The DNS  NSEC  Resource Record (RR) is used for
authenticated proof of the non-existence of DNS owner names and
types. The NSEC RR is based on the NXT RR as described in RFC 2535
, and is similar except for the name and typecode. The RDATA
format for the NXT RR has the limitation in that the RDATA could only
carry information about the existence of the first 127 types. RFC
2535 did reserve a bit to specify an extension mechanism, but the
mechanism was never actually defined.
In order to avoid needing to develop an extension mechanism into a
deployed base of DNSSEC aware servers and resolvers once the first
127 type codes are allocated, this document redefines the wire format
of the "Type Bit Map" field in the NSEC RDATA to cover the full RR
This document introduces a new format for the type bit map. The
properties of the type bit map format are that it can cover the full
possible range of typecodes, that it is relatively economical in the
amount of space it uses for the common case of a few types with an
owner name, that it can represent owner names with all possible types
present in packets of approximately 8.5 kilobytes, and that the
representation is simple to implement. Efficient searching of the
type bitmap for the presence of certain types is not a requirement.
For convenience and completeness, this document presents the syntax
and semantics for the NSEC RR based on the specification in RFC 2535
 and as updated by RFC 3755 , thereby not introducing changes
except for the syntax of the type bit map.
This document updates RFC 2535  and RFC 3755 .
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 BCP 14, RFC 2119 .
2. The NSEC Resource Record
The NSEC resource record lists two separate things: the owner name of
the next RRset in the canonical ordering of the zone, and the set of
RR types present at the NSEC RR's owner name. The complete set of
NSEC RRs in a zone indicate which RRsets exist in a zone, and form a
chain of owner names in the zone. This information is used to
provide authenticated denial of existence for DNS data, as described
in RFC 2535 .
The type value for the NSEC RR is 47.
The NSEC RR RDATA format is class independent and defined for all
The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
field. This is in the spirit of negative caching .
2.1. NSEC RDATA Wire Format
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 /
/ List of Type Bit Map(s) /
2.1.1. The Next Domain Name Field
The Next Domain Name field contains the owner name of the next RR in
the canonical ordering of the zone. 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).
A sender MUST NOT use DNS name compression on the Next Domain Name
field when transmitting an NSEC RR.
Owner names of RRsets that are 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
2.1.2. The List of Type Bit Map(s) Field
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.
Window blocks are present in the NSEC RR RDATA in increasing
"|" denotes concatenation
Type Bit Map(s) Field = ( Window Block # | Bitmap Length | Bitmap ) +
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, and 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
NSEC RR's owner name. If a bit is set to 0, it indicates that no
RRset of that type is present for the NSEC RR's owner name.
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 RFC 2929 
(section 3.1), 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 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.
2.1.3. Inclusion of Wildcard Names in NSEC RDATA
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.
RFC 2535  describes the impact of wildcards on authenticated
denial of existence.
2.2. The NSEC RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Next Domain Name field is represented as a domain name.
The List of Type Bit Map(s) Field is represented as a sequence of RR
type mnemonics. When the mnemonic is not known, the TYPE
representation as described in RFC 3597  (section 5) MUST be used.
2.3. NSEC RR Example
The following NSEC RR identifies the RRsets associated with
alfa.example.com. and the next authoritative name after
alfa.example.com. 86400 IN NSEC host.example.com. A MX RRSIG NSEC
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 resolver can authenticate this NSEC record, it
could be used to prove that beta.example.com does not exist, or could
be used to prove that there is no AAAA record associated with
alfa.example.com. Authenticated denial of existence is discussed in
RFC 2535 .
3. IANA Considerations
This document introduces no new IANA considerations, because all of
the protocol parameters used in this document have already been
assigned by RFC 3755 .
4. Security Considerations
The update of the RDATA format and encoding does not affect the
security of the use of NSEC RRs.
5.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Eastlake 3rd, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
 Eastlake 3rd, D., Brunner-Williams, E., and B. Manning, "Domain
Name System (DNS) IANA Considerations", BCP 42, RFC 2929,
 Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
Types", RFC 3597, September 2003.
 Weiler, S., "Legacy Resolver Compatibility for Delegation Signer
(DS)", RFC 3755, May 2004.
5.2. Informative References
 Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
 Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
 Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC
2308, March 1998.
The encoding described in this document was initially proposed by
Mark Andrews. Other encodings where proposed by David Blacka and
7. Author's Address
Jakob Schlyter (editor)
Stockholm SE-114 87
8. Full Copyright Statement
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