Extensible Provisioning Protocol (EPP) Mapping for DNS Time-to-Live (TTL) Values

Extensible Provisioning Protocol (EPP) Mapping for DNS Time-to-Live (TTL) Values ICANN

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ART regext EPP DNS TTL time-to-live This document describes an extension to the Extensible Provisioning Protocol (EPP) that allows EPP clients to manage the Time-to-Live (TTL) value for domain name delegation records.

Introduction The principal output of any domain name registry system is a DNS zone file, which contains the delegation record(s) for names registered within a zone (such as a top-level domain). These records typically include one or more NS records, but may also include DS records for domains secured with DNSSEC , and DNAME records for Internationalized Domain Name (IDN) variants . A and/or AAAA records may also be published for nameservers where they are required by DNS resolvers to avoid an infinite loop. Typically, the Time-to-Live (TTL) value (see ) of these records is determined by the registry operator. However, in some circumstances it may be desirable to allow the sponsoring client of a domain name to change the TTL values used for that domain's delegation: for example, to reduce the amount of time required to complete a change of DNS servers, DNSSEC deployment or key rollover, or to allow for fast rollback of such changes. This document describes an EPP extension to the domain name and host object mappings (described in and , respectively) that allows the sponsor of a domain name or host object to change the TTL values of the resource record(s) associated with that object. It also describes how EPP servers should handle TTLs specified by EPP clients and how both parties coordinate to manage TTL values in response to changes in operational or security requirements.

Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. In this document's examples, "C:" represents lines sent by a protocol client and "S:" represents lines returned by a protocol server. Indentation and white space in these examples are provided only to illustrate element relationships and are not required features of this protocol. A protocol client that is authorized to manage an existing object is described as a "sponsoring" client throughout this document. XML is case sensitive. Unless stated otherwise, the XML specifications and examples provided in this document MUST be interpreted in the character case presented in order to develop a conforming implementation. EPP uses XML namespaces to provide an extensible object management framework and to identify schemas required for XML instance parsing and validation. These namespaces and schema definitions are used to identify both the base protocol schema and the schemas for managed objects. The XML namespace prefixes used in these examples (such as the string ttl in ttl:create) are solely for illustrative purposes. A conforming implementation MUST NOT require the use of these or any other specific namespace prefixes. In accordance with Section 3.2.2.1 of XML Schema Part 2: Datatypes , the allowable lexical representations for the xs:boolean datatype are the strings "0" and "false" for the concept 'false' and the strings "1" and "true" for the concept 'true'. Implementations MUST support both styles of lexical representation.

Extension Elements This extension adds additional elements to the EPP domain and host mappings.

The <ttl:ttl> Element The <ttl:ttl> element is used to define TTL values for the DNS resource records associated with domain and host objects. <ttl:ttl> elements have the optional following attributes, depending on whether they appear in an EPP command or response:

"for": REQUIRED in both commands and responses, and specifies the DNS record type to which the TTL value pertains. This attribute MUST have one of the following values: "NS", "DS", "DNAME", "A", "AAAA" or "custom".
"custom": If the value of the "for" attribute is "custom", then the <ttl:ttl> element MUST also have a "custom" attribute containing a DNS record type conforming with the regular expression in . Additionally, the record type MUST be registered with IANA in .
"min": MUST NOT be present in EPP commands but MAY be present in EPP responses (see ). It is used by the server to indicate the lowest value that may be set.
"default": MUST NOT be present in EPP commands but MAY be present in EPP responses (see ). It is used by the server to indicate the default value.
"max": MUST NOT be present in EPP commands but MAY be present in EPP responses (see ). It is used by the server to indicate the highest value that may be set.

When present, the value of the "min" attribute MUST be lower than the value of the "max" attribute. The "default" attribute MUST be between the "min" and "max" values, inclusively.

Element Content The XML schema found in of this document restricts the content of <ttl:ttl> elements to be either:

a non-negative integer, indicating the value of the TTL in seconds, or
empty, in which case the server's default TTL for the given record type is to be applied.

Supported DNS Record Types To facilitate forward compatibility with future changes to the DNS protocol, this document does not enumerate or restrict the DNS record types that can be included in the "custom" attribute of the <ttl:ttl> element. The regular expression that is used to validate the values of the "custom" attribute is based on the expression found in , and it is intended to match both existing and future RRTYPE mnemonics. This eliminates the need to update this document in the event that new DNS records that exist above a zone cut () are specified. Nevertheless, EPP servers that implement this extension MUST restrict the DNS record types that are accepted in <create> and <update> commands, and included in <info> responses, allowing only those types that are (a) registered in and (b) appropriate for use above a zone cut. A server that receives a <create> or <update> command that attempts to set TTL values for inapplicable DNS record types MUST respond with a 2306 "Parameter value policy" error. As an illustrative example, a server MAY allow clients to specify TTL values for the following record types for domain objects:

NS;
DS (if the server also implements );
DNAME (if the server implements IDN variants using DNAME records).

Glue Records Glue records are described in . Servers that implement host objects MAY allow clients to specify TTL values for A and AAAA records for host objects. A server supporting host objects that receives a command that attempts to set TTL values for A and AAAA records on a domain object MUST respond with a 2306 "Parameter value policy" error. EPP servers that use the host attribute model (described in ) MAY allow clients to specify TTL values for A and AAAA records for domain objects.

The <ttl:info> Element The <ttl:info> element is used by clients to request that the server include additional information in <info> responses for domain and host objects. It has a single OPTIONAL "policy" attribute, which takes a boolean value with a default value of "false". The semantics of this element are described in . Below is an example of a <ttl:info> element with an explicit "policy" attribute: ]]>

Examples

Explicit TTL Value (<create> or <update> Command) 3600 ]]>

Explicit TTL Value (<info> Policy Mode) 3600 ]]>

Empty Value Indicating Default TTL (<create> or <update> Command, <info> Default Mode) ]]>

Custom Record Type (<create> or <update> Command, <info> Default Mode) 3600 ]]>

EPP Command Mapping

EPP Query Commands

EPP <info> Command This extension defines an additional element for EPP <info> commands and responses for domain and host objects. The EPP <info> command is extended to support two different modes:

The Default Mode (), which requests the inclusion of all non-default TTL values in the response; and
The Policy Mode (), which requests the inclusion of TTL information for all supported DNS record types in the response, along with the minimum, default, and maximum values for those records.

Default Mode If a server receives an <info> command for a domain or host object that includes a <ttl:info> element with a "policy" attribute that is "0" or "false", then the EPP response MUST contain <ttl:ttl> records for all DNS record types that have non-default TTL values. These elements MUST NOT have the "min", "default", and "max" attributes. Below is an example domain <info> command with a <ttl:info> element with a "policy" attribute that is "false": C: C: C: C: C: example.com C: C: C: C: C: C: C: ]]> Below is an example domain <info> response to a command with a <ttl:info> element with a "policy" attribute that is "false": S: S: S: S: Command completed successfully S: S: S: S: example.com S: EXAMPLE1-REP S: S: S: ns1.example.com S: ns1.example.net S: S: ClientX S: ClientX S: 2023-11-08T10:14:55.0Z S: 2024-11-08T10:14:55.0Z S: S: S: S: S: 172800 S: 300 S: S: S: S: 12345 S: 13 S: 2 S: 49FD46E6C4B45C55D4AC S: S: S: S: S: ABC-12345 S: 54322-XYZ S: S: S: ]]> Below is an example host <info> command with a <ttl:info> element with a "policy" attribute that is "false": C: C: C: C: C: ns1.example.com C: C: C: C: C: C: C: ]]> Below is an example host <info> response to a command with a <ttl:info> element with a "policy" attribute that is "false": S: S: S: S: Command completed successfully S: S: S: S: ns1.example.com S: NS1_EXAMPLE1-REP S: S: 192.0.2.2 S: 2001:db8::8:800:200c:417a S: ClientX S: ClientX S: 2023-11-08T10:14:55.0Z S: S: S: S: S: 172800 S: 86400 S: S: S: S: ABC-12345 S: 54322-XYZ S: S: S: ]]>

Policy Mode If a server receives an <info> command for a domain or host object that includes a <ttl:info> element with a "policy" attribute that is "1" or "true", then the EPP response MUST contain <ttl:ttl> records for all supported DNS record types, irrespective of whether those record types are actually in use by the object in question. These elements MUST have the "min", "default", and "max" attributes. Below is an example domain <info> command requesting the server policies: C: C: C: C: C: example.com C: C: C: C: C: C: C: ]]> Below is an example domain <info> response providing the server policies: S: S: S: S: Command completed successfully S: S: S: S: example.com S: EXAMPLE1-REP S: S: S: ns1.example.com S: ns1.example.net S: S: ClientX S: ClientX S: 2023-11-08T10:14:55.0Z S: 2024-11-08T10:14:55.0Z S: S: S: S: S: 172800 S: 300 S: S: S: S: 12345 S: 13 S: 2 S: 49FD46E6C4B45C55D4AC S: S: S: S: S: ABC-12345 S: 54322-XYZ S: S: S: ]]> Below is an example host <info> command requesting the server policies: C: C: C: C: C: ns1.example.com C: C: C: C: C: C: C: ]]> Below is an example host <info> response providing the server policies: S: S: S: S: Command completed successfully S: S: S: S: ns1.example.com S: NS1_EXAMPLE1-REP S: S: 192.0.2.2 S: 2001:db8::8:800:200c:417a S: ClientX S: ClientX S: 2023-11-08T10:14:55.0Z S: S: S: S: S: 172800 S: 86400 S: S: S: S: ABC-12345 S: 54322-XYZ S: S: S: ]]>

EPP Transform Commands

EPP <create> Command This extension defines an additional element for EPP <create> commands for domain and host objects. The <command> element of the <create> command MAY contain an <extension> element that MAY contain a <ttl:create> element. This element MUST contain one or more <ttl:ttl> records as described in . If an EPP server receives a <create> command containing a TTL value that is outside the server's permitted range, it MUST reject the command with a 2004 "Parameter value range error" response. Below is an example domain <create> command: C: C: C: C: C: example.com C: 1 C: C: ns1.example.com C: ns1.example.net C: C: C: C: C: C: C: C: C: 172800 C: 300 C: C: C: C: 12345 C: 13 C: 2 C: 49FD46E6C4B45C55D4AC C: C: C: C: ABC-12345 C: C: ]]> Below is an example host <create> command: C: C: C: C: C: ns1.example.com C: 192.0.2.2 C: 2001:db8::8:800:200c:417a C: C: C: C: C: C: 86400 C: C: C: ABC-12345 C: C: ]]>

EPP <update> Command This extension defines an additional element for EPP <update> commands for domain and host objects. The <command> element of the <update> command MAY contain an <extension> element that MAY contain a <ttl:update> element. This element MUST contain one or more <ttl:ttl> records as described in . If an EPP server receives an <update> command containing a TTL value that is outside the server's permitted range, it MUST reject the command with a 2004 "Parameter value range error" response. Below is an example domain <update> command: C: C: C: C: C: example.com C: C: C: C: C: C: C: 86400 C: C: C: ABC-12345 C: C: ]]> Below is an example host <update> command: C: C: C: C: C: ns1.example.com C: C: C: C: C: 86400 C: 3600 C: C: C: ABC-12345 C: C: ]]>

Server Processing of TTL Values

Permitted Record Types EPP servers MAY restrict the supported DNS record types. For example, a server MAY allow clients to specify TTL values for DS records only. A server that receives a <create> or <update> command that includes a restricted record type MUST respond with a 2306 "Parameter value policy" error. Clients can discover the DNS record types for which an EPP server permits TTL values to be changed by performing a Policy Mode <info> command, as outlined in .

Use of TTL Values in Delegation Records EPP servers that implement this extension SHOULD use the values provided by EPP clients for the TTL values of records published in the DNS for domain and (if supported) host objects. Server operators MAY disregard these values in order to address security and stability issues, as described in and . EPP servers that use the host attribute model SHOULD use any NS, A, and/or AAAA TTL values specified for the domain object when publishing NS, A, and/or AAAA records derived from host attributes.

Out-of-Band Changes to TTL Values In order to address operational or security issues, EPP server operators MAY make changes to TTL values out-of-band (that is, not in response to an <update> command received from the sponsoring client). Server operators MAY also implement automatic reset of TTL values, so that they revert to the default value a certain amount of time after an update has been made. If a TTL value is changed out-of-band, EPP server operators MAY notify the sponsoring client using the EPP Change Poll Extension , which provides a generalized method for EPP servers to notify clients of changes to objects under their sponsorship.

Operational Considerations

Operational Impact of TTL Values Registry operators must consider the balance between registrants' desire for changes to domains to be visible in the DNS quickly, and the increased DNS query traffic that short TTLs can bring. Registry operators SHOULD implement limits on the maximum and minimum accepted TTL values that are narrower than the values permitted in the XML schema in (which were chosen to allow any TTL permitted in DNS records). This is in order to prevent scenarios where an excessively high or low TTL causes operational issues on either side of the zone cut. describes how server operators MAY unilaterally change TTL values in order to address operational or security issues, or only permit changes for limited time periods (after which TTLs revert to the default).

When TTL Values Should Be Changed A common operational mistake is changing the DNS record TTLs during or after the planned change to the records themselves. This arises due to a misunderstanding about how TTLs work. It is RECOMMENDED that guidance be provided to users so they are aware that changes to a TTL are only effective in shortening transition periods if implemented a period of time (at least equal to the current TTL) before the planned change. The latency between receipt of the <update> command and the actual publication of the changes in the DNS should also be taken into consideration in this calculation.

Changes to Server Policy Registry operators may change their policies relating to TTL values from time to time. Previously configured TTL values may consequently fall outside a newly applied policy. This document places no obligation on EPP server operators in respect of these values, and server operators may, as part of a policy change, change the TTL values specified by clients for domain and host objects. describes how such out-of-band changes should be carried out.

Security Considerations

Fast Flux DNS Some malicious actors use a technique called "fast flux DNS" to rapidly change the DNS configuration for a zone in order to evade takedown and law enforcement activity. Server operators should take this into consideration when setting the lower limit on TTL values, since a short TTL on delegations may enhance the effectiveness of fast flux techniques on evasion. Client implementations that provide an interface for customers to configure TTL values for domain names should consider implementing controls to deter and mitigate abusive behavior, such as those outlined in the "Current and Possible Mitigation Alternatives" section of .

Compromised User Accounts An attacker who obtains access to a customer account at a domain registrar that supports this extension could make unauthorized changes to the NS and/or glue records for a domain, and then increase the associated TTLs so that the changes persist in caches for a long time after the attack has been detected. Client implementations that provide an interface for customers to configure TTL values for domain names should consider implementing upper limits in order to reduce the impact of account compromise, in addition to best practices relating to credential management, multi-factor authentication, risk-based access control, and so on.

IANA Considerations

XML Namespace This document uses URNs to describe XML namespaces and XML schemas conforming to a registry mechanism described in . The following URI assignments have been made by IANA: Registration for the TTL namespace:

URI:: urn:ietf:params:xml:ns:epp:ttl-1.0
Registrant Contact:: IESG
XML:: None. Namespace URIs do not represent an XML specification.

Registration for the TTL XML schema:

URI:: urn:ietf:params:xml:schema:epp:ttl-1.0
Registrant Contact:: IESG
XML:: See of this document.

EPP Extension Registry The EPP extension described in this document has been registered by IANA in the "Extensions for the Extensible Provisioning Protocol (EPP)" registry described in . The details of the registration are as follows:

Name of Extension:: Extensible Provisioning Protocol (EPP) Mapping for DNS Time-to-Live (TTL) Values
Document Status:: Standards Track
Reference:: RFC 9803
Registrant:: IESG
TLDs:: Any
IPR Disclosure:: None
Status:: Active
Notes:: None

Formal Syntax The formal syntax presented here is a complete schema representation of the extension suitable for automated validation of EPP XML instances. Extensible Provisioning Protocol v1.0 extension schema for Time-to-Live (TTL) Values for domain and host objects. ]]>

References Normative References XML Schema Part 2: Datatypes Second Edition W3C Recommendation Latest version available at . Resource Record (RR) TYPEs IANA Informative References SSAC Advisory on Fast Flux Hosting and DNS ICANN Security and Stability Advisory Committee (SSAC)

Acknowledgments The author wishes to thank the following people for their advice and feedback during the development of this document: