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 1.83 2001/02/04 11:11:55 djm 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 afgknqstvxACNPTX246
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 "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
378 Additionally, for protocol version 2 a comma-separated list of ciphers can
379 be specified in order of preference.
380 Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
382 .It Fl e Ar ch|^ch|none
383 Sets the escape character for sessions with a pty (default:
385 The escape character is only recognized at the beginning of a line.
386 The escape character followed by a dot
388 closes the connection, followed
389 by control-Z suspends the connection, and followed by itself sends the
390 escape character once.
391 Setting the character to
393 disables any escapes and makes the session fully transparent.
397 to go to background just before command execution.
400 is going to ask for passwords or passphrases, but the user
401 wants it in the background.
404 The recommended way to start X11 programs at a remote site is with
406 .Ic ssh -f host xterm .
408 Allows remote hosts to connect to local forwarded ports.
409 .It Fl i Ar identity_file
410 Selects the file from which the identity (private key) for
411 RSA or DSA authentication is read.
413 .Pa $HOME/.ssh/identity
414 in the user's home directory.
415 Identity files may also be specified on
416 a per-host basis in the configuration file.
417 It is possible to have multiple
419 options (and multiple identities specified in
420 configuration files).
422 Disables forwarding of Kerberos tickets and AFS tokens.
423 This may also be specified on a per-host basis in the configuration file.
424 .It Fl l Ar login_name
425 Specifies the user to log in as on the remote machine.
426 This also may be specified on a per-host basis in the configuration file.
430 (actually, prevents reading from stdin).
431 This must be used when
433 is run in the background.
434 A common trick is to use this to run X11 programs on a remote machine.
436 .Ic ssh -n shadows.cs.hut.fi emacs &
437 will start an emacs on shadows.cs.hut.fi, and the X11
438 connection will be automatically forwarded over an encrypted channel.
441 program will be put in the background.
442 (This does not work if
444 needs to ask for a password or passphrase; see also the
448 Do not execute a remote command.
449 This is useful if you just want to forward ports
450 (protocol version 2 only).
452 Can be used to give options in the format used in the config file.
453 This is useful for specifying options for which there is no separate
455 The option has the same format as a line in the configuration file.
457 Port to connect to on the remote host.
458 This can be specified on a
459 per-host basis in the configuration file.
461 Use a non-privileged port for outgoing connections.
462 This can be used if your firewall does
463 not permit connections from privileged ports.
464 Note that this option turns off
465 .Cm RhostsAuthentication
467 .Cm RhostsRSAAuthentication
471 Causes all warning and diagnostic messages to be suppressed.
472 Only fatal errors are displayed.
474 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
475 of SSH as a secure transport for other application (eg. sftp). The
476 subsystem is specified as the remote command.
478 Force pseudo-tty allocation.
479 This can be used to execute arbitrary
480 screen-based programs on a remote machine, which can be very useful,
481 e.g., when implementing menu services.
484 options force tty allocation, even if
488 Disable pseudo-tty allocation.
493 to print debugging messages about its progress.
495 debugging connection, authentication, and configuration problems.
498 options increases the verbosity.
501 Disables X11 forwarding.
503 Enables X11 forwarding.
504 This can also be specified on a per-host basis in a configuration file.
506 Requests compression of all data (including stdin, stdout, stderr, and
507 data for forwarded X11 and TCP/IP connections).
508 The compression algorithm is the same used by
512 can be controlled by the
515 Compression is desirable on modem lines and other
516 slow connections, but will only slow down things on fast networks.
517 The default value can be set on a host-by-host basis in the
518 configuration files; see the
521 .It Fl L Ar port:host:hostport
522 Specifies that the given port on the local (client) host is to be
523 forwarded to the given host and port on the remote side.
524 This works by allocating a socket to listen to
526 on the local side, and whenever a connection is made to this port, the
527 connection is forwarded over the secure channel, and a connection is
532 from the remote machine.
533 Port forwardings can also be specified in the configuration file.
534 Only root can forward privileged ports.
535 IPv6 addresses can be specified with an alternative syntax:
536 .Ar port/host/hostport
537 .It Fl R Ar port:host:hostport
538 Specifies that the given port on the remote (server) host is to be
539 forwarded to the given host and port on the local side.
540 This works by allocating a socket to listen to
542 on the remote side, and whenever a connection is made to this port, the
543 connection is forwarded over the secure channel, and a connection is
548 from the local machine.
549 Port forwardings can also be specified in the configuration file.
550 Privileged ports can be forwarded only when
551 logging in as root on the remote machine.
555 to try protocol version 2 only.
559 to use IPv4 addresses only.
563 to use IPv6 addresses only.
568 is not invoked with one of the standard program names
575 it uses this name as its
578 This is consistent with traditional
581 .Sh CONFIGURATION FILES
583 obtains configuration data from the following sources (in this order):
584 command line options, user's configuration file
585 .Pq Pa $HOME/.ssh/config ,
586 and system-wide configuration file
587 .Pq Pa /etc/ssh_config .
588 For each parameter, the first obtained value
590 The configuration files contain sections bracketed by
592 specifications, and that section is only applied for hosts that
593 match one of the patterns given in the specification.
594 The matched host name is the one given on the command line.
596 Since the first obtained value for each parameter is used, more
597 host-specific declarations should be given near the beginning of the
598 file, and general defaults at the end.
600 The configuration file has the following format:
602 Empty lines and lines starting with
606 Otherwise a line is of the format
607 .Dq keyword arguments .
609 keywords and their meanings are as follows (note that the
610 configuration files are case-sensitive):
613 Restricts the following declarations (up to the next
615 keyword) to be only for those hosts that match one of the patterns
616 given after the keyword.
620 can be used as wildcards in the
624 as a pattern can be used to provide global
625 defaults for all hosts.
628 argument given on the command line (i.e., the name is not converted to
629 a canonicalized host name before matching).
630 .It Cm AFSTokenPassing
631 Specifies whether to pass AFS tokens to remote host.
632 The argument to this keyword must be
639 passphrase/password querying will be disabled.
640 This option is useful in scripts and other batch jobs where you have no
641 user to supply the password.
647 If this flag is set to
649 ssh will additionally check the host ip address in the
652 This allows ssh to detect if a host key changed due to DNS spoofing.
653 If the option is set to
655 the check will not be executed.
657 Specifies the cipher to use for encrypting the session
658 in protocol version 1.
667 Specifies the ciphers allowed for protocol version 2
668 in order of preference.
669 Multiple ciphers must be comma-separated.
671 .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
673 Specifies whether to use compression.
678 .It Cm CompressionLevel
679 Specifies the compression level to use if compression is enable.
680 The argument must be an integer from 1 (fast) to 9 (slow, best).
681 The default level is 6, which is good for most applications.
682 The meaning of the values is the same as in
684 .It Cm ConnectionAttempts
685 Specifies the number of tries (one per second) to make before falling
686 back to rsh or exiting.
687 The argument must be an integer.
688 This may be useful in scripts if the connection sometimes fails.
689 .It Cm PubkeyAuthentication
690 Specifies whether to try public key authentication.
691 The argument to this keyword must be
695 Note that this option applies to protocol version 2 only.
697 Sets the escape character (default:
699 The escape character can also
700 be set on the command line.
701 The argument should be a single character,
703 followed by a letter, or
705 to disable the escape
706 character entirely (making the connection transparent for binary
709 Specifies that if connecting via
711 fails due to a connection refused error (there is no
713 listening on the remote host),
715 should automatically be used instead (after a suitable warning about
716 the session being unencrypted).
722 Specifies whether the connection to the authentication agent (if any)
723 will be forwarded to the remote machine.
731 Specifies whether X11 connections will be automatically redirected
732 over the secure channel and
742 Specifies whether remote hosts are allowed to connect to local
750 .It Cm GlobalKnownHostsFile
751 Specifies a file to use instead of
752 .Pa /etc/ssh_known_hosts .
754 Specifies an alias that should be used instead of the
755 real host name when looking up or saving the host key
756 in the known_hosts files.
757 This option is useful for tunneling ssh connections
758 or if you have multiple servers running on a single host.
760 Specifies the real host name to log into.
761 This can be used to specify nicknames or abbreviations for hosts.
762 Default is the name given on the command line.
763 Numeric IP addresses are also permitted (both on the command line and in
767 Specifies the file from which the user's RSA authentication identity
769 .Pa $HOME/.ssh/identity
770 in the user's home directory).
771 Additionally, any identities represented by the authentication agent
772 will be used for authentication.
773 The file name may use the tilde
774 syntax to refer to a user's home directory.
775 It is possible to have
776 multiple identity files specified in configuration files; all these
777 identities will be tried in sequence.
779 Specifies whether the system should send keepalive messages to the
781 If they are sent, death of the connection or crash of one
782 of the machines will be properly noticed.
783 However, this means that
784 connections will die if the route is down temporarily, and some people
789 (to send keepalives), and the client will notice
790 if the network goes down or the remote host dies.
791 This is important in scripts, and many users want it too.
793 To disable keepalives, the value should be set to
795 in both the server and the client configuration files.
796 .It Cm KerberosAuthentication
797 Specifies whether Kerberos authentication will be used.
798 The argument to this keyword must be
802 .It Cm KerberosTgtPassing
803 Specifies whether a Kerberos TGT will be forwarded to the server.
804 This will only work if the Kerberos server is actually an AFS kaserver.
805 The argument to this keyword must be
810 Specifies that a TCP/IP port on the local machine be forwarded over
811 the secure channel to given host:port from the remote machine.
812 The first argument must be a port number, and the second must be
814 Multiple forwardings may be specified, and additional
815 forwardings can be given on the command line.
816 Only the superuser can forward privileged ports.
818 Gives the verbosity level that is used when logging messages from
820 The possible values are:
821 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
823 .It Cm NumberOfPasswordPrompts
824 Specifies the number of password prompts before giving up.
825 The argument to this keyword must be an integer.
827 .It Cm PasswordAuthentication
828 Specifies whether to use password authentication.
829 The argument to this keyword must be
833 Note that this option applies to both protocol version 1 and 2.
835 Specifies the port number to connect on the remote host.
838 Specifies the protocol versions
840 should support in order of preference.
841 The possible values are
845 Multiple versions must be comma-separated.
850 tries version 1 and falls back to version 2
851 if version 1 is not available.
853 Specifies the command to use to connect to the server.
855 string extends to the end of the line, and is executed with
857 In the command string,
859 will be substituted by the host name to
863 The command can be basically anything,
864 and should read from its standard input and write to its standard output.
865 It should eventually connect an
867 server running on some machine, or execute
870 Host key management will be done using the
871 HostName of the host being connected (defaulting to the name typed by
875 is not available for connects with a proxy command.
878 Specifies that a TCP/IP port on the remote machine be forwarded over
879 the secure channel to given host:port from the local machine.
880 The first argument must be a port number, and the second must be
882 Multiple forwardings may be specified, and additional
883 forwardings can be given on the command line.
884 Only the superuser can forward privileged ports.
885 .It Cm RhostsAuthentication
886 Specifies whether to try rhosts based authentication.
888 declaration only affects the client side and has no effect whatsoever
890 Disabling rhosts authentication may reduce
891 authentication time on slow connections when rhosts authentication is
893 Most servers do not permit RhostsAuthentication because it
894 is not secure (see RhostsRSAAuthentication).
895 The argument to this keyword must be
899 .It Cm RhostsRSAAuthentication
900 Specifies whether to try rhosts based authentication with RSA host
902 This is the primary authentication method for most sites.
907 .It Cm RSAAuthentication
908 Specifies whether to try RSA authentication.
909 The argument to this keyword must be
913 RSA authentication will only be
914 attempted if the identity file exists, or an authentication agent is
916 Note that this option applies to protocol version 1 only.
917 .It Cm ChallengeResponseAuthentication
918 Specifies whether to use challenge response authentication.
919 Currently there is only support for
922 The argument to this keyword must be
928 .It Cm StrictHostKeyChecking
929 If this flag is set to
932 will never automatically add host keys to the
933 .Pa $HOME/.ssh/known_hosts
935 .Pa $HOME/.ssh/known_hosts2
936 files, and refuses to connect to hosts whose host key has changed.
937 This provides maximum protection against trojan horse attacks.
938 However, it can be somewhat annoying if you don't have good
939 .Pa /etc/ssh_known_hosts
941 .Pa /etc/ssh_known_hosts2
942 files installed and frequently
943 connect to new hosts.
944 This option forces the user to manually
946 If this flag is set to
949 will automatically add new host keys to the
950 user known hosts files.
951 If this flag is set to
954 will be added to the user known host files only after the user
955 has confirmed that is what they really want to do, and
957 will refuse to connect to hosts whose host key has changed.
959 known hosts will be verified automatically in all cases.
967 .It Cm UsePrivilegedPort
968 Specifies whether to use a privileged port for outgoing connections.
975 Note that setting this option to
978 .Cm RhostsAuthentication
980 .Cm RhostsRSAAuthentication
983 Specifies the user to log in as.
984 This can be useful if you have a different user name on different machines.
985 This saves the trouble of
986 having to remember to give the user name on the command line.
987 .It Cm UserKnownHostsFile
988 Specifies a file to use instead of
989 .Pa $HOME/.ssh/known_hosts .
991 Specifies that rlogin/rsh should be used for this host.
992 It is possible that the host does not at all support the
997 to immediately execute
999 All other options (except
1001 are ignored if this has been specified.
1002 The argument must be
1006 .It Cm XAuthLocation
1007 Specifies the location of the
1011 .Pa /usr/X11R6/bin/xauth .
1015 will normally set the following environment variables:
1020 variable indicates the location of the X11 server.
1021 It is automatically set by
1023 to point to a value of the form
1025 where hostname indicates
1026 the host where the shell runs, and n is an integer >= 1.
1028 uses this special value to forward X11 connections over the secure
1030 The user should normally not set DISPLAY explicitly, as that
1031 will render the X11 connection insecure (and will require the user to
1032 manually copy any required authorization cookies).
1034 Set to the path of the user's home directory.
1038 set for compatibility with systems that use this variable.
1040 Set to point the user's mailbox.
1044 as specified when compiling
1046 .It Ev SSH_AUTH_SOCK
1047 indicates the path of a unix-domain socket used to communicate with the
1050 Identifies the client end of the connection.
1051 The variable contains
1052 three space-separated values: client ip-address, client port number,
1053 and server port number.
1054 .It Ev SSH_ORIGINAL_COMMAND
1055 The variable contains the original command line if a forced command
1057 It can be used to extract the original arguments.
1059 This is set to the name of the tty (path to the device) associated
1060 with the current shell or command.
1061 If the current session has no tty,
1062 this variable is not set.
1064 The timezone variable is set to indicate the present timezone if it
1065 was set when the daemon was started (i.e., the daemon passes the value
1066 on to new connections).
1068 Set to the name of the user logging in.
1074 .Pa $HOME/.ssh/environment ,
1075 and adds lines of the format
1080 .It Pa $HOME/.ssh/known_hosts
1081 Records host keys for all hosts the user has logged into (that are not
1083 .Pa /etc/ssh_known_hosts ) .
1086 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1087 Contains the RSA and the DSA authentication identity of the user.
1089 contain sensitive data and should be readable by the user but not
1090 accessible by others (read/write/execute).
1093 ignores a private key file if it is accessible by others.
1094 It is possible to specify a passphrase when
1095 generating the key; the passphrase will be used to encrypt the
1096 sensitive part of this file using 3DES.
1097 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1098 Contains the public key for authentication (public part of the
1099 identity file in human-readable form).
1101 .Pa $HOME/.ssh/identity.pub
1102 file should be added to
1103 .Pa $HOME/.ssh/authorized_keys
1105 where you wish to log in using RSA authentication.
1107 .Pa $HOME/.ssh/id_dsa.pub
1108 file should be added to
1109 .Pa $HOME/.ssh/authorized_keys2
1111 where you wish to log in using DSA authentication.
1113 sensitive and can (but need not) be readable by anyone.
1115 never used automatically and are not necessary; they is only provided for
1116 the convenience of the user.
1117 .It Pa $HOME/.ssh/config
1118 This is the per-user configuration file.
1119 The format of this file is described above.
1120 This file is used by the
1123 This file does not usually contain any sensitive information,
1124 but the recommended permissions are read/write for the user, and not
1125 accessible by others.
1126 .It Pa $HOME/.ssh/authorized_keys
1127 Lists the RSA keys that can be used for logging in as this user.
1128 The format of this file is described in the
1131 In the simplest form the format is the same as the .pub
1132 identity files (that is, each line contains the number of bits in
1133 modulus, public exponent, modulus, and comment fields, separated by
1135 This file is not highly sensitive, but the recommended
1136 permissions are read/write for the user, and not accessible by others.
1137 .It Pa $HOME/.ssh/authorized_keys2
1138 Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1139 This file is not highly sensitive, but the recommended
1140 permissions are read/write for the user, and not accessible by others.
1141 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1142 Systemwide list of known host keys.
1143 .Pa /etc/ssh_known_hosts
1145 .Pa /etc/ssh_known_hosts2
1146 contains DSA or RSA keys for protocol version 2.
1147 These files should be prepared by the
1148 system administrator to contain the public host keys of all machines in the
1150 This file should be world-readable.
1152 public keys, one per line, in the following format (fields separated
1153 by spaces): system name, number of bits in modulus, public exponent,
1154 modulus, and optional comment field.
1155 When different names are used
1156 for the same machine, all such names should be listed, separated by
1158 The format is described on the
1162 The canonical system name (as returned by name servers) is used by
1164 to verify the client host when logging in; other names are needed because
1166 does not convert the user-supplied name to a canonical name before
1167 checking the key, because someone with access to the name servers
1168 would then be able to fool host authentication.
1169 .It Pa /etc/ssh_config
1170 Systemwide configuration file.
1171 This file provides defaults for those
1172 values that are not specified in the user's configuration file, and
1173 for those users who do not have a configuration file.
1174 This file must be world-readable.
1175 .It Pa $HOME/.rhosts
1176 This file is used in
1178 authentication to list the
1179 host/user pairs that are permitted to log in.
1180 (Note that this file is
1181 also used by rlogin and rsh, which makes using this file insecure.)
1182 Each line of the file contains a host name (in the canonical form
1183 returned by name servers), and then a user name on that host,
1184 separated by a space.
1185 One some machines this file may need to be
1186 world-readable if the user's home directory is on a NFS partition,
1190 Additionally, this file must be owned by the user,
1191 and must not have write permissions for anyone else.
1193 permission for most machines is read/write for the user, and not
1194 accessible by others.
1196 Note that by default
1198 will be installed so that it requires successful RSA host
1199 authentication before permitting \s+2.\s0rhosts authentication.
1200 If your server machine does not have the client's host key in
1201 .Pa /etc/ssh_known_hosts ,
1203 .Pa $HOME/.ssh/known_hosts .
1204 The easiest way to do this is to
1205 connect back to the client from the server machine using ssh; this
1206 will automatically add the host key to
1207 .Pa $HOME/.ssh/known_hosts .
1208 .It Pa $HOME/.shosts
1209 This file is used exactly the same way as
1212 having this file is to be able to use rhosts authentication with
1214 without permitting login with
1218 .It Pa /etc/hosts.equiv
1219 This file is used during
1220 .Pa \&.rhosts authentication.
1222 canonical hosts names, one per line (the full format is described on
1226 If the client host is found in this file, login is
1227 automatically permitted provided client and server user names are the
1229 Additionally, successful RSA host authentication is normally
1231 This file should only be writable by root.
1232 .It Pa /etc/shosts.equiv
1233 This file is processed exactly as
1234 .Pa /etc/hosts.equiv .
1235 This file may be useful to permit logins using
1237 but not using rsh/rlogin.
1239 Commands in this file are executed by
1241 when the user logs in just before the user's shell (or command) is started.
1244 manual page for more information.
1245 .It Pa $HOME/.ssh/rc
1246 Commands in this file are executed by
1248 when the user logs in just before the user's shell (or command) is
1252 manual page for more information.
1253 .It Pa $HOME/.ssh/environment
1254 Contains additional definitions for environment variables, see section
1257 .It Pa libcrypto.so.X.1
1258 A version of this library which includes support for the RSA algorithm
1259 is required for proper operation.
1262 OpenSSH is a derivative of the original and free
1263 ssh 1.2.12 release by Tatu Ylonen.
1264 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1265 Theo de Raadt and Dug Song
1266 removed many bugs, re-added newer features and
1268 Markus Friedl contributed the support for SSH
1269 protocol versions 1.5 and 2.0.