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. Maximum is 3.
190 .It Fl f Ar configuration_file
191 Specifies the name of the configuration file.
193 .Pa /etc/sshd_config .
195 refuses to start if there is no configuration file.
196 .It Fl g Ar login_grace_time
197 Gives the grace time for clients to authenticate themselves (default
199 If the client fails to authenticate the user within
200 this many seconds, the server disconnects and exits.
201 A value of zero indicates no limit.
202 .It Fl h Ar host_key_file
203 Specifies the file from which the RSA host key is read (default
204 .Pa /etc/ssh_host_key ) .
205 This option must be given if
207 is not run as root (as the normal
208 host file is normally not readable by anyone but root).
212 is being run from inetd.
215 from inetd because it needs to generate the server key before it can
216 respond to the client, and this may take tens of seconds.
217 Clients would have to wait too long if the key was regenerated every time.
218 However, with small key sizes (e.g., 512) using
222 .It Fl k Ar key_gen_time
223 Specifies how often the server key is regenerated (default 3600
224 seconds, or one hour).
225 The motivation for regenerating the key fairly
226 often is that the key is not stored anywhere, and after about an hour,
227 it becomes impossible to recover the key for decrypting intercepted
228 communications even if the machine is cracked into or physically
230 A value of zero indicates that the key will never be regenerated.
232 Specifies the port on which the server listens for connections
236 Nothing is sent to the system log.
237 Normally the beginning,
238 authentication, and termination of each connection is logged.
240 This option is used to specify the size of the field
243 structure that holds the remote host name.
244 If the resolved host name is longer than
246 the dotted decimal value will be used instead.
247 This allows hosts with very long host names that
248 overflow this field to still be uniquely identified.
251 indicates that only dotted decimal addresses
252 should be put into the
256 Do not print an error message if RSA support is missing.
257 .It Fl V Ar client_protocol_id
258 SSH2 compatibility mode.
259 When this option is specified
261 assumes the client has sent the supplied version string
263 Protocol Version Identification Exchange.
267 to use IPv4 addresses only.
271 to use IPv6 addresses only.
273 .Sh CONFIGURATION FILE
275 reads configuration data from
277 (or the file specified with
279 on the command line).
280 The file contains keyword-value pairs, one per line.
283 and empty lines are interpreted as comments.
285 The following keywords are possible.
287 .It Cm AFSTokenPassing
288 Specifies whether an AFS token may be forwarded to the server.
292 This keyword can be followed by a number of group names, separated
294 If specified, login is allowed only for users whose primary
295 group matches one of the patterns.
300 wildcards in the patterns.
301 Only group names are valid; a numerical group ID isn't recognized.
302 By default login is allowed regardless of the primary group.
305 This keyword can be followed by a number of user names, separated
307 If specified, login is allowed only for users names that
308 match one of the patterns.
313 wildcards in the patterns.
314 Only user names are valid; a numerical user ID isn't recognized.
315 By default login is allowed regardless of the user name.
318 Specifies the ciphers allowed for protocol version 2.
319 Multiple ciphers must be comma-separated.
321 .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
325 should check for new mail for interactive logins.
329 This keyword can be followed by a number of group names, separated
331 Users whose primary group matches one of the patterns
332 aren't allowed to log in.
337 wildcards in the patterns.
338 Only group names are valid; a numerical group ID isn't recognized.
339 By default login is allowed regardless of the primary group.
342 This keyword can be followed by a number of user names, separated
344 Login is disallowed for user names that match one of the patterns.
348 can be used as wildcards in the patterns.
349 Only user names are valid; a numerical user ID isn't recognized.
350 By default login is allowed regardless of the user name.
351 .It Cm DSAAuthentication
352 Specifies whether DSA authentication is allowed.
355 Note that this option applies to protocol version 2 only.
357 Specifies whether remote hosts are allowed to connect to ports
358 forwarded for the client.
366 Specifies the file containing the private DSA host key (default
367 .Pa /etc/ssh_host_dsa_key )
368 used by SSH protocol 2.0.
371 disables protocol 2.0 if this file is group/world-accessible.
373 Specifies the file containing the private RSA host key (default
374 .Pa /etc/ssh_host_key )
375 used by SSH protocols 1.3 and 1.5.
378 disables protocols 1.3 and 1.5 if this file is group/world-accessible.
384 files will not be used in authentication.
387 .Pa /etc/shosts.equiv
391 .It Cm IgnoreUserKnownHosts
394 should ignore the user's
395 .Pa $HOME/.ssh/known_hosts
397 .Cm RhostsRSAAuthentication .
401 Specifies whether the system should send keepalive messages to the
403 If they are sent, death of the connection or crash of one
404 of the machines will be properly noticed.
405 However, this means that
406 connections will die if the route is down temporarily, and some people
408 On the other hand, if keepalives are not sent,
409 sessions may hang indefinitely on the server, leaving
411 users and consuming server resources.
415 (to send keepalives), and the server will notice
416 if the network goes down or the client host reboots.
417 This avoids infinitely hanging sessions.
419 To disable keepalives, the value should be set to
421 in both the server and the client configuration files.
422 .It Cm KerberosAuthentication
423 Specifies whether Kerberos authentication is allowed.
424 This can be in the form of a Kerberos ticket, or if
425 .Cm PasswordAuthentication
426 is yes, the password provided by the user will be validated through
427 the Kerberos KDC. To use this option, the server needs a
428 Kerberos servtab which allows the verification of the KDC's identity.
431 .It Cm KerberosOrLocalPasswd
432 If set then if password authentication through Kerberos fails then
433 the password will be validated via any additional local mechanism
439 .It Cm KerberosTgtPassing
440 Specifies whether a Kerberos TGT may be forwarded to the server.
443 as this only works when the Kerberos KDC is actually an AFS kaserver.
444 .It Cm KerberosTicketCleanup
445 Specifies whether to automatically destroy the user's ticket cache
449 .It Cm KeyRegenerationInterval
450 The server key is automatically regenerated after this many seconds
451 (if it has been used).
452 The purpose of regeneration is to prevent
453 decrypting captured sessions by later breaking into the machine and
455 The key is never stored anywhere.
456 If the value is 0, the key is never regenerated.
457 The default is 3600 (seconds).
459 Specifies what local address
462 The default is to listen to all local addresses.
463 Multiple options of this type are permitted.
466 options must precede this option.
467 .It Cm LoginGraceTime
468 The server disconnects after this time if the user has not
469 successfully logged in.
470 If the value is 0, there is no time limit.
471 The default is 600 (seconds).
473 Gives the verbosity level that is used when logging messages from
475 The possible values are:
476 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
478 Logging with level DEBUG violates the privacy of users
479 and is not recommended.
481 Specifies the maximum number of concurrent unauthenticated connections to the
484 Additional connections will be dropped until authentication succeeds or the
486 expires for a connection.
489 Alternatively, random early drop can be enabled by specifying
490 the three colon separated values
494 will refuse connection attempts with a probabillity of
497 if there are currently
500 unauthenticated connections.
501 The probabillity increases linearly and all connection attempts
502 are refused if the number of unauthenticated connections reaches
505 .It Cm PasswordAuthentication
506 Specifies whether password authentication is allowed.
509 Note that this option applies to both protocol versions 1 and 2.
510 .It Cm PermitEmptyPasswords
511 When password authentication is allowed, it specifies whether the
512 server allows login to accounts with empty password strings.
515 .It Cm PermitRootLogin
516 Specifies whether the root can log in using
525 If this options is set to
527 only password authentication is disabled for root.
529 Root login with RSA authentication when the
532 specified will be allowed regardless of the value of this setting
533 (which may be useful for taking remote backups even if root login is
534 normally not allowed).
536 Specifies the file that contains the process identifier of the
540 .Pa /var/run/sshd.pid .
542 Specifies the port number that
546 Multiple options of this type are permitted.
552 when a user logs in interactively.
553 (On some systems it is also printed by the shell,
559 Specifies the protocol versions
562 The possible values are
566 Multiple versions must be comma-separated.
571 Random number generation uses other techniques.
572 .It Cm RhostsAuthentication
573 Specifies whether authentication using rhosts or /etc/hosts.equiv
575 Normally, this method should not be permitted because it is insecure.
576 .Cm RhostsRSAAuthentication
578 instead, because it performs RSA-based host authentication in addition
579 to normal rhosts or /etc/hosts.equiv authentication.
582 .It Cm RhostsRSAAuthentication
583 Specifies whether rhosts or /etc/hosts.equiv authentication together
584 with successful RSA host authentication is allowed.
587 .It Cm RSAAuthentication
588 Specifies whether pure RSA authentication is allowed.
591 Note that this option applies to protocol version 1 only.
593 Defines the number of bits in the server key.
594 The minimum value is 512, and the default is 768.
595 .It Cm SkeyAuthentication
598 authentication is allowed.
601 Note that s/key authentication is enabled only if
602 .Cm PasswordAuthentication
607 should check file modes and ownership of the
608 user's files and home directory before accepting login.
609 This is normally desirable because novices sometimes accidentally leave their
610 directory or files world-writable.
614 Configures an external subsystem (e.g. file transfer daemon).
615 Arguments should be a subsystem name and a command to execute upon subsystem request.
620 file transfer subsystem.
621 By default no subsystems are defined.
622 Note that this option applies to protocol version 2 only.
623 .It Cm SyslogFacility
624 Gives the facility code that is used when logging messages from
626 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
627 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
632 is used for interactive login sessions.
635 is never used for remote command execution.
638 .It Cm X11DisplayOffset
639 Specifies the first display number available for
644 from interfering with real X11 servers.
647 Specifies whether X11 forwarding is permitted.
650 Note that disabling X11 forwarding does not improve security in any
651 way, as users can always install their own forwarders.
653 Specifies the location of the
657 .Pa /usr/X11R6/bin/xauth .
660 When a user successfully logs in,
663 .Bl -enum -offset indent
665 If the login is on a tty, and no command has been specified,
666 prints last login time and
668 (unless prevented in the configuration file or by
669 .Pa $HOME/.hushlogin ;
674 If the login is on a tty, records login time.
678 if it exists, prints contents and quits
681 Changes to run with normal user privileges.
683 Sets up basic environment.
686 .Pa $HOME/.ssh/environment
689 Changes to user's home directory.
693 exists, runs it; else if
696 it; otherwise runs xauth.
699 files are given the X11
700 authentication protocol and cookie in standard input.
702 Runs user's shell or command.
704 .Sh AUTHORIZED_KEYS FILE FORMAT
706 .Pa $HOME/.ssh/authorized_keys
707 file lists the RSA keys that are
708 permitted for RSA authentication in SSH protocols 1.3 and 1.5
710 .Pa $HOME/.ssh/authorized_keys2
711 file lists the DSA keys that are
712 permitted for DSA authentication in SSH protocol 2.0.
713 Each line of the file contains one
714 key (empty lines and lines starting with a
718 Each line consists of the following fields, separated by
719 spaces: options, bits, exponent, modulus, comment.
721 is optional; its presence is determined by whether the line starts
722 with a number or not (the option field never starts with a number).
723 The bits, exponent, modulus and comment fields give the RSA key; the
724 comment field is not used for anything (but may be convenient for the
725 user to identify the key).
727 Note that lines in this file are usually several hundred bytes long
728 (because of the size of the RSA key modulus).
729 You don't want to type them in; instead, copy the
733 The options (if present) consist of comma-separated option
735 No spaces are permitted, except within double quotes.
736 The following option specifications are supported:
738 .It Cm from="pattern-list"
739 Specifies that in addition to RSA authentication, the canonical name
740 of the remote host must be present in the comma-separated list of
746 The list may also contain
747 patterns negated by prefixing them with
749 if the canonical host name matches a negated pattern, the key is not accepted.
751 of this option is to optionally increase security: RSA authentication
752 by itself does not trust the network or name servers or anything (but
753 the key); however, if somebody somehow steals the key, the key
754 permits an intruder to log in from anywhere in the world.
755 This additional option makes using a stolen key more difficult (name
756 servers and/or routers would have to be compromised in addition to
758 .It Cm command="command"
759 Specifies that the command is executed whenever this key is used for
761 The command supplied by the user (if any) is ignored.
762 The command is run on a pty if the connection requests a pty;
763 otherwise it is run without a tty.
764 A quote may be included in the command by quoting it with a backslash.
765 This option might be useful
766 to restrict certain RSA keys to perform just a specific operation.
767 An example might be a key that permits remote backups but nothing else.
768 Note that the client may specify TCP/IP and/or X11
769 forwarding unless they are explicitly prohibited.
770 .It Cm environment="NAME=value"
771 Specifies that the string is to be added to the environment when
772 logging in using this key.
773 Environment variables set this way
774 override other default environment values.
775 Multiple options of this type are permitted.
776 .It Cm no-port-forwarding
777 Forbids TCP/IP forwarding when this key is used for authentication.
778 Any port forward requests by the client will return an error.
779 This might be used, e.g., in connection with the
782 .It Cm no-X11-forwarding
783 Forbids X11 forwarding when this key is used for authentication.
784 Any X11 forward requests by the client will return an error.
785 .It Cm no-agent-forwarding
786 Forbids authentication agent forwarding when this key is used for
789 Prevents tty allocation (a request to allocate a pty will fail).
792 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
794 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
796 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
797 .Sh SSH_KNOWN_HOSTS FILE FORMAT
799 .Pa /etc/ssh_known_hosts ,
800 .Pa /etc/ssh_known_hosts2 ,
801 .Pa $HOME/.ssh/known_hosts ,
803 .Pa $HOME/.ssh/known_hosts2
804 files contain host public keys for all known hosts.
805 The global file should
806 be prepared by the administrator (optional), and the per-user file is
807 maintained automatically: whenever the user connects from an unknown host
808 its key is added to the per-user file.
810 Each line in these files contains the following fields: hostnames,
811 bits, exponent, modulus, comment.
812 The fields are separated by spaces.
814 Hostnames is a comma-separated list of patterns ('*' and '?' act as
815 wildcards); each pattern in turn is matched against the canonical host
816 name (when authenticating a client) or against the user-supplied
817 name (when authenticating a server).
818 A pattern may also be preceded by
820 to indicate negation: if the host name matches a negated
821 pattern, it is not accepted (by that line) even if it matched another
824 Bits, exponent, and modulus are taken directly from the RSA host key; they
825 can be obtained, e.g., from
826 .Pa /etc/ssh_host_key.pub .
827 The optional comment field continues to the end of the line, and is not used.
831 and empty lines are ignored as comments.
833 When performing host authentication, authentication is accepted if any
834 matching line has the proper key.
835 It is thus permissible (but not
836 recommended) to have several lines or different host keys for the same
838 This will inevitably happen when short forms of host names
839 from different domains are put in the file.
841 that the files contain conflicting information; authentication is
842 accepted if valid information can be found from either file.
844 Note that the lines in these files are typically hundreds of characters
845 long, and you definitely don't want to type in the host keys by hand.
846 Rather, generate them by a script
848 .Pa /etc/ssh_host_key.pub
849 and adding the host names at the front.
851 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
854 .It Pa /etc/sshd_config
855 Contains configuration data for
857 This file should be writable by root only, but it is recommended
858 (though not necessary) that it be world-readable.
859 .It Pa /etc/ssh_host_key
860 Contains the private part of the host key.
861 This file should only be owned by root, readable only by root, and not
862 accessible to others.
865 does not start if this file is group/world-accessible.
866 .It Pa /etc/ssh_host_key.pub
867 Contains the public part of the host key.
868 This file should be world-readable but writable only by
870 Its contents should match the private part.
872 really used for anything; it is only provided for the convenience of
873 the user so its contents can be copied to known hosts files.
874 These two files are created using
876 .It Pa /var/run/sshd.pid
877 Contains the process ID of the
879 listening for connections (if there are several daemons running
880 concurrently for different ports, this contains the pid of the one
882 The content of this file is not sensitive; it can be world-readable.
883 .It Pa $HOME/.ssh/authorized_keys
884 Lists the RSA keys that can be used to log into the user's account.
885 This file must be readable by root (which may on some machines imply
886 it being world-readable if the user's home directory resides on an NFS
888 It is recommended that it not be accessible by others.
889 The format of this file is described above.
890 Users will place the contents of their
892 files into this file, as described in
894 .It Pa $HOME/.ssh/authorized_keys2
895 Lists the DSA keys that can be used to log into the user's account.
896 This file must be readable by root (which may on some machines imply
897 it being world-readable if the user's home directory resides on an NFS
899 It is recommended that it not be accessible by others.
900 The format of this file is described above.
901 Users will place the contents of their
903 files into this file, as described in
905 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
906 These files are consulted when using rhosts with RSA host
907 authentication to check the public key of the host.
908 The key must be listed in one of these files to be accepted.
909 The client uses the same files
910 to verify that the remote host is the one it intended to connect.
911 These files should be writable only by root/the owner.
912 .Pa /etc/ssh_known_hosts
913 should be world-readable, and
914 .Pa $HOME/.ssh/known_hosts
915 can but need not be world-readable.
919 refuses to let anyone except root log in.
920 The contents of the file
921 are displayed to anyone trying to log in, and non-root connections are
923 The file should be world-readable.
924 .It Pa /etc/hosts.allow, /etc/hosts.deny
927 support, tcp-wrappers access controls may be defined here as described in
930 This file contains host-username pairs, separated by a space, one per
932 The given user on the corresponding host is permitted to log in
934 The same file is used by rlogind and rshd.
936 be writable only by the user; it is recommended that it not be
937 accessible by others.
939 If is also possible to use netgroups in the file.
941 name may be of the form +@groupname to specify all hosts or all users
945 this file is exactly the same as for
947 However, this file is
948 not used by rlogin and rshd, so using this permits access using SSH only.
949 .It Pa /etc/hosts.equiv
950 This file is used during
953 In the simplest form, this file contains host names, one per line.
955 those hosts are permitted to log in without a password, provided they
956 have the same user name on both machines.
957 The host name may also be
958 followed by a user name; such users are permitted to log in as
960 user on this machine (except root).
961 Additionally, the syntax
963 can be used to specify netgroups.
964 Negated entries start with
967 If the client host/user is successfully matched in this file, login is
968 automatically permitted provided the client and server user names are the
970 Additionally, successful RSA host authentication is normally required.
971 This file must be writable only by root; it is recommended
972 that it be world-readable.
974 .Sy "Warning: It is almost never a good idea to use user names in"
976 Beware that it really means that the named user(s) can log in as
978 which includes bin, daemon, adm, and other accounts that own critical
979 binaries and directories.
980 Using a user name practically grants the user root access.
981 The only valid use for user names that I can think
982 of is in negative entries.
984 Note that this warning also applies to rsh/rlogin.
985 .It Pa /etc/shosts.equiv
986 This is processed exactly as
987 .Pa /etc/hosts.equiv .
988 However, this file may be useful in environments that want to run both
990 .It Pa $HOME/.ssh/environment
991 This file is read into the environment at login (if it exists).
992 It can only contain empty lines, comment lines (that start with
994 and assignment lines of the form name=value.
995 The file should be writable
996 only by the user; it need not be readable by anyone else.
998 If this file exists, it is run with /bin/sh after reading the
999 environment files but before starting the user's shell or command.
1000 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1008 The primary purpose of this file is to run any initialization routines
1009 which may be needed before the user's home directory becomes
1010 accessible; AFS is a particular example of such an environment.
1012 This file will probably contain some initialization code followed by
1013 something similar to: "if read proto cookie; then echo add $DISPLAY
1014 $proto $cookie | xauth -q -; fi".
1016 If this file does not exist,
1019 does not exist either, xauth is used to store the cookie.
1021 This file should be writable only by the user, and need not be
1022 readable by anyone else.
1026 This can be used to specify
1027 machine-specific login-time initializations globally.
1028 This file should be writable only by root, and should be world-readable.
1032 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1033 but with bugs removed and newer features re-added.
1035 1.2.12 release, newer versions of the original ssh bore successively
1036 more restrictive licenses, and thus demand for a free version was born.
1038 This version of OpenSSH
1041 has all components of a restrictive nature (i.e., patents, see
1043 directly removed from the source code; any licensed or patented components
1047 has been updated to support SSH protocol 1.5 and 2, making it compatible with
1048 all other SSH clients and servers.
1050 contains added support for
1052 authentication and ticket passing.
1054 supports one-time password authentication with
1058 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1059 Niels Provos, Theo de Raadt, and Dug Song.
1061 The support for SSH protocol 2 was written by Markus Friedl.