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mnot@mnot.net https://www.mnot.net/

General Internet-Draft This memo defines a path prefix for "well-known locations",
"/.well-known/", in selected Uniform Resource Identifier (URI) schemes. In doing so, it obsoletes RFC 5785 and updates the URI schemes defined in RFC 7230 to reserve that space. It also updates RFC 7595 to track URI schemes that support well-known URIs in their registry.

Introduction Some applications on the Web require the discovery of information about an origin (sometimes called "site-wide metadata") before making a request. For example, the Robots Exclusion Protocol (http://www.robotstxt.org) specifies a way for automated processes to obtain permission to access resources; likewise, the Platform for Privacy Preferences tells user agents how to discover privacy policy before interacting with an origin server. While there are several ways to access per-resource metadata (e.g., HTTP header fields, PROPFIND in Web Distributed Authoring and Versioning (WebDAV) ), the perceived overhead (either in terms of client-perceived latency and/or deployment difficulties) associated with them often precludes their use in these scenarios. At the same time, it has become more popular to use HTTP as a substrate for non-Web protocols. Sometimes, such protocols need a way to locate one or more resources on a given host. When this happens, one solution is to designate a "well-known location" for data or services related to the origin overall, so that it can be easily located. However, this approach has the drawback of risking collisions, both with other such designated "well-known locations" and with resources that the origin has created (or wishes to create). Furthermore, defining well-known locations usurps the origin's control over its own URI space . To address these uses, this memo reserves a path prefix in HTTP, HTTPS, WebSocket (WS), and Secure WebSocket (WSS) URIs for these "well-known locations", "/.well-known/". Future specifications that need to define a resource for such metadata can register their use to avoid collisions and minimise impingement upon origins' URI space. Well-known URIs can also be used with other URI schemes, but only when those schemes' definitions explicitly allow it.

Notational Conventions 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.

Well-Known URIs A well-known URI is a URI whose path component begins with the characters "/.well-known/", provided that the scheme is explicitly defined to support well-known URIs. For example, if an application registers the name 'example', the corresponding well-known URI on 'http://www.example.com/' would be 'http://www.example.com/.well-known/example'. This specification updates the "http" and "https" schemes to support well-known URIs. Other existing schemes can use the appropriate process for updating their definitions; for example, does so for the "ws" and "wss" schemes. The "Uniform Resource Identifier (URI) Schemes" registry tracks which schemes support well-known URIs; see . Applications that wish to mint new well-known URIs MUST register them, following the procedures in , subject to the following requirements. Registered names MUST conform to the "segment-nz" production in . This means they cannot contain the "/" character. Registered names for a specific application SHOULD be correspondingly precise; "squatting" on generic terms is not encouraged. For example, if the Example application wants a well-known location for metadata, an appropriate registered name might be "example-metadata" or even "example.com-metadata", not "metadata". At a minimum, a registration will reference a specification that defines the format and associated media type(s) to be obtained by dereferencing the well-known URI, along with the URI scheme(s) that the well-known URI can be used with. If no URI schemes are explicitly specified, "http" and "https" are assumed. Typically, applications will use the default port for the given scheme; if an alternative port is used, it MUST be explicitly specified by the application in question. Registrations MAY also contain additional information, such as the syntax of additional path components, query strings, and/or fragment identifiers to be appended to the well-known URI, or protocol-specific details (e.g., HTTP method handling). Note that this specification defines neither how to determine the hostname to use to find the well-known URI for a particular application, nor the scope of the metadata discovered by dereferencing the well-known URI; both should be defined by the application itself. Also, this specification does not define a format or media type for the resource located at "/.well-known/", and clients should not expect a resource to exist at that location. Well-known URIs are rooted in the top of the path's hierarchy; they are not well-known by definition in other parts of the path. For example, "/.well-known/example" is a well-known URI, whereas "/foo/.well-known/example" is not. See also for Security Considerations regarding well-known locations.

Registering Well-Known URIs The "Well-Known URIs" registry is located at <https://www.iana.org/assignments/well-known-uris/>. Registration requests can be made by following the instructions located there or by sending an email to the <wellknown-uri-review@ietf.org> mailing list. Registration requests consist of at least the following information:

URI suffix:

The name requested for the well-known URI, relative to "/.well-known/"; e.g., "example".

Change controller:

For Standards Track RFCs, state "IETF". For others, give the name of the responsible party. Other details (e.g., email address, home page URI) may also be included.

Specification document(s):

Reference to the document that specifies the field, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant sections may also be included, but is not required.

Status:

One of "permanent" or "provisional". See guidance below.

Related information:

Optionally, citations to additional documents containing further relevant information.

General requirements for registered values are described in . Values defined by Standards Track RFCs and other open standards (in the sense of , Section 7.1.1) have a status of "permanent". Other values can also be registered as permanent, if the experts find that they are in use, in consultation with the community. Other values should be registered as "provisional". Provisional entries can be removed by the experts if -- in consultation with the community -- the experts find that they are not in use. The experts can change a provisional entry's status to permanent; in doing so, the experts should consider how widely used a value is and consult the community beforehand. Note that "consult the community" above refers to those responsible for the URI scheme(s) in question. Generally, this would take place on the mailing list(s) of the appropriate Working Group(s) (possibly concluded), or on <art@ietf.org> if no such list exists. Well-known URIs can be registered by third parties (including the expert(s)), if the expert(s) determine that an unregistered well-known URI is widely deployed and not likely to be registered in a timely manner otherwise. Such registrations still are subject to the requirements defined, including the need to reference a specification.

Security Considerations Applications minting new well-known URIs, as well as administrators deploying them, will need to consider several security-related issues, including (but not limited to) exposure of sensitive data, denial-of-service attacks (in addition to normal load issues), server and client authentication, vulnerability to DNS rebinding attacks, and attacks where limited access to a server grants the ability to affect how well-known URIs are served. contains some examples of potential security considerations that may be relevant to application protocols and administrators deploying them.

Protecting Well-Known Resources Because well-known locations effectively represent the entire origin, server operators should appropriately control the ability to write to them. This is especially true when more than one entity is
co-located on the same origin. Even for origins that are controlled by and represent a single entity, due care should be taken to assure that write access to well-known locations is not granted unwittingly, either externally through server configuration or locally through implementation permissions (e.g., on a filesystem).

Interaction with Web Browsing Applications using well-known URIs for "http" or "https" URLs need to be aware that well-known resources will be accessible to Web browsers, and therefore are able to be manipulated by content obtained from other parts of that origin. If an attacker is able to inject content (e.g., through a Cross-Site Scripting vulnerability), they will be able to make potentially arbitrary requests to the well-known resource. HTTP and HTTPS also use origins as a security boundary for many other mechanisms, including (but not limited to) cookies , Web Storage , and various capabilities. An application that defines well-known locations should not assume that it has sole access to these mechanisms or that it is the only application using the origin. Depending on the nature of the application, mitigations can include:

• Encrypting sensitive information
• Allowing flexibility in the use of identifiers (e.g., cookie names) to avoid collisions with other applications
• Using the 'HttpOnly' flag on cookies to assure that cookies are not exposed to browser scripting languages
• Using the 'Path' parameter on cookies to assure that they are not available to other parts of the origin
• Using X-Content-Type-Options: nosniff to assure that content under attacker control can't be coaxed into a form that is interpreted as active content by a Web browser

Other good practices include:

• Using an application-specific media type in the Content-Type header field, and requiring clients to fail if it is not used
• Using Content-Security-Policy to constrain the capabilities of active content (such as HTML ), thereby mitigating Cross-Site Scripting attacks
• Using Referrer-Policy to prevent sensitive data in URLs from being leaked in the Referer request header field
• Avoiding use of compression on any sensitive information (e.g., authentication tokens, passwords), as the scripting environment offered by Web browsers allows an attacker to repeatedly probe the compression space; if the attacker has access to the path of the communication, they can use this capability to recover that information.

Scoping Applications This memo does not specify the scope of applicability for the information obtained from a well-known URI, and does not specify how to discover a well-known URI for a particular application. Individual applications using this mechanism must define both aspects; if this is not specified, security issues can arise from implementation deviations and confusion about boundaries between applications. Applying metadata discovered in a well-known URI to resources other than those co-located on the same origin risks administrative as well as security issues. For example, allowing "https://example.com/.well-known/example" to apply policy to "https://department.example.com", "https://www.example.com", or even "https://www.example.com:8000" assumes a relationship between hosts where there might be none, thereby giving control to a potential attacker. Likewise, specifying that a well-known URI on a particular hostname is to be used to bootstrap a protocol can cause a large number of undesired requests. For example, if a well-known HTTPS URI is used to find policy about a separate service such as email, it can result in a flood of requests to Web servers, even if they don't implement the well-known URI. Such undesired requests can resemble a denial-of-service attack.

