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 1.112 2001/04/09 18:00:15 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH ssh 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 (SSH 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 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
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 the local addresses
491 The following forms may be used:
493 .Bl -item -offset indent -compact
497 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
502 .Ar host No | Ar IPv4_addr No : Ar port
508 .Ar host No | Ar IPv6_addr Oc : Ar port
516 will listen on the address and all prior
518 options specified. The default is to listen on all local
521 options are permitted. Additionally, any
523 options must precede this option for non port qualified addresses.
524 .It Cm LoginGraceTime
525 The server disconnects after this time if the user has not
526 successfully logged in.
527 If the value is 0, there is no time limit.
528 The default is 600 (seconds).
530 Gives the verbosity level that is used when logging messages from
532 The possible values are:
533 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
535 Logging with level DEBUG violates the privacy of users
536 and is not recommended.
538 Specifies the available MAC (message authentication code) algorithms.
539 The MAC algorithm is used in protocol version 2
540 for data integrity protection.
541 Multiple algorithms must be comma-separated.
545 ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
546 hmac-sha1-96,hmac-md5-96''
549 Specifies the maximum number of concurrent unauthenticated connections to the
552 Additional connections will be dropped until authentication succeeds or the
554 expires for a connection.
557 Alternatively, random early drop can be enabled by specifying
558 the three colon separated values
562 will refuse connection attempts with a probability of
565 if there are currently
568 unauthenticated connections.
569 The probability increases linearly and all connection attempts
570 are refused if the number of unauthenticated connections reaches
573 .It Cm PasswordAuthentication
574 Specifies whether password authentication is allowed.
577 Note that this option applies to both protocol versions 1 and 2.
578 .It Cm PermitEmptyPasswords
579 When password authentication is allowed, it specifies whether the
580 server allows login to accounts with empty password strings.
583 .It Cm PermitRootLogin
584 Specifies whether root can login using
588 .Dq without-password ,
589 .Dq forced-commands-only
595 If this option is set to
597 password authentication is disabled for root.
599 If this option is set to
600 .Dq forced-commands-only
601 root login with public key authentication will be allowed,
604 option has been specified
605 (which may be useful for taking remote backups even if root login is
606 normally not allowed). All other authentication methods are disabled
609 If this option is set to
611 root is not allowed to login.
613 Specifies the file that contains the process identifier of the
617 .Pa /var/run/sshd.pid .
619 Specifies the port number that
623 Multiple options of this type are permitted.
627 should print the date and time when the user last logged in.
635 when a user logs in interactively.
636 (On some systems it is also printed by the shell,
642 Specifies the protocol versions
645 The possible values are
649 Multiple versions must be comma-separated.
652 .It Cm PubkeyAuthentication
653 Specifies whether public key authentication is allowed.
656 Note that this option applies to protocol version 2 only.
657 .It Cm ReverseMappingCheck
660 should try to verify the remote host name and check that
661 the resolved host name for the remote IP address maps back to the
662 very same IP address.
665 .It Cm RhostsAuthentication
666 Specifies whether authentication using rhosts or /etc/hosts.equiv
668 Normally, this method should not be permitted because it is insecure.
669 .Cm RhostsRSAAuthentication
671 instead, because it performs RSA-based host authentication in addition
672 to normal rhosts or /etc/hosts.equiv authentication.
675 .It Cm RhostsRSAAuthentication
676 Specifies whether rhosts or /etc/hosts.equiv authentication together
677 with successful RSA host authentication is allowed.
680 .It Cm RSAAuthentication
681 Specifies whether pure RSA authentication is allowed.
684 Note that this option applies to protocol version 1 only.
686 Defines the number of bits in the server key.
687 The minimum value is 512, and the default is 768.
691 should check file modes and ownership of the
692 user's files and home directory before accepting login.
693 This is normally desirable because novices sometimes accidentally leave their
694 directory or files world-writable.
698 Configures an external subsystem (e.g., file transfer daemon).
699 Arguments should be a subsystem name and a command to execute upon subsystem
705 file transfer subsystem.
706 By default no subsystems are defined.
707 Note that this option applies to protocol version 2 only.
708 .It Cm SyslogFacility
709 Gives the facility code that is used when logging messages from
711 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
712 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
717 is used for interactive login sessions.
720 is never used for remote command execution.
723 .It Cm X11DisplayOffset
724 Specifies the first display number available for
729 from interfering with real X11 servers.
732 Specifies whether X11 forwarding is permitted.
735 Note that disabling X11 forwarding does not improve security in any
736 way, as users can always install their own forwarders.
738 Specifies the location of the
742 .Pa /usr/X11R6/bin/xauth .
745 When a user successfully logs in,
748 .Bl -enum -offset indent
750 If the login is on a tty, and no command has been specified,
751 prints last login time and
753 (unless prevented in the configuration file or by
754 .Pa $HOME/.hushlogin ;
759 If the login is on a tty, records login time.
763 if it exists, prints contents and quits
766 Changes to run with normal user privileges.
768 Sets up basic environment.
771 .Pa $HOME/.ssh/environment
774 Changes to user's home directory.
778 exists, runs it; else if
781 it; otherwise runs xauth.
784 files are given the X11
785 authentication protocol and cookie in standard input.
787 Runs user's shell or command.
789 .Sh AUTHORIZED_KEYS FILE FORMAT
791 .Pa $HOME/.ssh/authorized_keys
792 file lists the RSA keys that are
793 permitted for RSA authentication in SSH protocols 1.3 and 1.5
795 .Pa $HOME/.ssh/authorized_keys2
796 file lists the DSA and RSA keys that are
797 permitted for public key authentication (PubkeyAuthentication)
800 Each line of the file contains one
801 key (empty lines and lines starting with a
805 Each RSA public key consists of the following fields, separated by
806 spaces: options, bits, exponent, modulus, comment.
807 Each protocol version 2 public key consists of:
808 options, keytype, base64 encoded key, comment.
810 are optional; its presence is determined by whether the line starts
811 with a number or not (the option field never starts with a number).
812 The bits, exponent, modulus and comment fields give the RSA key for
813 protocol version 1; the
814 comment field is not used for anything (but may be convenient for the
815 user to identify the key).
816 For protocol version 2 the keytype is
821 Note that lines in this file are usually several hundred bytes long
822 (because of the size of the RSA key modulus).
823 You don't want to type them in; instead, copy the
829 The options (if present) consist of comma-separated option
831 No spaces are permitted, except within double quotes.
832 The following option specifications are supported:
834 .It Cm from="pattern-list"
835 Specifies that in addition to RSA authentication, the canonical name
836 of the remote host must be present in the comma-separated list of
842 The list may also contain
843 patterns negated by prefixing them with
845 if the canonical host name matches a negated pattern, the key is not accepted.
847 of this option is to optionally increase security: RSA authentication
848 by itself does not trust the network or name servers or anything (but
849 the key); however, if somebody somehow steals the key, the key
850 permits an intruder to log in from anywhere in the world.
851 This additional option makes using a stolen key more difficult (name
852 servers and/or routers would have to be compromised in addition to
854 .It Cm command="command"
855 Specifies that the command is executed whenever this key is used for
857 The command supplied by the user (if any) is ignored.
858 The command is run on a pty if the connection requests a pty;
859 otherwise it is run without a tty.
860 Note that if you want a 8-bit clean channel,
861 you must not request a pty or should specify
863 A quote may be included in the command by quoting it with a backslash.
864 This option might be useful
865 to restrict certain RSA keys to perform just a specific operation.
866 An example might be a key that permits remote backups but nothing else.
867 Note that the client may specify TCP/IP and/or X11
868 forwarding unless they are explicitly prohibited.
869 .It Cm environment="NAME=value"
870 Specifies that the string is to be added to the environment when
871 logging in using this key.
872 Environment variables set this way
873 override other default environment values.
874 Multiple options of this type are permitted.
875 .It Cm no-port-forwarding
876 Forbids TCP/IP forwarding when this key is used for authentication.
877 Any port forward requests by the client will return an error.
878 This might be used, e.g., in connection with the
881 .It Cm no-X11-forwarding
882 Forbids X11 forwarding when this key is used for authentication.
883 Any X11 forward requests by the client will return an error.
884 .It Cm no-agent-forwarding
885 Forbids authentication agent forwarding when this key is used for
888 Prevents tty allocation (a request to allocate a pty will fail).
889 .It Cm permitopen="host:port"
892 port forwarding such that it may only connect to the specified host and
895 options may be applied separated by commas. No pattern matching is
896 performed on the specified hostnames, they must be literal domains or
900 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
902 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
904 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
906 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
907 .Sh SSH_KNOWN_HOSTS FILE FORMAT
909 .Pa /etc/ssh_known_hosts ,
910 .Pa /etc/ssh_known_hosts2 ,
911 .Pa $HOME/.ssh/known_hosts ,
913 .Pa $HOME/.ssh/known_hosts2
914 files contain host public keys for all known hosts.
915 The global file should
916 be prepared by the administrator (optional), and the per-user file is
917 maintained automatically: whenever the user connects from an unknown host
918 its key is added to the per-user file.
920 Each line in these files contains the following fields: hostnames,
921 bits, exponent, modulus, comment.
922 The fields are separated by spaces.
924 Hostnames is a comma-separated list of patterns ('*' and '?' act as
925 wildcards); each pattern in turn is matched against the canonical host
926 name (when authenticating a client) or against the user-supplied
927 name (when authenticating a server).
928 A pattern may also be preceded by
930 to indicate negation: if the host name matches a negated
931 pattern, it is not accepted (by that line) even if it matched another
934 Bits, exponent, and modulus are taken directly from the RSA host key; they
935 can be obtained, e.g., from
936 .Pa /etc/ssh_host_key.pub .
937 The optional comment field continues to the end of the line, and is not used.
941 and empty lines are ignored as comments.
943 When performing host authentication, authentication is accepted if any
944 matching line has the proper key.
945 It is thus permissible (but not
946 recommended) to have several lines or different host keys for the same
948 This will inevitably happen when short forms of host names
949 from different domains are put in the file.
951 that the files contain conflicting information; authentication is
952 accepted if valid information can be found from either file.
954 Note that the lines in these files are typically hundreds of characters
955 long, and you definitely don't want to type in the host keys by hand.
956 Rather, generate them by a script
958 .Pa /etc/ssh_host_key.pub
959 and adding the host names at the front.
961 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
964 .It Pa /etc/sshd_config
965 Contains configuration data for
967 This file should be writable by root only, but it is recommended
968 (though not necessary) that it be world-readable.
969 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
970 These three files contain the private parts of the
971 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
972 These files should only be owned by root, readable only by root, and not
973 accessible to others.
976 does not start if this file is group/world-accessible.
977 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
978 These three files contain the public parts of the
979 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
980 These files should be world-readable but writable only by
982 Their contents should match the respective private parts.
984 really used for anything; they are provided for the convenience of
985 the user so their contents can be copied to known hosts files.
986 These files are created using
989 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
990 .It Pa /var/run/sshd.pid
991 Contains the process ID of the
993 listening for connections (if there are several daemons running
994 concurrently for different ports, this contains the pid of the one
996 The content of this file is not sensitive; it can be world-readable.
997 .It Pa $HOME/.ssh/authorized_keys
998 Lists the RSA keys that can be used to log into the user's account.
999 This file must be readable by root (which may on some machines imply
1000 it being world-readable if the user's home directory resides on an NFS
1002 It is recommended that it not be accessible by others.
1003 The format of this file is described above.
1004 Users will place the contents of their
1006 files into this file, as described in
1008 .It Pa $HOME/.ssh/authorized_keys2
1009 Lists the DSA keys that can be used to log into the user's account.
1010 This file must be readable by root (which may on some machines imply
1011 it being world-readable if the user's home directory resides on an NFS
1013 It is recommended that it not be accessible by others.
1014 The format of this file is described above.
1015 Users will place the contents of their
1017 files into this file, as described in
1019 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1020 These files are consulted when using rhosts with RSA host
1021 authentication to check the public key of the host.
1022 The key must be listed in one of these files to be accepted.
1023 The client uses the same files
1024 to verify that it is connecting to the correct remote host.
1025 These files should be writable only by root/the owner.
1026 .Pa /etc/ssh_known_hosts
1027 should be world-readable, and
1028 .Pa $HOME/.ssh/known_hosts
1029 can but need not be world-readable.
1031 If this file exists,
1033 refuses to let anyone except root log in.
1034 The contents of the file
1035 are displayed to anyone trying to log in, and non-root connections are
1037 The file should be world-readable.
1038 .It Pa /etc/hosts.allow, /etc/hosts.deny
1041 support, tcp-wrappers access controls may be defined here as described in
1042 .Xr hosts_access 5 .
1043 .It Pa $HOME/.rhosts
1044 This file contains host-username pairs, separated by a space, one per
1046 The given user on the corresponding host is permitted to log in
1048 The same file is used by rlogind and rshd.
1050 be writable only by the user; it is recommended that it not be
1051 accessible by others.
1053 If is also possible to use netgroups in the file.
1055 name may be of the form +@groupname to specify all hosts or all users
1057 .It Pa $HOME/.shosts
1059 this file is exactly the same as for
1061 However, this file is
1062 not used by rlogin and rshd, so using this permits access using SSH only.
1063 .It Pa /etc/hosts.equiv
1064 This file is used during
1067 In the simplest form, this file contains host names, one per line.
1069 those hosts are permitted to log in without a password, provided they
1070 have the same user name on both machines.
1071 The host name may also be
1072 followed by a user name; such users are permitted to log in as
1074 user on this machine (except root).
1075 Additionally, the syntax
1077 can be used to specify netgroups.
1078 Negated entries start with
1081 If the client host/user is successfully matched in this file, login is
1082 automatically permitted provided the client and server user names are the
1084 Additionally, successful RSA host authentication is normally required.
1085 This file must be writable only by root; it is recommended
1086 that it be world-readable.
1088 .Sy "Warning: It is almost never a good idea to use user names in"
1090 Beware that it really means that the named user(s) can log in as
1092 which includes bin, daemon, adm, and other accounts that own critical
1093 binaries and directories.
1094 Using a user name practically grants the user root access.
1095 The only valid use for user names that I can think
1096 of is in negative entries.
1098 Note that this warning also applies to rsh/rlogin.
1099 .It Pa /etc/shosts.equiv
1100 This is processed exactly as
1101 .Pa /etc/hosts.equiv .
1102 However, this file may be useful in environments that want to run both
1104 .It Pa $HOME/.ssh/environment
1105 This file is read into the environment at login (if it exists).
1106 It can only contain empty lines, comment lines (that start with
1108 and assignment lines of the form name=value.
1109 The file should be writable
1110 only by the user; it need not be readable by anyone else.
1111 .It Pa $HOME/.ssh/rc
1112 If this file exists, it is run with /bin/sh after reading the
1113 environment files but before starting the user's shell or command.
1114 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1122 The primary purpose of this file is to run any initialization routines
1123 which may be needed before the user's home directory becomes
1124 accessible; AFS is a particular example of such an environment.
1126 This file will probably contain some initialization code followed by
1127 something similar to: "if read proto cookie; then echo add $DISPLAY
1128 $proto $cookie | xauth -q -; fi".
1130 If this file does not exist,
1133 does not exist either, xauth is used to store the cookie.
1135 This file should be writable only by the user, and need not be
1136 readable by anyone else.
1140 This can be used to specify
1141 machine-specific login-time initializations globally.
1142 This file should be writable only by root, and should be world-readable.
1145 OpenSSH is a derivative of the original and free
1146 ssh 1.2.12 release by Tatu Ylonen.
1147 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1148 Theo de Raadt and Dug Song
1149 removed many bugs, re-added newer features and
1151 Markus Friedl contributed the support for SSH
1152 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob