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 1.90 2001/02/10 16:03:29 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 afgknqstvxACNPTX1246
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 or RSA algorithm is used
214 The client uses his private 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 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 host keys used in the protocol version 2 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.
356 The options are as follows:
359 Disables forwarding of the authentication agent connection.
361 Enables forwarding of the authentication agent connection.
362 This can also be specified on a per-host basis in a configuration file.
363 .It Fl c Ar blowfish|3des
364 Selects the cipher to use for encrypting the session.
367 It is believed to be secure.
369 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
370 It is presumably more secure than the
372 cipher which is no longer fully supported in
375 is a fast block cipher, it appears very secure and is much faster than
377 .It Fl c Ar cipher_spec
378 Additionally, for protocol version 2 a comma-separated list of ciphers can
379 be specified in order of preference.
382 for more information.
383 .It Fl e Ar ch|^ch|none
384 Sets the escape character for sessions with a pty (default:
386 The escape character is only recognized at the beginning of a line.
387 The escape character followed by a dot
389 closes the connection, followed
390 by control-Z suspends the connection, and followed by itself sends the
391 escape character once.
392 Setting the character to
394 disables any escapes and makes the session fully transparent.
398 to go to background just before command execution.
401 is going to ask for passwords or passphrases, but the user
402 wants it in the background.
405 The recommended way to start X11 programs at a remote site is with
407 .Ic ssh -f host xterm .
409 Allows remote hosts to connect to local forwarded ports.
410 .It Fl i Ar identity_file
411 Selects the file from which the identity (private key) for
412 RSA or DSA authentication is read.
414 .Pa $HOME/.ssh/identity
415 in the user's home directory.
416 Identity files may also be specified on
417 a per-host basis in the configuration file.
418 It is possible to have multiple
420 options (and multiple identities specified in
421 configuration files).
423 Disables forwarding of Kerberos tickets and AFS tokens.
424 This may also be specified on a per-host basis in the configuration file.
425 .It Fl l Ar login_name
426 Specifies the user to log in as on the remote machine.
427 This also may be specified on a per-host basis in the configuration file.
431 (actually, prevents reading from stdin).
432 This must be used when
434 is run in the background.
435 A common trick is to use this to run X11 programs on a remote machine.
437 .Ic ssh -n shadows.cs.hut.fi emacs &
438 will start an emacs on shadows.cs.hut.fi, and the X11
439 connection will be automatically forwarded over an encrypted channel.
442 program will be put in the background.
443 (This does not work if
445 needs to ask for a password or passphrase; see also the
449 Do not execute a remote command.
450 This is useful if you just want to forward ports
451 (protocol version 2 only).
453 Can be used to give options in the format used in the config file.
454 This is useful for specifying options for which there is no separate
456 The option has the same format as a line in the configuration file.
458 Port to connect to on the remote host.
459 This can be specified on a
460 per-host basis in the configuration file.
462 Use a non-privileged port for outgoing connections.
463 This can be used if your firewall does
464 not permit connections from privileged ports.
465 Note that this option turns off
466 .Cm RhostsAuthentication
468 .Cm RhostsRSAAuthentication
472 Causes all warning and diagnostic messages to be suppressed.
473 Only fatal errors are displayed.
475 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
476 of SSH as a secure transport for other application (eg. sftp). The
477 subsystem is specified as the remote command.
479 Force pseudo-tty allocation.
480 This can be used to execute arbitrary
481 screen-based programs on a remote machine, which can be very useful,
482 e.g., when implementing menu services.
485 options force tty allocation, even if
489 Disable pseudo-tty allocation.
494 to print debugging messages about its progress.
496 debugging connection, authentication, and configuration problems.
499 options increases the verbosity.
502 Disables X11 forwarding.
504 Enables X11 forwarding.
505 This can also be specified on a per-host basis in a configuration file.
507 Requests compression of all data (including stdin, stdout, stderr, and
508 data for forwarded X11 and TCP/IP connections).
509 The compression algorithm is the same used by
513 can be controlled by the
516 Compression is desirable on modem lines and other
517 slow connections, but will only slow down things on fast networks.
518 The default value can be set on a host-by-host basis in the
519 configuration files; see the
522 .It Fl L Ar port:host:hostport
523 Specifies that the given port on the local (client) host is to be
524 forwarded to the given host and port on the remote side.
