.\" (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 1.156 2002/06/10 17:45:20 stevesk Exp $
+.\" $OpenBSD: ssh.1,v 1.249 2006/01/15 17:37:05 jmc Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.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 1246AaCfgkMNnqsTtVvXxY
.Op Fl b Ar bind_address
.Op Fl c Ar cipher_spec
+.Oo Fl D\ \&
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port
+.Sm on
+.Oc
.Op Fl e Ar escape_char
+.Op Fl F Ar configfile
+.Bk -words
.Op Fl i Ar identity_file
+.Ek
+.Oo Fl L\ \&
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port : host : hostport
+.Sm on
+.Oc
+.Bk -words
.Op Fl l Ar login_name
+.Ek
.Op Fl m Ar mac_spec
+.Op Fl O Ar ctl_cmd
.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
+.Oo Fl R\ \&
.Sm off
-.Ar port :
-.Ar host :
-.Ar hostport
+.Oo Ar bind_address : Oc
+.Ar port : host : hostport
.Sm on
-.Xc
.Oc
-.Op Fl D Ar port
-.Ar hostname | user@hostname
+.Op Fl S Ar ctl_path
+.Bk -words
+.Op Fl w Ar tunnel : Ns Ar tunnel
+.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
+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.
+X11 connections and arbitrary TCP ports
+can also be forwarded over the secure channel.
.Pp
.Nm
connects and logs into the specified
-.Ar hostname .
+.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
-.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.
+depending on the protocol version used (see 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.
+If
+.Ar command
+is specified,
+it is executed on the remote host instead of a login shell.
.Pp
-The supported escapes (assuming the default
-.Ql ~ )
-are:
+The options are as follows:
.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 ~C
-Open command line (only useful for adding port forwardings using the
-.Fl L
-and
-.Fl R
-options)
-.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
+.It Fl 1
+Forces
.Nm
-creates a
-.Dq proxy
-X server on the server machine for forwarding the
-connections over the encrypted channel.
-.Pp
+to try protocol version 1 only.
+.It Fl 2
+Forces
.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
+to try protocol version 2 only.
+.It Fl 4
+Forces
.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,
+to use IPv4 addresses only.
+.It Fl 6
+Forces
.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.
+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 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.
+Use
+.Ar bind_address
+on the local machine as the source address
+of the connection.
+Only useful on systems with more than one address.
+.It Fl C
+Requests compression of all data (including stdin, stdout, stderr, and
+data for forwarded X11 and TCP 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 supported 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
+is a fast block cipher; it appears very secure and is much faster than
.Ar 3des .
.Ar des
is only supported in the
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
+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:
+3des-cbc,
+aes128-cbc,
+aes192-cbc,
+aes256-cbc,
+aes128-ctr,
+aes192-ctr,
+aes256-ctr,
+arcfour128,
+arcfour256,
+arcfour,
+blowfish-cbc,
+and
+cast128-cbc.
+The default is:
+.Bd -literal -offset indent
+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 Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port
+.Sm on
+.Xc
+Specifies a local
+.Dq dynamic
+application-level port forwarding.
+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 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.
+.Pp
+IPv6 addresses can be specified with an alternative syntax:
+.Sm off
+.Xo
+.Op Ar bind_address No /
+.Ar port
+.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 e Ar escape_char
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.
+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
.Ic ssh -f host xterm .
.It Fl g
Allows remote hosts to connect to local forwarded ports.
+.It Fl I Ar smartcard_device
+Specify the device
+.Nm
+should use to communicate with a smartcard used for storing the user's
+private RSA key.
+This option is only available if support for smartcard devices
+is compiled in (default is no support).
.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
+.Pa ~/.ssh/identity
for protocol version 1, and
-.Pa $HOME/.ssh/id_rsa
+.Pa ~/.ssh/id_rsa
and
-.Pa $HOME/.ssh/id_dsa
+.Pa ~/.ssh/id_dsa
for protocol version 2.
Identity files may also be specified on
a per-host basis in the configuration file.
.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.
+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.
+Multiple
+.Fl M
+options places
+.Nm
+into
+.Dq master
+mode with confirmation required before slave connections are accepted.
