Internet Engineering Task Force E. Lewis Internet-Draft ARIN February 4, 2003 Expires: August 4, 2003 Clarifying the Role of Wild Card Domains in the Domain Name System Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Abstract The definition of wild cards is recast from the original in RFC 1034, in words that are more specific and in line with RFC 2119. This document is meant to supplement the definition in RFC 1034 and to alter neither the spirit nor intent of that definition. 1 Introduction The first section of this document will give a crisp overview of what is begin defined, as well as the motivation for what amounts to a simple rewording of an original document. An example is included to help orient the reader. Wild card domain names are defined in Section 4.3.3. of RFC 1034 as "instructions for synthesizing RRs." [RFC1034] The meaning of this is that a specific, special domain name is used to construct responses in instances in which the query name is not otherwise represented in a zone. A wild card domain name has a specific range of influence on query names (QNAMEs) within a given class, which is rooted at the domain name containing the wild card label, and is limited by explicit entries, zone cuts and empty non-terminal domains (see section 1.3 of this document). Note that a wild card domain name has no special impact on the search for a query type (QTYPE). If a domain name is found that matches the QNAME (exact or a wild card) but the QTYPE is not found at that point, the proper response is that there is no data available. The search does not continue on to seek other wild cards that might match the QTYPE. To illustrate, a wild card owning an MX RR does not 'cover' other names in the zone that own an A RR. Why is this document needed? Empirical evidence suggests that the words in RFC 1034 are not clear enough. There exist a number of implementations that have strayed from the definition. There also exists a misconception of operators that the wild card can be used to add a specific RR type to all names, such as the MX RR example listed above. This document is also needed as input to efforts to extend DNS, such as the DNS Security Extensions [RFC 2535]. Lack of a clear base specification has proven to result in extension documents that have unpredictable consequences. (This is true in general, not just for DNS.) 1.1 Existence The notion that a domain name 'exists' will arise numerous times in this discussion. RFC 1034 raises the issue of existence in a number of places, usually in reference to non-existence and often in reference to processing involving wild card domain names. RFC 1034 does contain algorithms that describe how domain names impact the preparation of an answer and does define wild cards as a means of synthesizing answers. To help clarify the topic of wild cards, a positive definition of existence is needed. To complicate matters, though, there needs to be a recognition that existence is relative. To an authoritative server, a domain name exists if the domain name plays a role following the algorithms of preparing a response. To a resolver, a domain name exists if there is any data available corresponding to the name. The difference between the two is the synthesis of records according to a wild card. For the purposes of this document, the point of view of an authoritative server is adopted. A domain name is said to exist if it plays a role in the execution of the algorithms in RFC 1034. 1.2 An Example For example, consider this wild card domain name: *.example. Any query name under example. is a candidate to be matched (answered) by this wild card. Although any name is a candidate, not all queries will match. To further illustrate this, consider this example: $ORIGIN example. @ IN SOA NS NS * TXT "this is a wild card" MX 10 mailhost.example. host1 A 10.0.0.1 _ssh._tcp.host1 SRV _ssh._tcp.host2 SRV subdel NS The following queries would be synthesized from the wild card: QNAME=host3.example. QTYPE=MX, QCLASS=IN the answer will be a "host.example. IN MX ..." QNAME=host3.example. QTYPE=A, QCLASS=IN the answer will be a "host.example. IN NXT ..." because there is no A RR set at '*' The following queries would not be synthesized from the wild card: QNAME=host1.example., QTYPE=MX, QCLASS=IN because host1.example. exists QNAME=_telnet._tcp.host1.example., QTYPE=SRV, QCLASS=IN because _tcp.host1.example. exists (without data) QNAME=_telnet._tcp.host2.example., QTYPE=SRV, QCLASS=IN because host2.example. exists (without data) QNAME=host.subdel.example., QTYPE=A, QCLASS=IN because subdel.example. exists and is a zone cut To the server, the following domains are considered to exist in the zone: *, host1, _tcp.host1, _ssh._tcp.host1, host2, _tcp.host2, _ssh._tcp.host2, and subdel. To a resolver, many more domains appear to exist via the synthesis of the wild card. 1.3 Empty Non-terminals Empty non-terminals are domain names that have no data but have subdomains. This is defined in section 3.1 of RFC 1034: # The domain name space is a tree structure. Each node and leaf on the # tree corresponds to a resource set (which may be empty). The domain # system makes no distinctions between the uses of the interior nodes and # leaves, and this memo uses the term "node" to refer to both. The parenthesized "which may be empty" specifies that empty non-terminals are explicitly recognized. According to the definition of existence in this document, empty non-terminals do exist at the server. Carefully reading the above paragraph can lead to an interpretation that all possible domains exist - up to the suggested limit of 255 octets for a domain name [RFC 1035]. For example, www.example. may have an A RR, and as far as is practically concerned, is a leaf of the domain tree. But the definition can be taken to mean that sub.www.example. also exists, albeit with no data. By extension, all possible domains exist, from the root down. As RFC 1034 also defines "an authoritative name error indicating that the name does not exist" in section 4.3.1, this is not the intent of the original document. RFC1034's wording is to be clarified by adding the following paragraph: A node is considered to have an impact on the algorithms of 4.3.2 if it is a leaf node with any resource sets or an interior node, with or without a resource set, that has a subdomain that is a leaf node with a resource set. A QNAME and QCLASS matching an existing node never results in a response return code of authoritative name error. As an aside, an "authoritative name error" has been called NXDOMAIN in some RFCs, such as RFC 2136 [RFC 2136]. NXDOMAIN is the mnemonic assigned to such an error by at least one implementation of DNS. 1.3 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 the document entitled "Key words for use in RFCs to Indicate Requirement Levels." [RFC2119] Requirements are denoted by paragraphs that begin with with the following convention: 'R'.. 2 Defining the Wild Card Domain Name A wild card domain name is defined by having the initial label be: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal) This defines domain names that may play a role in being a wild card, that is, being a source for synthesized answers. Domain names conforming to this definition that appear in queries and RDATA sections do not have any special role. These cases will be described in more detail in following sections. R2.1 A domain name that is to be interpreted as a wild card MUST begin with a label of '0000 0001 0010 1010' in binary. The first octet is the normal label type and length for a 1 octet long label, the second octet is the ASCII representation [RFC 20] for the '*' character. In RFC 1034, ASCII encoding is assumed to be the character encoding. In the master file formats used in RFCs, a "*" is a legal representation for the wild card label. Even if the "*" is escaped, it is still interpreted as the wild card when it is the only character in the label. R2.2. A server MUST treat a wild card domain name as the basis of synthesized answers regardless of any "escape" sequences in the input format. RFC 1034 and RFC 1035 ignore the case in which a domain name might be "the*.example.com." The interpretation is that this domain name in a zone would only match queries for "the*.example.com" and not have any other role. Note: By virtue of this definition, a wild card domain name may have a subdomain. The subdomain (or sub-subdomain) itself may also be a wild card. E.g., *.*.example. is a wild card, so is *.sub.*.example. More discussion on this is given in Appendix A. 3 Defining Existence As described in the Introduction, a precise definition of existence is needed. R3.1 An authoritative server MUST treat a domain name as existing during the execution of the algorithms in RFC 1034 when the domain name conforms to the following definition. A domain name is defined to exist if the domain name owns data and/or has a subdomain that exists. Note that at a zone boundary, the domain name owns data, including the NS RR set. At the delegating server, the NS RR set is not authoritative, but that is of no consequence here. The domain name owns data, therefore, it exists. R3.2 An authoritative server MUST treat a domain name that has neither a resource record set nor a subdomain as nonexistent when executing the algorithm in section 4.3.2. of RFC 1034. 4 Impact of a Wild Card Domain In a Query Message When a wild card domain name appears in a question, e.g., the query name is "*.example.", the response in no way differs from any other query. In other words, the wild card label in a QNAME has no special meaning, and query processing will proceed using '*' as a literal query name. R4.1 A wild card domain name acting as a QNAME MUST be treated as any other QNAME, there MUST be no special processing accorded it. If a wild card domain name appears in the RDATA of a CNAME RR or any other RR that has a domain name in it, the same rule applies. In the instance of a CNAME RR, the wild card domain name is used in the same manner of as being the original QNAME. For other RR's, rules vary regarding what is done with the domain name(s) appearing in them, in no case does the wild card hold special meaning. R4.2 A wild card domain name appearing in any RR's RDATA MUST be treated as any other domain name in that situation, there MUST be no special processing accorded it. 5 Impact of a Wild Card Domain On a Response The description of how wild cards impact response generation is in RFC 1034, section 4.3.2. That passage contains the algorithm followed by a server in constructing a response. Within that algorithm step 3, part 'c' defines the behavior of the wild card. The algorithm is directly quoted in lines that begin with a '#' sign. Commentary is interleaved. [Note that are no requirements specifically listed in this section. The text here is explanatory and interpretative. There is no change to the algorithm specified in RFC 1034.] The context of part 'c' is that the search is progressing label by label through the QNAME. (Note that the data being searched is the authoritative data in the server, the cache is searched in step 4.) Step 3's part 'a' covers the case that the QNAME has been matched in full, regardless of the presence of a CNAME RR. Step 'b' covers crossing a cut point, resulting in a referral. All that is left is to look for the wild card. Step 3 of the algorithm also assumes that the search is looking in the zone closest to the answer, i.e., in the same class as QCLASS and as close to the authority as possible on this server. If the zone is not the authority, then a referral is given, possibly one indicating lameness. # c. If at some label, a match is impossible (i.e., the # corresponding label does not exist), look to see if a # the "*" label exists. The above paragraph refers to finding the domain name that exists in the zone and that most encloses the QNAME. Such a domain name will mark the boundary of candidate wild card domain names that might be used to synthesize an answer. (Remember that at this point, if the most enclosing name is the same as the QNAME, part 'a' would have recorded an exact match.) The existence of the enclosing name means that no wild card name higher in the tree is a candidate to answer the query. Once the closest enclosing node is identified, there's the matter of what exists below it. It may have subdomains, but none will be closer to the QNAME. One of the subdomains just might be a wild card. If it exists, this is the only wild card eligible to be used to synthesize an answer for the query. Even if the closest enclosing node conforms to the syntax rule in section 2 for being a wild card domain name, the closest enclosing node is not eligible to be a source of a synthesized answer. The only wild card domain name that is a candidate to synthesize an answer will be the "*" subdomain of the closest enclosing domain name. Three possibilities can happen. The "*" subdomain does not exist, the "*" subdomain does but does not have an RR set of the same type as the QTYPE, or it exists and has the desired RR set. For the sake of brevity, the closest enclosing node can be referred to as the "closest encloser." To illustrate, using the example in section 1.2 of this document, the following chart shows QNAMEs and the closest enclosers. In Appendix A there is another chart showing unusual cases. QNAME Closest Encloser Wild Card Source host3.example. example. *.example. _telnet._tcp.host1.example. _tcp.host1.example. no wild card _telnet._tcp.host2.example. host2.example. no wild card _telnet._tcp.host3.example. example. *.example. _chat._udp.host3.example. example. *.example. Note that host1.subdel.example. is in a subzone, so the search for it ends in a referral in part 'b', thus does not enter into finding a closest encloser. The fact that a closest encloser will be the only superdomain that can have a candidate wild card will have an impact when it comes to designing authenticated denial of existence proofs. (This concept is not introduced until DNS Security Extensions are considered in upcoming sections.) # If the "*" label does not exist, check whether the name # we are looking for is the original QNAME in the query # or a name we have followed due to a CNAME. If the name # is original, set an authoritative name error in the # response and exit. Otherwise just exit. The above passage says that if there is not even a wild card domain name to match at this point (failing to find an explicit answer elsewhere), we are to return an authoritative name error at this point. If we were following a CNAME, the specification is unclear, but seems to imply that a no error return code is appropriate, with just the CNAME RR (or sequence of CNAME RRs) in the answer section. # If the "*" label does exist, match RRs at that node # against QTYPE. If any match, copy them into the answer # section, but set the owner of the RR to be QNAME, and # not the node with the "*" label. Go to step 6. This final paragraph covers the role of the QTYPE in the process. Note that if no resource record set matches the QTYPE the result is that no data is copied, but the search still ceases ("Go to step 6."). 6 Authenticated Denial and Wild Cards In unsecured DNS, the only concern when there is no data to return to a query is whether the domain name from which the answer comes exists or not, whether or not a name error is indicated in the return code. In either case the answer section is empty or contained just a sequence of CNAME RR sets. In securing DNS, authenticated denial of existence is a service that is provided. The chosen solution to provide this service is to generate resource records indicating what is protected in a zone and to digitally sign these. The resource records that do this, as defined in RFC 2535, are NXT RRs. There are three points to consider when clarifying the topic of wild card domain names. One is the construction of the records. The second is the inclusion of records in responses. The third is the interpretation of the records in a response by the resolver. 6.1 Preparing Wild Card Domain Name Owned Non-existence Proofs During the creation of the authenticated denial records, the wild card domain name plays no special role, in the same manner as the wild card domain name playing no special role in a query. There is one consideration with regards to preparing non-existence proofs. R6.1 Any mechanism used to provide authenticated denial MUST reveal the closest enclosing existing domain for the query. If this is not provided, the resolver will not be able to ascertain the identity of an appropriate wild card domain name. 6.2 Role of Wild Cards in Answers There are three cases to address. The first is synthesizing from wild card domain name with data, the second is negatively synthesizing from an existing wild card, and the third is denying that neither an exact match, referral, nor wild card exist to answer the query. 6.2.1 Synthesizing From a Wild Card When preparing an answer from a wild card domain name, the answer needs to include proof that the exact match of the QNAME and QCLASS does not exist. This is needed because synthesis of the answer replaces the "*" label with the QNAME without securing the result. The resolver will realize that the answer was derived from a wild card, but cannot detect whether an exact match was maliciously omitted. R6.2 When synthesizing a positive answer from a wild card domain name, the answer MUST include proof that the exact match for the QNAME and QCLASS does not exist. 6.2.2. Synthesizing Negatively From a Wild Card When synthesizing a negative answer that is derived from a wild card, meaning that a wild card matched the QNAME (no exact match happened for QNAME) but that there is no match for QTYPE there, two negative answers are needed, possibly one. As in 6.2.1, a proof that the exact match failed is needed. A second proof is needed to show that the wild card domain name does not have the QTYPE. Depending on the method of authenticated denial, these this could be possible with one statement. R6.3 When synthesizing a negative answer from a wild card domain name, the answer MUST include proof that the exact match of the QNAME and QCLASS does not exist and that the QTYPE matches no RR set at the wild card. If this answer can be optimized, an implementation SHOULD reduce the number of records included in the response. 6.2.3. Answering With an Authoritative Name Error When answering with a result code of a name error, the answer needs to provide proof that neither the exact match for QNAME and QCLASS exists nor that a wild card domain name exists as a subdomain of the closest enclosing domain name. R6.4 When preparing a reply with an authoritative name error, the answer MUST include proof that the exact match for the QNAME and QCLASS does not exist and that no wild card is available to provide a match. 6.2.4. The Remaining Case When answering negatively because there is a match for QNAME and QCLASS but no match for the QTYPE, only a proof for that is needed. Just as the search does not proceed onto a search for the wild card in this case, neither does the construction of the negative answer proof. R6.5 When preparing a reply in which there is an exact match of the QNAME and QCLASS, but there is no RR set matching the QTYPE, the reply SHOULD NOT contain any proof regarding the wild card domain name. 6.3 Interpreting Negative Answers Involving Wild Cards There are two requirements for resolvers when it comes to handling negative answers generated as described in section 6.2. R6.6 A resolver MUST be able to identify negative answer data that indicate when a match for QNAME and QCLASS does not exist. R6.7 From a negative answer, a resolver MUST be able to determine the closest enclosing domain name in a negative answer and MUST be able to process a negative answer involving the one wild card domain name that is a candidate to provide a synthesized answer. 6.4 Authenticated Denial, Wild Card Domain Names, and Opt-In When considering the Opt-In proposal [WIP], it is wise to not combine a zone that adheres to both opt-in and that has a wild card domain name. The reason is rooted in that the synthesis of an answer is done by substituting the QNAME for the wild card domain name in the answer. Because this is unsecured, and the is ambiguity regarding whether a negative proof can be provided for the exact match (when it is outside the opt-in secured area), a definitive proof of authenticated denial is not possible. 7 Security Considerations This document is refining the specifications to make it more likely that security can be added to DNS. No functional additions are being made, just refining what is considered proper to allow the system, security of the system, and extending the system more predictable. 8 References Normative References [RFC 20] ASCII Format for Network Interchange, V.G. Cerf, Oct-16-1969 [RFC 1034] Domain Names - Concepts and Facilities, P.V. Mockapetris, Nov-01-1987 [RFC 1035] Domain Names - Implementation and Specification, P.V Mockapetris, Nov-01-1987 [RFC 2119] Key Words for Use in RFCs to Indicate Requirement Levels, S Bradner, March 1997 Non-normative References [RFC 2136] Dynamic Updates in the Domain Name System (DNS UPDATE), P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, April 1997 [RFC 2535] Domain Name System Security Extensions, D. Eastlake, March 1999 [WIP] DNSSEC Opt-In, Internet Draft, R. Arends, M. Kosters, D. Blacka, 2002 9 Other Contributing to This Document Others who have directly caused text to appear in the document: Paul Vixie and Olaf Kolkman. Many others have indirect influences on the content. 10 Editor Name: Edward Lewis Title: Research Engineer Affiliation: ARIN Email: edlewis@arin.net Phone: +1-703-227-9854 Appendix A: Subdomains of Wild Card Domain Names In reading the definition of section 2 carefully, it is possible to rationalize unusual names as legal. In the example given, *.example. could have subdomains of *.sub.*.example. and even the more direct *.*.example. (The implication here is that these domain names own explicit resource records sets.) Although defining these names is not easy to justify, it is important that implementations account for the possibility. This section will give some further guidance on handling these names. The first thing to realize is that by all definitions, subdomains of wild card domain names are legal. In analyzing them, one realizes that they cause no harm by their existence. Because of this, they are allowed to exist, i.e., there are no special case rules made to disallow them. The reason for not preventing these names is that the prevention would just introduce more code paths to put into implementations. The concept of "closest enclosing" existing names is important to keep in mind. It is also important to realize that a wild card domain name can be a closest encloser of a query name. For example, if *.*.example. is defined in a zone, and the query name is a.*.example., then the closest enclosing domain name is *.example. Keep in mind that the closest encloser is not eligible to be a source of synthesized answers, just the subdomain of it that has the first label "*". To illustrate this, the following chart shows some matches. Assume that the names *.example., *.*.example., and *.sub.*.example. are defined in the zone. QNAME Closest Encloser Wild Card Source a.example. example. *.example. b.a.example. example. *.example. a.*.example. *.example. *.*.example. b.a.*.example. *.example. *.*.example. b.a.*.*.example. *.*.example. no wild card a.sub.*.example. sub.*.example. *.sub.*.example. b.a.sub.*.example. sub.*.example. *.sub.*.example. a.*.sub.*.example. *.sub.*.example. no wild card *.a.example. example. *.example. a.sub.b.example. example. *.example. Recall that the closest encloser itself cannot be the wild card. Therefore the match for b.a.*.*.example. has no applicable wild card. Finally, if a query name is sub.*.example., any answer available will come from an exact name match for sub.*.example. No wild card synthesis is performed in this case. Full Copyright Statement Copyright (C) The Internet Society 2003. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Edward Lewis +1-703-227-9854 ARIN Research Engineer