| .\" -*- nroff -*- |
| .\" |
| .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| .\" All rights reserved |
| .\" |
| .\" As far as I am concerned, the code I have written for this software |
| .\" can be used freely for any purpose. Any derived versions of this |
| .\" software must be clearly marked as such, and if the derived work is |
| .\" incompatible with the protocol description in the RFC file, it must be |
| .\" called by a name other than "ssh" or "Secure Shell". |
| .\" |
| .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| .\" |
| .\" Redistribution and use in source and binary forms, with or without |
| .\" modification, are permitted provided that the following conditions |
| .\" are met: |
| .\" 1. Redistributions of source code must retain the above copyright |
| .\" notice, this list of conditions and the following disclaimer. |
| .\" 2. Redistributions in binary form must reproduce the above copyright |
| .\" notice, this list of conditions and the following disclaimer in the |
| .\" documentation and/or other materials provided with the distribution. |
| .\" |
| .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| .\" |
| .\" $OpenBSD: ssh_config.5,v 1.64 2005/10/30 08:43:47 jmc Exp $ |
| .Dd September 25, 1999 |
| .Dt SSH_CONFIG 5 |
| .Os |
| .Sh NAME |
| .Nm ssh_config |
| .Nd OpenSSH SSH client configuration files |
| .Sh SYNOPSIS |
| .Bl -tag -width Ds -compact |
| .It Pa ~/.ssh/config |
| .It Pa /etc/ssh/ssh_config |
| .El |
| .Sh DESCRIPTION |
| .Nm ssh |
| obtains configuration data from the following sources in |
| the following order: |
| .Bl -enum -offset indent -compact |
| .It |
| command-line options |
| .It |
| user's configuration file |
| .Pq Pa ~/.ssh/config |
| .It |
| system-wide configuration file |
| .Pq Pa /etc/ssh/ssh_config |
| .El |
| .Pp |
| For each parameter, the first obtained value |
| will be used. |
| The configuration files contain sections separated by |
| .Dq 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. |
| .Pp |
| 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. |
| .Pp |
| The configuration file has the following format: |
| .Pp |
| Empty lines and lines starting with |
| .Ql # |
| are comments. |
| .Pp |
| Otherwise a line is of the format |
| .Dq keyword arguments . |
| Configuration options may be separated by whitespace or |
| optional whitespace and exactly one |
| .Ql = ; |
| the latter format is useful to avoid the need to quote whitespace |
| when specifying configuration options using the |
| .Nm ssh , |
| .Nm scp |
| and |
| .Nm sftp |
| .Fl o |
| option. |
| .Pp |
| The possible |
| keywords and their meanings are as follows (note that |
| keywords are case-insensitive and arguments are case-sensitive): |
| .Bl -tag -width Ds |
| .It Cm Host |
| Restricts the following declarations (up to the next |
| .Cm Host |
| keyword) to be only for those hosts that match one of the patterns |
| given after the keyword. |
| .Ql \&* |
| and |
| .Ql \&? |
| can be used as wildcards in the |
| patterns. |
| A single |
| .Ql \&* |
| as a pattern can be used to provide global |
| defaults for all hosts. |
| The host is the |
| .Ar hostname |
| argument given on the command line (i.e., the name is not converted to |
| a canonicalized host name before matching). |
| .It Cm AddressFamily |
| Specifies which address family to use when connecting. |
| Valid arguments are |
| .Dq any , |
| .Dq inet |
| (use IPv4 only) or |
| .Dq inet6 |
| (use IPv6 only). |
| .It Cm BatchMode |
| If set to |
| .Dq 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 |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm BindAddress |
| Use the specified address on the local machine as the source address of |
| the connection. |
| Only useful on systems with more than one address. |
| Note that this option does not work if |
| .Cm UsePrivilegedPort |
| is set to |
| .Dq yes . |
| .It Cm ChallengeResponseAuthentication |
| Specifies whether to use challenge response authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm CheckHostIP |
| If this flag is set to |
| .Dq yes , |
| ssh will additionally check the host IP address in the |
| .Pa known_hosts |
| file. |
| This allows ssh to detect if a host key changed due to DNS spoofing. |
| If the option is set to |
| .Dq no , |
| the check will not be executed. |
| The default is |
| .Dq yes . |
| .It Cm Cipher |
| Specifies the cipher to use for encrypting the session |
| in protocol version 1. |
| Currently, |
| .Dq blowfish , |
| .Dq 3des , |
| and |
| .Dq des |
| are supported. |
| .Ar des |
| is only supported in the |
| .Nm ssh |
| client for interoperability with legacy protocol 1 implementations |
| that do not support the |
| .Ar 3des |
| cipher. |
| Its use is strongly discouraged due to cryptographic weaknesses. |
| The default is |
| .Dq 3des . |
| .It Cm Ciphers |
| Specifies the ciphers allowed for protocol version 2 |
| in order of preference. |
