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 1.153 2011/08/02 01:22:11 djm Exp $
37 .Dd $Mdocdate: August 2 2011 $
42 .Nd OpenSSH SSH client configuration files
45 .Nm /etc/ssh/ssh_config
48 obtains configuration data from the following sources in
51 .Bl -enum -offset indent -compact
55 user's configuration file
58 system-wide configuration file
59 .Pq Pa /etc/ssh/ssh_config
62 For each parameter, the first obtained value
64 The configuration files contain sections separated by
66 specifications, and that section is only applied for hosts that
67 match one of the patterns given in the specification.
68 The matched host name is the one given on the command line.
70 Since the first obtained value for each parameter is used, more
71 host-specific declarations should be given near the beginning of the
72 file, and general defaults at the end.
74 The configuration file has the following format:
76 Empty lines and lines starting with
79 Otherwise a line is of the format
80 .Dq keyword arguments .
81 Configuration options may be separated by whitespace or
82 optional whitespace and exactly one
84 the latter format is useful to avoid the need to quote whitespace
85 when specifying configuration options using the
92 Arguments may optionally be enclosed in double quotes
94 in order to represent arguments containing spaces.
97 keywords and their meanings are as follows (note that
98 keywords are case-insensitive and arguments are case-sensitive):
101 Restricts the following declarations (up to the next
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
105 If more than one pattern is provided, they should be separated by whitespace.
108 as a pattern can be used to provide global
109 defaults for all hosts.
112 argument given on the command line (i.e. the name is not converted to
113 a canonicalized host name before matching).
115 A pattern entry may be negated by prefixing it with an exclamation mark
117 If a negated entry is matched, then the
119 entry is ignored, regardless of whether any other patterns on the line
121 Negated matches are therefore useful to provide exceptions for wildcard
126 for more information on patterns.
128 Specifies which address family to use when connecting.
138 passphrase/password querying will be disabled.
139 This option is useful in scripts and other batch jobs where no user
140 is present to supply the password.
148 Use the specified address on the local machine as the source address of
150 Only useful on systems with more than one address.
151 Note that this option does not work if
152 .Cm UsePrivilegedPort
155 .It Cm ChallengeResponseAuthentication
156 Specifies whether to use challenge-response authentication.
157 The argument to this keyword must be
164 If this flag is set to
167 will additionally check the host IP address in the
170 This allows ssh to detect if a host key changed due to DNS spoofing.
171 If the option is set to
173 the check will not be executed.
177 Specifies the cipher to use for encrypting the session
178 in protocol version 1.
186 is only supported in the
188 client for interoperability with legacy protocol 1 implementations
189 that do not support the
192 Its use is strongly discouraged due to cryptographic weaknesses.
196 Specifies the ciphers allowed for protocol version 2
197 in order of preference.
198 Multiple ciphers must be comma-separated.
199 The supported ciphers are
214 .Bd -literal -offset 3n
215 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
216 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
219 .It Cm ClearAllForwardings
220 Specifies that all local, remote, and dynamic port forwardings
221 specified in the configuration files or on the command line be
223 This option is primarily useful when used from the
225 command line to clear port forwardings set in
226 configuration files, and is automatically set by
237 Specifies whether to use compression.
244 .It Cm CompressionLevel
245 Specifies the compression level to use if compression is enabled.
246 The argument must be an integer from 1 (fast) to 9 (slow, best).
247 The default level is 6, which is good for most applications.
248 The meaning of the values is the same as in
250 Note that this option applies to protocol version 1 only.
251 .It Cm ConnectionAttempts
252 Specifies the number of tries (one per second) to make before exiting.
253 The argument must be an integer.
254 This may be useful in scripts if the connection sometimes fails.
256 .It Cm ConnectTimeout
257 Specifies the timeout (in seconds) used when connecting to the
258 SSH server, instead of using the default system TCP timeout.
259 This value is used only when the target is down or really unreachable,
260 not when it refuses the connection.
262 Enables the sharing of multiple sessions over a single network connection.
266 will listen for connections on a control socket specified using the
269 Additional sessions can connect to this socket using the same
276 These sessions will try to reuse the master instance's network connection
277 rather than initiating new ones, but will fall back to connecting normally
278 if the control socket does not exist, or is not listening.
283 to listen for control connections, but require confirmation using the
285 program before they are accepted (see
291 ssh will continue without connecting to a master instance.
295 forwarding is supported over these multiplexed connections, however the
296 display and agent forwarded will be the one belonging to the master
297 connection i.e. it is not possible to forward multiple displays or agents.
299 Two additional options allow for opportunistic multiplexing: try to use a
300 master connection but fall back to creating a new one if one does not already
306 The latter requires confirmation like the
310 Specify the path to the control socket used for connection sharing as described
313 section above or the string
315 to disable connection sharing.
318 will be substituted by the first component of the local host name,
320 will be substituted by the local host name (including any domain name),
322 will be substituted by the target host name,
324 will be substituted by the original target host name
325 specified on the command line,
329 by the remote login username, and
331 by the username of the user running
333 It is recommended that any
335 used for opportunistic connection sharing include
336 at least %h, %p, and %r.
337 This ensures that shared connections are uniquely identified.
338 .It Cm ControlPersist
339 When used in conjunction with
341 specifies that the master connection should remain open
342 in the background (waiting for future client connections)
343 after the initial client connection has been closed.
346 then the master connection will not be placed into the background,
347 and will close as soon as the initial client connection is closed.
350 then the master connection will remain in the background indefinitely
351 (until killed or closed via a mechanism such as the
355 If set to a time in seconds, or a time in any of the formats documented in
357 then the backgrounded master connection will automatically terminate
358 after it has remained idle (with no client connections) for the
360 .It Cm DynamicForward
361 Specifies that a TCP port on the local machine be forwarded
362 over the secure channel, and the application
363 protocol is then used to determine where to connect to from the
368 .Oo Ar bind_address : Oc Ar port .
370 IPv6 addresses can be specified by enclosing addresses in square brackets.
371 By default, the local port is bound in accordance with the
376 may be used to bind the connection to a specific address.
381 indicates that the listening port be bound for local use only, while an
384 indicates that the port should be available from all interfaces.
386 Currently the SOCKS4 and SOCKS5 protocols are supported, and
388 will act as a SOCKS server.
389 Multiple forwardings may be specified, and
390 additional forwardings can be given on the command line.
391 Only the superuser can forward privileged ports.
392 .It Cm EnableSSHKeysign
393 Setting this option to
395 in the global client configuration file
396 .Pa /etc/ssh/ssh_config
397 enables the use of the helper program
400 .Cm HostbasedAuthentication .
407 This option should be placed in the non-hostspecific section.
410 for more information.
412 Sets the escape character (default:
414 The escape character can also
415 be set on the command line.
416 The argument should be a single character,
418 followed by a letter, or
420 to disable the escape
421 character entirely (making the connection transparent for binary
423 .It Cm ExitOnForwardFailure
426 should terminate the connection if it cannot set up all requested
427 dynamic, tunnel, local, and remote port forwardings.
435 Specifies whether the connection to the authentication agent (if any)
436 will be forwarded to the remote machine.
444 Agent forwarding should be enabled with caution.
445 Users with the ability to bypass file permissions on the remote host
446 (for the agent's Unix-domain socket)
447 can access the local agent through the forwarded connection.
448 An attacker cannot obtain key material from the agent,
449 however they can perform operations on the keys that enable them to
450 authenticate using the identities loaded into the agent.
452 Specifies whether X11 connections will be automatically redirected
453 over the secure channel and
463 X11 forwarding should be enabled with caution.
464 Users with the ability to bypass file permissions on the remote host
465 (for the user's X11 authorization database)
466 can access the local X11 display through the forwarded connection.
467 An attacker may then be able to perform activities such as keystroke monitoring
469 .Cm ForwardX11Trusted
470 option is also enabled.
471 .It Cm ForwardX11Timeout
472 Specify a timeout for untrusted X11 forwarding
473 using the format described in the
477 X11 connections received by
479 after this time will be refused.
480 The default is to disable untrusted X11 forwarding after twenty minutes has
482 .It Cm ForwardX11Trusted
483 If this option is set to
485 remote X11 clients will have full access to the original X11 display.
487 If this option is set to
489 remote X11 clients will be considered untrusted and prevented
490 from stealing or tampering with data belonging to trusted X11
494 token used for the session will be set to expire after 20 minutes.
495 Remote clients will be refused access after this time.
500 See the X11 SECURITY extension specification for full details on
501 the restrictions imposed on untrusted clients.
503 Specifies whether remote hosts are allowed to connect to local
507 binds local port forwardings to the loopback address.
508 This prevents other remote hosts from connecting to forwarded ports.
510 can be used to specify that ssh
511 should bind local port forwardings to the wildcard address,
512 thus allowing remote hosts to connect to forwarded ports.
519 .It Cm GlobalKnownHostsFile
520 Specifies one or more files to use for the global
521 host key database, separated by whitespace.
523 .Pa /etc/ssh/ssh_known_hosts ,
524 .Pa /etc/ssh/ssh_known_hosts2 .
525 .It Cm GSSAPIAuthentication
526 Specifies whether user authentication based on GSSAPI is allowed.
529 Note that this option applies to protocol version 2 only.
530 .It Cm GSSAPIKeyExchange
531 Specifies whether key exchange based on GSSAPI may be used. When using
532 GSSAPI key exchange the server need not have a host key.
535 Note that this option applies to protocol version 2 only.
536 .It Cm GSSAPIClientIdentity
537 If set, specifies the GSSAPI client identity that ssh should use when
538 connecting to the server. The default is unset, which means that the default
539 identity will be used.
540 .It Cm GSSAPIServerIdentity
541 If set, specifies the GSSAPI server identity that ssh should expect when
542 connecting to the server. The default is unset, which means that the
543 expected GSSAPI server identity will be determined from the target
545 .It Cm GSSAPIDelegateCredentials
546 Forward (delegate) credentials to the server.
549 Note that this option applies to protocol version 2 connections using GSSAPI.
550 .It Cm GSSAPIRenewalForcesRekey
553 then renewal of the client's GSSAPI credentials will force the rekeying of the
554 ssh connection. With a compatible server, this can delegate the renewed
555 credentials to a session on the server.
558 .It Cm GSSAPITrustDns
560 .Dq yes to indicate that the DNS is trusted to securely canonicalize
561 the name of the host being connected to. If
562 .Dq no, the hostname entered on the
563 command line will be passed untouched to the GSSAPI library.
566 This option only applies to protocol version 2 connections using GSSAPI.
567 .It Cm HashKnownHosts
570 should hash host names and addresses when they are added to
571 .Pa ~/.ssh/known_hosts .
572 These hashed names may be used normally by
576 but they do not reveal identifying information should the file's contents
580 Note that existing names and addresses in known hosts files
581 will not be converted automatically,
582 but may be manually hashed using
584 .It Cm HostbasedAuthentication
585 Specifies whether to try rhosts based authentication with public key
593 This option applies to protocol version 2 only and
595 .Cm RhostsRSAAuthentication .
596 .It Cm HostKeyAlgorithms
597 Specifies the protocol version 2 host key algorithms
598 that the client wants to use in order of preference.
599 The default for this option is:
600 .Bd -literal -offset 3n
601 ecdsa-sha2-nistp256-cert-v01@openssh.com,
602 ecdsa-sha2-nistp384-cert-v01@openssh.com,
603 ecdsa-sha2-nistp521-cert-v01@openssh.com,
604 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
605 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
606 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
610 If hostkeys are known for the destination host then this default is modified
611 to prefer their algorithms.
613 Specifies an alias that should be used instead of the
614 real host name when looking up or saving the host key
615 in the host key database files.
616 This option is useful for tunneling SSH connections
617 or for multiple servers running on a single host.
619 Specifies the real host name to log into.
620 This can be used to specify nicknames or abbreviations for hosts.
621 If the hostname contains the character sequence
623 then this will be replaced with the host name specified on the command line
624 (this is useful for manipulating unqualified names).
625 The default is the name given on the command line.
626 Numeric IP addresses are also permitted (both on the command line and in
629 .It Cm IdentitiesOnly
632 should only use the authentication identity files configured in the
637 offers more identities.
638 The argument to this keyword must be
642 This option is intended for situations where ssh-agent
643 offers many different identities.
647 Specifies a file from which the user's DSA, ECDSA or DSA authentication
651 for protocol version 1, and
656 for protocol version 2.
657 Additionally, any identities represented by the authentication agent
658 will be used for authentication.
660 will try to load certificate information from the filename obtained by
663 to the path of a specified
666 The file name may use the tilde
667 syntax to refer to a user's home directory or one of the following
670 (local user's home directory),
676 (remote host name) or
680 It is possible to have
681 multiple identity files specified in configuration files; all these
682 identities will be tried in sequence.
685 directives will add to the list of identities tried (this behaviour
686 differs from that of other configuration directives).
688 Specifies the IPv4 type-of-service or DSCP class for connections.
715 This option may take one or two arguments, separated by whitespace.
716 If one argument is specified, it is used as the packet class unconditionally.
717 If two values are specified, the first is automatically selected for
718 interactive sessions and the second for non-interactive sessions.
721 for interactive sessions and
723 for non-interactive sessions.
724 .It Cm KbdInteractiveAuthentication
725 Specifies whether to use keyboard-interactive authentication.
726 The argument to this keyword must be
732 .It Cm KbdInteractiveDevices
733 Specifies the list of methods to use in keyboard-interactive authentication.
734 Multiple method names must be comma-separated.
735 The default is to use the server specified list.
736 The methods available vary depending on what the server supports.
737 For an OpenSSH server,
738 it may be zero or more of:
744 Specifies the available KEX (Key Exchange) algorithms.
745 Multiple algorithms must be comma-separated.
747 .Bd -literal -offset indent
748 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
749 diffie-hellman-group-exchange-sha256,
750 diffie-hellman-group-exchange-sha1,
751 diffie-hellman-group14-sha1,
752 diffie-hellman-group1-sha1
755 Specifies a command to execute on the local machine after successfully
756 connecting to the server.
757 The command string extends to the end of the line, and is executed with
759 The following escape character substitutions will be performed:
761 (local user's home directory),
767 (host name as provided on the command line),
771 (remote user name) or
775 The command is run synchronously and does not have access to the
779 It should not be used for interactive commands.
781 This directive is ignored unless
782 .Cm PermitLocalCommand
785 Specifies that a TCP port on the local machine be forwarded over
786 the secure channel to the specified host and port from the remote machine.
787 The first argument must be
789 .Oo Ar bind_address : Oc Ar port
791 and the second argument must be
792 .Ar host : Ns Ar hostport .
793 IPv6 addresses can be specified by enclosing addresses in square brackets.
794 Multiple forwardings may be specified, and additional forwardings can be
795 given on the command line.
796 Only the superuser can forward privileged ports.
797 By default, the local port is bound in accordance with the
802 may be used to bind the connection to a specific address.
807 indicates that the listening port be bound for local use only, while an
810 indicates that the port should be available from all interfaces.
812 Gives the verbosity level that is used when logging messages from
814 The possible values are:
815 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
817 DEBUG and DEBUG1 are equivalent.
818 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
820 Specifies the MAC (message authentication code) algorithms
821 in order of preference.
822 The MAC algorithm is used in protocol version 2
823 for data integrity protection.
824 Multiple algorithms must be comma-separated.
826 .Bd -literal -offset indent
827 hmac-md5,hmac-sha1,umac-64@openssh.com,
828 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
829 hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,
832 .It Cm NoHostAuthenticationForLocalhost
833 This option can be used if the home directory is shared across machines.
834 In this case localhost will refer to a different machine on each of
835 the machines and the user will get many warnings about changed host keys.
836 However, this option disables host authentication for localhost.
837 The argument to this keyword must be
841 The default is to check the host key for localhost.
842 .It Cm NumberOfPasswordPrompts
843 Specifies the number of password prompts before giving up.
844 The argument to this keyword must be an integer.
846 .It Cm PasswordAuthentication
847 Specifies whether to use password authentication.
848 The argument to this keyword must be
854 .It Cm PermitLocalCommand
855 Allow local command execution via the
858 .Ic !\& Ns Ar command
867 .It Cm PKCS11Provider
868 Specifies which PKCS#11 provider to use.
869 The argument to this keyword is the PKCS#11 shared library
871 should use to communicate with a PKCS#11 token providing the user's
874 Specifies the port number to connect on the remote host.
876 .It Cm PreferredAuthentications
877 Specifies the order in which the client should try protocol 2
878 authentication methods.
879 This allows a client to prefer one method (e.g.\&
880 .Cm keyboard-interactive )
881 over another method (e.g.\&
884 .Bd -literal -offset indent
885 gssapi-with-mic,hostbased,publickey,
886 keyboard-interactive,password
889 Specifies the protocol versions
891 should support in order of preference.
892 The possible values are
896 Multiple versions must be comma-separated.
897 When this option is set to
900 will try version 2 and fall back to version 1
901 if version 2 is not available.
905 Specifies the command to use to connect to the server.
907 string extends to the end of the line, and is executed with
909 In the command string, any occurrence of
911 will be substituted by the host name to
916 by the remote user name.
917 The command can be basically anything,
918 and should read from its standard input and write to its standard output.
919 It should eventually connect an
921 server running on some machine, or execute
924 Host key management will be done using the
925 HostName of the host being connected (defaulting to the name typed by
927 Setting the command to
929 disables this option entirely.
932 is not available for connects with a proxy command.
934 This directive is useful in conjunction with
936 and its proxy support.
937 For example, the following directive would connect via an HTTP proxy at
939 .Bd -literal -offset 3n
940 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
942 .It Cm PubkeyAuthentication
943 Specifies whether to try public key authentication.
944 The argument to this keyword must be
950 This option applies to protocol version 2 only.
952 Specifies the maximum amount of data that may be transmitted before the
953 session key is renegotiated.
954 The argument is the number of bytes, with an optional suffix of
959 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
960 The default is between
964 depending on the cipher.
965 This option applies to protocol version 2 only.
967 Specifies that a TCP port on the remote machine be forwarded over
968 the secure channel to the specified host and port from the local machine.
969 The first argument must be
971 .Oo Ar bind_address : Oc Ar port
973 and the second argument must be
974 .Ar host : Ns Ar hostport .
975 IPv6 addresses can be specified by enclosing addresses in square brackets.
976 Multiple forwardings may be specified, and additional
977 forwardings can be given on the command line.
978 Privileged ports can be forwarded only when
979 logging in as root on the remote machine.
985 the listen port will be dynamically allocated on the server and reported
986 to the client at run time.
990 is not specified, the default is to only bind to loopback addresses.
995 or an empty string, then the forwarding is requested to listen on all
999 will only succeed if the server's
1001 option is enabled (see
1002 .Xr sshd_config 5 ) .
1004 Specifies whether to request a pseudo-tty for the session.
1005 The argument may be one of:
1007 (never request a TTY),
1009 (always request a TTY when standard input is a TTY),
1011 (always request a TTY) or
1013 (request a TTY when opening a login session).
1014 This option mirrors the
1020 .It Cm RhostsRSAAuthentication
1021 Specifies whether to try rhosts based authentication with RSA host
1023 The argument must be
1029 This option applies to protocol version 1 only and requires
1032 .It Cm RSAAuthentication
1033 Specifies whether to try RSA authentication.
1034 The argument to this keyword must be
1038 RSA authentication will only be
1039 attempted if the identity file exists, or an authentication agent is
1043 Note that this option applies to protocol version 1 only.
1045 Specifies what variables from the local
1047 should be sent to the server.
1048 Note that environment passing is only supported for protocol 2.
1049 The server must also support it, and the server must be configured to
1050 accept these environment variables.
1055 for how to configure the server.
1056 Variables are specified by name, which may contain wildcard characters.
1057 Multiple environment variables may be separated by whitespace or spread
1061 The default is not to send any environment variables.
1065 for more information on patterns.
1066 .It Cm ServerAliveCountMax
1067 Sets the number of server alive messages (see below) which may be
1070 receiving any messages back from the server.
1071 If this threshold is reached while server alive messages are being sent,
1072 ssh will disconnect from the server, terminating the session.
1073 It is important to note that the use of server alive messages is very
1077 The server alive messages are sent through the encrypted channel
1078 and therefore will not be spoofable.
1079 The TCP keepalive option enabled by
1082 The server alive mechanism is valuable when the client or
1083 server depend on knowing when a connection has become inactive.
1085 The default value is 3.
1087 .Cm ServerAliveInterval
1088 (see below) is set to 15 and
1089 .Cm ServerAliveCountMax
1090 is left at the default, if the server becomes unresponsive,
1091 ssh will disconnect after approximately 45 seconds.
1092 This option applies to protocol version 2 only.
1093 .It Cm ServerAliveInterval
1094 Sets a timeout interval in seconds after which if no data has been received
1097 will send a message through the encrypted
1098 channel to request a response from the server.
1100 is 0, indicating that these messages will not be sent to the server.
1101 This option applies to protocol version 2 only.
1102 .It Cm StrictHostKeyChecking
1103 If this flag is set to
1106 will never automatically add host keys to the
1107 .Pa ~/.ssh/known_hosts
1108 file, and refuses to connect to hosts whose host key has changed.
1109 This provides maximum protection against trojan horse attacks,
1110 though it can be annoying when the
1111 .Pa /etc/ssh/ssh_known_hosts
1112 file is poorly maintained or when connections to new hosts are
1114 This option forces the user to manually
1116 If this flag is set to
1118 ssh will automatically add new host keys to the
1119 user known hosts files.
