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 1.148 2002/02/18 17:55:20 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/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 protocol version 2
211 similar 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/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 a file from which the identity (private key) for
447 RSA or DSA authentication is read.
449 .Pa $HOME/.ssh/identity
450 for protocol version 1, and
451 .Pa $HOME/.ssh/id_rsa
453 .Pa $HOME/.ssh/id_dsa
454 for protocol version 2.
455 Identity files may also be specified on
456 a per-host basis in the configuration file.
457 It is possible to have multiple
459 options (and multiple identities specified in
460 configuration files).
461 .It Fl I Ar smartcard_device
462 Specifies which smartcard device to use. The argument is
465 should use to communicate with a smartcard used for storing the user's
468 Disables forwarding of Kerberos tickets and AFS tokens.
469 This may also be specified on a per-host basis in the configuration file.
470 .It Fl l Ar login_name
471 Specifies the user to log in as on the remote machine.
472 This also may be specified on a per-host basis in the configuration file.
474 Additionally, for protocol version 2 a comma-separated list of MAC
475 (message authentication code) algorithms can
476 be specified in order of preference.
479 keyword for more information.
483 (actually, prevents reading from stdin).
484 This must be used when
486 is run in the background.
487 A common trick is to use this to run X11 programs on a remote machine.
489 .Ic ssh -n shadows.cs.hut.fi emacs &
490 will start an emacs on shadows.cs.hut.fi, and the X11
491 connection will be automatically forwarded over an encrypted channel.
494 program will be put in the background.
495 (This does not work if
497 needs to ask for a password or passphrase; see also the
501 Do not execute a remote command.
502 This is useful for just forwarding ports
503 (protocol version 2 only).
505 Can be used to give options in the format used in the configuration file.
506 This is useful for specifying options for which there is no separate
509 Port to connect to on the remote host.
510 This can be specified on a
511 per-host basis in the configuration file.
513 Use a non-privileged port for outgoing connections.
514 This can be used if a firewall does
515 not permit connections from privileged ports.
516 Note that this option turns off
517 .Cm RhostsAuthentication
519 .Cm RhostsRSAAuthentication
523 Causes all warning and diagnostic messages to be suppressed.
525 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
526 of SSH as a secure transport for other applications (eg. sftp). The
527 subsystem is specified as the remote command.
529 Force pseudo-tty allocation.
530 This can be used to execute arbitrary
531 screen-based programs on a remote machine, which can be very useful,
532 e.g., when implementing menu services.
535 options force tty allocation, even if
539 Disable pseudo-tty allocation.
544 to print debugging messages about its progress.
546 debugging connection, authentication, and configuration problems.
549 options increases the verbosity.
552 Disables X11 forwarding.
554 Enables X11 forwarding.
555 This can also be specified on a per-host basis in a configuration file.
557 Requests compression of all data (including stdin, stdout, stderr, and
558 data for forwarded X11 and TCP/IP connections).
559 The compression algorithm is the same used by
563 can be controlled by the
566 Compression is desirable on modem lines and other
567 slow connections, but will only slow down things on fast networks.
568 The default value can be set on a host-by-host basis in the
569 configuration files; see the
572 .It Fl F Ar configfile
573 Specifies an alternative per-user configuration file.
574 If a configuration file is given on the command line,
575 the system-wide configuration file
576 .Pq Pa /etc/ssh/ssh_config
578 The default for the per-user configuration file is
579 .Pa $HOME/.ssh/config .
580 .It Fl L Ar port:host:hostport
581 Specifies that the given port on the local (client) host is to be
582 forwarded to the given host and port on the remote side.
583 This works by allocating a socket to listen to
585 on the local side, and whenever a connection is made to this port, the
586 connection is forwarded over the secure channel, and a connection is
591 from the remote machine.
592 Port forwardings can also be specified in the configuration file.
593 Only root can forward privileged ports.
594 IPv6 addresses can be specified with an alternative syntax:
595 .Ar port/host/hostport
596 .It Fl R Ar port:host:hostport
597 Specifies that the given port on the remote (server) host is to be
598 forwarded to the given host and port on the local side.
599 This works by allocating a socket to listen to
601 on the remote side, and whenever a connection is made to this port, the
602 connection is forwarded over the secure channel, and a connection is
607 from the local machine.
608 Port forwardings can also be specified in the configuration file.
609 Privileged ports can be forwarded only when
610 logging in as root on the remote machine.
611 IPv6 addresses can be specified with an alternative syntax:
612 .Ar port/host/hostport
616 application-level port forwarding.
617 This works by allocating a socket to listen to
619 on the local side, and whenever a connection is made to this port, the
620 connection is forwarded over the secure channel, and the application
621 protocol is then used to determine where to connect to from the
622 remote machine. Currently the SOCKS4 protocol is supported, and
624 will act as a SOCKS4 server.
625 Only root can forward privileged ports.
626 Dynamic port forwardings can also be specified in the configuration file.
630 to try protocol version 1 only.
634 to try protocol version 2 only.
638 to use IPv4 addresses only.
642 to use IPv6 addresses only.
644 .Sh CONFIGURATION FILES
646 obtains configuration data from the following sources in
648 command line options, user's configuration file
649 .Pq Pa $HOME/.ssh/config ,
650 and system-wide configuration file
651 .Pq Pa /etc/ssh/ssh_config .
652 For each parameter, the first obtained value
654 The configuration files contain sections bracketed by
656 specifications, and that section is only applied for hosts that
657 match one of the patterns given in the specification.
658 The matched host name is the one given on the command line.
660 Since the first obtained value for each parameter is used, more
661 host-specific declarations should be given near the beginning of the
662 file, and general defaults at the end.
664 The configuration file has the following format:
666 Empty lines and lines starting with
670 Otherwise a line is of the format
671 .Dq keyword arguments .
672 Configuration options may be separated by whitespace or
673 optional whitespace and exactly one
675 the latter format is useful to avoid the need to quote whitespace
676 when specifying configuration options using the
685 keywords and their meanings are as follows (note that
686 keywords are case-insensitive and arguments are case-sensitive):
689 Restricts the following declarations (up to the next
691 keyword) to be only for those hosts that match one of the patterns
692 given after the keyword.
696 can be used as wildcards in the
700 as a pattern can be used to provide global
701 defaults for all hosts.
704 argument given on the command line (i.e., the name is not converted to
705 a canonicalized host name before matching).
706 .It Cm AFSTokenPassing
707 Specifies whether to pass AFS tokens to remote host.
708 The argument to this keyword must be
712 This option applies to protocol version 1 only.
716 passphrase/password querying will be disabled.
717 This option is useful in scripts and other batch jobs where no user
718 is present to supply the password.
726 Specify the interface to transmit from on machines with multiple
727 interfaces or aliased addresses.
728 Note that this option does not work if
729 .Cm UsePrivilegedPort
733 If this flag is set to
735 ssh will additionally check the host IP address in the
738 This allows ssh to detect if a host key changed due to DNS spoofing.
739 If the option is set to
741 the check will not be executed.
745 Specifies the cipher to use for encrypting the session
746 in protocol version 1.
754 is only supported in the
756 client for interoperability with legacy protocol 1 implementations
757 that do not support the
759 cipher. Its use is strongly discouraged due to cryptographic
764 Specifies the ciphers allowed for protocol version 2
765 in order of preference.
766 Multiple ciphers must be comma-separated.
770 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
771 aes192-cbc,aes256-cbc''
773 .It Cm ClearAllForwardings
774 Specifies that all local, remote and dynamic port forwardings
775 specified in the configuration files or on the command line be
776 cleared. This option is primarily useful when used from the
778 command line to clear port forwardings set in
779 configuration files, and is automatically set by
790 Specifies whether to use compression.
797 .It Cm CompressionLevel
798 Specifies the compression level to use if compression is enabled.
799 The argument must be an integer from 1 (fast) to 9 (slow, best).
800 The default level is 6, which is good for most applications.
801 The meaning of the values is the same as in
803 Note that this option applies to protocol version 1 only.
804 .It Cm ConnectionAttempts
805 Specifies the number of tries (one per second) to make before falling
806 back to rsh or exiting.
807 The argument must be an integer.
808 This may be useful in scripts if the connection sometimes fails.
810 .It Cm DynamicForward
811 Specifies that a TCP/IP port on the local machine be forwarded
812 over the secure channel, and the application
813 protocol is then used to determine where to connect to from the
814 remote machine. The argument must be a port number.
815 Currently the SOCKS4 protocol is supported, and
817 will act as a SOCKS4 server.
818 Multiple forwardings may be specified, and
819 additional forwardings can be given on the command line. Only
820 the superuser can forward privileged ports.
822 Sets the escape character (default:
824 The escape character can also
825 be set on the command line.
826 The argument should be a single character,
828 followed by a letter, or
830 to disable the escape
831 character entirely (making the connection transparent for binary
834 Specifies that if connecting via
836 fails due to a connection refused error (there is no
838 listening on the remote host),
840 should automatically be used instead (after a suitable warning about
841 the session being unencrypted).
849 Specifies whether the connection to the authentication agent (if any)
850 will be forwarded to the remote machine.
858 Specifies whether X11 connections will be automatically redirected
859 over the secure channel and
869 Specifies whether remote hosts are allowed to connect to local
873 binds local port forwardings to the loopback addresss. This
874 prevents other remote hosts from connecting to forwarded ports.
876 can be used to specify that
878 should bind local port forwardings to the wildcard address,
879 thus allowing remote hosts to connect to forwarded ports.
886 .It Cm GlobalKnownHostsFile
887 Specifies a file to use for the global
888 host key database instead of
889 .Pa /etc/ssh/ssh_known_hosts .
890 .It Cm HostbasedAuthentication
891 Specifies whether to try rhosts based authentication with public key
899 This option applies to protocol version 2 only and
901 .Cm RhostsRSAAuthentication .
902 .It Cm HostKeyAlgorithms
903 Specifies the protocol version 2 host key algorithms
904 that the client wants to use in order of preference.
905 The default for this option is:
906 .Dq ssh-rsa,ssh-dss .
908 Specifies an alias that should be used instead of the
909 real host name when looking up or saving the host key
910 in the host key database files.
911 This option is useful for tunneling ssh connections
912 or for multiple servers running on a single host.
914 Specifies the real host name to log into.
915 This can be used to specify nicknames or abbreviations for hosts.
916 Default is the name given on the command line.
917 Numeric IP addresses are also permitted (both on the command line and in
921 Specifies a file from which the user's RSA or DSA authentication identity
922 is read. The default is
923 .Pa $HOME/.ssh/identity
924 for protocol version 1, and
925 .Pa $HOME/.ssh/id_rsa
927 .Pa $HOME/.ssh/id_dsa
928 for protocol version 2.
929 Additionally, any identities represented by the authentication agent
930 will be used for authentication.
931 The file name may use the tilde
932 syntax to refer to a user's home directory.
933 It is possible to have
934 multiple identity files specified in configuration files; all these
935 identities will be tried in sequence.
937 Specifies whether the system should send TCP keepalive messages to the
939 If they are sent, death of the connection or crash of one
940 of the machines will be properly noticed.
941 However, this means that
942 connections will die if the route is down temporarily, and some people
947 (to send keepalives), and the client will notice
948 if the network goes down or the remote host dies.
949 This is important in scripts, and many users want it too.
951 To disable keepalives, the value should be set to
953 .It Cm KerberosAuthentication
954 Specifies whether Kerberos authentication will be used.
955 The argument to this keyword must be
959 .It Cm KerberosTgtPassing
960 Specifies whether a Kerberos TGT will be forwarded to the server.
961 This will only work if the Kerberos server is actually an AFS kaserver.
962 The argument to this keyword must be
967 Specifies that a TCP/IP port on the local machine be forwarded over
968 the secure channel to the specified host and port from the remote machine.
969 The first argument must be a port number, and the second must be
971 IPv6 addresses can be specified with an alternative syntax:
973 Multiple forwardings may be specified, and additional
974 forwardings can be given on the command line.
975 Only the superuser can forward privileged ports.
977 Gives the verbosity level that is used when logging messages from
979 The possible values are:
980 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
981 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
982 and DEBUG3 each specify higher levels of verbose output.
984 Specifies the MAC (message authentication code) algorithms
985 in order of preference.
986 The MAC algorithm is used in protocol version 2
987 for data integrity protection.
988 Multiple algorithms must be comma-separated.
990 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
991 .It Cm NoHostAuthenticationForLocalhost
992 This option can be used if the home directory is shared across machines.
993 In this case localhost will refer to a different machine on each of
994 the machines and the user will get many warnings about changed host keys.
995 However, this option disables host authentication for localhost.
996 The argument to this keyword must be
1000 The default is to check the host key for localhost.
1001 .It Cm NumberOfPasswordPrompts
1002 Specifies the number of password prompts before giving up.
1003 The argument to this keyword must be an integer.
1005 .It Cm PasswordAuthentication
1006 Specifies whether to use password authentication.
1007 The argument to this keyword must be
1014 Specifies the port number to connect on the remote host.
1016 .It Cm PreferredAuthentications
1017 Specifies the order in which the client should try protocol 2
1018 authentication methods. This allows a client to prefer one method (e.g.
1019 .Cm keyboard-interactive )
1020 over another method (e.g.
1022 The default for this option is:
1023 .Dq hostbased,publickey,keyboard-interactive,password .
1025 Specifies the protocol versions
1027 should support in order of preference.
1028 The possible values are
1032 Multiple versions must be comma-separated.
1037 tries version 2 and falls back to version 1
1038 if version 2 is not available.
1040 Specifies the command to use to connect to the server.
1042 string extends to the end of the line, and is executed with
1044 In the command string,
1046 will be substituted by the host name to
1050 The command can be basically anything,
1051 and should read from its standard input and write to its standard output.
1052 It should eventually connect an
1054 server running on some machine, or execute
1057 Host key management will be done using the
1058 HostName of the host being connected (defaulting to the name typed by
1062 is not available for connects with a proxy command.
1064 .It Cm PubkeyAuthentication
1065 Specifies whether to try public key authentication.
1066 The argument to this keyword must be
1072 This option applies to protocol version 2 only.
1073 .It Cm RemoteForward
1074 Specifies that a TCP/IP port on the remote machine be forwarded over
1075 the secure channel to the specified host and port from the local machine.
1076 The first argument must be a port number, and the second must be
1078 IPv6 addresses can be specified with an alternative syntax:
1080 Multiple forwardings may be specified, and additional
1081 forwardings can be given on the command line.
1082 Only the superuser can forward privileged ports.
1083 .It Cm RhostsAuthentication
1084 Specifies whether to try rhosts based authentication.
1086 declaration only affects the client side and has no effect whatsoever
1088 Disabling rhosts authentication may reduce
1089 authentication time on slow connections when rhosts authentication is
1091 Most servers do not permit RhostsAuthentication because it
1093 .Cm RhostsRSAAuthentication ) .
1094 The argument to this keyword must be
1100 This option applies to protocol version 1 only.
1101 .It Cm RhostsRSAAuthentication
1102 Specifies whether to try rhosts based authentication with RSA host
1104 The argument must be
1110 This option applies to protocol version 1 only.
1111 .It Cm RSAAuthentication
1112 Specifies whether to try RSA authentication.
1113 The argument to this keyword must be
1117 RSA authentication will only be
1118 attempted if the identity file exists, or an authentication agent is
1122 Note that this option applies to protocol version 1 only.
1123 .It Cm ChallengeResponseAuthentication
1124 Specifies whether to use challenge response authentication.
1125 The argument to this keyword must be
1131 .It Cm SmartcardDevice
1132 Specifies which smartcard device to use. The argument to this keyword is
1135 should use to communicate with a smartcard used for storing the user's
1136 private RSA key. By default, no device is specified and smartcard support
1138 .It Cm StrictHostKeyChecking
1139 If this flag is set to
1142 will never automatically add host keys to the
1143 .Pa $HOME/.ssh/known_hosts
1144 file, and refuses to connect to hosts whose host key has changed.
1145 This provides maximum protection against trojan horse attacks,
1146 however, can be annoying when the
1147 .Pa /etc/ssh/ssh_known_hosts
1148 file is poorly maintained, or connections to new hosts are
1150 This option forces the user to manually
1152 If this flag is set to
1155 will automatically add new host keys to the
1156 user known hosts files.
1157 If this flag is set to
1160 will be added to the user known host files only after the user
1161 has confirmed that is what they really want to do, and
1163 will refuse to connect to hosts whose host key has changed.
1165 known hosts will be verified automatically in all cases.
1166 The argument must be
1173 .It Cm UsePrivilegedPort
1174 Specifies whether to use a privileged port for outgoing connections.
1175 The argument must be
1181 Note that this option must be set to
1184 .Cm RhostsAuthentication
1186 .Cm RhostsRSAAuthentication
1187 authentications are needed with older servers.
1189 Specifies the user to log in as.
1190 This can be useful when a different user name is used on different machines.
1191 This saves the trouble of
1192 having to remember to give the user name on the command line.
1193 .It Cm UserKnownHostsFile
1194 Specifies a file to use for the user
1195 host key database instead of
1196 .Pa $HOME/.ssh/known_hosts .
1198 Specifies that rlogin/rsh should be used for this host.
1199 It is possible that the host does not at all support the
1204 to immediately execute
1206 All other options (except
1208 are ignored if this has been specified.
1209 The argument must be
1213 .It Cm XAuthLocation
1214 Specifies the location of the
1218 .Pa /usr/X11R6/bin/xauth .
1222 will normally set the following environment variables:
1227 variable indicates the location of the X11 server.
1228 It is automatically set by
1230 to point to a value of the form
1232 where hostname indicates
1233 the host where the shell runs, and n is an integer >= 1.
1235 uses this special value to forward X11 connections over the secure
1237 The user should normally not set
1240 will render the X11 connection insecure (and will require the user to
1241 manually copy any required authorization cookies).
1243 Set to the path of the user's home directory.
1247 set for compatibility with systems that use this variable.
1249 Set to the path of the user's mailbox.
1253 as specified when compiling
1258 needs a passphrase, it will read the passphrase from the current
1259 terminal if it was run from a terminal.
1262 does not have a terminal associated with it but
1266 are set, it will execute the program specified by
1268 and open an X11 window to read the passphrase.
1269 This is particularly useful when calling
1274 (Note that on some machines it
1275 may be necessary to redirect the input from
1278 .It Ev SSH_AUTH_SOCK
1279 Identifies the path of a unix-domain socket used to communicate with the
1282 Identifies the client end of the connection.
1283 The variable contains
1284 three space-separated values: client ip-address, client port number,
1285 and server port number.
1286 .It Ev SSH_ORIGINAL_COMMAND
1287 The variable contains the original command line if a forced command
1289 It can be used to extract the original arguments.
1291 This is set to the name of the tty (path to the device) associated
1292 with the current shell or command.
1293 If the current session has no tty,
1294 this variable is not set.
1296 The timezone variable is set to indicate the present timezone if it
1297 was set when the daemon was started (i.e., the daemon passes the value
1298 on to new connections).
1300 Set to the name of the user logging in.
1306 .Pa $HOME/.ssh/environment ,
1307 and adds lines of the format
1312 .It Pa $HOME/.ssh/known_hosts
1313 Records host keys for all hosts the user has logged into that are not
1315 .Pa /etc/ssh/ssh_known_hosts .
1318 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1319 Contains the authentication identity of the user.
1320 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1322 contain sensitive data and should be readable by the user but not
1323 accessible by others (read/write/execute).
1326 ignores a private key file if it is accessible by others.
1327 It is possible to specify a passphrase when
1328 generating the key; the passphrase will be used to encrypt the
1329 sensitive part of this file using 3DES.
1330 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1331 Contains the public key for authentication (public part of the
1332 identity file in human-readable form).
1334 .Pa $HOME/.ssh/identity.pub
1335 file should be added to
1336 .Pa $HOME/.ssh/authorized_keys
1338 where the user wishes to log in using protocol version 1 RSA authentication.
1340 .Pa $HOME/.ssh/id_dsa.pub
1342 .Pa $HOME/.ssh/id_rsa.pub
1343 file should be added to
1344 .Pa $HOME/.ssh/authorized_keys
1346 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1348 sensitive and can (but need not) be readable by anyone.
1350 never used automatically and are not necessary; they are only provided for
1351 the convenience of the user.
1352 .It Pa $HOME/.ssh/config
1353 This is the per-user configuration file.
1354 The format of this file is described above.
1355 This file is used by the
1358 This file does not usually contain any sensitive information,
1359 but the recommended permissions are read/write for the user, and not
1360 accessible by others.
1361 .It Pa $HOME/.ssh/authorized_keys
1362 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1363 The format of this file is described in the
1366 In the simplest form the format is the same as the .pub
1368 This file is not highly sensitive, but the recommended
1369 permissions are read/write for the user, and not accessible by others.
1370 .It Pa /etc/ssh/ssh_known_hosts
1371 Systemwide list of known host keys.
1372 This file should be prepared by the
1373 system administrator to contain the public host keys of all machines in the
1375 This file should be world-readable.
1377 public keys, one per line, in the following format (fields separated
1378 by spaces): system name, public key and optional comment field.
1379 When different names are used
1380 for the same machine, all such names should be listed, separated by
1382 The format is described on the
1386 The canonical system name (as returned by name servers) is used by
1388 to verify the client host when logging in; other names are needed because
1390 does not convert the user-supplied name to a canonical name before
1391 checking the key, because someone with access to the name servers
1392 would then be able to fool host authentication.
1393 .It Pa /etc/ssh/ssh_config
1394 Systemwide configuration file.
1395 This file provides defaults for those
1396 values that are not specified in the user's configuration file, and
1397 for those users who do not have a configuration file.
1398 This file must be world-readable.
1399 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1400 These three files contain the private parts of the host keys
1402 .Cm RhostsRSAAuthentication
1404 .Cm HostbasedAuthentication .
1405 Since they are readable only by root
1407 must be setuid root if these authentication methods are desired.
1408 .It Pa $HOME/.rhosts
1409 This file is used in
1411 authentication to list the
1412 host/user pairs that are permitted to log in.
1413 (Note that this file is
1414 also used by rlogin and rsh, which makes using this file insecure.)
1415 Each line of the file contains a host name (in the canonical form
1416 returned by name servers), and then a user name on that host,
1417 separated by a space.
1418 On some machines this file may need to be
1419 world-readable if the user's home directory is on a NFS partition,
1423 Additionally, this file must be owned by the user,
1424 and must not have write permissions for anyone else.
1426 permission for most machines is read/write for the user, and not
1427 accessible by others.
1429 Note that by default
1431 will be installed so that it requires successful RSA host
1432 authentication before permitting \s+2.\s0rhosts authentication.
1433 If the server machine does not have the client's host key in
1434 .Pa /etc/ssh/ssh_known_hosts ,
1436 .Pa $HOME/.ssh/known_hosts .
1437 The easiest way to do this is to
1438 connect back to the client from the server machine using ssh; this
1439 will automatically add the host key to
1440 .Pa $HOME/.ssh/known_hosts .
1441 .It Pa $HOME/.shosts
1442 This file is used exactly the same way as
1445 having this file is to be able to use rhosts authentication with
1447 without permitting login with
1451 .It Pa /etc/hosts.equiv
1452 This file is used during
1453 .Pa \&.rhosts authentication.
1455 canonical hosts names, one per line (the full format is described on
1459 If the client host is found in this file, login is
1460 automatically permitted provided client and server user names are the
1462 Additionally, successful RSA host authentication is normally
1464 This file should only be writable by root.
1465 .It Pa /etc/shosts.equiv
1466 This file is processed exactly as
1467 .Pa /etc/hosts.equiv .
1468 This file may be useful to permit logins using
1470 but not using rsh/rlogin.
1471 .It Pa /etc/ssh/sshrc
1472 Commands in this file are executed by
1474 when the user logs in just before the user's shell (or command) is started.
1477 manual page for more information.
1478 .It Pa $HOME/.ssh/rc
1479 Commands in this file are executed by
1481 when the user logs in just before the user's shell (or command) is
1485 manual page for more information.
1486 .It Pa $HOME/.ssh/environment
1487 Contains additional definitions for environment variables, see section
1493 exits with the exit status of the remote command or with 255
1494 if an error occurred.
1496 OpenSSH is a derivative of the original and free
1497 ssh 1.2.12 release by Tatu Ylonen.
1498 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1499 Theo de Raadt and Dug Song
1500 removed many bugs, re-added newer features and
1502 Markus Friedl contributed the support for SSH
1503 protocol versions 1.5 and 2.0.
1520 .%T "SSH Protocol Architecture"
1521 .%N draft-ietf-secsh-architecture-09.txt
1523 .%O work in progress material
andersk / gssapi-openssh.git / blob