525 This works by allocating a socket to listen to
527 on the local side, and whenever a connection is made to this port, the
528 connection is forwarded over the secure channel, and a connection is
533 from the remote machine.
534 Port forwardings can also be specified in the configuration file.
535 Only root can forward privileged ports.
536 IPv6 addresses can be specified with an alternative syntax:
537 .Ar port/host/hostport
538 .It Fl R Ar port:host:hostport
539 Specifies that the given port on the remote (server) host is to be
540 forwarded to the given host and port on the local side.
541 This works by allocating a socket to listen to
543 on the remote side, and whenever a connection is made to this port, the
544 connection is forwarded over the secure channel, and a connection is
549 from the local machine.
550 Port forwardings can also be specified in the configuration file.
551 Privileged ports can be forwarded only when
552 logging in as root on the remote machine.
556 to try protocol version 1 only.
560 to try protocol version 2 only.
564 to use IPv4 addresses only.
568 to use IPv6 addresses only.
570 .Sh CONFIGURATION FILES
572 obtains configuration data from the following sources (in this order):
573 command line options, user's configuration file
574 .Pq Pa $HOME/.ssh/config ,
575 and system-wide configuration file
576 .Pq Pa /etc/ssh_config .
577 For each parameter, the first obtained value
579 The configuration files contain sections bracketed by
581 specifications, and that section is only applied for hosts that
582 match one of the patterns given in the specification.
583 The matched host name is the one given on the command line.
585 Since the first obtained value for each parameter is used, more
586 host-specific declarations should be given near the beginning of the
587 file, and general defaults at the end.
589 The configuration file has the following format:
591 Empty lines and lines starting with
595 Otherwise a line is of the format
596 .Dq keyword arguments .
598 keywords and their meanings are as follows (note that the
599 configuration files are case-sensitive):
602 Restricts the following declarations (up to the next
604 keyword) to be only for those hosts that match one of the patterns
605 given after the keyword.
609 can be used as wildcards in the
613 as a pattern can be used to provide global
614 defaults for all hosts.
617 argument given on the command line (i.e., the name is not converted to
618 a canonicalized host name before matching).
619 .It Cm AFSTokenPassing
620 Specifies whether to pass AFS tokens to remote host.
621 The argument to this keyword must be
628 passphrase/password querying will be disabled.
629 This option is useful in scripts and other batch jobs where you have no
630 user to supply the password.
636 If this flag is set to
638 ssh will additionally check the host ip address in the
641 This allows ssh to detect if a host key changed due to DNS spoofing.
642 If the option is set to
644 the check will not be executed.
646 Specifies the cipher to use for encrypting the session
647 in protocol version 1.
656 Specifies the ciphers allowed for protocol version 2
657 in order of preference.
658 Multiple ciphers must be comma-separated.
662 ``3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc,
663 aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
664 rijndael256-cbc,rijndael-cbc@lysator.liu.se''
667 Specifies whether to use compression.
672 .It Cm CompressionLevel
673 Specifies the compression level to use if compression is enable.
674 The argument must be an integer from 1 (fast) to 9 (slow, best).
675 The default level is 6, which is good for most applications.
676 The meaning of the values is the same as in
678 .It Cm ConnectionAttempts
679 Specifies the number of tries (one per second) to make before falling
680 back to rsh or exiting.
681 The argument must be an integer.
682 This may be useful in scripts if the connection sometimes fails.
683 .It Cm PubkeyAuthentication
684 Specifies whether to try public key authentication.
685 The argument to this keyword must be
689 Note that this option applies to protocol version 2 only.
691 Sets the escape character (default:
693 The escape character can also
694 be set on the command line.
695 The argument should be a single character,
697 followed by a letter, or
699 to disable the escape
700 character entirely (making the connection transparent for binary
703 Specifies that if connecting via
705 fails due to a connection refused error (there is no
707 listening on the remote host),
709 should automatically be used instead (after a suitable warning about
710 the session being unencrypted).
716 Specifies whether the connection to the authentication agent (if any)
717 will be forwarded to the remote machine.
725 Specifies whether X11 connections will be automatically redirected
726 over the secure channel and
736 Specifies whether remote hosts are allowed to connect to local
744 .It Cm GlobalKnownHostsFile
745 Specifies a file to use instead of
746 .Pa /etc/ssh_known_hosts .
748 Specifies an alias that should be used instead of the
749 real host name when looking up or saving the host key
750 in the known_hosts files.
