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$Id: control-spec-v0.txt 9411 2007-01-26 01:59:50Z nickm $ TC: A Tor control protocol (Version 0) -1. Deprecation THIS PROTOCOL IS DEPRECATED. It is still documented here because Tor 0.1.1.x happens to support much of it; but the support for v0 is not maintained, so you should expect it to rot in unpredictable ways. Support for v0 will be removed some time after Tor 0.1.2. 0. Scope This document describes an implementation-specific protocol that is used for other programs (such as frontend user-interfaces) to communicate with a locally running Tor process. It is not part of the Tor onion routing protocol. 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, but it must provide reliable in-order delivery. For security, the stream should not be accessible by untrusted parties. In TC, the client and server send typed variable-length messages to each other 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 "ERROR" message (code UNRECOGNIZED; see 3.1 below), and clients simply ignore the message. 2.1. Types and encodings All numbers are given in big-endian (network) order. OR identities are given in hexadecimal, in the same format as identity key fingerprints, but without spaces; see tor-spec.txt for more information. 3. Message types Message types are drawn from the following ranges: 0x0000-0xEFFF : Reserved for use by official versions of this spec. 0xF000-0xFFFF : Unallocated; usable by unofficial extensions. 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 Internal error [Something went wrong inside Tor, so that the client's request couldn't be fulfilled.] 0x0002 Unrecognized message type [The client sent a message type we don't understand.] 0x0003 Syntax error [The client sent a message body in a format we can't parse.] 0x0004 Unrecognized configuration key [The client tried to get or set a configuration option we don't recognize.] 0x0005 Invalid configuration value [The client tried to set a configuration option to an incorrect, ill-formed, or impossible value.] 0x0006 Unrecognized byte code [The client tried to set a byte code (in the body) that we don't recognize.] 0x0007 Unauthorized. [The client tried to send a command that requires authorization, but it hasn't sent a valid AUTHENTICATE message.] 0x0008 Failed authentication attempt [The client sent a well-formed authorization message.] 0x0009 Resource exhausted [The server didn't have enough of a given resource to fulfill a given request.] 0x000A No such stream 0x000B No such circuit 0x000C No such OR The rest of the body should be a human-readable description of the error. In general, new error codes should only be added when they don't fall under one of the existing error codes. 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 usually empty but may contain a message. 3.3. SETCONF (Type 0x0002) Change the value of a configuration variable. The body contains a list of newline-terminated key-value configuration lines. An individual key-value configuration line consists of the key, followed by a space, followed by the 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. When a configuration options takes multiple values, or when multiple configuration keys form a context-sensitive group (see below), then setting _any_ of the options in a SETCONF command is taken to reset all of the others. For example, if two ORBindAddress values are configured, and a SETCONF command arrives containing a single ORBindAddress value, the new command's value replaces the two old values. To _remove_ all settings for a given option entirely (and go back to its default value), send a single line containing the key and no value. 3.4. GETCONF (Type 0x0003) Request the value of a configuration variable. The body contains one or more NL-terminated strings for configuration keys. The server replies with a CONFVALUE message. If an option appears multiple times in the configuration, all of its key-value pairs are returned in order. Some options are context-sensitive, and depend on other options with different keywords. These cannot be fetched directly. Currently there is only one such option: clients should use the "HiddenServiceOptions" virtual keyword to get all HiddenServiceDir, HiddenServicePort, HiddenServiceNodes, and HiddenServiceExcludeNodes option settings. 3.5. CONFVALUE (Type 0x0004) Sent in response to a GETCONF message; contains a list of "Key Value\n" (A non-whitespace keyword, a single space, a non-NL value, a NL) strings. 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). Any events *not* listed in the SETEVENTS body are turned off; thus, 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] 0x00 Launched - circuit ID assigned to new circuit 0x01 Built - all hops finished, can now accept streams 0x02 Extended - one more hop has been completed 0x03 Failed - circuit closed (was not built) 0x04 Closed - circuit closed (was built) 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, new connection=5, new resolve request=6, stream detached from circuit and still retriable=7) 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 the last second Bytes read [4 octets] Bytes written [4 octets] 0x0005 -- Notice/warning/error occurred Message [NUL-terminated] <obsolete: use 0x0007-0x000B instead.> 0x0006 -- New descriptors available OR List [NUL-terminated, comma-delimited list of OR identity] 0x0007 -- Debug message occurred 0x0008 -- Info message occurred 0x0009 -- Notice message occurred 0x000A -- Warning message occurred 0x000B -- Error message 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 allowed to control this Tor process. The server responds with DONE or ERROR. The format of the 'cookie' is implementation-dependent; see 4.1 below for information on how the standard Tor implementation handles it. 3.9. SAVECONF (Type 0x0008) Sent from the client to the server. Instructs the server to write out its config options into its torrc. Server returns DONE if successful, or ERROR if it can't write the file or some other error occurs. 3.10. SIGNAL (Type 0x0009) Sent from the client to the server. The body contains one byte that indicates the action the client wishes the server to take. 1 (0x01) -- Reload: reload config items, refetch directory. 2 (0x02) -- Controlled shutdown: if server is an OP, exit immediately. If it's an OR, close listeners and exit after 30 seconds. 10 (0x0A) -- Dump stats: log information about open connections and circuits. 12 (0x0C) -- Debug: switch all open logs to loglevel debug. 15 (0x0F) -- Immediate shutdown: clean up and exit now. The server responds with DONE if the signal is recognized (or simply closes the socket if it was asked to close immediately), else ERROR. 3.11. MAPADDRESS (Type 0x000A) Sent from the client to the server. The body contains a sequence of address mappings, each consisting of the address to be mapped, a single space, the replacement address, and a NL character. Addresses may be IPv4 addresses, IPv6 addresses, or hostnames. The client sends this message to the server in order to tell it that future SOCKS requests for connections to the original address should be replaced with connections to the specified replacement address. If the addresses are well-formed, and the server is able to fulfill the request, the server replies with a single DONE message containing the source and destination addresses. If request is malformed, the server replies with a syntax error message. The server can't fulfill the request, it replies with an internal ERROR message. The client may decline to provide a body for the original address, and instead send a special null address ("0.0.0.0" for IPv4, "::0" for IPv6, or "." for hostname), signifying that the server should choose the original address itself, and return that address in the DONE message. The server should ensure that it returns an element of address space that is unlikely to be in actual use. If there is already an address mapped to the destination address, the server may reuse that mapping. If the original address is already mapped to a different address, the old mapping is removed. If the original address and the destination address are the same, the server removes any mapping in place for the original address. {Note: This feature is designed to be used to help Tor-ify applications that need to use SOCKS4 or hostname-less SOCKS5. There are three approaches to doing this: 1. Somehow make them use SOCKS4a or SOCKS5-with-hostnames instead. 2. Use tor-resolve (or another interface to Tor's resolve-over-SOCKS feature) to resolve the hostname remotely. This doesn't work with special addresses like x.onion or x.y.exit. 3. Use MAPADDRESS to map an IP address to the desired hostname, and then arrange to fool the application into thinking that the hostname has resolved to that IP. This functionality is designed to help implement the 3rd approach.} [XXXX When, if ever, can mappings expire? Should they expire?] [XXXX What addresses, if any, are safe to use?] 3.12 GETINFO (Type 0x000B) Sent from the client to the server. The message body is as for GETCONF: one or more NL-terminated strings. The server replies with an INFOVALUE message. Unlike GETCONF, this message is used for data that are not stored in the Tor configuration file, but instead. Recognized key and their values include: "version" -- The version of the server's software, including the name of the software. (example: "Tor 0.0.9.4") "desc/id/<OR identity>" or "desc/name/<OR nickname>" -- the latest server descriptor for a given OR, NUL-terminated. If no such OR is known, the corresponding value is an empty string. "network-status" -- a space-separated list of all known OR identities. This is in the same format as the router-status line in directories; see tor-spec.txt for details. "addr-mappings/all" "addr-mappings/config" "addr-mappings/cache" "addr-mappings/control" -- a NL-terminated list of address mappings, each in the form of "from-address" SP "to-address". The 'config' key returns those address mappings set in the configuration; the 'cache' key returns the mappings in the client-side DNS cache; the 'control' key returns the mappings set via the control interface; the 'all' target returns the mappings set through any mechanism. 3.13 INFOVALUE (Type 0x000C) Sent from the server to the client in response to a GETINFO message. Contains one or more items of the format: Key [(NUL-terminated string)] Value [(NUL-terminated string)] The keys match those given in the GETINFO message. 3.14 EXTENDCIRCUIT (Type 0x000D) Sent from the client to the server. The message body contains two fields: Circuit ID [4 octets] Path [NUL-terminated, comma-delimited string of OR nickname/identity] This request takes one of two forms: either the Circuit ID is zero, in which case it is a request for the server to build a new circuit according to the specified path, or the Circuit ID is nonzero, in which case it is a request for the server to extend an existing circuit with that ID according to the specified path. If the request is successful, the server sends a DONE message containing a message body consisting of the four-octet Circuit ID of the newly created circuit. 3.15 ATTACHSTREAM (Type 0x000E) Sent from the client to the server. The message body contains two fields: Stream ID [4 octets] Circuit ID [4 octets] This message informs the server that the specified stream should be associated with the specified circuit. Each stream may be associated with at most one circuit, and multiple streams may share the same circuit. Streams can only be attached to completed circuits (that is, circuits that have sent a circuit status 'built' event). If the circuit ID is 0, responsibility for attaching the given stream is returned to Tor. {Implementation note: By default, Tor automatically attaches streams to circuits itself, unless the configuration variable "__LeaveStreamsUnattached" is set to "1". Attempting to attach streams via TC when "__LeaveStreamsUnattached" is false may cause a race between Tor and the controller, as both attempt to attach streams to circuits.} 3.16 POSTDESCRIPTOR (Type 0x000F) Sent from the client to the server. The message body contains one field: Descriptor [NUL-terminated string] This message informs the server about a new descriptor. The descriptor, when parsed, must contain a number of well-specified fields, including fields for its nickname and identity. If there is an error in parsing the descriptor, the server must send an appropriate error message. If the descriptor is well-formed but the server chooses not to add it, it must reply with a DONE message whose body explains why the server was not added. 3.17 FRAGMENTHEADER (Type 0x0010) Sent in either direction. Used to encapsulate messages longer than 65535 bytes in length. Underlying type [2 bytes] Total Length [4 bytes] Data [Rest of message] A FRAGMENTHEADER message MUST be followed immediately by a number of FRAGMENT messages, such that lengths of the "Data" fields of the FRAGMENTHEADER and FRAGMENT messages add to the "Total Length" field of the FRAGMENTHEADER message. Implementations MUST NOT fragment messages of length less than 65536 bytes. Implementations MUST be able to process fragmented messages that not optimally packed. 3.18 FRAGMENT (Type 0x0011) Data [Entire message] See FRAGMENTHEADER for more information 3.19 REDIRECTSTREAM (Type 0x0012) Sent from the client to the server. The message body contains two fields: Stream ID [4 octets] Address [variable-length, NUL-terminated.] Tells the server to change the exit address on the specified stream. No remapping is performed on the new provided address. To be sure that the modified address will be used, this event must be sent after a new stream event is received, and before attaching this stream to a circuit. 3.20 CLOSESTREAM (Type 0x0013) Sent from the client to the server. The message body contains three fields: Stream ID [4 octets] Reason [1 octet] Flags [1 octet] Tells the server to close the specified stream. The reason should be one of the Tor RELAY_END reasons given in tor-spec.txt. Flags is not used currently. Tor may hold the stream open for a while to flush any data that is pending. 3.21 CLOSECIRCUIT (Type 0x0014) Sent from the client to the server. The message body contains two fields: Circuit ID [4 octets] Flags [1 octet] Tells the server to close the specified circuit. If the LSB of the flags field is nonzero, do not close the circuit unless it is unused. 4. Implementation notes 4.1. Authentication By default, the current Tor implementation trusts all local users. If the 'CookieAuthentication' option is true, Tor writes a "magic cookie" file named "control_auth_cookie" into its data directory. To authenticate, the controller must send the contents of this file. If the 'HashedControlPassword' option is set, it must contain the salted hash of a secret password. The salted hash is computed according to the S2K algorithm in RFC 2440 (OpenPGP), and prefixed with the s2k specifier. This is then encoded in hexadecimal, prefixed by the indicator sequence "16:". Thus, for example, the password 'foo' could encode to: 16:660537E3E1CD49996044A3BF558097A981F539FEA2F9DA662B4626C1C2 ++++++++++++++++**^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ salt hashed value indicator You can generate the salt of a password by calling 'tor --hash-password <password>' or by using the example code in the Python and Java controller libraries. To authenticate under this scheme, the controller sends Tor the original secret that was used to generate the password. 4.2. Don't let the buffer get too big. If you ask for lots of events, and 16MB of them queue up on the buffer, the Tor process will close the socket.