path: root/usr/src/man/man1/ssh.1

'\" te
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.\" Portions Copyright (c) 2009, Sun Microsystems, Inc. All Rights Reserved.
.TH ssh 1 "20 May 2009" "SunOS 5.11" "User Commands"
.SH NAME
ssh \- secure shell client (remote login program)
.SH SYNOPSIS
.LP
.nf
\fBssh\fR [\fB-l\fR \fIlogin_name\fR] \fIhostname\fR | \fIuser@hostname\fR [ \fIcommand\fR]
.fi

.LP
.nf
\fBssh\fR [\fB-afgknqstvxACNTX1246\fR] [\fB-b\fR \fIbind_address\fR] [\fB-m\fR \fImac_spec\fR] 
     [\fB-c\fR \fIcipher_spec\fR] [\fB-e\fR \fIescape_char\fR] [\fB-i\fR \fIidentity_file\fR] 
     [\fB-l\fR \fIlogin_name\fR] [\fB-F\fR \fIconfigfile\fR] [\fB-o\fR \fIoption\fR] [\fB-p\fR \fIport\fR] 
     [\fB-L\fR [\fIbind_address\fR\fB:\fR]\fIport\fR\fB:\fR\fIhost\fR\fB:\fR\fIhostport\fR]
     [\fB-R\fR [\fIbind_address\fR\fB:\fR]\fIport\fR\fB:\fR\fIhost\fR\fB:\fR\fIhostport\fR]
     [\fB-D\fR [\fIbind_address\fR\fB:\fR]\fIport\fR] \fIhostname\fR | \fIuser\fR\fB@\fR\fIhostname\fR [\fIcommand\fR]
.fi

