Modernizing HTTP Forward Proxy Functionality

1.1. History

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.¶

1.2. Problems

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.¶

1.3. Overview

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.¶

3.1. Example

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:¶

• The HTTP method is not altered.¶
• The request-related headers such as Content-Type are preserved, but the Host header (or :authority in HTTP/2 and HTTP/3) is altered.¶
• Certain characers in the target URI are percent-encoded during URI Template expansion.¶
• The scheme, which is implicit in the original request, is explicit in the transformed request. The scheme in this example is "https", indicating that the client is asking the proxy to establish a secure connection to the target.¶
• The client can add Proxy-* headers to communicate with the proxy.¶

4.1. In HTTP/1.1

In HTTP/1.1, the client uses the proxy by issuing a request as follows:¶

• The method SHALL be "GET".¶
• The request SHALL include a single Host header field containing the origin of the proxy.¶
• The request SHALL include a Connection header field with the value "Upgrade".¶
• The request SHALL include an "Upgrade" header field with the value "connect-tcp".¶
• The request's target SHALL be the URI derived from expansion of the proxy's URI Template.¶

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:¶

• The HTTP status code SHALL be 101 (Switching Protocols).¶
• The response SHALL include a Connection header field with the value "Upgrade".¶
• The response SHALL include a single Upgrade header field with the value "connect-tcp".¶

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.¶

Client                                                 Proxy

GET /proxy?target_host=192.0.2.1&tcp_port=443 HTTP/1.1
Host: example.com
Connection: Upgrade
Upgrade: connect-tcp

                            HTTP/1.1 101 Switching Protocols
                            Connection: Upgrade
                            Upgrade: connect-tcp

4.2. In HTTP/2 and HTTP/3

In HTTP/2 and HTTP/3, the client uses the proxy by issuing an "extended CONNECT" request as follows:¶

• The :method pseudo-header field SHALL be "CONNECT".¶
• The :protocol pseudo-header field SHALL be "connect-tcp".¶
• The :authority pseudo-header field SHALL contain the authority of the proxy.¶
• The :path and :scheme pseudo-header fields SHALL contain the path and scheme of the request URI derived from the proxy's URI Template.¶

From this point on, the request and response streams SHALL conform to all the usual requirements for non-extended CONNECT in this HTTP version.¶

5.1. Template expansion

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.¶

5.2. Sample exchanges

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:¶

POST /relay?target_uri=https%3A%2F%2Fexample.com%2Fgateway HTTP/1.1
Host: proxy.example.org
Proxy-Authorization: ...
Content-Type: message/ohttp-req
...

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:¶

CONNECT HTTP/2.0
:authority = request-proxy.example
:scheme = https
:path = /proxy?target_uri=https%3A%2F%2Ftransport-proxy.example%2Fproxy
    %3Ftarget_host%3Ddestination.example%26tcp_port%3D443
:protocol = connect-tcp
...

Modern TCP transport proxies support requests that offer multiple IP addresses:¶

CONNECT HTTP/2.0
:authority = request-proxy.example
:scheme = https
:path = /proxy?target_host=192.0.2.1,2001:db8::1&port=443
:protocol = connect-tcp
...