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,
49 This implementation of
51 supports both SSH protocol version 1 and 2 simultaneously.
55 .Ss SSH protocol version 1
57 Each host has a host-specific RSA key
58 (normally 1024 bits) used to identify the host.
60 the daemon starts, it generates a server RSA key (normally 768 bits).
61 This key is normally regenerated every hour if it has been used, and
62 is never stored on disk.
64 Whenever a client connects the daemon responds with its public
66 The client compares the
67 RSA host key against its own database to verify that it has not changed.
68 The client then generates a 256 bit random number.
70 random number using both the host key and the server key, and sends
71 the encrypted number to the server.
72 Both sides then use this
73 random number as a session key which is used to encrypt all further
74 communications in the session.
75 The rest of the session is encrypted
76 using a conventional cipher, currently Blowfish or 3DES, with 3DES
77 being used by default.
78 The client selects the encryption algorithm
79 to use from those offered by the server.
81 Next, the server and the client enter an authentication dialog.
82 The client tries to authenticate itself using
86 authentication combined with RSA host
87 authentication, RSA challenge-response authentication, or password
90 Rhosts authentication is normally disabled
91 because it is fundamentally insecure, but can be enabled in the server
92 configuration file if desired.
93 System security is not improved unless
99 are disabled (thus completely disabling
105 .Ss SSH protocol version 2
107 Version 2 works similar:
108 Each host has a host-specific DSA key used to identify the host.
109 However, when the daemon starts, it does not generate a server key.
110 Forward security is provided through a Diffie-Hellman key agreement.
111 This key agreement results in a shared session key.
112 The rest of the session is encrypted
113 using a symmetric cipher, currently
114 Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
115 The client selects the encryption algorithm
116 to use from those offered by the server.
117 Additionally, session integrity is provided
118 through a cryptographic message authentication code
119 (hmac-sha1 or hmac-md5).
121 Protocol version 2 provides a public key based
122 user authentication method (DSAAuthentication)
123 and conventional password authentication.
125 .Ss Command execution and data forwarding
127 If the client successfully authenticates itself, a dialog for
128 preparing the session is entered.
129 At this time the client may request
130 things like allocating a pseudo-tty, forwarding X11 connections,
131 forwarding TCP/IP connections, or forwarding the authentication agent
132 connection over the secure channel.
134 Finally, the client either requests a shell or execution of a command.
135 The sides then enter session mode.
136 In this mode, either side may send
137 data at any time, and such data is forwarded to/from the shell or
138 command on the server side, and the user terminal in the client side.
140 When the user program terminates and all forwarded X11 and other
141 connections have been closed, the server sends command exit status to
142 the client, and both sides exit.
145 can be configured using command-line options or a configuration
147 Command-line options override values specified in the
151 rereads its configuration file when it receives a hangup signal,
154 The options are as follows:
157 Specifies the number of bits in the server key (default 768).
161 The server sends verbose debug output to the system
162 log, and does not put itself in the background.
163 The server also will not fork and will only process one connection.
164 This option is only intended for debugging for the server.
165 .It Fl f Ar configuration_file
166 Specifies the name of the configuration file.
168 .Pa /etc/sshd_config .
170 refuses to start if there is no configuration file.
171 .It Fl g Ar login_grace_time
172 Gives the grace time for clients to authenticate themselves (default
174 If the client fails to authenticate the user within
175 this many seconds, the server disconnects and exits.
176 A value of zero indicates no limit.
177 .It Fl h Ar host_key_file
178 Specifies the file from which the RSA host key is read (default
179 .Pa /etc/ssh_host_key ) .
180 This option must be given if
182 is not run as root (as the normal
183 host file is normally not readable by anyone but root).
187 is being run from inetd.
190 from inetd because it needs to generate the server key before it can
191 respond to the client, and this may take tens of seconds.
192 Clients would have to wait too long if the key was regenerated every time.
193 However, with small key sizes (e.g., 512) using
197 .It Fl k Ar key_gen_time
198 Specifies how often the server key is regenerated (default 3600
199 seconds, or one hour).
200 The motivation for regenerating the key fairly
201 often is that the key is not stored anywhere, and after about an hour,
202 it becomes impossible to recover the key for decrypting intercepted
203 communications even if the machine is cracked into or physically
205 A value of zero indicates that the key will never be regenerated.
207 Specifies the port on which the server listens for connections
211 Nothing is sent to the system log.
212 Normally the beginning,
213 authentication, and termination of each connection is logged.