| Multiple ciphers must be comma-separated. |
| The supported ciphers are |
| .Dq 3des-cbc , |
| .Dq aes128-cbc , |
| .Dq aes192-cbc , |
| .Dq aes256-cbc , |
| .Dq aes128-ctr , |
| .Dq aes192-ctr , |
| .Dq aes256-ctr , |
| .Dq arcfour128 , |
| .Dq arcfour256 , |
| .Dq arcfour , |
| .Dq blowfish-cbc , |
| and |
| .Dq cast128-cbc . |
| The default is |
| .Bd -literal |
| ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, |
| arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, |
| aes192-ctr,aes256-ctr'' |
| .Ed |
| .It Cm 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 |
| .Nm ssh |
| command line to clear port forwardings set in |
| configuration files, and is automatically set by |
| .Xr scp 1 |
| and |
| .Xr sftp 1 . |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm Compression |
| Specifies whether to use compression. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm 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 |
| .Xr gzip 1 . |
| Note that this option applies to protocol version 1 only. |
| .It Cm 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. |
| .It Cm 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 really unreachable, |
| not when it refuses the connection. |
| .It Cm ControlMaster |
| Enables the sharing of multiple sessions over a single network connection. |
| When set to |
| .Dq yes |
| .Nm ssh |
| will listen for connections on a control socket specified using the |
| .Cm ControlPath |
| argument. |
| Additional sessions can connect to this socket using the same |
| .Cm ControlPath |
| with |
| .Cm ControlMaster |
| set to |
| .Dq no |
| (the default). |
| These sessions will try to reuse the master instance's network connection |
| rather than initiating new ones, but will fall back to connecting normally |
| if the control socket does not exist, or is not listening. |
| .Pp |
| Setting this to |
| .Dq ask |
| will cause |
| .Nm ssh |
| to listen for control connections, but require confirmation using the |
| .Ev SSH_ASKPASS |
| program before they are accepted (see |
| .Xr ssh-add 1 |
| for details). |
| If the |
| .Cm ControlPath |
| can not be opened, |
| .Nm ssh |
| will continue without connecting to a master instance. |
| .Pp |
| X11 and |
| .Xr ssh-agent 1 |
| forwarding is supported over these multiplexed connections, however the |
| display and agent fowarded will be the one belonging to the master |
| connection i.e. it is not possible to forward multiple displays or agents. |
| .Pp |
| Two additional options allow for opportunistic multiplexing: try to use a |
| master connection but fall back to creating a new one if one does not already |
| exist. |
| These options are: |
| .Dq auto |
| and |
| .Dq autoask . |
| The latter requires confirmation like the |
| .Dq ask |
| option. |
| .It Cm ControlPath |
| Specify the path to the control socket used for connection sharing as described |
| in the |
| .Cm ControlMaster |
| section above or the string |
| .Dq none |
| to disable connection sharing. |
| In the path, |
| .Ql %h |
| will be substituted by the target host name, |
| .Ql %p |
| the port and |
| .Ql %r |
| by the remote login username. |
| It is recommended that any |
| .Cm ControlPath |
| used for opportunistic connection sharing include |
| all three of these escape sequences. |
| This ensures that shared connections are uniquely identified. |
| .It Cm 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. |
| .Pp |
| The argument must be |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port . |
| .Sm on |
| IPv6 addresses can be specified by enclosing addresses in square brackets or |
| by using an alternative syntax: |
| .Oo Ar bind_address Ns / Oc Ns Ar port . |
| By default, the local port is bound in accordance with the |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .Pp |
| Currently the SOCKS4 and SOCKS5 protocols are supported, and |
| .Nm ssh |
| will act as a SOCKS server. |
| Multiple forwardings may be specified, and |
| additional forwardings can be given on the command line. |
| Only the superuser can forward privileged ports. |
| .It Cm EnableSSHKeysign |
| Setting this option to |
| .Dq yes |
| in the global client configuration file |
| .Pa /etc/ssh/ssh_config |
| enables the use of the helper program |
| .Xr ssh-keysign 8 |
| during |
| .Cm HostbasedAuthentication . |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option should be placed in the non-hostspecific section. |
| See |
| .Xr ssh-keysign 8 |
| for more information. |
| .It Cm EscapeChar |
| Sets the escape character (default: |
| .Ql ~ ) . |
| The escape character can also |
| be set on the command line. |
| The argument should be a single character, |
| .Ql ^ |
| followed by a letter, or |
| .Dq none |
| to disable the escape |
| character entirely (making the connection transparent for binary |
| data). |
| .It Cm ForwardAgent |
| Specifies whether the connection to the authentication agent (if any) |
| will be forwarded to the remote machine. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .Pp |
| 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. |
| .It Cm ForwardX11 |
| Specifies whether X11 connections will be automatically redirected |
| over the secure channel and |
| .Ev DISPLAY |
| set. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .Pp |
| X11 forwarding should be enabled with caution. |
| Users with the ability to bypass file permissions on the remote host |
| (for the user's X11 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 |
| if the |
| .Cm ForwardX11Trusted |
| option is also enabled. |
| .It Cm ForwardX11Trusted |
| If this option is set to |
| .Dq yes |
| then remote X11 clients will have full access to the original X11 display. |
| .Pp |
| If this option is set to |
| .Dq no |
| then remote X11 clients will be considered untrusted and prevented |
| from stealing or tampering with data belonging to trusted X11 |
| clients. |
| Furthermore, the |
| .Xr xauth 1 |
| token used for the session will be set to expire after 20 minutes. |
| Remote clients will be refused access after this time. |
| .Pp |
| The default is |
| .Dq no . |
| .Pp |
| See the X11 SECURITY extension specification for full details on |
| the restrictions imposed on untrusted clients. |
| .It Cm GatewayPorts |
| Specifies whether remote hosts are allowed to connect to local |
| forwarded ports. |
| By default, |
| .Nm ssh |
| binds local port forwardings to the loopback address. |
| This prevents other remote hosts from connecting to forwarded ports. |
| .Cm GatewayPorts |
| can be used to specify that |
| .Nm ssh |
| should bind local port forwardings to the wildcard address, |
| thus allowing remote hosts to connect to forwarded ports. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm GlobalKnownHostsFile |
| Specifies a file to use for the global |
| host key database instead of |
| .Pa /etc/ssh/ssh_known_hosts . |
| .It Cm GSSAPIAuthentication |
| Specifies whether user authentication based on GSSAPI is allowed. |
| The default is |
| .Dq no . |
| Note that this option applies to protocol version 2 only. |
| .It Cm GSSAPIDelegateCredentials |
| Forward (delegate) credentials to the server. |
| The default is |
| .Dq no . |
| Note that this option applies to protocol version 2 only. |
| .It Cm HashKnownHosts |
| Indicates that |
| .Nm ssh |
| should hash host names and addresses when they are added to |
| .Pa ~/.ssh/known_hosts . |
| These hashed names may be used normally by |
| .Nm ssh |
| and |
| .Nm sshd , |
| but they do not reveal identifying information should the file's contents |
| be disclosed. |
| The default is |
| .Dq no . |
| Note that hashing of names and addresses will not be retrospectively applied |
| to existing known hosts files, but these may be manually hashed using |
| .Xr ssh-keygen 1 . |
| .It Cm HostbasedAuthentication |
| Specifies whether to try rhosts based authentication with public key |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option applies to protocol version 2 only and |
| is similar to |
| .Cm RhostsRSAAuthentication . |
| .It Cm 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: |
| .Dq ssh-rsa,ssh-dss . |
| .It Cm 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. |
| .It Cm 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 |
| .Cm HostName |
| specifications). |
| .It Cm IdentityFile |
| Specifies a file from which the user's RSA or DSA authentication identity |
| is read. |
| The default is |
| .Pa ~/.ssh/identity |
| for protocol version 1, and |
| .Pa ~/.ssh/id_rsa |
| and |
| .Pa ~/.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. |
| .It Cm IdentitiesOnly |
| Specifies that |
| .Nm ssh |
| should only use the authentication identity files configured in the |
| .Nm |
| files, |
| even if the |
| .Nm ssh-agent |
| offers more identities. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| This option is intented for situations where |
| .Nm ssh-agent |
| offers many different identities. |
| The default is |
| .Dq no . |
| .It Cm KbdInteractiveDevices |
| Specifies the list of methods to use in keyboard-interactive authentication. |
| Multiple method names must be comma-separated. |
| The default is to use the server specified list. |
| .It Cm 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 |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port |
| .Sm on |
| and the second argument must be |
| .Ar host : Ns Ar hostport . |
| IPv6 addresses can be specified by enclosing addresses in square brackets or |
| by using an alternative syntax: |
| .Oo Ar bind_address Ns / Oc Ns Ar port |
| and |
| .Ar host Ns / Ns Ar hostport . |
| Multiple forwardings may be specified, and additional forwardings can be |
| given on the command line. |
| Only the superuser can forward privileged ports. |
