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NAME
ssh - secure shell client (remote login program)
SYNOPSIS
ssh [-l login_name] hostname | user@hostname [ command]
ssh [-afgknqstvxACNTX1246] [-b bind_address] [-m mac_spec]
[-c cipher_spec] [-e escape_char] [-i identity_file]
[-l login_name] [-F configfile] [-o option] [-p port]
[-L port:host:hostport] [-R port:host:hostport]
[-D port] hostname | user@hostname [command]
DESCRIPTION
ssh (Secure Shell) is a program for logging into a remote
machine and for executing commands on a remote machine. It
is intended to replace rlogin and rsh, and to provide secure
encrypted communications between two untrusted hosts over an
insecure network. X11 connections and arbitrary TCP/IP ports
can also be forwarded over the secure channel.
ssh connects and logs into the specified hostname. The user
must prove his or her identity to the remote machine using
one of several methods depending on the protocol version
used:
SSH protocol version 1
First, if the machine the user logs in from is listed in
/etc/hosts.equiv or /etc/shosts.equiv on the remote machine,
and the user names are the same on both sides, the user is
immediately permitted to log in. Second, if .rhosts or
.shosts exists in the user's home directory on the remote
machine and contains a line containing the name of the
client machine and the name of the user on that machine, the
user is permitted to log in. This form of authentication
alone is normally not allowed by the server because it is
not secure.
The second (and primary) authentication method is the rhosts
or hosts.equiv method combined with RSA-based host authenti-
cation. It means that if the login would be permitted by
$HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or
/etc/shosts.equiv, and if additionally the server can verify
the client's host key (see /etc/ssh_known_hosts in the FILES
section), only then is login permitted. This authentication
method closes security holes due to IP spoofing, DNS spoof-
ing, and routing spoofing.
Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts,
and the rlogin/rsh protocol in general, are inherently
insecure and should be disabled if security is desired.
As a third authentication method, ssh supports RSA-based
authentication. The scheme is based on public-key cryptogra-
phy. There are cryptosystems where encryption and decryption
are done using separate keys, and it is not possible to
derive the decryption key from the encryption key. RSA is
one such system. The idea is that each user creates a
public/private key pair for authentication purposes. The
server knows the public key, and only the user knows the
private key. The file $HOME/.ssh/authorized_keys lists the
public keys that are permitted for logging in. When the user
logs in, the ssh program tells the server which key pair it
would like to use for authentication. The server checks if
this key is permitted, and if so, sends the user (actually
the ssh program running on behalf of the user) a challenge
in the form of a random number, encrypted by the user's pub-
lic key. The challenge can only be decrypted using the
proper private key. The user's client then decrypts the
challenge using the private key, proving that he or she
knows the private key but without disclosing it to the
server.
ssh implements the RSA authentication protocol automati-
cally. The user creates his or her RSA key pair by running
ssh-keygen(1). This stores the private key in
$HOME/.ssh/identity and the public key in
$HOME/.ssh/identity.pub in the user's home directory. The
user should then copy the identity.pub to
$HOME/.ssh/authorized_keys in his or her home directory on
the remote machine (the authorized_keys file corresponds to
the conventional $HOME/.rhosts file, and has one key per
line, though the lines can be very long). After this, the
user can log in without giving the password. RSA authentica-
tion is much more secure than rhosts authentication.
The most convenient way to use RSA authentication can be
with an authentication agent. See ssh-agent(1) for more
information.
If other authentication methods fail, ssh prompts the user
for a password. The password is sent to the remote host for
checking. However, since all communications are encrypted,
the password cannot be seen by someone listening on the net-
work.
SSH protocol version 2
The SSH version 2 protocol supports multiple user authenti-
cation methods, some of which are similar to those available
with the SSH protocol version 1. These authentication
mechanisms are negotiated by the client and server, with the
client trying methods in the order specified in the Prefer-
redAuthentications client configuration option. The server
decides when enough authentication methods have passed suc-
cessfully so as to complete the authentication phase of the
protocol.
When a user connects by using protocol version 2, similar
authentication methods are available. Using the default
values for PreferredAuthentications, the client tries to
authenticate first by using the hostbased method. If this
method fails, public key authentication is attempted.
Finally, if this method fails, keyboard-interactive and
password authentication are tried.
The public key method is similar to RSA authentication
described in the previous section and allows the RSA or DSA
algorithm to be used: The client uses his or her private
key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to sign the
session identifier and sends the result to the server. The
server checks whether the matching public key is listed in
$HOME/.ssh/authorized_keys and grants access if both the key
is found and the signature is correct. The session identif-
ier is derived from a shared Diffie-Hellman value and is
only known to the client and the server.
If public key authentication fails or is not available, a
password can be sent encrypted to the remote host for prov-
ing the user's identity, or an extended prompt/reply proto-
col can be engaged.
Additionally, ssh supports hostbased or challenge response
authentication.
Protocol 2 provides additional mechanisms for confidential-
ity (the traffic is encrypted using 3DES, Blowfish, CAST128
or Arcfour) and integrity (hmac-sha1, hmac-md5). Protocol 1
lacks a strong mechanism for ensuring the integrity of the
connection.
Login session and remote execution
When the user's identity has been accepted by the server,
the server either executes the given command, or logs into
the machine and gives the user a normal shell on the remote
machine. All communication with the remote command or shell
is automatically encrypted.
If a pseudo-terminal has been allocated (normal login ses-
sion), the user can use the escape characters noted below.
If a pseudo-terminal has been allocated (normal login ses-
sion), the user can disconnect with ~., and suspend ssh with
~^Z. All forwarded connections can be listed with ~#. If the
session blocks waiting for forwarded X11 or TCP/IP connec-
tions to terminate, ssh can be backgrounded with ~&,
although this should not be used while the user shell is
active, as it can cause the shell to hang. All available
escapes can be listed with ~?.
A single tilde character can be sent as ~~, or by following
the tilde with a character other than those described above.
The escape character must always follow a newline to be
interpreted as special. The escape character can be changed
in configuration files or on the command line.
If no pseudo tty has been allocated, the session is tran-
sparent and can be used to reliably transfer binary data. On
most systems, setting the escape character to "none" also
makes the session transparent even if a tty is used.
The session terminates when the command or shell on the
remote machine exits and all X11 and TCP/IP connections have
been closed. The exit status of the remote program is
returned as the exit status of ssh.
Escape Characters
When a pseudo-terminal has been requested, ssh supports a
number of functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following
the tilde with a character other than those described below.
The escape character must always follow a newline to be
interpreted as special. The escape character can be changed
in configuration files using the EscapeChar configuration
directive or on the command line by the -e option.
The supported escapes, assuming the default ~, are:
~. Disconnect.
~^Z Background ssh.
~# List forwarded connections.
~& Background ssh at logout when waiting for forwarded
connection / X11 sessions to terminate.
~? Display a list of escape characters.
~B Send a break to the remote system. Only useful for
SSH protocol version 2 and if the peer supports it.
~C Open command line. Only useful for adding port for-
wardings using the -L and -R options).
~R Request rekeying of the connection. Only useful for
SSH protocol version 2 and if the peer supports it.
X11 and TCP forwarding
If the ForwardX11 variable is set to ``yes'' (or, see the
description of the -X and -x options described later) and
the user is using X11 (the DISPLAY environment variable is
set), the connection to the X11 display is automatically
forwarded to the remote side in such a way that any X11 pro-
grams started from the shell (or command) goes through the
encrypted channel, and the connection to the real X server
is made from the local machine. The user should not manually
set DISPLAY. Forwarding of X11 connections can be configured
on the command line or in configuration files.
The DISPLAY value set by ssh points to the server machine,
but with a display number greater than zero. This is normal
behavior, because ssh creates a "proxy" X11 server on the
server machine for forwarding the connections over the
encrypted channel.
ssh also automatically sets up Xauthority data on the server
machine. For this purpose, it generates a random authoriza-
tion cookie, store it in Xauthority on the server, and ver-
ify that any forwarded connections carry this cookie and
replace it by the real cookie when the connection is opened.
The real authentication cookie is never sent to the server
machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to "yes" (or, see the
description of the -A and -a options described later) and
the user is using an authentication agent, the connection to
the agent is automatically forwarded to the remote side.
Forwarding of arbitrary TCP/IP connections over the secure
channel can be specified either on the command line or in a
configuration file. One possible application of TCP/IP for-
warding is a secure connection to an electronic purse.
Another possible application is firewall traversal.
Server authentication
ssh automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
Host keys are stored in $HOME/.ssh/known_hosts in the user's
home directory. Additionally, the file /etc/ssh_known_hosts
is automatically checked for known hosts. The behavior of
ssh with respect to unknown host keys is controlled by the
StrictHostKeyChecking parameter. If a host's identification
ever changes, ssh warns about this and disables password
authentication to prevent a trojan horse from getting the
user's password. Another purpose of this mechanism is to
prevent attacks by intermediaries which could otherwise be
used to circumvent the encryption. The StrictHostKeyChecking
option can be used to prevent logins to machines whose host
key is not known or has changed.
However, when using key exchange protected by GSS-API, the
server can advertise a host key. The client automatically
adds this host key to its known hosts file,
$HOME/.ssh/known_hosts, regardless of the setting of the
StrictHostKeyChecking option, unless the advertised host key
collides with an existing known hosts entry.
When the user's GSS-API credentials expire, the client con-
tinues to be able to rekey the session using the server's
public host key to protect the key exchanges.
GSS-API user and server authentication
ssh uses the user's GSS-API credentials to authenticate the
client to the server wherever possible, if GssKeyEx and/or
GssAuthentication are set.
With GssKeyEx, one can have an SSHv2 server that has no host
public keys, so that only GssKeyEx can be used. With such
servers, rekeying fails if the client's credentials are
expired.
GSS-API user authentication has the disadvantage that it
does not obviate the need for SSH host keys, but its failure
does not impact rekeying. ssh can try other authentication
methods (such as public key, password, and so on) if GSS-API
authentication fails.
Delegation of GSS-API credentials can be quite useful, but
is not without danger. As with passwords, users should not
delegate GSS credentials to untrusted servers, since a
compromised server can use a user's delegated GSS creden-
tials to impersonate the user.
GSS-API user authorization is covered in gss_auth_rules(5).
Rekeying can be used to redelegate credentials when GssKeyEx
is "yes". (See ~R under Escape Characters above.)
OPTIONS
The following options are supported:
-1 Forces ssh to try protocol ver-
sion 1 only.
-2 Forces ssh to try protocol ver-
sion 2 only.
-4 Forces ssh to use IPv4 addresses
only.
-6 Forces ssh to use IPv6 addresses
only.
-a Disables forwarding of the
authentication agent connection.
-A Enables forwarding of the
authentication agent connection.
This can also be specified on a
per-host basis in a configura-
tion file.
Agent forwarding should be
enabled with caution. Users with
the ability to bypass file
permissions on the remote host
(for the agent's UNIX-domain
socket) can access the local
agent through the forwarded con-
nection. An attacker cannot
obtain key material from the
agent. However, the attacker can
perform operations on the keys
that enable the attacker to
authenticate using the identi-
ties loaded into the agent.
-b bind_address Specifies the interface to
transmit from on machines with
multiple interfaces or aliased
addresses.
-c blowfish | 3des | des Selects the cipher to use for
encrypting the session. 3des is
used by default. It is believed
to be secure. 3des (triple-des)
is an encrypt-decrypt-encrypt
triple with three different
keys. It is presumably more
secure than the des cipher,
which is no longer fully sup-
ported in ssh. blowfish is a
fast block cipher, it appears
very secure and is much faster
than 3des.
-c cipher_spec Additionally, for protocol ver-
sion 2 a comma-separated list of
ciphers can be specified in
order of preference. Protocol
version 2 supports 3DES, Blowf-
ish, and AES 128 in CBC mode.
See Ciphers in ssh_config(4) for
more information.
-C Requests compression of all data
(including stdin, stdout,
stderr, and data for forwarded
X11 and TCP/IP connections). The
compression algorithm is the
same used by gzip(1). The gzip
man page is available in the
SUNWsfman package. The "level"
can be controlled by the
CompressionLevel option (see
ssh_config(4)). Compression is
desirable on modem lines and
other slow connections, but only
slows down things on fast net-
works. The default value can be
set on a host-by-host basis in
the configuration files. See the
Compression option in
ssh_config(4).
-D port Specifies a local ``dynamic''
application-level port forward-
ing. This works by allocating a
socket to listen to port on the
local side, and whenever a con-
nection is made to this port,
the connection is forwarded over
the secure channel. The applica-
tion protocol is then used to
determine where to connect to
from the remote machine.
Currently, the SOCKS4 protocol
is supported and ssh acts as a
SOCKS4 server. Only root can
forward privileged ports.
Dynamic port forwardings can
also be specified in the confi-
guration file.
-e ch | ^ch | none Sets the escape character for
sessions with a pty (default:
`~'). The escape character is
only recognized at the beginning
of a line. The escape character
followed by a dot (".") closes
the connection. If followed by
Control-z, the escape character
suspends the connection. If fol-
lowed by itself, the escape
character sends itself once.
Setting the character to "none"
disables any escapes and makes
the session fully transparent.
-f Requests ssh to go to background
just before command execution.
This is useful if ssh is going
to ask for passwords or
passphrases, but the user wants
it in the background. This
implies the -n option. The
recommended way to start X11
programs at a remote site is
with something like ssh -f host
xterm.
-F configfile Specifies an alternative per-
user configuration file. If a
configuration file is given on
the command line, the system-
wide configuration file,
/etc/ssh_config, is ignored. The
default for the per-user confi-
guration file is
$HOME/.ssh/config.
-g Allows remote hosts to connect
to local forwarded ports.
-i identity_file Selects a file from which the
identity (private key) for RSA
or DSA authentication is read.
The default is
$HOME/.ssh/identity for protocol
version 1, and $HOME/.ssh/id_rsa
and $HOME/.ssh/id_dsa for proto-
col version 2. Identity files
can also be specified on a per-
host basis in the configuration
file. It is possible to have
multiple -i options (and multi-
ple identities specified in con-
figuration files).
-l login_name Specifies the user to log in as
on the remote machine. This also
can be specified on a per-host
basis in the configuration file.
-L port:host:hostport Specifies that the given port on
the local (client) host is to be
forwarded to the given host and
port on the remote side. This
works by allocating a socket to
listen to the port on the local
side. Then, whenever a connec-
tion is made to this port, the
connection is forwarded over the
secure channel and a connection
is made to host port hostport
from the remote machine. Port
forwardings can also be speci-
fied in the configuration file.
Only root can forward privileged
ports. IPv6 addresses can be
specified with an alternative
syntax: port/host/hostport.
-m mac_spec Additionally, for protocol ver-
sion 2 a comma-separated list of
MAC (message authentication
code) algorithms can be speci-
fied in order of preference. See
the MACs keyword for more infor-
mation.
-n Redirects stdin from /dev/null
(actually, prevents reading from
stdin). This must be used when
ssh is run in the background. A
common trick is to use this to
run X11 programs on a remote
machine. For example,
ssh -n shadows.cs.hut.fi emacs &
starts an emacs on
shadows.cs.hut.fi, and the X11
connection is automatically for-
warded over an encrypted chan-
nel. The ssh program is put in
the background. This does not
work if ssh needs to ask for a
password or passphrase. See also
the -f option.
-N Does not execute a remote com-
mand. This is useful if you just
want to forward ports (protocol
version 2 only).
-o option Can be used to give options in
the format used in the confi-
guration file. This is useful
for specifying options for which
there is no separate command-
line flag. The option has the
same format as a line in the
configuration file.
-p port Specifies the port to connect to
on the remote host. This can be
specified on a per-host basis in
the configuration file.
-P Obsoleted option. SSHv1 connec-
tions from privileged ports are
not supported.
-q Quiet mode. Causes all warning
and diagnostic messages to be
suppressed. Only fatal errors
are displayed.
-R port:host:hostport Specifies that the given port on
the remote (server) host is to
be forwarded to the given host
and port on the local side. This
works by allocating a socket to
listen to the port on the remote
side. Then, whenever a connec-
tion is made to this port, the
connection is forwarded over the
secure channel and a connection
is made to host port hostport
from the local machine. Port
forwardings can also be speci-
fied in the configuration file.
Privileged ports can be for-
warded only when logging in as
root on the remote machine.
-s Can be used to request invoca-
tion of a subsystem on the
remote system. Subsystems are a
feature of the SSH2 protocol
which facilitate the use of SSH
as a secure transport for other
applications, for example, sftp.
The subsystem is specified as
the remote command.
-t Forces pseudo-tty allocation.
This can be used to execute
arbitrary screen-based programs
on a remote machine, which can
be very useful, for example,
when implementing menu services.
Multiple -t options force allo-
cation, even if ssh has no local
tty.
-T Disables pseudo-tty allocation
(protocol version 2 only).
-v Verbose mode. Causes ssh to
print debugging messages about
its progress. This is helpful in
debugging connection, authenti-
cation, and configuration prob-
lems. Multiple -v options
increase the verbosity. Maximum
is 3.
-x Disables X11 forwarding.
-X Enables X11 forwarding. This can
also be specified on a per-host
basis in a configuration file.
X11 forwarding should be enabled
with caution. Users with the
ability to bypass file permis-
sions on the remote host (for
the user's X authorization data-
base) can access the local X11
display through the forwarded
connection. An attacker can then
be able to perform activities
such as keystroke monitoring.
ENVIRONMENT VARIABLES
ssh normally sets the following environment variables:
DISPLAY
The DISPLAY variable must be set for X11 display for-
warding to work.
SSH_ASKPASS
If ssh needs a passphrase, it reads the passphrase from
the current terminal if it was run from a terminal. If
ssh does not have a terminal associated with it but
DISPLAY and SSH_ASKPASS are set, it executes the program
specified by SSH_ASKPASS and opens an X11 window to read
the passphrase. This is particularly useful when calling
ssh from a .Xsession or related script. On some machines
it might be necessary to redirect the input from
/dev/null to make this work.
SSH_AUTH_SOCK
Indicates the path of a unix-domain socket used to com-
municate with the agent.
SSH_LANGS
A comma-separated list of IETF language tags (see
RFC3066) indicating the languages that the user can read
and write. Used for negotiation of the locale on the
server.
LANG, LC_ALL, LC_COLLATE, LC_CTYPE,
LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME
The values of these environment variables can be set in
remote sessions according to the locale settings on the
client side and availability of support for those
locales on the server side. Environment Variable Passing
(see RFC 4254) is used for passing them over to the
server side.
See the ENVIRONMENT VARIABLES section in the sshd(1M) man
page for more information on how locale setting can be
further changed depending on server side configuration.
EXIT STATUS
The status of the remote program is returned as the exit
status of ssh. 255 is returned if an error occurred at
anytime during the ssh connection, including the initial key
exchange.
FILES
$HOME/.ssh/known_hosts Records host keys for all hosts
the user has logged into that are
not in /etc/ssh/ssh_known_hosts.
See sshd(1M).
$HOME/.ssh/identity Contains the authentication iden-
$HOME/.ssh/id_dsa tity of the user. These files are
$HOME/.ssh/id_ssa for protocol 1 RSA, protocol 2
DSA, and protocol 2 RSA, respec-
tively. These files contain sensi-
tive data and should be readable
by the user but not accessible by
others (read/write/execute). ssh
ignores a private key file if it
is accessible by others. It is
possible to specify a passphrase
when generating the key. The
passphrase is used to encrypt the
sensitive part of this file using
3DES.
/etc/ssh/sshrc Commands in this file are executed
by ssh when the user logs in just
before the user's shell or command
is started. See sshd(1M) for more
information.
$HOME/.ssh/rc Commands in this file are executed
by ssh when the user logs in just
before the user's shell or command
is started. See sshd(1M) for more
information.
$HOME/.ssh/environment Contains additional definitions
for environment variables. See
ENVIRONMENT VARIABLES.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Availability | SUNWsshu |
|_____________________________|_____________________________|
| Interface Stability | See below. |
|_____________________________|_____________________________|
The command line syntax is Committed. The remote locale
selection through passing LC_* environment variables is
Uncommitted.
SEE ALSO
rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-
keygen(1), ssh-http-proxy-connect(1), ssh-socks5-proxy-
connect(1), telnet(1), sshd(1M), ssh_config(4),
sshd_config(4), attributes(5), gss_auth_rules(5), ker-
beros(5)
RFC 4254