]> Verisign Labs

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Applications REGEXT Working Group EPP The Extensible Provisioning Protocol (EPP) includes commands for clients to delete domain and host objects, both of which are used to publish information in the Domain Name System (DNS). EPP includes guidance concerning those deletions that is intended to avoid DNS resolution disruptions and maintain data consistency. However, operational relationships between objects can make that guidance difficult to implement. Some EPP clients have developed operational practices to delete those objects that have unintended impacts on DNS resolution and security. This document describes best practices to delete domain and host objects that reduce the risk of DNS resolution failure and maintain client-server data consistency.

Section 3.2.2 of RFC 5731 contains text that has led some domain name registrars (acting as EPP clients) to adopt an operational practice of re-naming name server host objects so that they can delete domain objects: "A domain object SHOULD NOT be deleted if subordinate host objects are associated with the domain object. For example, if domain "example.com" exists and host object "ns1.example.com" also exists, then domain "example.com" SHOULD NOT be deleted until host "ns1.example.com" has either been deleted or renamed to exist in a different superordinate domain." Similarly, Section 3.2.2 of RFC 5732 contains this text regarding deletion of host objects: "A host name object SHOULD NOT be deleted if the host object is associated with any other object. For example, if the host object is associated with a domain object, the host object SHOULD NOT be deleted until the existing association has been broken. Deleting a host object without first breaking existing associations can cause DNS resolution failure for domain objects that refer to the deleted host object." These recommendations create a dilemma when the sponsoring client for "example.com" intends to delete "example.com" but its associated host object "ns1.example.com" is also associated with domain objects sponsored by another client. It is advised not to delete the host object due to its associated domain objects. However, the associated domain objects cannot be directly updated because they are sponsored by another client. This situation affects all EPP operators that have implemented support for host objects. Section 3.2.5 of RFC 5732 describes host object renaming: "Host name changes can have an impact on associated objects that refer to the host object. A host name change SHOULD NOT require additional updates of associated objects to preserve existing associations, with one exception: changing an external host object that has associations with objects that are sponsored by a different client. Attempts to update such hosts directly MUST fail with EPP error code 2305. The change can be provisioned by creating a new external host with a new name and any needed new attributes, and subsequently updating the other objects sponsored by the client." Section 1.1 of RFC 5732 includes a description of external hosts. Note that the specific method used to rename the host object can create DNS delegation failures and introduce risks of loss of management control. If the new external host refers to an unregistered domain, then a malicious actor may register the domain and create the host object to gain control of DNS resolution for the domain previously associated with "ns1.example.com". If the new external host offers an authoritative DNS service but the domain is not assigned to an account, then a malicious actor may add the domain to a service account and gain control of (hijack) DNS resolution functionality. If the new external host offers recursive DNS service or no DNS service, then DNS requests for the domain will result in SERVFAIL messages or other errors. Aggressive re-queries by DNS resolvers may then create large numbers of spurious DNS queries for an unresolvable domain. Note that renaming a host object to a name of an external host is an operation that might not be possible to reverse. This document describes the rationale for the "SHOULD NOT be deleted" text, the risk associated with host object renaming, and the best practices that can be used to mitigate that risk.

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.

The primary consideration when deleting domain and host objects concerns the potential impact on DNS resolution. Deletion of a domain object will make all name servers associated with subordinate host objects unresolvable. Deletion of a host object will make any domain that has been delegated to the associated name server unresolvable. The text in RFCs 5731 and 5732 was written to encourage clients to take singular, discrete steps to delete objects in a way that avoids breaking DNS resolution functionality. Additionally, allowing host objects to exist after deletion of their superordinate domain object invites hijacking, as a malicious actor may re-register the domain object, potentially controlling resolution for the host objects and for their associated domain objects.

A server that implicitly deletes subordinate host objects in response to a request to delete a domain object can create a data inconsistency condition in which the EPP client and the EPP server have different views of what remains registered after processing a <delete> command. The text in RFCs 5731 and 5732 was written to encourage clients to take singular, discrete steps to delete objects in a way that maintains client-server data consistency.

Implementations of EPP can have dependencies on the hierarchical domain object/host object relationship, as can exist in a relational database. In such instances, deletion of a domain object without addressing the existing subordinate host objects can cause relational consistency and integrity issues. The text in RFCs 5731 and 5732 was written to reduce the risk of these issues arising as a result of implicit object deletion.

As described in RFC 5731, it's possible to delete a domain object that has associated host objects that are managed by other clients by renaming the host object to exist in a different superordinate domain. This is commonly required when the sponsoring client is unable to disassociate a host object from a domain object managed by another client because only the second client is authorized to make changes to their domain object and the EPP server requires host object disassociation to process a request to delete a domain object. For example: Domain object "domain1.example" is registered by ClientX. Domain object "domain2.example" is registered by ClientY. Subordinate host object "ns1.domain1.example" is registered by ClientX. Host object "ns1.domain1.example" is associated with domain object "domain1.example" by ClientX. Host object "ns1.domain1.example" is associated with domain object "domain2.example" by ClientY. ClientX wishes to delete domain object "domain1.example". It can modify domain object "domain1.example" to remove the association of host object "ns1.domain1.example", but ClientX cannot remove the association of host object "ns1.domain1.example" from domain object "domain2.example" because "domain2.example" is sponsored by ClientY and ClientX is unable to determine that relationship. Only ClientY can modify domain object "domain2.example", and if they do not do so ClientX will need to rename host object "ns1.domain1.example" so that "domain1.example" can be deleted. ClientX renames host object "ns1.domain1.example" to "ns1.example.org", creating an external host and meeting the EPP server's subordinate host object disassociation requirement. If domain "example.org" does not exist, this practice introduces a risk of DNS resolution hijacking if someone were to register the "example.org" domain and create a subordinate host object named "ns1.example.org". That name server would receive DNS queries for all domains delegated to it, allowing the operator of the name server to respond in potentially malicious ways.

As described above, this practice renames subordinate host objects to an external host in order to allow the deletion of the superordinate domain object. The external host is presumed to be non-existent by the deleting EPP client but no check for existence is typically performed.

The primary benefit is convenience for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic. Dependent domains may no longer resolve if all their hosts are renamed.

Malicious actors have registered these parent domains and created child host objects to take control of DNS resolution for associated domains . Sponsoring clients of the associated domains are not informed of the change. Associated domains may still resolve if they continue to be associated with existent hosts, in which case their partial vulnerability to hijacking is more difficult to detect.

Researchers have suggested that clients rename host objects to use the ".alt" pseudo-TLD or another non-DNS pseudo-TLD.

The primary benefit is convenience for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic. Dependent domains cannot be hijacked through the registration of these identifiers and delegation in the DNS.

The ".alt" pseudo-TLD is to be used "to signify that this is an alternative (non-DNS) namespace and should not be looked up in a DNS context" . Some EPP servers may restrict TLDs to valid IANA-delegated TLDs. These entries would mix DNS and non-DNS protocols, risk name collisions, create confusion, and potentially result in unpredictable resolver behaviors. These identifiers may be registered in non-DNS namespaces, potentially leading to hijacking vulnerabilities based in other systems.

Some domain registrars, acting as EPP clients, have maintained host objects with glue records pointing to prominent public recursive DNS services.

The primary benefit is convenience for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic.

Queries for the associated domains result in SERVFAIL or other failure responses. Some recursive name server implementations may aggressively re-query for these responses, potentially resulting in large numbers of queries for unresolvable domains .

Some domain registrars, acting as EPP clients, have begun renaming host objects to subdomains of "as112.arpa," such as "empty.as112.arpa" .

The primary benefit is convenience for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic. These subdomains are valid DNS names designed to sinkhole traffic.

This is a misuse of AS112, which is for reverse lookups on non-unique IPs, primarily so local admins can sinkhole non-global traffic . Unexpected AS112 traffic has previously caused problems with intrusion detection systems and firewalls . Local administrators can potentially hijack requests. AS112 infrastructure must be maintained.

Deleting EPP clients rename host objects to a sacrificial name server host object maintained by the client. The sacrificial name server should run a DNS resolution service capable of responding with an authoritative non-error, non-failure response for requests made for associated domains.

