The '_for-sale' Underscored and Globally Scoped DNS Node Name

Introduction Well-established services exist to determine whether a domain name is registered. However, the fact that a domain name exists does not necessarily mean it is unavailable; it may still be for sale. Some registrars and other entities offer mediation services between domain name holders and interested parties. For domain names that are not for sale, such services may be of limited value, whereas they may be beneficial for domain names that are clearly being offered for sale. This specification defines a lightweight method to ascertain whether a domain name, although registered, is available for purchase. It enables a domain name holder to add a reserved underscored leaf node name in the zone, indicating that the domain name is for sale. The TXT RR type created for this purpose MUST follow the formal definition of . Its content MAY contain a pointer, such as a Uniform Resource Identifier (URI) , or another string, allowing interested parties to obtain information or contact the domain name holder for further negotiations. With due caution, such information can also be incorporated into automated availability services. When checking a domain name for availability, the service may indicate whether it is for sale and provide a pointer to the seller's information. Note: In this document, the term "for sale" is used in a broad sense and MAY also refer to cases where the domain name is available for lease.

Terminology 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.

Rationale There are undoubtedly more ways to address this problem space. The reasons for the approach defined in this document are primarily accessibility and simplicity. The indicator can be easily turned on and off at will and moreover, it is immediately deployable and does not require significant changes in existing services. This allows for a smooth introduction of the concept.

Conventions

General Record Format Each '_for-sale' TXT record MUST begin with a version tag, optionally followed by a string containing content that follows a simple "tag=value" syntax. The formal definition of the record format, using ABNF , is as follows: ]]> See for more detailed format definitions per content tag type. Each '_for-sale' TXT record MUST NOT contain more than one tag-value pair. See for additional RRset limitations. The content value provides information to interested parties as explained in . In the absence of a tag-value pair, processors MAY assume that the domain is for sale. In such cases, processors SHOULD determine how to proceed. One possible approach is to indicate that the domain is for sale and to use traditional methods, such as WHOIS or RDAP, to obtain contact information: If a tag-value pair is present but invalid, this constitutes a syntax error and SHOULD be treated as if it were absent. In such cases, if the version tag itself is valid, processors MAY assume that the domain is for sale. For example: TXT records in the same RRset, but without a version tag, MUST NOT be interpreted or processed as a valid '_for-sale' indicator. However, they may still offer some additional information for humans when considered alongside a valid record. For example: If no TXT records at a leaf node contain a valid version tag, processors MUST consider the node name invalid and discard it. See for additional content limitations.

Content Tag Type Definitions A new registry for known content tags is created in , with this document registering the initial set. Implementations SHOULD process only registered tags they support, and MAY ignore any others. The following content tags are defined as the initial valid content tags.

fcod This content tag is intended to contain a code that is meaningful only to processors that understand its semantics. For example, a registry may allow registrars to enter a "for sale" URL into their system. From that URL, a unique code is generated. This code is inserted as the value of the "fcod=" content tag of the '_for-sale' TXT record of a domain name, as shown in the example below. When a user checks the availability of the domain name using a registry-provided tool (e.g., a web interface), the registry may use the code to redirect the user to the appropriate "for sale" URL, which may include a query component containing the domain name, for example: The rationale for this approach is that controlling parties retain authority over the redirection URLs, thereby preventing users from being sent to unintended or malicious destinations. The following example shows a base64-encoded string preceded by the prefix "ACME-" as the value of the content tag: Note: As an implementation consideration, when multiple parties are involved in the domain sale process and use the same mechanism, it may be difficult to identify the relevant content in an RRset. Adding a recognizable prefix to the content (e.g., "ACME-") is one possible approach. However, this is left to the implementor, as it is not enforced in this document. In this case, ACME would recognize its content tag and interpret it as intended. This example uses base64 encoding to avoid escaping and ensure printable characters, though this is also not required.

ftxt This content tag may contain human-readable text that conveys information to interested parties. For example: While a single visible character is the minimum, it is RECOMMENDED to provide more context.

furi This content tag may contain a human-readable and machine-parseable URI that conveys information to interested parties. While the syntax allows any URI scheme, only the following schemes are RECOMMENDED for use: http and https , mailto , and tel . The content value MUST contain exactly one URI. For example: URIs MUST conform to the syntax and encoding requirements specified in , including the percent-encoding of characters not allowed unencoded (for example, spaces MUST be encoded as %20 in a URL). See the section for possible risks.

Content Limitations The '_for-sale' TXT record MUST contain content deemed valid under this specification. Any text that suggests that the domain is not for sale is invalid content. If a domain name is not for sale, a '_for-sale' indicator is pointless and any existence of a valid '_for-sale' TXT record MAY therefore be regarded as an indication that the domain name is for sale. This specification does not dictate the exact use of any content values in the '_for-sale' TXT record. Parties - such as registries and registrars - MAY use it in their tools, perhaps even by defining specific requirements that the content value must meet. Content values can also be represented in a human-readable format for individuals to interpret. See the section for clarification. Since the content value in the TXT record has no strictly defined meaning, it is up to the processor of the content to decide how to handle it. See for operational guidelines. See for additional guidelines.

RRset Limitations This specification does not define restrictions on the number of TXT records in the RRset, but limiting it to one per content tag is RECOMMENDED. If this is not the case, the processor SHOULD determine which content to use. The RDATA of each TXT record MUST consist of a single character-string with a maximum length of 255 octets, in order to avoid the need to concatenate multiple character-strings during processing. The following example illustrates an invalid TXT record due to the presence of multiple character-strings: When multiple content TXT records present, the processor MAY select one or more of them. For example, a registry might extract content from an RRset that includes a recognizable "fcod=" content tag and use it to direct visitors to a sales page as part of its services. An individual, on the other hand, might extract a phone number (if present) from a "furi=" tag in the same RRset and use it to contact a potential seller. An example of such a combined record is provided in .

RR type Limitations Adding any resource record (RR) types under the '_for-sale' leaf, other than TXT (such as AAAA or HINFO), is unnecessary for the purposes of this document and therefore discouraged.

Wildcard Limitation Wildcards are only interpreted as leaf names, so _for-sale.*.example is not a valid wildcard and is non-conformant.

Placement of the Leaf Node Name The '_for-sale' leaf node name is primarily intended to indicate that a domain name is available for purchase. For that, the leaf node name is to be placed on the top-level domain, or any domain directly below. It can also be placed at a lower level, when that level is mentioned in the Public Suffix List . When the '_for-sale' leaf node name is placed elsewhere, the intent is ambiguous. illustrates this: Placements of TXT record

Name	Situation	Verdict
_for-sale.example.	root zone	For sale
_for-sale.aaa.example.	second level	For sale
_for-sale.acme.bbb.example.	bbb.example in PSL	For sale
_for-sale.www.ccc.example.	ccc.example not in PSL	See note 1
_for-sale.51.198.in-addr.arpa.	infrastructure TLD	See note 2
xyz._for-sale.example.	Invalid placement	non-conformant

Note 1: When the '_for-sale' leaf node name is placed in front of a label of a domain that is not in the PSL, it suggests that this label (and everything underneath) is for sale, and not the domain name as a whole. There may be use cases for this, but this situation is considered unusual in the context of this document. Processors MAY ignore such records. Note 2: If a '_for-sale' leaf node were to appear under the .arpa infrastructure top-level domain, it might be interpreted as an offer to sell IP address space. However, such use is explicitly out of scope for this document, and processors MUST ignore any such records.

Additional Examples

Example 1: Code Format A proprietary format, defined by a registry or registrar, without a clearly defined meaning to third parties. For example, it may be used to automatically redirect visitors to a web page, as described in :

Example 2: Free Text Format Free format text, with some additional unstructured information, aimed at being human-readable: The content in the following example could be malicious, but it is not in violation of this specification (see the ): ..." ]]>

Example 3: URI Format The holder of 'example.com' wishes to signal that the domain is for sale and adds this record to the 'example.com' zone: An interested party notices this signal and can visit the URI mentioned for further information. The TXT record may also be processed by automated tools, but see the section for possible risks. As an alternative, a mailto: URI could also be used: Or a telephone URI: There can be a use case for these URIs, especially since WHOIS (or RDAP) often has privacy restrictions. But see the section for possible downsides.

Example 4: Combinations An example of multiple valid TXT records from which a processor can choose:

Operational Guidelines DNS wildcards interact poorly with underscored names. Therefore, the use of wildcards is NOT RECOMMENDED when deploying this mechanism. However, wildcards may still be encountered in practice, especially with operators who are not implementing this mechanism. This is why the version tag is a REQUIRED element: it helps distinguish valid '_for-sale' records from unrelated TXT records. Nonetheless, any assumptions about the content of '_for-sale' TXT records SHOULD be made with caution, for example in cases where the use of wildcards inadvertently causes third-party property to be listed for sale. It is also RECOMMENDED that the content value be limited to visible ASCII characters, excluding the double quote (") and backslash (\). In ABNF syntax, this would be: Long TTLs are discouraged as they increase the risk of outdated data misleading buyers into thinking the domain is still available. Because the format of the content part is not strictly defined in this document, processors MAY apply the robustness principle of being liberal in what they accept. This also applies to space characters (%x20) immediately following the version tag. Alternatively, parties may mutually agree on a more strictly defined proprietary format for the content value to mitigate ambiguity.

IANA Considerations IANA has established the "Underscored and Globally Scoped DNS Node Names" registry . The underscored leaf node name defined in this specification should be added as follows: Entry for the "Underscored and Globally Scoped DNS Node Names" registry

RR Type	_NODE NAME	Reference
TXT	_for-sale	<this memo>

<NOTE TO RFC EDITOR: Adjust the text in the table above before publication with a citation for the (this) document making the addition as per RFC8552.> A registry group called "The '_for-sale' Underscored and Globally Scoped DNS Node Name" is to be created, along with a registry called "Content Tags" within it. This registry group will be maintained independently of IANA. The registry is publicly accessible at: The registry entries consist of content tags as defined in . The initial set of entries in this registry is as follows: Initial set of entries in the "Content Tags" registry

Tag Name	Reference	Status	Description
fcod	RFCXXXX	active	For Sale Proprietary Code
ftxt	RFCXXXX	active	For Sale Free Format Text
furi	RFCXXXX	active	For Sale URI

<NOTE TO RFC EDITOR: Adjust the text in the table above before publication with a citation for the (this) document making the addition as per RFC8552.> Future updates will be managed by the Designated Expert. Entries are assigned only for values that have been documented in a manner consistent with the "Specification Required" registration policy defined in . Newly defined content tags MUST NOT alter the semantics of existing content tags. The "status" column can have one of the following values:

active - the tag is in use in current implementations.
historic - the tag is deprecated and not expected to be used in current implementations.

This registry group is not maintained by IANA as per .

Privacy Considerations The use of the '_for-sale' leaf node name publicly indicates the intent to sell a domain name. Domain owners should be aware that this information is accessible to anyone querying the DNS and may have privacy implications. There is a risk of data scraping, such as email addresses and phone numbers.

Security Considerations One use of the TXT record type defined in this document is to parse the content it contains and to automatically publish certain information from it on a website or elsewhere. However, there is a risk if the domain name holder publishes a malicious URI or one that points to improper content. This may result in reputational damage for the party parsing the record. An even more serious scenario arises when the content of the TXT record is insufficiently validated and sanitized, potentially enabling attacks such as XSS or SQL injection. Therefore, it is RECOMMENDED that any parsing and publishing is conducted with the utmost care. There is also a risk that this method will be abused as a marketing tool, or to lure individuals into visiting certain sites or making contact by other means, without there being any intention to actually sell the domain name. Therefore, this method is best suited for use by professionals.

Implementation Status The concept described in this document is in use with the .nl ccTLD registry. See for example: The Dutch registry SIDN offers registrars the option to register a sales landing page via its registrar dashboard following the "fcod=" method. When this option is used, a unique code is generated, which can be included in the '_for-sale' record. If such a domain name is entered on the domain finder page of SIDN, a 'for sale' button is displayed accordingly. <NOTE TO RFC EDITOR: Please remove this section before publication.>

Acknowledgements The author would like to thank Thijs van den Hout, Caspar Schutijser, Melvin Elderman, Paul Bakker, Ben van Hartingsveldt, Jesse Davids, Juan Stelling and the ISE Editor for their valuable feedback.