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PROPOSED STANDARD
Errata Exist
Network Working Group                                          T. Ylonen
Request for Comments: 4254              SSH Communications Security Corp
Category: Standards Track                                C. Lonvick, Ed.
                                                     Cisco Systems, Inc.
                                                            January 2006


               The Secure Shell (SSH) Connection Protocol

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Secure Shell (SSH) is a protocol for secure remote login and other
   secure network services over an insecure network.

   This document describes the SSH Connection Protocol.  It provides
   interactive login sessions, remote execution of commands, forwarded
   TCP/IP connections, and forwarded X11 connections.  All of these
   channels are multiplexed into a single encrypted tunnel.

   The SSH Connection Protocol has been designed to run on top of the
   SSH transport layer and user authentication protocols.


















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Table of Contents

   1. Introduction ....................................................2
   2. Contributors ....................................................3
   3. Conventions Used in This Document ...............................3
   4. Global Requests .................................................4
   5. Channel Mechanism ...............................................5
      5.1. Opening a Channel ..........................................5
      5.2. Data Transfer ..............................................7
      5.3. Closing a Channel ..........................................9
      5.4. Channel-Specific Requests ..................................9
   6. Interactive Sessions ...........................................10
      6.1. Opening a Session .........................................10
      6.2. Requesting a Pseudo-Terminal ..............................11
      6.3. X11 Forwarding ............................................11
           6.3.1. Requesting X11 Forwarding ..........................11
           6.3.2. X11 Channels .......................................12
      6.4. Environment Variable Passing ..............................12
      6.5. Starting a Shell or a Command .............................13
      6.6. Session Data Transfer .....................................14
      6.7. Window Dimension Change Message ...........................14
      6.8. Local Flow Control ........................................14
      6.9. Signals ...................................................15
      6.10. Returning Exit Status ....................................15
   7. TCP/IP Port Forwarding .........................................16
      7.1. Requesting Port Forwarding ................................16
      7.2. TCP/IP Forwarding Channels ................................18
   8. Encoding of Terminal Modes .....................................19
   9. Summary of Message Numbers .....................................21
   10. IANA Considerations ...........................................21
   11. Security Considerations .......................................21
   12. References ....................................................22
      12.1. Normative References .....................................22
      12.2. Informative References ...................................22
   Authors' Addresses ................................................23
   Trademark Notice ..................................................23

1.  Introduction

   The SSH Connection Protocol has been designed to run on top of the
   SSH transport layer and user authentication protocols ([SSH-TRANS]
   and [SSH-USERAUTH]).  It provides interactive login sessions, remote
   execution of commands, forwarded TCP/IP connections, and forwarded
   X11 connections.

   The 'service name' for this protocol is "ssh-connection".





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   This document should be read only after reading the SSH architecture
   document [SSH-ARCH].  This document freely uses terminology and
   notation from the architecture document without reference or further
   explanation.

2.  Contributors

   The major original contributors of this set of documents have been:
   Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
   Communications Security Corp), and Markku-Juhani O. Saarinen
   (University of Jyvaskyla).  Darren Moffat was the original editor of
   this set of documents and also made very substantial contributions.

   Many people contributed to the development of this document over the
   years.  People who should be acknowledged include Mats Andersson, Ben
   Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
   Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
   Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
   Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
   Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
   Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
   Tadayoshi Kohno.  Listing their names here does not mean that they
   endorse this document, but that they have contributed to it.

3.  Conventions Used in This Document

   All documents related to the SSH protocols shall use the keywords
   "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
   requirements.  These keywords are to be interpreted as described in
   [RFC2119].

   The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
   FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
   APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
   this document when used to describe namespace allocation are to be
   interpreted as described in [RFC2434].

   Protocol fields and possible values to fill them are defined in this
   set of documents.  Protocol fields will be defined in the message
   definitions.  As an example, SSH_MSG_CHANNEL_DATA is defined as
   follows.

      byte      SSH_MSG_CHANNEL_DATA
      uint32    recipient channel
      string    data





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   Throughout these documents, when the fields are referenced, they will
   appear within single quotes.  When values to fill those fields are
   referenced, they will appear within double quotes.  Using the above
   example, possible values for 'data' are "foo" and "bar".

4.  Global Requests

   There are several kinds of requests that affect the state of the
   remote end globally, independent of any channels.  An example is a
   request to start TCP/IP forwarding for a specific port.  Note that
   both the client and server MAY send global requests at any time, and
   the receiver MUST respond appropriately.  All such requests use the
   following format.

      byte      SSH_MSG_GLOBAL_REQUEST
      string    request name in US-ASCII only
      boolean   want reply
      ....      request-specific data follows

   The value of 'request name' follows the DNS extensibility naming
   convention outlined in [SSH-ARCH].

