Network Working Group T. Ylonen
INTERNET-DRAFT T. Kivinen
draft-ietf-secsh-connect-09.txt M. Saarinen
Expires: 9 July, 2001 T. Rinne
S. Lehtinen
SSH Communications Security
9 January, 2001
SSH Connection Protocol
Status of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
SSH is a protocol for secure remote login and other secure network ser-
vices over an insecure network. This document describes the SSH Connec-
tion Protocol. It provides interactive login sessions, remote execution
of commands, forwarded TCP/IP connections, and forwarded X11 connec-
tions. 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. Global Requests . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Channel Mechanism . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Opening a Channel . . . . . . . . . . . . . . . . . . . . . 3
3.2. Data Transfer . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Closing a Channel . . . . . . . . . . . . . . . . . . . . . 5
3.4. Channel-Specific Requests . . . . . . . . . . . . . . . . . 5
4. Interactive Sessions . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Opening a Session . . . . . . . . . . . . . . . . . . . . . 6
4.2. Requesting a Pseudo-Terminal . . . . . . . . . . . . . . . . 7
4.3. X11 Forwarding . . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. Requesting X11 Forwarding . . . . . . . . . . . . . . . 7
4.3.2. X11 Channels . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Environment Variable Passing . . . . . . . . . . . . . . . . 8
4.5. Starting a Shell or a Command . . . . . . . . . . . . . . . 8
4.6. Session Data Transfer . . . . . . . . . . . . . . . . . . . 9
4.7. Window Dimension Change Message . . . . . . . . . . . . . . 9
4.8. Local Flow Control . . . . . . . . . . . . . . . . . . . . . 10
4.9. Signals . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.10. Returning Exit Status . . . . . . . . . . . . . . . . . . . 10
5. TCP/IP Port Forwarding . . . . . . . . . . . . . . . . . . . . . 11
5.1. Requesting Port Forwarding . . . . . . . . . . . . . . . . . 11
5.2. TCP/IP Forwarding Channels . . . . . . . . . . . . . . . . . 12
6. Encoding of Terminal Modes . . . . . . . . . . . . . . . . . . . 13
7. Summary of Message Numbers . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 15
9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
1. 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 (after user authentication) 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 explanation.
2. 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. All such requests use the
following format.
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byte SSH_MSG_GLOBAL_REQUEST
string request name (restricted to US-ASCII)
boolean want reply
... request-specific data follows
The recipient will respond to this message with SSH_MSG_REQUEST_SUCCESS,
SSH_MSG_REQUEST_FAILURE, or some request-specific continuation messages
if `want reply' is TRUE.
byte SSH_MSG_REQUEST_SUCCESS
If the recipient does not recognize or support the request, it simply
responds with SSH_MSG_REQUEST_FAILURE.
byte SSH_MSG_REQUEST_FAILURE
3. 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.
3.1. 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 (restricted to US-ASCII)
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 the SSH architecture
document, with similar extension mechanisms. `sender channel' is a local
identifier for the channel used by the sender of this message. `initial
window size' specifies how many bytes of channel data can be sent to the
sender of this message without adjusting the window. `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).
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The remote side then decides whether it can open the channel, and
responds with either
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
where `recipient channel' is the channel number given in the original
open request, and `sender channel' is the channel number allocated by
the other side, or
byte SSH_MSG_CHANNEL_OPEN_FAILURE
uint32 recipient channel
uint32 reason code
string additional textual information (ISO-10646 UTF-8
[RFC-2279])
string language tag (as defined in [RFC-1766])
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 additional
information to the user. If this is done, the client software should
take the precautions discussed in [SSH-ARCH].
The following reason codes are defined:
#define SSH_OPEN_ADMINISTRATIVELY_PROHIBITED 1
#define SSH_OPEN_CONNECT_FAILED 2
#define SSH_OPEN_UNKNOWN_CHANNEL_TYPE 3
#define SSH_OPEN_RESOURCE_SHORTAGE 4
3.2. 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.
Data transfer is done with messages of the following type.
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data
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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.
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.
#define SSH_EXTENDED_DATA_STDERR 1
3.3. 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 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 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 any data sent before this message is delivered to
the actual destination, if possible.
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3.4. Channel-Specific Requests
Many channel types 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 (restricted to US-ASCII)
boolean want reply
... type-specific data
If 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 are local to each channel type.
The client is allowed to send further messages without waiting for the
response to the request.
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.
4. 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.
4.1. 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
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to make it more difficult for a corrupt server to attack the client.
4.2. 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., 480)
uint32 terminal height, pixels (e.g., 640)
string encoded terminal modes
The encoding of terminal modes is described in Section ``Encoding of
Terminal Modes''. 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.
4.3. X11 Forwarding
4.3.1. Requesting X11 Forwarding
X11 forwarding may be requested for a session by sending
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 authentication 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, already opened forwardings should not be automatically
closed when the session channel is closed.
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.
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The `x11 authentication protocol' is the name of the X11 authentication
method used, i.e. "MIT-MAGIC-COOKIE-1".
X Protocol is documented in [SCHEIFLER].
4.3.2. 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.
4.4. Environment Variable Passing
Environment variables may be passed to the shell/command to be started
later. Typically, each machine will have a preconfigured set of
variables that it will allow. Since uncontrolled setting of environment
variables can be very dangerous, it is recommended that implementations
allow setting only variables whose names have been explicitly configured
to be allowed.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "env"
boolean want reply
string variable name
string variable value
4.5. 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
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This message will request the user's default shell (typically defined in
/etc/passwd in UNIX systems) to 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 the server to 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 from
arbitrary output from 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 to request and check the reply for these messages. The
client SHOULD ignore these messages.
4.6. 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.
4.7. 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
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uint32 terminal height, rows
uint32 terminal width, pixels
uint32 terminal height, pixels
No response SHOULD be sent to this message.
4.8. 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.
4.9. 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 names will be encoded as discussed in the "exit-signal"
SSH_MSG_CHANNEL_REQUEST.
4.10. 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.
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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 (ISO-10646 UTF-8)
string language tag (as defined in [RFC-1766])
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 names MAY be sent in the format "sig-name@xyz", where
`sig-name' and `xyz' may be anything a particular implementor wants
(except the `@' sign). However, it is suggested that if a `configure'
script is used, the non-standard signal names it finds be encoded as
"SIG@xyz.config.guess", where `SIG' is the signal name without the "SIG"
prefix, and `xyz' be the host type, as determined by `config.guess'.
The `error message' contains an additional 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 [SSH-ARCH].
5. TCP/IP Port Forwarding
5.1. 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
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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
`Address to bind' and `port number to bind' specify the IP address and
port to which the socket to be listened is bound. The address should be
"0.0.0.0" 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.
A port forwarding can be cancelled 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.
5.2. 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
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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
`Host to connect' and `port to connect' specify the TCP/IP host and port
where the recipient should connect the channel. `Host to connect' may
be either a domain name or a numeric IP address.
`Originator IP address' is the numeric IP address of the machine where
the connection request comes from, and `originator port' is the port on
the originator host from where the connection came from.
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.
6. Encoding of Terminal Modes
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.
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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.
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 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 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.
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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).
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.
7. Summary of Message Numbers
#define SSH_MSG_GLOBAL_REQUEST 80
#define SSH_MSG_REQUEST_SUCCESS 81
#define SSH_MSG_REQUEST_FAILURE 82
#define SSH_MSG_CHANNEL_OPEN 90
#define SSH_MSG_CHANNEL_OPEN_CONFIRMATION 91
#define SSH_MSG_CHANNEL_OPEN_FAILURE 92
#define SSH_MSG_CHANNEL_WINDOW_ADJUST 93
#define SSH_MSG_CHANNEL_DATA 94
#define SSH_MSG_CHANNEL_EXTENDED_DATA 95
#define SSH_MSG_CHANNEL_EOF 96
#define SSH_MSG_CHANNEL_CLOSE 97
#define SSH_MSG_CHANNEL_REQUEST 98
#define SSH_MSG_CHANNEL_SUCCESS 99
#define SSH_MSG_CHANNEL_FAILURE 100
8. 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.
This protocol can, however, be used to execute commands on remote
machines. The protocol also permits the server to run commands on the
client. Implementations may wish to disallow this to prevent an
attacker from coming from the server machine to the client machine.
X11 forwarding provides major security improvements over normal cookie-
based X11 forwarding. The cookie never needs to be transmitted in the
clear, and traffic is encrypted and integrity-protected. No useful
authentication data will remain on the server machine after the
connection has been closed. On the other hand, in some situations a
forwarded X11 connection might be used to get access to the local X
server across security perimeters.
Port forwardings can potentially allow an intruder to cross security
perimeters such as firewalls. They do not offer anything fundamentally
new that a user could not do otherwise; however, they make opening
tunnels very easy. Implementations should allow policy control over
what can be forwarded. Administrators should be able to deny
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 15]
INTERNET-DRAFT 9 January, 2001
forwardings where appropriate.
Since this protocol normally runs inside an encrypted tunnel, firewalls
will not be able to examine the traffic.
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.
9. Trademark Issues
SSH is a registered trademark and Secure Shell is a trademark of SSH
Communications Security Corp. SSH Communications Security Corp permits
the use of these trademarks as the name of this standard and protocol,
and permits their use to describe that a product conforms to this
standard, provided that the following acknowledgement is included where
the trademarks are used: ``SSH is a registered trademark and Secure
Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''. These trademarks may not be used as part of a product
name or in otherwise confusing manner without prior written permission
of SSH Communications Security Corp.
10. References
[RFC-1766] Alvestrand, H: "Tags for the Identification of Languages",
March 1995.
[RFC-1884] Hinden, R., and Deering, S: "IP Version 6 Addressing
Architecture", December 1995
[RFC-2279] Yergeau, F: "UTF-8, a transformation format of ISO 10646",
January 1998.
[SCHEIFLER] Scheifler, R. W., et al: "X Window System : The Complete
Reference to Xlib, X Protocol, Icccm, Xlfd", 3rd edition, Digital Press,
ISBN 1555580882, February 1992.
[POSIX] ISO/IEC Std 9945-1, ANSI/IEEE Std 1003.1 Information technology
-- Portable Operating System Interface (POSIX)-Part 1: System
Application Program Interface (API) [C Language], July 1996.
[SSH-ARCH] Ylonen, T., et al: "SSH Protocol Architecture", Internet-
Draft, draft-ietf-secsh-architecture-07.txt
[SSH-TRANS] Ylonen, T., et al: "SSH Transport Layer Protocol", Internet-
Draft, draft-ietf-secsh-transport-09.txt
[SSH-USERAUTH] Ylonen, T., et al: "SSH Authentication Protocol",
Internet-Draft, draft-ietf-secsh-userauth-09.txt
11. Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 16]
INTERNET-DRAFT 9 January, 2001
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: ylo@ssh.com
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: kivinen@ssh.com
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: tri@ssh.com
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: sjl@ssh.com
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 17]