/etc/ssh/ssh_config
DESCRIPTION
ssh obtains configuration data from the following sources in the follow-
ing 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 configu-
ration 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-spe-
cific 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 white-
space when specifying configuration options using the ssh, scp and sftp
-o option.
The possible keywords and their meanings are as follows (note that key-
words are case-insensitive and arguments are case-sensitive):
Host Restricts the following declarations (up to the next Host key-
word) 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
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
``yes''.
Cipher Specifies the cipher to use for encrypting the session in proto-
col 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
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The
default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''
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
command line to clear port forwardings set in configuration
files, and is automatically set by scp(1) and sftp(1). The argu-
ment 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.
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 and
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 dis-
able the escape character entirely (making the connection trans-
parent for binary data).
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 redi-
rected 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 dis-
play 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 the 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
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 con-
necting 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''.
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-rsa,ssh-dss''.
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 data-
base 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. Default is the
name given on the command line. Numeric IP addresses are also
permitted (both on the command line and in HostName specifica-
tions).
IdentityFile
Specifies a file from which the user's RSA or DSA authentication
identity is read. The default is $HOME/.ssh/identity for proto-
col 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 speci-
fied in configuration files; all these identities will be tried
in sequence.
IdentitiesOnly
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
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 com-
mand line. Only the superuser can forward privileged ports.
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 warn-
ings 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. Default is 3.
PasswordAuthentication
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
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,keyboard-interactive,password''.
Protocol
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
Specifies the command to use to connect to the server. The com-
mand 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 some-
where. 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
Specifies whether to try public key authentication. The argument
to this keyword must be ``yes'' or ``no''. The default is
The default is ``no''. This option applies to protocol version 1
only and requires ssh 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 authentica-
tion agent is running. The default is ``yes''. Note that this
option applies to protocol version 1 only.
ServerAliveInterval
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
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 ses-
sion. It is important to note that the use of server alive mes-
sages is very different from TCPKeepAlive (below). The server
alive messages are sent through the encrypted channel and there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion has become inactive.
The default value is 3. If, for example, ServerAliveInterval
(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
Specifies which smartcard device to use. The argument to this
keyword is the device ssh should use to communicate with a smart-
card 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 will never automatically add
host keys to the $HOME/.ssh/known_hosts file, and refuses to con-
nect 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
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
Specifies whether to use a privileged port for outgoing connec-
tions. 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'' for RhostsRSAAuthentication
with older servers.
User Specifies the user to log in as. This can be useful when a dif-
ferent user name is used on different machines. This saves the
trouble of having to remember to give the user name on the com-
mand line.
UserKnownHostsFile
Specifies a file to use for the user host key database instead of
$HOME/.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.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default
is /usr/X11R6/bin/xauth.
FILES
$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. This
file does not usually contain any sensitive information, but the
recommended permissions are 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.
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