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 1.146 2010/12/08 04:02:47 djm Exp $
37 .Dd $Mdocdate: December 8 2010 $
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.
76 package sets several options as standard in
77 .Pa /etc/ssh/ssh_config
78 which are not the default in
81 .Bl -bullet -offset indent -compact
83 .Cm SendEnv No LANG LC_*
85 .Cm HashKnownHosts No yes
87 .Cm GSSAPIAuthentication No yes
90 The configuration file has the following format:
92 Empty lines and lines starting with
95 Otherwise a line is of the format
96 .Dq keyword arguments .
97 Configuration options may be separated by whitespace or
98 optional whitespace and exactly one
100 the latter format is useful to avoid the need to quote whitespace
101 when specifying configuration options using the
108 Arguments may optionally be enclosed in double quotes
110 in order to represent arguments containing spaces.
113 keywords and their meanings are as follows (note that
114 keywords are case-insensitive and arguments are case-sensitive):
117 Restricts the following declarations (up to the next
119 keyword) to be only for those hosts that match one of the patterns
120 given after the keyword.
121 If more than one pattern is provided, they should be separated by whitespace.
124 as a pattern can be used to provide global
125 defaults for all hosts.
128 argument given on the command line (i.e. the name is not converted to
129 a canonicalized host name before matching).
133 for more information on patterns.
135 Specifies which address family to use when connecting.
145 passphrase/password querying will be disabled.
147 .Cm ServerAliveInterval
148 option will be set to 300 seconds by default.
149 This option is useful in scripts and other batch jobs where no user
150 is present to supply the password,
151 and where it is desirable to detect a broken network swiftly.
159 Use the specified address on the local machine as the source address of
161 Only useful on systems with more than one address.
162 Note that this option does not work if
163 .Cm UsePrivilegedPort
166 .It Cm ChallengeResponseAuthentication
167 Specifies whether to use challenge-response authentication.
168 The argument to this keyword must be
175 If this flag is set to
178 will additionally check the host IP address in the
181 This allows ssh to detect if a host key changed due to DNS spoofing.
182 If the option is set to
184 the check will not be executed.
188 Specifies the cipher to use for encrypting the session
189 in protocol version 1.
197 is only supported in the
199 client for interoperability with legacy protocol 1 implementations
200 that do not support the
203 Its use is strongly discouraged due to cryptographic weaknesses.
207 Specifies the ciphers allowed for protocol version 2
208 in order of preference.
209 Multiple ciphers must be comma-separated.
210 The supported ciphers are
225 .Bd -literal -offset 3n
226 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
227 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
230 .It Cm ClearAllForwardings
231 Specifies that all local, remote, and dynamic port forwardings
232 specified in the configuration files or on the command line be
234 This option is primarily useful when used from the
236 command line to clear port forwardings set in
237 configuration files, and is automatically set by
248 Specifies whether to use compression.
255 .It Cm CompressionLevel
256 Specifies the compression level to use if compression is enabled.
257 The argument must be an integer from 1 (fast) to 9 (slow, best).
258 The default level is 6, which is good for most applications.
259 The meaning of the values is the same as in
261 Note that this option applies to protocol version 1 only.
262 .It Cm ConnectionAttempts
263 Specifies the number of tries (one per second) to make before exiting.
264 The argument must be an integer.
265 This may be useful in scripts if the connection sometimes fails.
267 .It Cm ConnectTimeout
268 Specifies the timeout (in seconds) used when connecting to the
269 SSH server, instead of using the default system TCP timeout.
270 This value is used only when the target is down or really unreachable,
271 not when it refuses the connection.
273 Enables the sharing of multiple sessions over a single network connection.
277 will listen for connections on a control socket specified using the
280 Additional sessions can connect to this socket using the same
287 These sessions will try to reuse the master instance's network connection
288 rather than initiating new ones, but will fall back to connecting normally
289 if the control socket does not exist, or is not listening.
