draft-ietf-dnsop-reflectors-are-evil-00.txt   draft-ietf-dnsop-reflectors-are-evil-01.txt 
Network Working Group J. Damas Network Working Group J. Damas
Internet-Draft ISC Internet-Draft ISC
Expires: November 18, 2006 F. Neves Expires: December 27, 2006 F. Neves
Registro.br Registro.br
May 17, 2006 June 25, 2006
Preventing Use of Nameservers in Reflector Attacks Preventing Use of Nameservers in Reflector Attacks
draft-ietf-dnsop-reflectors-are-evil-00.txt draft-ietf-dnsop-reflectors-are-evil-01.txt
Status of this Memo Status of this Memo
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skipping to change at page 1, line 35 skipping to change at page 1, line 35
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This Internet-Draft will expire on November 18, 2006. This Internet-Draft will expire on December 27, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document describes the use of default configured recursive name This document describes the use of default configured recursive
servers as reflectors on DOS attacks. Recommended configuration as nameservers as reflectors on DOS attacks. Recommended configuration
measures to mitigate the attack are given. as measures to mitigate the attack are given.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4 3. Recommended Configuration . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
1. Introduction 1. Introduction
Recently, DNS [RFC1034] has been named as a major factor in the Recently, DNS [RFC1034] has been named as a major factor in the
generation of massive amounts of network traffic used in Denial of generation of massive amounts of network traffic used in Denial of
Service (DoS) attacks. These attacks, called reflector attacks, Service (DoS) attacks. These attacks, called reflector attacks, are
while not being due to any particular flaw in the design of the DNS not due to any particular flaw in the design of the DNS or its
or its implementations, have preferentially used DNS due to common implementations, asides perhaps the fact that DNS relies heavily on
default configurations that allow for easy use of public recursive UDP, the easy abuse of which is at the source of the problem. They
name servers that make use of such a default configuration. have preferentially used DNS due to common default configurations
that allow for easy use of public recursive nameservers that make use
of such a default configuration.
In addition, due to the small query-large response potential of the In addition, due to the small query-large response potential of the
DNS system it is easy to yield great amplification of the source DNS system it is easy to yield great amplification of the source
traffic as reflected traffic towards the victims. 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 In this document we describe the characteristics of the attack and
recommend DNS server configurations that alleviate the problem, while recommend DNS server configurations that specifically alleviate the
pointing to the only truly real solution to the problem, the wide- problem described, while pointing to the only truly real solution,
scale deployment of Ingress Filtering to prevent use of spoofed IP the wide-scale deployment of ingress filtering to prevent use of
addresses [BCP38]. spoofed IP addresses [BCP38].
2. Problem Description 2. Problem Description
Because of the fact that most of the DNS traffic is stateless by Because most DNS traffic is stateless by design, an attacker could
design an attacker could make use of the following scenario to start start a DoS attack in the following way:
a DOS attack using DNS packets:
1. The attacker starts by configuring a record (LRECORD) on an 1. The attacker starts by configuring a record (LRECORD) on any zone
undistinct zone he has access to (AZONE), normally with large he has access to (AZONE), normally with large RDATA and TTL.
RDATA and TTL.
2. Taking advantage of clients (ZCLIENTS) on non-BCP38 networks, the 2. Taking advantage of clients (ZCLIENTS) on non-BCP38 networks, the
attacker then crafts a query using the source address of their attacker then crafts a query using the source address of their
target victim and sends it to a Public Recursive Name Server target victim and sends it to a public recursive nameserver
(PRNS). (PRNS).
3. The PRNS proceeds with the resolution, caches the LRECORD and 3. Each PRNS proceeds with the resolution, caches the LRECORD and
finally sends it to the target. After this first packet, access finally sends it to the target. After this first lookup, access
to the authoritative name servers for AZONE is normally no longer to the authoritative name servers for AZONE is normally no longer
necessary. The LRECORD will remain cached for the duration of necessary. The LRECORD will remain cached for the duration of
the TTL at the PRNS even if the AZONE is corrected. the TTL at the PRNS even if the AZONE is corrected.
4. Cleanup of the AZONE might, depending on the implementation used 4. Cleanup of the AZONE might, depending on the implementation used
in the PRNS, afford a way to clean the cached LRECORD from the in the PRNS, afford a way to clean the cached LRECORD from the
PRNS. 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 a low volume Because the characteristics of the attack normally involve a low
of packets on all the kinds of actors besides the victim (AZONE, volume of packets amongst all the kinds of actors besides the victim
ZCLIENTS, PRNS), it's unlikely any one of them would notice their (AZONE, ZCLIENTS, PRNS), it's unlikely any one of them would notice
involvement based on traffic pattern changes. their involvement based on traffic pattern changes.
Taking advantage of PRNS that support EDNS0 [RFC2671], the Taking advantage of PRNS that support EDNS0 [RFC2671], the
amplification factor (response size / query size) could be around 80. amplification factor (response size / query size) could be around 80.
With this amplification factor a relatively small army of ZCLIENTS With this amplification factor a relatively small army of ZCLIENTS
and PRNS could generate gigabits of traffic towards the targetted and PRNS could generate gigabits of traffic towards the victim.
victim.
This amplification attack is possible because for historical reasons, Even if this attach is only really possible due to non-deployment of
out of times when the Internet was a much closer-knit community, some BCP 38, this amplification attack is easier to leverage because for
name server implementations have been made available with default historical reasons, out of times when the Internet was a much closer-
configurations that when used for recursive name servers made the knit community, some nameserver implementations have been made
server accessible to all hosts on the Internet. 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 For years this was a convenient and helpful configuration, enabling
wider availability of services. As the subject of this document wider availability of services. As this document aims to make
tries to make apparent, it is now much better to be conscious of ones apparent, it is now much better to be conscious of ones own
own name server services and focus the delivery of services on the nameserver services and focus the delivery of services on the
intended audience of those services, may them be a University Campus, intended audience of those services, be they a university campus, an
an Enterprise or an ISP's customers. The authors also want to draw enterprise or an ISP's customers. The authors also want to draw the
the attention of small network operators and private server managers attention of small network operators and private server managers who
who decide to operate name servers with the aim of optimizing their decide to operate nameservers with the aim of optimising their DNS
DNS service, as these are more likely to use default configurations service, as these are more likely to use default configurations as
as shipped by implementors. shipped by implementors.
3. Recommended Configuration 3. Recommended Configuration
From the description of the problem in the previous section it From the description of the problem in the previous section it
follows that the solution to this sort of attacks is the wide follows that the solution to this sort of attacks is the wide
deploying of ingress filtering in routers to prevent use of address deployment of ingress filtering [BCP38] in routers to prevent use of
spoofing as a viable course of action to elicit the attacks. address spoofing as a viable course of action to elicit the attacks.
Nonetheless, the fact remains that DNS servers acting as open Nonetheless, the fact remains that DNS servers acting as open
recursive servers provide an easy means to obtain great rates of recursive servers provide an easy means to obtain great rates of
amplification for attack traffic, requiring only a small amount of amplification for attack traffic, requiring only a small amount of
traffic from the attack sources to generate a vast amount of traffic traffic from the attack sources to generate a vast amount of traffic
towards the victim. 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 In this section we describe the Current Best Practice for operating
recursive name servers. Following these recommendations would reduce recursive name servers. Following these recommendations would reduce
the chances of having a given recursive name server be used for the the chances of having a given recursive name server be used for the
generation of an amplification attack. generation of an amplification attack.
The generic recommendation to name server operators is to use the The generic recommendation to name server operators is to use the
means provided by the implementation of choice to provide recursive means provided by the implementation of choice to provide recursive
name lookup service only to the intended clients. Client name lookup service only to the intended clients. Client
authentication can be usually done in several ways: authentication can be usually done in several ways:
o IP based authentication. Use the IP address of the sending host o IP based authentication. Use the IP address of the sending host
and filter them through and Access Control List (ACL) to service and filter them through and Access Control List (ACL) to service
only the intended clients. only the intended clients.
o Use TSIG [RFC2845]signed queries to authenticate the clients. o Use TSIG [RFC2845]signed queries to authenticate the clients.
This is a less error prone method, which allows server operators This is a less error prone method, which allows server operators
to provide service to clients who change IP address frequently to provide service to clients who change IP address frequently
(eg. roaming clients). The current drawback of this method is (e.g. roaming clients). The current drawback of this method is
that very few stub resolver implementations support TSIG signing that very few stub resolver implementations support TSIG signing
of outgoing queries. The effective use of this method implies in of outgoing queries. The effective use of this method implies in
most cases running a local instance of a caching nameserver or most cases running a local instance of a caching nameserver or
forwarder that will be able to TSIG sign the queries and send them forwarder that will be able to TSIG sign the queries and send them
on to the recursive name server of choice. on to the recursive name server of choice.
4. Security Considerations 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 This document does not create any new security issues for the DNS
protocol. protocol.
It's not excessive to repeat that, although recommended It's not excessive to repeat that, although recommended
configurations described in this document could alleviate the configurations described in this document could alleviate the
problem, the only solution to all kinds of source address spoofing problem, the only solution to all kinds of source address spoofing
problems is the wide-scale deployment of Ingress Filtering to prevent problems is the wide-scale deployment of Ingress Filtering to prevent
use of spoofed IP addresses [BCP38]. use of spoofed IP addresses [BCP38].
5. References 6. References
5.1. Normative References 6.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. RFC 2671, August 1999.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B. [RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000. (TSIG)", RFC 2845, May 2000.
5.2. Informative References 6.2. Informative References
[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering: [BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000. Address Spoofing", BCP 38, RFC 2827, May 2000.
Authors' Addresses Authors' Addresses
Joao Damas Joao Damas
Internet Systems Consortium, Inc. Internet Systems Consortium, Inc.
950 Charter Street 950 Charter Street
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