3. The NSEC3 Resource Record

The NSEC3 Resource Record (RR) provides authenticated denial of existence for DNS Resource Record Sets.

The NSEC3 RR lists RR types present at the original owner name of the NSEC3 RR. It includes the next hashed owner name in the hash order of the zone. The complete set of NSEC3 RRs in a zone indicates which RRSets exist for the original owner name of the RR and form a chain of hashed owner names in the zone. This information is used to provide authenticated denial of existence for DNS data. To provide protection against zone enumeration, the owner names used in the NSEC3 RR are cryptographic hashes of the original owner name prepended as a single label to the name of the zone. The NSEC3 RR indicates which hash function is used to construct the hash, which salt is used, and how many iterations of the hash function are performed over the original owner name. The hashing technique is described fully in Section 5.

Hashed owner names of unsigned delegations may be excluded from the chain. An NSEC3 RR whose span covers the hash of an owner name or "next closer" name of an unsigned delegation is referred to as an Opt-Out NSEC3 RR and is indicated by the presence of a flag.

The owner name for the NSEC3 RR is the base32 encoding of the hashed owner name prepended as a single label to the name of the zone.

The type value for the NSEC3 RR is 50.

The NSEC3 RR RDATA format is class independent and is described below.

The class MUST be the same as the class of the original owner name.

The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL field. This is in the spirit of negative caching [RFC2308].

3.1. RDATA Fields

3.1.1. Hash Algorithm

The Hash Algorithm field identifies the cryptographic hash algorithm used to construct the hash-value.

The values for this field are defined in the NSEC3 hash algorithm registry defined in Section 11.

3.1.2. Flags

The Flags field contains 8 one-bit flags that can be used to indicate different processing. All undefined flags must be zero. The only flag defined by this specification is the Opt-Out flag.

3.1.2.1. Opt-Out Flag

If the Opt-Out flag is set, the NSEC3 record covers zero or more unsigned delegations.

If the Opt-Out flag is clear, the NSEC3 record covers zero unsigned delegations.

The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations. It is the least significant bit in the Flags field. See Section 6 for details about the use of this flag.

3.1.3. Iterations

The Iterations field defines the number of additional times the hash function has been performed. More iterations result in greater resiliency of the hash value against dictionary attacks, but at a higher computational cost for both the server and resolver. See Section 5 for details of the use of this field, and Section 10.3 for limitations on the value.

3.1.4. Salt Length

The Salt Length field defines the length of the Salt field in octets, ranging in value from 0 to 255.

3.1.5. Salt

The Salt field is appended to the original owner name before hashing in order to defend against pre-calculated dictionary attacks. See Section 5 for details on how the salt is used.

3.1.6. Hash Length

The Hash Length field defines the length of the Next Hashed Owner Name field, ranging in value from 1 to 255 octets.

3.1.7. Next Hashed Owner Name

The Next Hashed Owner Name field contains the next hashed owner name in hash order. This value is in binary format. Given the ordered set of all hashed owner names, the Next Hashed Owner Name field contains the hash of an owner name that immediately follows the owner name of the given NSEC3 RR. The value of the Next Hashed Owner Name field in the last NSEC3 RR in the zone is the same as the hashed owner name of the first NSEC3 RR in the zone in hash order. Note that, unlike the owner name of the NSEC3 RR, the value of this field does not contain the appended zone name.

3.1.8. Type Bit Maps

The Type Bit Maps field identifies the RRSet types that exist at the original owner name of the NSEC3 RR.

3.2. NSEC3 RDATA Wire Format

The RDATA of the NSEC3 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Hash Alg.   |     Flags     |          Iterations           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Salt Length  |                     Salt                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Hash Length  |             Next Hashed Owner Name            /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

/ Type Bit Maps / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Hash Algorithm is a single octet.

Flags field is a single octet, the Opt-Out flag is the least significant bit, as shown below:

0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | |O| +-+-+-+-+-+-+-+-+

Iterations is represented as a 16-bit unsigned integer, with the most significant bit first.

Salt Length is represented as an unsigned octet. Salt Length represents the length of the Salt field in octets. If the value is zero, the following Salt field is omitted.

Salt, if present, is encoded as a sequence of binary octets. The length of this field is determined by the preceding Salt Length field.

Hash Length is represented as an unsigned octet. Hash Length represents the length of the Next Hashed Owner Name field in octets.

The next hashed owner name is not base32 encoded, unlike the owner name of the NSEC3 RR. It is the unmodified binary hash value. It does not include the name of the containing zone. The length of this field is determined by the preceding Hash Length field.

3.2.1. Type Bit Maps Encoding

The encoding of the Type Bit Maps field is the same as that used by the NSEC RR, described in [RFC4034]. It is explained and clarified here for clarity.

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 bitmap of the window block, and up to 32 octets (256 bits) of bitmap.

Blocks are present in the NSEC3 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 to 1, it indicates that an RRSet of that type is present for the original owner name of the NSEC3 RR. If a bit is set to 0, it indicates that no RRSet of that type is present for the original owner name of the NSEC3 RR.

Since bit 0 in window block 0 refers to the non-existing RR type 0, it MUST be set to 0. After verification, the validator MUST ignore the value of bit 0 in window block 0.

Bits representing Meta-TYPEs or QTYPEs as specified in Section 3.1 of [RFC2929] or within the range reserved for assignment only to QTYPEs and Meta-TYPEs MUST be set to 0, 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 the bitmap of each block is determined by the type code with the largest numerical value, within that block, among the set of RR types present at the original owner name of the NSEC3 RR. Trailing octets not specified MUST be interpreted as zero octets.

3.3. Presentation Format

The presentation format of the RDATA portion is as follows:

The Hash Algorithm field is represented as an unsigned decimal

integer. The value has a maximum of 255.

The Flags field is represented as an unsigned decimal integer.

The value has a maximum of 255.

The Iterations field is represented as an unsigned decimal

integer. The value is between 0 and 65535, inclusive.

The Salt Length field is not represented.
The Salt field is represented as a sequence of case-insensitive

hexadecimal digits. Whitespace is not allowed within the sequence. The Salt field is represented as "-" (without the quotes) when the Salt Length field has a value of 0.

The Hash Length field is not represented.
The Next Hashed Owner Name field is represented as an unpadded

sequence of case-insensitive base32 digits, without whitespace.

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 of [RFC3597] MUST be used.