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 1.132 2001/08/28 15:39:48 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl l Ar login_name
47 .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
60 .Op Fl F Ar configfile
77 .Ar hostname | user@hostname
81 (SSH client) is a program for logging into a remote machine and for
82 executing commands on a remote machine.
83 It is intended to replace
84 rlogin and rsh, and provide secure encrypted communications between
85 two untrusted hosts over an insecure network.
87 arbitrary TCP/IP ports can also be forwarded over the secure channel.
90 connects and logs into the specified
93 his/her identity to the remote machine using one of several methods
94 depending on the protocol version used:
96 .Ss SSH protocol version 1
98 First, if the machine the user logs in from is listed in
101 .Pa /etc/shosts.equiv
102 on the remote machine, and the user names are
103 the same on both sides, the user is immediately permitted to log in.
108 exists in the user's home directory on the
109 remote machine and contains a line containing the name of the client
110 machine and the name of the user on that machine, the user is
112 This form of authentication alone is normally not
113 allowed by the server because it is not secure.
115 The second authentication method is the
119 method combined with RSA-based host authentication.
120 It means that if the login would be permitted by
123 .Pa /etc/hosts.equiv ,
125 .Pa /etc/shosts.equiv ,
126 and if additionally the server can verify the client's
128 .Pa /etc/ssh_known_hosts
130 .Pa $HOME/.ssh/known_hosts
133 section), only then login is permitted.
134 This authentication method closes security holes due to IP
135 spoofing, DNS spoofing and routing spoofing.
136 [Note to the administrator:
137 .Pa /etc/hosts.equiv ,
139 and the rlogin/rsh protocol in general, are inherently insecure and should be
140 disabled if security is desired.]
142 As a third authentication method,
144 supports RSA based authentication.
145 The scheme is based on public-key cryptography: there are cryptosystems
146 where encryption and decryption are done using separate keys, and it
147 is not possible to derive the decryption key from the encryption key.
148 RSA is one such system.
149 The idea is that each user creates a public/private
150 key pair for authentication purposes.
151 The server knows the public key, and only the user knows the private key.
153 .Pa $HOME/.ssh/authorized_keys
154 lists the public keys that are permitted for logging
156 When the user logs in, the
158 program tells the server which key pair it would like to use for
160 The server checks if this key is permitted, and if
161 so, sends the user (actually the
163 program running on behalf of the user) a challenge, a random number,
164 encrypted by the user's public key.
165 The challenge can only be
166 decrypted using the proper private key.
167 The user's client then decrypts the
168 challenge using the private key, proving that he/she knows the private
169 key but without disclosing it to the server.
172 implements the RSA authentication protocol automatically.
173 The user creates his/her RSA key pair by running
175 This stores the private key in
176 .Pa $HOME/.ssh/identity
177 and the public key in
178 .Pa $HOME/.ssh/identity.pub
179 in the user's home directory.
180 The user should then copy the
183 .Pa $HOME/.ssh/authorized_keys
184 in his/her home directory on the remote machine (the
186 file corresponds to the conventional
188 file, and has one key
189 per line, though the lines can be very long).
190 After this, the user can log in without giving the password.
191 RSA authentication is much
192 more secure than rhosts authentication.
194 The most convenient way to use RSA authentication may be with an
195 authentication agent.
198 for more information.
200 If other authentication methods fail,
202 prompts the user for a password.
203 The password is sent to the remote
204 host for checking; however, since all communications are encrypted,
205 the password cannot be seen by someone listening on the network.
207 .Ss SSH protocol version 2
209 When a user connects using the protocol version 2
210 different authentication methods are available.
211 Using the default values for
212 .Cm PreferredAuthentications ,
213 the client will try to authenticate first using the hostbased method;
214 if this method fails public key authentication is attempted,
215 and finally if this method fails keyboard-interactive and
216 password authentication are tried.
218 The public key method is similar to RSA authentication described
219 in the previous section and allows the RSA or DSA algorithm to be used:
220 The client uses his private key,
221 .Pa $HOME/.ssh/id_dsa
223 .Pa $HOME/.ssh/id_rsa ,
224 to sign the session identifier and sends the result to the server.
225 The server checks whether the matching public key is listed in
226 .Pa $HOME/.ssh/authorized_keys
227 and grants access if both the key is found and the signature is correct.
228 The session identifier is derived from a shared Diffie-Hellman value
229 and is only known to the client and the server.
231 If public key authentication fails or is not available a password
232 can be sent encrypted to the remote host for proving the user's identity.
236 supports hostbased or challenge response authentication.
238 Protocol 2 provides additional mechanisms for confidentiality
239 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
240 and integrity (hmac-md5, hmac-sha1).
241 Note that protocol 1 lacks a strong mechanism for ensuring the
242 integrity of the connection.
244 .Ss Login session and remote execution
246 When the user's identity has been accepted by the server, the server
247 either executes the given command, or logs into the machine and gives
248 the user a normal shell on the remote machine.
249 All communication with
250 the remote command or shell will be automatically encrypted.
252 If a pseudo-terminal has been allocated (normal login session), the
253 user may use the escape characters noted below.
255 If no pseudo tty has been allocated, the
256 session is transparent and can be used to reliably transfer binary
258 On most systems, setting the escape character to
260 will also make the session transparent even if a tty is used.
262 The session terminates when the command or shell on the remote
263 machine exits and all X11 and TCP/IP connections have been closed.
264 The exit status of the remote program is returned as the exit status
268 .Ss Escape Characters
270 When a pseudo terminal has been requested, ssh supports a number of functions
271 through the use of an escape character.
273 A single tilde character can be sent as
275 or by following the tilde by a character other than those described below.
276 The escape character must always follow a newline to be interpreted as
278 The escape character can be changed in configuration files using the
280 configuration directive or on the command line by the
284 The supported escapes (assuming the default
293 List forwarded connections
295 Background ssh at logout when waiting for forwarded connection / X11 sessions
296 to terminate (protocol version 1 only)
298 Display a list of escape characters
300 Request rekeying of the connection (only useful for SSH protocol version 2
301 and if the peer supports it)
304 .Ss X11 and TCP forwarding
310 (or, see the description of the
314 options described later)
315 and the user is using X11 (the
317 environment variable is set), the connection to the X11 display is
318 automatically forwarded to the remote side in such a way that any X11
319 programs started from the shell (or command) will go through the
320 encrypted channel, and the connection to the real X server will be made
321 from the local machine.
322 The user should not manually set
324 Forwarding of X11 connections can be
325 configured on the command line or in configuration files.
331 will point to the server machine, but with a display number greater
333 This is normal, and happens because
337 X server on the server machine for forwarding the
338 connections over the encrypted channel.
341 will also automatically set up Xauthority data on the server machine.
342 For this purpose, it will generate a random authorization cookie,
343 store it in Xauthority on the server, and verify that any forwarded
344 connections carry this cookie and replace it by the real cookie when
345 the connection is opened.
346 The real authentication cookie is never
347 sent to the server machine (and no cookies are sent in the plain).
349 If the user is using an authentication agent, the connection to the agent
350 is automatically forwarded to the remote side unless disabled on
351 the command line or in a configuration file.
353 Forwarding of arbitrary TCP/IP connections over the secure channel can
354 be specified either on the command line or in a configuration file.
355 One possible application of TCP/IP forwarding is a secure connection to an
356 electronic purse; another is going through firewalls.
358 .Ss Server authentication
361 automatically maintains and checks a database containing
362 identifications for all hosts it has ever been used with.
363 Host keys are stored in
364 .Pa $HOME/.ssh/known_hosts
365 in the user's home directory.
366 Additionally, the file
367 .Pa /etc/ssh_known_hosts
368 is automatically checked for known hosts.
369 Any new hosts are automatically added to the user's file.
370 If a host's identification
373 warns about this and disables password authentication to prevent a
374 trojan horse from getting the user's password.
376 this mechanism is to prevent man-in-the-middle attacks which could
377 otherwise be used to circumvent the encryption.
379 .Cm StrictHostKeyChecking
380 option (see below) can be used to prevent logins to machines whose
381 host key is not known or has changed.
383 The options are as follows:
386 Disables forwarding of the authentication agent connection.
388 Enables forwarding of the authentication agent connection.
389 This can also be specified on a per-host basis in a configuration file.
390 .It Fl b Ar bind_address
391 Specify the interface to transmit from on machines with multiple
392 interfaces or aliased addresses.
393 .It Fl c Ar blowfish|3des|des
394 Selects the cipher to use for encrypting the session.
397 It is believed to be secure.
399 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
401 is a fast block cipher, it appears very secure and is much faster than
404 is only supported in the
406 client for interoperability with legacy protocol 1 implementations
407 that do not support the
409 cipher. Its use is strongly discouraged due to cryptographic
411 .It Fl c Ar cipher_spec
412 Additionally, for protocol version 2 a comma-separated list of ciphers can
413 be specified in order of preference.
416 for more information.
417 .It Fl e Ar ch|^ch|none
418 Sets the escape character for sessions with a pty (default:
420 The escape character is only recognized at the beginning of a line.
421 The escape character followed by a dot
423 closes the connection, followed
424 by control-Z suspends the connection, and followed by itself sends the
425 escape character once.
426 Setting the character to
428 disables any escapes and makes the session fully transparent.
432 to go to background just before command execution.
435 is going to ask for passwords or passphrases, but the user
436 wants it in the background.
439 The recommended way to start X11 programs at a remote site is with
441 .Ic ssh -f host xterm .
443 Allows remote hosts to connect to local forwarded ports.
444 .It Fl i Ar identity_file
445 Selects the file from which the identity (private key) for
446 RSA or DSA authentication is read.
448 .Pa $HOME/.ssh/identity
449 in the user's home directory.
450 Identity files may also be specified on
451 a per-host basis in the configuration file.
452 It is possible to have multiple
454 options (and multiple identities specified in
455 configuration files).
456 .It Fl I Ar smartcard_device
457 Specifies which smartcard device to use. The argument is
460 should use to communicate with a smartcard used for storing the user's
463 Disables forwarding of Kerberos tickets and AFS tokens.
464 This may also be specified on a per-host basis in the configuration file.
465 .It Fl l Ar login_name
466 Specifies the user to log in as on the remote machine.
467 This also may be specified on a per-host basis in the configuration file.
469 Additionally, for protocol version 2 a comma-separated list of MAC
470 (message authentication code) algorithms can
471 be specified in order of preference.
474 keyword for more information.
478 (actually, prevents reading from stdin).
479 This must be used when
481 is run in the background.
482 A common trick is to use this to run X11 programs on a remote machine.
484 .Ic ssh -n shadows.cs.hut.fi emacs &
485 will start an emacs on shadows.cs.hut.fi, and the X11
486 connection will be automatically forwarded over an encrypted channel.
489 program will be put in the background.
490 (This does not work if
492 needs to ask for a password or passphrase; see also the
496 Do not execute a remote command.
497 This is useful if you just want to forward ports
498 (protocol version 2 only).
500 Can be used to give options in the format used in the configuration file.
501 This is useful for specifying options for which there is no separate
504 Port to connect to on the remote host.
505 This can be specified on a
506 per-host basis in the configuration file.
508 Use a non-privileged port for outgoing connections.
509 This can be used if your firewall does
510 not permit connections from privileged ports.
511 Note that this option turns off
512 .Cm RhostsAuthentication
514 .Cm RhostsRSAAuthentication
518 Causes all warning and diagnostic messages to be suppressed.
519 Only fatal errors are displayed.
521 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
522 of SSH as a secure transport for other applications (eg. sftp). The
523 subsystem is specified as the remote command.
525 Force pseudo-tty allocation.
526 This can be used to execute arbitrary
527 screen-based programs on a remote machine, which can be very useful,
528 e.g., when implementing menu services.
531 options force tty allocation, even if
535 Disable pseudo-tty allocation.
540 to print debugging messages about its progress.
542 debugging connection, authentication, and configuration problems.
545 options increases the verbosity.
548 Disables X11 forwarding.
550 Enables X11 forwarding.
551 This can also be specified on a per-host basis in a configuration file.
553 Requests compression of all data (including stdin, stdout, stderr, and
554 data for forwarded X11 and TCP/IP connections).
555 The compression algorithm is the same used by
559 can be controlled by the
562 Compression is desirable on modem lines and other
563 slow connections, but will only slow down things on fast networks.
564 The default value can be set on a host-by-host basis in the
565 configuration files; see the
568 .It Fl F Ar configfile
569 Specifies an alternative per-user configuration file.
570 If a configuration file is given on the command line,
571 the system-wide configuration file
572 .Pq Pa /etc/ssh_config
574 The default for the per-user configuration file is
575 .Pa $HOME/.ssh/config .
576 .It Fl L Ar port:host:hostport
577 Specifies that the given port on the local (client) host is to be
578 forwarded to the given host and port on the remote side.
579 This works by allocating a socket to listen to
581 on the local side, and whenever a connection is made to this port, the
582 connection is forwarded over the secure channel, and a connection is
587 from the remote machine.
588 Port forwardings can also be specified in the configuration file.
589 Only root can forward privileged ports.
590 IPv6 addresses can be specified with an alternative syntax:
591 .Ar port/host/hostport
592 .It Fl R Ar port:host:hostport
593 Specifies that the given port on the remote (server) host is to be
594 forwarded to the given host and port on the local side.
595 This works by allocating a socket to listen to
597 on the remote side, and whenever a connection is made to this port, the
598 connection is forwarded over the secure channel, and a connection is
603 from the local machine.
604 Port forwardings can also be specified in the configuration file.
605 Privileged ports can be forwarded only when
606 logging in as root on the remote machine.
607 IPv6 addresses can be specified with an alternative syntax:
608 .Ar port/host/hostport
612 to try protocol version 1 only.
616 to try protocol version 2 only.
620 to use IPv4 addresses only.
624 to use IPv6 addresses only.
626 .Sh CONFIGURATION FILES
628 obtains configuration data from the following sources in
630 command line options, user's configuration file
631 .Pq Pa $HOME/.ssh/config ,
632 and system-wide configuration file
633 .Pq Pa /etc/ssh_config .
634 For each parameter, the first obtained value
636 The configuration files contain sections bracketed by
638 specifications, and that section is only applied for hosts that
639 match one of the patterns given in the specification.
640 The matched host name is the one given on the command line.
642 Since the first obtained value for each parameter is used, more
643 host-specific declarations should be given near the beginning of the
644 file, and general defaults at the end.
646 The configuration file has the following format:
648 Empty lines and lines starting with
652 Otherwise a line is of the format
653 .Dq keyword arguments .
654 Configuration options may be separated by whitespace or
655 optional whitespace and exactly one
657 the latter format is useful to avoid the need to quote whitespace
658 when specifying configuration options using the
667 keywords and their meanings are as follows (note that
668 keywords are case-insensitive and arguments are case-sensitive):
671 Restricts the following declarations (up to the next
673 keyword) to be only for those hosts that match one of the patterns
674 given after the keyword.
678 can be used as wildcards in the
682 as a pattern can be used to provide global
683 defaults for all hosts.
686 argument given on the command line (i.e., the name is not converted to
687 a canonicalized host name before matching).
688 .It Cm AFSTokenPassing
689 Specifies whether to pass AFS tokens to remote host.
690 The argument to this keyword must be
694 This option applies to protocol version 1 only.
698 passphrase/password querying will be disabled.
699 This option is useful in scripts and other batch jobs where you have no
700 user to supply the password.
708 Specify the interface to transmit from on machines with multiple
709 interfaces or aliased addresses.
710 Note that this option does not work if
711 .Cm UsePrivilegedPort
715 If this flag is set to
717 ssh will additionally check the host IP address in the
720 This allows ssh to detect if a host key changed due to DNS spoofing.
721 If the option is set to
723 the check will not be executed.
727 Specifies the cipher to use for encrypting the session
728 in protocol version 1.
736 is only supported in the
738 client for interoperability with legacy protocol 1 implementations
739 that do not support the
741 cipher. Its use is strongly discouraged due to cryptographic
746 Specifies the ciphers allowed for protocol version 2
747 in order of preference.
748 Multiple ciphers must be comma-separated.
752 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
753 aes192-cbc,aes256-cbc''
756 Specifies whether to use compression.
763 .It Cm CompressionLevel
764 Specifies the compression level to use if compression is enabled.
765 The argument must be an integer from 1 (fast) to 9 (slow, best).
766 The default level is 6, which is good for most applications.
767 The meaning of the values is the same as in
769 Note that this option applies to protocol version 1 only.
770 .It Cm ConnectionAttempts
771 Specifies the number of tries (one per second) to make before falling
772 back to rsh or exiting.
773 The argument must be an integer.
774 This may be useful in scripts if the connection sometimes fails.
777 Sets the escape character (default:
779 The escape character can also
780 be set on the command line.
781 The argument should be a single character,
783 followed by a letter, or
785 to disable the escape
786 character entirely (making the connection transparent for binary
789 Specifies that if connecting via
791 fails due to a connection refused error (there is no
793 listening on the remote host),
795 should automatically be used instead (after a suitable warning about
796 the session being unencrypted).
804 Specifies whether the connection to the authentication agent (if any)
805 will be forwarded to the remote machine.
813 Specifies whether X11 connections will be automatically redirected
814 over the secure channel and
824 Specifies whether remote hosts are allowed to connect to local
832 .It Cm GlobalKnownHostsFile
833 Specifies a file to use for the global
834 host key database instead of
835 .Pa /etc/ssh_known_hosts .
836 .It Cm HostbasedAuthentication
837 Specifies whether to try rhosts based authentication with public key
845 This option applies to protocol version 2 only and
847 .Cm RhostsRSAAuthentication .
848 .It Cm HostKeyAlgorithms
849 Specifies the protocol version 2 host key algorithms
850 that the client wants to use in order of preference.
851 The default for this option is:
854 Specifies an alias that should be used instead of the
855 real host name when looking up or saving the host key
856 in the host key database files.
857 This option is useful for tunneling ssh connections
858 or if you have multiple servers running on a single host.
860 Specifies the real host name to log into.
861 This can be used to specify nicknames or abbreviations for hosts.
862 Default is the name given on the command line.
863 Numeric IP addresses are also permitted (both on the command line and in
867 Specifies the file from which the user's RSA or DSA authentication identity
869 .Pa $HOME/.ssh/identity
870 in the user's home directory).
871 Additionally, any identities represented by the authentication agent
872 will be used for authentication.
873 The file name may use the tilde
874 syntax to refer to a user's home directory.
875 It is possible to have
876 multiple identity files specified in configuration files; all these
877 identities will be tried in sequence.
879 Specifies whether the system should send keepalive messages to the
881 If they are sent, death of the connection or crash of one
882 of the machines will be properly noticed.
883 However, this means that
884 connections will die if the route is down temporarily, and some people
889 (to send keepalives), and the client will notice
890 if the network goes down or the remote host dies.
891 This is important in scripts, and many users want it too.
893 To disable keepalives, the value should be set to
895 in both the server and the client configuration files.
896 .It Cm KerberosAuthentication
897 Specifies whether Kerberos authentication will be used.
898 The argument to this keyword must be
902 .It Cm KerberosTgtPassing
903 Specifies whether a Kerberos TGT will be forwarded to the server.
904 This will only work if the Kerberos server is actually an AFS kaserver.
905 The argument to this keyword must be
910 Specifies that a TCP/IP port on the local machine be forwarded over
911 the secure channel to given host:port from the remote machine.
912 The first argument must be a port number, and the second must be
914 Multiple forwardings may be specified, and additional
915 forwardings can be given on the command line.
916 Only the superuser can forward privileged ports.
918 Gives the verbosity level that is used when logging messages from
920 The possible values are:
921 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
924 Specifies the MAC (message authentication code) algorithms
925 in order of preference.
926 The MAC algorithm is used in protocol version 2
927 for data integrity protection.
928 Multiple algorithms must be comma-separated.
930 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
931 .It Cm NumberOfPasswordPrompts
932 Specifies the number of password prompts before giving up.
933 The argument to this keyword must be an integer.
935 .It Cm PasswordAuthentication
936 Specifies whether to use password authentication.
937 The argument to this keyword must be
944 Specifies the port number to connect on the remote host.
946 .It Cm PreferredAuthentications
947 Specifies the order in which the client should try protocol 2
948 authentication methods. This allows a client to prefer one method (e.g.
949 .Cm keyboard-interactive )
950 over another method (e.g.
952 The default for this option is:
953 .Dq hostbased,publickey,keyboard-interactive,password
955 Specifies the protocol versions
957 should support in order of preference.
958 The possible values are
962 Multiple versions must be comma-separated.
967 tries version 2 and falls back to version 1
968 if version 2 is not available.
970 Specifies the command to use to connect to the server.
972 string extends to the end of the line, and is executed with
974 In the command string,
976 will be substituted by the host name to
980 The command can be basically anything,
981 and should read from its standard input and write to its standard output.
982 It should eventually connect an
984 server running on some machine, or execute
987 Host key management will be done using the
988 HostName of the host being connected (defaulting to the name typed by
992 is not available for connects with a proxy command.
994 .It Cm PubkeyAuthentication
995 Specifies whether to try public key authentication.
996 The argument to this keyword must be
1002 This option applies to protocol version 2 only.
1003 .It Cm RemoteForward
1004 Specifies that a TCP/IP port on the remote machine be forwarded over
1005 the secure channel to given host:port from the local machine.
1006 The first argument must be a port number, and the second must be
1008 Multiple forwardings may be specified, and additional
1009 forwardings can be given on the command line.
1010 Only the superuser can forward privileged ports.
1011 .It Cm RhostsAuthentication
1012 Specifies whether to try rhosts based authentication.
1014 declaration only affects the client side and has no effect whatsoever
1016 Disabling rhosts authentication may reduce
1017 authentication time on slow connections when rhosts authentication is
1019 Most servers do not permit RhostsAuthentication because it
1021 .Cm RhostsRSAAuthentication ) .
1022 The argument to this keyword must be
1028 This option applies to protocol version 1 only.
1029 .It Cm RhostsRSAAuthentication
1030 Specifies whether to try rhosts based authentication with RSA host
1032 The argument must be
1038 This option applies to protocol version 1 only.
1039 .It Cm RSAAuthentication
1040 Specifies whether to try RSA authentication.
1041 The argument to this keyword must be
1045 RSA authentication will only be
1046 attempted if the identity file exists, or an authentication agent is
1050 Note that this option applies to protocol version 1 only.
1051 .It Cm ChallengeResponseAuthentication
1052 Specifies whether to use challenge response authentication.
1053 The argument to this keyword must be
1059 .It Cm SmartcardDevice
1060 Specifies which smartcard device to use. The argument to this keyword is
1063 should use to communicate with a smartcard used for storing the user's
1064 private RSA key. By default, no device is specified and smartcard support
1066 .It Cm StrictHostKeyChecking
1067 If this flag is set to
1070 will never automatically add host keys to the
1071 .Pa $HOME/.ssh/known_hosts
1072 file, and refuses to connect to hosts whose host key has changed.
1073 This provides maximum protection against trojan horse attacks.
1074 However, it can be somewhat annoying if you don't have good
1075 .Pa /etc/ssh_known_hosts
1076 files installed and frequently
1077 connect to new hosts.
1078 This option forces the user to manually
1080 If this flag is set to
1083 will automatically add new host keys to the
1084 user known hosts files.
1085 If this flag is set to
1088 will be added to the user known host files only after the user
1089 has confirmed that is what they really want to do, and
1091 will refuse to connect to hosts whose host key has changed.
1093 known hosts will be verified automatically in all cases.
1094 The argument must be
1101 .It Cm UsePrivilegedPort
1102 Specifies whether to use a privileged port for outgoing connections.
1103 The argument must be
1109 Note that you need to set this option to
1112 .Cm RhostsAuthentication
1114 .Cm RhostsRSAAuthentication
1117 Specifies the user to log in as.
1118 This can be useful if you have a different user name on different machines.
1119 This saves the trouble of
1120 having to remember to give the user name on the command line.
1121 .It Cm UserKnownHostsFile
1122 Specifies a file to use for the user
1123 host key database instead of
1124 .Pa $HOME/.ssh/known_hosts .
1126 Specifies that rlogin/rsh should be used for this host.
1127 It is possible that the host does not at all support the
1132 to immediately execute
1134 All other options (except
1136 are ignored if this has been specified.
1137 The argument must be
1141 .It Cm XAuthLocation
1142 Specifies the location of the
1146 .Pa /usr/X11R6/bin/xauth .
1150 will normally set the following environment variables:
1155 variable indicates the location of the X11 server.
1156 It is automatically set by
1158 to point to a value of the form
1160 where hostname indicates
1161 the host where the shell runs, and n is an integer >= 1.
1163 uses this special value to forward X11 connections over the secure
1165 The user should normally not set
1168 will render the X11 connection insecure (and will require the user to
1169 manually copy any required authorization cookies).
1171 Set to the path of the user's home directory.
1175 set for compatibility with systems that use this variable.
1177 Set to the path of the user's mailbox.
1181 as specified when compiling
1186 needs a passphrase, it will read the passphrase from the current
1187 terminal if it was run from a terminal.
1190 does not have a terminal associated with it but
1194 are set, it will execute the program specified by
1196 and open an X11 window to read the passphrase.
1197 This is particularly useful when calling
1202 (Note that on some machines it
1203 may be necessary to redirect the input from
1206 .It Ev SSH_AUTH_SOCK
1207 Identifies the path of a unix-domain socket used to communicate with the
1210 Identifies the client end of the connection.
1211 The variable contains
1212 three space-separated values: client ip-address, client port number,
1213 and server port number.
1214 .It Ev SSH_ORIGINAL_COMMAND
1215 The variable contains the original command line if a forced command
1217 It can be used to extract the original arguments.
1219 This is set to the name of the tty (path to the device) associated
1220 with the current shell or command.
1221 If the current session has no tty,
1222 this variable is not set.
1224 The timezone variable is set to indicate the present timezone if it
1225 was set when the daemon was started (i.e., the daemon passes the value
1226 on to new connections).
1228 Set to the name of the user logging in.
1234 .Pa $HOME/.ssh/environment ,
1235 and adds lines of the format
1240 .It Pa $HOME/.ssh/known_hosts
1241 Records host keys for all hosts the user has logged into that are not
1243 .Pa /etc/ssh_known_hosts .
1246 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1247 Contains the authentication identity of the user.
1248 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1250 contain sensitive data and should be readable by the user but not
1251 accessible by others (read/write/execute).
1254 ignores a private key file if it is accessible by others.
1255 It is possible to specify a passphrase when
1256 generating the key; the passphrase will be used to encrypt the
1257 sensitive part of this file using 3DES.
1258 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1259 Contains the public key for authentication (public part of the
1260 identity file in human-readable form).
1262 .Pa $HOME/.ssh/identity.pub
1263 file should be added to
1264 .Pa $HOME/.ssh/authorized_keys
1266 where you wish to log in using protocol version 1 RSA authentication.
1268 .Pa $HOME/.ssh/id_dsa.pub
1270 .Pa $HOME/.ssh/id_rsa.pub
1271 file should be added to
1272 .Pa $HOME/.ssh/authorized_keys
1274 where you wish to log in using protocol version 2 DSA/RSA authentication.
1276 sensitive and can (but need not) be readable by anyone.
1278 never used automatically and are not necessary; they are only provided for
1279 the convenience of the user.
1280 .It Pa $HOME/.ssh/config
1281 This is the per-user configuration file.
1282 The format of this file is described above.
1283 This file is used by the
1286 This file does not usually contain any sensitive information,
1287 but the recommended permissions are read/write for the user, and not
1288 accessible by others.
1289 .It Pa $HOME/.ssh/authorized_keys
1290 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1291 The format of this file is described in the
1294 In the simplest form the format is the same as the .pub
1296 This file is not highly sensitive, but the recommended
1297 permissions are read/write for the user, and not accessible by others.
1298 .It Pa /etc/ssh_known_hosts
1299 Systemwide list of known host keys.
1300 This file should be prepared by the
1301 system administrator to contain the public host keys of all machines in the
1303 This file should be world-readable.
1305 public keys, one per line, in the following format (fields separated
1306 by spaces): system name, public key and optional comment field.
1307 When different names are used
1308 for the same machine, all such names should be listed, separated by
1310 The format is described on the
1314 The canonical system name (as returned by name servers) is used by
1316 to verify the client host when logging in; other names are needed because
1318 does not convert the user-supplied name to a canonical name before
1319 checking the key, because someone with access to the name servers
1320 would then be able to fool host authentication.
1321 .It Pa /etc/ssh_config
1322 Systemwide configuration file.
1323 This file provides defaults for those
1324 values that are not specified in the user's configuration file, and
1325 for those users who do not have a configuration file.
1326 This file must be world-readable.
1327 .It Pa $HOME/.rhosts
1328 This file is used in
1330 authentication to list the
1331 host/user pairs that are permitted to log in.
1332 (Note that this file is
1333 also used by rlogin and rsh, which makes using this file insecure.)
1334 Each line of the file contains a host name (in the canonical form
1335 returned by name servers), and then a user name on that host,
1336 separated by a space.
1337 On some machines this file may need to be
1338 world-readable if the user's home directory is on a NFS partition,
1342 Additionally, this file must be owned by the user,
1343 and must not have write permissions for anyone else.
1345 permission for most machines is read/write for the user, and not
1346 accessible by others.
1348 Note that by default
1350 will be installed so that it requires successful RSA host
1351 authentication before permitting \s+2.\s0rhosts authentication.
1352 If your server machine does not have the client's host key in
1353 .Pa /etc/ssh_known_hosts ,
1355 .Pa $HOME/.ssh/known_hosts .
1356 The easiest way to do this is to
1357 connect back to the client from the server machine using ssh; this
1358 will automatically add the host key to
1359 .Pa $HOME/.ssh/known_hosts .
1360 .It Pa $HOME/.shosts
1361 This file is used exactly the same way as
1364 having this file is to be able to use rhosts authentication with
1366 without permitting login with
1370 .It Pa /etc/hosts.equiv
1371 This file is used during
1372 .Pa \&.rhosts authentication.
1374 canonical hosts names, one per line (the full format is described on
1378 If the client host is found in this file, login is
1379 automatically permitted provided client and server user names are the
1381 Additionally, successful RSA host authentication is normally
1383 This file should only be writable by root.
1384 .It Pa /etc/shosts.equiv
1385 This file is processed exactly as
1386 .Pa /etc/hosts.equiv .
1387 This file may be useful to permit logins using
1389 but not using rsh/rlogin.
1391 Commands in this file are executed by
1393 when the user logs in just before the user's shell (or command) is started.
1396 manual page for more information.
1397 .It Pa $HOME/.ssh/rc
1398 Commands in this file are executed by
1400 when the user logs in just before the user's shell (or command) is
1404 manual page for more information.
1405 .It Pa $HOME/.ssh/environment
1406 Contains additional definitions for environment variables, see section
1411 OpenSSH is a derivative of the original and free
1412 ssh 1.2.12 release by Tatu Ylonen.
1413 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1414 Theo de Raadt and Dug Song
1415 removed many bugs, re-added newer features and
1417 Markus Friedl contributed the support for SSH
1418 protocol versions 1.5 and 2.0.
1435 .%T "SSH Protocol Architecture"
1436 .%N draft-ietf-secsh-architecture-09.txt
1438 .%O work in progress material
andersk / openssh.git / blob