DNS Operation Working Group D.Senie Internet-Draft Amaranth Networks Inc. Expires March 10, 2007 A. Sullivan Afilias September 10, 2006 Considerations for the use of DNS Reverse Mapping draft-ietf-dnsop-reverse-mapping-considerations-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 10, 2007. Abstract Mapping of addresses to names is a feature of DNS. Many sites implement it, many others do not. Some applications attempt to use it as a part of a security strategy. The goal of this document is to encourage proper deployment of address to name mappings, and provide guidance for their use. 1. Introduction 1.1 Overview The Domain Name System has provision for providing mapping of IP addresses to host names. It is common practice to ensure both name to address, and address to name mappings are provided for networks. This Senie and Sullivan [Page 1] Internet-Draft Considerations for DNS Reverse Mapping April 2004 practice is documented, but without guidelines for those who control address blocks. This document encourages proper deployment of address to name mappings, and provides guidance for their use. 1.2 Terminology 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 RFC 2119 [RFC2119]. This document uses IPv4 CIDR block sizes and allocation strategy where there are differences and uses IPv4 terminology. Aside from the terminological differences, this document can and should be applied equally to IPv6 address spaces. 2. Background From the early days of the Domain Name System [RFC883] a special domain has been set aside for resolving mappings of IP addresses to domain names. This was refined in [RFC1035], describing the .IN- ADDR.ARPA domain in use today. For the in the IPv6 address space, .IP6.ARPA was added [RFC3152]. Although in what follows, the discussion uses IPv4 CIDR block sizes, allocation strategies, and terminology, this document can and should be applied to both address spaces. The assignment of blocks of IP Address space was delegated to three regional registries. Guidelines for the registries are specified in [RFC2050], which strictly requires regional registries to maintain IN-ADDR records only on the large blocks of space issued to ISPs and others. AfriNIC supports the maintenance of IN-ADDR, as long as the requests come from AfriNIC-active Local Internet Registries [AFRINIC]. The support includes support for the guidelines in [RFC2317]. ARIN's policy requires ISPs to maintain IN-ADDR for /16 or larger allocations. For smaller allocations, ARIN can provide IN-ADDR for /24 and shorter prefixes. [ARIN]. APNIC provides methods for ISPs to update IN-ADDR, and makes the maintenance of IN-ADDR records the responsibility of the associated Local Internet Registry. If an address block is not so associated, then the maintenance falls to the appropriate National Internet Registry or to APNIC [APNIC086]. The policy is worded slightly differently (and arguably more strongly) for IPv6 addresses [APNIC089]. RIPE's policy appears to require Local Internet Regisgtries to perform further delegation on blocks delegated by RIPE to them, but it does not actually make this a Senie and Sullivan [Page 2] Internet-Draft Considerations for DNS Reverse Mapping April 2004 requirement [RIPE302]. It does, however, explicitly permit registrants of address space to delegate authority for requesting reverse delegation. As we can see, the regional registries have their own policies for requirements for IN-ADDR maintenance. It should be noted, also, that many address blocks were allocated before the creation of the regional registries, and thus it is unclear whether any of the policies of the registries are binding on those who hold blocks from that era. Registries allocate address blocks on CIDR [RFC4632] boundaries. Unfortunately the IN-ADDR zones are based on classful allocations. Guidelines [RFC2317] for delegating on non-octet-aligned boundaries exist, but are not always implemented. 3. Issues surrounding IN-ADDR 3.1 Examples of effects of missing IN-ADDR Following are some examples of some of the uses to which IN-ADDR checks are put, and some of the difficulties that can be encountered because of missing IN-ADDR records. It is important to note that some of these strategies are at best often ineffective. Nevertheless, their failure in each case produces additional load on systems and additional latency in network activity. Some applications use DNS lookups for security checks. To ensure validity of claimed names, some applications will look up IN-ADDR records to get names, and then look up the resultant name to see if it maps back to the address originally known. Failure to resolve matching names is interpreted as a potential security concern. Some popular FTP sites will simply reject user sessions, even for anonymous FTP, if the IN-ADDR lookup fails or if the result of the IN-ADDR lookup when itself resolved, does not match. Some Telnet servers also implement this check. Web sites sometimes use IN-ADDR checks to verify whether the client is located within a certain geopolitical entity. This approach