Extensible Provisioning Protocol (EPP) mapping for DNS Time-To-Live (TTL) values

The principal output of any domain name provisioning 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 IDN variants (). Where glue (see ) is required, A and/or AAAA records may also be published for nameservers. Typically, the Time-To-Live value (TTL, 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) which 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 co-ordinate to manage TTL values in response to changes in operational or security requirements.

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 . In examples, "C:" represents lines sent by a protocol client and "S:" represents lines returned by a protocol server. Indentation and white space in 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, 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 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.

This extension adds additional elements to the EPP domain and host mappings. The <ttl:ttl> element is used to define TTL values for the DNS resource records associated with domain and host objects. The <ttl:ttl> element may have the following attributes, depending on whether it appears in a command or response frame:

"for", which is REQUIRED in both commands and responses, and which 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";
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.
"min", which MUST NOT be present in commands frames but MUST be present in response frames, and which is used by the server to indicate the lowest value that may be set;
"default", which MUST NOT be present in commands frames but MUST be present in response frames, and which is used by the server to indicate the default value;
"max", which MUST NOT be present in commands frames but MUST be present in response frames, and which is used by the server to indicate the highest value that may be set;

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.

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 "for" and "custom" attributes of <ttl:ttl> elements. The regular expression which is used to validate the values of the "custom" attribute is based on the expression found in , and is intended to match any existing and future RRTYPE mnemonic registered with the IANA: this eliminates the need to update this document in the event that a new DNS record that exists above a zone cut is specified. Nevertheless, EPP servers which 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 actually published in the DNS for domain and host objects. A server which receives a <create> or <update> command which includes an invalid DNS record type MUST respond with a 2004 "Parameter value range" 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 registry implements IDN variants using DNAME records).

Servers which implements host objects () MAY allow clients to specify TTL values for A and AAAA records for host objects. These servers MUST reject commands which attempt to set TTL values for these record types for domain objects. EPP servers which uses the "host attribute" model (described in ) MAY allow clients to specify TTL values for A and AAAA records for domain objects using a 2004 "Parameter value range" error.

<ttl:ttl for="NS">3600<ttl:ttl>

<ttl:ttl for="NS"/>

<ttl:ttl for="custom" custom="DELEG">3600<ttl:ttl>

This extension defines an additional element for EPP <info> responses for domain and host objects. When a server receives a domain or host <info> command from a client which included the extension namespace in the <svcExtension> element of the <login> command, the response MUST contain an <extension> element, which MUST contain a <ttl:infData> element. This element MUST contain one or more <ttl:ttl> records as described in . Example domain <info> response: S:<?xml version="1.0" encoding="utf-8" standalone="no"?> S:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> S: <response> S: <result code="1000"> S: <msg>Command completed successfully</msg> S: </result> S: <resData> S: <domain:infData S: xmlns:domain="urn:ietf:params:xml:ns:domain-1.0"> S: <domain:name>example.com</domain:name> S: <domain:roid>EXAMPLE1-REP</domain:roid> S: <domain:status s="ok"/> S: <domain:ns> S: <domain:hostObj>ns1.example.com</domain:hostObj> S: <domain:hostObj>ns1.example.net</domain:hostObj> S: </domain:ns> S: <domain:clID>ClientX</domain:clID> S: <domain:crID>ClientX</domain:crID> S: <domain:crDate>2023-11-08T10:14:55.0Z</domain:crDate> S: <domain:exDate>2024-11-08T10:14:55.0Z</domain:exDate> S: </domain:infData> S: </resData> S: <extension> S: <ttl:infData S: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> S: <ttl:ttl for="NS" S: min="3600" S: default="86400" S: max="172800">172800</ttl:ttl> S: <ttl:ttl for="DS" S: min="60" S: default="86400" S: max="172800">300</ttl:ttl> S: </ttl:infData> S: <secDNS:infData S: xmlns:secDNS="urn:ietf:params:xml:ns:secDNS-1.1"> S: <secDNS:dsData> S: <secDNS:keyTag>12345</secDNS:keyTag> S: <secDNS:alg>13</secDNS:alg> S: <secDNS:digestType>2</secDNS:digestType> S: <secDNS:digest>49FD46E6C4B45C55D4AC</secDNS:digest> S: </secDNS:dsData> S: </secDNS:infData> S: </extension> S: <trID> S: <clTRID>ABC-12345</clTRID> S: <svTRID>54322-XYZ</svTRID> S: </trID> S: </response> S:</epp> Example host <info> response: S:<?xml version="1.0" encoding="UTF-8" standalone="no"?> S:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> S: <response> S: <result code="1000"> S: <msg>Command completed successfully</msg> S: </result> S: <resData> S: <host:infData S: xmlns:host="urn:ietf:params:xml:ns:host-1.0"> S: <host:name>ns1.example.com</host:name> S: <host:roid>NS1_EXAMPLE1-REP</host:roid> S: <host:status s="ok"/> S: <host:addr ip="v4">192.0.2.2</host:addr> S: <host:addr ip="v6">1080::8:800:200C:417A</host:addr> S: <host:clID>ClientX</host:clID> S: <host:crID>ClientX</host:crID> S: <host:crDate>2023-11-08T10:14:55.0Z</host:crDate> S: </host:infData> S: </resData> S: <extension> S: <ttl:infData S: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> S: <ttl:ttl for="A" S: min="3600" S: default="86400" S: max="172800"/> S: <ttl:ttl for="AAAA" S: min="3600" S: default="86400" S: max="172800">86400</ttl:ttl> S: </ttl:infData> S: </extension> S: <trID> S: <clTRID>ABC-12345</clTRID> S: <svTRID>54322-XYZ</svTRID> S: </trID> S: </response> S:</epp>

This extension defines an additional element for EPP <create> commands for domain and host objects. The <command> element of the <create> command frame MAY contain an <extension> element which MAY contain a <ttl:create> element. This element MUST contain one or more <ttl:ttl> records as described in . Example domain <create> command: C:<?xml version="1.0" encoding="UTF-8" standalone="no"?> C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> C: <command> C: <create> C: <domain:create C: xmlns:domain="urn:ietf:params:xml:ns:domain-1.0"> C: <domain:name>example.com</domain:name> C: <domain:period unit="y">1</domain:period> C: <domain:ns> C: <domain:hostObj>ns1.example.com</domain:hostObj> C: <domain:hostObj>ns1.example.net</domain:hostObj> C: </domain:ns> C: <domain:authInfo> C: <domain:pw/> C: </domain:authInfo> C: </domain:create> C: </create> C: <extension> C: <ttl:create C: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> C: <ttl:ttl for="NS">172800</ttl:ttl> C: <ttl:ttl for="DS">300</ttl:ttl> C: </ttl:create> C: <secDNS:create C: xmlns:secDNS="urn:ietf:params:xml:ns:secDNS-1.1"> C: <secDNS:dsData> C: <secDNS:keyTag>12345</secDNS:keyTag> C: <secDNS:alg>13</secDNS:alg> C: <secDNS:digestType>2</secDNS:digestType> C: <secDNS:digest>49FD46E6C4B45C55D4AC</secDNS:digest> C: </secDNS:dsData> C: </secDNS:create> C: </extension> C: <clTRID>ABC-12345</clTRID> C: </command> C:</epp> Example host <create> command: C:<?xml version="1.0" encoding="UTF-8" standalone="no"?> C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> C: <command> C: <create> C: <host:create C: xmlns:host="urn:ietf:params:xml:ns:host-1.0"> C: <host:name>ns1.example.com</host:name> C: <host:addr ip="v4">192.0.2.2</host:addr> C: <host:addr ip="v6">1080::8:800:200C:417A</host:addr> C: </host:create> C: </create> C: <extension> C: <ttl:create C: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> C: <ttl:ttl for="A"/> C: <ttl:ttl for="AAAA">86400</ttl:ttl> C: </ttl:create> C: </extension> C: <clTRID>ABC-12345</clTRID> C: </command> C:</epp> 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 2306 "Parameter value policy error" response.

This extension defines an additional element for EPP <update> commands for domain and host objects. The <command> element of the <update> command frame MAY contain an <extension> element which MAY contain a <ttl:update> element. This element MUST contain one or more <ttl:ttl> records as described in . Example domain <update> command: C:<?xml version="1.0" encoding="UTF-8" standalone="no"?> C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> C: <command> C: <update> C: <domain:update C: xmlns:domain="urn:ietf:params:xml:ns:domain-1.0"> C: <domain:name>example.com</domain:name> C: </domain:update> C: </update> C: <extension> C: <ttl:update C: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> C: <ttl:ttl for="NS"/> C: </ttl:update> C: </extension> C: <clTRID>ABC-12345</clTRID> C: </command> C:</epp> Example host <update> command: C:<?xml version="1.0" encoding="UTF-8" standalone="no"?> C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"> C: <command> C: <update> C: <host:update C: xmlns:host="urn:ietf:params:xml:ns:host-1.0"> C: <host:name>ns1.example.com</host:name> C: </host:update> C: </update> C: <extension> C: <ttl:update C: xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"> C: <ttl:ttl for="A">86400</ttl:ttl> C: <ttl:ttl for="AAAA">3600</ttl:ttl> C: </ttl:update> C: </extension> C: <clTRID>ABC-12345</clTRID> C: </command> C:</epp> 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 2306 "Parameter value policy error" response.

Servers MAY restrict the supported DNS record types in accordance with their own operational needs. For example, a server MAY allow clients to specify TTL values for DS records only. A server which receives a <create> or <update> command which includes a restricted record type MUST respond with a 2306 "Parameter value policy" error.

EPP servers which implement this extension SHOULD use the values provided by EPP clients for the TTL values records published in the DNS for domain and and objects. EPP servers that use the "host attribute" model SHOULD use any A and/or AAAA TTL values specified for the domain object when publishing NS, A and AAAA records derived from host attributes.

EPP server operators MAY, in order to address operational or security issues, 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 generalised method for EPP servers to notify clients of changes to objects under their sponsorship.

Domain registry operators must strike a balance between, on the one hand, the desire of registrants for changes to their domains to be visible in the DNS quickly, and on the other, the increased DNS query traffic that short TTLs can bring. Historically, registry operators have used a global TTL value which was applied to all delegations within their zones, which could then be tuned to an optimum value. Domain 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 the (which were chosen to allow any TTL permitted in DNS records), 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).

A common operational mistake is changing of DNS record TTLs during or after the planned change to the records themselves. This arises due to a misunderstanding about how TTLs work. Implementations of this specification SHOULD ensure that the user understands 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.

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. Registry 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.

This document uses URNs to describe XML namespaces and XML schemas conforming to a registry mechanism described in . The following URI assignment has been made by IANA: Registration for the TTL namespace: URI: urn:ietf:params:xml:ns:epp:ttl-1.0 Registrant Contact: See the author of this document 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: See the author of this document XML: See the "" section of this document

The EPP extension described in this document has been registered by the 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: URL of this document Registrant Name and Email Address: See the author of this document TLDs: Any IPR Disclosure: None Status: Active Notes: None

The formal syntax presented here is a complete schema representation of the extension suitable for automated validation of EPP XML instances. <?xml version="1.0" encoding="UTF-8"?> <schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:ietf:params:xml:ns:epp:ttl-1.0" xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0" elementFormDefault="qualified"> <annotation> <documentation> Extensible Provisioning Protocol v1.0 extension schema for Time-To-Live (TTL) values for domain and host objects. </documentation> </annotation>  <element name="create" type="ttl:commandContainer"> <unique name="uniqueRRTypeForCreate"> <selector xpath="ttl:ttl"/> <field xpath="@for"/> </unique> </element> <element name="update" type="ttl:commandContainer"> <unique name="uniqueRRTypeForUpdate"> <selector xpath="ttl:ttl"/> <field xpath="@for"/> </unique> </element> <element name="infData" type="ttl:responseContainer"> <unique name="uniqueRRTypeForInfo"> <selector xpath="ttl:ttl"/> <field xpath="@for"/> </unique> </element> <complexType name="commandContainer"> <sequence> <element name="ttl" type="ttl:commandTTLType" minOccurs="1" maxOccurs="unbounded"/> </sequence> </complexType> <complexType name="responseContainer"> <sequence> <element name="ttl" type="ttl:responseTTLType" minOccurs="1" maxOccurs="unbounded"/> </sequence> </complexType> <complexType name="commandTTLType"> <simpleContent> <extension base="ttl:ttlOrNull"> <attribute name="for" type="ttl:rrType" use="required"/> </extension> </simpleContent> </complexType> <complexType name="responseTTLType"> <simpleContent> <extension base="ttl:ttlOrNull"> <attribute name="for" type="ttl:rrType" use="required"/> <attribute name="custom" type="ttl:customRRType"/> <attribute name="min" type="ttl:ttlValue" use="required"/> <attribute name="default" type="ttl:ttlValue" use="required"/> <attribute name="max" type="ttl:ttlValue" use="required"/> </extension> </simpleContent> </complexType>  <simpleType name="ttlOrNull"> <union memberTypes="ttl:emptyValue ttl:ttlValue"/> </simpleType>  <simpleType name="emptyValue"> <restriction base="token"> <length value="0"/> </restriction> </simpleType>  <simpleType name="ttlValue"> <restriction base="nonNegativeInteger"> <minInclusive value="0"/> <maxInclusive value="2147483647"/> </restriction> </simpleType>  <simpleType name="rrType"> <restriction base="token"> <enumeration value="NS" /> <enumeration value="DS" /> <enumeration value="DNAME" /> <enumeration value="A" /> <enumeration value="AAAA" /> <enumeration value="custom" /> </restriction> </simpleType>  <simpleType name="customRRType"> <restriction base="token"> <pattern value="A|[A-Z][A-Z0-9\-]*[A-Z0-9]"/> </restriction> </simpleType> </schema>

None currently reported - if you have implemented this extension, please let me know!

Changed the for attribute to be an enumeration and added the custom attribute.
Added the min, default and max attributes.
Apply feedback from Jim Gould.

Rolled back the "straw man" syntax from 02. ttl:ttl now has a for attribute which can be any DNS record type. describes how the set of supported record types may be limited.
Removed the global/explicit models and just use the explicit model.
Removed the cascading effect where a TTL set on a domain affects subordinate hosts.

Renamed the ttl:seconds XSD type to ttl:container, and the ttl:nonNegativeInteger type to ttl:ttlType, to permit multiple TTL values.
Converted XML instances from artwork to source code.

Incorporate feedback from Jim Gould.
Add wording to describe how TTL values are jointly managed by both clients and servers.
Fix minimum/maximum TTL value and schema namespace (thanks Patrick Mevzek).
Moved text on how the server should handle impermissible TTL values from the top of Section 4 to Sections 3.2.1 and 3.2.2 (thanks Rick Wilhelm).
Namespace changed from urn:ietf:params:xml:ns:ttl-1.0 to urn:ietf:params:xml:ns:epp:ttl-1.0.
Added discussion on EPP servers which use the host attribute model in (thanks Hugo Salgado).
Added a Change Log.

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

Hugo Salgado;
Patrick Mevzek;
Rick Wilhelm;
James Gould;
Marc Groeneweg;
Ties de Kock.