ssh_config.5

OpenSSH 是 SSH （Secure SHell） 协议的免费开源实现。它用安全、加密的网络连接工具代替了 telnet、ftp、 rlogin、rsh 和 rcp 工具。OpenSSH 支持

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 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 .
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
.Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar host : port
.Sm on
.Xc .
IPv6 addresses can be specified by enclosing addresses in square brackets or
by using an alternative syntax:
.Sm off
.Xo
.Op Ar bind_address No /
.Ar host No / Ar port
.Xc .
.Sm on
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 .
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).
Setting the command to
.Dq none
disables this option entirely.
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
.Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar host : port
.Sm on
.Xc .
IPv6 addresses can be specified by enclosing any addresses in square brackets
or by using the alternative syntax:
.Sm off
.Xo
.Op Ar bind_address No /
.Ar host No / Ar port
.Xc .
.Sm on
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.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 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 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
.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 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
.Dq yes
or
.Dq no .
The default is
.Dq no .
If set to
.Dq yes
.Nm ssh
must be setuid root.
Note that this option must be set to
.Dq yes
for
.Cm RhostsRSAAuthentication
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 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
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.
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
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 SEE ALSO
.Xr ssh 1
.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.