.SH DESCRIPTION
.sp
.LP
\fBssh\fR (Secure Shell) is a program for logging into a remote machine and for
executing commands on a remote machine. It is intended to replace \fBrlogin\fR
and \fBrsh\fR, and to 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.
.sp
.LP
\fBssh\fR connects and logs into the specified hostname. The user must prove
his or her identity to the remote machine using one of several methods
depending on the protocol version used:
.SS "SSH Protocol Version 1"
.sp
.LP
First, if the machine the user logs in from is listed in \fB/etc/hosts.equiv\fR
or \fB/etc/shosts.equiv\fR on the remote machine, and the user names are the
same on both sides, the user is immediately permitted to log in. Second,
if .\fBrhosts\fR or \fB\&.shosts\fR 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.
.sp
.LP
The second (and primary) authentication method is the \fBrhosts\fR or
\fBhosts.equiv\fR method combined with RSA-based host authentication. It means
that if the login would be permitted by \fB$HOME/.rhosts\fR,
\fB$HOME/.shosts\fR, \fB/etc/hosts.equiv\fR, or \fB/etc/shosts.equiv\fR, and if
additionally the server can verify the client's host key (see
\fB/etc/ssh_known_hosts\fR in the FILES section), only then is login permitted.
This authentication method closes security holes due to \fBIP\fR spoofing,
\fBDNS\fR spoofing, and routing spoofing.
.sp
.LP
\fBNote to the administrator:\fR \fB/etc/hosts.equiv\fR, \fB$HOME/.rhosts\fR,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.
.sp
.LP
As a third authentication method, \fBssh\fR supports \fBRSA\fR-based
authentication. The scheme is based on public-key cryptography. There are
cryptosystems where encryption and decryption are done using separate keys, and
it is not possible to derive the decryption key from the encryption key.
\fBRSA\fR is one such system. The idea is that each user creates a
public/private key pair for authentication purposes. The server knows the
public key, and only the user knows the private key. The file
\fB$HOME/.ssh/authorized_keys\fR lists the public keys that are permitted for
logging in. When the user logs in, the \fBssh\fR 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 \fBssh\fR program running
on behalf of the user) a challenge in the form of 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 or she knows the private key but without disclosing it to
the server.
.sp
.LP
\fBssh\fR implements the \fBRSA\fR authentication protocol automatically. The
user creates his or her \fBRSA\fR key pair by running \fBssh-keygen\fR(1). This
stores the private key in \fB$HOME/.ssh/identity\fR and the public key in
\fB$HOME/.ssh/identity.pub\fR in the user's home directory. The user should
then copy the \fBidentity.pub\fR to \fB$HOME/.ssh/authorized_keys\fR in his or
her home directory on the remote machine (the \fBauthorized_keys\fR file
corresponds to the conventional \fB$HOME/.rhosts\fR 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. \fBRSA\fR authentication is much more secure than
\fBrhosts\fR authentication.
.sp
.LP
The most convenient way to use \fBRSA\fR authentication can be with an
authentication agent. See \fBssh-agent\fR(1) for more information.
.sp
.LP
If other authentication methods fail, \fBssh\fR 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.
.SS "SSH Protocol Version 2"
.sp
.LP
The SSH version 2 protocol supports multiple user authentication methods, some
of which are similar to those available with the SSH protocol version 1. These
authentication mechanisms are negotiated by the client and server, with the
client trying methods in the order specified in the
\fBPreferredAuthentications\fR client configuration option. The server decides
when enough authentication methods have passed successfully so as to complete
the authentication phase of the protocol.
.sp
.LP
When a user connects by using protocol version 2, similar authentication
methods are available. Using the default values for
\fBPreferredAuthentications\fR, the client tries to authenticate first by using
the hostbased method. If this method fails, public key authentication is
attempted. Finally, if this method fails, keyboard-interactive and password
authentication are tried.
.sp
.LP
The public key method is similar to \fBRSA\fR authentication described in the
previous section and allows the \fBRSA\fR or \fBDSA\fR algorithm to be used:
The client uses his or her private key, \fB$HOME/.ssh/id_dsa\fR or
\fB$HOME/.ssh/id_rsa\fR, to sign the session identifier and sends the result to
the server. The server checks whether the matching public key is listed in
\fB$HOME/.ssh/authorized_keys\fR 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.
.sp
.LP
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, or an extended
prompt/reply protocol can be engaged.
.sp
.LP
Additionally, \fBssh\fR supports hostbased or challenge response
authentication.
.sp
.LP
Protocol 2 provides additional mechanisms for confidentiality (the traffic is
encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
(\fBhmac-sha1\fR, \fBhmac-md5\fR). Protocol 1 lacks a strong mechanism for
ensuring the integrity of the connection.
.SS "Login Session and Remote Execution"
.sp
.LP
When the user's identity has been accepted by the server, the server either
executes the specified 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 is automatically encrypted.
.sp
.LP
If a pseudo-terminal has been allocated (normal login session), the user can
use the escape characters noted below. If a pseudo-terminal has been allocated
(normal login session), the user can disconnect with \fB~.\fR, and suspend
\fBssh\fR with \fB~^Z\fR. All forwarded connections can be listed with
\fB~#\fR. If the session blocks waiting for forwarded X11 or TCP/IP connections
to terminate, \fBssh\fR can be backgrounded with \fB~&\fR, although this should
not be used while the user shell is active, as it can cause the shell to hang.
All available escapes can be listed with \fB~?\fR.
.sp
.LP
A single tilde character can be sent as \fB~~\fR, or by following the tilde
with a character other than those described above. The escape character must
always follow a newline to be interpreted as special. The escape character can
be changed in configuration files or on the command line.
.sp
.LP
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 "\fBnone\fR" also makes the session transparent even if a tty is used.
.sp
.LP
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 \fBssh\fR.
.SS "Escape Characters"
.sp
.LP
When a pseudo-terminal has been requested, \fBssh\fR supports a number of
functions through the use of an escape character.
.sp
.LP
A single tilde character can be sent as \fB~~\fR or by following the tilde with
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 \fBEscapeChar\fR configuration
directive or on the command line by the \fB-e\fR option.
.sp
.LP
The supported escapes, assuming the default \fB~\fR, are:
.sp
.ne 2
.mk
.na
\fB\fB~.\fR\fR
.ad
.RS 7n
.rt  
Disconnect.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~^Z\fR\fR
.ad
.RS 7n
.rt  
Background \fBssh\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~#\fR\fR
.ad
.RS 7n
.rt  
List forwarded connections.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~&\fR\fR
.ad
.RS 7n
.rt  
Background \fBssh\fR at logout when waiting for forwarded connection / X11
sessions to terminate.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~?\fR\fR
.ad
.RS 7n
.rt  
Display a list of escape characters.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~B\fR\fR
.ad
.RS 7n
.rt  
Send a break to the remote system. Only useful for SSH protocol version 2 and
if the peer supports it.
.RE

.sp
.ne 2
.mk
.na
\fB\fB~C\fR\fR
.ad
.RS 7n
.rt  
Open command line. Only useful for adding port forwardings using the \fB-L\fR
and \fB-R\fR options).
.RE

.sp
.ne 2
.mk
.na
\fB\fB~R\fR\fR
.ad
.RS 7n
.rt  
Request rekeying of the connection. Only useful for SSH protocol version 2 and
if the peer supports it.
.RE

.SS "X11 and TCP Forwarding"
.sp
.LP
If the \fBForwardX11\fR variable is set to ``\fByes\fR'' (or, see the
description of the \fB-X\fR and \fB-x\fR options described later) and the user
is using X11 (the \fBDISPLAY\fR 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) goes through the
encrypted channel, and the connection to the real X server is made from the
local machine. The user should not manually set \fBDISPLAY\fR. Forwarding of
X11 connections can be configured on the command line or in configuration
files.
.sp
.LP
The \fBDISPLAY\fR value set by \fBssh\fR points to the server machine, but with
a display number greater than zero. This is normal behavior, because \fBssh\fR
creates a "proxy" X11 server on the server machine for forwarding the
connections over the encrypted channel.
.sp
.LP
\fBssh\fR also automatically sets up \fBXauthority\fR data on the server
machine. For this purpose, it generates a random authorization cookie, store it
in \fBXauthority\fR 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).
.sp
.LP
If the \fBForwardAgent\fR variable is set to "\fByes\fR" (or, see the
description of the \fB-A\fR and \fB-a\fR options described later) and the user
is using an authentication agent, the connection to the agent is automatically
forwarded to the remote side.
.sp
.LP
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 possible application is firewall traversal.
.SS "Server Authentication"
.sp
.LP
\fBssh\fR automatically maintains and checks a database containing
identifications for all hosts it has ever been used with. Host keys are stored
in \fB$HOME/.ssh/known_hosts\fR in the user's home directory. Additionally, the
file \fB/etc/ssh_known_hosts\fR is automatically checked for known hosts. The
behavior of \fBssh\fR with respect to unknown host keys is controlled by the
\fBStrictHostKeyChecking\fR parameter. If a host's identification ever changes,
\fBssh\fR warns about this and disables password authentication to prevent a
trojan horse from getting the user's password. Another purpose of this
mechanism is to prevent attacks by intermediaries which could otherwise be used
to circumvent the encryption. The \fBStrictHostKeyChecking\fR option can be
used to prevent logins to machines whose host key is not known or has changed.
.sp
.LP
However, when using key exchange protected by GSS-API, the server can advertise
a host key. The client automatically adds this host key to its known hosts
file, \fB$HOME/.ssh/known_hosts\fR, regardless of the setting of the
\fBStrictHostKeyChecking\fR option, unless the advertised host key collides
with an existing known hosts entry.
.sp
.LP
When the user's GSS-API credentials expire, the client continues to be able to
rekey the session using the server's public host key to protect the key
exchanges.
.SS "GSS-API User and Server Authentication"
.sp
.LP
\fBssh\fR uses the user's GSS-API credentials to authenticate the client to the
server wherever possible, if \fBGssKeyEx\fR and/or \fBGssAuthentication\fR are
set.
.sp
.LP
With \fBGssKeyEx\fR, one can have an SSHv2 server that has no host public keys,
so that only \fBGssKeyEx\fR can be used. With such servers, rekeying fails if
the client's credentials are expired.
.sp
.LP
GSS-API user authentication has the disadvantage that it does not obviate the
need for SSH host keys, but its failure does not impact rekeying. \fBssh\fR can
try other authentication methods (such as public key, password, and so on) if
GSS-API authentication fails.
.sp
.LP
Delegation of GSS-API credentials can be quite useful, but is not without
danger. As with passwords, users should not delegate GSS credentials to
untrusted servers, since a compromised server can use a user's delegated GSS
credentials to impersonate the user.
.sp
.LP
GSS-API user authorization is covered in \fBgss_auth_rules\fR(5).
.sp
.LP
Rekeying can be used to redelegate credentials when \fBGssKeyEx\fR is
"\fByes\fR". (See \fB~R\fR under \fBEscape Characters\fR above.)
.SH OPTIONS
.sp
.LP
The following options are supported:
.sp
.ne 2
.mk
.na
\fB\fB-1\fR\fR
.ad
.sp .6
.RS 4n
Forces \fBssh\fR to try protocol version 1 only.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-2\fR\fR
.ad
.sp .6
.RS 4n
Forces \fBssh\fR to try protocol version 2 only.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-4\fR\fR
.ad
.sp .6
.RS 4n
Forces \fBssh\fR to use IPv4 addresses only.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-6\fR\fR
.ad
.sp .6
.RS 4n
Forces \fBssh\fR to use IPv6 addresses only.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-a\fR\fR
.ad
.sp .6
.RS 4n
Disables forwarding of the authentication agent connection.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-A\fR\fR
.ad
.sp .6
.RS 4n
Enables forwarding of the authentication agent connection. This can also be
specified on a per-host basis in a configuration file.
.sp
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, the attacker can perform
operations on the keys that enable the attacker to authenticate using the
identities loaded into the agent.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-b\fR \fIbind_address\fR\fR
.ad
.sp .6
.RS 4n
Specifies the interface to transmit from on machines with multiple interfaces
or aliased addresses.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-c\fR \fIcipher_spec\fR\fR
.ad
.sp .6
.RS 4n
Selects the cipher specification for encrypting the session.
.sp
For protocol version 1, \fIcipher_spec\fR is a single cipher. See the
\fBCipher\fR option in \fBssh_config\fR(4) for more information.
.sp
For protocol version 2, \fIcipher_spec\fR is a comma-separated list of ciphers
listed in order of preference. See the \fICiphers\fR option in
\fBssh_config\fR(4) for more information.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-C\fR\fR
.ad
.sp .6
.RS 4n
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 \fBgzip\fR(1). The \fBgzip\fR man page is available in the
\fBSUNWsfman\fR package. The "level" can be controlled by the
\fBCompressionLevel\fR option (see \fBssh_config\fR(4)). Compression is
desirable on modem lines and other slow connections, but only slows down things
on fast networks. The default value can be set on a host-by-host basis in the
configuration files. See the \fBCompression\fR option in \fBssh_config\fR(4).
.RE

.sp
.ne 2
.mk
.na
\fB\fB-D\fR [\fIbind_address\fR\fB:\fR]\fIport\fR\fR
.ad
.sp .6
.RS 4n
Specifies a local \fBdynamic\fR application-level port forwarding. This works
by allocating a socket to listen to port on the local side, optionally bound to
the specified \fIbind_address\fR. Whenever a connection is made to this port,
the connection is forwarded over the secure channel. The application protocol
is then used to determine where to connect to from the remote machine.
Currently, the \fBSOCKS4\fR and \fBSOCKS5\fR protocols are supported and
\fBssh\fR acts as a SOCKS server. Only a user with enough privileges can
forward privileged ports. Dynamic port forwardings can also be specified in the
configuration file.
.sp
IPv6 addresses can be specified with an alternative syntax:
\fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR or by enclosing the address in
square brackets. By default, the local port is bound in accordance with the
\fBGatewayPorts\fR setting. However, an explicit \fIbind_address\fR can be used
to bind the connection to a specific address. The \fIbind_address\fR of
\fBlocalhost\fR indicates that the listening port be bound for local use only,
while an empty address or \fB*\fR indicates that the port should be available
from all interfaces.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-e\fR \fIch\fR | ^\fIch\fR | none\fR
.ad
.sp .6
.RS 4n
Sets the escape character for sessions with a pty (default: `\fB~\fR'). The
escape character is only recognized at the beginning of a line. The escape
character followed by a dot (\fB\&.\fR) closes the connection. If followed by
CTRL-z, the escape character suspends the connection. If followed by itself,
the escape character sends itself once. Setting the character to \fBnone\fR
disables any escapes and makes the session fully transparent.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-f\fR\fR
.ad
.sp .6
.RS 4n
Requests \fBssh\fR to go to background just before command execution. This is
useful if \fBssh\fR is going to ask for passwords or passphrases, but the user
wants it in the background. This implies the \fB-n\fR option. The recommended
way to start X11 programs at a remote site is with something like \fBssh\fR
\fB-f\fR \fIhost\fR \fIxterm\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-F\fR \fIconfigfile\fR\fR
.ad
.sp .6
.RS 4n
Specifies an alternative per-user configuration file. If a configuration file
is specified on the command line, the system-wide configuration file,
\fB/etc/ssh_config\fR, is ignored. The default for the per-user configuration
file is \fB$HOME/.ssh/config\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-g\fR\fR
.ad
.sp .6
.RS 4n
Allows remote hosts to connect to local forwarded ports.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-i\fR \fIidentity_file\fR\fR
.ad
.sp .6
.RS 4n
Selects a file from which the identity (private key) for \fBRSA\fR or \fBDSA\fR
authentication is read. The default is \fB$HOME/.ssh/identity\fR for protocol
version 1, and \fB$HOME/.ssh/id_rsa\fR and \fB$HOME/.ssh/id_dsa\fR for protocol
version 2. Identity files can also be specified on a per-host basis in the
configuration file. It is possible to have multiple \fB-i\fR options (and
multiple identities specified in configuration files).
.RE

.sp
.ne 2
.mk
.na
\fB\fB-l\fR \fIlogin_name\fR\fR
.ad
.sp .6
.RS 4n
Specifies the user to log in as on the remote machine. This also can be
specified on a per-host basis in the configuration file.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-L\fR [\fIbind_address:\fR]\fIport\fR:\fIhost\fR:\fIhostport\fR\fR
.ad
.sp .6
.RS 4n
Specifies that the specified port on the local (client) host is to be forwarded
to the specified host and port on the remote side. This works by allocating a
socket to listen to the port on the local side, optionally bound to the
specified \fIbind_address\fR. Then, whenever a connection is made to this port,
the connection is forwarded over the secure channel and a connection is made to
host port \fIhostport\fR from the remote machine. Port forwardings can also be
specified in the configuration file. Only a user with enough privileges can
forward privileged ports. IPv6 addresses can be specified with an alternative
syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR\fB/\fR\fIhost\fR\fB/\fR\fIh
ostport\fR or by enclosing the address in square brackets.
.sp
By default, the local port is bound in accordance with the \fBGatewayPorts\fR
setting. However, an explicit \fIbind_address\fR can be used to bind the
connection to a specific address. The \fIbind_address\fR of \fBlocalhost\fR
indicates that the listening port be bound for local use only, while an empty
address or \fB*\fR indicates that the port should be available from all
interfaces.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-m\fR \fImac_spec\fR\fR
.ad
.sp .6
.RS 4n
Additionally, for protocol version 2 a comma-separated list of \fBMAC\fR
(message authentication code) algorithms can be specified in order of
preference. See the MACs keyword for more information.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-n\fR\fR
.ad
.sp .6
.RS 4n
Redirects \fBstdin\fR from \fB/dev/null\fR (actually, prevents reading from
\fBstdin\fR). This must be used when \fBssh\fR is run in the background. A
common trick is to use this to run X11 programs on a remote machine. For
example,
.sp
.in +2
.nf
ssh -n shadows.cs.hut.fi emacs &
.fi
.in -2
.sp

starts an \fBemacs\fR on \fBshadows.cs.hut.fi\fR, and the X11 connection is
automatically forwarded over an encrypted channel. The \fBssh\fR program is put
in the background. This does not work if \fBssh\fR needs to ask for a password
or passphrase. See also the \fB-f\fR option.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-N\fR\fR
.ad
.sp .6
.RS 4n
Does not execute a remote command. This is useful if you just want to forward
ports (protocol version 2 only).
.RE

.sp
.ne 2
.mk
.na
\fB\fB-o\fR \fIoption\fR\fR
.ad
.sp .6
.RS 4n
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. The option has the same format as a line in the configuration file.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-p\fR \fIport\fR\fR
.ad
.sp .6
.RS 4n
Specifies the port to connect to on the remote host. This can be specified on a
per-host basis in the configuration file.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-P\fR\fR
.ad
.sp .6
.RS 4n
Obsoleted option. SSHv1 connections from privileged ports are not supported.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-q\fR\fR
.ad
.sp .6
.RS 4n
Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only
fatal errors are displayed.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-R\fR [\fIbind_address\fR:]\fIport\fR:\fIhost\fR:\fIhostport\fR\fR
.ad
.sp .6
.RS 4n
Specifies that the specified port on the remote (server) host is to be
forwarded to the specified host and port on the local side. This works by
allocating a socket to listen to the port on the remote side. Then, whenever a
connection is made to this port, the connection is forwarded over the secure
channel and a connection is made to host port \fIhostport\fR from the local
machine. Port forwardings can also be specified in the configuration file.
Privileged ports can be forwarded only when logging in on the remote machine as
a user with enough privileges.
.sp
IPv6 addresses can be specified by enclosing the address in square braces or
using an alternative syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIhost\fR\fB/\fR
\fIport\fR\fB/\fR\fIhostport\fR.
.sp
By default, the listening socket on the server is bound to the loopback
interface only. This can be overridden by specifying a \fIbind_address\fR. An
empty \fIbind_address\fR, or the address \fB*\fR, indicates that the remote
socket should listen on all interfaces. Specifying a remote \fIbind_address\fR
only succeeds if the server's \fBGatewayPorts\fR option is enabled. See
\fBsshd_config\fR(4).
.RE

.sp
.ne 2
.mk
.na
\fB\fB-s\fR\fR
.ad
.sp .6
.RS 4n
Can 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, for example, \fBsftp\fR. The
subsystem is specified as the remote command.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-t\fR\fR
.ad
.sp .6
.RS 4n
Forces pseudo-tty allocation. This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very useful, for
example, when implementing menu services. Multiple \fB-t\fR options force
allocation, even if \fBssh\fR has no local \fBtty\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-T\fR\fR
.ad
.sp .6
.RS 4n
Disables pseudo-tty allocation (protocol version 2 only).
.RE

.sp
.ne 2
.mk
.na
\fB\fB-v\fR\fR
.ad
.sp .6
.RS 4n
Verbose mode. Causes \fBssh\fR to print debugging messages about its progress.
This is helpful in debugging connection, authentication, and configuration
problems. Multiple \fB-v\fR options increase the verbosity. Maximum is 3.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-x\fR\fR
.ad
.sp .6
.RS 4n
Disables X11 forwarding.
.RE

.sp
.ne 2
.mk
.na
\fB\fB-X\fR\fR
.ad
.sp .6
.RS 4n
Enables X11 forwarding. This can also be specified on a per-host basis in a
configuration file.
.sp
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
can then be able to perform activities such as keystroke monitoring.
.sp
For this reason, X11 forwarding might be subjected to X11 SECURITY extension
restrictions. Refer to the \fBForwardX11Trusted\fR directive in
\fBssh_config\fR(4) for more information.
.sp
If X11 forwarding is enabled, remote X11 clients is trusted by default. This
means that they have full access to the original X11 display.
.RE

.SH ENVIRONMENT VARIABLES
.sp
.LP
\fBssh\fR normally sets the following environment variables:
.sp
.ne 2
.mk
.na
\fB\fBDISPLAY\fR\fR
.ad
.sp .6
.RS 4n
The \fBDISPLAY\fR variable must be set for X11 display forwarding to work.
.RE

.sp
.ne 2
.mk
.na
\fB\fBSSH_ASKPASS\fR\fR
.ad
.sp .6
.RS 4n
If \fBssh\fR needs a passphrase, it reads the passphrase from the current
terminal if it was run from a terminal. If \fBssh\fR does not have a terminal
associated with it but \fBDISPLAY\fR and \fBSSH_ASKPASS\fR are set, it executes
the program specified by \fBSSH_ASKPASS\fR and opens an X11 window to read the
passphrase. This is particularly useful when calling \fBssh\fR from a .Xsession
or related script. On some machines it might be necessary to redirect the input
from \fB/dev/null\fR to make this work. The system is shipped with
\fB/usr/lib/ssh/ssh-askpass\fR which is the default value for \fBSSH_ASKPASS\fR
.RE

.sp
.ne 2
.mk
.na
\fB\fBSSH_AUTH_SOCK\fR\fR
.ad
.sp .6
.RS 4n
Indicates the path of a unix-domain socket used to communicate with the agent.
.RE

.sp
.ne 2
.mk
.na
\fB\fBSSH_LANGS\fR\fR
.ad
.sp .6
.RS 4n
A comma-separated list of IETF language tags (see RFC3066) indicating the
languages that the user can read and write. Used for negotiation of the locale
on the server.
.RE

.sp
.ne 2
.mk
.na
\fB\fBLANG\fR, \fBLC_ALL\fR, \fBLC_COLLATE\fR, \fBLC_CTYPE\fR,\fR
.ad
.br
.na
\fB\fBLC_MESSAGES\fR, \fBLC_MONETARY\fR, \fBLC_NUMERIC\fR, \fBLC_TIME\fR\fR
.ad
.sp .6
.RS 4n
The values of these environment variables can be set in remote sessions
according to the locale settings on the client side and availability of support
for those locales on the server side. Environment Variable Passing (see \fIRFC
4254\fR) is used for passing them over to the server side.
.RE

.sp
.LP
See the \fBENVIRONMENT VARIABLES\fR section in the \fBsshd\fR(1M) man page for
more information on how locale setting can be further changed depending on
server side configuration.
.SH EXIT STATUS
.sp
.LP
The status of the remote program is returned as the exit status of \fBssh\fR.
\fB255\fR is returned if an error occurred at anytime during the \fBssh\fR
connection, including the initial key exchange.
.SH FILES
.sp
.ne 2
.mk
.na
\fB\fB$HOME/.ssh/known_hosts\fR\fR
.ad
.RS 26n
.rt  
Records host keys for all hosts the user has logged into that are not in
\fB/etc/ssh/ssh_known_hosts\fR. See \fBsshd\fR(1M).
.RE

.sp
.ne 2
.mk
.na
\fB\fB$HOME/.ssh/identity\fR\fR
.ad
.br
.na
\fB\fB$HOME/.ssh/id_dsa\fR\fR
.ad
.br
.na
\fB\fB$HOME/.ssh/id_ssa\fR\fR
.ad
.RS 26n
.rt  
Contains the authentication identity of the user. These files are for protocol
1 \fBRSA\fR, protocol 2 \fBDSA\fR, and protocol 2 \fBRSA\fR, respectively.
These files contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute). \fBssh\fR ignores a private key file
if it is accessible by others. It is possible to specify a passphrase when
generating the key. The passphrase is used to encrypt the sensitive part of
this file using \fB3DES\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fB/etc/ssh/sshrc\fR\fR
.ad
.RS 26n
.rt  
Commands in this file are executed by \fBssh\fR when the user logs in just
before the user's shell or command is started. See \fBsshd\fR(1M) for more
information.
.RE

.sp
.ne 2
.mk
.na
\fB\fB$HOME/.ssh/rc\fR\fR
.ad
.RS 26n
.rt  
Commands in this file are executed by \fBssh\fR when the user logs in just
before the user's shell or command is started. See \fBsshd\fR(1M) for more
information.
.RE

.sp
.ne 2
.mk
.na
\fB\fB$HOME/.ssh/environment\fR\fR
.ad
.RS 26n
.rt  
Contains additional definitions for environment variables. See ENVIRONMENT
VARIABLES.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
tab() box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPEATTRIBUTE VALUE
_
Interface StabilitySee below.
.TE

.sp
.LP
The command line syntax is Committed. The remote locale selection through
passing \fBLC_*\fR environment variables is Uncommitted.
.SH SEE ALSO
.sp
.LP
\fBrlogin\fR(1), \fBrsh\fR(1), \fBscp\fR(1), \fBssh-add\fR(1),
\fBssh-agent\fR(1), \fBssh-keygen\fR(1), \fBssh-http-proxy-connect\fR(1),
\fBssh-socks5-proxy-connect\fR(1), \fBtelnet\fR(1), \fBsshd\fR(1M),
\fBssh_config\fR(4), \fBsshd_config\fR(4), \fBattributes\fR(5),
\fBgss_auth_rules\fR(5), \fBkerberos\fR(5), \fBprivileges\fR(5)
.sp
.LP
\fIRFC 1928\fR
.sp
.LP
\fIRFC 4254\fR