| Internet Engineering Task Force | P. M. Hallam-Baker |
| Internet-Draft | Comodo Group Inc. |
| Intended status: Standards Track | R. N. Stradling |
| Comodo CA Ltd. | |
| B. Laurie | |
| Google Inc. | |
| Sept 7, 2011 |
DNS Certification Authority Authorization (CAA) Resource Record
draft-ietf-pkix-caa-02
The Certification Authority Authorization (CAA) DNS Resource Record allows a DNS domain name holder to specify the certificate signing certificate(s) authorized to issue certificates for that domain. CAA resource records allow a public Certification Authority to implement additional controls to reduce the risk of unintended certificate mis-issue.
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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 RFC 2119 [RFC2119].
The following terms are used in this document:
The Certification Authority Authorization (CAA) DNS Resource Record allows a DNS domain name holder to specify the Certification Authorities authorized to issue certificates for that domain. Publication of CAA resource records allow a public Certification Authority (CA) to implement additional controls to reduce the risk of unintended certificate mis-issue.
Conformance with a published CAA record is a necessary but not sufficient condition for issue of a certificate. Before issuing a certificate, a PKIX CA is required to validate the request according to the policies set out in its Certificate Policy Statement. In the case of a public CA that validates certificate requests as a third party, the certificate will be typically issued under a public root certificate embedded in one or more relevant Relying Applications.
Criteria for inclusion of embedded root certificates in applications are outside the scope of this document but typically require the CA to publish a Certificate Practices Statement (CPS) that specifies how the requirements of the Certificate Policy (CP) are achieved and provide an annual audit statement of their performance against their CPS performed by an independent third party auditor.
It is the intention of the authors to propose the CAA record defined in this document as the basis for CA validation requirements to be proposed in organizations that publish validation requirements.
CAA records only describe the current state of Certification Authority certificate issue authority. Since a certificate is typically valid for at least a year, it is possible that a certificate that is not conformant with the CAA records currently published was conformant with the CAA records published at the time that it was issued. Thus Relying Applications MUST NOT use failure to conform to currently published CAA records specifying issue authorization as a certificate validity criteria.
A CAA RR publishes a CAA property entry that corresponds to the specified domain name. Multiple property entries MAY be associated with the same domain name by publishing multiple CAA RRs at that domain name. Each property entry MAY be tagged with one or more of the following flag values:
The following properties are defined:
The following example informs CAs that certificates must not be issued except by the holder of the domain name 'ca.example.net' or an authorized agent thereof. Since the policy is published at the Public Delegation Point, the policy applies to all subordinate domains under example.com.
$ORIGIN example.com
. CAA 1 issue "ca.example.net"
A certificate issuer MAY specify additional parameters that allow customers to specify additional parameters governing certificate issue. For example, the Certification Policy under which the certificate is to be issued or the authentication process to be used.
$ORIGIN example.com
. CAA 1 issue "ca.example.net; account=230123"
The syntax and semantics of such parameters is left to site policy and is outside the scope of this document.
Future versions of this specification MAY use the critical flag to introduce new semantics that MUST be understood for correct processing of the record, preventing Certification Authorities that do not recognize the record from issuing certificates.
In the following example, the property 'tbs' is flagged as critical. The example.net CA is not authorized to issue under either policy unless the processing rules for the 'tbs' property tag are understood.
$ORIGIN example.com
. CAA 1 issue "ca.example.net; policy=ev"
. CAA 129 tbs "Unknown"
Note that the above restrictions only apply to issue of certificates. Since the validity of an end entity certificate is typically a year or more it is quite possible that the CAA records published at a domain will change between the issue of the certificate and verification by a relying party.
Before issue of a certificate, a compliant CA MUST check for publication of a relevant CAA Resource Record(s) and if such record(s) are published, that the certificate requested is consistent with them. If the certificate requested is not consistent with the relevant CAA RRs, the CA MUST NOT issue the certificate.
The Issuer Authorization Set for a domain name consists of the set of all CAA Authorization Entries declared for the canonical form of the specified domain.
The Extended Issuer Authorization Set for a domain name consists of the Issuer Authorization Set for that domain name if it is non-empty. Otherwise the Extended Issuer Authorization Set for a domain name consists of the Issuer Authorization Set for the corresponding Public Delegation Point for that domain name.
If the Extended Issuer Authorization Set for a domain name is not empty, a Certification Authority MUST NOT issue a certificate unless it conforms to at least one authorization entry in the Extended Issuer Authorization Set.
The DNS defines the CNAME and DNAME mechanisms for specifying domain name aliases. The canonical name of a DNS name is the name that results from performing all DNS alias operations.
A Certification Authority MUST perform CNAME and DNAME processing as defined in the DNS specifications 1035 [RFC1035] to resolve CAA records.
Use of DNSSEC to authenticate CAA RRs is strongly recommended but not required. A CA MUST NOT issue certificates if doing so would conflict with the corresponding extended issuer authorization set whether the corresponding DNS records are signed or not.
Use of DNSSEC allows a CA to acquire and archive a non-repudiable proof that they were authorized to issue certificates for the domain.
A compliant CA SHOULD maintain an archive of the DNS transactions used to verify CAA eligibility.
In particular a CA SHOULD ensure that where DNSSEC data is available that the corresponding signature and NSEC/NSEC3 records are preserved so as to enable later compliance audits.
A CAA RR contains a single property entry consisting of a tag value pair. Each tag represents a property of the CAA record. The value of a CAA property is that specified in the corresponding value field.
A domain name MAY have multiple CAA RRs associated with it and a given property MAY be specified more than once.
The CAA data field contains one property entry. A property entry consists of the following data fields:
+0-1-2-3-4-5-6-7-|0-1-2-3-4-5-6-7-|
| Flags | Tag Length = n |
+----------------+----------------+...+---------------+
| Tag char 0 | Tag Char 1 |...| Tag Char n-1 |
+----------------+----------------+...+---------------+
+----------------+----------------+.....+---------------+
| Data byte 0 | Data byte 1 |.....| Data byte m-1 |
+----------------+----------------+.....+---------------+
Where n is the length specified in the tag length field and m is the remaining octets in the data field (m = d - n - 2) where d is the length of the data section.
The data fields are defined as follows:
The canonical presentation format of the CAA record is as follows:
CAA <flags> <tag> <data>
Where:
[TBS]
CAA Records provide an accountability control. They are intended to deter rather than prevent undesired behavior.
While a Certification Authority can choose to ignore published CAA records, doing so increases the both the probability that they will mis-issue a certificate and the consequences of doing so. Once it is known that a CA observes CAA records, malicious registration requests will disproportionately target the negligent CAs that do not, and so the mis-issue rate amongst the negligent CAs will increase. Since the CA could clearly have avoided the mis-issue by performing CAA processing, the likelihood of sanctions against the negligent CA is increased. Failure to observe CAA issue restrictions provides an objective criteria for excluding issuers from embedded roots of trust.
In contrast, a Certification Authority that processes CAA records correctly can reasonably claim that any residual mis-issue event could have been avoided had the Domain Name holder published appropriate CAA records.
Use of CAA records does not provide protection against mis-issue by an authorized Certification Authority.
Domain name holders SHOULD ensure that the CAs they authorize to issue certificates for their domains employ appropriate controls to ensure that certificates are only issued to authorized parties within their organization.
Such controls are most appropriately determined by the domain name holder and the authorized CA(s) directly and are thus out of scope of this document.
Suppression of the CAA record or insertion of a bogus CAA record could enable an attacker to obtain a certificate from a CA that was not authorized to issue for that domain name.
Applications performing CAA checking SHOULD mitigate the risk of suppresion or spoofing of CAA records by means of DNSSEC validation where present. In cases where DNSSEC validation is not available, CAA checking is of limited security value.
Since a certificate issued by a CA can be valid for several years, the consequences of a spoofing or suppression attack are much greater for Certification Authorities and so additional countermeasures are justified.
A CA MUST mitigate this risk by employing DNSSEC verification whenever possible and rejecting certificate requests in any case where it is not possible to verify the non-existence or contents of a relevant CAA record.
In cases where DNSSEC is not deployed in a corresponding domain, a CA SHOULD attempt to mitigate this risk by employing appropriate DNS security controls. For example all portions of the DNS lookup process SHOULD be performed against the authoritative name server. Cached data MUST NOT be relied on but MAY be used to support additional anti-spoofing or anti-suppression controls.
Introduction of a malformed or malicious CAA RR could in theory enable a Denial of Service attack.
This specific threat is not considered to add significantly to the risk of running an insecure DNS service.
A Certification Authority could make use of the critical flag to trick customers into publishing records which prevent competing Certification Authorities from issuing certificates even though the customer intends to authorize multiple providers.
In practice, such an attack would be of minimal effect since any competent competitor that found itself unable to issue certificates due to lack of support for a property marked critical is going to investigate the cause and report the reason to the customer who was deceived. It is thus unlikely that the attack would succeed and the attempt might lay the perpetrator open to civil or criminal sanctions.
IANA has assigned Resource Record Type 257 for the CAA Resource Record Type and added the line depicted below to the registry named Resource Record (RR) TYPEs and QTYPEs as defined in BCP 42 RFC 5395 [RFC5395] and located at http://www.iana.org/assignments/dns-parameters.
Value and meaning Reference
----------- --------------------------------------------- ---------
CAA 257 Certification Authority Restriction [RFC-THIS]
IANA has created the Certification Authority Authorization Properties registry with the following initial values:
Meaning Reference
----------- ----------------------------------------------- ---------
issue Authorization Entry by Domain [RFC-THIS]
auth Reserved
path Reserved
policy Reserved
Addition of tag identifiers requires a public specification and expert review as set out in RFC5395 [RFC5395]