3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh.1,v 1.124 2001/07/25 14:35:18 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl l Ar login_name
47 .Op Ar hostname | user@hostname
51 .Op Fl afgknqstvxACNPTX1246
52 .Op Fl b Ar bind_address
53 .Op Fl c Ar cipher_spec
54 .Op Fl e Ar escape_char
55 .Op Fl i Ar identity_file
56 .Op Fl l Ar login_name
76 .Op Ar hostname | user@hostname
80 (SSH client) is a program for logging into a remote machine and for
81 executing commands on a remote machine.
82 It is intended to replace
83 rlogin and rsh, and provide secure encrypted communications between
84 two untrusted hosts over an insecure network.
86 arbitrary TCP/IP ports can also be forwarded over the secure channel.
89 connects and logs into the specified
92 his/her identity to the remote machine using one of several methods
93 depending on the protocol version used:
95 .Ss SSH protocol version 1
97 First, if the machine the user logs in from is listed in
100 .Pa /etc/shosts.equiv
101 on the remote machine, and the user names are
102 the same on both sides, the user is immediately permitted to log in.
107 exists in the user's home directory on the
108 remote machine and contains a line containing the name of the client
109 machine and the name of the user on that machine, the user is
111 This form of authentication alone is normally not
112 allowed by the server because it is not secure.
114 The second authentication method is the
118 method combined with RSA-based host authentication.
119 It means that if the login would be permitted by
122 .Pa /etc/hosts.equiv ,
124 .Pa /etc/shosts.equiv ,
125 and if additionally the server can verify the client's
127 .Pa /etc/ssh_known_hosts
129 .Pa $HOME/.ssh/known_hosts
132 section), only then login is permitted.
133 This authentication method closes security holes due to IP
134 spoofing, DNS spoofing and routing spoofing.
135 [Note to the administrator:
136 .Pa /etc/hosts.equiv ,
138 and the rlogin/rsh protocol in general, are inherently insecure and should be
139 disabled if security is desired.]
141 As a third authentication method,
143 supports RSA based authentication.
144 The scheme is based on public-key cryptography: there are cryptosystems
145 where encryption and decryption are done using separate keys, and it
146 is not possible to derive the decryption key from the encryption key.
147 RSA is one such system.
148 The idea is that each user creates a public/private
149 key pair for authentication purposes.
150 The server knows the public key, and only the user knows the private key.
152 .Pa $HOME/.ssh/authorized_keys
153 lists the public keys that are permitted for logging
155 When the user logs in, the
157 program tells the server which key pair it would like to use for
159 The server checks if this key is permitted, and if
160 so, sends the user (actually the
162 program running on behalf of the user) a challenge, a random number,
163 encrypted by the user's public key.
164 The challenge can only be
165 decrypted using the proper private key.
166 The user's client then decrypts the
167 challenge using the private key, proving that he/she knows the private
168 key but without disclosing it to the server.
171 implements the RSA authentication protocol automatically.
172 The user creates his/her RSA key pair by running
174 This stores the private key in
175 .Pa $HOME/.ssh/identity
176 and the public key in
177 .Pa $HOME/.ssh/identity.pub
178 in the user's home directory.
179 The user should then copy the
182 .Pa $HOME/.ssh/authorized_keys
183 in his/her home directory on the remote machine (the
185 file corresponds to the conventional
187 file, and has one key
188 per line, though the lines can be very long).
189 After this, the user can log in without giving the password.
190 RSA authentication is much
191 more secure than rhosts authentication.
193 The most convenient way to use RSA authentication may be with an
194 authentication agent.
197 for more information.
199 If other authentication methods fail,
201 prompts the user for a password.
202 The password is sent to the remote
203 host for checking; however, since all communications are encrypted,
204 the password cannot be seen by someone listening on the network.
206 .Ss SSH protocol version 2
208 When a user connects using the protocol version 2
209 different authentication methods are available.
210 Using the default values for
211 .Cm PreferredAuthentications ,
212 the client will try to authenticate first using the hostbased method;
213 if this method fails public key authentication is attempted,
214 and finally if this method fails keyboard-interactive and
215 password authentication are tried.
217 The public key method is similar to RSA authentication described
218 in the previous section and allows the RSA or DSA algorithm to be used:
219 The client uses his private key,
220 .Pa $HOME/.ssh/id_dsa
222 .Pa $HOME/.ssh/id_rsa ,
223 to sign the session identifier and sends the result to the server.
224 The server checks whether the matching public key is listed in
225 .Pa $HOME/.ssh/authorized_keys
226 and grants access if both the key is found and the signature is correct.
227 The session identifier is derived from a shared Diffie-Hellman value
228 and is only known to the client and the server.
230 If public key authentication fails or is not available a password
231 can be sent encrypted to the remote host for proving the user's identity.
235 supports hostbased or challenge response authentication.
237 Protocol 2 provides additional mechanisms for confidentiality
238 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
239 and integrity (hmac-md5, hmac-sha1).
240 Note that protocol 1 lacks a strong mechanism for ensuring the
241 integrity of the connection.
243 .Ss Login session and remote execution
245 When the user's identity has been accepted by the server, the server
246 either executes the given command, or logs into the machine and gives
247 the user a normal shell on the remote machine.
248 All communication with
249 the remote command or shell will be automatically encrypted.
251 If a pseudo-terminal has been allocated (normal login session), the
252 user may use the escape characters noted below.
254 If no pseudo tty has been allocated, the
255 session is transparent and can be used to reliably transfer binary
257 On most systems, setting the escape character to
259 will also make the session transparent even if a tty is used.
261 The session terminates when the command or shell on the remote
262 machine exits and all X11 and TCP/IP connections have been closed.
263 The exit status of the remote program is returned as the exit status
267 .Ss Escape Characters
269 When a pseudo terminal has been requested, ssh supports a number of functions
270 through the use of an escape character.
272 A single tilde character can be sent as
274 or by following the tilde by a character other than those described below.
275 The escape character must always follow a newline to be interpreted as
277 The escape character can be changed in configuration files using the
279 configuration directive or on the command line by the
283 The supported escapes (assuming the default
292 List forwarded connections
294 Background ssh at logout when waiting for forwarded connection / X11 sessions
295 to terminate (protocol version 1 only)
297 Display a list of escape characters
299 Request rekeying of the connection (only useful for SSH protocol version 2
300 and if the peer supports it)
303 .Ss X11 and TCP forwarding
309 (or, see the description of the
313 options described later)
314 and the user is using X11 (the
316 environment variable is set), the connection to the X11 display is
317 automatically forwarded to the remote side in such a way that any X11
318 programs started from the shell (or command) will go through the
319 encrypted channel, and the connection to the real X server will be made
320 from the local machine.
321 The user should not manually set
323 Forwarding of X11 connections can be
324 configured on the command line or in configuration files.
330 will point to the server machine, but with a display number greater
332 This is normal, and happens because
336 X server on the server machine for forwarding the
337 connections over the encrypted channel.
340 will also automatically set up Xauthority data on the server machine.
341 For this purpose, it will generate a random authorization cookie,
342 store it in Xauthority on the server, and verify that any forwarded
343 connections carry this cookie and replace it by the real cookie when
344 the connection is opened.
345 The real authentication cookie is never
346 sent to the server machine (and no cookies are sent in the plain).
348 If the user is using an authentication agent, the connection to the agent
349 is automatically forwarded to the remote side unless disabled on
350 the command line or in a configuration file.
352 Forwarding of arbitrary TCP/IP connections over the secure channel can
353 be specified either on the command line or in a configuration file.
354 One possible application of TCP/IP forwarding is a secure connection to an
355 electronic purse; another is going through firewalls.
357 .Ss Server authentication
360 automatically maintains and checks a database containing
361 identifications for all hosts it has ever been used with.
362 Host keys are stored in
363 .Pa $HOME/.ssh/known_hosts
364 in the user's home directory.
365 Additionally, the file
366 .Pa /etc/ssh_known_hosts
367 is automatically checked for known hosts.
368 Any new hosts are automatically added to the user's file.
369 If a host's identification
372 warns about this and disables password authentication to prevent a
373 trojan horse from getting the user's password.
375 this mechanism is to prevent man-in-the-middle attacks which could
376 otherwise be used to circumvent the encryption.
378 .Cm StrictHostKeyChecking
379 option (see below) can be used to prevent logins to machines whose
380 host key is not known or has changed.
382 The options are as follows:
385 Disables forwarding of the authentication agent connection.
387 Enables forwarding of the authentication agent connection.
388 This can also be specified on a per-host basis in a configuration file.
389 .It Fl b Ar bind_address
390 Specify the interface to transmit from on machines with multiple
391 interfaces or aliased addresses.
392 .It Fl c Ar blowfish|3des
393 Selects the cipher to use for encrypting the session.
396 It is believed to be secure.
398 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
399 It is presumably more secure than the
401 cipher which is no longer fully supported in
404 is a fast block cipher, it appears very secure and is much faster than
406 .It Fl c Ar cipher_spec
407 Additionally, for protocol version 2 a comma-separated list of ciphers can
408 be specified in order of preference.
411 for more information.
412 .It Fl e Ar ch|^ch|none
413 Sets the escape character for sessions with a pty (default:
415 The escape character is only recognized at the beginning of a line.
416 The escape character followed by a dot
418 closes the connection, followed
419 by control-Z suspends the connection, and followed by itself sends the
420 escape character once.
421 Setting the character to
423 disables any escapes and makes the session fully transparent.
427 to go to background just before command execution.
430 is going to ask for passwords or passphrases, but the user
431 wants it in the background.
434 The recommended way to start X11 programs at a remote site is with
436 .Ic ssh -f host xterm .
438 Allows remote hosts to connect to local forwarded ports.
439 .It Fl i Ar identity_file
440 Selects the file from which the identity (private key) for
441 RSA or DSA authentication is read.
443 .Pa $HOME/.ssh/identity
444 in the user's home directory.
445 Identity files may also be specified on
446 a per-host basis in the configuration file.
447 It is possible to have multiple
449 options (and multiple identities specified in
450 configuration files).
452 Disables forwarding of Kerberos tickets and AFS tokens.
453 This may also be specified on a per-host basis in the configuration file.
454 .It Fl l Ar login_name
455 Specifies the user to log in as on the remote machine.
456 This also may be specified on a per-host basis in the configuration file.
458 Additionally, for protocol version 2 a comma-separated list of MAC
459 (message authentication code) algorithms can
460 be specified in order of preference.
463 keyword for more information.
467 (actually, prevents reading from stdin).
468 This must be used when
470 is run in the background.
471 A common trick is to use this to run X11 programs on a remote machine.
473 .Ic ssh -n shadows.cs.hut.fi emacs &
474 will start an emacs on shadows.cs.hut.fi, and the X11
475 connection will be automatically forwarded over an encrypted channel.
478 program will be put in the background.
479 (This does not work if
481 needs to ask for a password or passphrase; see also the
485 Do not execute a remote command.
486 This is useful if you just want to forward ports
487 (protocol version 2 only).
489 Can be used to give options in the format used in the config file.
490 This is useful for specifying options for which there is no separate
492 The option has the same format as a line in the configuration file.
494 Port to connect to on the remote host.
495 This can be specified on a
496 per-host basis in the configuration file.
498 Use a non-privileged port for outgoing connections.
499 This can be used if your firewall does
500 not permit connections from privileged ports.
501 Note that this option turns off
502 .Cm RhostsAuthentication
504 .Cm RhostsRSAAuthentication
508 Causes all warning and diagnostic messages to be suppressed.
509 Only fatal errors are displayed.
511 May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
512 of SSH as a secure transport for other applications (eg. sftp). The
513 subsystem is specified as the remote command.
515 Force pseudo-tty allocation.
516 This can be used to execute arbitrary
517 screen-based programs on a remote machine, which can be very useful,
518 e.g., when implementing menu services.
521 options force tty allocation, even if
525 Disable pseudo-tty allocation.
530 to print debugging messages about its progress.
532 debugging connection, authentication, and configuration problems.
535 options increases the verbosity.
538 Disables X11 forwarding.
540 Enables X11 forwarding.
541 This can also be specified on a per-host basis in a configuration file.
543 Requests compression of all data (including stdin, stdout, stderr, and
544 data for forwarded X11 and TCP/IP connections).
545 The compression algorithm is the same used by
549 can be controlled by the
552 Compression is desirable on modem lines and other
553 slow connections, but will only slow down things on fast networks.
554 The default value can be set on a host-by-host basis in the
555 configuration files; see the
558 .It Fl L Ar port:host:hostport
559 Specifies that the given port on the local (client) host is to be
560 forwarded to the given host and port on the remote side.
561 This works by allocating a socket to listen to
563 on the local side, and whenever a connection is made to this port, the
564 connection is forwarded over the secure channel, and a connection is
569 from the remote machine.
570 Port forwardings can also be specified in the configuration file.
571 Only root can forward privileged ports.
572 IPv6 addresses can be specified with an alternative syntax:
573 .Ar port/host/hostport
574 .It Fl R Ar port:host:hostport
575 Specifies that the given port on the remote (server) host is to be
576 forwarded to the given host and port on the local side.
577 This works by allocating a socket to listen to
579 on the remote side, and whenever a connection is made to this port, the
580 connection is forwarded over the secure channel, and a connection is
585 from the local machine.
586 Port forwardings can also be specified in the configuration file.
587 Privileged ports can be forwarded only when
588 logging in as root on the remote machine.
589 IPv6 addresses can be specified with an alternative syntax:
590 .Ar port/host/hostport
594 to try protocol version 1 only.
598 to try protocol version 2 only.
602 to use IPv4 addresses only.
606 to use IPv6 addresses only.
608 .Sh CONFIGURATION FILES
610 obtains configuration data from the following sources (in this order):
611 command line options, user's configuration file
612 .Pq Pa $HOME/.ssh/config ,
613 and system-wide configuration file
614 .Pq Pa /etc/ssh_config .
615 For each parameter, the first obtained value
617 The configuration files contain sections bracketed by
619 specifications, and that section is only applied for hosts that
620 match one of the patterns given in the specification.
621 The matched host name is the one given on the command line.
623 Since the first obtained value for each parameter is used, more
624 host-specific declarations should be given near the beginning of the
625 file, and general defaults at the end.
627 The configuration file has the following format:
629 Empty lines and lines starting with
633 Otherwise a line is of the format
634 .Dq keyword arguments .
636 keywords and their meanings are as follows (note that the
637 configuration files are case-sensitive):
640 Restricts the following declarations (up to the next
642 keyword) to be only for those hosts that match one of the patterns
643 given after the keyword.
647 can be used as wildcards in the
651 as a pattern can be used to provide global
652 defaults for all hosts.
655 argument given on the command line (i.e., the name is not converted to
656 a canonicalized host name before matching).
657 .It Cm AFSTokenPassing
658 Specifies whether to pass AFS tokens to remote host.
659 The argument to this keyword must be
663 This option applies to protocol version 1 only.
667 passphrase/password querying will be disabled.
668 This option is useful in scripts and other batch jobs where you have no
669 user to supply the password.
677 Specify the interface to transmit from on machines with multiple
678 interfaces or aliased addresses.
679 Note that this option does not work if
680 .Cm UsePrivilegedPort
684 If this flag is set to
686 ssh will additionally check the host IP address in the
689 This allows ssh to detect if a host key changed due to DNS spoofing.
690 If the option is set to
692 the check will not be executed.
696 Specifies the cipher to use for encrypting the session
697 in protocol version 1.
706 Specifies the ciphers allowed for protocol version 2
707 in order of preference.
708 Multiple ciphers must be comma-separated.
712 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
713 aes192-cbc,aes256-cbc''
716 Specifies whether to use compression.
723 .It Cm CompressionLevel
724 Specifies the compression level to use if compression is enabled.
725 The argument must be an integer from 1 (fast) to 9 (slow, best).
726 The default level is 6, which is good for most applications.
727 The meaning of the values is the same as in
729 Note that this option applies to protocol version 1 only.
730 .It Cm ConnectionAttempts
731 Specifies the number of tries (one per second) to make before falling
732 back to rsh or exiting.
733 The argument must be an integer.
734 This may be useful in scripts if the connection sometimes fails.
737 Sets the escape character (default:
739 The escape character can also
740 be set on the command line.
741 The argument should be a single character,
743 followed by a letter, or
745 to disable the escape
746 character entirely (making the connection transparent for binary
749 Specifies that if connecting via
751 fails due to a connection refused error (there is no
753 listening on the remote host),
755 should automatically be used instead (after a suitable warning about
756 the session being unencrypted).
764 Specifies whether the connection to the authentication agent (if any)
765 will be forwarded to the remote machine.
773 Specifies whether X11 connections will be automatically redirected
774 over the secure channel and
784 Specifies whether remote hosts are allowed to connect to local
792 .It Cm GlobalKnownHostsFile
793 Specifies a file to use for the global
794 host key database instead of
795 .Pa /etc/ssh_known_hosts .
796 .It Cm HostbasedAuthentication
797 Specifies whether to try rhosts based authentication with public key
805 This option applies to protocol version 2 only and
807 .Cm RhostsRSAAuthentication .
808 .It Cm HostKeyAlgorithms
809 Specifies the protocol version 2 host key algorithms
810 that the client wants to use in order of preference.
811 The default for this option is:
814 Specifies an alias that should be used instead of the
815 real host name when looking up or saving the host key
816 in the host key database files.
817 This option is useful for tunneling ssh connections
818 or if you have multiple servers running on a single host.
820 Specifies the real host name to log into.
821 This can be used to specify nicknames or abbreviations for hosts.
822 Default is the name given on the command line.
823 Numeric IP addresses are also permitted (both on the command line and in
827 Specifies the file from which the user's RSA or DSA authentication identity
829 .Pa $HOME/.ssh/identity
830 in the user's home directory).
831 Additionally, any identities represented by the authentication agent
832 will be used for authentication.
833 The file name may use the tilde
834 syntax to refer to a user's home directory.
835 It is possible to have
836 multiple identity files specified in configuration files; all these
837 identities will be tried in sequence.
839 Specifies whether the system should send keepalive messages to the
841 If they are sent, death of the connection or crash of one
842 of the machines will be properly noticed.
843 However, this means that
844 connections will die if the route is down temporarily, and some people
849 (to send keepalives), and the client will notice
850 if the network goes down or the remote host dies.
851 This is important in scripts, and many users want it too.
853 To disable keepalives, the value should be set to
855 in both the server and the client configuration files.
856 .It Cm KerberosAuthentication
857 Specifies whether Kerberos authentication will be used.
858 The argument to this keyword must be
862 .It Cm KerberosTgtPassing
863 Specifies whether a Kerberos TGT will be forwarded to the server.
864 This will only work if the Kerberos server is actually an AFS kaserver.
865 The argument to this keyword must be
870 Specifies that a TCP/IP port on the local machine be forwarded over
871 the secure channel to given host:port from the remote machine.
872 The first argument must be a port number, and the second must be
874 Multiple forwardings may be specified, and additional
875 forwardings can be given on the command line.
876 Only the superuser can forward privileged ports.
878 Gives the verbosity level that is used when logging messages from
880 The possible values are:
881 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
884 Specifies the MAC (message authentication code) algorithms
885 in order of preference.
886 The MAC algorithm is used in protocol version 2
887 for data integrity protection.
888 Multiple algorithms must be comma-separated.
890 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
891 .It Cm NumberOfPasswordPrompts
892 Specifies the number of password prompts before giving up.
893 The argument to this keyword must be an integer.
895 .It Cm PasswordAuthentication
896 Specifies whether to use password authentication.
897 The argument to this keyword must be
904 Specifies the port number to connect on the remote host.
906 .It Cm PreferredAuthentications
907 Specifies the order in which the client should try protocol 2
908 authentication methods. This allows a client to prefer one method (e.g.
909 .Cm keyboard-interactive )
910 over another method (e.g.
912 The default for this option is:
913 .Dq hostbased,publickey,keyboard-interactive,password
915 Specifies the protocol versions
917 should support in order of preference.
918 The possible values are
922 Multiple versions must be comma-separated.
927 tries version 2 and falls back to version 1
928 if version 2 is not available.
930 Specifies the command to use to connect to the server.
932 string extends to the end of the line, and is executed with
934 In the command string,
936 will be substituted by the host name to
940 The command can be basically anything,
941 and should read from its standard input and write to its standard output.
942 It should eventually connect an
944 server running on some machine, or execute
947 Host key management will be done using the
948 HostName of the host being connected (defaulting to the name typed by
952 is not available for connects with a proxy command.
954 .It Cm PubkeyAuthentication
955 Specifies whether to try public key authentication.
956 The argument to this keyword must be
962 This option applies to protocol version 2 only.
964 Specifies that a TCP/IP port on the remote machine be forwarded over
965 the secure channel to given host:port from the local machine.
966 The first argument must be a port number, and the second must be
968 Multiple forwardings may be specified, and additional
969 forwardings can be given on the command line.
970 Only the superuser can forward privileged ports.
971 .It Cm RhostsAuthentication
972 Specifies whether to try rhosts based authentication.
974 declaration only affects the client side and has no effect whatsoever
976 Disabling rhosts authentication may reduce
977 authentication time on slow connections when rhosts authentication is
979 Most servers do not permit RhostsAuthentication because it
981 .Cm RhostsRSAAuthentication ) .
982 The argument to this keyword must be
988 This option applies to protocol version 1 only.
989 .It Cm RhostsRSAAuthentication
990 Specifies whether to try rhosts based authentication with RSA host
998 This option applies to protocol version 1 only.
999 .It Cm RSAAuthentication
1000 Specifies whether to try RSA authentication.
1001 The argument to this keyword must be
1005 RSA authentication will only be
1006 attempted if the identity file exists, or an authentication agent is
1010 Note that this option applies to protocol version 1 only.
1011 .It Cm ChallengeResponseAuthentication
1012 Specifies whether to use challenge response authentication.
1013 The argument to this keyword must be
1019 .It Cm StrictHostKeyChecking
1020 If this flag is set to
1023 will never automatically add host keys to the
1024 .Pa $HOME/.ssh/known_hosts
1025 file, and refuses to connect to hosts whose host key has changed.
1026 This provides maximum protection against trojan horse attacks.
1027 However, it can be somewhat annoying if you don't have good
1028 .Pa /etc/ssh_known_hosts
1029 files installed and frequently
1030 connect to new hosts.
1031 This option forces the user to manually
1033 If this flag is set to
1036 will automatically add new host keys to the
1037 user known hosts files.
1038 If this flag is set to
1041 will be added to the user known host files only after the user
1042 has confirmed that is what they really want to do, and
1044 will refuse to connect to hosts whose host key has changed.
1046 known hosts will be verified automatically in all cases.
1047 The argument must be
1054 .It Cm UsePrivilegedPort
1055 Specifies whether to use a privileged port for outgoing connections.
1056 The argument must be
1062 Note that you need to set this option to
1065 .Cm RhostsAuthentication
1067 .Cm RhostsRSAAuthentication
1070 Specifies the user to log in as.
1071 This can be useful if you have a different user name on different machines.
1072 This saves the trouble of
1073 having to remember to give the user name on the command line.
1074 .It Cm UserKnownHostsFile
1075 Specifies a file to use for the user
1076 host key database instead of
1077 .Pa $HOME/.ssh/known_hosts .
1079 Specifies that rlogin/rsh should be used for this host.
1080 It is possible that the host does not at all support the
1085 to immediately execute
1087 All other options (except
1089 are ignored if this has been specified.
1090 The argument must be
1094 .It Cm XAuthLocation
1095 Specifies the location of the
1099 .Pa /usr/X11R6/bin/xauth .
1103 will normally set the following environment variables:
1108 variable indicates the location of the X11 server.
1109 It is automatically set by
1111 to point to a value of the form
1113 where hostname indicates
1114 the host where the shell runs, and n is an integer >= 1.
1116 uses this special value to forward X11 connections over the secure
1118 The user should normally not set
1121 will render the X11 connection insecure (and will require the user to
1122 manually copy any required authorization cookies).
1124 Set to the path of the user's home directory.
1128 set for compatibility with systems that use this variable.
1130 Set to point the user's mailbox.
1134 as specified when compiling
1139 needs a passphrase, it will read the passphrase from the current
1140 terminal if it was run from a terminal.
1143 does not have a terminal associated with it but
1147 are set, it will execute the program specified by
1149 and open an X11 window to read the passphrase.
1150 This is particularly useful when calling
1155 (Note that on some machines it
1156 may be necessary to redirect the input from
1159 .It Ev SSH_AUTH_SOCK
1160 indicates the path of a unix-domain socket used to communicate with the
1163 Identifies the client end of the connection.
1164 The variable contains
1165 three space-separated values: client ip-address, client port number,
1166 and server port number.
1167 .It Ev SSH_ORIGINAL_COMMAND
1168 The variable contains the original command line if a forced command
1170 It can be used to extract the original arguments.
1172 This is set to the name of the tty (path to the device) associated
1173 with the current shell or command.
1174 If the current session has no tty,
1175 this variable is not set.
1177 The timezone variable is set to indicate the present timezone if it
1178 was set when the daemon was started (i.e., the daemon passes the value
1179 on to new connections).
1181 Set to the name of the user logging in.
1187 .Pa $HOME/.ssh/environment ,
1188 and adds lines of the format
1193 .It Pa $HOME/.ssh/known_hosts
1194 Records host keys for all hosts the user has logged into (that are not
1196 .Pa /etc/ssh_known_hosts .
1199 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1200 Contains the authentication identity of the user.
1201 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1203 contain sensitive data and should be readable by the user but not
1204 accessible by others (read/write/execute).
1207 ignores a private key file if it is accessible by others.
1208 It is possible to specify a passphrase when
1209 generating the key; the passphrase will be used to encrypt the
1210 sensitive part of this file using 3DES.
1211 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1212 Contains the public key for authentication (public part of the
1213 identity file in human-readable form).
1215 .Pa $HOME/.ssh/identity.pub
1216 file should be added to
1217 .Pa $HOME/.ssh/authorized_keys
1219 where you wish to log in using protocol version 1 RSA authentication.
1221 .Pa $HOME/.ssh/id_dsa.pub
1223 .Pa $HOME/.ssh/id_rsa.pub
1224 file should be added to
1225 .Pa $HOME/.ssh/authorized_keys
1227 where you wish to log in using protocol version 2 DSA/RSA authentication.
1229 sensitive and can (but need not) be readable by anyone.
1231 never used automatically and are not necessary; they are only provided for
1232 the convenience of the user.
1233 .It Pa $HOME/.ssh/config
1234 This is the per-user configuration file.
1235 The format of this file is described above.
1236 This file is used by the
1239 This file does not usually contain any sensitive information,
1240 but the recommended permissions are read/write for the user, and not
1241 accessible by others.
1242 .It Pa $HOME/.ssh/authorized_keys
1243 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1244 The format of this file is described in the
1247 In the simplest form the format is the same as the .pub
1249 This file is not highly sensitive, but the recommended
1250 permissions are read/write for the user, and not accessible by others.
1251 .It Pa /etc/ssh_known_hosts
1252 Systemwide list of known host keys.
1253 This file should be prepared by the
1254 system administrator to contain the public host keys of all machines in the
1256 This file should be world-readable.
1258 public keys, one per line, in the following format (fields separated
1259 by spaces): system name, public key and optional comment field.
1260 When different names are used
1261 for the same machine, all such names should be listed, separated by
1263 The format is described on the
1267 The canonical system name (as returned by name servers) is used by
1269 to verify the client host when logging in; other names are needed because
1271 does not convert the user-supplied name to a canonical name before
1272 checking the key, because someone with access to the name servers
1273 would then be able to fool host authentication.
1274 .It Pa /etc/ssh_config
1275 Systemwide configuration file.
1276 This file provides defaults for those
1277 values that are not specified in the user's configuration file, and
1278 for those users who do not have a configuration file.
1279 This file must be world-readable.
1280 .It Pa $HOME/.rhosts
1281 This file is used in
1283 authentication to list the
1284 host/user pairs that are permitted to log in.
1285 (Note that this file is
1286 also used by rlogin and rsh, which makes using this file insecure.)
1287 Each line of the file contains a host name (in the canonical form
1288 returned by name servers), and then a user name on that host,
1289 separated by a space.
1290 On some machines this file may need to be
1291 world-readable if the user's home directory is on a NFS partition,
1295 Additionally, this file must be owned by the user,
1296 and must not have write permissions for anyone else.
1298 permission for most machines is read/write for the user, and not
1299 accessible by others.
1301 Note that by default
1303 will be installed so that it requires successful RSA host
1304 authentication before permitting \s+2.\s0rhosts authentication.
1305 If your server machine does not have the client's host key in
1306 .Pa /etc/ssh_known_hosts ,
1308 .Pa $HOME/.ssh/known_hosts .
1309 The easiest way to do this is to
1310 connect back to the client from the server machine using ssh; this
1311 will automatically add the host key to
1312 .Pa $HOME/.ssh/known_hosts .
1313 .It Pa $HOME/.shosts
1314 This file is used exactly the same way as
1317 having this file is to be able to use rhosts authentication with
1319 without permitting login with
1323 .It Pa /etc/hosts.equiv
1324 This file is used during
1325 .Pa \&.rhosts authentication.
1327 canonical hosts names, one per line (the full format is described on
1331 If the client host is found in this file, login is
1332 automatically permitted provided client and server user names are the
1334 Additionally, successful RSA host authentication is normally
1336 This file should only be writable by root.
1337 .It Pa /etc/shosts.equiv
1338 This file is processed exactly as
1339 .Pa /etc/hosts.equiv .
1340 This file may be useful to permit logins using
1342 but not using rsh/rlogin.
1344 Commands in this file are executed by
1346 when the user logs in just before the user's shell (or command) is started.
1349 manual page for more information.
1350 .It Pa $HOME/.ssh/rc
1351 Commands in this file are executed by
1353 when the user logs in just before the user's shell (or command) is
1357 manual page for more information.
1358 .It Pa $HOME/.ssh/environment
1359 Contains additional definitions for environment variables, see section
1364 OpenSSH is a derivative of the original and free
1365 ssh 1.2.12 release by Tatu Ylonen.
1366 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1367 Theo de Raadt and Dug Song
1368 removed many bugs, re-added newer features and
1370 Markus Friedl contributed the support for SSH
1371 protocol versions 1.5 and 2.0.
1388 .%T "SSH Protocol Architecture"
1389 .%N draft-ietf-secsh-architecture-07.txt
1391 .%O work in progress material
andersk / openssh.git / blob