ssh.1

.\"  -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $OpenBSD: ssh.1,v 1.148 2002/02/18 17:55:20 markus Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH SSH client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl l Ar login_name
.Ar hostname | user@hostname
.Op Ar command
.Pp
.Nm ssh
.Op Fl afgknqstvxACNPTX1246
.Op Fl b Ar bind_address
.Op Fl c Ar cipher_spec
.Op Fl e Ar escape_char
.Op Fl i Ar identity_file
.Op Fl l Ar login_name
.Op Fl m Ar mac_spec
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl F Ar configfile
.Oo Fl L Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Oo Fl R Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Op Fl D Ar port
.Ar hostname | user@hostname
.Op Ar command
.Sh DESCRIPTION
.Nm
(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.
.Pp
.Nm
connects and logs into the specified
.Ar hostname .
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used:
.Pp
.Ss SSH protocol version 1
.Pp
First, if the machine the user logs in from is listed in
.Pa /etc/hosts.equiv
or
.Pa /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
.Pa \&.rhosts
or
.Pa \&.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.
.Pp
The second authentication method is the
.Pa rhosts
or
.Pa hosts.equiv
method combined with RSA-based host authentication.
It means that if the login would be permitted by
.Pa $HOME/.rhosts ,
.Pa $HOME/.shosts ,
.Pa /etc/hosts.equiv ,
or
.Pa /etc/shosts.equiv ,
and if additionally the server can verify the client's
host key (see
.Pa /etc/ssh/ssh_known_hosts
and
.Pa $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:
.Pa /etc/hosts.equiv ,
.Pa $HOME/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
As a third authentication method,
.Nm
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
.Pa $HOME/.ssh/authorized_keys
lists the public keys that are permitted for logging
in.
When the user logs in, the
.Nm
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
.Nm
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.
.Pp
.Nm
implements the RSA authentication protocol automatically.
The user creates his/her RSA key pair by running
.Xr ssh-keygen 1 .
This stores the private key in
.Pa $HOME/.ssh/identity
and the public key in
.Pa $HOME/.ssh/identity.pub
in the user's home directory.
The user should then copy the
.Pa identity.pub
to
.Pa $HOME/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
.Pa $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.
.Pp
The most convenient way to use RSA authentication may be with an
authentication agent.
See
.Xr ssh-agent 1
for more information.
.Pp
If other authentication methods fail,
.Nm
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.
.Pp
.Ss SSH protocol version 2
.Pp
When a user connects using protocol version 2
similar authentication methods are available.
Using the default values for
.Cm 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.
.Pp
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,
.Pa $HOME/.ssh/id_dsa
or
.Pa $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
.Pa $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.
.Pp
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.
.Pp
Additionally,
.Nm
supports hostbased or challenge response authentication.
.Pp
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.
.Pp
.Ss Login session and remote execution
.Pp
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.
.Pp
If a pseudo-terminal has been allocated (normal login session), the
user may use the escape characters noted below.
.Pp
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
.Dq none
will also make the session transparent even if a tty is used.
.Pp
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
.Nm ssh .
.Pp
.Ss Escape Characters
.Pp
When a pseudo terminal has been requested, ssh supports a number of functions
through the use of an escape character.
.Pp
A single tilde character can be sent as
.Ic ~~
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
.Cm EscapeChar
configuration directive or on the command line by the
.Fl e
option.
.Pp
The supported escapes (assuming the default
.Ql ~ )
are:
.Bl -tag -width Ds
.It Cm ~.
Disconnect
.It Cm ~^Z
Background ssh
.It Cm ~#
List forwarded connections
.It Cm ~&
Background ssh at logout when waiting for forwarded connection / X11 sessions
to terminate
.It Cm ~?
Display a list of escape characters
.It Cm ~R
Request rekeying of the connection (only useful for SSH protocol version 2
and if the peer supports it)
.El
.Pp
.Ss X11 and TCP forwarding
.Pp
If the
.Cm ForwardX11
variable is set to
.Dq yes
(or, see the description of the
.Fl X
and
.Fl x
options described later)
and the user is using X11 (the
.Ev 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
.Ev DISPLAY .
Forwarding of X11 connections can be
configured on the command line or in configuration files.
.Pp
The
.Ev DISPLAY
value set by
.Nm
will point to the server machine, but with a display number greater
than zero.
This is normal, and happens because
.Nm
creates a
.Dq proxy
X server on the server machine for forwarding the
connections over the encrypted channel.
