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 .Dd September 25, 1999
42 .Nd secure shell daemon
47 .Op Fl f Ar config_file
48 .Op Fl g Ar login_grace_time
49 .Op Fl h Ar host_key_file
50 .Op Fl k Ar key_gen_time
53 .Op Fl V Ar client_protocol_id
56 (Secure Shell Daemon) is the daemon program for
58 Together these programs replace rlogin and rsh, and
59 provide secure encrypted communications between two untrusted hosts
60 over an insecure network.
61 The programs are intended to be as easy to
62 install and use as possible.
65 is the daemon that listens for connections from clients.
66 It is normally started at boot from
69 daemon for each incoming connection.
70 The forked daemons handle
71 key exchange, encryption, authentication, command execution,
73 This implementation of
75 supports both SSH protocol version 1 and 2 simultaneously.
79 .Ss SSH protocol version 1
81 Each host has a host-specific RSA key
82 (normally 1024 bits) used to identify the host.
84 the daemon starts, it generates a server RSA key (normally 768 bits).
85 This key is normally regenerated every hour if it has been used, and
86 is never stored on disk.
88 Whenever a client connects the daemon responds with its public
90 The client compares the
91 RSA host key against its own database to verify that it has not changed.
92 The client then generates a 256 bit random number.
94 random number using both the host key and the server key, and sends
95 the encrypted number to the server.
96 Both sides then use this
97 random number as a session key which is used to encrypt all further
98 communications in the session.
99 The rest of the session is encrypted
100 using a conventional cipher, currently Blowfish or 3DES, with 3DES
101 being used by default.
102 The client selects the encryption algorithm
103 to use from those offered by the server.
105 Next, the server and the client enter an authentication dialog.
106 The client tries to authenticate itself using
110 authentication combined with RSA host
111 authentication, RSA challenge-response authentication, or password
112 based authentication.
114 Rhosts authentication is normally disabled
115 because it is fundamentally insecure, but can be enabled in the server
116 configuration file if desired.
117 System security is not improved unless
123 are disabled (thus completely disabling
129 .Ss SSH protocol version 2
131 Version 2 works similarly:
132 Each host has a host-specific DSA key used to identify the host.
133 However, when the daemon starts, it does not generate a server key.
134 Forward security is provided through a Diffie-Hellman key agreement.
135 This key agreement results in a shared session key.
136 The rest of the session is encrypted
137 using a symmetric cipher, currently
138 Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
139 The client selects the encryption algorithm
140 to use from those offered by the server.
141 Additionally, session integrity is provided
142 through a cryptographic message authentication code
143 (hmac-sha1 or hmac-md5).
145 Protocol version 2 provides a public key based
146 user authentication method (DSAAuthentication)
147 and conventional password authentication.
149 .Ss Command execution and data forwarding
151 If the client successfully authenticates itself, a dialog for
152 preparing the session is entered.
153 At this time the client may request
154 things like allocating a pseudo-tty, forwarding X11 connections,
155 forwarding TCP/IP connections, or forwarding the authentication agent
156 connection over the secure channel.
158 Finally, the client either requests a shell or execution of a command.
159 The sides then enter session mode.
160 In this mode, either side may send
161 data at any time, and such data is forwarded to/from the shell or
162 command on the server side, and the user terminal in the client side.
164 When the user program terminates and all forwarded X11 and other
165 connections have been closed, the server sends command exit status to
166 the client, and both sides exit.
169 can be configured using command-line options or a configuration
171 Command-line options override values specified in the
175 rereads its configuration file when it receives a hangup signal,
178 The options are as follows:
181 Specifies the number of bits in the server key (default 768).
185 The server sends verbose debug output to the system
186 log, and does not put itself in the background.
187 The server also will not fork and will only process one connection.
188 This option is only intended for debugging for the server.
189 Multiple -d options increases the debugging level.
191 .It Fl f Ar configuration_file
192 Specifies the name of the configuration file.
194 .Pa /etc/sshd_config .
196 refuses to start if there is no configuration file.
197 .It Fl g Ar login_grace_time
198 Gives the grace time for clients to authenticate themselves (default
200 If the client fails to authenticate the user within
201 this many seconds, the server disconnects and exits.
202 A value of zero indicates no limit.
203 .It Fl h Ar host_key_file
204 Specifies the file from which the RSA host key is read (default
205 .Pa /etc/ssh_host_key ) .
206 This option must be given if
208 is not run as root (as the normal
209 host file is normally not readable by anyone but root).
213 is being run from inetd.
216 from inetd because it needs to generate the server key before it can
217 respond to the client, and this may take tens of seconds.
218 Clients would have to wait too long if the key was regenerated every time.
219 However, with small key sizes (e.g., 512) using
223 .It Fl k Ar key_gen_time
224 Specifies how often the server key is regenerated (default 3600
225 seconds, or one hour).
226 The motivation for regenerating the key fairly
227 often is that the key is not stored anywhere, and after about an hour,
228 it becomes impossible to recover the key for decrypting intercepted
229 communications even if the machine is cracked into or physically
231 A value of zero indicates that the key will never be regenerated.
233 Specifies the port on which the server listens for connections
237 Nothing is sent to the system log.
238 Normally the beginning,
239 authentication, and termination of each connection is logged.
241 This option is used to specify the size of the field
244 structure that holds the remote host name.
245 If the resolved host name is longer than
247 the dotted decimal value will be used instead.
248 This allows hosts with very long host names that
249 overflow this field to still be uniquely identified.
252 indicates that only dotted decimal addresses
253 should be put into the
257 Do not print an error message if RSA support is missing.
258 .It Fl V Ar client_protocol_id
259 SSH-2 compatibility mode.
260 When this option is specified
262 assumes the client has sent the supplied version string
264 Protocol Version Identification Exchange.
265 This option is not intended to be called directly.
269 to use IPv4 addresses only.
273 to use IPv6 addresses only.
275 .Sh CONFIGURATION FILE
277 reads configuration data from
279 (or the file specified with
281 on the command line).
282 The file contains keyword-value pairs, one per line.
285 and empty lines are interpreted as comments.
287 The following keywords are possible.
289 .It Cm AFSTokenPassing
290 Specifies whether an AFS token may be forwarded to the server.
294 This keyword can be followed by a number of group names, separated
296 If specified, login is allowed only for users whose primary
297 group matches one of the patterns.
302 wildcards in the patterns.
303 Only group names are valid; a numerical group ID isn't recognized.
304 By default login is allowed regardless of the primary group.
307 This keyword can be followed by a number of user names, separated
309 If specified, login is allowed only for users names that
310 match one of the patterns.
315 wildcards in the patterns.
316 Only user names are valid; a numerical user ID isn't recognized.
317 By default login is allowed regardless of the user name.
320 Specifies the ciphers allowed for protocol version 2.
321 Multiple ciphers must be comma-separated.
323 .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
327 should check for new mail for interactive logins.
331 This keyword can be followed by a number of group names, separated
333 Users whose primary group matches one of the patterns
334 aren't allowed to log in.
339 wildcards in the patterns.
340 Only group names are valid; a numerical group ID isn't recognized.
341 By default login is allowed regardless of the primary group.
344 This keyword can be followed by a number of user names, separated
346 Login is disallowed for user names that match one of the patterns.
350 can be used as wildcards in the patterns.
351 Only user names are valid; a numerical user ID isn't recognized.
352 By default login is allowed regardless of the user name.
353 .It Cm DSAAuthentication
354 Specifies whether DSA authentication is allowed.
357 Note that this option applies to protocol version 2 only.
359 Specifies whether remote hosts are allowed to connect to ports
360 forwarded for the client.
368 Specifies the file containing the private DSA host key (default
369 .Pa /etc/ssh_host_dsa_key )
370 used by SSH protocol 2.0.
373 disables protocol 2.0 if this file is group/world-accessible.
375 Specifies the file containing the private RSA host key (default
376 .Pa /etc/ssh_host_key )
377 used by SSH protocols 1.3 and 1.5.
380 disables protocols 1.3 and 1.5 if this file is group/world-accessible.
386 files will not be used in authentication.
389 .Pa /etc/shosts.equiv
393 .It Cm IgnoreUserKnownHosts
396 should ignore the user's
397 .Pa $HOME/.ssh/known_hosts
399 .Cm RhostsRSAAuthentication .
403 Specifies whether the system should send keepalive messages to the
405 If they are sent, death of the connection or crash of one
406 of the machines will be properly noticed.
407 However, this means that
408 connections will die if the route is down temporarily, and some people
410 On the other hand, if keepalives are not sent,
411 sessions may hang indefinitely on the server, leaving
413 users and consuming server resources.
417 (to send keepalives), and the server will notice
418 if the network goes down or the client host reboots.
419 This avoids infinitely hanging sessions.
421 To disable keepalives, the value should be set to
423 in both the server and the client configuration files.
424 .It Cm KerberosAuthentication
425 Specifies whether Kerberos authentication is allowed.
426 This can be in the form of a Kerberos ticket, or if
427 .Cm PasswordAuthentication
428 is yes, the password provided by the user will be validated through
430 To use this option, the server needs a
431 Kerberos servtab which allows the verification of the KDC's identity.
434 .It Cm KerberosOrLocalPasswd
435 If set then if password authentication through Kerberos fails then
436 the password will be validated via any additional local mechanism
441 .It Cm KerberosTgtPassing
442 Specifies whether a Kerberos TGT may be forwarded to the server.
445 as this only works when the Kerberos KDC is actually an AFS kaserver.
446 .It Cm KerberosTicketCleanup
447 Specifies whether to automatically destroy the user's ticket cache
451 .It Cm KeyRegenerationInterval
452 The server key is automatically regenerated after this many seconds
453 (if it has been used).
454 The purpose of regeneration is to prevent
455 decrypting captured sessions by later breaking into the machine and
457 The key is never stored anywhere.
458 If the value is 0, the key is never regenerated.
459 The default is 3600 (seconds).
461 Specifies what local address
464 The default is to listen to all local addresses.
465 Multiple options of this type are permitted.
468 options must precede this option.
469 .It Cm LoginGraceTime
470 The server disconnects after this time if the user has not
471 successfully logged in.
472 If the value is 0, there is no time limit.
473 The default is 600 (seconds).
475 Gives the verbosity level that is used when logging messages from
477 The possible values are:
478 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
480 Logging with level DEBUG violates the privacy of users
481 and is not recommended.
483 Specifies the maximum number of concurrent unauthenticated connections to the
486 Additional connections will be dropped until authentication succeeds or the
488 expires for a connection.
491 Alternatively, random early drop can be enabled by specifying
492 the three colon separated values
496 will refuse connection attempts with a probabillity of
499 if there are currently
502 unauthenticated connections.
503 The probabillity increases linearly and all connection attempts
504 are refused if the number of unauthenticated connections reaches
507 .It Cm PasswordAuthentication
508 Specifies whether password authentication is allowed.
511 Note that this option applies to both protocol versions 1 and 2.
512 .It Cm PermitEmptyPasswords
513 When password authentication is allowed, it specifies whether the
514 server allows login to accounts with empty password strings.
517 .It Cm PermitRootLogin
518 Specifies whether the root can log in using
527 If this options is set to
529 only password authentication is disabled for root.
531 Root login with RSA authentication when the
534 specified will be allowed regardless of the value of this setting
535 (which may be useful for taking remote backups even if root login is
536 normally not allowed).
538 Specifies the file that contains the process identifier of the
542 .Pa /var/run/sshd.pid .
544 Specifies the port number that
548 Multiple options of this type are permitted.
554 when a user logs in interactively.
555 (On some systems it is also printed by the shell,
561 Specifies the protocol versions
564 The possible values are
568 Multiple versions must be comma-separated.
573 Random number generation uses other techniques.
574 .It Cm RhostsAuthentication
575 Specifies whether authentication using rhosts or /etc/hosts.equiv
577 Normally, this method should not be permitted because it is insecure.
578 .Cm RhostsRSAAuthentication
580 instead, because it performs RSA-based host authentication in addition
581 to normal rhosts or /etc/hosts.equiv authentication.
584 .It Cm RhostsRSAAuthentication
585 Specifies whether rhosts or /etc/hosts.equiv authentication together
586 with successful RSA host authentication is allowed.
589 .It Cm RSAAuthentication
590 Specifies whether pure RSA authentication is allowed.
593 Note that this option applies to protocol version 1 only.
595 Defines the number of bits in the server key.
596 The minimum value is 512, and the default is 768.
597 .It Cm SkeyAuthentication
600 authentication is allowed.
603 Note that s/key authentication is enabled only if
604 .Cm PasswordAuthentication
609 should check file modes and ownership of the
610 user's files and home directory before accepting login.
611 This is normally desirable because novices sometimes accidentally leave their
612 directory or files world-writable.
616 Configures an external subsystem (e.g., file transfer daemon).
617 Arguments should be a subsystem name and a command to execute upon subsystem
623 file transfer subsystem.
624 By default no subsystems are defined.
625 Note that this option applies to protocol version 2 only.
626 .It Cm SyslogFacility
627 Gives the facility code that is used when logging messages from
629 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
630 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
635 is used for interactive login sessions.
638 is never used for remote command execution.
641 .It Cm X11DisplayOffset
642 Specifies the first display number available for
647 from interfering with real X11 servers.
650 Specifies whether X11 forwarding is permitted.
653 Note that disabling X11 forwarding does not improve security in any
654 way, as users can always install their own forwarders.
656 Specifies the location of the
660 .Pa /usr/X11R6/bin/xauth .
663 When a user successfully logs in,
666 .Bl -enum -offset indent
668 If the login is on a tty, and no command has been specified,
669 prints last login time and
671 (unless prevented in the configuration file or by
672 .Pa $HOME/.hushlogin ;
677 If the login is on a tty, records login time.
681 if it exists, prints contents and quits
684 Changes to run with normal user privileges.
686 Sets up basic environment.
689 .Pa $HOME/.ssh/environment
692 Changes to user's home directory.
696 exists, runs it; else if
699 it; otherwise runs xauth.
702 files are given the X11
703 authentication protocol and cookie in standard input.
705 Runs user's shell or command.
707 .Sh AUTHORIZED_KEYS FILE FORMAT
709 .Pa $HOME/.ssh/authorized_keys
710 file lists the RSA keys that are
711 permitted for RSA authentication in SSH protocols 1.3 and 1.5
713 .Pa $HOME/.ssh/authorized_keys2
714 file lists the DSA keys that are
715 permitted for DSA authentication in SSH protocol 2.0.
716 Each line of the file contains one
717 key (empty lines and lines starting with a
721 Each line consists of the following fields, separated by
722 spaces: options, bits, exponent, modulus, comment.
724 is optional; its presence is determined by whether the line starts
725 with a number or not (the option field never starts with a number).
726 The bits, exponent, modulus and comment fields give the RSA key; the
727 comment field is not used for anything (but may be convenient for the
728 user to identify the key).
730 Note that lines in this file are usually several hundred bytes long
731 (because of the size of the RSA key modulus).
732 You don't want to type them in; instead, copy the
736 The options (if present) consist of comma-separated option
738 No spaces are permitted, except within double quotes.
739 The following option specifications are supported:
741 .It Cm from="pattern-list"
742 Specifies that in addition to RSA authentication, the canonical name
743 of the remote host must be present in the comma-separated list of
749 The list may also contain
750 patterns negated by prefixing them with
752 if the canonical host name matches a negated pattern, the key is not accepted.
754 of this option is to optionally increase security: RSA authentication
755 by itself does not trust the network or name servers or anything (but
756 the key); however, if somebody somehow steals the key, the key
757 permits an intruder to log in from anywhere in the world.
758 This additional option makes using a stolen key more difficult (name
759 servers and/or routers would have to be compromised in addition to
761 .It Cm command="command"
762 Specifies that the command is executed whenever this key is used for
764 The command supplied by the user (if any) is ignored.
765 The command is run on a pty if the connection requests a pty;
766 otherwise it is run without a tty.
767 A quote may be included in the command by quoting it with a backslash.
768 This option might be useful
769 to restrict certain RSA keys to perform just a specific operation.
770 An example might be a key that permits remote backups but nothing else.
771 Note that the client may specify TCP/IP and/or X11
772 forwarding unless they are explicitly prohibited.
773 .It Cm environment="NAME=value"
774 Specifies that the string is to be added to the environment when
775 logging in using this key.
776 Environment variables set this way
777 override other default environment values.
778 Multiple options of this type are permitted.
779 .It Cm no-port-forwarding
780 Forbids TCP/IP forwarding when this key is used for authentication.
781 Any port forward requests by the client will return an error.
782 This might be used, e.g., in connection with the
785 .It Cm no-X11-forwarding
786 Forbids X11 forwarding when this key is used for authentication.
787 Any X11 forward requests by the client will return an error.
788 .It Cm no-agent-forwarding
789 Forbids authentication agent forwarding when this key is used for
792 Prevents tty allocation (a request to allocate a pty will fail).
795 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
797 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
799 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
800 .Sh SSH_KNOWN_HOSTS FILE FORMAT
802 .Pa /etc/ssh_known_hosts ,
803 .Pa /etc/ssh_known_hosts2 ,
804 .Pa $HOME/.ssh/known_hosts ,
806 .Pa $HOME/.ssh/known_hosts2
807 files contain host public keys for all known hosts.
808 The global file should
809 be prepared by the administrator (optional), and the per-user file is
810 maintained automatically: whenever the user connects from an unknown host
811 its key is added to the per-user file.
813 Each line in these files contains the following fields: hostnames,
814 bits, exponent, modulus, comment.
815 The fields are separated by spaces.
817 Hostnames is a comma-separated list of patterns ('*' and '?' act as
818 wildcards); each pattern in turn is matched against the canonical host
819 name (when authenticating a client) or against the user-supplied
820 name (when authenticating a server).
821 A pattern may also be preceded by
823 to indicate negation: if the host name matches a negated
824 pattern, it is not accepted (by that line) even if it matched another
827 Bits, exponent, and modulus are taken directly from the RSA host key; they
828 can be obtained, e.g., from
829 .Pa /etc/ssh_host_key.pub .
830 The optional comment field continues to the end of the line, and is not used.
834 and empty lines are ignored as comments.
836 When performing host authentication, authentication is accepted if any
837 matching line has the proper key.
838 It is thus permissible (but not
839 recommended) to have several lines or different host keys for the same
841 This will inevitably happen when short forms of host names
842 from different domains are put in the file.
844 that the files contain conflicting information; authentication is
845 accepted if valid information can be found from either file.
847 Note that the lines in these files are typically hundreds of characters
848 long, and you definitely don't want to type in the host keys by hand.
849 Rather, generate them by a script
851 .Pa /etc/ssh_host_key.pub
852 and adding the host names at the front.
854 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
857 .It Pa /etc/sshd_config
858 Contains configuration data for
860 This file should be writable by root only, but it is recommended
861 (though not necessary) that it be world-readable.
862 .It Pa /etc/ssh_host_key
863 Contains the private part of the host key.
864 This file should only be owned by root, readable only by root, and not
865 accessible to others.
868 does not start if this file is group/world-accessible.
869 .It Pa /etc/ssh_host_key.pub
870 Contains the public part of the host key.
871 This file should be world-readable but writable only by
873 Its contents should match the private part.
875 really used for anything; it is only provided for the convenience of
876 the user so its contents can be copied to known hosts files.
877 These two files are created using
879 .It Pa /var/run/sshd.pid
880 Contains the process ID of the
882 listening for connections (if there are several daemons running
883 concurrently for different ports, this contains the pid of the one
885 The content of this file is not sensitive; it can be world-readable.
886 .It Pa $HOME/.ssh/authorized_keys
887 Lists the RSA keys that can be used to log into the user's account.
888 This file must be readable by root (which may on some machines imply
889 it being world-readable if the user's home directory resides on an NFS
891 It is recommended that it not be accessible by others.
892 The format of this file is described above.
893 Users will place the contents of their
895 files into this file, as described in
897 .It Pa $HOME/.ssh/authorized_keys2
898 Lists the DSA keys that can be used to log into the user's account.
899 This file must be readable by root (which may on some machines imply
900 it being world-readable if the user's home directory resides on an NFS
902 It is recommended that it not be accessible by others.
903 The format of this file is described above.
904 Users will place the contents of their
906 files into this file, as described in
908 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
909 These files are consulted when using rhosts with RSA host
910 authentication to check the public key of the host.
911 The key must be listed in one of these files to be accepted.
912 The client uses the same files
913 to verify that the remote host is the one it intended to connect.
914 These files should be writable only by root/the owner.
915 .Pa /etc/ssh_known_hosts
916 should be world-readable, and
917 .Pa $HOME/.ssh/known_hosts
918 can but need not be world-readable.
922 refuses to let anyone except root log in.
923 The contents of the file
924 are displayed to anyone trying to log in, and non-root connections are
926 The file should be world-readable.
927 .It Pa /etc/hosts.allow, /etc/hosts.deny
930 support, tcp-wrappers access controls may be defined here as described in
933 This file contains host-username pairs, separated by a space, one per
935 The given user on the corresponding host is permitted to log in
937 The same file is used by rlogind and rshd.
939 be writable only by the user; it is recommended that it not be
940 accessible by others.
942 If is also possible to use netgroups in the file.
944 name may be of the form +@groupname to specify all hosts or all users
948 this file is exactly the same as for
950 However, this file is
951 not used by rlogin and rshd, so using this permits access using SSH only.
952 .It Pa /etc/hosts.equiv
953 This file is used during
956 In the simplest form, this file contains host names, one per line.
958 those hosts are permitted to log in without a password, provided they
959 have the same user name on both machines.
960 The host name may also be
961 followed by a user name; such users are permitted to log in as
963 user on this machine (except root).
964 Additionally, the syntax
966 can be used to specify netgroups.
967 Negated entries start with
970 If the client host/user is successfully matched in this file, login is
971 automatically permitted provided the client and server user names are the
973 Additionally, successful RSA host authentication is normally required.
974 This file must be writable only by root; it is recommended
975 that it be world-readable.
977 .Sy "Warning: It is almost never a good idea to use user names in"
979 Beware that it really means that the named user(s) can log in as
981 which includes bin, daemon, adm, and other accounts that own critical
982 binaries and directories.
983 Using a user name practically grants the user root access.
984 The only valid use for user names that I can think
985 of is in negative entries.
987 Note that this warning also applies to rsh/rlogin.
988 .It Pa /etc/shosts.equiv
989 This is processed exactly as
990 .Pa /etc/hosts.equiv .
991 However, this file may be useful in environments that want to run both
993 .It Pa $HOME/.ssh/environment
994 This file is read into the environment at login (if it exists).
995 It can only contain empty lines, comment lines (that start with
997 and assignment lines of the form name=value.
998 The file should be writable
999 only by the user; it need not be readable by anyone else.
1000 .It Pa $HOME/.ssh/rc
1001 If this file exists, it is run with /bin/sh after reading the
1002 environment files but before starting the user's shell or command.
1003 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1011 The primary purpose of this file is to run any initialization routines
1012 which may be needed before the user's home directory becomes
1013 accessible; AFS is a particular example of such an environment.
1015 This file will probably contain some initialization code followed by
1016 something similar to: "if read proto cookie; then echo add $DISPLAY
1017 $proto $cookie | xauth -q -; fi".
1019 If this file does not exist,
1022 does not exist either, xauth is used to store the cookie.
1024 This file should be writable only by the user, and need not be
1025 readable by anyone else.
1029 This can be used to specify
1030 machine-specific login-time initializations globally.
1031 This file should be writable only by root, and should be world-readable.
1035 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1036 but with bugs removed and newer features re-added.
1038 1.2.12 release, newer versions of the original ssh bore successively
1039 more restrictive licenses, and thus demand for a free version was born.
1041 This version of OpenSSH
1044 has all components of a restrictive nature (i.e., patents, see
1046 directly removed from the source code; any licensed or patented components
1050 has been updated to support SSH protocol 1.5 and 2, making it compatible with
1051 all other SSH clients and servers.
1053 contains added support for
1055 authentication and ticket passing.
1057 supports one-time password authentication with
1061 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1062 Niels Provos, Theo de Raadt, and Dug Song.
1064 The support for SSH protocol 2 was written by Markus Friedl.