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rfc:rfc4033

Network Working Group R. Arends Request for Comments: 4033 Telematica Instituut Obsoletes: 2535, 3008, 3090, 3445, 3655, 3658, R. Austein

         3755, 3757, 3845                                          ISC

Updates: 1034, 1035, 2136, 2181, 2308, 3225, M. Larson

       3007, 3597, 3226                                       VeriSign

Category: Standards Track D. Massey

                                             Colorado State University
                                                               S. Rose
                                                                  NIST
                                                            March 2005
             DNS Security Introduction and Requirements

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 The Domain Name System Security Extensions (DNSSEC) add data origin
 authentication and data integrity to the Domain Name System.  This
 document introduces these extensions and describes their capabilities
 and limitations.  This document also discusses the services that the
 DNS security extensions do and do not provide.  Last, this document
 describes the interrelationships between the documents that
 collectively describe DNSSEC.

Arends, et al. Standards Track [Page 1] RFC 4033 DNS Security Introduction and Requirements March 2005

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Definitions of Important DNSSEC Terms  . . . . . . . . . . .   3
 3.  Services Provided by DNS Security  . . . . . . . . . . . . .   7
     3.1.  Data Origin Authentication and Data Integrity  . . . .   7
     3.2.  Authenticating Name and Type Non-Existence . . . . . .   9
 4.  Services Not Provided by DNS Security  . . . . . . . . . . .   9
 5.  Scope of the DNSSEC Document Set and Last Hop Issues . . . .   9
 6.  Resolver Considerations  . . . . . . . . . . . . . . . . . .  10
 7.  Stub Resolver Considerations . . . . . . . . . . . . . . . .  11
 8.  Zone Considerations  . . . . . . . . . . . . . . . . . . . .  12
     8.1.  TTL Values vs. RRSIG Validity Period . . . . . . . . .  13
     8.2.  New Temporal Dependency Issues for Zones . . . . . . .  13
 9.  Name Server Considerations . . . . . . . . . . . . . . . . .  13
 10. DNS Security Document Family . . . . . . . . . . . . . . . .  14
 11. IANA Considerations  . . . . . . . . . . . . . . . . . . . .  15
 12. Security Considerations  . . . . . . . . . . . . . . . . . .  15
 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  17
 14. References . . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.1. Normative References . . . . . . . . . . . . . . . . .  17
     14.2. Informative References . . . . . . . . . . . . . . . .  18
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  20
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  21

1. Introduction

 This document introduces the Domain Name System Security Extensions
 (DNSSEC).  This document and its two companion documents ([RFC4034]
 and [RFC4035]) update, clarify, and refine the security extensions
 defined in [RFC2535] and its predecessors.  These security extensions
 consist of a set of new resource record types and modifications to
 the existing DNS protocol ([RFC1035]).  The new records and protocol
 modifications are not fully described in this document, but are
 described in a family of documents outlined in Section 10.  Sections
 3 and 4 describe the capabilities and limitations of the security
 extensions in greater detail.  Section 5 discusses the scope of the
 document set.  Sections 6, 7, 8, and 9 discuss the effect that these
 security extensions will have on resolvers, stub resolvers, zones,
 and name servers.
 This document and its two companions obsolete [RFC2535], [RFC3008],
 [RFC3090], [RFC3445], [RFC3655], [RFC3658], [RFC3755], [RFC3757], and
 [RFC3845].  This document set also updates but does not obsolete
 [RFC1034], [RFC1035], [RFC2136], [RFC2181], [RFC2308], [RFC3225],
 [RFC3007], [RFC3597], and the portions of [RFC3226] that deal with
 DNSSEC.

Arends, et al. Standards Track [Page 2] RFC 4033 DNS Security Introduction and Requirements March 2005

 The DNS security extensions provide origin authentication and
 integrity protection for DNS data, as well as a means of public key
 distribution.  These extensions do not provide confidentiality.

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.

Arends, et al. Standards Track [Page 3] RFC 4033 DNS Security Introduction and Requirements March 2005

 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.

Arends, et al. Standards Track [Page 4] RFC 4033 DNS Security Introduction and Requirements March 2005

 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.

Arends, et al. Standards Track [Page 5] RFC 4033 DNS Security Introduction and Requirements March 2005

 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.

Arends, et al. Standards Track [Page 6] RFC 4033 DNS Security Introduction and Requirements March 2005

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

Arends, et al. Standards Track [Page 7] RFC 4033 DNS Security Introduction and Requirements March 2005

 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.

Arends, et al. Standards Track [Page 8] RFC 4033 DNS Security Introduction and Requirements March 2005

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.

4. Services Not Provided by DNS Security

 DNS was originally designed with the assumptions that the DNS will
 return the same answer to any given query regardless of who may have
 issued the query, and that all data in the DNS is thus visible.
 Accordingly, DNSSEC is not designed to provide confidentiality,
 access control lists, or other means of differentiating between
 inquirers.
 DNSSEC provides no protection against denial of service attacks.
 Security-aware resolvers and security-aware name servers are
 vulnerable to an additional class of denial of service attacks based
 on cryptographic operations.  Please see Section 12 for details.
 The DNS security extensions provide data and origin authentication
 for DNS data.  The mechanisms outlined above are not designed to
 protect operations such as zone transfers and dynamic update
 ([RFC2136], [RFC3007]).  Message authentication schemes described in
 [RFC2845] and [RFC2931] address security operations that pertain to
 these transactions.

5. Scope of the DNSSEC Document Set and Last Hop Issues

 The specification in this document set defines the behavior for zone
 signers and security-aware name servers and resolvers in such a way
 that the validating entities can unambiguously determine the state of
 the data.
 A validating resolver can determine the following 4 states:
 Secure: The validating resolver has a trust anchor, has a chain of
    trust, and is able to verify all the signatures in the response.

Arends, et al. Standards Track [Page 9] RFC 4033 DNS Security Introduction and Requirements March 2005

 Insecure: The validating resolver has a trust anchor, a chain of
    trust, and, at some delegation point, signed proof of the
    non-existence of a DS record.  This indicates that subsequent
    branches in the tree are provably insecure.  A validating resolver
    may have a local policy to mark parts of the domain space as
    insecure.
 Bogus: The validating resolver has a trust anchor and a secure
    delegation indicating that subsidiary data is signed, but the
    response fails to validate for some reason: missing signatures,
    expired signatures, signatures with unsupported algorithms, data
    missing that the relevant NSEC RR says should be present, and so
    forth.
 Indeterminate: There is no trust anchor that would indicate that a
    specific portion of the tree is secure.  This is the default
    operation mode.
 This specification only defines how security-aware name servers can
 signal non-validating stub resolvers that data was found to be bogus
 (using RCODE=2, "Server Failure"; see [RFC4035]).
 There is a mechanism for security-aware name servers to signal
 security-aware stub resolvers that data was found to be secure (using
 the AD bit; see [RFC4035]).
 This specification does not define a format for communicating why
 responses were found to be bogus or marked as insecure.  The current
 signaling mechanism does not distinguish between indeterminate and
 insecure states.
 A method for signaling advanced error codes and policy between a
 security-aware stub resolver and security-aware recursive nameservers
 is a topic for future work, as is the interface between a security-
 aware resolver and the applications that use it.  Note, however, that
 the lack of the specification of such communication does not prohibit
 deployment of signed zones or the deployment of security aware
 recursive name servers that prohibit propagation of bogus data to the
 applications.

