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: sshd.8,v 1.76 2000/12/28 12:03:57 markus Exp $
38 .Dd September 25, 1999
43 .Nd secure shell daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
54 .Op Fl V Ar client_protocol_id
57 (Secure Shell Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256 bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
124 are disabled (thus completely disabling
130 .Ss SSH protocol version 2
132 Version 2 works similarly:
133 Each host has a host-specific DSA key used to identify the host.
134 However, when the daemon starts, it does not generate a server key.
135 Forward security is provided through a Diffie-Hellman key agreement.
136 This key agreement results in a shared session key.
137 The rest of the session is encrypted
138 using a symmetric cipher, currently
139 Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
140 The client selects the encryption algorithm
141 to use from those offered by the server.
142 Additionally, session integrity is provided
143 through a cryptographic message authentication code
144 (hmac-sha1 or hmac-md5).
146 Protocol version 2 provides a public key based
147 user authentication method (PubkeyAuthentication)
148 and conventional password authentication.
150 .Ss Command execution and data forwarding
152 If the client successfully authenticates itself, a dialog for
153 preparing the session is entered.
154 At this time the client may request
155 things like allocating a pseudo-tty, forwarding X11 connections,
156 forwarding TCP/IP connections, or forwarding the authentication agent
157 connection over the secure channel.
159 Finally, the client either requests a shell or execution of a command.
160 The sides then enter session mode.
161 In this mode, either side may send
162 data at any time, and such data is forwarded to/from the shell or
163 command on the server side, and the user terminal in the client side.
165 When the user program terminates and all forwarded X11 and other
166 connections have been closed, the server sends command exit status to
167 the client, and both sides exit.
170 can be configured using command-line options or a configuration
172 Command-line options override values specified in the
176 rereads its configuration file when it receives a hangup signal,
179 The options are as follows:
182 Specifies the number of bits in the server key (default 768).
186 The server sends verbose debug output to the system
187 log, and does not put itself in the background.
188 The server also will not fork and will only process one connection.
189 This option is only intended for debugging for the server.
190 Multiple -d options increases the debugging level.
192 .It Fl f Ar configuration_file
193 Specifies the name of the configuration file.
195 .Pa /etc/sshd_config .
197 refuses to start if there is no configuration file.
198 .It Fl g Ar login_grace_time
199 Gives the grace time for clients to authenticate themselves (default
201 If the client fails to authenticate the user within
202 this many seconds, the server disconnects and exits.
203 A value of zero indicates no limit.
204 .It Fl h Ar host_key_file
205 Specifies the file from which the host key is read (default
206 .Pa /etc/ssh_host_key ) .
207 This option must be given if
209 is not run as root (as the normal
210 host file is normally not readable by anyone but root).
211 It is possible to have multiple host key files for
212 the different protocol versions.
216 is being run from inetd.
219 from inetd because it needs to generate the server key before it can
220 respond to the client, and this may take tens of seconds.
221 Clients would have to wait too long if the key was regenerated every time.
222 However, with small key sizes (e.g., 512) using
226 .It Fl k Ar key_gen_time
227 Specifies how often the server key is regenerated (default 3600
228 seconds, or one hour).
229 The motivation for regenerating the key fairly
230 often is that the key is not stored anywhere, and after about an hour,
231 it becomes impossible to recover the key for decrypting intercepted
232 communications even if the machine is cracked into or physically
234 A value of zero indicates that the key will never be regenerated.
236 Specifies the port on which the server listens for connections
240 Nothing is sent to the system log.
241 Normally the beginning,
242 authentication, and termination of each connection is logged.
244 This option is used to specify the size of the field
247 structure that holds the remote host name.
248 If the resolved host name is longer than
250 the dotted decimal value will be used instead.
251 This allows hosts with very long host names that
252 overflow this field to still be uniquely identified.
255 indicates that only dotted decimal addresses
256 should be put into the
260 When this option is specified
262 will not detach and does not become a daemon.
263 This allows easy monitoring of
266 Do not print an error message if RSA support is missing.
267 .It Fl V Ar client_protocol_id
268 SSH-2 compatibility mode.
269 When this option is specified
271 assumes the client has sent the supplied version string
273 Protocol Version Identification Exchange.
274 This option is not intended to be called directly.
278 to use IPv4 addresses only.
282 to use IPv6 addresses only.
284 .Sh CONFIGURATION FILE
286 reads configuration data from
288 (or the file specified with
290 on the command line).
291 The file contains keyword-value pairs, one per line.
294 and empty lines are interpreted as comments.
296 The following keywords are possible.
298 .It Cm AFSTokenPassing
299 Specifies whether an AFS token may be forwarded to the server.
303 This keyword can be followed by a number of group names, separated
305 If specified, login is allowed only for users whose primary
306 group matches one of the patterns.
311 wildcards in the patterns.
312 Only group names are valid; a numerical group ID isn't recognized.
313 By default login is allowed regardless of the primary group.
315 .It Cm AllowTcpForwarding
316 Specifies whether TCP forwarding is permitted.
319 Note that disabling TCP forwarding does not improve security unless
320 users are also denied shell access, as they can always install their
324 This keyword can be followed by a number of user names, separated
326 If specified, login is allowed only for users names that
327 match one of the patterns.
332 wildcards in the patterns.
333 Only user names are valid; a numerical user ID isn't recognized.
334 By default login is allowed regardless of the user name.
337 Specifies the ciphers allowed for protocol version 2.
338 Multiple ciphers must be comma-separated.
340 .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
344 should check for new mail for interactive logins.
348 This keyword can be followed by a number of group names, separated
350 Users whose primary group matches one of the patterns
351 aren't allowed to log in.
356 wildcards in the patterns.
357 Only group names are valid; a numerical group ID isn't recognized.
358 By default login is allowed regardless of the primary group.
361 This keyword can be followed by a number of user names, separated
363 Login is disallowed for user names that match one of the patterns.
367 can be used as wildcards in the patterns.
368 Only user names are valid; a numerical user ID isn't recognized.
369 By default login is allowed regardless of the user name.
370 .It Cm PubkeyAuthentication
371 Specifies whether public key authentication is allowed.
374 Note that this option applies to protocol version 2 only.
376 Specifies whether remote hosts are allowed to connect to ports
377 forwarded for the client.
385 Specifies the file containing the private host keys (default
386 .Pa /etc/ssh_host_key )
387 used by SSH protocol versions 1 and 2.
390 if this file is group/world-accessible.
391 It is possible to have multiple host key files.
393 keys are used for version 1 and
397 are used for version 2 of the SSH protocol.
403 files will not be used in authentication.
406 .Pa /etc/shosts.equiv
410 .It Cm IgnoreUserKnownHosts
413 should ignore the user's
414 .Pa $HOME/.ssh/known_hosts
416 .Cm RhostsRSAAuthentication .
420 Specifies whether the system should send keepalive messages to the
422 If they are sent, death of the connection or crash of one
423 of the machines will be properly noticed.
424 However, this means that
425 connections will die if the route is down temporarily, and some people
427 On the other hand, if keepalives are not sent,
428 sessions may hang indefinitely on the server, leaving
430 users and consuming server resources.
434 (to send keepalives), and the server will notice
435 if the network goes down or the client host reboots.
436 This avoids infinitely hanging sessions.
438 To disable keepalives, the value should be set to
440 in both the server and the client configuration files.
441 .It Cm KerberosAuthentication
442 Specifies whether Kerberos authentication is allowed.
443 This can be in the form of a Kerberos ticket, or if
444 .Cm PasswordAuthentication
445 is yes, the password provided by the user will be validated through
447 To use this option, the server needs a
448 Kerberos servtab which allows the verification of the KDC's identity.
451 .It Cm KerberosOrLocalPasswd
452 If set then if password authentication through Kerberos fails then
453 the password will be validated via any additional local mechanism
458 .It Cm KerberosTgtPassing
459 Specifies whether a Kerberos TGT may be forwarded to the server.
462 as this only works when the Kerberos KDC is actually an AFS kaserver.
