| Internet-Draft | Modern HTTP Proxies | October 2022 |
| Schwartz | Expires 17 April 2023 | [Page] |
HTTP proxying features have long been part of the core HTTP specification. However, the core proxying functionality has several important deficiencies in modern HTTP environments. This specification defines alternative proxy service configurations for HTTP requests and TCP connections. These services are identified by URI Templates and designed for parallelism with DoH, MASQUE, and Oblivious HTTP.¶
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 17 April 2023.¶
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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.¶
An HTTP forward proxy (or just "proxy" in the HTTP standards) is an HTTP service that acts on behalf of the client as an intermediary for some or all HTTP requests. HTTP/1.0 defines the initial HTTP proxying mechanism: the client formats its request target in "absolute form" (i.e. with a full URI in the Request-Line) and delivers it to the proxy, which reissues it to the origin specified in the URI ([RFC1945], Section 5.1.2). In this specification, we call this behavior an "HTTP request proxy".¶
With the introduction of "https" URIs, a new proxying mechanism was needed to enable TLS connections to traverse the proxy. To enable this, HTTP/1.1 introduced the CONNECT method. In this method, the request target specifies a host and port number, and the proxy forwards TCP payloads between the client and this destination ([RFC9110], Section 9.3.6). In this specification, we call this behavior a "TCP transport proxy".¶
These two methods sufficed until the introduction of HTTP/3, which uses a UDP transport. The MASQUE effort has filled the gap by defining proxy mechanisms that are capable of proxying UDP datagrams [RFC9298], and more generally IP datagrams [I-D.ietf-masque-connect-ip]. The destination host and port number (if applicable) are encoded into the HTTP resource path, and end-to-end datagrams are wrapped into HTTP Datagrams [RFC9297] on the client-proxy path.¶
Classic HTTP request proxies and TCP transport proxies are identified by an origin, not a URI. The proxy service does not have a path of its own. This prevents any origin from hosting multiple distinct proxy services and makes it difficult to manage a proxy service in a fashion similar to other HTTP services.¶
In some circumstances, it may be possible to work around this limitation by hosting many origins on a single server (virtual-hosting). In HTTP/1.1, the "Host" header was introduced to support such virtual-hosting by distinguishing the hostname of the proxy (in the Host header) from the hostname of the destination (in the absolute-form request URI). However, in HTTP/2 and HTTP/3, this distinction no longer exists. As a result, classic HTTP request proxies are not compatible with virtual-hosting in HTTP/2 or HTTP/3.¶
Classic TCP transport proxies can be used with a host that is specified as a domain name or an IP address. However, because only a single IP address can be specified, Happy Eyeballs and cross-IP fallback can only be used when the host is a domain name. For requests to succeed, the client must know which address families are supported by the proxy.¶
This specification describes alternative protocols for HTTP request proxies and TCP transport proxies in HTTP. Like other modern HTTP access services such as DoH, CONNECT-UDP, and CONNECT-IP, the proxy is identified by a URI Template. Proxy interactions reuse standard HTTP components and semantics, avoiding changes to the core HTTP protocol.¶
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.¶
A modern HTTP request proxy is identified by a URI Template containing a variable named "target_uri". To convert an HTTP request into a proxied request, the client MUST substitute the request's URI into this variable, expand the template, and use the result as the new request URI.¶
HTTP headers work the same as in classic HTTP request proxies.¶
A modern HTTP request proxy is also suitable for use as an Oblivious HTTP relay, if it provides the required privacy guarantees.¶
Consider a proxy identified as "https://example.com/proxy{?target_uri}". Requests would then be transformed as follows:¶
Original request: PATCH /resource HTTP/1.1 Host: api.example Content-Type: application/example ... Transformed request: PATCH /proxy?target_uri=https%3A%2F%2Fapi.example%2Fresource HTTP/1.1 Host: example.com Content-Type: application/example Proxy-Authorization: ... ...¶
Notes on this example:¶
A modern TCP transport proxy for HTTP is identified by a URI Template [RFC6570] containing variables named "target_host" and "tcp_port". The client substitutes the destination host and port number into these variables to produce the request URI.¶
The "target_host" variable MUST be a domain name, an IP address literal, or a list of IP addresses. The "tcp_port" variable MUST be a single integer. If "target_host" is a list (as in Section 2.4.2 of [RFC6570]), the server SHOULD perform the same connection procedure as if these addresses had been returned in response to A and AAAA queries for a domain name.¶
In HTTP/1.1, the client uses the proxy by issuing a request as follows:¶
If the request is well-formed and permissible, the proxy MUST attempt the TCP connection before returning its response header. If the TCP connection is successful, the response SHALL be as follows:¶
If the request is malformed or impermissible, the proxy MUST return a 4XX error code. If the TCP connection failed, the proxy MUST NOT return a 101 or 2XX status code.¶
If the proxy observes an unclean shutdown from the client (e.g. a TCP RST or TLS error), it SHOULD send a TCP RST to the target. If the proxy receives a TCP RST from the target, it SHOULD send a TLS "internal_error" alert to the client, or set the TCP RST bit if TLS is not in use.¶
Consider a proxy identified as "https://example.com/proxy{?target_host,tcp_port}". To establish a TCP connection to 192.0.2.1:443, the following exchange would occur:¶
In HTTP/2 and HTTP/3, the client uses the proxy by issuing an "extended CONNECT" request as follows:¶
From this point on, the request and response streams SHALL conform to all the usual requirements for non-extended CONNECT in this HTTP version.¶
The names of the variables in the URI Template uniquely identify the capabilities of the proxy. Undefined variables are permitted in URI Templates, so a single template can be used for multiple purposes:¶
Combined HTTP request and TCP transport proxy:
https://example.com/proxy{?target_uri,target_host,tcp_port}
Combined HTTP, TCP, UDP, and IP proxy with DoH server:
https://proxy.example/{?target_uri,target_host,tcp_port,port,target,ipproto,dns}
Multipurpose templates can be useful when a single client may benefit from access to multiple complementary services (e.g. TCP and UDP), or when the proxy is used by a variety of clients with different needs.¶
A modern HTTP request proxy can be used as an Oblivious HTTP Relay. For example, suppose the relay is identified as "https://proxy.example.org/relay{?target_uri}", and the Oblivious HTTP Gateway is "https://example.com/gateway". The client would send requests to the proxy as follows:¶
If a modern HTTP request proxy supports HTTP/2 and Extended CONNECT, it is even possible to reach a modern TCP transport proxy through it:¶
Modern TCP transport proxies support requests that offer multiple IP addresses:¶
Modern HTTP proxies can make use of standard HTTP gateways and path-routing to ease implementation and allow use of shared infrastructure. However, current gateways might need modifications to support these services. A compatible gateway must:¶
IF APPROVED, IANA is requested to add the following entry to the HTTP Upgrade Token Registry:¶
TODO acknowledge.¶