.\" (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 1.150 2002/03/26 11:34:49 markus Exp $
+.\" $OpenBSD: ssh.1,v 1.208 2005/05/23 23:32:46 djm 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
+.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
-.Op Fl F Ar configfile
-.Oo Fl L 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
-.Oo Fl R Xo
-.Sm off
-.Ar port :
-.Ar host :
-.Ar hostport
-.Sm on
-.Xc
-.Oc
-.Op Fl D Ar port
-.Ar hostname | user@hostname
+.Op 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
+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/IP 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:
+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
-.Pp
-First, if the machine the user logs in from is listed in
+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, the user is immediately permitted to log in.
-Second, if
-.Pa \&.rhosts
+the same on both sides, or if the files
+.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
+.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
-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
+considered for log in.
+Additionally, if the server can verify the client's
host key (see
.Pa /etc/ssh/ssh_known_hosts
and
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.ssh/known_hosts
in the
.Sx FILES
-section), only then login is permitted.
+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 $HOME/.rhosts ,
+.Pa ~/.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,
+As a second authentication method,
.Nm
supports RSA based authentication.
The scheme is based on public-key cryptography: there are cryptosystems
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 $HOME/.ssh/authorized_keys
-lists the public keys that are permitted for logging
-in.
+.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
+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.
+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
+.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 $HOME/.ssh/authorized_keys
+.Pa ~/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
-.Pa $HOME/.rhosts
+.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.
-RSA authentication is much
-more secure than rhosts authentication.
.Pp
The most convenient way to use RSA authentication may be with an
authentication agent.
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
+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
+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
+.Pa ~/.ssh/id_dsa
or
-.Pa $HOME/.ssh/id_rsa ,
+.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 $HOME/.ssh/authorized_keys
+.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 for proving the user's identity.
+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 3DES, Blowfish, CAST128 or Arcfour)
-and integrity (hmac-md5, hmac-sha1).
+(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.
-.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.
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.
+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
+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.
+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 ~~
are:
.Bl -tag -width Ds
.It Cm ~.
-Disconnect
+Disconnect.
.It Cm ~^Z
-Background ssh
+Background
+.Nm ssh .
.It Cm ~#
-List forwarded connections
+List forwarded connections.
.It Cm ~&
-Background ssh at logout when waiting for forwarded connection / X11 sessions
-to terminate
+Background
+.Nm
+at logout when waiting for forwarded connection / X11 sessions to terminate.
.It Cm ~?
-Display a list of escape characters
+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 (only useful for adding port forwardings using the
+Open command line.
+Currently this allows the addition of port forwardings using the
.Fl L
and
.Fl R
-options)
+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)
+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
+(or see the description of the
.Fl X
and
.Fl x
.Ev DISPLAY
value set by
.Nm
-will point to the server machine, but with a display number greater
-than zero.
+will point to the server machine, but with a display number greater than zero.
This is normal, and happens because
.Nm
creates a
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.
+If the
+.Cm ForwardAgent
+variable is set to
+.Dq yes
+(or see the description of the
+.Fl A
+and
+.Fl a
+options described later) and
+the user is using an authentication agent, the connection to the agent
+is automatically forwarded to the remote side.
.Pp
Forwarding of arbitrary TCP/IP connections over the secure channel can
be specified either on the command line or in a configuration file.
One possible application of TCP/IP forwarding is a secure connection to an
electronic purse; another is going through firewalls.
-.Pp
.Ss Server authentication
-.Pp
.Nm
automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
Host keys are stored in
-.Pa $HOME/.ssh/known_hosts
+.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,
+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.
+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
+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 a
-Disables forwarding of the authentication agent connection.
+.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 to transmit from on machines with multiple
+Specify the interface address 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.
+.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
+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
+.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.
+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
+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 $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.
+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 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 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 X
Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
-.It Fl C
-Requests compression of all data (including stdin, stdout, stderr, and
-data for forwarded X11 and TCP/IP connections).
-The compression algorithm is the same used by
-.Xr gzip 1 ,
-and the
-.Dq level
-can be controlled by the
-.Cm CompressionLevel
-option (see below).
-Compression is desirable on modem lines and other
-slow connections, but will only slow down things on fast networks.
-The default value can be set on a host-by-host basis in the
-configuration files; see the
-.Cm Compression
-option below.
-.It Fl F Ar configfile
-Specifies an alternative per-user configuration file.
-If a configuration file is given on the command line,
-the system-wide configuration file
-.Pq Pa /etc/ssh/ssh_config
-will be ignored.
-The default for the per-user configuration file is
-.Pa $HOME/.ssh/config .
-.It Fl L Ar port:host:hostport
-Specifies that the given port on the local (client) host is to be
-forwarded to the given host and port on the remote side.
-This works by allocating a socket to listen to
-.Ar port
-on the local side, and whenever a connection is made to this port, the
-connection is forwarded over the secure channel, and a connection is
-made to
-.Ar host
-port
-.Ar hostport
-from the remote machine.
-Port forwardings can also be specified in the configuration file.
-Only root can forward privileged ports.
-IPv6 addresses can be specified with an alternative syntax:
-.Ar port/host/hostport
-.It Fl R Ar port:host:hostport
-Specifies that the given port on the remote (server) host is to be
-forwarded to the given host and port on the local side.
-This works by allocating a socket to listen to
-.Ar port
-on the remote side, and whenever a connection is made to this port, the
-connection is forwarded over the secure channel, and a connection is
-made to
-.Ar host
-port
-.Ar hostport
-from the local machine.
-Port forwardings can also be specified in the configuration file.
-Privileged ports can be forwarded only when
-logging in as root on the remote machine.
-IPv6 addresses can be specified with an alternative syntax:
-.Ar port/host/hostport
-.It Fl D Ar port
-Specifies a local
-.Dq dynamic
-application-level port forwarding.
-This works by allocating a socket to listen to
-.Ar port
-on the local side, and whenever a connection is made to this port, the
-connection is forwarded over the secure channel, and the application
-protocol is then used to determine where to connect to from the
-remote machine. Currently the SOCKS4 protocol is supported, and
-.Nm
-will act as a SOCKS4 server.
-Only root can forward privileged ports.
-Dynamic port forwardings can also be specified in the configuration file.
-.It Fl 1
-Forces
-.Nm
-to try protocol version 1 only.
-.It Fl 2
-Forces
-.Nm
-to try protocol version 2 only.
-.It Fl 4
-Forces
-.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
-.Nm
-to use IPv6 addresses only.
-.El
-.Sh CONFIGURATION FILES
-.Nm
-obtains configuration data from the following sources in
-the following order:
-command line options, user's configuration file
-.Pq Pa $HOME/.ssh/config ,
-and system-wide configuration file
-.Pq Pa /etc/ssh/ssh_config .
-For each parameter, the first obtained value
-will be used.
-The configuration files contain sections bracketed by
-.Dq Host
-specifications, and that section is only applied for hosts that
-match one of the patterns given in the specification.
-The matched host name is the one given on the command line.
-.Pp
-Since the first obtained value for each parameter is used, more
-host-specific declarations should be given near the beginning of the
-file, and general defaults at the end.
-.Pp
-The configuration file has the following format:
-.Pp
-Empty lines and lines starting with
-.Ql #
-are comments.
-.Pp
-Otherwise a line is of the format
-.Dq keyword arguments .
-Configuration options may be separated by whitespace or
-optional whitespace and exactly one
-.Ql = ;
-the latter format is useful to avoid the need to quote whitespace
-when specifying configuration options using the
-.Nm ssh ,
-.Nm scp
-and
-.Nm sftp
-.Fl o
-option.
-.Pp
-The possible
-keywords and their meanings are as follows (note that
-keywords are case-insensitive and arguments are case-sensitive):
-.Bl -tag -width Ds
-.It Cm Host
-Restricts the following declarations (up to the next
-.Cm Host
-keyword) to be only for those hosts that match one of the patterns
-given after the keyword.
-.Ql \&*
-and
-.Ql ?
-can be used as wildcards in the
-patterns.
-A single
-.Ql \&*
-as a pattern can be used to provide global
-defaults for all hosts.
-The host is the
-.Ar hostname
-argument given on the command line (i.e., the name is not converted to
-a canonicalized host name before matching).
-.It Cm AFSTokenPassing
-Specifies whether to pass AFS tokens to remote host.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-This option applies to protocol version 1 only.
-.It Cm BatchMode
-If set to
-.Dq yes ,
-passphrase/password querying will be disabled.
-This option is useful in scripts and other batch jobs where no user
-is present to supply the password.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm BindAddress
-Specify the interface to transmit from on machines with multiple
-interfaces or aliased addresses.
-Note that this option does not work if
-.Cm UsePrivilegedPort
-is set to
-.Dq yes .
-.It Cm CheckHostIP
-If this flag is set to
-.Dq yes ,
-ssh will additionally check the host IP address in the
-.Pa known_hosts
-file.
-This allows ssh to detect if a host key changed due to DNS spoofing.
-If the option is set to
-.Dq no ,
-the check will not be executed.
-The default is
-.Dq yes .
-.It Cm Cipher
-Specifies the cipher to use for encrypting the session
-in protocol version 1.
-Currently,
-.Dq blowfish ,
-.Dq 3des ,
-and
-.Dq des
-are supported.
-.Ar des
-is only supported in the
-.Nm
-client for interoperability with legacy protocol 1 implementations
-that do not support the
-.Ar 3des
-cipher. Its use is strongly discouraged due to cryptographic
-weaknesses.
-The default is
-.Dq 3des .
-.It Cm Ciphers
-Specifies the ciphers allowed for protocol version 2
-in order of preference.
-Multiple ciphers must be comma-separated.
-The default is
-.Pp
-.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
-.Ed
-.It Cm ClearAllForwardings
-Specifies that all local, remote and dynamic port forwardings
-specified in the configuration files or on the command line be
-cleared. This option is primarily useful when used from the
-.Nm
-command line to clear port forwardings set in
-configuration files, and is automatically set by
-.Xr scp 1
-and
-.Xr sftp 1 .
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm Compression
-Specifies whether to use compression.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm CompressionLevel
-Specifies the compression level to use if compression is enabled.
-The argument must be an integer from 1 (fast) to 9 (slow, best).
-The default level is 6, which is good for most applications.
-The meaning of the values is the same as in
-.Xr gzip 1 .
-Note that this option applies to protocol version 1 only.
-.It Cm ConnectionAttempts
-Specifies the number of tries (one per second) to make before falling
-back to rsh or exiting.
-The argument must be an integer.
-This may be useful in scripts if the connection sometimes fails.
-The default is 1.
-.It Cm DynamicForward
-Specifies that a TCP/IP port on the local machine be forwarded
-over the secure channel, and the application
-protocol is then used to determine where to connect to from the
-remote machine. The argument must be a port number.
-Currently the SOCKS4 protocol is supported, and
-.Nm
-will act as a SOCKS4 server.
-Multiple forwardings may be specified, and
-additional forwardings can be given on the command line. Only
-the superuser can forward privileged ports.
-.It Cm EscapeChar
-Sets the escape character (default:
-.Ql ~ ) .
-The escape character can also
-be set on the command line.
-The argument should be a single character,
-.Ql ^
-followed by a letter, or
-.Dq none
-to disable the escape
-character entirely (making the connection transparent for binary
-data).
-.It Cm FallBackToRsh
-Specifies that if connecting via
-.Nm
-fails due to a connection refused error (there is no
-.Xr sshd 8
-listening on the remote host),
-.Xr rsh 1
-should automatically be used instead (after a suitable warning about
-the session being unencrypted).
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm ForwardAgent
-Specifies whether the connection to the authentication agent (if any)
-will be forwarded to the remote machine.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm ForwardX11
-Specifies whether X11 connections will be automatically redirected
-over the secure channel and
-.Ev DISPLAY
-set.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm GatewayPorts
-Specifies whether remote hosts are allowed to connect to local
-forwarded ports.
-By default,
-.Nm
-binds local port forwardings to the loopback addresss. This
-prevents other remote hosts from connecting to forwarded ports.
-.Cm GatewayPorts
-can be used to specify that
-.Nm
-should bind local port forwardings to the wildcard address,
-thus allowing remote hosts to connect to forwarded ports.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm GlobalKnownHostsFile
-Specifies a file to use for the global
-host key database instead of
-.Pa /etc/ssh/ssh_known_hosts .
-.It Cm HostbasedAuthentication
-Specifies whether to try rhosts based authentication with public key
-authentication.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-This option applies to protocol version 2 only and
-is similar to
-.Cm RhostsRSAAuthentication .
-.It Cm HostKeyAlgorithms
-Specifies the protocol version 2 host key algorithms
-that the client wants to use in order of preference.
-The default for this option is:
-.Dq ssh-rsa,ssh-dss .
-.It Cm HostKeyAlias
-Specifies an alias that should be used instead of the
-real host name when looking up or saving the host key
-in the host key database files.
-This option is useful for tunneling ssh connections
-or for multiple servers running on a single host.
-.It Cm HostName
-Specifies the real host name to log into.
-This can be used to specify nicknames or abbreviations for hosts.
-Default is the name given on the command line.
-Numeric IP addresses are also permitted (both on the command line and in
-.Cm HostName
-specifications).
-.It Cm IdentityFile
-Specifies a file from which the user's RSA or DSA authentication identity
-is read. The default is
-.Pa $HOME/.ssh/identity
-for protocol version 1, and
-.Pa $HOME/.ssh/id_rsa
-and
-.Pa $HOME/.ssh/id_dsa
-for protocol version 2.
-Additionally, any identities represented by the authentication agent
-will be used for authentication.
-The file name may use the tilde
-syntax to refer to a user's home directory.
-It is possible to have
-multiple identity files specified in configuration files; all these
-identities will be tried in sequence.
-.It Cm KeepAlive
-Specifies whether the system should send TCP keepalive messages to the
-other side.
-If they are sent, death of the connection or crash of one
-of the machines will be properly noticed.
-However, this means that
-connections will die if the route is down temporarily, and some people
-find it annoying.
.Pp
-The default is
-.Dq yes
-(to send keepalives), and the client will notice
-if the network goes down or the remote host dies.
-This is important in scripts, and many users want it too.
+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
-To disable keepalives, the value should be set to
-.Dq no .
-.It Cm KerberosAuthentication
-Specifies whether Kerberos authentication will be used.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm KerberosTgtPassing
-Specifies whether a Kerberos TGT will be forwarded to the server.
-This will only work if the Kerberos server is actually an AFS kaserver.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm LocalForward
-Specifies that a TCP/IP port on the local machine be forwarded over
-the secure channel to the specified host and port from the remote machine.
-The first argument must be a port number, and the second must be
-.Ar host:port .
-IPv6 addresses can be specified with an alternative syntax:
-.Ar host/port .
-Multiple forwardings may be specified, and additional
-forwardings can be given on the command line.
-Only the superuser can forward privileged ports.
-.It Cm LogLevel
-Gives the verbosity level that is used when logging messages from
-.Nm ssh .
-The possible values are:
-QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
-The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
-and DEBUG3 each specify higher levels of verbose output.
-.It Cm MACs
-Specifies the MAC (message authentication code) algorithms
-in order of preference.
-The MAC algorithm is used in protocol version 2
-for data integrity protection.
-Multiple algorithms must be comma-separated.
-The default is
-.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
-.It Cm NoHostAuthenticationForLocalhost
-This option can be used if the home directory is shared across machines.
-In this case localhost will refer to a different machine on each of
-the machines and the user will get many warnings about changed host keys.
-However, this option disables host authentication for localhost.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-The default is to check the host key for localhost.
-.It Cm NumberOfPasswordPrompts
-Specifies the number of password prompts before giving up.
-The argument to this keyword must be an integer.
-Default is 3.
-.It Cm PasswordAuthentication
-Specifies whether to use password authentication.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-.It Cm Port
-Specifies the port number to connect on the remote host.
-Default is 22.
-.It Cm PreferredAuthentications
-Specifies the order in which the client should try protocol 2
-authentication methods. This allows a client to prefer one method (e.g.
-.Cm keyboard-interactive )
-over another method (e.g.
-.Cm password )
-The default for this option is:
-.Dq hostbased,publickey,keyboard-interactive,password .
-.It Cm Protocol
-Specifies the protocol versions
-.Nm
-should support in order of preference.
-The possible values are
-.Dq 1
-and
-.Dq 2 .
-Multiple versions must be comma-separated.
-The default is
-.Dq 2,1 .
-This means that
-.Nm
-tries version 2 and falls back to version 1
-if version 2 is not available.
-.It Cm ProxyCommand
-Specifies the command to use to connect to the server.
-The command
-string extends to the end of the line, and is executed with
-.Pa /bin/sh .
-In the command string,
-.Ql %h
-will be substituted by the host name to
-connect and
-.Ql %p
-by the port.
-The command can be basically anything,
-and should read from its standard input and write to its standard output.
-It should eventually connect an
-.Xr sshd 8
-server running on some machine, or execute
-.Ic sshd -i
-somewhere.
-Host key management will be done using the
-HostName of the host being connected (defaulting to the name typed by
-the user).
-Note that
-.Cm CheckHostIP
-is not available for connects with a proxy command.
-.Pp
-.It Cm PubkeyAuthentication
-Specifies whether to try public key authentication.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-This option applies to protocol version 2 only.
-.It Cm RemoteForward
-Specifies that a TCP/IP port on the remote machine be forwarded over
-the secure channel to the specified host and port from the local machine.
-The first argument must be a port number, and the second must be
-.Ar host:port .
-IPv6 addresses can be specified with an alternative syntax:
-.Ar host/port .
-Multiple forwardings may be specified, and additional
-forwardings can be given on the command line.
-Only the superuser can forward privileged ports.
-.It Cm RhostsAuthentication
-Specifies whether to try rhosts based authentication.
-Note that this
-declaration only affects the client side and has no effect whatsoever
-on security.
-Disabling rhosts authentication may reduce
-authentication time on slow connections when rhosts authentication is
-not used.
-Most servers do not permit RhostsAuthentication because it
-is not secure (see
-.Cm RhostsRSAAuthentication ) .
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-This option applies to protocol version 1 only.
-.It Cm RhostsRSAAuthentication
-Specifies whether to try rhosts based authentication with RSA host
-authentication.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-This option applies to protocol version 1 only.
-.It Cm RSAAuthentication
-Specifies whether to try RSA authentication.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-RSA authentication will only be
-attempted if the identity file exists, or an authentication agent is
-running.
-The default is
-.Dq yes .
-Note that this option applies to protocol version 1 only.
-.It Cm ChallengeResponseAuthentication
-Specifies whether to use challenge response authentication.
-The argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-.It Cm SmartcardDevice
-Specifies which smartcard device to use. The argument to this keyword is
-the device
-.Nm
-should use to communicate with a smartcard used for storing the user's
-private RSA key. By default, no device is specified and smartcard support
-is not activated.
-.It Cm StrictHostKeyChecking
-If this flag is set to
-.Dq yes ,
-.Nm
-will never automatically add host keys to the
-.Pa $HOME/.ssh/known_hosts
-file, and refuses to connect to hosts whose host key has changed.
-This provides maximum protection against trojan horse attacks,
-however, can be annoying when the
-.Pa /etc/ssh/ssh_known_hosts
-file is poorly maintained, or connections to new hosts are
-frequently made.
-This option forces the user to manually
-add all new hosts.
-If this flag is set to
-.Dq no ,
-.Nm
-will automatically add new host keys to the
-user known hosts files.
-If this flag is set to
-.Dq ask ,
-new host keys
-will be added to the user known host files only after the user
-has confirmed that is what they really want to do, and
+For this reason, X11 forwarding is subjected to X11 SECURITY extension
+restrictions by default.
+Please refer to the
.Nm
-will refuse to connect to hosts whose host key has changed.
-The host keys of
-known hosts will be verified automatically in all cases.
-The argument must be
-.Dq yes ,
-.Dq no
-or
-.Dq ask .
-The default is
-.Dq ask .
-.It Cm UsePrivilegedPort
-Specifies whether to use a privileged port for outgoing connections.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-Note that this option must be set to
-.Dq yes
-if
-.Cm RhostsAuthentication
-and
-.Cm RhostsRSAAuthentication
-authentications are needed with older servers.
-.It Cm User
-Specifies the user to log in as.
-This can be useful when a different user name is used on different machines.
-This saves the trouble of
-having to remember to give the user name on the command line.
-.It Cm UserKnownHostsFile
-Specifies a file to use for the user
-host key database instead of
-.Pa $HOME/.ssh/known_hosts .
-.It Cm UseRsh
-Specifies that rlogin/rsh should be used for this host.
-It is possible that the host does not at all support the
-.Nm
-protocol.
-This causes
-.Nm
-to immediately execute
-.Xr rsh 1 .
-All other options (except
-.Cm HostName )
-are ignored if this has been specified.
-The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm XAuthLocation
-Specifies the location of the
-.Xr xauth 1
-program.
-The default is
-.Pa /usr/X11R6/bin/xauth .
+.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 Ds
+.Bl -tag -width LOGNAME
.It Ev DISPLAY
The
.Ev DISPLAY
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.
+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.
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
.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.
+.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
is executed.
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 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 $HOME/.ssh/known_hosts
+.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 $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
+.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
It is possible to specify a passphrase when
generating the key; the passphrase will be used to encrypt the
sensitive part of this file using 3DES.
-.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
+.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 $HOME/.ssh/identity.pub
-file should be added to
-.Pa $HOME/.ssh/authorized_keys
+.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 $HOME/.ssh/id_dsa.pub
+.Pa ~/.ssh/id_dsa.pub
and
-.Pa $HOME/.ssh/id_rsa.pub
+.Pa ~/.ssh/id_rsa.pub
file should be added to
-.Pa $HOME/.ssh/authorized_keys
+.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
These files are
never used automatically and are not necessary; they are only provided for
the convenience of the user.
-.It Pa $HOME/.ssh/config
+.It Pa ~/.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
+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 .pub
+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.
When different names are used
for the same machine, all such names should be listed, separated by
commas.
-The format is described on the
+The format is described in the
.Xr sshd 8
manual page.
.Pp
would then be able to fool host authentication.
.It Pa /etc/ssh/ssh_config
Systemwide configuration file.
-This file provides defaults for those
-values that are not specified in the user's configuration file, and
-for those users who do not have a configuration file.
-This file must be world-readable.
+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 .
-Since they are readable only by root
+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
-must be setuid root if these authentication methods are desired.
-.It Pa $HOME/.rhosts
+is not setuid root.
+.It Pa ~/.rhosts
This file is used in
-.Pa \&.rhosts
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
authentication to list the
host/user pairs that are permitted to log in.
(Note that this file is
permission for most machines is read/write for the user, and not
accessible by others.
.Pp
-Note that by default
+Note that
.Xr sshd 8
-will be installed so that it requires successful RSA host
-authentication before permitting \s+2.\s0rhosts authentication.
+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 $HOME/.ssh/known_hosts .
+.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 $HOME/.ssh/known_hosts .
-.It Pa $HOME/.shosts
+.Pa ~/.ssh/known_hosts .
+.It Pa ~/.shosts
This file is used exactly the same way as
-.Pa \&.rhosts .
+.Pa .rhosts .
The purpose for
-having this file is to be able to use rhosts authentication with
-.Nm
-without permitting login with
-.Xr rlogin 1
+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
-.Pa \&.rhosts authentication.
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
+authentication.
It contains
-canonical hosts names, one per line (the full format is described on
-the
+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 RSA host authentication is normally
-required.
+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
See the
.Xr sshd 8
manual page for more information.
-.It Pa $HOME/.ssh/rc
+.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
See the
.Xr sshd 8
manual page for more information.
-.It Pa $HOME/.ssh/environment
+.It Pa ~/.ssh/environment
Contains additional definitions for environment variables, see section
.Sx ENVIRONMENT
above.
.Nm
exits with the exit status of the remote command or with 255
if an error occurred.
-.Sh AUTHORS
-OpenSSH is a derivative of the original and free
-ssh 1.2.12 release by Tatu Ylonen.
-Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
-Theo de Raadt and Dug Song
-removed many bugs, re-added newer features and
-created OpenSSH.
-Markus Friedl contributed the support for SSH
-protocol versions 1.5 and 2.0.
.Sh SEE ALSO
-.Xr rlogin 1 ,
+.Xr gzip 1 ,
.Xr rsh 1 ,
.Xr scp 1 ,
.Xr sftp 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
.%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