ssh [-l login_name] hostname | user@hostname [command]
ssh [-afgknqstvxACNPTX1246] [-b bind_address] [-c ci-
pher_spec]
[-e escape_char] [-i identity_file] [-l login_name] [-m
mac_spec]
[-o option] [-p port] [-F configfile] [-L
port:host:hostport] [-R
port:host:hostport] [-D port] hostname | user@hostname
[command]
DESCRIPTION
ssh (SSH client) is a program for logging into a remote ma-
chine 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/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 comM--
bined with RSA-based host authentication. It means that if
the login
would be permitted by $HOME/.rhosts, $HOME/.shosts,
/etc/hosts.equiv, or
As a third authentication method, ssh supports RSA based au-
thentication.
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 encryp-
tion 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 de-
crypts the chalM--
lenge 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 automatical-
ly. The user
creates his/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/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 authenM--
tication.
The most convenient way to use RSA authentication may be
with an authenM--
methods are available. Using the default values for
PreferredAuthentications, the client will try to authenti-
cate first using
the hostbased method; if this method fails public key au-
thentication is
attempted, and finally if this method fails keyboard-inter-
active and
password authentication are tried.
The public key method is similar to RSA authentication de-
scribed 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
authenticaM--
tion.
Protocol 2 provides additional mechanisms for confidentiali-
ty (the trafM--
fic 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 communica-
tion with the
remote command or shell will be automatically encrypted.
mote machine
exits and all X11 and TCP/IP connections have been closed.
The exit staM--
tus 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 funcM--
tions 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 characM--
ter can be changed in configuration files using the Es-
capeChar configuraM--
tion 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 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
display number greater than zero. This is normal, and hap-
pens 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 forward-
ed 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 dis-
abled on the comM--
mand 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 con-
nection to an
electronic purse; another is going through firewalls.
Server authentication
ssh automatically maintains and checks a database containing
identificaM--
tions for all hosts it has ever been used with. Host keys
are stored in
$HOME/.ssh/known_hosts in the user's home directory. Addi-
tionally, 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 dis-
ables password
authentication to prevent a trojan horse from getting the
user's passM--
word. Another purpose of this mechanism is to prevent man-
in-the-middle
attacks which could otherwise be used to circumvent the en-
cryption. The
-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 ses-
sion. 3des is
used by default. It is believed to be secure. 3des
(triple-des)
is an encrypt-decrypt-encrypt triple with three dif-
ferent keys.
blowfish is a fast block cipher, it appears very se-
cure and is
much faster than 3des. des is only supported in the
ssh client
for interoperability with legacy protocol 1 imple-
mentations 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-sepa-
rated 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 be-
ginning of a
line. The escape character followed by a dot (`.')
closes the
connection, followed by control-Z suspends the con-
nection, 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 back-
ground. This
implies -n. The recommended way to start X11 pro-
files may
also be specified on a per-host basis in the config-
uration file.
It is possible to have multiple -i options (and mul-
tiple identiM--
ties specified in configuration files).
-I smartcard_device
Specifies which smartcard device to use. The argu-
ment is the
device ssh should use to communicate with a smart-
card used for
storing the user's private RSA key.
-k Disables forwarding of Kerberos tickets and AFS to-
kens. This may
also be specified on a per-host basis in the config-
uration file.
-l login_name
Specifies the user to log in as on the remote ma-
chine. This also
may be specified on a per-host basis in the configu-
ration file.
-m mac_spec
Additionally, for protocol version 2 a comma-sepa-
rated 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 chan-
nel. 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
-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 Rhost-
sAuthentication and
RhostsRSAAuthentication for older servers.
-q Quiet mode. Causes all warning and diagnostic mes-
sages to be
suppressed.
-s May be used to request invocation of a subsystem on
the remote
system. Subsystems are a feature of the SSH2 proto-
col which
facilitate the use of SSH as a secure transport for
other appliM--
cations (eg. sftp). The subsystem is specified as
the remote comM--
mand.
-t Force pseudo-tty allocation. This can be used to
execute arbiM--
trary 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 lo-
cal tty.
-T Disable pseudo-tty allocation.
-v Verbose mode. Causes ssh to print debugging mes-
sages about its
progress. This is helpful in debugging connection,
authenticaM--
tion, and configuration problems. Multiple -v op-
tions 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 con-
nections). The
file. If a conM--
figuration file is given on the command line, the
system-wide
configuration file (/etc/ssh/ssh_config) will be ig-
nored. 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 forM--
ward privileged ports. IPv6 addresses can be speci-
fied 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 connecM--
tion is forwarded over the secure channel, and a
connection is
made to host port hostport from the local machine.
Port forwardM--
ings 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
-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 configuraM--
tion file and a system-wide configuration file. The file
format and conM--
figuration options are described in ssh_config(5).
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 spe-
cial 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
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 conM--
tains three space-separated values: client ip-ad-
dress, client
port number, and server port number.
SSH_ORIGINAL_COMMAND
The variable contains the original command line if a
forced comM--
mand is executed. It can be used to extract the
original arguM--
ments.
SSH_TTY
This is set to the name of the tty (path to the de-
vice) associM--
ated with the current shell or command. If the cur-
rent 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).
$HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
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/exe-
cute). Note
$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 authentica-
tion. The conM--
tents 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 ver-
sion 2 DSA/RSA
authentication. These files are not sensitive and
can (but need
not) be readable by anyone. These files are never
used automatiM--
cally and are not necessary; they are only provided
for the conM--
venience of the user.
$HOME/.ssh/config
This is the per-user configuration file. The file
format and
configuration options are described in ssh_con-
fig(5).
$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 for-
mat 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): sys-
tem name, pubM--
lic key and optional comment field. When different
names are
used for the same machine, all such names should be
fool host
authentication.
/etc/ssh/ssh_config
Systemwide configuration file. The file format and
configuration
options are described in ssh_config(5).
/etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
/etc/ssh/ssh_host_rsa_key
These three files contain the private parts of the
host keys and
are used for RhostsRSAAuthentication and Host-
basedAuthentication.
If the protocol version 1 RhostsRSAAuthentication
method is used,
ssh must be setuid root, since the host key is read-
able only by
root. For protocol version 2, ssh uses ssh-
keysign(8) to access
the host keys for HostbasedAuthentication. This
eliminates the
requirement that ssh be setuid root when that au-
thentication
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 partiM--
tion, because sshd(8) reads it as root. Additional-
ly, 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 oth-
ers.
Note that by default sshd(8) will be installed so
that it
This file is used exactly the same way as .rhosts.
The purpose
for having this file is to be able to use rhosts au-
thentication
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/shosts.equiv
This file is processed exactly as /etc/hosts.equiv.
This file
may be useful to permit logins using ssh but not us-
ing
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 start-
ed. 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 start-
ed. See the
sshd(8) manual page for more information.
$HOME/.ssh/environment
Contains additional definitions for environment
variables, see
section ENVIRONMENT above.
DIAGNOSTICS
ssh exits with the exit status of the remote command or with
255 if an
error occurred.
gen(1),
telnet(1), ssh_config(5), ssh-keysign(8), sshd(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehti-
nen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-12.txt,
January
2002, work in progress material.
BSD September 25, 1999
BSD
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