3. Services Provided by DNS Security
The Domain Name System (DNS) security extensions provide origin authentication and integrity assurance services for DNS data, including mechanisms for authenticated denial of existence of DNS data. These mechanisms are described below.
These mechanisms require changes to the DNS protocol. DNSSEC adds four new resource record types: Resource Record Signature (RRSIG), DNS Public Key (DNSKEY), Delegation Signer (DS), and Next Secure (NSEC). It also adds two new message header bits: Checking Disabled (CD) and Authenticated Data (AD). In order to support the larger DNS message sizes that result from adding the DNSSEC RRs, DNSSEC also requires EDNS0 support ([RFC2671]). Finally, DNSSEC requires support for the DNSSEC OK (DO) EDNS header bit ([RFC3225]) so that a security-aware resolver can indicate in its queries that it wishes to receive DNSSEC RRs in response messages.
These services protect against most of the threats to the Domain Name System described in [RFC3833]. Please see Section 12 for a discussion of the limitations of these extensions.
3.1. Data Origin Authentication and Data Integrity
DNSSEC provides authentication by associating cryptographically generated digital signatures with DNS RRsets. These digital signatures are stored in a new resource record, the RRSIG record. Typically, there will be a single private key that signs a zone's data, but multiple keys are possible. For example, there may be keys for each of several different digital signature algorithms. If a security-aware resolver reliably learns a zone's public key, it can authenticate that zone's signed data. An important DNSSEC concept is that the key that signs a zone's data is associated with the zone itself and not with the zone's authoritative name servers. (Public keys for DNS transaction authentication mechanisms may also appear in zones, as described in [RFC2931], but DNSSEC itself is concerned with object security of DNS data, not channel security of DNS transactions. The keys associated with transaction security may be stored in different RR types. See [RFC3755] for details.)
A security-aware resolver can learn a zone's public key either by having a trust anchor configured into the resolver or by normal DNS resolution. To allow the latter, public keys are stored in a new type of resource record, the DNSKEY RR. Note that the private keys used to sign zone data must be kept secure and should be stored offline when practical. To discover a public key reliably via DNS resolution, the target key itself has to be signed by either a configured authentication key or another key that has been authenticated previously. Security-aware resolvers authenticate zone information by forming an authentication chain from a newly learned public key back to a previously known authentication public key, which in turn either has been configured into the resolver or must have been learned and verified previously. Therefore, the resolver must be configured with at least one trust anchor.
If the configured trust anchor is a zone signing key, then it will authenticate the associated zone; if the configured key is a key signing key, it will authenticate a zone signing key. If the configured trust anchor is the hash of a key rather than the key itself, the resolver may have to obtain the key via a DNS query. To help security-aware resolvers establish this authentication chain, security-aware name servers attempt to send the signature(s) needed to authenticate a zone's public key(s) in the DNS reply message along with the public key itself, provided that there is space available in the message.
The Delegation Signer (DS) RR type simplifies some of the administrative tasks involved in signing delegations across organizational boundaries. The DS RRset resides at a delegation point in a parent zone and indicates the public key(s) corresponding to the private key(s) used to self-sign the DNSKEY RRset at the delegated child zone's apex. The administrator of the child zone, in turn, uses the private key(s) corresponding to one or more of the public keys in this DNSKEY RRset to sign the child zone's data. The typical authentication chain is therefore DNSKEY->[DS->DNSKEY]->RRset, where "" denotes zero or more DS->DNSKEY subchains. DNSSEC permits more complex authentication chains, such as additional layers of DNSKEY RRs signing other DNSKEY RRs within a zone.
A security-aware resolver normally constructs this authentication chain from the root of the DNS hierarchy down to the leaf zones based on configured knowledge of the public key for the root. Local policy, however, may also allow a security-aware resolver to use one or more configured public keys (or hashes of public keys) other than the root public key, may not provide configured knowledge of the root public key, or may prevent the resolver from using particular public keys for arbitrary reasons, even if those public keys are properly signed with verifiable signatures. DNSSEC provides mechanisms by which a security-aware resolver can determine whether an RRset's signature is "valid" within the meaning of DNSSEC. In the final analysis, however, authenticating both DNS keys and data is a matter of local policy, which may extend or even override the protocol extensions defined in this document set. See Section 5 for further discussion.
3.2. Authenticating Name and Type Non-Existence
The security mechanism described in Section 3.1 only provides a way to sign existing RRsets in a zone. The problem of providing negative responses with the same level of authentication and integrity requires the use of another new resource record type, the NSEC record. The NSEC record allows a security-aware resolver to authenticate a negative reply for either name or type non-existence with the same mechanisms used to authenticate other DNS replies. Use of NSEC records requires a canonical representation and ordering for domain names in zones. Chains of NSEC records explicitly describe the gaps, or "empty space", between domain names in a zone and list the types of RRsets present at existing names. Each NSEC record is signed and authenticated using the mechanisms described in Section 3.1.