2. Definitions of Important DNSSEC Terms

This section defines a number of terms used in this document set. Because this is intended to be useful as a reference while reading the rest of the document set, first-time readers may wish to skim this section quickly, read the rest of this document, and then come back to this section.

Authentication Chain: An alternating sequence of DNS public key (DNSKEY) RRsets and Delegation Signer (DS) RRsets forms a chain of signed data, with each link in the chain vouching for the next. A DNSKEY RR is used to verify the signature covering a DS RR and allows the DS RR to be authenticated. The DS RR contains a hash of another DNSKEY RR and this new DNSKEY RR is authenticated by matching the hash in the DS RR. This new DNSKEY RR in turn authenticates another DNSKEY RRset and, in turn, some DNSKEY RR in this set may be used to authenticate another DS RR, and so forth until the chain finally ends with a DNSKEY RR whose corresponding private key signs the desired DNS data. For example, the root DNSKEY RRset can be used to authenticate the DS RRset for "example." The "example." DS RRset contains a hash that matches some "example." DNSKEY, and this DNSKEY's corresponding private key signs the "example." DNSKEY RRset. Private key counterparts of the "example." DNSKEY RRset sign data records such as "www.example." and DS RRs for delegations such as "subzone.example."

Authentication Key: A public key that a security-aware resolver has verified and can therefore use to authenticate data. A security-aware resolver can obtain authentication keys in three ways. First, the resolver is generally configured to know about at least one public key; this configured data is usually either the public key itself or a hash of the public key as found in the DS RR (see "trust anchor"). Second, the resolver may use an authenticated public key to verify a DS RR and the DNSKEY RR to which the DS RR refers. Third, the resolver may be able to determine that a new public key has been signed by the private key corresponding to another public key that the resolver has verified. Note that the resolver must always be guided by local policy when deciding whether to authenticate a new public key, even if the local policy is simply to authenticate any new public key for which the resolver is able verify the signature.

Authoritative RRset: Within the context of a particular zone, an RRset is "authoritative" if and only if the owner name of the RRset lies within the subset of the name space that is at or below the zone apex and at or above the cuts that separate the zone from its children, if any. All RRsets at the zone apex are authoritative, except for certain RRsets at this domain name that, if present, belong to this zone's parent. These RRset could include a DS RRset, the NSEC RRset referencing this DS RRset (the "parental NSEC"), and RRSIG RRs associated with these RRsets, all of which are authoritative in the parent zone. Similarly, if this zone contains any delegation points, only the parental NSEC RRset, DS RRsets, and any RRSIG RRs associated with these RRsets are authoritative for this zone.

Delegation Point: Term used to describe the name at the parental side of a zone cut. That is, the delegation point for "foo.example" would be the foo.example node in the "example" zone (as opposed to the zone apex of the "foo.example" zone). See also zone apex.

Island of Security: Term used to describe a signed, delegated zone that does not have an authentication chain from its delegating parent. That is, there is no DS RR containing a hash of a DNSKEY RR for the island in its delegating parent zone (see [RFC4034]). An island of security is served by security-aware name servers and may provide authentication chains to any delegated child zones. Responses from an island of security or its descendents can only be authenticated if its authentication keys can be authenticated by some trusted means out of band from the DNS protocol.

Key Signing Key (KSK): An authentication key that corresponds to a private key used to sign one or more other authentication keys for a given zone. Typically, the private key corresponding to a key signing key will sign a zone signing key, which in turn has a corresponding private key that will sign other zone data. Local policy may require that the zone signing key be changed frequently, while the key signing key may have a longer validity period in order to provide a more stable secure entry point into the zone. Designating an authentication key as a key signing key is purely an operational issue: DNSSEC validation does not distinguish between key signing keys and other DNSSEC authentication keys, and it is possible to use a single key as both a key signing key and a zone signing key. Key signing keys are discussed in more detail in [RFC3757]. Also see zone signing key.

Non-Validating Security-Aware Stub Resolver: A security-aware stub resolver that trusts one or more security-aware recursive name servers to perform most of the tasks discussed in this document set on its behalf. In particular, a non-validating security-aware stub resolver is an entity that sends DNS queries, receives DNS responses, and is capable of establishing an appropriately secured channel to a security-aware recursive name server that will provide these services on behalf of the security-aware stub resolver. See also security-aware stub resolver, validating security-aware stub resolver.

Non-Validating Stub Resolver: A less tedious term for a non-validating security-aware stub resolver.

Security-Aware Name Server: An entity acting in the role of a name server (defined in section 2.4 of [RFC1034]) that understands the DNS security extensions defined in this document set. In particular, a security-aware name server is an entity that receives DNS queries, sends DNS responses, supports the EDNS0 ([RFC2671]) message size extension and the DO bit ([RFC3225]), and supports the RR types and message header bits defined in this document set.

Security-Aware Recursive Name Server: An entity that acts in both the security-aware name server and security-aware resolver roles. A more cumbersome but equivalent phrase would be "a security-aware name server that offers recursive service".

Security-Aware Resolver: An entity acting in the role of a resolver (defined in section 2.4 of [RFC1034]) that understands the DNS security extensions defined in this document set. In particular, a security-aware resolver is an entity that sends DNS queries, receives DNS responses, supports the EDNS0 ([RFC2671]) message size extension and the DO bit ([RFC3225]), and is capable of using the RR types and message header bits defined in this document set to provide DNSSEC services.

Security-Aware Stub Resolver: An entity acting in the role of a stub resolver (defined in section 5.3.1 of [RFC1034]) that has enough of an understanding the DNS security extensions defined in this document set to provide additional services not available from a security-oblivious stub resolver. Security-aware stub resolvers may be either "validating" or "non-validating", depending on whether the stub resolver attempts to verify DNSSEC signatures on its own or trusts a friendly security-aware name server to do so. See also validating stub resolver, non-validating stub resolver.

Security-Oblivious <anything>: An <anything> that is not "security-aware".

Signed Zone: A zone whose RRsets are signed and that contains properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records.

Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR. A validating security-aware resolver uses this public key or hash as a starting point for building the authentication chain to a signed DNS response. In general, a validating resolver will have to obtain the initial values of its trust anchors via some secure or trusted means outside the DNS protocol. Presence of a trust anchor also implies that the resolver should expect the zone to which the trust anchor points to be signed.

Unsigned Zone: A zone that is not signed.

Validating Security-Aware Stub Resolver: A security-aware resolver that sends queries in recursive mode but that performs signature validation on its own rather than just blindly trusting an upstream security-aware recursive name server. See also security-aware stub resolver, non-validating security-aware stub resolver.

Validating Stub Resolver: A less tedious term for a validating security-aware stub resolver.

Zone Apex: Term used to describe the name at the child's side of a zone cut. See also delegation point.

Zone Signing Key (ZSK): An authentication key that corresponds to a private key used to sign a zone. Typically, a zone signing key will be part of the same DNSKEY RRset as the key signing key whose corresponding private key signs this DNSKEY RRset, but the zone signing key is used for a slightly different purpose and may differ from the key signing key in other ways, such as validity lifetime. Designating an authentication key as a zone signing key is purely an operational issue; DNSSEC validation does not distinguish between zone signing keys and other DNSSEC authentication keys, and it is possible to use a single key as both a key signing key and a zone signing key. See also key signing key.