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 1.141 2001/11/08 17:49:53 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
78 .Ar hostname | user@hostname
82 (SSH client) is a program for logging into a remote machine and for
83 executing commands on a remote machine.
84 It is intended to replace
85 rlogin and rsh, and provide secure encrypted communications between
86 two untrusted hosts over an insecure network.
88 arbitrary TCP/IP ports can also be forwarded over the secure channel.
91 connects and logs into the specified
94 his/her identity to the remote machine using one of several methods
95 depending on the protocol version used:
97 .Ss SSH protocol version 1
99 First, if the machine the user logs in from is listed in
102 .Pa /etc/shosts.equiv
103 on the remote machine, and the user names are
104 the same on both sides, the user is immediately permitted to log in.
109 exists in the user's home directory on the
110 remote machine and contains a line containing the name of the client
111 machine and the name of the user on that machine, the user is
113 This form of authentication alone is normally not
114 allowed by the server because it is not secure.
116 The second authentication method is the
120 method combined with RSA-based host authentication.
121 It means that if the login would be permitted by
124 .Pa /etc/hosts.equiv ,
126 .Pa /etc/shosts.equiv ,
127 and if additionally the server can verify the client's
129 .Pa /etc/ssh_known_hosts
131 .Pa $HOME/.ssh/known_hosts
134 section), only then login is permitted.
135 This authentication method closes security holes due to IP
136 spoofing, DNS spoofing and routing spoofing.
137 [Note to the administrator:
138 .Pa /etc/hosts.equiv ,
140 and the rlogin/rsh protocol in general, are inherently insecure and should be
141 disabled if security is desired.]
143 As a third authentication method,
145 supports RSA based authentication.
146 The scheme is based on public-key cryptography: there are cryptosystems
147 where encryption and decryption are done using separate keys, and it
148 is not possible to derive the decryption key from the encryption key.
149 RSA is one such system.
150 The idea is that each user creates a public/private
151 key pair for authentication purposes.
152 The server knows the public key, and only the user knows the private key.
154 .Pa $HOME/.ssh/authorized_keys
155 lists the public keys that are permitted for logging
157 When the user logs in, the
159 program tells the server which key pair it would like to use for
161 The server checks if this key is permitted, and if
162 so, sends the user (actually the
164 program running on behalf of the user) a challenge, a random number,
165 encrypted by the user's public key.
166 The challenge can only be
167 decrypted using the proper private key.
168 The user's client then decrypts the
169 challenge using the private key, proving that he/she knows the private
170 key but without disclosing it to the server.
173 implements the RSA authentication protocol automatically.
174 The user creates his/her RSA key pair by running
176 This stores the private key in
177 .Pa $HOME/.ssh/identity
178 and the public key in
179 .Pa $HOME/.ssh/identity.pub
180 in the user's home directory.
181 The user should then copy the
184 .Pa $HOME/.ssh/authorized_keys
185 in his/her home directory on the remote machine (the
187 file corresponds to the conventional
189 file, and has one key
190 per line, though the lines can be very long).
191 After this, the user can log in without giving the password.
192 RSA authentication is much
193 more secure than rhosts authentication.
195 The most convenient way to use RSA authentication may be with an
196 authentication agent.
199 for more information.
201 If other authentication methods fail,
203 prompts the user for a password.
204 The password is sent to the remote
205 host for checking; however, since all communications are encrypted,
206 the password cannot be seen by someone listening on the network.
208 .Ss SSH protocol version 2
210 When a user connects using the protocol version 2
211 different authentication methods are available.
212 Using the default values for
213 .Cm PreferredAuthentications ,
214 the client will try to authenticate first using the hostbased method;
215 if this method fails public key authentication is attempted,
216 and finally if this method fails keyboard-interactive and
217 password authentication are tried.
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 below.
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
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 the command line or in a configuration file.
354 Forwarding of arbitrary TCP/IP connections over the secure channel can
355 be specified either on the 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|des
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.
402 is a fast block cipher, it appears very secure and is much faster than
405 is only supported in the
407 client for interoperability with legacy protocol 1 implementations
408 that do not support the
410 cipher. Its use is strongly discouraged due to cryptographic
412 .It Fl c Ar cipher_spec
413 Additionally, for protocol version 2 a comma-separated list of ciphers can
414 be specified in order of preference.
417 for more information.
418 .It Fl e Ar ch|^ch|none
419 Sets the escape character for sessions with a pty (default:
421 The escape character is only recognized at the beginning of a line.
422 The escape character followed by a dot
424 closes the connection, followed
425 by control-Z suspends the connection, and followed by itself sends the
426 escape character once.
427 Setting the character to
429 disables any escapes and makes the session fully transparent.
433 to go to background just before command execution.
436 is going to ask for passwords or passphrases, but the user
437 wants it in the background.
440 The recommended way to start X11 programs at a remote site is with
442 .Ic ssh -f host xterm .
444 Allows remote hosts to connect to local forwarded ports.
445 .It Fl i Ar identity_file
446 Selects the file from which the identity (private key) for
447 RSA or DSA authentication is read.
449 .Pa $HOME/.ssh/identity
450 in the user's home directory.
451 Identity files may also be specified on
452 a per-host basis in the configuration file.
453 It is possible to have multiple
455 options (and multiple identities specified in
456 configuration files).
457 .It Fl I Ar smartcard_device
458 Specifies which smartcard device to use. The argument is
461 should use to communicate with a smartcard used for storing the user's
464 Disables forwarding of Kerberos tickets and AFS tokens.
465 This may also be specified on a per-host basis in the configuration file.
466 .It Fl l Ar login_name
467 Specifies the user to log in as on the remote machine.
468 This also may be specified on a per-host basis in the configuration file.
470 Additionally, for protocol version 2 a comma-separated list of MAC
471 (message authentication code) algorithms can
472 be specified in order of preference.
475 keyword for more information.
479 (actually, prevents reading from stdin).
480 This must be used when
482 is run in the background.
483 A common trick is to use this to run X11 programs on a remote machine.
485 .Ic ssh -n shadows.cs.hut.fi emacs &
486 will start an emacs on shadows.cs.hut.fi, and the X11
487 connection will be automatically forwarded over an encrypted channel.
490 program will be put in the background.
491 (This does not work if
493 needs to ask for a password or passphrase; see also the
497 Do not execute a remote command.
498 This is useful for just forwarding ports
499 (protocol version 2 only).
501 Can be used to give options in the format used in the configuration file.
502 This is useful for specifying options for which there is no separate
505 Port to connect to on the remote host.
506 This can be specified on a
507 per-host basis in the configuration file.
509 Use a non-privileged port for outgoing connections.
510 This can be used if a firewall does
511 not permit connections from privileged ports.
512 Note that this option turns off
513 .Cm RhostsAuthentication
515 .Cm RhostsRSAAuthentication
519 Causes all warning and diagnostic messages to be suppressed.
520 Only fatal errors are displayed.
522 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
523 of SSH as a secure transport for other applications (eg. sftp). The
524 subsystem is specified as the remote command.
526 Force pseudo-tty allocation.
527 This can be used to execute arbitrary
528 screen-based programs on a remote machine, which can be very useful,
529 e.g., when implementing menu services.
532 options force tty allocation, even if
536 Disable pseudo-tty allocation.
541 to print debugging messages about its progress.
543 debugging connection, authentication, and configuration problems.
546 options increases the verbosity.
549 Disables X11 forwarding.
551 Enables X11 forwarding.
552 This can also be specified on a per-host basis in a configuration file.
554 Requests compression of all data (including stdin, stdout, stderr, and
555 data for forwarded X11 and TCP/IP connections).
556 The compression algorithm is the same used by
560 can be controlled by the
563 Compression is desirable on modem lines and other
564 slow connections, but will only slow down things on fast networks.
565 The default value can be set on a host-by-host basis in the
566 configuration files; see the
569 .It Fl F Ar configfile
570 Specifies an alternative per-user configuration file.
571 If a configuration file is given on the command line,
572 the system-wide configuration file
573 .Pq Pa /etc/ssh_config
575 The default for the per-user configuration file is
576 .Pa $HOME/.ssh/config .
577 .It Fl L Ar port:host:hostport
578 Specifies that the given port on the local (client) host is to be
579 forwarded to the given host and port on the remote side.
580 This works by allocating a socket to listen to
582 on the local side, and whenever a connection is made to this port, the
583 connection is forwarded over the secure channel, and a connection is
588 from the remote machine.
589 Port forwardings can also be specified in the configuration file.
590 Only root can forward privileged ports.
591 IPv6 addresses can be specified with an alternative syntax:
592 .Ar port/host/hostport
593 .It Fl R Ar port:host:hostport
594 Specifies that the given port on the remote (server) host is to be
595 forwarded to the given host and port on the local side.
596 This works by allocating a socket to listen to
598 on the remote side, and whenever a connection is made to this port, the
599 connection is forwarded over the secure channel, and a connection is
604 from the local machine.
605 Port forwardings can also be specified in the configuration file.
606 Privileged ports can be forwarded only when
607 logging in as root on the remote machine.
608 IPv6 addresses can be specified with an alternative syntax:
609 .Ar port/host/hostport
613 application-level port forwarding.
614 This works by allocating a socket to listen to
616 on the local side, and whenever a connection is made to this port, the
617 connection is forwarded over the secure channel, and the application
618 protocol is then used to determine where to connect to from the
619 remote machine. Currently the SOCKS4 protocol is supported, and
621 will act as a SOCKS4 server.
622 Only root can forward privileged ports.
623 Dynamic port forwardings can also be specified in the configuration file.
627 to try protocol version 1 only.
631 to try protocol version 2 only.
635 to use IPv4 addresses only.
639 to use IPv6 addresses only.
641 .Sh CONFIGURATION FILES
643 obtains configuration data from the following sources in
645 command line options, user's configuration file
646 .Pq Pa $HOME/.ssh/config ,
647 and system-wide configuration file
648 .Pq Pa /etc/ssh_config .
649 For each parameter, the first obtained value
651 The configuration files contain sections bracketed by
653 specifications, and that section is only applied for hosts that
654 match one of the patterns given in the specification.
655 The matched host name is the one given on the command line.
657 Since the first obtained value for each parameter is used, more
658 host-specific declarations should be given near the beginning of the
659 file, and general defaults at the end.
661 The configuration file has the following format:
663 Empty lines and lines starting with
667 Otherwise a line is of the format
668 .Dq keyword arguments .
669 Configuration options may be separated by whitespace or
670 optional whitespace and exactly one
672 the latter format is useful to avoid the need to quote whitespace
673 when specifying configuration options using the
682 keywords and their meanings are as follows (note that
683 keywords are case-insensitive and arguments are case-sensitive):
686 Restricts the following declarations (up to the next
688 keyword) to be only for those hosts that match one of the patterns
689 given after the keyword.
693 can be used as wildcards in the
697 as a pattern can be used to provide global
698 defaults for all hosts.
701 argument given on the command line (i.e., the name is not converted to
702 a canonicalized host name before matching).
703 .It Cm AFSTokenPassing
704 Specifies whether to pass AFS tokens to remote host.
705 The argument to this keyword must be
709 This option applies to protocol version 1 only.
713 passphrase/password querying will be disabled.
714 This option is useful in scripts and other batch jobs where no user
715 is present to supply the password.
723 Specify the interface to transmit from on machines with multiple
724 interfaces or aliased addresses.
725 Note that this option does not work if
726 .Cm UsePrivilegedPort
730 If this flag is set to
732 ssh will additionally check the host IP address in the
735 This allows ssh to detect if a host key changed due to DNS spoofing.
736 If the option is set to
738 the check will not be executed.
742 Specifies the cipher to use for encrypting the session
743 in protocol version 1.
751 is only supported in the
753 client for interoperability with legacy protocol 1 implementations
754 that do not support the
756 cipher. Its use is strongly discouraged due to cryptographic
761 Specifies the ciphers allowed for protocol version 2
762 in order of preference.
763 Multiple ciphers must be comma-separated.
767 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
768 aes192-cbc,aes256-cbc''
770 .It Cm ClearAllForwardings
771 Specifies that all local, remote and dynamic port forwardings
772 specified in the configuration files or on the command line be
773 cleared. This option is primarily useful when used from the
775 command line to clear port forwardings set in
776 configuration files, and is automatically set by
787 Specifies whether to use compression.
794 .It Cm CompressionLevel
795 Specifies the compression level to use if compression is enabled.
796 The argument must be an integer from 1 (fast) to 9 (slow, best).
797 The default level is 6, which is good for most applications.
798 The meaning of the values is the same as in
800 Note that this option applies to protocol version 1 only.
801 .It Cm ConnectionAttempts
802 Specifies the number of tries (one per second) to make before falling
803 back to rsh or exiting.
804 The argument must be an integer.
805 This may be useful in scripts if the connection sometimes fails.
807 .It Cm DynamicForward
808 Specifies that a TCP/IP port on the local machine be forwarded
809 over the secure channel, and the application
810 protocol is then used to determine where to connect to from the
811 remote machine. The argument must be a port number.
812 Currently the SOCKS4 protocol is supported, and
814 will act as a SOCKS4 server.
815 Multiple forwardings may be specified, and
816 additional forwardings can be given on the command line. Only
817 the superuser can forward privileged ports.
819 Sets the escape character (default:
821 The escape character can also
822 be set on the command line.
823 The argument should be a single character,
825 followed by a letter, or
827 to disable the escape
828 character entirely (making the connection transparent for binary
831 Specifies that if connecting via
833 fails due to a connection refused error (there is no
835 listening on the remote host),
837 should automatically be used instead (after a suitable warning about
838 the session being unencrypted).
846 Specifies whether the connection to the authentication agent (if any)
847 will be forwarded to the remote machine.
855 Specifies whether X11 connections will be automatically redirected
856 over the secure channel and
866 Specifies whether remote hosts are allowed to connect to local
870 binds local port forwardings to the loopback addresss. This
871 prevents other remote hosts from connecting to forwarded ports.
873 can be used to specify that
875 should bind local port forwardings to the wildcard address,
876 thus allowing remote hosts to connect to forwarded ports.
883 .It Cm GlobalKnownHostsFile
884 Specifies a file to use for the global
885 host key database instead of
886 .Pa /etc/ssh_known_hosts .
887 .It Cm HostbasedAuthentication
888 Specifies whether to try rhosts based authentication with public key
896 This option applies to protocol version 2 only and
898 .Cm RhostsRSAAuthentication .
899 .It Cm HostKeyAlgorithms
900 Specifies the protocol version 2 host key algorithms
901 that the client wants to use in order of preference.
902 The default for this option is:
905 Specifies an alias that should be used instead of the
906 real host name when looking up or saving the host key
907 in the host key database files.
908 This option is useful for tunneling ssh connections
909 or for multiple servers running on a single host.
911 Specifies the real host name to log into.
912 This can be used to specify nicknames or abbreviations for hosts.
913 Default is the name given on the command line.
914 Numeric IP addresses are also permitted (both on the command line and in
918 Specifies the file from which the user's RSA or DSA authentication identity
920 .Pa $HOME/.ssh/identity
921 in the user's home directory).
922 Additionally, any identities represented by the authentication agent
923 will be used for authentication.
924 The file name may use the tilde
925 syntax to refer to a user's home directory.
926 It is possible to have
927 multiple identity files specified in configuration files; all these
928 identities will be tried in sequence.
930 Specifies whether the system should send keepalive messages to the
932 If they are sent, death of the connection or crash of one
933 of the machines will be properly noticed.
934 However, this means that
935 connections will die if the route is down temporarily, and some people
940 (to send keepalives), and the client will notice
941 if the network goes down or the remote host dies.
942 This is important in scripts, and many users want it too.
944 To disable keepalives, the value should be set to
946 in both the server and the client configuration files.
947 .It Cm KerberosAuthentication
948 Specifies whether Kerberos authentication will be used.
949 The argument to this keyword must be
953 .It Cm KerberosTgtPassing
954 Specifies whether a Kerberos TGT will be forwarded to the server.
955 This will only work if the Kerberos server is actually an AFS kaserver.
956 The argument to this keyword must be
961 Specifies that a TCP/IP port on the local machine be forwarded over
962 the secure channel to the specified host and port from the remote machine.
963 The first argument must be a port number, and the second must be
965 IPv6 addresses can be specified with an alternative syntax:
967 Multiple forwardings may be specified, and additional
968 forwardings can be given on the command line.
969 Only the superuser can forward privileged ports.
971 Gives the verbosity level that is used when logging messages from
973 The possible values are:
974 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
977 Specifies the MAC (message authentication code) algorithms
978 in order of preference.
979 The MAC algorithm is used in protocol version 2
980 for data integrity protection.
981 Multiple algorithms must be comma-separated.
983 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
984 .It Cm NoHostAuthenticationForLocalhost
985 This option can be used if the home directory is shared across machines.
986 In this case localhost will refer to a different machine on each of
987 the machines and the user will get many warnings about changed host keys.
988 However, this option disables host authentication for localhost.
989 The argument to this keyword must be
993 The default is to check the host key for localhost.
994 .It Cm NumberOfPasswordPrompts
995 Specifies the number of password prompts before giving up.
996 The argument to this keyword must be an integer.
998 .It Cm PasswordAuthentication
999 Specifies whether to use password authentication.
1000 The argument to this keyword must be
1007 Specifies the port number to connect on the remote host.
1009 .It Cm PreferredAuthentications
1010 Specifies the order in which the client should try protocol 2
1011 authentication methods. This allows a client to prefer one method (e.g.
1012 .Cm keyboard-interactive )
1013 over another method (e.g.
1015 The default for this option is:
1016 .Dq hostbased,publickey,keyboard-interactive,password
1018 Specifies the protocol versions
1020 should support in order of preference.
1021 The possible values are
1025 Multiple versions must be comma-separated.
1030 tries version 2 and falls back to version 1
1031 if version 2 is not available.
1033 Specifies the command to use to connect to the server.
1035 string extends to the end of the line, and is executed with
1037 In the command string,
1039 will be substituted by the host name to
1043 The command can be basically anything,
1044 and should read from its standard input and write to its standard output.
1045 It should eventually connect an
1047 server running on some machine, or execute
1050 Host key management will be done using the
1051 HostName of the host being connected (defaulting to the name typed by
1055 is not available for connects with a proxy command.
1057 .It Cm PubkeyAuthentication
1058 Specifies whether to try public key authentication.
1059 The argument to this keyword must be
1065 This option applies to protocol version 2 only.
1066 .It Cm RemoteForward
1067 Specifies that a TCP/IP port on the remote machine be forwarded over
1068 the secure channel to the specified host and port from the local machine.
1069 The first argument must be a port number, and the second must be
1071 IPv6 addresses can be specified with an alternative syntax:
1073 Multiple forwardings may be specified, and additional
1074 forwardings can be given on the command line.
1075 Only the superuser can forward privileged ports.
1076 .It Cm RhostsAuthentication
1077 Specifies whether to try rhosts based authentication.
1079 declaration only affects the client side and has no effect whatsoever
1081 Disabling rhosts authentication may reduce
1082 authentication time on slow connections when rhosts authentication is
1084 Most servers do not permit RhostsAuthentication because it
1086 .Cm RhostsRSAAuthentication ) .
1087 The argument to this keyword must be
1093 This option applies to protocol version 1 only.
1094 .It Cm RhostsRSAAuthentication
1095 Specifies whether to try rhosts based authentication with RSA host
1097 The argument must be
1103 This option applies to protocol version 1 only.
1104 .It Cm RSAAuthentication
1105 Specifies whether to try RSA authentication.
1106 The argument to this keyword must be
1110 RSA authentication will only be
1111 attempted if the identity file exists, or an authentication agent is
1115 Note that this option applies to protocol version 1 only.
1116 .It Cm ChallengeResponseAuthentication
1117 Specifies whether to use challenge response authentication.
1118 The argument to this keyword must be
1124 .It Cm SmartcardDevice
1125 Specifies which smartcard device to use. The argument to this keyword is
1128 should use to communicate with a smartcard used for storing the user's
1129 private RSA key. By default, no device is specified and smartcard support
1131 .It Cm StrictHostKeyChecking
1132 If this flag is set to
1135 will never automatically add host keys to the
1136 .Pa $HOME/.ssh/known_hosts
1137 file, and refuses to connect to hosts whose host key has changed.
1138 This provides maximum protection against trojan horse attacks,
1139 however, can be annoying when the
1140 .Pa /etc/ssh_known_hosts
1141 file is poorly maintained, or connections to new hosts are
1143 This option forces the user to manually
1145 If this flag is set to
1148 will automatically add new host keys to the
1149 user known hosts files.
1150 If this flag is set to
1153 will be added to the user known host files only after the user
1154 has confirmed that is what they really want to do, and
1156 will refuse to connect to hosts whose host key has changed.
1158 known hosts will be verified automatically in all cases.
1159 The argument must be
1166 .It Cm UsePrivilegedPort
1167 Specifies whether to use a privileged port for outgoing connections.
1168 The argument must be
1174 Note that this option must be set to
1177 .Cm RhostsAuthentication
1179 .Cm RhostsRSAAuthentication
1180 authentications are needed with older servers.
1182 Specifies the user to log in as.
1183 This can be useful when a different user name is used on different machines.
1184 This saves the trouble of
1185 having to remember to give the user name on the command line.
1186 .It Cm UserKnownHostsFile
1187 Specifies a file to use for the user
1188 host key database instead of
1189 .Pa $HOME/.ssh/known_hosts .
1191 Specifies that rlogin/rsh should be used for this host.
1192 It is possible that the host does not at all support the
1197 to immediately execute
1199 All other options (except
1201 are ignored if this has been specified.
1202 The argument must be
1206 .It Cm XAuthLocation
1207 Specifies the location of the
1211 .Pa /usr/X11R6/bin/xauth .
1215 will normally set the following environment variables:
1220 variable indicates the location of the X11 server.
1221 It is automatically set by
1223 to point to a value of the form
1225 where hostname indicates
1226 the host where the shell runs, and n is an integer >= 1.
1228 uses this special value to forward X11 connections over the secure
1230 The user should normally not set
1233 will render the X11 connection insecure (and will require the user to
1234 manually copy any required authorization cookies).
1236 Set to the path of the user's home directory.
1240 set for compatibility with systems that use this variable.
1242 Set to the path of the user's mailbox.
1246 as specified when compiling
1251 needs a passphrase, it will read the passphrase from the current
1252 terminal if it was run from a terminal.
1255 does not have a terminal associated with it but
1259 are set, it will execute the program specified by
1261 and open an X11 window to read the passphrase.
1262 This is particularly useful when calling
1267 (Note that on some machines it
1268 may be necessary to redirect the input from
1271 .It Ev SSH_AUTH_SOCK
1272 Identifies the path of a unix-domain socket used to communicate with the
1275 Identifies the client end of the connection.
1276 The variable contains
1277 three space-separated values: client ip-address, client port number,
1278 and server port number.
1279 .It Ev SSH_ORIGINAL_COMMAND
1280 The variable contains the original command line if a forced command
1282 It can be used to extract the original arguments.
1284 This is set to the name of the tty (path to the device) associated
1285 with the current shell or command.
1286 If the current session has no tty,
1287 this variable is not set.
1289 The timezone variable is set to indicate the present timezone if it
1290 was set when the daemon was started (i.e., the daemon passes the value
1291 on to new connections).
1293 Set to the name of the user logging in.
1299 .Pa $HOME/.ssh/environment ,
1300 and adds lines of the format
1305 .It Pa $HOME/.ssh/known_hosts
1306 Records host keys for all hosts the user has logged into that are not
1308 .Pa /etc/ssh_known_hosts .
1311 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1312 Contains the authentication identity of the user.
1313 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1315 contain sensitive data and should be readable by the user but not
1316 accessible by others (read/write/execute).
1319 ignores a private key file if it is accessible by others.
1320 It is possible to specify a passphrase when
1321 generating the key; the passphrase will be used to encrypt the
1322 sensitive part of this file using 3DES.
1323 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1324 Contains the public key for authentication (public part of the
1325 identity file in human-readable form).
1327 .Pa $HOME/.ssh/identity.pub
1328 file should be added to
1329 .Pa $HOME/.ssh/authorized_keys
1331 where the user wishes to log in using protocol version 1 RSA authentication.
1333 .Pa $HOME/.ssh/id_dsa.pub
1335 .Pa $HOME/.ssh/id_rsa.pub
1336 file should be added to
1337 .Pa $HOME/.ssh/authorized_keys
1339 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1341 sensitive and can (but need not) be readable by anyone.
1343 never used automatically and are not necessary; they are only provided for
1344 the convenience of the user.
1345 .It Pa $HOME/.ssh/config
1346 This is the per-user configuration file.
1347 The format of this file is described above.
1348 This file is used by the
1351 This file does not usually contain any sensitive information,
1352 but the recommended permissions are read/write for the user, and not
1353 accessible by others.
1354 .It Pa $HOME/.ssh/authorized_keys
1355 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1356 The format of this file is described in the
1359 In the simplest form the format is the same as the .pub
1361 This file is not highly sensitive, but the recommended
1362 permissions are read/write for the user, and not accessible by others.
1363 .It Pa /etc/ssh_known_hosts
1364 Systemwide list of known host keys.
1365 This file should be prepared by the
1366 system administrator to contain the public host keys of all machines in the
1368 This file should be world-readable.
1370 public keys, one per line, in the following format (fields separated
1371 by spaces): system name, public key and optional comment field.
1372 When different names are used
1373 for the same machine, all such names should be listed, separated by
1375 The format is described on the
1379 The canonical system name (as returned by name servers) is used by
1381 to verify the client host when logging in; other names are needed because
1383 does not convert the user-supplied name to a canonical name before
1384 checking the key, because someone with access to the name servers
1385 would then be able to fool host authentication.
1386 .It Pa /etc/ssh_config
1387 Systemwide configuration file.
1388 This file provides defaults for those
1389 values that are not specified in the user's configuration file, and
1390 for those users who do not have a configuration file.
1391 This file must be world-readable.
1392 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1393 These three files contain the private parts of the host keys
1395 .Cm RhostsRSAAuthentication
1397 .Cm HostbasedAuthentication .
1398 Since they are readable only by root
1400 must be setuid root if these authentication methods are desired.
1401 .It Pa $HOME/.rhosts
1402 This file is used in
1404 authentication to list the
1405 host/user pairs that are permitted to log in.
1406 (Note that this file is
1407 also used by rlogin and rsh, which makes using this file insecure.)
1408 Each line of the file contains a host name (in the canonical form
1409 returned by name servers), and then a user name on that host,
1410 separated by a space.
1411 On some machines this file may need to be
1412 world-readable if the user's home directory is on a NFS partition,
1416 Additionally, this file must be owned by the user,
1417 and must not have write permissions for anyone else.
1419 permission for most machines is read/write for the user, and not
1420 accessible by others.
1422 Note that by default
1424 will be installed so that it requires successful RSA host
1425 authentication before permitting \s+2.\s0rhosts authentication.
1426 If the server machine does not have the client's host key in
1427 .Pa /etc/ssh_known_hosts ,
1429 .Pa $HOME/.ssh/known_hosts .
1430 The easiest way to do this is to
1431 connect back to the client from the server machine using ssh; this
1432 will automatically add the host key to
1433 .Pa $HOME/.ssh/known_hosts .
1434 .It Pa $HOME/.shosts
1435 This file is used exactly the same way as
1438 having this file is to be able to use rhosts authentication with
1440 without permitting login with
1444 .It Pa /etc/hosts.equiv
1445 This file is used during
1446 .Pa \&.rhosts authentication.
1448 canonical hosts names, one per line (the full format is described on
1452 If the client host is found in this file, login is
1453 automatically permitted provided client and server user names are the
1455 Additionally, successful RSA host authentication is normally
1457 This file should only be writable by root.
1458 .It Pa /etc/shosts.equiv
1459 This file is processed exactly as
1460 .Pa /etc/hosts.equiv .
1461 This file may be useful to permit logins using
1463 but not using rsh/rlogin.
1465 Commands in this file are executed by
1467 when the user logs in just before the user's shell (or command) is started.
1470 manual page for more information.
1471 .It Pa $HOME/.ssh/rc
1472 Commands in this file are executed by
1474 when the user logs in just before the user's shell (or command) is
1478 manual page for more information.
1479 .It Pa $HOME/.ssh/environment
1480 Contains additional definitions for environment variables, see section
1485 OpenSSH is a derivative of the original and free
1486 ssh 1.2.12 release by Tatu Ylonen.
1487 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1488 Theo de Raadt and Dug Song
1489 removed many bugs, re-added newer features and
1491 Markus Friedl contributed the support for SSH
1492 protocol versions 1.5 and 2.0.
1509 .%T "SSH Protocol Architecture"
1510 .%N draft-ietf-secsh-architecture-09.txt
1512 .%O work in progress material
andersk / gssapi-openssh.git / blob