Network Working Group Tatu Ylonen <ylo@ssh.fi>
INTERNET-DRAFT SSH Communications Security
draft-ietf-secsh-connect-01.txt July 30, 1997
Expires in six months
SSH Connection Protocol
Status of This memo
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Abstract
This document describes the SSH connection protocol. It multiplexes a
single encrypted tunnel into a number of channels (interactive sessions,
forwarded TCP/IP ports, X11 connections, etc). It is intended to run
above the SSH user authentication layer.
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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 . . . . . . . . . . . . . . . . 6
4.3. X11 Forwarding . . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. Requesting X11 Forwarding . . . . . . . . . . . . . . . 7
4.3.2. X11 Channels . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Authentication Agent Forwarding . . . . . . . . . . . . . . 8
4.4.1. Requesting Athentication Agent Forwarding . . . . . . . 8
4.4.2. Authentication Agent Channels . . . . . . . . . . . . . 8
4.5. Environment Variable Passing . . . . . . . . . . . . . . . . 8
4.6. Starting Shell or Command . . . . . . . . . . . . . . . . . 9
4.7. Session Data Transfer . . . . . . . . . . . . . . . . . . . 9
4.8. Window Change Message . . . . . . . . . . . . . . . . . . . 9
4.9. Local Flow Control . . . . . . . . . . . . . . . . . . . . . 10
4.10. Signals . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.11. Returning Exit Status . . . . . . . . . . . . . . . . . . . 10
5. TCP/IP Port Forwarding . . . . . . . . . . . . . . . . . . . . . 11
5.1. Requesting Port Forwarding . . . . . . . . . . . . . . . . . 11
5.2. TCP/IP Forwarding Channels . . . . . . . . . . . . . . . . . 11
6. FTP Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Encoding of Terminal Modes . . . . . . . . . . . . . . . . . . . 12
8. Summary of Message Numbers . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 16
10. Address of Author . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This protocol has been designed to run over the SSH transport layer and
user authentication protocols. The service name for this protocol
(after user authentication) is "ssh-connection". It provides
interactive login sessions, remote execution of commands, forwarded
TCP/IP connections, and forwarded X11 connections.
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.
byte SSH_MSG_GLOBAL_REQUEST
string request name
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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 the recipient does not recognize or support the request, it simply
responds with SSH_MSG_REQUEST_FAILURE.
byte SSH_MSG_REQUEST_SUCCESS
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 on the
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
uint32 sender_channel
uint32 initial_window_size
uint32 max_packet_size
... channel type specific data follows
The channel type is a name as described in the transport layer protocol,
with similar extension mechanisms (the domain name suffic convention).
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. Max_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
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byte SSH_MSG_CHANNEL_OPEN_CONFIRMATION
uint32 recipient_channel
uint32 sender_channel
uint32 initial_window_size
uint32 max_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
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 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 characters 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
Upon receiving this message, the recipient increases the number of bytes
it is allowed to send by the given amount.
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.
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.
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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 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. It is legal to send SSH_MSG_CHANNEL_CLOSE
without having sent or received SSH_MSG_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 be delivered to
the actual destination, if possible. This message should not normally
flush buffers.
3.4. Channel-Specific Requests
Many channel types have extensions that are specific to that particular
channel type. An example is requesting a pty for an interactive
session.
All channel-specific requests use the following format.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
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string request type
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 names are local to each channel type (it is recommended that
names with the same extension rules again be used).
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 command. The command may be a
shell, a program, 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. While this
message can be sent by either side, it is normally recommended for
clients not to permit opening new sessions to avoid a corrupt server
from attacking clients.
byte SSH_MSG_CHANNEL_OPEN
string "session"
uint32 sender_channel
uint32 initial_window_size
uint32 max_packet_size
4.2. Requesting a Pseudo-Terminal
A pseudo-terminal can be allocated for the session by sending the
following message.
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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''.
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.
4.3.2. X11 Channels
Forwarded X11 connections are normal channels, independent of the
session that originated them. X11 channels are opened with a normal
channel open request. The resulting channels are independent of the
session, and closing the session channel does not imply closing
forwarded X11 channels.
byte SSH_MSG_CHANNEL_OPEN
string "x11"
uint32 sender_channel
uint32 initial_window_size
uint32 max_packet_size
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string originator_string
The recipient should respond with open confirmation or open failure.
Originator_string is a free-form implementation-dependent description of
the X11 client that made the connection. It should typically contain
the IP address and port of the client, and may also contain user name or
other information if available. It should be in a format that is
understandable by a user.
4.4. Authentication Agent Forwarding
4.4.1. Requesting Athentication Agent Forwarding
Authentication agent forwarding may be requested for a session by
sending
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "auth-agent-req"
boolean want_reply
The server responds with either SSH_MSG_CHANNEL_SUCCESS or
SSH_MSG_CHANNEL_FAILURE (if want_reply is TRUE). The client is allowed
to send futher messages without waiting for the reponse to this message.
4.4.2. Authentication Agent Channels
When an application requests a connection to the authentication agent,
the following message is sent to the originator of the session.
byte SSH_MSG_CHANNEL_OPEN
string "auth-agent"
uint32 sender_channel
uint32 initial_window_size
uint32 max_packet_size
The recipient should respond with open confirmation or open failure.
4.5. 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
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4.6. Starting Shell or Command
Once the session has been set up, a shell or command is started at the
remote end. This can happen in any of a number of ways. Only one of
these requests can succeed per channel.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "shell"
boolean want_reply
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "exec"
boolean want_reply
string command
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "subsystem"
boolean want_reply
string subsystem_name
This last form executes a predefined subsystem. It expected that these
will include a general file transfer mechanism, and possibly other
features. Implementations may also allow configuring more such
mechanisms. Having a special message for them avoids the need to have
their paths and command names be supplied by the other side. This also
makes it easier to implement them in the same executable as the rest of
the protocol on platforms where that is desirable.
It is strongly recommended to request and check the reply for these
messages.
4.7. 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.
4.8. Window Change Message
When the window (terminal) size changes on the client side (client here
means the party who sent the create message for the session), it may
send a message to the other side to inform it of the new size.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "window-change"
boolean FALSE
uint32 terminal width, columns
uint32 terminal height, rows
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uint32 terminal width, pixels
uint32 terminal height, pixels
No response is sent to this message.
4.9. Local Flow Control
On many systems it is possible to determine if a pseudo-terminal is
using control-S control-Q flow control. When this is the case, 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 two
notifications. Initially, the server is responsible for flow control.
(Here, again, client means the side originating the session, and server
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 is allowed to ignore this message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "xon-xoff"
boolean FALSE
boolean client_can_do
If client_can_do is true, the client (originator) can do control-S
control-Q flow control locally.
No response is sent to this message.
4.10. 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 will ignore this message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "signal"
boolean FALSE
uint32 signal_number
4.11. Returning Exit Status
When the command running at the other end terminates, The following
message may be sent to return the exit status of the command. Returning
the status is optional, but recommended. No acknowledgement is sent for
this message. The channel needs to be closed with SSH_MSG_CHANNEL_CLOSE
after this message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
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string "exit-status"
string 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.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "exit-signal"
string FALSE
uint32 signal number
boolean core dumped
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, it if wishes to have 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
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.
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
uint32 port_number_to_bind
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.
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byte SSH_MSG_CHANNEL_OPEN
string "forwarded-tcpip"
uint32 sender_channel
uint32 initial_window_size
uint32 max_packet_size
string address_that_was_connected
uint32 port_that_was_connected
string originator_ip_address
uint32 originator_port
string originator_string
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 max_packet_size
string host_to_connect
uint32 port_to_connect
string originator_ip_address
uint32 originator_port
string originator_string
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. Originator_string
is a free-form description of where the connection came in a form that
can be displayed to the user.
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.
6. FTP Forwarding
XXX
7. 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
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opcode-argument pairs. It is terminated by opcode TTY_OP_END (0).
Opcodes 1-127 have one-byte arguments. Opcodes 128-159 have 32-bit
integer arguments (stored msb first). Opcodes 160-255 are not yet
defined, and cause parsing to stop (they should only be used after any
other data).
The client puts in the stream any modes it knows about, and the server
ignores 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.
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 UNIX 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.
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11 VDSUSP
Another suspend character.
12 VREPRINT
Reprints the current input line.
13 VWERASE
Erases a word left of cursor.
14 VLNEXT
More special input characters; these are probably not supported on
most systems.
15 VFLUSH
Character to flush output.
16 VSWTCH
???
17 VSTATUS
???
18 VDISCARD
???
30 IGNPAR
The ignore parity flag. The next byte should be 0 if this flag is
not set, and 1 if it is set.
31 PARMRK
Mark parity and framing errors.
32 INPCK
Enable checking of parity errors.
33 ISTRIP
Strip 8th bit off chars.
34 INLCR
Map NL into CR on input.
35 IGNCR
Ignore CR on input.
36 ICRNL
Map CR to NL on input.
37 IUCLC
???
38 IXON
Enable output flow control.
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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, DSUSP.
51 ICANON
Canonicalize input lines.
52 XCASE
???
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
???
74 ONOCR
???
75 ONLRET
???
90 CS7
7 bits.
91 CS8
8 bits.
92 PARENB
Parity enable.
93 PARODD
Odd parity, else even.
128 TTY_OP_ISPEED
Specifies the input baud rate in bits per second (as a 32-bit int,
msb first).
129 TTY_OP_OSPEED
Specifies the output baud rate in bits per second (as a 32-bt int,
msb first).
8. Summary of Message Numbers
#define SSH_MSG_GLOBAL_REQUEST 30
#define SSH_MSG_REQUEST_SUCCESS 31
#define SSH_MSG_REQUEST_FAILURE 32
#define SSH_MSG_CHANNEL_OPEN 33
#define SSH_MSG_CHANNEL_OPEN_CONFIRMATION 34
#define SSH_MSG_CHANNEL_OPEN_FAILURE 35
#define SSH_MSG_CHANNEL_WINDOW_ADJUST 36
#define SSH_MSG_CHANNEL_DATA 37
#define SSH_MSG_CHANNEL_EXTENDED_DATA 38
#define SSH_MSG_CHANNEL_EOF 39
#define SSH_MSG_CHANNEL_CLOSE 40
#define SSH_MSG_CHANNEL_REQUEST 41
#define SSH_MSG_CHANNEL_SUCCESS 42
#define SSH_MSG_CHANNEL_FAILURE 43
9. Security Considerations
This protocol is assumed to run on top of a secure, authenticated
protocol. User authentication and protection against network-level
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attacks are assumed to be provided by the underlying protocol.
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 couldn't 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
forwardings where appropriate.
Since this protocol normally runs inside an encrypted tunnel, firewalls
will not be able to examine the traffic.
10. Address of Author
Tatu Ylonen
SSH Communications Security Ltd.
Tekniikantie 12
FIN-02150 ESPOO
Finland
E-mail: ylo@ssh.fi
Tatu Ylonen <ylo@ssh.fi> [page 17]