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 1.112 2001/05/19 16:32:16 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl l Ar login_name
47 .Op Ar hostname | user@hostname
51 .Op Fl afgknqstvxACNPTX1246
52 .Op Fl b Ar bind_address
53 .Op Fl c Ar cipher_spec
54 .Op Fl e Ar escape_char
55 .Op Fl i Ar identity_file
56 .Op Fl l Ar login_name
76 .Op Ar hostname | user@hostname
80 (SSH client) is a program for logging into a remote machine and for
81 executing commands on a remote machine.
82 It is intended to replace
83 rlogin and rsh, and provide secure encrypted communications between
84 two untrusted hosts over an insecure network.
86 arbitrary TCP/IP ports can also be forwarded over the secure channel.
89 connects and logs into the specified
92 his/her identity to the remote machine using one of several methods
93 depending on the protocol version used:
95 .Ss SSH protocol version 1
97 First, if the machine the user logs in from is listed in
100 .Pa /etc/shosts.equiv
101 on the remote machine, and the user names are
102 the same on both sides, the user is immediately permitted to log in.
107 exists in the user's home directory on the
108 remote machine and contains a line containing the name of the client
109 machine and the name of the user on that machine, the user is
111 This form of authentication alone is normally not
112 allowed by the server because it is not secure.
114 The second authentication method is the
118 method combined with RSA-based host authentication.
119 It means that if the login would be permitted by
122 .Pa /etc/hosts.equiv ,
124 .Pa /etc/shosts.equiv ,
125 and if additionally the server can verify the client's
127 .Pa /etc/ssh_known_hosts
129 .Pa $HOME/.ssh/known_hosts
132 section), only then login is permitted.
133 This authentication method closes security holes due to IP
134 spoofing, DNS spoofing and routing spoofing.
135 [Note to the administrator:
136 .Pa /etc/hosts.equiv ,
138 and the rlogin/rsh protocol in general, are inherently insecure and should be
139 disabled if security is desired.]
141 As a third authentication method,
143 supports RSA based authentication.
144 The scheme is based on public-key cryptography: there are cryptosystems
145 where encryption and decryption are done using separate keys, and it
146 is not possible to derive the decryption key from the encryption key.
147 RSA is one such system.
148 The idea is that each user creates a public/private
149 key pair for authentication purposes.
150 The server knows the public key, and only the user knows the private key.
152 .Pa $HOME/.ssh/authorized_keys
153 lists the public keys that are permitted for logging
155 When the user logs in, the
157 program tells the server which key pair it would like to use for
159 The server checks if this key is permitted, and if
160 so, sends the user (actually the
162 program running on behalf of the user) a challenge, a random number,
163 encrypted by the user's public key.
164 The challenge can only be
165 decrypted using the proper private key.
166 The user's client then decrypts the
167 challenge using the private key, proving that he/she knows the private
168 key but without disclosing it to the server.
171 implements the RSA authentication protocol automatically.
172 The user creates his/her RSA key pair by running
174 This stores the private key in
175 .Pa $HOME/.ssh/identity
176 and the public key in
177 .Pa $HOME/.ssh/identity.pub
178 in the user's home directory.
179 The user should then copy the
182 .Pa $HOME/.ssh/authorized_keys
183 in his/her home directory on the remote machine (the
185 file corresponds to the conventional
187 file, and has one key
188 per line, though the lines can be very long).
189 After this, the user can log in without giving the password.
190 RSA authentication is much
191 more secure than rhosts authentication.
193 The most convenient way to use RSA authentication may be with an
194 authentication agent.
197 for more information.
199 If other authentication methods fail,
201 prompts the user for a password.
202 The password is sent to the remote
203 host for checking; however, since all communications are encrypted,
204 the password cannot be seen by someone listening on the network.
206 .Ss SSH protocol version 2
208 When a user connects using the protocol version 2
209 different authentication methods are available.
210 Using the default values for
211 .Cm PreferredAuthentications ,
212 the client will try to authenticate first using the public key method;
213 if this method fails password authentication is attempted,
214 and finally if this method fails keyboard-interactive authentication
216 If this method fails password authentication is
219 The public key method is similar to RSA authentication described
220 in the previous section and allows the RSA or DSA algorithm to be used:
221 The client uses his private key,
222 .Pa $HOME/.ssh/id_dsa
224 .Pa $HOME/.ssh/id_rsa ,
225 to sign the session identifier and sends the result to the server.
226 The server checks whether the matching public key is listed in
227 .Pa $HOME/.ssh/authorized_keys2
228 and grants access if both the key is found and the signature is correct.
229 The session identifier is derived from a shared Diffie-Hellman value
230 and is only known to the client and the server.
232 If public key authentication fails or is not available a password
233 can be sent encrypted to the remote host for proving the user's identity.
237 supports hostbased or challenge response authentication.
239 Protocol 2 provides additional mechanisms for confidentiality
240 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
241 and integrity (hmac-md5, hmac-sha1).
242 Note that protocol 1 lacks a strong mechanism for ensuring the
243 integrity of the connection.
245 .Ss Login session and remote execution
247 When the user's identity has been accepted by the server, the server
248 either executes the given command, or logs into the machine and gives
249 the user a normal shell on the remote machine.
250 All communication with
251 the remote command or shell will be automatically encrypted.
253 If a pseudo-terminal has been allocated (normal login session), the
254 user may use the escape characters noted below.
256 If no pseudo tty has been allocated, the
257 session is transparent and can be used to reliably transfer binary
259 On most systems, setting the escape character to
261 will also make the session transparent even if a tty is used.
263 The session terminates when the command or shell on the remote
264 machine exits and all X11 and TCP/IP connections have been closed.
265 The exit status of the remote program is returned as the exit status
269 .Ss Escape Characters
271 When a pseudo terminal has been requested, ssh supports a number of functions
272 through the use of an escape character.
274 A single tilde character can be sent as
276 (or by following the tilde by a character other than those described above).
277 The escape character must always follow a newline to be interpreted as
279 The escape character can be changed in configuration files using the
281 configuration directive or on the command line by the
285 The supported escapes (assuming the default
294 List forwarded connections
296 Background ssh at logout when waiting for forwarded connection / X11 sessions
297 to terminate (protocol version 1 only)
299 Display a list of escape characters
301 Request rekeying of the connection (only useful for SSH protocol version 2
302 and if the peer supports it)
305 .Ss X11 and TCP forwarding
311 (or, see the description of the
315 options described later)
316 and the user is using X11 (the
318 environment variable is set), the connection to the X11 display is
319 automatically forwarded to the remote side in such a way that any X11
320 programs started from the shell (or command) will go through the
321 encrypted channel, and the connection to the real X server will be made
322 from the local machine.
323 The user should not manually set
325 Forwarding of X11 connections can be
326 configured on the command line or in configuration files.
332 will point to the server machine, but with a display number greater
334 This is normal, and happens because
338 X server on the server machine for forwarding the
339 connections over the encrypted channel.
342 will also automatically set up Xauthority data on the server machine.
343 For this purpose, it will generate a random authorization cookie,
344 store it in Xauthority on the server, and verify that any forwarded
345 connections carry this cookie and replace it by the real cookie when
346 the connection is opened.
347 The real authentication cookie is never
348 sent to the server machine (and no cookies are sent in the plain).
350 If the user is using an authentication agent, the connection to the agent
351 is automatically forwarded to the remote side unless disabled on
352 command line or in a configuration file.
354 Forwarding of arbitrary TCP/IP connections over the secure channel can
355 be specified either on command line or in a configuration file.
356 One possible application of TCP/IP forwarding is a secure connection to an
357 electronic purse; another is going through firewalls.
359 .Ss Server authentication
362 automatically maintains and checks a database containing
363 identifications for all hosts it has ever been used with.
364 RSA host keys are stored in
365 .Pa $HOME/.ssh/known_hosts
367 host keys used in the protocol version 2 are stored in
368 .Pa $HOME/.ssh/known_hosts2
369 in the user's home directory.
370 Additionally, the files
371 .Pa /etc/ssh_known_hosts
373 .Pa /etc/ssh_known_hosts2
374 are automatically checked for known hosts.
375 Any new hosts are automatically added to the user's file.
376 If a host's identification
379 warns about this and disables password authentication to prevent a
380 trojan horse from getting the user's password.
382 this mechanism is to prevent man-in-the-middle attacks which could
383 otherwise be used to circumvent the encryption.
385 .Cm StrictHostKeyChecking
386 option (see below) can be used to prevent logins to machines whose
387 host key is not known or has changed.
389 The options are as follows:
392 Disables forwarding of the authentication agent connection.
394 Enables forwarding of the authentication agent connection.
395 This can also be specified on a per-host basis in a configuration file.
396 .It Fl b Ar bind_address
397 Specify the interface to transmit from on machines with multiple
398 interfaces or aliased addresses.
399 .It Fl c Ar blowfish|3des
400 Selects the cipher to use for encrypting the session.
403 It is believed to be secure.
405 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
406 It is presumably more secure than the
408 cipher which is no longer fully supported in
411 is a fast block cipher, it appears very secure and is much faster than
413 .It Fl c Ar cipher_spec
414 Additionally, for protocol version 2 a comma-separated list of ciphers can
415 be specified in order of preference.
418 for more information.
419 .It Fl e Ar ch|^ch|none
420 Sets the escape character for sessions with a pty (default:
422 The escape character is only recognized at the beginning of a line.
423 The escape character followed by a dot
425 closes the connection, followed
426 by control-Z suspends the connection, and followed by itself sends the
427 escape character once.
428 Setting the character to
430 disables any escapes and makes the session fully transparent.
434 to go to background just before command execution.
437 is going to ask for passwords or passphrases, but the user
438 wants it in the background.
441 The recommended way to start X11 programs at a remote site is with
443 .Ic ssh -f host xterm .
445 Allows remote hosts to connect to local forwarded ports.
446 .It Fl i Ar identity_file
447 Selects the file from which the identity (private key) for
448 RSA or DSA authentication is read.
450 .Pa $HOME/.ssh/identity
451 in the user's home directory.
452 Identity files may also be specified on
453 a per-host basis in the configuration file.
454 It is possible to have multiple
456 options (and multiple identities specified in
457 configuration files).
459 Disables forwarding of Kerberos tickets and AFS tokens.
460 This may also be specified on a per-host basis in the configuration file.
461 .It Fl l Ar login_name
462 Specifies the user to log in as on the remote machine.
463 This also may be specified on a per-host basis in the configuration file.
465 Additionally, for protocol version 2 a comma-separated list of MAC
466 (message authentication code) algorithms can
467 be specified in order of preference.
470 keyword for more information.
474 (actually, prevents reading from stdin).
475 This must be used when
477 is run in the background.
478 A common trick is to use this to run X11 programs on a remote machine.
480 .Ic ssh -n shadows.cs.hut.fi emacs &
481 will start an emacs on shadows.cs.hut.fi, and the X11
482 connection will be automatically forwarded over an encrypted channel.
485 program will be put in the background.
486 (This does not work if
488 needs to ask for a password or passphrase; see also the
492 Do not execute a remote command.
493 This is useful if you just want to forward ports
494 (protocol version 2 only).
496 Can be used to give options in the format used in the config file.
497 This is useful for specifying options for which there is no separate
499 The option has the same format as a line in the configuration file.
501 Port to connect to on the remote host.
502 This can be specified on a
503 per-host basis in the configuration file.
505 Use a non-privileged port for outgoing connections.
506 This can be used if your firewall does
507 not permit connections from privileged ports.
508 Note that this option turns off
509 .Cm RhostsAuthentication
511 .Cm RhostsRSAAuthentication
515 Causes all warning and diagnostic messages to be suppressed.
516 Only fatal errors are displayed.
518 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
519 of SSH as a secure transport for other applications (eg. sftp). The
520 subsystem is specified as the remote command.
522 Force pseudo-tty allocation.
523 This can be used to execute arbitrary
524 screen-based programs on a remote machine, which can be very useful,
525 e.g., when implementing menu services.
528 options force tty allocation, even if
532 Disable pseudo-tty allocation.
537 to print debugging messages about its progress.
539 debugging connection, authentication, and configuration problems.
542 options increases the verbosity.
545 Disables X11 forwarding.
547 Enables X11 forwarding.
548 This can also be specified on a per-host basis in a configuration file.
550 Requests compression of all data (including stdin, stdout, stderr, and
551 data for forwarded X11 and TCP/IP connections).
552 The compression algorithm is the same used by
556 can be controlled by the
559 Compression is desirable on modem lines and other
560 slow connections, but will only slow down things on fast networks.
561 The default value can be set on a host-by-host basis in the
562 configuration files; see the
565 .It Fl L Ar port:host:hostport
566 Specifies that the given port on the local (client) host is to be
567 forwarded to the given host and port on the remote side.
568 This works by allocating a socket to listen to
570 on the local side, and whenever a connection is made to this port, the
571 connection is forwarded over the secure channel, and a connection is
576 from the remote machine.
577 Port forwardings can also be specified in the configuration file.
578 Only root can forward privileged ports.
579 IPv6 addresses can be specified with an alternative syntax:
580 .Ar port/host/hostport
581 .It Fl R Ar port:host:hostport
582 Specifies that the given port on the remote (server) host is to be
583 forwarded to the given host and port on the local side.
584 This works by allocating a socket to listen to
586 on the remote side, and whenever a connection is made to this port, the
587 connection is forwarded over the secure channel, and a connection is
592 from the local machine.
593 Port forwardings can also be specified in the configuration file.
594 Privileged ports can be forwarded only when
595 logging in as root on the remote machine.
596 IPv6 addresses can be specified with an alternative syntax:
597 .Ar port/host/hostport
601 to try protocol version 1 only.
605 to try protocol version 2 only.
609 to use IPv4 addresses only.
613 to use IPv6 addresses only.
615 .Sh CONFIGURATION FILES
617 obtains configuration data from the following sources (in this order):
618 command line options, user's configuration file
619 .Pq Pa $HOME/.ssh/config ,
620 and system-wide configuration file
621 .Pq Pa /etc/ssh_config .
622 For each parameter, the first obtained value
624 The configuration files contain sections bracketed by
626 specifications, and that section is only applied for hosts that
627 match one of the patterns given in the specification.
628 The matched host name is the one given on the command line.
630 Since the first obtained value for each parameter is used, more
631 host-specific declarations should be given near the beginning of the
632 file, and general defaults at the end.
634 The configuration file has the following format:
636 Empty lines and lines starting with
640 Otherwise a line is of the format
641 .Dq keyword arguments .
643 keywords and their meanings are as follows (note that the
644 configuration files are case-sensitive):
647 Restricts the following declarations (up to the next
649 keyword) to be only for those hosts that match one of the patterns
650 given after the keyword.
654 can be used as wildcards in the
658 as a pattern can be used to provide global
659 defaults for all hosts.
662 argument given on the command line (i.e., the name is not converted to
663 a canonicalized host name before matching).
664 .It Cm AFSTokenPassing
665 Specifies whether to pass AFS tokens to remote host.
666 The argument to this keyword must be
670 This option applies to protocol version 1 only.
674 passphrase/password querying will be disabled.
675 This option is useful in scripts and other batch jobs where you have no
676 user to supply the password.
684 Specify the interface to transmit from on machines with multiple
685 interfaces or aliased addresses.
686 Note that this option does not work if
687 .Cm UsePrivilegedPort
691 If this flag is set to
693 ssh will additionally check the host IP address in the
696 This allows ssh to detect if a host key changed due to DNS spoofing.
697 If the option is set to
699 the check will not be executed.
703 Specifies the cipher to use for encrypting the session
704 in protocol version 1.
713 Specifies the ciphers allowed for protocol version 2
714 in order of preference.
715 Multiple ciphers must be comma-separated.
719 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
720 aes192-cbc,aes256-cbc''
723 Specifies whether to use compression.
730 .It Cm CompressionLevel
731 Specifies the compression level to use if compression is enabled.
732 The argument must be an integer from 1 (fast) to 9 (slow, best).
733 The default level is 6, which is good for most applications.
734 The meaning of the values is the same as in
736 Note that this option applies to protocol version 1 only.
737 .It Cm ConnectionAttempts
738 Specifies the number of tries (one per second) to make before falling
739 back to rsh or exiting.
740 The argument must be an integer.
741 This may be useful in scripts if the connection sometimes fails.
744 Sets the escape character (default:
746 The escape character can also
747 be set on the command line.
748 The argument should be a single character,
750 followed by a letter, or
752 to disable the escape
753 character entirely (making the connection transparent for binary
756 Specifies that if connecting via
758 fails due to a connection refused error (there is no
760 listening on the remote host),
762 should automatically be used instead (after a suitable warning about
763 the session being unencrypted).
771 Specifies whether the connection to the authentication agent (if any)
772 will be forwarded to the remote machine.
780 Specifies whether X11 connections will be automatically redirected
781 over the secure channel and
791 Specifies whether remote hosts are allowed to connect to local
799 .It Cm GlobalKnownHostsFile
800 Specifies a file to use for the protocol version 1 global
801 host key database instead of
802 .Pa /etc/ssh_known_hosts .
803 .It Cm GlobalKnownHostsFile2
804 Specifies a file to use for the protocol version 2 global
805 host key database instead of
806 .Pa /etc/ssh_known_hosts2 .
807 .It Cm HostbasedAuthentication
808 Specifies whether to try rhosts based authentication with public key
816 This option applies to protocol version 2 only and
818 .Cm RhostsRSAAuthentication .
819 .It Cm HostKeyAlgorithms
820 Specifies the protocol version 2 host key algorithms
821 that the client wants to use in order of preference.
822 The default for this option is:
825 Specifies an alias that should be used instead of the
826 real host name when looking up or saving the host key
827 in the host key database files.
828 This option is useful for tunneling ssh connections
829 or if you have multiple servers running on a single host.
831 Specifies the real host name to log into.
832 This can be used to specify nicknames or abbreviations for hosts.
833 Default is the name given on the command line.
834 Numeric IP addresses are also permitted (both on the command line and in
838 Specifies the file from which the user's RSA or DSA authentication identity
840 .Pa $HOME/.ssh/identity
841 in the user's home directory).
842 Additionally, any identities represented by the authentication agent
843 will be used for authentication.
844 The file name may use the tilde
845 syntax to refer to a user's home directory.
846 It is possible to have
847 multiple identity files specified in configuration files; all these
848 identities will be tried in sequence.
850 Specifies whether the system should send keepalive messages to the
852 If they are sent, death of the connection or crash of one
853 of the machines will be properly noticed.
854 However, this means that
855 connections will die if the route is down temporarily, and some people
860 (to send keepalives), and the client will notice
861 if the network goes down or the remote host dies.
862 This is important in scripts, and many users want it too.
864 To disable keepalives, the value should be set to
866 in both the server and the client configuration files.
867 .It Cm KerberosAuthentication
868 Specifies whether Kerberos authentication will be used.
869 The argument to this keyword must be
873 .It Cm KerberosTgtPassing
874 Specifies whether a Kerberos TGT will be forwarded to the server.
875 This will only work if the Kerberos server is actually an AFS kaserver.
876 The argument to this keyword must be
881 Specifies that a TCP/IP port on the local machine be forwarded over
882 the secure channel to given host:port from the remote machine.
883 The first argument must be a port number, and the second must be
885 Multiple forwardings may be specified, and additional
886 forwardings can be given on the command line.
887 Only the superuser can forward privileged ports.
889 Gives the verbosity level that is used when logging messages from
891 The possible values are:
892 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
895 Specifies the MAC (message authentication code) algorithms
896 in order of preference.
897 The MAC algorithm is used in protocol version 2
898 for data integrity protection.
899 Multiple algorithms must be comma-separated.
903 ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
904 hmac-sha1-96,hmac-md5-96''
906 .It Cm NumberOfPasswordPrompts
907 Specifies the number of password prompts before giving up.
908 The argument to this keyword must be an integer.
910 .It Cm PasswordAuthentication
911 Specifies whether to use password authentication.
912 The argument to this keyword must be
919 Specifies the port number to connect on the remote host.
921 .It Cm PreferredAuthentications
922 Specifies the order in which the client should try protocol 2
923 authentication methods. This allows a client to prefer one method (e.g.
924 .Cm keyboard-interactive )
925 over another method (e.g.
927 The default for this option is:
928 .Dq publickey,hostbased,password,keyboard-interactive
930 Specifies the protocol versions
932 should support in order of preference.
933 The possible values are
937 Multiple versions must be comma-separated.
942 tries version 2 and falls back to version 1
943 if version 2 is not available.
945 Specifies the command to use to connect to the server.
947 string extends to the end of the line, and is executed with
949 In the command string,
951 will be substituted by the host name to
955 The command can be basically anything,
956 and should read from its standard input and write to its standard output.
957 It should eventually connect an
959 server running on some machine, or execute
962 Host key management will be done using the
963 HostName of the host being connected (defaulting to the name typed by
967 is not available for connects with a proxy command.
969 .It Cm PubkeyAuthentication
970 Specifies whether to try public key authentication.
971 The argument to this keyword must be
977 This option applies to protocol version 2 only.
979 Specifies that a TCP/IP port on the remote machine be forwarded over
980 the secure channel to given host:port from the local machine.
981 The first argument must be a port number, and the second must be
983 Multiple forwardings may be specified, and additional
984 forwardings can be given on the command line.
985 Only the superuser can forward privileged ports.
986 .It Cm RhostsAuthentication
987 Specifies whether to try rhosts based authentication.
989 declaration only affects the client side and has no effect whatsoever
991 Disabling rhosts authentication may reduce
992 authentication time on slow connections when rhosts authentication is
994 Most servers do not permit RhostsAuthentication because it
996 .Cm RhostsRSAAuthentication ).
997 The argument to this keyword must be
1003 This option applies to protocol version 1 only.
1004 .It Cm RhostsRSAAuthentication
1005 Specifies whether to try rhosts based authentication with RSA host
1007 The argument must be
1013 This option applies to protocol version 1 only.
1014 .It Cm RSAAuthentication
1015 Specifies whether to try RSA authentication.
1016 The argument to this keyword must be
1020 RSA authentication will only be
1021 attempted if the identity file exists, or an authentication agent is
1025 Note that this option applies to protocol version 1 only.
1026 .It Cm ChallengeResponseAuthentication
1027 Specifies whether to use challenge response authentication.
1028 Currently there is only support for
1031 The argument to this keyword must be
1037 .It Cm StrictHostKeyChecking
1038 If this flag is set to
1041 will never automatically add host keys to the
1042 .Pa $HOME/.ssh/known_hosts
1044 .Pa $HOME/.ssh/known_hosts2
1045 files, and refuses to connect to hosts whose host key has changed.
1046 This provides maximum protection against trojan horse attacks.
1047 However, it can be somewhat annoying if you don't have good
1048 .Pa /etc/ssh_known_hosts
1050 .Pa /etc/ssh_known_hosts2
1051 files installed and frequently
1052 connect to new hosts.
1053 This option forces the user to manually
1055 If this flag is set to
1058 will automatically add new host keys to the
1059 user known hosts files.
1060 If this flag is set to
1063 will be added to the user known host files only after the user
1064 has confirmed that is what they really want to do, and
1066 will refuse to connect to hosts whose host key has changed.
1068 known hosts will be verified automatically in all cases.
1069 The argument must be
1076 .It Cm UsePrivilegedPort
1077 Specifies whether to use a privileged port for outgoing connections.
1078 The argument must be
1084 Note that you need to set this option to
1087 .Cm RhostsAuthentication
1089 .Cm RhostsRSAAuthentication
1092 Specifies the user to log in as.
1093 This can be useful if you have a different user name on different machines.
1094 This saves the trouble of
1095 having to remember to give the user name on the command line.
1096 .It Cm UserKnownHostsFile
1097 Specifies a file to use for the protocol version 1 user
1098 host key database instead of
1099 .Pa $HOME/.ssh/known_hosts .
1100 .It Cm UserKnownHostsFile2
1101 Specifies a file to use for the protocol version 2 user
1102 host key database instead of
1103 .Pa $HOME/.ssh/known_hosts2 .
1105 Specifies that rlogin/rsh should be used for this host.
1106 It is possible that the host does not at all support the
1111 to immediately execute
1113 All other options (except
1115 are ignored if this has been specified.
1116 The argument must be
1120 .It Cm XAuthLocation
1121 Specifies the location of the
1125 .Pa /usr/X11R6/bin/xauth .
1129 will normally set the following environment variables:
1134 variable indicates the location of the X11 server.
1135 It is automatically set by
1137 to point to a value of the form
1139 where hostname indicates
1140 the host where the shell runs, and n is an integer >= 1.
1142 uses this special value to forward X11 connections over the secure
1144 The user should normally not set
1147 will render the X11 connection insecure (and will require the user to
1148 manually copy any required authorization cookies).
1150 Set to the path of the user's home directory.
1154 set for compatibility with systems that use this variable.
1156 Set to point the user's mailbox.
1160 as specified when compiling
1162 .It Ev SSH_AUTH_SOCK
1163 indicates the path of a unix-domain socket used to communicate with the
1166 Identifies the client end of the connection.
1167 The variable contains
1168 three space-separated values: client ip-address, client port number,
1169 and server port number.
1170 .It Ev SSH_ORIGINAL_COMMAND
1171 The variable contains the original command line if a forced command
1173 It can be used to extract the original arguments.
1175 This is set to the name of the tty (path to the device) associated
1176 with the current shell or command.
1177 If the current session has no tty,
1178 this variable is not set.
1180 The timezone variable is set to indicate the present timezone if it
1181 was set when the daemon was started (i.e., the daemon passes the value
1182 on to new connections).
1184 Set to the name of the user logging in.
1190 .Pa $HOME/.ssh/environment ,
1191 and adds lines of the format
1196 .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1197 Records host keys for all hosts the user has logged into (that are not
1199 .Pa /etc/ssh_known_hosts
1200 for protocol version 1 or
1201 .Pa /etc/ssh_known_hosts2
1202 for protocol version 2).
1205 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1206 Contains the authentication identity of the user.
1207 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1209 contain sensitive data and should be readable by the user but not
1210 accessible by others (read/write/execute).
1213 ignores a private key file if it is accessible by others.
1214 It is possible to specify a passphrase when
1215 generating the key; the passphrase will be used to encrypt the
1216 sensitive part of this file using 3DES.
1217 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1218 Contains the public key for authentication (public part of the
1219 identity file in human-readable form).
1221 .Pa $HOME/.ssh/identity.pub
1222 file should be added to
1223 .Pa $HOME/.ssh/authorized_keys
1225 where you wish to log in using protocol version 1 RSA authentication.
1227 .Pa $HOME/.ssh/id_dsa.pub
1229 .Pa $HOME/.ssh/id_rsa.pub
1230 file should be added to
1231 .Pa $HOME/.ssh/authorized_keys2
1233 where you wish to log in using protocol version 2 DSA/RSA authentication.
1235 sensitive and can (but need not) be readable by anyone.
1237 never used automatically and are not necessary; they are only provided for
1238 the convenience of the user.
1239 .It Pa $HOME/.ssh/config
1240 This is the per-user configuration file.
1241 The format of this file is described above.
1242 This file is used by the
1245 This file does not usually contain any sensitive information,
1246 but the recommended permissions are read/write for the user, and not
1247 accessible by others.
1248 .It Pa $HOME/.ssh/authorized_keys
1249 Lists the RSA keys that can be used for logging in as this user.
1250 The format of this file is described in the
1253 In the simplest form the format is the same as the .pub
1254 identity files (that is, each line contains the number of bits in
1255 modulus, public exponent, modulus, and comment fields, separated by
1257 This file is not highly sensitive, but the recommended
1258 permissions are read/write for the user, and not accessible by others.
1259 .It Pa $HOME/.ssh/authorized_keys2
1260 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1261 This file is not highly sensitive, but the recommended
1262 permissions are read/write for the user, and not accessible by others.
1263 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1264 Systemwide list of known host keys.
1265 .Pa /etc/ssh_known_hosts
1267 .Pa /etc/ssh_known_hosts2
1268 contains RSA or DSA keys for protocol version 2.
1269 These files should be prepared by the
1270 system administrator to contain the public host keys of all machines in the
1272 This file should be world-readable.
1274 public keys, one per line, in the following format (fields separated
1275 by spaces): system name, number of bits in modulus, public exponent,
1276 modulus, and optional comment field.
1277 When different names are used
1278 for the same machine, all such names should be listed, separated by
1280 The format is described on the
1284 The canonical system name (as returned by name servers) is used by
1286 to verify the client host when logging in; other names are needed because
1288 does not convert the user-supplied name to a canonical name before
1289 checking the key, because someone with access to the name servers
1290 would then be able to fool host authentication.
1291 .It Pa /etc/ssh_config
1292 Systemwide configuration file.
1293 This file provides defaults for those
1294 values that are not specified in the user's configuration file, and
1295 for those users who do not have a configuration file.
1296 This file must be world-readable.
1297 .It Pa $HOME/.rhosts
1298 This file is used in
1300 authentication to list the
1301 host/user pairs that are permitted to log in.
1302 (Note that this file is
1303 also used by rlogin and rsh, which makes using this file insecure.)
1304 Each line of the file contains a host name (in the canonical form
1305 returned by name servers), and then a user name on that host,
1306 separated by a space.
1307 On some machines this file may need to be
1308 world-readable if the user's home directory is on a NFS partition,
1312 Additionally, this file must be owned by the user,
1313 and must not have write permissions for anyone else.
1315 permission for most machines is read/write for the user, and not
1316 accessible by others.
1318 Note that by default
1320 will be installed so that it requires successful RSA host
1321 authentication before permitting \s+2.\s0rhosts authentication.
1322 If your server machine does not have the client's host key in
1323 .Pa /etc/ssh_known_hosts ,
1325 .Pa $HOME/.ssh/known_hosts .
1326 The easiest way to do this is to
1327 connect back to the client from the server machine using ssh; this
1328 will automatically add the host key to
1329 .Pa $HOME/.ssh/known_hosts .
1330 .It Pa $HOME/.shosts
1331 This file is used exactly the same way as
1334 having this file is to be able to use rhosts authentication with
1336 without permitting login with
1340 .It Pa /etc/hosts.equiv
1341 This file is used during
1342 .Pa \&.rhosts authentication.
1344 canonical hosts names, one per line (the full format is described on
1348 If the client host is found in this file, login is
1349 automatically permitted provided client and server user names are the
1351 Additionally, successful RSA host authentication is normally
1353 This file should only be writable by root.
1354 .It Pa /etc/shosts.equiv
1355 This file is processed exactly as
1356 .Pa /etc/hosts.equiv .
1357 This file may be useful to permit logins using
1359 but not using rsh/rlogin.
1361 Commands in this file are executed by
1363 when the user logs in just before the user's shell (or command) is started.
1366 manual page for more information.
1367 .It Pa $HOME/.ssh/rc
1368 Commands in this file are executed by
1370 when the user logs in just before the user's shell (or command) is
1374 manual page for more information.
1375 .It Pa $HOME/.ssh/environment
1376 Contains additional definitions for environment variables, see section
1381 OpenSSH is a derivative of the original and free
1382 ssh 1.2.12 release by Tatu Ylonen.
1383 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1384 Theo de Raadt and Dug Song
1385 removed many bugs, re-added newer features and
1387 Markus Friedl contributed the support for SSH
1388 protocol versions 1.5 and 2.0.
1405 .%T "SSH Protocol Architecture"
1406 .%N draft-ietf-secsh-architecture-07.txt
1408 .%O work in progress material