ssh
[-afgknqstvxACNPTX1246
]
[-b bind_address
]
[-c cipher_spec
]
[-e escape_char
]
[-i identity_file
]
[-l login_name
]
[-m mac_spec
]
[-o option
]
[-p port
]
[-F configfile
]
[-L Xo Sm off
porthosthostport
Sm on
]
[-R Xo Sm off
porthosthostport
Sm on
]
[-D port
]
hostname | user@hostname
[command
]
DESCRIPTION
ssh
(SSH client) 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 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/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/ssh/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 authentication method is the
rhosts
or
hosts.equiv
method combined with RSA-based host authentication.
It means that if the login would be permitted by
$HOME/.rhosts
$HOME/.shosts
/etc/hosts.equiv
or
/etc/ssh/shosts.equiv
and if additionally the server can verify the client's
host key (see
/etc/ssh/ssh_known_hosts
and
$HOME/.ssh/known_hosts
in the
Sx FILES
section), only then login is permitted.
This authentication method closes security holes due to IP
spoofing, DNS spoofing 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 cryptography: 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, a random number,
encrypted by the user's public 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/she knows the private
key but without disclosing it to the server.
ssh
implements the RSA authentication protocol automatically.
The user creates his/her RSA key pair by running
ssh-keygen1.
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/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 authentication is much
more secure than rhosts authentication.
The most convenient way to use RSA authentication may be with an
authentication agent.
See
ssh-agent1
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 network.
SSH protocol version 2
When a user connects using protocol version 2
similar authentication methods are available.
Using the default values for
PreferredAuthentications
the client will try to authenticate first using the hostbased method;
if this method fails public key authentication is attempted,
and 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 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 identifier 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 proving the user's identity.
Additionally,
ssh
supports hostbased or challenge response authentication.
Protocol 2 provides additional mechanisms for confidentiality
(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
and integrity (hmac-md5, hmac-sha1).
Note 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 session), the
user may use the escape characters noted below.
If no pseudo tty has been allocated, the
session is transparent 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 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 by 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
~C
Open command line (only useful for adding port forwardings 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
programs 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, and happens 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 forwarding is a secure connection to an
electronic purse; another is going through firewalls.
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/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 purpose of
this mechanism is to prevent man-in-the-middle attacks 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.
The options are as follows:
-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 configuration file.
-b bind_address
Specify 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.
blowfish
is a fast block cipher, it appears very secure and is much faster than
3desdes
is only supported in the
ssh
client for interoperability with legacy protocol 1 implementations
that do not support the
3des
cipher. Its use is strongly discouraged due to cryptographic
weaknesses.
-c cipher_spec
Additionally, for protocol version 2 a comma-separated list of ciphers can
be specified in order of preference.
See
Ciphers
for more information.
-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, followed
by control-Z suspends the connection, and followed by itself sends the
escape character 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
-n
The recommended 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 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 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).
-I smartcard_device
Specifies which smartcard device to use. The argument is
the device
ssh
should use to communicate with a smartcard used for storing the user's
private RSA key.
-k
Disables forwarding of Kerberos tickets and AFS tokens.
This may also be specified on a per-host basis in the configuration file.
-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.
-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 keyword for more information.
-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
Do not execute a remote command.
This is useful for just forwarding 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.
-p port
Port to connect to on the remote host.
This can be specified on a
per-host basis in the configuration file.
-P
Use a non-privileged port for outgoing connections.
This can be used if a firewall does
not permit connections from privileged ports.
Note that this option turns off
RhostsAuthentication
and
RhostsRSAAuthentication
for older servers.
-q
Quiet mode.
Causes all warning and diagnostic messages to be suppressed.
Only fatal errors are displayed.
If a second
-q
is given then even fatal errors are suppressed.
-s
May be used to request invocation 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 (eg. sftp). The
subsystem is specified as the remote command.
-t
Force pseudo-tty allocation.
This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very useful,
e.g., when implementing menu services.
Multiple
-t
options force tty allocation, even if
ssh
has no local tty.
-T
Disable pseudo-tty allocation.
-v
Verbose mode.
Causes
ssh
to print debugging messages about its progress.
This is helpful in
debugging connection, authentication, and configuration problems.
Multiple
-v
options increases 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.
-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),
and the
``level''
can be controlled by the
CompressionLevel
option.
Compression is desirable on modem lines and other
slow connections, but will only slow down things on fast networks.
The default value can be set on a host-by-host basis in the
configuration files; see the
Compression
option.
-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/ssh_config
)
will be ignored.
The default for the per-user configuration file is
$HOME/.ssh/config
-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
port
on the local side, and 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 configuration file.
Only root can forward privileged ports.
IPv6 addresses can be specified with an alternative syntax:
port/host/hostport
-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
port
on the remote side, and 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 configuration file.
Privileged ports can be forwarded only when
logging in as root on the remote machine.
IPv6 addresses can be specified with an alternative syntax:
port/host/hostport
-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, and the application
protocol is then used to determine where to connect to from the
remote machine. Currently the SOCKS4 protocol is supported, and
ssh
will act as a SOCKS4 server.
Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration file.
-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.
CONFIGURATION FILES
ssh
may additionally obtain configuration data from
a per-user configuration file and a system-wide configuration file.
The file format and configuration options are described in
ssh_config5.
ENVIRONMENT
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 >= 1.
ssh
uses this special value to forward X11 connections over the secure
channel.
The user should normally not set
DISPLAY
explicitly, 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 the path of the user's mailbox.
PATH
Set to the default
PATH
as specified when compiling
ssh
SSH_ASKPASS
If
ssh
needs a passphrase, it will read 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 will execute the program specified by
SSH_ASKPASS
and open an X11 window to read the passphrase.
This is particularly useful when calling
ssh
from a
.Xsession
or related script.
(Note that on some machines it
may be necessary to redirect the input from
/dev/null
to make this work.)
SSH_AUTH_SOCK
Identifies the path of a unix-domain socket used to communicate with the
agent.
SSH_CLIENT
Identifies the client end of the connection.
The variable contains
three space-separated values: client ip-address, client port number,
and server port number.
SSH_ORIGINAL_COMMAND
The variable contains the original command line if a forced command
is executed.
It can be used to extract the original arguments.
SSH_TTY
This is set to the name of the tty (path to the device) 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 (i.e., the daemon passes the value
on to new connections).
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.
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(8).
Contains the authentication identity of the user.
They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
These files
contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute).
Note 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
sensitive part of this file using 3DES.
Contains the public key for authentication (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 the user wishes to log in using protocol version 1 RSA authentication.
The contents of the
$HOME/.ssh/id_dsa.pub
and
$HOME/.ssh/id_rsa.pub
file should be added to
$HOME/.ssh/authorized_keys
on all machines
where the user wishes to log in using protocol version 2 DSA/RSA authentication.
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 only provided for
the convenience of the user.
$HOME/.ssh/config
This is the per-user configuration file.
The file format and configuration options are described in
ssh_config5.
$HOME/.ssh/authorized_keys
Lists the public keys (RSA/DSA) that can be used for logging in as this user.
The format of this file is described in the
sshd(8)
manual page.
In the simplest form the format is the same as the .pub
identity files.
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.
This file should be prepared by the
system administrator to contain the public host keys of all machines in the
organization.
This file should be world-readable.
This file contains
public keys, one per line, in the following format (fields separated
by spaces): system name, public key and optional comment field.
When different names are used
for the same machine, all such names should be listed, separated by
commas.
The format is described on the
sshd(8)
manual page.
The canonical system name (as returned by name servers) is used by
sshd(8)
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, because someone with access to the name servers
would then be able to fool host authentication.
/etc/ssh/ssh_config
Systemwide configuration file.
The file format and configuration options are described in
ssh_config5.
These three files contain the private parts of the host keys
and are used for
RhostsRSAAuthentication
and
HostbasedAuthentication
If the protocol version 1
RhostsRSAAuthentication
method is used,
ssh
must be setuid root, since the host key is readable only by root.
For protocol version 2,
ssh
uses
ssh-keysign8
to access the host keys for
HostbasedAuthentication
This eliminates the requirement that
ssh
be setuid root when that authentication method is used.
By default
ssh
is not setuid root.
$HOME/.rhosts
This file is used in
.rhosts
authentication to list the
host/user pairs that are permitted to log in.
(Note 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 a NFS partition,
because
sshd(8)
reads it as root.
Additionally, this file must be owned by the user,
and must not have write permissions for anyone else.
The recommended
permission for most machines is read/write for the user, and not
accessible by others.
Note that by default
sshd(8)
will be installed so that it requires successful RSA host
authentication before permitting .rhosts authentication.
If the server machine does not have the client's host key in
/etc/ssh/ssh_known_hosts
it can be stored 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
or
rsh(1).
/etc/hosts.equiv
This file is used during
.rhosts authentication.
It contains
canonical hosts names, one per line (the full format is described on
the
sshd(8)
manual page).
If the client host is found in this file, login is
automatically permitted provided client and server user names are the
same.
Additionally, 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/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 the
sshd(8)
manual page 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 the
sshd(8)
manual page for more information.
$HOME/.ssh/environment
Contains additional definitions for environment variables, see section
Sx ENVIRONMENT
above.
DIAGNOSTICS
ssh
exits with the exit status of the remote command or with 255
if an error occurred.
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, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
T. Ylonen
T. Kivinen
M. Saarinen
T. Rinne
S. Lehtinen
"SSH Protocol Architecture"
draft-ietf-secsh-architecture-12.txt
January 2002
work in progress material