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, 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 responds with its public
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 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 or 3DES, with 3DES
72 being 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 option is specified
193 assumes the client has sent the supplied 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.
250 Specifies the ciphers allowed for protocol version 2.
251 Multiple ciphers must be comma-separated.
253 .Dq blowfish-cbc,3des-cbc,arcfour,cast128-cbc .
257 should check for new mail for interactive logins.
261 This keyword can be followed by a number of group names, separated
263 Users whose primary group matches one of the patterns
264 aren't allowed to log in.
269 wildcards in the patterns.
270 Only group names are valid, a numerical group ID isn't recognized.
271 By default login is allowed regardless of the primary group.
274 This keyword can be followed by a number of user names, separated
276 Login is disallowed for user names that match one of the patterns.
280 can be used as wildcards in the patterns.
281 Only user names are valid, a numerical user ID isn't recognized.
282 By default login is allowed regardless of the user name.
284 Specifies the file containing the private host key (default
285 .Pa /etc/ssh_host_key ) .
288 does not start if this file is group/world-accessible.
294 files will not be used in authentication.
297 .Pa /etc/shosts.equiv
301 .It Cm IgnoreUserKnownHosts
304 should ignore the user's
305 .Pa $HOME/.ssh/known_hosts
307 .Cm RhostsRSAAuthentication .
311 Specifies whether the system should send keepalive messages to the
313 If they are sent, death of the connection or crash of one
314 of the machines will be properly noticed.
315 However, this means that
316 connections will die if the route is down temporarily, and some people
318 On the other hand, if keepalives are not send,
319 sessions may hang indefinitely on the server, leaving
321 users and consuming server resources.
325 (to send keepalives), and the server will notice
326 if the network goes down or the client host reboots.
327 This avoids infinitely hanging sessions.
329 To disable keepalives, the value should be set to
331 in both the server and the client configuration files.
332 .It Cm KerberosAuthentication
333 Specifies whether Kerberos authentication is allowed.
334 This can be in the form of a Kerberos ticket, or if
335 .Cm PasswordAuthentication
336 is yes, the password provided by the user will be validated through
340 .It Cm KerberosOrLocalPasswd
341 If set then if password authentication through Kerberos fails then
342 the password will be validated via any additional local mechanism
348 .It Cm KerberosTgtPassing
349 Specifies whether a Kerberos TGT may be forwarded to the server.
352 as this only works when the Kerberos KDC is actually an AFS kaserver.
353 .It Cm KerberosTicketCleanup
354 Specifies whether to automatically destroy the user's ticket cache
358 .It Cm KeyRegenerationInterval
359 The server key is automatically regenerated after this many seconds
360 (if it has been used).
361 The purpose of regeneration is to prevent
362 decrypting captured sessions by later breaking into the machine and
364 The key is never stored anywhere.
365 If the value is 0, the key is never regenerated.
366 The default is 3600 (seconds).
368 Specifies what local address
371 The default is to listen to all local addresses.
372 Multiple options of this type are permitted.
375 options must precede this option.
376 .It Cm LoginGraceTime
377 The server disconnects after this time if the user has not
378 successfully logged in.
379 If the value is 0, there is no time limit.
380 The default is 600 (seconds).
382 Gives the verbosity level that is used when logging messages from
384 The possible values are:
385 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
387 Logging with level DEBUG violates the privacy of users
388 and is not recommended.
389 .It Cm PasswordAuthentication
390 Specifies whether password authentication is allowed.
393 .It Cm PermitEmptyPasswords
394 When password authentication is allowed, it specifies whether the
395 server allows login to accounts with empty password strings.
398 .It Cm PermitRootLogin
399 Specifies whether the root can log in using
408 If this options is set to
410 only password authentication is disabled for root.
412 Root login with RSA authentication when the
415 specified will be allowed regardless of the value of this setting
416 (which may be useful for taking remote backups even if root login is
417 normally not allowed).
419 Specifies the file that contains the process identifier of the
423 .Pa /var/run/sshd.pid .
425 Specifies the port number that
429 Multiple options of this type are permitted.
435 when a user logs in interactively.
436 (On some systems it is also printed by the shell,
442 Specifies the protocol versions
445 The possible values are
449 Multiple versions must be comma-separated.
454 Random number generation uses other techniques.
455 .It Cm RhostsAuthentication
456 Specifies whether authentication using rhosts or /etc/hosts.equiv
458 Normally, this method should not be permitted because it is insecure.
459 .Cm RhostsRSAAuthentication
461 instead, because it performs RSA-based host authentication in addition
462 to normal rhosts or /etc/hosts.equiv authentication.
465 .It Cm RhostsRSAAuthentication
466 Specifies whether rhosts or /etc/hosts.equiv authentication together
467 with successful RSA host authentication is allowed.
470 .It Cm RSAAuthentication
471 Specifies whether pure RSA authentication is allowed.
475 Defines the number of bits in the server key.
476 The minimum value is 512, and the default is 768.
477 .It Cm SkeyAuthentication
480 authentication is allowed.
483 Note that s/key authentication is enabled only if
484 .Cm PasswordAuthentication
489 should check file modes and ownership of the
490 user's files and home directory before accepting login.
491 This is normally desirable because novices sometimes accidentally leave their
492 directory or files world-writable.
495 .It Cm SyslogFacility
496 Gives the facility code that is used when logging messages from
498 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
499 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
507 .It Cm X11DisplayOffset
508 Specifies the first display number available for
513 from interfering with real X11 servers.
516 Specifies whether X11 forwarding is permitted.
519 Note that disabling X11 forwarding does not improve security in any
520 way, as users can always install their own forwarders.
523 When a user successfully logs in,
526 .Bl -enum -offset indent
528 If the login is on a tty, and no command has been specified,
529 prints last login time and
531 (unless prevented in the configuration file or by
532 .Pa $HOME/.hushlogin ;
537 If the login is on a tty, records login time.
541 if it exists, prints contents and quits
544 Changes to run with normal user privileges.
546 Sets up basic environment.
549 .Pa $HOME/.ssh/environment
552 Changes to user's home directory.
556 exists, runs it; else if
559 it; otherwise runs xauth.
562 files are given the X11
563 authentication protocol and cookie in standard input.
565 Runs user's shell or command.
567 .Sh AUTHORIZED_KEYS FILE FORMAT
569 .Pa $HOME/.ssh/authorized_keys
570 file lists the RSA keys that are
571 permitted for RSA authentication.
572 Each line of the file contains one
573 key (empty lines and lines starting with a
577 Each line consists of the following fields, separated by
578 spaces: options, bits, exponent, modulus, comment.
580 is optional; its presence is determined by whether the line starts
581 with a number or not (the option field never starts with a number).
582 The bits, exponent, modulus and comment fields give the RSA key; the
583 comment field is not used for anything (but may be convenient for the
584 user to identify the key).
586 Note that lines in this file are usually several hundred bytes long
587 (because of the size of the RSA key modulus).
588 You don't want to type them in; instead, copy the
592 The options (if present) consists of comma-separated option
594 No spaces are permitted, except within double quotes.
595 The following option specifications are supported:
597 .It Cm from="pattern-list"
598 Specifies that in addition to RSA authentication, the canonical name
599 of the remote host must be present in the comma-separated list of
605 The list may also contain
606 patterns negated by prefixing them with
608 if the canonical host name matches a negated pattern, the key is not accepted.
610 of this option is to optionally increase security: RSA authentication
611 by itself does not trust the network or name servers or anything (but
612 the key); however, if somebody somehow steals the key, the key
613 permits an intruder to log in from anywhere in the world.
614 This additional option makes using a stolen key more difficult (name
615 servers and/or routers would have to be compromised in addition to
617 .It Cm command="command"
618 Specifies that the command is executed whenever this key is used for
620 The command supplied by the user (if any) is ignored.
621 The command is run on a pty if the connection requests a pty;
622 otherwise it is run without a tty.
623 A quote may be included in the command by quoting it with a backslash.
624 This option might be useful
625 to restrict certain RSA keys to perform just a specific operation.
626 An example might be a key that permits remote backups but nothing else.
627 Notice that the client may specify TCP/IP and/or X11
628 forwardings unless they are explicitly prohibited.
629 .It Cm environment="NAME=value"
630 Specifies that the string is to be added to the environment when
631 logging in using this key.
632 Environment variables set this way
633 override other default environment values.
634 Multiple options of this type are permitted.
635 .It Cm no-port-forwarding
636 Forbids TCP/IP forwarding when this key is used for authentication.
637 Any port forward requests by the client will return an error.
638 This might be used, e.g., in connection with the
641 .It Cm no-X11-forwarding
642 Forbids X11 forwarding when this key is used for authentication.
643 Any X11 forward requests by the client will return an error.
644 .It Cm no-agent-forwarding
645 Forbids authentication agent forwarding when this key is used for
648 Prevents tty allocation (a request to allocate a pty will fail).
651 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
653 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
655 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
656 .Sh SSH_KNOWN_HOSTS FILE FORMAT
658 .Pa /etc/ssh_known_hosts
660 .Pa $HOME/.ssh/known_hosts
661 files contain host public keys for all known hosts.
662 The global file should
663 be prepared by the administrator (optional), and the per-user file is
664 maintained automatically: whenever the user connects an unknown host
665 its key is added to the per-user file.
667 Each line in these files contains the following fields: hostnames,
668 bits, exponent, modulus, comment.
669 The fields are separated by spaces.
671 Hostnames is a comma-separated list of patterns ('*' and '?' act as
672 wildcards); each pattern in turn is matched against the canonical host
673 name (when authenticating a client) or against the user-supplied
674 name (when authenticating a server).
675 A pattern may also be preceded by
677 to indicate negation: if the host name matches a negated
678 pattern, it is not accepted (by that line) even if it matched another
681 Bits, exponent, and modulus are taken directly from the host key; they
682 can be obtained, e.g., from
683 .Pa /etc/ssh_host_key.pub .
684 The optional comment field continues to the end of the line, and is not used.
688 and empty lines are ignored as comments.
690 When performing host authentication, authentication is accepted if any
691 matching line has the proper key.
692 It is thus permissible (but not
693 recommended) to have several lines or different host keys for the same
695 This will inevitably happen when short forms of host names
696 from different domains are put in the file.
698 that the files contain conflicting information; authentication is
699 accepted if valid information can be found from either file.
701 Note that the lines in these files are typically hundreds of characters
702 long, and you definitely don't want to type in the host keys by hand.
703 Rather, generate them by a script
705 .Pa /etc/ssh_host_key.pub
706 and adding the host names at the front.
708 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
711 .It Pa /etc/sshd_config
712 Contains configuration data for
714 This file should be writable by root only, but it is recommended
715 (though not necessary) that it be world-readable.
716 .It Pa /etc/ssh_host_key
717 Contains the private part of the host key.
718 This file should only be owned by root, readable only by root, and not
719 accessible to others.
722 does not start if this file is group/world-accessible.
723 .It Pa /etc/ssh_host_key.pub
724 Contains the public part of the host key.
725 This file should be world-readable but writable only by
727 Its contents should match the private part.
729 really used for anything; it is only provided for the convenience of
730 the user so its contents can be copied to known hosts files.
731 These two files are created using
733 .It Pa /var/run/sshd.pid
734 Contains the process ID of the
736 listening for connections (if there are several daemons running
737 concurrently for different ports, this contains the pid of the one
739 The contents of this file are not sensitive; it can be world-readable.
740 .It Pa $HOME/.ssh/authorized_keys
741 Lists the RSA keys that can be used to log into the user's account.
742 This file must be readable by root (which may on some machines imply
743 it being world-readable if the user's home directory resides on an NFS
745 It is recommended that it not be accessible by others.
746 The format of this file is described above.
747 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
748 These files are consulted when using rhosts with RSA host
749 authentication to check the public key of the host.
750 The key must be listed in one of these files to be accepted.
751 The client uses the same files
752 to verify that the remote host is the one we intended to connect.
753 These files should be writable only by root/the owner.
754 .Pa /etc/ssh_known_hosts
755 should be world-readable, and
756 .Pa $HOME/.ssh/known_hosts
757 can but need not be world-readable.
761 refuses to let anyone except root log in.
762 The contents of the file
763 are displayed to anyone trying to log in, and non-root connections are
765 The file should be world-readable.
766 .It Pa /etc/hosts.allow, /etc/hosts.deny
769 support, tcp-wrappers access controls may be defined here as described in
772 This file contains host-username pairs, separated by a space, one per
774 The given user on the corresponding host is permitted to log in
776 The same file is used by rlogind and rshd.
778 be writable only by the user; it is recommended that it not be
779 accessible by others.
781 If is also possible to use netgroups in the file.
783 name may be of the form +@groupname to specify all hosts or all users
787 this file is exactly the same as for
789 However, this file is
790 not used by rlogin and rshd, so using this permits access using SSH only.
792 This file is used during
795 In the simplest form, this file contains host names, one per line.
797 those hosts are permitted to log in without a password, provided they
798 have the same user name on both machines.
799 The host name may also be
800 followed by a user name; such users are permitted to log in as
802 user on this machine (except root).
803 Additionally, the syntax
805 can be used to specify netgroups.
806 Negated entries start with
809 If the client host/user is successfully matched in this file, login is
810 automatically permitted provided the client and server user names are the
812 Additionally, successful RSA host authentication is normally required.
813 This file must be writable only by root; it is recommended
814 that it be world-readable.
816 .Sy "Warning: It is almost never a good idea to use user names in"
818 Beware that it really means that the named user(s) can log in as
820 which includes bin, daemon, adm, and other accounts that own critical
821 binaries and directories.
822 Using a user name practically grants the user root access.
823 The only valid use for user names that I can think
824 of is in negative entries.
826 Note that this warning also applies to rsh/rlogin.
827 .It Pa /etc/shosts.equiv
828 This is processed exactly as
829 .Pa /etc/hosts.equiv .
830 However, this file may be useful in environments that want to run both
832 .It Pa $HOME/.ssh/environment
833 This file is read into the environment at login (if it exists).
834 It can only contain empty lines, comment lines (that start with
836 and assignment lines of the form name=value.
837 The file should be writable
838 only by the user; it need not be readable by anyone else.
840 If this file exists, it is run with /bin/sh after reading the
841 environment files but before starting the user's shell or command.
842 If X11 spoofing is in use, this will receive the "proto cookie" pair in
850 The primary purpose of this file is to run any initialization routines
851 which may be needed before the user's home directory becomes
852 accessible; AFS is a particular example of such an environment.
854 This file will probably contain some initialization code followed by
855 something similar to: "if read proto cookie; then echo add $DISPLAY
856 $proto $cookie | xauth -q -; fi".
858 If this file does not exist,
861 does not exist either, xauth is used to store the cookie.
863 This file should be writable only by the user, and need not be
864 readable by anyone else.
868 This can be used to specify
869 machine-specific login-time initializations globally.
870 This file should be writable only by root, and should be world-readable.
873 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
874 but with bugs removed and newer features re-added.
876 1.2.12 release, newer versions of the original ssh bore successively
877 more restrictive licenses, and thus demand for a free version was born.
878 This version of OpenSSH
881 has all components of a restrictive nature (i.e., patents)
882 directly removed from the source code; any licensed or patented components
886 has been updated to support ssh protocol 1.5, making it compatible with
887 all other ssh protocol 1 clients and servers.
889 contains added support for
891 authentication and ticket passing.
893 supports one-time password authentication with