ssh_config.5

OpenSSL Source code for SFTP, SSH, and many others

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.
.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 ssh
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 ssh
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.
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 ssh
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 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.
.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
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 ssh
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 ssh
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 XAuthLocation
Specifies the location of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Sh FILES
.Bl -tag -width Ds
.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 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.
.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.
.El
.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 ssh 1