ssh_config - OpenSSH SSH client configuration files
$HOME/.ssh/config
/etc/ssh/ssh_config
ssh obtains configuration data from the following sources in
the following
order:
1. command-line options
2. user's configuration file ($HOME/.ssh/config)
3. system-wide configuration file
(/etc/ssh/ssh_config)
For each parameter, the first obtained value will be used.
The configuration
files contain sections bracketed 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.
The configuration file has the following format:
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.
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. `*' and `?' can be used as
wildcards in
the patterns. 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).
AddressFamily [Toc] [Back]
Specifies which address family to use when connecting. Valid arguments
are ``any'', ``inet'' (Use IPv4 only) or
``inet6'' (Use
IPv6 only.)
BatchMode [Toc] [Back]
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 [Toc] [Back]
Specify the interface to transmit from on machines
with multiple
interfaces or aliased addresses. Note that this option does not
work if UsePrivilegedPort is set to ``yes''.
ChallengeResponseAuthentication [Toc] [Back]
Specifies whether to use challenge response authentication. The
argument to this keyword must be ``yes'' or ``no''.
The default
is ``yes''.
CheckHostIP [Toc] [Back]
If this flag is set to ``yes'', ssh 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
``yes''.
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
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 [Toc] [Back]
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'',
``arcfour'', ``blowfish-cbc'', and ``cast128-cbc''.
The default
is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''
ClearAllForwardings [Toc] [Back]
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
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 [Toc] [Back]
Specifies whether to use compression. The argument
must be
``yes'' or ``no''. The default is ``no''.
CompressionLevel [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
Enables the sharing of multiple sessions over a single network
connection. When set to ``yes'' ssh 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 reuse the master instance's network connection
rather than initiating new ones. 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-add(1) for details).
ControlPath [Toc] [Back]
Specify the path to the control socket used for connection sharing.
See ControlMaster above.
DynamicForward [Toc] [Back]
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 and
SOCKS5 protocols are supported, and ssh 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 [Toc] [Back]
Setting this option to ``yes'' in the global client
configuration
file /etc/ssh/ssh_config enables the use of the
helper program
ssh-keysign(8) during HostbasedAuthentication. The
argument must
be ``yes'' or ``no''. The default is ``no''. This
option should
be placed in the non-hostspecific section. See sshkeysign(8)
for more information.
EscapeChar [Toc] [Back]
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).
ForwardAgent [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
If this option is set to ``yes'' then remote X11
clients will
have full access to the original X11 display. If
this option is
set to ``no'' then remote X11 clients will be considered untrusted
and prevented from stealing or tampering with data belonging
to trusted X11 clients.
The default is ``no''.
See the X11 SECURITY extension specification for
full details on
the restrictions imposed on untrusted clients.
GatewayPorts [Toc] [Back]
Specifies whether remote hosts are allowed to connect to local
forwarded ports. By default, ssh 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 [Toc] [Back]
Specifies a file to use for the global host key
database instead
of /etc/ssh/ssh_known_hosts.
GSSAPIAuthentication [Toc] [Back]
Specifies whether user authentication based on GSSAPI is allowed.
The default is ``no''. Note that this option applies to protocol
version 2 only.
GSSAPIDelegateCredentials [Toc] [Back]
Forward (delegate) credentials to the server. The
default is
``no''. Note that this option applies to protocol
version 2 only.
HostbasedAuthentication [Toc] [Back]
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 [Toc] [Back]
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-rsa,ssh-dss''.
HostKeyAlias [Toc] [Back]
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 [Toc] [Back]
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 HostName
specifications).
IdentityFile [Toc] [Back]
Specifies a file from which the user's RSA or DSA
authentication
identity is read. The default is
$HOME/.ssh/identity for protocol
version 1, and $HOME/.ssh/id_rsa and
$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.
IdentitiesOnly [Toc] [Back]
Specifies that ssh should only use the authentication identity
files configured in the ssh_config files, even if
the ssh-agent
offers more identities. The argument to this keyword must be
``yes'' or ``no''. This option is intented for situations where
ssh-agent offers many different identities. The default is
``no''.
LocalForward [Toc] [Back]
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 host:port. IPv6 addresses can be
specified
with an alternative syntax: host/port. Multiple
forwardings may
be specified, and additional forwardings can be given on the command
line. Only the superuser can forward privileged ports.
LogLevel [Toc] [Back]
Gives the verbosity level that is used when logging
messages from
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.
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,hmacsha1,hmacripemd160,hmac-sha1-96,hmac-md5-96''.
NoHostAuthenticationForLocalhost [Toc] [Back]
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 [Toc] [Back]
Specifies the number of password prompts before giving up. The
argument to this keyword must be an integer. Default is 3.
PasswordAuthentication [Toc] [Back]
Specifies whether to use password authentication.
The argument
to this keyword must be ``yes'' or ``no''. The default is
``yes''.
Port Specifies the port number to connect on the remote
host. Default
is 22.
PreferredAuthentications [Toc] [Back]
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: ``hostbased,publickey,keyboardinteractive,password''.
Protocol [Toc] [Back]
Specifies the protocol versions ssh 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 [Toc] [Back]
Specifies the command to use to connect to the server. The command
string extends to the end of the line, and is
executed with
/bin/sh. 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.
PubkeyAuthentication [Toc] [Back]
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.
RemoteForward [Toc] [Back]
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 host:port. IPv6 addresses can be
specified with
an alternative syntax: host/port. Multiple forwardings may be
specified, and additional forwardings can be given
on the command
line. Only the superuser can forward privileged
ports.
RhostsRSAAuthentication [Toc] [Back]
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 to be setuid root.
RSAAuthentication [Toc] [Back]
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 [Toc] [Back]
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_config(5) for how to configure
the server.
Variables are specified by name, which may contain
the wildcard
characters `*' and `?'. Multiple environment variables may be
separated by whitespace or spread across multiple
SendEnv directives.
The default is not to send any environment
variables.
ServerAliveInterval [Toc] [Back]
Sets a timeout interval in seconds after which if no
data has
been received from the server, 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.
ServerAliveCountMax [Toc] [Back]
Sets the number of server alive messages (see above)
which may be
sent without ssh 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, (above) is set to 15, and ServerAliveCountMax is
left at the default,
if the server becomes unresponsive ssh will
disconnect after
approximately 45 seconds.
SmartcardDevice [Toc] [Back]
Specifies which smartcard device to use. The argument to this
keyword is the device 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.
StrictHostKeyChecking [Toc] [Back]
If this flag is set to ``yes'', ssh will never automatically add
host keys to the $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 /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
``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 [Toc] [Back]
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''.
UsePrivilegedPort [Toc] [Back]
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 must be setuid root.
Note that
this option must be set to ``yes'' forRhostsRSAAuthentication [Toc] [Back] 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 [Toc] [Back]
Specifies a file to use for the user host key
database instead of
$HOME/.ssh/known_hosts.
VerifyHostKeyDNS [Toc] [Back]
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 theStrictHostKeyChecking [Toc] [Back] option. The argument must be ``yes'', ``no'' or
``ask''. The
default is ``no''. Note that this option applies to
protocol
version 2 only.
XAuthLocation [Toc] [Back]
Specifies the full pathname of the xauth(1) program.
The default
is /usr/X11R6/bin/xauth.
$HOME/.ssh/config
This is the per-user configuration file. The format
of this file
is described above. This file is used by the ssh
client. Because
of the potential for abuse, this file must
have strict permissions:
read/write for the user, and not accessible by others.
/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.
ssh(1)
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.
OpenBSD 3.6 September 25, 1999
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