463 .It Cm KerberosTicketCleanup
464 Specifies whether to automatically destroy the user's ticket cache
468 .It Cm KeyRegenerationInterval
469 The server key is automatically regenerated after this many seconds
470 (if it has been used).
471 The purpose of regeneration is to prevent
472 decrypting captured sessions by later breaking into the machine and
474 The key is never stored anywhere.
475 If the value is 0, the key is never regenerated.
476 The default is 3600 (seconds).
478 Specifies what local address
481 The default is to listen to all local addresses.
482 Multiple options of this type are permitted.
485 options must precede this option.
486 .It Cm LoginGraceTime
487 The server disconnects after this time if the user has not
488 successfully logged in.
489 If the value is 0, there is no time limit.
490 The default is 600 (seconds).
492 Gives the verbosity level that is used when logging messages from
494 The possible values are:
495 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
497 Logging with level DEBUG violates the privacy of users
498 and is not recommended.
500 Specifies the maximum number of concurrent unauthenticated connections to the
503 Additional connections will be dropped until authentication succeeds or the
505 expires for a connection.
508 Alternatively, random early drop can be enabled by specifying
509 the three colon separated values
513 will refuse connection attempts with a probabillity of
516 if there are currently
519 unauthenticated connections.
520 The probabillity increases linearly and all connection attempts
521 are refused if the number of unauthenticated connections reaches
524 .It Cm PasswordAuthentication
525 Specifies whether password authentication is allowed.
528 Note that this option applies to both protocol versions 1 and 2.
529 .It Cm PermitEmptyPasswords
530 When password authentication is allowed, it specifies whether the
531 server allows login to accounts with empty password strings.
534 .It Cm PermitRootLogin
535 Specifies whether the root can log in using
544 If this options is set to
546 only password authentication is disabled for root.
548 Root login with RSA authentication when the
551 specified will be allowed regardless of the value of this setting
552 (which may be useful for taking remote backups even if root login is
553 normally not allowed).
555 Specifies the file that contains the process identifier of the
559 .Pa /var/run/sshd.pid .
561 Specifies the port number that
565 Multiple options of this type are permitted.
571 when a user logs in interactively.
572 (On some systems it is also printed by the shell,
578 Specifies the protocol versions
581 The possible values are
585 Multiple versions must be comma-separated.
590 Random number generation uses other techniques.
591 .It Cm RhostsAuthentication
592 Specifies whether authentication using rhosts or /etc/hosts.equiv
594 Normally, this method should not be permitted because it is insecure.
595 .Cm RhostsRSAAuthentication
597 instead, because it performs RSA-based host authentication in addition
598 to normal rhosts or /etc/hosts.equiv authentication.
601 .It Cm RhostsRSAAuthentication
602 Specifies whether rhosts or /etc/hosts.equiv authentication together
603 with successful RSA host authentication is allowed.
606 .It Cm RSAAuthentication
607 Specifies whether pure RSA authentication is allowed.
610 Note that this option applies to protocol version 1 only.
612 Defines the number of bits in the server key.
613 The minimum value is 512, and the default is 768.
614 .It Cm SkeyAuthentication
617 authentication is allowed.
620 Note that s/key authentication is enabled only if
621 .Cm PasswordAuthentication
626 should check file modes and ownership of the
627 user's files and home directory before accepting login.
628 This is normally desirable because novices sometimes accidentally leave their
629 directory or files world-writable.
633 Configures an external subsystem (e.g., file transfer daemon).
634 Arguments should be a subsystem name and a command to execute upon subsystem
640 file transfer subsystem.
641 By default no subsystems are defined.
642 Note that this option applies to protocol version 2 only.
643 .It Cm SyslogFacility
644 Gives the facility code that is used when logging messages from
646 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
647 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
652 is used for interactive login sessions.
655 is never used for remote command execution.
658 .It Cm X11DisplayOffset
659 Specifies the first display number available for
664 from interfering with real X11 servers.
667 Specifies whether X11 forwarding is permitted.
670 Note that disabling X11 forwarding does not improve security in any
671 way, as users can always install their own forwarders.
673 Specifies the location of the
677 .Pa /usr/X11R6/bin/xauth .
680 When a user successfully logs in,
683 .Bl -enum -offset indent
685 If the login is on a tty, and no command has been specified,
686 prints last login time and
688 (unless prevented in the configuration file or by
689 .Pa $HOME/.hushlogin ;
694 If the login is on a tty, records login time.
698 if it exists, prints contents and quits
701 Changes to run with normal user privileges.
703 Sets up basic environment.
706 .Pa $HOME/.ssh/environment
709 Changes to user's home directory.
713 exists, runs it; else if
716 it; otherwise runs xauth.
719 files are given the X11
720 authentication protocol and cookie in standard input.
722 Runs user's shell or command.
724 .Sh AUTHORIZED_KEYS FILE FORMAT
726 .Pa $HOME/.ssh/authorized_keys
727 file lists the RSA keys that are
728 permitted for RSA authentication in SSH protocols 1.3 and 1.5
730 .Pa $HOME/.ssh/authorized_keys2
731 file lists the DSA and RSA keys that are
732 permitted for public key authentication (PubkeyAuthentication)
735 Each line of the file contains one
736 key (empty lines and lines starting with a
740 Each RSA public key consists of the following fields, separated by
741 spaces: options, bits, exponent, modulus, comment.
742 Each protocol version 2 public key consists of:
743 options, keytype, base64 encoded key, comment.
745 are optional; its presence is determined by whether the line starts
746 with a number or not (the option field never starts with a number).
747 The bits, exponent, modulus and comment fields give the RSA key for
748 protocol version 1; the
749 comment field is not used for anything (but may be convenient for the
750 user to identify the key).
751 For protocol version 2 the keytype is
756 Note that lines in this file are usually several hundred bytes long
757 (because of the size of the RSA key modulus).
758 You don't want to type them in; instead, copy the
764 The options (if present) consist of comma-separated option
766 No spaces are permitted, except within double quotes.
767 The following option specifications are supported:
769 .It Cm from="pattern-list"
770 Specifies that in addition to RSA authentication, the canonical name
771 of the remote host must be present in the comma-separated list of
777 The list may also contain
778 patterns negated by prefixing them with
780 if the canonical host name matches a negated pattern, the key is not accepted.
782 of this option is to optionally increase security: RSA authentication
783 by itself does not trust the network or name servers or anything (but
784 the key); however, if somebody somehow steals the key, the key
785 permits an intruder to log in from anywhere in the world.
786 This additional option makes using a stolen key more difficult (name
787 servers and/or routers would have to be compromised in addition to
789 .It Cm command="command"
790 Specifies that the command is executed whenever this key is used for
792 The command supplied by the user (if any) is ignored.
793 The command is run on a pty if the connection requests a pty;
794 otherwise it is run without a tty.
795 A quote may be included in the command by quoting it with a backslash.
796 This option might be useful
797 to restrict certain RSA keys to perform just a specific operation.
798 An example might be a key that permits remote backups but nothing else.
799 Note that the client may specify TCP/IP and/or X11
800 forwarding unless they are explicitly prohibited.
801 .It Cm environment="NAME=value"
802 Specifies that the string is to be added to the environment when
803 logging in using this key.
804 Environment variables set this way
805 override other default environment values.
806 Multiple options of this type are permitted.
807 .It Cm no-port-forwarding
808 Forbids TCP/IP forwarding when this key is used for authentication.
809 Any port forward requests by the client will return an error.
810 This might be used, e.g., in connection with the
813 .It Cm no-X11-forwarding
814 Forbids X11 forwarding when this key is used for authentication.
815 Any X11 forward requests by the client will return an error.
816 .It Cm no-agent-forwarding
817 Forbids authentication agent forwarding when this key is used for
820 Prevents tty allocation (a request to allocate a pty will fail).
823 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
825 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
827 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
828 .Sh SSH_KNOWN_HOSTS FILE FORMAT
830 .Pa /etc/ssh_known_hosts ,
831 .Pa /etc/ssh_known_hosts2 ,
832 .Pa $HOME/.ssh/known_hosts ,
834 .Pa $HOME/.ssh/known_hosts2
835 files contain host public keys for all known hosts.
836 The global file should
837 be prepared by the administrator (optional), and the per-user file is
838 maintained automatically: whenever the user connects from an unknown host
839 its key is added to the per-user file.
841 Each line in these files contains the following fields: hostnames,
842 bits, exponent, modulus, comment.
843 The fields are separated by spaces.
845 Hostnames is a comma-separated list of patterns ('*' and '?' act as
846 wildcards); each pattern in turn is matched against the canonical host
847 name (when authenticating a client) or against the user-supplied
848 name (when authenticating a server).
849 A pattern may also be preceded by
851 to indicate negation: if the host name matches a negated
852 pattern, it is not accepted (by that line) even if it matched another
855 Bits, exponent, and modulus are taken directly from the RSA host key; they
856 can be obtained, e.g., from
857 .Pa /etc/ssh_host_key.pub .
858 The optional comment field continues to the end of the line, and is not used.
862 and empty lines are ignored as comments.
864 When performing host authentication, authentication is accepted if any
865 matching line has the proper key.
866 It is thus permissible (but not
867 recommended) to have several lines or different host keys for the same
869 This will inevitably happen when short forms of host names
870 from different domains are put in the file.
872 that the files contain conflicting information; authentication is
873 accepted if valid information can be found from either file.
875 Note that the lines in these files are typically hundreds of characters
876 long, and you definitely don't want to type in the host keys by hand.
877 Rather, generate them by a script
879 .Pa /etc/ssh_host_key.pub
880 and adding the host names at the front.
882 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
885 .It Pa /etc/sshd_config
886 Contains configuration data for
888 This file should be writable by root only, but it is recommended
889 (though not necessary) that it be world-readable.
890 .It Pa /etc/ssh_host_key
891 Contains the private part of the host key.
892 This file should only be owned by root, readable only by root, and not
893 accessible to others.
896 does not start if this file is group/world-accessible.
897 .It Pa /etc/ssh_host_key.pub
898 Contains the public part of the host key.
899 This file should be world-readable but writable only by
901 Its contents should match the private part.
903 really used for anything; it is only provided for the convenience of
904 the user so its contents can be copied to known hosts files.
905 These two files are created using
908 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
909 .It Pa /var/run/sshd.pid
910 Contains the process ID of the
912 listening for connections (if there are several daemons running
913 concurrently for different ports, this contains the pid of the one
915 The content of this file is not sensitive; it can be world-readable.
916 .It Pa $HOME/.ssh/authorized_keys
917 Lists the RSA keys that can be used to log into the user's account.
918 This file must be readable by root (which may on some machines imply
919 it being world-readable if the user's home directory resides on an NFS
921 It is recommended that it not be accessible by others.
922 The format of this file is described above.
923 Users will place the contents of their
925 files into this file, as described in
927 .It Pa $HOME/.ssh/authorized_keys2
928 Lists the DSA keys that can be used to log into the user's account.
929 This file must be readable by root (which may on some machines imply
930 it being world-readable if the user's home directory resides on an NFS
932 It is recommended that it not be accessible by others.
933 The format of this file is described above.
934 Users will place the contents of their
936 files into this file, as described in
938 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
939 These files are consulted when using rhosts with RSA host
940 authentication to check the public key of the host.
941 The key must be listed in one of these files to be accepted.
942 The client uses the same files
943 to verify that the remote host is the one it intended to connect.
944 These files should be writable only by root/the owner.
945 .Pa /etc/ssh_known_hosts
946 should be world-readable, and
947 .Pa $HOME/.ssh/known_hosts
948 can but need not be world-readable.
952 refuses to let anyone except root log in.
953 The contents of the file
954 are displayed to anyone trying to log in, and non-root connections are
956 The file should be world-readable.
957 .It Pa /etc/hosts.allow, /etc/hosts.deny
960 support, tcp-wrappers access controls may be defined here as described in
963 This file contains host-username pairs, separated by a space, one per
965 The given user on the corresponding host is permitted to log in
967 The same file is used by rlogind and rshd.
969 be writable only by the user; it is recommended that it not be
970 accessible by others.
972 If is also possible to use netgroups in the file.
974 name may be of the form +@groupname to specify all hosts or all users
978 this file is exactly the same as for
980 However, this file is
981 not used by rlogin and rshd, so using this permits access using SSH only.
982 .It Pa /etc/hosts.equiv
983 This file is used during
986 In the simplest form, this file contains host names, one per line.
988 those hosts are permitted to log in without a password, provided they
989 have the same user name on both machines.
990 The host name may also be
991 followed by a user name; such users are permitted to log in as
993 user on this machine (except root).
994 Additionally, the syntax
996 can be used to specify netgroups.
997 Negated entries start with
1000 If the client host/user is successfully matched in this file, login is
1001 automatically permitted provided the client and server user names are the
1003 Additionally, successful RSA host authentication is normally required.
1004 This file must be writable only by root; it is recommended
1005 that it be world-readable.
1007 .Sy "Warning: It is almost never a good idea to use user names in"
1009 Beware that it really means that the named user(s) can log in as
1011 which includes bin, daemon, adm, and other accounts that own critical
1012 binaries and directories.
1013 Using a user name practically grants the user root access.
1014 The only valid use for user names that I can think
1015 of is in negative entries.
1017 Note that this warning also applies to rsh/rlogin.
1018 .It Pa /etc/shosts.equiv
1019 This is processed exactly as
1020 .Pa /etc/hosts.equiv .
1021 However, this file may be useful in environments that want to run both
1023 .It Pa $HOME/.ssh/environment
1024 This file is read into the environment at login (if it exists).
1025 It can only contain empty lines, comment lines (that start with
1027 and assignment lines of the form name=value.
1028 The file should be writable
1029 only by the user; it need not be readable by anyone else.
1030 .It Pa $HOME/.ssh/rc
1031 If this file exists, it is run with /bin/sh after reading the
1032 environment files but before starting the user's shell or command.
1033 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1041 The primary purpose of this file is to run any initialization routines
1042 which may be needed before the user's home directory becomes
1043 accessible; AFS is a particular example of such an environment.
1045 This file will probably contain some initialization code followed by
1046 something similar to: "if read proto cookie; then echo add $DISPLAY
1047 $proto $cookie | xauth -q -; fi".
1049 If this file does not exist,
1052 does not exist either, xauth is used to store the cookie.
1054 This file should be writable only by the user, and need not be
1055 readable by anyone else.
1059 This can be used to specify
1060 machine-specific login-time initializations globally.
1061 This file should be writable only by root, and should be world-readable.
1065 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1066 but with bugs removed and newer features re-added.
1068 1.2.12 release, newer versions of the original ssh bore successively
1069 more restrictive licenses, and thus demand for a free version was born.
1071 This version of OpenSSH
1074 has all components of a restrictive nature (i.e., patents, see
1076 directly removed from the source code; any licensed or patented components
1080 has been updated to support SSH protocol 1.5 and 2, making it compatible with
1081 all other SSH clients and servers.
1083 contains added support for
1085 authentication and ticket passing.
1087 supports one-time password authentication with
1091 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1092 Niels Provos, Theo de Raadt, and Dug Song.
1094 The support for SSH protocol 2 was written by Markus Friedl.