2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: ssh_config.5,v 1.166 2013/06/27 14:05:37 jmc Exp $
43 .Nd OpenSSH SSH client configuration files
46 .Nm /etc/ssh/ssh_config
49 obtains configuration data from the following sources in
52 .Bl -enum -offset indent -compact
56 user's configuration file
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
63 For each parameter, the first obtained value
65 The configuration files contain sections separated by
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
75 The configuration file has the following format:
77 Empty lines and lines starting with
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
93 Arguments may optionally be enclosed in double quotes
95 in order to represent arguments containing spaces.
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
102 Restricts the following declarations (up to the next
104 keyword) to be only for those hosts that match one of the patterns
105 given after the keyword.
106 If more than one pattern is provided, they should be separated by whitespace.
109 as a pattern can be used to provide global
110 defaults for all hosts.
113 argument given on the command line (i.e. the name is not converted to
114 a canonicalized host name before matching).
116 A pattern entry may be negated by prefixing it with an exclamation mark
118 If a negated entry is matched, then the
120 entry is ignored, regardless of whether any other patterns on the line
122 Negated matches are therefore useful to provide exceptions for wildcard
127 for more information on patterns.
129 Specifies which address family to use when connecting.
139 passphrase/password querying will be disabled.
140 This option is useful in scripts and other batch jobs where no user
141 is present to supply the password.
149 Use the specified address on the local machine as the source address of
151 Only useful on systems with more than one address.
152 Note that this option does not work if
153 .Cm UsePrivilegedPort
156 .It Cm ChallengeResponseAuthentication
157 Specifies whether to use challenge-response authentication.
158 The argument to this keyword must be
165 If this flag is set to
168 will additionally check the host IP address in the
171 This allows ssh to detect if a host key changed due to DNS spoofing.
172 If the option is set to
174 the check will not be executed.
178 Specifies the cipher to use for encrypting the session
179 in protocol version 1.
187 is only supported in the
189 client for interoperability with legacy protocol 1 implementations
190 that do not support the
193 Its use is strongly discouraged due to cryptographic weaknesses.
197 Specifies the ciphers allowed for protocol version 2
198 in order of preference.
199 Multiple ciphers must be comma-separated.
200 The supported ciphers are
208 .Dq aes128-gcm@openssh.com ,
209 .Dq aes256-gcm@openssh.com ,
217 .Bd -literal -offset 3n
218 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
219 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
220 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
223 .It Cm ClearAllForwardings
224 Specifies that all local, remote, and dynamic port forwardings
225 specified in the configuration files or on the command line be
227 This option is primarily useful when used from the
229 command line to clear port forwardings set in
230 configuration files, and is automatically set by
241 Specifies whether to use compression.
248 .It Cm CompressionLevel
249 Specifies the compression level to use if compression is enabled.
250 The argument must be an integer from 1 (fast) to 9 (slow, best).
251 The default level is 6, which is good for most applications.
252 The meaning of the values is the same as in
254 Note that this option applies to protocol version 1 only.
255 .It Cm ConnectionAttempts
256 Specifies the number of tries (one per second) to make before exiting.
257 The argument must be an integer.
258 This may be useful in scripts if the connection sometimes fails.
260 .It Cm ConnectTimeout
261 Specifies the timeout (in seconds) used when connecting to the
262 SSH server, instead of using the default system TCP timeout.
263 This value is used only when the target is down or really unreachable,
264 not when it refuses the connection.
266 Enables the sharing of multiple sessions over a single network connection.
270 will listen for connections on a control socket specified using the
273 Additional sessions can connect to this socket using the same
280 These sessions will try to reuse the master instance's network connection
281 rather than initiating new ones, but will fall back to connecting normally
282 if the control socket does not exist, or is not listening.
287 to listen for control connections, but require confirmation using the
289 program before they are accepted (see
295 ssh will continue without connecting to a master instance.
299 forwarding is supported over these multiplexed connections, however the
300 display and agent forwarded will be the one belonging to the master
301 connection i.e. it is not possible to forward multiple displays or agents.
303 Two additional options allow for opportunistic multiplexing: try to use a
304 master connection but fall back to creating a new one if one does not already
310 The latter requires confirmation like the
314 Specify the path to the control socket used for connection sharing as described
317 section above or the string
319 to disable connection sharing.
