Network Working Group
P. FaltstromIAB Internet-Draft CiscoP. Faltstrom, Ed. Intended status: Informational October 23, 2006Standards Track R. Austein, Ed. Expires: April 26, 2007August 21, 2008 P. Koch, Ed. February 18, 2008 Design Choices When Expanding DNS draft-iab-dns-choices-04.txtdraft-iab-dns-choices-05 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 April 26, 2007.August 21, 2008. Copyright Notice Copyright (C) The Internet Society (2006).IETF Trust (2008). Abstract This note discusses how to extend the DNS with new data for a new application. DNS extension discussiondiscussions too often circulates aroundfocus on reuse of the TXT record.Resource Record Type. This document lists different mechanisms to accomplishextend the extension toDNS, and comes to the conclusionconcludes that the use of a new RRDNS Resource Record Type is the best solution. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Extension mechanisms . . . . . . . . . . . . . . . . . . . . . 45 3.1. Place selectors inside the RDATA of existing Resource Record Types . . . . . . . . . . . . . 4. . . . . . . . . . 5 3.2. Add a prefix to the owner name . . . . . . . . . . . . . . 5 3.3. Add a suffix to the owner name . . . . . . . . . . . . . . 6 3.4. Add a new Class . . . . . . . . . . . . . . . . . . . . . 67 3.5. Add a new Resource Record Type . . . . . . . . . . . . . . 7 4. Conclusion (whyZone boundaries are invisible to applications . . . . . . . . 8 5. Why adding a new RR typeResource Record Type is the answer)preferred solution . . . . . . . 8 5. Conclusion and Recommendation. . . . . . . . . . . . . . . . 10 6. New Resource Record Type. . . . 9 6. Conclusion and Recommendation . . . . . . . . . . . . . . . . 1112 7. Creating A New Resource Record Type . . . . . . . . . . . . . 13 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8.13 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9.13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 10.13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10.1.14 11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 10.2.14 11.2. Informative References . . . . . . . . . . . . . . . . . . 13 Author's Address .14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 1415 Intellectual Property and Copyright Statements . . . . . . . . . . 1516 1. Introduction The DNS stores multiple categories of data. The two most commonly used categories are infrastructure data for the DNS system itself (NS and SOA records)Resource Records) and data which have to do with mappings between domain names and IP addresses (A, AAAA and PTR).PTR Resource Records). There are other categories as well, some of which are tied to specific applications like email (MX),(MX Resource Records), while others are generic record typesResource Record Types used to convey information for multiple protocols (SRV, NAPTR).(SRV and NAPTR Resource Records). When storing data in the DNS for a new application, the data are usually tied to a "normal" domain name, so that the application can query for the data it wants, while minimizing the impact on existing applications. Historically, extending DNS to store application data for applicationstied to a domain name has been done in different ways at different times. MX recordsResource Records were created as a new resource record typeResource Record Type specifically designed to support electronic mail. SRV records are a generic type which use a prefixing scheme in combination with a base domain name. Records associated with ENUM use a suffixing scheme. NAPTR records add selection data inside the RDATA. It is clear that the way of addingmethods used to add new data types to the DNS hashave been inconsistent, and the purpose of this document is to attempt to clarify the implications of each of these methods, both for the applications that use them and for the rest of the DNS system. Many parts of thisDNS. This document talktalks extensively about use of wildcards, and manyDNS wildcards. Many people might think use of wildcards is not something that happens today. In reality thoug,though, wildcards are in use, especially for some specific usages likecertain application-specific data such as MX records.Resource Records. Because of this, the choice havehas to be made with existence of wildcards in mind. Another overall issue that have tomust be taken into account is what the new data in the DNS isare to describe. In some cases itthey might be completely new data. In other cases itthey might be meta-data that ismetadata tied to data that already existsexist in the DNS. An example of new data is key information for SSH and data used for fighting spam (meta data that is connected(metadata tied to theMX record).Resource Records). If the new data has connectionare tied to data that already existsexist in the DNS, an analysis should be made as to whether having (for example) A recordaddress records and SSH key information in different DNS zones is a problem, and if it is, whether the owner for the records by designspecification must require all of the related data to be in the same for both records.zone. This document does not talk about what one should store in the DNS. It also doesn't discuss whether DNS isshould be used for service discovery, or whether DNS is (also)should be used for storage of data specific for the service. In general, DNS is a protocol thatthat, apart from holding metadata that makes the DNS itself function (NS, SOA, DS RR Types etc)DNSSEC Resource Record Types, etc), only holds references to where services are locatedservice locations (SRV, NAPTR, A, AAAA RR types).Resource Record Types), but there are exceptions (such as MX Resource Records). 2. Background See RFC 2929[RFC2929] for a brief summary of DNS query structure. Readers interested in the full story should start with the base DNS specification in RFC 1035[RFC1035], and continue with the various documents whichthat update, clarify, and extend the base specification. When composing a query against theDNS system,query, the parameters actuallyused by the protocol are a triple: a DNS name, a DNS class, and a DNS record type.Type. Every resource record (RR)Resource Record matching a particular name, type and class is said to belong to the same resource record set (RRSet),"RRSet", and the whole RRSet is always returned to the client whichthat queries for it. Splitting an RRSet is a protocol violation, because it results in coherency problems with the DNS caching mechanism. Some discussions around extensions to the DNS include arguments around MTU size. Note that most of thediscussions aroundabout DNS and MTU size isare about the size of the whole DNS packet, andnot about the size of an RRSet explicitly. Aa single RRSet. Almost all DNS packetquery traffic is tocarried over UDP, where a DNS message must fit in the MTUwithin a single UDP packet. DNS response messages are almost always larger than DNS query messages, so message size when using UDP,issues are almost always about responses, not queries. The base DNS specification limits DNS messages over UDP to 512 octets; EDNS0 [RFC2671] specifies a mechanism by which a client can signal its willingness to receive larger responses, but deployment of EDNS0 is not universal, in part because of firewalls that block fragmented UDP packets or elseEDNS0. If a truncatedresponse is given back frommessage won't fit in a single packet, the name server (atreturns a truncated response, at which point the client can reissue themay retry using TCP. DNS queries over TCP are not subject to this length limitation, but TCP imposes significantly higher per- query overhead on name servers than UDP. It is also the case that the policies in deployed firewalls far too often is such that it blocks DNS over TCP).TCP, so using TCP might not in reality be an option. There are also risks (although possibly small) that a change of routing while a TCP flow is open create problems when the DNS servers are deployed in an anycast environment. 3. Extension mechanisms The DNS protocol is intended to be extensible to support new kinds of data. This section examines the various ways in which this sort of extension can be accomplished. 3.1. Place selectors inside the RDATA of existing Resource Record Types For a given query name, one might choose to have a single RRSet (all Resource Records sharing the same name, type, and class) shared by multiple applications, and have the different applications use selectors within the RRResource Record data (RDATA) to determine which records are intended for which applications. This sort of selector mechanism is usually referred to "subtyping", because it is in effect creating an additional type subsystem within a single DNS RR type.Resource Record Type. Examples of subtyping include NAPTR RRsResource Records (see RFC 3761[RFC3761]) and the original DNSSEC KEY RR type (RFC 2535 [RFC2535])Resource Record Type ([RFC2535]) (before it was updated by RFC 3445[RFC3445]). All DNS subtyping schemes share a common weakness: With subtyping schemes it is impossible for a client to query for just the data it wants. Instead, the client must fetch the entire RRSet, then select the RRsResource Records in which it is interested. Furthermore, since DNSSEC signatures operate on complete RRSets, the entire RRSet must be re- signedre-signed if any RRResource Record in it changes. As a result, each application that uses a subtyped RRResource Record incurs higher overhead than any of the applications would have incurred had they not been using a subtyping scheme. The fact the RRSet is always passed around as an indivisible unit increases the risk the RRSet will not fit in a UDP packet, which in turn increases the risk that the client will have to retry the query with TCP, which substantially increases the load on the name server. More precisely: Havinghaving one query fail over to TCP is not a big deal, but since the typical ratio of clients to servers in thetoday's deployed DNS systemis very high, having a substantial number of DNS messages fail over to TCP may cause the queried name servers to be overloaded by TCP overhead. To conclude, because of the limitationsBecause of the size limitations, using a subtyping scheme to list a large number of one RRSet is that not allservices tied tofor a single domain can be announced, but instead selected (byname risks triggering truncation and fallback to TCP, which may in turn force the zone administrator). This because alladministrator to announce only a subset of them can not be announced at the same time.available services. 3.2. Add a prefix to the owner name By adding an application-specific prefix to a domain name, we get a different name/class/type triples,triple, and therefore a different RRSets.RRSet. One problem with adding prefixes has to do with wildcards, especially if one has records like *.example.com. IN MX 1 mail.example.com. and one wants records tied to those names. Suppose one creates the prefix "_mail". One would then have to say something like _mail.*.example.com. IN X-FOO A B C D but DNS wildcards only work with the "*" as the leftmost token in the domain name.name (see also [RFC4592]). Even when a specific prefix is chosen, the data will still have to be stored in some RR type.Resource Record Type. This RR typeResource Record Type can either be a record typeType that canhas an appropriate format to store arbitrarythe data or a new RR type.Resource Record Type. This implies that some other selection mechanism has to be applied as well, such as ability to distinguish between the records in an RR setRRSet given they have the same RR type (see also draft-ietf-dnsext-wcard-clarify [wcardclarify] regarding use of wildcards and DNS).Resource Record Type. Because of this, one needs to both register a unique prefix and define what RR typeResource Record Type is to be used for this specific service. If the record has some relationship with another record in the zone, the fact that the two records can be in different zones might have implications on the trust the application havehas in the records. Example:For example: example.com. IN MX 10 mail.example.com. _foo.example.com. IN X-BAR "metadata for the mail service" In this example, the two records might be in two different zones, and because of this might be signed by two different organizations when using DNSSEC. 3.3. Add a suffix to the owner name Adding a suffix to a domain name changes the name/class/type triple, and therefore the RRSet. In this case, since the query name can be set to exactly the data one wants the size of the RRSet is minimized. The problem with adding a suffix is that it creates a parallel tree within the IN class. Further, there is no technical mechanism to ensure that the delegation for "example.com" and "example.com._bar" are made to the same organization. Furthermore, data associated with a single entity will now be stored in two different zones, such as "example.com" and "example.com._bar", which, depending on who controls "_bar", can create new synchronization and update authorization issues. One way of solving the administrative issues is by using the DNAME resource record typeResource Record Type specified in RFC 2672[RFC2672]. Even when using a different name, the data will still have to be stored in some RR type.Resource Record Type. This RR typeResource Record Type can either be a "kitchen-sink record" or a new RR type.Resource Record Type. This implies that some other mechanism has to be applied as well, with implications detailed in other parts of this note. In RFC 2163[RFC2163] an infix token is inserted directly below the TLD, but the result is the same asequivalent to adding a suffix to the owner name (and because of that creation(instead of creating a new TLD).TLD one is creating a second level domain). 3.4. Add a new Class DNS zones are class-specific in the sense that all the records in that zone share the same class as the zone's SOA record and the existence of a zone in one class does not guarantee the existence of the zone in any other class. In practice, only the IN class has ever seen widespread deployment, and the administrative overhead of deploying an additional class would almost certainly be prohibitive. Nevertheless, one could in theory use the DNS class mechanism to distinguish between different kinds of data. However, since the DNS delegation tree (represented by NS RRs)Resource Records) is itself tied to a specific class, attempting to resolve a query by crossing a class boundary may produce unexpected results,results because there is no guarantee that the name servers for the zone in the new class will be the same as the name servers in the IN class. The MIT Hesiod system used a scheme like this for storing data in the HS class, but only on a very small scale (within a single institution), and with an administrative fiat requiring that the delegation trees for the IN and HS trees be identical. Even when using a different class, the data will still have to be stored in some RR typeResource Record Type or another. This RR typeResource Record Type can either be a "kitchen-sink record" or a new RR type.Resource Record Type. This implies that some other mechanism has to be applied as well, with implications detailed in other parts of this note. 3.5. Add a new Resource Record Type When adding a new Resource Record typeType to the system, entities in four different roles have to be able to handle the new type:Type: 1. There must be a way to insert the new resource recordsResource Records into the zone of the Primary Master name server. For some server implementations, the user interface only accepts record typesTypes which it understands (perhaps so that the implementation can attempt to validate the data). Other implementations allow the zone administrator to enter an integer for the resource record typeResource Record Type code and the RDATA in Base64 or hexadecimal encoding (or even as raw data). RFC 3597[RFC3597] specifies a standard generic encoding for this purpose. 2. A slave authoritative name server must be able to do a zone transfer, receive the data from some other authoritative name server, and serve data from the zone even though the zone includes records of unknown types.Types. Historically, some implementations have had problems parsing stored copies of the zone file after restarting, but those problems have not been seen for a few years. 3. A full servicecaching resolver (most commonly a recursive name server) will cache the records which are responses to queries. As mentioned in RFC 3597[RFC3597],there are various pitfalls where a recursive name server might end up having problems. 4. The application mustpitfalls where a recursive name server might end up having problems. 4. The application must be able to get the RRSet with a new Resource Record Type. The application itself may understand the RDATA, but the resolver library might not. Support for a generic interface for retrieving arbitrary DNS Resource Record Types has been a requirement since 1989 (see [RFC1123] Section 18.104.22.168). Some stub resolver library implementations neglect to provide this functionality and cannot handle unknown Resource Record Types, but implementation of a new stub resolver library is not particularly difficult, and open source libraries that already provide this functionality are available. 4. Zone boundaries are invisible to applications Regardless of the possible choices above we have seen a number of cases where the application made assumptions about the structure of the namespace and the location where specific information resides. We take a small sidestep to argue against such approaches. The DNS namespace is a hierarchy, technically speaking. However, this only refers to the way names are built from multiple labels. DNS hierarchy neither follows nor implies administrative hierarchy. That said, it cannot be assumed that data attached to a node in the DNS tree is valid for the whole subtree. Technically, there are zone boundaries partitioning the namespace and administrative boundaries (or policy boundaries) may even exist elsewhere. The false assumption has lead to an approach called "tree climbing", where a query that does not receive a positive response (either the requested RRSet was missing or the name did not exist) is retried by repeatedly stripping off the leftmost label (climbing towards the root) until the root domain is reached. Sometimes these proposals try to avoid the query for the root or the TLD level, but still this approach has severe drawbacks: o Technically, the DNS was built as a query - response tool without any search capability [RFC3467]. Adding the search mechanism imposes additional burden on the technical infrastructure, in the worst case on TLD and root name servers. o For reasons similar to those outlined in RFC 1535, querying for information in a domain outside the control of the intended entity may lead to incorrect results and may also put security at risk. Finding the exact policy boundary is impossible without an explicit marker which does not exist at present. At best, software can detect zone boundaries (e.g., by looking for SOA Resource Records), but some TLD registries register names starting at the second level (e.g., CO.UK), and there are various other "registry" types at second, third or other level domains that cannot be ableidentified as such without policy knowledge external to get the record with a new resource record type. The application itself may understandthe RDATA, butDNS. To restate, the resolver library might not. Support forzone boundary is purely a generic interface for retrieving arbitraryboundary that exists in the DNS RR types has been a requirement since 1989 (see RFC 1123 [RFC1123] Section 22.214.171.124). Some stub resolver library implementations neglect to provide this functionalityfor administrative purposes, and cannot handle unknown RR types, but implementationapplications should be careful not to draw unwarranted conclusions from zone boundaries. A different way of a new stub resolver librarystating this is that the DNS does not particularly difficult, and open source librariessupport inheritance, e.g. a wildcard MX RRSet for a TLD will not be valid for any subdomain of that already provide this functionality are available. 4. Conclusion (whyparticular TLD. 5. Why adding a new RR typeResource Record Type is the answer)preferred solution By now, the astute reader willmight be be wondering howwhat conclusions to usedraw from all the issues presented so far. We will now attempt to clear up the reader's confusion by following the thought processes of a typical application designer who wishes to store data in the DNS, showing how such a designer almost inevitably hits upon the idea of just using TXT RR, andResource Record, why this is a bad thing, and insteadwhy a new RR type is toResource Record Type should be allocated.allocated instead. The overall problem with most solutions has to do with two main issues: o No semantics to prevent collision with other use o Space considerations (inin the DNS message)message A typical application designer is not interested in the DNS for its own sake, but rather as a distributed database in which application data can be stored. As a result, the designer of a new application is usually looking for the easiest way to add whatever new data the application needs to the DNS in a way that naturally associates the data with a DNS name. As explained in Section 3.4, using the DNS class system as an extension mechanism is not really an option, and in fact most users of the system don't even realize that the mechanism exists. As a practical matter, therefore any extension is likely to be within the IN class. Adding a new RR typeResource Record Type is the technically correct answer from the DNS protocol standpoint (more on this below), but doing so requires some DNS expertise, due to the issues listed in Section 3.5. As a result, this option is usually not considered. Note that according to RFC2929[RFC2929], some typesTypes require IETF Consensus, while others only require a specification. The application designer is thus left with the prospect of reusing some existing DNS typeType within the IN class, but when the designer looks at the existing types,Types, almost all of them have well-defined semantics, none of which quite match the needs of the new application. This has not completely prevented proposals to reusefrom reusing existing RR typesResource Record Types in ways incompatible with their defined semantics, but it does tend to steer application designers away from this approach. RRFor example, Resource Record Type 40 was registered for the SINK record type.Type. This RRResource Record Type was discussed in the DNSIND working group of the IETF, and it was decided at the 46th IETF to not move the I-D forward to become an RFC. Reason wasRFC because of the risk for large RR sets and the ability forof encouraging application creatorsdesigners to use the SINK RRResource Record Type instead of registering a new RR Type.Resource Record Type, which would result in infeasibly large SINK RRsets. Eliminating all of the above leaves the TXT RR typeResource Record Type in the IN class. The TXT RDATA format is free form text, and there are no existing semantics to get in the way. Furthermore, the TXT RRResource Record can obviously just be used as a bucket in which to carry around data to be used by some higher level parser, perhaps in some human readable programming or markup language. Thus, for many applications, TXT RRsResource Records are the "obvious" choice. Unfortunately, this conclusion, while understandable, is also wrong, for several reasons. The first reason why TXT RRsResource Records are not well suited to such use is precisely the lack of defined semantics that make them so attractive. Arguably, the TXT RRResource Record is misnamed, and should have been called the Local Container record, because the lack of defined semantics means that a TXT RRResource Record means precisely what the data producer says it means. This is fine, so long as TXT RRsResource Records are being used by human beings or by private agreement between data producer and data consumer. However, it becomes a problem once one starts using them for standardized protocols in which there is no prior relationship between data producer and data consumer. ReasonThe reason for this is that there is nothing to prevent collisions with some other incompatible use of TXT RRs.Resource Records. This is even worse than the general subtyping problem described in Section 3.1, because TXT RRsResource Records don't even have a standardized selector field in which to store the subtype. RFC1464[RFC1464] tried, but it was not a success. At best a definition of a subtype is reduced to hoping that whatever scheme one has come up with will not accidently conflict with somebody else's subtyping scheme, and that it will not be possible to mis-parse one application's use of TXT RRsResource Records as data intended for a different application. Any attempt to come up withimpose a standardized format within the TXT RRResource Record format would be at least fifteen years too late even if it were put into effect immediately.immediately; at best, one can restrict the syntax that a particular application uses within a TXT Resource Record and accept the risk that unrelated TXT Resource Record uses will collide with it. Using one of the naming modifications discussed in Section 3.2 and Section 3.3 would address the subtyping problem, but each of these approaches brings in new problems of its own. The prefix approach (such as(that for example SRV RRsResource Records use) does not work well with wildcards, which is a particular problem for mail-related applications, since MX RRsResource Records are probably the most common use of DNS wildcards. The suffix approach doesn't have wildcard issues, but, as noted previously, it does have synchronization and update authorization issues, since it works by creating a second subtree in a different part of the global DNS name space. The next reason why TXT RRsResource Records are not well suited to protocol use has to do with the limited data space available in a DNS message. As alluded to briefly in Section 3.1, typical DNS query traffic patterns involve a very large number of DNS clients sending queries to a relatively small number of DNS servers. Normal path MTU discovery schemes do little good here,here because, from the server's perspective, there isn't enough repeat traffic from any one client for it to be worth retaining state. UDP-based DNS is an idempotent query, whereas TCP-based DNS requires the server to keep state (in the form of TCP connection state, usually in the server's kernel) and roughly triples the traffic load. Thus, there's a strong incentive to keep DNS messages short enough to fit in a UDP datagram, preferably a UDP datagram short enough not to require IP fragmentation. Subtyping schemes are therefore again problematic,problematic because they produce larger Resource RRSets than necessary, but verbose text encodings of data are also wasteful, since the data they hold can usually be represented more compactly in a resource recordResource Record designed specifically to support the application's particular data needs. If the data that need to be carried are so large that there is no way to make them fit comfortably into the DNS regardless of encoding, it is probably better to move the data somewhere else, and just use the DNS as a pointer to the data, as with NAPTR. 5.6. Conclusion and Recommendation Given the problems detailed in Section 4,5, it is worth reexamining the oft-jumped-to conclusion that specifying a new RR typeResource Record Type is hard. Historically, this was indeed the case, but recent surveys suggest that support for unknown RR typesResource Record Types [RFC3597] is now widespread, and that lack of support for unknown typesTypes is mostly an issue for relatively old software that would probably need to be upgraded in any case as part of supporting a new application. One should also remember that deployed DNS software today should support DNSSEC, and software recent enough to do so will have higher chance of being able to alsolikely support RFC3597.both unknown Resource Record Types [RFC3597] and EDNS0 [RFC2671]. Of all the issues detailed in Section 3.5, provisioning the data is in some respects the most difficult. The problem here is less difficult for the authoritative name servers themselves than the front-end systems used to enter (and perhaps validate) the data. Hand editing does not work well for maintenance of large zones, so some sort of tool is necessary, and the tool may not be tightly coupled to the name server implementation itself. Note, however, that this provisioning problem exists to some degree with any new form of data to be stored in the DNS, regardless of data format, RRResource Record type, or naming scheme. Including the TXT Resource Record Type. Adapting front-end systems to support a new RRResource Record type may be a bit more difficult than reusing an existing type, but this appears to be a minor difference in degree rather than a difference in kind. Given the various issues described in this note, we believe that: o there is no magic solution which allows a completely painless addition of new data to the DNS, but o on the whole, the best solution is still to use the DNS type mechanism designed for precisely this purpose, and o of all the alternate solutions, the "obvious" approach of using TXT RRs is almost certainly the worst. This especially for the two reasons outlined above (lack of semantics and its implications, and size leading to the need to use TCP). 6. New Resource Record Type Creation of a new resource record type is specified in RFC 2929 [RFC2929]. Terminology is from RFC 2434 [RFC2434]. It is specified in RFC 2929 that not only standards track documents can specify new resource record types. Also experimental or informational RFC is ok, and for some numbers "...RFC or other permanent and readily available reference...". The following are the rules applicable at the time of writing from BCP 42 and BCP 26 for various ranges of Resource Record Types. Type number 1-32767 require IETF Consensus: IETF Consensus - New values are assigned through the IETF consensus process. Specifically, new assignments are made via RFCs approved by the IESG. Typically, the IESG will seek input on prospective assignments from appropriate persons (e.g., a relevant Working Group if one exists). Examples: A record is number 1, and AXFR number 252. Type number 32768-65279 require specification: Specification Required - Values and their meaning must be documented in an RFC or other permanent and readily available reference, in sufficient detail so that interoperability between independent implementationsmagic solution which allows a completely painless addition of new data to the DNS, but o on the whole, the best solution is possible. Nostill to use the DNS Resource Record types are registered in this range.Type number 65280-65535 ismechanism designed for private use: Private Use - For private or local use only, with the typeprecisely this purpose, and purpose defined byo of all the local site. No attemptalternate solutions, the "obvious" approach of using TXT Resource Records is madealmost certainly the worst. This especially for the two reasons outlined above (lack of semantics and its implications, and size leading to prevent multiple sites from usingthe same value in different (and incompatible) ways. There is noneed for IANAto review such assignments and assignments are not generally usefuluse TCP). 7. Creating A New Resource Record Type The process for interoperability.creating a new Resource records in this rangeRecord Type is not registered centrally at IANA, and collissions may exist. 7.specified in [I-D.ietf-dnsext-2929bis]. 8. IANA Considerations This document does not require any IANA actions. 8.9. Security Considerations DNS RRSets can be signed using DNSSEC. DNSSEC is almost certainly necessary for any application mechanism that stores authorization data in the DNS. DNSSEC signatures significantly increase the size of the messages transported, and because of this, the DNS message size issues discussed in Section 3.1 and Section 45 are more serious than they might at first appear. Adding new RR typesResource Record Types (as discussed in Section 3.5) might conceivably trigger bugs and other bad behavior in software whichthat is not compliant with RFC 3597[RFC3597], but most such software is old enough and insecure enough that it should be updated for other reasons in any case. Basic API support for retrieving arbitrary RR typesResource Record Types has been a requirement since 1989 (see RFC 1123[RFC1123]). Any new protocol that proposes to use the DNS to store data used to make authorization decisions would be well advised not only to use DNSSEC but also to encourage upgrades to DNS server software recent enough not to be riddled with well-known exploitable bugs. Because of this, support for new RRResource Record Types will not be as hard as people might think at first. 9.10. Acknowledgements This document has been created during a number of years, with input from many people. The question on how to expand and use the DNS is sensitive, and a document like this can not please everyone. The goal is instead to describe the architecture,architecture and given this IAB have based a number of recommendations.tradeoffs, and make some recommendations about best practices. People that have helped include: Dean Andersson, Loa Andersson, Mark Andrews, Rob Austein,John Angelmo, Roy Badami, Dan Bernstein, Alex Bligh, Nathaniel Borenstein, Stephane Bortzmeyer, Brian Carpenter, Leslie Daigle, Elwyn Davies, Mark Delany, Richard Draves, Martin Duerst, Donald Eastlake, Robert Elz, Jim Fenton, Tony Finch, Jim Gilroy, Olafur Gudmundsson, Eric Hall, Philip Hallam-Baker, Ted Hardie, Bob Hinden, Paul Hoffman, Geoff Houston, Christian Huitema, Johan Ihren, John Klensin, Peter Koch,Olaf Kolkman, Ben Laurie, William Leibzon, John Levine, Edward Lewis, David MacQuigg, Allison Manking, Bill Manning, Danny McPherson, David Meyer, Pekka Nikander, Masataka Ohta, Douglas Otis, Michael Patton, Jonathan Rosenberg, Anders Rundgren, Miriam Sapiro, Pekka Savola, Chip Sharp, James Snell, Dave Thaler, Michael Thomas, Paul Vixie, Sam Weiler, Florian Weimer, Bert Wijnen, and Dan WingWing. Members of the IAB when this document was made available were: Bernard Aboba,Loa Andesson, Brian Carpender,Andersson, Leslie Daigle, Elwyn Davies, Kevin Fall, Russ Housley, Olaf Kolkman, Barry Leiba, Kurtis Lindqvist, David Meyer,Danny McPherson, David Oran, Eric Rescorla, Dave Thaler, and Lixia Zhang. 10.11. References 10.1.11.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13,RFC 1035, STD 13, November 1987. [RFC1464] Rosenbaum, R., "Using the Domain Name System To Store Arbitrary String Attributes", RFC 1464, May 1993. [RFC2535] Eastlake,Eastlake 3rd, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR) Types", RFC 3597, September 2003. 10.2.11.2. Informative References [I-D.ietf-dnsext-2929bis] 3rd, D., "Domain Name System (DNS) IANA Considerations", draft-ietf-dnsext-2929bis-06 (work in progress), August 2007. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3,RFC 1123, STD 3, October 1989. [RFC2163] Allocchio, C., "Using the Internet DNS to Distribute MIXER Conformant Global Address Mapping (MCGAM)", RFC 2163, January 1998. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, June 1999.[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC 2672, August 1999. [RFC2929] Eastlake,Eastlake 3rd, D., Brunner-Williams, E., and B. Manning, "Domain Name System (DNS) IANA Considerations", BCP 42,RFC 2929, BCP 42, September 2000. [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, January 2002.[RFC3445] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource Record (RR)", RFC 3445, December 2002. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82,[RFC3467] Klensin, J., "Role of the Domain Name System (DNS)", RFC 3692, January 2004.3467, February 2003. [RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)", RFC 3761, April 2004. [wcardclarify][RFC4592] Lewis, E., "draft-ietf-dnsext-wcard-clarify-11.txt, The"The Role of Wild Card DomainsWildcards in the Domain Name System, work in progress", MarchSystem", RFC 4592, July 2006. Author's AddressAuthors' Addresses Internet Architecture Board Email: email@example.com Patrik Faltstrom Cisco(editor) Email: firstname.lastname@example.org Rob Austein (editor) Email: email@example.com Peter Koch (editor) Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The Internet Society (2006).IETF Trust (2008). 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 SOCIETYSOCIETY, THE IETF TRUST 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. 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