| By default, the local port is bound in accordance with the |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .It Cm LogLevel |
| Gives the verbosity level that is used when logging messages from |
| .Nm 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. |
| .It Cm 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 |
| .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
| .It Cm 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 |
| .Dq yes |
| or |
| .Dq no . |
| The default is to check the host key for localhost. |
| .It Cm NumberOfPasswordPrompts |
| Specifies the number of password prompts before giving up. |
| The argument to this keyword must be an integer. |
| Default is 3. |
| .It Cm PasswordAuthentication |
| Specifies whether to use password authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm Port |
| Specifies the port number to connect on the remote host. |
| Default is 22. |
| .It Cm PreferredAuthentications |
| Specifies the order in which the client should try protocol 2 |
| authentication methods. |
| This allows a client to prefer one method (e.g.\& |
| .Cm keyboard-interactive ) |
| over another method (e.g.\& |
| .Cm password ) |
| The default for this option is: |
| .Dq hostbased,publickey,keyboard-interactive,password . |
| .It Cm Protocol |
| Specifies the protocol versions |
| .Nm ssh |
| should support in order of preference. |
| The possible values are |
| .Dq 1 |
| and |
| .Dq 2 . |
| Multiple versions must be comma-separated. |
| The default is |
| .Dq 2,1 . |
| This means that |
| .Nm ssh |
| tries version 2 and falls back to version 1 |
| if version 2 is not available. |
| .It Cm 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 |
| .Pa /bin/sh . |
| In the command string, |
| .Ql %h |
| will be substituted by the host name to |
| connect and |
| .Ql %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 |
| .Xr sshd 8 |
| server running on some machine, or execute |
| .Ic 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). |
| Setting the command to |
| .Dq none |
| disables this option entirely. |
| Note that |
| .Cm CheckHostIP |
| is not available for connects with a proxy command. |
| .Pp |
| This directive is useful in conjunction with |
| .Xr nc 1 |
| and its proxy support. |
| For example, the following directive would connect via an HTTP proxy at |
| 192.0.2.0: |
| .Bd -literal -offset 3n |
| ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p |
| .Ed |
| .It Cm PubkeyAuthentication |
| Specifies whether to try public key authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| This option applies to protocol version 2 only. |
| .It Cm 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 |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port |
| .Sm on |
| and the second argument must be |
| .Ar host : Ns Ar hostport . |
| IPv6 addresses can be specified by enclosing addresses in square brackets |
| or by using an alternative syntax: |
| .Oo Ar bind_address Ns / Oc Ns Ar port |
| and |
| .Ar host Ns / Ns Ar hostport . |
| Multiple forwardings may be specified, and additional |
| forwardings can be given on the command line. |
| Only the superuser can forward privileged ports. |
| .Pp |
| If the |
| .Ar bind_address |
| is not specified, the default is to only bind to loopback addresses. |
| If the |
| .Ar bind_address |
| is |
| .Ql * |
| or an empty string, then the forwarding is requested to listen on all |
| interfaces. |
| Specifying a remote |
| .Ar bind_address |
| will only succeed if the server's |
| .Cm GatewayPorts |
| option is enabled (see |
| .Xr sshd_config 5 ) . |
| .It Cm RhostsRSAAuthentication |
| Specifies whether to try rhosts based authentication with RSA host |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option applies to protocol version 1 only and requires |
| .Nm ssh |
| to be setuid root. |
| .It Cm RSAAuthentication |
| Specifies whether to try RSA authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| RSA authentication will only be |
| attempted if the identity file exists, or an authentication agent is |
| running. |
| The default is |
| .Dq yes . |
| Note that this option applies to protocol version 1 only. |
| .It Cm SendEnv |
| Specifies what variables from the local |
| .Xr environ 7 |
| should be sent to the server. |
| Note that environment passing is only supported for protocol 2, the |
| server must also support it, and the server must be configured to |
| accept these environment variables. |
| Refer to |
| .Cm AcceptEnv |
| in |
| .Xr sshd_config 5 |
| for how to configure the server. |
| Variables are specified by name, which may contain the wildcard characters |
| .Ql \&* |
| and |
| .Ql \&? . |
| Multiple environment variables may be separated by whitespace or spread |
| across multiple |
| .Cm SendEnv |
| directives. |
| The default is not to send any environment variables. |
| .It Cm ServerAliveInterval |
| Sets a timeout interval in seconds after which if no data has been received |
| from the server, |
| .Nm ssh |
| will send a message through the encrypted |
| channel to request a response from the server. |
| The default |
| is 0, indicating that these messages will not be sent to the server. |
| This option applies to protocol version 2 only. |
| .It Cm ServerAliveCountMax |
| Sets the number of server alive messages (see above) which may be |
| sent without |
| .Nm ssh |
| receiving any messages back from the server. |
| If this threshold is reached while server alive messages are being sent, |
| .Nm ssh |
| will disconnect from the server, terminating the session. |
| It is important to note that the use of server alive messages is very |
| different from |
| .Cm TCPKeepAlive |
| (below). |
| The server alive messages are sent through the encrypted channel |
| and therefore will not be spoofable. |
| The TCP keepalive option enabled by |
| .Cm TCPKeepAlive |
| is spoofable. |
| The server alive mechanism is valuable when the client or |
| server depend on knowing when a connection has become inactive. |
| .Pp |
| The default value is 3. |
| If, for example, |
| .Cm ServerAliveInterval |
| (above) is set to 15, and |
| .Cm ServerAliveCountMax |
| is left at the default, if the server becomes unresponsive ssh |
| will disconnect after approximately 45 seconds. |
| .It Cm SmartcardDevice |
| Specifies which smartcard device to use. |
| The argument to this keyword is the device |
| .Nm 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. |
| .It Cm StrictHostKeyChecking |
| If this flag is set to |
| .Dq yes , |
| .Nm ssh |
| will never automatically add host keys to the |
| .Pa ~/.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 |
| .Pa /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 |
| .Dq no , |
| .Nm ssh |
| will automatically add new host keys to the |
| user known hosts files. |
| If this flag is set to |
| .Dq 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 |
| .Nm 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 |
| .Dq yes , |
| .Dq no |
| or |
| .Dq ask . |
| The default is |
| .Dq ask . |
| .It Cm 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 will be properly noticed. |
| However, this means that |
| connections will die if the route is down temporarily, and some people |
| find it annoying. |
| .Pp |
| The default is |
| .Dq yes |
| (to send TCP keepalive messages), 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. |
| .Pp |
| To disable TCP keepalive messages, the value should be set to |
| .Dq no . |
| .It Cm UsePrivilegedPort |
| Specifies whether to use a privileged port for outgoing connections. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| If set to |
| .Dq yes |
| .Nm ssh |
| must be setuid root. |
| Note that this option must be set to |
| .Dq yes |
| for |
| .Cm RhostsRSAAuthentication |
| with older servers. |
| .It Cm 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. |
| .It Cm UserKnownHostsFile |
| Specifies a file to use for the user |
| host key database instead of |
| .Pa ~/.ssh/known_hosts . |
| .It Cm VerifyHostKeyDNS |
| Specifies whether to verify the remote key using DNS and SSHFP resource |
| records. |
| If this option is set to |
| .Dq yes , |
| the client will implicitly trust keys that match a secure fingerprint |
| from DNS. |
| Insecure fingerprints will be handled as if this option was set to |
| .Dq ask . |
| If this option is set to |
| .Dq ask , |
| information on fingerprint match will be displayed, but the user will still |
| need to confirm new host keys according to the |
| .Cm StrictHostKeyChecking |
| option. |
| The argument must be |
| .Dq yes , |
| .Dq no |
| or |
| .Dq ask . |
| The default is |
| .Dq no . |
| Note that this option applies to protocol version 2 only. |
| .It Cm XAuthLocation |
| Specifies the full pathname of the |
| .Xr xauth 1 |
| program. |
| The default is |
| .Pa /usr/X11R6/bin/xauth . |
| .El |
| .Sh FILES |
| .Bl -tag -width Ds |
| .It Pa ~/.ssh/config |
| This is the per-user configuration file. |
| The format of this file is described above. |
| This file is used by the |
| .Nm ssh |
| client. |
| Because of the potential for abuse, this file must have strict permissions: |
| read/write for the user, and not accessible by others. |
| .It Pa /etc/ssh/ssh_config |
| Systemwide configuration file. |
| This file provides defaults for those |
| values that are not specified in the user's configuration file, and |
| for those users who do not have a configuration file. |
| This file must be world-readable. |
| .El |
| .Sh SEE ALSO |
| .Xr ssh 1 |
| .Sh AUTHORS |
| OpenSSH is a derivative of the original and free |
| ssh 1.2.12 release by Tatu Ylonen. |
| Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| Theo de Raadt and Dug Song |
| removed many bugs, re-added newer features and |
| created OpenSSH. |
| Markus Friedl contributed the support for SSH |
| protocol versions 1.5 and 2.0. |