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 1.218 2005/12/16 18:07:08 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgkMNnqsTtVvXxY
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
51 .Oo Ar bind_address : Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
58 .Op Fl i Ar identity_file
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
67 .Op Fl l Ar login_name
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
81 .Op Fl w Ar tunnel : Ns Ar tunnel
82 .Oo Ar user Ns @ Oc Ns Ar hostname
87 (SSH client) is a program for logging into a remote machine and for
88 executing commands on a remote machine.
89 It is intended to replace rlogin and rsh,
90 and provide secure encrypted communications between
91 two untrusted hosts over an insecure network.
92 X11 connections and arbitrary TCP/IP ports
93 can also be forwarded over the secure channel.
96 connects and logs into the specified
102 his/her identity to the remote machine using one of several methods
103 depending on the protocol version used.
109 is executed on the remote host instead of a login shell.
111 The options are as follows:
116 to try protocol version 1 only.
120 to try protocol version 2 only.
124 to use IPv4 addresses only.
128 to use IPv6 addresses only.
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
135 (for the agent's Unix-domain socket)
136 can access the local agent through the forwarded connection.
137 An attacker cannot obtain key material from the agent,
138 however they can perform operations on the keys that enable them to
139 authenticate using the identities loaded into the agent.
141 Disables forwarding of the authentication agent connection.
142 .It Fl b Ar bind_address
145 on the local machine as the source address
147 Only useful on systems with more than one address.
149 Requests compression of all data (including stdin, stdout, stderr, and
150 data for forwarded X11 and TCP/IP connections).
151 The compression algorithm is the same used by
155 can be controlled by the
157 option for protocol version 1.
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
164 .It Fl c Ar cipher_spec
165 Selects the cipher specification for encrypting the session.
167 Protocol version 1 allows specification of a single cipher.
168 The supported values are
174 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175 It is believed to be secure.
177 is a fast block cipher; it appears very secure and is much faster than
180 is only supported in the
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
186 Its use is strongly discouraged due to cryptographic weaknesses.
190 For protocol version 2
192 is a comma-separated list of ciphers
193 listed in order of preference.
194 The supported ciphers are
210 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212 aes192-ctr,aes256-ctr''
216 .Oo Ar bind_address : Oc
222 application-level port forwarding.
223 This works by allocating a socket to listen to
225 on the local side, optionally bound to the specified
227 Whenever a connection is made to this port, the
228 connection is forwarded over the secure channel, and the application
229 protocol is then used to determine where to connect to from the
231 Currently the SOCKS4 and SOCKS5 protocols are supported, and
233 will act as a SOCKS server.
234 Only root can forward privileged ports.
235 Dynamic port forwardings can also be specified in the configuration file.
237 IPv6 addresses can be specified with an alternative syntax:
240 .Op Ar bind_address No /
244 or by enclosing the address in square brackets.
245 Only the superuser can forward privileged ports.
246 By default, the local port is bound in accordance with the
251 may be used to bind the connection to a specific address.
256 indicates that the listening port be bound for local use only, while an
259 indicates that the port should be available from all interfaces.
260 .It Fl e Ar ch | ^ch | none
261 Sets the escape character for sessions with a pty (default:
263 The escape character is only recognized at the beginning of a line.
264 The escape character followed by a dot
266 closes the connection;
267 followed by control-Z suspends the connection;
268 and followed by itself sends the escape character once.
269 Setting the character to
271 disables any escapes and makes the session fully transparent.
272 .It Fl F Ar configfile
273 Specifies an alternative per-user configuration file.
274 If a configuration file is given on the command line,
275 the system-wide configuration file
276 .Pq Pa /etc/ssh/ssh_config
278 The default for the per-user configuration file is
283 to go to background just before command execution.
286 is going to ask for passwords or passphrases, but the user
287 wants it in the background.
290 The recommended way to start X11 programs at a remote site is with
292 .Ic ssh -f host xterm .
294 Allows remote hosts to connect to local forwarded ports.
295 .It Fl I Ar smartcard_device
296 Specifies which smartcard device to use.
297 The argument is the device
299 should use to communicate with a smartcard used for storing the user's
301 .It Fl i Ar identity_file
302 Selects a file from which the identity (private key) for
303 RSA or DSA authentication is read.
306 for protocol version 1, and
310 for protocol version 2.
311 Identity files may also be specified on
312 a per-host basis in the configuration file.
313 It is possible to have multiple
315 options (and multiple identities specified in
316 configuration files).
318 Disables forwarding (delegation) of GSSAPI credentials to the server.
321 .Oo Ar bind_address : Oc
322 .Ar port : host : hostport
325 Specifies that the given port on the local (client) host is to be
326 forwarded to the given host and port on the remote side.
327 This works by allocating a socket to listen to
329 on the local side, optionally bound to the specified
331 Whenever a connection is made to this port, the
332 connection is forwarded over the secure channel, and a connection is
337 from the remote machine.
338 Port forwardings can also be specified in the configuration file.
339 IPv6 addresses can be specified with an alternative syntax:
342 .Op Ar bind_address No /
343 .Ar port No / Ar host No /
347 or by enclosing the address in square brackets.
348 Only the superuser can forward privileged ports.
349 By default, the local port is bound in accordance with the
354 may be used to bind the connection to a specific address.
359 indicates that the listening port be bound for local use only, while an
362 indicates that the port should be available from all interfaces.
363 .It Fl l Ar login_name
364 Specifies the user to log in as on the remote machine.
365 This also may be specified on a per-host basis in the configuration file.
371 mode for connection sharing.
372 Refer to the description of
378 Additionally, for protocol version 2 a comma-separated list of MAC
379 (message authentication code) algorithms can
380 be specified in order of preference.
383 keyword for more information.
385 Do not execute a remote command.
386 This is useful for just forwarding ports
387 (protocol version 2 only).
391 (actually, prevents reading from stdin).
392 This must be used when
394 is run in the background.
395 A common trick is to use this to run X11 programs on a remote machine.
397 .Ic ssh -n shadows.cs.hut.fi emacs &
398 will start an emacs on shadows.cs.hut.fi, and the X11
399 connection will be automatically forwarded over an encrypted channel.
402 program will be put in the background.
403 (This does not work if
405 needs to ask for a password or passphrase; see also the
409 Control an active connection multiplexing master process.
412 option is specified, the
414 argument is interpreted and passed to the master process.
417 (check that the master process is running) and
419 (request the master to exit).
421 Can be used to give options in the format used in the configuration file.
422 This is useful for specifying options for which there is no separate
424 For full details of the options listed below, and their possible values, see
427 .Bl -tag -width Ds -offset indent -compact
431 .It ChallengeResponseAuthentication
435 .It ClearAllForwardings
438 .It ConnectionAttempts
446 .It ForwardX11Trusted
448 .It GlobalKnownHostsFile
449 .It GSSAPIAuthentication
450 .It GSSAPIDelegateCredentials
453 .It HostbasedAuthentication
454 .It HostKeyAlgorithms
459 .It KbdInteractiveDevices
464 .It NoHostAuthenticationForLocalhost
465 .It NumberOfPasswordPrompts
466 .It PasswordAuthentication
467 .It PermitLocalCommand
469 .It PreferredAuthentications
472 .It PubkeyAuthentication
474 .It RhostsRSAAuthentication
475 .It RSAAuthentication
477 .It ServerAliveInterval
478 .It ServerAliveCountMax
480 .It StrictHostKeyChecking
484 .It UsePrivilegedPort
486 .It UserKnownHostsFile
491 Port to connect to on the remote host.
492 This can be specified on a
493 per-host basis in the configuration file.
496 Causes all warning and diagnostic messages to be suppressed.
499 .Oo Ar bind_address : Oc
500 .Ar port : host : hostport
503 Specifies that the given port on the remote (server) host is to be
504 forwarded to the given host and port on the local side.
505 This works by allocating a socket to listen to
507 on the remote side, and whenever a connection is made to this port, the
508 connection is forwarded over the secure channel, and a connection is
513 from the local machine.
515 Port forwardings can also be specified in the configuration file.
516 Privileged ports can be forwarded only when
517 logging in as root on the remote machine.
518 IPv6 addresses can be specified by enclosing the address in square braces or
519 using an alternative syntax:
522 .Op Ar bind_address No /
523 .Ar host No / Ar port No /
528 By default, the listening socket on the server will be bound to the loopback
530 This may be overriden by specifying a
536 indicates that the remote socket should listen on all interfaces.
539 will only succeed if the server's
541 option is enabled (see
542 .Xr sshd_config 5 ) .
544 Specifies the location of a control socket for connection sharing.
545 Refer to the description of
553 May be used to request invocation of a subsystem on the remote system.
554 Subsystems are a feature of the SSH2 protocol which facilitate the use
555 of SSH as a secure transport for other applications (eg.\&
557 The subsystem is specified as the remote command.
559 Disable pseudo-tty allocation.
561 Force pseudo-tty allocation.
562 This can be used to execute arbitrary
563 screen-based programs on a remote machine, which can be very useful,
564 e.g., when implementing menu services.
567 options force tty allocation, even if
571 Display the version number and exit.
576 to print debugging messages about its progress.
578 debugging connection, authentication, and configuration problems.
581 options increase the verbosity.
583 .It Fl w Ar tunnel : Ns Ar tunnel
586 device on the client and server like the
591 Enables X11 forwarding.
592 This can also be specified on a per-host basis in a configuration file.
594 X11 forwarding should be enabled with caution.
595 Users with the ability to bypass file permissions on the remote host
596 (for the user's X authorization database)
597 can access the local X11 display through the forwarded connection.
598 An attacker may then be able to perform activities such as keystroke monitoring.
600 For this reason, X11 forwarding is subjected to X11 SECURITY extension
601 restrictions by default.
606 .Cm ForwardX11Trusted
609 for more information.
611 Disables X11 forwarding.
613 Enables trusted X11 forwarding.
614 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
617 .Ss SSH protocol version 1
618 The first authentication method is the
622 method combined with RSA-based host authentication.
623 If the machine the user logs in from is listed in
626 .Pa /etc/shosts.equiv
627 on the remote machine, and the user names are
628 the same on both sides, or if the files
632 exist in the user's home directory on the
633 remote machine and contain a line containing the name of the client
634 machine and the name of the user on that machine, the user is
635 considered for log in.
636 Additionally, if the server can verify the client's
638 .Pa /etc/ssh/ssh_known_hosts
640 .Pa ~/.ssh/known_hosts
643 section), only then is login permitted.
644 This authentication method closes security holes due to IP
645 spoofing, DNS spoofing and routing spoofing.
646 [Note to the administrator:
647 .Pa /etc/hosts.equiv ,
649 and the rlogin/rsh protocol in general, are inherently insecure and should be
650 disabled if security is desired.]
652 As a second authentication method,
654 supports RSA based authentication.
655 The scheme is based on public-key cryptography: there are cryptosystems
656 where encryption and decryption are done using separate keys, and it
657 is not possible to derive the decryption key from the encryption key.
658 RSA is one such system.
659 The idea is that each user creates a public/private
660 key pair for authentication purposes.
661 The server knows the public key, and only the user knows the private key.
664 .Pa ~/.ssh/authorized_keys
665 lists the public keys that are permitted for logging in.
666 When the user logs in, the
668 program tells the server which key pair it would like to use for
670 The server checks if this key is permitted, and if so,
671 sends the user (actually the
673 program running on behalf of the user) a challenge, a random number,
674 encrypted by the user's public key.
675 The challenge can only be decrypted using the proper private key.
676 The user's client then decrypts the challenge using the private key,
677 proving that he/she knows the private key
678 but without disclosing it to the server.
681 implements the RSA authentication protocol automatically.
682 The user creates his/her RSA key pair by running
684 This stores the private key in
686 and stores the public key in
687 .Pa ~/.ssh/identity.pub
688 in the user's home directory.
689 The user should then copy the
692 .Pa ~/.ssh/authorized_keys
693 in his/her home directory on the remote machine (the
695 file corresponds to the conventional
697 file, and has one key
698 per line, though the lines can be very long).
699 After this, the user can log in without giving the password.
701 The most convenient way to use RSA authentication may be with an
702 authentication agent.
705 for more information.
707 If other authentication methods fail,
709 prompts the user for a password.
710 The password is sent to the remote
711 host for checking; however, since all communications are encrypted,
712 the password cannot be seen by someone listening on the network.
713 .Ss SSH protocol version 2
714 When a user connects using protocol version 2,
715 similar authentication methods are available.
716 Using the default values for
717 .Cm PreferredAuthentications ,
718 the client will try to authenticate first using the hostbased method;
719 if this method fails, public key authentication is attempted,
720 and finally if this method fails, keyboard-interactive and
721 password authentication are tried.
723 The public key method is similar to RSA authentication described
724 in the previous section and allows the RSA or DSA algorithm to be used:
725 The client uses his private key,
729 to sign the session identifier and sends the result to the server.
730 The server checks whether the matching public key is listed in
731 .Pa ~/.ssh/authorized_keys
732 and grants access if both the key is found and the signature is correct.
733 The session identifier is derived from a shared Diffie-Hellman value
734 and is only known to the client and the server.
736 If public key authentication fails or is not available, a password
737 can be sent encrypted to the remote host to prove the user's identity.
741 supports hostbased or challenge response authentication.
743 Protocol 2 provides additional mechanisms for confidentiality
744 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour)
745 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
746 Note that protocol 1 lacks a strong mechanism for ensuring the
747 integrity of the connection.
748 .Ss Login session and remote execution
749 When the user's identity has been accepted by the server, the server
750 either executes the given command, or logs into the machine and gives
751 the user a normal shell on the remote machine.
752 All communication with
753 the remote command or shell will be automatically encrypted.
755 If a pseudo-terminal has been allocated (normal login session), the
756 user may use the escape characters noted below.
758 If no pseudo-tty has been allocated,
759 the session is transparent and can be used to reliably transfer binary data.
760 On most systems, setting the escape character to
762 will also make the session transparent even if a tty is used.
764 The session terminates when the command or shell on the remote
765 machine exits and all X11 and TCP/IP connections have been closed.
766 The exit status of the remote program is returned as the exit status of
770 may additionally obtain configuration data from
771 a per-user configuration file and a system-wide configuration file.
772 The file format and configuration options are described in
774 .Ss Escape Characters
775 When a pseudo-terminal has been requested,
777 supports a number of functions through the use of an escape character.
779 A single tilde character can be sent as
781 or by following the tilde by a character other than those described below.
782 The escape character must always follow a newline to be interpreted as
784 The escape character can be changed in configuration files using the
786 configuration directive or on the command line by the
790 The supported escapes (assuming the default
800 List forwarded connections.
804 at logout when waiting for forwarded connection / X11 sessions to terminate.
806 Display a list of escape characters.
808 Send a BREAK to the remote system
809 (only useful for SSH protocol version 2 and if the peer supports it).
812 Currently this allows the addition of port forwardings using the
817 It also allows the cancellation of existing remote port-forwardings
820 .Ic !\& Ns Ar command
821 allows the user to execute a local command if the
822 .Ic PermitLocalCommand
825 Basic help is available, using the
829 Request rekeying of the connection
830 (only useful for SSH protocol version 2 and if the peer supports it).
832 .Ss X11 and TCP forwarding
837 (or see the description of the
841 options described later)
842 and the user is using X11 (the
844 environment variable is set), the connection to the X11 display is
845 automatically forwarded to the remote side in such a way that any X11
846 programs started from the shell (or command) will go through the
847 encrypted channel, and the connection to the real X server will be made
848 from the local machine.
849 The user should not manually set
851 Forwarding of X11 connections can be
852 configured on the command line or in configuration files.
858 will point to the server machine, but with a display number greater than zero.
859 This is normal, and happens because
863 X server on the server machine for forwarding the
864 connections over the encrypted channel.
867 will also automatically set up Xauthority data on the server machine.
868 For this purpose, it will generate a random authorization cookie,
869 store it in Xauthority on the server, and verify that any forwarded
870 connections carry this cookie and replace it by the real cookie when
871 the connection is opened.
872 The real authentication cookie is never
873 sent to the server machine (and no cookies are sent in the plain).
879 (or see the description of the
883 options described later) and
884 the user is using an authentication agent, the connection to the agent
885 is automatically forwarded to the remote side.
887 Forwarding of arbitrary TCP/IP connections over the secure channel can
888 be specified either on the command line or in a configuration file.
889 One possible application of TCP/IP forwarding is a secure connection to an
890 electronic purse; another is going through firewalls.
891 .Ss Server authentication
893 automatically maintains and checks a database containing
894 identifications for all hosts it has ever been used with.
895 Host keys are stored in
896 .Pa ~/.ssh/known_hosts
897 in the user's home directory.
898 Additionally, the file
899 .Pa /etc/ssh/ssh_known_hosts
900 is automatically checked for known hosts.
901 Any new hosts are automatically added to the user's file.
902 If a host's identification ever changes,
904 warns about this and disables password authentication to prevent a
905 trojan horse from getting the user's password.
906 Another purpose of this mechanism is to prevent man-in-the-middle attacks
907 which could otherwise be used to circumvent the encryption.
909 .Cm StrictHostKeyChecking
910 option can be used to prevent logins to machines whose
911 host key is not known or has changed.
914 can be configured to verify host identification using fingerprint resource
915 records (SSHFP) published in DNS.
918 option can be used to control how DNS lookups are performed.
919 SSHFP resource records can be generated using
923 will normally set the following environment variables:
924 .Bl -tag -width LOGNAME
928 variable indicates the location of the X11 server.
929 It is automatically set by
931 to point to a value of the form
933 where hostname indicates
934 the host where the shell runs, and n is an integer \*(Ge 1.
936 uses this special value to forward X11 connections over the secure
938 The user should normally not set
941 will render the X11 connection insecure (and will require the user to
942 manually copy any required authorization cookies).
944 Set to the path of the user's home directory.
948 set for compatibility with systems that use this variable.
950 Set to the path of the user's mailbox.
954 as specified when compiling
959 needs a passphrase, it will read the passphrase from the current
960 terminal if it was run from a terminal.
963 does not have a terminal associated with it but
967 are set, it will execute the program specified by
969 and open an X11 window to read the passphrase.
970 This is particularly useful when calling
975 (Note that on some machines it
976 may be necessary to redirect the input from
980 Identifies the path of a unix-domain socket used to communicate with the
982 .It Ev SSH_CONNECTION
983 Identifies the client and server ends of the connection.
984 The variable contains
985 four space-separated values: client ip-address, client port number,
986 server ip-address and server port number.
987 .It Ev SSH_ORIGINAL_COMMAND
988 The variable contains the original command line if a forced command
990 It can be used to extract the original arguments.
992 This is set to the name of the tty (path to the device) associated
993 with the current shell or command.
994 If the current session has no tty,
995 this variable is not set.
997 This variable is set to indicate the present time zone if it
998 was set when the daemon was started (i.e., the daemon passes the value
999 on to new connections).
1001 Set to the name of the user logging in.
1007 .Pa ~/.ssh/environment ,
1008 and adds lines of the format
1010 to the environment if the file exists and if users are allowed to
1011 change their environment.
1012 For more information, see the
1013 .Cm PermitUserEnvironment
1018 .It Pa ~/.ssh/known_hosts
1019 Records host keys for all hosts the user has logged into that are not
1021 .Pa /etc/ssh/ssh_known_hosts .
1024 .It Pa ~/.ssh/identity, ~/.ssh/id_dsa, ~/.ssh/id_rsa
1025 Contains the authentication identity of the user.
1026 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1028 contain sensitive data and should be readable by the user but not
1029 accessible by others (read/write/execute).
1032 ignores a private key file if it is accessible by others.
1033 It is possible to specify a passphrase when
1034 generating the key; the passphrase will be used to encrypt the
1035 sensitive part of this file using 3DES.
1036 .It Pa ~/.ssh/identity.pub, ~/.ssh/id_dsa.pub, ~/.ssh/id_rsa.pub
1037 Contains the public key for authentication (public part of the
1038 identity file in human-readable form).
1040 .Pa ~/.ssh/identity.pub
1041 file should be added to the file
1042 .Pa ~/.ssh/authorized_keys
1044 where the user wishes to log in using protocol version 1 RSA authentication.
1046 .Pa ~/.ssh/id_dsa.pub
1048 .Pa ~/.ssh/id_rsa.pub
1049 file should be added to
1050 .Pa ~/.ssh/authorized_keys
1052 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1054 sensitive and can (but need not) be readable by anyone.
1056 never used automatically and are not necessary; they are only provided for
1057 the convenience of the user.
1058 .It Pa ~/.ssh/config
1059 This is the per-user configuration file.
1060 The file format and configuration options are described in
1062 Because of the potential for abuse, this file must have strict permissions:
1063 read/write for the user, and not accessible by others.
1064 .It Pa ~/.ssh/authorized_keys
1065 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1066 The format of this file is described in the
1069 In the simplest form the format is the same as the
1072 This file is not highly sensitive, but the recommended
1073 permissions are read/write for the user, and not accessible by others.
1074 .It Pa /etc/ssh/ssh_known_hosts
1075 Systemwide list of known host keys.
1076 This file should be prepared by the
1077 system administrator to contain the public host keys of all machines in the
1079 This file should be world-readable.
1081 public keys, one per line, in the following format (fields separated
1082 by spaces): system name, public key and optional comment field.
1083 When different names are used
1084 for the same machine, all such names should be listed, separated by
1086 The format is described in the
1090 The canonical system name (as returned by name servers) is used by
1092 to verify the client host when logging in; other names are needed because
1094 does not convert the user-supplied name to a canonical name before
1095 checking the key, because someone with access to the name servers
1096 would then be able to fool host authentication.
1097 .It Pa /etc/ssh/ssh_config
1098 Systemwide configuration file.
1099 The file format and configuration options are described in
1101 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1102 These three files contain the private parts of the host keys
1104 .Cm RhostsRSAAuthentication
1106 .Cm HostbasedAuthentication .
1107 If the protocol version 1
1108 .Cm RhostsRSAAuthentication
1111 must be setuid root, since the host key is readable only by root.
1112 For protocol version 2,
1116 to access the host keys for
1117 .Cm HostbasedAuthentication .
1118 This eliminates the requirement that
1120 be setuid root when that authentication method is used.
1125 This file is used in
1126 .Cm RhostsRSAAuthentication
1128 .Cm HostbasedAuthentication
1129 authentication to list the
1130 host/user pairs that are permitted to log in.
1131 (Note that this file is
1132 also used by rlogin and rsh, which makes using this file insecure.)
1133 Each line of the file contains a host name (in the canonical form
1134 returned by name servers), and then a user name on that host,
1135 separated by a space.
1136 On some machines this file may need to be
1137 world-readable if the user's home directory is on a NFS partition,
1141 Additionally, this file must be owned by the user,
1142 and must not have write permissions for anyone else.
1144 permission for most machines is read/write for the user, and not
1145 accessible by others.
1149 allows authentication only in combination with client host key
1150 authentication before permitting log in.
1151 If the server machine does not have the client's host key in
1152 .Pa /etc/ssh/ssh_known_hosts ,
1154 .Pa ~/.ssh/known_hosts .
1155 The easiest way to do this is to
1156 connect back to the client from the server machine using ssh; this
1157 will automatically add the host key to
1158 .Pa ~/.ssh/known_hosts .
1160 This file is used exactly the same way as
1163 having this file is to be able to use
1164 .Cm RhostsRSAAuthentication
1166 .Cm HostbasedAuthentication
1167 authentication without permitting login with
1171 .It Pa /etc/hosts.equiv
1172 This file is used during
1173 .Cm RhostsRSAAuthentication
1175 .Cm HostbasedAuthentication
1178 canonical hosts names, one per line (the full format is described in the
1181 If the client host is found in this file, login is
1182 automatically permitted provided client and server user names are the
1184 Additionally, successful client host key authentication is required.
1185 This file should only be writable by root.
1186 .It Pa /etc/shosts.equiv
1187 This file is processed exactly as
1188 .Pa /etc/hosts.equiv .
1189 This file may be useful to permit logins using
1191 but not using rsh/rlogin.
1192 .It Pa /etc/ssh/sshrc
1193 Commands in this file are executed by
1195 when the user logs in just before the user's shell (or command) is started.
1198 manual page for more information.
1200 Commands in this file are executed by
1202 when the user logs in just before the user's shell (or command) is
1206 manual page for more information.
1207 .It Pa ~/.ssh/environment
1208 Contains additional definitions for environment variables, see section
1214 exits with the exit status of the remote command or with 255
1215 if an error occurred.
1235 .%T "SSH Protocol Architecture"
1236 .%N draft-ietf-secsh-architecture-12.txt
1238 .%O work in progress material
1241 OpenSSH is a derivative of the original and free
1242 ssh 1.2.12 release by Tatu Ylonen.
1243 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1244 Theo de Raadt and Dug Song
1245 removed many bugs, re-added newer features and
1247 Markus Friedl contributed the support for SSH
1248 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob