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 1.104 2001/03/05 14:28:47 deraadt Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH 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 using a symmetric cipher, currently
138 Blowfish, 3DES, CAST128, Arcfour, 128 bit AES, or 256 bit AES.
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 (PubkeyAuthentication)
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,
177 by executing itself with the name it was started as, ie.
180 The options are as follows:
183 Specifies the number of bits in the server key (default 768).
187 The server sends verbose debug output to the system
188 log, and does not put itself in the background.
189 The server also will not fork and will only process one connection.
190 This option is only intended for debugging for the server.
191 Multiple -d options increases the debugging level.
193 .It Fl f Ar configuration_file
194 Specifies the name of the configuration file.
196 .Pa /etc/sshd_config .
198 refuses to start if there is no configuration file.
199 .It Fl g Ar login_grace_time
200 Gives the grace time for clients to authenticate themselves (default
202 If the client fails to authenticate the user within
203 this many seconds, the server disconnects and exits.
204 A value of zero indicates no limit.
205 .It Fl h Ar host_key_file
206 Specifies the file from which the host key is read (default
207 .Pa /etc/ssh_host_key ) .
208 This option must be given if
210 is not run as root (as the normal
211 host file is normally not readable by anyone but root).
212 It is possible to have multiple host key files for
213 the different protocol versions.
217 is being run from inetd.
220 from inetd because it needs to generate the server key before it can
221 respond to the client, and this may take tens of seconds.
222 Clients would have to wait too long if the key was regenerated every time.
223 However, with small key sizes (e.g., 512) using
227 .It Fl k Ar key_gen_time
228 Specifies how often the server key is regenerated (default 3600
229 seconds, or one hour).
230 The motivation for regenerating the key fairly
231 often is that the key is not stored anywhere, and after about an hour,
232 it becomes impossible to recover the key for decrypting intercepted
233 communications even if the machine is cracked into or physically
235 A value of zero indicates that the key will never be regenerated.
237 Specifies the port on which the server listens for connections
241 Nothing is sent to the system log.
242 Normally the beginning,
243 authentication, and termination of each connection is logged.
245 This option is used to specify the size of the field
248 structure that holds the remote host name.
249 If the resolved host name is longer than
251 the dotted decimal value will be used instead.
252 This allows hosts with very long host names that
253 overflow this field to still be uniquely identified.
256 indicates that only dotted decimal addresses
257 should be put into the
261 When this option is specified
263 will not detach and does not become a daemon.
264 This allows easy monitoring of
266 .It Fl V Ar client_protocol_id
267 SSH-2 compatibility mode.
268 When this option is specified
270 assumes the client has sent the supplied version string
272 Protocol Version Identification Exchange.
273 This option is not intended to be called directly.
277 to use IPv4 addresses only.
281 to use IPv6 addresses only.
283 .Sh CONFIGURATION FILE
285 reads configuration data from
287 (or the file specified with
289 on the command line).
290 The file contains keyword-value pairs, one per line.
293 and empty lines are interpreted as comments.
295 The following keywords are possible.
297 .It Cm AFSTokenPassing
298 Specifies whether an AFS token may be forwarded to the server.
302 This keyword can be followed by a list of group names, separated
304 If specified, login is allowed only for users whose primary
305 group or supplementary group list matches one of the patterns.
310 wildcards in the patterns.
311 Only group names are valid; a numerical group ID isn't recognized.
312 By default login is allowed regardless of the group list.
314 .It Cm AllowTcpForwarding
315 Specifies whether TCP forwarding is permitted.
318 Note that disabling TCP forwarding does not improve security unless
319 users are also denied shell access, as they can always install their
323 This keyword can be followed by a list of user names, separated
325 If specified, login is allowed only for users names that
326 match one of the patterns.
331 wildcards in the patterns.
332 Only user names are valid; a numerical user ID isn't recognized.
333 By default login is allowed regardless of the user name.
336 In some jurisdictions, sending a warning message before authentication
337 may be relevant for getting legal protection.
338 The contents of the specified file are sent to the remote user before
339 authentication is allowed.
340 This option is only available for protocol version 2.
342 .It Cm ChallengeResponseAuthentication
345 authentication is allowed.
346 Currently there is only support for
352 Specifies the ciphers allowed for protocol version 2.
353 Multiple ciphers must be comma-separated.
355 .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
359 should check for new mail for interactive logins.
363 This keyword can be followed by a number of group names, separated
365 Users whose primary group or supplementary group list matches
366 one of the patterns aren't allowed to log in.
371 wildcards in the patterns.
