DNS Security Working Group Donald E. Eastlake 3rd INTERNET-DRAFT CyberCash OBSOLETES RFC 2065 UPDATES RFC 1034, 1035 Expires: January 1998 July 1997 Domain Name System Security Extensions ------ ---- ------ -------- ---------- Status of This Document This draft, file name draft-ietf-dnssec-secext2-00.txt, is intended to become a Proposed Standard RFC obsoleting Proposed Standard RFC 2065. Distribution of this document is unlimited. Comments should be sent to the DNS Security Working Group mailing list <dns- security@tis.com> or to the author. This document is an Internet-Draft. 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. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (East USA), ftp.isi.edu (West USA), nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe), munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa). Donald E. Eastlake 3rd [Page 1]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Abstract Extensions to the Domain Name System (DNS) are described that provide data integrity and authentication to security aware resolvers or applications through the use of cryptographic digital signatures. These digital signatures are included in secured zones as resource records. Security can still be provided even through non-security aware DNS servers in many cases. The extensions also provide for the storage of authenticated public keys in the DNS. This storage of keys can support general public key distribution services as well as DNS security. The stored keys enable security aware resolvers to learn the authenticating key of zones in addition to those for which they are initially configured. Keys associated with DNS names can be retrieved to support other protocols. Provision is made for a variety of key types and algorithms. In addition, the security extensions provide for the optional authentication of DNS protocol transactions and requests. This document incorporates feedback from implementors and potential users to the existing Proposed Standard in RFC 2065. Acknowledgments The significant contributions of the following persons (in alphabetic order) to DNS security are gratefully acknowledged: Jim Galvin John Gilmore Olafur Gudmundsson Charlie Kaufman Edward Lewis Jeffrey I. Schiller Donald E. Eastlake 3rd [Page 2]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Table of Contents Status of This Document....................................1 Abstract...................................................2 Acknowledgments............................................2 Table of Contents..........................................3 1. Overview of Contents....................................5 2. Overview of the DNS Extensions.........................6 2.1 Services Not Provided..................................6 2.2 Key Distribution.......................................6 2.3 Data Origin Authentication and Integrity...............7 2.3.1 The SIG Resource Record..............................7 2.3.2 Authenticating Name and Type Non-existence...........8 2.3.3 Special Considerations With Time-to-Live.............8 2.3.4 Special Considerations at Delegation Points..........9 2.3.5 Special Considerations with CNAME....................9 2.3.6 Signers Other Than The Zone.........................10 2.4 DNS Transaction and Request Authentication............10 3. The KEY Resource Record................................11 3.1 KEY RDATA format......................................11 3.1.1 Object Types, DNS Names, and Keys...................12 3.1.2 The KEY RR Flag Field...............................12 3.1.3 Preference Field....................................14 3.1.4 Key Identifier Field................................14 3.1.5 The Protocol Octet..................................14 3.2 The KEY Algorithm Number Specification................15 3.2.1 The MD5/RSA Algorithm...............................15 3.3 Interaction of Flags, Algorithm, and Protocol Bytes...16 3.4 Determination of Zone Secure/Unsecured Status.........17 3.5 KEY RRs in the Construction of Responses..............18 4. The SIG Resource Record................................19 4.1 SIG RDATA Format......................................19 4.1.1 ....................................................19 4.1.2 Algorithm Number Field..............................20 4.1.3 Labels Field........................................20 4.1.4 Original TTL Field..................................20 4.1.5 Signature Expiration and Time Signed Fields.........21 4.1.6 Key Identifier Field................................21 4.1.7 Signer's Name Field.................................21 4.1.8 Signature Field.....................................22 4.1.8.1 Signature Data....................................22 4.1.8.2 MD5/RSA Algorithm Signature Calculation...........22 4.1.8.3 Transaction and Request SIGs......................23 4.2 SIG RRs in the Construction of Responses..............24 Donald E. Eastlake 3rd [Page 3]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4.3 Processing Responses and SIG RRs......................25 4.4 Signature Lifetime, Expiration, TTLs, and Validity....26 4.5 The Root Zone as Signer and Zone Skipping.............26 5. Non-existent Names and Types...........................27 5.1 The NXT Resource Record...............................27 5.2 NXT RDATA Format......................................28 5.3 Example...............................................28 5.4 Special Considerations at Delegation Points...........29 5.5 Zone Transfers........................................29 5.5.1 Full Zone Transfers.................................30 5.5.2 Incremental Zone Transfers..........................30 6. How to Resolve Securely and The AD and CD Bits.........32 6.1 The AD and CD Header Bits.............................32 6.2 Staticly Configured Keys..............................33 6.3 Chaining Through Zones................................34 6.4 Secure Time...........................................35 7. ASCII Representation of Security RRs...................36 7.1 Presentation of KEY RRs...............................36 7.2 Presentation of SIG RRs...............................37 7.3 Presentation of NXT RRs...............................38 8. Canonical Form and Order of Resource Records...........39 8.1 Canonical RR Form.....................................39 8.2 Canonical DNS Name Order..............................39 8.3 Canonical RR Ordering Within An RRset.................39 9. Conformance............................................40 9.1 Server Conformance....................................40 9.2 Resolver Conformance..................................40 10. Security Considerations...............................41 References................................................42 Author's Addresses........................................44 Expiration and File Name..................................44 Appendix A: Base 64 Encoding..............................45 Appendix B: Changes from RFC 2065.........................47 Donald E. Eastlake 3rd [Page 4]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 1. Overview of Contents This document standardizes extensions of the Domain Name System (DNS) protocol to support DNS security and public key distribution. It assumes that the reader is familiar with the Domain Name System, particularly as described in RFCs 1033, 1034, and 1035. An earlier version of these extensions appears in RFC 2065. This replacement incorporates implementation experience and requests from potential users. Section 2 provides an overview of the extensions and the key distribution, data origin authentication, and transaction and request security they provide. Section 3 discusses the KEY resource record, its structure, and use in DNS responses. These resource records represent the public keys of entities named in the DNS and are used for key distribution. Section 4 discusses the SIG digital signature resource record, its structure, and use in DNS responses. These resource records are used to authenticate other resource records in the DNS and optionally to authenticate DNS transactions and requests. Section 5 discusses the NXT resource record (RR) and its use in DNS responses. The NXT RR permits authenticated denial in the DNS of the existence of a name or of a particular type for an existing name. Section 6 discusses how a resolver can be configured with a starting key or keys and proceed to securely resolve DNS requests. Interactions between resolvers and servers are discussed for combinations of security aware and security non-aware. Two additional DNS header bits are defined for signaling between resolvers and servers. Section 7 describes the ASCII representation of the security resource records for use in master files and elsewhere. Section 8 defines the canonical form and order of RRs for DNS security puposes. Section 9 defines levels of conformance for resolvers and servers. Section 10 provides a few paragraphs on overall security considerations. Appendix A gives details of base 64 encoding which is used in the file representation of some RR's defined in this document. Appendix B summarizes changes between this draft and RFC 2065. Donald E. Eastlake 3rd [Page 5]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 2. Overview of the DNS Extensions The Domain Name System (DNS) protocol security extensions provide three distinct services: key distribution as described in Section 2.2 below, data origin authentication as described in Section 2.3 below, and transaction and request authentication, described in Section 2.4 below. Special considerations related to "time to live", CNAMEs, and delegation points are also discussed in Section 2.3. 2.1 Services Not Provided It is part of the design philosophy of the DNS that the data in it is public and that the DNS gives the same answers to all inquirers. Following this philosophy, no attempt has been made to include any sort of access control lists or other means to differentiate inquirers. No effort has been made to provide for any confidentiality for queries or responses. (This service may be available via IPSEC [RFC 1825] or TLS [draft-ietf-tls-*].) Protection is not provided against denial of service. 2.2 Key Distribution A resource record format is defined to associate keys with DNS names. This permits the DNS to be used as a public key distribution mechanism in support of the DNS data origin authentication and other security services. The syntax of a KEY resource record (RR) is described in Section 3. It includes an algorithm identifier, the actual public key parameter(s), and a variety of flags including those indicating the type of entity the key is associated with and/or asserting that there is no key associated with that entity. Under conditions described in Section 3.5, security aware DNS servers will automatically attempt to return KEY resources as additional information, along with those resource records actually requested, to minimize the number of queries needed. Donald E. Eastlake 3rd [Page 6]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 2.3 Data Origin Authentication and Integrity Authentication is provided by associating with resource record sets (RRsets) the DNS cryptographically generated digital signatures. Commonly, there will be a single private key that signs for an entire zone. If a security aware resolver reliably learns the public key of the zone, it can verify, for signed data read from that zone, that it was properly authorized and is current. The most secure implementation is for the zone private key to be kept off-line and used to re-sign all of the records in the zone periodically. This data origin authentication key belongs to the zone and not to the servers that store copies of the data. That means compromise of a server or even all servers for a zone will not necessarily affect the degree of assurance that a resolver has that it can determine whether data is genuine. A resolver can learn the public key of a zone either by reading it from the DNS or by having it staticly configured. To reliably learn the public key by reading it from the DNS, the key itself must be signed with a key the resolver trusts. The resolver must be configured with at least the public key of one zone as a starting point. From there, it can securely read the public keys of other zones, if the intervening zones in the DNS tree are secure and their signed keys accessible. Adding data origin authentication and integrity requires no change to the "on-the-wire" DNS protocol beyond the addition of the signature resource type and the key resource type needed for key distribution. (Data non-existence authentication also requires the NXT RR as described in 2.3.2.) This service can be supported by existing resolver and caching server implementations so long as they can support the additional resource types (see Section 9). The one exception is that CNAME referrals from a secure zone can not be authenticated if they are from non-security aware servers (see Section 2.3.5). If signatures are separately retrieved and verified when retrieving the information they authenticate, there will be more trips to the server and performance will suffer. Security aware servers mitigate that degradation by always attempting to send the signature(s) needed. 2.3.1 The SIG Resource Record The syntax of a SIG resource record (signature) is described in Section 4. It cryptographically binds the RR set being signed to the signer and a validity interval. Donald E. Eastlake 3rd [Page 7]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Every name in a secured zone will have associated with it at least one SIG resource record for each resource type under that name except for glue address RRs and delgation point NS RRs. A security aware server will attempt to return, with RRs retrieved, the corresponding SIGs. If a server is not security aware, the resolver must retrieve all the SIG records for a name and select the one or ones that sign the resource record set(s) that resolver is interested in. 2.3.2 Authenticating Name and Type Non-existence The above security mechanism provides only a way to sign existing RR sets in a zone. "Data origin" authentication is not obviously provided for the non-existence of a domain name in a zone or the non-existence of a type for an existing name. This gap is filled by the NXT RR which authenticatably asserts a range of non-existent names in a zone and the non-existence of types for the existing name just before that range. Section 5 below covers the NXT RR. 2.3.3 Special Considerations With Time-to-Live A digital signature will fail to verify if any change has occurred to the data between the time it was originally signed and the time the signature is verified. This conflicts with our desire to have the time-to-live (TTL) field of resource records tick down while they are cached. This could be avoided by leaving the time-to-live out of the digital signature, but that would allow unscrupulous servers to set arbitrarily long TTL values undetected. Instead, we include the "original" TTL in the signature and communicate that data along with the current TTL. Unscrupulous servers under this scheme can manipulate the TTL but a security aware resolver will bound the TTL value it uses at the original signed value. Separately, signatures include a time signed and an expiration time. A resolver that knows the absolute time can determine securely whether a signature has expired. It is not possible to rely solely on the signature expiration as a substitute for the TTL, however, since the TTL is primarily a database consistency mechanism and non-security aware servers that depend on TTL must still be supported. Donald E. Eastlake 3rd [Page 8]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 2.3.4 Special Considerations at Delegation Points DNS security would like to view each zone as a unit of data completely under the control of the zone owner with each entry (RRset) signed by a special private key held by the zone. But the DNS protocol views the leaf nodes in a zone, which are also the apex nodes of a subzone (i.e., delegation points), as "really" belonging to the subzone. These nodes occur in two master files and might have RRs signed by both the upper and lower zone's keys. A retrieval could get a mixture of these RRs and SIGs, especially since one server could be serving both the zone above and below a delegation point. There MUST be a zone KEY RR, signed by its superzone, for every subzone if the superzone is secure. In the case of an unsecured subzone which can not or will not be modified to add any security RRs, a KEY declaring the subzone to be unsecured MUST appear in and be signed by the superzone, if the superzone is secure. For all but one other RR type the data from the subzone is more authoritative so only the KEY RR in the superzone should be signed and the NS and any glue address RRs should only be signed in the subzone. The SOA and any other RRs that have the zone name as owner should appear only in the subzone and thus are signed only there. The NXT RR type is the exceptional case that will always appear differently and authoritatively in both the superzone and subzone, if both are secure, as described in Section 5. 2.3.5 Special Considerations with CNAME There is a problem when security related RRs with the same owner name as a CNAME RR are retrieved from a non-security-aware server. In particular, an initial retrieval for the CNAME or any other type may not retrieve any associated signature, KEY, or NXT RR. For retrieved types other than CNAME, it will retrieve that type at the target name of the CNAME (or chain of CNAMEs) and will also return the CNAME. In particular, a specific retrieval for type SIG will not get the SIG, if any, at the original CNAME domain name but rather a SIG at the target name. Security aware servers MUST be used to securely CNAME in DNS. Security aware servers must (1) allow KEY, SIG, and NXT RRs along with CNAME RRs, (2) suppress CNAME processing on retrieval of these types as well as on retrieval of the type CNAME, and (3) automatically return SIG RRs authenticating the CNAME or CNAMEs encountered in resolving a query. This is a change from the previous DNS standard [RFCs 1034/1035] which prohibited any other RR type at a node where a CNAME RR was present. Donald E. Eastlake 3rd [Page 9]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 2.3.6 Signers Other Than The Zone There are two cases where a SIG resource record is signed by other than the zone private key. One is for support of dynamic update [RFC 2136], or future requests which require authentication, where an entity is permitted to authenticate/update its records [RFC 2137]. The public key of the entity must be present in the DNS and be appropriately signed but the other RR(s) may be signed with the entity's key. The other is for support of transaction and request authentication as described in Section 2.4 immediately below. 2.4 DNS Transaction and Request Authentication The data origin authentication service described above protects retrieved resource records but provides no protection for DNS requests or for message headers. If header bits are falsely set by a bad server, there is little that can be done. However, it is possible to add transaction authentication. Such authentication means that a resolver can be sure it is at least getting messages from the server it thinks it queried and that the response is from the query it sent (i.e., that these messages have not been diddled in transit). This is accomplished by optionally adding a special SIG resource record at the end of the reply which digitally signs the concatenation of the server's response and the resolver's query. Requests can also be authenticated by including a special SIG RR at the end of the request. Authenticating requests serves no function in older DNS servers and requests with a non-empty additional information section are ignored by many older and current DNS servers. However, this syntax for signing requests is defined in connection with authenticating secure dynamic update requests [RFC 2137] or future requests requiring authentication. The private keys used in transaction and request security belong to the host composing the request or reply, not to the zone involved. The corresponding public key is normally stored in and retrieved from the DNS for verification. Because requests and replies are highly variable, message authentication SIGs can not be pre-calculated. Thus it will be necessary to keep the private key on-line, for example in software or in a directly connected piece of hardware. Donald E. Eastlake 3rd [Page 10]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 3. The KEY Resource Record The KEY resource record (RR) is used to store a key that is associated with a Domain Name System (DNS) name. It will be a public key as only public keys are stored in the DNS. This can be the public key of a zone, a host or other end entity, or a user. A KEY RR is, like any other RR, authenticated by a SIG RR. Security aware DNS implementations MUST be designed to handle at least two simultaneously valid keys of the same type associated with a name. The type number for the KEY RR is TBD [was 25 in RFC 2065 but may need to change due to changes in KEY RR]. 3.1 KEY RDATA format The RDATA for a KEY RR consists of flags, preference, key identifier, a protocol octet, the algorithm number, and the public key itself. The format is as follows: 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | preference | key identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | protocol | algorithm | / +---------------+---------------+ public key data / / (optional depending on flags/algorithm) / / / /-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| The KEY RR is not intended for storage of certificates and a separate certificate RR is being considered, to be defined in a separate document. The meaning of the KEY RR owner name, flags, preference, key identifier, and protocol octet are described in Sections 3.1.1 through 3.1.5 below. The flags and algorithm must be examined before any data following the algorithm octet as they control the existence and format of any following data. The algorithm and public key fields are described in Section 3.2. The format of the public key is algorithm dependent. KEY RRs do not expire but their authenticating SIG RR does as described in Section 4 below. Donald E. Eastlake 3rd [Page 11]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 3.1.1 Object Types, DNS Names, and Keys The public key in a KEY RR belongs to the object named in the owner name. This DNS name may refer to up to three different categories of things. For example, foo.host.example could be (1) a zone, (2) a host or other end entity , or (3) the mapping into a DNS name of the user or account foo@host.example. Thus, there are flags, as described below, in the KEY RR to indicate with which of these roles the owner name and public key are associated. Note that an appropriate zone KEY RR MUST occur at the apex node of a secure zone. Although the same name can be used for up to all three of these categories, in which case there would normally be three KEY RRs at that name with different flags, such overloading of a name is discouraged. It is also possible to use the same key for different things with the same name or even different names, but this is strongly discouraged. In particular, the use of a zone key as a non-zone key will usually require that the corresponding private key be kept on line and thereby become more vulnerable than if it were kept off line. In addition to the name type bits, there are additional flag bits, all described below. 3.1.2 The KEY RR Flag Field In the "flags" field: Bit 0 and 1 are the key "type" field. Bit 0 a one indicates that use of the key is prohibited for authentication. Bit 1 a one indicates that use of the key is prohibited for confidentiality. If this field is zero, then use of the key for authentication and/or confidentiality is permitted. Note that DNS security makes use of keys for authentication only. Confidentiality use flagging is provided for use of keys in other protocols. Implementations not intended to support key distribution for confidentiality MAY require that the confidentiality use prohibited bit be on for keys they serve. If both bits of this field are one, the "no key" value, there is no key information and the RR stops after the algorithm octet. By the use of this "no key" value, a signed KEY RR can authenticatably assert that, for example, a zone is not secured. See section 3.4 below. Bits 2-4 are reserved and must be zero. Bit 5 on indicates that this is a key associated with a "user" or Donald E. Eastlake 3rd [Page 12]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 "account" at an end entity, usually a host. The coding of the owner name is that used for the responsible individual mailbox in the SOA and RP RRs: The owner name is the user name as the name of a node under the entity name. For example, "j_random_user" on host.subdomain.example could have a public key associated through a KEY RR with name j_random_user.host.subdomain.example and the user bit a one. It could be used in a security protocol where authentication of a user was desired. This key might be useful in IP or other security for a user level service such a telnet, ftp, rlogin, etc. Bit 6 on indicates that this is a key associated with the non-zone "entity" whose name is the RR owner name. This will commonly be a host but could, in some parts of the DNS tree, be some other type of entity such as a telephone number [RFC 1530] or numeric IP address. This is the public key used in connection with DNS request and transaction authentication services if the owner name designates a DNS resolver or server host. It could also be used in an IP-security protocol where authentication at the host, rather than user, level was desired, such as routing, NTP, etc. Bit 7 is the "zone" bit and indicates that this is a zone key for the zone whose name is the KEY RR owner name. This is the public key used for the primary DNS security feature of data origin authentication. Bit 8 is reserved to be the IPSEC [RFC 1825] bit and indicates that this key is valid for use in conjunction with that security standard. This key could be used in connection with secured communication on behalf of an end entity or user whose name is the owner name of the KEY RR if the entity or user bits are on. The presence of a KEY resource with the IPSEC and entity bits on and the key type field (bits 0 and 1) set to indicate a key is present is an assertion that the host speaks IPSEC. Bit 9 is reserved to be the "email" bit and indicate that this key is valid for use in conjunction with MIME security multiparts. This key could be used in connection with secured communication on behalf of an end entity or user whose name is the owner name of the KEY RR if the entity or user bits are on. Bit 10 is reserved to be the TLS bit [draft-ietf-tls-*.txt] and indicates that this key is valid for use in conjunction with that security standard. This key could be used in connection with secured communication on behalf of an end entity or user whose name is the owner name of the KEY RR if the entity or user bits are on. The presence of a KEY resource with the TLS and entity bits on and the key type field (bits 0 and 1) set to indicate a key is present is an assertion that the host speaks TLS. Donald E. Eastlake 3rd [Page 13]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Bit 11 is reserved and must be zero. Bits 12-15 are the "signatory" field. If non-zero, they indicate that the key can validly sign RRs or updates of the same name. If the owner name is a wildcard, then RRs or updates with any name which is in the wildcard's scope can, in some cases, be signed. Fifteen different non-zero values are possible for this field and any differences in their meaning are reserved for definition in connection with DNS dynamic update [RFC 2137] or other new DNS commands. Zone keys always have authority to sign any RRs in the zone regardless of the value of this field. The signatory field, like all other aspects of the KEY RR, is only effective if the KEY RR is appropriately signed by a SIG RR. Bits 16-31 are reserved and must be zero. 3.1.3 Preference Field The "preference" field is an unsigned sixteen bit integer with octets in network order that is used to indicate the relative preference of different KEY RRs used for the same purpose. A smaller value is preferred to a larger value. (It is not generally significant for DNS origin authentication use, as zones SHOULD be signed by all the applicable zone keys, but is intended for use in keys retrieved for other protocols.) 3.1.4 Key Identifier Field The "key identifier" field is assigned to the key and matches the key identifier field in SIG RRs (1) whose signer name is the KEY RR owner name and (2) that were signed by the private key corresponding to the public key in the KEY RR. 3.1.5 The Protocol Octet It is anticipated that some keys stored in DNS will be used in conjunction with Internet protocols other than DNS (keys with zone bit or signatory field non-zero) and IPSEC/email/TLS (keys with IPSEC, email and/or TLS bit set). The protocol octet is provided to indicate that a key is valid for such use and, for end entity keys or the host part of user keys, that the secure version of that protocol is implemented on that entity or host. Values between 1 and 223 decimal inclusive are available for Donald E. Eastlake 3rd [Page 14]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 assignment by IANA for such protocols. The 31 values between 224 and 254 inclusive will not be assigned to a specific protocol and are available for experimental use under private bilateral agreement. Value 0 indicates, for a particular key, that it is not valid for any particular additional protocol beyond those indicated in the flag field. And value 255 indicates that the key is valid for all assigned protocols (those in the 1 to 223 range). It is intended that new uses of DNS stored keys would initially be implemented, and limited operational experience gained, using the experimental range of the protocol octet. If demand for widespread deployment for the indefinite future warrants, a value in the assigned range would then be designated for the protocol. Finally, (1) should the protocol become so widespread in conjunction with other protocols and with which it shares key values that duplicate RRs are a serious burden and (2) should the protocol provide substantial facilities not available in any protocol for which a flags field bit has been allocated, then one of the remaining flag field bits may be allocated to the protocol. When such a bit has been allocated, a key can be simultaneously indicated as valid for that protocol and the entity or host can be simultaneously flagged as implementing the secure version of that protocol, along with other protocols for which flag field bits have been assigned. 3.2 The KEY Algorithm Number Specification This octet is the key algorithm parallel to the same field for the SIG resource. The MD5/RSA algorithm described in this document is number 1. Numbers 2 through 253 are available for assignment should sufficient reason arise. However, the designation of a new algorithm could have a major impact on interoperability and requires an IETF standards action. Number 254 is reserved for private use and will never be assigned a specific algorithm. For number 254, the public key area for the KEY RR and the signature are in the SIG RR will actually begin with a length byte followed by an Object Identifier (ISO OID) of that length. The OID indicates the private algorithm in use and the remainder of the area is whatever is required by that algorithm. Values 0 and 255 are reserved. 