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 1.151 2002/05/06 23:34:33 millert 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,
655 feature-specific user configuration file(s) (see below),
656 user's configuration file
657 .Pq Pa $HOME/.ssh/config ,
658 and system-wide configuration file
659 .Pq Pa /etc/ssh/ssh_config .
660 For compatibility with other
662 versions, the following feature-specific user configuration files
663 will be processed after the command line options but before the user's
664 main configuration file, so options that other
666 versions may not support don't need to go in the main configuration file:
668 .It Pa $HOME/.ssh/config.gssapi
669 Read if GSSAPI authentication is supported. This is a good place for
670 the GssapiAuthentication and GssapiDelegateCredentials options.
671 .It Pa $HOME/.ssh/config.krb
672 Read if Kerberos authentication is supported. This is a good place
673 for the KerberosAuthentication and KerberosTgtPassing options.
674 .It Pa $HOME/.ssh/config.afs
675 Read if AFS token passing is supported. This is a good place for the
676 AfsTokenPassing option.
679 For each parameter, the first obtained value
681 The configuration files contain sections bracketed by
683 specifications, and that section is only applied for hosts that
684 match one of the patterns given in the specification.
685 The matched host name is the one given on the command line.
687 Since the first obtained value for each parameter is used, more
688 host-specific declarations should be given near the beginning of the
689 file, and general defaults at the end.
691 The configuration file has the following format:
693 Empty lines and lines starting with
697 Otherwise a line is of the format
698 .Dq keyword arguments .
699 Configuration options may be separated by whitespace or
700 optional whitespace and exactly one
702 the latter format is useful to avoid the need to quote whitespace
703 when specifying configuration options using the
712 keywords and their meanings are as follows (note that
713 keywords are case-insensitive and arguments are case-sensitive):
716 Restricts the following declarations (up to the next
718 keyword) to be only for those hosts that match one of the patterns
719 given after the keyword.
723 can be used as wildcards in the
727 as a pattern can be used to provide global
728 defaults for all hosts.
731 argument given on the command line (i.e., the name is not converted to
732 a canonicalized host name before matching).
733 .It Cm AFSTokenPassing
734 Specifies whether to pass AFS tokens to remote host.
735 The argument to this keyword must be
739 This option applies to protocol version 1 only.
743 passphrase/password querying will be disabled.
744 This option is useful in scripts and other batch jobs where no user
745 is present to supply the password.
753 Specify the interface to transmit from on machines with multiple
754 interfaces or aliased addresses.
755 Note that this option does not work if
756 .Cm UsePrivilegedPort
760 If this flag is set to
762 ssh will additionally check the host IP address in the
765 This allows ssh to detect if a host key changed due to DNS spoofing.
766 If the option is set to
768 the check will not be executed.
772 Specifies the cipher to use for encrypting the session
773 in protocol version 1.
781 is only supported in the
783 client for interoperability with legacy protocol 1 implementations
784 that do not support the
786 cipher. Its use is strongly discouraged due to cryptographic
791 Specifies the ciphers allowed for protocol version 2
792 in order of preference.
793 Multiple ciphers must be comma-separated.
797 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
798 aes192-cbc,aes256-cbc''
800 .It Cm ClearAllForwardings
801 Specifies that all local, remote and dynamic port forwardings
802 specified in the configuration files or on the command line be
803 cleared. This option is primarily useful when used from the
805 command line to clear port forwardings set in
806 configuration files, and is automatically set by
817 Specifies whether to use compression.
824 .It Cm CompressionLevel
825 Specifies the compression level to use if compression is enabled.
826 The argument must be an integer from 1 (fast) to 9 (slow, best).
827 The default level is 6, which is good for most applications.
828 The meaning of the values is the same as in
830 Note that this option applies to protocol version 1 only.
831 .It Cm ConnectionAttempts
832 Specifies the number of tries (one per second) to make before falling
833 back to rsh or exiting.
834 The argument must be an integer.
835 This may be useful in scripts if the connection sometimes fails.
837 .It Cm DynamicForward
838 Specifies that a TCP/IP port on the local machine be forwarded
839 over the secure channel, and the application
840 protocol is then used to determine where to connect to from the
841 remote machine. The argument must be a port number.
842 Currently the SOCKS4 protocol is supported, and
844 will act as a SOCKS4 server.
845 Multiple forwardings may be specified, and
846 additional forwardings can be given on the command line. Only
847 the superuser can forward privileged ports.
849 Sets the escape character (default:
851 The escape character can also
852 be set on the command line.
853 The argument should be a single character,
855 followed by a letter, or
857 to disable the escape
858 character entirely (making the connection transparent for binary
861 Specifies that if connecting via
863 fails due to a connection refused error (there is no
865 listening on the remote host),
867 should automatically be used instead (after a suitable warning about
868 the session being unencrypted).
876 Specifies whether the connection to the authentication agent (if any)
877 will be forwarded to the remote machine.
885 Specifies whether X11 connections will be automatically redirected
886 over the secure channel and
896 Specifies whether remote hosts are allowed to connect to local
900 binds local port forwardings to the loopback addresss. This
901 prevents other remote hosts from connecting to forwarded ports.
903 can be used to specify that
905 should bind local port forwardings to the wildcard address,
906 thus allowing remote hosts to connect to forwarded ports.
913 .It Cm GlobalKnownHostsFile
914 Specifies a file to use for the global
915 host key database instead of
916 .Pa /etc/ssh/ssh_known_hosts .
917 .It Cm GssapiAuthentication
918 Specifies whether authentication based on GSSAPI may be used, either using
919 the result of a successful key exchange, or using GSSAPI user
923 .It Cm GssapiDelegateCredentials
924 Specifies whether GSSAPI credentials will be delegated (forwarded) to
928 .It Cm HostbasedAuthentication
929 Specifies whether to try rhosts based authentication with public key
937 This option applies to protocol version 2 only and
939 .Cm RhostsRSAAuthentication .
940 .It Cm HostKeyAlgorithms
941 Specifies the protocol version 2 host key algorithms
942 that the client wants to use in order of preference.
943 The default for this option is:
944 .Dq ssh-rsa,ssh-dss .
946 Specifies an alias that should be used instead of the
947 real host name when looking up or saving the host key
948 in the host key database files.
949 This option is useful for tunneling ssh connections
950 or for multiple servers running on a single host.
952 Specifies the real host name to log into.
953 This can be used to specify nicknames or abbreviations for hosts.
954 Default is the name given on the command line.
955 Numeric IP addresses are also permitted (both on the command line and in
959 Specifies a file from which the user's RSA or DSA authentication identity
960 is read. The default is
961 .Pa $HOME/.ssh/identity
962 for protocol version 1, and
963 .Pa $HOME/.ssh/id_rsa
965 .Pa $HOME/.ssh/id_dsa
966 for protocol version 2.
967 Additionally, any identities represented by the authentication agent
968 will be used for authentication.
969 The file name may use the tilde
970 syntax to refer to a user's home directory.
971 It is possible to have
972 multiple identity files specified in configuration files; all these
973 identities will be tried in sequence.
975 Specifies whether the system should send TCP keepalive messages to the
977 If they are sent, death of the connection or crash of one
978 of the machines will be properly noticed.
979 However, this means that
980 connections will die if the route is down temporarily, and some people
985 (to send keepalives), and the client will notice
986 if the network goes down or the remote host dies.
987 This is important in scripts, and many users want it too.
989 To disable keepalives, the value should be set to
991 .It Cm KerberosAuthentication
992 Specifies whether Kerberos authentication will be used.
993 The argument to this keyword must be
997 .It Cm KerberosTgtPassing
998 Specifies whether a Kerberos TGT will be forwarded to the server.
999 This will only work if the Kerberos server is actually an AFS kaserver.
1000 The argument to this keyword must be
1005 Specifies that a TCP/IP port on the local machine be forwarded over
1006 the secure channel to the specified host and port from the remote machine.
1007 The first argument must be a port number, and the second must be
1009 IPv6 addresses can be specified with an alternative syntax:
1011 Multiple forwardings may be specified, and additional
1012 forwardings can be given on the command line.
1013 Only the superuser can forward privileged ports.
1015 Gives the verbosity level that is used when logging messages from
1017 The possible values are:
1018 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1019 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
1020 and DEBUG3 each specify higher levels of verbose output.
1022 Specifies the MAC (message authentication code) algorithms
1023 in order of preference.
1024 The MAC algorithm is used in protocol version 2
1025 for data integrity protection.
1026 Multiple algorithms must be comma-separated.
1028 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1029 .It Cm NoHostAuthenticationForLocalhost
1030 This option can be used if the home directory is shared across machines.
1031 In this case localhost will refer to a different machine on each of
1032 the machines and the user will get many warnings about changed host keys.
1033 However, this option disables host authentication for localhost.
1034 The argument to this keyword must be
1038 The default is to check the host key for localhost.
1039 .It Cm NumberOfPasswordPrompts
1040 Specifies the number of password prompts before giving up.
1041 The argument to this keyword must be an integer.
1043 .It Cm PasswordAuthentication
1044 Specifies whether to use password authentication.
1045 The argument to this keyword must be
1052 Specifies the port number to connect on the remote host.
1054 .It Cm PreferredAuthentications
1055 Specifies the order in which the client should try protocol 2
1056 authentication methods. This allows a client to prefer one method (e.g.
1057 .Cm keyboard-interactive )
1058 over another method (e.g.
1060 The default for this option is:
1061 .Dq hostbased,publickey,keyboard-interactive,password .
1063 Specifies the protocol versions
1065 should support in order of preference.
1066 The possible values are
1070 Multiple versions must be comma-separated.
1075 tries version 2 and falls back to version 1
1076 if version 2 is not available.
1078 Specifies the command to use to connect to the server.
1080 string extends to the end of the line, and is executed with
1082 In the command string,
1084 will be substituted by the host name to
1088 The command can be basically anything,
1089 and should read from its standard input and write to its standard output.
1090 It should eventually connect an
1092 server running on some machine, or execute
1095 Host key management will be done using the
1096 HostName of the host being connected (defaulting to the name typed by
1100 is not available for connects with a proxy command.
1102 .It Cm PubkeyAuthentication
1103 Specifies whether to try public key authentication.
1104 The argument to this keyword must be
1110 This option applies to protocol version 2 only.
1111 .It Cm RemoteForward
1112 Specifies that a TCP/IP port on the remote machine be forwarded over
1113 the secure channel to the specified host and port from the local machine.
1114 The first argument must be a port number, and the second must be
1116 IPv6 addresses can be specified with an alternative syntax:
1118 Multiple forwardings may be specified, and additional
1119 forwardings can be given on the command line.
1120 Only the superuser can forward privileged ports.
1121 .It Cm RhostsAuthentication
1122 Specifies whether to try rhosts based authentication.
1124 declaration only affects the client side and has no effect whatsoever
1126 Disabling rhosts authentication may reduce
1127 authentication time on slow connections when rhosts authentication is
1129 Most servers do not permit RhostsAuthentication because it
1131 .Cm RhostsRSAAuthentication ) .
1132 The argument to this keyword must be
1138 This option applies to protocol version 1 only.
1139 .It Cm RhostsRSAAuthentication
1140 Specifies whether to try rhosts based authentication with RSA host
1142 The argument must be
1148 This option applies to protocol version 1 only.
1149 .It Cm RSAAuthentication
1150 Specifies whether to try RSA authentication.
1151 The argument to this keyword must be
1155 RSA authentication will only be
1156 attempted if the identity file exists, or an authentication agent is
1160 Note that this option applies to protocol version 1 only.
1161 .It Cm ChallengeResponseAuthentication
1162 Specifies whether to use challenge response authentication.
1163 The argument to this keyword must be
1169 .It Cm SmartcardDevice
1170 Specifies which smartcard device to use. The argument to this keyword is
1173 should use to communicate with a smartcard used for storing the user's
1174 private RSA key. By default, no device is specified and smartcard support
1176 .It Cm StrictHostKeyChecking
1177 If this flag is set to
1180 will never automatically add host keys to the
1181 .Pa $HOME/.ssh/known_hosts
1182 file, and refuses to connect to hosts whose host key has changed.
1183 This provides maximum protection against trojan horse attacks,
1184 however, can be annoying when the
1185 .Pa /etc/ssh/ssh_known_hosts
1186 file is poorly maintained, or connections to new hosts are
1188 This option forces the user to manually
1190 If this flag is set to
1193 will automatically add new host keys to the
1194 user known hosts files.
1195 If this flag is set to
1198 will be added to the user known host files only after the user
1199 has confirmed that is what they really want to do, and
1201 will refuse to connect to hosts whose host key has changed.
1203 known hosts will be verified automatically in all cases.
1204 The argument must be
1211 .It Cm UsePrivilegedPort
1212 Specifies whether to use a privileged port for outgoing connections.
1213 The argument must be
1219 Note that this option must be set to
1222 .Cm RhostsAuthentication
1224 .Cm RhostsRSAAuthentication
1225 authentications are needed with older servers.
1227 Specifies the user to log in as.
1228 This can be useful when a different user name is used on different machines.
1229 This saves the trouble of
1230 having to remember to give the user name on the command line.
1231 .It Cm UserKnownHostsFile
1232 Specifies a file to use for the user
1233 host key database instead of
1234 .Pa $HOME/.ssh/known_hosts .
1236 Specifies that rlogin/rsh should be used for this host.
1237 It is possible that the host does not at all support the
1242 to immediately execute
1244 All other options (except
1246 are ignored if this has been specified.
1247 The argument must be
1251 .It Cm XAuthLocation
1252 Specifies the location of the
1256 .Pa /usr/X11R6/bin/xauth .
1260 will normally set the following environment variables:
1265 variable indicates the location of the X11 server.
1266 It is automatically set by
1268 to point to a value of the form
1270 where hostname indicates
1271 the host where the shell runs, and n is an integer >= 1.
1273 uses this special value to forward X11 connections over the secure
1275 The user should normally not set
1278 will render the X11 connection insecure (and will require the user to
1279 manually copy any required authorization cookies).
1281 Set to the path of the user's home directory.
1285 set for compatibility with systems that use this variable.
1287 Set to the path of the user's mailbox.
1291 as specified when compiling
1296 needs a passphrase, it will read the passphrase from the current
1297 terminal if it was run from a terminal.
1300 does not have a terminal associated with it but
1304 are set, it will execute the program specified by
1306 and open an X11 window to read the passphrase.
1307 This is particularly useful when calling
1312 (Note that on some machines it
1313 may be necessary to redirect the input from
1316 .It Ev SSH_AUTH_SOCK
1317 Identifies the path of a unix-domain socket used to communicate with the
1320 Identifies the client end of the connection.
1321 The variable contains
1322 three space-separated values: client ip-address, client port number,
1323 and server port number.
1324 .It Ev SSH_ORIGINAL_COMMAND
1325 The variable contains the original command line if a forced command
1327 It can be used to extract the original arguments.
1329 This is set to the name of the tty (path to the device) associated
1330 with the current shell or command.
1331 If the current session has no tty,
1332 this variable is not set.
1334 The timezone variable is set to indicate the present timezone if it
1335 was set when the daemon was started (i.e., the daemon passes the value
1336 on to new connections).
1338 Set to the name of the user logging in.
1344 .Pa $HOME/.ssh/environment ,
1345 and adds lines of the format
1350 .It Pa $HOME/.ssh/known_hosts
1351 Records host keys for all hosts the user has logged into that are not
1353 .Pa /etc/ssh/ssh_known_hosts .
1356 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1357 Contains the authentication identity of the user.
1358 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1360 contain sensitive data and should be readable by the user but not
1361 accessible by others (read/write/execute).
1364 ignores a private key file if it is accessible by others.
1365 It is possible to specify a passphrase when
1366 generating the key; the passphrase will be used to encrypt the
1367 sensitive part of this file using 3DES.
1368 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1369 Contains the public key for authentication (public part of the
1370 identity file in human-readable form).
1372 .Pa $HOME/.ssh/identity.pub
1373 file should be added to
1374 .Pa $HOME/.ssh/authorized_keys
1376 where the user wishes to log in using protocol version 1 RSA authentication.
1378 .Pa $HOME/.ssh/id_dsa.pub
1380 .Pa $HOME/.ssh/id_rsa.pub
1381 file should be added to
1382 .Pa $HOME/.ssh/authorized_keys
1384 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1386 sensitive and can (but need not) be readable by anyone.
1388 never used automatically and are not necessary; they are only provided for
1389 the convenience of the user.
1390 .It Pa $HOME/.ssh/config
1391 This is the per-user configuration file.
1392 The format of this file is described above.
1393 This file is used by the
1396 This file does not usually contain any sensitive information,
1397 but the recommended permissions are read/write for the user, and not
1398 accessible by others.
1399 .It Pa $HOME/.ssh/authorized_keys
1400 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1401 The format of this file is described in the
1404 In the simplest form the format is the same as the .pub
1406 This file is not highly sensitive, but the recommended
1407 permissions are read/write for the user, and not accessible by others.
1408 .It Pa /etc/ssh/ssh_known_hosts
1409 Systemwide list of known host keys.
1410 This file should be prepared by the
1411 system administrator to contain the public host keys of all machines in the
1413 This file should be world-readable.
1415 public keys, one per line, in the following format (fields separated
1416 by spaces): system name, public key and optional comment field.
1417 When different names are used
1418 for the same machine, all such names should be listed, separated by
1420 The format is described on the
1424 The canonical system name (as returned by name servers) is used by
1426 to verify the client host when logging in; other names are needed because
1428 does not convert the user-supplied name to a canonical name before
1429 checking the key, because someone with access to the name servers
1430 would then be able to fool host authentication.
1431 .It Pa /etc/ssh/ssh_config
1432 Systemwide configuration file.
1433 This file provides defaults for those
1434 values that are not specified in the user's configuration file, and
1435 for those users who do not have a configuration file.
1436 This file must be world-readable.
1437 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1438 These three files contain the private parts of the host keys
1440 .Cm RhostsRSAAuthentication
1442 .Cm HostbasedAuthentication .
1443 Since they are readable only by root
1445 must be setuid root if these authentication methods are desired.
1446 .It Pa $HOME/.rhosts
1447 This file is used in
1449 authentication to list the
1450 host/user pairs that are permitted to log in.
1451 (Note that this file is
1452 also used by rlogin and rsh, which makes using this file insecure.)
1453 Each line of the file contains a host name (in the canonical form
1454 returned by name servers), and then a user name on that host,
1455 separated by a space.
1456 On some machines this file may need to be
1457 world-readable if the user's home directory is on a NFS partition,
1461 Additionally, this file must be owned by the user,
1462 and must not have write permissions for anyone else.
1464 permission for most machines is read/write for the user, and not
1465 accessible by others.
1467 Note that by default
1469 will be installed so that it requires successful RSA host
1470 authentication before permitting \s+2.\s0rhosts authentication.
1471 If the server machine does not have the client's host key in
1472 .Pa /etc/ssh/ssh_known_hosts ,
1474 .Pa $HOME/.ssh/known_hosts .
1475 The easiest way to do this is to
1476 connect back to the client from the server machine using ssh; this
1477 will automatically add the host key to
1478 .Pa $HOME/.ssh/known_hosts .
1479 .It Pa $HOME/.shosts
1480 This file is used exactly the same way as
1483 having this file is to be able to use rhosts authentication with
1485 without permitting login with
1489 .It Pa /etc/hosts.equiv
1490 This file is used during
1491 .Pa \&.rhosts authentication.
1493 canonical hosts names, one per line (the full format is described on
1497 If the client host is found in this file, login is
1498 automatically permitted provided client and server user names are the
1500 Additionally, successful RSA host authentication is normally
1502 This file should only be writable by root.
1503 .It Pa /etc/shosts.equiv
1504 This file is processed exactly as
1505 .Pa /etc/hosts.equiv .
1506 This file may be useful to permit logins using
1508 but not using rsh/rlogin.
1509 .It Pa /etc/ssh/sshrc
1510 Commands in this file are executed by
1512 when the user logs in just before the user's shell (or command) is started.
1515 manual page for more information.
1516 .It Pa $HOME/.ssh/rc
1517 Commands in this file are executed by
1519 when the user logs in just before the user's shell (or command) is
1523 manual page for more information.
1524 .It Pa $HOME/.ssh/environment
1525 Contains additional definitions for environment variables, see section
1531 exits with the exit status of the remote command or with 255
1532 if an error occurred.
1534 OpenSSH is a derivative of the original and free
1535 ssh 1.2.12 release by Tatu Ylonen.
1536 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1537 Theo de Raadt and Dug Song
1538 removed many bugs, re-added newer features and
1540 Markus Friedl contributed the support for SSH
1541 protocol versions 1.5 and 2.0.
1557 .%T "SSH Protocol Architecture"
1558 .%N draft-ietf-secsh-architecture-12.txt
1560 .%O work in progress material
andersk / gssapi-openssh.git / blob