openssh/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 1.208 2005/05/23 23:32:46 djm Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH SSH client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Bk -words
.Op Fl 1246AaCfgkMNnqsTtVvXxY
.Op Fl b Ar bind_address
.Op Fl c Ar cipher_spec
.Op Fl D Ar port
.Op Fl e Ar escape_char
.Op Fl F Ar configfile
.Op Fl i Ar identity_file
.Oo Fl L\ \&
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Oc
.Op Fl l Ar login_name
.Op Fl m Ar mac_spec
.Op Fl O Ar ctl_cmd
.Op Fl o Ar option
.Op Fl p Ar port
.Oo Fl R\ \&
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Oc
.Op Fl S Ar ctl_path
.Oo Ar user Ns @ Oc Ns Ar hostname
.Op Ar command
.Ek
.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
(with optional
.Ar user
name).
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used.
.Pp
If
.Ar command
is specified,
.Ar command
is executed on the remote host instead of a login shell.
.Ss SSH protocol version 1
The first authentication method is the
.Em rhosts
or
.Em hosts.equiv
method combined with RSA-based host authentication.
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, or if the files
.Pa ~/.rhosts
or
.Pa ~/.shosts
exist in the user's home directory on the
remote machine and contain a line containing the name of the client
machine and the name of the user on that machine, the user is
considered for log in.
Additionally, if the server can verify the client's
host key (see
.Pa /etc/ssh/ssh_known_hosts
and
.Pa ~/.ssh/known_hosts
in the
.Sx FILES
section), only then is login 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 ~/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
As a second 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.
.Pp
The file
.Pa ~/.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 ~/.ssh/identity
and stores the public key in
.Pa ~/.ssh/identity.pub
in the user's home directory.
The user should then copy the
.Pa identity.pub
to
.Pa ~/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
.Pa ~/.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.
.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.
.Ss SSH protocol version 2
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 ~/.ssh/id_dsa
or
.Pa ~/.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 ~/.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 to prove 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 AES, 3DES, Blowfish, CAST128 or Arcfour)
and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
Note that protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.
.Ss Login session and remote execution
When the user's identity has been accepted by the server, the server
either executes the given command, or logs into the machine and gives
the user a normal shell on the remote machine.
All communication with
the remote command or shell will be automatically encrypted.
.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 .
.Ss Escape Characters
When a pseudo-terminal has been requested,
.Nm
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
.Nm ssh .
.It Cm ~#
List forwarded connections.
.It Cm ~&
Background
.Nm
at logout when waiting for forwarded connection / X11 sessions to terminate.
.It Cm ~?
Display a list of escape characters.
.It Cm ~B
Send a BREAK to the remote system
(only useful for SSH protocol version 2 and if the peer supports it).
.It Cm ~C
Open command line.
Currently this allows the addition of port forwardings using the
.Fl L
and
.Fl R
options (see below).
It also allows the cancellation of existing remote port-forwardings
using
.Fl KR Ar hostport .
Basic help is available, using the
.Fl h
option.
.It Cm ~R
Request rekeying of the connection
(only useful for SSH protocol version 2 and if the peer supports it).
.El
.Ss X11 and TCP forwarding
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
.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.
.Ss Server authentication
.Nm
automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
Host keys are stored in
.Pa ~/.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
.Nm
can be configured to verify host identification using fingerprint resource
records (SSHFP) published in DNS.
The
.Cm VerifyHostKeyDNS
option can be used to control how DNS lookups are performed.
SSHFP resource records can be generated using
.Xr ssh-keygen 1 .
.Pp
The options are as follows:
.Bl -tag -width Ds
.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.
.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 a
Disables forwarding of the authentication agent connection.
.It Fl b Ar bind_address
Specify the interface address to transmit from on machines with multiple
interfaces or aliased addresses.
.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 c Ar cipher_spec
Selects the cipher specification for encrypting the session.
.Pp
Protocol version 1 allows specification of a single cipher.
The suported values are
.Dq 3des ,
.Dq blowfish
and
.Dq des .
.Ar 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is believed to be secure.
.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.
The default is
.Dq 3des .
.Pp
For protocol version 2
.Ar cipher_spec
is a comma-separated list of ciphers
listed in order of preference.
The supported ciphers are
.Dq 3des-cbc ,
.Dq aes128-cbc ,
.Dq aes192-cbc ,
.Dq aes256-cbc ,
.Dq aes128-ctr ,
.Dq aes192-ctr ,
.Dq aes256-ctr ,
.Dq arcfour128 ,
.Dq arcfour256 ,
.Dq arcfour ,
.Dq blowfish-cbc ,
and
.Dq cast128-cbc .
The default is
.Bd -literal
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
    arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
    aes192-ctr,aes256-ctr''
.Ed
.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 and SOCKS5 protocols are supported, and
.Nm
will act as a SOCKS server.
Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration file.
.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 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 ~/.ssh/config .
.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 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 i Ar identity_file
Selects a file from which the identity (private key) for
RSA or DSA authentication is read.
The default is
.Pa ~/.ssh/identity
for protocol version 1, and
.Pa ~/.ssh/id_rsa
and
.Pa ~/.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 k
Disables forwarding (delegation) of GSSAPI credentials to the server.
.It Fl L Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Xc
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, optionally bound to the specified
.Ar bind_address .
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.
IPv6 addresses can be specified with an alternative syntax:
.Sm off
.Xo
.Op Ar bind_address No /
.Ar port No / Ar host No /
.Ar hostport
.Xc
.Sm on
or by enclosing the address in square brackets.
Only the superuser can forward privileged ports.
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Dq localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.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
Places the
.Nm
client into
.Dq master
mode for connection sharing.
Refer to the description of
.Cm ControlMaster
in
.Xr ssh_config 5
for details.
.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
Do not execute a remote command.
This is useful for just forwarding ports
(protocol version 2 only).
.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 O Ar ctl_cmd
Control an active connection multiplexing master process.
When the
.Fl O
option is specified, the
.Ar ctl_cmd
argument is interpreted and passed to the master process.
Valid commands are:
.Dq check
(check that the master process is running) and
.Dq exit
(request the master to exit).
.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.
For full details of the options listed below, and their possible values, see
.Xr ssh_config 5 .
.Pp
.Bl -tag -width Ds -offset indent -compact
.It AddressFamily
.It BatchMode
.It BindAddress
.It ChallengeResponseAuthentication
.It CheckHostIP
.It Cipher
.It Ciphers
.It ClearAllForwardings
.It Compression
.It CompressionLevel
.It ConnectionAttempts
.It ConnectTimeout
.It ControlMaster
.It ControlPath
.It DynamicForward
.It EscapeChar
.It ForwardAgent
.It ForwardX11
.It ForwardX11Trusted
.It GatewayPorts
.It GlobalKnownHostsFile
.It GSSAPIAuthentication
.It GSSAPIDelegateCredentials
.It HashKnownHosts
.It Host
.It HostbasedAuthentication
.It HostKeyAlgorithms
.It HostKeyAlias
.It HostName
.It IdentityFile
.It IdentitiesOnly
.It KbdInteractiveDevices
.It LocalForward
.It LogLevel
.It MACs
.It NoHostAuthenticationForLocalhost
.It NumberOfPasswordPrompts
.It PasswordAuthentication
.It Port
.It PreferredAuthentications
.It Protocol
.It ProxyCommand
.It PubkeyAuthentication
.It RemoteForward
.It RhostsRSAAuthentication
.It RSAAuthentication
.It SendEnv
.It ServerAliveInterval
.It ServerAliveCountMax
.It SmartcardDevice
.It StrictHostKeyChecking
.It TCPKeepAlive
.It UsePrivilegedPort
.It User
.It UserKnownHostsFile
.It VerifyHostKeyDNS
.It XAuthLocation
.El
.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 R Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Xc
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.
.Pp
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 by enclosing the address in square braces or
using an alternative syntax:
.Sm off
.Xo
.Op Ar bind_address No /
.Ar host No / Ar port No /
.Ar hostport
.Xc .
.Sm on
.Pp
By default, the listening socket on the server will be bound to the loopback
interface only.
This may be overriden by specifying a
.Ar bind_address .
An empty
.Ar bind_address ,
or the address
.Ql * ,
indicates that the remote socket should listen on all interfaces.
Specifying a remote
.Ar bind_address
will only succeed if the server's
.Cm GatewayPorts
option is enabled (see
.Xr sshd_config 5 ) .
.It Fl S Ar ctl_path
Specifies the location of a control socket for connection sharing.
Refer to the description of
.Cm ControlPath
and
.Cm ControlMaster
in
.Xr ssh_config 5
for details.
.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.\&
.Xr sftp 1 ) .
The subsystem is specified as the remote command.
.It Fl T
Disable pseudo-tty allocation.
.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 V
Display the version number and exit.
.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 increase the verbosity.
The maximum is 3.
.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.
.Pp
For this reason, X11 forwarding is subjected to X11 SECURITY extension
restrictions by default.
Please refer to the
.Nm
.Fl Y
option and the
.Cm ForwardX11Trusted
directive in
.Xr ssh_config 5
for more information.
.It Fl x
Disables X11 forwarding.
.It Fl Y
Enables trusted X11 forwarding.
Trusted X11 forwardings are not subjected to the X11 SECURITY extension
controls.
.El
.Sh CONFIGURATION FILES
.Nm
may additionally obtain configuration data from
a per-user configuration file and a system-wide configuration file.
The file format and configuration options are described in
.Xr ssh_config 5 .
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
.Bl -tag -width LOGNAME
.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 \*(Ge 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_CONNECTION
Identifies the client and server ends of the connection.
The variable contains
four space-separated values: client ip-address, client port number,
server ip-address and server port number.
.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 ~/.ssh/environment ,
and adds lines of the format
.Dq VARNAME=value
to the environment if the file exists and if users are allowed to
change their environment.
For more information, see the
.Cm PermitUserEnvironment
option in
.Xr sshd_config 5 .
.Sh FILES
.Bl -tag -width Ds
.It Pa ~/.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 ~/.ssh/identity, ~/.ssh/id_dsa, ~/.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 ~/.ssh/identity.pub, ~/.ssh/id_dsa.pub, ~/.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 ~/.ssh/identity.pub
file should be added to the file
.Pa ~/.ssh/authorized_keys
on all machines
where the user wishes to log in using protocol version 1 RSA authentication.
The contents of the
.Pa ~/.ssh/id_dsa.pub
and
.Pa ~/.ssh/id_rsa.pub
file should be added to
.Pa ~/.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 ~/.ssh/config
This is the per-user configuration file.
The file format and configuration options are described in
.Xr ssh_config 5 .
Because of the potential for abuse, this file must have strict permissions:
read/write for the user, and not accessible by others.
.It Pa ~/.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
.Pa .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 in 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.
The file format and configuration options are described in
.Xr ssh_config 5 .
.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 .
If the protocol version 1
.Cm RhostsRSAAuthentication
method is used,
.Nm
must be setuid root, since the host key is readable only by root.
For protocol version 2,
.Nm
uses
.Xr ssh-keysign 8
to access the host keys for
.Cm HostbasedAuthentication .
This eliminates the requirement that
.Nm
be setuid root when that authentication method is used.
By default
.Nm
is not setuid root.
.It Pa ~/.rhosts
This file is used in
.Cm RhostsRSAAuthentication
and
.Cm HostbasedAuthentication
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
.Xr sshd 8
allows authentication only in combination with client host key
authentication before permitting log in.
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 ~/.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 ~/.ssh/known_hosts .
.It Pa ~/.shosts
This file is used exactly the same way as
.Pa .rhosts .
The purpose for
having this file is to be able to use
.Cm RhostsRSAAuthentication
and
.Cm HostbasedAuthentication
authentication without permitting login with
.Xr rlogin
or
.Xr rsh 1 .
.It Pa /etc/hosts.equiv
This file is used during
.Cm RhostsRSAAuthentication
and
.Cm HostbasedAuthentication
authentication.
It contains
canonical hosts names, one per line (the full format is described in 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 client host key authentication is 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 ~/.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 ~/.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 SEE ALSO
.Xr gzip 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 hosts.equiv 5 ,
.Xr ssh_config 5 ,
.Xr ssh-keysign 8 ,
.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-12.txt
.%D January 2002
.%O work in progress material
.Re
.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.