For each parameter, the first obtained value
will be used.
The configuration files contain sections separated by
``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.
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.
Empty lines and lines starting with
`#'
are comments.
Otherwise a line is of the format
``keyword arguments''
Configuration options may be separated by whitespace or
optional whitespace and exactly one
`='
;
the latter format is useful to avoid the need to quote whitespace
when specifying configuration options using the
ssh
scp
and
sftp
-o
option.
Arguments may optionally be enclosed in double quotes
()
in order to represent arguments containing spaces.
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
- Host
-
Restricts the following declarations (up to the next
Host
keyword) to be only for those hosts that match one of the patterns
given after the keyword.
If more than one pattern is provided, they should be separated by whitepsace.
A single
`*'
as a pattern can be used to provide global
defaults for all hosts.
The host is the
hostname
argument given on the command line (i.e. the name is not converted to
a canonicalized host name before matching).
See
Sx PATTERNS
for more information on patterns.
- AddressFamily
-
Specifies which address family to use when connecting.
Valid arguments are
``any''
``inet''
(use IPv4 only), or
``inet6''
(use IPv6 only).
- BatchMode
-
If set to
``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
``yes''
or
``no''
The default is
``no''
- BindAddress
-
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
UsePrivilegedPort
is set to
``yes''
- ChallengeResponseAuthentication
-
Specifies whether to use challenge-response authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''
- CheckHostIP
-
If this flag is set to
``yes''
ssh(1)
will additionally check the host IP address in the
known_hosts
file.
This allows ssh to detect if a host key changed due to DNS spoofing.
If the option is set to
``no''
the check will not be executed.
The default is
``no''
- Cipher
-
Specifies the cipher to use for encrypting the session
in protocol version 1.
Currently,
``blowfish''
``3des''
and
``des''
are supported.
des
is only supported in the
ssh(1)
client for interoperability with legacy protocol 1 implementations
that do not support the
3des
cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
``3des''
- Ciphers
-
Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
The supported ciphers are
``3des-cbc''
``aes128-cbc''
``aes192-cbc''
``aes256-cbc''
``aes128-ctr''
``aes192-ctr''
``aes256-ctr''
``arcfour128''
``arcfour256''
``arcfour''
``blowfish-cbc''
and
``cast128-cbc''
The default is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr
- 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
ssh(1)
command line to clear port forwardings set in
configuration files, and is automatically set by
scp(1)
and
sftp(1).
The argument must be
``yes''
or
``no''
The default is
``no''
- Compression
-
Specifies whether to use compression.
The argument must be
``yes''
or
``no''
The default is
``no''
- 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
gzip(1).
Note that this option applies to protocol version 1 only.
- ConnectionAttempts
-
Specifies the number of tries (one per second) to make before exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
The default is 1.
- 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.
- ControlMaster
-
Enables the sharing of multiple sessions over a single network connection.
When set to
``yes''
ssh(1)
will listen for connections on a control socket specified using the
ControlPath
argument.
Additional sessions can connect to this socket using the same
ControlPath
with
ControlMaster
set to
``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.
Setting this to
``ask''
will cause ssh
to listen for control connections, but require confirmation using the
SSH_ASKPASS
program before they are accepted (see
ssh-add1
for details).
If the
ControlPath
cannot be opened,
ssh will continue without connecting to a master instance.
X11 and
ssh-agent1
forwarding is supported over these multiplexed connections, however the
display and agent forwarded will be the one belonging to the master
connection i.e. it is not possible to forward multiple displays or agents.
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:
``auto''
and
``autoask''
The latter requires confirmation like the
``ask''
option.
- ControlPath
-
Specify the path to the control socket used for connection sharing as described
in the
ControlMaster
section above or the string
``none''
to disable connection sharing.
In the path,
`%l'
will be substituted by the local host name,
`%h'
will be substituted by the target host name,
`%p'
the port, and
`%r'
by the remote login username.
It is recommended that any
ControlPath
used for opportunistic connection sharing include
at least %h, %p, and %r.
This ensures that shared connections are uniquely identified.
- DynamicForward
-
Specifies that a TCP 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
[bind_address : port
]
IPv6 addresses can be specified by enclosing addresses in square brackets or
by using an alternative syntax:
[bind_address / port
]
By default, the local port is bound in accordance with the
GatewayPorts
setting.
However, an explicit
bind_address
may be used to bind the connection to a specific address.
The
bind_address
of
``localhost''
indicates that the listening port be bound for local use only, while an
empty address or
`*'
indicates that the port should be available from all interfaces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and
ssh(1)
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.
- EnableSSHKeysign
-
Setting this option to
``yes''
in the global client configuration file
/etc/ssh/ssh_config
enables the use of the helper program
ssh-keysign8
during
HostbasedAuthentication
The argument must be
``yes''
or
``no''
The default is
``no''
This option should be placed in the non-hostspecific section.
See
ssh-keysign8
for more information.
- EscapeChar
-
Sets the escape character (default:
`~'
) .
The escape character can also
be set on the command line.
The argument should be a single character,
`^'
followed by a letter, or
``none''
to disable the escape
character entirely (making the connection transparent for binary
data).
- ExitOnForwardFailure
-
Specifies whether
ssh(1)
should terminate the connection if it cannot set up all requested
dynamic, tunnel, local, and remote port forwardings.
The argument must be
``yes''
or
``no''
The default is
``no''
- ForwardAgent
-
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument must be
``yes''
or
``no''
The default is
``no''
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.
- ForwardX11
-
Specifies whether X11 connections will be automatically redirected
over the secure channel and
DISPLAY
set.
The argument must be
``yes''
or
``no''
The default is
``no''
X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(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
if the
ForwardX11Trusted
option is also enabled.
- ForwardX11Trusted
-
If this option is set to
``yes''
remote X11 clients will have full access to the original X11 display.
If this option is set to
``no''
remote X11 clients will be considered untrusted and prevented
from stealing or tampering with data belonging to trusted X11
clients.
Furthermore, the
xauth(1)
token used for the session will be set to expire after 20 minutes.
Remote clients will be refused access after this time.
The default is
``no''
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.
- GatewayPorts
-
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
By default,
ssh(1)
binds local port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
GatewayPorts
can be used to specify that ssh
should bind local port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
``yes''
or
``no''
The default is
``no''
- GlobalKnownHostsFile
-
Specifies a file to use for the global
host key database instead of
/etc/ssh/ssh_known_hosts
- GSSAPIAuthentication
-
Specifies whether user authentication based on GSSAPI is allowed.
The default is
``no''
Note that this option applies to protocol version 2 only.
- GSSAPIDelegateCredentials
-
Forward (delegate) credentials to the server.
The default is
``no''
Note that this option applies to protocol version 2 only.
- HashKnownHosts
-
Indicates that
ssh(1)
should hash host names and addresses when they are added to
~/.ssh/known_hosts
These hashed names may be used normally by
ssh(1)
and
sshd(8),
but they do not reveal identifying information should the file's contents
be disclosed.
The default is
``no''
Note that existing names and addresses in known hosts files
will not be converted automatically,
but may be manually hashed using
ssh-keygen1.
- HostbasedAuthentication
-
Specifies whether to try rhosts based authentication with public key
authentication.
The argument must be
``yes''
or
``no''
The default is
``no''
This option applies to protocol version 2 only and
is similar to
RhostsRSAAuthentication
- 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:
``ssh-dss,ssh-rsa''
- 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.
- HostName
-
Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
The default is the name given on the command line.
Numeric IP addresses are also permitted (both on the command line and in
HostName
specifications).
- IdentitiesOnly
-
Specifies that
ssh(1)
should only use the authentication identity files configured in the
~/.ssh/config
files,
even if
ssh-agent1
offers more identities.
The argument to this keyword must be
``yes''
or
``no''
This option is intended for situations where ssh-agent
offers many different identities.
The default is
``no''
- IdentityFile
-
Specifies a file from which the user's RSA or DSA authentication identity
is read.
The default is
~/.ssh/identity
for protocol version 1, and
~/.ssh/id_rsa
and
~/.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 or one of the following
escape characters:
`%d'
(local user's home directory),
`%u'
(local user name),
`%l'
(local host name),
`%h'
(remote host name) or
`%r'
(remote user name).
It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
- KbdInteractiveAuthentication
-
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''
- 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.
The methods available vary depending on what the server supports.
For an OpenSSH server,
it may be zero or more of:
``bsdauth''
``pam''
and
``skey''
- LocalCommand
-
Specifies a command to execute on the local machine after successfully
connecting to the server.
The command string extends to the end of the line, and is executed with
the user's shell.
The following escape character substitutions will be performed:
`%d'
(local user's home directory),
`%h'
(remote host name),
`%l'
(local host name),
`%n'
(host name as provided on the command line),
`%p'
(remote port),
`%r'
(remote user name) or
`%u'
(local user name).
This directive is ignored unless
PermitLocalCommand
has been enabled.
- LocalForward
-
Specifies that a TCP 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
[bind_address : port
]
and the second argument must be
host : hostport
IPv6 addresses can be specified by enclosing addresses in square brackets or
by using an alternative syntax:
[bind_address / port
]
and
host / 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
GatewayPorts
setting.
However, an explicit
bind_address
may be used to bind the connection to a specific address.
The
bind_address
of
``localhost''
indicates that the listening port be bound for local use only, while an
empty address or
`*'
indicates that the port should be available from all interfaces.
- LogLevel
-
Gives the verbosity level that is used when logging messages from
ssh(1).
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.
- 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:
hmac-md5,hmac-sha1,[email protected],
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
- 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
``yes''
or
``no''
The default is to check the host key for localhost.
- NumberOfPasswordPrompts
-
Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
The default is 3.
- PasswordAuthentication
-
Specifies whether to use password authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''
- PermitLocalCommand
-
Allow local command execution via the
LocalCommand
option or using the
! command
escape sequence in
ssh(1).
The argument must be
``yes''
or
``no''
The default is
``no''
- Port
-
Specifies the port number to connect on the remote host.
The default is 22.
- PreferredAuthentications
-
Specifies the order in which the client should try protocol 2
authentication methods.
This allows a client to prefer one method (e.g.
keyboard-interactive
over another method (e.g.
password
The default for this option is:
Do gssapi-with-mic ,
hostbased,
publickey,
keyboard-interactive,
password
Dc .
- Protocol
-
Specifies the protocol versions
ssh(1)
should support in order of preference.
The possible values are
`1'
and
`2'
Multiple versions must be comma-separated.
The default is
``2,1''
This means that ssh
tries version 2 and falls back to version 1
if version 2 is not available.
- 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
the user's shell.
In the command string,
`%h'
will be substituted by the host name to
connect and
`%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
sshd(8)
server running on some machine, or execute
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
``none''
disables this option entirely.
Note that
CheckHostIP
is not available for connects with a proxy command.
This directive is useful in conjunction with
nc(1)
and its proxy support.
For example, the following directive would connect via an HTTP proxy at
192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
- PubkeyAuthentication
-
Specifies whether to try public key authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''
This option applies to protocol version 2 only.
- RekeyLimit
-
Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated.
The argument is the number of bytes, with an optional suffix of
`K'
`M'
or
`G'
to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
The default is between
`1G'
and
`4G'
depending on the cipher.
This option applies to protocol version 2 only.
- RemoteForward
-
Specifies that a TCP 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
[bind_address : port
]
and the second argument must be
host : hostport
IPv6 addresses can be specified by enclosing addresses in square brackets
or by using an alternative syntax:
[bind_address / port
]
and
host / hostport
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
If the
bind_address
is not specified, the default is to only bind to loopback addresses.
If the
bind_address
is
`*'
or an empty string, then the forwarding is requested to listen on all
interfaces.
Specifying a remote
bind_address
will only succeed if the server's
GatewayPorts
option is enabled (see
sshd_config5).
- RhostsRSAAuthentication
-
Specifies whether to try rhosts based authentication with RSA host
authentication.
The argument must be
``yes''
or
``no''
The default is
``no''
This option applies to protocol version 1 only and requires
ssh(1)
to be setuid root.
- RSAAuthentication
-
Specifies whether to try RSA authentication.
The argument to this keyword must be
``yes''
or
``no''
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
The default is
``yes''
Note that this option applies to protocol version 1 only.
- SendEnv
-
Specifies what variables from the local
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
AcceptEnv
in
sshd_config5
for how to configure the server.
Variables are specified by name, which may contain wildcard characters.
Multiple environment variables may be separated by whitespace or spread
across multiple
SendEnv
directives.
The default is not to send any environment variables.
See
Sx PATTERNS
for more information on patterns.
- ServerAliveCountMax
-
Sets the number of server alive messages (see below) which may be
sent without
ssh(1)
receiving any messages back from the server.
If this threshold is reached while server alive messages are being sent,
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
TCPKeepAlive
(below).
The server alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
TCPKeepAlive
is spoofable.
The server alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.
The default value is 3.
If, for example,
ServerAliveInterval
(see below) is set to 15 and
ServerAliveCountMax
is left at the default, if the server becomes unresponsive,
ssh will disconnect after approximately 45 seconds.
This option applies to protocol version 2 only.
- ServerAliveInterval
-
Sets a timeout interval in seconds after which if no data has been received
from the server,
ssh(1)
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.
- SmartcardDevice
-
Specifies which smartcard device to use.
The argument to this keyword is the device
ssh(1)
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.
- StrictHostKeyChecking
-
If this flag is set to
``yes''
ssh(1)
will never automatically add host keys to the
~/.ssh/known_hosts
file, and refuses to connect to hosts whose host key has changed.
This provides maximum protection against trojan horse attacks,
though it can be annoying when the
/etc/ssh/ssh_known_hosts
file is poorly maintained or when connections to new hosts are
frequently made.
This option forces the user to manually
add all new hosts.
If this flag is set to
``no''
ssh will automatically add new host keys to the
user known hosts files.
If this flag is set to
``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
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
``yes''
``no''
or
``ask''
The default is
``ask''
- 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.
The default is
``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.
To disable TCP keepalive messages, the value should be set to
``no''
- Tunnel
-
Request
tun(4)
device forwarding between the client and the server.
The argument must be
``yes''
``point-to-point''
(layer 3),
``ethernet''
(layer 2),
or
``no''
Specifying
``yes''
requests the default tunnel mode, which is
``point-to-point''
The default is
``no''
- TunnelDevice
-
Specifies the
tun(4)
devices to open on the client
(local_tun
)
and the server
(remote_tun
)
The argument must be
local_tun [: remote_tun
]
The devices may be specified by numerical ID or the keyword
``any''
which uses the next available tunnel device.
If
remote_tun
is not specified, it defaults to
``any''
The default is
``any:any''
- UsePrivilegedPort
-
Specifies whether to use a privileged port for outgoing connections.
The argument must be
``yes''
or
``no''
The default is
``no''
If set to
``yes''
ssh(1)
must be setuid root.
Note that this option must be set to
``yes''
for
RhostsRSAAuthentication
with older servers.
- 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.
- UserKnownHostsFile
-
Specifies a file to use for the user
host key database instead of
~/.ssh/known_hosts
- VerifyHostKeyDNS
-
Specifies whether to verify the remote key using DNS and SSHFP resource
records.
If this option is set to
``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
``ask''
If this option is set to
``ask''
information on fingerprint match will be displayed, but the user will still
need to confirm new host keys according to the
StrictHostKeyChecking
option.
The argument must be
``yes''
``no''
or
``ask''
The default is
``no''
Note that this option applies to protocol version 2 only.
See also
Sx VERIFYING HOST KEYS
in
ssh(1).
- VersionAddendum
-
Specifies a string to append to the regular version string to identify
OS- or site-specific modifications.
The default is
``FreeBSD-20080901''
- VisualHostKey
-
If this flag is set to
``yes''
an ASCII art representation of the remote host key fingerprint is
printed additionally to the hex fingerprint string.
If this flag is set to
``no''
only the hex fingerprint string will be printed.
The default is
``no''
- XAuthLocation
-
Specifies the full pathname of the
xauth(1)
program.
The default is
/usr/local/bin/xauth
The following pattern
would match any host in the 192.168.0.[0-9] network range:
