ssh_config - OpenSSH SSH client configuration files

NAME

     ssh_config - OpenSSH SSH client configuration files

SYNOPSIS

     ~/.ssh/config
     /etc/ssh/ssh_config

DESCRIPTION

ssh(1) obtains configuration data from the following sources in the
following order:

1. command-line options
2. user’s configuration file (~/.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 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.

Note that the Debian openssh-client package sets several options as
standard in /etc/ssh/ssh_config which are not the default in ssh(1):

· SendEnv LANG LC_*
· HashKnownHosts yes
· GSSAPIAuthentication yes

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.
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 whitespace. 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 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.
In addition, the ServerAliveInterval option will be set to 300
seconds by default. This option is useful in scripts and other
batch jobs where no user is present to supply the password, and
where it is desirable to detect a broken network swiftly. 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
“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(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-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour

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-add(1) for details).
If the ControlPath cannot be opened, ssh will continue without
connecting to a master instance.

X11 and ssh-agent(1) 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-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 ssh-keysign(8) 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 “yes” (Debian-specific).

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.

GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When
using GSSAPI key exchange the server need not have a host key.
The default is “no”. Note that this option applies to protocol
version 2 only.

GSSAPIClientIdentity
If set, specifies the GSSAPI client identity that ssh should use
when connecting to the server. The default is unset, which means
that the default identity will be used.

GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is
“no”. Note that this option applies to protocol version 2
connections using GSSAPI.

GSSAPIRenewalForcesRekey
If set to “yes” then renewal of the client’s GSSAPI credentials
will force the rekeying of the ssh connection. With a compatible
server, this can delegate the renewed credentials to a session on
the server. The default is “no”.

GSSAPITrustDns
Set to “yes to indicate that the DNS is trusted to securely
canonicalize” the name of the host being connected to. If “no,
the hostname entered on the” command line will be passed
untouched to the GSSAPI library. The default is “no”. This
option only applies to protocol version 2 connections using
GSSAPI.

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-keygen(1). Use of this option may
break facilities such as tab-completion that rely on being able
to read unhashed host names from ~/.ssh/known_hosts.

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
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-agent(1)
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. ssh(1)
will try to load certificate information from the filename
obtained by appending -cert.pub to the path of a specified
IdentityFile.

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).

The command is run synchronously and does not have access to the
session of the ssh(1) that spawned it. It should not be used for
interactive commands.

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,umac-64@openssh.com,
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”.

PKCS11Provider
Specifies which PKCS#11 provider to use. The argument to this
keyword is the PKCS#11 shared libary ssh(1) should use to
communicate with a PKCS#11 token providing the user’s private RSA
key.

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:
“gssapi-with-mic,hostbased,publickey,keyboard-interactive,password”.

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. When this option is set to
“2,1” ssh will try version 2 and fall back to version 1 if
version 2 is not available. The default is ‘2’.

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. Privileged ports can be
forwarded only when logging in as root on the remote machine.

If the port argument is ‘0’, the listen port will be dynamically
allocated on the server and reported to the client at run time.

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_config(5)).

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_config(5) 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 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; in protocol version 1 there is no mechanism to
request a response from the server to the server alive messages,
so disconnection is the responsibility of the TCP stack.

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, or 300 if the BatchMode option is set. This option
applies to protocol version 2 only. ProtocolKeepAlives and
SetupTimeOut are Debian-specific compatibility aliases for this
option.

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. This
option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection
dies. As such, you probably want the ServerAliveInterval option
as well. 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”.

UseBlacklistedKeys
Specifies whether ssh(1) should use keys recorded in its
blacklist of known-compromised keys (see ssh-vulnkey(1)) for
authentication. If “yes”, then attempts to use compromised keys
for authentication will be logged but accepted. It is strongly
recommended that this be used only to install new authorized keys
on the remote system, and even then only with the utmost care.
If “no”, then attempts to use compromised keys for authentication
will be prevented. The default is “no”.

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.

PATTERNS

     A pattern consists of zero or more non-whitespace characters, ‘*’ (a
     wildcard that matches zero or more characters), or ‘?’ (a wildcard that
     matches exactly one character).  For example, to specify a set of
     declarations for any host in the “.co.uk” set of domains, the following
     pattern could be used:

           Host *.co.uk

     The following pattern would match any host in the 192.168.0.[0-9] network
     range:

           Host 192.168.0.?

     A pattern-list is a comma-separated list of patterns.  Patterns within
     pattern-lists may be negated by preceding them with an exclamation mark
     (‘!’).  For example, to allow a key to be used from anywhere within an
     organisation except from the “dialup” pool, the following entry (in
     authorized_keys) could be used:

           from="!*.dialup.example.com,*.example.com"

FILES

     ~/.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.  It may be group-writable provided that the group in
             question contains only the user.

     /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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