3.2.1 The MD5/RSA Algorithm If the type field does not have the "no key" value and the algorithm field is 1, indicating the MD5/RSA algorithm, the public key field is structured as follows: Donald E. Eastlake 3rd [Page 15]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | pub exp length | public key exponent / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / +- modulus / | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/ To promote interoperability, the exponent and modulus are each currently limited to 4096 bits in length. The public key exponent is a variable length unsigned integer. Its length in octets is represented as a two octet unsigned integer. The public key modulus field is a multiprecision unsigned integer. The length of the modulus can be determined from the RDLENGTH and the preceding RDATA fields including the exponent. Leading zero octets are permitted in the exponent and modulus. 3.3 Interaction of Flags, Algorithm, and Protocol Bytes Various combinations of the no-key type value, algorithm byte, protocol byte, and any protocol indicating flags (such as the reserved IPSEC flag) are possible. (Note that the zone flag bit being on or the signatory field being non-zero is effectively a DNS protocol flag on.) The meaning of these combinations is indicated below: NK = no key type value AL = algorithm byte PR = protocols indicated by protocol byte or protocol flags x represents any valid non-zero value(s). AL PR NK Meaning 0 0 0 Illegal, claims key but has bad algorithm field. 0 0 1 Specifies total lack of security for owner. 0 x 0 Illegal, claims key but has bad algorithm field. 0 x 1 Specified protocols unsecured, others may be secure. x 0 0 Useless. Gives key but no protocols to use it. x 0 1 Useless. Denies key but for no protocols. x x 0 Specifies key for protocols and asserts that those protocols are implemented with security. x x 1 Algorithm not understood for protocol. (remember, in reference to the above table, that a protocol byte of 255 means all protocols with protocol byte values assigned) Donald E. Eastlake 3rd [Page 16]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 3.4 Determination of Zone Secure/Unsecured Status A zone KEY RR with the "no-key" type field value (both bits 0 and 1 on) indicates that the zone named is unsecured while a zone KEY RR with a key present indicates that the zone named is secure. It is possible for conflicting zone KEY RRs to be present. Zone KEY RRs, like all RRs, are only trusted if they are authenticated by a SIG RR whose signer field is a signer for which the resolver has a public key they trust. Untrusted zone KEY RRs can be ignored in determining the security status of the zone. There can be multiple sets of trusted zone KEY RRs for a zone with each set having a different signer. Zones can be (1) secure, indicating that any retrieved RR must be authenticated by a SIG RR or it will be discarded as bogus, (2) unsecured, indicating that SIG RRs are not expected or required for RRs retrieved from the zone, or (3) experimentally secure, which indicates that SIG RRs might or might not be present but must be checked if found. The status of a zone is determined as follows: 1. If, for a zone, every zone KEY RR signed by a signer trusted by the resolver says there is no key for that zone, it is unsecured. 2. If, for every trusted zone KEY RR signer for a zone, there is at least one no-key KEY RR and for at least one trusted zone KEY RR signer there are one or more key specifying KEY RR(s), then that zone is only experimentally secure. Both authenticated and non- authenticated RRs for it should be accepted by the resolver. 3. If any trusted zone KEY RR signer for the zone has only key specifying KEY RR(s) for the zone, then it is secure and only authenticated RRs from it will be accepted. Examples: (1) A resolver only trusts signatures by the superzone within the DNS hierarcy so it will look only at the KEY RRs that are signed by the superzone. If it finds only no-key KEY RRs, it will assume the zone is not secure. If it finds only key specifying KEY RRs, it will assume the zone is secure and reject any unsigned responses. If it finds both, it will assume the zone is experimentally secure (2) A resolver trusts the superzone of zone Z (to which it got securely from its local zone) and a third party, cert-auth.xx. When considering data from zone Z, it may be signed by the superzone of Z, by cert-auth.xx, by both, or by neither. The following table indicates whether zone Z will be considered secure, experimentally secure, or unsecured, depending on the signed zone KEY RRs for Z; Donald E. Eastlake 3rd [Page 17]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 c e r t - a u t h . x x | None | NoKeys | Mixed | Keys | S --+-----------+-----------+----------+----------+ u None | illegal | unsecured | experim. | secure | p +-----------+-----------+----------+----------+ e NoKeys | unsecured | unsecured | experim. | secure | r +-----------+-----------+----------+----------+ Z Mixed | experim. | experim. | experim. | secure | o +-----------+-----------+----------+----------+ n Keys | secure | secure | secure | secure | e +-----------+-----------+----------+----------+ 3.5 KEY RRs in the Construction of Responses An explicit request for KEY RRs does not cause any special additional information processing except, of course, for the corresponding SIG RR from a security aware server. Security aware DNS servers MUST include KEY RRs as additional information in responses, where a KEY is available, in the following cases: (1) On the retrieval of NS RRs, the zone key KEY RR(s) for the zone served by these name servers MUST be included as additional information if space is available. There will always be at least one such KEY RR in a secure zone, even if it has the no-key type value to indicate that the subzone is unsecured. If not all additional information will fit, the KEY RR(s) have higher priority than type A or AAAA glue RRs. If such a KEY RR does not fit on a retrieval, the retrieval must be considered truncated. (2) On retrieval of type A or AAAA RRs, the end entity KEY RR(s) MUST be included if space is available. On inclusion of A or AAAA RRs as additional information, KEY RRs with the same name will also be included but with lower priority than the A or AAAA RRs. Donald E. Eastlake 3rd [Page 18]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4. The SIG Resource Record The SIG or "signature" resource record (RR) is the fundamental way that data is authenticated in the secure Domain Name System (DNS). As such it is the heart of the security provided. The SIG RR unforgably authenticates an RRset of a particular type, class, and name and binds it to a time interval and the signer's domain name. This is done using cryptographic techniques and the signer's private key. The signer is frequently the owner of the zone from which the RR originated. The SIG RR is only intended to be meaningful to DNS security. The type number for the SIG RR type is 24. 4.1 SIG RDATA Format The RDATA portion of a SIG RR is as shown below. The integrity of the RDATA information is protected by the signature field. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | type covered | algorithm | labels | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | original TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | signature expiration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time signed | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | key identifier | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ signer's name + | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/ / / / signature / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.1.1 The "type covered" is the type of the other RRs covered by this SIG. Donald E. Eastlake 3rd [Page 19]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4.1.2 Algorithm Number Field This octet is as described in section 3.2. 4.1.3 Labels Field The "labels" octet is an unsigned count of how many labels there are in the original SIG RR owner name not counting the null label for root and not counting any initial "*" for a wildcard. If a secured retrieval is the result of wild card substitution, it is necessary for the resolver to use the original form of the name in verifying the digital signature. This field makes it easy to determine the original form. If, on retrieval, the RR appears to have a longer name than indicated by "labels", the resolver can tell it is the result of wildcard substitution. If the RR owner name appears to be shorter than the labels count, the SIG RR must be considered corrupt and ignored. The maximum number of labels allowed in the current DNS is 127 but the entire octet is reserved and would be required should DNS names ever be expanded to 255 labels. The following table gives some examples. The value of "labels" is at the top, the retrieved owner name on the left, and the table entry is the name to use in signature verification except that "bad" means the RR is corrupt. labels= | 0 | 1 | 2 | 3 | 4 | --------+-----+------+--------+----------+----------+ .| . | bad | bad | bad | bad | d.| *. | d. | bad | bad | bad | c.d.| *. | *.d. | c.d. | bad | bad | b.c.d.| *. | *.d. | *.c.d. | b.c.d. | bad | a.b.c.d.| *. | *.d. | *.c.d. | *.b.c.d. | a.b.c.d. | 4.1.4 Original TTL Field The "original TTL" field is included in the RDATA portion to avoid (1) authentication problems that caching servers would otherwise cause by decrementing the real TTL field and (2) security problems that unscrupulous servers could otherwise cause by manipulating the real TTL field. This original TTL is protected by the signature while the current TTL field is not. NOTE: The "original TTL" must be restored into the covered RRs when the signature is verified. This implies that all RRs for a particular type, name, and class must have the same TTL to start with. Donald E. Eastlake 3rd [Page 20]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4.1.5 Signature Expiration and Time Signed Fields The SIG is valid until the "signature expiration" time which is an unsigned number of seconds since the start of 1 January 1970, GMT, ignoring leap seconds. (See also Section 4.4.) Ring arithmetic is used as for DNS SOA serial numbers [RFC 1982] which means that the expiration date can never be much more than 130 years in the future. The "time signed" field is an unsigned number of seconds since the start of 1 January 1970, GMT, ignoring leap seconds. SIG RRs SHOULD NOT have a date signed more than a few days in the future. To prevent misordering of network requests to update a zone dynamically, monotonically increasing "time signed" dates may be necessary. SOA serial numbers for secure zones MUST not only be advanced when their data is updated but also when new SIG RRs are inserted (ie, the zone or any part of it is re-signed). A SIG RR may have an expiration date numerically less than the time signed if time is near the 32 bit wrap around point and/or the signature is long lived. 4.1.6 Key Identifier Field The "key identifier" is a two octet quantity that is used to efficiently select between multiple keys which may be applicable and thus check that a public key about to be used for the computationally expensive effort to check the signature is possibly valid. It is assigned by the entity creating the KEY RRs, appears in the corresponding KEY RR, and MUST be unique among valid KEY RRs of the same name and algorithm. 4.1.7 Signer's Name Field The "signer's name" field is the domain name of the signer generating the SIG RR. This is the owner of the public KEY RR that can be used to verify the signature. It is frequently the zone which contained the RRset being authenticated. The signer's name may be compressed with standard DNS name compression when being transmitted over the network. Donald E. Eastlake 3rd [Page 21]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4.1.8 Signature Field The structure of the "signature" field is described below. 4.1.8.1 Signature Data The actual signature portion of the SIG RR binds the other RDATA fields to all of the "type covered" RRs with that owner name and class. This covered RRset is thereby authenticated. To accomplish this, a data sequence is constructed as follows: data = RDATA | RR(s)... where "|" is concatenation, RDATA is wire format of all the RDATA fields in the SIG RR itself including the canonical form of the signers name before and not including the signature, and RR(s) are all the RR(s) of the type covered with the same owner name and class as the SIG RR in canonical form and order as defined in section 8. How this data sequence is processed into the signature is algorithm dependent. SIGs SHOULD NOT be included in a zone for any "meta-type" such as ANY, AXFR, etc. 4.1.8.2 MD5/RSA Algorithm Signature Calculation For the MD5/RSA algorithm, the signature is as follows hash = MD5 ( data ) signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n) where MD5 is the message digest algorithm documented in RFC 1321, "|" is concatenation, "e" is the private key exponent of the signer, and "n" is the modulus of the signer's public key. 01, FF, and 00 are fixed octets of the corresponding hexadecimal value. "prefix" is the ASN.1 BER MD5 algorithm designator prefix specified in PKCS1, that is, hex 3020300c06082a864886f70d020505000410 [NETSEC]. This prefix is included to make it easier to use RSAREF (or similar packages such as EuroRef). The FF octet MUST be repeated the maximum number of times such that the value of the quantity being exponentiated is one octet shorter than the value of n. (The above specifications are identical to the corresponding part of Donald E. Eastlake 3rd [Page 22]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Public Key Cryptographic Standard #1 [PKCS1].) The size of n, including most and least significant bits (which will be 1) MUST be not less than 512 bits and not more than 4096 bits. n and e SHOULD be chosen such that the public exponent is small. Leading zero bytes are permitted in the MD5/RSA algorithm signature. A public exponent of 3 minimizes the effort needed to decode a signature. Use of 3 as the public exponent may be weak for confidentiality uses since, if the same data can be collected encrypted under three different keys with an exponent of 3 then, using the Chinese Remainder Theorem [NETSEC], the original plain text can be easily recovered. This weakness is not significant for DNS security because we seek only authentication, not confidentiality. 4.1.8.3 Transaction and Request SIGs A response message from a security aware server may optionally contain a special SIG as the last item in the additional information section to authenticate the transaction. This SIG has a "type covered" field of zero, which is not a valid RR type. It is calculated by using a "data" (see Section 4.1.8.1) of the entire preceding DNS reply message, including DNS header but not the IP header and before the reply RR counts have been adjusted for the inclusion of any transaction SIG, concatenated with the entire DNS query message that produced this response, including the query's DNS header and any request SIGs but not its IP header. That is data = full response (less transaction SIG) | full query Verification of the transaction SIG (which is signed by the server host key, not the zone key) by the requesting resolver shows that the query and response were not tampered with in transit, that the response corresponds to the intended query, and that the response comes from the queried server. A DNS request may be optionally signed by including one or more SIGs at the end of the query. Such SIGs are identified by having a "type covered" field of zero. They sign the preceding DNS request message including DNS header but not including the IP header or any request SIGs at the end and before the request RR counts have been adjusted for the inclusions of any request SIG. WARNING: Request SIGs are unnecessary for currently defined queries and will cause almost all existing DNS servers to completely ignore a query. However, such SIGs are needed to authenticate some DNS secure Donald E. Eastlake 3rd [Page 23]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 dynamic update requests [RFC 2137] and may in the future been needed for other requests. Except where needed to authenticate an update or similar privileged request, servers are not required to check request SIGs. 4.2 SIG RRs in the Construction of Responses Security aware DNS servers MUST, for every authenticated RR the query will return, attempt to send the available SIG RRs which authenticate the requested RR. The following rules apply to the inclusion of SIG RRs in responses: 1. when an RRset is placed in a response, its SIG RR has a higher priority for inclusion than other additional RRs that may need to be included. If space does not permit its inclusion, the response MUST be considered truncated except as provided in 2 below. 2. when a SIG RR is present in the zone for an additional information section RR, the response MUST NOT be considered truncated merely because space does not permit the inclusion of its SIG RR. 3. SIGs to authenticate non-authoritative data (glue records and NS RRs for subzones) are unnecessary and MUST NOT be sent. (Note that KEYs given for a subzone in that subzone's superzone is controlling so the superzone's signature on the KEY MUST be included (unless the KEY was additional information and the SIG did not fit).) 4. If a SIG covers any RR that would be in the answer section of the response, its automatic inclusion MUST be in the answer section. If it covers an RR that would appear in the authority section, its automatic inclusion MUST be in the authority section. If it covers an RR that would appear in the additional information section it MUST appear in the additional information section. This is a change in the existing standard [RFCs 10334/1035] which contemplates only NS and SOA RRs in the authority section. 5. Optionally, DNS transactions may be authenticated by a SIG RR at the end of the response in the additional information section (Section 4.1.8.3). Such SIG RRs are signed by the DNS server originating the response. Although the signer field MUST be the name of the originating server host, the owner name, class, TTL, and original TTL, are meaningless. The class and TTL fields SHOULD be zero. To conserve space, the owner name SHOULD be Donald E. Eastlake 3rd [Page 24]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 root (a single zero octet). If transaction authentication is desired, that SIG RR must be considered the highest priority for inclusion. 4.3 Processing Responses and SIG RRs The following rules apply to the processing of SIG RRs included in a response: 1. a security aware resolver that receives a response from what it believes to be a security aware server via a secure communication with the AD bit (see Section 6.1) set, MAY choose to accept the RRs as received without verifying the zone SIG RRs. 2. in other cases, a security aware resolver SHOULD verify the SIG RRs for the RRs of interest. This may involve initiating additional queries for SIG or KEY RRs, especially in the case of getting a response from an server that does not implement security. (As explained in 2.3.5 above, it will not be possible to secure CNAMEs being served up by non-secure resolvers.) NOTE: Implementers might expect the above SHOULD to be a MUST. However, local policy or the calling application may not require the security services. 3. If SIG RRs are received in response to a user query explicitly specifying the SIG type, no special processing is required. If the message does not pass integrity checks or the SIG does not check against the signed RRs, the SIG RR is invalid and should be ignored. If all of the SIG RR(s) purporting to authenticate an RRset are invalid, then the RRset is not authenticated. If the SIG RR is the last RR in a response in the additional information section and has a type covered of zero, it is a transaction signature of the response and the query that produced the response. It MAY be optionally checked and the message rejected if the checks fail. But even if the checks succeed, such a transaction authentication SIG does NOT authenticate any RRs in the message. Only a proper SIG RR signed by the zone or a key tracing its authority to the zone or to static resolver configuration can authenticate RRs. If a resolver does not implement transaction and/or request SIGs, it MUST ignore them without error. If all checks indicate that the SIG RR is valid then RRs verified by it should be considered authenticated. Donald E. Eastlake 3rd [Page 25]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 4.4 Signature Lifetime, Expiration, TTLs, and Validity Security aware servers must not consider SIG RRs to authenticate anything after their expiration time and not consider any RR to be authenticated after all its signatures have expired. Within that constraint, servers should continue to follow DNS TTL aging. Thus authoritative servers should continue to follow the zone refresh and expire parameters and a non-authoritative server should count down the TTL and discard RRs when the TTL is zero. In addition, when RRs are transmitted in a query response, the TTL should be trimmed so that current time plus the TTL does not extend beyond the signature expiration time. Thus, in general, the TTL on a transmitted RR would be min(sigExpTim,max(zoneMinTTL,min(originalTTL,currentTTL))) When signatures are generated, signature expiration times must be set far enough in the future that it is quite certain that new signatures can be generated before the old ones expire. However, setting expiration too far into the future could, if bad data or signatures were ever generated, mean a long time to flush such badness. It is recommended that signature lifetime be a small multiple of the TTL (ie, 4 to 16 times the TTL) but not less than a reasonable maximum re-signing interval and not less than the zone expiry time. 4.5 The Root Zone as Signer and Zone Skipping It should be noted that in DNS the root is a zone unto itself. Thus the root zone key should only be seen signing itself or signing RRs with names one level below root, such as .aq, .edu, or .arpa. Implementations SHOULD reject as bogus any purported root signature of records with a name more than one level below root. The root zone contains the root KEY RR signed by a SIG RR under the root key itself. The signing authority of a zone key ends with the delegation points within that zone. Implementations SHOULD reject purported signatures on data below the apex of a subzone (or even lower level data) by the superzone's key. Donald E. Eastlake 3rd [Page 26]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 5. Non-existent Names and Types The SIG RR mechanism described in Section 4 above provides strong authentication of RRs that exist in a zone. But is it not clear above how to authenticatably deny the existence of a name in a zone or a type for an existent name. The nonexistence of a name in a zone is indicated by the NXT ("next") RR for a name interval containing the nonexistent name. A NXT RR and its SIG are returned in the authority section, along with the error, if the server is security aware. The same is true for a non-existent type under an existing name. This is a change in the existing standard [RFCs 1034/1035] which contemplates only NS and SOA RRs in the authority section. NXT RRs will also be returned if an explicit query is made for the NXT type. The existence of a complete set of NXT records in a zone means that any query for any name and any type to a security aware server serving the zone will result in an reply containing at least one signed RR unless it is a query for delegation point NS or glue A or AAAA RRs. 5.1 The NXT Resource Record The NXT resource record is used to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and to indicate what RR types are present for an existing name. The owner name of the NXT RR is an existing name in the zone. It's RDATA is a "next" name and a type bit map. The presence of the NXT RR means that no name between its owner name and the name in its RDATA area exists and that no other types exist under its owner name. This implies a canonical ordering of all domain names in a zone as described in Section 8. There is a potential problem with the last NXT in a zone as it wants to have an owner name which is the last existing name in canonical order, which is easy, but it is not obvious what name to put in its RDATA to indicate the entire remainder of the name space. This is handled by treating the name space as circular and putting the zone name in the RDATA of the last NXT in a zone. The NXT RRs for a zone SHOULD be automatically calculated and added to the zone when SIGs are added. The NXT RR's TTL SHOULD NOT exceed the zone minimum TTL. The type number for the NXT RR is 30. Donald E. Eastlake 3rd [Page 27]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 5.2 NXT RDATA Format The RDATA for an NXT RR consists simply of a domain name followed by a bit map, as shown below. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | next domain name / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | type bit map / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The NXT RR type bit map is one bit per RR type present for the owner name similar to the WKS RR socket bit map. The first bit represents RR type zero (an illegal type which should not be present.) A one bit indicates that at least one RR of that type is present for the owner name. A zero indicates that no such RR is present. All bits not specified because they are beyond the end of the bit map are assumed to be zero. Note that bit 30, for NXT, will always be on so the minimum bit map length is actually four octets. The NXT bit map should be printed as a list of RR type mnemonics or decimal numbers similar to the WKS RR. The domain name may be compressed with standard DNS name compression when being transmitted over the network. The size of the bit map can be inferred from the RDLENGTH and the length of the next domain name. 5.3 Example Assume zone foo.nil has entries for big.foo.nil, medium.foo.nil. small.foo.nil. tiny.foo.nil. Then a query to a security aware server for huge.foo.nil would produce an error reply with an RCODE of NXDOMAIN and the authority section data including something like the following: Donald E. Eastlake 3rd [Page 28]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 big.foo.nil. NXT medium.foo.nil. A MX SIG NXT big.foo.nil. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3 19960102030405 ;signature expiration 19951211100908 ;time signed 21435 ;key identifier foo.nil. ;signer MxFcby9k/yvedMfQgKzhH5er0Mu/vILz45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoU 1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature (640 bits) ) Note that this response implies that big.foo.nil is an existing name in the zone and thus has other RR types associated with it than NXT. However, only the NXT (and its SIG) RR appear in the response to this query for huge.foo.nil, which is a non-existent name. 5.4 Special Considerations at Delegation Points A name (other than root) which is the head of a zone also appears as the leaf in a superzone. If both are secure, there will always be two different NXT RRs with the same name. They can be distinguished by their signers and next domain name fields. Security aware servers should return the correct NXT automatically when required to authenticate the non-existence of a name and both NXTs, if available, on explicit query for type NXT. Non-security aware servers will never automatically return an NXT and some old implementations may only return the NXT from the subzone on explicit queries. 5.5 Zone Transfers The sections below descrie how full and incremental zone transfers are secured. SIG RRs secure all authoritative RRs transferred for both full and incremental [RFC 1995] zone transfers. NXT RRs are an essential elements in secure zone transfers and assure that every authoritative name and type will be present; however, if there are multiple SIGs with the same name and type covered a subset of the SIGs could be sent as long as at least one is present and, in the case of unsigned delegation point NS or glue A or AAAA RRs a subset of these RRs could be sent as long as at least one of each type is included. When an incremental or full zone transfer request is received with the same or newer version number than that of Donald E. Eastlake 3rd [Page 29]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 the server's copy of the zone is received, it is replied to with just the SOA RR of the server's current version and the SIG(s) verifying that SOA RR. 5.5.1 Full Zone Transfers Even if all SIGs in a transfer verify, this does not indicate that the entire zone has been transfered. However the NXT name chain can be used to verify that RRs for all authoritative names have been transfered and the NXT type bit map can be used to verify that at least one of all RR types for those names have been transfered. In combination, the above checks provide security checking for all signed RRs in a full zone transfer of a secure zone. To provide server authentication that a complete transfer has occured, transaction authentication SHOULD be used on all full zone transfers. This provides strong protection for the entire zone in transit. 5.5.2 Incremental Zone Transfers Individual RRs in an incremental (IXFR) transfer [RFC 1995] can be verified in the same way as for a full zone transfer and the integrity of the NXT name chain and correctness of the NXT type bits for the zone after the incremental RR deletes and adds can check each disjoint area of the zone updated. But the completeness of an incremental transfer can not because usually neither the deleted RR section nor the added RR section has a compete NXT chain. As a result, a server which securely supports IXFR must handle IXFR SIG RRs for each incremental transfer set that it maintains. The IXFR SIG is calculated over the incremental zone update collection of RRs in the order in which it is tranmitted: old SOA, then deleted RRs, then new SOA and added RRs. It is recommended that, within each section, RRs be ordered as specified in Section 8. If condensation of adjacent incremental update sets is done by the zone owner, the original IXFR SIG for each set included in the condensation must be discarded and a new on IXFR SIG calculated to cover the resulting condensed set. The IXFR SIG really belongs to the zone as a whole, not to the zone name. Although it should be correct for the zone name, the labels field of an IXFR SIG is otherwise meaningless. The IXFR SIG is only Donald E. Eastlake 3rd [Page 30]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 sent as part of an incremental zone transfer. After validation of the IXFR SIG, the transfered RRs MAY be considered valid without verification of the internal SIGs. Donald E. Eastlake 3rd [Page 31]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 6. How to Resolve Securely and The AD and CD Bits Retrieving or resolving secure data from the Domain Name System (DNS) involves starting with one or more trusted public keys for one or more zones. With trusted keys, a resolver willing to perform cryptography can progress securely through the secure DNS zone structure to the zone of interest as described in Section 6.3. Such trusted public keys would normally be configured in a manner similar to that described in Section 6.2. However, as a practical matter, a security aware resolver would still gain some confidence in the results it returns even if it was not configured with any keys but trusted what it got from a local well known server as a starting point. Data stored at a security aware server needs to be internally categorized as Authenticated, Pending, or Insecure. There is also a fourth transient state of Bad which indicates that all SIG checks have explicitly failed on the data. Such Bad data is not retained at a security aware server. Authenticated means that the data has a valid SIG under a KEY traceable via a chain of zero or more SIG and KEY RRs to a KEY staticly configured at the resolver. Pending data has no authenticated SIGs and at least one additional SIG the resolver is still trying to authenticate. Insecure data is data which it is known can never be either Authenticated or found Bad because it is in or has been reached via a unsecured zone. Behavior in terms of control of and flagging based on such data labels is described in Section 6.1. The proper validation of signatures requires a reasonably secure shared opinion of the absolute time between resolvers and servers as described in Section 6.4. 