Network Working Group                                           J. Damas
Internet-Draft                                                       ISC
Expires: November 18, 2006                                      F. Neves
                                                            May 17, 2006

           Preventing Use of Nameservers in Reflector Attacks

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes the use of default configured recursive name
   servers as reflectors on DOS attacks.  Recommended configuration as
   measures to mitigate the attack are given.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7

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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,
   while not being due to any particular flaw in the design of the DNS
   or its implementations, have preferentially used DNS due to common
   default configurations that allow for easy use of public recursive
   name servers 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.

   In this document we describe the characteristics of the attack and
   recommend DNS server configurations that alleviate the problem, while
   pointing to the only truly real solution to the problem, the wide-
   scale deployment of Ingress Filtering to prevent use of spoofed IP
   addresses [BCP38].

2.  Problem Description

   Because of the fact that most of the DNS traffic is stateless by
   design an attacker could make use of the following scenario to start
   a DOS attack using DNS packets:

   1.  The attacker starts by configuring a record (LRECORD) on an
       undistinct zone he has access to (AZONE), normally with large
       RDATA and TTL.
   2.  Taking advantage of clients (ZCLIENTS) on non-BCP38 networks, the
       attacker then crafts a query using the source address of their
       target victim and sends it to a Public Recursive Name Server
   3.  The PRNS proceeds with the resolution, caches the LRECORD and
       finally sends it to the target.  After this first packet, access
       to the authoritative name servers for AZONE is normally no longer
       necessary.  The LRECORD will remain cached for the duration of
       the TTL at the PRNS even if the AZONE is corrected.
   4.  Cleanup of the AZONE might, depending on the implementation used
       in the PRNS, afford a way to clean the cached LRECORD from the

   Because the characteristics of the attack normally use a low volume
   of packets on all the kinds of actors besides the victim (AZONE,
   ZCLIENTS, PRNS), it's unlikely any one of them would notice their
   involvement based on traffic pattern changes.

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   Taking advantage of PRNS that support EDNS0 [RFC2671], the
   amplification factor (response size / query size) could be around 80.
   With this amplification factor a relatively small army of ZCLIENTS
   and PRNS could generate gigabits of traffic towards the targetted

   This amplification attack is possible because for historical reasons,
   out of times when the Internet was a much closer-knit community, some
   name server implementations have been made available with default
   configurations that when used for recursive name servers 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 the subject of this document
   tries to make apparent, it is now much better to be conscious of ones
   own name server services and focus the delivery of services on the
   intended audience of those services, may them be a University Campus,
   an Enterprise or an ISP's customers.  The authors also want to draw
   the attention of small network operators and private server managers
   who decide to operate name servers with the aim of optimizing their
   DNS service, as these are more likely to use default configurations
   as shipped by implementors.

3.  Recommended Configuration

   From the description of the problem in the previous section it
   follows that the solution to this sort of attacks is the wide
   deploying of ingress filtering in routers to prevent use of address
   spoofing as a viable course of action to elicit the attacks.

   Nonetheless, the fact remains that DNS servers acting as open
   recursive servers provide an easy means to obtain great rates of
   amplification for attack traffic, requiring only a small amount of
   traffic from the attack sources to generate a vast amount of traffic
   towards the victim.

   In this section we describe the Current Best Practice for operating
   recursive name servers.  Following these recommendations would reduce
   the chances of having a given recursive name server be used for the
   generation of an amplification attack.

   The generic recommendation to name server operators is to use the
   means provided by the implementation of choice to provide recursive
   name lookup service only to the intended clients.  Client
   authentication can be usually done in several ways:

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   o  IP based authentication.  Use the IP address of the sending host
      and filter them through and Access Control List (ACL) to service
      only the intended clients.

   o  Use TSIG [RFC2845]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
      (eg. roaming clients).  The current drawback of this method is
      that very few stub resolver implementations support TSIG 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 name server of choice.

4.  Security Considerations

   This document does not create any new security issues for the DNS

   It's not excessive to repeat that, although recommended
   configurations described in this document could alleviate the
   problem, the only solution to all kinds of source address spoofing
   problems is the wide-scale deployment of Ingress Filtering to prevent
   use of spoofed IP addresses [BCP38].

5.  References

5.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [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.

5.2.  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.

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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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