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 1.210 2005/09/19 11:37:34 djm Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
47 .Op Fl 1246AaCfgkMNnqsTtVvXxY
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
52 .Oo Ar bind_address : Oc
56 .Op Fl e Ar escape_char
57 .Op Fl F Ar configfile
58 .Op Fl i Ar identity_file
61 .Oo Ar bind_address : Oc
62 .Ar port : host : hostport
65 .Op Fl l Ar login_name
72 .Oo Ar bind_address : Oc
73 .Ar port : host : hostport
77 .Oo Ar user Ns @ Oc Ns Ar 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 rlogin and rsh,
85 and provide secure encrypted communications between
86 two untrusted hosts over an insecure network.
87 X11 connections and arbitrary TCP/IP ports
88 can also be forwarded over the secure channel.
91 connects and logs into the specified
97 his/her identity to the remote machine using one of several methods
98 depending on the protocol version used.
104 is executed on the remote host instead of a login shell.
105 .Ss SSH protocol version 1
106 The first authentication method is the
110 method combined with RSA-based host authentication.
111 If the machine the user logs in from is listed in
114 .Pa /etc/shosts.equiv
115 on the remote machine, and the user names are
116 the same on both sides, or if the files
120 exist in the user's home directory on the
121 remote machine and contain a line containing the name of the client
122 machine and the name of the user on that machine, the user is
123 considered for log in.
124 Additionally, if the server can verify the client's
126 .Pa /etc/ssh/ssh_known_hosts
128 .Pa ~/.ssh/known_hosts
131 section), only then is login permitted.
132 This authentication method closes security holes due to IP
133 spoofing, DNS spoofing and routing spoofing.
134 [Note to the administrator:
135 .Pa /etc/hosts.equiv ,
137 and the rlogin/rsh protocol in general, are inherently insecure and should be
138 disabled if security is desired.]
140 As a second authentication method,
142 supports RSA based authentication.
143 The scheme is based on public-key cryptography: there are cryptosystems
144 where encryption and decryption are done using separate keys, and it
145 is not possible to derive the decryption key from the encryption key.
146 RSA is one such system.
147 The idea is that each user creates a public/private
148 key pair for authentication purposes.
149 The server knows the public key, and only the user knows the private key.
152 .Pa ~/.ssh/authorized_keys
153 lists the public keys that are permitted for logging in.
154 When the user logs in, the
156 program tells the server which key pair it would like to use for
158 The server checks if this key is permitted, and if so,
159 sends the user (actually the
161 program running on behalf of the user) a challenge, a random number,
162 encrypted by the user's public key.
163 The challenge can only be decrypted using the proper private key.
164 The user's client then decrypts the challenge using the private key,
165 proving that he/she knows the private key
166 but without disclosing it to the server.
169 implements the RSA authentication protocol automatically.
170 The user creates his/her RSA key pair by running
172 This stores the private key in
174 and stores the public key in
175 .Pa ~/.ssh/identity.pub
176 in the user's home directory.
177 The user should then copy the
180 .Pa ~/.ssh/authorized_keys
181 in his/her home directory on the remote machine (the
183 file corresponds to the conventional
185 file, and has one key
186 per line, though the lines can be very long).
187 After this, the user can log in without giving the password.
189 The most convenient way to use RSA authentication may be with an
190 authentication agent.
193 for more information.
195 If other authentication methods fail,
197 prompts the user for a password.
198 The password is sent to the remote
199 host for checking; however, since all communications are encrypted,
200 the password cannot be seen by someone listening on the network.
201 .Ss SSH protocol version 2
202 When a user connects using protocol version 2,
203 similar authentication methods are available.
204 Using the default values for
205 .Cm PreferredAuthentications ,
206 the client will try to authenticate first using the hostbased method;
207 if this method fails, public key authentication is attempted,
208 and finally if this method fails, keyboard-interactive and
209 password authentication are tried.
211 The public key method is similar to RSA authentication described
212 in the previous section and allows the RSA or DSA algorithm to be used:
213 The client uses his private key,
217 to sign the session identifier and sends the result to the server.
218 The server checks whether the matching public key is listed in
219 .Pa ~/.ssh/authorized_keys
220 and grants access if both the key is found and the signature is correct.
221 The session identifier is derived from a shared Diffie-Hellman value
222 and is only known to the client and the server.
224 If public key authentication fails or is not available, a password
225 can be sent encrypted to the remote host to prove the user's identity.
229 supports hostbased or challenge response authentication.
231 Protocol 2 provides additional mechanisms for confidentiality
232 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour)
233 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
234 Note that protocol 1 lacks a strong mechanism for ensuring the
235 integrity of the connection.
236 .Ss Login session and remote execution
237 When the user's identity has been accepted by the server, the server
238 either executes the given command, or logs into the machine and gives
239 the user a normal shell on the remote machine.
240 All communication with
241 the remote command or shell will be automatically encrypted.
243 If a pseudo-terminal has been allocated (normal login session), the
244 user may use the escape characters noted below.
246 If no pseudo-tty has been allocated,
247 the session is transparent and can be used to reliably transfer binary data.
248 On most systems, setting the escape character to
250 will also make the session transparent even if a tty is used.
252 The session terminates when the command or shell on the remote
253 machine exits and all X11 and TCP/IP connections have been closed.
254 The exit status of the remote program is returned as the exit status of
256 .Ss Escape Characters
257 When a pseudo-terminal has been requested,
259 supports a number of functions through the use of an escape character.
261 A single tilde character can be sent as
263 or by following the tilde by a character other than those described below.
264 The escape character must always follow a newline to be interpreted as
266 The escape character can be changed in configuration files using the
268 configuration directive or on the command line by the
272 The supported escapes (assuming the default
282 List forwarded connections.
286 at logout when waiting for forwarded connection / X11 sessions to terminate.
288 Display a list of escape characters.
290 Send a BREAK to the remote system
291 (only useful for SSH protocol version 2 and if the peer supports it).
294 Currently this allows the addition of port forwardings using the
299 It also allows the cancellation of existing remote port-forwardings
302 Basic help is available, using the
306 Request rekeying of the connection
307 (only useful for SSH protocol version 2 and if the peer supports it).
309 .Ss X11 and TCP forwarding
314 (or see the description of the
318 options described later)
319 and the user is using X11 (the
321 environment variable is set), the connection to the X11 display is
322 automatically forwarded to the remote side in such a way that any X11
323 programs started from the shell (or command) will go through the
324 encrypted channel, and the connection to the real X server will be made
325 from the local machine.
326 The user should not manually set
328 Forwarding of X11 connections can be
329 configured on the command line or in configuration files.
335 will point to the server machine, but with a display number greater than zero.
336 This is normal, and happens because
340 X server on the server machine for forwarding the
341 connections over the encrypted channel.
344 will also automatically set up Xauthority data on the server machine.
345 For this purpose, it will generate a random authorization cookie,
346 store it in Xauthority on the server, and verify that any forwarded
347 connections carry this cookie and replace it by the real cookie when
348 the connection is opened.
349 The real authentication cookie is never
350 sent to the server machine (and no cookies are sent in the plain).
356 (or see the description of the
360 options described later) and
361 the user is using an authentication agent, the connection to the agent
362 is automatically forwarded to the remote side.
364 Forwarding of arbitrary TCP/IP connections over the secure channel can
365 be specified either on the command line or in a configuration file.
366 One possible application of TCP/IP forwarding is a secure connection to an
367 electronic purse; another is going through firewalls.
368 .Ss Server authentication
370 automatically maintains and checks a database containing
371 identifications for all hosts it has ever been used with.
372 Host keys are stored in
373 .Pa ~/.ssh/known_hosts
374 in the user's home directory.
375 Additionally, the file
376 .Pa /etc/ssh/ssh_known_hosts
377 is automatically checked for known hosts.
378 Any new hosts are automatically added to the user's file.
379 If a host's identification ever changes,
381 warns about this and disables password authentication to prevent a
382 trojan horse from getting the user's password.
383 Another purpose of this mechanism is to prevent man-in-the-middle attacks
384 which could otherwise be used to circumvent the encryption.
386 .Cm StrictHostKeyChecking
387 option can be used to prevent logins to machines whose
388 host key is not known or has changed.
391 can be configured to verify host identification using fingerprint resource
392 records (SSHFP) published in DNS.
395 option can be used to control how DNS lookups are performed.
396 SSHFP resource records can be generated using
399 The options are as follows:
404 to try protocol version 1 only.
408 to try protocol version 2 only.
412 to use IPv4 addresses only.
416 to use IPv6 addresses only.
418 Enables forwarding of the authentication agent connection.
419 This can also be specified on a per-host basis in a configuration file.
421 Agent forwarding should be enabled with caution.
422 Users with the ability to bypass file permissions on the remote host
423 (for the agent's Unix-domain socket)
424 can access the local agent through the forwarded connection.
425 An attacker cannot obtain key material from the agent,
426 however they can perform operations on the keys that enable them to
427 authenticate using the identities loaded into the agent.
429 Disables forwarding of the authentication agent connection.
430 .It Fl b Ar bind_address
433 on the local machine as the source address
435 Only useful on systems with more than one address.
437 Requests compression of all data (including stdin, stdout, stderr, and
438 data for forwarded X11 and TCP/IP connections).
439 The compression algorithm is the same used by
443 can be controlled by the
445 option for protocol version 1.
446 Compression is desirable on modem lines and other
447 slow connections, but will only slow down things on fast networks.
448 The default value can be set on a host-by-host basis in the
449 configuration files; see the
452 .It Fl c Ar cipher_spec
453 Selects the cipher specification for encrypting the session.
455 Protocol version 1 allows specification of a single cipher.
456 The suported values are
462 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
463 It is believed to be secure.
465 is a fast block cipher; it appears very secure and is much faster than
468 is only supported in the
470 client for interoperability with legacy protocol 1 implementations
471 that do not support the
474 Its use is strongly discouraged due to cryptographic weaknesses.
478 For protocol version 2
480 is a comma-separated list of ciphers
481 listed in order of preference.
482 The supported ciphers are
498 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
499 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
500 aes192-ctr,aes256-ctr''
504 .Oo Ar bind_address : Oc
510 application-level port forwarding.
511 This works by allocating a socket to listen to
513 on the local side, optionally bound to the specified
515 Whenever a connection is made to this port, the
516 connection is forwarded over the secure channel, and the application
517 protocol is then used to determine where to connect to from the
519 Currently the SOCKS4 and SOCKS5 protocols are supported, and
521 will act as a SOCKS server.
522 Only root can forward privileged ports.
523 Dynamic port forwardings can also be specified in the configuration file.
525 IPv6 addresses can be specified with an alternative syntax:
528 .Op Ar bind_address No /
532 or by enclosing the address in square brackets.
533 Only the superuser can forward privileged ports.
534 By default, the local port is bound in accordance with the
539 may be used to bind the connection to a specific address.
544 indicates that the listening port be bound for local use only, while an
547 indicates that the port should be available from all interfaces.
548 .It Fl e Ar ch | ^ch | none
549 Sets the escape character for sessions with a pty (default:
551 The escape character is only recognized at the beginning of a line.
552 The escape character followed by a dot
554 closes the connection;
555 followed by control-Z suspends the connection;
556 and followed by itself sends the escape character once.
557 Setting the character to
559 disables any escapes and makes the session fully transparent.
560 .It Fl F Ar configfile
561 Specifies an alternative per-user configuration file.
562 If a configuration file is given on the command line,
563 the system-wide configuration file
564 .Pq Pa /etc/ssh/ssh_config
566 The default for the per-user configuration file is
571 to go to background just before command execution.
574 is going to ask for passwords or passphrases, but the user
575 wants it in the background.
578 The recommended way to start X11 programs at a remote site is with
580 .Ic ssh -f host xterm .
582 Allows remote hosts to connect to local forwarded ports.
583 .It Fl I Ar smartcard_device
584 Specifies which smartcard device to use.
585 The argument is the device
587 should use to communicate with a smartcard used for storing the user's
589 .It Fl i Ar identity_file
590 Selects a file from which the identity (private key) for
591 RSA or DSA authentication is read.
594 for protocol version 1, and
598 for protocol version 2.
599 Identity files may also be specified on
600 a per-host basis in the configuration file.
601 It is possible to have multiple
603 options (and multiple identities specified in
604 configuration files).
606 Disables forwarding (delegation) of GSSAPI credentials to the server.
609 .Oo Ar bind_address : Oc
610 .Ar port : host : hostport
613 Specifies that the given port on the local (client) host is to be
614 forwarded to the given host and port on the remote side.
615 This works by allocating a socket to listen to
617 on the local side, optionally bound to the specified
619 Whenever a connection is made to this port, the
620 connection is forwarded over the secure channel, and a connection is
625 from the remote machine.
626 Port forwardings can also be specified in the configuration file.
627 IPv6 addresses can be specified with an alternative syntax:
630 .Op Ar bind_address No /
631 .Ar port No / Ar host No /
635 or by enclosing the address in square brackets.
636 Only the superuser can forward privileged ports.
637 By default, the local port is bound in accordance with the
642 may be used to bind the connection to a specific address.
647 indicates that the listening port be bound for local use only, while an
650 indicates that the port should be available from all interfaces.
651 .It Fl l Ar login_name
652 Specifies the user to log in as on the remote machine.
653 This also may be specified on a per-host basis in the configuration file.
659 mode for connection sharing.
660 Refer to the description of
666 Additionally, for protocol version 2 a comma-separated list of MAC
667 (message authentication code) algorithms can
668 be specified in order of preference.
671 keyword for more information.
673 Do not execute a remote command.
674 This is useful for just forwarding ports
675 (protocol version 2 only).
679 (actually, prevents reading from stdin).
680 This must be used when
682 is run in the background.
683 A common trick is to use this to run X11 programs on a remote machine.
685 .Ic ssh -n shadows.cs.hut.fi emacs &
686 will start an emacs on shadows.cs.hut.fi, and the X11
687 connection will be automatically forwarded over an encrypted channel.
690 program will be put in the background.
691 (This does not work if
693 needs to ask for a password or passphrase; see also the
697 Control an active connection multiplexing master process.
700 option is specified, the
702 argument is interpreted and passed to the master process.
705 (check that the master process is running) and
707 (request the master to exit).
709 Can be used to give options in the format used in the configuration file.
710 This is useful for specifying options for which there is no separate
712 For full details of the options listed below, and their possible values, see
715 .Bl -tag -width Ds -offset indent -compact
719 .It ChallengeResponseAuthentication
723 .It ClearAllForwardings
726 .It ConnectionAttempts
734 .It ForwardX11Trusted
736 .It GlobalKnownHostsFile
737 .It GSSAPIAuthentication
738 .It GSSAPIDelegateCredentials
741 .It HostbasedAuthentication
742 .It HostKeyAlgorithms
747 .It KbdInteractiveDevices
751 .It NoHostAuthenticationForLocalhost
752 .It NumberOfPasswordPrompts
753 .It PasswordAuthentication
755 .It PreferredAuthentications
758 .It PubkeyAuthentication
760 .It RhostsRSAAuthentication
761 .It RSAAuthentication
763 .It ServerAliveInterval
764 .It ServerAliveCountMax
766 .It StrictHostKeyChecking
768 .It UsePrivilegedPort
770 .It UserKnownHostsFile
775 Port to connect to on the remote host.
776 This can be specified on a
777 per-host basis in the configuration file.
780 Causes all warning and diagnostic messages to be suppressed.
783 .Oo Ar bind_address : Oc
784 .Ar port : host : hostport
787 Specifies that the given port on the remote (server) host is to be
788 forwarded to the given host and port on the local side.
789 This works by allocating a socket to listen to
791 on the remote side, and whenever a connection is made to this port, the
792 connection is forwarded over the secure channel, and a connection is
797 from the local machine.
799 Port forwardings can also be specified in the configuration file.
800 Privileged ports can be forwarded only when
801 logging in as root on the remote machine.
802 IPv6 addresses can be specified by enclosing the address in square braces or
803 using an alternative syntax:
806 .Op Ar bind_address No /
807 .Ar host No / Ar port No /
812 By default, the listening socket on the server will be bound to the loopback
814 This may be overriden by specifying a
820 indicates that the remote socket should listen on all interfaces.
823 will only succeed if the server's
825 option is enabled (see
826 .Xr sshd_config 5 ) .
828 Specifies the location of a control socket for connection sharing.
829 Refer to the description of
837 May be used to request invocation of a subsystem on the remote system.
838 Subsystems are a feature of the SSH2 protocol which facilitate the use
839 of SSH as a secure transport for other applications (eg.\&
841 The subsystem is specified as the remote command.
843 Disable pseudo-tty allocation.
845 Force pseudo-tty allocation.
846 This can be used to execute arbitrary
847 screen-based programs on a remote machine, which can be very useful,
848 e.g., when implementing menu services.
851 options force tty allocation, even if
855 Display the version number and exit.
860 to print debugging messages about its progress.
862 debugging connection, authentication, and configuration problems.
865 options increase the verbosity.
868 Enables X11 forwarding.
869 This can also be specified on a per-host basis in a configuration file.
871 X11 forwarding should be enabled with caution.
872 Users with the ability to bypass file permissions on the remote host
873 (for the user's X authorization database)
874 can access the local X11 display through the forwarded connection.
875 An attacker may then be able to perform activities such as keystroke monitoring.
877 For this reason, X11 forwarding is subjected to X11 SECURITY extension
878 restrictions by default.
883 .Cm ForwardX11Trusted
886 for more information.
888 Disables X11 forwarding.
890 Enables trusted X11 forwarding.
891 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
894 .Sh CONFIGURATION FILES
896 may additionally obtain configuration data from
897 a per-user configuration file and a system-wide configuration file.
898 The file format and configuration options are described in
902 will normally set the following environment variables:
903 .Bl -tag -width LOGNAME
907 variable indicates the location of the X11 server.
908 It is automatically set by
910 to point to a value of the form
912 where hostname indicates
913 the host where the shell runs, and n is an integer \*(Ge 1.
915 uses this special value to forward X11 connections over the secure
917 The user should normally not set
920 will render the X11 connection insecure (and will require the user to
921 manually copy any required authorization cookies).
923 Set to the path of the user's home directory.
927 set for compatibility with systems that use this variable.
929 Set to the path of the user's mailbox.
933 as specified when compiling
938 needs a passphrase, it will read the passphrase from the current
939 terminal if it was run from a terminal.
942 does not have a terminal associated with it but
946 are set, it will execute the program specified by
948 and open an X11 window to read the passphrase.
949 This is particularly useful when calling
954 (Note that on some machines it
955 may be necessary to redirect the input from
959 Identifies the path of a unix-domain socket used to communicate with the
961 .It Ev SSH_CONNECTION
962 Identifies the client and server ends of the connection.
963 The variable contains
964 four space-separated values: client ip-address, client port number,
965 server ip-address and server port number.
966 .It Ev SSH_ORIGINAL_COMMAND
967 The variable contains the original command line if a forced command
969 It can be used to extract the original arguments.
971 This is set to the name of the tty (path to the device) associated
972 with the current shell or command.
973 If the current session has no tty,
974 this variable is not set.
976 The timezone variable is set to indicate the present timezone if it
977 was set when the daemon was started (i.e., the daemon passes the value
978 on to new connections).
980 Set to the name of the user logging in.
986 .Pa ~/.ssh/environment ,
987 and adds lines of the format
989 to the environment if the file exists and if users are allowed to
990 change their environment.
991 For more information, see the
992 .Cm PermitUserEnvironment
997 .It Pa ~/.ssh/known_hosts
998 Records host keys for all hosts the user has logged into that are not
1000 .Pa /etc/ssh/ssh_known_hosts .
1003 .It Pa ~/.ssh/identity, ~/.ssh/id_dsa, ~/.ssh/id_rsa
1004 Contains the authentication identity of the user.
1005 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1007 contain sensitive data and should be readable by the user but not
1008 accessible by others (read/write/execute).
1011 ignores a private key file if it is accessible by others.
1012 It is possible to specify a passphrase when
1013 generating the key; the passphrase will be used to encrypt the
1014 sensitive part of this file using 3DES.
1015 .It Pa ~/.ssh/identity.pub, ~/.ssh/id_dsa.pub, ~/.ssh/id_rsa.pub
1016 Contains the public key for authentication (public part of the
1017 identity file in human-readable form).
1019 .Pa ~/.ssh/identity.pub
1020 file should be added to the file
1021 .Pa ~/.ssh/authorized_keys
1023 where the user wishes to log in using protocol version 1 RSA authentication.
1025 .Pa ~/.ssh/id_dsa.pub
1027 .Pa ~/.ssh/id_rsa.pub
1028 file should be added to
1029 .Pa ~/.ssh/authorized_keys
1031 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1033 sensitive and can (but need not) be readable by anyone.
1035 never used automatically and are not necessary; they are only provided for
1036 the convenience of the user.
1037 .It Pa ~/.ssh/config
1038 This is the per-user configuration file.
1039 The file format and configuration options are described in
1041 Because of the potential for abuse, this file must have strict permissions:
1042 read/write for the user, and not accessible by others.
1043 .It Pa ~/.ssh/authorized_keys
1044 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1045 The format of this file is described in the
1048 In the simplest form the format is the same as the
1051 This file is not highly sensitive, but the recommended
1052 permissions are read/write for the user, and not accessible by others.
1053 .It Pa /etc/ssh/ssh_known_hosts
1054 Systemwide list of known host keys.
1055 This file should be prepared by the
1056 system administrator to contain the public host keys of all machines in the
1058 This file should be world-readable.
1060 public keys, one per line, in the following format (fields separated
1061 by spaces): system name, public key and optional comment field.
1062 When different names are used
1063 for the same machine, all such names should be listed, separated by
1065 The format is described in the
1069 The canonical system name (as returned by name servers) is used by
1071 to verify the client host when logging in; other names are needed because
1073 does not convert the user-supplied name to a canonical name before
1074 checking the key, because someone with access to the name servers
1075 would then be able to fool host authentication.
1076 .It Pa /etc/ssh/ssh_config
1077 Systemwide configuration file.
1078 The file format and configuration options are described in
1080 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1081 These three files contain the private parts of the host keys
1083 .Cm RhostsRSAAuthentication
1085 .Cm HostbasedAuthentication .
1086 If the protocol version 1
1087 .Cm RhostsRSAAuthentication
1090 must be setuid root, since the host key is readable only by root.
1091 For protocol version 2,
1095 to access the host keys for
1096 .Cm HostbasedAuthentication .
1097 This eliminates the requirement that
1099 be setuid root when that authentication method is used.
1104 This file is used in
1105 .Cm RhostsRSAAuthentication
1107 .Cm HostbasedAuthentication
1108 authentication to list the
1109 host/user pairs that are permitted to log in.
1110 (Note that this file is
1111 also used by rlogin and rsh, which makes using this file insecure.)
1112 Each line of the file contains a host name (in the canonical form
1113 returned by name servers), and then a user name on that host,
1114 separated by a space.
1115 On some machines this file may need to be
1116 world-readable if the user's home directory is on a NFS partition,
1120 Additionally, this file must be owned by the user,
1121 and must not have write permissions for anyone else.
1123 permission for most machines is read/write for the user, and not
1124 accessible by others.
1128 allows authentication only in combination with client host key
1129 authentication before permitting log in.
1130 If the server machine does not have the client's host key in
1131 .Pa /etc/ssh/ssh_known_hosts ,
1133 .Pa ~/.ssh/known_hosts .
1134 The easiest way to do this is to
1135 connect back to the client from the server machine using ssh; this
1136 will automatically add the host key to
1137 .Pa ~/.ssh/known_hosts .
1139 This file is used exactly the same way as
1142 having this file is to be able to use
1143 .Cm RhostsRSAAuthentication
1145 .Cm HostbasedAuthentication
1146 authentication without permitting login with
1150 .It Pa /etc/hosts.equiv
1151 This file is used during
1152 .Cm RhostsRSAAuthentication
1154 .Cm HostbasedAuthentication
1157 canonical hosts names, one per line (the full format is described in the
1160 If the client host is found in this file, login is
1161 automatically permitted provided client and server user names are the
1163 Additionally, successful client host key authentication is required.
1164 This file should only be writable by root.
1165 .It Pa /etc/shosts.equiv
1166 This file is processed exactly as
1167 .Pa /etc/hosts.equiv .
1168 This file may be useful to permit logins using
1170 but not using rsh/rlogin.
1171 .It Pa /etc/ssh/sshrc
1172 Commands in this file are executed by
1174 when the user logs in just before the user's shell (or command) is started.
1177 manual page for more information.
1179 Commands in this file are executed by
1181 when the user logs in just before the user's shell (or command) is
1185 manual page for more information.
1186 .It Pa ~/.ssh/environment
1187 Contains additional definitions for environment variables, see section
1193 exits with the exit status of the remote command or with 255
1194 if an error occurred.
1214 .%T "SSH Protocol Architecture"
1215 .%N draft-ietf-secsh-architecture-12.txt
1217 .%O work in progress material
1220 OpenSSH is a derivative of the original and free
1221 ssh 1.2.12 release by Tatu Ylonen.
1222 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1223 Theo de Raadt and Dug Song
1224 removed many bugs, re-added newer features and
1226 Markus Friedl contributed the support for SSH
1227 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob