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 1.110 2001/04/08 16:01:36 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
496 host|IPv4_addr|IPv6_addr
502 [host|IPv6_addr]:port
509 will listen on the address and all prior
511 options specified. The default is to listen on all local
514 options are permitted. Additionally, any
516 options must precede this option for non port qualified addresses.
517 .It Cm LoginGraceTime
518 The server disconnects after this time if the user has not
519 successfully logged in.
520 If the value is 0, there is no time limit.
521 The default is 600 (seconds).
523 Gives the verbosity level that is used when logging messages from
525 The possible values are:
526 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
528 Logging with level DEBUG violates the privacy of users
529 and is not recommended.
531 Specifies the available MAC (message authentication code) algorithms.
532 The MAC algorithm is used in protocol version 2
533 for data integrity protection.
534 Multiple algorithms must be comma-separated.
538 ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
539 hmac-sha1-96,hmac-md5-96''
542 Specifies the maximum number of concurrent unauthenticated connections to the
545 Additional connections will be dropped until authentication succeeds or the
547 expires for a connection.
550 Alternatively, random early drop can be enabled by specifying
551 the three colon separated values
555 will refuse connection attempts with a probability of
558 if there are currently
561 unauthenticated connections.
562 The probability increases linearly and all connection attempts
563 are refused if the number of unauthenticated connections reaches
566 .It Cm PasswordAuthentication
567 Specifies whether password authentication is allowed.
570 Note that this option applies to both protocol versions 1 and 2.
571 .It Cm PermitEmptyPasswords
572 When password authentication is allowed, it specifies whether the
573 server allows login to accounts with empty password strings.
576 .It Cm PermitRootLogin
577 Specifies whether root can login using
581 .Dq without-password ,
582 .Dq forced-commands-only
588 If this option is set to
590 password authentication is disabled for root.
592 If this option is set to
593 .Dq forced-commands-only
594 root login with public key authentication will be allowed,
597 option has been specified
598 (which may be useful for taking remote backups even if root login is
599 normally not allowed). All other authentication methods are disabled
602 If this option is set to
604 root is not allowed to login.
606 Specifies the file that contains the process identifier of the
610 .Pa /var/run/sshd.pid .
612 Specifies the port number that
616 Multiple options of this type are permitted.
620 should print the date and time when the user last logged in.
628 when a user logs in interactively.
629 (On some systems it is also printed by the shell,
635 Specifies the protocol versions
638 The possible values are
642 Multiple versions must be comma-separated.
645 .It Cm PubkeyAuthentication
646 Specifies whether public key authentication is allowed.
649 Note that this option applies to protocol version 2 only.
650 .It Cm ReverseMappingCheck
653 should try to verify the remote host name and check that
654 the resolved host name for the remote IP address maps back to the
655 very same IP address.
658 .It Cm RhostsAuthentication
659 Specifies whether authentication using rhosts or /etc/hosts.equiv
661 Normally, this method should not be permitted because it is insecure.
662 .Cm RhostsRSAAuthentication
664 instead, because it performs RSA-based host authentication in addition
665 to normal rhosts or /etc/hosts.equiv authentication.
668 .It Cm RhostsRSAAuthentication
669 Specifies whether rhosts or /etc/hosts.equiv authentication together
670 with successful RSA host authentication is allowed.
673 .It Cm RSAAuthentication
674 Specifies whether pure RSA authentication is allowed.
677 Note that this option applies to protocol version 1 only.
679 Defines the number of bits in the server key.
680 The minimum value is 512, and the default is 768.
684 should check file modes and ownership of the
685 user's files and home directory before accepting login.
686 This is normally desirable because novices sometimes accidentally leave their
687 directory or files world-writable.
691 Configures an external subsystem (e.g., file transfer daemon).
692 Arguments should be a subsystem name and a command to execute upon subsystem
698 file transfer subsystem.
699 By default no subsystems are defined.
700 Note that this option applies to protocol version 2 only.
701 .It Cm SyslogFacility
702 Gives the facility code that is used when logging messages from
704 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
705 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
710 is used for interactive login sessions.
713 is never used for remote command execution.
716 .It Cm X11DisplayOffset
717 Specifies the first display number available for
722 from interfering with real X11 servers.
725 Specifies whether X11 forwarding is permitted.
728 Note that disabling X11 forwarding does not improve security in any
729 way, as users can always install their own forwarders.
731 Specifies the location of the
735 .Pa /usr/X11R6/bin/xauth .
738 When a user successfully logs in,
741 .Bl -enum -offset indent
743 If the login is on a tty, and no command has been specified,
744 prints last login time and
746 (unless prevented in the configuration file or by
747 .Pa $HOME/.hushlogin ;
752 If the login is on a tty, records login time.
756 if it exists, prints contents and quits
759 Changes to run with normal user privileges.
761 Sets up basic environment.
764 .Pa $HOME/.ssh/environment
767 Changes to user's home directory.
771 exists, runs it; else if
774 it; otherwise runs xauth.
777 files are given the X11
778 authentication protocol and cookie in standard input.
780 Runs user's shell or command.
782 .Sh AUTHORIZED_KEYS FILE FORMAT
784 .Pa $HOME/.ssh/authorized_keys
785 file lists the RSA keys that are
786 permitted for RSA authentication in SSH protocols 1.3 and 1.5
788 .Pa $HOME/.ssh/authorized_keys2
789 file lists the DSA and RSA keys that are
790 permitted for public key authentication (PubkeyAuthentication)
793 Each line of the file contains one
794 key (empty lines and lines starting with a
798 Each RSA public key consists of the following fields, separated by
799 spaces: options, bits, exponent, modulus, comment.
800 Each protocol version 2 public key consists of:
801 options, keytype, base64 encoded key, comment.
803 are optional; its presence is determined by whether the line starts
804 with a number or not (the option field never starts with a number).
805 The bits, exponent, modulus and comment fields give the RSA key for
806 protocol version 1; the
807 comment field is not used for anything (but may be convenient for the
808 user to identify the key).
809 For protocol version 2 the keytype is
814 Note that lines in this file are usually several hundred bytes long
815 (because of the size of the RSA key modulus).
816 You don't want to type them in; instead, copy the
822 The options (if present) consist of comma-separated option
824 No spaces are permitted, except within double quotes.
825 The following option specifications are supported:
827 .It Cm from="pattern-list"
828 Specifies that in addition to RSA authentication, the canonical name
829 of the remote host must be present in the comma-separated list of
835 The list may also contain
836 patterns negated by prefixing them with
838 if the canonical host name matches a negated pattern, the key is not accepted.
840 of this option is to optionally increase security: RSA authentication
841 by itself does not trust the network or name servers or anything (but
842 the key); however, if somebody somehow steals the key, the key
843 permits an intruder to log in from anywhere in the world.
844 This additional option makes using a stolen key more difficult (name
845 servers and/or routers would have to be compromised in addition to
847 .It Cm command="command"
848 Specifies that the command is executed whenever this key is used for
850 The command supplied by the user (if any) is ignored.
851 The command is run on a pty if the connection requests a pty;
852 otherwise it is run without a tty.
853 Note that if you want a 8-bit clean channel,
854 you must not request a pty or should specify
856 A quote may be included in the command by quoting it with a backslash.
857 This option might be useful
858 to restrict certain RSA keys to perform just a specific operation.
859 An example might be a key that permits remote backups but nothing else.
860 Note that the client may specify TCP/IP and/or X11
861 forwarding unless they are explicitly prohibited.
862 .It Cm environment="NAME=value"
863 Specifies that the string is to be added to the environment when
864 logging in using this key.
865 Environment variables set this way
866 override other default environment values.
867 Multiple options of this type are permitted.
868 .It Cm no-port-forwarding
869 Forbids TCP/IP forwarding when this key is used for authentication.
870 Any port forward requests by the client will return an error.
871 This might be used, e.g., in connection with the
874 .It Cm no-X11-forwarding
875 Forbids X11 forwarding when this key is used for authentication.
876 Any X11 forward requests by the client will return an error.
877 .It Cm no-agent-forwarding
878 Forbids authentication agent forwarding when this key is used for
881 Prevents tty allocation (a request to allocate a pty will fail).
882 .It Cm permitopen="host:port"
885 port-forwading such that it may only connect to the specified host and
888 options may be applied seperated by commas. No pattern matching is
889 performed on the specified hostnames, they must be literal domains or
893 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
895 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
897 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
899 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
900 .Sh SSH_KNOWN_HOSTS FILE FORMAT
902 .Pa /etc/ssh_known_hosts ,
903 .Pa /etc/ssh_known_hosts2 ,
904 .Pa $HOME/.ssh/known_hosts ,
906 .Pa $HOME/.ssh/known_hosts2
907 files contain host public keys for all known hosts.
908 The global file should
909 be prepared by the administrator (optional), and the per-user file is
910 maintained automatically: whenever the user connects from an unknown host
911 its key is added to the per-user file.
913 Each line in these files contains the following fields: hostnames,
914 bits, exponent, modulus, comment.
915 The fields are separated by spaces.
917 Hostnames is a comma-separated list of patterns ('*' and '?' act as
918 wildcards); each pattern in turn is matched against the canonical host
919 name (when authenticating a client) or against the user-supplied
920 name (when authenticating a server).
921 A pattern may also be preceded by
923 to indicate negation: if the host name matches a negated
924 pattern, it is not accepted (by that line) even if it matched another
927 Bits, exponent, and modulus are taken directly from the RSA host key; they
928 can be obtained, e.g., from
929 .Pa /etc/ssh_host_key.pub .
930 The optional comment field continues to the end of the line, and is not used.
934 and empty lines are ignored as comments.
936 When performing host authentication, authentication is accepted if any
937 matching line has the proper key.
938 It is thus permissible (but not
939 recommended) to have several lines or different host keys for the same
941 This will inevitably happen when short forms of host names
942 from different domains are put in the file.
944 that the files contain conflicting information; authentication is
945 accepted if valid information can be found from either file.
947 Note that the lines in these files are typically hundreds of characters
948 long, and you definitely don't want to type in the host keys by hand.
949 Rather, generate them by a script
951 .Pa /etc/ssh_host_key.pub
952 and adding the host names at the front.
954 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
957 .It Pa /etc/sshd_config
958 Contains configuration data for
960 This file should be writable by root only, but it is recommended
961 (though not necessary) that it be world-readable.
962 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
963 These three files contain the private parts of the
964 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
965 These files should only be owned by root, readable only by root, and not
966 accessible to others.
969 does not start if this file is group/world-accessible.
970 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
971 These three files contain the public parts of the
972 (SSH1, SSH2 DSA, and SSH2 RSA) host keys.
973 These files should be world-readable but writable only by
975 Their contents should match the respective private parts.
977 really used for anything; they are provided for the convenience of
978 the user so their contents can be copied to known hosts files.
979 These files are created using
982 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
983 .It Pa /var/run/sshd.pid
984 Contains the process ID of the
986 listening for connections (if there are several daemons running
987 concurrently for different ports, this contains the pid of the one
989 The content of this file is not sensitive; it can be world-readable.
990 .It Pa $HOME/.ssh/authorized_keys
991 Lists the RSA keys that can be used to log into the user's account.
992 This file must be readable by root (which may on some machines imply
993 it being world-readable if the user's home directory resides on an NFS
995 It is recommended that it not be accessible by others.
996 The format of this file is described above.
997 Users will place the contents of their
999 files into this file, as described in
1001 .It Pa $HOME/.ssh/authorized_keys2
1002 Lists the DSA keys that can be used to log into the user's account.
1003 This file must be readable by root (which may on some machines imply
1004 it being world-readable if the user's home directory resides on an NFS
1006 It is recommended that it not be accessible by others.
1007 The format of this file is described above.
1008 Users will place the contents of their
1010 files into this file, as described in
1012 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1013 These files are consulted when using rhosts with RSA host
1014 authentication to check the public key of the host.
1015 The key must be listed in one of these files to be accepted.
1016 The client uses the same files
1017 to verify that it is connecting to the correct remote host.
1018 These files should be writable only by root/the owner.
1019 .Pa /etc/ssh_known_hosts
1020 should be world-readable, and
1021 .Pa $HOME/.ssh/known_hosts
1022 can but need not be world-readable.
1024 If this file exists,
1026 refuses to let anyone except root log in.
1027 The contents of the file
1028 are displayed to anyone trying to log in, and non-root connections are
1030 The file should be world-readable.
1031 .It Pa /etc/hosts.allow, /etc/hosts.deny
1034 support, tcp-wrappers access controls may be defined here as described in
1035 .Xr hosts_access 5 .
1036 .It Pa $HOME/.rhosts
1037 This file contains host-username pairs, separated by a space, one per
1039 The given user on the corresponding host is permitted to log in
1041 The same file is used by rlogind and rshd.
1043 be writable only by the user; it is recommended that it not be
1044 accessible by others.
1046 If is also possible to use netgroups in the file.
1048 name may be of the form +@groupname to specify all hosts or all users
1050 .It Pa $HOME/.shosts
1052 this file is exactly the same as for
1054 However, this file is
1055 not used by rlogin and rshd, so using this permits access using SSH only.
1056 .It Pa /etc/hosts.equiv
1057 This file is used during
1060 In the simplest form, this file contains host names, one per line.
1062 those hosts are permitted to log in without a password, provided they
1063 have the same user name on both machines.
1064 The host name may also be
1065 followed by a user name; such users are permitted to log in as
1067 user on this machine (except root).
1068 Additionally, the syntax
1070 can be used to specify netgroups.
1071 Negated entries start with
1074 If the client host/user is successfully matched in this file, login is
1075 automatically permitted provided the client and server user names are the
1077 Additionally, successful RSA host authentication is normally required.
1078 This file must be writable only by root; it is recommended
1079 that it be world-readable.
1081 .Sy "Warning: It is almost never a good idea to use user names in"
1083 Beware that it really means that the named user(s) can log in as
1085 which includes bin, daemon, adm, and other accounts that own critical
1086 binaries and directories.
1087 Using a user name practically grants the user root access.
1088 The only valid use for user names that I can think
1089 of is in negative entries.
1091 Note that this warning also applies to rsh/rlogin.
1092 .It Pa /etc/shosts.equiv
1093 This is processed exactly as
1094 .Pa /etc/hosts.equiv .
1095 However, this file may be useful in environments that want to run both
1097 .It Pa $HOME/.ssh/environment
1098 This file is read into the environment at login (if it exists).
1099 It can only contain empty lines, comment lines (that start with
1101 and assignment lines of the form name=value.
1102 The file should be writable
1103 only by the user; it need not be readable by anyone else.
1104 .It Pa $HOME/.ssh/rc
1105 If this file exists, it is run with /bin/sh after reading the
1106 environment files but before starting the user's shell or command.
1107 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1115 The primary purpose of this file is to run any initialization routines
1116 which may be needed before the user's home directory becomes
1117 accessible; AFS is a particular example of such an environment.
1119 This file will probably contain some initialization code followed by
1120 something similar to: "if read proto cookie; then echo add $DISPLAY
1121 $proto $cookie | xauth -q -; fi".
1123 If this file does not exist,
1126 does not exist either, xauth is used to store the cookie.
1128 This file should be writable only by the user, and need not be
1129 readable by anyone else.
1133 This can be used to specify
1134 machine-specific login-time initializations globally.
1135 This file should be writable only by root, and should be world-readable.
1138 OpenSSH is a derivative of the original and free
1139 ssh 1.2.12 release by Tatu Ylonen.
1140 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1141 Theo de Raadt and Dug Song
1142 removed many bugs, re-added newer features and
1144 Markus Friedl contributed the support for SSH
1145 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob