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 1.205 2005/03/07 23:41:54 jmc 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
51 .Op Fl e Ar escape_char
52 .Op Fl F Ar configfile
53 .Op Fl i Ar identity_file
56 .Oo Ar bind_address : Oc
57 .Ar port : host : hostport
60 .Op Fl l Ar login_name
67 .Oo Ar bind_address : Oc
68 .Ar port : host : hostport
72 .Oo Ar user Ns @ Oc Ns Ar hostname
77 (SSH client) is a program for logging into a remote machine and for
78 executing commands on a remote machine.
79 It is intended to replace rlogin and rsh,
80 and provide secure encrypted communications between
81 two untrusted hosts over an insecure network.
82 X11 connections and arbitrary TCP/IP ports
83 can also be forwarded over the secure channel.
86 connects and logs into the specified
92 his/her identity to the remote machine using one of several methods
93 depending on the protocol version used.
99 is executed on the remote host instead of a login shell.
100 .Ss SSH protocol version 1
101 The first authentication method is the
105 method combined with RSA-based host authentication.
106 If the machine the user logs in from is listed in
109 .Pa /etc/shosts.equiv
110 on the remote machine, and the user names are
111 the same on both sides, or if the files
115 exist in the user's home directory on the
116 remote machine and contain a line containing the name of the client
117 machine and the name of the user on that machine, the user is
118 considered for log in.
119 Additionally, if the server can verify the client's
121 .Pa /etc/ssh/ssh_known_hosts
123 .Pa $HOME/.ssh/known_hosts
126 section), only then is login permitted.
127 This authentication method closes security holes due to IP
128 spoofing, DNS spoofing and routing spoofing.
129 [Note to the administrator:
130 .Pa /etc/hosts.equiv ,
132 and the rlogin/rsh protocol in general, are inherently insecure and should be
133 disabled if security is desired.]
135 As a second authentication method,
137 supports RSA based authentication.
138 The scheme is based on public-key cryptography: there are cryptosystems
139 where encryption and decryption are done using separate keys, and it
140 is not possible to derive the decryption key from the encryption key.
141 RSA is one such system.
142 The idea is that each user creates a public/private
143 key pair for authentication purposes.
144 The server knows the public key, and only the user knows the private key.
147 .Pa $HOME/.ssh/authorized_keys
148 lists the public keys that are permitted for logging in.
149 When the user logs in, the
151 program tells the server which key pair it would like to use for
153 The server checks if this key is permitted, and if so,
154 sends the user (actually the
156 program running on behalf of the user) a challenge, a random number,
157 encrypted by the user's public key.
158 The challenge can only be decrypted using the proper private key.
159 The user's client then decrypts the challenge using the private key,
160 proving that he/she knows the private key
161 but without disclosing it to the server.
164 implements the RSA authentication protocol automatically.
165 The user creates his/her RSA key pair by running
167 This stores the private key in
168 .Pa $HOME/.ssh/identity
169 and stores the public key in
170 .Pa $HOME/.ssh/identity.pub
171 in the user's home directory.
172 The user should then copy the
175 .Pa $HOME/.ssh/authorized_keys
176 in his/her home directory on the remote machine (the
178 file corresponds to the conventional
180 file, and has one key
181 per line, though the lines can be very long).
182 After this, the user can log in without giving the password.
184 The most convenient way to use RSA authentication may be with an
185 authentication agent.
188 for more information.
190 If other authentication methods fail,
192 prompts the user for a password.
193 The password is sent to the remote
194 host for checking; however, since all communications are encrypted,
195 the password cannot be seen by someone listening on the network.
196 .Ss SSH protocol version 2
197 When a user connects using protocol version 2,
198 similar authentication methods are available.
199 Using the default values for
200 .Cm PreferredAuthentications ,
201 the client will try to authenticate first using the hostbased method;
202 if this method fails, public key authentication is attempted,
203 and finally if this method fails, keyboard-interactive and
204 password authentication are tried.
206 The public key method is similar to RSA authentication described
207 in the previous section and allows the RSA or DSA algorithm to be used:
208 The client uses his private key,
209 .Pa $HOME/.ssh/id_dsa
211 .Pa $HOME/.ssh/id_rsa ,
212 to sign the session identifier and sends the result to the server.
213 The server checks whether the matching public key is listed in
214 .Pa $HOME/.ssh/authorized_keys
215 and grants access if both the key is found and the signature is correct.
216 The session identifier is derived from a shared Diffie-Hellman value
217 and is only known to the client and the server.
219 If public key authentication fails or is not available, a password
220 can be sent encrypted to the remote host to prove the user's identity.
224 supports hostbased or challenge response authentication.
226 Protocol 2 provides additional mechanisms for confidentiality
227 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour)
228 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
229 Note that protocol 1 lacks a strong mechanism for ensuring the
230 integrity of the connection.
231 .Ss Login session and remote execution
232 When the user's identity has been accepted by the server, the server
233 either executes the given command, or logs into the machine and gives
234 the user a normal shell on the remote machine.
235 All communication with
236 the remote command or shell will be automatically encrypted.
238 If a pseudo-terminal has been allocated (normal login session), the
239 user may use the escape characters noted below.
241 If no pseudo-tty has been allocated,
242 the session is transparent and can be used to reliably transfer binary data.
243 On most systems, setting the escape character to
245 will also make the session transparent even if a tty is used.
247 The session terminates when the command or shell on the remote
248 machine exits and all X11 and TCP/IP connections have been closed.
249 The exit status of the remote program is returned as the exit status of
251 .Ss Escape Characters
252 When a pseudo-terminal has been requested,
254 supports a number of functions through the use of an escape character.
256 A single tilde character can be sent as
258 or by following the tilde by a character other than those described below.
259 The escape character must always follow a newline to be interpreted as
261 The escape character can be changed in configuration files using the
263 configuration directive or on the command line by the
267 The supported escapes (assuming the default
277 List forwarded connections.
281 at logout when waiting for forwarded connection / X11 sessions to terminate.
283 Display a list of escape characters.
285 Send a BREAK to the remote system
286 (only useful for SSH protocol version 2 and if the peer supports it).
289 Currently this allows the addition of port forwardings using the
294 It also allows the cancellation of existing remote port-forwardings
297 Basic help is available, using the
301 Request rekeying of the connection
302 (only useful for SSH protocol version 2 and if the peer supports it).
304 .Ss X11 and TCP forwarding
309 (or see the description of the
313 options described later)
314 and the user is using X11 (the
316 environment variable is set), the connection to the X11 display is
317 automatically forwarded to the remote side in such a way that any X11
318 programs started from the shell (or command) will go through the
319 encrypted channel, and the connection to the real X server will be made
320 from the local machine.
321 The user should not manually set
323 Forwarding of X11 connections can be
324 configured on the command line or in configuration files.
330 will point to the server machine, but with a display number greater than zero.
331 This is normal, and happens because
335 X server on the server machine for forwarding the
336 connections over the encrypted channel.
339 will also automatically set up Xauthority data on the server machine.
340 For this purpose, it will generate a random authorization cookie,
341 store it in Xauthority on the server, and verify that any forwarded
342 connections carry this cookie and replace it by the real cookie when
343 the connection is opened.
344 The real authentication cookie is never
345 sent to the server machine (and no cookies are sent in the plain).
351 (or see the description of the
355 options described later) and
356 the user is using an authentication agent, the connection to the agent
357 is automatically forwarded to the remote side.
359 Forwarding of arbitrary TCP/IP connections over the secure channel can
360 be specified either on the command line or in a configuration file.
361 One possible application of TCP/IP forwarding is a secure connection to an
362 electronic purse; another is going through firewalls.
363 .Ss Server authentication
365 automatically maintains and checks a database containing
366 identifications for all hosts it has ever been used with.
367 Host keys are stored in
368 .Pa $HOME/.ssh/known_hosts
369 in the user's home directory.
370 Additionally, the file
371 .Pa /etc/ssh/ssh_known_hosts
372 is automatically checked for known hosts.
373 Any new hosts are automatically added to the user's file.
374 If a host's identification ever changes,
376 warns about this and disables password authentication to prevent a
377 trojan horse from getting the user's password.
378 Another purpose of this mechanism is to prevent man-in-the-middle attacks
379 which could otherwise be used to circumvent the encryption.
381 .Cm StrictHostKeyChecking
382 option can be used to prevent logins to machines whose
383 host key is not known or has changed.
386 can be configured to verify host identification using fingerprint resource
387 records (SSHFP) published in DNS.
390 option can be used to control how DNS lookups are performed.
391 SSHFP resource records can be generated using
394 The options are as follows:
399 to try protocol version 1 only.
403 to try protocol version 2 only.
407 to use IPv4 addresses only.
411 to use IPv6 addresses only.
413 Enables forwarding of the authentication agent connection.
414 This can also be specified on a per-host basis in a configuration file.
416 Agent forwarding should be enabled with caution.
417 Users with the ability to bypass file permissions on the remote host
418 (for the agent's Unix-domain socket)
419 can access the local agent through the forwarded connection.
420 An attacker cannot obtain key material from the agent,
421 however they can perform operations on the keys that enable them to
422 authenticate using the identities loaded into the agent.
424 Disables forwarding of the authentication agent connection.
425 .It Fl b Ar bind_address
426 Specify the interface to transmit from on machines with multiple
427 interfaces or aliased addresses.
429 Requests compression of all data (including stdin, stdout, stderr, and
430 data for forwarded X11 and TCP/IP connections).
431 The compression algorithm is the same used by
435 can be controlled by the
437 option for protocol version 1.
438 Compression is desirable on modem lines and other
439 slow connections, but will only slow down things on fast networks.
440 The default value can be set on a host-by-host basis in the
441 configuration files; see the
444 .It Fl c Ar cipher_spec
445 Selects the cipher specification for encrypting the session.
447 Protocol version 1 allows specification of a single cipher.
448 The suported values are
454 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
455 It is believed to be secure.
457 is a fast block cipher; it appears very secure and is much faster than
460 is only supported in the
462 client for interoperability with legacy protocol 1 implementations
463 that do not support the
466 Its use is strongly discouraged due to cryptographic weaknesses.
470 For protocol version 2
472 is a comma-separated list of ciphers
473 listed in order of preference.
474 The supported ciphers are
488 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
489 aes192-cbc,aes256-cbc''
494 application-level port forwarding.
495 This works by allocating a socket to listen to
497 on the local side, and whenever a connection is made to this port, the
498 connection is forwarded over the secure channel, and the application
499 protocol is then used to determine where to connect to from the
501 Currently the SOCKS4 and SOCKS5 protocols are supported, and
503 will act as a SOCKS server.
504 Only root can forward privileged ports.
505 Dynamic port forwardings can also be specified in the configuration file.
506 .It Fl e Ar ch | ^ch | none
507 Sets the escape character for sessions with a pty (default:
509 The escape character is only recognized at the beginning of a line.
510 The escape character followed by a dot
512 closes the connection;
513 followed by control-Z suspends the connection;
514 and followed by itself sends the escape character once.
515 Setting the character to
517 disables any escapes and makes the session fully transparent.
518 .It Fl F Ar configfile
519 Specifies an alternative per-user configuration file.
520 If a configuration file is given on the command line,
521 the system-wide configuration file
522 .Pq Pa /etc/ssh/ssh_config
524 The default for the per-user configuration file is
525 .Pa $HOME/.ssh/config .
529 to go to background just before command execution.
532 is going to ask for passwords or passphrases, but the user
533 wants it in the background.
536 The recommended way to start X11 programs at a remote site is with
538 .Ic ssh -f host xterm .
540 Allows remote hosts to connect to local forwarded ports.
541 .It Fl I Ar smartcard_device
542 Specifies which smartcard device to use.
543 The argument is the device
545 should use to communicate with a smartcard used for storing the user's
547 .It Fl i Ar identity_file
548 Selects a file from which the identity (private key) for
549 RSA or DSA authentication is read.
551 .Pa $HOME/.ssh/identity
552 for protocol version 1, and
553 .Pa $HOME/.ssh/id_rsa
555 .Pa $HOME/.ssh/id_dsa
556 for protocol version 2.
557 Identity files may also be specified on
558 a per-host basis in the configuration file.
559 It is possible to have multiple
561 options (and multiple identities specified in
562 configuration files).
564 Disables forwarding (delegation) of GSSAPI credentials to the server.
567 .Oo Ar bind_address : Oc
568 .Ar port : host : hostport
571 Specifies that the given port on the local (client) host is to be
572 forwarded to the given host and port on the remote side.
573 This works by allocating a socket to listen to
575 on the local side, optionally bound to the specified
577 Whenever a connection is made to this port, the
578 connection is forwarded over the secure channel, and a connection is
583 from the remote machine.
584 Port forwardings can also be specified in the configuration file.
585 IPv6 addresses can be specified with an alternative syntax:
588 .Op Ar bind_address No /
589 .Ar port No / Ar host No /
593 or by enclosing the address in square brackets.
594 Only the superuser can forward privileged ports.
595 By default, the local port is bound in accordance with the
600 may be used to bind the connection to a specific address.
605 indicates that the listening port be bound for local use only, while an
608 indicates that the port should be available from all interfaces.
609 .It Fl l Ar login_name
610 Specifies the user to log in as on the remote machine.
611 This also may be specified on a per-host basis in the configuration file.
617 mode for connection sharing.
618 Refer to the description of
624 Additionally, for protocol version 2 a comma-separated list of MAC
625 (message authentication code) algorithms can
626 be specified in order of preference.
629 keyword for more information.
631 Do not execute a remote command.
632 This is useful for just forwarding ports
633 (protocol version 2 only).
637 (actually, prevents reading from stdin).
638 This must be used when
640 is run in the background.
641 A common trick is to use this to run X11 programs on a remote machine.
643 .Ic ssh -n shadows.cs.hut.fi emacs &
644 will start an emacs on shadows.cs.hut.fi, and the X11
645 connection will be automatically forwarded over an encrypted channel.
648 program will be put in the background.
649 (This does not work if
651 needs to ask for a password or passphrase; see also the
655 Control an active connection multiplexing master process.
658 option is specified, the
660 argument is interpreted and passed to the master process.
663 (check that the master process is running) and
665 (request the master to exit).
667 Can be used to give options in the format used in the configuration file.
668 This is useful for specifying options for which there is no separate
670 For full details of the options listed below, and their possible values, see
673 .Bl -tag -width Ds -offset indent -compact
677 .It ChallengeResponseAuthentication
681 .It ClearAllForwardings
684 .It ConnectionAttempts
692 .It ForwardX11Trusted
694 .It GlobalKnownHostsFile
695 .It GSSAPIAuthentication
696 .It GSSAPIDelegateCredentials
699 .It HostbasedAuthentication
700 .It HostKeyAlgorithms
705 .It KbdInteractiveDevices
709 .It NoHostAuthenticationForLocalhost
710 .It NumberOfPasswordPrompts
711 .It PasswordAuthentication
713 .It PreferredAuthentications
716 .It PubkeyAuthentication
718 .It RhostsRSAAuthentication
719 .It RSAAuthentication
721 .It ServerAliveInterval
722 .It ServerAliveCountMax
724 .It StrictHostKeyChecking
726 .It UsePrivilegedPort
728 .It UserKnownHostsFile
733 Port to connect to on the remote host.
734 This can be specified on a
735 per-host basis in the configuration file.
738 Causes all warning and diagnostic messages to be suppressed.
741 .Oo Ar bind_address : Oc
742 .Ar port : host : hostport
745 Specifies that the given port on the remote (server) host is to be
746 forwarded to the given host and port on the local side.
747 This works by allocating a socket to listen to
749 on the remote side, and whenever a connection is made to this port, the
750 connection is forwarded over the secure channel, and a connection is
755 from the local machine.
757 Port forwardings can also be specified in the configuration file.
758 Privileged ports can be forwarded only when
759 logging in as root on the remote machine.
760 IPv6 addresses can be specified by enclosing the address in square braces or
761 using an alternative syntax:
764 .Op Ar bind_address No /
765 .Ar host No / Ar port No /
770 By default, the listening socket on the server will be bound to the loopback
772 This may be overriden by specifying a
778 indicates that the remote socket should listen on all interfaces.
781 will only succeed if the server's
783 option is enabled (see
784 .Xr sshd_config 5 ) .
786 Specifies the location of a control socket for connection sharing.
787 Refer to the description of
795 May be used to request invocation of a subsystem on the remote system.
796 Subsystems are a feature of the SSH2 protocol which facilitate the use
797 of SSH as a secure transport for other applications (eg.\&
799 The subsystem is specified as the remote command.
801 Disable pseudo-tty allocation.
803 Force pseudo-tty allocation.
804 This can be used to execute arbitrary
805 screen-based programs on a remote machine, which can be very useful,
806 e.g., when implementing menu services.
809 options force tty allocation, even if
813 Display the version number and exit.
818 to print debugging messages about its progress.
820 debugging connection, authentication, and configuration problems.
823 options increase the verbosity.
826 Enables X11 forwarding.
827 This can also be specified on a per-host basis in a configuration file.
829 X11 forwarding should be enabled with caution.
830 Users with the ability to bypass file permissions on the remote host
831 (for the user's X authorization database)
832 can access the local X11 display through the forwarded connection.
833 An attacker may then be able to perform activities such as keystroke monitoring.
835 For this reason, X11 forwarding is subjected to X11 SECURITY extension
836 restrictions by default.
841 .Cm ForwardX11Trusted
844 for more information.
846 Disables X11 forwarding.
848 Enables trusted X11 forwarding.
849 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
852 .Sh CONFIGURATION FILES
854 may additionally obtain configuration data from
855 a per-user configuration file and a system-wide configuration file.
856 The file format and configuration options are described in
860 will normally set the following environment variables:
861 .Bl -tag -width LOGNAME
865 variable indicates the location of the X11 server.
866 It is automatically set by
868 to point to a value of the form
870 where hostname indicates
871 the host where the shell runs, and n is an integer \*(Ge 1.
873 uses this special value to forward X11 connections over the secure
875 The user should normally not set
878 will render the X11 connection insecure (and will require the user to
879 manually copy any required authorization cookies).
881 Set to the path of the user's home directory.
885 set for compatibility with systems that use this variable.
887 Set to the path of the user's mailbox.
891 as specified when compiling
896 needs a passphrase, it will read the passphrase from the current
897 terminal if it was run from a terminal.
900 does not have a terminal associated with it but
904 are set, it will execute the program specified by
906 and open an X11 window to read the passphrase.
907 This is particularly useful when calling
912 (Note that on some machines it
913 may be necessary to redirect the input from
917 Identifies the path of a unix-domain socket used to communicate with the
919 .It Ev SSH_CONNECTION
920 Identifies the client and server ends of the connection.
921 The variable contains
922 four space-separated values: client ip-address, client port number,
923 server ip-address and server port number.
924 .It Ev SSH_ORIGINAL_COMMAND
925 The variable contains the original command line if a forced command
927 It can be used to extract the original arguments.
929 This is set to the name of the tty (path to the device) associated
930 with the current shell or command.
931 If the current session has no tty,
932 this variable is not set.
934 The timezone variable is set to indicate the present timezone if it
935 was set when the daemon was started (i.e., the daemon passes the value
936 on to new connections).
938 Set to the name of the user logging in.
944 .Pa $HOME/.ssh/environment ,
945 and adds lines of the format
947 to the environment if the file exists and if users are allowed to
948 change their environment.
949 For more information, see the
950 .Cm PermitUserEnvironment
955 .It Pa $HOME/.ssh/known_hosts
956 Records host keys for all hosts the user has logged into that are not
958 .Pa /etc/ssh/ssh_known_hosts .
961 .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
962 Contains the authentication identity of the user.
963 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
965 contain sensitive data and should be readable by the user but not
966 accessible by others (read/write/execute).
969 ignores a private key file if it is accessible by others.
970 It is possible to specify a passphrase when
971 generating the key; the passphrase will be used to encrypt the
972 sensitive part of this file using 3DES.
973 .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
974 Contains the public key for authentication (public part of the
975 identity file in human-readable form).
977 .Pa $HOME/.ssh/identity.pub
978 file should be added to the file
979 .Pa $HOME/.ssh/authorized_keys
981 where the user wishes to log in using protocol version 1 RSA authentication.
983 .Pa $HOME/.ssh/id_dsa.pub
985 .Pa $HOME/.ssh/id_rsa.pub
986 file should be added to
987 .Pa $HOME/.ssh/authorized_keys
989 where the user wishes to log in using protocol version 2 DSA/RSA authentication.
991 sensitive and can (but need not) be readable by anyone.
993 never used automatically and are not necessary; they are only provided for
994 the convenience of the user.
995 .It Pa $HOME/.ssh/config
996 This is the per-user configuration file.
997 The file format and configuration options are described in
999 Because of the potential for abuse, this file must have strict permissions:
1000 read/write for the user, and not accessible by others.
1001 .It Pa $HOME/.ssh/authorized_keys
1002 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1003 The format of this file is described in the
1006 In the simplest form the format is the same as the
1009 This file is not highly sensitive, but the recommended
1010 permissions are read/write for the user, and not accessible by others.
1011 .It Pa /etc/ssh/ssh_known_hosts
1012 Systemwide list of known host keys.
1013 This file should be prepared by the
1014 system administrator to contain the public host keys of all machines in the
1016 This file should be world-readable.
1018 public keys, one per line, in the following format (fields separated
1019 by spaces): system name, public key and optional comment field.
1020 When different names are used
1021 for the same machine, all such names should be listed, separated by
1023 The format is described in the
1027 The canonical system name (as returned by name servers) is used by
1029 to verify the client host when logging in; other names are needed because
1031 does not convert the user-supplied name to a canonical name before
1032 checking the key, because someone with access to the name servers
1033 would then be able to fool host authentication.
1034 .It Pa /etc/ssh/ssh_config
1035 Systemwide configuration file.
1036 The file format and configuration options are described in
1038 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1039 These three files contain the private parts of the host keys
1041 .Cm RhostsRSAAuthentication
1043 .Cm HostbasedAuthentication .
1044 If the protocol version 1
1045 .Cm RhostsRSAAuthentication
1048 must be setuid root, since the host key is readable only by root.
1049 For protocol version 2,
1053 to access the host keys for
1054 .Cm HostbasedAuthentication .
1055 This eliminates the requirement that
1057 be setuid root when that authentication method is used.
1061 .It Pa $HOME/.rhosts
1062 This file is used in
1063 .Cm RhostsRSAAuthentication
1065 .Cm HostbasedAuthentication
1066 authentication to list the
1067 host/user pairs that are permitted to log in.
1068 (Note that this file is
1069 also used by rlogin and rsh, which makes using this file insecure.)
1070 Each line of the file contains a host name (in the canonical form
1071 returned by name servers), and then a user name on that host,
1072 separated by a space.
1073 On some machines this file may need to be
1074 world-readable if the user's home directory is on a NFS partition,
1078 Additionally, this file must be owned by the user,
1079 and must not have write permissions for anyone else.
1081 permission for most machines is read/write for the user, and not
1082 accessible by others.
1086 allows authentication only in combination with client host key
1087 authentication before permitting log in.
1088 If the server machine does not have the client's host key in
1089 .Pa /etc/ssh/ssh_known_hosts ,
1091 .Pa $HOME/.ssh/known_hosts .
1092 The easiest way to do this is to
1093 connect back to the client from the server machine using ssh; this
1094 will automatically add the host key to
1095 .Pa $HOME/.ssh/known_hosts .
1096 .It Pa $HOME/.shosts
1097 This file is used exactly the same way as
1100 having this file is to be able to use
1101 .Cm RhostsRSAAuthentication
1103 .Cm HostbasedAuthentication
1104 authentication without permitting login with
1108 .It Pa /etc/hosts.equiv
1109 This file is used during
1110 .Cm RhostsRSAAuthentication
1112 .Cm HostbasedAuthentication
1115 canonical hosts names, one per line (the full format is described in the
1118 If the client host is found in this file, login is
1119 automatically permitted provided client and server user names are the
1121 Additionally, successful client host key authentication is required.
1122 This file should only be writable by root.
1123 .It Pa /etc/shosts.equiv
1124 This file is processed exactly as
1125 .Pa /etc/hosts.equiv .
1126 This file may be useful to permit logins using
1128 but not using rsh/rlogin.
1129 .It Pa /etc/ssh/sshrc
1130 Commands in this file are executed by
1132 when the user logs in just before the user's shell (or command) is started.
1135 manual page for more information.
1136 .It Pa $HOME/.ssh/rc
1137 Commands in this file are executed by
1139 when the user logs in just before the user's shell (or command) is
1143 manual page for more information.
1144 .It Pa $HOME/.ssh/environment
1145 Contains additional definitions for environment variables, see section
1151 exits with the exit status of the remote command or with 255
1152 if an error occurred.
1172 .%T "SSH Protocol Architecture"
1173 .%N draft-ietf-secsh-architecture-12.txt
1175 .%O work in progress material
1178 OpenSSH is a derivative of the original and free
1179 ssh 1.2.12 release by Tatu Ylonen.
1180 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1181 Theo de Raadt and Dug Song
1182 removed many bugs, re-added newer features and
1184 Markus Friedl contributed the support for SSH
1185 protocol versions 1.5 and 2.0.
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