294 to listen for control connections, but require confirmation using the
296 program before they are accepted (see
302 ssh will continue without connecting to a master instance.
306 forwarding is supported over these multiplexed connections, however the
307 display and agent forwarded will be the one belonging to the master
308 connection i.e. it is not possible to forward multiple displays or agents.
310 Two additional options allow for opportunistic multiplexing: try to use a
311 master connection but fall back to creating a new one if one does not already
317 The latter requires confirmation like the
321 Specify the path to the control socket used for connection sharing as described
324 section above or the string
326 to disable connection sharing.
329 will be substituted by the local host name,
331 will be substituted by the target host name,
335 by the remote login username.
336 It is recommended that any
338 used for opportunistic connection sharing include
339 at least %h, %p, and %r.
340 This ensures that shared connections are uniquely identified.
341 .It Cm ControlPersist
342 When used in conjunction with
344 specifies that the master connection should remain open
345 in the background (waiting for future client connections)
346 after the initial client connection has been closed.
349 then the master connection will not be placed into the background,
350 and will close as soon as the initial client connection is closed.
353 then the master connection will remain in the background indefinitely
354 (until killed or closed via a mechanism such as the
358 If set to a time in seconds, or a time in any of the formats documented in
360 then the backgrounded master connection will automatically terminate
361 after it has remained idle (with no client connections) for the
363 .It Cm DynamicForward
364 Specifies that a TCP port on the local machine be forwarded
365 over the secure channel, and the application
366 protocol is then used to determine where to connect to from the
371 .Oo Ar bind_address : Oc Ar port .
373 IPv6 addresses can be specified by enclosing addresses in square brackets.
374 By default, the local port is bound in accordance with the
379 may be used to bind the connection to a specific address.
384 indicates that the listening port be bound for local use only, while an
387 indicates that the port should be available from all interfaces.
389 Currently the SOCKS4 and SOCKS5 protocols are supported, and
391 will act as a SOCKS server.
392 Multiple forwardings may be specified, and
393 additional forwardings can be given on the command line.
394 Only the superuser can forward privileged ports.
395 .It Cm EnableSSHKeysign
396 Setting this option to
398 in the global client configuration file
399 .Pa /etc/ssh/ssh_config
400 enables the use of the helper program
403 .Cm HostbasedAuthentication .
410 This option should be placed in the non-hostspecific section.
413 for more information.
415 Sets the escape character (default:
417 The escape character can also
418 be set on the command line.
419 The argument should be a single character,
421 followed by a letter, or
423 to disable the escape
424 character entirely (making the connection transparent for binary
426 .It Cm ExitOnForwardFailure
429 should terminate the connection if it cannot set up all requested
430 dynamic, tunnel, local, and remote port forwardings.
438 Specifies whether the connection to the authentication agent (if any)
439 will be forwarded to the remote machine.
447 Agent forwarding should be enabled with caution.
448 Users with the ability to bypass file permissions on the remote host
449 (for the agent's Unix-domain socket)
450 can access the local agent through the forwarded connection.
451 An attacker cannot obtain key material from the agent,
452 however they can perform operations on the keys that enable them to
453 authenticate using the identities loaded into the agent.
455 Specifies whether X11 connections will be automatically redirected
456 over the secure channel and
466 X11 forwarding should be enabled with caution.
467 Users with the ability to bypass file permissions on the remote host
468 (for the user's X11 authorization database)
469 can access the local X11 display through the forwarded connection.
470 An attacker may then be able to perform activities such as keystroke monitoring
472 .Cm ForwardX11Trusted
473 option is also enabled.
474 .It Cm ForwardX11Timeout
475 Specify a timeout for untrusted X11 forwarding
476 using the format described in the
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.
504 See the X11 SECURITY extension specification for full details on
505 the restrictions imposed on untrusted clients.
507 Specifies whether remote hosts are allowed to connect to local
511 binds local port forwardings to the loopback address.
512 This prevents other remote hosts from connecting to forwarded ports.
514 can be used to specify that ssh
515 should bind local port forwardings to the wildcard address,
516 thus allowing remote hosts to connect to forwarded ports.
523 .It Cm GlobalKnownHostsFile
524 Specifies a file to use for the global
525 host key database instead of
526 .Pa /etc/ssh/ssh_known_hosts .
527 .It Cm GSSAPIAuthentication
528 Specifies whether user authentication based on GSSAPI is allowed.
531 Note that this option applies to protocol version 2 only.
532 .It Cm GSSAPIKeyExchange
533 Specifies whether key exchange based on GSSAPI may be used. When using
534 GSSAPI key exchange the server need not have a host key.
537 Note that this option applies to protocol version 2 only.
538 .It Cm GSSAPIClientIdentity
539 If set, specifies the GSSAPI client identity that ssh should use when
540 connecting to the server. The default is unset, which means that the default
541 identity will be used.
542 .It Cm GSSAPIServerIdentity
543 If set, specifies the GSSAPI server identity that ssh should expect when
544 connecting to the server. The default is unset, which means that the
545 expected GSSAPI server identity will be determined from the target
547 .It Cm GSSAPIDelegateCredentials
548 Forward (delegate) credentials to the server.
551 Note that this option applies to protocol version 2 connections using GSSAPI.
552 .It Cm GSSAPIRenewalForcesRekey
555 then renewal of the client's GSSAPI credentials will force the rekeying of the
556 ssh connection. With a compatible server, this can delegate the renewed
557 credentials to a session on the server.
560 .It Cm GSSAPITrustDns
562 .Dq yes to indicate that the DNS is trusted to securely canonicalize
563 the name of the host being connected to. If
564 .Dq no, the hostname entered on the
565 command line will be passed untouched to the GSSAPI library.
568 This option only applies to protocol version 2 connections using GSSAPI.
569 .It Cm HashKnownHosts
572 should hash host names and addresses when they are added to
573 .Pa ~/.ssh/known_hosts .
574 These hashed names may be used normally by
578 but they do not reveal identifying information should the file's contents
582 Note that existing names and addresses in known hosts files
583 will not be converted automatically,
584 but may be manually hashed using
586 Use of this option may break facilities such as tab-completion that rely
587 on being able to read unhashed host names from
588 .Pa ~/.ssh/known_hosts .
589 .It Cm HostbasedAuthentication
590 Specifies whether to try rhosts based authentication with public key
598 This option applies to protocol version 2 only and
600 .Cm RhostsRSAAuthentication .
601 .It Cm HostKeyAlgorithms
602 Specifies the protocol version 2 host key algorithms
603 that the client wants to use in order of preference.
604 The default for this option is:
605 .Bd -literal -offset 3n
606 ecdsa-sha2-nistp256-cert-v01@openssh.com,
607 ecdsa-sha2-nistp384-cert-v01@openssh.com,
608 ecdsa-sha2-nistp521-cert-v01@openssh.com,
609 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
610 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
611 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
615 If hostkeys are known for the destination host then this default is modified
616 to prefer their algorithms.
618 Specifies an alias that should be used instead of the
619 real host name when looking up or saving the host key
620 in the host key database files.
621 This option is useful for tunneling SSH connections
622 or for multiple servers running on a single host.
624 Specifies the real host name to log into.
625 This can be used to specify nicknames or abbreviations for hosts.
626 If the hostname contains the character sequence
628 then this will be replaced with the host name specified on the commandline
629 (this is useful for manipulating unqualified names).
630 The default is the name given on the command line.
631 Numeric IP addresses are also permitted (both on the command line and in
634 .It Cm IdentitiesOnly
637 should only use the authentication identity files configured in the
642 offers more identities.
643 The argument to this keyword must be
647 This option is intended for situations where ssh-agent
648 offers many different identities.
652 Specifies a file from which the user's DSA, ECDSA or DSA authentication
656 for protocol version 1, and
661 for protocol version 2.
662 Additionally, any identities represented by the authentication agent
663 will be used for authentication.
665 will try to load certificate information from the filename obtained by
668 to the path of a specified
671 The file name may use the tilde
672 syntax to refer to a user's home directory or one of the following
675 (local user's home directory),
681 (remote host name) or
685 It is possible to have
686 multiple identity files specified in configuration files; all these
687 identities will be tried in sequence.
689 Specifies the IPv4 type-of-service or DSCP class for connections.
716 This option may take one or two arguments, separated by whitespace.
717 If one argument is specified, it is used as the packet class unconditionally.
718 If two values are specified, the first is automatically selected for
719 interactive sessions and the second for non-interactive sessions.
722 for interactive sessions and
724 for non-interactive sessions.
725 .It Cm KbdInteractiveAuthentication
726 Specifies whether to use keyboard-interactive authentication.
727 The argument to this keyword must be
733 .It Cm KbdInteractiveDevices
734 Specifies the list of methods to use in keyboard-interactive authentication.
735 Multiple method names must be comma-separated.
736 The default is to use the server specified list.
737 The methods available vary depending on what the server supports.
738 For an OpenSSH server,
739 it may be zero or more of:
745 Specifies the available KEX (Key Exchange) algorithms.
746 Multiple algorithms must be comma-separated.
748 .Bd -literal -offset indent
749 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
750 diffie-hellman-group-exchange-sha256,
751 diffie-hellman-group-exchange-sha1,
752 diffie-hellman-group14-sha1,
753 diffie-hellman-group1-sha1
756 Specifies a command to execute on the local machine after successfully
757 connecting to the server.
758 The command string extends to the end of the line, and is executed with
760 The following escape character substitutions will be performed:
762 (local user's home directory),
768 (host name as provided on the command line),
772 (remote user name) or
776 The command is run synchronously and does not have access to the
780 It should not be used for interactive commands.
782 This directive is ignored unless
783 .Cm PermitLocalCommand
786 Specifies that a TCP port on the local machine be forwarded over
787 the secure channel to the specified host and port from the remote machine.
788 The first argument must be
790 .Oo Ar bind_address : Oc Ar port
792 and the second argument must be
793 .Ar host : Ns Ar hostport .
794 IPv6 addresses can be specified by enclosing addresses in square brackets.
795 Multiple forwardings may be specified, and additional forwardings can be
796 given on the command line.
797 Only the superuser can forward privileged ports.
798 By default, the local port is bound in accordance with the
803 may be used to bind the connection to a specific address.
808 indicates that the listening port be bound for local use only, while an
811 indicates that the port should be available from all interfaces.
813 Gives the verbosity level that is used when logging messages from
815 The possible values are:
816 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
818 DEBUG and DEBUG1 are equivalent.
819 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
821 Specifies the MAC (message authentication code) algorithms
822 in order of preference.
823 The MAC algorithm is used in protocol version 2
824 for data integrity protection.
825 Multiple algorithms must be comma-separated.
827 .Bd -literal -offset indent
828 hmac-md5,hmac-sha1,umac-64@openssh.com,
829 hmac-ripemd160,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.
953 The argument is the number of bytes, with an optional suffix of
958 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
959 The default is between
963 depending on the cipher.
964 This option applies to protocol version 2 only.
966 Specifies that a TCP port on the remote machine be forwarded over
967 the secure channel to the specified host and port from the local machine.
968 The first argument must be
970 .Oo Ar bind_address : Oc Ar port
972 and the second argument must be
973 .Ar host : Ns Ar hostport .
974 IPv6 addresses can be specified by enclosing addresses in square brackets.
975 Multiple forwardings may be specified, and additional
976 forwardings can be given on the command line.
977 Privileged ports can be forwarded only when
978 logging in as root on the remote machine.
984 the listen port will be dynamically allocated on the server and reported
985 to the client at run time.
989 is not specified, the default is to only bind to loopback addresses.
994 or an empty string, then the forwarding is requested to listen on all
998 will only succeed if the server's
1000 option is enabled (see
1001 .Xr sshd_config 5 ) .
1002 .It Cm RhostsRSAAuthentication
1003 Specifies whether to try rhosts based authentication with RSA host
1005 The argument must be
1011 This option applies to protocol version 1 only and requires
1014 .It Cm RSAAuthentication
1015 Specifies whether to try RSA authentication.
1016 The argument to this keyword must be
1020 RSA authentication will only be
1021 attempted if the identity file exists, or an authentication agent is
1025 Note that this option applies to protocol version 1 only.
1027 Specifies what variables from the local
1029 should be sent to the server.
1030 Note that environment passing is only supported for protocol 2.
1031 The server must also support it, and the server must be configured to
1032 accept these environment variables.
1037 for how to configure the server.
1038 Variables are specified by name, which may contain wildcard characters.
1039 Multiple environment variables may be separated by whitespace or spread
1043 The default is not to send any environment variables.
1047 for more information on patterns.
1048 .It Cm ServerAliveCountMax
1049 Sets the number of server alive messages (see below) which may be
1052 receiving any messages back from the server.
1053 If this threshold is reached while server alive messages are being sent,
1054 ssh will disconnect from the server, terminating the session.
1055 It is important to note that the use of server alive messages is very
1059 The server alive messages are sent through the encrypted channel
1060 and therefore will not be spoofable.
1061 The TCP keepalive option enabled by
1064 The server alive mechanism is valuable when the client or
1065 server depend on knowing when a connection has become inactive.
1067 The default value is 3.
1069 .Cm ServerAliveInterval
1070 (see below) is set to 15 and
1071 .Cm ServerAliveCountMax
1072 is left at the default, if the server becomes unresponsive,
1073 ssh will disconnect after approximately 45 seconds.
1074 This option applies to protocol version 2 only; in protocol version
1075 1 there is no mechanism to request a response from the server to the
1076 server alive messages, so disconnection is the responsibility of the TCP
1078 .It Cm ServerAliveInterval
1079 Sets a timeout interval in seconds after which if no data has been received
1082 will send a message through the encrypted
1083 channel to request a response from the server.
1085 is 0, indicating that these messages will not be sent to the server,
1089 This option applies to protocol version 2 only.
1090 .Cm ProtocolKeepAlives
1093 are Debian-specific compatibility aliases for this option.
1094 .It Cm StrictHostKeyChecking
1095 If this flag is set to
1098 will never automatically add host keys to the
1099 .Pa ~/.ssh/known_hosts
1100 file, and refuses to connect to hosts whose host key has changed.
1101 This provides maximum protection against trojan horse attacks,
1102 though it can be annoying when the
1103 .Pa /etc/ssh/ssh_known_hosts
1104 file is poorly maintained or when connections to new hosts are
1106 This option forces the user to manually
1108 If this flag is set to
1110 ssh will automatically add new host keys to the
1111 user known hosts files.
1112 If this flag is set to
1115 will be added to the user known host files only after the user
1116 has confirmed that is what they really want to do, and
1117 ssh will refuse to connect to hosts whose host key has changed.
1119 known hosts will be verified automatically in all cases.
1120 The argument must be
1128 Specifies whether the system should send TCP keepalive messages to the
1130 If they are sent, death of the connection or crash of one
1131 of the machines will be properly noticed.
1132 This option only uses TCP keepalives (as opposed to using ssh level
1133 keepalives), so takes a long time to notice when the connection dies.
1134 As such, you probably want
1136 .Cm ServerAliveInterval
1138 However, this means that
1139 connections will die if the route is down temporarily, and some people
1144 (to send TCP keepalive messages), and the client will notice
1145 if the network goes down or the remote host dies.
1146 This is important in scripts, and many users want it too.
1148 To disable TCP keepalive messages, the value should be set to
1153 device forwarding between the client and the server.
1154 The argument must be
1164 requests the default tunnel mode, which is
1165 .Dq point-to-point .
1171 devices to open on the client
1176 The argument must be
1178 .Ar local_tun Op : Ar remote_tun .
1180 The devices may be specified by numerical ID or the keyword
1182 which uses the next available tunnel device.
1185 is not specified, it defaults to
1189 .It Cm UseBlacklistedKeys
1192 should use keys recorded in its blacklist of known-compromised keys (see
1197 then attempts to use compromised keys for authentication will be logged but
1199 It is strongly recommended that this be used only to install new authorized
1200 keys on the remote system, and even then only with the utmost care.
1203 then attempts to use compromised keys for authentication will be prevented.
1206 .It Cm UsePrivilegedPort
1207 Specifies whether to use a privileged port for outgoing connections.
1208 The argument must be
1217 must be setuid root.
1218 Note that this option must be set to
1221 .Cm RhostsRSAAuthentication
1224 Specifies the user to log in as.
1225 This can be useful when a different user name is used on different machines.
1226 This saves the trouble of
1227 having to remember to give the user name on the command line.
1228 .It Cm UserKnownHostsFile
1229 Specifies a file to use for the user
1230 host key database instead of
1231 .Pa ~/.ssh/known_hosts .
1232 .It Cm VerifyHostKeyDNS
1233 Specifies whether to verify the remote key using DNS and SSHFP resource
1235 If this option is set to
1237 the client will implicitly trust keys that match a secure fingerprint
1239 Insecure fingerprints will be handled as if this option was set to
1241 If this option is set to
1243 information on fingerprint match will be displayed, but the user will still
1244 need to confirm new host keys according to the
1245 .Cm StrictHostKeyChecking
1247 The argument must be
1254 Note that this option applies to protocol version 2 only.
1257 .Sx VERIFYING HOST KEYS
1260 .It Cm VisualHostKey
1261 If this flag is set to
1263 an ASCII art representation of the remote host key fingerprint is
1264 printed in addition to the hex fingerprint string at login and
1265 for unknown host keys.
1266 If this flag is set to
1268 no fingerprint strings are printed at login and
1269 only the hex fingerprint string will be printed for unknown host keys.
1272 .It Cm XAuthLocation
1273 Specifies the full pathname of the
1277 .Pa /usr/X11R6/bin/xauth .
1282 consists of zero or more non-whitespace characters,
1284 (a wildcard that matches zero or more characters),
1287 (a wildcard that matches exactly one character).
1288 For example, to specify a set of declarations for any host in the
1291 the following pattern could be used:
1295 The following pattern
1296 would match any host in the 192.168.0.[0-9] network range:
1298 .Dl Host 192.168.0.?
1302 is a comma-separated list of patterns.
1303 Patterns within pattern-lists may be negated
1304 by preceding them with an exclamation mark
1307 to allow a key to be used from anywhere within an organisation
1311 the following entry (in authorized_keys) could be used:
1313 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1316 .It Pa ~/.ssh/config
1317 This is the per-user configuration file.
1318 The format of this file is described above.
1319 This file is used by the SSH client.
1320 Because of the potential for abuse, this file must have strict permissions:
1321 read/write for the user, and not accessible by others.
1322 It may be group-writable provided that the group in question contains only
1324 .It Pa /etc/ssh/ssh_config
1325 Systemwide configuration file.
1326 This file provides defaults for those
1327 values that are not specified in the user's configuration file, and
1328 for those users who do not have a configuration file.
1329 This file must be world-readable.
1334 OpenSSH is a derivative of the original and free
1335 ssh 1.2.12 release by Tatu Ylonen.
1336 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1337 Theo de Raadt and Dug Song
1338 removed many bugs, re-added newer features and
1340 Markus Friedl contributed the support for SSH
1341 protocol versions 1.5 and 2.0.