Hidden Capabilities Applications using well-known locations should consider that some server administrators might be unaware of their existence (especially on operating systems that hide directories whose names begin with "."). This means that if an attacker has write access to the
.well-known directory, they would be able to control its contents, possibly without the administrator realising it.

IANA Considerations

The Well-Known URI Registry This specification updates the registration procedures for the "Well-Known URI" registry, first defined in ; see . Well-known URIs are registered on the advice of one or more experts, with a Specification Required (using terminology from ). The experts' primary considerations in evaluating registration requests are:

• Conformance to the requirements in
• The availability and stability of the specifying document
• The considerations outlined in

IANA will direct the senders of any incoming registry requests to this document and, if defined, the processes established by the expert(s); typically, this will mean referring them to the registry Web page. Per this document, IANA has:

• Updated the registration procedure to Specification Required.
• Added a "Status" column to the registry and marked all of the existing registrations as "permanent".

The Uniform Resource Identifier (URI) Schemes Registry This specification adds a field to the registration template of the "Uniform Resource Identifier (URI) Schemes" registry, with the name "Well-Known URI Support" and a default value of "-". If a URI scheme explicitly has been specified to use well-known URIs as per , the value changes to a reference to that specification. Initial values not equal to "-" are given in . Rows in URI Scheme Registry with Nonempty New Column

URI Scheme	Well-Known URI Support
coap
coap+tcp
coap+ws
coaps
coaps+tcp
coaps+ws
http	[RFC8615]
https	[RFC8615]
ws
wss

References Normative References This document defines the concept of an "origin", which is often used as the scope of authority or privilege by user agents. Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites. In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string. It also defines an HTTP header field, named "Origin", that indicates which origins are associated with an HTTP request. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Informative References WHATWG n.d. WHATWG n.d. The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Web Distributed Authoring and Versioning (WebDAV) consists of a set of methods, headers, and content-types ancillary to HTTP/1.1 for the management of resource properties, creation and management of resource collections, URL namespace manipulation, and resource locking (collision avoidance). RFC 2518 was published in February 1999, and this specification obsoletes RFC 2518 with minor revisions mostly due to interoperability experience. [STANDARDS-TRACK] Section 1.1.1 of RFC 3986 defines URI syntax as "a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme." In other words, the structure of a URI is defined by its scheme. While it is common for schemes to further delegate their substructure to the URI's owner, publishing independent standards that mandate particular forms of URI substructure is inappropriate, because that essentially usurps ownership. This document further describes this problematic practice and provides some acceptable alternatives for use in standards. RFC 5785 defines a path prefix, "/.well-known/", that can be used by well-known URIs. It was specifically defined for the "http" and "https" URI schemes. The present memo formally updates RFC 6455, which defines the URI schemes defined for the WebSocket Protocol, to extend the use of these well-known URIs to those URI schemes. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. This memo documents the process used by the Internet community for the standardization of protocols and procedures. It defines the stages in the standardization process, the requirements for moving a document between stages and the types of documents used during this process. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. All RFCs are required to have a Security Considerations section. Historically, such sections have been relatively weak. This document provides guidelines to RFC authors on how to write a good Security Considerations section. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK] This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. [STANDARDS-TRACK]

Frequently Asked Questions

Aren't well-known locations bad for the Web?

They are, but for various reasons -- both technical and social -- they are sometimes necessary. This memo defines a "sandbox" for them, to reduce the risks of collision and to minimise the impact upon preexisting URIs on sites.

Why "/.well-known?"

It's short, descriptive, and according to search indices, not widely used.

What impact does this have on existing mechanisms, such as P3P and robots.txt?

None, until they choose to use this mechanism.

Why aren't per-directory well-known locations defined?

Allowing every URI path segment to have a well-known location (e.g., "/images/.well-known/") would increase the risks of colliding with a preexisting URI on a site, and generally these solutions are found not to scale well because they're too "chatty".

Changes from RFC 5785

• Allowed non-Web well-known locations
• Adjusted IANA instructions
• Updated references
• Made various other clarifications
• Tracked supporting schemes in the "Uniform Resource Identifier (URI) Schemes" registry