6. Resolver Considerations

 A security-aware resolver has to be able to perform cryptographic
 functions necessary to verify digital signatures using at least the
 mandatory-to-implement algorithm(s).  Security-aware resolvers must
 also be capable of forming an authentication chain from a newly
 learned zone back to an authentication key, as described above.  This
 process might require additional queries to intermediate DNS zones to

Arends, et al. Standards Track [Page 10] RFC 4033 DNS Security Introduction and Requirements March 2005

 obtain necessary DNSKEY, DS, and RRSIG records.  A security-aware
 resolver should be configured with at least one trust anchor as the
 starting point from which it will attempt to establish authentication
 chains.
 If a security-aware resolver is separated from the relevant
 authoritative name servers by a recursive name server or by any sort
 of intermediary device that acts as a proxy for DNS, and if the
 recursive name server or intermediary device is not security-aware,
 the security-aware resolver may not be capable of operating in a
 secure mode.  For example, if a security-aware resolver's packets are
 routed through a network address translation (NAT) device that
 includes a DNS proxy that is not security-aware, the security-aware
 resolver may find it difficult or impossible to obtain or validate
 signed DNS data.  The security-aware resolver may have a particularly
 difficult time obtaining DS RRs in such a case, as DS RRs do not
 follow the usual DNS rules for ownership of RRs at zone cuts.  Note
 that this problem is not specific to NATs: any security-oblivious DNS
 software of any kind between the security-aware resolver and the
 authoritative name servers will interfere with DNSSEC.
 If a security-aware resolver must rely on an unsigned zone or a name
 server that is not security aware, the resolver may not be able to
 validate DNS responses and will need a local policy on whether to
 accept unverified responses.
 A security-aware resolver should take a signature's validation period
 into consideration when determining the TTL of data in its cache, to
 avoid caching signed data beyond the validity period of the
 signature.  However, it should also allow for the possibility that
 the security-aware resolver's own clock is wrong.  Thus, a
 security-aware resolver that is part of a security-aware recursive
 name server will have to pay careful attention to the DNSSEC
 "checking disabled" (CD) bit ([RFC4034]).  This is in order to avoid
 blocking valid signatures from getting through to other
 security-aware resolvers that are clients of this recursive name
 server.  See [RFC4035] for how a secure recursive server handles
 queries with the CD bit set.

7. Stub Resolver Considerations

 Although not strictly required to do so by the protocol, most DNS
 queries originate from stub resolvers.  Stub resolvers, by
 definition, are minimal DNS resolvers that use recursive query mode
 to offload most of the work of DNS resolution to a recursive name
 server.  Given the widespread use of stub resolvers, the DNSSEC

Arends, et al. Standards Track [Page 11] RFC 4033 DNS Security Introduction and Requirements March 2005

 architecture has to take stub resolvers into account, but the
 security features needed in a stub resolver differ in some respects
 from those needed in a security-aware iterative resolver.
 Even a security-oblivious stub resolver may benefit from DNSSEC if
 the recursive name servers it uses are security-aware, but for the
 stub resolver to place any real reliance on DNSSEC services, the stub
 resolver must trust both the recursive name servers in question and
 the communication channels between itself and those name servers.
 The first of these issues is a local policy issue: in essence, a
 security-oblivious stub resolver has no choice but to place itself at
 the mercy of the recursive name servers that it uses, as it does not
 perform DNSSEC validity checks on its own.  The second issue requires
 some kind of channel security mechanism; proper use of DNS
 transaction authentication mechanisms such as SIG(0) ([RFC2931]) or
 TSIG ([RFC2845]) would suffice, as would appropriate use of IPsec.
 Particular implementations may have other choices available, such as
 operating system specific interprocess communication mechanisms.
 Confidentiality is not needed for this channel, but data integrity
 and message authentication are.
 A security-aware stub resolver that does trust both its recursive
 name servers and its communication channel to them may choose to
 examine the setting of the Authenticated Data (AD) bit in the message
 header of the response messages it receives.  The stub resolver can
 use this flag bit as a hint to find out whether the recursive name
 server was able to validate signatures for all of the data in the
 Answer and Authority sections of the response.
 There is one more step that a security-aware stub resolver can take
 if, for whatever reason, it is not able to establish a useful trust
 relationship with the recursive name servers that it uses: it can
 perform its own signature validation by setting the Checking Disabled
 (CD) bit in its query messages.  A validating stub resolver is thus
 able to treat the DNSSEC signatures as trust relationships between
 the zone administrators and the stub resolver itself.

8. Zone Considerations

 There are several differences between signed and unsigned zones.  A
 signed zone will contain additional security-related records (RRSIG,
 DNSKEY, DS, and NSEC records).  RRSIG and NSEC records may be
 generated by a signing process prior to serving the zone.  The RRSIG
 records that accompany zone data have defined inception and
 expiration times that establish a validity period for the signatures
 and the zone data the signatures cover.

Arends, et al. Standards Track [Page 12] RFC 4033 DNS Security Introduction and Requirements March 2005

8.1. TTL Values vs. RRSIG Validity Period

 It is important to note the distinction between a RRset's TTL value
 and the signature validity period specified by the RRSIG RR covering
 that RRset.  DNSSEC does not change the definition or function of the
 TTL value, which is intended to maintain database coherency in
 caches.  A caching resolver purges RRsets from its cache no later
 than the end of the time period specified by the TTL fields of those
 RRsets, regardless of whether the resolver is security-aware.
 The inception and expiration fields in the RRSIG RR ([RFC4034]), on
 the other hand, specify the time period during which the signature
 can be used to validate the covered RRset.  The signatures associated
 with signed zone data are only valid for the time period specified by
 these fields in the RRSIG RRs in question.  TTL values cannot extend
 the validity period of signed RRsets in a resolver's cache, but the
 resolver may use the time remaining before expiration of the
 signature validity period of a signed RRset as an upper bound for the
 TTL of the signed RRset and its associated RRSIG RR in the resolver's
 cache.

8.2. New Temporal Dependency Issues for Zones

 Information in a signed zone has a temporal dependency that did not
 exist in the original DNS protocol.  A signed zone requires regular
 maintenance to ensure that each RRset in the zone has a current valid
 RRSIG RR.  The signature validity period of an RRSIG RR is an
 interval during which the signature for one particular signed RRset
 can be considered valid, and the signatures of different RRsets in a
 zone may expire at different times.  Re-signing one or more RRsets in
 a zone will change one or more RRSIG RRs, which will in turn require
 incrementing the zone's SOA serial number to indicate that a zone
 change has occurred and re-signing the SOA RRset itself.  Thus,
 re-signing any RRset in a zone may also trigger DNS NOTIFY messages
 and zone transfer operations.

9. Name Server Considerations

 A security-aware name server should include the appropriate DNSSEC
 records (RRSIG, DNSKEY, DS, and NSEC) in all responses to queries
 from resolvers that have signaled their willingness to receive such
 records via use of the DO bit in the EDNS header, subject to message
 size limitations.  Because inclusion of these DNSSEC RRs could easily
 cause UDP message truncation and fallback to TCP, a security-aware
 name server must also support the EDNS "sender's UDP payload"
 mechanism.

Arends, et al. Standards Track [Page 13] RFC 4033 DNS Security Introduction and Requirements March 2005

 If possible, the private half of each DNSSEC key pair should be kept
 offline, but this will not be possible for a zone for which DNS
 dynamic update has been enabled.  In the dynamic update case, the
 primary master server for the zone will have to re-sign the zone when
 it is updated, so the private key corresponding to the zone signing
 key will have to be kept online.  This is an example of a situation
 in which the ability to separate the zone's DNSKEY RRset into zone
 signing key(s) and key signing key(s) may be useful, as the key
 signing key(s) in such a case can still be kept offline and may have
 a longer useful lifetime than the zone signing key(s).
 By itself, DNSSEC is not enough to protect the integrity of an entire
 zone during zone transfer operations, as even a signed zone contains
 some unsigned, nonauthoritative data if the zone has any children.
 Therefore, zone maintenance operations will require some additional
 mechanisms (most likely some form of channel security, such as TSIG,
 SIG(0), or IPsec).

