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Network Working Group                                           J. DamasRequest for Comments: 5358                                           ISCBCP: 140                                                        F. NevesCategory: Best Current Practice                              Registro.br                                                            October 2008Preventing Use of Recursive Nameservers in Reflector AttacksStatus 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.Abstract   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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Document Terminology  . . . . . . . . . . . . . . . . . . . . .23.  Problem Description . . . . . . . . . . . . . . . . . . . . . .24.  Recommended Configuration . . . . . . . . . . . . . . . . . . .45.  Security Considerations . . . . . . . . . . . . . . . . . . . .56.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .57.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .57.1.  Normative References  . . . . . . . . . . . . . . . . . . .57.2.  Informative References  . . . . . . . . . . . . . . . . . .6Damas & Neves            Best Current Practice                  [Page 1]

RFC 5358        Preventing Rec. NS in Reflector Attacks     October 20081.  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",   "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   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       amplification.   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 ofBCP38, 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 whoDamas & 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      server.   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   reference.   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.  References7.1.  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   US   Phone: +1 650 423 1300   EMail: Joao_Damas@isc.org   URI:http://www.isc.org/   Frederico A. C. Neves   NIC.br / Registro.br   Av. das Nacoes Unidas, 11541, 7   Sao Paulo, SP  04578-000   BR   Phone: +55 11 5509 3511   EMail: fneves@registro.br   URI:http://registro.br/Damas & Neves            Best Current Practice                  [Page 6]

RFC 5358        Preventing Rec. NS in Reflector Attacks     October 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.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 inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   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   ietf-ipr@ietf.org.Damas & Neves            Best Current Practice                  [Page 7]