ssh_config(4) (man pages section 4: File Formats)

man pages section 4: File Formats

ssh_config(4)

Name | Synopsis | Description | See Also

Name

ssh_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 is 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-specific 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:

Empty lines and lines starting with # are comments.
Non-commented lines are of the form:
keyword arguments
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, blowfish, 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 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 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 connection 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 falling 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 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 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.

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 warning 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 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 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 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 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 example). 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 identifying 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 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 is 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 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.

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 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 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-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 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 example, 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 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 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.

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 alternative 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. Specifying a remote bind_address only succeeds if the server's GatewayPorts option is enabled. See sshd_config(4).

RhostsAuthentication

Specifies whether to try rhosts-based authentication. 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 argument 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.

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 disconnects 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 disconnects 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 inconvenience 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 disabled, 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 connections. The argument must be yes or no. The default is yes. 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.