372 Only group names are valid; a numerical group ID isn't recognized.
373 By default login is allowed regardless of the group list.
376 This keyword can be followed by a number of user names, separated
378 Login is disallowed for user names that match one of the patterns.
382 can be used as wildcards in the patterns.
383 Only user names are valid; a numerical user ID isn't recognized.
384 By default login is allowed regardless of the user name.
386 Specifies whether remote hosts are allowed to connect to ports
387 forwarded for the client.
395 Specifies the file containing the private host keys (default
396 .Pa /etc/ssh_host_key )
397 used by SSH protocol versions 1 and 2.
400 will refuse to use a file if it is group/world-accessible.
401 It is possible to have multiple host key files.
403 keys are used for version 1 and
407 are used for version 2 of the SSH protocol.
413 files will not be used in authentication.
416 .Pa /etc/shosts.equiv
420 .It Cm IgnoreUserKnownHosts
423 should ignore the user's
424 .Pa $HOME/.ssh/known_hosts
426 .Cm RhostsRSAAuthentication .
430 Specifies whether the system should send keepalive messages to the
432 If they are sent, death of the connection or crash of one
433 of the machines will be properly noticed.
434 However, this means that
435 connections will die if the route is down temporarily, and some people
437 On the other hand, if keepalives are not sent,
438 sessions may hang indefinitely on the server, leaving
440 users and consuming server resources.
444 (to send keepalives), and the server will notice
445 if the network goes down or the client host reboots.
446 This avoids infinitely hanging sessions.
448 To disable keepalives, the value should be set to
450 in both the server and the client configuration files.
451 .It Cm KerberosAuthentication
452 Specifies whether Kerberos authentication is allowed.
453 This can be in the form of a Kerberos ticket, or if
454 .Cm PasswordAuthentication
455 is yes, the password provided by the user will be validated through
457 To use this option, the server needs a
458 Kerberos servtab which allows the verification of the KDC's identity.
461 .It Cm KerberosOrLocalPasswd
462 If set then if password authentication through Kerberos fails then
463 the password will be validated via any additional local mechanism
468 .It Cm KerberosTgtPassing
469 Specifies whether a Kerberos TGT may be forwarded to the server.
472 as this only works when the Kerberos KDC is actually an AFS kaserver.
473 .It Cm KerberosTicketCleanup
474 Specifies whether to automatically destroy the user's ticket cache
478 .It Cm KeyRegenerationInterval
479 The server key is automatically regenerated after this many seconds
480 (if it has been used).
481 The purpose of regeneration is to prevent
482 decrypting captured sessions by later breaking into the machine and
484 The key is never stored anywhere.
485 If the value is 0, the key is never regenerated.
486 The default is 3600 (seconds).
488 Specifies what local address
491 The default is to listen to all local addresses.
492 Multiple options of this type are permitted.
495 options must precede this option.
496 .It Cm LoginGraceTime
497 The server disconnects after this time if the user has not
498 successfully logged in.
499 If the value is 0, there is no time limit.
500 The default is 600 (seconds).
502 Gives the verbosity level that is used when logging messages from
504 The possible values are:
505 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
507 Logging with level DEBUG violates the privacy of users
508 and is not recommended.
510 Specifies the available MAC (message authentication code) algorithms.
511 The MAC algorithm is used in protocol version 2
512 for data integrity protection.
513 Multiple algorithms must be comma-separated.
517 ``hmac-sha1,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,
518 hmac-sha1-96,hmac-md5-96''
521 Specifies the maximum number of concurrent unauthenticated connections to the
524 Additional connections will be dropped until authentication succeeds or the
526 expires for a connection.
529 Alternatively, random early drop can be enabled by specifying
530 the three colon separated values
534 will refuse connection attempts with a probability of
537 if there are currently
540 unauthenticated connections.
541 The probability increases linearly and all connection attempts
542 are refused if the number of unauthenticated connections reaches
545 .It Cm PasswordAuthentication
546 Specifies whether password authentication is allowed.
549 Note that this option applies to both protocol versions 1 and 2.
550 .It Cm PermitEmptyPasswords
551 When password authentication is allowed, it specifies whether the
552 server allows login to accounts with empty password strings.
555 .It Cm PermitRootLogin
556 Specifies whether root can login using
560 .Dq without-password ,
561 .Dq forced-commands-only
567 If this option is set to
569 password authentication is disabled for root.
571 If this option is set to
572 .Dq forced-commands-only
573 root login with public key authentication will be allowed,
576 option has been specified
577 (which may be useful for taking remote backups even if root login is
578 normally not allowed). All other authentication methods are disabled
581 If this option is set to
583 root is not allowed to login.
585 Specifies the file that contains the process identifier of the
589 .Pa /var/run/sshd.pid .
591 Specifies the port number that
595 Multiple options of this type are permitted.
601 when a user logs in interactively.
602 (On some systems it is also printed by the shell,
608 Specifies the protocol versions
611 The possible values are
615 Multiple versions must be comma-separated.
618 .It Cm PubkeyAuthentication
619 Specifies whether public key authentication is allowed.
622 Note that this option applies to protocol version 2 only.
623 .It Cm ReverseMappingCheck
626 should try to verify the remote host name and check that
627 the resolved host name for the remote IP address maps back to the
628 very same IP address.
631 .It Cm RhostsAuthentication
632 Specifies whether authentication using rhosts or /etc/hosts.equiv
634 Normally, this method should not be permitted because it is insecure.
635 .Cm RhostsRSAAuthentication
637 instead, because it performs RSA-based host authentication in addition
638 to normal rhosts or /etc/hosts.equiv authentication.
641 .It Cm RhostsRSAAuthentication
642 Specifies whether rhosts or /etc/hosts.equiv authentication together
643 with successful RSA host authentication is allowed.
646 .It Cm RSAAuthentication
647 Specifies whether pure RSA authentication is allowed.
650 Note that this option applies to protocol version 1 only.
652 Defines the number of bits in the server key.
653 The minimum value is 512, and the default is 768.
657 should check file modes and ownership of the
658 user's files and home directory before accepting login.
659 This is normally desirable because novices sometimes accidentally leave their
660 directory or files world-writable.
664 Configures an external subsystem (e.g., file transfer daemon).
665 Arguments should be a subsystem name and a command to execute upon subsystem
671 file transfer subsystem.
672 By default no subsystems are defined.
673 Note that this option applies to protocol version 2 only.
674 .It Cm SyslogFacility
675 Gives the facility code that is used when logging messages from
677 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
678 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
683 is used for interactive login sessions.
686 is never used for remote command execution.
689 .It Cm X11DisplayOffset
690 Specifies the first display number available for
695 from interfering with real X11 servers.
698 Specifies whether X11 forwarding is permitted.
701 Note that disabling X11 forwarding does not improve security in any
702 way, as users can always install their own forwarders.
704 Specifies the location of the
708 .Pa /usr/X11R6/bin/xauth .
711 When a user successfully logs in,
714 .Bl -enum -offset indent
716 If the login is on a tty, and no command has been specified,
717 prints last login time and
719 (unless prevented in the configuration file or by
720 .Pa $HOME/.hushlogin ;
725 If the login is on a tty, records login time.
729 if it exists, prints contents and quits
732 Changes to run with normal user privileges.
734 Sets up basic environment.
737 .Pa $HOME/.ssh/environment
740 Changes to user's home directory.
744 exists, runs it; else if
747 it; otherwise runs xauth.
750 files are given the X11
751 authentication protocol and cookie in standard input.
753 Runs user's shell or command.
755 .Sh AUTHORIZED_KEYS FILE FORMAT
757 .Pa $HOME/.ssh/authorized_keys
758 file lists the RSA keys that are
759 permitted for RSA authentication in SSH protocols 1.3 and 1.5
761 .Pa $HOME/.ssh/authorized_keys2
762 file lists the DSA and RSA keys that are
763 permitted for public key authentication (PubkeyAuthentication)
766 Each line of the file contains one
767 key (empty lines and lines starting with a
771 Each RSA public key consists of the following fields, separated by
772 spaces: options, bits, exponent, modulus, comment.
773 Each protocol version 2 public key consists of:
774 options, keytype, base64 encoded key, comment.
776 are optional; its presence is determined by whether the line starts
777 with a number or not (the option field never starts with a number).
778 The bits, exponent, modulus and comment fields give the RSA key for
779 protocol version 1; the
780 comment field is not used for anything (but may be convenient for the
781 user to identify the key).
782 For protocol version 2 the keytype is
787 Note that lines in this file are usually several hundred bytes long
788 (because of the size of the RSA key modulus).
789 You don't want to type them in; instead, copy the
795 The options (if present) consist of comma-separated option
797 No spaces are permitted, except within double quotes.
798 The following option specifications are supported:
800 .It Cm from="pattern-list"
801 Specifies that in addition to RSA authentication, the canonical name
802 of the remote host must be present in the comma-separated list of
808 The list may also contain
809 patterns negated by prefixing them with
811 if the canonical host name matches a negated pattern, the key is not accepted.
