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 1.157 2002/06/19 00:27:55 deraadt 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 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).
356 If the user is using an authentication agent, the connection to the agent
357 is automatically forwarded to the remote side unless disabled on
358 the command line or in a configuration file.
360 Forwarding of arbitrary TCP/IP connections over the secure channel can
361 be specified either on the command line or in a configuration file.
362 One possible application of TCP/IP forwarding is a secure connection to an
363 electronic purse; another is going through firewalls.
365 .Ss Server authentication
368 automatically maintains and checks a database containing
369 identifications for all hosts it has ever been used with.
370 Host keys are stored in
371 .Pa $HOME/.ssh/known_hosts
372 in the user's home directory.
373 Additionally, the file
374 .Pa /etc/ssh/ssh_known_hosts
375 is automatically checked for known hosts.
376 Any new hosts are automatically added to the user's file.
377 If a host's identification
380 warns about this and disables password authentication to prevent a
381 trojan horse from getting the user's password.
383 this mechanism is to prevent man-in-the-middle attacks which could
384 otherwise be used to circumvent the encryption.
386 .Cm StrictHostKeyChecking
387 option (see below) can be used to prevent logins to machines whose
388 host key is not known or has changed.
390 The options are as follows:
393 Disables forwarding of the authentication agent connection.
395 Enables forwarding of the authentication agent connection.
396 This can also be specified on a per-host basis in a configuration file.
397 .It Fl b Ar bind_address
398 Specify the interface to transmit from on machines with multiple
399 interfaces or aliased addresses.
400 .It Fl c Ar blowfish|3des|des
401 Selects the cipher to use for encrypting the session.
404 It is believed to be secure.
406 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
408 is a fast block cipher, it appears very secure and is much faster than
411 is only supported in the
413 client for interoperability with legacy protocol 1 implementations
414 that do not support the
416 cipher. Its use is strongly discouraged due to cryptographic
418 .It Fl c Ar cipher_spec
419 Additionally, for protocol version 2 a comma-separated list of ciphers can
420 be specified in order of preference.
423 for more information.
424 .It Fl e Ar ch|^ch|none
425 Sets the escape character for sessions with a pty (default:
427 The escape character is only recognized at the beginning of a line.
428 The escape character followed by a dot
430 closes the connection, followed
431 by control-Z suspends the connection, and followed by itself sends the
432 escape character once.
433 Setting the character to
435 disables any escapes and makes the session fully transparent.
439 to go to background just before command execution.
442 is going to ask for passwords or passphrases, but the user
443 wants it in the background.
446 The recommended way to start X11 programs at a remote site is with
448 .Ic ssh -f host xterm .
450 Allows remote hosts to connect to local forwarded ports.
451 .It Fl i Ar identity_file
452 Selects a file from which the identity (private key) for
453 RSA or DSA authentication is read.
455 .Pa $HOME/.ssh/identity
456 for protocol version 1, and
457 .Pa $HOME/.ssh/id_rsa
459 .Pa $HOME/.ssh/id_dsa
460 for protocol version 2.
461 Identity files may also be specified on
462 a per-host basis in the configuration file.
463 It is possible to have multiple
465 options (and multiple identities specified in
466 configuration files).
467 .It Fl I Ar smartcard_device
468 Specifies which smartcard device to use. The argument is
471 should use to communicate with a smartcard used for storing the user's
474 Disables forwarding of Kerberos tickets and AFS tokens.
475 This may also be specified on a per-host basis in the configuration file.
476 .It Fl l Ar login_name
477 Specifies the user to log in as on the remote machine.
478 This also may be specified on a per-host basis in the configuration file.
480 Additionally, for protocol version 2 a comma-separated list of MAC
481 (message authentication code) algorithms can
482 be specified in order of preference.
485 keyword for more information.
489 (actually, prevents reading from stdin).
490 This must be used when
492 is run in the background.
493 A common trick is to use this to run X11 programs on a remote machine.
495 .Ic ssh -n shadows.cs.hut.fi emacs &
496 will start an emacs on shadows.cs.hut.fi, and the X11
497 connection will be automatically forwarded over an encrypted channel.
500 program will be put in the background.
501 (This does not work if
503 needs to ask for a password or passphrase; see also the
507 Do not execute a remote command.
508 This is useful for just forwarding ports
509 (protocol version 2 only).
511 Can be used to give options in the format used in the configuration file.
512 This is useful for specifying options for which there is no separate
515 Port to connect to on the remote host.
516 This can be specified on a
517 per-host basis in the configuration file.
519 Use a non-privileged port for outgoing connections.
520 This can be used if a firewall does
521 not permit connections from privileged ports.
522 Note that this option turns off
523 .Cm RhostsAuthentication
525 .Cm RhostsRSAAuthentication
529 Causes all warning and diagnostic messages to be suppressed.
531 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
532 of SSH as a secure transport for other applications (eg. sftp). The
533 subsystem is specified as the remote command.
535 Force pseudo-tty allocation.
536 This can be used to execute arbitrary
537 screen-based programs on a remote machine, which can be very useful,
538 e.g., when implementing menu services.
541 options force tty allocation, even if
545 Disable pseudo-tty allocation.
550 to print debugging messages about its progress.
552 debugging connection, authentication, and configuration problems.
555 options increases the verbosity.
558 Disables X11 forwarding.
560 Enables X11 forwarding.
561 This can also be specified on a per-host basis in a configuration file.
563 Requests compression of all data (including stdin, stdout, stderr, and
564 data for forwarded X11 and TCP/IP connections).
565 The compression algorithm is the same used by
569 can be controlled by the
572 Compression is desirable on modem lines and other
573 slow connections, but will only slow down things on fast networks.
574 The default value can be set on a host-by-host basis in the
575 configuration files; see the
578 .It Fl F Ar configfile
579 Specifies an alternative per-user configuration file.
580 If a configuration file is given on the command line,
581 the system-wide configuration file
582 .Pq Pa /etc/ssh/ssh_config
584 The default for the per-user configuration file is
585 .Pa $HOME/.ssh/config .
586 .It Fl L Ar port:host:hostport
587 Specifies that the given port on the local (client) host is to be
588 forwarded to the given host and port on the remote side.
589 This works by allocating a socket to listen to
591 on the local side, and whenever a connection is made to this port, the
592 connection is forwarded over the secure channel, and a connection is
597 from the remote machine.
598 Port forwardings can also be specified in the configuration file.
599 Only root can forward privileged ports.
600 IPv6 addresses can be specified with an alternative syntax:
601 .Ar port/host/hostport
602 .It Fl R Ar port:host:hostport
603 Specifies that the given port on the remote (server) host is to be
604 forwarded to the given host and port on the local side.
605 This works by allocating a socket to listen to
607 on the remote side, and whenever a connection is made to this port, the
608 connection is forwarded over the secure channel, and a connection is
613 from the local machine.
614 Port forwardings can also be specified in the configuration file.
615 Privileged ports can be forwarded only when
616 logging in as root on the remote machine.
617 IPv6 addresses can be specified with an alternative syntax:
618 .Ar port/host/hostport
622 application-level port forwarding.
623 This works by allocating a socket to listen to
625 on the local side, and whenever a connection is made to this port, the
626 connection is forwarded over the secure channel, and the application
627 protocol is then used to determine where to connect to from the
628 remote machine. Currently the SOCKS4 protocol is supported, and
630 will act as a SOCKS4 server.
631 Only root can forward privileged ports.
632 Dynamic port forwardings can also be specified in the configuration file.
636 to try protocol version 1 only.
640 to try protocol version 2 only.
644 to use IPv4 addresses only.
648 to use IPv6 addresses only.
650 .Sh CONFIGURATION FILES
652 obtains configuration data from the following sources in
654 command line options, user's configuration file
655 .Pq Pa $HOME/.ssh/config ,
656 and system-wide configuration file
657 .Pq Pa /etc/ssh/ssh_config .
658 For each parameter, the first obtained value
660 The configuration files contain sections bracketed by
662 specifications, and that section is only applied for hosts that
663 match one of the patterns given in the specification.
664 The matched host name is the one given on the command line.
666 Since the first obtained value for each parameter is used, more
667 host-specific declarations should be given near the beginning of the
668 file, and general defaults at the end.
670 The configuration file has the following format:
672 Empty lines and lines starting with
676 Otherwise a line is of the format
677 .Dq keyword arguments .
678 Configuration options may be separated by whitespace or
679 optional whitespace and exactly one
681 the latter format is useful to avoid the need to quote whitespace
682 when specifying configuration options using the
691 keywords and their meanings are as follows (note that
692 keywords are case-insensitive and arguments are case-sensitive):
695 Restricts the following declarations (up to the next
697 keyword) to be only for those hosts that match one of the patterns
698 given after the keyword.
702 can be used as wildcards in the
706 as a pattern can be used to provide global
707 defaults for all hosts.
710 argument given on the command line (i.e., the name is not converted to
711 a canonicalized host name before matching).
712 .It Cm AFSTokenPassing
713 Specifies whether to pass AFS tokens to remote host.
714 The argument to this keyword must be
718 This option applies to protocol version 1 only.
722 passphrase/password querying will be disabled.
723 This option is useful in scripts and other batch jobs where no user
724 is present to supply the password.
732 Specify the interface to transmit from on machines with multiple
733 interfaces or aliased addresses.
734 Note that this option does not work if
735 .Cm UsePrivilegedPort
738 .It Cm ChallengeResponseAuthentication
739 Specifies whether to use challenge response authentication.
740 The argument to this keyword must be
747 If this flag is set to
749 ssh will additionally check the host IP address in the
752 This allows ssh to detect if a host key changed due to DNS spoofing.
753 If the option is set to
755 the check will not be executed.
759 Specifies the cipher to use for encrypting the session
760 in protocol version 1.
768 is only supported in the
770 client for interoperability with legacy protocol 1 implementations
771 that do not support the
773 cipher. Its use is strongly discouraged due to cryptographic
778 Specifies the ciphers allowed for protocol version 2
779 in order of preference.
780 Multiple ciphers must be comma-separated.
784 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
785 aes192-cbc,aes256-cbc''
787 .It Cm ClearAllForwardings
788 Specifies that all local, remote and dynamic port forwardings
789 specified in the configuration files or on the command line be
790 cleared. This option is primarily useful when used from the
792 command line to clear port forwardings set in
793 configuration files, and is automatically set by
804 Specifies whether to use compression.
811 .It Cm CompressionLevel
812 Specifies the compression level to use if compression is enabled.
813 The argument must be an integer from 1 (fast) to 9 (slow, best).
814 The default level is 6, which is good for most applications.
815 The meaning of the values is the same as in
817 Note that this option applies to protocol version 1 only.
818 .It Cm ConnectionAttempts
819 Specifies the number of tries (one per second) to make before exiting.
820 The argument must be an integer.
821 This may be useful in scripts if the connection sometimes fails.
823 .It Cm DynamicForward
824 Specifies that a TCP/IP port on the local machine be forwarded
825 over the secure channel, and the application
826 protocol is then used to determine where to connect to from the
827 remote machine. The argument must be a port number.
828 Currently the SOCKS4 protocol is supported, and
830 will act as a SOCKS4 server.
831 Multiple forwardings may be specified, and
832 additional forwardings can be given on the command line. Only
833 the superuser can forward privileged ports.
835 Sets the escape character (default:
837 The escape character can also
838 be set on the command line.
839 The argument should be a single character,
841 followed by a letter, or
843 to disable the escape
844 character entirely (making the connection transparent for binary
847 Specifies whether the connection to the authentication agent (if any)
848 will be forwarded to the remote machine.
856 Specifies whether X11 connections will be automatically redirected
857 over the secure channel and
867 Specifies whether remote hosts are allowed to connect to local
871 binds local port forwardings to the loopback address. This
872 prevents other remote hosts from connecting to forwarded ports.
874 can be used to specify that
876 should bind local port forwardings to the wildcard address,
877 thus allowing remote hosts to connect to forwarded ports.
884 .It Cm GlobalKnownHostsFile
885 Specifies a file to use for the global
886 host key database instead of
887 .Pa /etc/ssh/ssh_known_hosts .
888 .It Cm HostbasedAuthentication
889 Specifies whether to try rhosts based authentication with public key
897 This option applies to protocol version 2 only and
899 .Cm RhostsRSAAuthentication .
900 .It Cm HostKeyAlgorithms
901 Specifies the protocol version 2 host key algorithms
902 that the client wants to use in order of preference.
903 The default for this option is:
904 .Dq ssh-rsa,ssh-dss .
906 Specifies an alias that should be used instead of the
907 real host name when looking up or saving the host key
908 in the host key database files.
909 This option is useful for tunneling ssh connections
910 or for multiple servers running on a single host.
912 Specifies the real host name to log into.
913 This can be used to specify nicknames or abbreviations for hosts.
914 Default is the name given on the command line.
915 Numeric IP addresses are also permitted (both on the command line and in
919 Specifies a file from which the user's RSA or DSA authentication identity
920 is read. The default is
921 .Pa $HOME/.ssh/identity
922 for protocol version 1, and
923 .Pa $HOME/.ssh/id_rsa
925 .Pa $HOME/.ssh/id_dsa
926 for protocol version 2.
927 Additionally, any identities represented by the authentication agent
928 will be used for authentication.
929 The file name may use the tilde
930 syntax to refer to a user's home directory.
931 It is possible to have
932 multiple identity files specified in configuration files; all these
933 identities will be tried in sequence.
935 Specifies whether the system should send TCP keepalive messages to the
937 If they are sent, death of the connection or crash of one
938 of the machines will be properly noticed.
939 However, this means that
940 connections will die if the route is down temporarily, and some people
945 (to send keepalives), and the client will notice
946 if the network goes down or the remote host dies.
947 This is important in scripts, and many users want it too.
949 To disable keepalives, the value should be set to
951 .It Cm KerberosAuthentication
952 Specifies whether Kerberos authentication will be used.
953 The argument to this keyword must be
957 .It Cm KerberosTgtPassing
958 Specifies whether a Kerberos TGT will be forwarded to the server.
959 This will only work if the Kerberos server is actually an AFS kaserver.
960 The argument to this keyword must be
965 Specifies that a TCP/IP port on the local machine be forwarded over
966 the secure channel to the specified host and port from the remote machine.
967 The first argument must be a port number, and the second must be
969 IPv6 addresses can be specified with an alternative syntax:
971 Multiple forwardings may be specified, and additional
972 forwardings can be given on the command line.
973 Only the superuser can forward privileged ports.
975 Gives the verbosity level that is used when logging messages from
977 The possible values are:
978 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
979 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
980 and DEBUG3 each specify higher levels of verbose output.
982 Specifies the MAC (message authentication code) algorithms
983 in order of preference.
984 The MAC algorithm is used in protocol version 2
985 for data integrity protection.
986 Multiple algorithms must be comma-separated.
988 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
989 .It Cm NoHostAuthenticationForLocalhost
990 This option can be used if the home directory is shared across machines.
991 In this case localhost will refer to a different machine on each of
992 the machines and the user will get many warnings about changed host keys.
993 However, this option disables host authentication for localhost.
994 The argument to this keyword must be
998 The default is to check the host key for localhost.
999 .It Cm NumberOfPasswordPrompts
1000 Specifies the number of password prompts before giving up.
1001 The argument to this keyword must be an integer.
1003 .It Cm PasswordAuthentication
1004 Specifies whether to use password authentication.
1005 The argument to this keyword must be
1012 Specifies the port number to connect on the remote host.
1014 .It Cm PreferredAuthentications
1015 Specifies the order in which the client should try protocol 2
1016 authentication methods. This allows a client to prefer one method (e.g.
1017 .Cm keyboard-interactive )
1018 over another method (e.g.
1020 The default for this option is:
1021 .Dq hostbased,publickey,keyboard-interactive,password .
1023 Specifies the protocol versions
1025 should support in order of preference.
1026 The possible values are
1030 Multiple versions must be comma-separated.
1035 tries version 2 and falls back to version 1
1036 if version 2 is not available.
1038 Specifies the command to use to connect to the server.
1040 string extends to the end of the line, and is executed with
1042 In the command string,
1044 will be substituted by the host name to
1048 The command can be basically anything,
1049 and should read from its standard input and write to its standard output.
1050 It should eventually connect an
1052 server running on some machine, or execute
1055 Host key management will be done using the
1056 HostName of the host being connected (defaulting to the name typed by
1060 is not available for connects with a proxy command.
1062 .It Cm PubkeyAuthentication
1063 Specifies whether to try public key authentication.
1064 The argument to this keyword must be
1070 This option applies to protocol version 2 only.
1071 .It Cm RemoteForward
1072 Specifies that a TCP/IP port on the remote machine be forwarded over
1073 the secure channel to the specified host and port from the local machine.
1074 The first argument must be a port number, and the second must be
1076 IPv6 addresses can be specified with an alternative syntax:
1078 Multiple forwardings may be specified, and additional
1079 forwardings can be given on the command line.
1080 Only the superuser can forward privileged ports.
1081 .It Cm RhostsAuthentication
1082 Specifies whether to try rhosts based authentication.
1084 declaration only affects the client side and has no effect whatsoever
1086 Most servers do not permit RhostsAuthentication because it
1088 .Cm RhostsRSAAuthentication ) .
1089 The argument to this keyword must be
1095 This option applies to protocol version 1 only.
1096 .It Cm RhostsRSAAuthentication
1097 Specifies whether to try rhosts based authentication with RSA host
1099 The argument must be
1105 This option applies to protocol version 1 only and requires
1108 .It Cm RSAAuthentication
1109 Specifies whether to try RSA authentication.
1110 The argument to this keyword must be
1114 RSA authentication will only be
1115 attempted if the identity file exists, or an authentication agent is
1119 Note that this option applies to protocol version 1 only.
1120 .It Cm SmartcardDevice
1121 Specifies which smartcard device to use. The argument to this keyword is
1124 should use to communicate with a smartcard used for storing the user's
1125 private RSA key. By default, no device is specified and smartcard support
1127 .It Cm StrictHostKeyChecking
1128 If this flag is set to
1131 will never automatically add host keys to the
1132 .Pa $HOME/.ssh/known_hosts
1133 file, and refuses to connect to hosts whose host key has changed.
1134 This provides maximum protection against trojan horse attacks,
1135 however, can be annoying when the
1136 .Pa /etc/ssh/ssh_known_hosts
1137 file is poorly maintained, or connections to new hosts are
1139 This option forces the user to manually
1141 If this flag is set to
1144 will automatically add new host keys to the
1145 user known hosts files.
1146 If this flag is set to
1149 will be added to the user known host files only after the user
1150 has confirmed that is what they really want to do, and
1152 will refuse to connect to hosts whose host key has changed.
1154 known hosts will be verified automatically in all cases.
1155 The argument must be
1162 .It Cm UsePrivilegedPort
1163 Specifies whether to use a privileged port for outgoing connections.
1164 The argument must be
1170 Note that this option must be set to
1173 .Cm RhostsAuthentication
1175 .Cm RhostsRSAAuthentication
1176 authentications are needed with older servers.
1178 Specifies the user to log in as.
1179 This can be useful when a different user name is used on different machines.
1180 This saves the trouble of
1181 having to remember to give the user name on the command line.
1182 .It Cm UserKnownHostsFile
1183 Specifies a file to use for the user
1184 host key database instead of
1185 .Pa $HOME/.ssh/known_hosts .
1186 .It Cm XAuthLocation
1187 Specifies the location of the
1191 .Pa /usr/X11R6/bin/xauth .
1195 will normally set the following environment variables:
1200 variable indicates the location of the X11 server.
1201 It is automatically set by
1203 to point to a value of the form
1205 where hostname indicates
1206 the host where the shell runs, and n is an integer >= 1.
1208 uses this special value to forward X11 connections over the secure
1210 The user should normally not set
1213 will render the X11 connection insecure (and will require the user to
1214 manually copy any required authorization cookies).
1216 Set to the path of the user's home directory.
1220 set for compatibility with systems that use this variable.
1222 Set to the path of the user's mailbox.
1226 as specified when compiling
1231 needs a passphrase, it will read the passphrase from the current
1232 terminal if it was run from a terminal.
1235 does not have a terminal associated with it but
1239 are set, it will execute the program specified by
1241 and open an X11 window to read the passphrase.
1242 This is particularly useful when calling
1247 (Note that on some machines it
1248 may be necessary to redirect the input from
1251 .It Ev SSH_AUTH_SOCK
1252 Identifies the path of a unix-domain socket used to communicate with the
1255 Identifies the client end of the connection.
1256 The variable contains
1257 three space-separated values: client ip-address, client port number,
1258 and server port number.
1259 .It Ev SSH_ORIGINAL_COMMAND
1260 The variable contains the original command line if a forced command
1262 It can be used to extract the original arguments.
1264 This is set to the name of the tty (path to the device) associated
1265 with the current shell or command.
1266 If the current session has no tty,
1267 this variable is not set.
1269 The timezone variable is set to indicate the present timezone if it
1270 was set when the daemon was started (i.e., the daemon passes the value
1271 on to new connections).
1273 Set to the name of the user logging in.
1279 .Pa $HOME/.ssh/environment ,
1280 and adds lines of the format
1285 .It Pa $HOME/.ssh/known_hosts
1286 Records host keys for all hosts the user has logged into that are not
1288 .Pa /etc/ssh/ssh_known_hosts .
1291 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1292 Contains the authentication identity of the user.
1293 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1295 contain sensitive data and should be readable by the user but not
1296 accessible by others (read/write/execute).
1299 ignores a private key file if it is accessible by others.
1300 It is possible to specify a passphrase when
1301 generating the key; the passphrase will be used to encrypt the
1302 sensitive part of this file using 3DES.
1303 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1304 Contains the public key for authentication (public part of the
1305 identity file in human-readable form).
1307 .Pa $HOME/.ssh/identity.pub
1308 file should be added to
1309 .Pa $HOME/.ssh/authorized_keys
1311 where the user wishes to log in using protocol version 1 RSA authentication.
1313 .Pa $HOME/.ssh/id_dsa.pub
1315 .Pa $HOME/.ssh/id_rsa.pub
1316 file should be added to
1317 .Pa $HOME/.ssh/authorized_keys
1319 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1321 sensitive and can (but need not) be readable by anyone.
1323 never used automatically and are not necessary; they are only provided for
1324 the convenience of the user.
1325 .It Pa $HOME/.ssh/config
1326 This is the per-user configuration file.
1327 The format of this file is described above.
1328 This file is used by the
1331 This file does not usually contain any sensitive information,
1332 but the recommended permissions are read/write for the user, and not
1333 accessible by others.
1334 .It Pa $HOME/.ssh/authorized_keys
1335 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1336 The format of this file is described in the
1339 In the simplest form the format is the same as the .pub
1341 This file is not highly sensitive, but the recommended
1342 permissions are read/write for the user, and not accessible by others.
1343 .It Pa /etc/ssh/ssh_known_hosts
1344 Systemwide list of known host keys.
1345 This file should be prepared by the
1346 system administrator to contain the public host keys of all machines in the
1348 This file should be world-readable.
1350 public keys, one per line, in the following format (fields separated
1351 by spaces): system name, public key and optional comment field.
1352 When different names are used
1353 for the same machine, all such names should be listed, separated by
1355 The format is described on the
1359 The canonical system name (as returned by name servers) is used by
1361 to verify the client host when logging in; other names are needed because
1363 does not convert the user-supplied name to a canonical name before
1364 checking the key, because someone with access to the name servers
1365 would then be able to fool host authentication.
1366 .It Pa /etc/ssh/ssh_config
1367 Systemwide configuration file.
1368 This file provides defaults for those
1369 values that are not specified in the user's configuration file, and
1370 for those users who do not have a configuration file.
1371 This file must be world-readable.
1372 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1373 These three files contain the private parts of the host keys
1375 .Cm RhostsRSAAuthentication
1377 .Cm HostbasedAuthentication .
1378 If the protocol version 1
1379 .Cm RhostsRSAAuthentication
1382 must be setuid root, since the host key is readable only by root.
1383 For protocol version 2,
1387 to access the host keys for
1388 .Cm HostbasedAuthentication .
1389 This eliminates the requirement that
1391 be setuid root when that authentication method is used.
1395 .It Pa $HOME/.rhosts
1396 This file is used in
1398 authentication to list the
1399 host/user pairs that are permitted to log in.
1400 (Note that this file is
1401 also used by rlogin and rsh, which makes using this file insecure.)
1402 Each line of the file contains a host name (in the canonical form
1403 returned by name servers), and then a user name on that host,
1404 separated by a space.
1405 On some machines this file may need to be
1406 world-readable if the user's home directory is on a NFS partition,
1410 Additionally, this file must be owned by the user,
1411 and must not have write permissions for anyone else.
1413 permission for most machines is read/write for the user, and not
1414 accessible by others.
1416 Note that by default
1418 will be installed so that it requires successful RSA host
1419 authentication before permitting \s+2.\s0rhosts authentication.
1420 If the server machine does not have the client's host key in
1421 .Pa /etc/ssh/ssh_known_hosts ,
1423 .Pa $HOME/.ssh/known_hosts .
1424 The easiest way to do this is to
1425 connect back to the client from the server machine using ssh; this
1426 will automatically add the host key to
1427 .Pa $HOME/.ssh/known_hosts .
1428 .It Pa $HOME/.shosts
1429 This file is used exactly the same way as
1432 having this file is to be able to use rhosts authentication with
1434 without permitting login with
1438 .It Pa /etc/hosts.equiv
1439 This file is used during
1440 .Pa \&.rhosts authentication.
1442 canonical hosts names, one per line (the full format is described on
1446 If the client host is found in this file, login is
1447 automatically permitted provided client and server user names are the
1449 Additionally, successful RSA host authentication is normally
1451 This file should only be writable by root.
1452 .It Pa /etc/shosts.equiv
1453 This file is processed exactly as
1454 .Pa /etc/hosts.equiv .
1455 This file may be useful to permit logins using
1457 but not using rsh/rlogin.
1458 .It Pa /etc/ssh/sshrc
1459 Commands in this file are executed by
1461 when the user logs in just before the user's shell (or command) is started.
1464 manual page for more information.
1465 .It Pa $HOME/.ssh/rc
1466 Commands in this file are executed by
1468 when the user logs in just before the user's shell (or command) is
1472 manual page for more information.
1473 .It Pa $HOME/.ssh/environment
1474 Contains additional definitions for environment variables, see section
1480 exits with the exit status of the remote command or with 255
1481 if an error occurred.
1483 OpenSSH is a derivative of the original and free
1484 ssh 1.2.12 release by Tatu Ylonen.
1485 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1486 Theo de Raadt and Dug Song
1487 removed many bugs, re-added newer features and
1489 Markus Friedl contributed the support for SSH
1490 protocol versions 1.5 and 2.0.
1507 .%T "SSH Protocol Architecture"
1508 .%N draft-ietf-secsh-architecture-12.txt
1510 .%O work in progress material
andersk / openssh.git / blob