+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
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
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 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 LocalCommand
+.It LocalForward
+.It LogLevel
+.It MACs
+.It NoHostAuthenticationForLocalhost
+.It NumberOfPasswordPrompts
+.It PasswordAuthentication
+.It PermitLocalCommand
+.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 Tunnel
+.It TunnelDevice
+.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 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 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. sftp). The
-subsystem is specified as the remote command.
+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
options force tty allocation, even if
.Nm
has no local tty.
-.It Fl T
-Disable pseudo-tty allocation.
+.It Fl V
+Display the version number and exit.
.It Fl v
Verbose mode.
Causes
debugging connection, authentication, and configuration problems.
Multiple
.Fl v
-options increases the verbosity.
-Maximum is 3.
-.It Fl x
-Disables X11 forwarding.
+options increase the verbosity.
+The maximum is 3.
+.It Fl w Ar tunnel : Ns Ar tunnel
+Requests a
+.Xr tun 4
+device on the client
+(first
+.Ar tunnel
+arg)
+and server
+(second
+.Ar tunnel
+arg).
+The devices may be specified by numerical ID or the keyword
+.Dq any ,
+which uses the next available tunnel device.
+See also the
+.Cm Tunnel
+directive in
+.Xr ssh_config 5 .
.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
+.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
-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
+.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
+.Pp
.Nm
-to try protocol version 1 only.
-.It Fl 2
-Forces
+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 .
+.Pp
.Nm
-to try protocol version 2 only.
-.It Fl 4
-Forces
+exits with the exit status of the remote command or with 255
+if an error occurred.
+.Sh AUTHENTICATION
+The OpenSSH SSH client supports SSH protocols 1 and 2.
+Protocol 2 is the default, with
.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
+falling back to protocol 1 if it detects protocol 2 is unsupported.
+These settings may be altered using the
+.Cm Protocol
+option in
+.Xr ssh_config 5 ,
+or enforced using the
+.Fl 1
+and
+.Fl 2
+options (see above).
+Both protocols support similar authentication methods,
+but protocol 2 is preferred since
+it provides additional mechanisms for confidentiality
+(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
+and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
+Protocol 1 lacks a strong mechanism for ensuring the
+integrity of the connection.
+.Pp
+The methods available for authentication are:
+host-based authentication,
+public key authentication,
+challenge-response authentication,
+and password authentication.
+Authentication methods are tried in the order specified above,
+though protocol 2 has a configuration option to change the default order:
+.Cm PreferredAuthentications .
+.Pp
+Host-based authentication works as follows:
+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 login.
+Additionally, the server
+.Em must
+be able to verify the client's
+host key (see the description of
+.Pa /etc/ssh/ssh_known_hosts
+and
+.Pa ~/.ssh/known_hosts ,
+below)
+for login to be 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
+Public key authentication works as follows:
+The scheme is based on public-key cryptography,
+using cryptosystems
+where encryption and decryption are done using separate keys,
+and it is unfeasible to derive the decryption key from the encryption key.
+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.
.Nm
-to use IPv6 addresses only.
-.El
-.Sh CONFIGURATION FILES
+implements public key authentication protocol automatically,
+using either the RSA or DSA algorithms.
+Protocol 1 is restricted to using only RSA keys,
+but protocol 2 may use either.
+The
+.Sx HISTORY
+section of
+.Xr ssl 8
+contains a brief discussion of the two algorithms.
+.Pp
+The file
+.Pa ~/.ssh/authorized_keys
+lists the public keys that are permitted for logging in.
+When the user logs in, the
.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.
+program tells the server which key pair it would like to use for
+authentication.
+The client proves that it has access to the private key
+and the server checks that the corresponding public key
+is authorized to accept the account.
.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.
+The user creates his/her key pair by running
+.Xr ssh-keygen 1 .
+This stores the private key in
+.Pa ~/.ssh/identity
+(protocol 1),
+.Pa ~/.ssh/id_dsa
+(protocol 2 DSA),
+or
+.Pa ~/.ssh/id_rsa
+(protocol 2 RSA)
+and stores the public key in
+.Pa ~/.ssh/identity.pub
+(protocol 1),
+.Pa ~/.ssh/id_dsa.pub
+(protocol 2 DSA),
+or
+.Pa ~/.ssh/id_rsa.pub
+(protocol 2 RSA)
+in the user's home directory.
+The user should then copy the public key
+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 configuration file has the following format:
+The most convenient way to use public key authentication may be with an
+authentication agent.
+See
+.Xr ssh-agent 1
+for more information.
.Pp
-Empty lines and lines starting with
-.Ql #
-are comments.
+Challenge-response authentication works as follows:
+The server sends an arbitrary
+.Qq challenge
+text, and prompts for a response.
+Protocol 2 allows multiple challenges and responses;
+protocol 1 is restricted to just one challenge/response.
+Examples of challenge-response authentication include
+BSD Authentication (see
+.Xr login.conf 5 )
+and PAM (some non-OpenBSD systems).
.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
+Finally, 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
+.Nm
+automatically maintains and checks a database containing
+identification 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
+server spoofing or man-in-the-middle attacks,
+which could otherwise be used to circumvent the encryption.
+The
+.Cm StrictHostKeyChecking
+option can be used to control 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
+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 connections have been closed.
+.Sh 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 possible
-keywords and their meanings are as follows (note that
-keywords are case-insensitive and arguments are case-sensitive):
+The supported escapes (assuming the default
+.Ql ~ )
+are:
.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 \&*
+.It Cm ~.
+Disconnect.
+.It Cm ~^Z
+Background
+.Nm .
+.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
-.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 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 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 ,
+.Fl R
+options (see above).
+It also allows the cancellation of existing remote port-forwardings
+using
+.Fl KR Ar hostport .
+.Ic !\& Ns Ar command
+allows the user to execute a local command if the
+.Ic PermitLocalCommand
+option is enabled in
+.Xr ssh_config 5 .
+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
+.Sh TCP FORWARDING
+Forwarding of arbitrary TCP connections over the secure channel can
+be specified either on the command line or in a configuration file.
+One possible application of TCP forwarding is a secure connection to a
+mail server; another is going through firewalls.
+.Pp
+In the example below, we look at encrypting communication between
+an IRC client and server, even though the IRC server does not directly
+support encrypted communications.
+This works as follows:
+the user connects to the remote host using
+.Nm ,
+specifying a port to be used to forward connections
+to the remote server.
+After that it is possible to start the service which is to be encrypted
+on the client machine,
+connecting to the same local port,
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
+will encrypt and forward the connection.
.Pp
-.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
+The following example tunnels an IRC session from client machine
+.Dq 127.0.0.1
+(localhost)
+to remote server
+.Dq server.example.com :
+.Bd -literal -offset 4n
+$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
+$ irc -c '#users' -p 1234 pinky 127.0.0.1
.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 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 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,
+.Pp
+This tunnels a connection to IRC server
+.Dq server.example.com ,
+joining channel
+.Dq #users ,
+nickname
+.Dq pinky ,
+using port 1234.
+It doesn't matter which port is used,
+as long as it's greater than 1023
+(remember, only root can open sockets on privileged ports)
+and doesn't conflict with any ports already in use.
+The connection is forwarded to port 6667 on the remote server,
+since that's the standard port for IRC services.
+.Pp
+The
+.Fl f
+option backgrounds
.Nm
-binds local port forwardings to the loopback address. This
-prevents other remote hosts from connecting to forwarded ports.
-.Cm GatewayPorts
-can be used to specify that
+and the remote command
+.Dq sleep 10
+is specified to allow an amount of time
+(10 seconds, in the example)
+to start the service which is to be tunnelled.
+If no connections are made within the time specified,
.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
+will exit.
+.Sh X11 FORWARDING
+If the
+.Cm ForwardX11
+variable is set to
.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
+(or see the description of the
+.Fl X ,
+.Fl x ,
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.
+.Fl Y
+options above)
+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
-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
+The
+.Ev DISPLAY
+value set by
.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
+will point to the server machine, but with a display number greater than zero.
+This is normal, and happens because
.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.
+creates a
+.Dq proxy
+X server on the server machine for forwarding the
+connections over the encrypted channel.
.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.
-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 no .
-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 no .
-This option applies to protocol version 1 only and requires
.Nm
-to be setuid root.
-.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 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
+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
-if
-.Cm RhostsAuthentication
+(or see the description of the
+.Fl A
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 XAuthLocation
-Specifies the location of the
-.Xr xauth 1
-program.
-The default is
-.Pa /usr/X11R6/bin/xauth .
-.El
+.Fl a
+options above) and
+the user is using an authentication agent, the connection to the agent
+is automatically forwarded to the remote side.
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
-.Bl -tag -width Ds
+.Bl -tag -width "SSH_ORIGINAL_COMMAND"
.It Ev DISPLAY
The
.Ev DISPLAY
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.
+.Dq hostname:n ,
+where
+.Dq hostname
+indicates the host where the shell runs, and
+.Sq n
+is an integer \*(Ge 1.
.Nm
uses this special value to forward X11 connections over the secure
channel.
Set to the default
.Ev PATH ,
as specified when compiling
-.Nm ssh .
+.Nm .
.It Ev SSH_ASKPASS
If
.Nm
This is particularly useful when calling
.Nm
from a
-.Pa .Xsession
+.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.
+Identifies the path of a
+.Ux Ns -domain
+socket used to communicate with the agent.
+.It Ev SSH_CONNECTION
+Identifies the client and server ends of the connection.
The variable contains
-three space-separated values: client ip-address, client port number,
-and server port number.
+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
+This 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
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
+This variable is set to indicate the present time zone if it
was set when the daemon was started (i.e., the daemon passes the value
on to new connections).
.It Ev USER
Additionally,
.Nm
reads
-.Pa $HOME/.ssh/environment ,
+.Pa ~/.ssh/environment ,
and adds lines of the format
.Dq VARNAME=value
-to the environment.
+to the environment if the file exists and 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 $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.
+.Bl -tag -width Ds -compact
+.It ~/.rhosts
+This file is used for host-based authentication (see above).
+On some machines this file may need to be
+world-readable if the user's home directory is on an 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
+.It ~/.shosts
+This file is used in exactly the same way as
+.Pa .rhosts ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It ~/.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.
+This file is not highly sensitive, but the recommended
+permissions are read/write for the user, and not accessible by others.
+.Pp
+.It ~/.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.
+.Pp
+.It ~/.ssh/environment
+Contains additional definitions for environment variables; see
+.Sx ENVIRONMENT ,
+above.
+.Pp
+.It ~/.ssh/identity
+.It ~/.ssh/id_dsa
+.It ~/.ssh/id_rsa
+Contains the private key for authentication.
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.
+will simply ignore 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
+generating the key which 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.
+.Pp
+.It ~/.ssh/identity.pub
+.It ~/.ssh/id_dsa.pub
+.It ~/.ssh/id_rsa.pub
+Contains the public key for 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
+They 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
+.It ~/.ssh/known_hosts
+Contains a list of host keys for all hosts the user has logged into
+that are not already in the systemwide list of known host keys.
+See
.Xr sshd 8
-to verify the client host when logging in; other names are needed because
+for further details of the format of this file.
+.Pp
+.It ~/.ssh/rc
+Commands in this file are executed by
.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.
+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.
+.Pp
+.It /etc/hosts.equiv
+This file is for host-based authentication (see above).
+It should only be writable by root.
+.Pp
+.It /etc/shosts.equiv
+This file is used in exactly the same way as
+.Pa hosts.equiv ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
.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
+The file format and configuration options are described in
+.Xr ssh_config 5 .
+.Pp
+.It /etc/ssh/ssh_host_key
+.It /etc/ssh/ssh_host_dsa_key
+.It /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,
+and are used for host-based authentication.
+If protocol version 1 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
+to access the host keys,
+eliminating the requirement that
.Nm
-be setuid root when that authentication method is used.
+be setuid root when host-based authentication is used.
By default
.Nm
is not setuid root.
-.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
-.Nm rlogin
-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
+.It /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.
+It should be world-readable.
+See
.Xr sshd 8
-manual page for more information.
-.It Pa $HOME/.ssh/rc
+for further details of the format of this file.
+.Pp
+.It /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.
+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 rsh 1 ,
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
-.Xr telnet 1 ,
-.Xr ssh-keysign 8,
+.Xr ssh-keyscan 1 ,
+.Xr hosts.equiv 5 ,
+.Xr ssh_config 5 ,
+.Xr ssh-keysign 8 ,
.Xr sshd 8
.Rs
.%A T. Ylonen
.%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.
andersk / openssh.git / blobdiff