.Pp
.Nm
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).
.Pp
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.
.Pp
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.
.Pp
.Ss Server authentication
.Pp
.Nm
automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
Host keys are stored in
.Pa $HOME/.ssh/known_hosts
in the user's home directory.
Additionally, the file
.Pa /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,
.Nm
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
.Cm StrictHostKeyChecking
option (see below) can be used to prevent logins to machines whose
host key is not known or has changed.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a
Disables forwarding of the authentication agent connection.
.It Fl A
Enables forwarding of the authentication agent connection.
This can also be specified on a per-host basis in a configuration file.
.It Fl b Ar bind_address
Specify the interface to transmit from on machines with multiple
interfaces or aliased addresses.
.It Fl c Ar blowfish|3des|des
Selects the cipher to use for encrypting the session.
.Ar 3des
is used by default.
It is believed to be secure.
.Ar 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
.Ar blowfish
is a fast block cipher, it appears very secure and is much faster than
.Ar 3des .
.Ar des
is only supported in the
.Nm
client for interoperability with legacy protocol 1 implementations
that do not support the
.Ar 3des
cipher.  Its use is strongly discouraged due to cryptographic
weaknesses.
.It Fl c Ar cipher_spec
Additionally, for protocol version 2 a comma-separated list of ciphers can
be specified in order of preference.
See
.Cm Ciphers
for more information.
.It Fl e Ar ch|^ch|none
Sets the escape character for sessions with a pty (default:
.Ql ~ ) .
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot
.Pq Ql \&.
closes the connection, followed
by control-Z suspends the connection, and followed by itself sends the
escape character once.
Setting the character to
.Dq none
disables any escapes and makes the session fully transparent.
.It Fl f
Requests
.Nm
to go to background just before command execution.
This is useful if
.Nm
is going to ask for passwords or passphrases, but the user
wants it in the background.
This implies
.Fl n .
The recommended way to start X11 programs at a remote site is with
something like
.Ic ssh -f host xterm .
.It Fl g
Allows remote hosts to connect to local forwarded ports.
.It Fl i Ar identity_file
Selects a file from which the identity (private key) for
RSA or DSA authentication is read.
The default is
.Pa $HOME/.ssh/identity
for protocol version 1, and
.Pa $HOME/.ssh/id_rsa
and
.Pa $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
.Fl i
options (and multiple identities specified in
configuration files).
.It Fl I Ar smartcard_device
Specifies which smartcard device to use. The argument is
the device
.Nm
should use to communicate with a smartcard used for storing the user's
private RSA key.
.It Fl k
Disables forwarding of Kerberos tickets and AFS tokens.
This may also be specified on a per-host basis in the configuration file.
.It Fl l Ar 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.
.It Fl m Ar 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
.Cm MACs
keyword for more information.
.It Fl n
Redirects stdin from
.Pa /dev/null
(actually, prevents reading from stdin).
This must be used when
.Nm
is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
.Ic 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
.Nm
program will be put in the background.
(This does not work if
.Nm
needs to ask for a password or passphrase; see also the
.Fl f
option.)
.It Fl N
Do not execute a remote command.
This is useful for just forwarding ports
(protocol version 2 only).
.It Fl o Ar 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.
.It Fl p Ar port
Port to connect to on the remote host.
This can be specified on a
per-host basis in the configuration file.
.It Fl 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
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication
for older servers.
.It Fl q
Quiet mode.
Causes all warning and diagnostic messages to be suppressed.
.It Fl 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.
.It Fl 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
.Fl t
options force tty allocation, even if
.Nm
has no local tty.
.It Fl T
Disable pseudo-tty allocation.
.It Fl v
Verbose mode.
Causes
.Nm
to print debugging messages about its progress.
This is helpful in
debugging connection, authentication, and configuration problems.
Multiple
.Fl v
options increases the verbosity.
Maximum is 3.
.It Fl x
Disables X11 forwarding.
.It Fl X
Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
.It Fl 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
.Xr gzip 1 ,
and the
.Dq level
can be controlled by the
.Cm CompressionLevel
option (see below).
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
.Cm Compression
option below.
.It Fl F Ar configfile
Specifies an alternative per-user configuration file.
If a configuration file is given on the command line,
the system-wide configuration file
.Pq Pa /etc/ssh/ssh_config
will be ignored.
