Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


Network Working Group J. Damas Request for Comments: 5358 ISC BCP: 140 F. Neves Category: Best Current Practice

                                                          October 2008
    Preventing Use of Recursive Nameservers in Reflector Attacks

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.


 This document describes ways to prevent the use of default configured
 recursive nameservers as reflectors in Denial of Service (DoS)
 attacks.  It provides recommended configuration as measures to
 mitigate the attack.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Document Terminology  . . . . . . . . . . . . . . . . . . . . . 2
 3.  Problem Description . . . . . . . . . . . . . . . . . . . . . . 2
 4.  Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
 6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
   7.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

Damas & Neves Best Current Practice [Page 1] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

1. Introduction

 Recently, DNS [RFC1034] has been named as a major factor in the
 generation of massive amounts of network traffic used in Denial of
 Service (DoS) attacks.  These attacks, called reflector attacks, are
 not due to any particular flaw in the design of the DNS or its
 implementations, except that DNS relies heavily on UDP, the easy
 abuse of which is at the source of the problem.  The attacks have
 preferentially used DNS due to common default configurations that
 allow for easy use of open recursive nameservers that make use of
 such a default configuration.
 In addition, due to the small query-large response potential of the
 DNS system, it is easy to yield great amplification of the source
 traffic as reflected traffic towards the victims.
 DNS authoritative servers that do not provide recursion to clients
 can also be used as amplifiers; however, the amplification potential
 is greatly reduced when authoritative servers are used.  It is also
 impractical to restrict access to authoritative servers to a subset
 of the Internet, since their normal operation relies on them being
 able to serve a wide audience; hence, the opportunities to mitigate
 the scale of an attack by modifying authoritative server
 configurations are limited.  This document's recommendations are
 concerned with recursive nameservers only.
 In this document we describe the characteristics of the attack and
 recommend DNS server configurations that specifically alleviate the
 problem described, while pointing to the only real solution: the
 wide-scale deployment of ingress filtering to prevent use of spoofed
 IP addresses [BCP38].

2. Document Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [RFC2119].

3. Problem Description

 Because most DNS traffic is stateless by design, an attacker could
 start a DoS attack in the following way:
 1.  The attacker starts by configuring a record on any zone he has
     access to, normally with large RDATA and Time to Live (TTL).

Damas & Neves Best Current Practice [Page 2] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

 2.  Taking advantage of clients on non-BCP38 networks, the attacker
     then crafts a query using the source address of their target
     victim and sends it to an open recursive nameserver.
 3.  Each open recursive nameserver proceeds with the resolution,
     caches the record, and finally sends it to the target.  After
     this first lookup, access to the authoritative nameservers is
     normally no longer necessary.  The record will remain cached at
     the open recursive nameserver for the duration of the TTL, even
     if it's deleted from the zone.
 4.  Cleanup of the zone might, depending on the implementation used
     in the open recursive nameserver, afford a way to clean the
     cached record from the open recursive nameserver.  This would
     possibly involve queries luring the open recursive nameserver to
     lookup information for the same name that is being used in the
 Because the characteristics of the attack normally involve a low
 volume of packets amongst all the kinds of actors besides the victim,
 it's unlikely any one of them would notice their involvement based on
 traffic pattern changes.
 Taking advantage of an open recursive nameserver that supports EDNS0
 [RFC2671], the amplification factor (response packet size / query
 packet size) could be around 80.  With this amplification factor, a
 relatively small army of clients and open recursive nameservers could
 generate gigabits of traffic towards the victim.
 With the increasing length of authoritative DNS responses derived
 from deployment of DNSSEC [RFC4033] and NAPTR resource records as
 used in ENUM services, authoritative servers will eventually be more
 useful as actors in this sort of amplification attack.
 Even if this amplification attack is only possible due to non-
 deployment of BCP38, it is easier to leverage because of historical
 reasons.  When the Internet was a much closer-knit community, some
 nameserver implementations were made available with default
 configurations that, when used for recursive nameservers, made the
 server accessible to all hosts on the Internet.
 For years this was a convenient and helpful configuration, enabling
 wider availability of services.  As this document aims to make
 apparent, it is now much better to be conscious of one's own
 nameserver services and focus the delivery of services on the
 intended audience of those services -- be they a university campus,
 an enterprise, or an ISP's customers.  The target audience also
 includes operators of small networks and private server managers who

Damas & Neves Best Current Practice [Page 3] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

 decide to operate nameservers with the aim of optimising their DNS
 service, as these are more likely to use default configurations as
 shipped by implementors.

4. Recommended Configuration

 In this section we describe the Best Current Practice for operating
 recursive nameservers.  Following these recommendations would reduce
 the chances of any given recursive nameserver being used for the
 generation of an amplification attack.
 The generic recommendation to nameserver operators is to use the
 means provided by the implementation of choice to provide recursive
 name lookup service to only the intended clients.  Client
 authorization can usually be done in several ways:
 o  IP address based authorization.  Use the IP source address of the
    DNS queries and filter them through an Access Control List (ACL)
    to service only the intended clients.  This is easily applied if
    the recursive nameserver's service area is a reasonably fixed IP
    address range that is protected against external address spoofing,
    usually the local network.
 o  Incoming interface based selection.  Use the incoming interface
    for the query as a discriminator to select which clients are to be
    served.  This is of particular applicability for SOHO (Small
    Office, Home Office) devices, such as broadband routers that
    include embedded recursive nameservers.
 o  TSIG [RFC2845] or SIG(0) [RFC2931] signed queries to authenticate
    the clients.  This is a less error prone method that 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.
 o  For mobile users, use a local caching nameserver running on the
    mobile device or use a Virtual Private Network to a trusted
 In nameservers that do not need to be providing recursive service,
 for instance servers that are meant to be authoritative only, turn
 recursion off completely.  In general, it is a good idea to keep
 recursive and authoritative services separate as much as practical.
 This, of course, depends on local circumstances.

Damas & Neves Best Current Practice [Page 4] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

 Even with all these recommendations, network operators should
 consider deployment of ingress filtering [BCP38] in routers to
 prevent use of address spoofing as a viable course of action.  In
 situations where more complex network setups are in place, "Ingress
 Filtering for Multihomed Network" [BCP84] maybe a useful additional
 By default, nameservers SHOULD NOT offer recursive service to
 external networks.

5. Security Considerations

 This document does not create any new security issues for the DNS
 protocol, it deals with a weakness in implementations.
 Deployment of SIG(0) transaction security [RFC2931] should consider
 the caveats with SIG(0) computational expense as it uses public key
 cryptography rather than the symmetric keys used by TSIG [RFC2845].
 In addition, the identification of the appropriate keys needs similar
 mechanisms as those for deploying TSIG or, alternatively, the use of
 DNSSEC [RFC4033] signatures (RRSIGs) over the KEY RRs if published in
 DNS.  This will in turn require the appropriate management of DNSSEC
 trust anchors.

6. Acknowledgments

 The authors would like to acknowledge the helpful input and comments
 of Joe Abley, Olafur Gudmundsson, Pekka Savola, Andrew Sullivan, and
 Tim Polk.

7. References

7.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            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)",
            RFC 2671, August 1999.
 [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D., and B.
            Wellington, "Secret Key Transaction Authentication for DNS
            (TSIG)", RFC 2845, May 2000.

Damas & Neves Best Current Practice [Page 5] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

 [RFC2931]  Eastlake, D., "DNS Request and Transaction Signatures
            (SIG(0)s)", RFC 2931, September 2000.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, March 2005.

7.2. Informative References

 [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, May 2000.
 [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
            Networks", BCP 84, RFC 3704, March 2004.

Authors' Addresses

 Joao Damas
 Internet Systems Consortium, Inc.
 950 Charter Street
 Redwood City, CA  94063
 Phone: +1 650 423 1300
 Frederico A. C. Neves /
 Av. das Nacoes Unidas, 11541, 7
 Sao Paulo, SP  04578-000
 Phone: +55 11 5509 3511

Damas & Neves Best Current Practice [Page 6] RFC 5358 Preventing Rec. NS in Reflector Attacks October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at

Damas & Neves Best Current Practice [Page 7]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5358.txt · Last modified: 2008/10/16 16:21 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki