r11895@catbus: nickm | 2007-02-23 15:12:58 -0500

[tor.git] / doc / spec / 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.

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

  ; 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" / "GUARDS" / "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.

  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" / "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.

    "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 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 of all known OR identities.
      This is in the same format as the router-status line in 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.)

    "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.

    "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:
         ServerID  SP (Status-with-time / Status) CRLF

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

      [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.]

    "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/..."
      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.)

  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 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: 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" ("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.)

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] 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
   the new address as a response.

4.1.3. OR Connection status changed

  The syntax is:
    "650" SP "ORCONN" SP (ServerID / Target) SP ORStatus [ SP "REASON="
             Reason ] [ SP "NCIRCS=" NumCircuits ]

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

  NEW is for incoming connections, and LAUNCHED is for outgoing
  connections. CONNECTED means the TLS handshake has finished (in
  either direction). FAILED means a connection is being closed that
  hasn't finished its handshake, and CLOSED is for connections that
  have handshaked.

  A ServerID is specified unless it's a NEW connection, in which
  case we don't know what server it is yet, so we use Address:Port.

  If extended events are enabled (see 3.19), optional reason and
  circuit counting information is provided for CLOSED and FAILED
  events. 

      Reason = "MISC" / "DONE" / "CONNECTREFUSED" /
               "IDENTITY" / "CONNECTRESET" / "TIMEOUT" / "NOROUTE" /
               "IOERROR" 

  NumCircuits counts both established and pending circuits.


4.1.4. Bandwidth used in the last second

  The syntax is:
     "650" SP "BW" SP BytesRead SP BytesWritten *(SP Type "=" Num)
     BytesRead = 1*DIGIT
     BytesWritten = 1*DIGIT
     Type = "DIR" / "OR" / "EXIT" / "APP" / ...
     Num = 1*DIGIT

  BytesRead and BytesWritten are the totals. In Tor 0.1.x.y-alpha
  and later, we also include a breakdown of the connection types
  that used bandwidth this second (not implemented yet).

4.1.5. Log messages

  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 = DQUOTE ISOTime DQUOTE / "NEVER"

  Expiry is expressed as the local time (rather than GMT).

  [XXX We should rename this to ADDRESSMAP. -RD]
    [Why? Surely it can't be worth the compatibility issues. -NM]

4.1.8. Descriptors uploaded to us in our role as authoritative dirserver

  Syntax:
     "650" "+" "AUTHDIR_NEWDESCS" CRLF Action CRLF Message CRLF
       Descriptor CRLF "." CRLF
     Action = "ACCEPTED" / "DROPPED" / "REJECTED"
     Message = Text

4.1.9. Our descriptor changed

  Syntax:
     "650" SP "DESCCHANGED"

  [First added in 0.1.2.2-alpha.]

4.1.10. Status events

  Status events (STATUS_GENERAL, STATUS_CLIENT, and STATUS_SERVER) are sent
  based on occurrences in the Tor process pertaining to the general state of
  the program.  Generally, they correspond to log messages of severity Notice
  or higher.  They differ from log messages in that their format is a
  specified interface.

  Syntax:
     "650" SP StatusType SP StatusSeverity SP StatusAction
                                         [SP StatusArguments] CRLF

     StatusType = "STATUS_GENERAL" / "STATUS_CLIENT" / "STATUS_SERVER"
     StatusSeverity = "NOTICE" / "WARN" / "ERR"
     StatusAction = 1*ALPHA
     StatusArguments = StatusArgument *(SP StatusArgument)
     StatusArgument = StatusKeyword '=' StatusValue
     StatusKeyword = 1*(ALNUM / "_")
     StatusValue = 1*(ALNUM / '_')  / QuotedString

     Action is a string, and Arguments is a series of keyword=value
     pairs on the same line.  Values may be space-terminated strings,
     or quoted strings.

     These events are always produced with EXTENDED_EVENTS and
     VERBOSE_NAMES; see the explanations in the USEFEATURE section
     for details.

