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 1.131 2001/08/22 17:45:16 stevesk 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
76 .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 hostbased method;
213 if this method fails public key authentication is attempted,
214 and finally if this method fails keyboard-interactive and
215 password authentication are tried.
217 The public key method is similar to RSA authentication described
218 in the previous section and allows the RSA or DSA algorithm to be used:
219 The client uses his private key,
220 .Pa $HOME/.ssh/id_dsa
222 .Pa $HOME/.ssh/id_rsa ,
223 to sign the session identifier and sends the result to the server.
224 The server checks whether the matching public key is listed in
225 .Pa $HOME/.ssh/authorized_keys
226 and grants access if both the key is found and the signature is correct.
227 The session identifier is derived from a shared Diffie-Hellman value
228 and is only known to the client and the server.
230 If public key authentication fails or is not available a password
231 can be sent encrypted to the remote host for proving the user's identity.
235 supports hostbased or challenge response authentication.
237 Protocol 2 provides additional mechanisms for confidentiality
238 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
239 and integrity (hmac-md5, hmac-sha1).
240 Note that protocol 1 lacks a strong mechanism for ensuring the
241 integrity of the connection.
243 .Ss Login session and remote execution
245 When the user's identity has been accepted by the server, the server
246 either executes the given command, or logs into the machine and gives
247 the user a normal shell on the remote machine.
248 All communication with
249 the remote command or shell will be automatically encrypted.
251 If a pseudo-terminal has been allocated (normal login session), the
252 user may use the escape characters noted below.
254 If no pseudo tty has been allocated, the
255 session is transparent and can be used to reliably transfer binary
257 On most systems, setting the escape character to
259 will also make the session transparent even if a tty is used.
261 The session terminates when the command or shell on the remote
262 machine exits and all X11 and TCP/IP connections have been closed.
263 The exit status of the remote program is returned as the exit status
267 .Ss Escape Characters
269 When a pseudo terminal has been requested, ssh supports a number of functions
270 through the use of an escape character.
272 A single tilde character can be sent as
274 or by following the tilde by a character other than those described below.
275 The escape character must always follow a newline to be interpreted as
277 The escape character can be changed in configuration files using the
279 configuration directive or on the command line by the
283 The supported escapes (assuming the default
292 List forwarded connections
294 Background ssh at logout when waiting for forwarded connection / X11 sessions
295 to terminate (protocol version 1 only)
297 Display a list of escape characters
299 Request rekeying of the connection (only useful for SSH protocol version 2
300 and if the peer supports it)
303 .Ss X11 and TCP forwarding
309 (or, see the description of the
313 options described later)
314 and the user is using X11 (the
316 environment variable is set), the connection to the X11 display is
317 automatically forwarded to the remote side in such a way that any X11
318 programs started from the shell (or command) will go through the
319 encrypted channel, and the connection to the real X server will be made
320 from the local machine.
321 The user should not manually set
323 Forwarding of X11 connections can be
324 configured on the command line or in configuration files.
330 will point to the server machine, but with a display number greater
332 This is normal, and happens because
336 X server on the server machine for forwarding the
337 connections over the encrypted channel.
340 will also automatically set up Xauthority data on the server machine.
341 For this purpose, it will generate a random authorization cookie,
342 store it in Xauthority on the server, and verify that any forwarded
343 connections carry this cookie and replace it by the real cookie when
344 the connection is opened.
345 The real authentication cookie is never
346 sent to the server machine (and no cookies are sent in the plain).
348 If the user is using an authentication agent, the connection to the agent
349 is automatically forwarded to the remote side unless disabled on
350 the command line or in a configuration file.
352 Forwarding of arbitrary TCP/IP connections over the secure channel can
353 be specified either on the command line or in a configuration file.
354 One possible application of TCP/IP forwarding is a secure connection to an
355 electronic purse; another is going through firewalls.
357 .Ss Server authentication
360 automatically maintains and checks a database containing
361 identifications for all hosts it has ever been used with.
362 Host keys are stored in
363 .Pa $HOME/.ssh/known_hosts
364 in the user's home directory.
365 Additionally, the file
366 .Pa /etc/ssh_known_hosts
367 is automatically checked for known hosts.
368 Any new hosts are automatically added to the user's file.
369 If a host's identification
372 warns about this and disables password authentication to prevent a
373 trojan horse from getting the user's password.
375 this mechanism is to prevent man-in-the-middle attacks which could
376 otherwise be used to circumvent the encryption.
378 .Cm StrictHostKeyChecking
379 option (see below) can be used to prevent logins to machines whose
380 host key is not known or has changed.
382 The options are as follows:
385 Disables forwarding of the authentication agent connection.
387 Enables forwarding of the authentication agent connection.
388 This can also be specified on a per-host basis in a configuration file.
389 .It Fl b Ar bind_address
390 Specify the interface to transmit from on machines with multiple
391 interfaces or aliased addresses.
392 .It Fl c Ar blowfish|3des|des
393 Selects the cipher to use for encrypting the session.
396 It is believed to be secure.
398 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
400 is a fast block cipher, it appears very secure and is much faster than
403 is only supported in the
405 client for interoperability with legacy protocol 1 implementations
406 that do not support the
408 cipher. Its use is strongly discouraged due to cryptographic
410 .It Fl c Ar cipher_spec
411 Additionally, for protocol version 2 a comma-separated list of ciphers can
412 be specified in order of preference.
415 for more information.
416 .It Fl e Ar ch|^ch|none
417 Sets the escape character for sessions with a pty (default:
419 The escape character is only recognized at the beginning of a line.
420 The escape character followed by a dot
422 closes the connection, followed
423 by control-Z suspends the connection, and followed by itself sends the
424 escape character once.
425 Setting the character to
427 disables any escapes and makes the session fully transparent.
431 to go to background just before command execution.
434 is going to ask for passwords or passphrases, but the user
435 wants it in the background.
438 The recommended way to start X11 programs at a remote site is with
440 .Ic ssh -f host xterm .
442 Allows remote hosts to connect to local forwarded ports.
443 .It Fl i Ar identity_file
444 Selects the file from which the identity (private key) for
445 RSA or DSA authentication is read.
447 .Pa $HOME/.ssh/identity
448 in the user's home directory.
449 Identity files may also be specified on
450 a per-host basis in the configuration file.
451 It is possible to have multiple
453 options (and multiple identities specified in
454 configuration files).
455 .It Fl I Ar smartcard_device
456 Specifies which smartcard device to use. The argument is
459 should use to communicate with a smartcard used for storing the user's
462 Disables forwarding of Kerberos tickets and AFS tokens.
463 This may also be specified on a per-host basis in the configuration file.
464 .It Fl l Ar login_name
465 Specifies the user to log in as on the remote machine.
466 This also may be specified on a per-host basis in the configuration file.
468 Additionally, for protocol version 2 a comma-separated list of MAC
469 (message authentication code) algorithms can
470 be specified in order of preference.
473 keyword for more information.
477 (actually, prevents reading from stdin).
478 This must be used when
480 is run in the background.
481 A common trick is to use this to run X11 programs on a remote machine.
483 .Ic ssh -n shadows.cs.hut.fi emacs &
484 will start an emacs on shadows.cs.hut.fi, and the X11
485 connection will be automatically forwarded over an encrypted channel.
488 program will be put in the background.
489 (This does not work if
491 needs to ask for a password or passphrase; see also the
495 Do not execute a remote command.
496 This is useful if you just want to forward ports
497 (protocol version 2 only).
499 Can be used to give options in the format used in the configuration file.
500 This is useful for specifying options for which there is no separate
503 Port to connect to on the remote host.
504 This can be specified on a
505 per-host basis in the configuration file.
507 Use a non-privileged port for outgoing connections.
508 This can be used if your firewall does
509 not permit connections from privileged ports.
510 Note that this option turns off
511 .Cm RhostsAuthentication
513 .Cm RhostsRSAAuthentication
517 Causes all warning and diagnostic messages to be suppressed.
518 Only fatal errors are displayed.
520 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
521 of SSH as a secure transport for other applications (eg. sftp). The
522 subsystem is specified as the remote command.
524 Force pseudo-tty allocation.
525 This can be used to execute arbitrary
526 screen-based programs on a remote machine, which can be very useful,
527 e.g., when implementing menu services.
530 options force tty allocation, even if
534 Disable pseudo-tty allocation.
539 to print debugging messages about its progress.
541 debugging connection, authentication, and configuration problems.
544 options increases the verbosity.
547 Disables X11 forwarding.
549 Enables X11 forwarding.
550 This can also be specified on a per-host basis in a configuration file.
552 Requests compression of all data (including stdin, stdout, stderr, and
553 data for forwarded X11 and TCP/IP connections).
554 The compression algorithm is the same used by
558 can be controlled by the
561 Compression is desirable on modem lines and other
562 slow connections, but will only slow down things on fast networks.
563 The default value can be set on a host-by-host basis in the
564 configuration files; see the
567 .It Fl L Ar port:host:hostport
568 Specifies that the given port on the local (client) host is to be
569 forwarded to the given host and port on the remote side.
570 This works by allocating a socket to listen to
572 on the local side, and whenever a connection is made to this port, the
573 connection is forwarded over the secure channel, and a connection is
578 from the remote machine.
579 Port forwardings can also be specified in the configuration file.
580 Only root can forward privileged ports.
581 IPv6 addresses can be specified with an alternative syntax:
582 .Ar port/host/hostport
583 .It Fl R Ar port:host:hostport
584 Specifies that the given port on the remote (server) host is to be
585 forwarded to the given host and port on the local side.
586 This works by allocating a socket to listen to
588 on the remote side, and whenever a connection is made to this port, the
589 connection is forwarded over the secure channel, and a connection is
594 from the local machine.
595 Port forwardings can also be specified in the configuration file.
596 Privileged ports can be forwarded only when
597 logging in as root on the remote machine.
598 IPv6 addresses can be specified with an alternative syntax:
599 .Ar port/host/hostport
603 to try protocol version 1 only.
607 to try protocol version 2 only.
611 to use IPv4 addresses only.
615 to use IPv6 addresses only.
617 .Sh CONFIGURATION FILES
619 obtains configuration data from the following sources in
621 command line options, user's configuration file
622 .Pq Pa $HOME/.ssh/config ,
623 and system-wide configuration file
624 .Pq Pa /etc/ssh_config .
625 For each parameter, the first obtained value
627 The configuration files contain sections bracketed by
629 specifications, and that section is only applied for hosts that
630 match one of the patterns given in the specification.
631 The matched host name is the one given on the command line.
633 Since the first obtained value for each parameter is used, more
634 host-specific declarations should be given near the beginning of the
635 file, and general defaults at the end.
637 The configuration file has the following format:
639 Empty lines and lines starting with
643 Otherwise a line is of the format
644 .Dq keyword arguments .
645 Configuration options may be separated by whitespace or
646 optional whitespace and exactly one
648 the latter format is useful to avoid the need to quote whitespace
649 when specifying configuration options using the
658 keywords and their meanings are as follows (note that
659 keywords are case-insensitive and arguments are case-sensitive):
662 Restricts the following declarations (up to the next
664 keyword) to be only for those hosts that match one of the patterns
665 given after the keyword.
669 can be used as wildcards in the
673 as a pattern can be used to provide global
674 defaults for all hosts.
677 argument given on the command line (i.e., the name is not converted to
678 a canonicalized host name before matching).
679 .It Cm AFSTokenPassing
680 Specifies whether to pass AFS tokens to remote host.
681 The argument to this keyword must be
685 This option applies to protocol version 1 only.
689 passphrase/password querying will be disabled.
690 This option is useful in scripts and other batch jobs where you have no
691 user to supply the password.
699 Specify the interface to transmit from on machines with multiple
700 interfaces or aliased addresses.
701 Note that this option does not work if
702 .Cm UsePrivilegedPort
706 If this flag is set to
708 ssh will additionally check the host IP address in the
711 This allows ssh to detect if a host key changed due to DNS spoofing.
712 If the option is set to
714 the check will not be executed.
718 Specifies the cipher to use for encrypting the session
719 in protocol version 1.
727 is only supported in the
729 client for interoperability with legacy protocol 1 implementations
730 that do not support the
732 cipher. Its use is strongly discouraged due to cryptographic
737 Specifies the ciphers allowed for protocol version 2
738 in order of preference.
739 Multiple ciphers must be comma-separated.
743 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
744 aes192-cbc,aes256-cbc''
747 Specifies whether to use compression.
754 .It Cm CompressionLevel
755 Specifies the compression level to use if compression is enabled.
756 The argument must be an integer from 1 (fast) to 9 (slow, best).
757 The default level is 6, which is good for most applications.
758 The meaning of the values is the same as in
760 Note that this option applies to protocol version 1 only.
761 .It Cm ConnectionAttempts
762 Specifies the number of tries (one per second) to make before falling
763 back to rsh or exiting.
764 The argument must be an integer.
765 This may be useful in scripts if the connection sometimes fails.
768 Sets the escape character (default:
770 The escape character can also
771 be set on the command line.
772 The argument should be a single character,
774 followed by a letter, or
776 to disable the escape
777 character entirely (making the connection transparent for binary
780 Specifies that if connecting via
782 fails due to a connection refused error (there is no
784 listening on the remote host),
786 should automatically be used instead (after a suitable warning about
787 the session being unencrypted).
795 Specifies whether the connection to the authentication agent (if any)
796 will be forwarded to the remote machine.
804 Specifies whether X11 connections will be automatically redirected
805 over the secure channel and
815 Specifies whether remote hosts are allowed to connect to local
823 .It Cm GlobalKnownHostsFile
824 Specifies a file to use for the global
825 host key database instead of
826 .Pa /etc/ssh_known_hosts .
827 .It Cm HostbasedAuthentication
828 Specifies whether to try rhosts based authentication with public key
836 This option applies to protocol version 2 only and
838 .Cm RhostsRSAAuthentication .
839 .It Cm HostKeyAlgorithms
840 Specifies the protocol version 2 host key algorithms
841 that the client wants to use in order of preference.
842 The default for this option is:
845 Specifies an alias that should be used instead of the
846 real host name when looking up or saving the host key
847 in the host key database files.
848 This option is useful for tunneling ssh connections
849 or if you have multiple servers running on a single host.
851 Specifies the real host name to log into.
852 This can be used to specify nicknames or abbreviations for hosts.
853 Default is the name given on the command line.
854 Numeric IP addresses are also permitted (both on the command line and in
858 Specifies the file from which the user's RSA or DSA authentication identity
860 .Pa $HOME/.ssh/identity
861 in the user's home directory).
862 Additionally, any identities represented by the authentication agent
863 will be used for authentication.
864 The file name may use the tilde
865 syntax to refer to a user's home directory.
866 It is possible to have
867 multiple identity files specified in configuration files; all these
868 identities will be tried in sequence.
870 Specifies whether the system should send keepalive messages to the
872 If they are sent, death of the connection or crash of one
873 of the machines will be properly noticed.
874 However, this means that
875 connections will die if the route is down temporarily, and some people
880 (to send keepalives), and the client will notice
881 if the network goes down or the remote host dies.
882 This is important in scripts, and many users want it too.
884 To disable keepalives, the value should be set to
886 in both the server and the client configuration files.
887 .It Cm KerberosAuthentication
888 Specifies whether Kerberos authentication will be used.
889 The argument to this keyword must be
893 .It Cm KerberosTgtPassing
894 Specifies whether a Kerberos TGT will be forwarded to the server.
895 This will only work if the Kerberos server is actually an AFS kaserver.
896 The argument to this keyword must be
901 Specifies that a TCP/IP port on the local machine be forwarded over
902 the secure channel to given host:port from the remote machine.
903 The first argument must be a port number, and the second must be
905 Multiple forwardings may be specified, and additional
906 forwardings can be given on the command line.
907 Only the superuser can forward privileged ports.
909 Gives the verbosity level that is used when logging messages from
911 The possible values are:
912 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
915 Specifies the MAC (message authentication code) algorithms
916 in order of preference.
917 The MAC algorithm is used in protocol version 2
918 for data integrity protection.
919 Multiple algorithms must be comma-separated.
921 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
922 .It Cm NumberOfPasswordPrompts
923 Specifies the number of password prompts before giving up.
924 The argument to this keyword must be an integer.
926 .It Cm PasswordAuthentication
927 Specifies whether to use password authentication.
928 The argument to this keyword must be
935 Specifies the port number to connect on the remote host.
937 .It Cm PreferredAuthentications
938 Specifies the order in which the client should try protocol 2
939 authentication methods. This allows a client to prefer one method (e.g.
940 .Cm keyboard-interactive )
941 over another method (e.g.
943 The default for this option is:
944 .Dq hostbased,publickey,keyboard-interactive,password
946 Specifies the protocol versions
948 should support in order of preference.
949 The possible values are
953 Multiple versions must be comma-separated.
958 tries version 2 and falls back to version 1
959 if version 2 is not available.
961 Specifies the command to use to connect to the server.
963 string extends to the end of the line, and is executed with
965 In the command string,
967 will be substituted by the host name to
971 The command can be basically anything,
972 and should read from its standard input and write to its standard output.
973 It should eventually connect an
975 server running on some machine, or execute
978 Host key management will be done using the
979 HostName of the host being connected (defaulting to the name typed by
983 is not available for connects with a proxy command.
985 .It Cm PubkeyAuthentication
986 Specifies whether to try public key authentication.
987 The argument to this keyword must be
993 This option applies to protocol version 2 only.
995 Specifies that a TCP/IP port on the remote machine be forwarded over
996 the secure channel to given host:port from the local machine.
997 The first argument must be a port number, and the second must be
999 Multiple forwardings may be specified, and additional
1000 forwardings can be given on the command line.
1001 Only the superuser can forward privileged ports.
1002 .It Cm RhostsAuthentication
1003 Specifies whether to try rhosts based authentication.
1005 declaration only affects the client side and has no effect whatsoever
1007 Disabling rhosts authentication may reduce
1008 authentication time on slow connections when rhosts authentication is
1010 Most servers do not permit RhostsAuthentication because it
1012 .Cm RhostsRSAAuthentication ) .
1013 The argument to this keyword must be
1019 This option applies to protocol version 1 only.
1020 .It Cm RhostsRSAAuthentication
1021 Specifies whether to try rhosts based authentication with RSA host
1023 The argument must be
1029 This option applies to protocol version 1 only.
1030 .It Cm RSAAuthentication
1031 Specifies whether to try RSA authentication.
1032 The argument to this keyword must be
1036 RSA authentication will only be
1037 attempted if the identity file exists, or an authentication agent is
1041 Note that this option applies to protocol version 1 only.
1042 .It Cm ChallengeResponseAuthentication
1043 Specifies whether to use challenge response authentication.
1044 The argument to this keyword must be
1050 .It Cm SmartcardDevice
1051 Specifies which smartcard device to use. The argument to this keyword is
1054 should use to communicate with a smartcard used for storing the user's
1055 private RSA key. By default, no device is specified and smartcard support
1057 .It Cm StrictHostKeyChecking
1058 If this flag is set to
1061 will never automatically add host keys to the
1062 .Pa $HOME/.ssh/known_hosts
1063 file, and refuses to connect to hosts whose host key has changed.
1064 This provides maximum protection against trojan horse attacks.
1065 However, it can be somewhat annoying if you don't have good
1066 .Pa /etc/ssh_known_hosts
1067 files installed and frequently
1068 connect to new hosts.
1069 This option forces the user to manually
1071 If this flag is set to
1074 will automatically add new host keys to the
1075 user known hosts files.
1076 If this flag is set to
1079 will be added to the user known host files only after the user
1080 has confirmed that is what they really want to do, and
1082 will refuse to connect to hosts whose host key has changed.
1084 known hosts will be verified automatically in all cases.
1085 The argument must be
1092 .It Cm UsePrivilegedPort
1093 Specifies whether to use a privileged port for outgoing connections.
1094 The argument must be
1100 Note that you need to set this option to
1103 .Cm RhostsAuthentication
1105 .Cm RhostsRSAAuthentication
1108 Specifies the user to log in as.
1109 This can be useful if you have a different user name on different machines.
1110 This saves the trouble of
1111 having to remember to give the user name on the command line.
1112 .It Cm UserKnownHostsFile
1113 Specifies a file to use for the user
1114 host key database instead of
1115 .Pa $HOME/.ssh/known_hosts .
1117 Specifies that rlogin/rsh should be used for this host.
1118 It is possible that the host does not at all support the
1123 to immediately execute
1125 All other options (except
1127 are ignored if this has been specified.
1128 The argument must be
1132 .It Cm XAuthLocation
1133 Specifies the location of the
1137 .Pa /usr/X11R6/bin/xauth .
1141 will normally set the following environment variables:
1146 variable indicates the location of the X11 server.
1147 It is automatically set by
1149 to point to a value of the form
1151 where hostname indicates
1152 the host where the shell runs, and n is an integer >= 1.
1154 uses this special value to forward X11 connections over the secure
1156 The user should normally not set
1159 will render the X11 connection insecure (and will require the user to
1160 manually copy any required authorization cookies).
1162 Set to the path of the user's home directory.
1166 set for compatibility with systems that use this variable.
1168 Set to the path of the user's mailbox.
1172 as specified when compiling
1177 needs a passphrase, it will read the passphrase from the current
1178 terminal if it was run from a terminal.
1181 does not have a terminal associated with it but
1185 are set, it will execute the program specified by
1187 and open an X11 window to read the passphrase.
1188 This is particularly useful when calling
1193 (Note that on some machines it
1194 may be necessary to redirect the input from
1197 .It Ev SSH_AUTH_SOCK
1198 Identifies the path of a unix-domain socket used to communicate with the
1201 Identifies the client end of the connection.
1202 The variable contains
1203 three space-separated values: client ip-address, client port number,
1204 and server port number.
1205 .It Ev SSH_ORIGINAL_COMMAND
1206 The variable contains the original command line if a forced command
1208 It can be used to extract the original arguments.
1210 This is set to the name of the tty (path to the device) associated
1211 with the current shell or command.
1212 If the current session has no tty,
1213 this variable is not set.
1215 The timezone variable is set to indicate the present timezone if it
1216 was set when the daemon was started (i.e., the daemon passes the value
1217 on to new connections).
1219 Set to the name of the user logging in.
1225 .Pa $HOME/.ssh/environment ,
1226 and adds lines of the format
1231 .It Pa $HOME/.ssh/known_hosts
1232 Records host keys for all hosts the user has logged into that are not
1234 .Pa /etc/ssh_known_hosts .
1237 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1238 Contains the authentication identity of the user.
1239 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1241 contain sensitive data and should be readable by the user but not
1242 accessible by others (read/write/execute).
1245 ignores a private key file if it is accessible by others.
1246 It is possible to specify a passphrase when
1247 generating the key; the passphrase will be used to encrypt the
1248 sensitive part of this file using 3DES.
1249 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1250 Contains the public key for authentication (public part of the
1251 identity file in human-readable form).
1253 .Pa $HOME/.ssh/identity.pub
1254 file should be added to
1255 .Pa $HOME/.ssh/authorized_keys
1257 where you wish to log in using protocol version 1 RSA authentication.
1259 .Pa $HOME/.ssh/id_dsa.pub
1261 .Pa $HOME/.ssh/id_rsa.pub
1262 file should be added to
1263 .Pa $HOME/.ssh/authorized_keys
1265 where you wish to log in using protocol version 2 DSA/RSA authentication.
1267 sensitive and can (but need not) be readable by anyone.
1269 never used automatically and are not necessary; they are only provided for
1270 the convenience of the user.
1271 .It Pa $HOME/.ssh/config
1272 This is the per-user configuration file.
1273 The format of this file is described above.
1274 This file is used by the
1277 This file does not usually contain any sensitive information,
1278 but the recommended permissions are read/write for the user, and not
1279 accessible by others.
1280 .It Pa $HOME/.ssh/authorized_keys
1281 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1282 The format of this file is described in the
1285 In the simplest form the format is the same as the .pub
1287 This file is not highly sensitive, but the recommended
1288 permissions are read/write for the user, and not accessible by others.
1289 .It Pa /etc/ssh_known_hosts
1290 Systemwide list of known host keys.
1291 This file should be prepared by the
1292 system administrator to contain the public host keys of all machines in the
1294 This file should be world-readable.
1296 public keys, one per line, in the following format (fields separated
1297 by spaces): system name, public key and optional comment field.
1298 When different names are used
1299 for the same machine, all such names should be listed, separated by
1301 The format is described on the
1305 The canonical system name (as returned by name servers) is used by
1307 to verify the client host when logging in; other names are needed because
1309 does not convert the user-supplied name to a canonical name before
1310 checking the key, because someone with access to the name servers
1311 would then be able to fool host authentication.
1312 .It Pa /etc/ssh_config
1313 Systemwide configuration file.
1314 This file provides defaults for those
1315 values that are not specified in the user's configuration file, and
1316 for those users who do not have a configuration file.
1317 This file must be world-readable.
1318 .It Pa $HOME/.rhosts
1319 This file is used in
1321 authentication to list the
1322 host/user pairs that are permitted to log in.
1323 (Note that this file is
1324 also used by rlogin and rsh, which makes using this file insecure.)
1325 Each line of the file contains a host name (in the canonical form
1326 returned by name servers), and then a user name on that host,
1327 separated by a space.
1328 On some machines this file may need to be
1329 world-readable if the user's home directory is on a NFS partition,
1333 Additionally, this file must be owned by the user,
1334 and must not have write permissions for anyone else.
1336 permission for most machines is read/write for the user, and not
1337 accessible by others.
1339 Note that by default
1341 will be installed so that it requires successful RSA host
1342 authentication before permitting \s+2.\s0rhosts authentication.
1343 If your server machine does not have the client's host key in
1344 .Pa /etc/ssh_known_hosts ,
1346 .Pa $HOME/.ssh/known_hosts .
1347 The easiest way to do this is to
1348 connect back to the client from the server machine using ssh; this
1349 will automatically add the host key to
1350 .Pa $HOME/.ssh/known_hosts .
1351 .It Pa $HOME/.shosts
1352 This file is used exactly the same way as
1355 having this file is to be able to use rhosts authentication with
1357 without permitting login with
1361 .It Pa /etc/hosts.equiv
1362 This file is used during
1363 .Pa \&.rhosts authentication.
1365 canonical hosts names, one per line (the full format is described on
1369 If the client host is found in this file, login is
1370 automatically permitted provided client and server user names are the
1372 Additionally, successful RSA host authentication is normally
1374 This file should only be writable by root.
1375 .It Pa /etc/shosts.equiv
1376 This file is processed exactly as
1377 .Pa /etc/hosts.equiv .
1378 This file may be useful to permit logins using
1380 but not using rsh/rlogin.
1382 Commands in this file are executed by
1384 when the user logs in just before the user's shell (or command) is started.
1387 manual page for more information.
1388 .It Pa $HOME/.ssh/rc
1389 Commands in this file are executed by
1391 when the user logs in just before the user's shell (or command) is
1395 manual page for more information.
1396 .It Pa $HOME/.ssh/environment
1397 Contains additional definitions for environment variables, see section
1402 OpenSSH is a derivative of the original and free
1403 ssh 1.2.12 release by Tatu Ylonen.
1404 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1405 Theo de Raadt and Dug Song
1406 removed many bugs, re-added newer features and
1408 Markus Friedl contributed the support for SSH
1409 protocol versions 1.5 and 2.0.
1426 .%T "SSH Protocol Architecture"
1427 .%N draft-ietf-secsh-architecture-09.txt
1429 .%O work in progress material
andersk / openssh.git / blob