DNS Extensions R. Arends Internet-Draft Telematica Instituut Expires: September 1, 2003 M. Larson VeriSign R. Austein ISC D. Massey USC/ISI S. Rose NIST March 3, 2003 Protocol Modifications for the DNS Security Extensions draft-ietf-dnsext-dnssec-protocol-01 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. This Internet-Draft will expire on September 1, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document is part of a family of documents which describes the DNS Security Extensions (DNSSEC). The DNS Security Extensions are a collection of new resource records and protocol modifications which add data origin authentication and data integrity to the DNS. This Arends, et al. Expires September 1, 2003 [Page 1] Internet-Draft DNSSEC Protocol Modifications March 2003 document describes the DNSSEC protocol modifications. This document defines the concept of a signed zone, along with the requirements for serving and resolving using DNSSEC. These techniques allow a security-aware resolver to authenticate both DNS resource records and authoritative DNS error indications. This document obsoletes RFC 2535 and incorporates changes from all updates to RFC 2535. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Background and Related Documents . . . . . . . . . . . . . . 4 1.2 Reserved Words . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Editors' Notes . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.1 Open Technical Issues . . . . . . . . . . . . . . . . . . . 4 1.3.2 Technical Changes or Corrections . . . . . . . . . . . . . . 4 1.3.3 Typos and Minor Corrections . . . . . . . . . . . . . . . . 5 2. Zone Signing . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Including KEY RRs in a Zone . . . . . . . . . . . . . . . . 6 2.2 Including SIG RRs in a Zone . . . . . . . . . . . . . . . . 7 2.3 Including NXT RRs in a Zone . . . . . . . . . . . . . . . . 8 2.4 Including DS RRs in a Zone . . . . . . . . . . . . . . . . . 8 2.5 Changes to the CNAME Resource Record. . . . . . . . . . . . 8 2.6 Example of a Secure Zone . . . . . . . . . . . . . . . . . . 8 3. Serving . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 Including SIG RRs in a Response . . . . . . . . . . . . . . 10 3.2 Including KEY RRs In a Response . . . . . . . . . . . . . . 11 3.3 Including NXT RRs In a Response . . . . . . . . . . . . . . 11 3.3.1 Case 1: Query Name Exists, but RR Type Not Present . . . . . 12 3.3.2 Case 2: Query Name Does Not Exist, and No Wildcard Matches . 12 3.3.3 Case 3: Query Name Does Not Exist, but Wildcard Matches . . 13 3.4 Including DS RRs In a Response . . . . . . . . . . . . . . . 13 3.5 Responding to Queries for DS RRs . . . . . . . . . . . . . . 13 3.6 Responding to Queries for Type AXFR or IXFR . . . . . . . . 15 3.7 Setting the AD and CD Bits in a Response . . . . . . . . . . 15 3.8 Example DNSSEC Responses . . . . . . . . . . . . . . . . . . 16 4. Resolving . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1 Recursive Name Servers . . . . . . . . . . . . . . . . . . . 23 4.2 Stub resolvers . . . . . . . . . . . . . . . . . . . . . . . 24 5. Authenticating DNS Responses . . . . . . . . . . . . . . . . 25 5.1 Authenticating Referrals . . . . . . . . . . . . . . . . . . 26 5.2 Authenticating an RRSet Using a SIG RR . . . . . . . . . . . 27 5.2.1 Checking the SIG RR Validity . . . . . . . . . . . . . . . . 27 5.2.2 Reconstructing the Signed Data . . . . . . . . . . . . . . . 28 5.2.3 Checking the Signature . . . . . . . . . . . . . . . . . . . 30 5.2.4 Authenticating A Wildcard Expanded RRset Positive Response . 31 5.3 Authenticated Denial of Existence . . . . . . . . . . . . . 31 Arends, et al. Expires September 1, 2003 [Page 2] Internet-Draft DNSSEC Protocol Modifications March 2003 5.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.4.1 Example of Re-Constructing the Original Owner Name . . . . . 32 5.4.2 Examples of Authenticating a Response . . . . . . . . . . . 33 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 34 7. Security Considerations . . . . . . . . . . . . . . . . . . 35 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 Normative References . . . . . . . . . . . . . . . . . . . . 37 Informative References . . . . . . . . . . . . . . . . . . . 38 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 38 A. Algorithm For Handling Wildcard Expansion . . . . . . . . . 40 Full Copyright Statement . . . . . . . . . . . . . . . . . . 41 Arends, et al. Expires September 1, 2003 [Page 3] Internet-Draft DNSSEC Protocol Modifications March 2003 1. Introduction The DNS Security Extensions (DNSSEC) modify several aspects of the DNS protocol. Section 2 defines the concept of a signed zone and lists the requirements for zone signing. Section 3 describes the modifications to authoritative name server behavior necessary to handle signed zones. Section 4 describes the behavior of entities which include security-aware resolver functions Finally, Section 5 defines how to use DNSSEC RRs to authenticate a response. 1.1 Background and Related Documents The reader is assumed to be familiar with the basic DNS concepts described in RFC1034 [1] and RFC1035 [2]. This document is part of a family of documents which define the DNS security extensions (DNSSEC). The DNS Security Extensions are a collection of new resource records and protocol modifications which add data origin authentication and data integrity to the DNS. An introduction to DNSSEC and definition of common terms can be found in [9]. A definition of the DNSSEC resource records can be found in [10]. This document defines the DNSSEC protocol modifications. 1.2 Reserved Words 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. [4]. 1.3 Editors' Notes 1.3.1 Open Technical Issues Use of NXT RRs throughout this document set will have to be modified if DNSSEC-Opt-In [11] becomes part of DNSSEC. The use of the NXT record requires input from the working group. This text describes the NXT record as it was defined in RFC 2535, and substantial portions of this document would need to be updated to incorporate opt-in. The updates will be made if the WG adopts opt-in. Use of the AD bit requires input from the working group. Since the AD bit usage is not resolved, this text attempts to capture current ideas and drafts, but further input from the working group is required. 1.3.2 Technical Changes or Corrections Please report technical corrections to dnssec-editors@east.isi.edu. Arends, et al. Expires September 1, 2003 [Page 4] Internet-Draft DNSSEC Protocol Modifications March 2003 To assist the editors, please indicate the text in error and point out the RFC that defines the correct behavior. For a technical change where no RFC that defines the correct behavior, or if there's more than one applicable RFC and the definitions conflict, please post the issue to namedroppers. An example correction to dnssec-editors might be: Page X says "DNSSEC RRs SHOULD be automatically returned in responses." This was true in RFC 2535, but RFC 3225 (Section 3, 3rd paragraph) says the DNSSEC RR types MUST NOT be included in responses unless the resolver indicated support for DNSSEC. 1.3.3 Typos and Minor Corrections Please report any typos corrections to dnssec-editors@east.isi.edu. To assist the editors, please provide enough context for us to find the incorrect text quickly. An example message to dnssec-editors might be: page X says "the DNSSEC standard has been in development for over 1 years". It should read "over 10 years". Arends, et al. Expires September 1, 2003 [Page 5] Internet-Draft DNSSEC Protocol Modifications March 2003 2. Zone Signing DNSSEC defines the concept of a signed zone. A signed zone includes KEY, SIG, NXT and (optionally) DS records according to the rules specified in Section 2.1, Section 2.2, Section 2.3 and Section 2.4, respectively. Any zone which does not include these records according to the rules in this section must be considered unsigned. Editors' note: Should this be "MUST be considered unsigned"? DNSSEC requires a change to the definition of the CNAME resource record. Section 2.5 changes the CNAME RR to allow SIG and NXT RRs to appear at the same owner name as a CNAME RR. Section 2.6 shows a sample signed zone. 2.1 Including KEY RRs in a Zone Editors' note: Unresolved inconsistency between paragraphs in this section, regarding non-zone KEY RRs at the zone apex. SHOULD NOT, or MUST NOT? To sign a zone, the zone's administrator generates one or more public/private key pairs and uses the private key(s) to sign authoritative RRsets in the zone. For each private key used to create SIG RRs, there SHOULD be a corresponding KEY RR stored at the zone apex. All KEY RRs at the zone apex MUST be zone keys. (A zone key KEY RR has the Zone Key bit of the Flags RDATA field set to one. See Section 2.1.1 of [10].) Zone key KEY RRs MUST appear only at the zone apex. A signed zone MUST have at least one zone key KEY RR in its apex KEY RRset. The KEY RRset at the zone apex MUST be self-signed by at least one private key whose corresponding public key is a zone key stored in the apex KEY RRset. Editors' note: The requirement listed in this paragraph may not be necessary anymore, since the KEY RRset is self-signed anyway (because the whole zone is signed). This is probably a pre-DS relic, but we spotted this a few hours before the I-D deadline and were too chicken to remove it on such short notice.... Other public keys associated with other DNS operations can be stored in KEY RRs that are not marked as zone keys. Non-zone key KEY RRs MUST NOT appear at delegation names. Non-zone key KEY RRs also SHOULD NOT appear at the zone apex, because large KEY RRsets add processing time at resolvers. Non-zone key KEY RRs MAY appear at any other name in the zone. Arends, et al. Expires September 1, 2003 [Page 6] Internet-Draft DNSSEC Protocol Modifications March 2003 2.2 Including SIG RRs in a Zone For each authoritative RRset in the zone (which excludes NS RRsets at delegation points and glue RRsets), there MUST be at least one SIG record that meets all of the following requirements: o The SIG owner name is equal to the RRset owner name; o The SIG class is equal to the RRset class; o The SIG Type Covered field is equal to the RRset type; o The SIG Original TTL field is equal to the TTL of the RRset; o The SIG RR's TTL is equal to the TTL of the RRset; o The SIG Labels field is equal to the number of labels in the RRset owner name, not counting the null root label or any wildcard label; o The SIG Signer's Name field is equal to the name of the zone containing the RRset; and o The SIG Algorithm, Signer's Name, and Key Tag fields identify a zone key KEY record at the zone apex. The process for constructing the SIG RR for a given RRset is described in [10]. An RRset MAY have multiple SIG RRs associated with it. A SIG RR itself MUST NOT be signed, since signing a SIG RRset would add no value and would create an unterminated dependency loop in the signing process. The NS RRset which appears at the zone apex name MUST be signed, but the NS RRsets which appear at delegation points (that is, the NS RRsets in the parent zone which delegate the name to the child zone's name servers) MUST NOT be signed. Glue address RRsets associated with delegations MUST NOT be signed. The difference between the set of owner names which require SIG records and the set of owner names which require NXT records is subtle and worth highlighting. SIG records are present at the owner names of all authoritative RRsets. NXT records are present at the owner names of all names for which the signed zone is authoritative and also at the owner names of delegations from the signed zone to its children. Neither NXT nor SIG records are present (in the parent zone) at the owner names of glue address RRsets. Note, however, that Arends, et al. Expires September 1, 2003 [Page 7] Internet-Draft DNSSEC Protocol Modifications March 2003 this distinction is for the most part only visible during the zone signing process, because NXT RRsets are authoritative data, and therefore are signed, thus any owner name which has an NXT RRset will have SIG RRs as well in the signed zone. 2.3 Including NXT RRs in a Zone Each owner name in the zone MUST have an NXT resource record, except for the owner names of any glue address RRsets. The process for constructing the NXT RR for a given name is described in [10]. 2.4 Including DS RRs in a Zone The DS resource record establishes authentication chains between DNS zones. A DS RRset SHOULD be present at a delegation point when the child zone is signed. The DS RRset MAY contain multiple records, each referencing a key used by the child zone to sign its apex KEY RRset. All DS RRsets in a zone MUST be signed and DS RRsets MUST NOT appear at non-delegation points nor at a zone's apex. A DS RR SHOULD point to a KEY RR which is present in the child's apex KEY RRset, and the child's apex KEY RRset SHOULD be signed by the corresponding private key. Construction of a DS RR requires knowledge of the corresponding KEY RR in the child zone, which implies communication between the child and parent zones. This communication is an operational matter not covered by this document. 2.5 Changes to the CNAME Resource Record. If a CNAME RRset is present at a name in a signed zone, appropriate SIG and NXT RRsets are REQUIRED at that name. Other types MUST NOT be present at that name. This is a modification to the original CNAME definition given in [1]. The original definition of the CNAME RR did not allow any other types to co-exist with a CNAME record, but a signed zone requires NXT and SIG RRsets for every authoritative name. To resolve this conflict, the definition of the CNAME resource record is hereby modified to allow for the co-existence of NXT and SIG RRsets. 2.6 Example of a Secure Zone {{secure zone here}} Editors' note: Zone file example deferred pending hackery to add zone files in a format usable by xml2rfc. Goal here is to show a Arends, et al. Expires September 1, 2003 [Page 8] Internet-Draft DNSSEC Protocol Modifications March 2003 (small) complete signed zone. The apex KEY set includes two KEY RRs, and the KEY RDATA Flags indicate that each of these KEY RRs is a zone key. The first zone KEY is used to sign the apex KEY RRset, and a DS record for this key is provided to the parent zone. The second zone KEY is used to sign all the other RRsets in the zone. A non-zone KEY RR is also stored at "host1.example.com"; this KEY might be used by SIG(0) to authenticate transactions from this host. The zone includes a wildcard entry "*.a.example.com". Note that the "*.a.example.com" name is used in constructing NXT chains, and that the SIG covering the "*.a.example.com" MX RRset has a label count of 3. The zone also includes two delegations. The delegation to "insecure.example.com" includes an NS RRset, glue address records, and an NXT RR, but note that only the NXT RRset is signed. The "secure.example.com" delegation provides a DS RR, and note that only NXT and DS RRsets are signed. Arends, et al. Expires September 1, 2003 [Page 9] Internet-Draft DNSSEC Protocol Modifications March 2003 3. Serving This section describes the behavior of a security-aware authoritative name server. A security-aware authoritative name server MUST support the EDNS0 [6] message size extension, MUST support a message size of at least 1220 octets, and SHOULD support a message size of 4000 octets [8]. Since functions specific to security-aware recursive name servers included components of both resolving and serving, issues specific to security-aware recursive name servers are described in Section 4. Upon receiving a relevant query which has the EDNS [6] OPT pseudo-RR DO [7] bit set to one, a security-aware authoritative name server for a signed zone MUST include additional SIG, NXT, and DS RRs according to the following rules: o SIG RRs which can be used to authenticate a response MUST be included in the response automatically according to the rules in Section 3.1; o NXT RRs which can be used to provide authenticated denial of existence MUST be included in the response automatically according to the rules in Section 3.3; o Either DS RRs or an NXT RR proving that no DS RRs exist MUST be included in referrals automatically according to the rules in Section 3.4. DNSSEC does not change the DNS zone transfer protocol. Zone transfer requirements are reviewed in Section 3.6. A security-aware name server which receives a DNS query which does not include the EDNS OPT pseudo-RR, or which has the DO bit set to zero, MUST treat the SIG, KEY, and NXT RRs as it would any other RRset, and MUST NOT perform any of the additional processing described above. Since the DS RR type has the peculiar property of only existing in the parent zone at delegation points, DS RRs always require some special processing, as described in Section 3.5. 3.1 Including SIG RRs in a Response When a query has the DO bit set to one, the authoritative name server SHOULD attempt to send SIG RRs which can be used to authenticate the RRsets in the response. Inclusion of SIG RRs in a response is subject to the following rules: o When a signed RRset is placed in the Answer section, its SIG RRs are also placed in the Answer section. The SIG RRs have a higher Arends, et al. Expires September 1, 2003 [Page 10] Internet-Draft DNSSEC Protocol Modifications March 2003 priority for inclusion than any other RRsets which may need to be included. If space does not permit the inclusion of these SIG RRs, the response MUST be considered truncated, and the TC bit MUST be set. o When a signed RRset is placed in the Authority section, its SIG RRs are also placed in the Authority section. The SIG RRs have a higher priority for inclusion than any other RRsets that may need to be included. If space does not permit the inclusion of these SIG RRs, the response MUST be considered truncated, and the TC bit MUST be set. o When a signed RRset is placed in the Additional section, its SIG RRs are also placed in the Additional section. If space does not permit the inclusion of these SIG RRs, the response MUST NOT be considered truncated just because these SIG RRs didn't fit. 3.2 Including KEY RRs In a Response When a query has the DO bit set to one and requests the SOA or NS RRs at the apex of a signed zone, then a security-aware authoritative name server for that zone SHOULD return the KEY RRset with the same name in the Additional section. If Additional section processing results in more data than will fit in the response message, address glue RRs have higher priority than KEY RRs. SIG RR(s) associated with the KEY RRset SHOULD also be included in the Additional section (see Section 3.1). Editors' note: Didn't the WG decide that DS RR removes the need for Additional section processing for KEY RRs? If so, this subsection should be deleted. 3.3 Including NXT RRs In a Response Editors' note: This whole section uses the phrase "query name exists", which is somewhat ambiguous when discussing internal nodes with no RRs. We are reasonably certain that, as used here, the phrase only refers to names which are the owner name for least one RR. Better phrasing needed. When a query has the DO bit set to one, security-aware authoritative name servers for a signed zone MUST include NXT RRs in each of the following cases: Case 1: The query name exists, but the requested RR type does not exist. Arends, et al. Expires September 1, 2003 [Page 11] Internet-Draft DNSSEC Protocol Modifications March 2003 Case 2: The query name does not exist, and no wildcard can be expanded to answer the query. Case 3: The query name does not exist, but a wildcard can be expanded to positively answer the query. Note that, in each case, a set of NXT RRs is included to provide authenticated denial of existence. NXT RRs are also included in a referral response when no DS RR is present; in this case, the NXT RR proves that no DS RR exists for the delegation. Referrals are discussed in more detail in Section 3.4. 3.3.1 Case 1: Query Name Exists, but RR Type Not Present If the query name exists but the requested RR type is not present at the name, then the NXT RR associated with the query name MUST be included in the Authority section. Any SIG(s) associated with the NXT RRset are also included in the Authority section (see Section 3.1) If space does not permit the inclusion of the NXT RR (or its associate SIG RRs), the response MUST be considered truncated and the TC bit MUST be set. Note that, since the query name exists, no wildcard expansion applies to this query, and a single NXT RR suffices to prove the requested type does not exist. 3.3.2 Case 2: Query Name Does Not Exist, and No Wildcard Matches If the query name does not exist, and no wildcard expansion matches the query, then the Authority section of the response MUST include the following NXT RRs: o An NXT RR proving that there was no exact match for the name; and o An NXT RR proving that there was no wildcard which would have matched the query. If space does not permit the inclusion of these NXT RRs, the response MUST be considered truncated, and the TC bit MUST be set. Any SIG(s) associated with the NXT RRsets MUST also be included in the Authority section (see Section 3.1). Editors' note: Should lack of space to include the SIGs cause the packet to be considered truncated? Appendix A provides an algorithm which computes the appropriate NXT RRs to prove that no wildcard matches a given query name. Arends, et al. Expires September 1, 2003 [Page 12] Internet-Draft DNSSEC Protocol Modifications March 2003 3.3.3 Case 3: Query Name Does Not Exist, but Wildcard Matches If the query name does not exist, but a wildcard expansion can be used to return a positive match to the query, then the wildcard- expanded answer and any SIG RRs associated with the wildcard RR MUST be returned in the Answer section. The Authority section of the response MUST include the following NXT RRs: o An NXT RR which proves that there were no exact matches for the QNAME and QTYPE; and o An NXT RR which proves that there are no closer wildcard entries which could have been expanded to match the query. If space does not permit inclusion of these NXT RRs, the response MUST be considered truncated, and the TC bit MUST be set. Any SIG RRs associated with the NXT RRsets MUST also be included in the response. Editors' note: Should lack of space to include the SIGs cause the packet to be considered truncated? Appendix A provides an algorithm which computes the appropriate NXT RRs to prove that no closer wildcard matches the query name. 3.4 Including DS RRs In a Response When a query has the DO bit set to one, and a DS RR exists at the query name, an authoritative security-aware name server returning a referral for the delegation MUST include both the NS RRset and also the DS RRset and its associated SIG RR(s). The NS RRset MUST be placed before the DS RRset and its associated SIG RRs. When a query has the DO bit set to one, and no DS RR exists at the query name, an authoritative security-aware name server returning a referral for the delegation MUST include both the NS RRset and also the NXT RR and associated SIG RR(s) which proves that the DS RRset does not exist. The NS RRset MUST be placed before the NXT RRset and its associated SIG RR(s). This increases the size of referral messages, and may cause some or all glue RRs to be omitted. If space does not permit the inclusion of the DS or NXT RRset and associated SIG RRs, the response MUST be considered truncated, and the TC bit MUST be set. 3.5 Responding to Queries for DS RRs The DS record is the first resource record type which appears only on Arends, et al. Expires September 1, 2003 [Page 13] Internet-Draft DNSSEC Protocol Modifications March 2003 the parent zone's side of a zone cut. In other words, the DS record for the delegation of "example.com" is only stored in the "com" zone. This introduces novel name server behavior, since the name server for the child zone is authoritative for the name by the normal DNS rules but the child zone does not contain the DS RR. A name server's response to a DS query depends on whether the name server is authoritative for the parent zone, the child zone, or both, as described below. If a name server is authoritative for the parent zone, and receives a query for the DS record at the delegated name, then the name server MUST return the DS RRset from the parent zone. This rule applies regardless of whether or not the name server is also authoritative for the child zone. If the name server is authoritative for the child zone, is not authoritative for the parent zone, and receives a query for the DS record at the delegated name, there is no obvious response, because the child zone is not authoritative for the DS record at the child zone's apex, and the authoritative DS RR is only stored at the parent. If the name server allows recursion, and the RD bit is set in the query, the name server MAY perform recursion to find the DS record for the delegated name from the parent zone, and MAY return the DS record from its cache. In this case, the AA bit MUST NOT be set in the response. If the name server does not perform recursion to find the DS RR, the name server MUST reply with: RCODE: NOERROR AA bit: set Answer Section: Empty Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)] In other words, a name server which is authoritative for the child zone but not for the parent zone answers as if the DS record does not exist. Note that security-aware resolvers will query the parent zone at delegation points, and thus will not be affected by this behavior. For example, suppose that "example.com" is a delegation point, and a name server receives a query for the "example.com" DS RRset. o If the name server is authoritative for "com", the name server MUST reply with the "example.com" DS RRset from the "com" zone. o If the name server is authoritative for "example.com", is not Arends, et al. Expires September 1, 2003 [Page 14] Internet-Draft DNSSEC Protocol Modifications March 2003 authoritative for "com", and the RD bit is set to one in the query, the name server MAY perform recursion to find the "example.com" DS record. If the name server does not use recursion to obtain the DS RR, the name server MUST reply as though the DS RR did not exist: RCODE: NOERROR AA bit: set Answer Section: Empty Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)] 3.6 Responding to Queries for Type AXFR or IXFR DNSSEC does not change the DNS zone transfer process. A signed zone will contain SIG, KEY, NXT, and DS resource records, but these records have no special meaning with respect to a zone transfer operation, and these RRs are treated as any other resource record type. An authoritative name server is not required to verify that a zone is properly signed before sending or accepting a zone transfer. However, an authoritative name server MAY choose to reject the entire zone transfer if the zone fails meets any of the signing requirements described in Section Section 2. The primary objective of a zone transfer to ensure that all authoritative name servers have identical copies of the zone. An authoritative name server which chooses to perform its own zone validation MUST NOT selectively reject some RRs and accept others. Note that the DS RR appears only in the parental side of a delegation, and is authoritative data in the parent zone. For example, the DS RR for "example.com" is stored in the "com" zone (the parent zone) rather than in the "example.com" zone (the child zone). As with any other authoritative RRset, the "example.com" DS RR MUST be included the "com" zone transfer. Note that authoritative NXT RRs appear in both the parent and child zones at a delegated name, and that the NXT RRs for the delegated name in the parent and child zones are never identical to each other. As with any other authoritative RRset, the parental NXT RR at a delegated name MUST be included zone transfers of the parent zone, while the NXT at the zone apex of the child zone MUST be included in zone transfers of the child zone. 3.7 Setting the AD and CD Bits in a Response Editors' note: This section seems a little lost here. Perhaps we Arends, et al. Expires September 1, 2003 [Page 15] Internet-Draft DNSSEC Protocol Modifications March 2003 should rearrange the section ordering slightly, or provide a pointer to this subsection at the beginning of Section 3. DNSSEC allocates two new bits in the DNS message header: The CD (Checking Disabled) bit and the AD (Authentic Data) bit. The CD bit is set in query messages by the resolver, and MUST be copied into the response. If the CD bit is set to one, it indicates that the resolver is willing to perform authentication, and thus that the name server need not perform authentication on the RRsets in the response. Regardless of the setting of the CD bit, the name server MAY choose whether or not to perform authentication according to the local name server policy. The CD bit MAY be used in constructing the local name server policy. If local name server policy does perform authentication, any RRsets rejected by the local authentication policy MUST NOT be returned in a response (regardless of the CD bit). The AD bit is set by name servers, and indicates the data in the response has been authenticated by the name server, according to the local name server policy. The AD bit MUST NOT be set on a response unless all of the RRsets in the Answer and Authority sections have met the name server's local authentication policy. A resolver MUST NOT trust the AD bit unless it communicates with the name server over a secure transport mechanism and is explicitly configured to trust the name server's policy. 3.8 Example DNSSEC Responses The examples in this section use the following example zone to demonstrate the formation of replies by an authoritative name server. The zone has two name servers, a single child, and a wildcard MX RR. The zone is completely signed and has a full NXT chain. Arends, et al. Expires September 1, 2003 [Page 16] Internet-Draft DNSSEC Protocol Modifications March 2003 example.com. SOA (...) SIG SOA ... NS a.example.com. NS b.example.com. SIG NS ... MX 10 a.example.com SIG MX ... KEY ... SIG KEY ... NXT *.example.com. * MX 10 a.example.com. SIG MX ... NXT a.example.com. a A 10.10.10.1 SIG A ... NXT b.example.com. b A 10.10.10.2 SIG A ... NXT c.example.com. c CNAME a.example.com. SIG CNAME NXT sub.example.com. sub NS ns.sub.example.com. SIG NS DS ... SIG DS NXT *.example.com. ns.sub A 10.10.10.3 sub-nosig NS ns.sub-nosig.example.com. NXT example.com. ns.sub-nosig A 10.10.10.4 Arends, et al. Expires September 1, 2003 [Page 17] Internet-Draft DNSSEC Protocol Modifications March 2003 A query to the authoritative name server for this zone for QNAME="c.example.com", QCLASS=IN, QTYPE=A would produce: Flags: QR=1, AA=1, RCODE=0 (NOERROR) EDNS: DO=1, size=4000 QUERY: c.example.com. IN A ANSWER: c.example.com. IN A a.example.com IN SIG CNAME a.example.com. IN A 10.10.10.1 IN SIG A AUTHORITY: example.com. IN NS a.example.com. IN NS b.example.com. IN SIG NS ... ADDITIONAL: a.example.com. IN A 10.10.10.1 IN SIG A ... b.example.com. IN A 10.10.10.2 IN SIG A ... example.com. IN KEY ... IN SIG KEY ... A query for QNAME="www.sub.example.com", QCLASS=IN, QTYPE=A would results in a referral to a signed zone. The resolver can determine that "sub.example.com" is signed because of the presence of the DS RR with the hash of the "sub.example.com" zone key. Flags: QR=1, AA=1, RCODE=0 (NOERROR) EDNS: DO=1, size=4000 QUERY: www.sub.example.com. IN A ANSWER: ;; empty AUTHORITY: sub.example.com. IN NS ns.sub.example.com. IN DS ... IN SIG DS ... ADDITIONAL: ns.sub.example.com. IN A 10.10.10.3 Arends, et al. Expires September 1, 2003 [Page 18] Internet-Draft DNSSEC Protocol Modifications March 2003 A query for QNAME="www.sub-nosig.example.com", QCLASS=IN, QTYPE=A would result in a referral to an unsigned zone. The resolver knows not to expect DNSSEC RRs from "sub-nosig.example.com", because the DS bit in the NXT RR bitmap in the referral is not set. Even if DNSSEC RRs are present in responses from "sub-nosig.example.com" name servers, the resolver will not be able to construct a authentication chain, since there is a break between "sub-nosig.example.com" and its delegating parent zone. Flags: QR=1, AA=1, RCODE=0 (NOERROR) EDNS: DO=1, size=4000 QUERY: www.sub-nosig.example.com. IN A ANSWER: ;; empty AUTHORITY: sub-nosig.example.com. IN NS ns.sub-nosig.example.com. IN NXT ;; (DS bit not set) IN SIG NXT ... ADDITIONAL: ns.sub-nosig.example.com. IN A 10.10.10.4 A query for QNAME="f.example.com", QCLASS=IN, QTYPE=A returns a name error, because the name does not exist and is not covered by wildcard expansion. Therefore, the name server must present proof that the name does not exist, and that no wildcard expansion is present which could have been used to answer the query. Flags: QR=1, AA=1, RCODE=3 (NXDOMAIN) EDNS: DO=1, size=4000 QUERY: f.example.com. IN A ANSWER: ;; empty AUTHORITY: example.com. IN SOA ... IN SIG SOA ... c.example.com. IN NXT sub.example.com. ... IN SIG NXT ... *.example.com. IN NXT a.example.com. ... IN SIG NXT ... ADDITIONAL: example.com. IN KEY ... IN SIG KEY ... Arends, et al. Expires September 1, 2003 [Page 19] Internet-Draft DNSSEC Protocol Modifications March 2003 A query for QNAME="f.example.com" QCLASS=IN, QTYPE=MX returns an MX RR synthesized via wildcard expansion. The name server must prove that no exact match exists. Flags: QR=1, AA=1, RCODE=0 (NOERROR) EDNS: DO=1, size=4000 QUERY: f.example.com. IN MX ANSWER: f.example.com. IN MX 10 a.example.com. IN SIG MX ... AUTHORITY: example.com. IN NS a.example.com. IN NS b.example.com. IN SIG NS ... c.example.com. IN NXT sub.example.com. IN SIG NXT ... ADDITIONAL: a.example.com. IN A 10.10.10.1 IN SIG A ... b.example.com. IN A 10.10.10.2 IN SIG A ... example.com. IN KEY ... IN SIG KEY ... If these responses came from a recursive name server which had all of the necessary RRsets in its cache instead of from an authoritative server, the only differences would be the TTLs and the header flags. The AA bit would not be set, and the AD bit would be set if (and only if) all the RRsets in a response passed the security policy checks of the recursive name server. Arends, et al. Expires September 1, 2003 [Page 20] Internet-Draft DNSSEC Protocol Modifications March 2003 4. Resolving Editors' note: This section is still very rough, and some of the text here duplicates text from other portions of this document. This needs to be fixed (one way or another) during final editing. Suggestions for better text would be welcome. This section describes the behavior of entities which include security-aware resolver functions. In many cases such functions will be part of a security-aware recursive name server, but a stand-alone security-aware resolver has many of the same requirements. Functions specific to security-aware recursive name servers are described in a separate subsection. A security-aware resolver MUST include an EDNS [6] OPT pseudo-RR with the DO [7] bit set to one when sending queries. A security-aware resolver MUST support a message size of at least 1220 octets, SHOULD support a message size of 4000 octets, and MUST advertise the supported message size using the "sender's UDP payload size" field in the EDNS OPT pseudo-RR. A security-aware resolver MUST handle fragmented UDP packets correctly regardless of whether any such fragmented packets were received via IPv4 or IPv6. Please see [8] for discussion of these requirements. A security-aware resolver MUST support the signature verification mechanisms described in Section 5, and MUST apply them to every received response except when: o The security-aware resolver is part of a security-aware recursive name server, and the response is the result of recursion on behalf of a query received with the CD bit set; o The response is the result of a query generated directly via some form of application interface which instructed the security-aware resolver not to perform validation for this query; or o Validation for this query has been disabled by local policy. A security-aware resolver's support for signature verification MUST include support for verification of wildcard owner names. A security-aware resolver MUST attempt to retrieve missing DS, KEY, or SIG RRs via explicit queries if the resolver needs these RRs in order to perform signature verification. A security-aware resolver MUST attempt to retrieve missing a NXT RR which the resolver needs to authenticate a NODATA response. In Arends, et al. Expires September 1, 2003 [Page 21] Internet-Draft DNSSEC Protocol Modifications March 2003 general it is not possible for a resolver to retrieve missing NXT RRs, since the resolver will have no way of knowing the owner name of the missing NXT RR, but in the specific case of a NODATA response, the resolver does know the name of the missing NXT RR, and must therefore attempt to retrieve it. A security-aware resolver MUST be able to determine whether or not it should expect a particular RRset to be signed. More precisely, a security-aware resolver must be able to distinguish between three cases: 1. An RRset for which the resolver is able to build a chain of signed KEY and DS RRs from a trusted starting point to the RRset. In this case, the RRset should be signed, and is subject to signature validation as described above. 2. An RRset for which the resolver knows that it has no chain of signed KEY and DS RRs from any trusted starting point to the RRset. This can occur when the target RRset lies in an unsigned zone or in a descendent of an unsigned zone. In this case, the RRset may or may not be signed, but the resolver will not be able to verify the signature. 3. An RRset for which the resolver is not able to determine whether or not the RRset should be signed, because the resolver is not able to obtain the necessary DNSSEC RRs. This can occur due when the security-aware resolver is not able to contact security-aware name servers for the relevant zones. A security-aware resolver MUST be capable of being preconfigured with at least one trusted public key, and SHOULD be capable of being preconfigured with multiple trusted public keys. Since a security- aware resolver will not be able to validate signatures without such a preconfigured trusted key, the resolver SHOULD have some reasonably robust mechanism for obtaining such keys when it boots. Editors' note: Should support for multiple public keys be a MUST rather than a SHOULD? A security-aware resolver SHOULD cache each response as a single atomic entry, indexed by the triple , with the single atomic entry containing the entire answer, including the named RRset and any associated DNSSEC RRs. The resolver SHOULD discard the entire atomic entry when any of the RRs contained in it expire. Editors' note: This is implementation advice which came out of discussions at various workshops and investigations into possible implementation issues with the (as yet unsettled) opt-in proposal. Arends, et al. Expires September 1, 2003 [Page 22] Internet-Draft DNSSEC Protocol Modifications March 2003 All of this advice has been discussed in WG meetings, and as far as the editors know these recommendations are not controversial, but it is up to the WG to decide whether this sort of implementation advice belongs in this document. 4.1 Recursive Name Servers As explained in [9], a security-aware recursive name server is an entity which acts in both the security-aware name server and security-aware resolver roles. This section uses the terms "name server side" and "resolver side" to refer to the code within a security-aware recursive name server which implements the security- aware name server role and the code which implements the security- aware resolver role, respectively. The resolver side of a security-aware recursive name server MUST set the DO bit when sending requests, regardless of the state of the DO bit in the initiating request received by the name server side. If the initiating request does not have the DO bit set, the name server side MUST remove any DNSSEC RRs from the response sent to the initiating resolver, but the resolver side MUST follow the usual rules for caching which would apply to any security-aware resolver. A security-aware recursive name server SHOULD NOT attempt to answer a query by piecing together cached data it received in response to previous queries that requested different QNAMEs, QTYPEs, or QCLASSes. A security-aware recursive name server SHOULD NOT use NXT RRs from one negative response to synthesize a response for a different query. A security-aware recursive name server SHOULD NOT use a previous wildcard expansion to generate a response to a different query. Editors' note: Should any of the SHOULD NOTs in this paragraph be MUST NOTs instead? The name server side of a security-aware recursive name server MUST pass the sense of the CD bit to the resolver side along with the rest of an initiating query, so that the resolver side will know whether whether or not it is required to verify the response data it returns to the name server side. Editors' note: What should a security-aware recursive name server do if it receives a query with CD=1 and DO=0? Arends, et al. Expires September 1, 2003 [Page 23] Internet-Draft DNSSEC Protocol Modifications March 2003 4.2 Stub resolvers A security-aware stub resolver MUST include an EDNS [6] OPT pseudo-RR with the DO [7] bit set to one when sending queries. A security-aware stub resolver MUST support a message size of at least 1220 octets, SHOULD support a message size of 4000 octets, and MUST advertise the supported message size using the "sender's UDP payload size" field in the EDNS OPT pseudo-RR. A security-aware stub resolver MUST handle fragmented UDP packets correctly regardless of whether any such fragmented packets were received via IPv4 or IPv6. Please see [8] for discussion of these requirements. A security-aware stub resolver MUST support the DNSSEC RR types, at least to the extent of not mishandling responses just because they contain DNSSEC RRs. A security-aware stub resolver MAY include the DNSSEC RRs returned by a security-aware recursive name server as part of the data that it the stub resolver hands back to the application which invoked it, but is not required to do so. A security-aware stub resolver SHOULD NOT set the CD bit when sending queries, since, by definition, a security-aware stub resolver does not validate signatures and thus depends on the security-aware recursive name server to perform validation on its behalf. Editors' note: Should this SHOULD NOT be a MUST NOT? A security-aware stub resolver MUST NOT place any reliance on signature validation allegedly performed on its behalf except when the security-aware stub resolver obtained the data in question from a trusted security-aware recursive name server via a secure channel. Arends, et al. Expires September 1, 2003 [Page 24] Internet-Draft DNSSEC Protocol Modifications March 2003 5. Authenticating DNS Responses In order to use DNSSEC RRs for authentication, a security-aware resolver requires preconfigured knowledge of at least one authenticated KEY RR. The process for obtaining and authenticating this initial KEY RR is achieved via some external mechanism. For example, a resolver could use some off-line authenticated exchange to obtain a zone's KEY RR or obtain a DS RR that identifies and authenticates a zone's KEY RR. The remainder of this section assumes that the resolver has somehow obtained an initial set of authenticated KEY RRs. An initial KEY RR can be used to authenticate a zone's apex KEY RRset. To authenticate an apex KEY RRset using an initial key, the resolver MUST: 1. Verify that the initial KEY RR appears in the apex KEY RRset, and verify that the KEY RR has the Zone Key Flag (KEY RDATA bit 7) set to one. 2. Verify that there is some SIG RR which covers the apex KEY RRset, and that the combination of the SIG RR and the initial KEY RR authenticates the KEY RRset. The process for using a SIG RR to authenticate an RRset is described in Section 5.2. Once the resolver has authenticated the apex KEY RRset using an initial KEY RR, delegations from that zone can be authenticated using DS RRs. This allows a resolver to start from an initial key, and use DS RRsets to proceed recursively down the DNS tree obtaining other apex KEY RRsets. If the resolver were preconfigured with a root KEY RR, and if every delegation had a DS RR associated with it, then the resolver could obtain any apex KEY RRset. The process of using DS RRs to authenticate referrals is described in Section 5.1. Once the resolver has authenticated a zone's apex KEY RRset, Section 5.2 shows how the resolver can use KEY RRs in the apex KEY RRset and SIG RRs from the zone to authenticate any other RRsets in the zone. Section 5.3 shows how the resolver can use authenticated NXT RRsets from the zone to prove that an RRset is not present in the zone. When a resolver indicates support for DNSSEC, a security-aware name server should attempt to provide the necessary KEY, SIG, NXT, and DS RRsets in a response (see Section 3). However, a security-aware resolver may still receive a response which that lacks the appropriate DNSSEC RRs, whether due to configuration issues such as a security-oblivious recursive name server which accidently interfere with DNSSEC RRs or due to a deliberate attack in which an adversary forges a response, strips DNSSEC RRs from a response, or modifies a Arends, et al. Expires September 1, 2003 [Page 25] Internet-Draft DNSSEC Protocol Modifications March 2003 query so that DNSSEC RRs appear not to be requested. The absence of DNSSEC data in a response MUST NOT by itself be taken as an indication that no authentication information exists. A resolver SHOULD expect authentication information from signed zones. A resolver SHOULD believe that a zone is signed if the resolver has been configured with public key information for the zone, or if the zone's parent is signed and the delegation from the parent contains a DS RRset. 5.1 Authenticating Referrals Once the apex KEY RRset for a signed parent zone has been authenticated, DS RRsets can be used to authenticate the delegation to a signed child zone. A DS RR identifies a KEY RR in the child zone's apex KEY RRset, and contains a cryptographic digest of the child zone's KEY RR. A strong cryptographic digest algorithm ensures that an adversary can not easily generate a KEY RR that matches the digest. Thus, authenticating the digest allows a resolver to authenticate the matching KEY RR. The resolver can then use this child KEY RR to authenticate the entire child apex KEY RRset. Given a DS RR for a delegation, the child zone's apex KEY RRset can be authenticated if all of the following hold: o The DS RR has been authenticated using some KEY RR in the parent's apex KEY RRset (see Section 5.2); o The Algorithm and Key Tag in the DS RR match the Algorithm field and the key tag of a KEY RR in the child zone's apex KEY RRset which, when hashed using the digest algorithm specified in the DS RR's Digest Type field, results in a digest value which matches the Digest field of the DS RR; and o The matching KEY RR in the child zone has the Zone Flag bit set to one, the corresponding private key has signed the child zone's apex KEY RRset, and the resulting SIG RR authenticates the child zone's apex KEY RRset. If the referral from the parent zone did not contain a DS RRset, the response should have included a signed NXT RRset proving that no DS RRset exists for the delegated name (see Section 3.4). A security- aware resolver MUST send the parent a query for the DS RRset if the referral includes neither a DS RRset nor a NXT RRset proving the nonexistence of the DS RRset (see Section 4). If the resolver authenticates an NXT RRset which proves that no DS RRset is present for this zone, then there is no authentication path Arends, et al. Expires September 1, 2003 [Page 26] Internet-Draft DNSSEC Protocol Modifications March 2003 leading from the parent to the child. If the resolver has an initial KEY RR which belongs to the child zone or to any delegation below the child zone, this initial KEY RR MAY be used to re-establish an authentication path. If no such initial KEY RR exists, the resolver can not authenticate RRsets at or below the child zone. Note that, for a signed delegation, there are two NXT RRs associated with the delegated name. One NXT RR resides in the parent zone, and can be used to prove whether a DS RRset exists for the delegated name. The second NXT RR resides in the child zone, and identifies which RRsets are present at the apex of the child zone. The parent NXT RR and child NXT RR can always be distinguished, since the SOA bit will be set in the child NXT RR and clear in the parent NXT RR. A security-aware resolver MUST use the parent NXT RR when attempting to prove that a DS RRset does not exist. 5.2 Authenticating an RRSet Using a SIG RR A resolver can use a SIG RR and its corresponding KEY RR to attempt to authenticate RRsets. The resolver first checks the SIG RR to verify that it covers the RRset, has a valid time interval, and identifies a valid KEY RR. The resolver then constructs the canonical form of the signed data by appending the SIG RDATA (excluding the Signature Field) with the canonical form of the covered RRset. Finally, resolver uses the public key and signature to authenticate the signed data. Section 5.2.1, Section 5.2.2, and Section 5.2.3 describe each step in detail. 5.2.1 Checking the SIG RR Validity A security-aware resolver can use a SIG RR to authenticate an RRset if all of the following conditions hold: o The SIG RR and the RRset MUST have the same owner name and the same class; o The SIG RR's Signer's Name field MUST be the name of the zone that contains the RRset; o The SIG RR's Type Covered field MUST equal the RRset's type; o The number of labels in the RRset owner name MUST be greater than or equal to the value in the SIG RR's Labels field; o The resolver's notion of the current time MUST be less than or equal to the time listed in the SIG RR's Expiration field; o The resolver's notion of the current time MUST be greater than or Arends, et al. Expires September 1, 2003 [Page 27] Internet-Draft DNSSEC Protocol Modifications March 2003 equal to the time listed in the SIG RR's Inception field; o The SIG RR's Signer's Name, Algorithm, and Key Tag fields MUST match the owner name, algorithm, and key tag for some KEY RR in the zone's apex KEY RRset; o The matching KEY RR MUST be present in the zone's apex KEY RRset, and MUST have the Zone Flag bit (KEY RDATA Flag bit 7) set to one. It is possible for more than one KEY RR to match the conditions above. In this case, the resolver can not predetermine which KEY RR to use to authenticate the signature, MUST try each matching KEY RR until the resolver has either validated the signature or has run out of matching keys to try. Note that this authentication process is only meaningful if the resolver authenticates the KEY RR before using it to validate signatures. The matching KEY RR is considered to be authentic if: o The apex KEY RRset containing the KEY RR is considered authentic; or o The RRset covered by the SIG RR is the apex KEY RRset itself, and the KEY RR either matches an authenticated DS RR from the parent zone or matches a DS RR or KEY RR which the resolver has been preconfigured to believe to be authentic. 5.2.2 Reconstructing the Signed Data Once the SIG RR has met the validity requirements described in Section 5.2.1, the resolver needs to reconstruct the original signed data. The original signed data includes SIG RDATA (excluding the Signature field) and the canonical form of the RRset. Aside from being ordered, the canonical form of the RRset might also differ from the received RRset due to DNS name compression, decremented TTLs, or wildcard expansion. The resolver should use the following to reconstruct the original signed data: signed_data = SIG_RDATA | RR(1) | RR(2)... where "|" denotes concatenation SIG_RDATA is the wire format of the SIG RDATA fields with the Signature field excluded and the Signer's Name in canonical form. RR(i) = name | class | type | OrigTTL | RDATA length | RDATA Arends, et al. Expires September 1, 2003 [Page 28] Internet-Draft DNSSEC Protocol Modifications March 2003 name is calculated according to the function below class is the RRset's class type is the RRset type and all RRs in the class OrigTTL is the value from the SIG Original TTL field All names in the RDATA field are in canonical form The set of all RR(i) is sorted into canonical order. To calculate the name: let sig_labels = the value of the SIG Labels field let fqdn = RRset's fully qualified domain name in canonical form let fqdn_labels = RRset's fully qualified domain name in canonical form if sig_labels = fqdn_labels, name = fqdn if sig_labels < fqdn_labels, name = "*." | the leftmost sig_label labels of the fqdn if sig_labels > fqdn the SIG RR did not pass the necessary validation checks and MUST NOT be used to authenticate this RRset. Editors' note: The algorithm above needs to be cross-checked very carefully against the definitions in [10]. Section 5.4.1 gives an example of original name calculation. The canonical forms for names and RRsets are defined in [10]. NXT RRsets at a delegation boundary require special processing. There are two distinct NXT RRsets associated with a signed delegated name. One NXT RRset resides in the parent zone, and specifies which RRset are present at the parent zone. The second NXT RRset resides at the child zone, and identifies which RRsets are present at the apex in the child zone. The parent NXT RRset and child NXT RRset can always be distinguished since only the child NXT RRs will specify an SOA RRset exists at the name. When reconstructing the original NXT RRset for the delegation from the parent zone, the NXT RRs MUST NOT Arends, et al. Expires September 1, 2003 [Page 29] Internet-Draft DNSSEC Protocol Modifications March 2003 be combined with NXT RRs from the child zone, and when reconstructing the original NXT RRset for the apex of the child zone, the NXT RRs MUST NOT be combined with NXT RRs from the parent zone. Note also that each of the two NXT RRsets at a delegation point has a corresponding SIG RR with an owner name matching the delegated name, and each of these SIG RRs is authoritative data associated with the same zone which contains the corresponding NXT RRset. If necessary, a resolver can tell these SIG RRs apart by checking the Signer's Name field. 5.2.3 Checking the Signature Once the resolver has validated the SIG RR as described in Section 5.2.1 and reconstructed the original signed data as described in Section 5.2.2, the resolver can attempt to use the cryptographic signature to authenticate the signed data, and thus (finally!) authenticate the RRset. The Algorithm field in the SIG RR identifies the cryptographic algorithm to generate the signature. The signature itself is contained in the Signature field of the SIG RDATA, and the public key to used generate the signature is contained in the Public Key field of the matching KEY RR(s) (found in Section 5.2.1). [10] provides a list of algorithm types, and provides pointers to the documents that define each algorithm's use. Note that it is possible for more than one KEY RR to match the conditions in Section 5.2.1. In this case, the resolver can only determine which KEY RR by trying each matching key until the resolver either succeeds in validating the signature or runs out of keys to try. If the Labels field of the SIG RR is not equal to the number of labels in the RRset's fully qualified owner name, then the RRset is either invalid or the result of wildcard expansion. The resolver MUST verify that wildcard expansion was applied properly before considering the RRset to be authentic. Section 5.2.4 describes how to determine whether a wildcard was applied properly. If other SIG RRs also cover this SIG RR, the local resolver security policy determines whether the resolver also needs to test these SIG RRs, and determines how to resolve conflicts if these SIG RRs lead to differing results. If the resolver accepts the RRset as authentic, the resolver MUST set the SIG RR's TTL and the TTL of each RR in the authenticated RRset to the minimum of: Arends, et al. Expires September 1, 2003 [Page 30] Internet-Draft DNSSEC Protocol Modifications March 2003 o The RRset's TTL as received in the response; o The SIG RR's TTL as received in the response; and o The value in the SIG RR's Original TTL field. 5.2.4 Authenticating A Wildcard Expanded RRset Positive Response If the number of labels in an RRset's fully qualified domain name is greater than the Labels field in the covering SIG RDATA, then the RRset and its covering SIG RR were created as a result of wildcard expansion. Once the resolver has verified the signature as described in Section 5.2, the resolver must take additional steps to verify the non-existence of an exact match or closer wildcard match for the query. Section 5.3 discusses these steps. Note that the response received by the resolver should include all NXT RRs needed to authenticate the response (see Section 3.3). 5.3 Authenticated Denial of Existence A resolver can use authenticated NXT RRs to prove that an RRset is not present in a signed zone. Security-aware name servers should automatically include any necessary NXT RRs for signed zones in their responses to security-aware resolvers. Security-aware resolvers MUST first authenticate NXT RRsets according to the standard RRset authentication rules described in Section 5.2, then apply the NXT RRsets as follows: o If the requested RR name matches the owner name of an authenticated NXT RR, then the NXT RR's type bit map field lists all RR types present at that owner name, and a resolver can prove that the requested RR type does not exist by checking for the RR type in the bit map. Since the existence of the authenticated NXT RR proves that the owner name exists in the zone, wildcard expansion could not have been used to match the requested RR owner name and type. o If the requested RR name would appear after an authenticated NXT RR owner name and before the name listed in that NXT RR's Next Domain Name field according to the canonical DNS name order defined in [10], then no exact match for the requested RR name exists in the zone. However, it is possible that a wildcard could be used to match the requested RR owner name and type, so proving that the requested RRset does not exist also requires proving that no possible wildcard exists which could have been used to generate Arends, et al. Expires September 1, 2003 [Page 31] Internet-Draft DNSSEC Protocol Modifications March 2003 a positive response. To prove non-existence of an RRset, the resolver must be able to verify both that the queried RRset does not exist and that no relevant wildcard RRset exists. Proving this may require more than one NXT RRset from the zone. If the complete set of necessary NXT RRsets is not present in a response (perhaps due to truncation), then a security-aware resolver MUST resend the query in order to attempt to obtain the full collection of NXT RRs necessary to verify non- existence of the requested RRset. As with all DNS operations, however, the resolver MUST bound the work it puts into answering any particular query. 5.4 Example 5.4.1 Example of Re-Constructing the Original Owner Name Suppose that a security-aware resolver receives a response containing an answer RRset with an owner name of is "www.a.b.c.example.com". This fully qualified domain name has 6 labels: "www", "a", "b", "c", "example", and "com". What name the resolver should use when reconstructing the original signed data depends on the value of the SIG RR's Labels field. If the value of the SIG RR's Labels field is 6, then the SIG RR's Labels field matches the number of labels in the owner name, and the resolver should assume that this RRset is not the result of wildcard expansion. The resolver should therefore use "www.a.b.c.example.com" as the owner name when reconstructing the original signed data for the signature check. If the value of the SIG RR's Labels field is less than 6, then the SIG RR's Labels count is less than the number of labels in the RRset's owner name, and the resolver should assume that this RRset is the result of wildcard expansion. The resolver should therefore reconstruct the original owner name by replacing the labels which appear to be the result of wildcard expansion with a single "*." label. For example, if the SIG RR's Labels field is 3, the resolver should reconstruct the original owner name by prepending "*." to the last 3 labels of the owner name of the answer RRset. Thus, the resolver should use "*.c.example.com" as the owner name when reconstructing the original signed data. If the value of the SIG RR's Labels field is greater than 6, then this SIG RR cannot possibly be valid for the answer RRset, and there is no point in attempting to validate the signature. Arends, et al. Expires September 1, 2003 [Page 32] Internet-Draft DNSSEC Protocol Modifications March 2003 5.4.2 Examples of Authenticating a Response Editors' note: Eventually this will be an example of the authentication process for "www.example.com", starting from an initial root key. Editors' note: Eventually this will be an example of the authentication process for non-existent "www.a.b.c.example.com", starting from an initial root key. Arends, et al. Expires September 1, 2003 [Page 33] Internet-Draft DNSSEC Protocol Modifications March 2003 6. IANA Considerations This document introduces no IANA considerations. [10] contains a complete review of the IANA considerations introduced by DNSSEC. Editors' note: This may not be true anymore, since the AD and CD bit definitions are now in this document rather than in [10]. Arends, et al. Expires September 1, 2003 [Page 34] Internet-Draft DNSSEC Protocol Modifications March 2003 7. Security Considerations This document describes how the DNS security extensions use public key cryptography to sign and authenticate DNS resource record sets. At this time, at least two substantial elements of the DNSSEC specification have yet to be decided by the working group. The open opt-in issue would change elements such as what RRsets must be signed, would impact how wildcards are used, and would replace authenticated denial of existence with authenticated denial of security. Handling of the AD bit is also undecided. The AD bit (as currently defined) is used to indicate the security status of RRsets in the response. These items clearly raise security considerations and will be addressed here as these issues are resolved in the working group. DNSSEC introduces a number of denial of service issues. These issues will also be addressed in a future version of these security considerations. Please see [9] for general security considerations related to DNSSEC. Arends, et al. Expires September 1, 2003 [Page 35] Internet-Draft DNSSEC Protocol Modifications March 2003 8. Acknowledgements This document was created from the input and ideas of several members of the DNS Extensions Working Group and working group mailing list. The co-authors of this draft would like to express their thanks for the comments and suggestions received during the revision of these security extension specifications. Arends, et al. Expires September 1, 2003 [Page 36] Internet-Draft DNSSEC Protocol Modifications March 2003 Normative References [1] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [2] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [3] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, August 1996. [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [5] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [6] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [7] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225, December 2001. [8] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver message size requirements", RFC 3226, December 2001. [9] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "DNS Security Introduction and Requirements", draft-ietf- dnsext-dnssec-intro-05 (work in progress), February 2003. [10] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Resource Records for DNS Security Extensions", draft-ietf- dnsext-dnssec-records-04 (work in progress), February 2003. [11] Kosters, M., Blacka, D. and R. Arends, "DNSSEC Opt-in", draft- ietf-dnsext-dnssec-opt-in-04 (work in progress), February 2003. Arends, et al. Expires September 1, 2003 [Page 37] Internet-Draft DNSSEC Protocol Modifications March 2003 Informative References [12] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [13] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC 2930, September 2000. [14] Eastlake, D., "DNS Request and Transaction Signatures ( SIG(0)s)", RFC 2931, September 2000. [15] Gudmundsson, O., "Delegation Signer Resource Record", draft- ietf-dnsext-delegation-signer-12 (work in progress), December 2002. Authors' Addresses Roy Arends Telematica Instituut Drienerlolaan 5 7522 NB Enschede NL EMail: roy.arends@telin.nl Matt Larson VeriSign, Inc. 21345 Ridgetop Circle Dulles, VA 20166-6503 USA EMail: mlarson@verisign.com Rob Austein Internet Software Consortium 40 Gavin Circle Reading, MA 01867 USA EMail: sra@isc.org Arends, et al. Expires September 1, 2003 [Page 38] Internet-Draft DNSSEC Protocol Modifications March 2003 Dan Massey USC Information Sciences Institute 3811 N. Fairfax Drive Arlington, VA 22203 USA EMail: masseyd@isi.edu Scott Rose National Institute for Standards and Technology 100 Bureau Drive Gaithersburg, MD 20899-8920 USA EMail: scott.rose@nist.gov Arends, et al. Expires September 1, 2003 [Page 39] Internet-Draft DNSSEC Protocol Modifications March 2003 Appendix A. Algorithm For Handling Wildcard Expansion For zone (Z) and a name (N) that may occur in Z, the following algorithm finds all wildcard RRsets that match N or returns an NXT RRset that proves no wildcard expansion matches N. The algorithm was written for clarity, not efficiency: 0. INPUT: a name (N) and a zone (Z). INIT: NXT_SET = NULL 1. Construct S = sequence of all names in Z, sorted into canonical order. 2. If N exists in S There is an exact match for N. Return all RRsets associated with N Else Add the name that would immediately precede N in S to NXT_SET. EndIf 3. Replace the leftmost label of N with * 4. If N exists in S There is a positive wildcard match for N. Return all RRsets associated with N Else Add the NXT for name that would immediately precede N in S to NXT_SET. EndIf 5. Remove the leading * from N. 6. If N exists in S There is a name that terminates the wildcard search. Add the NXT for N to NXT_SET and return NXT_SET. Else Goto Step 3 EndIf Note: the algorithm is guaranteed to terminate since eventually there will be a match or N will be reduced to zone name itself and the zone name must exist in S. Arends, et al. Expires September 1, 2003 [Page 40] Internet-Draft DNSSEC Protocol Modifications March 2003 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. Arends, et al. Expires September 1, 2003 [Page 41]