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 1.76 2001/01/07 11:28:06 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 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 Force pseudo-tty allocation.
475 This can be used to execute arbitrary
476 screen-based programs on a remote machine, which can be very useful,
477 e.g., when implementing menu services.
480 options force tty allocation, even if
484 Disable pseudo-tty allocation.
489 to print debugging messages about its progress.
491 debugging connection, authentication, and configuration problems.
494 options increases the verbosity.
497 Disables X11 forwarding.
499 Enables X11 forwarding.
500 This can also be specified on a per-host basis in a configuration file.
502 Requests compression of all data (including stdin, stdout, stderr, and
503 data for forwarded X11 and TCP/IP connections).
504 The compression algorithm is the same used by
508 can be controlled by the
511 Compression is desirable on modem lines and other
512 slow connections, but will only slow down things on fast networks.
513 The default value can be set on a host-by-host basis in the
514 configuration files; see the
517 .It Fl L Ar port:host:hostport
518 Specifies that the given port on the local (client) host is to be
519 forwarded to the given host and port on the remote side.
520 This works by allocating a socket to listen to
522 on the local side, and whenever a connection is made to this port, the
523 connection is forwarded over the secure channel, and a connection is
528 from the remote machine.
529 Port forwardings can also be specified in the configuration file.
530 Only root can forward privileged ports.
531 IPv6 addresses can be specified with an alternative syntax:
532 .Ar port/host/hostport
533 .It Fl R Ar port:host:hostport
534 Specifies that the given port on the remote (server) host is to be
535 forwarded to the given host and port on the local side.
536 This works by allocating a socket to listen to
538 on the remote side, and whenever a connection is made to this port, the
539 connection is forwarded over the secure channel, and a connection is
544 from the local machine.
545 Port forwardings can also be specified in the configuration file.
546 Privileged ports can be forwarded only when
547 logging in as root on the remote machine.
551 to try protocol version 2 only.
555 to use IPv4 addresses only.
559 to use IPv6 addresses only.
564 is not invoked with one of the standard program names
571 it uses this name as its
574 This is consistent with traditional
577 .Sh CONFIGURATION FILES
579 obtains configuration data from the following sources (in this order):
580 command line options, user's configuration file
581 .Pq Pa $HOME/.ssh/config ,
582 and system-wide configuration file
583 .Pq Pa /etc/ssh_config .
584 For each parameter, the first obtained value
586 The configuration files contain sections bracketed by
588 specifications, and that section is only applied for hosts that
589 match one of the patterns given in the specification.
590 The matched host name is the one given on the command line.
592 Since the first obtained value for each parameter is used, more
593 host-specific declarations should be given near the beginning of the
594 file, and general defaults at the end.
596 The configuration file has the following format:
598 Empty lines and lines starting with
602 Otherwise a line is of the format
603 .Dq keyword arguments .
605 keywords and their meanings are as follows (note that the
606 configuration files are case-sensitive):
609 Restricts the following declarations (up to the next
611 keyword) to be only for those hosts that match one of the patterns
612 given after the keyword.
616 can be used as wildcards in the
620 as a pattern can be used to provide global
621 defaults for all hosts.
624 argument given on the command line (i.e., the name is not converted to
625 a canonicalized host name before matching).
626 .It Cm AFSTokenPassing
627 Specifies whether to pass AFS tokens to remote host.
628 The argument to this keyword must be
635 passphrase/password querying will be disabled.
636 This option is useful in scripts and other batch jobs where you have no
637 user to supply the password.
643 If this flag is set to
645 ssh will additionally check the host ip address in the
648 This allows ssh to detect if a host key changed due to DNS spoofing.
649 If the option is set to
651 the check will not be executed.
653 Specifies the cipher to use for encrypting the session
654 in protocol version 1.
663 Specifies the ciphers allowed for protocol version 2
664 in order of preference.
665 Multiple ciphers must be comma-separated.
667 .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
669 Specifies whether to use compression.
674 .It Cm CompressionLevel
675 Specifies the compression level to use if compression is enable.
676 The argument must be an integer from 1 (fast) to 9 (slow, best).
677 The default level is 6, which is good for most applications.
678 The meaning of the values is the same as in
680 .It Cm ConnectionAttempts
681 Specifies the number of tries (one per second) to make before falling
682 back to rsh or exiting.
683 The argument must be an integer.
684 This may be useful in scripts if the connection sometimes fails.
685 .It Cm PubkeyAuthentication
686 Specifies whether to try public key authentication.
687 The argument to this keyword must be
691 Note that this option applies to protocol version 2 only.
693 Sets the escape character (default:
695 The escape character can also
696 be set on the command line.
697 The argument should be a single character,
699 followed by a letter, or
701 to disable the escape
702 character entirely (making the connection transparent for binary
705 Specifies that if connecting via
707 fails due to a connection refused error (there is no
709 listening on the remote host),
711 should automatically be used instead (after a suitable warning about
712 the session being unencrypted).
718 Specifies whether the connection to the authentication agent (if any)
719 will be forwarded to the remote machine.
727 Specifies whether X11 connections will be automatically redirected
728 over the secure channel and
738 Specifies whether remote hosts are allowed to connect to local
746 .It Cm GlobalKnownHostsFile
747 Specifies a file to use instead of
748 .Pa /etc/ssh_known_hosts .
750 Specifies an alias that should be used instead of the
751 real host name when looking up or saving the host key
752 the kown_hosts files.
753 This option is useful for tunneling ssh connection
754 or if you have multiple servers running on a single host.
756 Specifies the real host name to log into.
757 This can be used to specify nicknames or abbreviations for hosts.
758 Default is the name given on the command line.
759 Numeric IP addresses are also permitted (both on the command line and in
763 Specifies the file from which the user's RSA authentication identity
765 .Pa $HOME/.ssh/identity
766 in the user's home directory).
767 Additionally, any identities represented by the authentication agent
768 will be used for authentication.
769 The file name may use the tilde
770 syntax to refer to a user's home directory.
771 It is possible to have
772 multiple identity files specified in configuration files; all these
773 identities will be tried in sequence.
775 Specifies whether the system should send keepalive messages to the
777 If they are sent, death of the connection or crash of one
778 of the machines will be properly noticed.
779 However, this means that
780 connections will die if the route is down temporarily, and some people
785 (to send keepalives), and the client will notice
786 if the network goes down or the remote host dies.
787 This is important in scripts, and many users want it too.
789 To disable keepalives, the value should be set to
791 in both the server and the client configuration files.
792 .It Cm KerberosAuthentication
793 Specifies whether Kerberos authentication will be used.
794 The argument to this keyword must be
798 .It Cm KerberosTgtPassing
799 Specifies whether a Kerberos TGT will be forwarded to the server.
800 This will only work if the Kerberos server is actually an AFS kaserver.
801 The argument to this keyword must be
806 Specifies that a TCP/IP port on the local machine be forwarded over
807 the secure channel to given host:port from the remote machine.
808 The first argument must be a port number, and the second must be
810 Multiple forwardings may be specified, and additional
811 forwardings can be given on the command line.
812 Only the superuser can forward privileged ports.
814 Gives the verbosity level that is used when logging messages from
816 The possible values are:
817 QUIET, FATAL, ERROR, NOTICE, VERBOSE and DEBUG.
818 The default is NOTICE.
819 .It Cm NumberOfPasswordPrompts
820 Specifies the number of password prompts before giving up.
821 The argument to this keyword must be an integer.
823 .It Cm PasswordAuthentication
824 Specifies whether to use password authentication.
825 The argument to this keyword must be
829 Note that this option applies to both protocol version 1 and 2.
831 Specifies the port number to connect on the remote host.
834 Specifies the protocol versions
836 should support in order of preference.
837 The possible values are
841 Multiple versions must be comma-separated.
846 tries version 1 and falls back to version 2
847 if version 1 is not available.
849 Specifies the command to use to connect to the server.
851 string extends to the end of the line, and is executed with
853 In the command string,
855 will be substituted by the host name to
859 The command can be basically anything,
860 and should read from its standard input and write to its standard output.
861 It should eventually connect an
863 server running on some machine, or execute
866 Host key management will be done using the
867 HostName of the host being connected (defaulting to the name typed by
871 is not available for connects with a proxy command.
874 Specifies that a TCP/IP port on the remote machine be forwarded over
875 the secure channel to given host:port from the local machine.
876 The first argument must be a port number, and the second must be
878 Multiple forwardings may be specified, and additional
879 forwardings can be given on the command line.
880 Only the superuser can forward privileged ports.
881 .It Cm RhostsAuthentication
882 Specifies whether to try rhosts based authentication.
884 declaration only affects the client side and has no effect whatsoever
886 Disabling rhosts authentication may reduce
887 authentication time on slow connections when rhosts authentication is
889 Most servers do not permit RhostsAuthentication because it
890 is not secure (see RhostsRSAAuthentication).
891 The argument to this keyword must be
895 .It Cm RhostsRSAAuthentication
896 Specifies whether to try rhosts based authentication with RSA host
898 This is the primary authentication method for most sites.
903 .It Cm RSAAuthentication
904 Specifies whether to try RSA authentication.
905 The argument to this keyword must be
909 RSA authentication will only be
910 attempted if the identity file exists, or an authentication agent is
912 Note that this option applies to protocol version 1 only.
913 .It Cm SkeyAuthentication
914 Specifies whether to use
917 The argument to this keyword must be
923 .It Cm StrictHostKeyChecking
924 If this flag is set to
927 ssh will never automatically add host keys to the
928 .Pa $HOME/.ssh/known_hosts
930 .Pa $HOME/.ssh/known_hosts2
931 files, and refuses to connect hosts whose host key has changed.
932 This provides maximum protection against trojan horse attacks.
933 However, it can be somewhat annoying if you don't have good
934 .Pa /etc/ssh_known_hosts
936 .Pa /etc/ssh_known_hosts2
937 files installed and frequently
939 Basically this option forces the user to manually
941 Normally this option is disabled, and new hosts
942 will automatically be added to the known host files.
944 known hosts will be verified automatically in either case.
949 .It Cm UsePrivilegedPort
950 Specifies whether to use a privileged port for outgoing connections.
957 Note that setting this option to
960 .Cm RhostsAuthentication
962 .Cm RhostsRSAAuthentication
965 Specifies the user to log in as.
966 This can be useful if you have a different user name on different machines.
967 This saves the trouble of
968 having to remember to give the user name on the command line.
969 .It Cm UserKnownHostsFile
970 Specifies a file to use instead of
971 .Pa $HOME/.ssh/known_hosts .
973 Specifies that rlogin/rsh should be used for this host.
974 It is possible that the host does not at all support the
979 to immediately execute
981 All other options (except
983 are ignored if this has been specified.
989 Specifies the location of the
993 .Pa /usr/X11R6/bin/xauth .
997 will normally set the following environment variables:
1002 variable indicates the location of the X11 server.
1003 It is automatically set by
1005 to point to a value of the form
1007 where hostname indicates
1008 the host where the shell runs, and n is an integer >= 1.
1010 uses this special value to forward X11 connections over the secure
1012 The user should normally not set DISPLAY explicitly, as that
1013 will render the X11 connection insecure (and will require the user to
1014 manually copy any required authorization cookies).
1016 Set to the path of the user's home directory.
1020 set for compatibility with systems that use this variable.
1022 Set to point the user's mailbox.
1026 as specified when compiling
1028 .It Ev SSH_AUTH_SOCK
1029 indicates the path of a unix-domain socket used to communicate with the
1032 Identifies the client end of the connection.
1033 The variable contains
1034 three space-separated values: client ip-address, client port number,
1035 and server port number.
1036 .It Ev SSH_ORIGINAL_COMMAND
1037 The variable contains the original command line if a forced command
1039 It can be used to extract the original arguments.
1041 This is set to the name of the tty (path to the device) associated
1042 with the current shell or command.
1043 If the current session has no tty,
1044 this variable is not set.
1046 The timezone variable is set to indicate the present timezone if it
1047 was set when the daemon was started (i.e., the daemon passes the value
1048 on to new connections).
1050 Set to the name of the user logging in.
1056 .Pa $HOME/.ssh/environment ,
1057 and adds lines of the format
1062 .It Pa $HOME/.ssh/known_hosts
1063 Records host keys for all hosts the user has logged into (that are not
1065 .Pa /etc/ssh_known_hosts ) .
1068 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1069 Contains the RSA and the DSA authentication identity of the user.
1071 contain sensitive data and should be readable by the user but not
1072 accessible by others (read/write/execute).
1075 ignores a private key file if it is accessible by others.
1076 It is possible to specify a passphrase when
1077 generating the key; the passphrase will be used to encrypt the
1078 sensitive part of this file using 3DES.
1079 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1080 Contains the public key for authentication (public part of the
1081 identity file in human-readable form).
1083 .Pa $HOME/.ssh/identity.pub
1084 file should be added to
1085 .Pa $HOME/.ssh/authorized_keys
1087 where you wish to log in using RSA authentication.
1089 .Pa $HOME/.ssh/id_dsa.pub
1090 file should be added to
1091 .Pa $HOME/.ssh/authorized_keys2
1093 where you wish to log in using DSA authentication.
1095 sensitive and can (but need not) be readable by anyone.
1097 never used automatically and are not necessary; they is only provided for
1098 the convenience of the user.
1099 .It Pa $HOME/.ssh/config
1100 This is the per-user configuration file.
1101 The format of this file is described above.
1102 This file is used by the
1105 This file does not usually contain any sensitive information,
1106 but the recommended permissions are read/write for the user, and not
1107 accessible by others.
