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321 lines
12 KiB
Plaintext
321 lines
12 KiB
Plaintext
$Id$
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TC: A Tor control protocol
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0. Scope
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This document describes an implementation-specific protocol that is used
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for other programs (such as frontend user-interfaces) to communicate
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with a locally running Tor process. It is not part of the Tor onion
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routing protocol.
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We're trying to be pretty extensible here, but not infinitely
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forward-compatible.
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1. Protocol outline
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TC is a bidirectional message-based protocol. It assumes an underlying
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stream for communication between a controlling process (the "client") and
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a Tor process (the "server"). The stream may be implemented via TCP,
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TLS-over-TCP, a Unix-domain socket, or so on, but it must provide
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reliable in-order delivery. For security, the stream should not be
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accessible by untrusted parties.
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In TC, the client and server send typed variable-length messages to each
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other over the underlying stream. By default, all messages from the server
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are in response to messages from the client. Some client requests, however,
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will cause the server to send messages to the client indefinitely far into
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the future.
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Servers respond to messages in the order they're received.
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2. Message format
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The messages take the following format:
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Length [2 octets; big-endian]
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Type [2 octets; big-endian]
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Body [Length octets]
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Upon encountering a recognized Type, implementations behave as described in
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section 3 below. If the type is not recognized, servers respond with a
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"STAT" message (code UNRECOGNIZED; see 3.1 below), and clients simply ignore
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the message.
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3. Message types
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3.1. ERROR (Type 0x0000)
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Sent in response to a message that could not be processed as requested.
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The body of the message begins with a 2-byte error code. The following
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values are defined:
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0x0000 Unspecified error
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[]
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0x0001 Internal error
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[Something went wrong inside Tor, so that the client's
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request couldn't be fulfilled.]
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0x0002 Unrecognized message type
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[The client sent a message type we don't understand.]
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0x0003 Syntax error
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[The client sent a message body in a format we can't parse.]
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0x0004 Unrecognized configuration key
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[The client tried to get or set a configuration option we don't
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recognize.]
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0x0005 Invalid configuration value
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[The client tried to set a configuration option to an
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incorrect, ill-formed, or impossible value.]
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0x0006 Unrecognized byte code
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[The client tried to set a byte code (in the body) that
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we don't recognize.]
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0x0007 Unauthorized.
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[The client tried to send a command that requires
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authorization, but it hasn't sent a valid AUTHENTICATE
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message.]
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0x0008 Failed authentication attempt
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[The client sent a well-formed authorization message.]
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0x0009 Resource exhausted
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[The server didn't have enough of a given resource to
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fulfill a given request.]
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The rest of the body should be a human-readable description of the error.
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In general, new error codes should only be added when they don't fall under
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one of the existing error codes.
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3.2. DONE (Type 0x0001)
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Sent from server to client in response to a request that was successfully
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completed, with no more information needed. The body is empty.
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3.3. SETCONF (Type 0x0002)
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Change the value of a configuration variable. The body contains a list of
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newline-terminated key-value configuration lines.
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The server behaves as though it had just read the key-value pair in its
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configuration file.
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The server responds with a DONE message on success, or an ERROR message on
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failure.
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When a configuration options takes multiple values, or when multiple
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configuration keys form a context-sensitive group (see below), then
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setting _any_ of the options in a SETCONF command is taken to reset all of
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the others. For example, if two ORBindAddress values are configured,
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and a SETCONF command arrives containing a single ORBindAddress value, the
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new command's value replaces the two old values.
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To _remove_ all settings for a given option entirely (and go back to its
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default value), send a single line containing the key and no value.
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3.4. GETCONF (Type 0x0003)
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Request the value of a configuration variable. The body contains one or
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more NL-terminated strings for configuration keys. The server replies
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with a CONFVALUE message.
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If an option appears multiple times in the configuration, all of its
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key-value pairs are returned in order.
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Some options are context-sensitive, and depend on other options with
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different keywords. These cannot be fetched directly. Currently there
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is only one such option: clients should use the "HiddenServiceOptions"
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virtual keyword to get all HiddenServiceDir, HiddenServicePort,
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HiddenServiceNodes, and HiddenServiceExcludeNodes option settings.
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3.5. CONFVALUE (Type 0x0004)
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Sent in response to a GETCONF message; contains a list of "Key Value\n"
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(A non-whitespace keyword, a single space, a non-NL value, a NL)
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strings.
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3.6. SETEVENTS (Type 0x0005)
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Request the server to inform the client about interesting events.
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The body contains a list of 2-byte event codes (see "event" below).
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Sending SETEVENTS with an empty body turns off all event reporting.
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The server responds with a DONE message on success, and an ERROR message
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if one of the event codes isn't recognized. (On error, the list of active
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event codes isn't changed.)
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3.7. EVENT (Type 0x0006)
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Sent from the server to the client when an event has occurred and the
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client has requested that kind of event. The body contains a 2-byte
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event code followed by additional event-dependent information. Event
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codes are:
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0x0001 -- Circuit status changed
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Status [1 octet]
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(Launched=0,Built=1,Extended=2,Failed=3,Closed=4)
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Circuit ID [4 octets]
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(Must be unique to Tor process/time)
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Path [NUL-terminated comma-separated string]
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(For extended/failed, is the portion of the path that is
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built)
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0x0002 -- Stream status changed
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Status [1 octet]
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(Sent connect=0,sent resolve=1,succeeded=2,failed=3,
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closed=4)
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Stream ID [4 octets]
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(Must be unique to Tor process/time)
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Target (NUL-terminated address-port string]
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0x0003 -- OR Connection status changed
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Status [1 octet]
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(Launched=0,connected=1,failed=2,closed=3)
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OR nickname/identity [NUL-terminated]
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0x0004 -- Bandwidth used in the last second
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Bytes read [4 octets]
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Bytes written [4 octets]
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0x0005 -- Notice/warning/error occurred
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Message [NUL-terminated]
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3.8. AUTHENTICATE (Type 0x0007)
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Sent from the client to the server. Contains a 'magic cookie' to prove
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that client is really the admin for this Tor process. The server responds
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with DONE or ERROR.
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3.9. SAVECONF (Type 0x0008)
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Sent from the client to the server. Instructs the server to write out
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its config options into its torrc. Server returns DONE if successful, or
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ERROR if it can't write the file or some other error occurs.
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3.10. SIGNAL (Type 0x0009)
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Sent from the client to the server. The body contains one byte that
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indicates the action the client wishes the server to take.
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0x01 -- Reload: reload config items, refetch directory.
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0x02 -- Controlled shutdown: if server is an OP, exit immediately.
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If it's an OR, close listeners and exit after 30 seconds.
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0x10 -- Dump stats: log information about open connections and
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circuits.
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0x12 -- Debug: switch all open logs to loglevel debug.
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0x15 -- Immediate shutdown: clean up and exit now.
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The server responds with DONE if the signal is recognized (or simply
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closes the socket if it was asked to close immediately), else ERROR.
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3.11. MAPADDRESS (Type 0x000A)
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[Proposal; not finalized]
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Sent from the client to the server. The body contains:
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Original address type [1 octet]
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Original address [Variable length]
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Replacement address type [1 octet]
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Replacement address [Variable length]
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Addresses types can be:
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[0x01] for an IPv4 address (4 octets)
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[0x02] for an IPv6 address (16 octets)
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[0x03] for a hostname (variable-length, NUL-terminated)
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The client sends this message to the server in order to tell it that future
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SOCKS requests for connections to the original address should be replaced
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with connections to the specified replacement address. If the addresses
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are well-formed, and the server is able to fulfill the request, the server
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replies with a single ADDRESSMAPPED message containing the source and
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destination addresses. If request is malformed, the server replies with
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a syntax error message. The server can't fulfill the request, it replies
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with an internal ERROR message.
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The client may decline to provide a body for the original address, and
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instead send a special null address (0.0.0.0 for IPv4, ::0 for IPv6, or
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"." for hostname). This signifies that the server should choose the
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original address itself, and return that address in the ADDRESSMAPPED
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message. The server should ensure that an element of address space
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that is unlikely to be in actual use. If there is already an address
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mapped to the destination address, the server may reuse that mapping.
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If the original address is already mapped to a different address, the old
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mapping is removed. If the original address and the destination address
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are the same, the server removes any mapping in place for the original
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address.
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{Note: This feature is designed to be used to help Tor-ify applications
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that need to use SOCKS4 or hostname-less SOCKS5. There are three
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approaches to doing this:
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1. Somehow make them use SOCKS4a or SOCKS5-with-hostnames instead.
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2. Use tor-resolve (or another interface to Tor's resolve-over-SOCKS
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feature) to resolve the hostname remotely. This doesn't work
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with special addresses like x.onion or x.y.exit.
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3. Use MAPADDRESS to map an IP address to the desired hostname, and then
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arrange to fool the application into thinking that the hostname
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has resolved to that IP.
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This functionality is designed to help implement the 3rd approach.}
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[XXXX When, if ever, can mappings expire? Should they expire?]
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[XXXX What addresses, if any, are safe to use?]
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3.12 ADDRESSMAPPED (Type 0x000B)
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[Proposal; not finalized]
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Same format as MAPADDRESS.
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This message is sent from the server to the client in response to a
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MAPADDRESS or GETALLMAPPINGS message.
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3.13 GETALLMAPPINGS (Type 0x000C)
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[Proposal; not finalized]
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Sent from the client to the server. The server replies by sending an
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ADDRESSMAPPED message for each current address mapping set by MAPADDRESS or
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otherwise, followed by a DONE message.
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4. Implementation notes
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4.1. There are four ways we could authenticate, for now:
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1) Listen on 127.0.0.1; trust all local users.
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2) Write a named socket in tor's data-directory or in some other location;
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rely on the OS to ensure that only authorized users can open it. (NOTE:
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the Linux unix(7) man page suggests that some BSDs don't enforce
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authorization.) If the OS has named sockets, and implements
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authentication, trust all users who can read Tor's data directory.
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3) Write a random magic cookie to the FS in Tor's data-directory; use that
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magic cookie for authentication. Trust all users who can read Tor's data
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directory.
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4) Store a salted-and-hashed passphrase in Tor's configuration. Use the
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passphrase for authentication. Trust all users who know the passphrase.
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On Win32, our only options are 1, 3, and 4. Since the semantics for 2
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and 3 are so similar, we chose to not support 2, and just always bind
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on 127.0.0.1. We've implemented 1, 3, and 4.
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By default, the Tor client accepts authentication approach #1. If
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the controller wants Tor to demand more authentication, it should use
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setconf and saveconf to configure Tor to demand more next time.
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4.2. Don't let the buffer get too big.
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If you ask for lots of events, and 16MB of them queue up on the buffer,
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the Tor process will close the socket.
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