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 1.64 2000/10/16 21:46:31 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH secure shell client (remote login program)
46 .Op Fl l Ar login_name
47 .Op Ar hostname | user@hostname
51 .Op Fl afgknqtvxACNPTX246
52 .Op Fl c Ar cipher_spec
53 .Op Fl e Ar escape_char
54 .Op Fl i Ar identity_file
55 .Op Fl l Ar login_name
74 .Op Ar hostname | user@hostname
78 (Secure Shell) is a program for logging into a remote machine and for
79 executing commands on a remote machine.
80 It is intended to replace
81 rlogin and rsh, and provide secure encrypted communications between
82 two untrusted hosts over an insecure network.
84 arbitrary TCP/IP ports can also be forwarded over the secure channel.
87 connects and logs into the specified
90 his/her identity to the remote machine using one of several methods
91 depending on the protocol version used:
93 .Ss SSH protocol version 1
95 First, if the machine the user logs in from is listed in
99 on the remote machine, and the user names are
100 the same on both sides, the user is immediately permitted to log in.
105 exists in the user's home directory on the
106 remote machine and contains a line containing the name of the client
107 machine and the name of the user on that machine, the user is
109 This form of authentication alone is normally not
110 allowed by the server because it is not secure.
112 The second (and primary) authentication method is the
116 method combined with RSA-based host authentication.
117 It means that if the login would be permitted by
120 .Pa /etc/hosts.equiv ,
122 .Pa /etc/shosts.equiv ,
123 and if additionally the server can verify the client's
125 .Pa /etc/ssh_known_hosts
127 .Pa $HOME/.ssh/known_hosts
130 section), only then login is permitted.
131 This authentication method closes security holes due to IP
132 spoofing, DNS spoofing and routing spoofing.
133 [Note to the administrator:
134 .Pa /etc/hosts.equiv ,
136 and the rlogin/rsh protocol in general, are inherently insecure and should be
137 disabled if security is desired.]
139 As a third authentication method,
141 supports RSA based authentication.
142 The scheme is based on public-key cryptography: there are cryptosystems
143 where encryption and decryption are done using separate keys, and it
144 is not possible to derive the decryption key from the encryption key.
145 RSA is one such system.
146 The idea is that each user creates a public/private
147 key pair for authentication purposes.
148 The server knows the public key, and only the user knows the private key.
150 .Pa $HOME/.ssh/authorized_keys
151 lists the public keys that are permitted for logging
153 When the user logs in, the
155 program tells the server which key pair it would like to use for
157 The server checks if this key is permitted, and if
158 so, sends the user (actually the
160 program running on behalf of the user) a challenge, a random number,
161 encrypted by the user's public key.
162 The challenge can only be
163 decrypted using the proper private key.
164 The user's client then decrypts the
165 challenge using the private key, proving that he/she knows the private
166 key but without disclosing it to the server.
169 implements the RSA authentication protocol automatically.
170 The user creates his/her RSA key pair by running
172 This stores the private key in
173 .Pa $HOME/.ssh/identity
174 and the public key in
175 .Pa $HOME/.ssh/identity.pub
176 in the user's home directory.
177 The user should then copy the
180 .Pa $HOME/.ssh/authorized_keys
181 in his/her home directory on the remote machine (the
183 file corresponds to the conventional
185 file, and has one key
186 per line, though the lines can be very long).
187 After this, the user can log in without giving the password.
188 RSA authentication is much
189 more secure than rhosts authentication.
191 The most convenient way to use RSA authentication may be with an
192 authentication agent.
195 for more information.
197 If other authentication methods fail,
199 prompts the user for a password.
200 The password is sent to the remote
201 host for checking; however, since all communications are encrypted,
202 the password cannot be seen by someone listening on the network.
204 .Ss SSH protocol version 2
206 When a user connects using the protocol version 2
207 different authentication methods are available:
208 At first, the client attempts to authenticate using the public key method.
209 If this method fails password authentication is tried.
211 The public key method is similar to RSA authentication described
212 in the previous section except that the DSA algorithm is used
213 instead of the patented RSA algorithm.
214 The client uses his private DSA key
215 .Pa $HOME/.ssh/id_dsa
216 to sign the session identifier and sends the result to the server.
217 The server checks whether the matching public key is listed in
218 .Pa $HOME/.ssh/authorized_keys2
219 and grants access if both the key is found and the signature is correct.
220 The session identifier is derived from a shared Diffie-Hellman value
221 and is only known to the client and the server.
223 If public key authentication fails or is not available a password
224 can be sent encrypted to the remote host for proving the user's identity.
225 This protocol 2 implementation does not yet support Kerberos or
226 S/Key authentication.
228 Protocol 2 provides additional mechanisms for confidentiality
229 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
230 and integrity (hmac-sha1, hmac-md5).
231 Note that protocol 1 lacks a strong mechanism for ensuring the
232 integrity of the connection.
234 .Ss Login session and remote execution
236 When the user's identity has been accepted by the server, the server
237 either executes the given command, or logs into the machine and gives
238 the user a normal shell on the remote machine.
239 All communication with
240 the remote command or shell will be automatically encrypted.
242 If a pseudo-terminal has been allocated (normal login session), the
243 user can disconnect with
249 All forwarded connections can be listed with
252 the session blocks waiting for forwarded X11 or TCP/IP
253 connections to terminate, it can be backgrounded with
255 (this should not be used while the user shell is active, as it can cause the
257 All available escapes can be listed with
260 A single tilde character can be sent as
262 (or by following the tilde by a character other than those described above).
263 The escape character must always follow a newline to be interpreted as
265 The escape character can be changed in configuration files
266 or on the command line.
268 If no pseudo tty has been allocated, the
269 session is transparent and can be used to reliably transfer binary
271 On most systems, setting the escape character to
273 will also make the session transparent even if a tty is used.
275 The session terminates when the command or shell in on the remote
276 machine exists and all X11 and TCP/IP connections have been closed.
277 The exit status of the remote program is returned as the exit status
281 .Ss X11 and TCP forwarding
283 If the user is using X11 (the
285 environment variable is set), the connection to the X11 display is
286 automatically forwarded to the remote side in such a way that any X11
287 programs started from the shell (or command) will go through the
288 encrypted channel, and the connection to the real X server will be made
289 from the local machine.
290 The user should not manually set
292 Forwarding of X11 connections can be
293 configured on the command line or in configuration files.
299 will point to the server machine, but with a display number greater
301 This is normal, and happens because
305 X server on the server machine for forwarding the
306 connections over the encrypted channel.
309 will also automatically set up Xauthority data on the server machine.
310 For this purpose, it will generate a random authorization cookie,
311 store it in Xauthority on the server, and verify that any forwarded
312 connections carry this cookie and replace it by the real cookie when
313 the connection is opened.
314 The real authentication cookie is never
315 sent to the server machine (and no cookies are sent in the plain).
317 If the user is using an authentication agent, the connection to the agent
318 is automatically forwarded to the remote side unless disabled on
319 command line or in a configuration file.
321 Forwarding of arbitrary TCP/IP connections over the secure channel can
322 be specified either on command line or in a configuration file.
323 One possible application of TCP/IP forwarding is a secure connection to an
324 electronic purse; another is going trough firewalls.
326 .Ss Server authentication
329 automatically maintains and checks a database containing
330 identifications for all hosts it has ever been used with.
331 RSA host keys are stored in
332 .Pa $HOME/.ssh/known_hosts
334 DSA host keys are stored in
335 .Pa $HOME/.ssh/known_hosts2
336 in the user's home directory.
337 Additionally, the files
338 .Pa /etc/ssh_known_hosts
340 .Pa /etc/ssh_known_hosts2
341 are automatically checked for known hosts.
342 Any new hosts are automatically added to the user's file.
343 If a host's identification
346 warns about this and disables password authentication to prevent a
347 trojan horse from getting the user's password.
349 this mechanism is to prevent man-in-the-middle attacks which could
350 otherwise be used to circumvent the encryption.
352 .Cm StrictHostKeyChecking
353 option (see below) can be used to prevent logins to machines whose
354 host key is not known or has changed.
358 Disables forwarding of the authentication agent connection.
360 Enables forwarding of the authentication agent connection.
361 This can also be specified on a per-host basis in a configuration file.
362 .It Fl c Ar blowfish|3des
363 Selects the cipher to use for encrypting the session.
366 It is believed to be secure.
368 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
369 It is presumably more secure than the
371 cipher which is no longer fully supported in
374 is a fast block cipher, it appears very secure and is much faster than
376 .It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
377 Additionally, for protocol version 2 a comma-separated list of ciphers can
378 be specified in order of preference.
379 Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
381 .It Fl e Ar ch|^ch|none
382 Sets the escape character for sessions with a pty (default:
384 The escape character is only recognized at the beginning of a line.
385 The escape character followed by a dot
387 closes the connection, followed
388 by control-Z suspends the connection, and followed by itself sends the
389 escape character once.
390 Setting the character to
392 disables any escapes and makes the session fully transparent.
396 to go to background just before command execution.
399 is going to ask for passwords or passphrases, but the user
400 wants it in the background.
403 The recommended way to start X11 programs at a remote site is with
405 .Ic ssh -f host xterm .
407 Allows remote hosts to connect to local forwarded ports.
408 .It Fl i Ar identity_file
409 Selects the file from which the identity (private key) for
410 RSA authentication is read.
412 .Pa $HOME/.ssh/identity
413 in the user's home directory.
414 Identity files may also be specified on
415 a per-host basis in the configuration file.
416 It is possible to have multiple
418 options (and multiple identities specified in
419 configuration files).
421 Disables forwarding of Kerberos tickets and AFS tokens.
422 This may also be specified on a per-host basis in the configuration file.
423 .It Fl l Ar login_name
424 Specifies the user to log in as on the remote machine.
425 This also may be specified on a per-host basis in the configuration file.
429 (actually, prevents reading from stdin).
430 This must be used when
432 is run in the background.
433 A common trick is to use this to run X11 programs on a remote machine.
435 .Ic ssh -n shadows.cs.hut.fi emacs &
436 will start an emacs on shadows.cs.hut.fi, and the X11
437 connection will be automatically forwarded over an encrypted channel.
440 program will be put in the background.
441 (This does not work if
443 needs to ask for a password or passphrase; see also the
447 Do not execute a remote command.
448 This is usefull if you just want to forward ports
449 (protocol version 2 only).
451 Can be used to give options in the format used in the config file.
452 This is useful for specifying options for which there is no separate
454 The option has the same format as a line in the configuration file.
456 Port to connect to on the remote host.
457 This can be specified on a
458 per-host basis in the configuration file.
460 Use a non-privileged port for outgoing connections.
461 This can be used if your firewall does
462 not permit connections from privileged ports.
463 Note that this option turns off
464 .Cm RhostsAuthentication
466 .Cm RhostsRSAAuthentication .
469 Causes all warning and diagnostic messages to be suppressed.
470 Only fatal errors are displayed.
472 Force pseudo-tty allocation.
473 This can be used to execute arbitrary
474 screen-based programs on a remote machine, which can be very useful,
475 e.g., when implementing menu services.
477 Disable pseudo-tty allocation (protocol version 2 only).
482 to print debugging messages about its progress.
484 debugging connection, authentication, and configuration problems.
485 The verbose mode is also used to display
487 challenges, if the user entered "s/key" as password.
488 Multiple -v options increases the verbosity.
491 Disables X11 forwarding.
493 Enables X11 forwarding.
494 This can also be specified on a per-host basis in a configuration file.
496 Requests compression of all data (including stdin, stdout, stderr, and
497 data for forwarded X11 and TCP/IP connections).
498 The compression algorithm is the same used by
502 can be controlled by the
505 Compression is desirable on modem lines and other
506 slow connections, but will only slow down things on fast networks.
507 The default value can be set on a host-by-host basis in the
508 configuration files; see the
511 .It Fl L Ar port:host:hostport
512 Specifies that the given port on the local (client) host is to be
513 forwarded to the given host and port on the remote side.
514 This works by allocating a socket to listen to
516 on the local side, and whenever a connection is made to this port, the
517 connection is forwarded over the secure channel, and a connection is
522 from the remote machine.
523 Port forwardings can also be specified in the configuration file.
524 Only root can forward privileged ports.
525 IPv6 addresses can be specified with an alternative syntax:
526 .Ar port/host/hostport
527 .It Fl R Ar port:host:hostport
528 Specifies that the given port on the remote (server) host is to be
529 forwarded to the given host and port on the local side.
530 This works by allocating a socket to listen to
532 on the remote side, and whenever a connection is made to this port, the
533 connection is forwarded over the secure channel, and a connection is
538 from the local machine.
539 Port forwardings can also be specified in the configuration file.
540 Privileged ports can be forwarded only when
541 logging in as root on the remote machine.
545 to try protocol version 2 only.
549 to use IPv4 addresses only.
553 to use IPv6 addresses only.
555 .Sh CONFIGURATION FILES
557 obtains configuration data from the following sources (in this order):
558 command line options, user's configuration file
559 .Pq Pa $HOME/.ssh/config ,
560 and system-wide configuration file
561 .Pq Pa /etc/ssh_config .
562 For each parameter, the first obtained value
564 The configuration files contain sections bracketed by
566 specifications, and that section is only applied for hosts that
567 match one of the patterns given in the specification.
568 The matched host name is the one given on the command line.
570 Since the first obtained value for each parameter is used, more
571 host-specific declarations should be given near the beginning of the
572 file, and general defaults at the end.
574 The configuration file has the following format:
576 Empty lines and lines starting with
580 Otherwise a line is of the format
581 .Dq keyword arguments .
583 keywords and their meanings are as follows (note that the
584 configuration files are case-sensitive):
587 Restricts the following declarations (up to the next
589 keyword) to be only for those hosts that match one of the patterns
590 given after the keyword.
594 can be used as wildcards in the
598 as a pattern can be used to provide global
599 defaults for all hosts.
602 argument given on the command line (i.e., the name is not converted to
603 a canonicalized host name before matching).
604 .It Cm AFSTokenPassing
605 Specifies whether to pass AFS tokens to remote host.
606 The argument to this keyword must be
613 passphrase/password querying will be disabled.
614 This option is useful in scripts and other batch jobs where you have no
615 user to supply the password.
621 If this flag is set to
623 ssh will additionally check the host ip address in the
626 This allows ssh to detect if a host key changed due to DNS spoofing.
627 If the option is set to
629 the check will not be executed.
631 Specifies the cipher to use for encrypting the session
632 in protocol version 1.
641 Specifies the ciphers allowed for protocol version 2
642 in order of preference.
643 Multiple ciphers must be comma-separated.
645 .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour .
647 Specifies whether to use compression.
652 .It Cm CompressionLevel
653 Specifies the compression level to use if compression is enable.
654 The argument must be an integer from 1 (fast) to 9 (slow, best).
655 The default level is 6, which is good for most applications.
656 The meaning of the values is the same as in
658 .It Cm ConnectionAttempts
659 Specifies the number of tries (one per second) to make before falling
660 back to rsh or exiting.
661 The argument must be an integer.
662 This may be useful in scripts if the connection sometimes fails.
663 .It Cm DSAAuthentication
664 Specifies whether to try DSA authentication.
665 The argument to this keyword must be
669 DSA authentication will only be
670 attempted if a DSA identity file exists.
671 Note that this option applies to protocol version 2 only.
673 Sets the escape character (default:
675 The escape character can also
676 be set on the command line.
677 The argument should be a single character,
679 followed by a letter, or
681 to disable the escape
682 character entirely (making the connection transparent for binary
685 Specifies that if connecting via
687 fails due to a connection refused error (there is no
689 listening on the remote host),
691 should automatically be used instead (after a suitable warning about
692 the session being unencrypted).
698 Specifies whether the connection to the authentication agent (if any)
699 will be forwarded to the remote machine.
707 Specifies whether X11 connections will be automatically redirected
708 over the secure channel and
718 Specifies whether remote hosts are allowed to connect to local
726 .It Cm GlobalKnownHostsFile
727 Specifies a file to use instead of
728 .Pa /etc/ssh_known_hosts .
730 Specifies the real host name to log into.
731 This can be used to specify nicknames or abbreviations for hosts.
732 Default is the name given on the command line.
733 Numeric IP addresses are also permitted (both on the command line and in
737 Specifies the file from which the user's RSA authentication identity
739 .Pa $HOME/.ssh/identity
740 in the user's home directory).
741 Additionally, any identities represented by the authentication agent
742 will be used for authentication.
743 The file name may use the tilde
744 syntax to refer to a user's home directory.
745 It is possible to have
746 multiple identity files specified in configuration files; all these
747 identities will be tried in sequence.
749 Specifies the file from which the user's DSA authentication identity
751 .Pa $HOME/.ssh/id_dsa
752 in the user's home directory).
753 The file name may use the tilde
754 syntax to refer to a user's home directory.
755 It is possible to have
756 multiple identity files specified in configuration files; all these
757 identities will be tried in sequence.
759 Specifies whether the system should send keepalive messages to the
761 If they are sent, death of the connection or crash of one
762 of the machines will be properly noticed.
763 However, this means that
764 connections will die if the route is down temporarily, and some people
769 (to send keepalives), and the client will notice
770 if the network goes down or the remote host dies.
771 This is important in scripts, and many users want it too.
773 To disable keepalives, the value should be set to
775 in both the server and the client configuration files.
776 .It Cm KerberosAuthentication
777 Specifies whether Kerberos authentication will be used.
778 The argument to this keyword must be
782 .It Cm KerberosTgtPassing
783 Specifies whether a Kerberos TGT will be forwarded to the server.
784 This will only work if the Kerberos server is actually an AFS kaserver.
785 The argument to this keyword must be
790 Specifies that a TCP/IP port on the local machine be forwarded over
791 the secure channel to given host:port from the remote machine.
792 The first argument must be a port number, and the second must be
794 Multiple forwardings may be specified, and additional
795 forwardings can be given on the command line.
796 Only the superuser can forward privileged ports.
798 Gives the verbosity level that is used when logging messages from
800 The possible values are:
801 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
803 .It Cm NumberOfPasswordPrompts
804 Specifies the number of password prompts before giving up.
805 The argument to this keyword must be an integer.
807 .It Cm PasswordAuthentication
808 Specifies whether to use password authentication.
809 The argument to this keyword must be
813 Note that this option applies to both protocol version 1 and 2.
815 Specifies the port number to connect on the remote host.
818 Specifies the protocol versions
820 should support in order of preference.
821 The possible values are
825 Multiple versions must be comma-separated.
830 tries version 1 and falls back to version 2
831 if version 1 is not available.
833 Specifies the command to use to connect to the server.
835 string extends to the end of the line, and is executed with
837 In the command string,
839 will be substituted by the host name to
843 The command can be basically anything,
844 and should read from its standard input and write to its standard output.
845 It should eventually connect an
847 server running on some machine, or execute
850 Host key management will be done using the
851 HostName of the host being connected (defaulting to the name typed by
855 is not available for connects with a proxy command.
858 Specifies that a TCP/IP port on the remote machine be forwarded over
859 the secure channel to given host:port from the local machine.
860 The first argument must be a port number, and the second must be
862 Multiple forwardings may be specified, and additional
863 forwardings can be given on the command line.
864 Only the superuser can forward privileged ports.
865 .It Cm RhostsAuthentication
866 Specifies whether to try rhosts based authentication.
868 declaration only affects the client side and has no effect whatsoever
870 Disabling rhosts authentication may reduce
871 authentication time on slow connections when rhosts authentication is
873 Most servers do not permit RhostsAuthentication because it
874 is not secure (see RhostsRSAAuthentication).
875 The argument to this keyword must be
879 .It Cm RhostsRSAAuthentication
880 Specifies whether to try rhosts based authentication with RSA host
882 This is the primary authentication method for most sites.
887 .It Cm RSAAuthentication
888 Specifies whether to try RSA authentication.
889 The argument to this keyword must be
893 RSA authentication will only be
894 attempted if the identity file exists, or an authentication agent is
896 Note that this option applies to protocol version 1 only.
897 .It Cm SkeyAuthentication
898 Specifies whether to use
901 The argument to this keyword must be
907 .It Cm StrictHostKeyChecking
908 If this flag is set to
911 ssh will never automatically add host keys to the
912 .Pa $HOME/.ssh/known_hosts
914 .Pa $HOME/.ssh/known_hosts2
915 files, and refuses to connect hosts whose host key has changed.
916 This provides maximum protection against trojan horse attacks.
917 However, it can be somewhat annoying if you don't have good
918 .Pa /etc/ssh_known_hosts
920 .Pa /etc/ssh_known_hosts2
921 files installed and frequently
923 Basically this option forces the user to manually
925 Normally this option is disabled, and new hosts
926 will automatically be added to the known host files.
928 known hosts will be verified automatically in either case.
933 .It Cm UsePrivilegedPort
934 Specifies whether to use a privileged port for outgoing connections.
941 Note that setting this option to
944 .Cm RhostsAuthentication
946 .Cm RhostsRSAAuthentication .
948 Specifies the user to log in as.
949 This can be useful if you have a different user name on different machines.
950 This saves the trouble of
951 having to remember to give the user name on the command line.
952 .It Cm UserKnownHostsFile
953 Specifies a file to use instead of
954 .Pa $HOME/.ssh/known_hosts .
956 Specifies that rlogin/rsh should be used for this host.
957 It is possible that the host does not at all support the
962 to immediately execute
964 All other options (except
966 are ignored if this has been specified.
972 Specifies the location of the
976 .Pa /usr/X11R6/bin/xauth .
980 will normally set the following environment variables:
985 variable indicates the location of the X11 server.
986 It is automatically set by
988 to point to a value of the form
990 where hostname indicates
991 the host where the shell runs, and n is an integer >= 1.
993 uses this special value to forward X11 connections over the secure
995 The user should normally not set DISPLAY explicitly, as that
996 will render the X11 connection insecure (and will require the user to
997 manually copy any required authorization cookies).
999 Set to the path of the user's home directory.
1003 set for compatibility with systems that use this variable.
1005 Set to point the user's mailbox.
1009 as specified when compiling
1011 .It Ev SSH_AUTH_SOCK
1012 indicates the path of a unix-domain socket used to communicate with the
1015 Identifies the client end of the connection.
1016 The variable contains
1017 three space-separated values: client ip-address, client port number,
1018 and server port number.
1020 This is set to the name of the tty (path to the device) associated
1021 with the current shell or command.
1022 If the current session has no tty,
1023 this variable is not set.
1025 The timezone variable is set to indicate the present timezone if it
1026 was set when the daemon was started (i.e., the daemon passes the value
1027 on to new connections).
1029 Set to the name of the user logging in.
1035 .Pa $HOME/.ssh/environment ,
1036 and adds lines of the format
1041 .It Pa $HOME/.ssh/known_hosts
1042 Records host keys for all hosts the user has logged into (that are not
1044 .Pa /etc/ssh_known_hosts ) .
1047 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1048 Contains the RSA and the DSA authentication identity of the user.
1050 contain sensitive data and should be readable by the user but not
1051 accessible by others (read/write/execute).
1054 ignores a private key file if it is accessible by others.
1055 It is possible to specify a passphrase when
1056 generating the key; the passphrase will be used to encrypt the
1057 sensitive part of this file using 3DES.
1058 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1059 Contains the public key for authentication (public part of the
1060 identity file in human-readable form).
1062 .Pa $HOME/.ssh/identity.pub
1063 file should be added to
1064 .Pa $HOME/.ssh/authorized_keys
1066 where you wish to log in using RSA authentication.
1068 .Pa $HOME/.ssh/id_dsa.pub
1069 file should be added to
1070 .Pa $HOME/.ssh/authorized_keys2
1072 where you wish to log in using DSA authentication.
1074 sensitive and can (but need not) be readable by anyone.
1076 never used automatically and are not necessary; they is only provided for
1077 the convenience of the user.
1078 .It Pa $HOME/.ssh/config
1079 This is the per-user configuration file.
1080 The format of this file is described above.
1081 This file is used by the
1084 This file does not usually contain any sensitive information,
1085 but the recommended permissions are read/write for the user, and not
1086 accessible by others.
1087 .It Pa $HOME/.ssh/authorized_keys
1088 Lists the RSA keys that can be used for logging in as this user.
1089 The format of this file is described in the
1092 In the simplest form the format is the same as the .pub
1093 identity files (that is, each line contains the number of bits in
1094 modulus, public exponent, modulus, and comment fields, separated by
1096 This file is not highly sensitive, but the recommended
1097 permissions are read/write for the user, and not accessible by others.
1098 .It Pa $HOME/.ssh/authorized_keys2
1099 Lists the DSA keys that can be used for logging in as this user.
1100 This file is not highly sensitive, but the recommended
1101 permissions are read/write for the user, and not accessible by others.
1102 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1103 Systemwide list of known host keys.
1104 .Pa /etc/ssh_known_hosts
1106 .Pa /etc/ssh_known_hosts2
1108 These files should be prepared by the
1109 system administrator to contain the public host keys of all machines in the
1111 This file should be world-readable.
1113 public keys, one per line, in the following format (fields separated
1114 by spaces): system name, number of bits in modulus, public exponent,
1115 modulus, and optional comment field.
1116 When different names are used
1117 for the same machine, all such names should be listed, separated by
1119 The format is described on the
1123 The canonical system name (as returned by name servers) is used by
1125 to verify the client host when logging in; other names are needed because
1127 does not convert the user-supplied name to a canonical name before
1128 checking the key, because someone with access to the name servers
1129 would then be able to fool host authentication.
1130 .It Pa /etc/ssh_config
1131 Systemwide configuration file.
1132 This file provides defaults for those
1133 values that are not specified in the user's configuration file, and
1134 for those users who do not have a configuration file.
1135 This file must be world-readable.
1136 .It Pa $HOME/.rhosts
1137 This file is used in
1139 authentication to list the
1140 host/user pairs that are permitted to log in.
1141 (Note that this file is
1142 also used by rlogin and rsh, which makes using this file insecure.)
1143 Each line of the file contains a host name (in the canonical form
1144 returned by name servers), and then a user name on that host,
1145 separated by a space.
1146 One some machines this file may need to be
1147 world-readable if the user's home directory is on a NFS partition,
1151 Additionally, this file must be owned by the user,
1152 and must not have write permissions for anyone else.
1154 permission for most machines is read/write for the user, and not
1155 accessible by others.
1157 Note that by default
1159 will be installed so that it requires successful RSA host
1160 authentication before permitting \s+2.\s0rhosts authentication.
1161 If your server machine does not have the client's host key in
1162 .Pa /etc/ssh_known_hosts ,
1164 .Pa $HOME/.ssh/known_hosts .
1165 The easiest way to do this is to
1166 connect back to the client from the server machine using ssh; this
1167 will automatically add the host key to
1168 .Pa $HOME/.ssh/known_hosts .
1169 .It Pa $HOME/.shosts
1170 This file is used exactly the same way as
1173 having this file is to be able to use rhosts authentication with
1175 without permitting login with
1179 .It Pa /etc/hosts.equiv
1180 This file is used during
1181 .Pa \&.rhosts authentication.
1183 canonical hosts names, one per line (the full format is described on
1187 If the client host is found in this file, login is
1188 automatically permitted provided client and server user names are the
1190 Additionally, successful RSA host authentication is normally
1192 This file should only be writable by root.
1193 .It Pa /etc/shosts.equiv
1194 This file is processed exactly as
1195 .Pa /etc/hosts.equiv .
1196 This file may be useful to permit logins using
1198 but not using rsh/rlogin.
1200 Commands in this file are executed by
1202 when the user logs in just before the user's shell (or command) is started.
1205 manual page for more information.
1206 .It Pa $HOME/.ssh/rc
1207 Commands in this file are executed by
1209 when the user logs in just before the user's shell (or command) is
1213 manual page for more information.
1214 .It Pa $HOME/.ssh/environment
1215 Contains additional definitions for environment variables, see section
1218 .It Pa libcrypto.so.X.1
1219 A version of this library which includes support for the RSA algorithm
1220 is required for proper operation.
1224 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1225 but with bugs removed and newer features re-added.
1227 1.2.12 release, newer versions of the original ssh bore successively
1228 more restrictive licenses, and thus demand for a free version was born.
1230 This version of OpenSSH
1233 has all components of a restrictive nature (i.e., patents, see
1235 directly removed from the source code; any licensed or patented components
1239 has been updated to support SSH protocol 1.5 and 2, making it compatible with
1240 all other SSH clients and servers.
1242 contains added support for
1244 authentication and ticket passing.
1246 supports one-time password authentication with
1250 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1251 Niels Provos, Theo de Raadt, and Dug Song.
1253 The support for SSH protocol 2 was written by Markus Friedl.