ssh_config(4) File Formats ssh_config(4)NAMEssh_config - ssh configuration file
SYNOPSIS
/etc/ssh/ssh_config
$HOME/.ssh/config
DESCRIPTION
The first ssh_config path, above, provides the system-wide defaults for
ssh(1). The second version is user-specific defaults for ssh.
ssh obtains configuration data from the following sources (in this
order): command line options, user's configuration file
($HOME/.ssh/config), and 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 applied only 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, 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 and syntax:
o Empty lines and lines starting with # are comments.
o Non-commented lines are of the form:
keyword arguments
o Configuration options can be separated by whitespace or
optional whitespace and exactly one equal sign. The latter
format allows you to avoid the need to quote whitespace when
specifying configuration options using the -o option to ssh,
scp, and sftp.
The possible keywords and their meanings are listed below. Note that
keywords are case-insensitive and arguments are case-sensitive.
BatchMode
The argument must be yes or no. If set to yes, passphrase/password
querying will be disabled. This option is useful in scripts and
other batch jobs where you have no user to supply the password.
BindAddress
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.
CheckHostIP
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.
Cipher
Specifies the cipher to use for encrypting the session in protocol
version 1; blowfish and 3des are the only valid values.
Ciphers
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The default
is:
aes128-ctr, aes128-cbc, arcfour, 3des-cbc, blowfish-cbc,
aes192-ctr, aes192-cbc, aes256-ctr, 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 argument must
be yes or no. The default is no.
Compression
Specifies whether to use compression. The argument must be yes or
no. Defaults to 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. kNote
that this option applies to protocol version 1 only.
ConnectionAttempts
Specifies the number of tries (one per second) to make before fall‐
ing back to rsh or exiting. The argument must be an integer. This
can 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 truly unreachable, not when
it refuses the connection.
DynamicForward
Specifies that a TCP/IP port on the local machine be forwarded over
the secure channel. The application protocol is then used to deter‐
mine where to connect to from the remote machine. The argument must
be a port number. Currently the SOCKS4 protocol is supported, and
ssh will act as a SOCKS4 server. Multiple forwardings can be speci‐
fied and additional forwardings can be specified on the command
line. Only the superuser can forward privileged ports.
EscapeChar
Sets the escape character. The default is tilde (~). 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).
FallBackToRsh
Specifies that if connecting with ssh fails due to a connection
refused error (there is no sshd(8) listening on the remote host),
rsh(1) should automatically be used instead (after a suitable warn‐
ing about the session being unencrypted). The argument must be yes
or 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 he can perform operations on the keys that
enable him 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 abil‐
ity to bypass file permissions on the remote host (for the user's X
authorization database) can access the local X11 display through
the forwarded connection. An attacker might then be able to perform
activities such as keystroke monitoring.
GatewayPorts
Specifies whether remote hosts are allowed to connect to local for‐
warded 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
Specifies a file to use instead of /etc/ssh/ssh_known_hosts.
GSSAPIAuthentication
Enables/disables GSS-API user authentication. The default is yes.
GSSAPIDelegateCredentials
Enables/disables GSS-API credential forwarding. The default is no.
GSSAPIKeyExchange
Enables/disables GSS-API-authenticated key exchanges. The default
is yes.
This option is intended primarily to allow users to disable the use
of GSS-API key exchange for SSHv2 when it would otherwise be
selected and then fail (due to server misconfiguration, for exam‐
ple). SSHv2 key exchange failure always results in disconnection.
This option also enables the use of the GSS-API to authenticate the
user to the server after the key exchange. Note that GSS-API key
exchange can succeed but the subsequent authentication using the
GSS-API fail if the server does not authorize the user's GSS prin‐
cipal name to the target user account.
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. An asterisk (*) and a question mark (?) can be
used as wildcards in the patterns. A single asterisk as a pattern
can be used to provide global defaults for all hosts. The host is
the host name argument given on the command line (that is, the name
is not converted to a canonicalized host name before matching).
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 spec‐
ify 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
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 authen‐
tication agent will be used for authentication. The file name can
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.
IgnoreIfUnknown
Specifies a comma-separated list of ssh_config parameters, which,
if unknown to ssh(1), are to be ignored by ssh.
This parameter is primarily intended to be used in the per-user
ssh_config, ~/.ssh/config. While this parameter can also be used in
the system wide /etc/ssh/ssh_config file, it is generally useless
as the capabilities of the ssh(1) client on that host should match
that file.
KeepAlive
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 die if the route is down temporarily, which
can be a source of annoyance.
The default is yes (to send keepalives), which means the client
notices if the network goes down or the remote host dies. This is
important in scripts, and many users want it too. To disable
keepalives, the value should be set to no in both the server and
the client configuration files.
LocalForward
Specifies that a TCP/IP port on the local machine be forwarded over
the secure channel to a given host: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 syn‐
tax: host/port. Multiple forwardings can be specified and addi‐
tional forwardings can be given on the command line. Only the supe‐
ruser can forward privileged ports.
LogLevel
Gives the verbosity level that is used when logging messages from
ssh. The possible values are: FATAL, ERROR, QUIET, 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-sepa‐
rated. The default is hmac-md5,hmac-sha1,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 authentica‐
tion 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 attempts before giving up for password and
keyboard-interactive methods. Attempts for each method are counted
separately. 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. Note that this option applies to
both protocol versions 1 and 2. The default is yes.
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
(for example, keyboard-interactive) over another method (for exam‐
ple, password). The default for this option is: hostbased,pub‐
lickey,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 avail‐
able.
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
/bin/sh. In the command string, %h is substituted by the host name
to connect and %p by the port. The string can be any valid command,
and should read from its standard input and write to its standard
output. It should eventually connect an sshd(1M) 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). 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 yes. This option
applies to protocol version 2 only.
RemoteForward
Specifies that a TCP/IP port on the remote machine be forwarded
over the secure channel to a given host: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 alterna‐
tive syntax: host/port. You can specify multiple forwardings and
give additional forwardings on the command line. Only the superuser
can forward privileged ports.
RhostsAuthentication
Specifies whether to try rhosts-based authentication. Note that
this declaration affects only the client side and has no effect
whatsoever on security. Disabling rhosts authentication can reduce
authentication time on slow connections when rhosts authentication
is not used. Most servers do not permit RhostsAuthentication
because it is not secure (see RhostsRSAAuthentication). The argu‐
ment to this keyword must be yes or no. This option applies only to
the protocol version 1 and requires that ssh be setuid root and
that UsePrivilegedPort be set to yes.
RhostsRSAAuthentication
Specifies whether to try rhosts-based authentication with RSA host
authentication. This is the primary authentication method for most
sites. The argument must be yes or no. This option applies only to
the protocol version 1 and requires that ssh be setuid root and
that UsePrivilegedPort be set to yes.
StrictHostKeyChecking
If this flag is set to yes, ssh will never automatically add host
keys to the $HOME/.ssh/known_hosts file, and will refuse to connect
hosts whose host key has changed. This provides maximum protection
against trojan horse attacks. However, it can be a source of incon‐
venience if you do not have good /etc/ssh/ssh_known_hosts files
installed and frequently connect new hosts. This option forces the
user to manually add any new hosts. Normally this option is dis‐
abled, and new hosts will automatically be added to the known host
files. The host keys of known hosts will be verified automatically
in either case. The argument must be yes or no or ask. The default
is ask.
UseOpenSSLEngine
Specifies whether ssh should use the OpenSSL PKCS#11 engine for
offloading cryptographic operations to the Cryptographic Framework.
Cryptographic operations are accelerated according to the the
available installed plug-ins. When no suitable plug-ins are present
this option does not have an effect. The default is yes.
UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connec‐
tions. The argument must be yes or no. The default is yes. Note
that setting this option to no turns off RhostsAuthentication and
RhostsRSAAuthentication. If set to yes ssh must be setuid root.
Defaults to no.
User
Specifies the user to log in as. This can be useful if you have
different user names on different machines. This saves you the
trouble of having to remember to enter the user name on the command
line.
UserKnownHostsFile
Specifies a file to use instead of $HOME/.ssh/known_hosts.
UseRsh
Specifies that rlogin or rsh should be used for this host. It is
possible that the host does not support the ssh protocol. This
causes ssh to immediately execute rsh(1). All other options (except
HostName) are ignored if this has been specified. The argument must
be yes or no.
XAuthLocation
Specifies the location of the xauth(1) program. The default is
/usr/openwin/bin/xauth.
SEE ALSOssh(1), ssh-http-proxy-connect(1), ssh-socks5-proxy-connect(1),
sshd(1M), sshd_config(4), kerberos(5)SunOS 5.10 5 Jan 2012 ssh_config(4)