6.1 The AD and CD Header Bits Two previously unused bits are allocated out of the DNS query/response format header. The AD (authentic data) bit indicates in a response that the data included has been verified by the server providing it. The CD (checking disabled) bit indicates in a query that non-verified data is acceptable to the resolver sending the query. These bits are allocated from the must-be-zero Z field as follows: Donald E. Eastlake 3rd [Page 32]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |QR| Opcode |AA|TC|RD|RA| Z|AD|CD| RCODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QDCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ANCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NSCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ARCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ These bits are zero in old servers and resolvers. Thus the responses of old servers are not flagged as authenticated to security aware resolvers and queries from non-security aware resolvers do not assert the checking disabled bit and thus will be answered by security aware servers only with valid data. Aware resolvers MUST NOT trust the AD bit unless they trust the server they are talking to and either have a secure path to it or use DNS transaction security. Any security aware resolver willing to do cryptography SHOULD assert the CD bit on all queries to reduce DNS latency time by allowing security aware servers to answer before they have resolved the validity of data. Security aware servers NEVER return Bad data. For non-security aware resolvers or security aware resolvers requesting service by having the CD bit clear, security aware servers MUST return only Authenticated or Insecure data with the AD bit set in the response. Security aware resolvers will know if data is Insecure versus Authentic by the absence of SIG RRs. Security aware servers MAY return Pending data to security aware resolvers requesting the service by clearing the AD bit in the response. The AD bit MUST NOT be set on a response unless all of the RRs in the response are either Authenticated or Insecure. 6.2 Staticly Configured Keys The public key to authenticate a zone MUST be defined in local configuration files before that zone is loaded at the primary server. While it might seem logical for everyone to start with a key for the root zone and staticly configure this in every resolver, this has problems. The logistics of updating every DNS resolver in the world Donald E. Eastlake 3rd [Page 33]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 when the root key changes would be excessive. It may be some time before there even is a root key. Furthermore, many organizations will explicitly wish their "interior" DNS implementations to completely trust only their own zone. Such interior resolvers can then go through the organization's zone servers to access data outsize the organization's domain and should only be configured with the key for the organization's DNS apex. Host resolvers that are not part of a larger organization will likely be configures with a key for the domain of their local ISP whose recursive secure DNS caching server they use. 6.3 Chaining Through Zones Starting with one or more trusted keys for any zone, it should be possible to retrieve signed keys for its subzones which have a key and, if the zone is not root, for its superzone. Every authoritative secure zone server MUST also include the KEY RR for a super-zone signed by the secure zone via static configuration. This makes it possible to climb the tree of zones if one starts below root. A secure sub-zone is indicated by a KEY RR with non-null key information appearing with the NS RRs for the sub-zone. These make it possible to descend within the tree of zones. A resolver should keep track of the number of successive secure zones traversed from a starting point to any secure zone it can reach. In general, the lower such a distance number is, the greater the confidence in the data. Data configured via a boot file directive should be given a distance number of zero. If a query encounters different data for the same query with different distance values, that with a larger value should be ignored unless some other local policy covers the case and requires a different trust model. A security conscious resolver should completely refuse to step from a secure zone into a unsecured zone unless the unsecured zone is certified to be non-secure by the presence of an authenticated KEY RR for the unsecured zone with the no-key type value. Otherwise the resolver is getting bogus or spoofed data. If legitimate unsecured zones are encountered in traversing the DNS tree, then no zone can be trusted as secure that can be reached only via information from such non-secure zones. Since the unsecured zone data could have been spoofed, the "secure" zone reach via it could be counterfeit. The "distance" to data in such zones or zones reached via such zones could be set to 256 or more as this exceeds the largest possible distance through secure zones in the DNS. Nevertheless, continuing to apply secure checks within "secure" zones reached via unsecured zones is a good practice and will, as a Donald E. Eastlake 3rd [Page 34]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 practical matter, provide some small increase in security. 6.4 Secure Time Coordinated interpretation of the time fields in SIG RRs requires that reasonably consistent time be available to the hosts implementing the DNS security extensions. A variety of time synchronization protocols exist including the Network Time Protocol (NTP, RFC 1305). If such protocols are used, they MUST be used securely so that time can not be spoofed. Otherwise, for example, a host could get its clock turned back and might then believe old SIG and KEY RRs which were valid but no longer are. Donald E. Eastlake 3rd [Page 35]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 7. ASCII Representation of Security RRs This section discusses the format for master file and other ASCII presentation of the three DNS security resource records. The algorithm field in KEY and SIG RRs can be represented as either an unsigned integer or symbolicly. The following initial symbols are defined as indicated: 001 RSAMD5 253 NULL (obsolete, see RFC 2065) 254 PRIVATE The key identifier field in KEY and SIG PRs is represented as an unsigned integer. 7.1 Presentation of KEY RRs KEY RRs may appear as single logical lines in a zone data master file [RFC 1033]. The flag field is represented as an unsigned integer or a sequence of mnemonics as follows: BIT Mnemonic Explanation 0 NOAUTH authentication use prohibited 1 NOCONF confidentiality use prohibited 5 USERK user key 6 ENTITYK end entity key 7 ZONEK zone key 8 IPSECK IPSEC key 9 MAILK email key 10 TLSK TLS key 12-15 signatory field, values 0 to 15 can be represented by SIG0, SIG1, ... SIG15 The preference field appear as unsigned integers. The protocol octet can be represented as either an unsigned integer or symbolicly. The following initial symbols are defined: 000 NONE 255 ALL Note that if the type field has the "no key" value, nothing appears after the algorithm octet. The remaining public key portion is represented in base 64 (see Donald E. Eastlake 3rd [Page 36]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Appendix A) and may be divided up into any number of white space separated substrings, down to single base 64 digits, which are concatenated to obtain the full signature. These substrings can span lines using the standard parenthesis. Note that the public key may have internal sub-fields but these do not appear in the master file representation. For example, with algorithm 1 there is a public exponent size, then a public exponent, and then a modulus. With algorithm 254, there will be an OID size, an OID, and algorithm dependent information. But in both cases only a single logical base 64 string will appear in the master file. 7.2 Presentation of SIG RRs A SIG RR may be represented as a single logical line in a zone data file [RFC 1033] but there are some special considerations as described below. (It does not make sense to include a transaction or request authenticating SIG RR in a file as they are a transient authentication that covers data including an ephemeral transaction number and so must be calculated in real time.) There is no particular problem with the signer, covered type, and times. The time fields appears in the form YYYYMMDDHHMMSS where YYYY is the year, the first MM is the month number (01-12), DD is the day of the month (01-31), HH is the hour in 24 hours notation (00-23), the second MM is the minute (00-59), and SS is the second (00-59). The original TTL field appears as an unsigned integer. If the original TTL, which applies to the type signed, is the same as the TTL of the SIG RR itself, it may be omitted. The date field which follows it is larger than the maximum possible TTL so there is no ambiguity. The "labels" field appears as an unsigned integer. The key identifier appears as unsigned decimal numbers. However, the signature itself can be very long. It is the last data field and is represented in base 64 (see Appendix A) and may be divided up into any number of white space separated substrings, down to single base 64 digits, which are concatenated to obtain the full signature. These substrings can be split between lines using the standard parenthesis. Donald E. Eastlake 3rd [Page 37]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 7.3 Presentation of NXT RRs NXT RRs do not appear in original unsigned zone master files since they should be derived from the zone as it is being siged. If a signed file with NXTs added is printed, they appear as the next domain name followed by the RR type present bits in the same format as the WKS RR. Donald E. Eastlake 3rd [Page 38]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 8. Canonical Form and Order of Resource Records This section describes the canonical form of resource records (RRs), their default name order, and their intra-RRset order, for purposes of domain name system (DNS) security. A canonical name order is necessary to construct the NXT name chain. A canonical form and ordering within an RRset is necessary in construsting SIG RRs. There is no requirement in DNS security for a canonical ordering of types within a name so none is defined. 