Telematica Instituut

Drienerlolaan 5 7522 NB Enschede +31534850485 NL roy.arends@telin.nl

Internet Systems Consortium

950 Charter Street Redwood City CA 94063 USA sra@isc.org

VeriSign, Inc.

21345 Ridgetop Circle Dulles VA 20166-6503 USA mlarson@verisign.com

USC Information Sciences Institute

3811 N. Fairfax Drive Arlington VA 22203 USA masseyd@isi.edu

National Institute for Standards and Technology

100 Bureau Drive Gaithersburg MD 20899-8920 USA scott.rose@nist.gov

This document is part of a family of documents that describes the DNS Security Extensions (DNSSEC). The DNS Security Extensions are a collection of resource records and protocol modifications that provide source authentication for the DNS. This document defines the public key (DNSKEY), delegation signer (DS), resource record digital signature (RRSIG), and authenticated denial of existence (NSEC) resource records. The purpose and format of each resource record is described in detail, and an example of each resource record is given. This document obsoletes RFC 2535 and incorporates changes from all updates to RFC 2535. Internet DNS Extensions

The DNS Security Extensions (DNSSEC) introduce four new DNS resource record types: DNS Public Key (DNSKEY), Resource Record Signature (RRSIG), Next Secure (NSEC), and Delegation Signer (DS). This document defines the purpose of each resource record (RR), the RR's RDATA format, and its presentation format (ASCII representation).

This document is part of a family of documents that define DNSSEC, which should be read together as a set. contains an introduction to DNSSEC and definition of common terms; the reader is assumed to be familiar with this document. also contains a list of other documents updated by and obsoleted by this document set. defines the DNSSEC protocol operations. The reader is also assumed to be familiar with the basic DNS concepts described in , , and the subsequent documents that update them, particularly and . This document defines the DNSSEC resource records. All numeric DNS type codes given in this document are decimal integers.

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 .

DNSSEC uses public key cryptography to sign and authenticate DNS resource record sets (RRsets). The public keys are stored in DNSKEY resource records and are used in the DNSSEC authentication process described in : A zone signs its authoritative RRsets using a private key and stores the corresponding public key in a DNSKEY RR. A resolver can then use the public key to validate signatures covering the RRsets in the zone, and thus authenticate them. The DNSKEY RR is not intended as a record for storing arbitrary public keys and MUST NOT be used to store certificates or public keys that do not directly relate to the DNS infrastructure. The Type value for the DNSKEY RR type is 48. The DNSKEY RR is class independent. The DNSKEY RR has no special TTL requirements.

The RDATA for a DNSKEY RR consists of a 2 octet Flags Field, a 1 octet Protocol Field, a 1 octet Algorithm Field, and the Public Key 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Protocol | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / / Public Key / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Bit 7 of the Flags field is the Zone Key flag. If bit 7 has value 1, then the DNSKEY record holds a DNS zone key and the DNSKEY RR's owner name MUST be the name of a zone. If bit 7 has value 0, then the DNSKEY record holds some other type of DNS public key and MUST NOT be used to verify RRSIGs that cover RRsets. Bit 15 of the Flags field is the Secure Entry Point flag, described in . If bit 15 has value 1, then the DNSKEY record holds a key intended for use as a secure entry point. This flag is only intended to be to a hint to zone signing or debugging software as to the intended use of this DNSKEY record; validators MUST NOT alter their behavior during the signature validation process in any way based on the setting of this bit. This also means a DNSKEY RR with the SEP bit set would also need the Zone Key flag set in order to legally be able to generate signatures. A DNSKEY RR with the SEP set and the Zone Key flag not set MUST NOT be used to verify RRSIGs that cover RRsets. Bits 0-6 and 8-14 are reserved: these bits MUST have value 0 upon creation of the DNSKEY RR, and MUST be ignored upon reception.

The Protocol Field MUST have value 3 and the DNSKEY RR MUST be treated as invalid during signature verification if found to be some value other than 3.

The Algorithm field identifies the public key's cryptographic algorithm and determines the format of the Public Key field. A list of DNSSEC algorithm types can be found in

The Public Key Field holds the public key material. The format depends on the algorithm of the key being stored and are described in separate documents.

Although the Protocol Field always has value 3, it is retained for backward compatibility with early versions of the KEY record.

The presentation format of the RDATA portion is as follows: The Flag field MUST be represented as an unsigned decimal integer. Given the currently defined flags, the possible values are: 0, 256, or 257. The Protocol Field MUST be represented as an unsigned decimal integer with a value of 3. The Algorithm field MUST be represented either as an unsigned decimal integer or as an algorithm mnemonic as specified in . The Public Key field MUST be represented as a Base64 encoding of the Public Key. Whitespace is allowed within the Base64 text. For a definition of Base64 encoding, see .

The following DNSKEY RR stores a DNS zone key for example.com. example.com. 86400 IN DNSKEY 256 3 5 ( AQPSKmynfzW4kyBv015MUG2DeIQ3 Cbl+BBZH4b/0PY1kxkmvHjcZc8no kfzj31GajIQKY+5CptLr3buXA10h WqTkF7H6RfoRqXQeogmMHfpftf6z Mv1LyBUgia7za6ZEzOJBOztyvhjL 742iU/TpPSEDhm2SNKLijfUppn1U aNvv4w== ) The first four text fields specify the owner name, TTL, Class, and RR type (DNSKEY). Value 256 indicates that the Zone Key bit (bit 7) in the Flags field has value 1. Value 3 is the fixed Protocol value. Value 5 indicates the public key algorithm. identifies algorithm type 5 as RSA/SHA1 and indicates that the format of the RSA/SHA1 public key field is defined in . The remaining text is a Base64 encoding of the public key.

DNSSEC uses public key cryptography to sign and authenticate DNS resource record sets (RRsets). Digital signatures are stored in RRSIG resource records and are used in the DNSSEC authentication process described in . A validator can use these RRSIG RRs to authenticate RRsets from the zone. The RRSIG RR MUST only be used to carry verification material (digital signatures) used to secure DNS operations. An RRSIG record contains the signature for an RRset with a particular name, class, and type. The RRSIG RR specifies a validity interval for the signature and uses the Algorithm, the Signer's Name, and the Key Tag to identify the DNSKEY RR containing the public key that a validator can use to verify the signature. Because every authoritative RRset in a zone must be protected by a digital signature, RRSIG RRs must be present for names containing a CNAME RR. This is a change to the traditional DNS specification that stated that if a CNAME is present for a name, it is the only type allowed at that name. A RRSIG and NSEC (see ) MUST exist for the same name as a CNAME resource record in a signed zone. The Type value for the RRSIG RR type is 46. The RRSIG RR is class independent. An RRSIG RR MUST have the same class as the RRset it covers. The TTL value of an RRSIG RR MUST match the TTL value of the RRset it covers. This is an exception to the rules for TTL values of individual RRs within a RRset: individual RRSIG with the same owner name will have different TTL values if the RRsets they cover have different TTL values.

The RDATA for an RRSIG RR consists of a 2 octet Type Covered field, a 1 octet Algorithm field, a 1 octet Labels field, a 4 octet Original TTL field, a 4 octet Signature Expiration field, a 4 octet Signature Inception field, a 2 octet Key tag, the Signer's Name field, and 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Signature Inception | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key Tag | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Signer's Name / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / / Signature / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Type Covered field identifies the type of the RRset that is covered by this RRSIG record.

The Algorithm Number field identifies the cryptographic algorithm used to create the signature. A list of DNSSEC algorithm types can be found in

The Labels field specifies the number of labels in the original RRSIG RR owner name. The significance of this field is that a validator uses it to determine if the answer was synthesized from a wildcard. If so, it can be used to determine what owner name was used in generating the signature. To validate a signature, the validator needs the original owner name that was used to create the signature. If the original owner name contains a wildcard label ("*"), the owner name may have been expanded by the server during the response process, in which case the validator will need to reconstruct the original owner name in order to validate the signature. describes how to use the Labels field to reconstruct the original owner name. The value of the Labels field MUST NOT count either the null (root) label that terminates the owner name or the wildcard label (if present). The value of the Labels field MUST be less than or equal to the number of labels in the RRSIG owner name. For example, "www.example.com." has a Labels field value of 3, and "*.example.com." has a Labels field value of 2. Root (".") has a Labels field value of 0. Although the wildcard label is not included in the count stored in the Labels field of the RRSIG RR, the wildcard label is part of the RRset's owner name when generating or verifying the signature.

The Original TTL field specifies the TTL of the covered RRset as it appears in the authoritative zone. The Original TTL field is necessary because a caching resolver decrements the TTL value of a cached RRset. In order to validate a signature, a validator requires the original TTL. describes how to use the Original TTL field value to reconstruct the original TTL.

The Signature Expiration and Inception fields specify a validity period for the signature. The RRSIG record MUST NOT be used for authentication prior to the inception date and MUST NOT be used for authentication after the expiration date. The Signature Expiration and Inception field values specify a date and time in the form of a 32-bit unsigned number of seconds elapsed since 1 January 1970 00:00:00 UTC, ignoring leap seconds, in network byte order. The longest interval which can be expressed by this format without wrapping is approximately 136 years. An RRSIG RR can have an Expiration field value which is numerically smaller than the Inception field value if the expiration field value is near the 32-bit wrap-around point or if the signature is long lived. Because of this, all comparisons involving these fields MUST use "Serial number arithmetic" as defined in . As a direct consequence, the values contained in these fields cannot refer to dates more than 68 years in either the past or the future.

The Key Tag field contains the key tag value of the DNSKEY RR that validates this signature, in network byte order. explains how to calculate Key Tag values.

The Signer's Name field value identifies the owner name of the DNSKEY RR which a validator is supposed to use to validate this signature. The Signer's Name field MUST contain the name of the zone of the covered RRset. A sender MUST NOT use DNS name compression on the Signer's Name field when transmitting a RRSIG RR.

The Signature field contains the cryptographic signature that covers the RRSIG RDATA (excluding the Signature field) and the RRset specified by the RRSIG owner name, RRSIG class, and RRSIG Type Covered field. The format of this field depends on the algorithm in use and these formats are described in separate companion documents.

A signature covers the RRSIG RDATA (excluding the Signature Field) and covers the data RRset specified by the RRSIG owner name, RRSIG class, and RRSIG Type Covered fields. The RRset is in canonical form (see ) and the set RR(1),...RR(n) is signed as follows: signature = sign(RRSIG_RDATA | RR(1) | RR(2)... ) where "|" denotes concatenation; RRSIG_RDATA is the wire format of the RRSIG RDATA fields with the Signer's Name field in canonical form and the Signature field excluded; RR(i) = owner | type | class | TTL | RDATA length | RDATA "owner" is the fully qualified owner name of the RRset in canonical form (for RRs with wildcard owner names, the wildcard label is included in the owner name); Each RR MUST have the same owner name as the RRSIG RR; Each RR MUST have the same class as the RRSIG RR; Each RR in the RRset MUST have the RR type listed in the RRSIG RR's Type Covered field; Each RR in the RRset MUST have the TTL listed in the RRSIG Original TTL Field; Any DNS names in the RDATA field of each RR MUST be in canonical form; and The RRset MUST be sorted in canonical order. See and for details on canonical form and ordering of RRsets.

The presentation format of the RDATA portion is as follows: The Type Covered field is represented as a RR type mnemonic. When the mnemonic is not known, the TYPE representation as described in (section 5) MUST be used. The Algorithm field value MUST be represented either as an unsigned decimal integer or as an algorithm mnemonic as specified in . The Labels field value MUST be represented as an unsigned decimal integer. The Original TTL field value MUST be represented as an unsigned decimal integer. The Signature Expiration Time and Inception Time field values MUST be represented either as an unsigned decimal integer indicating seconds since 1 January 1970 00:00:00 UTC, or in the form YYYYMMDDHHmmSS in UTC, where: YYYY is the year (0001-9999, but see ); 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); mm is the minute (00-59); and SS is the second (00-59). Note that it is always be possible to distinguish between these two formats, because the YYYYMMDDHHmmSS format will always be exactly 14 digits, while the decimal representation of a 32-bit unsigned integer can never be longer than 10 digits. The Key Tag field MUST be represented as an unsigned decimal integer. The Signer's Name field value MUST be represented as a domain name. The Signature field is represented as a Base64 encoding of the signature. Whitespace is allowed within the Base64 text. See .

The following RRSIG RR stores the signature for the A RRset of host.example.com: host.example.com. 86400 IN RRSIG A 5 3 86400 20030322173103 ( 20030220173103 2642 example.com. oJB1W6WNGv+ldvQ3WDG0MQkg5IEhjRip8WTr PYGv07h108dUKGMeDPKijVCHX3DDKdfb+v6o B9wfuh3DTJXUAfI/M0zmO/zz8bW0Rznl8O3t GNazPwQKkRN20XPXV6nwwfoXmJQbsLNrLfkG J5D6fwFm8nN+6pBzeDQfsS3Ap3o= ) The first four fields specify the owner name, TTL, Class, and RR type (RRSIG). The "A" represents the Type Covered field. The value 5 identifies the algorithm used (RSA/SHA1) to create the signature. The value 3 is the number of Labels in the original owner name. The value 86400 in the RRSIG RDATA is the Original TTL for the covered A RRset. 20030322173103 and 20030220173103 are the expiration and inception dates, respectively. 2642 is the Key Tag, and example.com. is the Signer's Name. The remaining text is a Base64 encoding of the signature. Note that combination of RRSIG RR owner name, class, and Type Covered indicate that this RRSIG covers the "host.example.com" A RRset. The Label value of 3 indicates that no wildcard expansion was used. The Algorithm, Signer's Name, and Key Tag indicate this signature can be authenticated using an example.com zone DNSKEY RR whose algorithm is 5 and key tag is 2642.

The NSEC resource record lists two separate things: the next owner name (in the canonical ordering of the zone) which contains authoritative data or a delegation point NS RRset, and the set of RR types present at the NSEC RR's owner name. The complete set of NSEC RRs in a zone both indicate which authoritative RRsets exist in a zone and also form a chain of authoritative owner names in the zone. This information is used to provide authenticated denial of existence for DNS data, as described in . Because every authoritative name in a zone must be part of the NSEC chain, NSEC RRs must be present for names containing a CNAME RR. This is a change to the traditional DNS specification that stated that if a CNAME is present for a name, it is the only type allowed at that name. An RRSIG (see ) and NSEC MUST exist for the same name as a CNAME resource record in a signed zone. See for discussion of how a zone signer determines precisely which NSEC RRs it needs to include in a zone. The type value for the NSEC RR is 47. The NSEC RR is class independent. The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL field. This is in the spirit of negative caching ().

The RDATA of the NSEC RR is 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 Maps / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Next Domain field contains the next owner name (in the canonical ordering of the zone) which has authoritative data or contains a delegation point NS RRset; see for an explanation of canonical ordering. The value of the Next Domain Name field in the last NSEC record in the zone is the name of the zone apex (the owner name of the zone's SOA RR). This indicates that the owner name of the NSEC RR is the last name in the canonical ordering of the zone. A sender MUST NOT use DNS name compression on the Next Domain Name field when transmitting an NSEC RR. Owner names of RRsets not authoritative for the given zone (such as glue records) MUST NOT be listed in the Next Domain Name unless at least one authoritative RRset exists at the same owner name.

The Type Bit Maps field identifies the RRset types which exist at the NSEC RR's owner name. The RR type space is split into 256 window blocks, each representing the low-order 8 bits of the 16-bit RR type space. Each block that has at least one active RR type is encoded using a single octet window number (from 0 to 255), a single octet bitmap length (from 1 to 32) indicating the number of octets used for the window block's bitmap, and up to 32 octets (256 bits) of bitmap. Blocks are present in the NSEC RR RDATA in increasing numerical order.

Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+ where "|" denotes concatenation.

Each bitmap encodes the low-order 8 bits of RR types within the window block, in network bit order. The first bit is bit 0. For window block 0, bit 1 corresponds to RR type 1 (A), bit 2 corresponds to RR type 2 (NS), and so forth. For window block 1, bit 1 corresponds to RR type 257, bit 2 to RR type 258. If a bit is set, it indicates that an RRset of that type is present for the NSEC RR's owner name. If a bit is clear, it indicates that no RRset of that type is present for the NSEC RR's owner name. Bits representing pseudo-types MUST be clear, since they do not appear in zone data. If encountered, they MUST be ignored upon reading. Blocks with no types present MUST NOT be included. Trailing zero octets in the bitmap MUST be omitted. The length of each block's bitmap is determined by the type code with the largest numerical value, within that block, among the set of RR types present at the NSEC RR's owner name. Trailing zero octets not specified MUST be interpreted as zero octets. The bitmap for the NSEC RR at a delegation point requires special attention. Bits corresponding to the delegation NS RRset and the RR types for which the parent zone has authoritative data MUST be set; bits corresponding to any non-NS RRset for which the parent is not authoritative MUST be clear. A zone MUST NOT include an NSEC RR for any domain name that only holds glue records.

If a wildcard owner name appears in a zone, the wildcard label ("*") is treated as a literal symbol and is treated the same as any other owner name for purposes of generating NSEC RRs. Wildcard owner names appear in the Next Domain Name field without any wildcard expansion. describes the impact of wildcards on authenticated denial of existence.

The presentation format of the RDATA portion is as follows: The Next Domain Name field is represented as a domain name. The Type Bit Maps field is represented as a sequence of RR type mnemonics. When the mnemonic is not known, the TYPE representation as described in (section 5) MUST be used.

The following NSEC RR identifies the RRsets associated with alfa.example.com. and identifies the next authoritative name after alfa.example.com.

alfa.example.com. 86400 IN NSEC host.example.com. ( A MX RRSIG NSEC TYPE1234 )

The first four text fields specify the name, TTL, Class, and RR type (NSEC). The entry host.example.com. is the next authoritative name after alfa.example.com. in canonical order. The A, MX, RRSIG, NSEC, and TYPE1234 mnemonics indicate there are A, MX, RRSIG, NSEC, and TYPE1234 RRsets associated with the name alfa.example.com. The RDATA section of the NSEC RR above would be encoded as:

0x04 'h' 'o' 's' 't' 0x07 'e' 'x' 'a' 'm' 'p' 'l' 'e' 0x03 'c' 'o' 'm' 0x00 0x00 0x06 0x40 0x01 0x00 0x00 0x00 0x03 0x04 0x1b 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x20

Assuming that the validator can authenticate this NSEC record, it could be used to prove that beta.example.com does not exist, or could be used to prove there is no AAAA record associated with alfa.example.com. Authenticated denial of existence is discussed in .

The DS Resource Record refers to a DNSKEY RR and is used in the DNS DNSKEY authentication process. A DS RR refers to a DNSKEY RR by storing the key tag, algorithm number, and a digest of the DNSKEY RR. Note that while the digest should be sufficient to identify the public key, storing the key tag and key algorithm helps make the identification process more efficient. By authenticating the DS record, a resolver can authenticate the DNSKEY RR to which the DS record points. The key authentication process is described in . The DS RR and its corresponding DNSKEY RR have the same owner name, but they are stored in different locations. The DS RR appears only on the upper (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). The corresponding DNSKEY RR is stored in the "example.com" zone (the child zone). This simplifies DNS zone management and zone signing, but introduces special response processing requirements for the DS RR; these are described in . The type number for the DS record is 43. The DS resource record is class independent. The DS RR has no special TTL requirements.

The RDATA for a DS RR consists of a 2 octet Key Tag field, a one octet Algorithm field, a one octet Digest Type field, and a Digest 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key Tag | Algorithm | Digest Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / / Digest / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Key Tag field lists the key tag of the DNSKEY RR referred to by the DS record, in network byte order. The Key Tag used by the DS RR is identical to the Key Tag used by RRSIG RRs. describes how to compute a Key Tag.

The Algorithm field lists the algorithm number of the DNSKEY RR referred to by the DS record. The algorithm number used by the DS RR is identical to the algorithm number used by RRSIG and DNSKEY RRs. lists the algorithm number types.

The DS RR refers to a DNSKEY RR by including a digest of that DNSKEY RR. The Digest Type field identifies the algorithm used to construct the digest. lists the possible digest algorithm types.

The DS record refers to a DNSKEY RR by including a digest of that DNSKEY RR. The digest is calculated by concatenating the canonical form of the fully qualified owner name of the DNSKEY RR with the DNSKEY RDATA, and then applying the digest algorithm. digest = digest_algorithm( DNSKEY owner name | DNSKEY RDATA); "|" denotes concatenation DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key. The size of the digest may vary depending on the digest algorithm and DNSKEY RR size. As of the time of writing, the only defined digest algorithm is SHA-1, which produces a 20 octet digest.

The DS RR links the authentication chain across zone boundaries, so the DS RR requires extra care in processing. The DNSKEY RR referred to in the DS RR MUST be a DNSSEC zone key. The DNSKEY RR Flags MUST have Flags bit 7 set. If the DNSKEY flags do not indicate a DNSSEC zone key, the DS RR (and DNSKEY RR it references) MUST NOT be used in the validation process.

The presentation format of the RDATA portion is as follows: The Key Tag field MUST be represented as an unsigned decimal integer. The Algorithm field MUST be represented either as an unsigned decimal integer or as an algorithm mnemonic specified in . The Digest Type field MUST be represented as an unsigned decimal integer. The Digest MUST be represented as a sequence of case-insensitive hexadecimal digits. Whitespace is allowed within the hexadecimal text.

The following example shows a DNSKEY RR and its corresponding DS RR. dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz fwJr1AYtsmx3TGkJaNXVbfi/ 2pHm822aJ5iI9BMzNXxeYCmZ DRD99WYwYqUSdjMmmAphXdvx egXd/M5+X7OrzKBaMbCVdFLU Uh6DhweJBjEVv5f2wwjM9Xzc nOf+EPbtG9DMBmADjFDc2w/r ljwvFw== ) ; key id = 60485 dskey.example.com. 86400 IN DS 60485 5 1 ( 2BB183AF5F22588179A53B0A 98631FAD1A292118 ) The first four text fields specify the name, TTL, Class, and RR type (DS). Value 60485 is the key tag for the corresponding "dskey.example.com." DNSKEY RR, and value 5 denotes the algorithm used by this "dskey.example.com." DNSKEY RR. The value 1 is the algorithm used to construct the digest, and the rest of the RDATA text is the digest in hexadecimal.

This section defines a canonical form for resource records, a canonical ordering of DNS names, and a canonical ordering of resource records within an RRset. A canonical name order is required to construct the NSEC name chain. A canonical RR form and ordering within an RRset are required to construct and verify RRSIG RRs.

For purposes of DNS security, owner names are ordered by treating individual labels as unsigned left-justified octet strings. The absence of a octet sorts before a zero value octet, and upper case US-ASCII letters are treated as if they were lower case US-ASCII letters. To compute the canonical ordering of a set of DNS names, start by sorting the names according to their most significant (rightmost) labels. For names in which the most significant label is identical, continue sorting according to their next most significant label, and so forth. For example, the following names are sorted in canonical DNS name order. The most significant label is "example". At this level, "example" sorts first, followed by names ending in "a.example", then names ending "z.example". The names within each level are sorted in the same way. example a.example yljkjljk.a.example Z.a.example zABC.a.EXAMPLE z.example \001.z.example *.z.example \200.z.example

For purposes of DNS security, the canonical form of an RR is the wire format of the RR where: Every domain name in the RR is fully expanded (no DNS name compression) and fully qualified; All uppercase US-ASCII letters in the owner name of the RR are replaced by the corresponding lowercase US-ASCII letters; If the type of the RR is NS, MD, MF, CNAME, SOA, MB, MG, MR, PTR, HINFO, MINFO, MX, HINFO, RP, AFSDB, RT, SIG, PX, NXT, NAPTR, KX, SRV, DNAME, A6, RRSIG or NSEC, all uppercase US-ASCII letters in the DNS names contained within the RDATA are replaced by the corresponding lowercase US-ASCII letters; If the owner name of the RR is a wildcard name, the owner name is in its original unexpanded form, including the "*" label (no wildcard substitution); and The RR's TTL is set to its original value as it appears in the originating authoritative zone or the Original TTL field of the covering RRSIG RR.

For purposes of DNS security, RRs with the same owner name, class, and type are sorted by treating the RDATA portion of the canonical form of each RR as a left-justified unsigned octet sequence where the absence of an octet sorts before a zero octet. specifies that an RRset is not allowed to contain duplicate records (multiple RRs with the same owner name, class, type, and RDATA). Therefore, if an implementation detects duplicate RRs when putting the RRset in canonical form, the implementation MUST treat this as a protocol error. If the implementation chooses to handle this protocol error in the spirit of the robustness principle (being liberal in what it accepts), the implementation MUST remove all but one of the duplicate RR(s) for purposes of calculating the canonical form of the RRset.

This document introduces no new IANA considerations, because all of the protocol parameters used in this document have already been assigned by previous specifications. However, since the evolution of DNSSEC has been long and somewhat convoluted, this section attempts to describe the current state of the IANA registries and other protocol parameters which are (or once were) related to DNSSEC. Please refer to for additional IANA considerations. assigned types 24, 25, and 30 to the SIG, KEY, and NXT RRs, respectively. assigned DNS Resource Record Type 43 to DS. assigned types 46, 47, and 48 to the RRSIG, NSEC, and DNSKEY RRs, respectively. also marked type 30 (NXT) as Obsolete, and restricted use of types 24 (SIG) and 25 (KEY) to the "SIG(0)" transaction security protocol described in and the transaction KEY Resource Record described in . created an IANA registry for DNSSEC Resource Record Algorithm field numbers, and assigned values 1-4 and 252-255. assigned value 5. altered this registry to include flags for each entry regarding its use with the DNS security extensions. Each algorithm entry could refer to an algorithm that can be used for zone signing, transaction security (see ) or both. Values 6-251 are available for assignment by IETF standards action (). See for a full listing of the DNS Security Algorithm Numbers entries at the time of writing and their status of use in DNSSEC. created an IANA registry for DNSSEC DS Digest Types, and assigned value 0 to reserved and value 1 to SHA-1. created an IANA Registry for KEY Protocol Values, but re-assigned all values other than 3 to reserved and closed this IANA registry. The registry remains closed, and all KEY and DNSKEY records are required to have Protocol Octet value of 3. created an IANA registry for the DNSSEC KEY and DNSKEY RR flag bits. Initially, this registry only contains assignments for bit 7 (the ZONE bit) and bit 15 (the Secure Entry Point flag (SEP) bit, see ). As also stated in , bits 0-6 and 8-14 are available for assignment by IETF Standards Action.

This document describes the format of four DNS resource records used by the DNS security extensions, and presents an algorithm for calculating a key tag for a public key. Other than the items described below, the resource records themselves introduce no security considerations. Please see and for additional security considerations related to the use of these records. The DS record points to a DNSKEY RR using a cryptographic digest, the key algorithm type and a key tag. The DS record is intended to identify an existing DNSKEY RR, but it is theoretically possible for an attacker to generate a DNSKEY that matches all the DS fields. The probability of constructing such a matching DNSKEY depends on the type of digest algorithm in use. The only currently defined digest algorithm is SHA-1, and the working group believes that constructing a public key which would match the algorithm, key tag, and SHA-1 digest given in a DS record would be a sufficiently difficult problem that such an attack is not a serious threat at this time. The key tag is used to help select DNSKEY resource records efficiently, but it does not uniquely identify a single DNSKEY resource record. It is possible for two distinct DNSKEY RRs to have the same owner name, the same algorithm type, and the same key tag. An implementation which uses only the key tag to select a DNSKEY RR might select the wrong public key in some circumstances. Please see for further details. The table of algorithms in and the key tag calculation algorithms in include the RSA/MD5 algorithm for completeness, but the RSA/MD5 algorithm is NOT RECOMMENDED, as explained in .

This document was created from the input and ideas of the members of the DNS Extensions Working Group and working group mailing list. The editors would like to express their thanks for the comments and suggestions received during the revision of these security extension specifications. While explicitly listing everyone who has contributed during the decade during which DNSSEC has been under development would be an impossible task, includes a list of some of the participants who were kind enough to comment on these documents.

Information Sciences Institute (ISI) USC/ISI

4676 Admiralty Way Marina del Rey CA 90291 US +1 213 822 1511

University of Melbourne, Computer Science

Parkville Victoria 3052 AU kre@munnari.OZ.AU

RGnet, Inc.

10361 NE Sasquatch Lane Bainbridge Island WA 98110 US randy@psg.com

Harvard University

1350 Mass. Ave. Cambridge MA 02138 - +1 617 495 3864 -

General keyword Internet Software Consortium

Star Route Box 159A Woodside CA 94062 +1 415 747 0204 paul@vix.com

Bellcore

445 South Street Morristown NJ 07960 +1 201 829 4514 set@thumper.bellcore.com

Cisco Systems

170 West Tasman Drive San Jose CA 95134-1706 +1 914 528 0090 yakov@cisco.com

Digital Equipment Corp.

110 Spitbrook Rd ZK3-3/U14 Nashua NH 03062-2698 +1 603 881 0400 bound@zk3.dec.com

Applications domain name domain name system Computer Science

Parkville Victoria 3052 Australia. kre@munnari.OZ.AU e

RGnet, Inc.

5147 Crystal Springs Drive Bainbridge Island Washington 98110 United States. NE randy@psg.com

Applications DNS domain name system CSIRO - Mathematical and Information Sciences

Locked Bag 17 North Ryde NSW 2113 AUSTRALIA +61 2 9325 3148 Mark.Andrews@cmis.csiro.au

Applications domain name system DNS IBM

65 Shindegan Hill Road RR #1 Carmel NY 10512 US +1 914 276 2668 +1 914 784 3833 dee3@us.ibm.com

Internet Software Consortium

950 Charter Street Redwood City CA 94063 US +1 650 779 7001 vixie@isc.org

IBM

65 Shindegan Hill Road RR #1 Carmel NY 10512 US +1 914 784 7913 +1 914 784 3833 dee3@us.ibm.com

IBM

65 Shindegan Hill Road RR #1 Carmel NY 10512 US +1 914 276 2668 +1 914 784 3833 dee3@us.ibm.com

A standard method for storing RSA keys and and RSA/MD5 based signatures in the Domain Name System is described which utilizes DNS KEY and SIG resource records. IBM

65 Shindegan Hill Road RR #1 Carmel NY 10512 US +1 914 276 2668 +1 914 784 3833 dee3@us.ibm.com

A standard method for storing Diffie-Hellman keys in the Domain Name System is described which utilizes DNS KEY resource records.

The DNS security extensions are designed to be independent of the underlying cryptographic algorithms. The DNSKEY, RRSIG, and DS resource records all use a DNSSEC Algorithm Number to identify the cryptographic algorithm in use by the resource record. The DS resource record also specifies a Digest Algorithm Number to identify the digest algorithm used to construct the DS record. The currently defined Algorithm and Digest Types are listed below. Additional Algorithm or Digest Types could be added as advances in cryptography warrant. A DNSSEC aware resolver or name server MUST implement all MANDATORY algorithms.

