Network Working Group                                          T. Ylonen
Internet-Draft                          SSH Communications Security Corp
Expires: December 1, 2004                                    C. Lonvick, Ed.
Internet-Draft                                        Cisco Systems, Inc
                                                            June 2,
Expires: April 24, 2005                                 October 24, 2004

                        SSH Connection Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with subject to all provisions
   of Section 10 section 3 of RFC2026. RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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.

Table of Contents

   1.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   4.  Global Requests  . . . . . . . . . . . . . . . . . . . . . . .  3
   5.  Channel Mechanism  . . . . . . . . . . . . . . . . . . . . . .  4
     5.1   Opening a Channel  . . . . . . . . . . . . . . . . . . . .  4  5
     5.2   Data Transfer  . . . . . . . . . . . . . . . . . . . . . .  6
     5.3   Closing a Channel  . . . . . . . . . . . . . . . . . . . .  6  7
     5.4   Channel-Specific Requests  . . . . . . . . . . . . . . . .  7  8
   6.  Interactive Sessions . . . . . . . . . . . . . . . . . . . . .  8
     6.1   Opening a Session  . . . . . . . . . . . . . . . . . . . .  8  9
     6.2   Requesting a Pseudo-Terminal . . . . . . . . . . . . . . .  8  9
     6.3   X11 Forwarding . . . . . . . . . . . . . . . . . . . . . .  9
       6.3.1   Requesting X11 Forwarding  . . . . . . . . . . . . . .  9
       6.3.2   X11 Channels . . . . . . . . . . . . . . . . . . . . . 10
     6.4   Environment Variable Passing . . . . . . . . . . . . . . . 10 11
     6.5   Starting a Shell or a Command  . . . . . . . . . . . . . . 10 11
     6.6   Session Data Transfer  . . . . . . . . . . . . . . . . . . 11 12
     6.7   Window Dimension Change Message  . . . . . . . . . . . . . 12
     6.8   Local Flow Control . . . . . . . . . . . . . . . . . . . . 12 13
     6.9   Signals  . . . . . . . . . . . . . . . . . . . . . . . . . 12 13
     6.10  Returning Exit Status  . . . . . . . . . . . . . . . . . . 13
   7.  TCP/IP Port Forwarding . . . . . . . . . . . . . . . . . . . . 14 15
     7.1   Requesting Port Forwarding . . . . . . . . . . . . . . . . 14 15
     7.2   TCP/IP Forwarding Channels . . . . . . . . . . . . . . . . 15 16
   8.  Encoding of Terminal Modes . . . . . . . . . . . . . . . . . . 16 17
   9.  Summary of Message Numbers . . . . . . . . . . . . . . . . . . 18 19
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 18 19
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 18 19
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 19 20
   12.1  Normative References . . . . . . . . . . . . . . . . . . . . 19 20
   12.2  Informative References . . . . . . . . . . . . . . . . . . . 19
       Authors' Addresses 20
       Author's Address . . . . . . . . . . . . . . . . . . . . . . 20 . 21
       Intellectual Property and Copyright Statements . . . . . . . . 21 22

1.  Contributors

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

   Additional contributors to this document include [need list].
   Listing their names here does not mean that they endorse this
   document, but that they have contributed to it.

   Comments on this internet draft should be sent to the IETF SECSH
   working group, details at: Note: This paragraph
   will be removed before this document progresses to become an RFC.

2.  Introduction

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

   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

3.  Conventions Used in This Document


   All documents related to the SSH protocols shall use the keywords
   "SHOULD NOT", "RECOMMENDED", "MAY", and "MAY" that appear in this document "OPTIONAL" to describe
   requirements.  These keywords are to be interpreted as described in

   The used data types keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
   APPROVAL", "IETF CONSENSUS", and terminology are specified "STANDARDS ACTION" that appear in the architecture
   document [SSH-ARCH].

   The architecture
   this document also discusses the algorithm naming
   conventions that MUST be when used with the SSH protocols. to describe namespace allocation are to be
   interpreted as described in [RFC2434].

4.  Global Requests

   There are several kinds of requests that affect the state of the
   remote end "globally", globally, independent of any channels.  An example is a
   request to start TCP/IP forwarding for a specific port.  All such
   requests use the following format.

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

   Request names follow

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

   The recipient will respond to this message with

     .....     response specific data

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

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

5.  Channel Mechanism

   All terminal sessions, forwarded connections, etc. 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 'channel type' is a name as described in the SSH architecture
   document, [SSH-ARCH] and
   [SSH-NUMBERS], with similar extension mechanisms.  `sender  The 'sender
   channel' is a local identifier for the channel used by the sender of
   this message.
   `initial  The 'initial window size' specifies how many bytes of
   channel data can be sent to the sender of this message without
   adjusting the window.
   `Maximum  The 'maximum packet size' specifies the
   maximum size of an individual data packet that can be sent to the sender (for
   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

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

   where `recipient

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

      uint32    recipient channel
      uint32    reason code
      string    additional textual information    description in ISO-10646 UTF-8 encoding [RFC2279] [RFC3629]
      string    language tag as defined in [RFC3066]

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

   The following reason codes 'reason code' values are defined:


      description                             reason code
      -----------                             -----------
     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 0x00000001
   to 0xFDFFFFFF MUST be done through the IETF CONSENSUS method as
   described in [RFC2434].  The SSH_MSG_CHANNEL_OPEN 'reason code'
   values in the range of 0xFE000000 to 0xFEFFFFFF are reserved for
   PRIVATE USE.  The SSH_MSG_CHANNEL_OPEN 'reason code' values in the
   range of 0xFF000000 to 0xFFFFFFFF are also reserved for PRIVATE USE
   as described in [RFC2434].  As is noted, the actual instructions to
   the IANA is in [SSH-NUMBERS].

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.

     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.

   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 the current window size.  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.

   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 the data.  The available types
   and their interpretation depend on the type of the channel.

     uint32    recipient_channel
     uint32    data_type_code
     string    data

   Data sent with these messages consumes the same window as ordinary

   Currently, only the following type is defined.


            data                       data_type_code
            ----                       --------------
      SSH_EXTENDED_DATA_STDERR                1

5.3  Closing a Channel

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

     byte      SSH_MSG_CHANNEL_EOF
     uint32    recipient_channel

   No explicit response is sent to this message; however, 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 a 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

     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 any data sent before this message is delivered
   to the actual destination, if possible.

5.4  Channel-Specific Requests

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

   All channel-specific requests use the following format.

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

   If want reply 'want reply' is FALSE, no response will be sent to the request.
   Otherwise, the recipient responds with either SSH_MSG_CHANNEL_SUCCESS
   or 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.  Request types  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

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

     uint32    recipient_channel

     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

6.2  Requesting a Pseudo-Terminal

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

     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 encoding of 'encoded terminal modes is modes' are described in Section Encoding of
   Terminal Modes (Section 8). 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

     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 RECOMMENDED that the 'x11 authentication cookie cookie' that is sent
   be a fake, random cookie, and that the cookie is 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,
   closed.  However, already opened forwardings should not be
   automatically closed when the session channel is closed.

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

   The x11 'x11 authentication cookie 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. "")
     uint32    originator port

   The recipient should respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION

   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.

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

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.

     uint32    recipient channel
     string    "shell"
     boolean   want reply

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

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

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

     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 scripts, etc.  This spurious output from the shell may
   be filtered out either at the server or at the client.

   The server SHOULD not 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 to request and check the reply for these messages.
   The client SHOULD ignore these messages.

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

     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 '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.

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

   No response is sent to this message.

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.

     uint32    recipient channel
     string    "signal"
     boolean   FALSE
     string    signal name without the "SIG" prefix.

   Signal names will be encoded as discussed in the "exit-signal"

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 acknowledgment 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.

     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.
      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 as defined in [RFC3066]

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


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

   The `error 'error message' contains an additional textual explanation of the
   error message.  The message may consist of multiple lines.  The
   client software MAY display this message to the user.  If this is
   done, the client software should take the precautions discussed in

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.

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


   The 'address to bind' and `port 'port number to bind' specify the IP
   address and port to which the socket to be listened is bound.  The
   address should be "" if connections are allowed from anywhere.
   (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 'want reply' TRUE
   then the server allocates the next available unprivileged port number
   and replies with the following message, otherwise there is no
   response specific data.

     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.

     string    "cancel-tcpip-forward"
     boolean   want reply
     string    address_to_bind (e.g. "")
     uint32    port number to bind
   Client implementations SHOULD reject these messages; they are
   normally only sent by the client.

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

     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 be sent also 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 'port to connect' specify the TCP/IP host
   and port where the recipient should connect the channel.  `Host  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
   where the connection request comes from, and `originator the 'originator port' is
   the port on the originator host from where the connection came from. 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.

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

8.  Encoding of Terminal Modes

   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 tty mode description is a stream of bytes.  The stream consists
   of opcode-argument pairs.  It is terminated by opcode TTY_OP_END (0).
   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 client SHOULD put in the stream any modes it knows about, 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 following opcodes have been defined.  The naming of opcodes
   mostly follows the POSIX terminal mode flags.

   opcode  argument       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
   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
   9     VSTOP       Pauses output (normally control-S).

   10    VSUSP       Suspends the current program.
   11    VDSUSP      Another suspend character.
   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
                      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 set,
                      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
   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
   74    ONOCR       Translate newline to carriage
                      return-newline (output).

   75    ONLRET      Newline performs a carriage return
   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

     SSH_MSG_GLOBAL_REQUEST                  80
     SSH_MSG_REQUEST_SUCCESS                 81
     SSH_MSG_REQUEST_FAILURE                 82
     SSH_MSG_CHANNEL_OPEN                    90
     SSH_MSG_CHANNEL_DATA                    94
     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.

12.  References

12.1  Normative References

              Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
              I-D draft-ietf-architecture-16.txt, May draft-ietf-architecture-17.txt, October 2004.

              Ylonen, T. and C. Lonvick, "SSH Transport Layer Protocol",
              I-D draft-ietf-transport-18.txt, May draft-ietf-transport-19.txt, October 2004.

              Ylonen, T. and C. Lonvick, "SSH Authentication Protocol",
              I-D draft-ietf-userauth-21.txt, May draft-ietf-userauth-22.txt, October 2004.

              Ylonen, T. and C. Lonvick, "SSH Protocol Assigned
              Numbers", I-D draft-ietf-assignednumbers-06.txt, May draft-ietf-assignednumbers-07.txt, October

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

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

12.2  Informative References

   [RFC1884]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 1884, December 1995.

   [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2279, January 2434,
              October 1998.

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

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

   [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
   Fredrikinkatu 42
   HELSINKI  FIN-00100


Author's Address

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


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