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 1.116 2001/06/23 02:34:31 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl l Ar login_name
47 .Op Ar hostname | user@hostname
51 .Op Fl afgknqstvxACNPTX1246
52 .Op Fl b Ar bind_address
53 .Op Fl c Ar cipher_spec
54 .Op Fl e Ar escape_char
55 .Op Fl i Ar identity_file
56 .Op Fl l Ar login_name
76 .Op Ar hostname | user@hostname
80 (SSH client) is a program for logging into a remote machine and for
81 executing commands on a remote machine.
82 It is intended to replace
83 rlogin and rsh, and provide secure encrypted communications between
84 two untrusted hosts over an insecure network.
86 arbitrary TCP/IP ports can also be forwarded over the secure channel.
89 connects and logs into the specified
92 his/her identity to the remote machine using one of several methods
93 depending on the protocol version used:
95 .Ss SSH protocol version 1
97 First, if the machine the user logs in from is listed in
100 .Pa /etc/shosts.equiv
101 on the remote machine, and the user names are
102 the same on both sides, the user is immediately permitted to log in.
107 exists in the user's home directory on the
108 remote machine and contains a line containing the name of the client
109 machine and the name of the user on that machine, the user is
111 This form of authentication alone is normally not
112 allowed by the server because it is not secure.
114 The second authentication method is the
118 method combined with RSA-based host authentication.
119 It means that if the login would be permitted by
122 .Pa /etc/hosts.equiv ,
124 .Pa /etc/shosts.equiv ,
125 and if additionally the server can verify the client's
127 .Pa /etc/ssh_known_hosts
129 .Pa $HOME/.ssh/known_hosts
132 section), only then login is permitted.
133 This authentication method closes security holes due to IP
134 spoofing, DNS spoofing and routing spoofing.
135 [Note to the administrator:
136 .Pa /etc/hosts.equiv ,
138 and the rlogin/rsh protocol in general, are inherently insecure and should be
139 disabled if security is desired.]
141 As a third authentication method,
143 supports RSA based authentication.
144 The scheme is based on public-key cryptography: there are cryptosystems
145 where encryption and decryption are done using separate keys, and it
146 is not possible to derive the decryption key from the encryption key.
147 RSA is one such system.
148 The idea is that each user creates a public/private
149 key pair for authentication purposes.
150 The server knows the public key, and only the user knows the private key.
152 .Pa $HOME/.ssh/authorized_keys
153 lists the public keys that are permitted for logging
155 When the user logs in, the
157 program tells the server which key pair it would like to use for
159 The server checks if this key is permitted, and if
160 so, sends the user (actually the
162 program running on behalf of the user) a challenge, a random number,
163 encrypted by the user's public key.
164 The challenge can only be
165 decrypted using the proper private key.
166 The user's client then decrypts the
167 challenge using the private key, proving that he/she knows the private
168 key but without disclosing it to the server.
171 implements the RSA authentication protocol automatically.
172 The user creates his/her RSA key pair by running
174 This stores the private key in
175 .Pa $HOME/.ssh/identity
176 and the public key in
177 .Pa $HOME/.ssh/identity.pub
178 in the user's home directory.
179 The user should then copy the
182 .Pa $HOME/.ssh/authorized_keys
183 in his/her home directory on the remote machine (the
185 file corresponds to the conventional
187 file, and has one key
188 per line, though the lines can be very long).
189 After this, the user can log in without giving the password.
190 RSA authentication is much
191 more secure than rhosts authentication.
193 The most convenient way to use RSA authentication may be with an
194 authentication agent.
197 for more information.
199 If other authentication methods fail,
201 prompts the user for a password.
202 The password is sent to the remote
203 host for checking; however, since all communications are encrypted,
204 the password cannot be seen by someone listening on the network.
206 .Ss SSH protocol version 2
208 When a user connects using the protocol version 2
209 different authentication methods are available.
210 Using the default values for
211 .Cm PreferredAuthentications ,
212 the client will try to authenticate first using the public key method;
213 if this method fails password authentication is attempted,
214 and finally if this method fails keyboard-interactive authentication
216 If this method fails password authentication is
219 The public key method is similar to RSA authentication described
220 in the previous section and allows the RSA or DSA algorithm to be used:
221 The client uses his private key,
222 .Pa $HOME/.ssh/id_dsa
224 .Pa $HOME/.ssh/id_rsa ,
225 to sign the session identifier and sends the result to the server.
226 The server checks whether the matching public key is listed in
227 .Pa $HOME/.ssh/authorized_keys
228 and grants access if both the key is found and the signature is correct.
229 The session identifier is derived from a shared Diffie-Hellman value
230 and is only known to the client and the server.
232 If public key authentication fails or is not available a password
233 can be sent encrypted to the remote host for proving the user's identity.
237 supports hostbased or challenge response authentication.
239 Protocol 2 provides additional mechanisms for confidentiality
240 (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
241 and integrity (hmac-md5, hmac-sha1).
242 Note that protocol 1 lacks a strong mechanism for ensuring the
243 integrity of the connection.
245 .Ss Login session and remote execution
247 When the user's identity has been accepted by the server, the server
248 either executes the given command, or logs into the machine and gives
249 the user a normal shell on the remote machine.
250 All communication with
251 the remote command or shell will be automatically encrypted.
253 If a pseudo-terminal has been allocated (normal login session), the
254 user may use the escape characters noted below.
256 If no pseudo tty has been allocated, the
257 session is transparent and can be used to reliably transfer binary
259 On most systems, setting the escape character to
261 will also make the session transparent even if a tty is used.
263 The session terminates when the command or shell on the remote
264 machine exits and all X11 and TCP/IP connections have been closed.
265 The exit status of the remote program is returned as the exit status
269 .Ss Escape Characters
271 When a pseudo terminal has been requested, ssh supports a number of functions
272 through the use of an escape character.
274 A single tilde character can be sent as
276 (or by following the tilde by a character other than those described above).
277 The escape character must always follow a newline to be interpreted as
279 The escape character can be changed in configuration files using the
281 configuration directive or on the command line by the
285 The supported escapes (assuming the default
294 List forwarded connections
296 Background ssh at logout when waiting for forwarded connection / X11 sessions
297 to terminate (protocol version 1 only)
299 Display a list of escape characters
301 Request rekeying of the connection (only useful for SSH protocol version 2
302 and if the peer supports it)
305 .Ss X11 and TCP forwarding
311 (or, see the description of the
315 options described later)
316 and the user is using X11 (the
318 environment variable is set), the connection to the X11 display is
319 automatically forwarded to the remote side in such a way that any X11
320 programs started from the shell (or command) will go through the
321 encrypted channel, and the connection to the real X server will be made
322 from the local machine.
323 The user should not manually set
325 Forwarding of X11 connections can be
326 configured on the command line or in configuration files.
332 will point to the server machine, but with a display number greater
334 This is normal, and happens because
338 X server on the server machine for forwarding the
339 connections over the encrypted channel.
342 will also automatically set up Xauthority data on the server machine.
343 For this purpose, it will generate a random authorization cookie,
344 store it in Xauthority on the server, and verify that any forwarded
345 connections carry this cookie and replace it by the real cookie when
346 the connection is opened.
347 The real authentication cookie is never
348 sent to the server machine (and no cookies are sent in the plain).
350 If the user is using an authentication agent, the connection to the agent
351 is automatically forwarded to the remote side unless disabled on
352 command line or in a configuration file.
354 Forwarding of arbitrary TCP/IP connections over the secure channel can
355 be specified either on command line or in a configuration file.
356 One possible application of TCP/IP forwarding is a secure connection to an
357 electronic purse; another is going through firewalls.
359 .Ss Server authentication
362 automatically maintains and checks a database containing
363 identifications for all hosts it has ever been used with.
364 Host keys are stored in
365 .Pa $HOME/.ssh/known_hosts
366 in the user's home directory.
367 Additionally, the file
368 .Pa /etc/ssh_known_hosts
369 is automatically checked for known hosts.
370 Any new hosts are automatically added to the user's file.
371 If a host's identification
374 warns about this and disables password authentication to prevent a
375 trojan horse from getting the user's password.
377 this mechanism is to prevent man-in-the-middle attacks which could
378 otherwise be used to circumvent the encryption.
380 .Cm StrictHostKeyChecking
381 option (see below) can be used to prevent logins to machines whose
382 host key is not known or has changed.
384 The options are as follows:
387 Disables forwarding of the authentication agent connection.
389 Enables forwarding of the authentication agent connection.
390 This can also be specified on a per-host basis in a configuration file.
391 .It Fl b Ar bind_address
392 Specify the interface to transmit from on machines with multiple
393 interfaces or aliased addresses.
394 .It Fl c Ar blowfish|3des
395 Selects the cipher to use for encrypting the session.
398 It is believed to be secure.
400 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
401 It is presumably more secure than the
403 cipher which is no longer fully supported in
406 is a fast block cipher, it appears very secure and is much faster than
408 .It Fl c Ar cipher_spec
409 Additionally, for protocol version 2 a comma-separated list of ciphers can
410 be specified in order of preference.
413 for more information.
414 .It Fl e Ar ch|^ch|none
415 Sets the escape character for sessions with a pty (default:
417 The escape character is only recognized at the beginning of a line.
418 The escape character followed by a dot
420 closes the connection, followed
421 by control-Z suspends the connection, and followed by itself sends the
422 escape character once.
423 Setting the character to
425 disables any escapes and makes the session fully transparent.
429 to go to background just before command execution.
432 is going to ask for passwords or passphrases, but the user
433 wants it in the background.
436 The recommended way to start X11 programs at a remote site is with
438 .Ic ssh -f host xterm .
440 Allows remote hosts to connect to local forwarded ports.
441 .It Fl i Ar identity_file
442 Selects the file from which the identity (private key) for
443 RSA or DSA authentication is read.
445 .Pa $HOME/.ssh/identity
446 in the user's home directory.
447 Identity files may also be specified on
448 a per-host basis in the configuration file.
449 It is possible to have multiple
451 options (and multiple identities specified in
452 configuration files).
454 Disables forwarding of Kerberos tickets and AFS tokens.
455 This may also be specified on a per-host basis in the configuration file.
456 .It Fl l Ar login_name
457 Specifies the user to log in as on the remote machine.
458 This also may be specified on a per-host basis in the configuration file.
460 Additionally, for protocol version 2 a comma-separated list of MAC
461 (message authentication code) algorithms can
462 be specified in order of preference.
465 keyword for more information.
469 (actually, prevents reading from stdin).
470 This must be used when
472 is run in the background.
473 A common trick is to use this to run X11 programs on a remote machine.
475 .Ic ssh -n shadows.cs.hut.fi emacs &
476 will start an emacs on shadows.cs.hut.fi, and the X11
477 connection will be automatically forwarded over an encrypted channel.
480 program will be put in the background.
481 (This does not work if
483 needs to ask for a password or passphrase; see also the
487 Do not execute a remote command.
488 This is useful if you just want to forward ports
489 (protocol version 2 only).
491 Can be used to give options in the format used in the config file.
492 This is useful for specifying options for which there is no separate
494 The option has the same format as a line in the configuration file.
496 Port to connect to on the remote host.
497 This can be specified on a
498 per-host basis in the configuration file.
500 Use a non-privileged port for outgoing connections.
501 This can be used if your firewall does
502 not permit connections from privileged ports.
503 Note that this option turns off
504 .Cm RhostsAuthentication
506 .Cm RhostsRSAAuthentication
510 Causes all warning and diagnostic messages to be suppressed.
511 Only fatal errors are displayed.
513 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
514 of SSH as a secure transport for other applications (eg. sftp). The
515 subsystem is specified as the remote command.
517 Force pseudo-tty allocation.
518 This can be used to execute arbitrary
519 screen-based programs on a remote machine, which can be very useful,
520 e.g., when implementing menu services.
523 options force tty allocation, even if
527 Disable pseudo-tty allocation.
532 to print debugging messages about its progress.
534 debugging connection, authentication, and configuration problems.
537 options increases the verbosity.
540 Disables X11 forwarding.
542 Enables X11 forwarding.
543 This can also be specified on a per-host basis in a configuration file.
545 Requests compression of all data (including stdin, stdout, stderr, and
546 data for forwarded X11 and TCP/IP connections).
547 The compression algorithm is the same used by
551 can be controlled by the
554 Compression is desirable on modem lines and other
555 slow connections, but will only slow down things on fast networks.
556 The default value can be set on a host-by-host basis in the
557 configuration files; see the
560 .It Fl L Ar port:host:hostport
561 Specifies that the given port on the local (client) host is to be
562 forwarded to the given host and port on the remote side.
563 This works by allocating a socket to listen to
565 on the local side, and whenever a connection is made to this port, the
566 connection is forwarded over the secure channel, and a connection is
571 from the remote machine.
572 Port forwardings can also be specified in the configuration file.
573 Only root can forward privileged ports.
574 IPv6 addresses can be specified with an alternative syntax:
575 .Ar port/host/hostport
576 .It Fl R Ar port:host:hostport
577 Specifies that the given port on the remote (server) host is to be
578 forwarded to the given host and port on the local side.
579 This works by allocating a socket to listen to
581 on the remote 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 local machine.
588 Port forwardings can also be specified in the configuration file.
589 Privileged ports can be forwarded only when
590 logging in as root on the remote machine.
591 IPv6 addresses can be specified with an alternative syntax:
592 .Ar port/host/hostport
596 to try protocol version 1 only.
600 to try protocol version 2 only.
604 to use IPv4 addresses only.
608 to use IPv6 addresses only.
610 .Sh CONFIGURATION FILES
612 obtains configuration data from the following sources (in this order):
613 command line options, user's configuration file
614 .Pq Pa $HOME/.ssh/config ,
615 and system-wide configuration file
616 .Pq Pa /etc/ssh_config .
617 For each parameter, the first obtained value
619 The configuration files contain sections bracketed by
621 specifications, and that section is only applied for hosts that
622 match one of the patterns given in the specification.
623 The matched host name is the one given on the command line.
625 Since the first obtained value for each parameter is used, more
626 host-specific declarations should be given near the beginning of the
627 file, and general defaults at the end.
629 The configuration file has the following format:
631 Empty lines and lines starting with
635 Otherwise a line is of the format
636 .Dq keyword arguments .
638 keywords and their meanings are as follows (note that the
639 configuration files are case-sensitive):
642 Restricts the following declarations (up to the next
644 keyword) to be only for those hosts that match one of the patterns
645 given after the keyword.
649 can be used as wildcards in the
653 as a pattern can be used to provide global
654 defaults for all hosts.
657 argument given on the command line (i.e., the name is not converted to
658 a canonicalized host name before matching).
659 .It Cm AFSTokenPassing
660 Specifies whether to pass AFS tokens to remote host.
661 The argument to this keyword must be
665 This option applies to protocol version 1 only.
669 passphrase/password querying will be disabled.
670 This option is useful in scripts and other batch jobs where you have no
671 user to supply the password.
679 Specify the interface to transmit from on machines with multiple
680 interfaces or aliased addresses.
681 Note that this option does not work if
682 .Cm UsePrivilegedPort
686 If this flag is set to
688 ssh will additionally check the host IP address in the
691 This allows ssh to detect if a host key changed due to DNS spoofing.
692 If the option is set to
694 the check will not be executed.
698 Specifies the cipher to use for encrypting the session
699 in protocol version 1.
708 Specifies the ciphers allowed for protocol version 2
709 in order of preference.
710 Multiple ciphers must be comma-separated.
714 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
715 aes192-cbc,aes256-cbc''
718 Specifies whether to use compression.
725 .It Cm CompressionLevel
726 Specifies the compression level to use if compression is enabled.
727 The argument must be an integer from 1 (fast) to 9 (slow, best).
728 The default level is 6, which is good for most applications.
729 The meaning of the values is the same as in
731 Note that this option applies to protocol version 1 only.
732 .It Cm ConnectionAttempts
733 Specifies the number of tries (one per second) to make before falling
734 back to rsh or exiting.
735 The argument must be an integer.
736 This may be useful in scripts if the connection sometimes fails.
739 Sets the escape character (default:
741 The escape character can also
742 be set on the command line.
743 The argument should be a single character,
745 followed by a letter, or
747 to disable the escape
748 character entirely (making the connection transparent for binary
751 Specifies that if connecting via
753 fails due to a connection refused error (there is no
755 listening on the remote host),
757 should automatically be used instead (after a suitable warning about
758 the session being unencrypted).
766 Specifies whether the connection to the authentication agent (if any)
767 will be forwarded to the remote machine.
775 Specifies whether X11 connections will be automatically redirected
776 over the secure channel and
786 Specifies whether remote hosts are allowed to connect to local
794 .It Cm GlobalKnownHostsFile
795 Specifies a file to use for the global
796 host key database instead of
797 .Pa /etc/ssh_known_hosts .
798 .It Cm HostbasedAuthentication
799 Specifies whether to try rhosts based authentication with public key
807 This option applies to protocol version 2 only and
809 .Cm RhostsRSAAuthentication .
810 .It Cm HostKeyAlgorithms
811 Specifies the protocol version 2 host key algorithms
812 that the client wants to use in order of preference.
813 The default for this option is:
816 Specifies an alias that should be used instead of the
817 real host name when looking up or saving the host key
818 in the host key database files.
819 This option is useful for tunneling ssh connections
820 or if you have multiple servers running on a single host.
822 Specifies the real host name to log into.
823 This can be used to specify nicknames or abbreviations for hosts.
824 Default is the name given on the command line.
825 Numeric IP addresses are also permitted (both on the command line and in
829 Specifies the file from which the user's RSA or DSA authentication identity
831 .Pa $HOME/.ssh/identity
832 in the user's home directory).
833 Additionally, any identities represented by the authentication agent
834 will be used for authentication.
835 The file name may use the tilde
836 syntax to refer to a user's home directory.
837 It is possible to have
838 multiple identity files specified in configuration files; all these
839 identities will be tried in sequence.
841 Specifies whether the system should send keepalive messages to the
843 If they are sent, death of the connection or crash of one
844 of the machines will be properly noticed.
845 However, this means that
846 connections will die if the route is down temporarily, and some people
851 (to send keepalives), and the client will notice
852 if the network goes down or the remote host dies.
853 This is important in scripts, and many users want it too.
855 To disable keepalives, the value should be set to
857 in both the server and the client configuration files.
858 .It Cm KerberosAuthentication
859 Specifies whether Kerberos authentication will be used.
860 The argument to this keyword must be
864 .It Cm KerberosTgtPassing
865 Specifies whether a Kerberos TGT will be forwarded to the server.
866 This will only work if the Kerberos server is actually an AFS kaserver.
867 The argument to this keyword must be
872 Specifies that a TCP/IP port on the local machine be forwarded over
873 the secure channel to given host:port from the remote machine.
874 The first argument must be a port number, and the second must be
876 Multiple forwardings may be specified, and additional
877 forwardings can be given on the command line.
878 Only the superuser can forward privileged ports.
880 Gives the verbosity level that is used when logging messages from
882 The possible values are:
883 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
886 Specifies the MAC (message authentication code) algorithms
887 in order of preference.
888 The MAC algorithm is used in protocol version 2
889 for data integrity protection.
890 Multiple algorithms must be comma-separated.
892 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
893 .It Cm NumberOfPasswordPrompts
894 Specifies the number of password prompts before giving up.
895 The argument to this keyword must be an integer.
897 .It Cm PasswordAuthentication
898 Specifies whether to use password authentication.
899 The argument to this keyword must be
906 Specifies the port number to connect on the remote host.
908 .It Cm PreferredAuthentications
909 Specifies the order in which the client should try protocol 2
910 authentication methods. This allows a client to prefer one method (e.g.
911 .Cm keyboard-interactive )
912 over another method (e.g.
914 The default for this option is:
915 .Dq publickey,hostbased,password,keyboard-interactive
917 Specifies the protocol versions
919 should support in order of preference.
920 The possible values are
924 Multiple versions must be comma-separated.
929 tries version 2 and falls back to version 1
930 if version 2 is not available.
932 Specifies the command to use to connect to the server.
934 string extends to the end of the line, and is executed with
936 In the command string,
938 will be substituted by the host name to
942 The command can be basically anything,
943 and should read from its standard input and write to its standard output.
944 It should eventually connect an
946 server running on some machine, or execute
949 Host key management will be done using the
950 HostName of the host being connected (defaulting to the name typed by
954 is not available for connects with a proxy command.
956 .It Cm PubkeyAuthentication
957 Specifies whether to try public key authentication.
958 The argument to this keyword must be
964 This option applies to protocol version 2 only.
966 Specifies that a TCP/IP port on the remote machine be forwarded over
967 the secure channel to given host:port from the local machine.
968 The first argument must be a port number, and the second must be
970 Multiple forwardings may be specified, and additional
971 forwardings can be given on the command line.
972 Only the superuser can forward privileged ports.
973 .It Cm RhostsAuthentication
974 Specifies whether to try rhosts based authentication.
976 declaration only affects the client side and has no effect whatsoever
978 Disabling rhosts authentication may reduce
979 authentication time on slow connections when rhosts authentication is
981 Most servers do not permit RhostsAuthentication because it
983 .Cm RhostsRSAAuthentication ) .
984 The argument to this keyword must be
990 This option applies to protocol version 1 only.
991 .It Cm RhostsRSAAuthentication
992 Specifies whether to try rhosts based authentication with RSA host
1000 This option applies to protocol version 1 only.
1001 .It Cm RSAAuthentication
1002 Specifies whether to try RSA authentication.
1003 The argument to this keyword must be
1007 RSA authentication will only be
1008 attempted if the identity file exists, or an authentication agent is
1012 Note that this option applies to protocol version 1 only.
1013 .It Cm ChallengeResponseAuthentication
1014 Specifies whether to use challenge response authentication.
1015 Currently there is only support for
1018 The argument to this keyword must be
1024 .It Cm StrictHostKeyChecking
1025 If this flag is set to
1028 will never automatically add host keys to the
1029 .Pa $HOME/.ssh/known_hosts
1030 file, and refuses to connect to hosts whose host key has changed.
1031 This provides maximum protection against trojan horse attacks.
1032 However, it can be somewhat annoying if you don't have good
1033 .Pa /etc/ssh_known_hosts
1034 files installed and frequently
1035 connect to new hosts.
1036 This option forces the user to manually
1038 If this flag is set to
1041 will automatically add new host keys to the
1042 user known hosts files.
1043 If this flag is set to
1046 will be added to the user known host files only after the user
1047 has confirmed that is what they really want to do, and
1049 will refuse to connect to hosts whose host key has changed.
1051 known hosts will be verified automatically in all cases.
1052 The argument must be
1059 .It Cm UsePrivilegedPort
1060 Specifies whether to use a privileged port for outgoing connections.
1061 The argument must be
1067 Note that you need to set this option to
1070 .Cm RhostsAuthentication
1072 .Cm RhostsRSAAuthentication
1075 Specifies the user to log in as.
1076 This can be useful if you have a different user name on different machines.
1077 This saves the trouble of
1078 having to remember to give the user name on the command line.
1079 .It Cm UserKnownHostsFile
1080 Specifies a file to use for the user
1081 host key database instead of
1082 .Pa $HOME/.ssh/known_hosts .
1084 Specifies that rlogin/rsh should be used for this host.
1085 It is possible that the host does not at all support the
1090 to immediately execute
1092 All other options (except
1094 are ignored if this has been specified.
1095 The argument must be
1099 .It Cm XAuthLocation
1100 Specifies the location of the
1104 .Pa /usr/X11R6/bin/xauth .
1108 will normally set the following environment variables:
1113 variable indicates the location of the X11 server.
1114 It is automatically set by
1116 to point to a value of the form
1118 where hostname indicates
1119 the host where the shell runs, and n is an integer >= 1.
1121 uses this special value to forward X11 connections over the secure
1123 The user should normally not set
1126 will render the X11 connection insecure (and will require the user to
1127 manually copy any required authorization cookies).
1129 Set to the path of the user's home directory.
1133 set for compatibility with systems that use this variable.
1135 Set to point the user's mailbox.
1139 as specified when compiling
1141 .It Ev SSH_AUTH_SOCK
1142 indicates the path of a unix-domain socket used to communicate with the
1145 Identifies the client end of the connection.
1146 The variable contains
1147 three space-separated values: client ip-address, client port number,
1148 and server port number.
1149 .It Ev SSH_ORIGINAL_COMMAND
1150 The variable contains the original command line if a forced command
1152 It can be used to extract the original arguments.
1154 This is set to the name of the tty (path to the device) associated
1155 with the current shell or command.
1156 If the current session has no tty,
1157 this variable is not set.
1159 The timezone variable is set to indicate the present timezone if it
1160 was set when the daemon was started (i.e., the daemon passes the value
1161 on to new connections).
1163 Set to the name of the user logging in.
1169 .Pa $HOME/.ssh/environment ,
1170 and adds lines of the format
1175 .It Pa $HOME/.ssh/known_hosts
1176 Records host keys for all hosts the user has logged into (that are not
1178 .Pa /etc/ssh_known_hosts .
1181 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1182 Contains the authentication identity of the user.
1183 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1185 contain sensitive data and should be readable by the user but not
1186 accessible by others (read/write/execute).
1189 ignores a private key file if it is accessible by others.
1190 It is possible to specify a passphrase when
1191 generating the key; the passphrase will be used to encrypt the
1192 sensitive part of this file using 3DES.
1193 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1194 Contains the public key for authentication (public part of the
1195 identity file in human-readable form).
1197 .Pa $HOME/.ssh/identity.pub
1198 file should be added to
1199 .Pa $HOME/.ssh/authorized_keys
1201 where you wish to log in using protocol version 1 RSA authentication.
1203 .Pa $HOME/.ssh/id_dsa.pub
1205 .Pa $HOME/.ssh/id_rsa.pub
1206 file should be added to
1207 .Pa $HOME/.ssh/authorized_keys
1209 where you wish to log in using protocol version 2 DSA/RSA authentication.
1211 sensitive and can (but need not) be readable by anyone.
1213 never used automatically and are not necessary; they are only provided for
1214 the convenience of the user.
1215 .It Pa $HOME/.ssh/config
1216 This is the per-user configuration file.
1217 The format of this file is described above.
1218 This file is used by the
1221 This file does not usually contain any sensitive information,
1222 but the recommended permissions are read/write for the user, and not
1223 accessible by others.
1224 .It Pa $HOME/.ssh/authorized_keys
1225 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1226 The format of this file is described in the
1229 In the simplest form the format is the same as the .pub
1231 This file is not highly sensitive, but the recommended
1232 permissions are read/write for the user, and not accessible by others.
1233 .It Pa /etc/ssh_known_hosts
1234 Systemwide list of known host keys.
1235 This file should be prepared by the
1236 system administrator to contain the public host keys of all machines in the
1238 This file should be world-readable.
1240 public keys, one per line, in the following format (fields separated
1241 by spaces): system name, public key and optional comment field.
1242 When different names are used
1243 for the same machine, all such names should be listed, separated by
1245 The format is described on the
1249 The canonical system name (as returned by name servers) is used by
1251 to verify the client host when logging in; other names are needed because
1253 does not convert the user-supplied name to a canonical name before
1254 checking the key, because someone with access to the name servers
1255 would then be able to fool host authentication.
1256 .It Pa /etc/ssh_config
1257 Systemwide configuration file.
1258 This file provides defaults for those
1259 values that are not specified in the user's configuration file, and
1260 for those users who do not have a configuration file.
1261 This file must be world-readable.
1262 .It Pa $HOME/.rhosts
1263 This file is used in
1265 authentication to list the
1266 host/user pairs that are permitted to log in.
1267 (Note that this file is
1268 also used by rlogin and rsh, which makes using this file insecure.)
1269 Each line of the file contains a host name (in the canonical form
1270 returned by name servers), and then a user name on that host,
1271 separated by a space.
1272 On some machines this file may need to be
1273 world-readable if the user's home directory is on a NFS partition,
1277 Additionally, this file must be owned by the user,
1278 and must not have write permissions for anyone else.
1280 permission for most machines is read/write for the user, and not
1281 accessible by others.
1283 Note that by default
1285 will be installed so that it requires successful RSA host
1286 authentication before permitting \s+2.\s0rhosts authentication.
1287 If your server machine does not have the client's host key in
1288 .Pa /etc/ssh_known_hosts ,
1290 .Pa $HOME/.ssh/known_hosts .
1291 The easiest way to do this is to
1292 connect back to the client from the server machine using ssh; this
1293 will automatically add the host key to
1294 .Pa $HOME/.ssh/known_hosts .
1295 .It Pa $HOME/.shosts
1296 This file is used exactly the same way as
1299 having this file is to be able to use rhosts authentication with
1301 without permitting login with
1305 .It Pa /etc/hosts.equiv
1306 This file is used during
1307 .Pa \&.rhosts authentication.
1309 canonical hosts names, one per line (the full format is described on
1313 If the client host is found in this file, login is
1314 automatically permitted provided client and server user names are the
1316 Additionally, successful RSA host authentication is normally
1318 This file should only be writable by root.
1319 .It Pa /etc/shosts.equiv
1320 This file is processed exactly as
1321 .Pa /etc/hosts.equiv .
1322 This file may be useful to permit logins using
1324 but not using rsh/rlogin.
1326 Commands in this file are executed by
1328 when the user logs in just before the user's shell (or command) is started.
1331 manual page for more information.
1332 .It Pa $HOME/.ssh/rc
1333 Commands in this file are executed by
1335 when the user logs in just before the user's shell (or command) is
1339 manual page for more information.
1340 .It Pa $HOME/.ssh/environment
1341 Contains additional definitions for environment variables, see section
1346 OpenSSH is a derivative of the original and free
1347 ssh 1.2.12 release by Tatu Ylonen.
1348 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1349 Theo de Raadt and Dug Song
1350 removed many bugs, re-added newer features and
1352 Markus Friedl contributed the support for SSH
1353 protocol versions 1.5 and 2.0.
1370 .%T "SSH Protocol Architecture"
1371 .%N draft-ietf-secsh-architecture-07.txt
1373 .%O work in progress material
andersk / openssh.git / blob