To use DNSSEC RRs for authentication, a security-aware resolver
requires configured knowledge of at least one authenticated DNSKEY or
DS RR. The process for obtaining and authenticating this initial
trust anchor is achieved via some external mechanism. For example, a
resolver could use some off-line authenticated exchange to obtain a
zone's DNSKEY RR or to obtain a DS RR that identifies and
authenticates a zone's DNSKEY RR. The remainder of this section
assumes that the resolver has somehow obtained an initial set of
trust anchors.
An initial DNSKEY RR can be used to authenticate a zone's apex DNSKEY
RRset. To authenticate an apex DNSKEY RRset by using an initial key,
the resolver MUST:
1. verify that the initial DNSKEY RR appears in the apex DNSKEY
RRset, and that the DNSKEY RR has the Zone Key Flag (DNSKEY RDATA
bit 7) set; and
2. verify that there is some RRSIG RR that covers the apex DNSKEY
RRset, and that the combination of the RRSIG RR and the initial
DNSKEY RR authenticates the DNSKEY RRset. The process for using
an RRSIG RR to authenticate an RRset is described in Section 5.3.
Once the resolver has authenticated the apex DNSKEY RRset by using an
initial DNSKEY RR, delegations from that zone can be authenticated by
using DS RRs. This allows a resolver to start from an initial key
and use DS RRsets to proceed recursively down the DNS tree, obtaining
other apex DNSKEY RRsets. If the resolver were configured with a
root DNSKEY RR, and if every delegation had a DS RR associated with
it, then the resolver could obtain and validate any apex DNSKEY
RRset. The process of using DS RRs to authenticate referrals is
described in Section 5.2.
Section 5.3 shows how the resolver can use DNSKEY RRs in the apex
DNSKEY RRset and RRSIG RRs from the zone to authenticate any other
RRsets in the zone once the resolver has authenticated a zone's apex
DNSKEY RRset. Section 5.4 shows how the resolver can use
authenticated NSEC RRsets from the zone to prove that an RRset is not
present in the zone.
When a resolver indicates support for DNSSEC (by setting the DO bit),
a security-aware name server should attempt to provide the necessary
DNSKEY, RRSIG, NSEC, and DS RRsets in a response (see Section 3).
However, a security-aware resolver may still receive a response that
lacks the appropriate DNSSEC RRs, whether due to configuration issues
such as an upstream security-oblivious recursive name server that
accidentally interferes with DNSSEC RRs or due to a deliberate attack
in which an adversary forges a response, strips DNSSEC RRs from a
response, or modifies a query so that DNSSEC RRs appear not to be
requested. The absence of DNSSEC data in a response MUST NOT by
itself be taken as an indication that no authentication information
exists.
A resolver SHOULD expect authentication information from signed
zones. A resolver SHOULD believe that a zone is signed if the
resolver has been configured with public key information for the
zone, or if the zone's parent is signed and the delegation from the
parent contains a DS RRset.
5.1. Special Considerations for Islands of Security
Islands of security (see [RFC4033]) are signed zones for which it is
not possible to construct an authentication chain to the zone from
its parent. Validating signatures within an island of security
requires that the validator have some other means of obtaining an
initial authenticated zone key for the island. If a validator cannot
obtain such a key, it SHOULD switch to operating as if the zones in
the island of security are unsigned.
All the normal processes for validating responses apply to islands of
security. The only difference between normal validation and
validation within an island of security is in how the validator
obtains a trust anchor for the authentication chain.
5.2. Authenticating Referrals
Once the apex DNSKEY RRset for a signed parent zone has been
authenticated, DS RRsets can be used to authenticate the delegation
to a signed child zone. A DS RR identifies a DNSKEY RR in the child
zone's apex DNSKEY RRset and contains a cryptographic digest of the
child zone's DNSKEY RR. Use of a strong cryptographic digest
algorithm ensures that it is computationally infeasible for an
adversary to generate a DNSKEY RR that matches the digest. Thus,
authenticating the digest allows a resolver to authenticate the
matching DNSKEY RR. The resolver can then use this child DNSKEY RR
to authenticate the entire child apex DNSKEY RRset.
Given a DS RR for a delegation, the child zone's apex DNSKEY RRset
can be authenticated if all of the following hold:
o The DS RR has been authenticated using some DNSKEY RR in the
parent's apex DNSKEY RRset (see Section 5.3).
o The Algorithm and Key Tag in the DS RR match the Algorithm field
and the key tag of a DNSKEY RR in the child zone's apex DNSKEY
RRset, and, when the DNSKEY RR's owner name and RDATA are hashed
using the digest algorithm specified in the DS RR's Digest Type
field, the resulting digest value matches the Digest field of the
DS RR.
o The matching DNSKEY RR in the child zone has the Zone Flag bit
set, the corresponding private key has signed the child zone's
apex DNSKEY RRset, and the resulting RRSIG RR authenticates the
child zone's apex DNSKEY RRset.
If the referral from the parent zone did not contain a DS RRset, the
response should have included a signed NSEC RRset proving that no DS
RRset exists for the delegated name (see Section 3.1.4). A
security-aware resolver MUST query the name servers for the parent
zone for the DS RRset if the referral includes neither a DS RRset nor
a NSEC RRset proving that the DS RRset does not exist (see Section
4).
If the validator authenticates an NSEC RRset that proves that no DS
RRset is present for this zone, then there is no authentication path
leading from the parent to the child. If the resolver has an initial
DNSKEY or DS RR that belongs to the child zone or to any delegation
below the child zone, this initial DNSKEY or DS RR MAY be used to
re-establish an authentication path. If no such initial DNSKEY or DS
RR exists, the validator cannot authenticate RRsets in or below the
child zone.
If the validator does not support any of the algorithms listed in an
authenticated DS RRset, then the resolver has no supported
authentication path leading from the parent to the child. The
resolver should treat this case as it would the case of an
authenticated NSEC RRset proving that no DS RRset exists, as
described above.
Note that, for a signed delegation, there are two NSEC RRs associated
with the delegated name. One NSEC RR resides in the parent zone and
can be used to prove whether a DS RRset exists for the delegated
name. The second NSEC RR resides in the child zone and identifies
which RRsets are present at the apex of the child zone. The parent
NSEC RR and child NSEC RR can always be distinguished because the SOA
bit will be set in the child NSEC RR and clear in the parent NSEC RR.
A security-aware resolver MUST use the parent NSEC RR when attempting
to prove that a DS RRset does not exist.
If the resolver does not support any of the algorithms listed in an
authenticated DS RRset, then the resolver will not be able to verify
the authentication path to the child zone. In this case, the
resolver SHOULD treat the child zone as if it were unsigned.
5.3. Authenticating an RRset with an RRSIG RR
A validator can use an RRSIG RR and its corresponding DNSKEY RR to
attempt to authenticate RRsets. The validator first checks the RRSIG
RR to verify that it covers the RRset, has a valid time interval, and
identifies a valid DNSKEY RR. The validator then constructs the
canonical form of the signed data by appending the RRSIG RDATA
(excluding the Signature Field) with the canonical form of the
covered RRset. Finally, the validator uses the public key and
signature to authenticate the signed data. Sections 5.3.1, 5.3.2,
and 5.3.3 describe each step in detail.
A security-aware resolver can use an RRSIG RR to authenticate an
RRset if all of the following conditions hold:
o The RRSIG RR and the RRset MUST have the same owner name and the
same class.
o The RRSIG RR's Signer's Name field MUST be the name of the zone
that contains the RRset.
o The RRSIG RR's Type Covered field MUST equal the RRset's type.
o The number of labels in the RRset owner name MUST be greater than
or equal to the value in the RRSIG RR's Labels field.
o The validator's notion of the current time MUST be less than or
equal to the time listed in the RRSIG RR's Expiration field.
o The validator's notion of the current time MUST be greater than or
equal to the time listed in the RRSIG RR's Inception field.
o The RRSIG RR's Signer's Name, Algorithm, and Key Tag fields MUST
match the owner name, algorithm, and key tag for some DNSKEY RR in
the zone's apex DNSKEY RRset.
o The matching DNSKEY RR MUST be present in the zone's apex DNSKEY
RRset, and MUST have the Zone Flag bit (DNSKEY RDATA Flag bit 7)
set.
It is possible for more than one DNSKEY RR to match the conditions
above. In this case, the validator cannot predetermine which DNSKEY
RR to use to authenticate the signature, and it MUST try each
matching DNSKEY RR until either the signature is validated or the
validator has run out of matching public keys to try.
Note that this authentication process is only meaningful if the
validator authenticates the DNSKEY RR before using it to validate
signatures. The matching DNSKEY RR is considered to be authentic if:
o the apex DNSKEY RRset containing the DNSKEY RR is considered
authentic; or
o the RRset covered by the RRSIG RR is the apex DNSKEY RRset itself,
and the DNSKEY RR either matches an authenticated DS RR from the
parent zone or matches a trust anchor.
5.3.2. Reconstructing the Signed Data
Once the RRSIG RR has met the validity requirements described in
Section 5.3.1, the validator has to reconstruct the original signed
data. The original signed data includes RRSIG RDATA (excluding the
Signature field) and the canonical form of the RRset. Aside from
being ordered, the canonical form of the RRset might also differ from
the received RRset due to DNS name compression, decremented TTLs, or
wildcard expansion. The validator should use the following to
reconstruct the original signed data:
signed_data = RRSIG_RDATA | RR(1) | RR(2)... where
"|" denotes concatenation
RRSIG_RDATA is the wire format of the RRSIG RDATA fields
with the Signature field excluded and the Signer's Name
in canonical form.
RR(i) = name | type | class | OrigTTL | RDATA length | RDATA
name is calculated according to the function below
class is the RRset's class
type is the RRset type and all RRs in the class
OrigTTL is the value from the RRSIG Original TTL field
All names in the RDATA field are in canonical form
The set of all RR(i) is sorted into canonical order.
To calculate the name:
let rrsig_labels = the value of the RRSIG Labels field
let fqdn = RRset's fully qualified domain name in
canonical form
let fqdn_labels = Label count of the fqdn above.
if rrsig_labels = fqdn_labels,
name = fqdn
if rrsig_labels < fqdn_labels,
name = "*." | the rightmost rrsig_label labels of the
fqdn
if rrsig_labels > fqdn_labels
the RRSIG RR did not pass the necessary validation
checks and MUST NOT be used to authenticate this
RRset.
The canonical forms for names and RRsets are defined in [RFC4034].
NSEC RRsets at a delegation boundary require special processing.
There are two distinct NSEC RRsets associated with a signed delegated
name. One NSEC RRset resides in the parent zone, and specifies which
RRsets are present at the parent zone. The second NSEC RRset resides
at the child zone and identifies which RRsets are present at the apex
in the child zone. The parent NSEC RRset and child NSEC RRset can
always be distinguished as only a child NSEC RR will indicate that an
SOA RRset exists at the name. When reconstructing the original NSEC
RRset for the delegation from the parent zone, the NSEC RRs MUST NOT
be combined with NSEC RRs from the child zone. When reconstructing
the original NSEC RRset for the apex of the child zone, the NSEC RRs
MUST NOT be combined with NSEC RRs from the parent zone.
Note that each of the two NSEC RRsets at a delegation point has a
corresponding RRSIG RR with an owner name matching the delegated
name, and each of these RRSIG RRs is authoritative data associated
with the same zone that contains the corresponding NSEC RRset. If
necessary, a resolver can tell these RRSIG RRs apart by checking the
Signer's Name field.
5.3.3. Checking the Signature
Once the resolver has validated the RRSIG RR as described in Section
5.3.1 and reconstructed the original signed data as described in
Section 5.3.2, the validator can attempt to use the cryptographic
signature to authenticate the signed data, and thus (finally!)
authenticate the RRset.
The Algorithm field in the RRSIG RR identifies the cryptographic
algorithm used to generate the signature. The signature itself is
contained in the Signature field of the RRSIG RDATA, and the public
key used to verify the signature is contained in the Public Key field
of the matching DNSKEY RR(s) (found in Section 5.3.1). [RFC4034]
provides a list of algorithm types and provides pointers to the
documents that define each algorithm's use.
Note that it is possible for more than one DNSKEY RR to match the
conditions in Section 5.3.1. In this case, the validator can only
determine which DNSKEY RR is correct by trying each matching public
key until the validator either succeeds in validating the signature
or runs out of keys to try.
If the Labels field of the RRSIG RR is not equal to the number of
labels in the RRset's fully qualified owner name, then the RRset is
either invalid or the result of wildcard expansion. The resolver
MUST verify that wildcard expansion was applied properly before
considering the RRset to be authentic. Section 5.3.4 describes how
to determine whether a wildcard was applied properly.
If other RRSIG RRs also cover this RRset, the local resolver security
policy determines whether the resolver also has to test these RRSIG
RRs and how to resolve conflicts if these RRSIG RRs lead to differing
results.
If the resolver accepts the RRset as authentic, the validator MUST
set the TTL of the RRSIG RR and each RR in the authenticated RRset to
a value no greater than the minimum of:
o the RRset's TTL as received in the response;
o the RRSIG RR's TTL as received in the response;
o the value in the RRSIG RR's Original TTL field; and
o the difference of the RRSIG RR's Signature Expiration time and the
current time.
If the number of labels in an RRset's owner name is greater than the
Labels field of the covering RRSIG RR, then the RRset and its
covering RRSIG RR were created as a result of wildcard expansion.
Once the validator has verified the signature, as described in
Section 5.3, it must take additional steps to verify the non-
existence of an exact match or closer wildcard match for the query.
Section 5.4 discusses these steps.
Note that the response received by the resolver should include all
NSEC RRs needed to authenticate the response (see Section 3.1.3).
A resolver can use authenticated NSEC RRs to prove that an RRset is
not present in a signed zone. Security-aware name servers should
automatically include any necessary NSEC RRs for signed zones in
their responses to security-aware resolvers.
Denial of existence is determined by the following rules:
o If the requested RR name matches the owner name of an
authenticated NSEC RR, then the NSEC RR's type bit map field lists
all RR types present at that owner name, and a resolver can prove
that the requested RR type does not exist by checking for the RR
type in the bit map. If the number of labels in an authenticated
NSEC RR's owner name equals the Labels field of the covering RRSIG
RR, then the existence of the NSEC RR proves that wildcard
expansion could not have been used to match the request.
o If the requested RR name would appear after an authenticated NSEC
RR's owner name and before the name listed in that NSEC RR's Next
Domain Name field according to the canonical DNS name order
defined in [RFC4034], then no RRsets with the requested name exist
in the zone. However, it is possible that a wildcard could be
used to match the requested RR owner name and type, so proving
that the requested RRset does not exist also requires proving that
no possible wildcard RRset exists that could have been used to
generate a positive response.
In addition, security-aware resolvers MUST authenticate the NSEC
RRsets that comprise the non-existence proof as described in Section
5.3.
To prove the non-existence of an RRset, the resolver must be able to
verify both that the queried RRset does not exist and that no
relevant wildcard RRset exists. Proving this may require more than
one NSEC RRset from the zone. If the complete set of necessary NSEC
RRsets is not present in a response (perhaps due to message
truncation), then a security-aware resolver MUST resend the query in
order to attempt to obtain the full collection of NSEC RRs necessary
to verify the non-existence of the requested RRset. As with all DNS
operations, however, the resolver MUST bound the work it puts into
answering any particular query.
Since a validated NSEC RR proves the existence of both itself and its
corresponding RRSIG RR, a validator MUST ignore the settings of the
NSEC and RRSIG bits in an NSEC RR.
If for whatever reason none of the RRSIGs can be validated, the
response SHOULD be considered BAD. If the validation was being done
to service a recursive query, the name server MUST return RCODE 2 to
the originating client. However, it MUST return the full response if
and only if the original query had the CD bit set. Also see Section
4.7 on caching responses that do not validate.
Appendix C shows an example of the authentication process.
