SSH_CONFIG(4)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/con‐
fig), and system-wide configuration file (/etc/ssh/ssh_config). For
each parameter, the first obtained value is used. The configuration
files contain sections bracketed by Host specifications, and that sec‐
tion 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 com‐
mand 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 white space or
optional whitespace and exactly one equal sign. The latter
format allows you to avoid the need to quote white space
when specifying configuration options using the -o option to
ssh, scp, and sftp.
The possible keywords and their meanings are listed in the following
list.Keywords are case-insensitive and arguments are case-sensitive.
BatchMode
The argument must be yes or no. If set to yes, passphrase/password
querying is 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. This option does not work if
UsePrivilegedPort is set to yes.
CheckHostIP
If this flag is set to yes, ssh additionally checks 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 is not executed.
Cipher
Specifies the cipher to use for encrypting the session in protocol
version 1. Only a single cipher can be specified. Currently, blow‐
fish, 3des, and des are supported. 3des (triple-des) is an encrypt-
decrypt-encrypt triple with three different keys. It is believed to
be secure. blowfish is a fast block cipher. It appears very secure
and is much faster than 3des. 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 discour‐
aged 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 cipher list contains all supported ciphers in this
order:
aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, aes128-cbc,
aes192-cbc, aes256-cbc, arcfour, 3des-cbc,blowfish-cbc
While CBC modes are not considered as secure as other modes in con‐
nection with the SSH protocol 2 they are present at the back of the
default client cipher list for backward compatibility with SSH
servers that do not support other cipher modes.
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. 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.
DisableBanner
If set to yes, disables the display of the banner message. If set
to in-exec-mode, disables the display of banner message when in
remote command mode only.
The default value is no, which means that the banner is displayed
unless the log level is QUIET, FATAL, or ERROR. See also the Banner
option in sshd_config(4). This option applies to protocol version 2
only.
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 [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 can be used to bind the connection to a spe‐
cific 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 inter‐
faces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh
acts as a SOCKS server. Multiple forwardings can be specified and
additional forwardings can be specified on the command line. Only a
user with enough privileges 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(1M) 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) is 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 are 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. See the ForwardX11Trusted
option for more information how to prevent this.
ForwardX11Trusted
If this option is set to yes, remote X11 clients have full access
to the original X11 display. This option is set to yes by default.
If this option is set to no, remote X11 clients are considered
untrusted and prevented from stealing or tampering with data
belonging to trusted X11 clients. Furthermore, the xauth(1) token
used for the session is set to expire after 20 minutes. Remote
clients are refused access after this time.
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 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. 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 principal name to the
target user account.
HashKnownHosts
Indicates that ssh(1), should hash host names and addresses when
they are added to ~/.ssh/known_hosts. These hashed names can be
used normally by ssh(1) and sshd(1M), but they do not reveal iden‐
tifying information should the file's contents be disclosed. The
default is no. Existing names and addresses in known hosts files
are not be converted automatically, but can be manually hashed
using ssh-keygen(1).
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 is used for authentication. The file name can use
the tilde syntax to refer to a user's home directory. It is possi‐
ble to have multiple identity files specified in configuration
files; all these identities is 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.
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 is 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 [bind_address:]port and the second must
be host:port. IPv6 addresses can be specified by enclosing
addresses in square brackets or by using an alternative syntax:
[bind_address/]port and host/port. Multiple forwardings can be
specified and additional forwardings can be given on the command
line. Only a user with enough privileges can forward privileged
ports. By default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit bind_address can 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. 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 lev‐
els 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 refers to a different machine on
each of the machines and the user gets 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 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. This option applies to both proto‐
col 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 is
done using the HostName of the host being connected (defaulting to
the name typed by the user). CheckHostIP is not available for con‐
nects 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.
RekeyLimit
Specifies the maximum amount of data that can 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/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 [bind_address:]port and the
second argument must be host:port. IPv6 addresses can be specified
by enclosing addresses in square brackets or by using an alterna‐
tive syntax: [bind_address/]port and host/port. You can specify
multiple forwardings and give additional forwardings on the command
line. Only a user with enough privileges 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. Spec‐
ifying a remote bind_address only succeeds if the server's Gateway‐
Ports option is enabled. See sshd_config(4).
RhostsAuthentication
Specifies whether to try rhosts-based authentication. This declara‐
tion affects only the client side and has no effect whatsoever on
security. Disabling rhosts authentication can reduce authentica‐
tion time on slow connections when rhosts authentication is not
used. Most servers do not permit RhostsAuthentication because it is
not secure (see RhostsRSAAuthentication). The argument to this key‐
word must be yes or no. This option applies only to the protocol
version 1 and requires that ssh be setuid root and that UsePrivi‐
legedPort 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.
ServerAliveCountMax
Sets the number of server alive messages which can be sent without
ssh(1) receiving messages back from the server. If this threshold
is reached while server alive messages are being sent, ssh discon‐
nects from the server, terminating the session. The use of server
alive messages differs from TCPKeepAlive. Server alive messages are
sent through the encrypted channel and are not spoofable. The TCP
keep alive 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 is set
to 15 and ServerAliveCountMax is left at the default, ssh discon‐
nects in 45-60 seconds if the server becomes unresponsive. 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) sends a message through the
encrypted channel to request a response from the server. The
default is 0, indicating that these messages are not sent to the
server. This option applies to protocol version 2 only.
StrictHostKeyChecking
If this flag is set to yes, ssh never automatically adds host keys
to the $HOME/.ssh/known_hosts file, and refuses 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 are automatically added to the known host
files. The host keys of known hosts are 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 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. Setting
this option to no turns off RhostsAuthentication and RhostsRSAAu‐
thentication. 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 ALSOrsh(1), ssh(1), ssh-http-proxy-connect(1), ssh-keygen(1), ssh-
socks5-proxy-connect(1), sshd(1M), sshd_config(4), kerberos(5)
RFC 4252
Jan 17, 2013 SSH_CONFIG(4)
