wdiff draft-ietf-dnsop-reflectors-are-evil-00.txt draft-ietf-dnsop-reflectors-are-evil-01.txt

Network Working Group J. Damas
Internet-Draft ISC
Expires: November 18, December 27, 2006 F. Neves
Registro.br
May 17,
June 25, 2006

Preventing Use of Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-00.txt
draft-ietf-dnsop-reflectors-are-evil-01.txt

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Abstract

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5.
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
5.2. 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 5 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 7
Intellectual Property and Copyright Statements . . . . . . . . . . 7 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,
while are
not being due to any particular flaw in the design of the DNS or its
implementations, asides 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 public recursive
name servers 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 authority servers which do not provide recursion to clients can
also be used as amplifiers; however, the amplification potential is
greatly reduced when authority servers are used. It is also not
practical to restrict access to authority 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 authority 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,
problem described, while pointing to the only truly real solution to solution,
the problem, the wide-
scale wide-scale deployment of Ingress Filtering 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 design, an attacker could make use of the following scenario to
start a DOS DoS attack using DNS packets: in the following way:

1. The attacker starts by configuring a record (LRECORD) on an
undistinct 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 using the source address of their
target victim and sends it to a Public Recursive Name Server public recursive nameserver
(PRNS).
3. The Each PRNS proceeds with the resolution, caches the LRECORD and
finally sends it to the target. After this first packet, lookup, access
to the authoritative name servers nameservers 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. This would possibly involve queries luring the PRNS to
lookup information for the same name that is being used in the
amplification.

Because the characteristics of the attack normally use involve a low
volume of packets on amongst 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.

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

Even if this attach is only really possible due to non-deployment of
BCP 38, this amplification attack is possible easier to leverage because for
historical reasons, out of times when the Internet was a much closer-knit closer-
knit community, some
name server nameserver implementations have been made
available with default configurations that when used for recursive name servers
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 the subject of this document
tries aims to make
apparent, it is now much better to be conscious of ones own name server
nameserver services and focus the delivery of services on the
intended audience of those services, may them be they a University Campus, university campus, an Enterprise
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 nameservers with the aim of optimizing 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 this sort of attacks is the wide
deploying
deployment of ingress filtering [BCP38] 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.

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 name servers. nameservers. Following these recommendations would reduce
the chances of having a given recursive name server nameserver be used for the
generation of an amplification attack.

The generic recommendation to name server 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 Use TSIG [RFC2845]signed [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.
(e.g. 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 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.

4. Acknowledgments

Joe Abley, 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 all kinds of source address spoofing
problems is the wide-scale deployment of Ingress Filtering to prevent
use of spoofed IP addresses [BCP38].

6. References

5.1.

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.

5.2.

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.

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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