The DNSKEY, RRSIG, and DS RRs use an 8-bit number used to identify the security algorithm being used. These values are stored in the "Algorithm number" field in the resource record RDATA. Some algorithms are usable only for zone signing (DNSSEC), some only for transaction security mechanisms (SIG(0) and TSIG), and some for both. Those usable for zone signing may appear in DNSKEY, RRSIG, and DS RRs. Those usable for transaction security would be present in SIG(0) and KEY RRs as described in Zone Value Algorithm [Mnemonic] Signing References Status ----- -------------------- --------- ---------- --------- 0 reserved 1 RSA/MD5 [RSAMD5] n [RFC2537] NOT RECOMMENDED 2 Diffie-Hellman [DH] n [RFC2539] - 3 DSA/SHA-1 [DSA] y [RFC2536] OPTIONAL 4 Elliptic Curve [ECC] TBA - 5 RSA/SHA-1 [RSASHA1] y [RFC3110] MANDATORY 252 Indirect [INDIRECT] n - 253 Private [PRIVATEDNS] y see below OPTIONAL 254 Private [PRIVATEOID] y see below OPTIONAL 255 reserved 6 - 251 Available for assignment by IETF Standards Action.

Algorithm number 253 is reserved for private use and will never be assigned to a specific algorithm. The public key area in the DNSKEY RR and the signature area in the RRSIG RR begin with a wire encoded domain name, which MUST NOT be compressed. The domain name indicates the private algorithm to use and the remainder of the public key area is determined by that algorithm. Entities should only use domain names they control to designate their private algorithms. Algorithm number 254 is reserved for private use and will never be assigned to a specific algorithm. The public key area in the DNSKEY RR and the signature area in the RRSIG RR begin with an unsigned length byte followed by a BER encoded 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. Entities should only use OIDs they control to designate their private algorithms.

A "Digest Type" field in the DS resource record types identifies the cryptographic digest algorithm used by the resource record. The following table lists the currently defined digest algorithm types. VALUE Algorithm STATUS 0 Reserved - 1 SHA-1 MANDATORY 2-255 Unassigned -

The Key Tag field in the RRSIG and DS resource record types provides a mechanism for selecting a public key efficiently. In most cases, a combination of owner name, algorithm, and key tag can efficiently identify a DNSKEY record. Both the RRSIG and DS resource records have corresponding DNSKEY records. The Key Tag field in the RRSIG and DS records can be used to help select the corresponding DNSKEY RR efficiently when more than one candidate DNSKEY RR is available. However, it is essential to note that the key tag is not a unique identifier. It is theoretically possible for two distinct DNSKEY RRs to have the same owner name, the same algorithm, and the same key tag. The key tag is used to limit the possible candidate keys, but it does not uniquely identify a DNSKEY record. Implementations MUST NOT assume that the key tag uniquely identifies a DNSKEY RR. The key tag is the same for all DNSKEY algorithm types except algorithm 1 (please see for the definition of the key tag for algorithm 1). The key tag algorithm is the sum of the wire format of the DNSKEY RDATA broken into 2 octet groups. First the RDATA (in wire format) is treated as a series of 2 octet groups, these groups are then added together ignoring any carry bits. A reference implementation of the key tag algorithm is as an ANSI C function is given below with the RDATA portion of the DNSKEY RR is used as input. It is not necessary to use the following reference code verbatim, but the numerical value of the Key Tag MUST be identical to what the reference implementation would generate for the same input. Please note that the algorithm for calculating the Key Tag is almost but not completely identical to the familiar ones-complement checksum used in many other Internet protocols. Key Tags MUST be calculated using the algorithm described here rather than the ones complement checksum. The following ANSI C reference implementation calculates the value of a Key Tag. This reference implementation applies to all algorithm types except algorithm 1 (see ). The input is the wire format of the RDATA portion of the DNSKEY RR. The code is written for clarity, not efficiency.

/* * Assumes that int is at least 16 bits. * First octet of the key tag is the most significant 8 bits of the * return value; * Second octet of the key tag is the least significant 8 bits of the * return value. */ unsigned int keytag ( unsigned char key[], /* the RDATA part of the DNSKEY RR */ unsigned int keysize /* the RDLENGTH */ ) { unsigned long ac; /* assumed to be 32 bits or larger */ int i; /* loop index */ for ( ac = 0, i = 0; i < keysize; ++i ) ac += (i & 1) ? key[i] : key[i] << 8; ac += (ac >> 16) & 0xFFFF; return ac & 0xFFFF; }

The key tag for algorithm 1 (RSA/MD5) is defined differently than the key tag for all other algorithms, for historical reasons. For a DNSKEY RR with algorithm 1, the key tag is defined to be the most significant 16 bits of the least significant 24 bits in the public key modulus (in other words, the 4th to last and 3rd to last octets of the public key modulus). Please note that Algorithm 1 is NOT RECOMMENDED.