Network Working Group                                           J. Damas
Internet-Draft                                                       ISC
Intended status: Informational Best Current                                   F. Neves
Expires: August 17, 2007
                                                       February 13,
Expires: January 10, 2008                                   July 9, 2007

      Preventing Use of Recursive Nameservers in Reflector Attacks

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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 on DOS in Denial of Service (DoS)
   attacks.  Recommended configuration as measures to mitigate the
   attack are given.

Table of Contents

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

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 (LRECORD) on any 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 for LRECORD using the source address
       of their target victim and sends it to an open recursive
       nameserver (ORNS).
   3.  Each ORNS proceeds with the resolution, caches the LRECORD and
       finally sends it to the target.  After this first lookup, access
       to the authoritative nameservers for AZONE is normally no longer
       necessary.  The LRECORD will remain cached for the duration of
       the TTL at the ORNS even if the AZONE is corrected.
   4.  Cleanup of the AZONE might, depending on the implementation used
       in the ORNS, afford a way to clean the cached LRECORD from the
       ORNS.  This would possibly involve queries luring the ORNS 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
   (AZONE, ZCLIENTS, ORNS), it's unlikely any one of them would notice
   their involvement based on traffic pattern changes.

   Taking advantage of ORNS 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 ZCLIENTS and ORNS could generate gigabits of traffic towards the

   Even if this attack is only really possible due to non-deployment of
   BCP 38, 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 authors target audience also want to draw the
   attention 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
   shipped by implementors.


4.  Recommended Configuration

   From the description of the problem in the previous section it
   follows that the solution to these sort of attacks is the wide
   deployment of ingress filtering [BCP38] in routers to prevent use of
   address spoofing as a viable course of action to prevent the attacks.
   In situations were more complex network setups are in place, "Ingress
   Filtering for Multihomed Network" [BCP84] maybe a useful additional

   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.

   The authors also want to note that with

   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, stressing even more the need for
   deployment of BCP 38.

   In this section we describe the Current 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
   authentication can be usually done in several ways:

   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  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.

   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.

   By default, nameservers SHOULD not NOT offer recursive service to
   external networks.

4.  Acknowledgments

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

5.  Security Considerations

   This document does not create any new security issues for the DNS
   protocol, it deals with a weakness in implementations.

   It's not excessive to repeat that, although recommended
   configurations described in this document could alleviate the
   problem, the only solution to source address spoofing problems is the
   wide-scale deployment of Ingress Filtering to prevent use of spoofed
   IP addresses [BCP38], [BCP84].

6.  Acknowledgments

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

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.


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

   Frederico A. C. Neves /
   Av. das Nacoes Unidas, 11541, 7
   Sao Paulo, SP  04578-000

   Phone: +55 11 5509 3511

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