   The recipient will respond to this message with
   SSH_MSG_REQUEST_SUCCESS or SSH_MSG_REQUEST_FAILURE if 'want reply' is
   TRUE.

      byte      SSH_MSG_REQUEST_SUCCESS
      ....     response specific data

   Usually, the 'response specific data' is non-existent.

   If the recipient does not recognize or support the request, it simply
   responds with SSH_MSG_REQUEST_FAILURE.

      byte      SSH_MSG_REQUEST_FAILURE

   In general, the reply messages do not include request type
   identifiers.  To make it possible for the originator of a request to
   identify to which request each reply refers, it is REQUIRED that
   replies to SSH_MSG_GLOBAL_REQUESTS MUST be sent in the same order as
   the corresponding request messages.  For channel requests, replies
   that relate to the same channel MUST also be replied to in the right
   order.  However, channel requests for distinct channels MAY be
   replied to out-of-order.







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5.  Channel Mechanism

   All terminal sessions, forwarded connections, etc., are channels.
   Either side may open a channel.  Multiple channels are multiplexed
   into a single connection.

   Channels are identified by numbers at each end.  The number referring
   to a channel may be different on each side.  Requests to open a
   channel contain the sender's channel number.  Any other channel-
   related messages contain the recipient's channel number for the
   channel.

   Channels are flow-controlled.  No data may be sent to a channel until
   a message is received to indicate that window space is available.

5.1.  Opening a Channel

   When either side wishes to open a new channel, it allocates a local
   number for the channel.  It then sends the following message to the
   other side, and includes the local channel number and initial window
   size in the message.

      byte      SSH_MSG_CHANNEL_OPEN
      string    channel type in US-ASCII only
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size
      ....      channel type specific data follows

   The 'channel type' is a name, as described in [SSH-ARCH] and
   [SSH-NUMBERS], with similar extension mechanisms.  The 'sender
   channel' is a local identifier for the channel used by the sender of
   this message.  The 'initial window size' specifies how many bytes of
   channel data can be sent to the sender of this message without
   adjusting the window.  The 'maximum packet size' specifies the
   maximum size of an individual data packet that can be sent to the
   sender.  For example, one might want to use smaller packets for
   interactive connections to get better interactive response on slow
   links.

   The remote side then decides whether it can open the channel, and
   responds with either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
   SSH_MSG_CHANNEL_OPEN_FAILURE.








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      byte      SSH_MSG_CHANNEL_OPEN_CONFIRMATION
      uint32    recipient channel
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size
      ....      channel type specific data follows

   The 'recipient channel' is the channel number given in the original
   open request, and 'sender channel' is the channel number allocated by
   the other side.

      byte      SSH_MSG_CHANNEL_OPEN_FAILURE
      uint32    recipient channel
      uint32    reason code
      string    description in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

   If the recipient of the SSH_MSG_CHANNEL_OPEN message does not support
   the specified 'channel type', it simply responds with
   SSH_MSG_CHANNEL_OPEN_FAILURE.  The client MAY show the 'description'
   string to the user.  If this is done, the client software should take
   the precautions discussed in [SSH-ARCH].

   The SSH_MSG_CHANNEL_OPEN_FAILURE 'reason code' values are defined in
   the following table.  Note that the values for the 'reason code' are
   given in decimal format for readability, but they are actually uint32
   values.

             Symbolic name                           reason code
             -------------                           -----------
            SSH_OPEN_ADMINISTRATIVELY_PROHIBITED          1
            SSH_OPEN_CONNECT_FAILED                       2
            SSH_OPEN_UNKNOWN_CHANNEL_TYPE                 3
            SSH_OPEN_RESOURCE_SHORTAGE                    4

   Requests for assignments of new SSH_MSG_CHANNEL_OPEN 'reason code'
   values (and associated 'description' text) in the range of 0x00000005
   to 0xFDFFFFFF MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].  The IANA will not assign Channel Connection
   Failure 'reason code' values in the range of 0xFE000000 to
   0xFFFFFFFF.  Channel Connection Failure 'reason code' values in that
   range are left for PRIVATE USE, as described in [RFC2434].

   While it is understood that the IANA will have no control over the
   range of 0xFE000000 to 0xFFFFFFFF, this range will be split in two
   parts and administered by the following conventions.