1108 .It Pa $HOME/.ssh/authorized_keys
1109 Lists the RSA keys that can be used for logging in as this user.
1110 The format of this file is described in the
1113 In the simplest form the format is the same as the .pub
1114 identity files (that is, each line contains the number of bits in
1115 modulus, public exponent, modulus, and comment fields, separated by
1117 This file is not highly sensitive, but the recommended
1118 permissions are read/write for the user, and not accessible by others.
1119 .It Pa $HOME/.ssh/authorized_keys2
1120 Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1121 This file is not highly sensitive, but the recommended
1122 permissions are read/write for the user, and not accessible by others.
1123 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1124 Systemwide list of known host keys.
1125 .Pa /etc/ssh_known_hosts
1127 .Pa /etc/ssh_known_hosts2
1128 contains DSA or RSA keys for protocol version 2.
1129 These files should be prepared by the
1130 system administrator to contain the public host keys of all machines in the
1132 This file should be world-readable.
1134 public keys, one per line, in the following format (fields separated
1135 by spaces): system name, number of bits in modulus, public exponent,
1136 modulus, and optional comment field.
1137 When different names are used
1138 for the same machine, all such names should be listed, separated by
1140 The format is described on the
1144 The canonical system name (as returned by name servers) is used by
1146 to verify the client host when logging in; other names are needed because
1148 does not convert the user-supplied name to a canonical name before
1149 checking the key, because someone with access to the name servers
1150 would then be able to fool host authentication.
1151 .It Pa /etc/ssh_config
1152 Systemwide configuration file.
1153 This file provides defaults for those
1154 values that are not specified in the user's configuration file, and
1155 for those users who do not have a configuration file.
1156 This file must be world-readable.
1157 .It Pa $HOME/.rhosts
1158 This file is used in
1160 authentication to list the
1161 host/user pairs that are permitted to log in.
1162 (Note that this file is
1163 also used by rlogin and rsh, which makes using this file insecure.)
1164 Each line of the file contains a host name (in the canonical form
1165 returned by name servers), and then a user name on that host,
1166 separated by a space.
1167 One some machines this file may need to be
1168 world-readable if the user's home directory is on a NFS partition,
1172 Additionally, this file must be owned by the user,
1173 and must not have write permissions for anyone else.
1175 permission for most machines is read/write for the user, and not
1176 accessible by others.
1178 Note that by default
1180 will be installed so that it requires successful RSA host
1181 authentication before permitting \s+2.\s0rhosts authentication.
1182 If your server machine does not have the client's host key in
1183 .Pa /etc/ssh_known_hosts ,
1185 .Pa $HOME/.ssh/known_hosts .
1186 The easiest way to do this is to
1187 connect back to the client from the server machine using ssh; this
1188 will automatically add the host key to
1189 .Pa $HOME/.ssh/known_hosts .
1190 .It Pa $HOME/.shosts
1191 This file is used exactly the same way as
1194 having this file is to be able to use rhosts authentication with
1196 without permitting login with
1200 .It Pa /etc/hosts.equiv
1201 This file is used during
1202 .Pa \&.rhosts authentication.
1204 canonical hosts names, one per line (the full format is described on
1208 If the client host is found in this file, login is
1209 automatically permitted provided client and server user names are the
1211 Additionally, successful RSA host authentication is normally
1213 This file should only be writable by root.
1214 .It Pa /etc/shosts.equiv
1215 This file is processed exactly as
1216 .Pa /etc/hosts.equiv .
1217 This file may be useful to permit logins using
1219 but not using rsh/rlogin.
1221 Commands in this file are executed by
1223 when the user logs in just before the user's shell (or command) is started.
1226 manual page for more information.
1227 .It Pa $HOME/.ssh/rc
1228 Commands in this file are executed by
1230 when the user logs in just before the user's shell (or command) is
1234 manual page for more information.
1235 .It Pa $HOME/.ssh/environment
1236 Contains additional definitions for environment variables, see section
1239 .It Pa libcrypto.so.X.1
1240 A version of this library which includes support for the RSA algorithm
1241 is required for proper operation.
1245 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1246 but with bugs removed and newer features re-added.
1248 1.2.12 release, newer versions of the original ssh bore successively
1249 more restrictive licenses, and thus demand for a free version was born.
1251 This version of OpenSSH
1254 has all components of a restrictive nature (i.e., patents, see
1256 directly removed from the source code; any licensed or patented components
1260 has been updated to support SSH protocol 1.5 and 2, making it compatible with
1261 all other SSH clients and servers.
1263 contains added support for
1265 authentication and ticket passing.
1267 supports one-time password authentication with
1271 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1272 Niels Provos, Theo de Raadt, and Dug Song.
1274 The support for SSH protocol 2 was written by Markus Friedl.