     Controllers MUST tolerate unrecognized actions, MUST tolerate
     unrecognized arguments, MUST tolerate missing arguments, and MUST
     tolerate arguments that arrive in any order.

     Each event description below is accompanied by a recommendation for
     controllers.  These recommendations are suggestions only; no controller
     is required to implement them.

  Actions for STATUS_GENERAL events can be as follows:

     CLOCK_JUMPED
     "TIME=NUM"
       Tor spent enough time without CPU cycles that it has closed all
       its circuits and will establish them anew. This typically
       happens when a laptop goes to sleep and then wakes up again. It
       also happens when the system is swapping so heavily that Tor is
       starving. The "time" argument specifies the number of seconds Tor
       thinks it was unconscious for (or alternatively, the number of
       seconds it went back in time).

       This status event is sent as NOTICE severity normally, but WARN
       severity if Tor is acting as a server currently.

       {Recommendation for controller: ignore it, since we don't really
       know what the user should do anyway. Hm.}

     DANGEROUS_VERSION
     "CURRENT=version"
     "REASON=NEW/OLD/UNRECOMMENDED"
     "RECOMMENDED=\"version, version, ...\""
       Tor has found that directory servers don't recommend its version of
       the Tor software.  RECOMMENDED is a comma-and-space-separated string
       of Tor versions that are recommended.  REASON is NEW if this version
       of Tor is newer than any recommended version, OLD if this version of
       Tor is older than any recommended version, and UNRECOMMENDED if
       some recommended versions of Tor are newer and some are old than this
       version.

       {Controllers may want to suggest that the user upgrade OLD or
       UNRECOMMENDED versions.  NEW versions may be known-insecure, or may
       simply be development versions.}

     TOO_MANY_CONNECTIONS
     "CURRENT=NUM"
       Tor has reached its ulimit -n or whatever the native limit is on file
       descriptors or sockets.  CURRENT is the number of sockets Tor
       currently has open.  The user should really do something about
       this. The "current" argument shows the number of connections currently
       open.

       {Controllers may recommend that the user increase the limit, or
       increase it for them.  Recommendations should be phrased in an
       OS-appropriate way and automated when possible.}

     BUG
     "REASON=STRING"
       Tor has encountered a situation that its developers never expected,
       and the developers would like to learn that it happened. Perhaps
       the controller can explain this to the user and encourage her to
       file a bug report?

       {Controllers should log bugs, but shouldn't annoy the user in case a
       bug appears frequently.}

     CLOCK_SKEWED
       SKEW="+" / "-" SECONDS
       SOURCE="DIRSERV:IP:Port" / "NETWORKSTATUS:IP:PORT"
         If "SKEW" is present, it's an estimate of how far we are from the
         time declared in the source.  If the source is a DIRSERV, we got
         the current time from a connection to a dirserver.  If the source is
         a NETWORKSTATUS, we decided we're skewed because we got a
         networkstatus from far in the future.

         {Controllers may want to warn the user if the skew is high, or if
         multiple skew messages appear at severity WARN.  Controllers
         shouldn't blindly adjust the clock, since the more accurate source
         of skew info (DIRSERV) is currently unauthenticated.}

     BAD_LIBEVENT
     "METHOD=" libevent method
     "VERSION=" libevent version
     "BADNESS=" "BROKEN" / "BUGGY" / "SLOW"
     "RECOVERED=" "NO" / "YES"
        Tor knows about bugs in using the configured event method in this
        version of libevent.  "BROKEN" libevents won't work at all;
        "BUGGY" libevents might work okay; "SLOW" libevents will work
        fine, but not quickly.  If "RECOVERED" is YES, Tor managed to
        switch to a more reliable (but probably slower!) libevent method.

        {Controllers may want to warn the user if this event occurs, though
        generally it's the fault of whoever built the Tor binary and there's
        not much the user can do besides upgrade libevent or upgrade the
        binary.}

     DIR_ALL_UNREACHABLE
       Tor believes that none of the known directory servers are
       reachable -- this is most likely because the local network is
       down or otherwise not working, and might help to explain for the
       user why Tor appears to be broken.

       {Controllers may want to warn the user if this event occurs; further
       action is generally not possible.}

  Actions for STATUS_CLIENT events can be as follows:

     ENOUGH_DIR_INFO
       Tor now knows enough network-status documents and enough server
       descriptors that it's going to start trying to build circuits now.

       {Controllers may want to use this event to decide when to indicate
       progress to their users, but should not interrupt the user's browsing
       to tell them so.}

     NOT_ENOUGH_DIR_INFO
       We discarded expired statuses and router descriptors to fall
       below the desired threshold of directory information. We won't
       try to build any circuits until ENOUGH_DIR_INFO occurs again.

       {Controllers may want to use this event to decide when to indicate
       progress to their users, but should not interrupt the user's browsing
       to tell them so.}

     CIRCUIT_ESTABLISHED
       Tor is able to establish circuits for client use. This event will
       only be sent if we just built a circuit that changed our mind --
       that is, prior to this event we didn't know whether we could
       establish circuits.

       {Suggested use: controllers can notify their users that Tor is
       ready for use as a client once they see this status event. [Perhaps
       controllers should also have a timeout if too much time passes and
       this event hasn't arrived, to give tips on how to troubleshoot.
       On the other hand, hopefully Tor will send further status events
       if it can identify the problem.]}

     CIRCUIT_NOT_ESTABLISHED
     "REASON=" "EXTERNAL_ADDRESS" / "DIR_ALL_UNREACHABLE" / "CLOCK_JUMPED"
       We are no longer confident that we can build circuits. The "reason"
       keyword provides an explanation: which other status event type caused
       our lack of confidence.

       {Controllers may want to use this event to decide when to indicate
       progress to their users, but should not interrupt the user's browsing
       to do so.}
       [Note: only REASON=CLOCK_JUMPED is implemented currently.]

     DANGEROUS_SOCKS
     "PROTOCOL=SOCKS4/SOCKS5"
     "ADDRESS=IP:port"
       A connection was made to Tor's SOCKS port using one of the SOCKS
       approaches that doesn't support hostnames -- only raw IP addresses.
       If the client application got this address from gethostbyname(),
       it may be leaking target addresses via DNS.

       {Controllers should warn their users when this occurs, unless they
       happen to know that the application using Tor is in fact doing so
       correctly (e.g., because it is part of a distributed bundle).}

     SOCKS_UNKNOWN_PROTOCOL
       "DATA=string"
       A connection was made to Tor's SOCKS port that tried to use it
       for something other than the SOCKS protocol. Perhaps the user is
       using Tor as an HTTP proxy?   The DATA is the first few characters
       sent to Tor on the SOCKS port.

       {Controllers may want to warn their users when this occurs: it
       indicates a misconfigured application.}

     SOCKS_BAD_HOSTNAME
      "HOSTNAME=QuotedString"
       Some application gave us a funny-looking hostname. Perhaps
       it is broken? In any case it won't work with Tor and the user
       should know.

       {Controllers may want to warn their users when this occurs: it
       usually indicates a misconfigured application.}

  Actions for STATUS_SERVER can be as follows:

     EXTERNAL_ADDRESS
     "ADDRESS=IP"
     "HOSTNAME=NAME"
     "METHOD=CONFIGURED/DIRSERV/RESOLVED/INTERFACE/GETHOSTNAME"
       Our best idea for our externally visible IP has changed to 'IP'.
       If 'HOSTNAME' is present, we got the new IP by resolving 'NAME'.  If the
       method is 'CONFIGURED', the IP was given verbatim as a configuration
       option.  If the method is 'RESOLVED', we resolved the Address
       configuration option to get the IP.  If the method is 'GETHOSTNAME',
       we resolved our hostname to get the IP.  If the method is 'INTERFACE',
       we got the address of one of our network interfaces to get the IP.  If
       the method is 'DIRSERV', a directory server told us a guess for what
       our IP might be.

