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Network Working Group                                           J. Damas
Internet-Draft                                                       ISC
Intended status: Best Current                                   F. Neves
Practice                                                     Registro.br
Expires: June 5, 2008                                   December 3, 2007

      Preventing Use of Recursive Nameservers in Reflector Attacks

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Document Terminology  . . . . . . . . . . . . . . . . . . . . . 3
   3.  Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Recommended Configuration . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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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, aside perhaps the fact that DNS relies heavily on
   UDP, the easy abuse of which is at the source of the problem.  They
   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 which 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
   not practical 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, and 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 truly 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 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 (ORNS).
   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 for the
       duration of the TTL at the open recursive nameserver 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 open recursive nameserver that support 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 and NAPTR as used in ENUM services,
   authoritative servers will eventually be more useful as actors in
   this sort of amplification attack.

   Even if this attack is only really possible due to non-deployment of
   BCP38, this amplification attack is easier to leverage because for
   historical reasons, from times when the Internet was a much closer-
   knit community, some nameserver implementations have been 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 ones 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 network operators and private server managers who
   decide to operate nameservers with the aim of optimising their DNS
   service, as these are more likely to use default configurations as

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   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 having a given recursive nameserver be 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 only to the intended clients.  Client
   authorization can be usually 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 name server'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 devices,
      such as broadband routers that include embedded recursive name

   o  Use TSIG [RFC2845] or SIG(0) [RFC2931] signed queries to
      authenticate the clients.  This is a less error prone method,
      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.

   o  For mobile users use a local caching name server 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 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.

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.  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",
              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.

   [RFC2931]  Eastlake, D., "DNS Request and Transaction Signatures (
              SIG(0)s)", RFC 2931, September 2000.

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8.2.  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
   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

   Phone: +55 11 5509 3511
   Email: fneves@registro.br
   URI:   http://registro.br/

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