Internet Engineering Task Force (IETF) M. Davids Internet-Draft SIDN Labs Intended status: Best Current Practice 26 April 2025 Expires: 28 October 2025 Registration of the "_for-sale" Underscored and Globally Scoped DNS Node Name draft-davids-forsalereg-06 Abstract This document defines an operational convention for using the reserved DNS leaf node name "_for-sale" to indicate that the parent domain name is available for purchase. This approach offers the advantage of easy deployment without affecting ongoing operations. As such, the method can be applied to a domain name that is still in full use. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 28 October 2025. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. Davids Expires 28 October 2025 [Page 1] Internet-Draft forsalereg April 2025 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. General Record Format . . . . . . . . . . . . . . . . . . 3 3.2. Content limitations . . . . . . . . . . . . . . . . . . . 4 3.3. RRset limitations . . . . . . . . . . . . . . . . . . . . 5 3.4. RR Type limitations . . . . . . . . . . . . . . . . . . . 5 3.5. TTL limitation . . . . . . . . . . . . . . . . . . . . . 5 3.6. Wildcard limitation . . . . . . . . . . . . . . . . . . . 5 3.7. CNAME limitation . . . . . . . . . . . . . . . . . . . . 5 3.8. Placement of leaf node name . . . . . . . . . . . . . . . 5 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Example 1: A URI . . . . . . . . . . . . . . . . . . . . 6 4.2. Example 2: Various other approaches . . . . . . . . . . . 7 5. Operational Guidelines . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 11.1. Normative References . . . . . . . . . . . . . . . . . . 9 11.2. Informative References . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction Well-established services [RFC3912][RFC9083] 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. Davids Expires 28 October 2025 [Page 2] Internet-Draft forsalereg April 2025 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 and universal 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 [RFC8552] in the zone, indicating that the domain name is for sale. The TXT RR type [RFC1035] created for this purpose MUST follow the formal definition of Section 3.1. Its content MAY contain a pointer, such as a Uniform Resource Identifier (URI) [RFC8820], 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. 1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. 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. 3. Conventions 3.1. General Record Format The "_for-sale" TXT record MUST start with a version tag, possibly followed by a string. Davids Expires 28 October 2025 [Page 3] Internet-Draft forsalereg April 2025 The formal definition of the record format, using ABNF [RFC5234][RFC7405], is as follows: forsale-record = forsale-version forsale-content forsale-version = %s"v=FORSALE1;" ; version tag, case sensitive, no spaces forsale-content = 0*244OCTET ; referred to as content or data Records 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: _for-sale.example.com. IN TXT "I am for sale" _for-sale.example.com. IN TXT "v=FORSALE1;fscode=NGYyYjEyZWY" If no TXT records at a leaf node contain a version tag, processors MUST consider the node name invalid and discard it. 3.2. Content limitations The TXT [RFC8553] (Section 2.1) record MUST contain any valid content, ranging from an empty string to meaningful text or URIs. 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 in the "_for-sale" TXT record, or the lack of any such content. Parties - such as registries and registrars - MAY use it in their tools, perhaps even by defining specific requirements that the content must meet. Content can also be represented in a human-readable format for individuals to interpret. See the Examples section for clarification. Since the content of TXT record has no defined meaning, it is up to the processor of the content to decide how to handle it. See Section 5 for additional guidelines. Davids Expires 28 October 2025 [Page 4] Internet-Draft forsalereg April 2025 3.3. RRset limitations This specification does not define any restrictions on the number of TXT records in the RRset, but limiting it to one is RECOMMENDED. It is also RECOMMENDED that the length of the RDATA [RFC9499] per TXT record does not exceed 255 octets. If this is not the case, the processor SHOULD determine which content to use. For example, a registry might select content that includes a recognizable code, which can be used to direct visitors to a sales page as part of its services, whereas an individual might simply extract a phone number (if present) and use it to contact a potential seller. 3.4. RR Type limitations Adding any resource record (RR) types under the "_for-sale" leaf other than TXT is NOT RECOMMENDED. Such records MUST be ignored for the purposes of this document. 3.5. TTL limitation A TTL longer than 86400 is NOT RECOMMENDED. Long TTLs increase the risk of outdated information persisting, potentially misleading buyers into believing the domain is still available for purchase. 3.6. Wildcard limitation The "_for-sale" leaf node name SHOULD NOT be a wildcard, but processors MAY still analyze it if it is. 3.7. CNAME limitation The "_for-sale" leaf node name MAY be an alias, but if that is the case, the CNAME record it is associated with it SHOULD also be named "_for-sale", for example: _for-sale.example.com. IN CNAME _for-sale.example.org. However, processors MAY follow the CNAME pointers in other cases as well. 3.8. Placement of leaf node name The "_for-sale" leaf node name MAY be placed on the top level domain, or any domain directly below, with the exception of the .arpa infrastructure top-level domain. Davids Expires 28 October 2025 [Page 5] Internet-Draft forsalereg April 2025 It MAY also be placed at a lower level, but only when that level is mentioned in the Public Suffix List [PSL]. Any other placement of the record MUST NOT be regarded as a signal that the domain above it is for sale. Table 1 provides further clarification. +===============================+====================+==========+ | 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 | Other | Invalid | +-------------------------------+--------------------+----------+ | _for-sale.51.198.in-addr.arpa | infrastructure TLD | Invalid | +-------------------------------+--------------------+----------+ Table 1: Allowed placements of TXT record 4. Examples 4.1. Example 1: A URI The holder of 'example.com' wishes to signal that the domain is for sale and adds this record to the 'example.com' zone: _for-sale.example.com. IN TXT "v=FORSALE1;https://buy.example.com/" 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 Security Considerations section for possible risks. As an alternative, a mailto: URI could also be used: _for-sale.example.com. IN TXT "v=FORSALE1;mailto:owner@example.com" Or a telephone URI: _for-sale.example.com. IN TXT "v=FORSALE1;tel:+1-201-555-0123" Davids Expires 28 October 2025 [Page 6] Internet-Draft forsalereg April 2025 There can be a use case for these URIs, especially since WHOIS (or RDAP) often has privacy restrictions. But see the Privacy Considerations section for possible downsides. 4.2. Example 2: Various other approaches Free format text, with some additional unstructured information, aimed at being human-readable: _for-sale.example.com. IN TXT "v=FORSALE1;$500, info[at]example.com" A proprietary format, defined by a registry or registrar to automatically redirect visitors to a web page, but without a clearly defined meaning to third parties: _for-sale.example.com. IN TXT "v=FORSALE1;fscode=aHR0cHM...wbGUuY29t" The content in the following example could be malicious, but it is not in violation of this specification (see Section 8): _for-sale.example.com. IN TXT "v=FORSALE1;" 5. 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. It is also RECOMMENDED that the content string be limited to visible ASCII characters, excluding the double quote (") and backslash (\). In ABNF syntax, this would be: forsale-content = 0*244recommended-char recommended-char = %x20-21 / %x23-5B / %x5D-7E 6. IANA Considerations IANA has established the "Underscored and Globally Scoped DNS Node Names" registry [RFC8552][IANA]. The underscored leaf node name defined in this specification should be added as follows: Davids Expires 28 October 2025 [Page 7] Internet-Draft forsalereg April 2025 +=========+============+===========+ | RR Type | _NODE NAME | Reference | +=========+============+===========+ | TXT | _for-sale | TBD | +---------+------------+-----------+ Table 2: Entry for the "Underscored and Globally Scoped DNS Node Names" registry This specification does not require the creation of an IANA registry for record fields. 7. 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. 8. 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 occurs when the content of the TXT record is not validated and sanitized sufficiently, opening doors to - for example - XSS attacks among other things. 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 particular domain name. Therefore, this method is best suited for use by professionals. Davids Expires 28 October 2025 [Page 8] Internet-Draft forsalereg April 2025 9. Implementation Status The concept described in this document is in use with the .nl ccTLD registry. See for example: https://www.sidn.nl/en/whois?q=example.nl 10. Acknowledgements The author would like to thank Thijs van den Hout, Caspar Schutijser, Melvin Elderman, Paul Bakker and Ben van Hartingsveldt for their valuable feedback. 11. References 11.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8552] Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019, . 11.2. Informative References Davids Expires 28 October 2025 [Page 9] Internet-Draft forsalereg April 2025 [IANA] IANA, "Underscored and Globally Scoped DNS Node Names", . [PSL] Mozilla Foundation, "Public Suffix List", . [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, DOI 10.17487/RFC3912, September 2004, . [RFC8553] Crocker, D., "DNS Attrleaf Changes: Fixing Specifications That Use Underscored Node Names", BCP 222, RFC 8553, DOI 10.17487/RFC8553, March 2019, . [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190, RFC 8820, DOI 10.17487/RFC8820, June 2020, . [RFC9083] Hollenbeck, S. and A. Newton, "JSON Responses for the Registration Data Access Protocol (RDAP)", STD 95, RFC 9083, DOI 10.17487/RFC9083, June 2021, . [RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219, RFC 9499, DOI 10.17487/RFC9499, March 2024, . Author's Address Marco Davids SIDN Labs Meander 501 6825 MD Arnhem Netherlands Phone: +31 26 352 5500 Email: marco.davids@sidn.nl Davids Expires 28 October 2025 [Page 10]