8.1 Canonical RR Form For purposes of DNS security, the canonical form for an RR is the wire format of the RR with domain names (1) fully expanded (no name compression via pointers), (2) all domain name letters set to lower case, and (3) the original TTL substituted for the current TTL. 8.2 Canonical DNS Name Order For purposes of DNS security, the canonical ordering of owner names is to sort labels as unsigned left justified octet strings where the absence of a octet sorts before a zero value octet and upper case letters are treated as lower case letters. Names are then sorted by sorting on the highest level label and then, within those names with the same highest level label by the next lower label, etc. down to leaf node labels. Within a zone, the zone name itself always exists and all other names are the zone name with some prefix of lower level labels. Thus the zone name itself always sorts first. Example: foo.example a.foo.example yljkjljk.a.foo.example Z.a.foo.example zABC.a.FOO.EXAMPLE z.foo.example *.z.foo.example \200.z.foo.example 8.3 Canonical RR Ordering Within An RRset Within any particular owner name and type, RRs are sorted by RDATA as a left justified unsigned octet sequence where the absence of an octet sorts before the zero octet. Donald E. Eastlake 3rd [Page 39]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 9. Conformance Levels of server and resolver conformance are defined below. 9.1 Server Conformance Two levels of server conformance for DNS security are defined as follows: BASIC: Basic server compliance is the ability to store and retrieve (including zone transfer) SIG, KEY, and NXT RRs. Any secondary, caching, or other server for a secure zone MUST have at least basic compliance and even then some things, such as secure CNAMEs, will not work without full compliance. FULL: Full server compliance adds the following to basic compliance: (1) ability to read SIG, KEY, and NXT RRs in zone files and (2) ability, given a zone file and private key, to add appropriate SIG and NXT RRs, possibly via a separate application, (3) proper automatic inclusion of SIG, KEY, and NXT RRs in responses, (4) suppression of CNAME following on retrieval of the security type RRs, (5) recognize the CD query header bit and set the AD query header bit, as appropriate, and (6) proper handling of the two NXT RRs at delegation points. Primary servers for secure zones MUST be fully compliant and for complete secure operation, all secondary, caching, and other servers handling the zone SHOULD be fully compliant as well. 9.2 Resolver Conformance Two levels of resolver compliance (including the resolver portion of a server) are defined for DNS Security: BASIC: A basic compliance resolver can handle SIG, KEY, and NXT RRs when they are explicitly requested. FULL: A fully compliant resolver (1) understands KEY, SIG, and NXT RRs including verification of SIGs, (2) maintains appropriate information in its local caches and database to indicate which RRs have been authenticated and to what extent they have been authenticated, (3) performs additional queries as necessary to attempt to obtain KEY, SIG, or NXT RRs from non-security aware servers, (4) normally sets the CD query header bit on its queries. Donald E. Eastlake 3rd [Page 40]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 10. Security Considerations This document describes extensions to the Domain Name System (DNS) protocol to provide data integrity and origin authentication, public key distribution, and optional transaction and request security. It should be noted that, at most, these extensions guarantee the validity of resource records, including KEY resource records, retrieved from the DNS. They do not magically solve other security problems. For example, using secure DNS you can have high confidence in the IP address you retrieve for a host name; however, this does not stop someone for substituting an unauthorized host at that address or capturing packets sent to that address and falsely responding with packets apparently from that address. Any reasonably complete security system will require the protection of many additional facets of the Internet. Donald E. Eastlake 3rd [Page 41]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 References [NETSEC] - Network Security: PRIVATE Communications in a PUBLIC World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall Series in Computer Networking and Distributed Communications, 1995. [PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc., 3 June 1991, Version 1.4. [RFC 1032] - M. Stahl, "Domain Administrators Guide", November 1987. [RFC 1033] - M. Lottor, "Domain Administrators Operations Guide", November 1987. [RFC 1034] - P. Mockapetris, "Domain Names - Concepts and Facilities", STD 13, November 1987. [RFC 1035] - P. Mockapetris, "Domain Names - Implementation and Specifications", STD 13, November 1987. [RFC 1305] - Mills, D., "Network Time Protocol (v3)", March 1992. [RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April 1992. [RFC 1530] - Malamud, C., and M. Rose, "Principles of Operation for the TPC.INT Subdomain: General Principles and Policy", October 1993. [RFC 1750] - D. Eastlake, S. Crocker, and J. Schiller, "Randomness Requirements for Security", December 1994. [RFC 1825] - Atkinson, R., "Security Architecture for the Internet Protocol", August 1995. [RFC 1982] - R. Elz, R. Bush, "Serial Number Arithmetic", 09/03/1996. [RFC 1995] - Ohta, M., "Incremental Zone Transfer in DNS", August 1996. [RFC 2065] - D. Eastlake, C. Kaufman, "Domain Name System Security Extensions", 01/03/1997. [RFC 2136] - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", 04/21/1997. [RFC 2137] - D. Eastlake, "Secure Domain Name System Dynamic Update", 04/21/1997. [RSA FAQ] - RSADSI Frequently Asked Questions periodic posting. Donald E. Eastlake 3rd [Page 42]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 draft-ietf-tls-*.txt Donald E. Eastlake 3rd [Page 43]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Author's Addresses Donald E. Eastlake 3rd CyberCash, Inc. 318 Acton Street Carlisle, MA 01741 USA Telephone: +1 508-287-4877 +1 508-371-7148(fax) +1 703-620-4200(main office, Reston, Virginia, USA) EMail: dee@cybercash.com Expiration and File Name This draft expires January 1998. Its file name is draft-ietf-dnssec-secext2-00.txt. Donald E. Eastlake 3rd [Page 44]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Appendix A: Base 64 Encoding The following encoding technique is taken from RFC 1521 by N. Borenstein and N. Freed. It is reproduced here in an edited form for convenience. A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.) The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base 64 alphabet. Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. Table 1: The Base 64 Alphabet Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w (pad) = 15 P 32 g 49 x 16 Q 33 h 50 y Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a quantity. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Padding at the end of the data is performed using the '=' character. Since all base 64 input is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be Donald E. Eastlake 3rd [Page 45]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character. Donald E. Eastlake 3rd [Page 46]
INTERNET-DRAFT DNS Protocol Security Extensions July 1997 Appendix B: Changes from RFC 2065 This section sumarizes the most important changes that have been made since RFC 2065. 1. Most of Section 7 of RFC 2065 called "Operational Considerations", has been split off into a separate document. 2. The KEY RR has been changed by (2a) expanding the flags field to 32 bits, (2b) eliminating the "experimental" flag as unnecessary, (2c) defining a number of additional flags, (2d) adding the preference and key identifier fields, and (2e) for the RSA MD5 algorithm, fixing the exponent size at two octets and increasing the maximum required key modulus size implementation to 4096 bits. Section 3.4 describing the meaning of various combintions of "no- key" and key present KEY RRs has been added. 3. The SIG RR has been changed by (3a) claifying that signature expiration and date signed used serial number ring arithmetic, and (3b) changing the definition of the key footprint/identifier. In addition, the SIG AXFR has been eliminated. 4. The key footprint has been changed from algorithm specific to user determined and renamed the "key identifier", due to implementors requests for assured "footprint" uniqueness. It now occurs in both the SIG and KEY RRs. 5. Both the KEY and SIG RR definitions have been simplified by eliminating the "null" algorithm 253 as defined in RFC 2065. That algorithm had been included because at the time it was thought it might be useful in DNS dynamic update. It was in fact not so used and it is dropped to simplify DNS security. 6. The NXT RRs in a zone have been extended to cover all names, including wildcards as literal names without expansion. 7. Information on the canonical form and ordering of RRs has been moved into a separate section, number 8. 8. A subsection covering incremental and full zone transfer has been added. Donald E. Eastlake 3rd [Page 47]
