ssh(1)
NAME
ssh - OpenSSH secure shell client (remote login program)
SYNOPSIS
ssh [-l login_name] [ hostname | user@hostname] [ com-
mand]
ssh -afgknqtvxACNPTX246 [-c cipher_spec] [-e escape_char]
[-i identity_file] [-l login_name] [-o option] [-p port] [-L
port:host:hostport] [-R port:host:hostport] [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 may 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
When a user connects using the protocol version 2, different
authentication methods are available. At first, the client
attempts to authenticate using the public key method. If
this method fails, password authentication is tried.
The public key method is similar to RSA authentication
described in the previous section except that the DSA
algorithm is used instead of the patented RSA algorithm. The
client uses his private DSA key $HOME/.ssh/id_dsa 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 iden-
tifier is derived from a shared Diffie-Hellman value and is
known only 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. This protocol 2 implementation does
not yet support Kerberos or S/Key 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). Notice that
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
will be automatically encrypted.
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 by 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" will
also make the session transparent even if a tty is used.
The session terminates when the command or shell in 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.
X11 and TCP forwarding
If 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) will go through
the encrypted channel, and the connection to the real X
server will be 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 will point to the server
machine, but with a display number greater than zero. This
is normal behavior, because ssh creates a "proxy" X server
on the server machine for forwarding the connections over
the encrypted channel.
ssh will also automatically set up Xauthority data on the
server machine. For this purpose, it will generate a random
authorization cookie, store it in Xauthority on the server,
and verify 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 user is using an authentication agent, the connection
to the agent is automatically forwarded to the remote side
unless disabled on the command line or in a configuration
file.
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 going through firewalls.
Server authentication
ssh automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
RSA 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. Any new hosts are automatically added to the user's
file. 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 pur-
pose of this mechanism is to prevent man-in-the-middle
attacks which could otherwise be used to circumvent the
encryption. The StrictHostKeyChecking option (see below) can
be used to prevent logins to machines whose host key is not
known or has changed.
OPTIONS
The following options are supported:
-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 con-
nection.
-A Enables forwarding of the authentication agent connec-
tion. This can also be specified on a per-host basis
in a configuration file.
-c blowfish | 3des
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 3des-cbc,blowfish-cbc,aes128-cbc
Additionally, for protocol version 2 a comma-separated
list of ciphers can be specified in order of prefer-
ence. Protocol version 2 supports 3DES, Blowfish, and
AES 128 in CBC mode.
-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 below). Compres-
sion is desirable on modem lines and other slow con-
nections, but will only slow down things on fast net-
works. The default value can be set on a host-by-host
basis in the configuration files. See the Compress
option below.
-e ch | ^ch | none
Sets the escape character for sessions with a pty
(default: `~'). The escape character is only recog-
nized at the beginning of a line. The escape character
followed by a dot (".") closes the connection. If fol-
lowed by control-Z, the escape character suspends the
connection. If followed by itself, the escape charac-
ter 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 recom-
mended way to start X11 programs at a remote site is
with something like ssh -f host xterm.
-g Allows remote hosts to connect to local forwarded
ports.
-i identity_file
Selects the file from which the identity (private key)
for RSA authentication is read. Default is
$HOME/.ssh/identity in the user's home directory.
Identity files may 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 may 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
connection 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 confi-
guration file. Only root can forward privileged ports.
IPv6 addresses can be specified with an alternative
syntax: port/host/hostport.
-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 &
will start an emacs on shadows.cs.hut.fi, and the X11
connection will be automatically forwarded over an
encrypted channel. The ssh program will be 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 con-
figuration file.
-P Uses a non-privileged port for outgoing connections.
This can be used if your firewall does not permit con-
nections from privileged ports. Notice that this
option turns off RhostsAuthentication and RhostsRSAAu-
thentication.
-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 connection 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 specified in the confi-
guration file. Privileged ports can be forwarded only
when logging in as root on the remote machine.
-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.
-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 con-
nection, authentication, and configuration problems.
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.
ENVIRONMENT VARIABLES
ssh will normally set the following environment variables:
DISPLAY
The DISPLAY variable indicates the location of the X11
server. It is automatically set by ssh to point to a
value of the form hostname:n where hostname indicates
the host where the shell runs, and n is an integer
greater than or equal to 1. ssh uses this special
value to forward X11 connections over the secure chan-
nel. The user should normally not set DISPLAY expli-
citly, as that will render the X11 connection insecure
(and will require the user to manually copy any
required authorization cookies).
HOME Set to the path of the user's home directory.
LOGNAME
Synonym for USER. Set for compatibility with systems
that use this variable.
MAIL Set to point to the user's mailbox.
PATH Set to the default PATH, as specified when compiling
ssh.
SSH_AUTH_SOCK
Indicates the path of a unix-domain socket used to
communicate with the agent.
SSH_CLIENT
Identifies the client end of the connection. The vari-
able contains three space-separated values: client
ip-address, client port number, and server port
number.
SSH_TTY
This is set to the name of the tty (path to the dev-
ice) associated with the current shell or command. If
the current session has no tty, this variable is not
set.
TZ The timezone variable is set to indicate the present
timezone if it was set when the daemon was started,
that is, the daemon passes the value on to new connec-
tions.
USER Set to the name of the user logging in.
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_known_hosts. See
sshd(1M).
$HOME/.ssh/identity
$HOME/.ssh/id_dsa
Contains the RSA and the DSA authentication identity
of the user. These files contain sensitive data and
should be readable by the user but not accessible by
others (read/write/execute). Notice that 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 will be used to encrypt the sensi-
tive part of this file using 3DES.
$HOME/.ssh/identity.pub
$HOME/.ssh/id_dsa.pub
Contains the public key for authentication, that is,
the public part of the identity file in human-readable
form. The contents of the $HOME/.ssh/identity.pub file
should be added to $HOME/.ssh/authorized_keys on all
machines where you wish to log in using RSA authenti-
cation. The contents of the $HOME/.ssh/id_dsa.pub file
should be added to $HOME/.ssh/authorized_keys on all
machines where you wish to log in using DSA authenti-
cation. These files are not sensitive and can, but
need not, be readable by anyone. These files are never
used automatically and are not necessary. They are
provided only for the convenience of the user.
$HOME/.ssh/config
This is the per-user configuration file. The format of
this file is described above. This file is used by the
ssh client. This file does not usually contain any
sensitive information, but the recommended permissions
are read/write for the user and not accessible by oth-
ers.
$HOME/.ssh/authorized_keys
Lists the DSA keys that can be used for logging in as
this user. This file is not highly sensitive, but the
recommended permissions are read/write for the user
and not accessible by others.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys.
/etc/ssh_known_hosts contains RSA keys. This file
should be prepared by the system administrator to con-
tain the public host keys of all machines in the
organization and should be world-readable. The file
contains public keys, one per line, in the following
format, with fields separated by spaces: system name,
number of bits in modulus, public exponent, modulus,
and optional comment field. When different names are
used for the same machine, all such names should be
listed, separated by commas. See sshd(1M).
The canonical system name (as returned by name
servers) is used by sshd(1M) to verify the client host
when logging in. Other names are needed because ssh
does not convert the user-supplied name to a canonical
name before checking the key, to prevent someone with
access to the name servers from being able able to
fool host authentication.
/etc/ssh/ssh_config
Systemwide configuration file. This file provides
defaults for those values that are not specified in
the user's configuration file, and for those users who
do not have a configuration file.
This file must be world-readable.
$HOME/.rhosts
This file is used in .rhosts authentication to list
the host/user pairs that are permitted to log in.
(Notice that this file is also used by rlogin and rsh,
which makes using this file insecure.) Each line of
the file contains a host name (in the canonical form
returned by name servers), and then a user name on
that host, separated by a space. On some machines,
this file may need to be world-readable if the user's
home directory is on an NFS partition, because
sshd(1M) reads it as root. Additionally, this file
must be owned by the user and must not have write per-
missions for anyone else. The recommended permission
for most machines is read/write for the user and not
accessible by others.
Notice that, by default, sshd(1M) will be installed so
that it requires successful RSA host authentication
before permitting .rhosts authentication. If your
server machine does not have the client's host key in
/etc/ssh_known_hosts, you can store it in
$HOME/.ssh/known_hosts. The easiest way to do this is
to connect back to the client from the server machine
using ssh. This will automatically add the host key to
$HOME/.ssh/known_hosts.
$HOME/.shosts
This file is used exactly the same way as .rhosts. The
purpose for having this file is to be able to use
rhosts authentication with ssh without permitting
login with rlogin(1) or rsh(1).
/etc/hosts.equiv
This file is used during .rhosts authentication. It
contains canonical hosts names, one per line. (See
sshd(1M) for the full format description.). If the
client host is found in this file, login is automati-
cally permitted, provided that client and server user
names are the same. In addition, successful RSA host
authentication is normally required. This file should
only be writable by root.
/etc/ssh/shosts.equiv
This file is processed exactly as /etc/hosts.equiv.
This file may be useful to permit logins using ssh but
not using rsh or rlogin.
/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 |
|_____________________________|_____________________________|
SEE ALSO
rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-
keygen(1), telnet(1), sshd(1M), ssh_config(4), attributes(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.
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.
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