751 This option is useful for tunneling ssh connections
752 or if you have multiple servers running on a single host.
754 Specifies the real host name to log into.
755 This can be used to specify nicknames or abbreviations for hosts.
756 Default is the name given on the command line.
757 Numeric IP addresses are also permitted (both on the command line and in
761 Specifies the file from which the user's RSA authentication identity
763 .Pa $HOME/.ssh/identity
764 in the user's home directory).
765 Additionally, any identities represented by the authentication agent
766 will be used for authentication.
767 The file name may use the tilde
768 syntax to refer to a user's home directory.
769 It is possible to have
770 multiple identity files specified in configuration files; all these
771 identities will be tried in sequence.
773 Specifies whether the system should send keepalive messages to the
775 If they are sent, death of the connection or crash of one
776 of the machines will be properly noticed.
777 However, this means that
778 connections will die if the route is down temporarily, and some people
783 (to send keepalives), and the client will notice
784 if the network goes down or the remote host dies.
785 This is important in scripts, and many users want it too.
787 To disable keepalives, the value should be set to
789 in both the server and the client configuration files.
790 .It Cm KerberosAuthentication
791 Specifies whether Kerberos authentication will be used.
792 The argument to this keyword must be
796 .It Cm KerberosTgtPassing
797 Specifies whether a Kerberos TGT will be forwarded to the server.
798 This will only work if the Kerberos server is actually an AFS kaserver.
799 The argument to this keyword must be
804 Specifies that a TCP/IP port on the local machine be forwarded over
805 the secure channel to given host:port from the remote machine.
806 The first argument must be a port number, and the second must be
808 Multiple forwardings may be specified, and additional
809 forwardings can be given on the command line.
810 Only the superuser can forward privileged ports.
812 Gives the verbosity level that is used when logging messages from
814 The possible values are:
815 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
817 .It Cm NumberOfPasswordPrompts
818 Specifies the number of password prompts before giving up.
819 The argument to this keyword must be an integer.
821 .It Cm PasswordAuthentication
822 Specifies whether to use password authentication.
823 The argument to this keyword must be
827 Note that this option applies to both protocol version 1 and 2.
829 Specifies the port number to connect on the remote host.
832 Specifies the protocol versions
834 should support in order of preference.
835 The possible values are
839 Multiple versions must be comma-separated.
844 tries version 1 and falls back to version 2
845 if version 1 is not available.
847 Specifies the command to use to connect to the server.
849 string extends to the end of the line, and is executed with
851 In the command string,
853 will be substituted by the host name to
857 The command can be basically anything,
858 and should read from its standard input and write to its standard output.
859 It should eventually connect an
861 server running on some machine, or execute
864 Host key management will be done using the
865 HostName of the host being connected (defaulting to the name typed by
869 is not available for connects with a proxy command.
872 Specifies that a TCP/IP port on the remote machine be forwarded over
873 the secure channel to given host:port from the local machine.
874 The first argument must be a port number, and the second must be
876 Multiple forwardings may be specified, and additional
877 forwardings can be given on the command line.
878 Only the superuser can forward privileged ports.
879 .It Cm RhostsAuthentication
880 Specifies whether to try rhosts based authentication.
882 declaration only affects the client side and has no effect whatsoever
884 Disabling rhosts authentication may reduce
885 authentication time on slow connections when rhosts authentication is
887 Most servers do not permit RhostsAuthentication because it
888 is not secure (see RhostsRSAAuthentication).
889 The argument to this keyword must be
893 .It Cm RhostsRSAAuthentication
894 Specifies whether to try rhosts based authentication with RSA host
896 This is the primary authentication method for most sites.
901 .It Cm RSAAuthentication
902 Specifies whether to try RSA authentication.
903 The argument to this keyword must be
907 RSA authentication will only be
908 attempted if the identity file exists, or an authentication agent is
910 Note that this option applies to protocol version 1 only.
911 .It Cm ChallengeResponseAuthentication
912 Specifies whether to use challenge response authentication.
913 Currently there is only support for
916 The argument to this keyword must be
922 .It Cm StrictHostKeyChecking
923 If this flag is set to
926 will never automatically add host keys to the
927 .Pa $HOME/.ssh/known_hosts
929 .Pa $HOME/.ssh/known_hosts2
930 files, and refuses to connect to hosts whose host key has changed.
931 This provides maximum protection against trojan horse attacks.
932 However, it can be somewhat annoying if you don't have good
933 .Pa /etc/ssh_known_hosts
935 .Pa /etc/ssh_known_hosts2
936 files installed and frequently
937 connect to new hosts.
938 This option forces the user to manually
940 If this flag is set to
943 will automatically add new host keys to the
944 user known hosts files.
945 If this flag is set to
948 will be added to the user known host files only after the user
949 has confirmed that is what they really want to do, and
951 will refuse to connect to hosts whose host key has changed.
953 known hosts will be verified automatically in all cases.
961 .It Cm UsePrivilegedPort
962 Specifies whether to use a privileged port for outgoing connections.
969 Note that setting this option to
972 .Cm RhostsAuthentication
974 .Cm RhostsRSAAuthentication
977 Specifies the user to log in as.
978 This can be useful if you have a different user name on different machines.
979 This saves the trouble of
980 having to remember to give the user name on the command line.
981 .It Cm UserKnownHostsFile
982 Specifies a file to use instead of
983 .Pa $HOME/.ssh/known_hosts .
985 Specifies that rlogin/rsh should be used for this host.
986 It is possible that the host does not at all support the
991 to immediately execute
993 All other options (except
995 are ignored if this has been specified.
1000 .It Cm XAuthLocation
1001 Specifies the location of the
1005 .Pa /usr/X11R6/bin/xauth .
1009 will normally set the following environment variables:
1014 variable indicates the location of the X11 server.
1015 It is automatically set by
1017 to point to a value of the form
1019 where hostname indicates
1020 the host where the shell runs, and n is an integer >= 1.
1022 uses this special value to forward X11 connections over the secure
1024 The user should normally not set DISPLAY explicitly, as that
1025 will render the X11 connection insecure (and will require the user to
1026 manually copy any required authorization cookies).
1028 Set to the path of the user's home directory.
1032 set for compatibility with systems that use this variable.
1034 Set to point the user's mailbox.
1038 as specified when compiling
1040 .It Ev SSH_AUTH_SOCK
1041 indicates the path of a unix-domain socket used to communicate with the
1044 Identifies the client end of the connection.
1045 The variable contains
1046 three space-separated values: client ip-address, client port number,
1047 and server port number.
1048 .It Ev SSH_ORIGINAL_COMMAND
1049 The variable contains the original command line if a forced command
1051 It can be used to extract the original arguments.
1053 This is set to the name of the tty (path to the device) associated
1054 with the current shell or command.
1055 If the current session has no tty,
1056 this variable is not set.
1058 The timezone variable is set to indicate the present timezone if it
1059 was set when the daemon was started (i.e., the daemon passes the value
1060 on to new connections).
1062 Set to the name of the user logging in.
1068 .Pa $HOME/.ssh/environment ,
1069 and adds lines of the format
1074 .It Pa $HOME/.ssh/known_hosts
1075 Records host keys for all hosts the user has logged into (that are not
1077 .Pa /etc/ssh_known_hosts ) .
1080 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1081 Contains the RSA and the DSA authentication identity of the user.
1083 contain sensitive data and should be readable by the user but not
1084 accessible by others (read/write/execute).
1087 ignores a private key file if it is accessible by others.
1088 It is possible to specify a passphrase when
1089 generating the key; the passphrase will be used to encrypt the
1090 sensitive part of this file using 3DES.
1091 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1092 Contains the public key for authentication (public part of the
1093 identity file in human-readable form).
1095 .Pa $HOME/.ssh/identity.pub
1096 file should be added to
1097 .Pa $HOME/.ssh/authorized_keys
1099 where you wish to log in using RSA authentication.
1101 .Pa $HOME/.ssh/id_dsa.pub
1102 file should be added to
1103 .Pa $HOME/.ssh/authorized_keys2
1105 where you wish to log in using DSA authentication.
1107 sensitive and can (but need not) be readable by anyone.
1109 never used automatically and are not necessary; they are only provided for
1110 the convenience of the user.
1111 .It Pa $HOME/.ssh/config
1112 This is the per-user configuration file.
1113 The format of this file is described above.
1114 This file is used by the
1117 This file does not usually contain any sensitive information,
1118 but the recommended permissions are read/write for the user, and not
1119 accessible by others.
1120 .It Pa $HOME/.ssh/authorized_keys
1121 Lists the RSA keys that can be used for logging in as this user.
1122 The format of this file is described in the
1125 In the simplest form the format is the same as the .pub
1126 identity files (that is, each line contains the number of bits in
1127 modulus, public exponent, modulus, and comment fields, separated by
1129 This file is not highly sensitive, but the recommended
1130 permissions are read/write for the user, and not accessible by others.
1131 .It Pa $HOME/.ssh/authorized_keys2
1132 Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1133 This file is not highly sensitive, but the recommended
1134 permissions are read/write for the user, and not accessible by others.
1135 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1136 Systemwide list of known host keys.
1137 .Pa /etc/ssh_known_hosts
1139 .Pa /etc/ssh_known_hosts2
1140 contains DSA or RSA keys for protocol version 2.
1141 These files should be prepared by the
1142 system administrator to contain the public host keys of all machines in the
1144 This file should be world-readable.
1146 public keys, one per line, in the following format (fields separated
1147 by spaces): system name, number of bits in modulus, public exponent,
1148 modulus, and optional comment field.
1149 When different names are used
1150 for the same machine, all such names should be listed, separated by
1152 The format is described on the
1156 The canonical system name (as returned by name servers) is used by
1158 to verify the client host when logging in; other names are needed because
1160 does not convert the user-supplied name to a canonical name before
1161 checking the key, because someone with access to the name servers
1162 would then be able to fool host authentication.
1163 .It Pa /etc/ssh_config
1164 Systemwide configuration file.
1165 This file provides defaults for those
1166 values that are not specified in the user's configuration file, and
1167 for those users who do not have a configuration file.
1168 This file must be world-readable.
1169 .It Pa $HOME/.rhosts
1170 This file is used in
1172 authentication to list the
1173 host/user pairs that are permitted to log in.
1174 (Note that this file is
1175 also used by rlogin and rsh, which makes using this file insecure.)
1176 Each line of the file contains a host name (in the canonical form
1177 returned by name servers), and then a user name on that host,
1178 separated by a space.
1179 One some machines this file may need to be
1180 world-readable if the user's home directory is on a NFS partition,
1184 Additionally, this file must be owned by the user,
1185 and must not have write permissions for anyone else.
1187 permission for most machines is read/write for the user, and not
1188 accessible by others.
1190 Note that by default
1192 will be installed so that it requires successful RSA host
1193 authentication before permitting \s+2.\s0rhosts authentication.
1194 If your server machine does not have the client's host key in
1195 .Pa /etc/ssh_known_hosts ,
1197 .Pa $HOME/.ssh/known_hosts .
1198 The easiest way to do this is to
1199 connect back to the client from the server machine using ssh; this
1200 will automatically add the host key to
1201 .Pa $HOME/.ssh/known_hosts .
1202 .It Pa $HOME/.shosts
1203 This file is used exactly the same way as
1206 having this file is to be able to use rhosts authentication with
1208 without permitting login with
1212 .It Pa /etc/hosts.equiv
1213 This file is used during
1214 .Pa \&.rhosts authentication.
1216 canonical hosts names, one per line (the full format is described on
1220 If the client host is found in this file, login is
1221 automatically permitted provided client and server user names are the
1223 Additionally, successful RSA host authentication is normally
1225 This file should only be writable by root.
1226 .It Pa /etc/shosts.equiv
1227 This file is processed exactly as
1228 .Pa /etc/hosts.equiv .
1229 This file may be useful to permit logins using
1231 but not using rsh/rlogin.
1233 Commands in this file are executed by
1235 when the user logs in just before the user's shell (or command) is started.
1238 manual page for more information.
1239 .It Pa $HOME/.ssh/rc
1240 Commands in this file are executed by
1242 when the user logs in just before the user's shell (or command) is
1246 manual page for more information.
1247 .It Pa $HOME/.ssh/environment
1248 Contains additional definitions for environment variables, see section
1251 .It Pa libcrypto.so.X.1
1252 A version of this library which includes support for the RSA algorithm
1253 is required for proper operation.
1256 OpenSSH is a derivative of the original and free
1257 ssh 1.2.12 release by Tatu Ylonen.
1258 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1259 Theo de Raadt and Dug Song
1260 removed many bugs, re-added newer features and
1262 Markus Friedl contributed the support for SSH
1263 protocol versions 1.5 and 2.0.