ssh [-1246AaCfgkNnqsTtVvXxY] [-b bind_address] [-c ci-
pher_spec] [-D port]
[-e escape_char] [-F configfile] [-i identity_file]
[-L port:host:hostport] [-l login_name] [-m mac_spec]
[-o option]
[-p port] [-R port:host:hostport] [user@]hostname [com-
mand]
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 untrust-
ed hosts over an insecure network. X11 connections and ar-
bitrary TCP/IP
ports can also be forwarded over the secure channel.
ssh connects and logs into the specified hostname (with op-
tional user
name). The user must prove his/her identity to the remote
machine using
one of several methods depending on the protocol version
used.
If command is specified, command is executed on the remote
host instead
of a login shell.
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 com-
bined with RSA-based host authentication. It means that if
the login
would be permitted by $HOME/.rhosts, $HOME/.shosts,
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 per-
mitted for logging in. When the user logs in, the ssh pro-
gram tells the
server which key pair it would like to use for authentica-
tion. The serv-
er checks if this key is permitted, and if so, sends the us-
er (actually
the ssh program running on behalf of the user) a challenge,
a random num-
ber, encrypted by the user's public key. The challenge can
only be de-
crypted using the proper private key. The user's client
then decrypts
the challenge using the private key, proving that he/she
knows the pri-
vate 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 stores 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.
When a user connects using protocol version 2, similar au-
thentication
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 serv-
er.
If public key authentication fails or is not available, a
password can be
sent encrypted to the remote host to prove the user's iden-
tity.
Additionally, ssh supports hostbased or challenge response
authentica-
tion.
Protocol 2 provides additional mechanisms for confidentiali-
ty (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 ei-
ther 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
The session terminates when the command or shell on the re-
mote 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 Es-
capeChar 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.
~B Send a BREAK to the remote system (only useful for
SSH protocol
version 2 and if the peer supports it).
~C Open command line (only useful for adding port for-
wardings using
the -L and -R options).
~R Request rekeying of the connection (only useful for
SSH protocol
version 2 and if the peer supports it).
X11 and TCP forwarding
If the ForwardX11 variable is set to ``yes'' (or see the de-
scription of
the -X and -x options described later) and the user is using
The DISPLAY value set by ssh will point to the server ma-
chine, but with a
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 ForwardAgent variable is set to ``yes'' (or see the
description of
the -A and -a options described later) and the user is using
an authenti-
cation agent, the connection to the agent is automatically
forwarded to
the remote side.
Forwarding of arbitrary TCP/IP connections over the secure
channel can be
specified either on the command line or in a configuration
file. One
possible application of TCP/IP 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
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. 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 pass-
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
-A Enables forwarding of the authentication agent con-
nection. This
can also be specified on a per-host basis in a con-
figuration
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 identi-
ties loaded into
the agent.
-a Disables forwarding of the authentication agent con-
nection.
-b bind_address
Specify the interface to transmit from on machines
with multiple
interfaces or aliased addresses.
-C Requests compression of all data (including stdin,
stdout,
stderr, and data for forwarded X11 and TCP/IP con-
nections). 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 mo-
dem 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 op-
tion.
-c blowfish | 3des | des
Selects the cipher to use for encrypting the ses-
sion. 3des is
-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.
-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 and SOCKS5
protocols are
supported, and ssh will act as a SOCKS server. Only
root can
forward privileged ports. Dynamic port forwardings
can also be
specified in the configuration file.
-e ch | ^ch | none
Sets the escape character for sessions with a pty
(default: `~').
The escape character is only recognized at the 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 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 ig-
nored. The
default for the per-user configuration file is
$HOME/.ssh/config.
-f Requests ssh to go to background just before command
ment is the de-
vice ssh should use to communicate with a smartcard
used for
storing the user's private RSA key.
-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 al-
so be specified on a per-host basis in the configu-
ration file.
It is possible to have multiple -i options (and mul-
tiple identi-
ties specified in configuration files).
-k Disables forwarding (delegation) of GSSAPI creden-
tials to the
server.
-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 speci-
fied with an
alternative syntax: port/host/hostport.
-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
chine. For example, ssh -n shadows.cs.hut.fi emacs
& will start
an emacs on shadows.cs.hut.fi, and the X11 connec-
tion will be au-
tomatically 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.)
-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. For full details
of the op-
tions listed below, and their possible values, see
ssh_config(5).
AddressFamily
BatchMode
BindAddress
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectionTimeout
DynamicForward
EscapeChar
ForwardAgent
ForwardX11
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
Host
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
HostName
IdentityFile
IdentitiesOnly
LocalForward
LogLevel
MACs
StrictHostKeyChecking
TCPKeepAlive
UsePrivilegedPort
User
UserKnownHostsFile
VerifyHostKeyDNS
XAuthLocation
-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 mes-
sages to be
suppressed.
-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.
-s May be used to request invocation of a subsystem on
the remote
system. Subsystems are a feature of the SSH2 proto-
col which fa-
cilitate the use of SSH as a secure transport for
other applica-
tions (eg. sftp(1)). The subsystem is specified as
the remote
command.
-T Disable pseudo-tty allocation.
-t Force pseudo-tty allocation. This can be used to
execute arbi-
tions increase
the verbosity. The maximum is 3.
-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 lo-
cal X11 display
through the forwarded connection. An attacker may
then be able
to perform activities such as keystroke monitoring.
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding.
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 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 manu-
ally copy any required authorization cookies).
HOME Set to the path of the user's home directory.
LOGNAME Synonym for USER; set for compatibility with sys-
tems that use
this variable.
SSH_ASKPASS
and open an X11 window to read the passphrase.
This is particu-
larly useful when calling ssh from a .Xsession or
related
script. (Note that on some machines it may be nec-
essary 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 con-
nection. The
variable contains four space-separated values:
client ip-ad-
dress, 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 de-
vice) 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 ex-
ists and if
users are allowed to change their environment. For more in-
formation, see
the PermitUserEnvironment option in sshd_config(5).
user but not accessible by others (read/write/exe-
cute). Note
that ssh ignores a private key file if it is acces-
sible by oth-
ers. It is possible to specify a passphrase when
generating the
key; the passphrase will be used to encrypt the sen-
sitive 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 the
file
$HOME/.ssh/authorized_keys on all machines where the
user wishes
to log in using protocol version 1 RSA authentica-
tion. 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 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 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_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
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 in 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 au-
thentication.
/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
Note that by default sshd(8) will be installed so
that it re-
quires 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 us-
ing 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 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
in 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 au-
thentication 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.
ssh exits with the exit status of the remote command or with
255 if an
error occurred.
SEE ALSO
gzip(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1),
ssh-keygen(1), telnet(1), hosts.equiv(5), 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.
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 fea-
tures and
created OpenSSH. Markus Friedl contributed the support for
SSH protocol
versions 1.5 and 2.0.
OpenBSD 3.5 September 25, 1999
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