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User Commands ssh(1)
NAME
ssh - OpenSSH 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] host-
name | 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.
SunOS 5.10 Last change: 6 Nov 2004 1
User Commands ssh(1)
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
SunOS 5.10 Last change: 6 Nov 2004 2
User Commands ssh(1)
successfully 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 con-
nections 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
SunOS 5.10 Last change: 6 Nov 2004 3
User Commands ssh(1)
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.
SunOS 5.10 Last change: 6 Nov 2004 4
User Commands ssh(1)
~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.
SunOS 5.10 Last change: 6 Nov 2004 5
User Commands ssh(1)
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
StrictHostKeyCheckingKey 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).
SunOS 5.10 Last change: 6 Nov 2004 6
User Commands ssh(1)
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 version 1 only.
-2
Forces ssh to try protocol version 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 connec-
tion.
-A
Enables forwarding of the authentication agent connec-
tion. This can also be specified on a per-host basis in
a configuration 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 connection.
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 iden-
tities loaded into the agent.
SunOS 5.10 Last change: 6 Nov 2004 7
User Commands ssh(1)
-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 supported
in ssh. blowfish is a fast block cipher, it appears very
secure and is much faster than 3des.
-c cipher_spec
Additionally, for protocol version 2 a comma-separated
list of ciphers can be specified in order of preference.
Protocol version 2 supports 3DES, Blowfish, and AES 128
in CBC mode. See Ciphers in ssh_config(4) for more
information.
Additionally, for protocol version 2 a comma-separated
list of ciphers can be specified in order of preference.
Protocol version 2 supports AES with 128-bit keys in
counter and CBC modes, RC4, and 3DES and Blowfish in CBC
mode. See ssh_config(4).
-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 networks. The default
value can be set on a host-by-host basis in the confi-
guration files. See the Compression option in
ssh_config(4).
SunOS 5.10 Last change: 6 Nov 2004 8
User Commands ssh(1)
-D port
Specifies a local ``dynamic'' application-level port
forwarding. This works by allocating a socket to listen
to port on the local side, and whenever a connection is
made to this port, the connection is forwarded over the
secure channel. The application 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 configuration 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 fol-
lowed by a dot (".") closes the connection. If followed
by Control-z, the escape character suspends the connec-
tion. If followed 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 some-
thing 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 configuration file
is $HOME/.ssh/config.
-g
Allows remote hosts to connect to local forwarded ports.
SunOS 5.10 Last change: 6 Nov 2004 9
User Commands ssh(1)
-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 protocol
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 multiple identities
specified in configuration 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 specified 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 version 2 a comma-separated
list of MAC (message authentication code) algorithms can
be specified in order of preference. See the MACs key-
word for more information.
-n
Redirects stdin from /dev/null (actually, prevents read-
ing 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,
SunOS 5.10 Last change: 6 Nov 2004 10
User Commands ssh(1)
ssh -n shadows.cs.hut.fi emacs &
starts an emacs on shadows.cs.hut.fi, and the X11 con-
nection is automatically forwarded over an encrypted
channel. 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 command. 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
configuration 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 confi-
guration file.
-P
Obsoleted option. SSHv1 connections 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)
SunOS 5.10 Last change: 6 Nov 2004 11
User Commands ssh(1)
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
connection is made to this port, the connection is for-
warded over the secure channel and a connection is made
to host port hostport from the local machine. Port for-
wardings can also be specified in the configuration
file. Privileged ports can be forwarded only when log-
ging in as root on the remote machine.
-s
Can be used to request invocation of a subsystem on the
remote system. Subsystems are a feature of the SSH2 pro-
tocol which facilitate the use of SSH as a secure tran-
sport for other applications, for example, sftp. The
subsystem is specified as the remote command.
-t
Forces pseudo-tty allocation. This can be used to exe-
cute arbitrary screen-based programs on a remote
machine, which can be very useful, for example, when
implementing menu services. Multiple -t options force
allocation, 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 connec-
tion, authentication, and configuration problems. Multi-
ple -v options increase the verbosity. Maximum is 3.
-x
Disables X11 forwarding.
SunOS 5.10 Last change: 6 Nov 2004 12
User Commands ssh(1)
-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 permissions on the
remote host (for the user's X authorization database)
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.
SunOS 5.10 Last change: 6 Nov 2004 13
User Commands ssh(1)
LANG, LC_ALL, LC_CTYPE, LC_COLLATE,
LC_TIME, LC_NUMERIC, LC_MONETARY, LC_MESSAGES
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.
Additionally, ssh reads $HOME/.ssh/environment and adds
lines of the format VARNAME=value to the environment
EXIT STATUS
The following exit values are returned:
0 Successful completion.
1 An error occurred.
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
$HOME/.ssh/id_dsa
$HOME/.ssh/id_ssa
Contains the authentication identity of the user. These
files are for protocol 1 RSA, protocol 2 DSA, and proto-
col 2 RSA, respectively. These files contain sensitive
data and should be readable by the user but not accessi-
ble 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.
SunOS 5.10 Last change: 6 Nov 2004 14
User Commands ssh(1)
/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 vari-
ables. 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 Evolving. The remote locale
selection through passing LC_* environment variables is
Unstable.
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), attri-
butes(5), gss_auth_rules(5), kerberos(5)
To view license terms, attribution, and copyright for
OpenSSH, the default path is
/var/sadm/pkg/SUNWsshdr/install/copyright. If the Solaris
operating environment has been installed anywhere other than
the default, modify the given path to access the file at the
installed location.
SunOS 5.10 Last change: 6 Nov 2004 15
User Commands ssh(1)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12
release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus
Friedl, Niels Provos, Theo de Raadt and Dug Song removed
many bugs, added newer features and created Open SSH. Markus
Friedl contributed the support for SSH protocol versions 1.4
and 2.0.
SunOS 5.10 Last change: 6 Nov 2004 16
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