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RFC 4254

The Secure Shell (SSH) Connection Protocol

Pages: 24
Proposed Standard
Errata
Updated by:  8308

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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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   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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   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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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.
Top   ToC   RFC4254 - Page 20
          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).
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          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.
Top   ToC   RFC4254 - Page 22

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.
Top   ToC   RFC4254 - Page 23
   [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.
Top   ToC   RFC4254 - Page 24
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