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This document describes ways to prevent the use of default configured recursive nameservers as reflectors in Denial of Service (DoS) attacks. Recommended configuration as measures to mitigate the attack are given.
2. Document Terminology
3. Problem Description
4. Recommended Configuration
5. Security Considerations
7. IANA Considerations
8.1. Normative References
8.2. Informative References
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
Recently, DNS [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) 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] (Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” May 2000.).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
Because most DNS traffic is stateless by design, an attacker could start a DoS attack in the following way:
Because the characteristics of the attack normally involve a low volume of packets amongst all the kinds of actors besides the victim, it's unlikely any one of them would notice their involvement based on traffic pattern changes.
Taking advantage of open recursive nameserver that support EDNS0 [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.), the amplification factor (response packet size / query packet size) could be around 80. With this amplification factor a relatively small army of clients and open recursive nameservers could generate gigabits of traffic towards the victim.
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.
Even if this attack is only really possible due to non-deployment of BCP38, 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 target audience also includes operators of small networks 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.
In this section we describe the 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 authorization can be usually done in several ways:
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.
Even with all these recommendations network operators should consider deployment of ingress filtering [BCP38] (Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” May 2000.) in routers to prevent use of address spoofing as a viable course of action. In situations where more complex network setups are in place, "Ingress Filtering for Multihomed Network" [BCP84] (Baker, F. and P. Savola, “Ingress Filtering for Multihomed Networks,” March 2004.) maybe a useful additional reference.
By default, nameservers SHOULD NOT offer recursive service to external networks.
This document does not create any new security issues for the DNS protocol, it deals with a weakness in implementations.
Deployment of SIG(0) transaction security should consider the caveats with SIG(0) computational expense as it uses public key cryptography rather than the symmetric keys used by TSIG. In addition, the identification of the appropriate keys needs similar mechanisms to those for deploying TSIG, or alternatively, the use of DNSSEC signatures (RRSIGs) over the KEY RRs if published in DNS. This will in turn require the appropriate management of DNSSEC trust anchors.
The authors would like to acknowledge the helpful input and comments of Joe Abley, Olafur Gudmundsson, Pekka Savola, Andrew Sullivan and Tim Polk.
This document does not define a registry and does not require any IANA action.
|[RFC1034]||Mockapetris, P., “Domain names - concepts and facilities,” STD 13, RFC 1034, November 1987 (TXT).|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[RFC2671]||Vixie, P., “Extension Mechanisms for DNS (EDNS0),” RFC 2671, August 1999 (TXT).|
|[RFC2845]||Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington, “Secret Key Transaction Authentication for DNS (TSIG),” RFC 2845, May 2000 (TXT).|
|[RFC2931]||Eastlake, D., “DNS Request and Transaction Signatures ( SIG(0)s),” RFC 2931, September 2000 (TXT).|
|[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 (TXT).|
|[BCP84]||Baker, F. and P. Savola, “Ingress Filtering for Multihomed Networks,” BCP 84, RFC 3704, March 2004 (TXT).|
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