Network Working Group J. Damas
Internet-Draft ISC
Intended status: Informational F. Neves
Expires: August 17, 2007 Registro.br
February 13, 2007
Preventing Use of Recursive Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-03.txt
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Copyright (C) The IETF Trust (2007).
Abstract
This document describes ways to prevent the use of default configured
recursive nameservers 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. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6
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. 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
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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
amplification.
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
victim.
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 also want to draw the
attention 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.
3. 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
reference.
Nonetheless, the fact remains that DNS servers acting as open
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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 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 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
servers.
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.
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By default, nameservers SHOULD 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'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. References
6.1. 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.
[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
6.2. 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.
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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/
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