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{{Use American English|date=January 2018}}
{{Use American English|date=January 2018}}
{{Use mdy dates|date=January 2018}}
{{Use mdy dates|date=January 2018}}
'''DNS Certification Authority Authorization''' ('''CAA''') is an [[Internet security]] mechanism which uses [[resource record]]s in the [[Domain Name System]] (DNS) to allow [[domain name]] holders to specify which [[certificate authorities]] are authorized to issue [[Public key certificate|certificates]] for that domain, and which types of certificates they are able to issue.<ref name="rfc6844">{{Cite IETF|title=DNS Certification Authority Authorization (CAA) Resource Record|rfc=6844|last1=Hallam-Baker|first1=Phillip|authorlink1=Phillip Hallam-Baker|last2=Stradling|first2=Rob|date=January 2013|publisher=[[Internet Engineering Task Force|IETF]]|doi=10.17487/RFC6844|issn=2070-1721}}</ref><ref>{{Cite web|url=https://www.websecurity.symantec.com/security-topics/what-is-certificate-authority-authorization|title=What is Certificate Authority Authorization (CAA)?|website=[[Symantec]]|access-date=January 8, 2018}}</ref> In September 2017, the [[CA/Browser Forum]] made it mandatory for publicly trusted [[certificate authorities]] to check a domain's CAA records before issuing a certificate.<ref name="cab2017">{{Cite web|url=https://cabforum.org/pipermail/public/2017-March/009988.html|title=Results on Ballot 187 - Make CAA Checking Mandatory|last=Hall|first=Kirk|date=March 8, 2017|publisher=[[CA/Browser Forum]]|access-date=January 7, 2018}}</ref> CAA is specified by {{IETF RFC|6844}}, a proposed [[Internet Standard]] written by [[Phillip Hallam-Baker]] and Rob Stradling in January 2013, which defines a new "CAA" [[Domain Name System|DNS]] [[resource record]] type containing [[name-value pair]]s of the [[certificate authorities]] and whether they are able to issue [[wildcard certificate]]s or only specific-[[subdomain]] certificates.<ref name="rfc6844" /> Unlike [[DNS-based Authentication of Named Entities]] (DANE), it is a purely administrative control, and does not prevent a malicious [[certificate authority]] from issuing fraudulent certificates. Third parties monitoring [[certificate authority]] behaviour might check newly issued certificates against the domain's CAA records, but must be aware that a domain's CAA records may have changed between the time the certificate was issued and the time the third-party makes checks them.
'''DNS Certification Authority Authorization''' ('''CAA''') is an [[Internet security]] policy mechanism which uses [[resource record]]s in the [[Domain Name System]] (DNS) to allow [[domain name]] holders to specify which [[certificate authorities]] are authorized to issue [[Public key certificate|certificates]] for that domain, and which types of certificates they are able to issue.<ref name="rfc6844">{{Cite IETF|title=DNS Certification Authority Authorization (CAA) Resource Record|rfc=6844|last1=Hallam-Baker|first1=Phillip|authorlink1=Phillip Hallam-Baker|last2=Stradling|first2=Rob|date=January 2013|publisher=[[Internet Engineering Task Force|IETF]]|doi=10.17487/RFC6844|issn=2070-1721}}</ref><ref>{{Cite web|url=https://www.websecurity.symantec.com/security-topics/what-is-certificate-authority-authorization|title=What is Certificate Authority Authorization (CAA)?|website=[[Symantec]]|access-date=January 8, 2018}}</ref> In September 2017, the [[CA/Browser Forum]] made it mandatory for publicly trusted [[certificate authorities]] to check a domain's CAA records before issuing a certificate.<ref name="cab2017">{{Cite web|url=https://cabforum.org/pipermail/public/2017-March/009988.html|title=Results on Ballot 187 - Make CAA Checking Mandatory|last=Hall|first=Kirk|date=March 8, 2017|publisher=[[CA/Browser Forum]]|access-date=January 7, 2018}}</ref> CAA is specified by {{IETF RFC|6844}}, a proposed [[Internet Standard]] written by [[Phillip Hallam-Baker]] and Rob Stradling in January 2013, which defines a new "CAA" [[Domain Name System|DNS]] [[resource record]] type containing [[name-value pair]]s of the [[certificate authorities]] and whether they are able to issue [[wildcard certificate]]s or only specific-[[subdomain]] certificates.<ref name="rfc6844" /> It is a purely administrative control, and does not prevent a malicious [[certificate authority]] from issuing fraudulent certificates. Third parties monitoring [[certificate authority]] behaviour might check newly issued certificates against the domain's CAA records, but must be aware that a domain's CAA records may have changed between the time the certificate was issued and the time the third-party makes checks them.


== Record structure ==
== Record structure ==


Each CAA [[resource record]] contains a [[flag (computing)|flags byte]] and a property. The flags byte contains signals which influence the interpretation of the record. The property consists of a "tag" which allows selection between several kinds of CAA record, and a "value" string whose meaning depends on the choice of tag.<ref name="rfc6844" />
Each CAA [[resource record]] contains a [[flag (computing)|flags byte]] and a property. The property consists of a "tag" which allows selection between several kinds of CAA record, and a "value" [[String (computer science)|string]] whose meaning depends on the choice of tag.<ref name="rfc6844" /> {{As of|2018}}, only one flag is defined, the ''issuer critical'' flag, which is represented by the [[most significant bit]] of the flag's byte. If the flag is set (that is, the flags byte's value is equal to or above 128), this indicates that a CA which does not understand or does not implement the property tag in this record should refuse to issue a certificate for the domain.<ref name="rfc6844" /> This is similar to the way [[X.509#Structure of a certificate|critical extensions in X.509 certificates]] work.

Currently only one flag is defined, the ''issuer critical'' flag, which is represented by the [[most significant bit]] of the flag's byte. If the ''issuer-critical'' flag's value is 1 (that is, a flags byte equal to 128), this indicates that a CA which does not understand or does not implement the property tag in this record should refuse to issue a certificate for the domain.<ref name="rfc6844" /> This is similar to the way [[X.509#Structure of a certificate|critical extensions in X.509 certificates]] work.


Three property tags are defined:<ref name="rfc6844" />
Three property tags are defined:<ref name="rfc6844" />

Revision as of 05:42, 11 January 2018

DNS Certification Authority Authorization (CAA) is an Internet security policy mechanism which uses resource records in the Domain Name System (DNS) to allow domain name holders to specify which certificate authorities are authorized to issue certificates for that domain, and which types of certificates they are able to issue.[1][2] In September 2017, the CA/Browser Forum made it mandatory for publicly trusted certificate authorities to check a domain's CAA records before issuing a certificate.[3] CAA is specified by RFC 6844, a proposed Internet Standard written by Phillip Hallam-Baker and Rob Stradling in January 2013, which defines a new "CAA" DNS resource record type containing name-value pairs of the certificate authorities and whether they are able to issue wildcard certificates or only specific-subdomain certificates.[1] It is a purely administrative control, and does not prevent a malicious certificate authority from issuing fraudulent certificates. Third parties monitoring certificate authority behaviour might check newly issued certificates against the domain's CAA records, but must be aware that a domain's CAA records may have changed between the time the certificate was issued and the time the third-party makes checks them.

Record structure

Each CAA resource record contains a flags byte and a property. The property consists of a "tag" which allows selection between several kinds of CAA record, and a "value" string whose meaning depends on the choice of tag.[1] As of 2018, only one flag is defined, the issuer critical flag, which is represented by the most significant bit of the flag's byte. If the flag is set (that is, the flags byte's value is equal to or above 128), this indicates that a CA which does not understand or does not implement the property tag in this record should refuse to issue a certificate for the domain.[1] This is similar to the way critical extensions in X.509 certificates work.

Three property tags are defined:[1]

issue
This property authorizes the holder of the domain specified in the "value" field to issue certificates for the domain for which the property is published.
issuewild
This property acts like issue but allows wildcard certificates.
iodef
This property specifies a method for CAs to report to the domain holder when a certificate is issued. Not all CAs support this tag, so there is no guarantee that all certificate issuances will be reported.

Support

As of 2018, CAA records are supported by BIND (since version 9.10.1B),[4] Knot DNS (since version 2.2.0),[5][6] ldns (since version 1.6.17),[5] NSD (as of version 4.0.1)[5][7], OpenDNSSEC,[5] PowerDNS (since version 4.0.0),[5] Simple DNS Plus (since version 6.0),[5] tinydns[5] and Windows Server 2016.[5] Many hosted DNS providers also support CAA records, including Amazon Route 53,[8] Cloudflare, DNS Made Easy and Google Cloud DNS.[5]

Mandatory checking

Initially the implementation of CAA was voluntary. However, in March 2017 the CA/Browser Forum voted in favor of making CAA checking mandatory for all certificate authorities by September 2017.[3] At least one certificate authority, Comodo, failed to meet the deadline.[9]

Examples

In the following examples, assume that we want to control certificate issuance for the domain example.com. To signify that only the Let's Encrypt certificate authority can issue certificates to the domain, as well as all of its subdomains, one may use the CAA record: 

example.com.	IN	CAA	0 issue "letsencrypt.org"

To disallow issuance for a specific subdomain, nocerts.example.com, one would allow issuance only to an empty issuer list: 

nocerts.example.com.	IN	CAA	0 issue ";"

In this case, in a request to issue a certificate to nocerts.example.com, the relevant CA will stop its search for CAA records at the subdomain.

To signify that the CA may report certificate issues or policy violations by email to caa@example.com, or through RID messages to caa.example.com, the iodef property may be applied as follows: 

nocerts.example.com.	IN	CAA	0 issue ";"
nocerts.example.com.	IN	CAA	0 iodef "mailto:caa@example.com"
nocerts.example.com.	IN	CAA	0 iodef "https://caa.example.com"

If, in the original example, we would like to allow issuance to specific domains, but disallow issuance of any wildcard certificates, the issuewild property may be applied: 

example.com.	IN	CAA	0 issue "letsencrypt.org"
example.com.	IN	CAA	0 issuewild ";"

The critical flag is intended for usage when future extensions of the protocol, which add new properties that may impact issuance. If, for example, a future version of CAA would include the tag "future", then the record set: 

example.com.	IN	CAA	0 issue "letsencrypt.org"
example.com.	IN	CAA	128 future "Some value"

would bar a certificate authority from issuing a certificate if it does not understand how to parse the record, for instance if it has not yet updated its parsing engine to the new version.

References

  1. ^ a b c d e Hallam-Baker, Phillip; Stradling, Rob (January 2013). DNS Certification Authority Authorization (CAA) Resource Record. IETF. doi:10.17487/RFC6844. ISSN 2070-1721. RFC 6844.
  2. ^ "What is Certificate Authority Authorization (CAA)?". Symantec. Retrieved January 8, 2018.
  3. ^ a b Hall, Kirk (March 8, 2017). "Results on Ballot 187 - Make CAA Checking Mandatory". CA/Browser Forum. Retrieved January 7, 2018.
  4. ^ Risk, Vicky (August 29, 2014). "Certificate Authority Authorization Records". Internet Systems Consortium. Retrieved January 7, 2018.
  5. ^ a b c d e f g h i "Who Supports CAA Records?". SSLMate. Retrieved January 7, 2018.
  6. ^ Včelak, Jan (April 26, 2016). "Knot DNS 2.2.0 release". Retrieved January 7, 2018.
  7. ^ "Name Server Daemon (NSD) Releases". NLnet Labs. January 27, 2014. Retrieved January 7, 2018.
  8. ^ "Amazon Route 53 now supports CAA records". Amazon Web Services. August 21, 2017. Retrieved January 7, 2018.
  9. ^ Cimpanu, Catalin (September 11, 2017). "Comodo Caught Breaking New CAA Standard One Day After It Went Into Effect". Bleeping Computer. Retrieved January 8, 2018.

See also

source: https://en.wikipedia.org/w/index.php?title=DNS_Certification_Authority_Authorization&diff=prev&oldid=819774393

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