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3. The RRSIG Resource Record

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 [RFC4035]. 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 [RFC1034], which stated that if a CNAME is present for a name, it is the only type allowed at that name. A RRSIG and NSEC (see Section 4) 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 [RFC2181] rules for TTL values of individual RRs within a RRset: individual RRSIG RRs with the same owner name will have different TTL values if the RRsets they cover have different TTL values.

3.1. RRSIG RDATA Wire Format \​

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                          /
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.1. The Type Covered Field \​

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

3.1.2. The Algorithm Number Field \​

The Algorithm Number field identifies the cryptographic algorithm used to create the signature. A list of DNSSEC algorithm types can be found in Appendix A.1

3.1.3. The Labels Field \​

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 whether 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 have to reconstruct the original owner name in order to validate the signature. [RFC4035] 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 the signature is generated or verified.

3.1.4. Original TTL Field \​

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. [RFC4035] describes how to use the Original TTL field value to reconstruct the original TTL.

3.1.5. Signature Expiration and Inception Fields \​

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 that can be expressed by this format without wrapping is approximately 136 years. An RRSIG RR can have an Expiration field value that 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 [RFC1982]. 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.

3.1.6. The Key Tag Field \​

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

3.1.7. The Signer's Name Field \​

The Signer's Name field value identifies the owner name of the DNSKEY RR that 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.

3.1.8. The Signature Field \​

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.

3.2. The RRSIG RR Presentation Format \​

The presentation format of the RDATA portion is as follows:

The Type Covered field is represented as an RR type mnemonic. When the mnemonic is not known, the TYPE representation as described in [RFC3597], 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 Appendix A.1.

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 and Inception 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)
  • MM is the month number (01-12)
  • DD is the day of the month (01-31)
  • HH is the hour, in 24 hour notation (00-23)
  • mm is the minute (00-59)
  • SS is the second (00-59)

Implementations that present these fields to humans SHOULD use the YYYYMMDDHHmmSS format.

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 MUST be represented as a Base64 encoding of the signature. Whitespace is allowed within the Base64 text. For a definition of Base64 encoding, see [RFC3548].

3.3. RRSIG RR Example \​

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 20050322173103 (
                                20050220173103 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. Value 5 is the Algorithm field. Value 3 is the Labels field. Value 86400 is the Original TTL field, which was the TTL value used for the A RRset. The time values 20050322173103 and 20050220173103 are the Signature Expiration and Inception dates, respectively. Value 2642 is the Key Tag, and example.com. is the Signer's Name. The remaining text is a Base64 encoding of the Signature field.

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