MAN page from Trustix openssh-clients-3.6.1p2-5tr.i586.rpm
Section: File Formats (5)Index
- OpenSSH SSH client configuration files
obtains configuration data from the following sources inthe following order:
- command-line options
- user's configuration file($HOME/.ssh/config)
- system-wide configuration file(/etc/ssh/ssh_config)
For each parameter, the first obtained valuewill be used.The configuration files contain sections bracketed by``Host''specifications, and that section is only applied for hosts thatmatch 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, morehost-specific declarations should be given near the beginning of thefile, 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 oroptional whitespace and exactly one`=';the latter format is useful to avoid the need to quote whitespacewhen specifying configuration options using thessh scpandsftp-ooption.
The possiblekeywords and their meanings are as follows (note thatkeywords are case-insensitive and arguments are case-sensitive):
- Restricts the following declarations (up to the nextHostkeyword) to be only for those hosts that match one of the patternsgiven after the keyword.`*'and`?'can be used as wildcards in thepatterns.A single`*'as a pattern can be used to provide globaldefaults for all hosts.The host is thehostnameargument given on the command line (i.e., the name is not converted toa canonicalized host name before matching).
- 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.
- If set to``yes'' passphrase/password querying will be disabled.This option is useful in scripts and other batch jobs where no useris present to supply the password.The argument must be``yes''or``no'' The default is``no''
- Specify the interface to transmit from on machines with multipleinterfaces or aliased addresses.Note that this option does not work ifUsePrivilegedPortis set to``yes''
- Specifies whether to use challenge response authentication.The argument to this keyword must be``yes''or``no'' The default is``yes''
- If this flag is set to``yes'' ssh will additionally check the host IP address in theknown_hostsfile.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''
- Specifies the cipher to use for encrypting the sessionin protocol version 1.Currently,``blowfish'' ``3des'' and``des''are supported.desis only supported in thesshclient for interoperability with legacy protocol 1 implementationsthat do not support the3descipher.Its use is strongly discouraged due to cryptographic weaknesses.The default is``3des''
- Specifies the ciphers allowed for protocol version 2in order of preference.Multiple ciphers must be comma-separated.The default is
- Specifies that all local, remote and dynamic port forwardingsspecified in the configuration files or on the command line becleared.This option is primarily useful when used from thesshcommand line to clear port forwardings set inconfiguration files, and is automatically set byscp(1)andsftp(1).The argument must be``yes''or``no'' The default is``no''
- Specifies whether to use compression.The argument must be``yes''or``no'' The default is``no''
- 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 ingzip(1).Note that this option applies to protocol version 1 only.
- 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.
- Specifies that a TCP/IP port on the local machine be forwardedover the secure channel, and the applicationprotocol is then used to determine where to connect to from theremote machine.The argument must be a port number.Currently the SOCKS4 protocol is supported, andsshwill act as a SOCKS4 server.Multiple forwardings may be specified, andadditional forwardings can be given on the command line.Only the superuser can forward privileged ports.
- Sets the escape character (default:`~') .The escape character can alsobe set on the command line.The argument should be a single character,`^'followed by a letter, or``none''to disable the escapecharacter entirely (making the connection transparent for binarydata).
- 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 toauthenticate using the identities loaded into the agent.
- Specifies whether X11 connections will be automatically redirectedover the secure channel andDISPLAYset.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.
- Specifies whether remote hosts are allowed to connect to localforwarded ports.By default,sshbinds local port forwardings to the loopback address.This prevents other remote hosts from connecting to forwarded ports.GatewayPortscan be used to specify thatsshshould 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''
- Specifies a file to use for the globalhost key database instead of/etc/ssh/ssh_known_hosts
- Specifies whether to try rhosts based authentication with public keyauthentication.The argument must be``yes''or``no'' The default is``no'' This option applies to protocol version 2 only andis similar toRhostsRSAAuthentication
- Specifies the protocol version 2 host key algorithmsthat the client wants to use in order of preference.The default for this option is:``ssh-rsa,ssh-dss''
- Specifies an alias that should be used instead of thereal host name when looking up or saving the host keyin the host key database files.This option is useful for tunneling ssh connectionsor for multiple servers running on a single host.
- 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 inHostNamespecifications).
- Specifies a file from which the user's RSA or DSA authentication identityis read. The default is$HOME/.ssh/identityfor protocol version 1, and$HOME/.ssh/id_rsaand$HOME/.ssh/id_dsafor protocol version 2.Additionally, any identities represented by the authentication agentwill be used for authentication.The file name may use the tildesyntax to refer to a user's home directory.It is possible to havemultiple identity files specified in configuration files; all theseidentities will be tried in sequence.
- Specifies whether the system should send TCP keepalive messages to theother side.If they are sent, death of the connection or crash of oneof the machines will be properly noticed.However, this means thatconnections will die if the route is down temporarily, and some peoplefind it annoying.
The default is``yes''(to send keepalives), and the client will noticeif 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''
- Specifies whether Kerberos authentication will be used.The argument to this keyword must be``yes''or``no''
- 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''
- Specifies that a TCP/IP port on the local machine be forwarded overthe secure channel to the specified host and port from the remote machine.The first argument must be a port number, and the second must behost:port IPv6 addresses can be specified with an alternative syntax:host/port Multiple forwardings may be specified, and additionalforwardings can be given on the command line.Only the superuser can forward privileged ports.
- Gives the verbosity level that is used when logging messages fromssh 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 2for data integrity protection.Multiple algorithms must be comma-separated.The default is``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''
- 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 ofthe 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.
- Specifies the number of password prompts before giving up.The argument to this keyword must be an integer.Default is 3.
- Specifies whether to use password authentication.The argument to this keyword must be``yes''or``no'' The default is``yes''
- Specifies the port number to connect on the remote host.Default is 22.
- Specifies the order in which the client should try protocol 2authentication 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''
- Specifies the protocol versionssshshould 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 thatsshtries version 2 and falls back to version 1if version 2 is not available.
- Specifies the command to use to connect to the server.The commandstring 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 toconnect 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 ansshd(8)server running on some machine, or executesshd -isomewhere.Host key management will be done using theHostName of the host being connected (defaulting to the name typed bythe user).Setting the command to``none''disables this option entirely.Note thatCheckHostIPis not available for connects with a proxy command.
- 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.
- Specifies that a TCP/IP port on the remote machine be forwarded overthe secure channel to the specified host and port from the local machine.The first argument must be a port number, and the second must behost:port IPv6 addresses can be specified with an alternative syntax:host/port Multiple forwardings may be specified, and additionalforwardings can be given on the command line.Only the superuser can forward privileged ports.
- Specifies whether to try rhosts based authentication.Note that thisdeclaration only affects the client side and has no effect whatsoeveron security.Most servers do not permit RhostsAuthentication because itis not secure (seeRhostsRSAAuthentication ) The argument to this keyword must be``yes''or``no'' The default is``no'' This option applies to protocol version 1 only and requiressshto be setuid root andUsePrivilegedPortto be set to``yes''
- Specifies whether to try rhosts based authentication with RSA hostauthentication.The argument must be``yes''or``no'' The default is``no'' This option applies to protocol version 1 only and requiressshto be setuid root.
- Specifies whether to try RSA authentication.The argument to this keyword must be``yes''or``no'' RSA authentication will only beattempted if the identity file exists, or an authentication agent isrunning.The default is``yes'' Note that this option applies to protocol version 1 only.
- Specifies which smartcard device to use. The argument to this keyword isthe devicesshshould use to communicate with a smartcard used for storing the user'sprivate RSA key. By default, no device is specified and smartcard supportis not activated.
- If this flag is set to``yes'' sshwill never automatically add host keys to the$HOME/.ssh/known_hostsfile, and refuses to connect to hosts whose host key has changed.This provides maximum protection against trojan horse attacks,however, can be annoying when the/etc/ssh/ssh_known_hostsfile is poorly maintained, or connections to new hosts arefrequently made.This option forces the user to manuallyadd all new hosts.If this flag is set to``no'' sshwill automatically add new host keys to theuser known hosts files.If this flag is set to``ask'' new host keyswill be added to the user known host files only after the userhas confirmed that is what they really want to do, andsshwill refuse to connect to hosts whose host key has changed.The host keys ofknown hosts will be verified automatically in all cases.The argument must be``yes'' ``no''or``ask'' The default is``ask''
- Specifies whether to use a privileged port for outgoing connections.The argument must be``yes''or``no'' The default is``no'' If set to``yes''sshmust be setuid root.Note that this option must be set to``yes''ifRhostsAuthenticationandRhostsRSAAuthenticationauthentications are needed with older servers.
- Specifies the user to log in as.This can be useful when a different user name is used on different machines.This saves the trouble ofhaving to remember to give the user name on the command line.
- Specifies a file to use for the userhost key database instead of$HOME/.ssh/known_hosts
- Specifies the full pathname of thexauth(1)program.The default isundefined
- This is the per-user configuration file.The format of this file is described above.This file is used by thesshclient.This file does not usually contain any sensitive information,but the recommended permissions are read/write for the user, and notaccessible by others.
- Systemwide configuration file.This file provides defaults for thosevalues that are not specified in the user's configuration file, andfor those users who do not have a configuration file.This file must be world-readable.
OpenSSH is a derivative of the original and freessh 1.2.12 release by Tatu Ylonen.Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,Theo de Raadt and Dug Songremoved many bugs, re-added newer features andcreated OpenSSH.Markus Friedl contributed the support for SSHprotocol versions 1.5 and 2.0.
- SEE ALSO
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