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 1.91 2001/02/11 12:59:25 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
75 .Op Ar hostname | user@hostname
79 (Secure Shell) is a program for logging into a remote machine and for
80 executing commands on a remote machine.
81 It is intended to replace
82 rlogin and rsh, and provide secure encrypted communications between
83 two untrusted hosts over an insecure network.
85 arbitrary TCP/IP ports can also be forwarded over the secure channel.
88 connects and logs into the specified
91 his/her identity to the remote machine using one of several methods
92 depending on the protocol version used:
94 .Ss SSH protocol version 1
96 First, if the machine the user logs in from is listed in
100 on the remote machine, and the user names are
101 the same on both sides, the user is immediately permitted to log in.
106 exists in the user's home directory on the
107 remote machine and contains a line containing the name of the client
108 machine and the name of the user on that machine, the user is
110 This form of authentication alone is normally not
111 allowed by the server because it is not secure.
113 The second (and primary) authentication method is the
117 method combined with RSA-based host authentication.
118 It means that if the login would be permitted by
121 .Pa /etc/hosts.equiv ,
123 .Pa /etc/shosts.equiv ,
124 and if additionally the server can verify the client's
126 .Pa /etc/ssh_known_hosts
128 .Pa $HOME/.ssh/known_hosts
131 section), only then login is permitted.
132 This authentication method closes security holes due to IP
133 spoofing, DNS spoofing and routing spoofing.
134 [Note to the administrator:
135 .Pa /etc/hosts.equiv ,
137 and the rlogin/rsh protocol in general, are inherently insecure and should be
138 disabled if security is desired.]
140 As a third authentication method,
142 supports RSA based authentication.
143 The scheme is based on public-key cryptography: there are cryptosystems
144 where encryption and decryption are done using separate keys, and it
145 is not possible to derive the decryption key from the encryption key.
146 RSA is one such system.
147 The idea is that each user creates a public/private
148 key pair for authentication purposes.
149 The server knows the public key, and only the user knows the private key.
151 .Pa $HOME/.ssh/authorized_keys
152 lists the public keys that are permitted for logging
154 When the user logs in, the
156 program tells the server which key pair it would like to use for
158 The server checks if this key is permitted, and if
159 so, sends the user (actually the
161 program running on behalf of the user) a challenge, a random number,
162 encrypted by the user's public key.
163 The challenge can only be
164 decrypted using the proper private key.
165 The user's client then decrypts the
166 challenge using the private key, proving that he/she knows the private
167 key but without disclosing it to the server.
170 implements the RSA authentication protocol automatically.
171 The user creates his/her RSA key pair by running
173 This stores the private key in
174 .Pa $HOME/.ssh/identity
175 and the public key in
176 .Pa $HOME/.ssh/identity.pub
177 in the user's home directory.
178 The user should then copy the
181 .Pa $HOME/.ssh/authorized_keys
182 in his/her home directory on the remote machine (the
184 file corresponds to the conventional
186 file, and has one key
187 per line, though the lines can be very long).
188 After this, the user can log in without giving the password.
189 RSA authentication is much
190 more secure than rhosts authentication.
192 The most convenient way to use RSA authentication may be with an
193 authentication agent.
196 for more information.
198 If other authentication methods fail,
200 prompts the user for a password.
201 The password is sent to the remote
202 host for checking; however, since all communications are encrypted,
203 the password cannot be seen by someone listening on the network.
205 .Ss SSH protocol version 2
207 When a user connects using the protocol version 2
208 different authentication methods are available:
209 At first, the client attempts to authenticate using the public key method.
210 If this method fails password authentication is tried.
212 The public key method is similar to RSA authentication described
213 in the previous section except that the DSA or RSA algorithm is used
215 The client uses his private key
216 .Pa $HOME/.ssh/id_dsa
217 to sign the session identifier and sends the result to the server.
218 The server checks whether the matching public key is listed in
219 .Pa $HOME/.ssh/authorized_keys2
220 and grants access if both the key is found and the signature is correct.
221 The session identifier is derived from a shared Diffie-Hellman value
222 and is only known to the client and the server.
224 If public key authentication fails or is not available a password
225 can be sent encrypted to the remote host for proving the user's identity.
