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Preventing Use of Recursive Nameservers in Reflector Attacks
RFC 5358 also known as BCP 140

Document Type RFC - Best Current Practice (October 2008)
Authors Frederico Neves , Joao da Silva Damas
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Ron Bonica
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RFC 5358
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

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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).

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   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

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   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.

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   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.

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   [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

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RFC 5358        Preventing Rec. NS in Reflector Attacks     October 2008

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