The default for the per-user configuration file is
.Pa $HOME/.ssh/config .
.It Fl L Ar 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
.Ar 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
.Ar host
port
.Ar 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:
.Ar port/host/hostport
.It Fl R Ar 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
.Ar 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
.Ar host
port
.Ar 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:
.Ar port/host/hostport
.It Fl D Ar port
Specifies a local
.Dq dynamic
application-level port forwarding.
This works by allocating a socket to listen to
.Ar 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
.Nm
will act as a SOCKS4 server.
Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration file.
.It Fl 1
Forces
.Nm
to try protocol version 1 only.
.It Fl 2
Forces
.Nm
to try protocol version 2 only.
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.El
.Sh CONFIGURATION FILES
.Nm
obtains configuration data from the following sources in
the following order:
command line options, user's configuration file
.Pq Pa $HOME/.ssh/config ,
and system-wide configuration file
.Pq Pa /etc/ssh/ssh_config .
For each parameter, the first obtained value
will be used.
The configuration files contain sections bracketed by
.Dq Host
specifications, and that section is only applied for hosts that
match one of the patterns given in the specification.
The matched host name is the one given on the command line.
.Pp
Since the first obtained value for each parameter is used, more
host-specific declarations should be given near the beginning of the
file, and general defaults at the end.
.Pp
The configuration file has the following format:
.Pp
Empty lines and lines starting with
.Ql #
are comments.
.Pp
Otherwise a line is of the format
.Dq keyword arguments .
Configuration options may be separated by whitespace or
optional whitespace and exactly one
.Ql = ;
the latter format is useful to avoid the need to quote whitespace
when specifying configuration options using the
.Nm ssh ,
.Nm scp
and
.Nm sftp
.Fl o
option.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm Host
Restricts the following declarations (up to the next
.Cm Host
keyword) to be only for those hosts that match one of the patterns
given after the keyword.
.Ql \&*
and
.Ql ?
can be used as wildcards in the
patterns.
A single
.Ql \&*
as a pattern can be used to provide global
defaults for all hosts.
The host is the
.Ar hostname
argument given on the command line (i.e., the name is not converted to
a canonicalized host name before matching).
.It Cm AFSTokenPassing
Specifies whether to pass AFS tokens to remote host.
The argument to this keyword must be
.Dq yes
or
.Dq no .
This option applies to protocol version 1 only.
.It Cm BatchMode
If set to
.Dq yes ,
passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where no user
is present to supply the password.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm BindAddress
Specify the interface to transmit from on machines with multiple
interfaces or aliased addresses.
Note that this option does not work if
.Cm UsePrivilegedPort
is set to
.Dq yes .
.It Cm CheckHostIP
If this flag is set to
.Dq yes ,
ssh will additionally check the host IP address in the
.Pa known_hosts
file.
This allows ssh to detect if a host key changed due to DNS spoofing.
If the option is set to
.Dq no ,
the check will not be executed.
The default is
.Dq yes .
.It Cm Cipher
Specifies the cipher to use for encrypting the session
in protocol version 1.
Currently,
.Dq blowfish ,
.Dq 3des ,
and
.Dq des
are supported.
.Ar des
is only supported in the
.Nm
client for interoperability with legacy protocol 1 implementations
that do not support the
.Ar 3des
cipher.  Its use is strongly discouraged due to cryptographic
weaknesses.
The default is
.Dq 3des .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
The default is
.Pp
.Bd -literal
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
    aes192-cbc,aes256-cbc''
.Ed
.It Cm ClearAllForwardings
Specifies that all local, remote and dynamic port forwardings
specified in the configuration files or on the command line be
cleared.  This option is primarily useful when used from the
.Nm
command line to clear port forwardings set in
configuration files, and is automatically set by
.Xr scp 1
and
.Xr sftp 1 .
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm Compression
Specifies whether to use compression.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm CompressionLevel
Specifies the compression level to use if compression is enabled.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications.
The meaning of the values is the same as in
.Xr gzip 1 .
Note that this option applies to protocol version 1 only.
.It Cm ConnectionAttempts
Specifies the number of tries (one per second) to make before falling
back to rsh or exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
The default is 1.
.It Cm DynamicForward
Specifies that a TCP/IP port on the local machine be forwarded
over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine.  The argument must be a port number.
Currently the SOCKS4 protocol is supported, and
.Nm
will act as a SOCKS4 server.
Multiple forwardings may be specified, and
additional forwardings can be given on the command line.  Only
the superuser can forward privileged ports.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
The escape character can also
be set on the command line.
The argument should be a single character,
.Ql ^
followed by a letter, or
.Dq none
to disable the escape
character entirely (making the connection transparent for binary
data).
.It Cm FallBackToRsh
Specifies that if connecting via
.Nm
fails due to a connection refused error (there is no
.Xr sshd 8
listening on the remote host),
.Xr rsh 1
should automatically be used instead (after a suitable warning about
the session being unencrypted).
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm ForwardAgent
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
.Ev DISPLAY
set.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
By default,
.Nm
binds local port forwardings to the loopback addresss.  This
prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that
.Nm
should bind local port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm GlobalKnownHostsFile
Specifies a file to use for the global
host key database instead of
.Pa /etc/ssh/ssh_known_hosts .
.It Cm HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key
authentication.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
This option applies to protocol version 2 only and
is similar to
.Cm RhostsRSAAuthentication .
.It Cm HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms
that the client wants to use in order of preference.
The default for this option is:
.Dq ssh-rsa,ssh-dss .
.It Cm HostKeyAlias
Specifies an alias that should be used instead of the
real host name when looking up or saving the host key
in the host key database files.
This option is useful for tunneling ssh connections
or for multiple servers running on a single host.
.It Cm HostName
Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
Default is the name given on the command line.
Numeric IP addresses are also permitted (both on the command line and in
.Cm HostName
specifications).
.It Cm IdentityFile
Specifies a file from which the user's RSA or DSA authentication identity
is read. The default is
.Pa $HOME/.ssh/identity
for protocol version 1, and
.Pa $HOME/.ssh/id_rsa
and
.Pa $HOME/.ssh/id_dsa
for protocol version 2.
Additionally, any identities represented by the authentication agent
will be used for authentication.
The file name may use the tilde
syntax to refer to a user's home directory.
It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
.It Cm KeepAlive
Specifies whether the system should send TCP keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
.Pp
The default is
.Dq yes
(to send keepalives), and the client will notice
if the network goes down or the remote host dies.
This is important in scripts, and many users want it too.
.Pp
To disable keepalives, the value should be set to
.Dq no .
.It Cm KerberosAuthentication
Specifies whether Kerberos authentication will be used.
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT will be forwarded to the server.
This will only work if the Kerberos server is actually an AFS kaserver.
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm LocalForward
Specifies that a TCP/IP port on the local machine be forwarded over
the secure channel to the specified host and port from the remote machine.
The first argument must be a port number, and the second must be
.Ar host:port .
IPv6 addresses can be specified with an alternative syntax:
.Ar host/port .
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm ssh .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
The default is INFO.  DEBUG and DEBUG1 are equivalent.  DEBUG2
and DEBUG3 each specify higher levels of verbose output.
.It Cm MACs
Specifies the MAC (message authentication code) algorithms
in order of preference.
The MAC algorithm is used in protocol version 2
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
.It Cm NoHostAuthenticationForLocalhost
This option can be used if the home directory is shared across machines.
In this case localhost will refer to a different machine on each of
the machines and the user will get many warnings about changed host keys.
However, this option disables host authentication for localhost.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is to check the host key for localhost.
.It Cm NumberOfPasswordPrompts
Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
Default is 3.
.It Cm PasswordAuthentication
Specifies whether to use password authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm Port
Specifies the port number to connect on the remote host.
Default is 22.
.It Cm PreferredAuthentications
Specifies the order in which the client should try protocol 2
authentication methods. This allows a client to prefer one method (e.g.
.Cm keyboard-interactive )
over another method (e.g.
.Cm password )
The default for this option is:
.Dq hostbased,publickey,keyboard-interactive,password .
.It Cm Protocol
Specifies the protocol versions
.Nm
should support in order of preference.
The possible values are
.Dq 1
and
.Dq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
This means that
.Nm
tries version 2 and falls back to version 1
if version 2 is not available.
.It Cm ProxyCommand
Specifies the command to use to connect to the server.
The command
string extends to the end of the line, and is executed with
.Pa /bin/sh .
In the command string,
.Ql %h
will be substituted by the host name to
connect and
.Ql %p
by the port.
The command can be basically anything,
and should read from its standard input and write to its standard output.
It should eventually connect an
.Xr sshd 8
server running on some machine, or execute
.Ic sshd -i
somewhere.
Host key management will be done using the
HostName of the host being connected (defaulting to the name typed by
the user).
Note that
.Cm CheckHostIP
is not available for connects with a proxy command.
.Pp
.It Cm PubkeyAuthentication
Specifies whether to try public key authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
This option applies to protocol version 2 only.
.It Cm RemoteForward
Specifies that a TCP/IP port on the remote machine be forwarded over
the secure channel to the specified host and port from the local machine.
The first argument must be a port number, and the second must be
.Ar host:port .
IPv6 addresses can be specified with an alternative syntax:
.Ar host/port .
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.It Cm RhostsAuthentication
Specifies whether to try rhosts based authentication.
Note that this
declaration only affects the client side and has no effect whatsoever
on security.
Disabling rhosts authentication may reduce
authentication time on slow connections when rhosts authentication is
not used.
Most servers do not permit RhostsAuthentication because it
is not secure (see
.Cm RhostsRSAAuthentication ) .
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
This option applies to protocol version 1 only.
.It Cm RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host
authentication.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
This option applies to protocol version 1 only.
.It Cm RSAAuthentication
Specifies whether to try RSA authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
The default is
.Dq yes .
Note that this option applies to protocol version 1 only.
.It Cm ChallengeResponseAuthentication
Specifies whether to use challenge response authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm SmartcardDevice
Specifies which smartcard device to use. The argument to this keyword is
the device
.Nm
should use to communicate with a smartcard used for storing the user's
private RSA key. By default, no device is specified and smartcard support
is not activated.
.It Cm StrictHostKeyChecking
If this flag is set to
.Dq yes ,
.Nm
will never automatically add host keys to the
.Pa $HOME/.ssh/known_hosts
file, and refuses to connect to hosts whose host key has changed.
This provides maximum protection against trojan horse attacks,
however, can be annoying when the
.Pa /etc/ssh/ssh_known_hosts
file is poorly maintained, or connections to new hosts are
frequently made.
This option forces the user to manually
add all new hosts.
If this flag is set to
.Dq no ,
.Nm
will automatically add new host keys to the
user known hosts files.
If this flag is set to
.Dq ask ,
new host keys
will be added to the user known host files only after the user
has confirmed that is what they really want to do, and
.Nm
will refuse to connect to hosts whose host key has changed.
The host keys of
known hosts will be verified automatically in all cases.
The argument must be
.Dq yes ,
.Dq no
or
.Dq ask .
The default is
.Dq ask .
.It Cm UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
Note that this option must be set to
.Dq yes
if
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication
authentications are needed with older servers.
.It Cm User
Specifies the user to log in as.
This can be useful when a different user name is used on different machines.
This saves the trouble of
having to remember to give the user name on the command line.
.It Cm UserKnownHostsFile
Specifies a file to use for the user
host key database instead of
.Pa $HOME/.ssh/known_hosts .
.It Cm UseRsh
Specifies that rlogin/rsh should be used for this host.
It is possible that the host does not at all support the
.Nm
protocol.
This causes
.Nm
to immediately execute
.Xr rsh 1 .
All other options (except
.Cm HostName )
are ignored if this has been specified.
The argument must be
.Dq yes
or
.Dq no .
.It Cm XAuthLocation
Specifies the location of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
.Bl -tag -width Ds
.It Ev DISPLAY
The
.Ev DISPLAY
variable indicates the location of the X11 server.
It is automatically set by
.Nm
to point to a value of the form
.Dq hostname:n
where hostname indicates
the host where the shell runs, and n is an integer >= 1.
.Nm
uses this special value to forward X11 connections over the secure
channel.
The user should normally not set
.Ev DISPLAY
explicitly, as that
will render the X11 connection insecure (and will require the user to
manually copy any required authorization cookies).
.It Ev HOME
Set to the path of the user's home directory.
.It Ev LOGNAME
Synonym for
.Ev USER ;
set for compatibility with systems that use this variable.
.It Ev MAIL
Set to the path of the user's mailbox.
.It Ev PATH
Set to the default
.Ev PATH ,
as specified when compiling
.Nm ssh .
.It Ev SSH_ASKPASS
If
.Nm
needs a passphrase, it will read the passphrase from the current
terminal if it was run from a terminal.
If
.Nm
does not have a terminal associated with it but
.Ev DISPLAY
and
.Ev SSH_ASKPASS
are set, it will execute the program specified by
.Ev SSH_ASKPASS
and open an X11 window to read the passphrase.
This is particularly useful when calling
.Nm
from a
.Pa .Xsession
or related script.
(Note that on some machines it
may be necessary to redirect the input from
.Pa /dev/null
to make this work.)
.It Ev SSH_AUTH_SOCK
Identifies the path of a unix-domain socket used to communicate with the
agent.
.It Ev 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.
.It Ev 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.
.It Ev 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.
.It Ev 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).
.It Ev USER
Set to the name of the user logging in.
.El
.Pp
Additionally,
.Nm
reads
.Pa $HOME/.ssh/environment ,
and adds lines of the format
.Dq VARNAME=value
to the environment.
.Sh FILES
.Bl -tag -width Ds
.It Pa $HOME/.ssh/known_hosts
Records host keys for all hosts the user has logged into that are not
in
.Pa /etc/ssh/ssh_known_hosts .
See
.Xr sshd 8 .
.It Pa $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
.Nm
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.
.It Pa $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
.Pa $HOME/.ssh/identity.pub
file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where the user wishes to log in using protocol version 1 RSA authentication.
The contents of the
.Pa $HOME/.ssh/id_dsa.pub
and
.Pa $HOME/.ssh/id_rsa.pub
file should be added to
.Pa $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.
.It Pa $HOME/.ssh/config
This is the per-user configuration file.
The format of this file is described above.
This file is used by the
.Nm
client.
This file does not usually contain any sensitive information,
but the recommended permissions are read/write for the user, and not
accessible by others.
.It Pa $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
.Xr 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.
.It Pa /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
.Xr sshd 8
manual page.
.Pp
The canonical system name (as returned by name servers) is used by
.Xr sshd 8
to verify the client host when logging in; other names are needed because
.Nm
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.
.It Pa /etc/ssh/ssh_config
Systemwide configuration file.
This file provides defaults for those
values that are not specified in the user's configuration file, and
for those users who do not have a configuration file.
This file must be world-readable.
.It Pa /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
.Cm RhostsRSAAuthentication
and
.Cm HostbasedAuthentication .
Since they are readable only by root
.Nm
must be setuid root if these authentication methods are desired.
.It Pa $HOME/.rhosts
This file is used in
.Pa \&.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
.Xr 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.
.Pp
Note that by default
.Xr sshd 8
will be installed so that it requires successful RSA host
authentication before permitting \s+2.\s0rhosts authentication.
If the server machine does not have the client's host key in
.Pa /etc/ssh/ssh_known_hosts ,
it can be stored in
.Pa $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
.Pa $HOME/.ssh/known_hosts .
.It Pa $HOME/.shosts
This file is used exactly the same way as
.Pa \&.rhosts .
The purpose for
having this file is to be able to use rhosts authentication with
.Nm
without permitting login with
.Xr rlogin 1
or
.Xr rsh 1 .
.It Pa /etc/hosts.equiv
This file is used during
.Pa \&.rhosts authentication.
It contains
canonical hosts names, one per line (the full format is described on
the
.Xr 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.
.It Pa /etc/shosts.equiv
This file is processed exactly as
.Pa /etc/hosts.equiv .
This file may be useful to permit logins using
.Nm
but not using rsh/rlogin.
.It Pa /etc/ssh/sshrc
Commands in this file are executed by
.Nm
when the user logs in just before the user's shell (or command) is started.
See the
.Xr sshd 8
manual page for more information.
.It Pa $HOME/.ssh/rc
Commands in this file are executed by
.Nm
when the user logs in just before the user's shell (or command) is
started.
See the
.Xr sshd 8
manual page for more information.
.It Pa $HOME/.ssh/environment
Contains additional definitions for environment variables, see section
.Sx ENVIRONMENT
above.
.El
.Sh DIAGNOSTICS
.Nm
exits with the exit status of the remote command or with 255
if an error occurred.
.Sh 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.
.Sh SEE ALSO
.Xr rlogin 1 ,
.Xr rsh 1 ,
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr telnet 1 ,
.Xr sshd 8
.Rs
.%A T. Ylonen
.%A T. Kivinen
.%A M. Saarinen
.%A T. Rinne
.%A S. Lehtinen
.%T "SSH Protocol Architecture"
.%N draft-ietf-secsh-architecture-09.txt
.%D July 2001
.%O work in progress material
.Re