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 1.106 2001/04/22 13:32:27 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 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 (SSH client) 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
218 .Pa $HOME/.ssh/id_rsa ,
219 to sign the session identifier and sends the result to the server.
220 The server checks whether the matching public key is listed in
221 .Pa $HOME/.ssh/authorized_keys2
222 and grants access if both the key is found and the signature is correct.
223 The session identifier is derived from a shared Diffie-Hellman value
224 and is only known to the client and the server.
226 If public key authentication fails or is not available a password
227 can be sent encrypted to the remote host for proving the user's identity.
228 This protocol 2 implementation does not yet support Kerberos or
229 S/Key authentication.
231 Protocol 2 provides additional mechanisms for confidentiality
232 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
233 and integrity (hmac-md5, hmac-sha1).
234 Note that protocol 1 lacks a strong mechanism for ensuring the
235 integrity of the connection.
237 .Ss Login session and remote execution
239 When the user's identity has been accepted by the server, the server
240 either executes the given command, or logs into the machine and gives
241 the user a normal shell on the remote machine.
242 All communication with
243 the remote command or shell will be automatically encrypted.
245 If a pseudo-terminal has been allocated (normal login session), the
246 user may use the escape characters noted below.
248 If no pseudo tty has been allocated, the
249 session is transparent and can be used to reliably transfer binary
251 On most systems, setting the escape character to
253 will also make the session transparent even if a tty is used.
255 The session terminates when the command or shell on the remote
256 machine exits and all X11 and TCP/IP connections have been closed.
257 The exit status of the remote program is returned as the exit status
261 .Ss Escape Characters
263 When a pseudo terminal has been requested, ssh supports a number of functions
264 through the use of an escape character.
266 A single tilde character can be sent as
268 (or by following the tilde by a character other than those described above).
269 The escape character must always follow a newline to be interpreted as
271 The escape character can be changed in configuration files using the
273 configuration directive or on the command line by the
277 The supported escapes (assuming the default
286 List forwarded connections
288 Background ssh at logout when waiting for forwarded connection / X11 sessions
289 to terminate (this does not currently work for SSH protocol version 2)
291 Display a list of escape characters
293 Request rekeying of the connection (only useful for SSH protocol version 2
294 and if the peer supports it)
297 .Ss X11 and TCP forwarding
299 If the user is using X11 (the
301 environment variable is set), the connection to the X11 display is
302 automatically forwarded to the remote side in such a way that any X11
303 programs started from the shell (or command) will go through the
304 encrypted channel, and the connection to the real X server will be made
305 from the local machine.
306 The user should not manually set
308 Forwarding of X11 connections can be
309 configured on the command line or in configuration files.
315 will point to the server machine, but with a display number greater
317 This is normal, and happens because
321 X server on the server machine for forwarding the
322 connections over the encrypted channel.
325 will also automatically set up Xauthority data on the server machine.
326 For this purpose, it will generate a random authorization cookie,
327 store it in Xauthority on the server, and verify that any forwarded
328 connections carry this cookie and replace it by the real cookie when
329 the connection is opened.
330 The real authentication cookie is never
331 sent to the server machine (and no cookies are sent in the plain).
333 If the user is using an authentication agent, the connection to the agent
334 is automatically forwarded to the remote side unless disabled on
335 command line or in a configuration file.
337 Forwarding of arbitrary TCP/IP connections over the secure channel can
338 be specified either on command line or in a configuration file.
339 One possible application of TCP/IP forwarding is a secure connection to an
340 electronic purse; another is going through firewalls.
342 .Ss Server authentication
345 automatically maintains and checks a database containing
346 identifications for all hosts it has ever been used with.
347 RSA host keys are stored in
348 .Pa $HOME/.ssh/known_hosts
350 host keys used in the protocol version 2 are stored in
351 .Pa $HOME/.ssh/known_hosts2
352 in the user's home directory.
353 Additionally, the files
354 .Pa /etc/ssh_known_hosts
356 .Pa /etc/ssh_known_hosts2
357 are automatically checked for known hosts.
358 Any new hosts are automatically added to the user's file.
359 If a host's identification
362 warns about this and disables password authentication to prevent a
363 trojan horse from getting the user's password.
365 this mechanism is to prevent man-in-the-middle attacks which could
366 otherwise be used to circumvent the encryption.
368 .Cm StrictHostKeyChecking
369 option (see below) can be used to prevent logins to machines whose
370 host key is not known or has changed.
372 The options are as follows:
375 Disables forwarding of the authentication agent connection.
377 Enables forwarding of the authentication agent connection.
378 This can also be specified on a per-host basis in a configuration file.
379 .It Fl c Ar blowfish|3des
380 Selects the cipher to use for encrypting the session.
383 It is believed to be secure.
385 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
386 It is presumably more secure than the
388 cipher which is no longer fully supported in
391 is a fast block cipher, it appears very secure and is much faster than
393 .It Fl c Ar cipher_spec
394 Additionally, for protocol version 2 a comma-separated list of ciphers can
395 be specified in order of preference.
398 for more information.
399 .It Fl e Ar ch|^ch|none
400 Sets the escape character for sessions with a pty (default:
402 The escape character is only recognized at the beginning of a line.
403 The escape character followed by a dot
405 closes the connection, followed
406 by control-Z suspends the connection, and followed by itself sends the
407 escape character once.
408 Setting the character to
410 disables any escapes and makes the session fully transparent.
414 to go to background just before command execution.
417 is going to ask for passwords or passphrases, but the user
418 wants it in the background.
421 The recommended way to start X11 programs at a remote site is with
423 .Ic ssh -f host xterm .
425 Allows remote hosts to connect to local forwarded ports.
426 .It Fl i Ar identity_file
427 Selects the file from which the identity (private key) for
428 RSA or DSA authentication is read.
430 .Pa $HOME/.ssh/identity
431 in the user's home directory.
432 Identity files may also be specified on
433 a per-host basis in the configuration file.
434 It is possible to have multiple
436 options (and multiple identities specified in
437 configuration files).
439 Disables forwarding of Kerberos tickets and AFS tokens.
440 This may also be specified on a per-host basis in the configuration file.
441 .It Fl l Ar login_name
442 Specifies the user to log in as on the remote machine.
443 This also may be specified on a per-host basis in the configuration file.
445 Additionally, for protocol version 2 a comma-separated list of MAC
446 (message authentication code) algorithms can
447 be specified in order of preference.
450 keyword for more information.
454 (actually, prevents reading from stdin).
455 This must be used when
457 is run in the background.
458 A common trick is to use this to run X11 programs on a remote machine.
460 .Ic ssh -n shadows.cs.hut.fi emacs &
461 will start an emacs on shadows.cs.hut.fi, and the X11
462 connection will be automatically forwarded over an encrypted channel.
465 program will be put in the background.
466 (This does not work if
468 needs to ask for a password or passphrase; see also the
472 Do not execute a remote command.
473 This is useful if you just want to forward ports
474 (protocol version 2 only).
476 Can be used to give options in the format used in the config file.
477 This is useful for specifying options for which there is no separate
479 The option has the same format as a line in the configuration file.
481 Port to connect to on the remote host.
482 This can be specified on a
483 per-host basis in the configuration file.
485 Use a non-privileged port for outgoing connections.
486 This can be used if your firewall does
487 not permit connections from privileged ports.
488 Note that this option turns off
489 .Cm RhostsAuthentication
491 .Cm RhostsRSAAuthentication
495 Causes all warning and diagnostic messages to be suppressed.
496 Only fatal errors are displayed.
498 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
499 of SSH as a secure transport for other application (eg. sftp). The
500 subsystem is specified as the remote command.
502 Force pseudo-tty allocation.
503 This can be used to execute arbitrary
504 screen-based programs on a remote machine, which can be very useful,
505 e.g., when implementing menu services.
508 options force tty allocation, even if
512 Disable pseudo-tty allocation.
517 to print debugging messages about its progress.
519 debugging connection, authentication, and configuration problems.
522 options increases the verbosity.
525 Disables X11 forwarding.
527 Enables X11 forwarding.
528 This can also be specified on a per-host basis in a configuration file.
530 Requests compression of all data (including stdin, stdout, stderr, and
531 data for forwarded X11 and TCP/IP connections).
532 The compression algorithm is the same used by
536 can be controlled by the
539 Compression is desirable on modem lines and other
540 slow connections, but will only slow down things on fast networks.
541 The default value can be set on a host-by-host basis in the
542 configuration files; see the
545 .It Fl L Ar port:host:hostport
546 Specifies that the given port on the local (client) host is to be
547 forwarded to the given host and port on the remote side.
548 This works by allocating a socket to listen to
550 on the local side, and whenever a connection is made to this port, the
551 connection is forwarded over the secure channel, and a connection is
556 from the remote machine.
557 Port forwardings can also be specified in the configuration file.
558 Only root can forward privileged ports.
559 IPv6 addresses can be specified with an alternative syntax:
560 .Ar port/host/hostport
561 .It Fl R Ar port:host:hostport
562 Specifies that the given port on the remote (server) host is to be
563 forwarded to the given host and port on the local side.
564 This works by allocating a socket to listen to
566 on the remote side, and whenever a connection is made to this port, the
567 connection is forwarded over the secure channel, and a connection is
572 from the local machine.
573 Port forwardings can also be specified in the configuration file.
574 Privileged ports can be forwarded only when
575 logging in as root on the remote machine.
579 to try protocol version 1 only.
583 to try protocol version 2 only.
587 to use IPv4 addresses only.
591 to use IPv6 addresses only.
593 .Sh CONFIGURATION FILES
595 obtains configuration data from the following sources (in this order):
596 command line options, user's configuration file
597 .Pq Pa $HOME/.ssh/config ,
598 and system-wide configuration file
599 .Pq Pa /etc/ssh_config .
600 For each parameter, the first obtained value
602 The configuration files contain sections bracketed by
604 specifications, and that section is only applied for hosts that
605 match one of the patterns given in the specification.
606 The matched host name is the one given on the command line.
608 Since the first obtained value for each parameter is used, more
609 host-specific declarations should be given near the beginning of the
610 file, and general defaults at the end.
612 The configuration file has the following format:
614 Empty lines and lines starting with
618 Otherwise a line is of the format
619 .Dq keyword arguments .
621 keywords and their meanings are as follows (note that the
622 configuration files are case-sensitive):
625 Restricts the following declarations (up to the next
627 keyword) to be only for those hosts that match one of the patterns
628 given after the keyword.
632 can be used as wildcards in the
636 as a pattern can be used to provide global
637 defaults for all hosts.
640 argument given on the command line (i.e., the name is not converted to
641 a canonicalized host name before matching).
642 .It Cm AFSTokenPassing
643 Specifies whether to pass AFS tokens to remote host.
644 The argument to this keyword must be
651 passphrase/password querying will be disabled.
652 This option is useful in scripts and other batch jobs where you have no
653 user to supply the password.
661 If this flag is set to
663 ssh will additionally check the host IP address in the
666 This allows ssh to detect if a host key changed due to DNS spoofing.
667 If the option is set to
669 the check will not be executed.
673 Specifies the cipher to use for encrypting the session
674 in protocol version 1.
683 Specifies the ciphers allowed for protocol version 2
684 in order of preference.
685 Multiple ciphers must be comma-separated.
689 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
690 aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
691 rijndael256-cbc,rijndael-cbc@lysator.liu.se''
694 Specifies whether to use compression.
701 .It Cm CompressionLevel
702 Specifies the compression level to use if compression is enabled.
703 The argument must be an integer from 1 (fast) to 9 (slow, best).
704 The default level is 6, which is good for most applications.
705 The meaning of the values is the same as in
707 .It Cm ConnectionAttempts
708 Specifies the number of tries (one per second) to make before falling
709 back to rsh or exiting.
710 The argument must be an integer.
711 This may be useful in scripts if the connection sometimes fails.
713 .It Cm PubkeyAuthentication
714 Specifies whether to try public key authentication.
715 The argument to this keyword must be
721 Note that this option applies to protocol version 2 only.
723 Sets the escape character (default:
725 The escape character can also
726 be set on the command line.
727 The argument should be a single character,
729 followed by a letter, or
731 to disable the escape
732 character entirely (making the connection transparent for binary
735 Specifies that if connecting via
737 fails due to a connection refused error (there is no
739 listening on the remote host),
741 should automatically be used instead (after a suitable warning about
742 the session being unencrypted).
750 Specifies whether the connection to the authentication agent (if any)
751 will be forwarded to the remote machine.
759 Specifies whether X11 connections will be automatically redirected
760 over the secure channel and
770 Specifies whether remote hosts are allowed to connect to local
778 .It Cm GlobalKnownHostsFile
779 Specifies a file to use for the protocol version 1 global
780 host key database instead of
781 .Pa /etc/ssh_known_hosts .
782 .It Cm GlobalKnownHostsFile2
783 Specifies a file to use for the protocol version 2 global
784 host key database instead of
785 .Pa /etc/ssh_known_hosts2 .
787 Specifies an alias that should be used instead of the
788 real host name when looking up or saving the host key
789 in the known_hosts files.
790 This option is useful for tunneling ssh connections
791 or if you have multiple servers running on a single host.
792 .It Cm HostKeyAlgorithms
793 Specfies the protocol version 2 host key algorithms
794 that the client wants to use in order of preference.
795 The default for this option is:
798 Specifies the real host name to log into.
799 This can be used to specify nicknames or abbreviations for hosts.
800 Default is the name given on the command line.
801 Numeric IP addresses are also permitted (both on the command line and in
805 Specifies the file from which the user's RSA authentication identity
807 .Pa $HOME/.ssh/identity
808 in the user's home directory).
809 Additionally, any identities represented by the authentication agent
810 will be used for authentication.
811 The file name may use the tilde
812 syntax to refer to a user's home directory.
813 It is possible to have
814 multiple identity files specified in configuration files; all these
815 identities will be tried in sequence.
817 Specifies whether the system should send keepalive messages to the
819 If they are sent, death of the connection or crash of one
820 of the machines will be properly noticed.
821 However, this means that
822 connections will die if the route is down temporarily, and some people
827 (to send keepalives), and the client will notice
828 if the network goes down or the remote host dies.
829 This is important in scripts, and many users want it too.
831 To disable keepalives, the value should be set to
833 in both the server and the client configuration files.
834 .It Cm KerberosAuthentication
835 Specifies whether Kerberos authentication will be used.
836 The argument to this keyword must be
840 .It Cm KerberosTgtPassing
841 Specifies whether a Kerberos TGT will be forwarded to the server.
842 This will only work if the Kerberos server is actually an AFS kaserver.
843 The argument to this keyword must be
848 Specifies that a TCP/IP port on the local machine be forwarded over
849 the secure channel to given host:port from the remote machine.
850 The first argument must be a port number, and the second must be
852 Multiple forwardings may be specified, and additional
853 forwardings can be given on the command line.
854 Only the superuser can forward privileged ports.
856 Gives the verbosity level that is used when logging messages from
858 The possible values are:
859 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
862 Specifies the MAC (message authentication code) algorithms
863 in order of preference.
864 The MAC algorithm is used in protocol version 2
865 for data integrity protection.
866 Multiple algorithms must be comma-separated.
870 ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
871 hmac-sha1-96,hmac-md5-96''
873 .It Cm NumberOfPasswordPrompts
874 Specifies the number of password prompts before giving up.
875 The argument to this keyword must be an integer.
877 .It Cm PasswordAuthentication
878 Specifies whether to use password authentication.
879 The argument to this keyword must be
885 Note that this option applies to both protocol version 1 and 2.
887 Specifies the port number to connect on the remote host.
889 .It Cm PreferredAuthentications
890 Specifies the order in which the client should try protocol 2
891 authentication methods. This allows a client to prefer one method (e.g.
892 .Cm keyboard-interactive )
893 over another method (e.g.
895 The default for this option is:
896 .Dq publickey, password, keyboard-interactive
898 Specifies the protocol versions
900 should support in order of preference.
901 The possible values are
905 Multiple versions must be comma-separated.
910 tries version 2 and falls back to version 1
911 if version 2 is not available.
913 Specifies the command to use to connect to the server.
915 string extends to the end of the line, and is executed with
917 In the command string,
919 will be substituted by the host name to
923 The command can be basically anything,
924 and should read from its standard input and write to its standard output.
925 It should eventually connect an
927 server running on some machine, or execute
930 Host key management will be done using the
931 HostName of the host being connected (defaulting to the name typed by
935 is not available for connects with a proxy command.
938 Specifies that a TCP/IP port on the remote machine be forwarded over
939 the secure channel to given host:port from the local machine.
940 The first argument must be a port number, and the second must be
942 Multiple forwardings may be specified, and additional
943 forwardings can be given on the command line.
944 Only the superuser can forward privileged ports.
945 .It Cm RhostsAuthentication
946 Specifies whether to try rhosts based authentication.
948 declaration only affects the client side and has no effect whatsoever
950 Disabling rhosts authentication may reduce
951 authentication time on slow connections when rhosts authentication is
953 Most servers do not permit RhostsAuthentication because it
954 is not secure (see RhostsRSAAuthentication).
955 The argument to this keyword must be
961 .It Cm RhostsRSAAuthentication
962 Specifies whether to try rhosts based authentication with RSA host
970 .It Cm RSAAuthentication
971 Specifies whether to try RSA authentication.
972 The argument to this keyword must be
976 RSA authentication will only be
977 attempted if the identity file exists, or an authentication agent is
981 Note that this option applies to protocol version 1 only.
982 .It Cm ChallengeResponseAuthentication
983 Specifies whether to use challenge response authentication.
984 Currently there is only support for
987 The argument to this keyword must be
993 .It Cm StrictHostKeyChecking
994 If this flag is set to
997 will never automatically add host keys to the
998 .Pa $HOME/.ssh/known_hosts
1000 .Pa $HOME/.ssh/known_hosts2
1001 files, and refuses to connect to hosts whose host key has changed.
1002 This provides maximum protection against trojan horse attacks.
1003 However, it can be somewhat annoying if you don't have good
1004 .Pa /etc/ssh_known_hosts
1006 .Pa /etc/ssh_known_hosts2
1007 files installed and frequently
1008 connect to new hosts.
1009 This option forces the user to manually
1011 If this flag is set to
1014 will automatically add new host keys to the
1015 user known hosts files.
1016 If this flag is set to
1019 will be added to the user known host files only after the user
1020 has confirmed that is what they really want to do, and
1022 will refuse to connect to hosts whose host key has changed.
1024 known hosts will be verified automatically in all cases.
1025 The argument must be
1032 .It Cm UsePrivilegedPort
1033 Specifies whether to use a privileged port for outgoing connections.
1034 The argument must be
1040 Note that setting this option to
1043 .Cm RhostsAuthentication
1045 .Cm RhostsRSAAuthentication
1048 Specifies the user to log in as.
1049 This can be useful if you have a different user name on different machines.
1050 This saves the trouble of
1051 having to remember to give the user name on the command line.
1052 .It Cm UserKnownHostsFile
1053 Specifies a file to use for the protocol version 1 user
1054 host key database instead of
1055 .Pa $HOME/.ssh/known_hosts .
1056 .It Cm UserKnownHostsFile2
1057 Specifies a file to use for the protocol version 2 user
1058 host key database instead of
1059 .Pa $HOME/.ssh/known_hosts2 .
1061 Specifies that rlogin/rsh should be used for this host.
1062 It is possible that the host does not at all support the
1067 to immediately execute
1069 All other options (except
1071 are ignored if this has been specified.
1072 The argument must be
1076 .It Cm XAuthLocation
1077 Specifies the location of the
1081 .Pa /usr/X11R6/bin/xauth .
1085 will normally set the following environment variables:
1090 variable indicates the location of the X11 server.
1091 It is automatically set by
1093 to point to a value of the form
1095 where hostname indicates
1096 the host where the shell runs, and n is an integer >= 1.
1098 uses this special value to forward X11 connections over the secure
1100 The user should normally not set DISPLAY explicitly, as that
1101 will render the X11 connection insecure (and will require the user to
1102 manually copy any required authorization cookies).
1104 Set to the path of the user's home directory.
1108 set for compatibility with systems that use this variable.
1110 Set to point the user's mailbox.
1114 as specified when compiling
1116 .It Ev SSH_AUTH_SOCK
1117 indicates the path of a unix-domain socket used to communicate with the
1120 Identifies the client end of the connection.
1121 The variable contains
1122 three space-separated values: client ip-address, client port number,
1123 and server port number.
1124 .It Ev SSH_ORIGINAL_COMMAND
1125 The variable contains the original command line if a forced command
1127 It can be used to extract the original arguments.
1129 This is set to the name of the tty (path to the device) associated
1130 with the current shell or command.
1131 If the current session has no tty,
1132 this variable is not set.
1134 The timezone variable is set to indicate the present timezone if it
1135 was set when the daemon was started (i.e., the daemon passes the value
1136 on to new connections).
1138 Set to the name of the user logging in.
1144 .Pa $HOME/.ssh/environment ,
1145 and adds lines of the format
1150 .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1151 Records host keys for all hosts the user has logged into (that are not
1153 .Pa /etc/ssh_known_hosts
1154 for protocol version 1 or
1155 .Pa /etc/ssh_known_hosts2
1156 for protocol version 2).
1159 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1160 Contains the authentication identity of the user.
1161 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1163 contain sensitive data and should be readable by the user but not
1164 accessible by others (read/write/execute).
1167 ignores a private key file if it is accessible by others.
1168 It is possible to specify a passphrase when
1169 generating the key; the passphrase will be used to encrypt the
1170 sensitive part of this file using 3DES.
1171 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1172 Contains the public key for authentication (public part of the
1173 identity file in human-readable form).
1175 .Pa $HOME/.ssh/identity.pub
1176 file should be added to
1177 .Pa $HOME/.ssh/authorized_keys
1179 where you wish to log in using protocol version 1 RSA authentication.
1181 .Pa $HOME/.ssh/id_dsa.pub
1183 .Pa $HOME/.ssh/id_rsa.pub
1184 file should be added to
1185 .Pa $HOME/.ssh/authorized_keys2
1187 where you wish to log in using protocol version 2 DSA/RSA authentication.
1189 sensitive and can (but need not) be readable by anyone.
1191 never used automatically and are not necessary; they are only provided for
1192 the convenience of the user.
1193 .It Pa $HOME/.ssh/config
1194 This is the per-user configuration file.
1195 The format of this file is described above.
1196 This file is used by the
1199 This file does not usually contain any sensitive information,
1200 but the recommended permissions are read/write for the user, and not
1201 accessible by others.
1202 .It Pa $HOME/.ssh/authorized_keys
1203 Lists the RSA keys that can be used for logging in as this user.
1204 The format of this file is described in the
1207 In the simplest form the format is the same as the .pub
1208 identity files (that is, each line contains the number of bits in
1209 modulus, public exponent, modulus, and comment fields, separated by
1211 This file is not highly sensitive, but the recommended
1212 permissions are read/write for the user, and not accessible by others.
1213 .It Pa $HOME/.ssh/authorized_keys2
1214 Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1215 This file is not highly sensitive, but the recommended
1216 permissions are read/write for the user, and not accessible by others.
1217 .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1218 Systemwide list of known host keys.
1219 .Pa /etc/ssh_known_hosts
1221 .Pa /etc/ssh_known_hosts2
1222 contains DSA or RSA keys for protocol version 2.
1223 These files should be prepared by the
1224 system administrator to contain the public host keys of all machines in the
1226 This file should be world-readable.
1228 public keys, one per line, in the following format (fields separated
1229 by spaces): system name, number of bits in modulus, public exponent,
1230 modulus, and optional comment field.
1231 When different names are used
1232 for the same machine, all such names should be listed, separated by
1234 The format is described on the
1238 The canonical system name (as returned by name servers) is used by
1240 to verify the client host when logging in; other names are needed because
1242 does not convert the user-supplied name to a canonical name before
1243 checking the key, because someone with access to the name servers
1244 would then be able to fool host authentication.
1245 .It Pa /etc/ssh_config
1246 Systemwide configuration file.
1247 This file provides defaults for those
1248 values that are not specified in the user's configuration file, and
1249 for those users who do not have a configuration file.
1250 This file must be world-readable.
1251 .It Pa $HOME/.rhosts
1252 This file is used in
1254 authentication to list the
1255 host/user pairs that are permitted to log in.
1256 (Note that this file is
1257 also used by rlogin and rsh, which makes using this file insecure.)
1258 Each line of the file contains a host name (in the canonical form
1259 returned by name servers), and then a user name on that host,
1260 separated by a space.
1261 On some machines this file may need to be
1262 world-readable if the user's home directory is on a NFS partition,
1266 Additionally, this file must be owned by the user,
1267 and must not have write permissions for anyone else.
1269 permission for most machines is read/write for the user, and not
1270 accessible by others.
1272 Note that by default
1274 will be installed so that it requires successful RSA host
1275 authentication before permitting \s+2.\s0rhosts authentication.
1276 If your server machine does not have the client's host key in
1277 .Pa /etc/ssh_known_hosts ,
1279 .Pa $HOME/.ssh/known_hosts .
1280 The easiest way to do this is to
1281 connect back to the client from the server machine using ssh; this
1282 will automatically add the host key to
1283 .Pa $HOME/.ssh/known_hosts .
1284 .It Pa $HOME/.shosts
1285 This file is used exactly the same way as
1288 having this file is to be able to use rhosts authentication with
1290 without permitting login with
1294 .It Pa /etc/hosts.equiv
1295 This file is used during
1296 .Pa \&.rhosts authentication.
1298 canonical hosts names, one per line (the full format is described on
1302 If the client host is found in this file, login is
1303 automatically permitted provided client and server user names are the
1305 Additionally, successful RSA host authentication is normally
1307 This file should only be writable by root.
1308 .It Pa /etc/shosts.equiv
1309 This file is processed exactly as
1310 .Pa /etc/hosts.equiv .
1311 This file may be useful to permit logins using
1313 but not using rsh/rlogin.
1315 Commands in this file are executed by
1317 when the user logs in just before the user's shell (or command) is started.
1320 manual page for more information.
1321 .It Pa $HOME/.ssh/rc
1322 Commands in this file are executed by
1324 when the user logs in just before the user's shell (or command) is
1328 manual page for more information.
1329 .It Pa $HOME/.ssh/environment
1330 Contains additional definitions for environment variables, see section
1335 OpenSSH is a derivative of the original and free
1336 ssh 1.2.12 release by Tatu Ylonen.
1337 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1338 Theo de Raadt and Dug Song
1339 removed many bugs, re-added newer features and
1341 Markus Friedl contributed the support for SSH
1342 protocol versions 1.5 and 2.0.
1359 .%T "SSH Protocol Architecture"
1360 .%N draft-ietf-secsh-architecture-07.txt
1362 .%O work in progress material
andersk / openssh.git / blob