322 will be substituted by the first component of the local host name,
324 will be substituted by the local host name (including any domain name),
326 will be substituted by the target host name,
328 will be substituted by the original target host name
329 specified on the command line,
333 by the remote login username, and
335 by the username of the user running
337 It is recommended that any
339 used for opportunistic connection sharing include
340 at least %h, %p, and %r.
341 This ensures that shared connections are uniquely identified.
342 .It Cm ControlPersist
343 When used in conjunction with
345 specifies that the master connection should remain open
346 in the background (waiting for future client connections)
347 after the initial client connection has been closed.
350 then the master connection will not be placed into the background,
351 and will close as soon as the initial client connection is closed.
354 then the master connection will remain in the background indefinitely
355 (until killed or closed via a mechanism such as the
359 If set to a time in seconds, or a time in any of the formats documented in
361 then the backgrounded master connection will automatically terminate
362 after it has remained idle (with no client connections) for the
364 .It Cm DynamicForward
365 Specifies that a TCP port on the local machine be forwarded
366 over the secure channel, and the application
367 protocol is then used to determine where to connect to from the
372 .Oo Ar bind_address : Oc Ar port .
374 IPv6 addresses can be specified by enclosing addresses in square brackets.
375 By default, the local port is bound in accordance with the
380 may be used to bind the connection to a specific address.
385 indicates that the listening port be bound for local use only, while an
388 indicates that the port should be available from all interfaces.
390 Currently the SOCKS4 and SOCKS5 protocols are supported, and
392 will act as a SOCKS server.
393 Multiple forwardings may be specified, and
394 additional forwardings can be given on the command line.
395 Only the superuser can forward privileged ports.
396 .It Cm EnableSSHKeysign
397 Setting this option to
399 in the global client configuration file
400 .Pa /etc/ssh/ssh_config
401 enables the use of the helper program
404 .Cm HostbasedAuthentication .
411 This option should be placed in the non-hostspecific section.
414 for more information.
416 Sets the escape character (default:
418 The escape character can also
419 be set on the command line.
420 The argument should be a single character,
422 followed by a letter, or
424 to disable the escape
425 character entirely (making the connection transparent for binary
427 .It Cm ExitOnForwardFailure
430 should terminate the connection if it cannot set up all requested
431 dynamic, tunnel, local, and remote port forwardings.
439 Specifies whether the connection to the authentication agent (if any)
440 will be forwarded to the remote machine.
448 Agent forwarding should be enabled with caution.
449 Users with the ability to bypass file permissions on the remote host
450 (for the agent's Unix-domain socket)
451 can access the local agent through the forwarded connection.
452 An attacker cannot obtain key material from the agent,
453 however they can perform operations on the keys that enable them to
454 authenticate using the identities loaded into the agent.
456 Specifies whether X11 connections will be automatically redirected
457 over the secure channel and
467 X11 forwarding should be enabled with caution.
468 Users with the ability to bypass file permissions on the remote host
469 (for the user's X11 authorization database)
470 can access the local X11 display through the forwarded connection.
471 An attacker may then be able to perform activities such as keystroke monitoring
473 .Cm ForwardX11Trusted
474 option is also enabled.
475 .It Cm ForwardX11Timeout
476 Specify a timeout for untrusted X11 forwarding
477 using the format described in the
478 TIME FORMATS section of
480 X11 connections received by
482 after this time will be refused.
483 The default is to disable untrusted X11 forwarding after twenty minutes has
485 .It Cm ForwardX11Trusted
486 If this option is set to
488 remote X11 clients will have full access to the original X11 display.
490 If this option is set to
492 remote X11 clients will be considered untrusted and prevented
493 from stealing or tampering with data belonging to trusted X11
497 token used for the session will be set to expire after 20 minutes.
498 Remote clients will be refused access after this time.
503 See the X11 SECURITY extension specification for full details on
504 the restrictions imposed on untrusted clients.
506 Specifies whether remote hosts are allowed to connect to local
510 binds local port forwardings to the loopback address.
511 This prevents other remote hosts from connecting to forwarded ports.
513 can be used to specify that ssh
514 should bind local port forwardings to the wildcard address,
515 thus allowing remote hosts to connect to forwarded ports.
522 .It Cm GlobalKnownHostsFile
523 Specifies one or more files to use for the global
524 host key database, separated by whitespace.
526 .Pa /etc/ssh/ssh_known_hosts ,
527 .Pa /etc/ssh/ssh_known_hosts2 .
528 .It Cm GSSAPIAuthentication
529 Specifies whether user authentication based on GSSAPI is allowed.
532 Note that this option applies to protocol version 2 only.
533 .It Cm GSSAPIDelegateCredentials
534 Forward (delegate) credentials to the server.
537 Note that this option applies to protocol version 2 only.
538 .It Cm HashKnownHosts
541 should hash host names and addresses when they are added to
542 .Pa ~/.ssh/known_hosts .
543 These hashed names may be used normally by
547 but they do not reveal identifying information should the file's contents
551 Note that existing names and addresses in known hosts files
552 will not be converted automatically,
553 but may be manually hashed using
555 .It Cm HostbasedAuthentication
556 Specifies whether to try rhosts based authentication with public key
564 This option applies to protocol version 2 only and
566 .Cm RhostsRSAAuthentication .
567 .It Cm HostKeyAlgorithms
568 Specifies the protocol version 2 host key algorithms
569 that the client wants to use in order of preference.
570 The default for this option is:
571 .Bd -literal -offset 3n
572 ecdsa-sha2-nistp256-cert-v01@openssh.com,
573 ecdsa-sha2-nistp384-cert-v01@openssh.com,
574 ecdsa-sha2-nistp521-cert-v01@openssh.com,
575 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
576 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
577 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
581 If hostkeys are known for the destination host then this default is modified
582 to prefer their algorithms.
584 Specifies an alias that should be used instead of the
585 real host name when looking up or saving the host key
586 in the host key database files.
587 This option is useful for tunneling SSH connections
588 or for multiple servers running on a single host.
590 Specifies the real host name to log into.
591 This can be used to specify nicknames or abbreviations for hosts.
592 If the hostname contains the character sequence
594 then this will be replaced with the host name specified on the command line
595 (this is useful for manipulating unqualified names).
596 The default is the name given on the command line.
597 Numeric IP addresses are also permitted (both on the command line and in
600 .It Cm IdentitiesOnly
603 should only use the authentication identity files configured in the
610 offers more identities.
611 The argument to this keyword must be
615 This option is intended for situations where ssh-agent
616 offers many different identities.
620 Specifies a file from which the user's DSA, ECDSA or RSA authentication
624 for protocol version 1, and
629 for protocol version 2.
630 Additionally, any identities represented by the authentication agent
631 will be used for authentication unless
635 will try to load certificate information from the filename obtained by
638 to the path of a specified
641 The file name may use the tilde
642 syntax to refer to a user's home directory or one of the following
645 (local user's home directory),
651 (remote host name) or
655 It is possible to have
656 multiple identity files specified in configuration files; all these
657 identities will be tried in sequence.
660 directives will add to the list of identities tried (this behaviour
661 differs from that of other configuration directives).
664 may be used in conjunction with
666 to select which identities in an agent are offered during authentication.
668 Specifies a pattern-list of unknown options to be ignored if they are
669 encountered in configuration parsing.
670 This may be used to suppress errors if
672 contains options that are unrecognised by
674 It is recommended that
676 be listed early in the configuration file as it will not be applied
677 to unknown options that appear before it.
679 Specifies the IPv4 type-of-service or DSCP class for connections.
706 This option may take one or two arguments, separated by whitespace.
707 If one argument is specified, it is used as the packet class unconditionally.
708 If two values are specified, the first is automatically selected for
709 interactive sessions and the second for non-interactive sessions.
712 for interactive sessions and
714 for non-interactive sessions.
715 .It Cm KbdInteractiveAuthentication
716 Specifies whether to use keyboard-interactive authentication.
717 The argument to this keyword must be
723 .It Cm KbdInteractiveDevices
724 Specifies the list of methods to use in keyboard-interactive authentication.
725 Multiple method names must be comma-separated.
726 The default is to use the server specified list.
727 The methods available vary depending on what the server supports.
728 For an OpenSSH server,
729 it may be zero or more of:
735 Specifies the available KEX (Key Exchange) algorithms.
736 Multiple algorithms must be comma-separated.
738 .Bd -literal -offset indent
739 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
740 diffie-hellman-group-exchange-sha256,
741 diffie-hellman-group-exchange-sha1,
742 diffie-hellman-group14-sha1,
743 diffie-hellman-group1-sha1
746 Specifies a command to execute on the local machine after successfully
747 connecting to the server.
748 The command string extends to the end of the line, and is executed with
750 The following escape character substitutions will be performed:
752 (local user's home directory),
758 (host name as provided on the command line),
762 (remote user name) or
766 The command is run synchronously and does not have access to the
770 It should not be used for interactive commands.
772 This directive is ignored unless
773 .Cm PermitLocalCommand
776 Specifies that a TCP port on the local machine be forwarded over
777 the secure channel to the specified host and port from the remote machine.
778 The first argument must be
780 .Oo Ar bind_address : Oc Ar port
782 and the second argument must be
783 .Ar host : Ns Ar hostport .
784 IPv6 addresses can be specified by enclosing addresses in square brackets.
785 Multiple forwardings may be specified, and additional forwardings can be
786 given on the command line.
787 Only the superuser can forward privileged ports.
788 By default, the local port is bound in accordance with the
793 may be used to bind the connection to a specific address.
798 indicates that the listening port be bound for local use only, while an
801 indicates that the port should be available from all interfaces.
803 Gives the verbosity level that is used when logging messages from
805 The possible values are:
806 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
808 DEBUG and DEBUG1 are equivalent.
809 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
811 Specifies the MAC (message authentication code) algorithms
812 in order of preference.
813 The MAC algorithm is used in protocol version 2
814 for data integrity protection.
815 Multiple algorithms must be comma-separated.
816 The algorithms that contain
818 calculate the MAC after encryption (encrypt-then-mac).
819 These are considered safer and their use recommended.
821 .Bd -literal -offset indent
822 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
823 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
824 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
825 hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
826 hmac-md5-96-etm@openssh.com,
827 hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
828 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
829 hmac-sha1-96,hmac-md5-96
831 .It Cm NoHostAuthenticationForLocalhost
832 This option can be used if the home directory is shared across machines.
833 In this case localhost will refer to a different machine on each of
834 the machines and the user will get many warnings about changed host keys.
835 However, this option disables host authentication for localhost.
836 The argument to this keyword must be
840 The default is to check the host key for localhost.
841 .It Cm NumberOfPasswordPrompts
842 Specifies the number of password prompts before giving up.
843 The argument to this keyword must be an integer.
845 .It Cm PasswordAuthentication
846 Specifies whether to use password authentication.
847 The argument to this keyword must be
853 .It Cm PermitLocalCommand
854 Allow local command execution via the
857 .Ic !\& Ns Ar command
866 .It Cm PKCS11Provider
867 Specifies which PKCS#11 provider to use.
868 The argument to this keyword is the PKCS#11 shared library
870 should use to communicate with a PKCS#11 token providing the user's
873 Specifies the port number to connect on the remote host.
875 .It Cm PreferredAuthentications
876 Specifies the order in which the client should try protocol 2
877 authentication methods.
878 This allows a client to prefer one method (e.g.\&
879 .Cm keyboard-interactive )
880 over another method (e.g.\&
883 .Bd -literal -offset indent
884 gssapi-with-mic,hostbased,publickey,
885 keyboard-interactive,password
888 Specifies the protocol versions
890 should support in order of preference.
891 The possible values are
895 Multiple versions must be comma-separated.
896 When this option is set to
899 will try version 2 and fall back to version 1
900 if version 2 is not available.
904 Specifies the command to use to connect to the server.
906 string extends to the end of the line, and is executed with
908 In the command string, any occurrence of
910 will be substituted by the host name to
915 by the remote user name.
916 The command can be basically anything,
917 and should read from its standard input and write to its standard output.
918 It should eventually connect an
920 server running on some machine, or execute
923 Host key management will be done using the
924 HostName of the host being connected (defaulting to the name typed by
926 Setting the command to
928 disables this option entirely.
931 is not available for connects with a proxy command.
933 This directive is useful in conjunction with
935 and its proxy support.
936 For example, the following directive would connect via an HTTP proxy at
938 .Bd -literal -offset 3n
939 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
941 .It Cm PubkeyAuthentication
942 Specifies whether to try public key authentication.
943 The argument to this keyword must be
949 This option applies to protocol version 2 only.
951 Specifies the maximum amount of data that may be transmitted before the
952 session key is renegotiated, optionally followed a maximum amount of
953 time that may pass before the session key is renegotiated.
954 The first argument is specified in bytes and may have a suffix of
959 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
960 The default is between
964 depending on the cipher.
965 The optional second value is specified in seconds and may use any of the
966 units documented in the
967 TIME FORMATS section of
969 The default value for
973 which means that rekeying is performed after the cipher's default amount
974 of data has been sent or received and no time based rekeying is done.
975 This option applies to protocol version 2 only.
977 Specifies that a TCP port on the remote machine be forwarded over
978 the secure channel to the specified host and port from the local machine.
979 The first argument must be
981 .Oo Ar bind_address : Oc Ar port
983 and the second argument must be
984 .Ar host : Ns Ar hostport .
985 IPv6 addresses can be specified by enclosing addresses in square brackets.
986 Multiple forwardings may be specified, and additional
987 forwardings can be given on the command line.
988 Privileged ports can be forwarded only when
989 logging in as root on the remote machine.
995 the listen port will be dynamically allocated on the server and reported
996 to the client at run time.
1000 is not specified, the default is to only bind to loopback addresses.
1005 or an empty string, then the forwarding is requested to listen on all
1009 will only succeed if the server's
1011 option is enabled (see
1012 .Xr sshd_config 5 ) .
1014 Specifies whether to request a pseudo-tty for the session.
1015 The argument may be one of:
1017 (never request a TTY),
1019 (always request a TTY when standard input is a TTY),
1021 (always request a TTY) or
1023 (request a TTY when opening a login session).
1024 This option mirrors the
1030 .It Cm RhostsRSAAuthentication
1031 Specifies whether to try rhosts based authentication with RSA host
1033 The argument must be
1039 This option applies to protocol version 1 only and requires
1042 .It Cm RSAAuthentication
1043 Specifies whether to try RSA authentication.
1044 The argument to this keyword must be
1048 RSA authentication will only be
1049 attempted if the identity file exists, or an authentication agent is
1053 Note that this option applies to protocol version 1 only.
1055 Specifies what variables from the local
1057 should be sent to the server.
1058 Note that environment passing is only supported for protocol 2.
1059 The server must also support it, and the server must be configured to
1060 accept these environment variables.
1065 for how to configure the server.
1066 Variables are specified by name, which may contain wildcard characters.
1067 Multiple environment variables may be separated by whitespace or spread
1071 The default is not to send any environment variables.
1075 for more information on patterns.
1076 .It Cm ServerAliveCountMax
1077 Sets the number of server alive messages (see below) which may be
1080 receiving any messages back from the server.
1081 If this threshold is reached while server alive messages are being sent,
1082 ssh will disconnect from the server, terminating the session.
1083 It is important to note that the use of server alive messages is very
1087 The server alive messages are sent through the encrypted channel
1088 and therefore will not be spoofable.
1089 The TCP keepalive option enabled by
1092 The server alive mechanism is valuable when the client or
1093 server depend on knowing when a connection has become inactive.
1095 The default value is 3.
1097 .Cm ServerAliveInterval
1098 (see below) is set to 15 and
1099 .Cm ServerAliveCountMax
1100 is left at the default, if the server becomes unresponsive,
1101 ssh will disconnect after approximately 45 seconds.
1102 This option applies to protocol version 2 only.
1103 .It Cm ServerAliveInterval
1104 Sets a timeout interval in seconds after which if no data has been received
1107 will send a message through the encrypted
1108 channel to request a response from the server.
1110 is 0, indicating that these messages will not be sent to the server.
1111 This option applies to protocol version 2 only.
1112 .It Cm StrictHostKeyChecking
1113 If this flag is set to
1116 will never automatically add host keys to the
1117 .Pa ~/.ssh/known_hosts
1118 file, and refuses to connect to hosts whose host key has changed.
1119 This provides maximum protection against trojan horse attacks,
1120 though it can be annoying when the
1121 .Pa /etc/ssh/ssh_known_hosts
1122 file is poorly maintained or when connections to new hosts are
1124 This option forces the user to manually
1126 If this flag is set to
1128 ssh will automatically add new host keys to the
1129 user known hosts files.
1130 If this flag is set to
1133 will be added to the user known host files only after the user
1134 has confirmed that is what they really want to do, and
1135 ssh will refuse to connect to hosts whose host key has changed.
1137 known hosts will be verified automatically in all cases.
1138 The argument must be
1146 Specifies whether the system should send TCP keepalive messages to the
1148 If they are sent, death of the connection or crash of one
1149 of the machines will be properly noticed.
1150 However, this means that
1151 connections will die if the route is down temporarily, and some people
1156 (to send TCP keepalive messages), and the client will notice
1157 if the network goes down or the remote host dies.
1158 This is important in scripts, and many users want it too.
1160 To disable TCP keepalive messages, the value should be set to
1165 device forwarding between the client and the server.
1166 The argument must be
1176 requests the default tunnel mode, which is
1177 .Dq point-to-point .
1183 devices to open on the client
1188 The argument must be
1190 .Ar local_tun Op : Ar remote_tun .
1192 The devices may be specified by numerical ID or the keyword
1194 which uses the next available tunnel device.
1197 is not specified, it defaults to
1201 .It Cm UsePrivilegedPort
1202 Specifies whether to use a privileged port for outgoing connections.
1203 The argument must be
1212 must be setuid root.
1213 Note that this option must be set to
1216 .Cm RhostsRSAAuthentication
1219 Specifies the user to log in as.
1220 This can be useful when a different user name is used on different machines.
1221 This saves the trouble of
1222 having to remember to give the user name on the command line.
1223 .It Cm UserKnownHostsFile
1224 Specifies one or more files to use for the user
1225 host key database, separated by whitespace.
1227 .Pa ~/.ssh/known_hosts ,
1228 .Pa ~/.ssh/known_hosts2 .
1229 .It Cm VerifyHostKeyDNS
1230 Specifies whether to verify the remote key using DNS and SSHFP resource
1232 If this option is set to
1234 the client will implicitly trust keys that match a secure fingerprint
1236 Insecure fingerprints will be handled as if this option was set to
1238 If this option is set to
1240 information on fingerprint match will be displayed, but the user will still
1241 need to confirm new host keys according to the
1242 .Cm StrictHostKeyChecking
1244 The argument must be
1251 if compiled with LDNS and
1254 Note that this option applies to protocol version 2 only.
1256 See also VERIFYING HOST KEYS in
1258 .It Cm VersionAddendum
1259 Specifies a string to append to the regular version string to identify
1260 OS- or site-specific modifications.
1262 .Dq FreeBSD-20131111 .
1263 .It Cm VisualHostKey
1264 If this flag is set to
1266 an ASCII art representation of the remote host key fingerprint is
1267 printed in addition to the hex fingerprint string at login and
1268 for unknown host keys.
1269 If this flag is set to
1271 no fingerprint strings are printed at login and
1272 only the hex fingerprint string will be printed for unknown host keys.
1275 .It Cm XAuthLocation
1276 Specifies the full pathname of the
1280 .Pa /usr/local/bin/xauth .
1285 consists of zero or more non-whitespace characters,
1287 (a wildcard that matches zero or more characters),
1290 (a wildcard that matches exactly one character).
1291 For example, to specify a set of declarations for any host in the
1294 the following pattern could be used:
1298 The following pattern
1299 would match any host in the 192.168.0.[0-9] network range:
1301 .Dl Host 192.168.0.?
1305 is a comma-separated list of patterns.
1306 Patterns within pattern-lists may be negated
1307 by preceding them with an exclamation mark
1310 to allow a key to be used from anywhere within an organisation
1314 the following entry (in authorized_keys) could be used:
1316 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1319 .It Pa ~/.ssh/config
1320 This is the per-user configuration file.
1321 The format of this file is described above.
1322 This file is used by the SSH client.
1323 Because of the potential for abuse, this file must have strict permissions:
1324 read/write for the user, and not accessible by others.
1325 .It Pa /etc/ssh/ssh_config
1326 Systemwide configuration file.
1327 This file provides defaults for those
1328 values that are not specified in the user's configuration file, and
1329 for those users who do not have a configuration file.
1330 This file must be world-readable.
1335 OpenSSH is a derivative of the original and free
1336 ssh 1.2.12 release by Tatu Ylonen.
1337 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1338 Theo de Raadt and Dug Song
1339 removed many bugs, re-added newer features and
1341 Markus Friedl contributed the support for SSH
1342 protocol versions 1.5 and 2.0.