226 This protocol 2 implementation does not yet support Kerberos or
227 S/Key authentication.
229 Protocol 2 provides additional mechanisms for confidentiality
230 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
231 and integrity (hmac-sha1, hmac-md5).
232 Note that protocol 1 lacks a strong mechanism for ensuring the
233 integrity of the connection.
235 .Ss Login session and remote execution
237 When the user's identity has been accepted by the server, the server
238 either executes the given command, or logs into the machine and gives
239 the user a normal shell on the remote machine.
240 All communication with
241 the remote command or shell will be automatically encrypted.
243 If a pseudo-terminal has been allocated (normal login session), the
244 user can disconnect with
250 All forwarded connections can be listed with
253 the session blocks waiting for forwarded X11 or TCP/IP
254 connections to terminate, it can be backgrounded with
256 (this should not be used while the user shell is active, as it can cause the
258 All available escapes can be listed with
261 A single tilde character can be sent as
263 (or by following the tilde by a character other than those described above).
264 The escape character must always follow a newline to be interpreted as
266 The escape character can be changed in configuration files
267 or on the command line.
269 If no pseudo tty has been allocated, the
270 session is transparent and can be used to reliably transfer binary
272 On most systems, setting the escape character to
274 will also make the session transparent even if a tty is used.
276 The session terminates when the command or shell on the remote
277 machine exists and all X11 and TCP/IP connections have been closed.
278 The exit status of the remote program is returned as the exit status
282 .Ss X11 and TCP forwarding
284 If the user is using X11 (the
286 environment variable is set), the connection to the X11 display is
287 automatically forwarded to the remote side in such a way that any X11
288 programs started from the shell (or command) will go through the
289 encrypted channel, and the connection to the real X server will be made
290 from the local machine.
291 The user should not manually set
293 Forwarding of X11 connections can be
294 configured on the command line or in configuration files.
300 will point to the server machine, but with a display number greater
302 This is normal, and happens because
306 X server on the server machine for forwarding the
307 connections over the encrypted channel.
310 will also automatically set up Xauthority data on the server machine.
311 For this purpose, it will generate a random authorization cookie,
312 store it in Xauthority on the server, and verify that any forwarded
313 connections carry this cookie and replace it by the real cookie when
314 the connection is opened.
315 The real authentication cookie is never
316 sent to the server machine (and no cookies are sent in the plain).
318 If the user is using an authentication agent, the connection to the agent
319 is automatically forwarded to the remote side unless disabled on
320 command line or in a configuration file.
322 Forwarding of arbitrary TCP/IP connections over the secure channel can
323 be specified either on command line or in a configuration file.
324 One possible application of TCP/IP forwarding is a secure connection to an
325 electronic purse; another is going trough firewalls.
327 .Ss Server authentication
330 automatically maintains and checks a database containing
331 identifications for all hosts it has ever been used with.
332 RSA host keys are stored in
333 .Pa $HOME/.ssh/known_hosts
335 host keys used in the protocol version 2 are stored in
336 .Pa $HOME/.ssh/known_hosts2
337 in the user's home directory.
338 Additionally, the files
339 .Pa /etc/ssh_known_hosts
341 .Pa /etc/ssh_known_hosts2
342 are automatically checked for known hosts.
343 Any new hosts are automatically added to the user's file.
344 If a host's identification
347 warns about this and disables password authentication to prevent a
348 trojan horse from getting the user's password.
350 this mechanism is to prevent man-in-the-middle attacks which could
351 otherwise be used to circumvent the encryption.
353 .Cm StrictHostKeyChecking
354 option (see below) can be used to prevent logins to machines whose
355 host key is not known or has changed.
357 The options are as follows:
360 Disables forwarding of the authentication agent connection.
362 Enables forwarding of the authentication agent connection.
363 This can also be specified on a per-host basis in a configuration file.
364 .It Fl c Ar blowfish|3des
365 Selects the cipher to use for encrypting the session.
368 It is believed to be secure.
370 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
371 It is presumably more secure than the
373 cipher which is no longer fully supported in
376 is a fast block cipher, it appears very secure and is much faster than
378 .It Fl c Ar cipher_spec
379 Additionally, for protocol version 2 a comma-separated list of ciphers can
380 be specified in order of preference.
383 for more information.
384 .It Fl e Ar ch|^ch|none
385 Sets the escape character for sessions with a pty (default:
387 The escape character is only recognized at the beginning of a line.
388 The escape character followed by a dot
390 closes the connection, followed
391 by control-Z suspends the connection, and followed by itself sends the
392 escape character once.
393 Setting the character to
395 disables any escapes and makes the session fully transparent.
399 to go to background just before command execution.
402 is going to ask for passwords or passphrases, but the user
403 wants it in the background.
406 The recommended way to start X11 programs at a remote site is with
408 .Ic ssh -f host xterm .
410 Allows remote hosts to connect to local forwarded ports.
411 .It Fl i Ar identity_file
412 Selects the file from which the identity (private key) for
413 RSA or DSA authentication is read.
415 .Pa $HOME/.ssh/identity
416 in the user's home directory.
417 Identity files may also be specified on
418 a per-host basis in the configuration file.
419 It is possible to have multiple
421 options (and multiple identities specified in
422 configuration files).
424 Disables forwarding of Kerberos tickets and AFS tokens.
425 This may also be specified on a per-host basis in the configuration file.
426 .It Fl l Ar login_name
427 Specifies the user to log in as on the remote machine.
428 This also may be specified on a per-host basis in the configuration file.
430 Additionally, for protocol version 2 a comma-separated list of MAC
431 (message authentication code) algorithms can
432 be specified in order of preference.
435 keyword for more information.
439 (actually, prevents reading from stdin).
440 This must be used when
442 is run in the background.
443 A common trick is to use this to run X11 programs on a remote machine.
445 .Ic ssh -n shadows.cs.hut.fi emacs &
446 will start an emacs on shadows.cs.hut.fi, and the X11
447 connection will be automatically forwarded over an encrypted channel.
450 program will be put in the background.
451 (This does not work if
453 needs to ask for a password or passphrase; see also the
457 Do not execute a remote command.
458 This is useful if you just want to forward ports
459 (protocol version 2 only).
461 Can be used to give options in the format used in the config file.
462 This is useful for specifying options for which there is no separate
464 The option has the same format as a line in the configuration file.
466 Port to connect to on the remote host.
467 This can be specified on a
468 per-host basis in the configuration file.
470 Use a non-privileged port for outgoing connections.
471 This can be used if your firewall does
472 not permit connections from privileged ports.
473 Note that this option turns off
474 .Cm RhostsAuthentication
476 .Cm RhostsRSAAuthentication
480 Causes all warning and diagnostic messages to be suppressed.
481 Only fatal errors are displayed.
483 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
484 of SSH as a secure transport for other application (eg. sftp). The
485 subsystem is specified as the remote command.
487 Force pseudo-tty allocation.
488 This can be used to execute arbitrary
489 screen-based programs on a remote machine, which can be very useful,
490 e.g., when implementing menu services.
493 options force tty allocation, even if
497 Disable pseudo-tty allocation.
502 to print debugging messages about its progress.
504 debugging connection, authentication, and configuration problems.
507 options increases the verbosity.
510 Disables X11 forwarding.
512 Enables X11 forwarding.
513 This can also be specified on a per-host basis in a configuration file.
515 Requests compression of all data (including stdin, stdout, stderr, and
516 data for forwarded X11 and TCP/IP connections).
517 The compression algorithm is the same used by
521 can be controlled by the
524 Compression is desirable on modem lines and other
525 slow connections, but will only slow down things on fast networks.
526 The default value can be set on a host-by-host basis in the
527 configuration files; see the
530 .It Fl L Ar port:host:hostport
531 Specifies that the given port on the local (client) host is to be
532 forwarded to the given host and port on the remote side.
533 This works by allocating a socket to listen to
535 on the local side, and whenever a connection is made to this port, the
536 connection is forwarded over the secure channel, and a connection is
541 from the remote machine.
542 Port forwardings can also be specified in the configuration file.
543 Only root can forward privileged ports.
544 IPv6 addresses can be specified with an alternative syntax:
545 .Ar port/host/hostport
546 .It Fl R Ar port:host:hostport
547 Specifies that the given port on the remote (server) host is to be
548 forwarded to the given host and port on the local side.
549 This works by allocating a socket to listen to
551 on the remote side, and whenever a connection is made to this port, the
552 connection is forwarded over the secure channel, and a connection is
557 from the local machine.
558 Port forwardings can also be specified in the configuration file.
559 Privileged ports can be forwarded only when
560 logging in as root on the remote machine.
564 to try protocol version 1 only.
568 to try protocol version 2 only.
572 to use IPv4 addresses only.
576 to use IPv6 addresses only.
578 .Sh CONFIGURATION FILES
580 obtains configuration data from the following sources (in this order):
581 command line options, user's configuration file
582 .Pq Pa $HOME/.ssh/config ,
583 and system-wide configuration file
584 .Pq Pa /etc/ssh_config .
585 For each parameter, the first obtained value
587 The configuration files contain sections bracketed by
589 specifications, and that section is only applied for hosts that
590 match one of the patterns given in the specification.
591 The matched host name is the one given on the command line.
593 Since the first obtained value for each parameter is used, more
594 host-specific declarations should be given near the beginning of the
595 file, and general defaults at the end.
597 The configuration file has the following format:
599 Empty lines and lines starting with
603 Otherwise a line is of the format
604 .Dq keyword arguments .
606 keywords and their meanings are as follows (note that the
607 configuration files are case-sensitive):
610 Restricts the following declarations (up to the next
612 keyword) to be only for those hosts that match one of the patterns
613 given after the keyword.
617 can be used as wildcards in the
621 as a pattern can be used to provide global
622 defaults for all hosts.
625 argument given on the command line (i.e., the name is not converted to
626 a canonicalized host name before matching).
627 .It Cm AFSTokenPassing
628 Specifies whether to pass AFS tokens to remote host.
629 The argument to this keyword must be
636 passphrase/password querying will be disabled.
637 This option is useful in scripts and other batch jobs where you have no
638 user to supply the password.
644 If this flag is set to
646 ssh will additionally check the host ip address in the
649 This allows ssh to detect if a host key changed due to DNS spoofing.
650 If the option is set to
652 the check will not be executed.
654 Specifies the cipher to use for encrypting the session
655 in protocol version 1.
664 Specifies the ciphers allowed for protocol version 2
665 in order of preference.
666 Multiple ciphers must be comma-separated.
670 ``3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc,
671 aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
672 rijndael256-cbc,rijndael-cbc@lysator.liu.se''
675 Specifies whether to use compression.
680 .It Cm CompressionLevel
681 Specifies the compression level to use if compression is enable.
682 The argument must be an integer from 1 (fast) to 9 (slow, best).
683 The default level is 6, which is good for most applications.
684 The meaning of the values is the same as in
686 .It Cm ConnectionAttempts
687 Specifies the number of tries (one per second) to make before falling
688 back to rsh or exiting.
689 The argument must be an integer.
690 This may be useful in scripts if the connection sometimes fails.
691 .It Cm PubkeyAuthentication
692 Specifies whether to try public key authentication.
693 The argument to this keyword must be
697 Note that this option applies to protocol version 2 only.
699 Sets the escape character (default:
701 The escape character can also
702 be set on the command line.
703 The argument should be a single character,
705 followed by a letter, or
707 to disable the escape
708 character entirely (making the connection transparent for binary
711 Specifies that if connecting via
713 fails due to a connection refused error (there is no
715 listening on the remote host),
717 should automatically be used instead (after a suitable warning about
718 the session being unencrypted).
724 Specifies whether the connection to the authentication agent (if any)
725 will be forwarded to the remote machine.
733 Specifies whether X11 connections will be automatically redirected
734 over the secure channel and
744 Specifies whether remote hosts are allowed to connect to local
752 .It Cm GlobalKnownHostsFile
753 Specifies a file to use instead of
754 .Pa /etc/ssh_known_hosts .
756 Specifies an alias that should be used instead of the
757 real host name when looking up or saving the host key
758 in the known_hosts files.
759 This option is useful for tunneling ssh connections
760 or if you have multiple servers running on a single host.
762 Specifies the real host name to log into.
763 This can be used to specify nicknames or abbreviations for hosts.
764 Default is the name given on the command line.
765 Numeric IP addresses are also permitted (both on the command line and in
769 Specifies the file from which the user's RSA authentication identity
771 .Pa $HOME/.ssh/identity
772 in the user's home directory).
773 Additionally, any identities represented by the authentication agent
774 will be used for authentication.
775 The file name may use the tilde
776 syntax to refer to a user's home directory.
777 It is possible to have
778 multiple identity files specified in configuration files; all these
779 identities will be tried in sequence.
781 Specifies whether the system should send keepalive messages to the
783 If they are sent, death of the connection or crash of one
784 of the machines will be properly noticed.
785 However, this means that
786 connections will die if the route is down temporarily, and some people
791 (to send keepalives), and the client will notice
792 if the network goes down or the remote host dies.
793 This is important in scripts, and many users want it too.
795 To disable keepalives, the value should be set to
797 in both the server and the client configuration files.
798 .It Cm KerberosAuthentication
799 Specifies whether Kerberos authentication will be used.
800 The argument to this keyword must be
804 .It Cm KerberosTgtPassing
805 Specifies whether a Kerberos TGT will be forwarded to the server.
806 This will only work if the Kerberos server is actually an AFS kaserver.
807 The argument to this keyword must be
812 Specifies that a TCP/IP port on the local machine be forwarded over
813 the secure channel to given host:port from the remote machine.
814 The first argument must be a port number, and the second must be
816 Multiple forwardings may be specified, and additional
817 forwardings can be given on the command line.
818 Only the superuser can forward privileged ports.
820 Gives the verbosity level that is used when logging messages from
822 The possible values are:
823 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
826 Specifies the MAC (message authentication code) algorithms
827 in order of preference.
828 The MAC algorithm is used in protocol version 2
829 for data integrity protection.
830 Multiple algorithms must be comma-separated.
834 ``hmac-sha1,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,
835 hmac-sha1-96,hmac-md5-96''
837 .It Cm NumberOfPasswordPrompts
838 Specifies the number of password prompts before giving up.
839 The argument to this keyword must be an integer.
841 .It Cm PasswordAuthentication
842 Specifies whether to use password authentication.
843 The argument to this keyword must be
847 Note that this option applies to both protocol version 1 and 2.
849 Specifies the port number to connect on the remote host.
852 Specifies the protocol versions
854 should support in order of preference.
855 The possible values are
859 Multiple versions must be comma-separated.
864 tries version 1 and falls back to version 2
865 if version 1 is not available.
867 Specifies the command to use to connect to the server.
869 string extends to the end of the line, and is executed with
871 In the command string,
873 will be substituted by the host name to
877 The command can be basically anything,
878 and should read from its standard input and write to its standard output.
879 It should eventually connect an
881 server running on some machine, or execute
884 Host key management will be done using the
885 HostName of the host being connected (defaulting to the name typed by
889 is not available for connects with a proxy command.
892 Specifies that a TCP/IP port on the remote machine be forwarded over
893 the secure channel to given host:port from the local machine.
894 The first argument must be a port number, and the second must be
896 Multiple forwardings may be specified, and additional
897 forwardings can be given on the command line.
898 Only the superuser can forward privileged ports.
899 .It Cm RhostsAuthentication
900 Specifies whether to try rhosts based authentication.
902 declaration only affects the client side and has no effect whatsoever
904 Disabling rhosts authentication may reduce
905 authentication time on slow connections when rhosts authentication is
907 Most servers do not permit RhostsAuthentication because it
908 is not secure (see RhostsRSAAuthentication).
909 The argument to this keyword must be
913 .It Cm RhostsRSAAuthentication
914 Specifies whether to try rhosts based authentication with RSA host
916 This is the primary authentication method for most sites.
921 .It Cm RSAAuthentication
922 Specifies whether to try RSA authentication.
923 The argument to this keyword must be
927 RSA authentication will only be
928 attempted if the identity file exists, or an authentication agent is
930 Note that this option applies to protocol version 1 only.
931 .It Cm ChallengeResponseAuthentication
932 Specifies whether to use challenge response authentication.
933 Currently there is only support for
936 The argument to this keyword must be
942 .It Cm StrictHostKeyChecking
943 If this flag is set to
946 will never automatically add host keys to the
947 .Pa $HOME/.ssh/known_hosts
949 .Pa $HOME/.ssh/known_hosts2
950 files, and refuses to connect to hosts whose host key has changed.
951 This provides maximum protection against trojan horse attacks.
952 However, it can be somewhat annoying if you don't have good
953 .Pa /etc/ssh_known_hosts
955 .Pa /etc/ssh_known_hosts2
956 files installed and frequently
957 connect to new hosts.
958 This option forces the user to manually
960 If this flag is set to
963 will automatically add new host keys to the
964 user known hosts files.
965 If this flag is set to
968 will be added to the user known host files only after the user
969 has confirmed that is what they really want to do, and
971 will refuse to connect to hosts whose host key has changed.
973 known hosts will be verified automatically in all cases.
981 .It Cm UsePrivilegedPort
982 Specifies whether to use a privileged port for outgoing connections.
989 Note that setting this option to
992 .Cm RhostsAuthentication
994 .Cm RhostsRSAAuthentication
997 Specifies the user to log in as.
998 This can be useful if you have a different user name on different machines.
999 This saves the trouble of
1000 having to remember to give the user name on the command line.
1001 .It Cm UserKnownHostsFile
1002 Specifies a file to use instead of
1003 .Pa $HOME/.ssh/known_hosts .
1005 Specifies that rlogin/rsh should be used for this host.
1006 It is possible that the host does not at all support the
1011 to immediately execute
1013 All other options (except
1015 are ignored if this has been specified.
1016 The argument must be
1020 .It Cm XAuthLocation
1021 Specifies the location of the
1025 .Pa /usr/X11R6/bin/xauth .
1029 will normally set the following environment variables:
1034 variable indicates the location of the X11 server.
1035 It is automatically set by
1037 to point to a value of the form
1039 where hostname indicates
1040 the host where the shell runs, and n is an integer >= 1.
1042 uses this special value to forward X11 connections over the secure
1044 The user should normally not set DISPLAY explicitly, as that
1045 will render the X11 connection insecure (and will require the user to
1046 manually copy any required authorization cookies).
1048 Set to the path of the user's home directory.
1052 set for compatibility with systems that use this variable.
1054 Set to point the user's mailbox.
1058 as specified when compiling
1060 .It Ev SSH_AUTH_SOCK
1061 indicates the path of a unix-domain socket used to communicate with the
1064 Identifies the client end of the connection.
1065 The variable contains
1066 three space-separated values: client ip-address, client port number,
1067 and server port number.
1068 .It Ev SSH_ORIGINAL_COMMAND
1069 The variable contains the original command line if a forced command
1071 It can be used to extract the original arguments.
1073 This is set to the name of the tty (path to the device) associated
1074 with the current shell or command.
1075 If the current session has no tty,
1076 this variable is not set.
1078 The timezone variable is set to indicate the present timezone if it
1079 was set when the daemon was started (i.e., the daemon passes the value
1080 on to new connections).
1082 Set to the name of the user logging in.
1088 .Pa $HOME/.ssh/environment ,
1089 and adds lines of the format
1094 .It Pa $HOME/.ssh/known_hosts
1095 Records host keys for all hosts the user has logged into (that are not
1097 .Pa /etc/ssh_known_hosts ) .
1100 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1101 Contains the RSA and the DSA authentication identity of the user.
1103 contain sensitive data and should be readable by the user but not
1104 accessible by others (read/write/execute).
1107 ignores a private key file if it is accessible by others.
1108 It is possible to specify a passphrase when
1109 generating the key; the passphrase will be used to encrypt the
1110 sensitive part of this file using 3DES.
1111 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1112 Contains the public key for authentication (public part of the
1113 identity file in human-readable form).
1115 .Pa $HOME/.ssh/identity.pub
1116 file should be added to
1117 .Pa $HOME/.ssh/authorized_keys
1119 where you wish to log in using RSA authentication.
1121 .Pa $HOME/.ssh/id_dsa.pub
1122 file should be added to
1123 .Pa $HOME/.ssh/authorized_keys2
1125 where you wish to log in using DSA authentication.
1127 sensitive and can (but need not) be readable by anyone.
1129 never used automatically and are not necessary; they are only provided for
1130 the convenience of the user.
1131 .It Pa $HOME/.ssh/config
1132 This is the per-user configuration file.
1133 The format of this file is described above.
1134 This file is used by the
1137 This file does not usually contain any sensitive information,
1138 but the recommended permissions are read/write for the user, and not
1139 accessible by others.
1140 .It Pa $HOME/.ssh/authorized_keys
1141 Lists the RSA keys that can be used for logging in as this user.
1142 The format of this file is described in the
1145 In the simplest form the format is the same as the .pub
1146 identity files (that is, each line contains the number of bits in
1147 modulus, public exponent, modulus, and comment fields, separated by
1149 This file is not highly sensitive, but the recommended
1150 permissions are read/write for the user, and not accessible by others.
1151 .It Pa $HOME/.ssh/authorized_keys2
1152 Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1153 This file is not highly sensitive, but the recommended
1154 permissions are read/write for the user, and not accessible by others.
1155 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1156 Systemwide list of known host keys.
1157 .Pa /etc/ssh_known_hosts
1159 .Pa /etc/ssh_known_hosts2
1160 contains DSA or RSA keys for protocol version 2.
1161 These files should be prepared by the
1162 system administrator to contain the public host keys of all machines in the
1164 This file should be world-readable.
1166 public keys, one per line, in the following format (fields separated
1167 by spaces): system name, number of bits in modulus, public exponent,
1168 modulus, and optional comment field.
1169 When different names are used
1170 for the same machine, all such names should be listed, separated by
1172 The format is described on the
1176 The canonical system name (as returned by name servers) is used by
1178 to verify the client host when logging in; other names are needed because
1180 does not convert the user-supplied name to a canonical name before
1181 checking the key, because someone with access to the name servers
1182 would then be able to fool host authentication.
1183 .It Pa /etc/ssh_config
1184 Systemwide configuration file.
1185 This file provides defaults for those
1186 values that are not specified in the user's configuration file, and
1187 for those users who do not have a configuration file.
1188 This file must be world-readable.
1189 .It Pa $HOME/.rhosts
1190 This file is used in
1192 authentication to list the
1193 host/user pairs that are permitted to log in.
1194 (Note that this file is
1195 also used by rlogin and rsh, which makes using this file insecure.)
1196 Each line of the file contains a host name (in the canonical form
1197 returned by name servers), and then a user name on that host,
1198 separated by a space.
1199 One some machines this file may need to be
1200 world-readable if the user's home directory is on a NFS partition,
1204 Additionally, this file must be owned by the user,
1205 and must not have write permissions for anyone else.
1207 permission for most machines is read/write for the user, and not
1208 accessible by others.
1210 Note that by default
1212 will be installed so that it requires successful RSA host
1213 authentication before permitting \s+2.\s0rhosts authentication.
1214 If your server machine does not have the client's host key in
1215 .Pa /etc/ssh_known_hosts ,
1217 .Pa $HOME/.ssh/known_hosts .
1218 The easiest way to do this is to
1219 connect back to the client from the server machine using ssh; this
1220 will automatically add the host key to
1221 .Pa $HOME/.ssh/known_hosts .
1222 .It Pa $HOME/.shosts
1223 This file is used exactly the same way as
1226 having this file is to be able to use rhosts authentication with
1228 without permitting login with
1232 .It Pa /etc/hosts.equiv
1233 This file is used during
1234 .Pa \&.rhosts authentication.
1236 canonical hosts names, one per line (the full format is described on
1240 If the client host is found in this file, login is
1241 automatically permitted provided client and server user names are the
1243 Additionally, successful RSA host authentication is normally
1245 This file should only be writable by root.
1246 .It Pa /etc/shosts.equiv
1247 This file is processed exactly as
1248 .Pa /etc/hosts.equiv .
1249 This file may be useful to permit logins using
1251 but not using rsh/rlogin.
1253 Commands in this file are executed by
1255 when the user logs in just before the user's shell (or command) is started.
1258 manual page for more information.
1259 .It Pa $HOME/.ssh/rc
1260 Commands in this file are executed by
1262 when the user logs in just before the user's shell (or command) is
1266 manual page for more information.
1267 .It Pa $HOME/.ssh/environment
1268 Contains additional definitions for environment variables, see section
1271 .It Pa libcrypto.so.X.1
1272 A version of this library which includes support for the RSA algorithm
1273 is required for proper operation.
1276 OpenSSH is a derivative of the original and free
1277 ssh 1.2.12 release by Tatu Ylonen.
1278 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1279 Theo de Raadt and Dug Song
1280 removed many bugs, re-added newer features and
1282 Markus Friedl contributed the support for SSH
1283 protocol versions 1.5 and 2.0.