation of an amplification attack. generation of an amplification attack.
The generic recommendation to nameserver operators is to use the The generic recommendation to nameserver operators is to use the
means provided by the implementation of choice to provide recursive means provided by the implementation of choice to provide recursive
name lookup service only to the intended clients. Client name lookup service only to the intended clients. Client
authentication can be usually done in several ways: authentication can be usually done in several ways:
o IP based authentication. Use the IP address of the sending host o IP based authentication. Use the IP address of the sending host
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running a local instance of a caching nameserver or forwarder that running a local instance of a caching nameserver or forwarder that
will be able to TSIG sign the queries and send them on to the will be able to TSIG sign the queries and send them on to the
recursive nameserver of choice. recursive nameserver of choice.
In nameservers that do not need to be providing recursive service, In nameservers that do not need to be providing recursive service,
for instance servers that are meant to be authoritative only, turn for instance servers that are meant to be authoritative only, turn
recursion off completely. In general, it is a good idea to keep recursion off completely. In general, it is a good idea to keep
recursive and authoritative services separate as much as practical. recursive and authoritative services separate as much as practical.
This, of course, depends on local circumstances. This, of course, depends on local circumstances.
By default, nameservers SHOULD not offer recursive service to By default, nameservers SHOULD NOT offer recursive service to
external networks. external networks.
4. Acknowledgments
The authors would like to acknowledge the helpful input and comments
of Joe Abley, Olafur Gudmundsson, Pekka Savola, and Andrew Sullivan.
5. Security Considerations 5. Security Considerations
This document does not create any new security issues for the DNS This document does not create any new security issues for the DNS
protocol. protocol, it deals with a weakness in implementations.
It's not excessive to repeat that, although recommended It's not excessive to repeat that, although recommended
configurations described in this document could alleviate the configurations described in this document could alleviate the
problem, the only solution to source address spoofing problems is the problem, the only solution to source address spoofing problems is the
wide-scale deployment of Ingress Filtering to prevent use of spoofed wide-scale deployment of Ingress Filtering to prevent use of spoofed
IP addresses [BCP38], [BCP84]. IP addresses [BCP38], [BCP84].
6. References 6. Acknowledgments
6.1. Normative References The authors would like to acknowledge the helpful input and comments
of Joe Abley, Olafur Gudmundsson, Pekka Savola, and Andrew Sullivan.
7. IANA Considerations
This document does not define a registry and does not require any
IANA action.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. RFC 2671, August 1999.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B. [RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000. (TSIG)", RFC 2845, May 2000.
[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures ( [RFC2931] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000. SIG(0)s)", RFC 2931, September 2000.
6.2. Informative References 8.2. Informative References
[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering: [BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000. Address Spoofing", BCP 38, RFC 2827, May 2000.
[BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004. Networks", BCP 84, RFC 3704, March 2004.
Authors' Addresses Authors' Addresses
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