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   o  The range of 0xFE000000 to 0xFEFFFFFF is to be used in conjunction
      with locally assigned channels.  For example, if a channel is
      proposed with a 'channel type' of "example_session@example.com",
      but fails, then the response will contain either a 'reason code'
      assigned by the IANA (as listed above and in the range of
      0x00000001 to 0xFDFFFFFF) or a locally assigned value in the range
      of 0xFE000000 to 0xFEFFFFFF.  Naturally, if the server does not
      understand the proposed 'channel type', even if it is a locally
      defined 'channel type', then the 'reason code' MUST be 0x00000003,
      as described above, if the 'reason code' is sent.  If the server
      does understand the 'channel type', but the channel still fails to
      open, then the server SHOULD respond with a locally assigned
      'reason code' value consistent with the proposed, local 'channel
      type'.  It is assumed that practitioners will first attempt to use
      the IANA assigned 'reason code' values and then document their
      locally assigned 'reason code' values.

   o  There are no restrictions or suggestions for the range starting
      with 0xFF.  No interoperability is expected for anything used in
      this range.  Essentially, it is for experimentation.

5.2.  Data Transfer

   The window size specifies how many bytes the other party can send
   before it must wait for the window to be adjusted.  Both parties use
   the following message to adjust the window.

      byte      SSH_MSG_CHANNEL_WINDOW_ADJUST
      uint32    recipient channel
      uint32    bytes to add

   After receiving this message, the recipient MAY send the given number
   of bytes more than it was previously allowed to send; the window size
   is incremented.  Implementations MUST correctly handle window sizes
   of up to 2^32 - 1 bytes.  The window MUST NOT be increased above
   2^32 - 1 bytes.

   Data transfer is done with messages of the following type.

      byte      SSH_MSG_CHANNEL_DATA
      uint32    recipient channel
      string    data

   The maximum amount of data allowed is determined by the maximum
   packet size for the channel, and the current window size, whichever
   is smaller.  The window size is decremented by the amount of data
   sent.  Both parties MAY ignore all extra data sent after the allowed
   window is empty.



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   Implementations are expected to have some limit on the SSH transport
   layer packet size (any limit for received packets MUST be 32768 bytes
   or larger, as described in [SSH-TRANS]).  The implementation of the
   SSH connection layer

   o  MUST NOT advertise a maximum packet size that would result in
      transport packets larger than its transport layer is willing to
      receive.

   o  MUST NOT generate data packets larger than its transport layer is
      willing to send, even if the remote end would be willing to accept
      very large packets.

   Additionally, some channels can transfer several types of data.  An
   example of this is stderr data from interactive sessions.  Such data
   can be passed with SSH_MSG_CHANNEL_EXTENDED_DATA messages, where a
   separate integer specifies the type of data.  The available types and
   their interpretation depend on the type of channel.

      byte      SSH_MSG_CHANNEL_EXTENDED_DATA
      uint32    recipient channel
      uint32    data_type_code
      string    data

   Data sent with these messages consumes the same window as ordinary
   data.

   Currently, only the following type is defined.  Note that the value
   for the 'data_type_code' is given in decimal format for readability,
   but the values are actually uint32 values.

               Symbolic name                  data_type_code
               -------------                  --------------
             SSH_EXTENDED_DATA_STDERR               1

   Extended Channel Data Transfer 'data_type_code' values MUST be
   assigned sequentially.  Requests for assignments of new Extended
   Channel Data Transfer 'data_type_code' values and their associated
   Extended Channel Data Transfer 'data' strings, in the range of
   0x00000002 to 0xFDFFFFFF, MUST be done through the IETF CONSENSUS
   method as described in [RFC2434].  The IANA will not assign Extended
   Channel Data Transfer 'data_type_code' values in the range of
   0xFE000000 to 0xFFFFFFFF.  Extended Channel Data Transfer
   'data_type_code' values in that range are left for PRIVATE USE, as
   described in [RFC2434].  As is noted, the actual instructions to the
   IANA are in [SSH-NUMBERS].





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5.3.  Closing a Channel

   When a party will no longer send more data to a channel, it SHOULD
   send SSH_MSG_CHANNEL_EOF.

      byte      SSH_MSG_CHANNEL_EOF
      uint32    recipient channel

   No explicit response is sent to this message.  However, the
   application may send EOF to whatever is at the other end of the
   channel.  Note that the channel remains open after this message, and
   more data may still be sent in the other direction.  This message
   does not consume window space and can be sent even if no window space
   is available.

   When either party wishes to terminate the channel, it sends
   SSH_MSG_CHANNEL_CLOSE.  Upon receiving this message, a party MUST
   send back an SSH_MSG_CHANNEL_CLOSE unless it has already sent this
   message for the channel.  The channel is considered closed for a
   party when it has both sent and received SSH_MSG_CHANNEL_CLOSE, and
   the party may then reuse the channel number.  A party MAY send
   SSH_MSG_CHANNEL_CLOSE without having sent or received
   SSH_MSG_CHANNEL_EOF.

      byte      SSH_MSG_CHANNEL_CLOSE
      uint32    recipient channel

   This message does not consume window space and can be sent even if no
   window space is available.

   It is RECOMMENDED that all data sent before this message be delivered
   to the actual destination, if possible.

5.4.  Channel-Specific Requests

   Many 'channel type' values have extensions that are specific to that
   particular 'channel type'.  An example is requesting a pty (pseudo
   terminal) for an interactive session.

   All channel-specific requests use the following format.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    request type in US-ASCII characters only
      boolean   want reply
      ....      type-specific data follows





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   If 'want reply' is FALSE, no response will be sent to the request.
   Otherwise, the recipient responds with either
   SSH_MSG_CHANNEL_SUCCESS, SSH_MSG_CHANNEL_FAILURE, or request-specific
   continuation messages.  If the request is not recognized or is not
   supported for the channel, SSH_MSG_CHANNEL_FAILURE is returned.

   This message does not consume window space and can be sent even if no
   window space is available.  The values of 'request type' are local to
   each channel type.

   The client is allowed to send further messages without waiting for
   the response to the request.

   'request type' names follow the DNS extensibility naming convention
   outlined in [SSH-ARCH] and [SSH-NUMBERS].

      byte      SSH_MSG_CHANNEL_SUCCESS
      uint32    recipient channel


      byte      SSH_MSG_CHANNEL_FAILURE
      uint32    recipient channel

   These messages do not consume window space and can be sent even if no
   window space is available.

6.  Interactive Sessions

   A session is a remote execution of a program.  The program may be a
   shell, an application, a system command, or some built-in subsystem.
   It may or may not have a tty, and may or may not involve X11
   forwarding.  Multiple sessions can be active simultaneously.

6.1.  Opening a Session

   A session is started by sending the following message.

      byte      SSH_MSG_CHANNEL_OPEN
      string    "session"
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size

   Client implementations SHOULD reject any session channel open
   requests to make it more difficult for a corrupt server to attack the
   client.





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6.2.  Requesting a Pseudo-Terminal

   A pseudo-terminal can be allocated for the session by sending the
   following message.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "pty-req"
      boolean   want_reply
      string    TERM environment variable value (e.g., vt100)
      uint32    terminal width, characters (e.g., 80)
      uint32    terminal height, rows (e.g., 24)
      uint32    terminal width, pixels (e.g., 640)
      uint32    terminal height, pixels (e.g., 480)
      string    encoded terminal modes

   The 'encoded terminal modes' are described in Section 8.  Zero
   dimension parameters MUST be ignored.  The character/row dimensions
   override the pixel dimensions (when nonzero).  Pixel dimensions refer
   to the drawable area of the window.

   The dimension parameters are only informational.

   The client SHOULD ignore pty requests.

6.3.  X11 Forwarding

6.3.1.  Requesting X11 Forwarding

   X11 forwarding may be requested for a session by sending a
   SSH_MSG_CHANNEL_REQUEST message.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "x11-req"
      boolean   want reply
      boolean   single connection
      string    x11 authentication protocol
      string    x11 authentication cookie
      uint32    x11 screen number

   It is RECOMMENDED that the 'x11 authentication cookie' that is sent
   be a fake, random cookie, and that the cookie be checked and replaced
   by the real cookie when a connection request is received.

   X11 connection forwarding should stop when the session channel is
   closed.  However, already opened forwardings should not be
   automatically closed when the session channel is closed.



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   If 'single connection' is TRUE, only a single connection should be
   forwarded.  No more connections will be forwarded after the first, or
   after the session channel has been closed.

   The 'x11 authentication protocol' is the name of the X11
   authentication method used, e.g., "MIT-MAGIC-COOKIE-1".

   The 'x11 authentication cookie' MUST be hexadecimal encoded.

   The X Protocol is documented in [SCHEIFLER].

6.3.2.  X11 Channels

   X11 channels are opened with a channel open request.  The resulting
   channels are independent of the session, and closing the session
   channel does not close the forwarded X11 channels.

      byte      SSH_MSG_CHANNEL_OPEN
      string    "x11"
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size
      string    originator address (e.g., "192.168.7.38")
      uint32    originator port

   The recipient should respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION
   or SSH_MSG_CHANNEL_OPEN_FAILURE.

   Implementations MUST reject any X11 channel open requests if they
   have not requested X11 forwarding.

6.4.  Environment Variable Passing

   Environment variables may be passed to the shell/command to be
   started later.  Uncontrolled setting of environment variables in a
   privileged process can be a security hazard.  It is recommended that
   implementations either maintain a list of allowable variable names or
   only set environment variables after the server process has dropped
   sufficient privileges.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "env"
      boolean   want reply
      string    variable name
      string    variable value





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6.5.  Starting a Shell or a Command

   Once the session has been set up, a program is started at the remote
   end.  The program can be a shell, an application program, or a
   subsystem with a host-independent name.  Only one of these requests
   can succeed per channel.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "shell"
      boolean   want reply

   This message will request that the user's default shell (typically
   defined in /etc/passwd in UNIX systems) be started at the other end.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "exec"
      boolean   want reply
      string    command

   This message will request that the server start the execution of the
   given command.  The 'command' string may contain a path.  Normal
   precautions MUST be taken to prevent the execution of unauthorized
   commands.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "subsystem"
      boolean   want reply
      string    subsystem name

   This last form executes a predefined subsystem.  It is expected that
   these will include a general file transfer mechanism, and possibly
   other features.  Implementations may also allow configuring more such
   mechanisms.  As the user's shell is usually used to execute the
   subsystem, it is advisable for the subsystem protocol to have a
   "magic cookie" at the beginning of the protocol transaction to
   distinguish it from arbitrary output generated by shell
   initialization scripts, etc.  This spurious output from the shell may
   be filtered out either at the server or at the client.

   The server SHOULD NOT halt the execution of the protocol stack when
   starting a shell or a program.  All input and output from these
   SHOULD be redirected to the channel or to the encrypted tunnel.

   It is RECOMMENDED that the reply to these messages be requested and
   checked.  The client SHOULD ignore these messages.



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   Subsystem names follow the DNS extensibility naming convention
   outlined in [SSH-NUMBERS].

6.6.  Session Data Transfer

   Data transfer for a session is done using SSH_MSG_CHANNEL_DATA and
   SSH_MSG_CHANNEL_EXTENDED_DATA packets and the window mechanism.  The
   extended data type SSH_EXTENDED_DATA_STDERR has been defined for
   stderr data.

6.7.  Window Dimension Change Message

   When the window (terminal) size changes on the client side, it MAY
   send a message to the other side to inform it of the new dimensions.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "window-change"
      boolean   FALSE
      uint32    terminal width, columns
      uint32    terminal height, rows
      uint32    terminal width, pixels
      uint32    terminal height, pixels

   A response SHOULD NOT be sent to this message.

6.8.  Local Flow Control

   On many systems, it is possible to determine if a pseudo-terminal is
   using control-S/control-Q flow control.  When flow control is
   allowed, it is often desirable to do the flow control at the client
   end to speed up responses to user requests.  This is facilitated by
   the following notification.  Initially, the server is responsible for
   flow control.  (Here, again, client means the side originating the
   session, and server means the other side.)

   The message below is used by the server to inform the client when it
   can or cannot perform flow control (control-S/control-Q processing).
   If 'client can do' is TRUE, the client is allowed to do flow control
   using control-S and control-Q.  The client MAY ignore this message.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "xon-xoff"
      boolean   FALSE
      boolean   client can do

   No response is sent to this message.



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6.9.  Signals

   A signal can be delivered to the remote process/service using the
   following message.  Some systems may not implement signals, in which
   case they SHOULD ignore this message.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "signal"
      boolean   FALSE
      string    signal name (without the "SIG" prefix)

   'signal name' values will be encoded as discussed in the passage
   describing SSH_MSG_CHANNEL_REQUEST messages using "exit-signal" in
   this section.

6.10.  Returning Exit Status

   When the command running at the other end terminates, the following
   message can be sent to return the exit status of the command.
   Returning the status is RECOMMENDED.  No acknowledgement is sent for
   this message.  The channel needs to be closed with
   SSH_MSG_CHANNEL_CLOSE after this message.

   The client MAY ignore these messages.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "exit-status"
      boolean   FALSE
      uint32    exit_status

   The remote command may also terminate violently due to a signal.
   Such a condition can be indicated by the following message.  A zero
   'exit_status' usually means that the command terminated successfully.

      byte      SSH_MSG_CHANNEL_REQUEST
      uint32    recipient channel
      string    "exit-signal"
      boolean   FALSE
      string    signal name (without the "SIG" prefix)
      boolean   core dumped
      string    error message in ISO-10646 UTF-8 encoding
      string    language tag [RFC3066]







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RFC 4254                SSH Connection Protocol             January 2006


   The 'signal name' is one of the following (these are from [POSIX]).

            ABRT
            ALRM
            FPE
            HUP
            ILL
            INT
            KILL
            PIPE
            QUIT
            SEGV
            TERM
            USR1
            USR2

   Additional 'signal name' values MAY be sent in the format
   "sig-name@xyz", where "sig-name" and "xyz" may be anything a
   particular implementer wants (except the "@" sign).  However, it is
   suggested that if a 'configure' script is used, any non-standard
   'signal name' values it finds be encoded as "SIG@xyz.config.guess",
   where "SIG" is the 'signal name' without the "SIG" prefix, and "xyz"
   is the host type, as determined by "config.guess".

   The 'error message' contains an additional textual explanation of the
   error message.  The message may consist of multiple lines separated
   by CRLF (Carriage Return - Line Feed) pairs.  The client software MAY
   display this message to the user.  If this is done, the client
   software should take the precautions discussed in [SSH-ARCH].

7.  TCP/IP Port Forwarding

7.1.  Requesting Port Forwarding

   A party need not explicitly request forwardings from its own end to
   the other direction.  However, if it wishes that connections to a
   port on the other side be forwarded to the local side, it must
   explicitly request this.

      byte      SSH_MSG_GLOBAL_REQUEST
      string    "tcpip-forward"
      boolean   want reply
      string    address to bind (e.g., "0.0.0.0")
      uint32    port number to bind







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RFC 4254                SSH Connection Protocol             January 2006


   The 'address to bind' and 'port number to bind' specify the IP
   address (or domain name) and port on which connections for forwarding
   are to be accepted.  Some strings used for 'address to bind' have
   special-case semantics.

   o  "" means that connections are to be accepted on all protocol
      families supported by the SSH implementation.

   o  "0.0.0.0" means to listen on all IPv4 addresses.

   o  "::" means to listen on all IPv6 addresses.

   o  "localhost" means to listen on all protocol families supported by
      the SSH implementation on loopback addresses only ([RFC3330] and
      [RFC3513]).

   o  "127.0.0.1" and "::1" indicate listening on the loopback
      interfaces for IPv4 and IPv6, respectively.

   Note that the client can still filter connections based on
   information passed in the open request.

   Implementations should only allow forwarding privileged ports if the
   user has been authenticated as a privileged user.

   Client implementations SHOULD reject these messages; they are
   normally only sent by the client.

   If a client passes 0 as port number to bind and has 'want reply' as
   TRUE, then the server allocates the next available unprivileged port
   number and replies with the following message; otherwise, there is no
   response-specific data.

      byte     SSH_MSG_REQUEST_SUCCESS
      uint32   port that was bound on the server

   A port forwarding can be canceled with the following message.  Note
   that channel open requests may be received until a reply to this
   message is received.

      byte      SSH_MSG_GLOBAL_REQUEST
      string    "cancel-tcpip-forward"
      boolean   want reply
      string    address_to_bind (e.g., "127.0.0.1")
      uint32    port number to bind

   Client implementations SHOULD reject these messages; they are
   normally only sent by the client.



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RFC 4254                SSH Connection Protocol             January 2006


7.2.  TCP/IP Forwarding Channels

   When a connection comes to a port for which remote forwarding has
   been requested, a channel is opened to forward the port to the other
   side.

      byte      SSH_MSG_CHANNEL_OPEN
      string    "forwarded-tcpip"
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size
      string    address that was connected
      uint32    port that was connected
      string    originator IP address
      uint32    originator port

   Implementations MUST reject these messages unless they have
   previously requested a remote TCP/IP port forwarding with the given
   port number.

   When a connection comes to a locally forwarded TCP/IP port, the
   following packet is sent to the other side.  Note that these messages
   MAY also be sent for ports for which no forwarding has been
   explicitly requested.  The receiving side must decide whether to
   allow the forwarding.

      byte      SSH_MSG_CHANNEL_OPEN
      string    "direct-tcpip"
      uint32    sender channel
      uint32    initial window size
      uint32    maximum packet size
      string    host to connect
      uint32    port to connect
      string    originator IP address
      uint32    originator port

   The 'host to connect' and 'port to connect' specify the TCP/IP host
   and port where the recipient should connect the channel.  The 'host
   to connect' may be either a domain name or a numeric IP address.

   The 'originator IP address' is the numeric IP address of the machine
   from where the connection request originates, and the 'originator
   port' is the port on the host from where the connection originated.

   Forwarded TCP/IP channels are independent of any sessions, and
   closing a session channel does not in any way imply that forwarded
   connections should be closed.




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   Client implementations SHOULD reject direct TCP/IP open requests for
   security reasons.

8.  Encoding of Terminal Modes

   All 'encoded terminal modes' (as passed in a pty request) are encoded
   into a byte stream.  It is intended that the coding be portable
   across different environments.  The stream consists of opcode-
   argument pairs wherein the opcode is a byte value.  Opcodes 1 to 159
   have a single uint32 argument.  Opcodes 160 to 255 are not yet
   defined, and cause parsing to stop (they should only be used after
   any other data).  The stream is terminated by opcode TTY_OP_END
   (0x00).

   The client SHOULD put any modes it knows about in the stream, and the
   server MAY ignore any modes it does not know about.  This allows some
   degree of machine-independence, at least between systems that use a
   POSIX-like tty interface.  The protocol can support other systems as
   well, but the client may need to fill reasonable values for a number
   of parameters so the server pty gets set to a reasonable mode (the
   server leaves all unspecified mode bits in their default values, and
   only some combinations make sense).

   The naming of opcode values mostly follows the POSIX terminal mode
   flags.  The following opcode values have been defined.  Note that the
   values given below are in decimal format for readability, but they
   are actually byte values.

          opcode  mnemonic       description
          ------  --------       -----------
          0     TTY_OP_END  Indicates end of options.
          1     VINTR       Interrupt character; 255 if none.  Similarly
                             for the other characters.  Not all of these
                             characters are supported on all systems.
          2     VQUIT       The quit character (sends SIGQUIT signal on
                             POSIX systems).
          3     VERASE      Erase the character to left of the cursor.
          4     VKILL       Kill the current input line.
          5     VEOF        End-of-file character (sends EOF from the
                             terminal).
          6     VEOL        End-of-line character in addition to
                             carriage return and/or linefeed.
          7     VEOL2       Additional end-of-line character.
          8     VSTART      Continues paused output (normally
                             control-Q).
          9     VSTOP       Pauses output (normally control-S).
          10    VSUSP       Suspends the current program.
          11    VDSUSP      Another suspend character.



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RFC 4254                SSH Connection Protocol             January 2006


          12    VREPRINT    Reprints the current input line.
          13    VWERASE     Erases a word left of cursor.
          14    VLNEXT      Enter the next character typed literally,
                             even if it is a special character
          15    VFLUSH      Character to flush output.
          16    VSWTCH      Switch to a different shell layer.
          17    VSTATUS     Prints system status line (load, command,
                             pid, etc).
          18    VDISCARD    Toggles the flushing of terminal output.
          30    IGNPAR      The ignore parity flag.  The parameter
                             SHOULD be 0 if this flag is FALSE,
                             and 1 if it is TRUE.
          31    PARMRK      Mark parity and framing errors.
          32    INPCK       Enable checking of parity errors.
          33    ISTRIP      Strip 8th bit off characters.
          34    INLCR       Map NL into CR on input.
          35    IGNCR       Ignore CR on input.
          36    ICRNL       Map CR to NL on input.
          37    IUCLC       Translate uppercase characters to
                             lowercase.
          38    IXON        Enable output flow control.
          39    IXANY       Any char will restart after stop.
          40    IXOFF       Enable input flow control.
          41    IMAXBEL     Ring bell on input queue full.
          50    ISIG        Enable signals INTR, QUIT, [D]SUSP.
          51    ICANON      Canonicalize input lines.
          52    XCASE       Enable input and output of uppercase
                             characters by preceding their lowercase
                             equivalents with "\".
          53    ECHO        Enable echoing.
          54    ECHOE       Visually erase chars.
          55    ECHOK       Kill character discards current line.
          56    ECHONL      Echo NL even if ECHO is off.
          57    NOFLSH      Don't flush after interrupt.
          58    TOSTOP      Stop background jobs from output.
          59    IEXTEN      Enable extensions.
          60    ECHOCTL     Echo control characters as ^(Char).
          61    ECHOKE      Visual erase for line kill.
          62    PENDIN      Retype pending input.
          70    OPOST       Enable output processing.
          71    OLCUC       Convert lowercase to uppercase.
          72    ONLCR       Map NL to CR-NL.
          73    OCRNL       Translate carriage return to newline
                             (output).
          74    ONOCR       Translate newline to carriage
                             return-newline (output).
          75    ONLRET      Newline performs a carriage return
                             (output).



Ylonen & Lonvick            Standards Track                    [Page 20]

RFC 4254                SSH Connection Protocol             January 2006


          90    CS7         7 bit mode.
          91    CS8         8 bit mode.
          92    PARENB      Parity enable.
          93    PARODD      Odd parity, else even.

          128 TTY_OP_ISPEED  Specifies the input baud rate in
                              bits per second.
          129 TTY_OP_OSPEED  Specifies the output baud rate in
                              bits per second.

9.  Summary of Message Numbers

   The following is a summary of messages and their associated message
   number.

            SSH_MSG_GLOBAL_REQUEST                  80
            SSH_MSG_REQUEST_SUCCESS                 81
            SSH_MSG_REQUEST_FAILURE                 82
            SSH_MSG_CHANNEL_OPEN                    90
            SSH_MSG_CHANNEL_OPEN_CONFIRMATION       91
            SSH_MSG_CHANNEL_OPEN_FAILURE            92
            SSH_MSG_CHANNEL_WINDOW_ADJUST           93
            SSH_MSG_CHANNEL_DATA                    94
            SSH_MSG_CHANNEL_EXTENDED_DATA           95
            SSH_MSG_CHANNEL_EOF                     96
            SSH_MSG_CHANNEL_CLOSE                   97
            SSH_MSG_CHANNEL_REQUEST                 98
            SSH_MSG_CHANNEL_SUCCESS                 99
            SSH_MSG_CHANNEL_FAILURE                100

10.  IANA Considerations

   This document is part of a set.  The IANA considerations for the SSH
   protocol as defined in [SSH-ARCH], [SSH-TRANS], [SSH-USERAUTH], and
   this document, are detailed in [SSH-NUMBERS].

11.  Security Considerations

   This protocol is assumed to run on top of a secure, authenticated
   transport.  User authentication and protection against network-level
   attacks are assumed to be provided by the underlying protocols.

   Full security considerations for this protocol are provided in
   [SSH-ARCH].  Specific to this document, it is RECOMMENDED that
   implementations disable all the potentially dangerous features (e.g.,
   agent forwarding, X11 forwarding, and TCP/IP forwarding) if the host
   key has changed without notice or explanation.




Ylonen & Lonvick            Standards Track                    [Page 21]

RFC 4254                SSH Connection Protocol             January 2006


12.  References

12.1.  Normative References

   [SSH-ARCH]     Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Protocol Architecture", RFC 4251, January 2006.

   [SSH-TRANS]    Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Transport Layer Protocol", RFC 4253, January
                  2006.

   [SSH-USERAUTH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Authentication Protocol", RFC 4252, January
                  2006.

   [SSH-NUMBERS]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Protocol Assigned Numbers", RFC 4250, January
                  2006.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26, RFC
                  2434, October 1998.

   [RFC3066]      Alvestrand, H., "Tags for the Identification of
                  Languages", BCP 47, RFC 3066, January 2001.

   [RFC3629]      Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.

12.2.  Informative References

   [RFC3330]      IANA, "Special-Use IPv4 Addresses", RFC 3330,
                  September 2002.

   [RFC3513]      Hinden, R. and S. Deering, "Internet Protocol Version
                  6 (IPv6) Addressing Architecture", RFC 3513, April
                  2003.

   [SCHEIFLER]    Scheifler, R., "X Window System : The Complete
                  Reference to Xlib, X Protocol, Icccm, Xlfd, 3rd
                  edition.", Digital Press ISBN 1555580882, February
                  1992.






Ylonen & Lonvick            Standards Track                    [Page 22]

RFC 4254                SSH Connection Protocol             January 2006


   [POSIX]        ISO/IEC, 9945-1., "Information technology -- Portable
                  Operating System Interface  (POSIX)-Part 1: System
                  Application Program Interface (API) C Language", ANSI/
                  IEE Std 1003.1, July 1996.

Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Valimotie 17
   00380 Helsinki
   Finland

   EMail: ylo@ssh.com


   Chris Lonvick (editor)
   Cisco Systems, Inc.
   12515 Research Blvd.
   Austin  78759
   USA

   EMail: clonvick@cisco.com

Trademark Notice

   "ssh" is a registered trademark in the United States and/or other
   countries.























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RFC 4254                SSH Connection Protocol             January 2006


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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