10. DNS Security Document Family

 The DNSSEC document set can be partitioned into several main groups,
 under the larger umbrella of the DNS base protocol documents.
 The "DNSSEC protocol document set" refers to the three documents that
 form the core of the DNS security extensions:
 1.  DNS Security Introduction and Requirements (this document)
 2.  Resource Records for DNS Security Extensions [RFC4034]
 3.  Protocol Modifications for the DNS Security Extensions [RFC4035]
 Additionally, any document that would add to or change the core DNS
 Security extensions would fall into this category.  This includes any
 future work on the communication between security-aware stub
 resolvers and upstream security-aware recursive name servers.
 The "Digital Signature Algorithm Specification" document set refers
 to the group of documents that describe how specific digital
 signature algorithms should be implemented to fit the DNSSEC resource
 record format.  Each document in this set deals with a specific
 digital signature algorithm.  Please see the appendix on "DNSSEC
 Algorithm and Digest Types" in [RFC4034] for a list of the algorithms
 that were defined when this core specification was written.
 The "Transaction Authentication Protocol" document set refers to the
 group of documents that deal with DNS message authentication,
 including secret key establishment and verification.  Although not

Arends, et al. Standards Track [Page 14] RFC 4033 DNS Security Introduction and Requirements March 2005

 strictly part of the DNSSEC specification as defined in this set of
 documents, this group is noted because of its relationship to DNSSEC.
 The final document set, "New Security Uses", refers to documents that
 seek to use proposed DNS Security extensions for other security
 related purposes.  DNSSEC does not provide any direct security for
 these new uses but may be used to support them.  Documents that fall
 in this category include those describing the use of DNS in the
 storage and distribution of certificates ([RFC2538]).

11. IANA Considerations

 This overview document introduces no new IANA considerations.  Please
 see [RFC4034] for a complete review of the IANA considerations
 introduced by DNSSEC.

12. Security Considerations

 This document introduces DNS security extensions and describes the
 document set that contains the new security records and DNS protocol
 modifications.  The extensions provide data origin authentication and
 data integrity using digital signatures over resource record sets.
 This section discusses the limitations of these extensions.
 In order for a security-aware resolver to validate a DNS response,
 all zones along the path from the trusted starting point to the zone
 containing the response zones must be signed, and all name servers
 and resolvers involved in the resolution process must be
 security-aware, as defined in this document set.  A security-aware
 resolver cannot verify responses originating from an unsigned zone,
 from a zone not served by a security-aware name server, or for any
 DNS data that the resolver is only able to obtain through a recursive
 name server that is not security-aware.  If there is a break in the
 authentication chain such that a security-aware resolver cannot
 obtain and validate the authentication keys it needs, then the
 security-aware resolver cannot validate the affected DNS data.
 This document briefly discusses other methods of adding security to a
 DNS query, such as using a channel secured by IPsec or using a DNS
 transaction authentication mechanism such as TSIG ([RFC2845]) or
 SIG(0) ([RFC2931]), but transaction security is not part of DNSSEC
 per se.
 A non-validating security-aware stub resolver, by definition, does
 not perform DNSSEC signature validation on its own and thus is
 vulnerable both to attacks on (and by) the security-aware recursive
 name servers that perform these checks on its behalf and to attacks
 on its communication with those security-aware recursive name

Arends, et al. Standards Track [Page 15] RFC 4033 DNS Security Introduction and Requirements March 2005

 servers.  Non-validating security-aware stub resolvers should use
 some form of channel security to defend against the latter threat.
 The only known defense against the former threat would be for the
 security-aware stub resolver to perform its own signature validation,
 at which point, again by definition, it would no longer be a
 non-validating security-aware stub resolver.
 DNSSEC does not protect against denial of service attacks.  DNSSEC
 makes DNS vulnerable to a new class of denial of service attacks
 based on cryptographic operations against security-aware resolvers
 and security-aware name servers, as an attacker can attempt to use
 DNSSEC mechanisms to consume a victim's resources.  This class of
 attacks takes at least two forms.  An attacker may be able to consume
 resources in a security-aware resolver's signature validation code by
 tampering with RRSIG RRs in response messages or by constructing
 needlessly complex signature chains.  An attacker may also be able to
 consume resources in a security-aware name server that supports DNS
 dynamic update, by sending a stream of update messages that force the
 security-aware name server to re-sign some RRsets in the zone more
 frequently than would otherwise be necessary.
 Due to a deliberate design choice, DNSSEC does not provide
 confidentiality.
 DNSSEC introduces the ability for a hostile party to enumerate all
 the names in a zone by following the NSEC chain.  NSEC RRs assert
 which names do not exist in a zone by linking from existing name to
 existing name along a canonical ordering of all the names within a
 zone.  Thus, an attacker can query these NSEC RRs in sequence to
 obtain all the names in a zone.  Although this is not an attack on
 the DNS itself, it could allow an attacker to map network hosts or
 other resources by enumerating the contents of a zone.
 DNSSEC introduces significant additional complexity to the DNS and
 thus introduces many new opportunities for implementation bugs and
 misconfigured zones.  In particular, enabling DNSSEC signature
 validation in a resolver may cause entire legitimate zones to become
 effectively unreachable due to DNSSEC configuration errors or bugs.
 DNSSEC does not protect against tampering with unsigned zone data.
 Non-authoritative data at zone cuts (glue and NS RRs in the parent
 zone) are not signed.  This does not pose a problem when validating
 the authentication chain, but it does mean that the non-authoritative
 data itself is vulnerable to tampering during zone transfer
 operations.  Thus, while DNSSEC can provide data origin
 authentication and data integrity for RRsets, it cannot do so for
 zones, and other mechanisms (such as TSIG, SIG(0), or IPsec) must be
 used to protect zone transfer operations.

Arends, et al. Standards Track [Page 16] RFC 4033 DNS Security Introduction and Requirements March 2005

 Please see [RFC4034] and [RFC4035] for additional security
 considerations.

13. Acknowledgements

 This document was created from the input and ideas of the members of
 the DNS Extensions Working Group.  Although explicitly listing
 everyone who has contributed during the decade in which DNSSEC has
 been under development would be impossible, the editors would
 particularly like to thank the following people for their
 contributions to and comments on this document set: Jaap Akkerhuis,
 Mark Andrews, Derek Atkins, Roy Badami, Alan Barrett, Dan Bernstein,
 David Blacka, Len Budney, Randy Bush, Francis Dupont, Donald
 Eastlake, Robert Elz, Miek Gieben, Michael Graff, Olafur Gudmundsson,
 Gilles Guette, Andreas Gustafsson, Jun-ichiro Itojun Hagino, Phillip
 Hallam-Baker, Bob Halley, Ted Hardie, Walter Howard, Greg Hudson,
 Christian Huitema, Johan Ihren, Stephen Jacob, Jelte Jansen, Simon
 Josefsson, Andris Kalnozols, Peter Koch, Olaf Kolkman, Mark Kosters,
 Suresh Krishnaswamy, Ben Laurie, David Lawrence, Ted Lemon, Ed Lewis,
 Ted Lindgreen, Josh Littlefield, Rip Loomis, Bill Manning, Russ
 Mundy, Thomas Narten, Mans Nilsson, Masataka Ohta, Mike Patton, Rob
 Payne, Jim Reid, Michael Richardson, Erik Rozendaal, Marcos Sanz,
 Pekka Savola, Jakob Schlyter, Mike StJohns, Paul Vixie, Sam Weiler,
 Brian Wellington, and Suzanne Woolf.
 No doubt the above list is incomplete.  We apologize to anyone we
 left out.

14. References

14.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC2535]  Eastlake 3rd, D., "Domain Name System Security
            Extensions", RFC 2535, March 1999.
 [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
            2671, August 1999.
 [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
            3225, December 2001.

Arends, et al. Standards Track [Page 17] RFC 4033 DNS Security Introduction and Requirements March 2005

 [RFC3226]  Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
            message size requirements", RFC 3226, December 2001.
 [RFC3445]  Massey, D. and S. Rose, "Limiting the Scope of the KEY
            Resource Record (RR)", RFC 3445, December 2002.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for DNS Security Extensions", RFC
            4034, March 2005.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, March 2005.

14.2. Informative References

 [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
            "Dynamic Updates in the Domain Name System (DNS UPDATE)",
            RFC 2136, April 1997.
 [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
            Specification", RFC 2181, July 1997.
 [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
            NCACHE)", RFC 2308, March 1998.
 [RFC2538]  Eastlake 3rd, D. and O. Gudmundsson, "Storing Certificates
            in the Domain Name System (DNS)", RFC 2538, March 1999.
 [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
            Wellington, "Secret Key Transaction Authentication for DNS
            (TSIG)", RFC 2845, May 2000.
 [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
            ( SIG(0)s )", RFC 2931, September 2000.
 [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
            Update", RFC 3007, November 2000.
 [RFC3008]  Wellington, B., "Domain Name System Security (DNSSEC)
            Signing Authority", RFC 3008, November 2000.
 [RFC3090]  Lewis, E., "DNS Security Extension Clarification on Zone
            Status", RFC 3090, March 2001.
 [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
            (RR) Types", RFC 3597, September 2003.

Arends, et al. Standards Track [Page 18] RFC 4033 DNS Security Introduction and Requirements March 2005

 [RFC3655]  Wellington, B. and O. Gudmundsson, "Redefinition of DNS
            Authenticated Data (AD) bit", RFC 3655, November 2003.
 [RFC3658]  Gudmundsson, O., "Delegation Signer (DS) Resource Record
            (RR)", RFC 3658, December 2003.
 [RFC3755]  Weiler, S., "Legacy Resolver Compatibility for Delegation
            Signer (DS)", RFC 3755, May 2004.
 [RFC3757]  Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
            System KEY (DNSKEY) Resource Record (RR) Secure Entry
            Point (SEP) Flag", RFC 3757, April 2004.
 [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
            Name System (DNS)", RFC 3833, August 2004.
 [RFC3845]  Schlyter, J., "DNS Security (DNSSEC) NextSECure (NSEC)
            RDATA Format", RFC 3845, August 2004.

Arends, et al. Standards Track [Page 19] RFC 4033 DNS Security Introduction and Requirements March 2005

Authors' Addresses

 Roy Arends
 Telematica Instituut
 Brouwerijstraat 1
 7523 XC  Enschede
 NL
 EMail: roy.arends@telin.nl
 Rob Austein
 Internet Systems Consortium
 950 Charter Street
 Redwood City, CA  94063
 USA
 EMail: sra@isc.org
 Matt Larson
 VeriSign, Inc.
 21345 Ridgetop Circle
 Dulles, VA  20166-6503
 USA
 EMail: mlarson@verisign.com
 Dan Massey
 Colorado State University
 Department of Computer Science
 Fort Collins, CO 80523-1873
 EMail: massey@cs.colostate.edu
 Scott Rose
 National Institute for Standards and Technology
 100 Bureau Drive
 Gaithersburg, MD  20899-8920
 USA
 EMail: scott.rose@nist.gov

Arends, et al. Standards Track [Page 20] RFC 4033 DNS Security Introduction and Requirements March 2005

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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 Internet Society.

Arends, et al. Standards Track [Page 21]

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