has sometimes been employed for downloads of crypto software, for example, where export of that software is restricted to certain locales. Site operators may choose to refuse to allow the connection in the event they are not able to perform these checks. Credit card anti-fraud systems also sometimes use similar methods for geographic placement purposes, and may generate false alarms in the event the reverse resolution is not possible. Senie and Sullivan [Page 3] Internet-Draft Considerations for DNS Reverse Mapping April 2004 The popular TCP Wrappers program found on most Unix and Linux systems has options to enforce IN-ADDR checks and to reject any client that does not resolve. The program also has a way to check to see that the name given by a PTR record then resolves back to the same IP address. In the event that the checks fail, connections may be terminated. Poor or missing implementation of IN-ADDR on dialup, CDPD and other such client-oriented portions of the Internet results in higher latency for queries (due to lack of negative caching), and higher name server load and DNS traffic. Some anti-spam (anti junk email) systems use IN-ADDR to verify the presence of a PTR record, or validate the PTR value points back to the same address as the system originating the mail. Some mail servers have the ability to perform such checks at the time of negotiation, and to reject all mail from hosts that do not have matching IN-ADDR entries for their hostnames. These PTR checks sometimes include databases of well-known conventions for "generic naming" conventions (for example, PTR records for dynamically- assigned hostnames and IP addresses), and sometimes allow complicated rules for quarantining or filtering mail from unknown or suspect sources. Even very large ISPs may reserve the right to refuse mail from hosts without a reverse mapping. Many web servers look up the IN-ADDR of visitors to be used in log analysis. This adds to the server load, but in the case of IN-ADDR unavailability, it can lead to delayed responses for users. Morever, some statistics packages perform such lookups in retrospect, and missing reverse mapping will prevent such packages from working as expected. Traceroutes with descriptive IN-ADDR naming proves useful when debugging problems spanning large areas. When this information is missing, the traceroutes take longer, and it takes additional steps to determine that network is the cause of problems. 3.2 The difficulty with blanket policies Some users have reported difficulty in ensuring correct IN-ADDR management by their upstream providers. Users without many choices among providers, especially, can become the needless victim of aggressive IN-ADDR checks. IN-ADDR tests may also give the administrator a false sense of security. There is little evidence that an IN-ADDR test provides much in the way of security, and may make troubleshooting in the case Senie and Sullivan [Page 4] Internet-Draft Considerations for DNS Reverse Mapping April 2004 of DNS failure more difficult. It is possible that there be multiple PTRs at a single IN-ADDR node. In extreme cases, these multiple PTRs could cause a DNS response to exceed the UDP limit, and fall back to TCP. Such a case could be one where the advantages of reverse mapping are exceeded by the disadvantages of the additional burden. This may be of particular significance for "mass vitual hosting" systems, where many hostnames are associated with a single IP. 4. Recommendations 4.1 Delegation Recommendations Regional Registries and any Local Registries to whom they delegate SHOULD establish and convey a policy to those to whom they delegate blocks that IN-ADDR mappings are required. Policies SHOULD require those receiving delegations to provide IN-ADDR service, to delegate to downstream customers, or both. Network operators SHOULD define and implement policies and procedures which delegate IN-ADDR to their clients who wish to run their own IN- ADDR DNS services, and provide IN-ADDR services for those who do not have the resources to do it themselves. Such delegation mechanisms MUST permit the downstream customer to implement and comply with IETF recommendations application of IN-ADDR to CIDR [RFC2317]. All IP address space assigned and in use SHOULD be resolved by IN- ADDR records. All PTR records MUST use canonical names. All IP addresses in use within a block should have an IN-ADDR mapping. Those addresses not in use, and those that are not valid for use (zeros or ones broadcast addresses within a CIDR block) need not have mappings, although it may be useful to indicate that a given block is unassigned. It should be noted that due to CIDR, many addresses that appear to be otherwise valid host addresses may actually be zeroes or ones broadcast addresses. As such, attempting to audit a site's degree of compliance can only be done with knowledge of the internal routing structure of the site. However, any host that originates an IP packet necessarily will have a valid host address, and ought therefore to have a reverse mapping. 4.2 Application Recommendations Applications SHOULD NOT rely on IN-ADDR for proper operation, Senie and Sullivan [Page 5] Internet-Draft Considerations for DNS Reverse Mapping April 2004 although they MAY provide reduced functionality in the absense of reverse mapping. The use of IN-ADDR, sometimes in conjunction with a lookup of the name resulting from the PTR record provides no real security, can lead to erroneous results and generally just increases load on DNS servers. Further, in cases where address block holders fail to properly configure IN-ADDR, users of those blocks are penalized. 4.3 Usage and deployment considerations Site administrators are encouraged to think carefully before adopting any test of reverse delegation, particularly when that test is intended to improve security. The use of IN-ADDR either with or without the lookup of the name resulting from the PTR record does not usually improve security, and should not be a default policy. In particular, some users contine to report difficulty in ensuring correct management of IN-ADDR by upstream providers. This situation can be corrected by the adoption by those providers of the recommendations in this document; but until such adoption has happened, complete connection rejection on the basis of missing IN- ADDR should be regarded as a last resort. At the same time, site administrators are cautioned that administrators at other sites sometimes use reverse mappings as one of several pieces of evidence in evaluating connection traffic, particularly in the context of mail systems and anti-spam efforts. Administrators are advised to keep in mind the effects of adding a very large number of PTR records for a given IN-ADDR entry. In particular, sites where a very large number of "virtual" host names resolve to the same host may, if the foregoing advice is followed too rigourously, force DNS responses to use TCP. Such cases should be treated as unusual exceptions to the usual rule that IN-ADDR entries are to be added for hosts on the Internet. 5. Security Considerations This document has no negative impact on security. While it could be argued that lack of PTR record capabilities provides a degree of anonymity, this is really not valid. Trace routes, whois lookups and other sources will still provide methods for discovering identity. By recommending applications avoid using IN-ADDR as a security mechanism this document points out that this practice, despite its use by many applications, is an ineffective form of security. Applications should use better mechanisms of authentication. 6. IANA Considerations Senie and Sullivan [Page 6] Internet-Draft Considerations for DNS Reverse Mapping April 2004 There are no IANA considerations or implications that arise from this document. 7. References 7.1 Normative References [RFC883] Mockapetris, P.V., "Domain names: Implementation specification," RFC883, November 1983. [RFC1035] Mockapetris, P.V., "Domain Names: Implementation Specification," RFC 1035, November 1987. [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", RFC 2026, BCP 9, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [RFC2050] Hubbard, K., M. Kosters, D. Conrad, D. Karrenberg, J. Postel, "Internet Registry IP Allocation Guidelines", RFC2050, BCP 12, Novebmer 1996. [RFC2317] Eidnes, H., G. de Groot, P. Vixie, "Classless IN-ADDR.ARPA delegation," RFC 2317, March 1998. [RFC3152] R. Bush, "Delegation of IP6.ARPA," RFC 3152, BCP 49, August 2001. [RFC4632] Fuller, V., T. Li, "Classless Inter-Domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan," RFC 4632, August 2006. 7.2 Informative References [AFRINIC] "AfriNIC Policy for reverse delegation on Allocated IP addresses," version Draft.01a. March 31, 2004. [APNIC086] "Policies For IPv4 Address Space Management in the Asia Pacific Region," APNIC-086, version 006. December 13, 2005. [APNIC089] "IPv6 Address Allocation and Assignment Policy," APNIC-089, version 003. May 26, 2005. [ARIN] "ARIN Number Resource Policy Manual", Version 2006.1. February 17 2006. Senie and Sullivan [Page 7] Internet-Draft Considerations for DNS Reverse Mapping April 2004 [RIPE302]Kolkman, O. and L. Vegoda, "Policy for Reverse Address Delegation of IPv4 and IPv6 Address Space in the RIPE NCC Service Region" ripe-185, October 26, 1998. 8. Acknowledgements Thanks to Steven Champion, Peter Koch and Gary Miller for their input, and to many people who encouraged the writing of this document. 9. Authors' Addresses Daniel Senie Amaranth Networks Inc. 324 Still River Road Bolton, MA 01740 Phone: +1 978 779 5100 EMail: dts@senie.com Andrew Sullivan Afilias 204-4141 Yonge Street Toronto, ON, CA M2P 2A8 Phone: +1 416 673 4110 EMail: andrew@ca.afilias.info 9. Full Copyright Statement Copyright (C) The Internet Society 2006. This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Senie and Sullivan [Page 8] Internet-Draft Considerations for DNS Reverse Mapping April 2004 Intellectual Property By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org." Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Senie and Sullivan [Page 9]