1120 If this flag is set to
1123 will be added to the user known host files only after the user
1124 has confirmed that is what they really want to do, and
1125 ssh will refuse to connect to hosts whose host key has changed.
1127 known hosts will be verified automatically in all cases.
1128 The argument must be
1136 Specifies whether the system should send TCP keepalive messages to the
1138 If they are sent, death of the connection or crash of one
1139 of the machines will be properly noticed.
1140 However, this means that
1141 connections will die if the route is down temporarily, and some people
1146 (to send TCP keepalive messages), and the client will notice
1147 if the network goes down or the remote host dies.
1148 This is important in scripts, and many users want it too.
1150 To disable TCP keepalive messages, the value should be set to
1155 device forwarding between the client and the server.
1156 The argument must be
1166 requests the default tunnel mode, which is
1167 .Dq point-to-point .
1173 devices to open on the client
1178 The argument must be
1180 .Ar local_tun Op : Ar remote_tun .
1182 The devices may be specified by numerical ID or the keyword
1184 which uses the next available tunnel device.
1187 is not specified, it defaults to
1191 .It Cm UsePrivilegedPort
1192 Specifies whether to use a privileged port for outgoing connections.
1193 The argument must be
1202 must be setuid root.
1203 Note that this option must be set to
1206 .Cm RhostsRSAAuthentication
1209 Specifies the user to log in as.
1210 This can be useful when a different user name is used on different machines.
1211 This saves the trouble of
1212 having to remember to give the user name on the command line.
1213 .It Cm UserKnownHostsFile
1214 Specifies one or more files to use for the user
1215 host key database, separated by whitespace.
1217 .Pa ~/.ssh/known_hosts ,
1218 .Pa ~/.ssh/known_hosts2 .
1219 .It Cm VerifyHostKeyDNS
1220 Specifies whether to verify the remote key using DNS and SSHFP resource
1222 If this option is set to
1224 the client will implicitly trust keys that match a secure fingerprint
1226 Insecure fingerprints will be handled as if this option was set to
1228 If this option is set to
1230 information on fingerprint match will be displayed, but the user will still
1231 need to confirm new host keys according to the
1232 .Cm StrictHostKeyChecking
1234 The argument must be
1241 Note that this option applies to protocol version 2 only.
1244 .Sx VERIFYING HOST KEYS
1247 .It Cm VisualHostKey
1248 If this flag is set to
1250 an ASCII art representation of the remote host key fingerprint is
1251 printed in addition to the hex fingerprint string at login and
1252 for unknown host keys.
1253 If this flag is set to
1255 no fingerprint strings are printed at login and
1256 only the hex fingerprint string will be printed for unknown host keys.
1259 .It Cm XAuthLocation
1260 Specifies the full pathname of the
1264 .Pa /usr/X11R6/bin/xauth .
1269 consists of zero or more non-whitespace characters,
1271 (a wildcard that matches zero or more characters),
1274 (a wildcard that matches exactly one character).
1275 For example, to specify a set of declarations for any host in the
1278 the following pattern could be used:
1282 The following pattern
1283 would match any host in the 192.168.0.[0-9] network range:
1285 .Dl Host 192.168.0.?
1289 is a comma-separated list of patterns.
1290 Patterns within pattern-lists may be negated
1291 by preceding them with an exclamation mark
1294 to allow a key to be used from anywhere within an organisation
1298 the following entry (in authorized_keys) could be used:
1300 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1303 .It Pa ~/.ssh/config
1304 This is the per-user configuration file.
1305 The format of this file is described above.
1306 This file is used by the SSH client.
1307 Because of the potential for abuse, this file must have strict permissions:
1308 read/write for the user, and not accessible by others.
1309 .It Pa /etc/ssh/ssh_config
1310 Systemwide configuration file.
1311 This file provides defaults for those
1312 values that are not specified in the user's configuration file, and
1313 for those users who do not have a configuration file.
1314 This file must be world-readable.
1319 OpenSSH is a derivative of the original and free
1320 ssh 1.2.12 release by Tatu Ylonen.
1321 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1322 Theo de Raadt and Dug Song
1323 removed many bugs, re-added newer features and
1325 Markus Friedl contributed the support for SSH
1326 protocol versions 1.5 and 2.0.