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 1.164 2002/08/29 16:02:54 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl l Ar login_name
47 .Ar hostname | user@hostname
51 .Op Fl afgknqstvxACNTX1246
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 Open command line (only useful for adding port forwardings using the
307 Request rekeying of the connection (only useful for SSH protocol version 2
308 and if the peer supports it)
311 .Ss X11 and TCP forwarding
317 (or, see the description of the
321 options described later)
322 and the user is using X11 (the
324 environment variable is set), the connection to the X11 display is
325 automatically forwarded to the remote side in such a way that any X11
326 programs started from the shell (or command) will go through the
327 encrypted channel, and the connection to the real X server will be made
328 from the local machine.
329 The user should not manually set
331 Forwarding of X11 connections can be
332 configured on the command line or in configuration files.
338 will point to the server machine, but with a display number greater
340 This is normal, and happens because
344 X server on the server machine for forwarding the
345 connections over the encrypted channel.
348 will also automatically set up Xauthority data on the server machine.
349 For this purpose, it will generate a random authorization cookie,
350 store it in Xauthority on the server, and verify that any forwarded
351 connections carry this cookie and replace it by the real cookie when
352 the connection is opened.
353 The real authentication cookie is never
354 sent to the server machine (and no cookies are sent in the plain).
360 (or, see the description of the
364 options described later) and
365 the user is using an authentication agent, the connection to the agent
366 is automatically forwarded to the remote side.
368 Forwarding of arbitrary TCP/IP connections over the secure channel can
369 be specified either on the command line or in a configuration file.
370 One possible application of TCP/IP forwarding is a secure connection to an
371 electronic purse; another is going through firewalls.
373 .Ss Server authentication
376 automatically maintains and checks a database containing
377 identifications for all hosts it has ever been used with.
378 Host keys are stored in
379 .Pa $HOME/.ssh/known_hosts
380 in the user's home directory.
381 Additionally, the file
382 .Pa /etc/ssh/ssh_known_hosts
383 is automatically checked for known hosts.
384 Any new hosts are automatically added to the user's file.
385 If a host's identification
388 warns about this and disables password authentication to prevent a
389 trojan horse from getting the user's password.
391 this mechanism is to prevent man-in-the-middle attacks which could
392 otherwise be used to circumvent the encryption.
394 .Cm StrictHostKeyChecking
395 option can be used to prevent logins to machines whose
396 host key is not known or has changed.
398 The options are as follows:
401 Disables forwarding of the authentication agent connection.
403 Enables forwarding of the authentication agent connection.
404 This can also be specified on a per-host basis in a configuration file.
405 .It Fl b Ar bind_address
406 Specify the interface to transmit from on machines with multiple
407 interfaces or aliased addresses.
408 .It Fl c Ar blowfish|3des|des
409 Selects the cipher to use for encrypting the session.
412 It is believed to be secure.
414 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
416 is a fast block cipher, it appears very secure and is much faster than
419 is only supported in the
421 client for interoperability with legacy protocol 1 implementations
422 that do not support the
424 cipher. Its use is strongly discouraged due to cryptographic
426 .It Fl c Ar cipher_spec
427 Additionally, for protocol version 2 a comma-separated list of ciphers can
428 be specified in order of preference.
431 for more information.
432 .It Fl e Ar ch|^ch|none
433 Sets the escape character for sessions with a pty (default:
435 The escape character is only recognized at the beginning of a line.
436 The escape character followed by a dot
438 closes the connection, followed
439 by control-Z suspends the connection, and followed by itself sends the
440 escape character once.
441 Setting the character to
443 disables any escapes and makes the session fully transparent.
447 to go to background just before command execution.
450 is going to ask for passwords or passphrases, but the user
451 wants it in the background.
454 The recommended way to start X11 programs at a remote site is with
456 .Ic ssh -f host xterm .
458 Allows remote hosts to connect to local forwarded ports.
459 .It Fl i Ar identity_file
460 Selects a file from which the identity (private key) for
461 RSA or DSA authentication is read.
463 .Pa $HOME/.ssh/identity
464 for protocol version 1, and
465 .Pa $HOME/.ssh/id_rsa
467 .Pa $HOME/.ssh/id_dsa
468 for protocol version 2.
469 Identity files may also be specified on
470 a per-host basis in the configuration file.
471 It is possible to have multiple
473 options (and multiple identities specified in
474 configuration files).
475 .It Fl I Ar smartcard_device
476 Specifies which smartcard device to use. The argument is
479 should use to communicate with a smartcard used for storing the user's
482 Disables forwarding of Kerberos tickets and AFS tokens.
483 This may also be specified on a per-host basis in the configuration file.
484 .It Fl l Ar login_name
485 Specifies the user to log in as on the remote machine.
486 This also may be specified on a per-host basis in the configuration file.
488 Additionally, for protocol version 2 a comma-separated list of MAC
489 (message authentication code) algorithms can
490 be specified in order of preference.
493 keyword for more information.
497 (actually, prevents reading from stdin).
498 This must be used when
500 is run in the background.
501 A common trick is to use this to run X11 programs on a remote machine.
503 .Ic ssh -n shadows.cs.hut.fi emacs &
504 will start an emacs on shadows.cs.hut.fi, and the X11
505 connection will be automatically forwarded over an encrypted channel.
508 program will be put in the background.
509 (This does not work if
511 needs to ask for a password or passphrase; see also the
515 Do not execute a remote command.
516 This is useful for just forwarding ports
517 (protocol version 2 only).
519 Can be used to give options in the format used in the configuration file.
520 This is useful for specifying options for which there is no separate
523 Port to connect to on the remote host.
524 This can be specified on a
525 per-host basis in the configuration file.
528 Causes all warning and diagnostic messages to be suppressed.
530 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
531 of SSH as a secure transport for other applications (eg. sftp). The
532 subsystem is specified as the remote command.
534 Force pseudo-tty allocation.
535 This can be used to execute arbitrary
536 screen-based programs on a remote machine, which can be very useful,
537 e.g., when implementing menu services.
540 options force tty allocation, even if
544 Disable pseudo-tty allocation.
549 to print debugging messages about its progress.
551 debugging connection, authentication, and configuration problems.
554 options increases the verbosity.
557 Disables X11 forwarding.
559 Enables X11 forwarding.
560 This can also be specified on a per-host basis in a configuration file.
562 Requests compression of all data (including stdin, stdout, stderr, and
563 data for forwarded X11 and TCP/IP connections).
564 The compression algorithm is the same used by
568 can be controlled by the
571 Compression is desirable on modem lines and other
572 slow connections, but will only slow down things on fast networks.
573 The default value can be set on a host-by-host basis in the
574 configuration files; see the
577 .It Fl F Ar configfile
578 Specifies an alternative per-user configuration file.
579 If a configuration file is given on the command line,
580 the system-wide configuration file
581 .Pq Pa /etc/ssh/ssh_config
583 The default for the per-user configuration file is
584 .Pa $HOME/.ssh/config .
585 .It Fl L Ar port:host:hostport
586 Specifies that the given port on the local (client) host is to be
587 forwarded to the given host and port on the remote side.
588 This works by allocating a socket to listen to
590 on the local side, and whenever a connection is made to this port, the
591 connection is forwarded over the secure channel, and a connection is
596 from the remote machine.
597 Port forwardings can also be specified in the configuration file.
598 Only root can forward privileged ports.
599 IPv6 addresses can be specified with an alternative syntax:
600 .Ar port/host/hostport
601 .It Fl R Ar port:host:hostport
602 Specifies that the given port on the remote (server) host is to be
603 forwarded to the given host and port on the local side.
604 This works by allocating a socket to listen to
606 on the remote side, and whenever a connection is made to this port, the
607 connection is forwarded over the secure channel, and a connection is
612 from the local machine.
613 Port forwardings can also be specified in the configuration file.
614 Privileged ports can be forwarded only when
615 logging in as root on the remote machine.
616 IPv6 addresses can be specified with an alternative syntax:
617 .Ar port/host/hostport
621 application-level port forwarding.
622 This works by allocating a socket to listen to
624 on the local side, and whenever a connection is made to this port, the
625 connection is forwarded over the secure channel, and the application
626 protocol is then used to determine where to connect to from the
627 remote machine. Currently the SOCKS4 protocol is supported, and
629 will act as a SOCKS4 server.
630 Only root can forward privileged ports.
631 Dynamic port forwardings can also be specified in the configuration file.
635 to try protocol version 1 only.
639 to try protocol version 2 only.
643 to use IPv4 addresses only.
647 to use IPv6 addresses only.
649 .Sh CONFIGURATION FILES
651 may additionally obtain configuration data from
652 a per-user configuration file and a system-wide configuration file.
653 The file format and configuration options are described in
657 will normally set the following environment variables:
662 variable indicates the location of the X11 server.
663 It is automatically set by
665 to point to a value of the form
667 where hostname indicates
668 the host where the shell runs, and n is an integer >= 1.
670 uses this special value to forward X11 connections over the secure
672 The user should normally not set
675 will render the X11 connection insecure (and will require the user to
676 manually copy any required authorization cookies).
678 Set to the path of the user's home directory.
682 set for compatibility with systems that use this variable.
684 Set to the path of the user's mailbox.
688 as specified when compiling
693 needs a passphrase, it will read the passphrase from the current
694 terminal if it was run from a terminal.
697 does not have a terminal associated with it but
701 are set, it will execute the program specified by
703 and open an X11 window to read the passphrase.
704 This is particularly useful when calling
709 (Note that on some machines it
710 may be necessary to redirect the input from
714 Identifies the path of a unix-domain socket used to communicate with the
717 Identifies the client end of the connection.
718 The variable contains
719 three space-separated values: client ip-address, client port number,
720 and server port number.
721 .It Ev SSH_ORIGINAL_COMMAND
722 The variable contains the original command line if a forced command
724 It can be used to extract the original arguments.
726 This is set to the name of the tty (path to the device) associated
727 with the current shell or command.
728 If the current session has no tty,
729 this variable is not set.
731 The timezone variable is set to indicate the present timezone if it
732 was set when the daemon was started (i.e., the daemon passes the value
733 on to new connections).
735 Set to the name of the user logging in.
741 .Pa $HOME/.ssh/environment ,
742 and adds lines of the format
744 to the environment if the file exists and if users are allowed to
745 change their environment.
747 .Cm PermitUserEnvironment
752 .It Pa $HOME/.ssh/known_hosts
753 Records host keys for all hosts the user has logged into that are not
755 .Pa /etc/ssh/ssh_known_hosts .
758 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
759 Contains the authentication identity of the user.
760 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
762 contain sensitive data and should be readable by the user but not
763 accessible by others (read/write/execute).
766 ignores a private key file if it is accessible by others.
767 It is possible to specify a passphrase when
768 generating the key; the passphrase will be used to encrypt the
769 sensitive part of this file using 3DES.
770 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
771 Contains the public key for authentication (public part of the
772 identity file in human-readable form).
774 .Pa $HOME/.ssh/identity.pub
775 file should be added to
776 .Pa $HOME/.ssh/authorized_keys
778 where the user wishes to log in using protocol version 1 RSA authentication.
780 .Pa $HOME/.ssh/id_dsa.pub
782 .Pa $HOME/.ssh/id_rsa.pub
783 file should be added to
784 .Pa $HOME/.ssh/authorized_keys
786 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
788 sensitive and can (but need not) be readable by anyone.
790 never used automatically and are not necessary; they are only provided for
791 the convenience of the user.
792 .It Pa $HOME/.ssh/config
793 This is the per-user configuration file.
794 The file format and configuration options are described in
796 .It Pa $HOME/.ssh/authorized_keys
797 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
798 The format of this file is described in the
801 In the simplest form the format is the same as the .pub
803 This file is not highly sensitive, but the recommended
804 permissions are read/write for the user, and not accessible by others.
805 .It Pa /etc/ssh/ssh_known_hosts
806 Systemwide list of known host keys.
807 This file should be prepared by the
808 system administrator to contain the public host keys of all machines in the
810 This file should be world-readable.
812 public keys, one per line, in the following format (fields separated
813 by spaces): system name, public key and optional comment field.
814 When different names are used
815 for the same machine, all such names should be listed, separated by
817 The format is described on the
821 The canonical system name (as returned by name servers) is used by
823 to verify the client host when logging in; other names are needed because
825 does not convert the user-supplied name to a canonical name before
826 checking the key, because someone with access to the name servers
827 would then be able to fool host authentication.
828 .It Pa /etc/ssh/ssh_config
829 Systemwide configuration file.
830 The file format and configuration options are described in
832 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
833 These three files contain the private parts of the host keys
835 .Cm RhostsRSAAuthentication
837 .Cm HostbasedAuthentication .
838 If the protocol version 1
839 .Cm RhostsRSAAuthentication
842 must be setuid root, since the host key is readable only by root.
843 For protocol version 2,
847 to access the host keys for
848 .Cm HostbasedAuthentication .
849 This eliminates the requirement that
851 be setuid root when that authentication method is used.
858 authentication to list the
859 host/user pairs that are permitted to log in.
860 (Note that this file is
861 also used by rlogin and rsh, which makes using this file insecure.)
862 Each line of the file contains a host name (in the canonical form
863 returned by name servers), and then a user name on that host,
864 separated by a space.
865 On some machines this file may need to be
866 world-readable if the user's home directory is on a NFS partition,
870 Additionally, this file must be owned by the user,
871 and must not have write permissions for anyone else.
873 permission for most machines is read/write for the user, and not
874 accessible by others.
878 will be installed so that it requires successful RSA host
879 authentication before permitting \s+2.\s0rhosts authentication.
880 If the server machine does not have the client's host key in
881 .Pa /etc/ssh/ssh_known_hosts ,
883 .Pa $HOME/.ssh/known_hosts .
884 The easiest way to do this is to
885 connect back to the client from the server machine using ssh; this
886 will automatically add the host key to
887 .Pa $HOME/.ssh/known_hosts .
889 This file is used exactly the same way as
892 having this file is to be able to use rhosts authentication with
894 without permitting login with
898 .It Pa /etc/hosts.equiv
899 This file is used during
900 .Pa \&.rhosts authentication.
902 canonical hosts names, one per line (the full format is described on
906 If the client host is found in this file, login is
907 automatically permitted provided client and server user names are the
909 Additionally, successful RSA host authentication is normally
911 This file should only be writable by root.
912 .It Pa /etc/shosts.equiv
913 This file is processed exactly as
914 .Pa /etc/hosts.equiv .
915 This file may be useful to permit logins using
917 but not using rsh/rlogin.
918 .It Pa /etc/ssh/sshrc
919 Commands in this file are executed by
921 when the user logs in just before the user's shell (or command) is started.
924 manual page for more information.
926 Commands in this file are executed by
928 when the user logs in just before the user's shell (or command) is
932 manual page for more information.
933 .It Pa $HOME/.ssh/environment
934 Contains additional definitions for environment variables, see section
940 exits with the exit status of the remote command or with 255
941 if an error occurred.
943 OpenSSH is a derivative of the original and free
944 ssh 1.2.12 release by Tatu Ylonen.
945 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
946 Theo de Raadt and Dug Song
947 removed many bugs, re-added newer features and
949 Markus Friedl contributed the support for SSH
950 protocol versions 1.5 and 2.0.
968 .%T "SSH Protocol Architecture"
969 .%N draft-ietf-secsh-architecture-12.txt
971 .%O work in progress material
andersk / openssh.git / blob