tor/doc/control-spec.txt

$Id$

                            TC: A Tor control protocol

0. Scope

(8 Aug 2004) This document describes an implementation-specific protocol to
be implemented in a future version of Tor.  It is not part of the Tor onion
routing protocol.

The protocol described in this document is used for other programs (such as
frontend user-interfaces) to communicate with a locally running Tor process.

We're trying to be pretty extensible here, but not infinitely
forward-compatible.

1. Protocol outline

TC is a bidirectional message-based protocol.  It assumes an underlying
stream for communication between a controlling process (the "client") and
a Tor process (the "server").  The stream may be implemented via TCP,
TLS-over-TCP, a Unix-domain socket, or so on.  For security, the stream
should not be observable by untrusted parties.

In TC, the client and server send typed variable-length messages to one
another over the underlying stream.  By default, all messages from the server
are in response to messages from the client.  Some client requests, however,
will cause the server to send messages to the client indefinitely far into
the future.

Servers respond to messages in the order they're received.

2. Message format

The messages take the following format:

   Length [2 octets; big-endian]
   Type   [2 octets; big-endian]
   Body   [Length octets]

Upon encountering a recognized Type, implementations behave as described in
section 3 below.  If the type is not recognized, servers respond with an
"STAT" message (code UNRECOGNIZED; see 3.1 below), and clients simply ignore
the message.

3. Message types

3.1. ERROR (Type 0x0000)

  Sent in response to a message that could not be processed as requested.

  The body of the message begins with a 2-byte error code.  The following
  values are defined:
        0x0000 Unspecified error
        0x0001 Unrecognized message type
        0x0002 Unrecognized configuration key
        0x0003 Invalid configuration value
        0x0004 Unrecognized event code
        0x0005 Unauthorized user
        0x0006 Failed authentication attempt

  The rest of the body should be a human-readable description of the error.

3.2. DONE (Type 0x0001)

  Sent from server to client in response to a request that was successfully
  completed, with no more information needed.  The body is empty.

3.3. SETCONF (Type 0x0002)

  Change the value of a configuration variable. The body contains
  two nul-terminated strings: a configuration key and a configuration value.
  The server behaves as though it had just read the key-value pair in its
  configuration file.  The server responds with a DONE message on success,
  or an ERROR message on failure.

3.4. GETCONF (Type 0x0003)

  Request the value of a configuration variable.  The body contains one or
  more nul-terminated strings for configuration keys.  The server replies
  with a CONFVALUE message.

3.5. CONFVALUE (Type 0x0004)

  Sent in response to a GETCONF message; contains a list of nul-terminated
  key strings followed by nul-terminated value strings.

  [XXXX note that you'll get more keys than you expect with things like
  loglevel.]

3.6. SETEVENTS (Type 0x0005)

  Request the server to inform the client about interesting events.
  The body contains a list of 2-byte event codes (see "event" below).
  Sending SETEVENTS with an empty body turns off all event reporting.

  The server responds with a DONE message on success, and an ERROR message
  if one of the event codes isn't recognized.  (On error, the list of active
  event codes isn't changed.)

3.7. EVENT (Type 0x0006)

  Sent from the server to the client when an event has occurred, and the
  client has requested that kind of event.  The body contains a 2-byte
  event code, followed by additional event-dependent information.  Event
  codes are:
      0x0001 -- Circuit status changed

                Status [1 octet]
                   (Launched=0,Built=1,Extended=2,Failed=3,Closed=4)
                Circuit ID [4 octets]
                   (Must be unique to Tor process/time)
                Path [NUL-terminated comma-separated string]
                   (For extended/failed, is the portion of the path that is
                   built)

      0x0002 -- Stream status changed

                Status [1 octet]
                   (Sent connect=0,sent resolve=1,succeeded=2,failed=3,
                    closed=4)
                Stream ID [4 octets]
                   (Must be unique to Tor process/time)
                Target (NUL-terminated address-port string]

      0x0003 -- OR Connection status changed

                Status [1 octet]
                   (Launched=0,connected=1,failed=2,closed=3)
                OR nickname/identity [NUL-terminated]

      0x0004 -- Bandwidth used in last N seconds. (N=1? 5?)

                Bytes read [4 octets]
                Bytes written [4 octets]

      0x0005 -- Warning/error occurred

                Message [NUL-terminated]

3.8. AUTHENTICATE (Type 0x0007)

  Sent from the client to the server.  Contains a 'magic cookie' to prove
  that client is really the admin for this Tor process.  The server responds
  with DONE or ERROR.

4. Implementation notes

There are four ways we could authenticate, for now:

 1) Listen on 127.0.0.1; trust all local users.

 2) Write a named socket in tor's data-directory or in some other location;
    rely on the OS to ensure that only authorized users can open it.  (NOTE:
    the Linux unix(7) man page suggests that some BSDs don't enforce
    authorization.)  If the OS has named sockets, and implements
    authentication, trust all users who can read Tor's data directory.

 3) Write a random magic cookie to the FS in Tor's data-directory; use that
    magic cookie for authentication.  Trust all users who can read Tor's data
    directory.

 4) Store a salted-and-hashed passphrase in Tor's configuration.  Use the
    passphrase for authentication.  Trust all users who know the passphrase.


On Win32, our only options are 1, 3, and 4.  Since the semantics for 2 and 3
are so similar, I'm recommending that we not support 2, and just always bind
on 127.0.0.1.  I've implemented 3 and 4; 1 would be trivial.  -NM

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