5 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
7 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8 .\" All rights reserved
10 .\" Created: Sat Apr 22 21:55:14 1995 ylo
14 .Dd September 25, 1999
19 .Nd secure shell daemon
24 .Op Fl f Ar config_file
25 .Op Fl g Ar login_grace_time
26 .Op Fl h Ar host_key_file
27 .Op Fl k Ar key_gen_time
29 .Op Fl V Ar client_protocol_id
32 (Secure Shell Daemon) is the daemon program for
34 Together these programs replace rlogin and rsh programs, and
35 provide secure encrypted communications between two untrusted hosts
36 over an insecure network.
37 The programs are intended to be as easy to
38 install and use as possible.
41 is the daemon that listens for connections from clients.
42 It is normally started at boot from
45 daemon for each incoming connection.
46 The forked daemons handle
47 key exchange, encryption, authentication, command execution,
52 Each host has a host-specific RSA key
53 (normally 1024 bits) used to identify the host.
55 the daemon starts, it generates a server RSA key (normally 768 bits).
56 This key is normally regenerated every hour if it has been used, and
57 is never stored on disk.
59 Whenever a client connects the daemon, the daemon sends its host
60 and server public keys to the client.
61 The client compares the
62 host key against its own database to verify that it has not changed.
63 The client then generates a 256 bit random number.
65 random number using both the host key and the server key, and sends
66 the encrypted number to the server.
67 Both sides then start to use this
68 random number as a session key which is used to encrypt all further
69 communications in the session.
70 The rest of the session is encrypted
71 using a conventional cipher, currently Blowfish and 3DES, with 3DES
72 being is used by default.
73 The client selects the encryption algorithm
74 to use from those offered by the server.
76 Next, the server and the client enter an authentication dialog.
77 The client tries to authenticate itself using
81 authentication combined with RSA host
82 authentication, RSA challenge-response authentication, or password
85 Rhosts authentication is normally disabled
86 because it is fundamentally insecure, but can be enabled in the server
87 configuration file if desired.
88 System security is not improved unless
94 are disabled (thus completely disabling
100 If the client successfully authenticates itself, a dialog for
101 preparing the session is entered.
102 At this time the client may request
103 things like allocating a pseudo-tty, forwarding X11 connections,
104 forwarding TCP/IP connections, or forwarding the authentication agent
105 connection over the secure channel.
107 Finally, the client either requests a shell or execution of a command.
108 The sides then enter session mode.
109 In this mode, either side may send
110 data at any time, and such data is forwarded to/from the shell or
111 command on the server side, and the user terminal in the client side.
113 When the user program terminates and all forwarded X11 and other
114 connections have been closed, the server sends command exit status to
115 the client, and both sides exit.
118 can be configured using command-line options or a configuration
120 Command-line options override values specified in the
124 rereads its configuration file when it receives a hangup signal,
127 The options are as follows:
130 Specifies the number of bits in the server key (default 768).
134 The server sends verbose debug output to the system
135 log, and does not put itself in the background.
136 The server also will not fork and will only process one connection.
137 This option is only intended for debugging for the server.
138 .It Fl f Ar configuration_file
139 Specifies the name of the configuration file.
141 .Pa /etc/sshd_config .
143 refuses to start if there is no configuration file.
144 .It Fl g Ar login_grace_time
145 Gives the grace time for clients to authenticate themselves (default
147 If the client fails to authenticate the user within
148 this many seconds, the server disconnects and exits.
149 A value of zero indicates no limit.
150 .It Fl h Ar host_key_file
151 Specifies the file from which the host key is read (default
152 .Pa /etc/ssh_host_key ) .
153 This option must be given if
155 is not run as root (as the normal
156 host file is normally not readable by anyone but root).
160 is being run from inetd.
163 from inetd because it needs to generate the server key before it can
164 respond to the client, and this may take tens of seconds.
165 Clients would have to wait too long if the key was regenerated every time.
166 However, with small key sizes (e.g., 512) using
170 .It Fl k Ar key_gen_time
171 Specifies how often the server key is regenerated (default 3600
172 seconds, or one hour).
173 The motivation for regenerating the key fairly
174 often is that the key is not stored anywhere, and after about an hour,
175 it becomes impossible to recover the key for decrypting intercepted
176 communications even if the machine is cracked into or physically
178 A value of zero indicates that the key will never be regenerated.
180 Specifies the port on which the server listens for connections
184 Nothing is sent to the system log.
185 Normally the beginning,
186 authentication, and termination of each connection is logged.
188 Do not print an error message if RSA support is missing.
189 .It Fl V Ar client_protocol_id
190 SSH2 compatibility mode.
191 When this options is specified
193 assumes the client has sent the given version string
195 Protocol Version Identification Exchange.
199 to use IPv4 addresses only.
203 to use IPv6 addresses only.
205 .Sh CONFIGURATION FILE
207 reads configuration data from
209 (or the file specified with
211 on the command line).
212 The file contains keyword-value pairs, one per line.
215 and empty lines are interpreted as comments.
217 The following keywords are possible.
219 .It Cm AFSTokenPassing
220 Specifies whether an AFS token may be forwarded to the server.
224 This keyword can be followed by a number of group names, separated
226 If specified, login is allowed only for users whose primary
227 group matches one of the patterns.
232 wildcards in the patterns.
233 Only group names are valid, a numerical group ID isn't recognized.
234 By default login is allowed regardless of the primary group.
237 This keyword can be followed by a number of user names, separated
239 If specified, login is allowed only for users names that
240 match one of the patterns.
245 wildcards in the patterns.
246 Only user names are valid, a numerical user ID isn't recognized.
247 By default login is allowed regardless of the user name.
252 should check for new mail for interactive logins.
256 This keyword can be followed by a number of group names, separated
258 Users whose primary group matches one of the patterns
259 aren't allowed to log in.
264 wildcards in the patterns.
265 Only group names are valid, a numerical group ID isn't recognized.
266 By default login is allowed regardless of the primary group.
269 This keyword can be followed by a number of user names, separated
271 Login is disallowed for user names that match one of the patterns.
275 can be used as wildcards in the patterns.
276 Only user names are valid, a numerical user ID isn't recognized.
277 By default login is allowed regardless of the user name.
279 Specifies the file containing the private host key (default
280 .Pa /etc/ssh_host_key ) .
283 does not start if this file is group/world-accessible.
289 files will not be used in authentication.
292 .Pa /etc/shosts.equiv
296 .It Cm IgnoreUserKnownHosts
299 should ignore the user's
300 .Pa $HOME/.ssh/known_hosts
302 .Cm RhostsRSAAuthentication .
306 Specifies whether the system should send keepalive messages to the
308 If they are sent, death of the connection or crash of one
309 of the machines will be properly noticed.
310 However, this means that
311 connections will die if the route is down temporarily, and some people
313 On the other hand, if keepalives are not send,
314 sessions may hang indefinitely on the server, leaving
316 users and consuming server resources.
320 (to send keepalives), and the server will notice
321 if the network goes down or the client host reboots.
322 This avoids infinitely hanging sessions.
324 To disable keepalives, the value should be set to
326 in both the server and the client configuration files.
327 .It Cm KerberosAuthentication
328 Specifies whether Kerberos authentication is allowed.
329 This can be in the form of a Kerberos ticket, or if
330 .Cm PasswordAuthentication
331 is yes, the password provided by the user will be validated through
335 .It Cm KerberosOrLocalPasswd
336 If set then if password authentication through Kerberos fails then
337 the password will be validated via any additional local mechanism
343 .It Cm KerberosTgtPassing
344 Specifies whether a Kerberos TGT may be forwarded to the server.
347 as this only works when the Kerberos KDC is actually an AFS kaserver.
348 .It Cm KerberosTicketCleanup
349 Specifies whether to automatically destroy the user's ticket cache
353 .It Cm KeyRegenerationInterval
354 The server key is automatically regenerated after this many seconds
355 (if it has been used).
356 The purpose of regeneration is to prevent
357 decrypting captured sessions by later breaking into the machine and
359 The key is never stored anywhere.
360 If the value is 0, the key is never regenerated.
361 The default is 3600 (seconds).
363 Specifies what local address
366 The default is to listen to all local addresses.
367 Multiple options of this type are permitted.
370 options must precede this option.
371 .It Cm LoginGraceTime
372 The server disconnects after this time if the user has not
373 successfully logged in.
374 If the value is 0, there is no time limit.
375 The default is 600 (seconds).
377 Gives the verbosity level that is used when logging messages from
379 The possible values are:
380 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
382 Logging with level DEBUG violates the privacy of users
383 and is not recommended.
384 .It Cm PasswordAuthentication
385 Specifies whether password authentication is allowed.
388 .It Cm PermitEmptyPasswords
389 When password authentication is allowed, it specifies whether the
390 server allows login to accounts with empty password strings.
393 .It Cm PermitRootLogin
394 Specifies whether the root can log in using
403 If this options is set to
405 only password authentication is disabled for root.
407 Root login with RSA authentication when the
410 specified will be allowed regardless of the value of this setting
411 (which may be useful for taking remote backups even if root login is
412 normally not allowed).
414 Specifies the port number that
418 Multiple options of this type are permitted.
424 when a user logs in interactively.
425 (On some systems it is also printed by the shell,
432 Random number generation uses other techniques.
433 .It Cm RhostsAuthentication
434 Specifies whether authentication using rhosts or /etc/hosts.equiv
436 Normally, this method should not be permitted because it is insecure.
437 .Cm RhostsRSAAuthentication
439 instead, because it performs RSA-based host authentication in addition
440 to normal rhosts or /etc/hosts.equiv authentication.
443 .It Cm RhostsRSAAuthentication
444 Specifies whether rhosts or /etc/hosts.equiv authentication together
445 with successful RSA host authentication is allowed.
448 .It Cm RSAAuthentication
449 Specifies whether pure RSA authentication is allowed.
453 Defines the number of bits in the server key.
454 The minimum value is 512, and the default is 768.
455 .It Cm SkeyAuthentication
458 authentication is allowed.
461 Note that s/key authentication is enabled only if
462 .Cm PasswordAuthentication
467 should check file modes and ownership of the
468 user's files and home directory before accepting login.
469 This is normally desirable because novices sometimes accidentally leave their
470 directory or files world-writable.
473 .It Cm SyslogFacility
474 Gives the facility code that is used when logging messages from
476 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
477 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
485 .It Cm X11DisplayOffset
486 Specifies the first display number available for
491 from interfering with real X11 servers.
494 Specifies whether X11 forwarding is permitted.
497 Note that disabling X11 forwarding does not improve security in any
498 way, as users can always install their own forwarders.
501 When a user successfully logs in,
504 .Bl -enum -offset indent
506 If the login is on a tty, and no command has been specified,
507 prints last login time and
509 (unless prevented in the configuration file or by
510 .Pa $HOME/.hushlogin ;
515 If the login is on a tty, records login time.
519 if it exists, prints contents and quits
522 Changes to run with normal user privileges.
524 Sets up basic environment.
527 .Pa $HOME/.ssh/environment
530 Changes to user's home directory.
534 exists, runs it; else if
537 it; otherwise runs xauth.
540 files are given the X11
541 authentication protocol and cookie in standard input.
543 Runs user's shell or command.
545 .Sh AUTHORIZED_KEYS FILE FORMAT
547 .Pa $HOME/.ssh/authorized_keys
548 file lists the RSA keys that are
549 permitted for RSA authentication.
550 Each line of the file contains one
551 key (empty lines and lines starting with a
555 Each line consists of the following fields, separated by
556 spaces: options, bits, exponent, modulus, comment.
558 is optional; its presence is determined by whether the line starts
559 with a number or not (the option field never starts with a number).
560 The bits, exponent, modulus and comment fields give the RSA key; the
561 comment field is not used for anything (but may be convenient for the
562 user to identify the key).
564 Note that lines in this file are usually several hundred bytes long
565 (because of the size of the RSA key modulus).
566 You don't want to type them in; instead, copy the
570 The options (if present) consists of comma-separated option
572 No spaces are permitted, except within double quotes.
573 The following option specifications are supported:
575 .It Cm from="pattern-list"
576 Specifies that in addition to RSA authentication, the canonical name
577 of the remote host must be present in the comma-separated list of
583 The list may also contain
584 patterns negated by prefixing them with
586 if the canonical host name matches a negated pattern, the key is not accepted.
588 of this option is to optionally increase security: RSA authentication
589 by itself does not trust the network or name servers or anything (but
590 the key); however, if somebody somehow steals the key, the key
591 permits an intruder to log in from anywhere in the world.
592 This additional option makes using a stolen key more difficult (name
593 servers and/or routers would have to be compromised in addition to
595 .It Cm command="command"
596 Specifies that the command is executed whenever this key is used for
598 The command supplied by the user (if any) is ignored.
599 The command is run on a pty if the connection requests a pty;
600 otherwise it is run without a tty.
601 A quote may be included in the command by quoting it with a backslash.
602 This option might be useful
603 to restrict certain RSA keys to perform just a specific operation.
604 An example might be a key that permits remote backups but nothing else.
605 Notice that the client may specify TCP/IP and/or X11
606 forwardings unless they are explicitly prohibited.
607 .It Cm environment="NAME=value"
608 Specifies that the string is to be added to the environment when
609 logging in using this key.
610 Environment variables set this way
611 override other default environment values.
612 Multiple options of this type are permitted.
613 .It Cm no-port-forwarding
614 Forbids TCP/IP forwarding when this key is used for authentication.
615 Any port forward requests by the client will return an error.
616 This might be used, e.g., in connection with the
619 .It Cm no-X11-forwarding
620 Forbids X11 forwarding when this key is used for authentication.
621 Any X11 forward requests by the client will return an error.
622 .It Cm no-agent-forwarding
623 Forbids authentication agent forwarding when this key is used for
626 Prevents tty allocation (a request to allocate a pty will fail).
629 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
631 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
633 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
634 .Sh SSH_KNOWN_HOSTS FILE FORMAT
636 .Pa /etc/ssh_known_hosts
638 .Pa $HOME/.ssh/known_hosts
639 files contain host public keys for all known hosts.
640 The global file should
641 be prepared by the administrator (optional), and the per-user file is
642 maintained automatically: whenever the user connects an unknown host
643 its key is added to the per-user file.
645 Each line in these files contains the following fields: hostnames,
646 bits, exponent, modulus, comment.
647 The fields are separated by spaces.
649 Hostnames is a comma-separated list of patterns ('*' and '?' act as
650 wildcards); each pattern in turn is matched against the canonical host
651 name (when authenticating a client) or against the user-supplied
652 name (when authenticating a server).
653 A pattern may also be preceded by
655 to indicate negation: if the host name matches a negated
656 pattern, it is not accepted (by that line) even if it matched another
659 Bits, exponent, and modulus are taken directly from the host key; they
660 can be obtained, e.g., from
661 .Pa /etc/ssh_host_key.pub .
662 The optional comment field continues to the end of the line, and is not used.
666 and empty lines are ignored as comments.
668 When performing host authentication, authentication is accepted if any
669 matching line has the proper key.
670 It is thus permissible (but not
671 recommended) to have several lines or different host keys for the same
673 This will inevitably happen when short forms of host names
674 from different domains are put in the file.
676 that the files contain conflicting information; authentication is
677 accepted if valid information can be found from either file.
679 Note that the lines in these files are typically hundreds of characters
680 long, and you definitely don't want to type in the host keys by hand.
681 Rather, generate them by a script
683 .Pa /etc/ssh_host_key.pub
684 and adding the host names at the front.
686 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
689 .It Pa /etc/sshd_config
690 Contains configuration data for
692 This file should be writable by root only, but it is recommended
693 (though not necessary) that it be world-readable.
694 .It Pa /etc/ssh_host_key
695 Contains the private part of the host key.
696 This file should only be owned by root, readable only by root, and not
697 accessible to others.
700 does not start if this file is group/world-accessible.
701 .It Pa /etc/ssh_host_key.pub
702 Contains the public part of the host key.
703 This file should be world-readable but writable only by
705 Its contents should match the private part.
707 really used for anything; it is only provided for the convenience of
708 the user so its contents can be copied to known hosts files.
709 These two files are created using
711 .It Pa /var/run/sshd.pid
712 Contains the process ID of the
714 listening for connections (if there are several daemons running
715 concurrently for different ports, this contains the pid of the one
717 The contents of this file are not sensitive; it can be world-readable.
718 .It Pa $HOME/.ssh/authorized_keys
719 Lists the RSA keys that can be used to log into the user's account.
720 This file must be readable by root (which may on some machines imply
721 it being world-readable if the user's home directory resides on an NFS
723 It is recommended that it not be accessible by others.
724 The format of this file is described above.
725 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
726 These files are consulted when using rhosts with RSA host
727 authentication to check the public key of the host.
728 The key must be listed in one of these files to be accepted.
729 The client uses the same files
730 to verify that the remote host is the one we intended to connect.
731 These files should be writable only by root/the owner.
732 .Pa /etc/ssh_known_hosts
733 should be world-readable, and
734 .Pa $HOME/.ssh/known_hosts
735 can but need not be world-readable.
739 refuses to let anyone except root log in.
740 The contents of the file
741 are displayed to anyone trying to log in, and non-root connections are
743 The file should be world-readable.
744 .It Pa /etc/hosts.allow, /etc/hosts.deny
747 support, tcp-wrappers access controls may be defined here as described in
750 This file contains host-username pairs, separated by a space, one per
752 The given user on the corresponding host is permitted to log in
754 The same file is used by rlogind and rshd.
756 be writable only by the user; it is recommended that it not be
757 accessible by others.
759 If is also possible to use netgroups in the file.
761 name may be of the form +@groupname to specify all hosts or all users
765 this file is exactly the same as for
767 However, this file is
768 not used by rlogin and rshd, so using this permits access using SSH only.
770 This file is used during
773 In the simplest form, this file contains host names, one per line.
775 those hosts are permitted to log in without a password, provided they
776 have the same user name on both machines.
777 The host name may also be
778 followed by a user name; such users are permitted to log in as
780 user on this machine (except root).
781 Additionally, the syntax
783 can be used to specify netgroups.
784 Negated entries start with
787 If the client host/user is successfully matched in this file, login is
788 automatically permitted provided the client and server user names are the
790 Additionally, successful RSA host authentication is normally required.
791 This file must be writable only by root; it is recommended
792 that it be world-readable.
794 .Sy "Warning: It is almost never a good idea to use user names in"
796 Beware that it really means that the named user(s) can log in as
798 which includes bin, daemon, adm, and other accounts that own critical
799 binaries and directories.
800 Using a user name practically grants the user root access.
801 The only valid use for user names that I can think
802 of is in negative entries.
804 Note that this warning also applies to rsh/rlogin.
805 .It Pa /etc/shosts.equiv
806 This is processed exactly as
807 .Pa /etc/hosts.equiv .
808 However, this file may be useful in environments that want to run both
810 .It Pa $HOME/.ssh/environment
811 This file is read into the environment at login (if it exists).
812 It can only contain empty lines, comment lines (that start with
814 and assignment lines of the form name=value.
815 The file should be writable
816 only by the user; it need not be readable by anyone else.
818 If this file exists, it is run with /bin/sh after reading the
819 environment files but before starting the user's shell or command.
820 If X11 spoofing is in use, this will receive the "proto cookie" pair in
828 The primary purpose of this file is to run any initialization routines
829 which may be needed before the user's home directory becomes
830 accessible; AFS is a particular example of such an environment.
832 This file will probably contain some initialization code followed by
833 something similar to: "if read proto cookie; then echo add $DISPLAY
834 $proto $cookie | xauth -q -; fi".
836 If this file does not exist,
839 does not exist either, xauth is used to store the cookie.
841 This file should be writable only by the user, and need not be
842 readable by anyone else.
846 This can be used to specify
847 machine-specific login-time initializations globally.
848 This file should be writable only by root, and should be world-readable.
851 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
852 but with bugs removed and newer features re-added.
854 1.2.12 release, newer versions of the original ssh bore successively
855 more restrictive licenses, and thus demand for a free version was born.
856 This version of OpenSSH
859 has all components of a restrictive nature (i.e., patents, see
861 directly removed from the source code; any licensed or patented components
865 has been updated to support ssh protocol 1.5, making it compatible with
866 all other ssh protocol 1 clients and servers.
868 contains added support for
870 authentication and ticket passing.
872 supports one-time password authentication with
876 The libraries described in
878 are required for proper operation.