1. Technology
Linux / Unix Command: ssh_config
Command Library

NAME

ssh_config - OpenSSH SSH client configuration files  

SYNOPSIS

$HOME/.ssh/config
/etc/ssh/ssh_config

 

DESCRIPTION

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).
AFSTokenPassing
Specifies whether to pass AFS tokens to remote host. The argument to this keyword must be ``yes'' or ``no'' This option applies to protocol version 1 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
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
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 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
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 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.
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 protocol is supported, and ssh will act as a SOCKS4 server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
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).
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 X 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.

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 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
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. 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 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.
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 will die if the route is down temporarily, and some people find it annoying.

The default is ``yes'' (to send keepalives), 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 keepalives, the value should be set to ``no''

KerberosAuthentication
Specifies whether Kerberos authentication will be used. The argument to this keyword must be ``yes'' or ``no''
KerberosTgtPassing
Specifies whether a Kerberos TGT will be forwarded to the server. This will only work if the Kerberos server is actually an AFS kaserver. The argument to this keyword must be ``yes'' or ``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 command line. Only the superuser can forward privileged ports.
LogLevel
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,hmac-sha1,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. 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 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). 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 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.
RhostsAuthentication
Specifies whether to try rhosts based authentication. Note that this declaration only affects the client side and has no effect whatsoever on security. Most servers do not permit RhostsAuthentication because it is not secure (see RhostsRSAAuthentication ) The argument to this keyword must be ``yes'' or ``no'' The default is ``no'' This option applies to protocol version 1 only and requires ssh to be setuid root and UsePrivilegedPort to be set to ``yes''
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 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.
SmartcardDevice
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
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''
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 must be setuid root. Note that this option must be set to ``yes'' if RhostsAuthentication and RhostsRSAAuthentication authentications are needed 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 $HOME/.ssh/known_hosts
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default is /usr/X11R6/bin/xauth

 

SEE ALSO

ssh(1)


Important: Use the man command (% man) to see how a command is used on your particular computer.

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