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Applications and Real-Time HTTP proxy status This document defines the next-hop-aliases HTTP Proxy-Status Parameter. This parameter carries the list of aliases and canonical names an intermediary received during DNS resolution as part establishing a connection to the next hop. About This Document Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at .

Introduction The Proxy-Status HTTP response field allows intermediaries to convey information about how they handled the request in HTTP responses sent to clients. It defines a set of parameters that provide information, such as the name of the next hop. defines a next-hop parameter, which can contain a hostname, IP address, or alias of the next hop. This parameter can contain only one such item, so it cannot be used to communicate a chain of aliases encountered during DNS resolution when connecting to the next hop. Knowing the full chain of names that were used during DNS resolution via CNAME records is particularly useful for clients of forward proxies, in which the client is requesting to connect to a specific target hostname using the CONNECT method or UDP proxying . CNAME records can be used to "cloak" hosts that perform tracking or malicious activity behind more innocuous hostnames, and clients such as web browsers use the chain of DNS names to influence behavior like cookie usage policies or blocking of malicious hosts. This document allows clients to receive the CNAME chain of DNS names for the next hop by including the list of names in a new next-hop-aliases Proxy-Status parameter.

Requirements 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.

next-hop-aliases Parameter The next-hop-aliases parameter's value is a String that contains one or more DNS names in a comma-separated list. The items in the list include all alias names and canonical names received in CNAME records during the course of resolving the next hop's hostname using DNS, not including the original requested hostname itself. The names SHOULD appear in the order in which they were received in DNS. If there are multiple CNAME records in the chain, the first name in the next-hop-aliases list would be the value in the CNAME record for the original hostname, and the final name in the next-hop-aliases list would be the name that ultimately resolved to one or more addresses. The list of DNS names in next-hop-aliases uses a comma (",") as a separator between names. Note that if a comma is included in a name itself, the comma must be encoded as described in . For example, consider a proxy "proxy.example.net" that receives the following records when performing DNS resolution for the next hop "host.example.com": The proxy could include the following proxy status in its response: This indicates that proxy.example.net, which used the IP address "2001:db8::1" as the next hop for this request, encountered the names "tracker.example.com" and "service1.example.com" in the DNS resolution chain. Note that while this example includes both the next-hop and next-hop-aliases parameters, next-hop-aliases can be included without including next-hop. The next-hop-aliases parameter only applies when DNS was used to resolve the next hop's name, and does not apply in all situations. Clients can use the information in this parameter to determine how to use the connection established through the proxy, but need to gracefully handle situations in which this parameter is not present. The proxy MAY send the empty string ("") as the value of next-hop-aliases to indicate that no CNAME records were encountered when resolving the next hop's name.

Encoding special characters DNS names commonly just contain alphanumeric characters and hyphens ("-"), although they are allowed to contain any character (), including a comma. To prevent commas or other special characters in names leading to incorrect parsing, any characters that appear in names in this list that do not belong to the set of URI Unreserved Characters () MUST be percent-encoded as defined in . For example, consider the DNS name comma,name.example.com. This name would be encoded within a next-hop-aliases parameter as follows: It is also possible for a DNS name to include a period character (".") within a label, instead of as a label separator. In this case, the period character MUST be first escaped as "\.". Since the "\" character itself will be percent-encoded, the name "dot\.label.example.com" would be encoded within a next-hop-aliases parameter as follows: Upon parsing this name, "dot%5C.label" MUST be treated as a single label. Similarly the "\" character in a label MUST be escaped as "\\". Other uses of "\" MUST NOT appear in the label after percent-decoding.

Implementation Considerations In order to include the next-hop-aliases parameter, a proxy needs to have access to the chain of alias names and canonical names received in CNAME records. Implementations ought to note that the full chain of names might not available in common DNS resolution APIs, such as getaddrinfo. getaddrinfo does have an option for AI_CANONNAME, but this will only return the last name in the chain (the canonical name), not the alias names. An implementation MAY include incomplete information in the next-hop-aliases parameter to accommodate cases where it is unable to include the full chain, such as only including the canonical name if the implementation can only use getaddrinfo as described above.

Security Considerations The next-hop-aliases parameter does not include any DNSSEC information or imply that DNSSEC was used. The information included in the parameter can only be trusted to be valid insofar as the client trusts the proxy to provide accurate information. This information is intended to be used as a hint, and SHOULD NOT be used for making security decisions about the identity of a resource accessed through the proxy. Inspecting CNAME chains can be used to detect cloaking of trackers or malicious hosts. However, the CNAME records could be omitted by a recursive or authoritative resolver that is trying to hide this form of cloaking. In particular, recursive or authoritative resolvers can omit these records for both clients directly performing DNS name resolution and proxies performing DNS name resolution on behalf of client. A malicious proxy could also choose to not report these CNAME chains in order to hide the cloaking. In general, clients can trust information included (or not included) in the next-hop-aliases parameter to the degree that the proxy and any resolvers used by the proxy are trusted.

IANA Considerations This document registers the "next-hop-aliases" parameter in the "HTTP Proxy-Status Parameters" registry <>.

Name:

next-hop-aliases

Description:

A string containing one or more DNS aliases or canonical names used to establish a proxied connection to the next hop.

Reference:

This document

References Normative References This document defines the Proxy-Status HTTP response field to convey the details of an intermediary's response handling, including generated errors. This RFC is the revised basic definition of The Domain Name System. It obsoletes RFC-882. This memo describes the domain style names and their used for host address look up and electronic mail forwarding. It discusses the clients and servers in the domain name system and the protocol used between them. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. This document describes how to proxy UDP in HTTP, similar to how the HTTP CONNECT method allows proxying TCP in HTTP. More specifically, this document defines a protocol that allows an HTTP client to create a tunnel for UDP communications through an HTTP server that acts as a proxy. 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. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. 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] Informative References 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 RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication.