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.  (Version 0 is used by Tor
  versions 0.1.0.x; the protocol in this document only works with Tor
  versions in the 0.1.1.x series and later.)

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 RFC 2234.
  The protocol itself is loosely based on SMTP (see RFC 2821).

  We use the following nonterminals from RFC 2822: 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.

  Wherever CRLF is specified to be accepted from the controller, Tor MAY also
  accept LF.  Tor, however, MUST NOT generate LF instead of CRLF.
  Controllers SHOULD always send CRLF.

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 = SyncReply / AsyncReply
    SyncReply = *(MidReplyLine / DataReplyLine) EndReplyLine
    AsyncReply = *(MidReplyLine / DataReplyLine) EndReplyLine

    MidReplyLine = StatusCode "-" ReplyLine
    DataReplyLine = StatusCode "+" ReplyLine Data
    EndReplyLine = StatusCode SP ReplyLine
    ReplyLine = [ReplyText] CRLF
    ReplyText = XXXX
    StatusCode = 3DIGIT

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

  [Compatibility note:  versions of Tor before 0.2.0.3-alpha sometimes
  generate AsyncReplies of the form "*(MidReplyLine / DataReplyLine)".
  This is incorrect, but controllers that need to work with these
  versions of Tor should be prepared to get multi-line AsyncReplies with
  the final line (usually "650 OK") omitted.]

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

  ; A "=" indicates that the given nickname is canonical; a "~" indicates
  ; that the given nickname is not canonical.
  LongName = Fingerprint [ ( "=" / "~" ) Nickname ]

  ; How a controller tells Tor about a particular OR.  There are four
  ; possible formats:
  ;    $Digest -- The router whose identity key hashes to the given digest.
  ;        This is the preferred way to refer to an OR.
  ;    $Digest~Name -- The router whose identity key hashes to the given
  ;        digest, but only if the router has the given nickname.
  ;    $Digest=Name -- The router whose identity key hashes to the given
  ;        digest, but only if the router is Named and has the given
  ;        nickname.
  ;    Name -- The Named router with the given nickname, or, if no such
  ;        router exists, any router whose nickname matches the one given.
  ;        This is not a safe way to refer to routers, since Named status
  ;        could under some circumstances change over time.
  ServerSpec = LongName / Nickname

  ; 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 RFC 2821 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.
  If some of the listed keywords can't be found, Tor replies with a
  "552 Unrecognized option" message. Otherwise, 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" /
         "AUTHDIR_NEWDESCS" / "DESCCHANGED" / "STATUS_GENERAL" /
         "STATUS_CLIENT" / "STATUS_SERVER" / "GUARD" / "NS" / "STREAM_BW"

  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.

  Each event is described in more detail in Section 4.1.

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.  Tor closes the connection on an
  authentication failure.

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

  Before the client has authenticated, no command other than PROTOCOLINFO,
  AUTHENTICATE, or QUIT is valid.  If the controller sends any other command,
  or sends a malformed command, or sends an unsuccessful AUTHENTICATE
  command, or sends PROTOCOLINFO more than once, Tor sends an error reply and
  closes the connection.

  (Versions of Tor before 0.1.2.16 and 0.2.0.4-alpha did not close the
  connection after an authentication failure.)

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" / "NEWNYM" /
              "CLEARDNSCACHE"

  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)
      CLEARDNSCACHE -- Forget the client-side cached IPs for all hostnames.
      NEWNYM    -- Switch to clean circuits, so new application requests
                   don't share any circuits with old ones.  Also clears
                   the client-side DNS cache.  (Tor MAY rate-limit its
                   response to this signal.)

  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, or a 551 or 552 error.

  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").

    ["exit-policy/prepend" -- The default exit policy lines that Tor will
      *prepend* to the ExitPolicy config option.
     -- Never implemented. Useful?]

    "exit-policy/default" -- The default exit policy lines that Tor will
      *append* to the ExitPolicy config option.

    "desc/id/<OR identity>" or "desc/name/<OR nickname>" -- the latest
      server descriptor for a given OR, NUL-terminated.

    "extra-info/digest/<digest>"  -- the extrainfo document whose digest (in
      hex) is <digest>.  Only available if we're downloading extra-info
      documents.

    "ns/id/<OR identity>" or "ns/name/<OR nickname>" -- the latest network
      status info for a given OR.  Network status info is as given in
      dir-spec.txt, and reflects the current beliefs of this Tor about the
      router in question. Like directory clients, controllers MUST
      tolerate unrecognized flags and lines.  The published date and
      descriptor digest are those believed to be best by this Tor,
      not necessarily those for a descriptor that Tor currently has.
      [First implemented in 0.1.2.3-alpha.]

    "ns/all" -- Network status info (v2 directory style) for all ORs we
      have an opinion about, joined by newlines. [First implemented
      in 0.1.2.3-alpha.]

    "desc/all-recent" -- the latest server descriptor for every router that
      Tor knows about.

    "network-status" -- a space-separated list (v1 directory style)
      of all known OR identities. This is in the same format as the
      router-status line in v1 directories; see dir-spec-v1.txt section
      3 for details.  (If VERBOSE_NAMES is enabled, the output will
      not conform to dir-spec-v1.txt; instead, the result will be a
      space-separated list of LongName, each preceded by a "!" if it is
      believed to be not running.)

    "address-mappings/all"
    "address-mappings/config"
    "address-mappings/cache"
    "address-mappings/control" -- a \r\n-separated list of address
      mappings, each in the form of "from-address to-address expiry".
      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.
      Expiry is formatted as with ADDRMAP events, except that "expiry" is
      always a time in GMT or the string "NEVER"; see section 4.1.7.
      First introduced in 0.2.0.3-alpha.

    "addr-mappings/*" -- as for address-mappings/*, but without the
      expiry portion of the value.  Use of this value is deprecated
      since 0.2.0.3-alpha; use address-mappings instead.

    "address" -- the best guess at our external IP address. If we
      have no guess, return a 551 error. (Added in 0.1.2.2-alpha)

    "fingerprint" -- the contents of the fingerprint file that Tor
      writes as a server, or a 551 if we're not a server currently.
      (Added in 0.1.2.3-alpha)

    "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

    "entry-guards"
      A series of lines listing the currently chosen entry guards, if any.
      Each is of the form:
         ServerID2 SP Status [SP ISOTime] CRLF

         Status-with-time = ("unlisted") SP ISOTime
         Status = ("up" / "never-connected" / "down" /
                      "unusable" / "unlisted" )

         ServerID2 = Nickname / 40*HEXDIG

      [From 0.1.1.4-alpha to 0.1.1.10-alpha, this was called "helper-nodes".
       Tor still supports calling it that for now, but support will be
       removed in 0.1.3.x.]

      [Older versions of Tor (before 0.1.2.x-final) generated 'down' instead
       of unlisted/unusable.  Current Tors never generate 'down'.]

      [XXXX ServerID2 differs from ServerID in not prefixing fingerprints
       with a $.  This is an implementation error.  It would be nice to add
       the $ back in if we can do so without breaking compatibility.]

    "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 "/*"

    "events/names"
      A space-separated list of all the events supported by this version of
      Tor's SETEVENTS.

    "features/names"
      A space-separated list of all the events supported by this version of
      Tor's USEFEATURE.

    "next-circuit/IP:port"
      XXX todo.

    "dir/status/authority"
    "dir/status/fp/<F>"
    "dir/status/fp/<F1>+<F2>+<F3>"
    "dir/status/all"
    "dir/server/fp/<F>"
    "dir/server/fp/<F1>+<F2>+<F3>"
    "dir/server/d/<D>"
    "dir/server/d/<D1>+<D2>+<D3>"
    "dir/server/authority"
    "dir/server/all"
      A series of lines listing directory contents, provided according to the
      specification for the URLs listed in Section 4.4 of dir-spec.txt.  Note
      that Tor MUST NOT provide private information, such as descriptors for
      routers not marked as general-purpose.  When asked for 'authority'
      information for which this Tor is not authoritative, Tor replies with
      an empty string.

    "status/circuit-established"
    "status/enough-dir-info"
    "status/good-server-descriptor"
    "status/..."
      These provide the current internal Tor values for various Tor
      states. See Section 4.1.10 for explanations. (Only a few of the
      status events are available as getinfo's currently. Let us know if
      you want more exposed.)<
    "status/reachability/or"
      0 or 1, depending on whether we've found our ORPort reachable.
    "status/reachability/dir"
      0 or 1, depending on whether we've found our DirPort reachable.
    "status/reachability"
      "OR=" ("0"/"1") SP "DIR=" ("0"/"1")
      Combines status/reachability/*; controllers MUST ignore unrecognized
      elements in this entry.

    "status/version/recommended" -- List of currently recommended versions
    "status/version/current" -- Status of the current version. One of:
        new, old, unrecommended, recommended, new in series, obsolete.
    "status/version/num-versioning" -- Number of versioning authorities
    "status/version/num-concurring" -- Number of versioning authorities
        agreeing on the status of the current version

  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
                      ServerSpec *("," ServerSpec)
                      [SP "purpose=" Purpose] CRLF

  This request takes one of two forms: either the CircuitID is zero, in
  which case it is a request for the server to build a new circuit according
  to the specified path, or the CircuitID 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 CircuitID is 0 and "purpose=" is specified, then the circuit's
  purpose is set. Two choices are recognized: "general" and
  "controller". If not specified, circuits are created as "general".

  If the request is successful, the server sends a reply containing a
  message body consisting of the CircuitID of the (maybe newly created)
  circuit. The syntax is "250" SP "EXTENDED" SP CircuitID CRLF.

3.11. SETCIRCUITPURPOSE

  Sent from the client to the server.  The format is:
      "SETCIRCUITPURPOSE" SP CircuitID SP Purpose CRLF

  This changes the circuit's purpose. See EXTENDCIRCUIT above for details.

3.12. SETROUTERPURPOSE

  Sent from the client to the server.  The format is:
      "SETROUTERPURPOSE" SP NicknameOrKey SP Purpose CRLF

  This changes the descriptor's purpose. See +POSTDESCRIPTOR below
  for details.

3.13. ATTACHSTREAM

  Sent from the client to the server.  The syntax is:
     "ATTACHSTREAM" SP StreamID SP CircuitID [SP "HOP=" HopNum] 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.

  If HOP=HopNum is specified, Tor will choose the HopNumth hop in the
  circuit as the exit node, rather than the last node in the circuit.
  Hops are 1-indexed; generally, it is not permitted to attach to hop 1.

  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: Tor will close unattached streams by itself,
  roughly two minutes after they are born. Let the developers know if
  that turns out to be a problem.}

  {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.}

  {Implementation note: You can try to attachstream to a stream that
  has already sent a connect or resolve request but hasn't succeeded
  yet, in which case Tor will detach the stream from its current circuit
  before proceeding with the new attach request.}

3.14. POSTDESCRIPTOR

  Sent from the client to the server. The syntax is:
    "+POSTDESCRIPTOR" [SP "purpose=" Purpose] CRLF Descriptor CRLF "." CRLF

  This message informs the server about a new descriptor. If Purpose is
  specified, it must be either "general" or "controller", else we
  return a 552 error.

  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".

3.15. REDIRECTSTREAM

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

  Tells the server to change the exit address on the specified stream.  If
  Port is specified, changes the destination port as well.  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.

3.16. 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.

  Tor replies with "250 OK" on success, or a 512 if there aren't enough
  arguments, or a 552 if it doesn't recognize the StreamID or reason.

3.17. 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.

  Tor replies with "250 OK" on success, or a 512 if there aren't enough
  arguments, or a 552 if it doesn't recognize the CircuitID.

3.18. QUIT

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

3.19. USEFEATURE

  The syntax is:

    "USEFEATURE" *(SP FeatureName) CRLF
    FeatureName = 1*(ALPHA / DIGIT / "_" / "-")

  Sometimes extensions to the controller protocol break compatibility with
  older controllers.  In this case, whenever possible, the extensions are
  first included in Tor disabled by default, and only enabled on a given
  controller connection when the "USEFEATURE" command is given.  Once a
  "USEFEATURE" command is given, it applies to all subsequent interactions on
  the same connection; to disable an enabled feature, a new controller
  connection must be opened.

  This is a forward-compatibility mechanism; each feature will eventually
  become a regular part of the control protocol in some future version of Tor.
  Tor will ignore a request to use any feature that is already on by default.
  Tor will give a "552" error if any requested feature is not recognized.

  Feature names are case-insensitive.

  EXTENDED_EVENTS

     Same as passing 'EXTENDED' to SETEVENTS; this is the preferred way to
     request the extended event syntax.

     This will not be always-enabled until at least XXX (or, at least two
     stable releases after XXX, the release where it was first used for
     anything.)

  VERBOSE_NAMES

     Instead of ServerID as specified above, the controller should
     identify ORs by LongName in events and GETINFO results.  This format is
     strictly more informative: rather than including Nickname for
     known Named routers and Fingerprint for unknown or unNamed routers, the
     LongName format includes a Fingerprint, an indication of Named status,
     and a Nickname (if one is known).

     This will not be always-enabled until at least 0.1.4.x (or at least two
     stable releases after 0.1.2.2-alpha, the release where it was first
     available.)

3.20. RESOLVE

  The syntax is
    "RESOLVE" *Option *Address CRLF
    Option = "mode=reverse"
    Address = a hostname or IPv4 address

  This command launches a remote hostname lookup request for every specified
  request (or reverse lookup if "mode=reverse" is specified).  Note that the
  request is done in the background: to see the answers, your controller will
  need to listen for ADDRMAP events; see 4.1.7 below.

  [Added in Tor 0.2.0.3-alpha]

3.21. PROTOCOLINFO

  The syntax is:
    "PROTOCOLINFO" *(SP PIVERSION) CRLF

  The server reply format is:
    "250-PROTOCOLINFO" SP PIVERSION CRLF *InfoLine "250 OK" CRLF

    InfoLine = AuthLine / VersionLine / OtherLine

     AuthLine = "250-AUTH" SP "METHODS=" AuthMethod *(",")AuthMethod
                       *(SP "COOKIEFILE=" AuthCookieFile) CRLF
     VersionLine = "250-VERSION" SP "Tor=" TorVersion [SP Arguments] CRLF

     AuthMethod =
      "NULL"           / ; No authentication is required
      "HASHEDPASSWORD" / ; A controller must supply the original password
      "COOKIE"         / ; A controller must supply the contents of a cookie

     AuthCookieFile = QuotedString
     TorVersion = QuotedString

     OtherLine = "250-" Keyword [SP Arguments] CRLF

    PIVERSION: 1*DIGIT

  Tor MAY give its InfoLines in any order; controllers MUST ignore InfoLines
  with keywords they do not recognize.  Controllers MUST ignore extraneous
  data on any InfoLine.

  PIVERSION is there in case we drastically change the syntax one day. For
  now it should always be "1".  Controllers MAY provide a list of the
  protocolinfo versions they support; Tor MAY select a version that the
  controller does not support.

  AuthMethod is used to specify one or more control authentication
  methods that Tor currently accepts.

  AuthCookieFile specifies the absolute path and filename of the
  authentication cookie that Tor is expecting and is provided iff
  the METHODS field contains the method "COOKIE".  Controllers MUST handle
  escape sequences inside this string.

  The VERSION line contains the Tor version.

  [Unlike other commands besides AUTHENTICATE, PROTOCOLINFO may be used (but
  only once!) before AUTHENTICATE.]

  [PROTOCOLINFO was not supported before Tor 0.2.0.5-alpha.]

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 MUST tolerate more arguments in an asynchonous reply than
  expected, and MUST 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
  must 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. Additional lines will be of the
  form
      "650" ("-"/" ") KEYWORD ["=" ARGUMENTS] CRLF
  Additional arguments will be of the form
      SP KEYWORD ["=" ( QuotedString / * 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]
          [SP "REASON=" Reason [SP "REMOTE_REASON=" Reason]] CRLF

      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)

      Reason = "NONE" / "TORPROTOCOL" / "INTERNAL" / "REQUESTED" /
               "HIBERNATING" / "RESOURCELIMIT" / "CONNECTFAILED" /
               "OR_IDENTITY" / "OR_CONN_CLOSED" / "TIMEOUT" /
               "FINISHED" / "DESTROYED" / "NOPATH" / "NOSUCHSERVICE"

   The path is provided only when the circuit has been extended at least one
   hop.

   The "REASON" field is provided only for FAILED and CLOSED events, and only
   if extended events are enabled (see 3.19).  Clients MUST accept reasons
   not listed above.  Reasons are as given in tor-spec.txt, except for:

      NOPATH          (Not enough nodes to make circuit)

   The "REMOTE_REASON" field is provided only when we receive a DESTROY or
   TRUNCATE cell, and only if extended events are enabled.  It contains the
   actual reason given by the remote OR for closing the circuit. Clients MUST
   accept reasons not listed above.  Reasons are as listed in tor-spec.txt.

4.1.2. Stream status changed

    The syntax is:

      "650" SP "STREAM" SP StreamID SP StreamStatus SP CircID SP Target
          [SP "REASON=" Reason [ SP "REMOTE_REASON=" Reason ]]
          [SP "SOURCE=" Source] [ SP "SOURCE_ADDR=" Address ":" Port ]
          CRLF

      StreamStatus =
               "NEW"          / ; New request to connect
               "NEWRESOLVE"   / ; New request to resolve an address
               "REMAP"        / ; Address re-mapped to another
               "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.

      Reason = "MISC" / "RESOLVEFAILED" / "CONNECTREFUSED" /
               "EXITPOLICY" / "DESTROY" / "DONE" / "TIMEOUT" /
               "HIBERNATING" / "INTERNAL"/ "RESOURCELIMIT" /
               "CONNRESET" / "TORPROTOCOL" / "NOTDIRECTORY" / "END"

   The "REASON" field is provided only for FAILED, CLOSED, and DETACHED
   events, and only if extended events are enabled (see 3.19).  Clients MUST
   accept reasons not listed above.  Reasons are as given in tor-spec.txt,
   except for:

      END          (We received a RELAY_END cell from the other side of thise
                    stream.)
      [XXXX document more.]

   The "REMOTE_REASON" field is provided only when we receive a RELAY_END
   cell, and only if extended events are enabled.  It contains the actual
   reason given by the remote OR for closing the stream. Clients MUST accept
   reasons not listed above.  Reasons are as listed in tor-spec.txt.

   "REMAP" events include a Source if extended events are enabled:
      Source = "CACHE" / "EXIT"
   Clients MUST accept sources not listed above.  "CACHE" is given if
   the Tor client decided to remap the address because of a cached
   answer, and "EXIT" is given if the remote node we queried gave us