| Internet-Draft | Private use top-level domain | July 2025 |
| Davies, et al. | Expires 3 January 2026 | [Page] |
This document describes the "internal" top-level domain for use in private applications.¶
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There are certain circumstances in which private network operators may wish to use their own domain naming scheme that is not intended to be used or accessible by the global domain name system (DNS), such as within closed corporate or home networks.¶
The "internal" top-level domain provides this purpose in the DNS. Such domains will not resolve in the global DNS, but can be configured within closed networks as the network operator sees fit. It fulfils a similar purpose as private-use IP address ranges that are set aside (e.g. [RFC1918]).¶
Network operators have been using different names for private-use DNS for many years. This usage has been uncoordinated and can result in incompatibilities or harm to Internet users. For example, an organization might choose to use a name for this purpose that has not been assigned to them, that would later appear in the global DNS thereby causing name collisions and undefined behavior for users.¶
If an organization determines that it requires a private-use DNS namespace, it should either use sub-domains of a global DNS name that is under its organizational and operational control, or use the "internal" top-level domain. This document does not offer guidance on when a network operators should choose the "internal" top-level domain instead of a sub-domain of a global DNS name. This decision will depend on multiple factors such as network design or organizational needs, and is outside the scope of this publication.¶
DNSSEC validating resolvers relying on the global DNS trust anchor will fail to resolve names ending in "internal".¶
Other namespaces are reserved for similar purposes, which superficially may seem to serve the same purpose as the "internal" domain, but are intended for different use cases.¶
The "local" namespace [RFC6762] is reserved for use with the multicast DNS protocol. This protocol allows for resolution between devices on a local network. This namespace does not use typical DNS zones for name allocation, and instead uses the multicast DNS protocol to negotiate names and resolve conflicts. It is expected "internal" will be used for applications where names are specified in locally-configured zones.¶
The "alt" namespace [RFC9476] is reserved for contexts where identifiers are used that may look like domain names, but do not use the DNS protocol for resolution. This is in contrast to the "internal" domain which is to be used with the DNS protocol, but in limited private-use network scope.¶
The "home.arpa" namespace [RFC8375] is reserved for use within residential networks, including with the Home Networking Control Protocol [RFC7788].¶
The document requires no IANA actions. For the reasons stated above, the "internal" top-level domain is reserved from being used in the global DNS.¶
While the namespace is designated for private use, there is no guarantee that the names utilized in this namespace will not leak into the broader Internet. Since usage may appear in log files, email headers, and the like; users should not rely on the confidentiality of the "internal" namespace.¶
Users should not expect that names in the "internal" namespace are globally unique; it is assumed that many of the same names will be used for entirely different purposes on different networks. This is similar to the use of the "local" namespace in the multicast DNS protocol - just as there are many different devices named "printer.local", there may be many different servers named "accounting.internal". Users should be aware of this when performing operations requiring authenticity, such as entering credentials.¶
Users should also not assume the appearance of such names is indicative of the true source of transmissions. When diagnosing network issues, the appearance of such addresses must be interpreted with the associated context to ascertain the private network with which the name is being used. A name within the "internal" namespace can never be used by itself to identify the origin of a communication.¶
The lack of global uniqueness also has implications for HTTP cookies; a cookie set for "accounting.internal" on one network may be sent to a different "accounting.internal" if the user changes their local network. This may be mitigated by adding the Secure flag to the cookie. It is expected that Certificate Authorities will not issue certificates for the "internal" namespace as it does not resolve in the global DNS. If an organization wants to use HTTP over TLS with names in the "internal" namespace, they will also need an internal, private CA. The details of this are outside the scope of this document.¶
This reservation is the result of a community deliberation on this topic over many years, most notably [SAC113]. The SAC113 advisory recommended the establishment of a single top-level domain for private-use applications. DNS records within this top-level domain will not be resolvable in contexts outside of a private network.¶
A selection process [IANA-Assessment] determined "internal" was the best suited string given the requirement that a single string be selected for this purpose, and subsequently reserved for this purpose in July 2024. [ICANN-Board-Resolution]¶
I-D source is maintained at: https://github.com/kjd/draft-davies-internal-tld¶