       {Controllers may want to record this info and display it to the user.}

     CHECKING_REACHABILITY
     "ORADDRESS=IP:port"
     "DIRADDRESS=IP:port"
       We're going to start testing the reachability of our external OR port
       or directory port.

       {This event could effect the controller's idea of server status, but
       the controller should not interrupt the user to tell them so.}

     REACHABILITY_SUCCEEDED
     "ORADDRESS=IP:port"
     "DIRADDRESS=IP:port"
       We successfully verified the reachability of our external OR port or
       directory port.

       {This event could effect the controller's idea of server status, but
       the controller should not interrupt the user to tell them so.}

     GOOD_SERVER_DESCRIPTOR
       We successfully uploaded our server descriptor to each of the
       directory authorities, with no complaints.

       {This event could effect the controller's idea of server status, but
       the controller should not interrupt the user to tell them so.}

     NAMESERVER_STATUS
     "NS=addr"
     "STATUS=" "UP" / "DOWN"
     "ERR=" message
        One of our nameservers has changed status.
        // actually notice

       {This event could effect the controller's idea of server status, but
       the controller should not interrupt the user to tell them so.}

     NAMESERVER_ALL_DOWN
        All of our nameservers have gone down.

        {This is a problem; if it happens often without the nameservers
        coming up again, the user needs to configure more or better
        nameservers.}

     DNS_HIJACKED
        Our DNS provider is providing an address when it should be saying
        "NOTFOUND"; Tor will treat the address as a synonym for "NOTFOUND".

        {This is an annoyance; controllers may want to tell admins that their
        DNS provider is not to be trusted.}

     DNS_USELESS
        Our DNS provider is giving a hijacked address instead of well-known
        websites; Tor will not try to be an exit node.

        {Controllers could warn the admin if the server is running as an
        exit server: the admin needs to configure a good DNS server.
        Alternatively, this happens a lot in some restrictive environments
        (hotels, universities, coffeeshops) when the user hasn't registered.}

     BAD_SERVER_DESCRIPTOR
     "DIRAUTH=addr:port"
     "REASON=string"
        A directory authority rejected our descriptor.  Possible reasons
        include malformed descriptors, incorrect keys, highly skewed clocks,
        and so on.

        {Controllers should warn the admin, and try to cope if they can.}

     ACCEPTED_SERVER_DESCRIPTOR
     "DIRAUTH=addr:port"
        A single directory authority accepted our descriptor.
        // actually notice

       {This event could effect the controller's idea of server status, but
       the controller should not interrupt the user to tell them so.}

     REACHABILITY_FAILED
     "ORADDRESS=IP:port"
     "DIRADDRESS=IP:port"
       We failed to connect to our external OR port or directory port
       successfully.

       {This event could effect the controller's idea of server status.  The
       controller should warn the admin and suggest reasonable steps to take.}

4.1.11. Our set of guard nodes has changed

  Syntax:
     "650" SP "GUARDS" SP Type SP Name SP Status ... CRLF
     Type = "ENTRY"
     Name = The (possibly verbose) nickname of the guard affected.
     Status = "NEW" | "UP" | "DOWN" | "BAD" | "GOOD" | "DROPPED"

  [explain states. XXX]

4.1.12. Network status has changed

  Syntax:
     "650" "+" "NS" CRLF  1*NetworkStatus "." CRLF

  [First added in 0.1.2.3-alpha]

4.1.13. Bandwidth used on an application stream

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

  BytesRead and BytesWritten are the number of bytes read and written since
  the last STREAM_BW event on a stream.  These events are generated about
  once per second per stream; No event is generated for streams that have not
  read or written.

  These events apply only to streams entering Tor (such as on a SOCKSPort,
  TransPort, or so on).  They are not generated for exiting streams.

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, encoded in hexadecimal.

  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 with v0 control protocol.

  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 0.1.3.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.