813 of this option is to optionally increase security: RSA authentication
814 by itself does not trust the network or name servers or anything (but
815 the key); however, if somebody somehow steals the key, the key
816 permits an intruder to log in from anywhere in the world.
817 This additional option makes using a stolen key more difficult (name
818 servers and/or routers would have to be compromised in addition to
820 .It Cm command="command"
821 Specifies that the command is executed whenever this key is used for
823 The command supplied by the user (if any) is ignored.
824 The command is run on a pty if the connection requests a pty;
825 otherwise it is run without a tty.
826 Note that if you want a 8-bit clean channel,
827 you must not request a pty or should specify
829 A quote may be included in the command by quoting it with a backslash.
830 This option might be useful
831 to restrict certain RSA keys to perform just a specific operation.
832 An example might be a key that permits remote backups but nothing else.
833 Note that the client may specify TCP/IP and/or X11
834 forwarding unless they are explicitly prohibited.
835 .It Cm environment="NAME=value"
836 Specifies that the string is to be added to the environment when
837 logging in using this key.
838 Environment variables set this way
839 override other default environment values.
840 Multiple options of this type are permitted.
841 .It Cm no-port-forwarding
842 Forbids TCP/IP forwarding when this key is used for authentication.
843 Any port forward requests by the client will return an error.
844 This might be used, e.g., in connection with the
847 .It Cm no-X11-forwarding
848 Forbids X11 forwarding when this key is used for authentication.
849 Any X11 forward requests by the client will return an error.
850 .It Cm no-agent-forwarding
851 Forbids authentication agent forwarding when this key is used for
854 Prevents tty allocation (a request to allocate a pty will fail).
857 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
859 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
861 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
862 .Sh SSH_KNOWN_HOSTS FILE FORMAT
864 .Pa /etc/ssh_known_hosts ,
865 .Pa /etc/ssh_known_hosts2 ,
866 .Pa $HOME/.ssh/known_hosts ,
868 .Pa $HOME/.ssh/known_hosts2
869 files contain host public keys for all known hosts.
870 The global file should
871 be prepared by the administrator (optional), and the per-user file is
872 maintained automatically: whenever the user connects from an unknown host
873 its key is added to the per-user file.
875 Each line in these files contains the following fields: hostnames,
876 bits, exponent, modulus, comment.
877 The fields are separated by spaces.
879 Hostnames is a comma-separated list of patterns ('*' and '?' act as
880 wildcards); each pattern in turn is matched against the canonical host
881 name (when authenticating a client) or against the user-supplied
882 name (when authenticating a server).
883 A pattern may also be preceded by
885 to indicate negation: if the host name matches a negated
886 pattern, it is not accepted (by that line) even if it matched another
889 Bits, exponent, and modulus are taken directly from the RSA host key; they
890 can be obtained, e.g., from
891 .Pa /etc/ssh_host_key.pub .
892 The optional comment field continues to the end of the line, and is not used.
896 and empty lines are ignored as comments.
898 When performing host authentication, authentication is accepted if any
899 matching line has the proper key.
900 It is thus permissible (but not
901 recommended) to have several lines or different host keys for the same
903 This will inevitably happen when short forms of host names
904 from different domains are put in the file.
906 that the files contain conflicting information; authentication is
907 accepted if valid information can be found from either file.
909 Note that the lines in these files are typically hundreds of characters
910 long, and you definitely don't want to type in the host keys by hand.
911 Rather, generate them by a script
913 .Pa /etc/ssh_host_key.pub
914 and adding the host names at the front.
916 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
919 .It Pa /etc/sshd_config
920 Contains configuration data for
922 This file should be writable by root only, but it is recommended
923 (though not necessary) that it be world-readable.
924 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
925 These three files contain the private parts of the
926 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
927 These files should only be owned by root, readable only by root, and not
928 accessible to others.
931 does not start if this file is group/world-accessible.
932 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
933 There three files contain the public parts of the
934 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
935 These files should be world-readable but writable only by
937 Their contents should match the respective private parts.
939 really used for anything; they are provided for the convenience of
940 the user so their contents can be copied to known hosts files.
941 These files are created using
944 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
945 .It Pa /var/run/sshd.pid
946 Contains the process ID of the
948 listening for connections (if there are several daemons running
949 concurrently for different ports, this contains the pid of the one
951 The content of this file is not sensitive; it can be world-readable.
952 .It Pa $HOME/.ssh/authorized_keys
953 Lists the RSA keys that can be used to log into the user's account.
954 This file must be readable by root (which may on some machines imply
955 it being world-readable if the user's home directory resides on an NFS
957 It is recommended that it not be accessible by others.
958 The format of this file is described above.
959 Users will place the contents of their
961 files into this file, as described in
963 .It Pa $HOME/.ssh/authorized_keys2
964 Lists the DSA keys that can be used to log into the user's account.
965 This file must be readable by root (which may on some machines imply
966 it being world-readable if the user's home directory resides on an NFS
968 It is recommended that it not be accessible by others.
969 The format of this file is described above.
970 Users will place the contents of their
972 files into this file, as described in
974 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
975 These files are consulted when using rhosts with RSA host
976 authentication to check the public key of the host.
977 The key must be listed in one of these files to be accepted.
978 The client uses the same files
979 to verify that it is connecting to the correct remote host.
980 These files should be writable only by root/the owner.
981 .Pa /etc/ssh_known_hosts
982 should be world-readable, and
983 .Pa $HOME/.ssh/known_hosts
984 can but need not be world-readable.
988 refuses to let anyone except root log in.
989 The contents of the file
990 are displayed to anyone trying to log in, and non-root connections are
992 The file should be world-readable.
993 .It Pa /etc/hosts.allow, /etc/hosts.deny
996 support, tcp-wrappers access controls may be defined here as described in
999 This file contains host-username pairs, separated by a space, one per
1001 The given user on the corresponding host is permitted to log in
1003 The same file is used by rlogind and rshd.
1005 be writable only by the user; it is recommended that it not be
1006 accessible by others.
1008 If is also possible to use netgroups in the file.
1010 name may be of the form +@groupname to specify all hosts or all users
1012 .It Pa $HOME/.shosts
1014 this file is exactly the same as for
1016 However, this file is
1017 not used by rlogin and rshd, so using this permits access using SSH only.
1018 .It Pa /etc/hosts.equiv
1019 This file is used during
1022 In the simplest form, this file contains host names, one per line.
1024 those hosts are permitted to log in without a password, provided they
1025 have the same user name on both machines.
1026 The host name may also be
1027 followed by a user name; such users are permitted to log in as
1029 user on this machine (except root).
1030 Additionally, the syntax
1032 can be used to specify netgroups.
1033 Negated entries start with
1036 If the client host/user is successfully matched in this file, login is
1037 automatically permitted provided the client and server user names are the
1039 Additionally, successful RSA host authentication is normally required.
1040 This file must be writable only by root; it is recommended
1041 that it be world-readable.
1043 .Sy "Warning: It is almost never a good idea to use user names in"
1045 Beware that it really means that the named user(s) can log in as
1047 which includes bin, daemon, adm, and other accounts that own critical
1048 binaries and directories.
1049 Using a user name practically grants the user root access.
1050 The only valid use for user names that I can think
1051 of is in negative entries.
1053 Note that this warning also applies to rsh/rlogin.
1054 .It Pa /etc/shosts.equiv
1055 This is processed exactly as
1056 .Pa /etc/hosts.equiv .
1057 However, this file may be useful in environments that want to run both
1059 .It Pa $HOME/.ssh/environment
1060 This file is read into the environment at login (if it exists).
1061 It can only contain empty lines, comment lines (that start with
1063 and assignment lines of the form name=value.
1064 The file should be writable
1065 only by the user; it need not be readable by anyone else.
1066 .It Pa $HOME/.ssh/rc
1067 If this file exists, it is run with /bin/sh after reading the
1068 environment files but before starting the user's shell or command.
1069 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1077 The primary purpose of this file is to run any initialization routines
1078 which may be needed before the user's home directory becomes
1079 accessible; AFS is a particular example of such an environment.
1081 This file will probably contain some initialization code followed by
1082 something similar to: "if read proto cookie; then echo add $DISPLAY
1083 $proto $cookie | xauth -q -; fi".
1085 If this file does not exist,
1088 does not exist either, xauth is used to store the cookie.
1090 This file should be writable only by the user, and need not be
1091 readable by anyone else.
1095 This can be used to specify
1096 machine-specific login-time initializations globally.
1097 This file should be writable only by root, and should be world-readable.
1100 OpenSSH is a derivative of the original and free
1101 ssh 1.2.12 release by Tatu Ylonen.
1102 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1103 Theo de Raadt and Dug Song
1104 removed many bugs, re-added newer features and
1106 Markus Friedl contributed the support for SSH
1107 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob