Network Working Group J. Damas
Internet-Draft ISC
Expires: November 18, 2006 F. Neves
Registro.br
May 17, 2006
Preventing Use of Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
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
(PRNS).
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
PRNS.
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
victim.
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
protocol.
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
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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