.\" (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_config.5,v 1.9 2003/05/15 14:55:25 djm Exp $
+.\" $OpenBSD: ssh_config.5,v 1.64 2005/10/30 08:43:47 jmc Exp $
.Dd September 25, 1999
.Dt SSH_CONFIG 5
.Os
.Nd OpenSSH SSH client configuration files
.Sh SYNOPSIS
.Bl -tag -width Ds -compact
-.It Pa $HOME/.ssh/config
+.It Pa ~/.ssh/config
.It Pa /etc/ssh/ssh_config
.El
.Sh DESCRIPTION
command-line options
.It
user's configuration file
-.Pq Pa $HOME/.ssh/config
+.Pq Pa ~/.ssh/config
.It
system-wide configuration file
.Pq Pa /etc/ssh/ssh_config
.Pp
For each parameter, the first obtained value
will be used.
-The configuration files contain sections bracketed by
+The configuration files contain sections separated by
.Dq Host
specifications, and that section is only applied for hosts that
match one of the patterns given in the specification.
given after the keyword.
.Ql \&*
and
-.Ql ?
+.Ql \&?
can be used as wildcards in the
patterns.
A single
.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 AddressFamily
+Specifies which address family to use when connecting.
+Valid arguments are
+.Dq any ,
+.Dq inet
+(use IPv4 only) or
+.Dq inet6
+(use IPv6 only).
.It Cm BatchMode
If set to
.Dq yes ,
The default is
.Dq no .
.It Cm BindAddress
-Specify the interface to transmit from on machines with multiple
-interfaces or aliased addresses.
+Use the specified address on the local machine as the source address of
+the connection.
+Only useful on systems with more than one address.
Note that this option does not work if
.Cm UsePrivilegedPort
is set to
Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
+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
-.Pp
.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
+ ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
+ arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
+ aes192-ctr,aes256-ctr''
.Ed
.It Cm ClearAllForwardings
Specifies that all local, remote and dynamic port forwardings
The default is 1.
.It Cm ConnectTimeout
Specifies the timeout (in seconds) used when connecting to the ssh
-server, instead of using the default system TCP timeout. This value is
-used only when the target is down or really unreachable, not when it
-refuses the connection.
+server, instead of using the default system TCP timeout.
+This value is used only when the target is down or really unreachable,
+not when it refuses the connection.
+.It Cm ControlMaster
+Enables the sharing of multiple sessions over a single network connection.
+When set to
+.Dq yes
+.Nm ssh
+will listen for connections on a control socket specified using the
+.Cm ControlPath
+argument.
+Additional sessions can connect to this socket using the same
+.Cm ControlPath
+with
+.Cm ControlMaster
+set to
+.Dq no
+(the default).
+These sessions will try to reuse the master instance's network connection
+rather than initiating new ones, but will fall back to connecting normally
+if the control socket does not exist, or is not listening.
+.Pp
+Setting this to
+.Dq ask
+will cause
+.Nm ssh
+to listen for control connections, but require confirmation using the
+.Ev SSH_ASKPASS
+program before they are accepted (see
+.Xr ssh-add 1
+for details).
+If the
+.Cm ControlPath
+can not be opened,
+.Nm ssh
+will continue without connecting to a master instance.
+.Pp
+X11 and
+.Xr ssh-agent 1
+forwarding is supported over these multiplexed connections, however the
+display and agent fowarded will be the one belonging to the master
+connection i.e. it is not possible to forward multiple displays or agents.
+.Pp
+Two additional options allow for opportunistic multiplexing: try to use a
+master connection but fall back to creating a new one if one does not already
+exist.
+These options are:
+.Dq auto
+and
+.Dq autoask .
+The latter requires confirmation like the
+.Dq ask
+option.
+.It Cm ControlPath
+Specify the path to the control socket used for connection sharing as described
+in the
+.Cm ControlMaster
+section above or the string
+.Dq none
+to disable connection sharing.
+In the path,
+.Ql %h
+will be substituted by the target host name,
+.Ql %p
+the port and
+.Ql %r
+by the remote login username.
+It is recommended that any
+.Cm ControlPath
+used for opportunistic connection sharing include
+all three of these escape sequences.
+This ensures that shared connections are uniquely identified.
.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
+.Pp
+The argument must be
+.Sm off
+.Oo Ar bind_address : Oc Ar port .
+.Sm on
+IPv6 addresses can be specified by enclosing addresses in square brackets or
+by using an alternative syntax:
+.Oo Ar bind_address Ns / Oc Ns Ar port .
+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.
+.Pp
+Currently the SOCKS4 and SOCKS5 protocols are supported, and
.Nm ssh
-will act as a SOCKS4 server.
+will act as a SOCKS 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 EnableSSHKeysign
+Setting this option to
+.Dq yes
+in the global client configuration file
+.Pa /etc/ssh/ssh_config
+enables the use of the helper program
+.Xr ssh-keysign 8
+during
+.Cm HostbasedAuthentication .
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq no .
+This option should be placed in the non-hostspecific section.
+See
+.Xr ssh-keysign 8
+for more information.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
.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)
+(for the user's X11 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.
+An attacker may then be able to perform activities such as keystroke monitoring
+if the
+.Cm ForwardX11Trusted
+option is also enabled.
+.It Cm ForwardX11Trusted
+If this option is set to
+.Dq yes
+then remote X11 clients will have full access to the original X11 display.
+.Pp
+If this option is set to
+.Dq no
+then remote X11 clients will be considered untrusted and prevented
+from stealing or tampering with data belonging to trusted X11
+clients.
+Furthermore, the
+.Xr xauth 1
+token used for the session will be set to expire after 20 minutes.
+Remote clients will be refused access after this time.
+.Pp
+The default is
+.Dq no .
+.Pp
+See the X11 SECURITY extension specification for full details on
+the restrictions imposed on untrusted clients.
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
Specifies a file to use for the global
host key database instead of
.Pa /etc/ssh/ssh_known_hosts .
+.It Cm GSSAPIAuthentication
+Specifies whether user authentication based on GSSAPI is allowed.
+The default is
+.Dq no .
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPIDelegateCredentials
+Forward (delegate) credentials to the server.
+The default is
+.Dq no .
+Note that this option applies to protocol version 2 only.
+.It Cm HashKnownHosts
+Indicates that
+.Nm ssh
+should hash host names and addresses when they are added to
+.Pa ~/.ssh/known_hosts .
+These hashed names may be used normally by
+.Nm ssh
+and
+.Nm sshd ,
+but they do not reveal identifying information should the file's contents
+be disclosed.
+The default is
+.Dq no .
+Note that hashing of names and addresses will not be retrospectively applied
+to existing known hosts files, but these may be manually hashed using
+.Xr ssh-keygen 1 .
.It Cm HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key
authentication.
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
+is read.
+The default is
+.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.
Additionally, any identities represented by the authentication agent
will be used for authentication.
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.
-.Pp
-To disable keepalives, the value should be set to
-.Dq no .
-.It Cm KerberosAuthentication
-Specifies whether Kerberos authentication will be used.
+.It Cm IdentitiesOnly
+Specifies that
+.Nm ssh
+should only use the authentication identity files configured in the
+.Nm
+files,
+even if the
+.Nm ssh-agent
+offers more identities.
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
+This option is intented for situations where
+.Nm ssh-agent
+offers many different identities.
+The default is
.Dq no .
+.It Cm KbdInteractiveDevices
+Specifies the list of methods to use in keyboard-interactive authentication.
+Multiple method names must be comma-separated.
+The default is to use the server specified list.
.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.
+The first argument must be
+.Sm off
+.Oo Ar bind_address : Oc Ar port
+.Sm on
+and the second argument must be
+.Ar host : Ns Ar hostport .
+IPv6 addresses can be specified by enclosing addresses in square brackets or
+by using an alternative syntax:
+.Oo Ar bind_address Ns / Oc Ns Ar port
+and
+.Ar host Ns / Ns Ar hostport .
+Multiple forwardings may be specified, and additional forwardings can be
+given on the command line.
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 Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm ssh .
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.
+authentication methods.
+This allows a client to prefer one method (e.g.\&
.Cm keyboard-interactive )
-over another method (e.g.
+over another method (e.g.\&
.Cm password )
The default for this option is:
.Dq hostbased,publickey,keyboard-interactive,password .
.Cm CheckHostIP
is not available for connects with a proxy command.
.Pp
+This directive is useful in conjunction with
+.Xr nc 1
+and its proxy support.
+For example, the following directive would connect via an HTTP proxy at
+192.0.2.0:
+.Bd -literal -offset 3n
+ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
+.Ed
.It Cm PubkeyAuthentication
Specifies whether to try public key authentication.
The argument to this keyword must be
.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 .
+The first argument must be
+.Sm off
+.Oo Ar bind_address : Oc Ar port
+.Sm on
+and the second argument must be
+.Ar host : Ns Ar hostport .
+IPv6 addresses can be specified by enclosing addresses in square brackets
+or by using an alternative syntax:
+.Oo Ar bind_address Ns / Oc Ns Ar port
+and
+.Ar host Ns / Ns Ar hostport .
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 and requires
-.Nm ssh
-to be setuid root and
-.Cm UsePrivilegedPort
-to be set to
-.Dq yes .
+.Pp
+If the
+.Ar bind_address
+is not specified, the default is to only bind to loopback addresses.
+If the
+.Ar bind_address
+is
+.Ql *
+or an empty string, then the forwarding is requested to 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 Cm RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host
authentication.
The default is
.Dq yes .
Note that this option applies to protocol version 1 only.
+.It Cm SendEnv
+Specifies what variables from the local
+.Xr environ 7
+should be sent to the server.
+Note that environment passing is only supported for protocol 2, the
+server must also support it, and the server must be configured to
+accept these environment variables.
+Refer to
+.Cm AcceptEnv
+in
+.Xr sshd_config 5
+for how to configure the server.
+Variables are specified by name, which may contain the wildcard characters
+.Ql \&*
+and
+.Ql \&? .
+Multiple environment variables may be separated by whitespace or spread
+across multiple
+.Cm SendEnv
+directives.
+The default is not to send any environment variables.
+.It Cm ServerAliveInterval
+Sets a timeout interval in seconds after which if no data has been received
+from the server,
+.Nm ssh
+will send a message through the encrypted
+channel to request a response from the server.
+The default
+is 0, indicating that these messages will not be sent to the server.
+This option applies to protocol version 2 only.
+.It Cm ServerAliveCountMax
+Sets the number of server alive messages (see above) which may be
+sent without
+.Nm ssh
+receiving any messages back from the server.
+If this threshold is reached while server alive messages are being sent,
+.Nm ssh
+will disconnect from the server, terminating the session.
+It is important to note that the use of server alive messages is very
+different from
+.Cm TCPKeepAlive
+(below).
+The server alive messages are sent through the encrypted channel
+and therefore will not be spoofable.
+The TCP keepalive option enabled by
+.Cm TCPKeepAlive
+is spoofable.
+The server alive mechanism is valuable when the client or
+server depend on knowing when a connection has become inactive.
+.Pp
+The default value is 3.
+If, for example,
+.Cm ServerAliveInterval
+(above) is set to 15, and
+.Cm ServerAliveCountMax
+is left at the default, if the server becomes unresponsive ssh
+will disconnect after approximately 45 seconds.
.It Cm SmartcardDevice
-Specifies which smartcard device to use. The argument to this keyword is
-the device
+Specifies which smartcard device to use.
+The argument to this keyword is the device
.Nm ssh
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.
+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 ssh
will never automatically add host keys to the
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.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
.Dq ask .
The default is
.Dq ask .
+.It Cm TCPKeepAlive
+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 TCP keepalive messages), 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.
+.Pp
+To disable TCP keepalive messages, the value should be set to
+.Dq no .
.It Cm UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections.
The argument must be
must be setuid root.
Note that this option must be set to
.Dq yes
-if
-.Cm RhostsAuthentication
-and
+for
.Cm RhostsRSAAuthentication
-authentications are needed with older servers.
+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.
.It Cm UserKnownHostsFile
Specifies a file to use for the user
host key database instead of
-.Pa $HOME/.ssh/known_hosts .
+.Pa ~/.ssh/known_hosts .
.It Cm VerifyHostKeyDNS
Specifies whether to verify the remote key using DNS and SSHFP resource
records.
+If this option is set to
+.Dq yes ,
+the client will implicitly trust keys that match a secure fingerprint
+from DNS.
+Insecure fingerprints will be handled as if this option was set to
+.Dq ask .
+If this option is set to
+.Dq ask ,
+information on fingerprint match will be displayed, but the user will still
+need to confirm new host keys according to the
+.Cm StrictHostKeyChecking
+option.
+The argument must be
+.Dq yes ,
+.Dq no
+or
+.Dq ask .
The default is
.Dq no .
+Note that this option applies to protocol version 2 only.
.It Cm XAuthLocation
Specifies the full pathname of the
.Xr xauth 1
.El
.Sh FILES
.Bl -tag -width Ds
-.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 ssh
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.
+Because of the potential for abuse, this file must have strict permissions:
+read/write for the user, and not accessible by others.
.It Pa /etc/ssh/ssh_config
Systemwide configuration file.
This file provides defaults for those
for those users who do not have a configuration file.
This file must be world-readable.
.El
+.Sh SEE ALSO
+.Xr ssh 1
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
-.Sh SEE ALSO
-.Xr ssh 1
andersk / openssh.git / blobdiff