ssh
SSH(1) BSD General Commands Manual SSH(1)
NAME
ssh - OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-l login_name] hostname | user@hostname [command]
ssh [-afgknqstvxACNTX1246] [-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 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 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 com-
bined 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 FILES section), only then login is permit-
ted. This authentication method closes security holes due to IP spoof-
ing, 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 chal-
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 automatically. 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 authen-
tication.
The most convenient way to use RSA authentication may be with an authen-
tication agent. See ssh-agent(1) for more information.
If other authentication methods fail, ssh prompts the user for a pass-
word. 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 authentica-
tion.
Protocol 2 provides additional mechanisms for confidentiality (the traf-
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 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 sta-
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 func-
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 charac-
ter can be changed in configuration files using the EscapeChar configura-
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 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. For-
warding 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 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 for-
warded 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 forwarding is a secure connection to an
electronic purse; another is going through firewalls.
Server authentication
ssh automatically maintains and checks a database containing identifica-
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. 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 pass-
word. 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.
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 they can perform operations on the keys
that enable them to authenticate using the identities loaded into
the agent.
-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 3des. des 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 identi-
ties 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 for-
warding ports (protocol version 2 only).
-o option
Can be used to give options in the format used in the configura-
tion 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.
-q Quiet mode. Causes all warning and diagnostic messages to be
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 appli-
cations (eg. sftp). The subsystem is specified as the remote com-
mand.
-t Force pseudo-tty allocation. This can be used to execute arbi-
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 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, authentica-
tion, 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.
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 may then be able
to perform activities such as keystroke monitoring.
-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 for
protocol version 1. 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 con-
figuration 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 for-
ward 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 connec-
tion is forwarded over the secure channel, and a connection is
made to host port hostport from the local machine. Port forward-
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 on the local
side, and whenever a connection is made to this port, the connec-
tion 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 privi-
leged 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 configura-
tion file and a system-wide configuration file. The file format and con-
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 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_CONNECTION
Identifies the client and server ends of the connection. The
variable contains four space-separated values: client ip-address,
client port number, server ip-address and server port number.
SSH_ORIGINAL_COMMAND
The variable contains the original command line if a forced com-
mand is executed. It can be used to extract the original argu-
ments.
SSH_TTY
This is set to the name of the tty (path to the device) associ-
ated 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 if the file exists and if
users are allowed to change their environment. See the
PermitUserEnvironment option in sshd_config(5).
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/execute). Note
that ssh ignores a private key file if it is accessible by oth-
ers. 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.
$HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
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 con-
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 version 2 DSA/RSA
authentication. These files are not sensitive and can (but need
not) be readable by anyone. These files are never used automati-
cally and are not necessary; they are only provided for the con-
venience of the user.
$HOME/.ssh/config
This is the per-user configuration file. The file format and
configuration options are described in ssh_config(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 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, pub-
lic key and optional comment field. When different names are
used for the same machine, all such names should be listed, sepa-
rated 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_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 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-keysign(8) 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 parti-
tion, 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 con-
nect 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 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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
SEE ALSO
rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
telnet(1), ssh_config(5), ssh-keysign(8), sshd(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
2002, work in progress material.
BSD September 25, 1999 BSD
