ssh.1

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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
.Cm ForwardAgent
variable is set to
.Dq yes
(or, see the description of the
.Fl A
and
.Fl a
options described later) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.
.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 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.
.Pp
Agent forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the agent's Unix-domain socket)
can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent.
.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 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.
.Pp
X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user's X authorization database)
can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring.
.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 for protocol version 1.
Compression is desirable on modem lines and other
slow connections, but will only slow down things on fast networks.
The default value can be set on a host-by-host basis in the
configuration files; see the
.Cm Compression
option.
.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.