draft-ietf-dnsop-reflectors-are-evil-01.txt   draft-ietf-dnsop-reflectors-are-evil-02.txt 
Network Working Group J. Damas Network Working Group J. Damas
Internet-Draft ISC Internet-Draft ISC
Expires: December 27, 2006 F. Neves Expires: March 19, 2007 F. Neves
Registro.br Registro.br
June 25, 2006 September 15, 2006
Preventing Use of Nameservers in Reflector Attacks Preventing Use of Recursive Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-01.txt draft-ietf-dnsop-reflectors-are-evil-02.txt
Status of this Memo Status of this Memo
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This Internet-Draft will expire on December 27, 2006. This Internet-Draft will expire on March 19, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document describes the use of default configured recursive This document describes the use of default configured recursive
nameservers as reflectors on DOS attacks. Recommended configuration nameservers as reflectors on DOS attacks. Recommended configuration
as measures to mitigate the attack are given. 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. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4 3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 6
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, asides perhaps the fact that DNS relies heavily on implementations, asides perhaps the fact that DNS relies heavily on
UDP, the easy abuse of which is at the source of the problem. They UDP, the easy abuse of which is at the source of the problem. They
have preferentially used DNS due to common default configurations have preferentially used DNS due to common default configurations
that allow for easy use of public recursive nameservers that make use that allow for easy use of open recursive nameservers that make use
of such a default configuration. of such a default configuration.
In addition, due to the small query-large response potential of the In addition, due to the small query-large response potential of the
DNS system it is easy to yield great amplification of the source DNS system it is easy to yield great amplification of the source
traffic as reflected traffic towards the victims. traffic as reflected traffic towards the victims.
DNS authority servers which do not provide recursion to clients can DNS authority servers which do not provide recursion to clients can
also be used as amplifiers; however, the amplification potential is also be used as amplifiers; however, the amplification potential is
greatly reduced when authority servers are used. It is also not greatly reduced when authority servers are used. It is also not
practical to restrict access to authority servers to a subset of the practical to restrict access to authority servers to a subset of the
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2. Problem Description 2. 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 using the source address of their attacker then crafts a query using the source address of their
target victim and sends it to a public recursive nameserver target victim and sends it to a open recursive nameserver (ORNS).
(PRNS). 3. Each ORNS proceeds with the resolution, caches the LRECORD and
3. Each PRNS 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 PRNS 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
in the PRNS, afford a way to clean the cached LRECORD from the in the ORNS, afford a way to clean the cached LRECORD from the
PRNS. This would possibly involve queries luring the PRNS to ORNS. This would possibly involve queries luring the ORNS to
lookup information for the same name that is being used in the lookup information for the same name that is being used in the
amplification. amplification.
Because the characteristics of the attack normally involve a low Because the characteristics of the attack normally involve a low
volume of packets amongst all the kinds of actors besides the victim volume of packets amongst all the kinds of actors besides the victim
(AZONE, ZCLIENTS, PRNS), it's unlikely any one of them would notice (AZONE, ZCLIENTS, ORNS), it's unlikely any one of them would notice
their involvement based on traffic pattern changes. their involvement based on traffic pattern changes.
Taking advantage of PRNS that support EDNS0 [RFC2671], the Taking advantage of ORNS that support EDNS0 [RFC2671], the
amplification factor (response size / query size) could be around 80. amplification factor (response packet size / query packet size) could
With this amplification factor a relatively small army of ZCLIENTS be around 80. With this amplification factor a relatively small army
and PRNS could generate gigabits of traffic towards the victim. of ZCLIENTS and ORNS could generate gigabits of traffic towards the
victim.
Even if this attach is only really possible due to non-deployment of Even if this attach is only really possible due to non-deployment of
BCP 38, this amplification attack is easier to leverage because for BCP 38, this amplification attack is easier to leverage because for
historical reasons, out of times when the Internet was a much closer- historical reasons, out of 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
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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
and filter them through and Access Control List (ACL) to service and filter them through and Access Control List (ACL) to service
only the intended clients. only the intended clients.
o Use TSIG [RFC2845] signed queries to authenticate the clients. o Incoming Interface based selection. Use the incoming interface
This is a less error prone method, which allows server operators for the query as a discriminator to select which clients are to be
to provide service to clients who change IP address frequently served. This is of particular applicability for SOHO devices,
(e.g. roaming clients). The current drawback of this method is such as broadband routers that include embedded recursive name
that very few stub resolver implementations support TSIG signing servers.
of outgoing queries. The effective use of this method implies in
most cases running a local instance of a caching nameserver or o Use TSIG [RFC2845] or SIG(0) [RFC2931] signed queries to
forwarder that will be able to TSIG sign the queries and send them authenticate the clients. This is a less error prone method,
on to the recursive nameserver of choice. which allows server operators to provide service to clients who
change IP address frequently (e.g. roaming clients). The current
drawback of this method is that very few stub resolver
implementations support TSIG or SIG(0) signing of outgoing
queries. The effective use of this method implies in most cases
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
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.
4. Acknowledgments 4. Acknowledgments
Joe Abley, Andrew Sullivan Joe Abley, Andrew Sullivan
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[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.
[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 (
SIG(0)s)", RFC 2931, September 2000.
6.2. Informative References 6.2. 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.
Authors' Addresses Authors' Addresses
Joao Damas Joao Damas
Internet Systems Consortium, Inc. Internet Systems Consortium, Inc.
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