RFC 4254 PROPOSED STANDARD SSH · CHANNELS · TUNNELING
The Secure Shell (SSH) Connection Protocol
How one encrypted SSH connection carries shells, commands and tunnels at the same time.
RFC EDITOR
Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006, <https://www.rfc-editor.org/info/rfc4254>.
BIBTEX
@misc{rfc4254,
series = {Request for Comments},
number = 4254,
howpublished = {RFC 4254},
publisher = {RFC Editor},
doi = {10.17487/RFC4254},
url = {https://www.rfc-editor.org/info/rfc4254},
author = {T. Ylonen and C. Lonvick},
title = {{The Secure Shell (SSH) Connection Protocol}},
pagetotal = 24,
year = 2006,
month = jan,
} RFC text
Contents
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.
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".
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
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.
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.
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.
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.
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].
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
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.
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.
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
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.
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.
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]
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
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.
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.
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.
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).
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.
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.
[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.
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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AI ELI5
On this page
PROBLEMA secure, authenticated SSH link is useless until something defines what to do over it, and people need shells, commands and tunnels at the same time.
MECHANISMNumbered channels are multiplexed over the one encrypted connection. Each channel has its own send window, and typed requests such as pty-req, shell, exec and tcpip-forward set it up.
WHY IT MATTERSIt is the layer that every SSH client and server uses for logins, remote commands, file-transfer subsystems and port forwarding.
§1Introduction
SSH is built in layers. RFC 4253 provides an encrypted, integrity-protected transport, and RFC 4252 proves who the user is. This document, the Connection Protocol, sits on top of both and answers the practical question: now that there is a safe pipe between two machines, what do people actually do with it?
The RFC's answer is that everything is a channel. An interactive login, a one-off remote command, a forwarded TCP connection and a forwarded X11 window are all channels, and many of them are multiplexed into the single encrypted connection. The abstract lists the four services: interactive login sessions, remote execution of commands, forwarded TCP/IP connections and forwarded X11 connections.
The protocol is symmetric where it can be. Either side may open a channel and either side may send global requests, although in practice the client opens most of them. The RFC borrows its field notation from the architecture document: byte, uint32, boolean and string.
The connection protocol does no encryption or authentication of its own. Section 11 states that it is assumed to run on top of a secure, authenticated transport.
Imagine one phone line that has been made completely private. This document explains how you can have lots of separate conversations on that single line at once: one to type commands, one to send a file, one to share a window.
- sessionA shell, a command or a subsystem, optionally with a pseudo-terminal and X11 forwarding.
- x11A forwarded X11 connection, opened independently of the session that requested it.
- forwarded-tcpipA connection that arrived at a port the other side asked us to listen on (remote forwarding).
- direct-tcpipA connection that arrived at a local forwarded port, to be connected onward by the recipient.
- Connection protocolRFC 4254: channels, sessions, forwarding
- User authenticationRFC 4252
- Transport layerRFC 4253: encryption, integrity, server authentication
- TCPReliable byte stream
§2Global Requests
Most messages in this protocol are about one particular channel, but some affect the connection as a whole. These are global requests. The example the RFC gives is asking the other side to start listening on a TCP port for forwarding. Either side may send a global request at any time, and the receiver must respond appropriately.
A request is an SSH_MSG_GLOBAL_REQUEST carrying a request name (US-ASCII), a want reply boolean and request-specific data. If want reply is TRUE, the receiver answers with SSH_MSG_REQUEST_SUCCESS (which may carry response-specific data) or SSH_MSG_REQUEST_FAILURE (which carries nothing). A receiver that does not recognise or support the request simply answers with failure.
Because the replies do not say which request they answer, the RFC requires that replies to global requests be sent in the same order as the requests. Replies to channel requests for the same channel must also stay in order, but replies for distinct channels may be sent out of order.
Request names follow the DNS extensibility naming convention from the architecture document, which is how vendors can add private requests such as name@example.com without asking anyone.
Sometimes you need to ask the other computer for something that is not about any one conversation, like "please start listening at this door". The answer is always yes or no, and the answers come back in the order you asked.
§3The Channel Mechanism
Each end gives every channel a number, and the two numbers need not match. A message that opens a channel carries the sender's number; every other channel message carries the recipient's number, so each side only ever uses its own numbering to look things up.
To open a channel, a side allocates a local number and sends SSH_MSG_CHANNEL_OPEN with a channel type (such as "session"), its sender channel, an initial window size and a maximum packet size, plus any type-specific data. The other side answers with SSH_MSG_CHANNEL_OPEN_CONFIRMATION, which echoes the original number as the recipient channel and adds its own, or with SSH_MSG_CHANNEL_OPEN_FAILURE.
A failure carries a reason code, a UTF-8 description and a language tag. Four codes are defined:
1
SSH_OPEN_ADMINISTRATIVELY_PROHIBITED
2
SSH_OPEN_CONNECT_FAILED
3
SSH_OPEN_UNKNOWN_CHANNEL_TYPE
4
SSH_OPEN_RESOURCE_SHORTAGE
Codes 0xFE000000 to 0xFFFFFFFF are never assigned by IANA and are left for private use. If a server does not understand the channel type at all, it must use code 3.
Once a channel exists, a side can send SSH_MSG_CHANNEL_REQUEST to ask for channel-specific things, such as a pseudo-terminal. The request names are local to each channel type, and a request that is not recognised gets SSH_MSG_CHANNEL_FAILURE.
Each conversation gets a number. When you ask to start one, you say "I will call it number 5", and your friend answers "OK, I will call it number 9". After that you each use the number the other person picked when you speak to them.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
MSG | message number | 1 byte | 91 | byte SSH_MSG_CHANNEL_OPEN_CONFIRMATION, value 91 |
RECIPIENT | recipient channel | 4 bytes | uint32: the channel number given in the original open request | |
SENDER | sender channel | 4 bytes | uint32: the channel number allocated by the confirming side | |
WINDOW | initial window size | 4 bytes | uint32: bytes that may be sent to the confirming side before a window adjust | |
MAXPACKET | maximum packet size | 4 bytes | uint32: the largest single data packet the confirming side will accept |
§4Flow Control, Data and Closing
Channels are flow-controlled. No data may be sent on a channel until the other side has said there is room for it. The room is the window: the initial window size in the open message says how many bytes may be sent before the sender must wait. SSH_MSG_CHANNEL_WINDOW_ADJUST adds more bytes to the window, which may never exceed 232 - 1.
Payload travels in SSH_MSG_CHANNEL_DATA. The amount in any one message is limited by both the channel's maximum packet size and the current window, whichever is smaller, and the window shrinks by the amount sent. A second form, SSH_MSG_CHANNEL_EXTENDED_DATA, adds a data_type_code so that one channel can carry several kinds of data. Only one is defined: type 1, SSH_EXTENDED_DATA_STDERR. Extended data uses the same window as ordinary data.
Closing is a two-step affair. A party that will send no more data should send SSH_MSG_CHANNEL_EOF, but the channel stays open and data can still flow the other way. To end the channel, a party sends SSH_MSG_CHANNEL_CLOSE; the receiver must send one back unless it already has. Only when a party has both sent and received a close may it reuse the channel number.
EOF, CLOSE, CHANNEL_REQUEST and the success and failure replies do not consume window space, so they can be sent even when the window is empty.
Your friend has a bucket that can only hold so much. You may only pour in as much as they said, and when they empty it they tell you "you can pour some more". When you are done talking you say so, and the conversation only really ends once both of you have said goodbye.
| Number | Message | Purpose |
|---|---|---|
| 80 | SSH_MSG_GLOBAL_REQUEST | Connection-wide request |
| 81 | SSH_MSG_REQUEST_SUCCESS | Global request accepted |
| 82 | SSH_MSG_REQUEST_FAILURE | Global request refused or unknown |
| 90 | SSH_MSG_CHANNEL_OPEN | Ask to open a channel |
| 91 | SSH_MSG_CHANNEL_OPEN_CONFIRMATION | Channel opened |
| 92 | SSH_MSG_CHANNEL_OPEN_FAILURE | Channel refused, with reason code |
| 93 | SSH_MSG_CHANNEL_WINDOW_ADJUST | Add bytes to the send window |
| 94 | SSH_MSG_CHANNEL_DATA | Ordinary channel data |
| 95 | SSH_MSG_CHANNEL_EXTENDED_DATA | Typed data such as stderr |
| 96 | SSH_MSG_CHANNEL_EOF | No more data from this side |
| 97 | SSH_MSG_CHANNEL_CLOSE | Terminate the channel |
| 98 | SSH_MSG_CHANNEL_REQUEST | Channel-specific request |
| 99 | SSH_MSG_CHANNEL_SUCCESS | Channel request accepted |
| 100 | SSH_MSG_CHANNEL_FAILURE | Channel request refused or unknown |
§5Interactive Sessions
A session is the remote execution of a program: a shell, an application, a single command or a built-in subsystem. It begins with SSH_MSG_CHANNEL_OPEN of type "session", and then a series of channel requests configure it.
pty-req
Allocates a pseudo-terminal. Carries the TERM value (for example vt100), the width and height in characters and in pixels, and encoded terminal modes.
x11-req
Requests X11 forwarding. The RFC recommends sending a fake, random authentication cookie and swapping in the real one when a connection arrives.
env
Passes an environment variable name and value. The RFC warns that setting variables in a privileged process is a hazard.
shell / exec / subsystem
Starts the user's default shell, runs a given command, or runs a named subsystem. Only one of these can succeed per channel.
While the program runs, its input and output travel as channel data, with standard error carried as extended data type 1. The client can send window-change when the terminal is resized, and signal to deliver a POSIX signal such as INT or TERM. The server can send xon-xoff to say whether the client may handle Ctrl-S/Ctrl-Q flow control.
When the command ends, the server sends exit-status (a uint32) or exit-signal (a signal name, a core-dumped flag and an error message), and then the channel is closed. The RFC recommends returning the status. Zero usually means success.
When you log in to another computer, your keyboard and screen are connected to a program over there. You can say what kind of screen you have, ask it to start a shell, and when the program finishes it tells you whether it went well.
ClientServer
- CHANNEL_OPEN "session"sender channel, initial window, max packet size
- CHANNEL_OPEN_CONFIRMATIONadds the server's own channel number
- CHANNEL_REQUEST "pty-req"TERM, size, encoded terminal modes
- CHANNEL_SUCCESS
- CHANNEL_REQUEST "shell"
- CHANNEL_SUCCESS
- CHANNEL_DATAkeystrokes; the server replies with data in the other direction
- CHANNEL_REQUEST "exit-status"no reply wanted
- CHANNEL_EOF, CHANNEL_CLOSE
- CHANNEL_CLOSEchannel is closed once both sides have sent and received CLOSE
§6TCP/IP Port Forwarding
The same tunnel can carry ordinary TCP connections, which is how SSH is used to protect protocols that have no security of their own. There are two directions.
Local forwarding. The side that wants the traffic listens locally. When a connection arrives, it opens a channel of type "direct-tcpip" carrying the host and port to connect to, plus the originator's IP address and port. The recipient decides whether to allow it and makes the TCP connection.
Remote forwarding. To have the other side do the listening, a party sends the global request "tcpip-forward" with an address to bind and a port. When a connection arrives there, the listener opens a "forwarded-tcpip" channel back. Receivers must reject such channels unless they asked for that forwarding. If the port requested is 0 and a reply is wanted, the server picks the next available unprivileged port and returns it in SSH_MSG_REQUEST_SUCCESS. A "cancel-tcpip-forward" request stops the listening.
The address to bind has special cases: an empty string means all protocol families, 0.0.0.0 all IPv4 addresses, :: all IPv6 addresses, and localhost, 127.0.0.1 and ::1 loopback only.
Forwarded channels are independent of any session. Closing a session channel does not close forwarded connections.
Say there is a door in one room that you cannot reach. Your friend opens a secret tunnel, and anything you hand to your friend at one end comes out the door at the other end. Either of you can be the one waiting at the door.
§7Terminal Modes and Message Numbers
The pty-req message ends with a string of encoded terminal modes, meant to be portable across systems. It is a stream of opcode and argument pairs. Opcodes 1 to 159 each take one uint32 argument; opcodes 160 to 255 are undefined and stop parsing; and opcode 0, TTY_OP_END, ends the stream.
The names mostly follow POSIX terminal flags. Examples are 1 VINTR (the interrupt character), 53 ECHO (enable echoing), 51 ICANON (canonicalise input lines), 128 TTY_OP_ISPEED and 129 TTY_OP_OSPEED (the input and output baud rates). The client should list every mode it knows about, and the server may ignore any it does not understand.
Section 9 gives the message number for every message. They sit in a block from 80 to 100, which leaves the lower numbers to the transport and authentication layers.
When your remote screen is set up, you also send a list of settings, such as whether typed letters should show up on screen. The computer on the other side ignores any setting it does not know.
§8Security Considerations
This protocol assumes it is running on a secure, authenticated transport. User authentication and protection against network attackers are provided by the layers below it, and full security considerations are in the architecture document, RFC 4251.
The one recommendation specific to this document is that implementations should disable all potentially dangerous features, such as agent forwarding, X11 forwarding and TCP/IP forwarding, if the host key has changed without notice or explanation.
Smaller cautions are scattered through the text. Clients should reject incoming session channel opens, direct-tcpip opens and tcpip-forward requests, since those are normally sent by clients to servers, and this makes it harder for a corrupt server to attack the client. X11 channel opens must be rejected if X11 forwarding was never requested. Servers should take care before setting environment variables sent by the client and before executing commands, and should only allow forwarding of privileged ports for a user authenticated as privileged.
Remote text such as a channel open failure description or an exit-signal error message may be shown to the user, but only with the precautions from the architecture document, because it is untrusted input from the other end.
This protocol trusts that the private line underneath is already safe. It still says to be careful: if the other computer suddenly looks different than before, switch off the risky extras like sharing windows and secret tunnels.
§9References
[1]RFC 4251Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006.January 2006[2]RFC 4253Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006.January 2006[3]RFC 4252Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, January 2006.January 2006[4]RFC 4250Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006.January 2006[5]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997See alsoRFC 8308Bider, D. and M. Baushke, "Extension Negotiation in the Secure Shell (SSH) Protocol", RFC 8308, March 2018.March 2018