Associated domains are not able to be hijacked, remain in the zone, and have valid DNS records and a responsive DNS service. The service may provide responses that indicate problems with a domain's delegation, such as non-existence or include controlled interruption IP addresses .

This requires that the client maintain the registration of the sacrificial name server's superordinate domain. The client may consider long registration periods and the use of registrar and registry lock services to maintain and protect the superordinate domain and the host object. Failures to maintain these registrations have allowed domain hijacks .

A new community-wide service could be created explicitly intended for use for renaming host records. This would require maintenance of name servers capable of authoritatively responding with NXDOMAIN or a controlled interruption IP addresses for all queries without delegating domains or records. This service could use a new special-use TLD created either through ICANN or IETF processes (e.g., ".sacrificial"), as an IAB request that IANA delegate a second-level domain (SLD) for ".arpa" (e.g., "sacrificial-nameserver.arpa"), or as a contracted sinkhole service by ICANN or other DNS ecosystem actors.

This is a convenient for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic. The associated domains are not vulnerable to hijacking. This would provide an well-understood, industry-standard solution, allowing registrars and registrants to easily identify associated domains that have been affected. Infrastructure operators could monitor traffic to identify affected associated domains that result in significant traffic and attempt to contact registrars and registrants. Economies of scale would allow reduced overall costs to the industry (in contrast to each client running an independent service).

Some entity must maintain the infrastructure for the service.

The recommendations in RFC 5731 are intended to maintain consistency. However, they are not requirements. EPP servers may relax their constraints and allow sponsoring clients to delete host objects and disassociate them from domain objects sponsored by other clients.

This is a convenient for the deleting EPP client. The deleting EPP client is not required to maintain an authoritative DNS service or receive traffic. The associated domains are not vulnerable to hijacking.

This could result in domains with no remaining name servers being removed from the zone or domains with only one remaining name server. Deletions could potentially affect large numbers of associated domains, placing strain on domain registries.

As noted previously, EPP servers can allow clients to delete host objects and disassociate them from domain objects.

EPP servers may require that the EPP client explicitly request the deletion of the host object before taking this action.

This would give EPP clients greater flexibility with respect to deletion. For example, they may choose only to exercise deletions that have no impact on other clients.

This change would require additional development on the part of EPP servers and clients. It may only be relevant if the EPP client is also provided additional deletion details.

The EPP server may provide the deleting EPP client with additional details of the affected objects. The deleting EPP client may receive a message that deletion of the host object would affect domain objects sponsored by another client and may receive details about those objects.

The deleting EPP client would be able to better understand and assess the potential harms of a host object deletion. Depending on the content of the message, the deleting EPP client might choose additional actions, such as delaying the deletion until manual approval can be obtained, renaming the host objects, or informing affected EPP clients.

This change would require additional development on the part of EPP servers and clients. There may be scalability concerns if large numbers of domain objects are updated in a single transaction. The EPP server must determine the relevant information to provide for the EPP client's assessment.

The sponsoring clients of affected domain objects may also be informed of the change.

Updates would help achieve the goals of client-server data consistency and minimal interruptions to resolution. The sponsoring clients of affected domain objects would be able to update their database to reflect the change and would be able to inform the domain's registrant. The sponsoring clients could also quickly or automatically update the affected domains to use another authoritative host.

This change would require additional development on the part of EPP servers and clients. There may be scalability concerns if large numbers of domain objects are updated in a single transaction.

EPP servers can allow for the explicit deletion of a domain with subordinate host objects associated with other domains as long as the associations can be restored by the <restore> operation described in RFC 3915 . Allowing for the explicit deletion of a domain name that performs a cascade purge of the subordinate host objects and the association with other domains is unrecoverable and would be a risk to malicious or accidental actions. Instead of purging the subordinate hosts objects, the server can keep them with the "pendingDelete" status and keep the associations with other domains, but they will not be available in DNS. This will provide for a preview of the operation that is restorable. If the action was malicious, accidental, or had negative side effects, the domain, its subordinate host objects, and the associations with other domains can be restored with the <restore> operation in RFC 3915 during the redemption period. The purge of the domain will correspond with the purging of the subordinate hosts objects and the associations at the end of the pending delete period in RFC 3915. Due to the potential large number of associations, the server can asynchronously update (e.g., add and remove from DNS) and purge the associations.

This practice enables the clients to directly delete the domains that they need to without the requirement to rename the subordinate host objects, since the server will fully support restoration of the associations during the redemption period. The management of the domain and the subordinate hosts will be simplified for the client by supporting the explicit deletion of the domain with the capability of mitigating a destructive malicious or accidental action.

By making it easier for a client to explicitly delete a domain having subordinates hosts with associations, there is higher risk of inadvertent side effects in a single delete command. There is existing risk in EPP of inadvertent side effects, such as adding the "clientHold" status to the domain that will impact the DNS resolution of the subordinate hosts and the associated delegations. The ability to easily rollback the command is key to minimize the impact of the side effects. Another issue is the potential size of the database transaction to disable, re-enable, or purge the subordinate host associations, since there is no limit with the number of associations to delegated domains. Servers can break-up the disable, re-enable, or purge of the subordinate host associations into smaller transactions by implementing it asynchronously.

Different EPP servers may enforce different glue policies. One common policy, also referred to as the "wide" glue policy, allows publishing of glue records for any name server residing in the same EPP zone. For instance, an EPP client can publish A/AAAA records for an existing delegation ns3.example.com even if the superordinate domain object example.com is not delegated to it. The existence of these glue records was one rationale to prohibit deletion of domain objects with linked subordinate host objects, since the domains using ns3.example.com may rely on the glue records provided by the EPP server. To prevent this dependence, EPP servers can adopt a more restrictive glue policy, also referred to as a "narrow" glue policy, wherein only glue records for host objects that are in the NS object set for the superordinate domain are published. For instance, an EPP client can only publish A/AAAA records for ns3.example.com if and only if example.com is delegated to it.

While the adoption of narrow glue policy does not prevent creation of sacrificial nameservers it reduces the number of domains that may be exposed as a result of a domain object deletion. Furthermore, the narrow glue policy makes explicit the dependence and thus prevents domains from implicitly relying on glue records in the parent zone thereby limiting the potential impact of allowing these domains to be deleted and reducing the overall state stored in the EPP server.

That said, the adoption of narrow glue policy can mean significant updates to EPP server and client implementations. Domains that are configured to implicitly depend on the wide glue may not resolve correctly on removal of the wide glue. Moreover, the removal of glue can affect the resolution performance of domains that previously used the wide glue but now need additional queries to be resolved.

TBD.

This document does not contain any instructions for IANA.

This document describes guidance found in RFCs 5731 and 5732 regarding the deletion of domain and host objects by EPP clients. That guidance sometimes requires that host objects be renamed such that they become "external" hosts (see Section 1.1 of RFC 5731 ) in order to meet an EPP server's requirements for host object disassociation prior to domain object deletion. Host object renaming can introduce a risk of DNS resolution hijacking under certain operational conditions. This document provides guidance that is intended to reduce the risk of DNS resolution failure or hijacking as part of the process of deleting EPP domain or host objects. Child domains that depend on host objects associated with domain objects sponsored by another EPP client for DNS resolution may be protected from hijacking through the use of DNSSEC. Their resolution may be protected from the effects of deletion by using host objects associated with multiple domain objects. DNSSEC and multiple host objects may interfere with the use of controlled interruption IP addresses to alert registrants to DNS changes. EPP clients can periodically scan sponsored domains for association with sacrificial name servers and alert end users concerning those domains.

The authors would like to thank the following people for their contributions to this document: James Gould, James Mitchell, Matthew Thomas, Peter Thomassen.

References Normative References Informative References

RFC Editor: Please remove this section before publication. This section lists substantial changes to the document as it is being worked on. 00:

1. Initial version.

01:

1. Restructured practice sections to describe pros and cons, combined "allow delete" practices.
2. Added Gautam Akiwate as co-author.
3. Added narrow/wide glue explanation.