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 1.94 2001/02/12 16:16:24 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
265 .It Fl V Ar client_protocol_id
266 SSH-2 compatibility mode.
267 When this option is specified
269 assumes the client has sent the supplied version string
271 Protocol Version Identification Exchange.
272 This option is not intended to be called directly.
276 to use IPv4 addresses only.
280 to use IPv6 addresses only.
282 .Sh CONFIGURATION FILE
284 reads configuration data from
286 (or the file specified with
288 on the command line).
289 The file contains keyword-value pairs, one per line.
292 and empty lines are interpreted as comments.
294 The following keywords are possible.
296 .It Cm AFSTokenPassing
297 Specifies whether an AFS token may be forwarded to the server.
301 This keyword can be followed by a list of group names, separated
303 If specified, login is allowed only for users whose primary
304 group or supplementary group list matches one of the patterns.
309 wildcards in the patterns.
310 Only group names are valid; a numerical group ID isn't recognized.
311 By default login is allowed regardless of the group list.
313 .It Cm AllowTcpForwarding
314 Specifies whether TCP forwarding is permitted.
317 Note that disabling TCP forwarding does not improve security unless
318 users are also denied shell access, as they can always install their
322 This keyword can be followed by a list of user names, separated
324 If specified, login is allowed only for users names that
325 match one of the patterns.
330 wildcards in the patterns.
331 Only user names are valid; a numerical user ID isn't recognized.
332 By default login is allowed regardless of the user name.
335 In some jurisdictions, sending a warning message before authentication
336 may be relevant for getting legal protection.
337 The contents of the specified file are sent to the remote user before
338 authentication is allowed.
339 This option is only available for protocol version 2.
342 Specifies the ciphers allowed for protocol version 2.
343 Multiple ciphers must be comma-separated.
345 .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
349 should check for new mail for interactive logins.
353 This keyword can be followed by a number of group names, separated
355 Users whose primary group or supplementary group list matches
356 one of the patterns aren't allowed to log in.
361 wildcards in the patterns.
362 Only group names are valid; a numerical group ID isn't recognized.
363 By default login is allowed regardless of the group list.
366 This keyword can be followed by a number of user names, separated
368 Login is disallowed for user names that match one of the patterns.
372 can be used as wildcards in the patterns.
373 Only user names are valid; a numerical user ID isn't recognized.
374 By default login is allowed regardless of the user name.
375 .It Cm PubkeyAuthentication
376 Specifies whether public key authentication is allowed.
379 Note that this option applies to protocol version 2 only.
381 Specifies whether remote hosts are allowed to connect to ports
382 forwarded for the client.
390 Specifies the file containing the private host keys (default
391 .Pa /etc/ssh_host_key )
392 used by SSH protocol versions 1 and 2.
395 will refuse to use a file if it is group/world-accessible.
396 It is possible to have multiple host key files.
398 keys are used for version 1 and
402 are used for version 2 of the SSH protocol.
408 files will not be used in authentication.
411 .Pa /etc/shosts.equiv
415 .It Cm IgnoreUserKnownHosts
418 should ignore the user's
419 .Pa $HOME/.ssh/known_hosts
421 .Cm RhostsRSAAuthentication .
425 Specifies whether the system should send keepalive messages to the
427 If they are sent, death of the connection or crash of one
428 of the machines will be properly noticed.
429 However, this means that
430 connections will die if the route is down temporarily, and some people
432 On the other hand, if keepalives are not sent,
433 sessions may hang indefinitely on the server, leaving
435 users and consuming server resources.
439 (to send keepalives), and the server will notice
440 if the network goes down or the client host reboots.
441 This avoids infinitely hanging sessions.
443 To disable keepalives, the value should be set to
445 in both the server and the client configuration files.
446 .It Cm KerberosAuthentication
447 Specifies whether Kerberos authentication is allowed.
448 This can be in the form of a Kerberos ticket, or if
449 .Cm PasswordAuthentication
450 is yes, the password provided by the user will be validated through
452 To use this option, the server needs a
453 Kerberos servtab which allows the verification of the KDC's identity.
456 .It Cm KerberosOrLocalPasswd
457 If set then if password authentication through Kerberos fails then
458 the password will be validated via any additional local mechanism
463 .It Cm KerberosTgtPassing
464 Specifies whether a Kerberos TGT may be forwarded to the server.
467 as this only works when the Kerberos KDC is actually an AFS kaserver.
468 .It Cm KerberosTicketCleanup
469 Specifies whether to automatically destroy the user's ticket cache
473 .It Cm KeyRegenerationInterval
474 The server key is automatically regenerated after this many seconds
475 (if it has been used).
476 The purpose of regeneration is to prevent
477 decrypting captured sessions by later breaking into the machine and
479 The key is never stored anywhere.
480 If the value is 0, the key is never regenerated.
481 The default is 3600 (seconds).
483 Specifies what local address
486 The default is to listen to all local addresses.
487 Multiple options of this type are permitted.
490 options must precede this option.
491 .It Cm LoginGraceTime
492 The server disconnects after this time if the user has not
493 successfully logged in.
494 If the value is 0, there is no time limit.
495 The default is 600 (seconds).
497 Gives the verbosity level that is used when logging messages from
499 The possible values are:
500 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
502 Logging with level DEBUG violates the privacy of users
503 and is not recommended.
505 Specifies the available MAC (message authentication code) algorithms.
506 The MAC algorithm is used in protocol version 2
507 for data integrity protection.
508 Multiple algorithms must be comma-separated.
512 ``hmac-sha1,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,
513 hmac-sha1-96,hmac-md5-96''
516 Specifies the maximum number of concurrent unauthenticated connections to the
519 Additional connections will be dropped until authentication succeeds or the
521 expires for a connection.
524 Alternatively, random early drop can be enabled by specifying
525 the three colon separated values
529 will refuse connection attempts with a probability of
532 if there are currently
535 unauthenticated connections.
536 The probability increases linearly and all connection attempts
537 are refused if the number of unauthenticated connections reaches
540 .It Cm PasswordAuthentication
541 Specifies whether password authentication is allowed.
544 Note that this option applies to both protocol versions 1 and 2.
545 .It Cm PermitEmptyPasswords
546 When password authentication is allowed, it specifies whether the
547 server allows login to accounts with empty password strings.
550 .It Cm PermitRootLogin
551 Specifies whether the root can log in using
555 .Dq without-password ,
556 .Dq forced-commands-only
562 If this option is set to
564 password authentication is disabled for root.
566 If this option is set to
567 .Dq forced-commands-only
568 root login with public key authentication will be allowed,
571 option has been specified
572 (which may be useful for taking remote backups even if root login is
573 normally not allowed). All other authentication methods are disabled
576 Specifies the file that contains the process identifier of the
580 .Pa /var/run/sshd.pid .
582 Specifies the port number that
586 Multiple options of this type are permitted.
592 when a user logs in interactively.
593 (On some systems it is also printed by the shell,
599 Specifies the protocol versions
602 The possible values are
606 Multiple versions must be comma-separated.
611 Random number generation uses other techniques.
612 .It Cm ReverseMappingCheck
615 should try to verify the remote host name and check that
616 the resolved host name for the remote IP address maps back to the
617 very same IP address.
620 .It Cm RhostsAuthentication
621 Specifies whether authentication using rhosts or /etc/hosts.equiv
623 Normally, this method should not be permitted because it is insecure.
624 .Cm RhostsRSAAuthentication
626 instead, because it performs RSA-based host authentication in addition
627 to normal rhosts or /etc/hosts.equiv authentication.
630 .It Cm RhostsRSAAuthentication
631 Specifies whether rhosts or /etc/hosts.equiv authentication together
632 with successful RSA host authentication is allowed.
635 .It Cm RSAAuthentication
636 Specifies whether pure RSA authentication is allowed.
639 Note that this option applies to protocol version 1 only.
641 Defines the number of bits in the server key.
642 The minimum value is 512, and the default is 768.
643 .It Cm ChallengeResponseAuthentication
646 authentication is allowed.
647 Currently there is only support for
655 should check file modes and ownership of the
656 user's files and home directory before accepting login.
657 This is normally desirable because novices sometimes accidentally leave their
658 directory or files world-writable.
662 Configures an external subsystem (e.g., file transfer daemon).
663 Arguments should be a subsystem name and a command to execute upon subsystem
669 file transfer subsystem.
670 By default no subsystems are defined.
671 Note that this option applies to protocol version 2 only.
672 .It Cm SyslogFacility
673 Gives the facility code that is used when logging messages from
675 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
676 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
681 is used for interactive login sessions.
684 is never used for remote command execution.
687 .It Cm X11DisplayOffset
688 Specifies the first display number available for
693 from interfering with real X11 servers.
696 Specifies whether X11 forwarding is permitted.
699 Note that disabling X11 forwarding does not improve security in any
700 way, as users can always install their own forwarders.
702 Specifies the location of the
706 .Pa /usr/X11R6/bin/xauth .
709 When a user successfully logs in,
712 .Bl -enum -offset indent
714 If the login is on a tty, and no command has been specified,
715 prints last login time and
717 (unless prevented in the configuration file or by
718 .Pa $HOME/.hushlogin ;
723 If the login is on a tty, records login time.
727 if it exists, prints contents and quits
730 Changes to run with normal user privileges.
732 Sets up basic environment.
735 .Pa $HOME/.ssh/environment
738 Changes to user's home directory.
742 exists, runs it; else if
745 it; otherwise runs xauth.
748 files are given the X11
749 authentication protocol and cookie in standard input.
751 Runs user's shell or command.
753 .Sh AUTHORIZED_KEYS FILE FORMAT
755 .Pa $HOME/.ssh/authorized_keys
756 file lists the RSA keys that are
757 permitted for RSA authentication in SSH protocols 1.3 and 1.5
759 .Pa $HOME/.ssh/authorized_keys2
760 file lists the DSA and RSA keys that are
761 permitted for public key authentication (PubkeyAuthentication)
764 Each line of the file contains one
765 key (empty lines and lines starting with a
769 Each RSA public key consists of the following fields, separated by
770 spaces: options, bits, exponent, modulus, comment.
771 Each protocol version 2 public key consists of:
772 options, keytype, base64 encoded key, comment.
774 are optional; its presence is determined by whether the line starts
775 with a number or not (the option field never starts with a number).
776 The bits, exponent, modulus and comment fields give the RSA key for
777 protocol version 1; the
778 comment field is not used for anything (but may be convenient for the
779 user to identify the key).
780 For protocol version 2 the keytype is
785 Note that lines in this file are usually several hundred bytes long
786 (because of the size of the RSA key modulus).
787 You don't want to type them in; instead, copy the
793 The options (if present) consist of comma-separated option
795 No spaces are permitted, except within double quotes.
796 The following option specifications are supported:
798 .It Cm from="pattern-list"
799 Specifies that in addition to RSA authentication, the canonical name
800 of the remote host must be present in the comma-separated list of
806 The list may also contain
807 patterns negated by prefixing them with
809 if the canonical host name matches a negated pattern, the key is not accepted.
811 of this option is to optionally increase security: RSA authentication
812 by itself does not trust the network or name servers or anything (but
813 the key); however, if somebody somehow steals the key, the key
814 permits an intruder to log in from anywhere in the world.
815 This additional option makes using a stolen key more difficult (name
816 servers and/or routers would have to be compromised in addition to
818 .It Cm command="command"
819 Specifies that the command is executed whenever this key is used for
821 The command supplied by the user (if any) is ignored.
822 The command is run on a pty if the connection requests a pty;
823 otherwise it is run without a tty.
824 Note that if you want a 8-bit clean channel,
825 you must not request a pty or should specify
827 A quote may be included in the command by quoting it with a backslash.
828 This option might be useful
829 to restrict certain RSA keys to perform just a specific operation.
830 An example might be a key that permits remote backups but nothing else.
831 Note that the client may specify TCP/IP and/or X11
832 forwarding unless they are explicitly prohibited.
833 .It Cm environment="NAME=value"
834 Specifies that the string is to be added to the environment when
835 logging in using this key.
836 Environment variables set this way
837 override other default environment values.
838 Multiple options of this type are permitted.
839 .It Cm no-port-forwarding
840 Forbids TCP/IP forwarding when this key is used for authentication.
841 Any port forward requests by the client will return an error.
842 This might be used, e.g., in connection with the
845 .It Cm no-X11-forwarding
846 Forbids X11 forwarding when this key is used for authentication.
847 Any X11 forward requests by the client will return an error.
848 .It Cm no-agent-forwarding
849 Forbids authentication agent forwarding when this key is used for
852 Prevents tty allocation (a request to allocate a pty will fail).
855 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
857 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
859 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
860 .Sh SSH_KNOWN_HOSTS FILE FORMAT
862 .Pa /etc/ssh_known_hosts ,
863 .Pa /etc/ssh_known_hosts2 ,
864 .Pa $HOME/.ssh/known_hosts ,
866 .Pa $HOME/.ssh/known_hosts2
867 files contain host public keys for all known hosts.
868 The global file should
869 be prepared by the administrator (optional), and the per-user file is
870 maintained automatically: whenever the user connects from an unknown host
871 its key is added to the per-user file.
873 Each line in these files contains the following fields: hostnames,
874 bits, exponent, modulus, comment.
875 The fields are separated by spaces.
877 Hostnames is a comma-separated list of patterns ('*' and '?' act as
878 wildcards); each pattern in turn is matched against the canonical host
879 name (when authenticating a client) or against the user-supplied
880 name (when authenticating a server).
881 A pattern may also be preceded by
883 to indicate negation: if the host name matches a negated
884 pattern, it is not accepted (by that line) even if it matched another
887 Bits, exponent, and modulus are taken directly from the RSA host key; they
888 can be obtained, e.g., from
889 .Pa /etc/ssh_host_key.pub .
890 The optional comment field continues to the end of the line, and is not used.
894 and empty lines are ignored as comments.
896 When performing host authentication, authentication is accepted if any
897 matching line has the proper key.
898 It is thus permissible (but not
899 recommended) to have several lines or different host keys for the same
901 This will inevitably happen when short forms of host names
902 from different domains are put in the file.
904 that the files contain conflicting information; authentication is
905 accepted if valid information can be found from either file.
907 Note that the lines in these files are typically hundreds of characters
908 long, and you definitely don't want to type in the host keys by hand.
909 Rather, generate them by a script
911 .Pa /etc/ssh_host_key.pub
912 and adding the host names at the front.
914 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
917 .It Pa /etc/sshd_config
918 Contains configuration data for
920 This file should be writable by root only, but it is recommended
921 (though not necessary) that it be world-readable.
922 .It Pa /etc/ssh_host_key
923 Contains the private part of the host key.
924 This file should only be owned by root, readable only by root, and not
925 accessible to others.
928 does not start if this file is group/world-accessible.
929 .It Pa /etc/ssh_host_key.pub
930 Contains the public part of the host key.
931 This file should be world-readable but writable only by
933 Its contents should match the private part.
935 really used for anything; it is only provided for the convenience of
936 the user so its contents can be copied to known hosts files.
937 These two files are created using
940 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
941 .It Pa /var/run/sshd.pid
942 Contains the process ID of the
944 listening for connections (if there are several daemons running
945 concurrently for different ports, this contains the pid of the one
947 The content of this file is not sensitive; it can be world-readable.
948 .It Pa $HOME/.ssh/authorized_keys
949 Lists the RSA keys that can be used to log into the user's account.
950 This file must be readable by root (which may on some machines imply
951 it being world-readable if the user's home directory resides on an NFS
953 It is recommended that it not be accessible by others.
954 The format of this file is described above.
955 Users will place the contents of their
957 files into this file, as described in
959 .It Pa $HOME/.ssh/authorized_keys2
960 Lists the DSA keys that can be used to log into the user's account.
961 This file must be readable by root (which may on some machines imply
962 it being world-readable if the user's home directory resides on an NFS
964 It is recommended that it not be accessible by others.
965 The format of this file is described above.
966 Users will place the contents of their
968 files into this file, as described in
970 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
971 These files are consulted when using rhosts with RSA host
972 authentication to check the public key of the host.
973 The key must be listed in one of these files to be accepted.
974 The client uses the same files
975 to verify that the remote host is the one it intended to connect.
976 These files should be writable only by root/the owner.
977 .Pa /etc/ssh_known_hosts
978 should be world-readable, and
979 .Pa $HOME/.ssh/known_hosts
980 can but need not be world-readable.
984 refuses to let anyone except root log in.
985 The contents of the file
986 are displayed to anyone trying to log in, and non-root connections are
988 The file should be world-readable.
989 .It Pa /etc/hosts.allow, /etc/hosts.deny
992 support, tcp-wrappers access controls may be defined here as described in
995 This file contains host-username pairs, separated by a space, one per
997 The given user on the corresponding host is permitted to log in
999 The same file is used by rlogind and rshd.
1001 be writable only by the user; it is recommended that it not be
1002 accessible by others.
1004 If is also possible to use netgroups in the file.
1006 name may be of the form +@groupname to specify all hosts or all users
1008 .It Pa $HOME/.shosts
1010 this file is exactly the same as for
1012 However, this file is
1013 not used by rlogin and rshd, so using this permits access using SSH only.
1014 .It Pa /etc/hosts.equiv
1015 This file is used during
1018 In the simplest form, this file contains host names, one per line.
1020 those hosts are permitted to log in without a password, provided they
1021 have the same user name on both machines.
1022 The host name may also be
1023 followed by a user name; such users are permitted to log in as
1025 user on this machine (except root).
1026 Additionally, the syntax
1028 can be used to specify netgroups.
1029 Negated entries start with
1032 If the client host/user is successfully matched in this file, login is
1033 automatically permitted provided the client and server user names are the
1035 Additionally, successful RSA host authentication is normally required.
1036 This file must be writable only by root; it is recommended
1037 that it be world-readable.
1039 .Sy "Warning: It is almost never a good idea to use user names in"
1041 Beware that it really means that the named user(s) can log in as
1043 which includes bin, daemon, adm, and other accounts that own critical
1044 binaries and directories.
1045 Using a user name practically grants the user root access.
1046 The only valid use for user names that I can think
1047 of is in negative entries.
1049 Note that this warning also applies to rsh/rlogin.
1050 .It Pa /etc/shosts.equiv
1051 This is processed exactly as
1052 .Pa /etc/hosts.equiv .
1053 However, this file may be useful in environments that want to run both
1055 .It Pa $HOME/.ssh/environment
1056 This file is read into the environment at login (if it exists).
1057 It can only contain empty lines, comment lines (that start with
1059 and assignment lines of the form name=value.
1060 The file should be writable
1061 only by the user; it need not be readable by anyone else.
1062 .It Pa $HOME/.ssh/rc
1063 If this file exists, it is run with /bin/sh after reading the
1064 environment files but before starting the user's shell or command.
1065 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1073 The primary purpose of this file is to run any initialization routines
1074 which may be needed before the user's home directory becomes
1075 accessible; AFS is a particular example of such an environment.
1077 This file will probably contain some initialization code followed by
1078 something similar to: "if read proto cookie; then echo add $DISPLAY
1079 $proto $cookie | xauth -q -; fi".
1081 If this file does not exist,
1084 does not exist either, xauth is used to store the cookie.
1086 This file should be writable only by the user, and need not be
1087 readable by anyone else.
1091 This can be used to specify
1092 machine-specific login-time initializations globally.
1093 This file should be writable only by root, and should be world-readable.
1096 OpenSSH is a derivative of the original and free
1097 ssh 1.2.12 release by Tatu Ylonen.
1098 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1099 Theo de Raadt and Dug Song
1100 removed many bugs, re-added newer features and
1102 Markus Friedl contributed the support for SSH
1103 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob