tor/doc/control-spec.txt

$Id$

                   TC: A Tor control protocol (Version 1)

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.

  This protocol replaces version 0 of TC, which is now deprecated.  For
  reference, TC is described in "control-spec-v0.txt".  Implementors are
  recommended to avoid using TC directly, but instead to use a library that
  can easily be updated to use the newer protocol.

1. Protocol outline

  TC is a bidirectional message-based protocol.  It assumes an underlying
  stream for communication between a controlling process (the "client"
  or "controller") and a Tor process (or "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 messages to each other over the
  underlying stream.  The client sends "commands" and the server sends
  "replies".

  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.  Such
  "asynchronous" replies are marked as such.

  Servers respond to messages in the order messages are received.

2. Message format

2.1. Description format

  The message formats listed below use ABNF as described in RFC2234.
  The protocol itself is loosely based on SMTP (see RFC 2821).

  We use the following nonterminals from RFC2822: atom, qcontent

  We define the following general-use nonterminals:

     String = DQUOTE *qcontent DQUOTE

  There are explicitly no limits on line length.  All 8-bit characters are
  permitted unless explicitly disallowed.

2.2. Commands from controller to Tor

    Command = Keyword Arguments CRLF / "+" Keyword Arguments CRLF Data
    Keyword = 1*ALPHA
    Arguments = *(SP / VCHAR)

  Specific commands and their arguments are described below in section 3.

2.3. Replies from Tor to the controller

    Reply = *(MidReplyLine / DataReplyLine) EndReplyLine

    MidReplyLine = "-" ReplyLine
    DataReplyLine = "+" ReplyLine Data
    EndReplyLine = SP ReplyLine
    ReplyLine = StatusCode [ SP ReplyText ]  CRLF
    ReplyText = XXXX
    StatusCode = XXXX

  Specific replies are mentioned below in section 3, and described more fully
  in section 4.

2.4. General-use tokens

  ; Identifiers for servers.
  ServerID = Nickname / Fingerprint
  Nickname = 1*19 NicknameChar
  NicknameChar = "a"-"z" / "A"-"Z" / "0" - "9"
  Fingerprint = "$" 40*HEXDIG

  ; Unique identifiers for streams or circuits.  Currently, Tor only
  ; uses digits, but this may change
  StreamID = 1*16 IDChar
  CircuitID = 1*16 IDChar
  IDChar = ALPHA / DIGIT

  Address = ip4-address / ip6-address / hostname   (XXXX Define these)


  ; A "Data" section is a sequence of octets concluded by the terminating
  ; sequence CRLF "." CRLF.  The terminating sequence may not appear in the
  ; body of the data.  Leading periods on lines in the data are escaped with
  ; an additional leading period as in RFC2821 section 4.5.2
  Data = *DataLine "." CRLF
  DataLine = CRLF / "." 1*LineItem CRLF / NonDotItem *LineItem CRLF
  LineItem = NonCR / 1*CR NonCRLF
  NonDotItem = NonDotCR / 1*CR NonCRLF

3. Commands

  All commands and other keywords are case-insensitive.

3.1. SETCONF

  Change the value of one or more configuration variables.  The syntax is:

    "SETCONF" 1*(SP keyword ["=" String]) CRLF

  Tor behaves as though it had just read each of the key-value pairs
  from its configuration file.  Keywords with no corresponding values have
  their configuration values reset to 0 or NULL (use RESETCONF if you want
  to set it back to its default).  SETCONF is all-or-nothing: if there
  is an error in any of the configuration settings, Tor sets none of them.

  Tor responds with a "250 configuration values set" reply on success.
  Tor responds with a "513 syntax error in configuration values" reply on
  syntax error, or a "553 impossible configuration setting" reply on a
  semantic error.

  When a configuration option takes multiple values, or when multiple
  configuration keys form a context-sensitive group (see GETCONF 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.

3.2. RESETCONF

  Remove all settings for a given configuration option entirely, assign
  its default value (if any), and then assign the String provided.
  Typically the String is left empty, to simply set an option back to
  its default. The syntax is:

    "RESETCONF" 1*(SP keyword ["=" String]) CRLF

  Otherwise it behaves like SETCONF above.

3.3. GETCONF

  Request the value of a configuration variable.  The syntax is:

    "GETCONF" 1*(SP keyword) CRLF

  If all of the listed keywords exist in the Tor configuration, Tor replies
  with a series of reply lines of the form:
      250 keyword=value
  If any option is set to a 'default' value semantically different from an
  empty string, Tor may reply with a reply line of the form:
      250 keyword

  If some of the listed keywords can't be found, Tor replies with a
   "552 unknown configuration keyword" 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.

3.4. SETEVENTS

  Request the server to inform the client about interesting events.  The
  syntax is:

     "SETEVENTS" [SP "EXTENDED"] *(SP EventCode) CRLF

     EventCode = "CIRC" / "STREAM" / "ORCONN" / "BW" / "DEBUG" /
         "INFO" / "NOTICE" / "WARN" / "ERR" / "NEWDESC" / "ADDRMAP"

  Any events *not* listed in the SETEVENTS line are turned off; thus, sending
  SETEVENTS with an empty body turns off all event reporting.

  The server responds with a "250 OK" reply on success, and a "552
  Unrecognized event" reply if one of the event codes isn't recognized.  (On
  error, the list of active event codes isn't changed.)

  If the flag string "EXTENDED" is provided, Tor may provide extra
  information with events for this connection; see 4.1 for more information.
  NOTE: All events on a given connection will be provided in extended format,
  or none.
  NOTE: "EXTENDED" is only supported in Tor 0.1.1.9-alpha or later.

3.5. AUTHENTICATE

  Sent from the client to the server.  The syntax is:
     "AUTHENTICATE" [ SP 1*HEXDIG / QuotedString ] CRLF

  The server responds with "250 OK" on success or "515 Bad authentication" if
  the authentication cookie is incorrect.

  The format of the 'cookie' is implementation-dependent; see 5.1 below for
  information on how the standard Tor implementation handles it.

  If Tor requires authentication and the controller has not yet sent an
  AUTHENTICATE message, Tor sends a "514 authentication required" reply to
  any other kind of message.

3.6. SAVECONF

  Sent from the client to the server.  The syntax is:
     "SAVECONF" CRLF

  Instructs the server to write out its config options into its torrc. Server
  returns "250 OK" if successful, or "551 Unable to write configuration
  to disk" if it can't write the file or some other error occurs.

3.7. SIGNAL

  Sent from the client to the server. The syntax is:

     "SIGNAL" SP Signal CRLF

     Signal = "RELOAD" / "SHUTDOWN" / "DUMP" / "DEBUG" / "HALT" /
              "HUP" / "INT" / "USR1" / "USR2" / "TERM"

  The meaning of the signals are:

      RELOAD    -- Reload: reload config items, refetch directory. (like HUP)
      SHUTDOWN  -- Controlled shutdown: if server is an OP, exit immediately.
                   If it's an OR, close listeners and exit after 30 seconds.
                   (like INT)
      DUMP      -- Dump stats: log information about open connections and
                   circuits. (like USR1)
      DEBUG     -- Debug: switch all open logs to loglevel debug. (like USR2)
      HALT      -- Immediate shutdown: clean up and exit now. (like TERM)

  The server responds with "250 OK" if the signal is recognized (or simply
  closes the socket if it was asked to close immediately), or "552
  Unrecognized signal" if the signal is unrecognized.

3.8. MAPADDRESS

  Sent from the client to the server.  The syntax is:

    "MAPADDRESS" 1*(Address "=" Address SP) CRLF

  The first address in each pair is an "original" address; the second is a
  "replacement" address.  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 250 message:
    250-OldAddress1=NewAddress1
    250 OldAddress2=NewAddress2

  containing the source and destination addresses.  If request is malformed,
  the server replies with "512 syntax error in command argument".  If the server
  can't fulfill the request, it replies with "451 resource exhausted."

  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 reply.  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.

  Example:
    C: MAPADDRESS 0.0.0.0=tor.eff.org 1.2.3.4=tor.freehaven.net
    S: 250-127.192.10.10=tor.eff.org
    S: 250 1.2.3.4=tor.freehaven.net

  {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.}

  Mappings set by the controller last until the Tor process exits:
  they never expire. If the controller wants the mapping to last only
  a certain time, then it must explicitly un-map the address when that
  time has elapsed.

3.9. GETINFO

  Sent from the client to the server.  The syntax is as for GETCONF:
    "GETINFO" 1*(SP keyword) CRLF
  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, and that may be longer than a single line.  On success,
  one ReplyLine is sent for each requested value, followed by a final 250 OK
  ReplyLine.  If a value fits on a single line, the format is:
      250-keyword=value
  If a value must be split over multiple lines, the format is:
      250+keyword=
      value
      .
  Recognized keys and their values include:

    "version" -- The version of the server's software, including the name
      of the software. (example: "Tor 0.0.9.4")

    "config-file" -- The location of Tor's configuration file ("torrc").

    "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 space-separated list of address mappings, each
      in the form of "from-address=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.

    "circuit-status"
      A series of lines as for a circuit status event. Each line is of the form:
         CircuitID SP CircStatus SP Path CRLF

    "stream-status"
      A series of lines as for a stream status event.  Each is of the form:
         StreamID SP StreamStatus SP CircID SP Target CRLF

    "orconn-status"
      A series of lines as for an OR connection status event.  Each is of the
      form:
         ServerID SP ORStatus CRLF

    "helper-nodes"
      A series of lines listing the currently chosen helper nodes, if any.
      Each is of the form:
         ServerID  SP ((("down" / "unlisted") ISOTime) / "up")  CRLF

    "accounting/enabled"
    "accounting/hibernating"
    "accounting/bytes"
    "accounting/bytes-left"
    "accounting/interval-start"
    "accounting/interval-wake"
    "accounting/interval-end"
      Information about accounting status.  If accounting is enabled,
      "enabled" is 1; otherwise it is 0.  The "hibernating" field is "hard"
      if we are accepting no data; "soft" if we're accepting no new
      connections, and "awake" if we're not hibernating at all.  The "bytes"
      and "bytes-left" fields contain (read-bytes SP write-bytes), for the
      start and the rest of the interval respectively.  The 'interval-start'
      and 'interval-end' fields are the borders of the current interval; the
      'interval-wake' field is the time within the current interval (if any)
      where we plan[ned] to start being active.

    "config/names"
      A series of lines listing the available configuration options. Each is
      of the form:
         OptionName SP OptionType [ SP Documentation ] CRLF
         OptionName = Keyword
         OptionType = "Integer" / "TimeInterval" / "DataSize" / "Float" /
           "Boolean" / "Time" / "CommaList" / "Dependant" / "Virtual" /
           "String" / "LineList"
         Documentation = Text

    "info/names"
      A series of lines listing the available GETINFO options.  Each is of
      one of these forms:
         OptionName SP Documentation CRLF
         OptionPrefix SP Documentation CRLF
         OptionPrefix = OptionName "/*"

  Examples:
     C: GETINFO version desc/name/moria1
     S: 250+desc/name/moria=
     S: [Descriptor for moria]
     S: .
     S: 250-version=Tor 0.1.1.0-alpha-cvs
     S: 250 OK

3.10. EXTENDCIRCUIT

  Sent from the client to the server.  The format is:
      "EXTENDCIRCUIT" SP CircuitID SP ServerID *("," ServerID) CRLF

  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 reply containing a message
  body consisting of the Circuit ID of the (maybe newly created) circuit.
  The syntax is "250" SP "EXTENDED" SP CircuitID CRLF.

3.11. ATTACHSTREAM

  Sent from the client to the server.  The syntax is:
     "ATTACHSTREAM" SP StreamID SP CircuitID CRLF

  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 or are listed as built in a
  GETINFO circuit-status request).

  If the circuit ID is 0, responsibility for attaching the given stream is
  returned to Tor.

  Tor responds with "250 OK" if it can attach the stream, 552 if the circuit
  or stream didn't exist, or 551 if the stream couldn't be attached for
  another reason.

  {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.}

3.12. POSTDESCRIPTOR

  Sent from the client to the server. The syntax is:
    "+POSTDESCRIPTOR" CRLF Descriptor CRLF "." CRLF

  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 a "554
  Invalid descriptor" reply.  If the descriptor is well-formed but the server
  chooses not to add it, it must reply with a 251 message whose body explains
  why the server was not added.  If the descriptor is added, Tor replies with
  "250 OK".

3.13. REDIRECTSTREAM

  Sent from the client to the server. The syntax is:
    "REDIRECTSTREAM" SP StreamID SP Address CRLF

  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.

  Tor replies with "250 OK" on success.

3.14. CLOSESTREAM

  Sent from the client to the server.  The syntax is:

    "CLOSESTREAM" SP StreamID SP Reason *(SP Flag) CRLF

  Tells the server to close the specified stream.  The reason should be one
  of the Tor RELAY_END reasons given in tor-spec.txt, as a decimal.  Flags is
  not used currently; Tor servers SHOULD ignore unrecognized flags.  Tor may
  hold the stream open for a while to flush any data that is pending.

3.15. CLOSECIRCUIT

   The syntax is:
     CLOSECIRCUIT SP CircuitID *(SP Flag) CRLF
     Flag = "IfUnused"

  Tells the server to close the specified circuit.   If "IfUnused" is
  provided, do not close the circuit unless it is unused.

  Other flags may be defined in the future; Tor SHOULD ignore unrecognized
  flags.

3.16. QUIT

  Tells the server to hang up on this controller connection. This command
  can be used before authenticating.

4. Replies

  Reply codes follow the same 3-character format as used by SMTP, with the
  first character defining a status, the second character defining a
  subsystem, and the third designating fine-grained information.

  The TC protocol currently uses the following first characters:

    2yz   Positive Completion Reply
       The command was successful; a new request can be started.

    4yz   Temporary Negative Completion reply
       The command was unsuccessful but might be reattempted later.

    5yz   Permanent Negative Completion Reply
       The command was unsuccessful; the client should not try exactly
       that sequence of commands again.

    6yz   Asynchronous Reply
       Sent out-of-order in response to an earlier SETEVENTS command.

  The following second characters are used:

    x0z   Syntax
       Sent in response to ill-formed or nonsensical commands.

    x1z   Protocol
       Refers to operations of the Tor Control protocol.

    x5z   Tor
       Refers to actual operations of Tor system.

  The following codes are defined:

     250 OK
     251 Operation was unnecessary
         [Tor has declined to perform the operation, but no harm was done.]

     451 Resource exhausted

     500 Syntax error: protocol

     510 Unrecognized command
     511 Unimplemented command
     512 Syntax error in command argument
     513 Unrecognized command argument
     514 Authentication required
     515 Bad authentication

     550 Unspecified Tor error

     551 Internal error
               [Something went wrong inside Tor, so that the client's
                request couldn't be fulfilled.]

     552 Unrecognized entity
               [A configuration key, a stream ID, circuit ID, event,
                mentioned in the command did not actually exist.]

     553 Invalid configuration value
         [The client tried to set a configuration option to an
           incorrect, ill-formed, or impossible value.]

     554 Invalid descriptor

     555 Unmanaged entity

     650 Asynchronous event notification

  Unless specified to have specific contents, the human-readable messages
  in error replies should not be relied upon to match those in this document.

4.1. Asynchronous events

  These replies can be sent after a corresponding SETEVENTS command has been
  received.  They will not be interleaved with other Reply elements, but they
  can appear between a command and its corresponding reply.  For example,
  this sequence is possible:

     C: SETEVENTS CIRC
     S: 250 OK
     C: GETCONF SOCKSPORT ORPORT
     S: 650 CIRC 1000 EXTENDED moria1,moria2
     S: 250-SOCKSPORT=9050
     S: 250 ORPORT=0

  But this sequence is disallowed:
     C: SETEVENTS CIRC
     S: 250 OK
     C: GETCONF SOCKSPORT ORPORT
     S: 250-SOCKSPORT=9050
     S: 650 CIRC 1000 EXTENDED moria1,moria2
     S: 250 ORPORT=0

  Clients SHOULD tolerate more arguments in an asynchonous reply than
  expected, and SHOULD tolerate more lines in an asynchronous reply than
  expected.  For instance, a client that expects a CIRC message like:
      650 CIRC 1000 EXTENDED moria1,moria2
  should tolerate:
      650-CIRC 1000 EXTENDED moria1,moria2 0xBEEF
      650-EXTRAMAGIC=99
      650 ANONYMITY=high

  If clients ask for extended events, then each event line as specified below
  will be followed by additional extensions.  Clients that do so MUST
  tolerate additional arguments and lines.  Additional lines will be of the
  form
      "650" ("-"/" ") KEYWORD ["=" ARGUMENTS] CRLF
  Additional arguments will be of the form
      SP KEYWORD ["=" ( QuoutedString / * NonSpDquote ) ]
  Such clients MUST tolerate lines with keywords they do not recognize.

4.1.1. Circuit status changed

   The syntax is:

     "650" SP "CIRC" SP CircuitID SP CircStatus SP Path

      CircStatus =
               "LAUNCHED" / ; circuit ID assigned to new circuit
               "BUILT"    / ; all hops finished, can now accept streams
               "EXTENDED" / ; one more hop has been completed
               "FAILED"   / ; circuit closed (was not built)
               "CLOSED"     ; circuit closed (was built)

      Path = ServerID *("," ServerID)

4.1.2. Stream status changed

    The syntax is:

      "650" SP "STREAM" SP StreamID SP StreamStatus SP CircID SP Target SP 

      StreamStatus =
               "NEW"          / ; New request to connect
               "NEWRESOLVE"   / ; New request to resolve an address
               "SENTCONNECT"  / ; Sent a connect cell along a circuit
               "SENTRESOLVE"  / ; Sent a resolve cell along a circuit
               "SUCCEEDED"    / ; Received a reply; stream established
               "FAILED"       / ; Stream failed and not retriable.
               "CLOSED"       / ; Stream closed
               "DETACHED"       ; Detached from circuit; still retriable.

       Target = Address ":" Port

  The circuit ID designates which circuit this stream is attached to.  If
  the stream is unattached, the circuit ID "0" is given.

4.1.3. OR Connection status changed

  The syntax is:
    "650" SP "ORCONN" SP ServerID SP ORStatus

    ORStatus = "LAUNCHED" / "CONNECTED" / "FAILED" / "CLOSED"

4.1.4. Bandwidth used in the last second

  The syntax is:
     "650" SP "BW" SP BytesRead SP BytesWritten
     BytesRead = 1*DIGIT
     BytesWritten = 1*DIGIT

4.1.5. Log message

  The syntax is:
     "650" SP Severity SP ReplyText
  or
     "650+" Severity CRLF Data

     Severity = "DEBUG" / "INFO" / "NOTICE" / "WARN"/ "ERR"

4.1.6. New descriptors available

  Syntax:
     "650" SP "NEWDESC" 1*(SP ServerID)

4.1.7. New Address mapping

  Syntax:
     "650" SP "ADDRMAP" SP Address SP Address SP Expiry
     Expiry = DQOUTE ISOTime DQUOTE / "NEVER"

5. Implementation notes

5.1. 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.

5.2. 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.

5.3. Backward compatibility

  For backward compatibility with the "version 0" control protocol, Tor checks
  whether the third octet the first command is zero.  If it is, Tor
  assumes that version 0 is in use.  This feature is deprecated, and will be
  removed in the 0.1.2.x Tor development series.

  In order to detect which version of the protocol is supported controllers
  should send the sequence [00 00 0D 0A].  This is a valid and unrecognized
  command in both protocol versions, and implementations can detect which
  error they have received.