commit d168142144067342fa096b2157c7fb5b8ab40bdc Author: David Fifield david@bamsoftware.com Date: Mon Apr 9 00:12:49 2012 -0700
WebSocket transport doc. --- doc/websocket-transport.txt | 218 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 218 insertions(+)
diff --git a/doc/websocket-transport.txt b/doc/websocket-transport.txt new file mode 100644 index 0000000..80d8b5e --- /dev/null +++ b/doc/websocket-transport.txt @@ -0,0 +1,218 @@ +Title: WebSocket pluggable transport +Author: David Fifield + +Overview + + This proposal describes the "websocket" pluggable transport for Tor. + It uses the WebSocket protocol now implemented by many web browsers. + It is mostly a straightforward description of proxying WebSocket to + plain TCP, with special consideration for a base64 encoding for agents + that don't support binary WebSocket frames. + +Motivation + + The WebSocket protocol is used by the "flash proxy" system that uses + web browsers as temporary proxies; browsers may connect to a relay + that supports this pluggable transport. Additionally, if WebSocket has + a lot of non-Tor use, it becomes a good target for tunneling, perhaps + in conjunction with a lower layer of obfuscation. WebSocket commonly + works over HTTP ports that are likely to get through a firewall. + +WebSocket overview + + WebSocket is a protocol (rather, several mostly compatible protocols) + aimed at exposing socket-like functionality to JavaScript in web + browsers. It is partially aimed at supplanting techniques such as HTTP + long polling for client–server communication. WebSocket provides + bidirectional communication between a client and server, sufficient to + tunnel Tor traffic. A WebSocket session begins with an HTTP Upgrade + handshake. The socket carries data broken into variable-length + "messages" which are further broken into "frames." There are + distinguished frame opcodes that serve to send either data or control + information. Frames sent by the client (but not the server) are XORed + with a repeating 32-bit mask that is randomly generated per-frame. + + Broadly speaking, there are two versions of WebSocket: the older + "hixie" protocol, and the newer "hybi" protocol which is now RFC 6455. + There are subprotocols within these two versions that differ only in + small ways: "hixie-75" and "hixie-76"; and "hybi-7", "hybi-10", and + "hybi-17". The older "hybi" sockets were supported by Firefox 4 and + Opera 11, but were later disabled because of problems with interaction + with reverse HTTP proxies. Current versions of Firefox and Chrome + support "hybi" sockets, while Safari only supports "hixie". + + The "hybi" sockets support text frames and binary frames. Text frames + may only include UTF-8–encoded text; it is an error if payload doesn't + decode. Binary frames may contain any binary data. However, not all + web browsers support binary frames; they were first added to Firefox + in version 11. The "hixie" sockets have only text frames. + +Method name + + The method name of the transport is "websocket". For example, these + are possible torrc configurations for a client and server, + respectively: + +UseBridges 1 +ClientTransportPlugin websocket exec /usr/libexec/tor-websocket-proxy --client --managed +Bridge websocket 198.51.100.1 + +ServerTransportPlugin websocket exec /usr/libexec/tor-websocket-proxy --server --managed + +The base64 subprotocol + + The most convenient way to tunnel data over WebSocket is with binary + frames, but not all web browsers support binary frames. To work around + this, the "base64" subprotocol encodes binary data as base64 within + text frames. A client that knows it does not support binary frames + requests the base64 subprotocol by including "base64" in the value of + the Sec-WebSocket-Protocol header field. A server that also supports + this subprotocol by sending the value "base64" (and only "base64") in + the Sec-WebSocket-Protocol header field of its response. See under + "Examples" for example of handshakes like this. + + The base64 encoding is applied at the message level, not the frame + level. This means, in particular, that any '=' padding occurs only at + the end of a message, not at the end of each of its constituent + frames. So, for example, the 5-byte message "Hello", whose base64 + encoding is "SGVsbG8=", may be sent as one text frame as follows: + + 0x81 0x08 "SGVsbG8=" + + or, for example, as two frames (one of 2 bytes and one of 6 bytes): + + 0x01 0x02 "SG" 0x81 0x06 "sbG8=" + + When sent by a client, all frames including these must be masked. If + the masking key is 0x12345678, then the message may be sent as one + frame like this: + + 0x81 0x18 0x12 0x34 0x56 0x78 0x41 0x73 0x00 0x0b 0x70 0x73 0x6e 0x45 + +Examples + + Here are examples of WebSocket handshakes and the beginning of data + transfer. The data is the beginning of a Tor connection (i.e., it + begins with a TLS handshake). Data are shown using C string syntax. "> + " at the beginning of a line indicates client-to-server communication; + "< " is server-to-client. "[...]" indicates contents omitted for + brevity. Newlines in the presentation are not significant. This + section is non-normative. + + Using "hybi"/RFC 6455 WebSocket with binary frames: + +> GET / HTTP/1.1\r\n +> Host: 192.0.2.1:80\r\n +> Origin: https://example.com%5Cr%5Cn +> Sec-WebSocket-Version: 13\r\n +> Sec-WebSocket-Key: mzo2xSF9N8VUxuefqO0RSw==\r\n +> Connection: Upgrade\r\n +> Upgrade: websocket\r\n +> \r\n +< HTTP/1.1 101 Switching Protocols\r\n +< Upgrade: websocket\r\n +< Connection: Upgrade\r\n +< Sec-WebSocket-Accept: fM0KjD7ixoxkl4PEXU6tNaTveSg=\r\n +< \r\n +> \x82\xfe\x01\x04\xc9\xd6\xdd\x29\xdf\xd5\xde\x29\x36\xd7[...] +< \x16\x03\x01\x00\x31\x02\x00\x00\x2d\x03[...] + + Using "hybi"/RFC 6455 WebSocket with the base64 subprotocol: + +> GET / HTTP/1.1\r\n +> Host: 192.0.2.1:80\r\n +> Origin: https://example.com%5Cr%5Cn +> Sec-WebSocket-Version: 13\r\n +> Sec-WebSocket-Protocol: base64\r\n +> Sec-WebSocket-Key: k5Ybhw0XBDeBfmda1J9ooQ==\r\n +> Connection: Upgrade\r\n +> Upgrade: websocket\r\n +> \r\n +< HTTP/1.1 101 Switching Protocols\r\n +< Upgrade: websocket\r\n +< Connection: Upgrade\r\n +< Sec-WebSocket-Accept: LYWpflPUHdal8U1BLPXWR3iqUrI=\r\n +< Sec-WebSocket-Protocol: base64\r\n +< \r\n +> \x81\xfe\x01\x58\xbd\x94\x2a\x31\xfb\xf3\x67\x75\xfc\xc4[...] +< \x81\x7e\x04\xd0FgMBADECAA[...] + +Considerations specific to pluggable transports + + Endpoints must implement WebSocket according to RFC 6455; for example, + a server MUST close the connection if it receives an unmasked frame + from a client, and a client MUST close the connection if it receives a + masked frame from a server (RFC 6455 section 5.1). There are also + additional requirements for WebSocket when used as a Tor pluggable + transport. + + Clients MUST implement the RFC 6455 version of the protocol and use it + for all connections. Servers MUST implement the RFC 6455 version of + the protocol and MAY also implement earlier versions. That is, a + server MAY check a client HTTP request to see if it matches an earlier + version of the protocol, and MAY begin communicating using that + protocol. Section 4.4 of RFC 6455 discusses supporting multiple + versions of the protocol. + + Servers MUST support binary frames (opcode 2). Servers MAY also + support text frames (opcode 1). Servers supporting text frames MUST + implement the base64 subprotocol and accept it when requested by a + client in the Sec-WebSocket-Protocol header field. Text frames MUST + NOT be sent by either side if the base64 subprotocol has not been + negotiated. Any endpoint receiving a text frame when base64 has not + been negotiated, or a text message that cannot be decoded as base64, + MUST close the connection. + + A client MUST NOT proceed after receiving any HTTP response status + code other than 101. In particular, it MUST NOT follow redirections + such as 301. + + Endpoints SHOULD respond to Ping frames with a single Pong frame, but + nothing in this specification requires the sending of Ping frames. + + Endpoints SHOULD limit the size of messages they send. All messages + SHOULD be sent in a single frame. + + Endpoints MUST limit the size of messages and frames that they will + buffer. Upon receiving such a message (when the sum of the length of + already-buffered data and the length of the next frame exceeds the + limit), the endpoint MUST close the connection and SHOULD do so with a + status code of 1009 (see RFC 6455 section 7.4.1). Endpoints MUST be + capable of receiving messages containing 1500 octets of binary data; + this may require buffering up to 2026 bytes of UTF-8–encoded base64 + text. + +Questions/extensions + + WebSocket also has a TLS-wrapped version, identified by using the + "wss" (as opposed to "ws") URL scheme. An advantage of this when + tunneling through a browser is that the TLS handshake will be exactly + that of a browser. However, this probably requires the certificates of + relays' server transport plugins to be trusted by browsers. + +References + + "Pluggable transports for circumvention" + https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/180-pluggable... + + RFC 6455, "The WebSocket Protocol" (a.k.a. hybi-17) + https://tools.ietf.org/html/rfc6455 + + "The WebSocket protocol (draft-ietf-hybi-thewebsocketprotocol-10)" + (a.k.a. hybi-10) + https://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-10 + + "The WebSocket protocol (draft-ietf-hybi-thewebsocketprotocol-7)" + (a.k.a. hybi-7) + https://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-7 + + "The WebSocket protocol (draft-ietf-hybi-thewebsocketprotocol-00)" + (a.k.a. hybi-00, hixie-76) + https://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-00 + + "The Web Socket protocol (draft-hixie-thewebsocketprotocol-75)" + (a.k.a. hixie-75) + https://tools.ietf.org/html/draft-hixie-thewebsocketprotocol-75 + + Browser support matrix + http://autobahn.ws/testsuite/reports/clients/index.html