215 Do not print an error message if RSA support is missing.
216 .It Fl V Ar client_protocol_id
217 SSH2 compatibility mode.
218 When this option is specified
220 assumes the client has sent the supplied version string
222 Protocol Version Identification Exchange.
226 to use IPv4 addresses only.
230 to use IPv6 addresses only.
232 .Sh CONFIGURATION FILE
234 reads configuration data from
236 (or the file specified with
238 on the command line).
239 The file contains keyword-value pairs, one per line.
242 and empty lines are interpreted as comments.
244 The following keywords are possible.
246 .It Cm AFSTokenPassing
247 Specifies whether an AFS token may be forwarded to the server.
251 This keyword can be followed by a number of group names, separated
253 If specified, login is allowed only for users whose primary
254 group matches one of the patterns.
259 wildcards in the patterns.
260 Only group names are valid, a numerical group ID isn't recognized.
261 By default login is allowed regardless of the primary group.
264 This keyword can be followed by a number of user names, separated
266 If specified, login is allowed only for users names that
267 match one of the patterns.
272 wildcards in the patterns.
273 Only user names are valid, a numerical user ID isn't recognized.
274 By default login is allowed regardless of the user name.
277 Specifies the ciphers allowed for protocol version 2.
278 Multiple ciphers must be comma-separated.
280 .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
284 should check for new mail for interactive logins.
288 This keyword can be followed by a number of group names, separated
290 Users whose primary group matches one of the patterns
291 aren't allowed to log in.
296 wildcards in the patterns.
297 Only group names are valid, a numerical group ID isn't recognized.
298 By default login is allowed regardless of the primary group.
301 This keyword can be followed by a number of user names, separated
303 Login is disallowed for user names that match one of the patterns.
307 can be used as wildcards in the patterns.
308 Only user names are valid, a numerical user ID isn't recognized.
309 By default login is allowed regardless of the user name.
310 .It Cm DSAAuthentication
311 Specifies whether DSA authentication is allowed.
314 Note that this option applies to protocol version 2 only.
316 Specifies whether remote hosts are allowed to connect to ports
317 forwarded for the client.
325 Specifies the file containing the private DSA host key (default
326 .Pa /etc/ssh_host_dsa_key )
327 used by SSH protocol 2.0.
330 disables protocol 2.0 if this file is group/world-accessible.
332 Specifies the file containing the private RSA host key (default
333 .Pa /etc/ssh_host_key )
334 used by SSH protocols 1.3 and 1.5.
337 disables protocols 1.3 and 1.5 if this file is group/world-accessible.
343 files will not be used in authentication.
346 .Pa /etc/shosts.equiv
350 .It Cm IgnoreUserKnownHosts
353 should ignore the user's
354 .Pa $HOME/.ssh/known_hosts
356 .Cm RhostsRSAAuthentication .
360 Specifies whether the system should send keepalive messages to the
362 If they are sent, death of the connection or crash of one
363 of the machines will be properly noticed.
364 However, this means that
365 connections will die if the route is down temporarily, and some people
367 On the other hand, if keepalives are not sent,
368 sessions may hang indefinitely on the server, leaving
370 users and consuming server resources.
374 (to send keepalives), and the server will notice
375 if the network goes down or the client host reboots.
376 This avoids infinitely hanging sessions.
378 To disable keepalives, the value should be set to
380 in both the server and the client configuration files.
381 .It Cm KerberosAuthentication
382 Specifies whether Kerberos authentication is allowed.
383 This can be in the form of a Kerberos ticket, or if
384 .Cm PasswordAuthentication
385 is yes, the password provided by the user will be validated through
389 .It Cm KerberosOrLocalPasswd
390 If set then if password authentication through Kerberos fails then
391 the password will be validated via any additional local mechanism
397 .It Cm KerberosTgtPassing
398 Specifies whether a Kerberos TGT may be forwarded to the server.
401 as this only works when the Kerberos KDC is actually an AFS kaserver.
402 .It Cm KerberosTicketCleanup
403 Specifies whether to automatically destroy the user's ticket cache
407 .It Cm KeyRegenerationInterval
408 The server key is automatically regenerated after this many seconds
409 (if it has been used).
410 The purpose of regeneration is to prevent
411 decrypting captured sessions by later breaking into the machine and
413 The key is never stored anywhere.
414 If the value is 0, the key is never regenerated.
415 The default is 3600 (seconds).
417 Specifies what local address
420 The default is to listen to all local addresses.
421 Multiple options of this type are permitted.
424 options must precede this option.
425 .It Cm LoginGraceTime
426 The server disconnects after this time if the user has not
427 successfully logged in.
428 If the value is 0, there is no time limit.
429 The default is 600 (seconds).
431 Gives the verbosity level that is used when logging messages from
433 The possible values are:
434 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
436 Logging with level DEBUG violates the privacy of users
437 and is not recommended.
438 .It Cm PasswordAuthentication
439 Specifies whether password authentication is allowed.
442 Note that this option applies to both protocol version 1 and 2.
443 .It Cm PermitEmptyPasswords
444 When password authentication is allowed, it specifies whether the
445 server allows login to accounts with empty password strings.
448 .It Cm PermitRootLogin
449 Specifies whether the root can log in using
458 If this options is set to
460 only password authentication is disabled for root.
462 Root login with RSA authentication when the
465 specified will be allowed regardless of the value of this setting
466 (which may be useful for taking remote backups even if root login is
467 normally not allowed).
469 Specifies the file that contains the process identifier of the
473 .Pa /var/run/sshd.pid .
475 Specifies the port number that
479 Multiple options of this type are permitted.
485 when a user logs in interactively.
486 (On some systems it is also printed by the shell,
492 Specifies the protocol versions
495 The possible values are
499 Multiple versions must be comma-separated.
504 Random number generation uses other techniques.
505 .It Cm RhostsAuthentication
506 Specifies whether authentication using rhosts or /etc/hosts.equiv
508 Normally, this method should not be permitted because it is insecure.
509 .Cm RhostsRSAAuthentication
511 instead, because it performs RSA-based host authentication in addition
512 to normal rhosts or /etc/hosts.equiv authentication.
515 .It Cm RhostsRSAAuthentication
516 Specifies whether rhosts or /etc/hosts.equiv authentication together
517 with successful RSA host authentication is allowed.
520 .It Cm RSAAuthentication
521 Specifies whether pure RSA authentication is allowed.
524 Note that this option applies to protocol version 1 only.
526 Defines the number of bits in the server key.
527 The minimum value is 512, and the default is 768.
528 .It Cm SkeyAuthentication
531 authentication is allowed.
534 Note that s/key authentication is enabled only if
535 .Cm PasswordAuthentication
540 should check file modes and ownership of the
541 user's files and home directory before accepting login.
542 This is normally desirable because novices sometimes accidentally leave their
543 directory or files world-writable.
546 .It Cm SyslogFacility
547 Gives the facility code that is used when logging messages from
549 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
550 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
555 is used for interactive login sessions.
558 is not never for remote command execution.
561 .It Cm X11DisplayOffset
562 Specifies the first display number available for
567 from interfering with real X11 servers.
570 Specifies whether X11 forwarding is permitted.
573 Note that disabling X11 forwarding does not improve security in any
574 way, as users can always install their own forwarders.
576 Specifies the location of the
580 .Pa /usr/X11R6/bin/xauth .
583 When a user successfully logs in,
586 .Bl -enum -offset indent
588 If the login is on a tty, and no command has been specified,
589 prints last login time and
591 (unless prevented in the configuration file or by
592 .Pa $HOME/.hushlogin ;
597 If the login is on a tty, records login time.
601 if it exists, prints contents and quits
604 Changes to run with normal user privileges.
606 Sets up basic environment.
609 .Pa $HOME/.ssh/environment
612 Changes to user's home directory.
616 exists, runs it; else if
619 it; otherwise runs xauth.
622 files are given the X11
623 authentication protocol and cookie in standard input.
625 Runs user's shell or command.
627 .Sh AUTHORIZED_KEYS FILE FORMAT
629 .Pa $HOME/.ssh/authorized_keys
630 file lists the RSA keys that are
631 permitted for RSA authentication in SSH protocols 1.3 and 1.5
633 .Pa $HOME/.ssh/authorized_keys2
634 file lists the DSA keys that are
635 permitted for DSA authentication in SSH protocol 2.0.
636 Each line of the file contains one
637 key (empty lines and lines starting with a
641 Each line consists of the following fields, separated by
642 spaces: options, bits, exponent, modulus, comment.
644 is optional; its presence is determined by whether the line starts
645 with a number or not (the option field never starts with a number).
646 The bits, exponent, modulus and comment fields give the RSA key; the
647 comment field is not used for anything (but may be convenient for the
648 user to identify the key).
650 Note that lines in this file are usually several hundred bytes long
651 (because of the size of the RSA key modulus).
652 You don't want to type them in; instead, copy the
656 The options (if present) consists of comma-separated option
658 No spaces are permitted, except within double quotes.
659 The following option specifications are supported:
661 .It Cm from="pattern-list"
662 Specifies that in addition to RSA authentication, the canonical name
663 of the remote host must be present in the comma-separated list of
669 The list may also contain
670 patterns negated by prefixing them with
672 if the canonical host name matches a negated pattern, the key is not accepted.
674 of this option is to optionally increase security: RSA authentication
675 by itself does not trust the network or name servers or anything (but
676 the key); however, if somebody somehow steals the key, the key
677 permits an intruder to log in from anywhere in the world.
678 This additional option makes using a stolen key more difficult (name
679 servers and/or routers would have to be compromised in addition to
681 .It Cm command="command"
682 Specifies that the command is executed whenever this key is used for
684 The command supplied by the user (if any) is ignored.
685 The command is run on a pty if the connection requests a pty;
686 otherwise it is run without a tty.
687 A quote may be included in the command by quoting it with a backslash.
688 This option might be useful
689 to restrict certain RSA keys to perform just a specific operation.
690 An example might be a key that permits remote backups but nothing else.
691 Note that the client may specify TCP/IP and/or X11
692 forwarding unless they are explicitly prohibited.
693 .It Cm environment="NAME=value"
694 Specifies that the string is to be added to the environment when
695 logging in using this key.
696 Environment variables set this way
697 override other default environment values.
698 Multiple options of this type are permitted.
699 .It Cm no-port-forwarding
700 Forbids TCP/IP forwarding when this key is used for authentication.
701 Any port forward requests by the client will return an error.
702 This might be used, e.g., in connection with the
705 .It Cm no-X11-forwarding
706 Forbids X11 forwarding when this key is used for authentication.
707 Any X11 forward requests by the client will return an error.
708 .It Cm no-agent-forwarding
709 Forbids authentication agent forwarding when this key is used for
712 Prevents tty allocation (a request to allocate a pty will fail).
715 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
717 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
719 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
720 .Sh SSH_KNOWN_HOSTS FILE FORMAT
722 .Pa /etc/ssh_known_hosts ,
723 .Pa /etc/ssh_known_hosts2 ,
724 .Pa $HOME/.ssh/known_hosts ,
726 .Pa $HOME/.ssh/known_hosts2
727 files contain host public keys for all known hosts.
728 The global file should
729 be prepared by the administrator (optional), and the per-user file is
730 maintained automatically: whenever the user connects an unknown host
731 its key is added to the per-user file.
733 Each line in these files contains the following fields: hostnames,
734 bits, exponent, modulus, comment.
735 The fields are separated by spaces.
737 Hostnames is a comma-separated list of patterns ('*' and '?' act as
738 wildcards); each pattern in turn is matched against the canonical host
739 name (when authenticating a client) or against the user-supplied
740 name (when authenticating a server).
741 A pattern may also be preceded by
743 to indicate negation: if the host name matches a negated
744 pattern, it is not accepted (by that line) even if it matched another
747 Bits, exponent, and modulus are taken directly from the RSA host key; they
748 can be obtained, e.g., from
749 .Pa /etc/ssh_host_key.pub .
750 The optional comment field continues to the end of the line, and is not used.
754 and empty lines are ignored as comments.
756 When performing host authentication, authentication is accepted if any
757 matching line has the proper key.
758 It is thus permissible (but not
759 recommended) to have several lines or different host keys for the same
761 This will inevitably happen when short forms of host names
762 from different domains are put in the file.
764 that the files contain conflicting information; authentication is
765 accepted if valid information can be found from either file.
767 Note that the lines in these files are typically hundreds of characters
768 long, and you definitely don't want to type in the host keys by hand.
769 Rather, generate them by a script
771 .Pa /etc/ssh_host_key.pub
772 and adding the host names at the front.
774 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
777 .It Pa /etc/sshd_config
778 Contains configuration data for
780 This file should be writable by root only, but it is recommended
781 (though not necessary) that it be world-readable.
782 .It Pa /etc/ssh_host_key
783 Contains the private part of the host key.
784 This file should only be owned by root, readable only by root, and not
785 accessible to others.
788 does not start if this file is group/world-accessible.
789 .It Pa /etc/ssh_host_key.pub
790 Contains the public part of the host key.
791 This file should be world-readable but writable only by
793 Its contents should match the private part.
795 really used for anything; it is only provided for the convenience of
796 the user so its contents can be copied to known hosts files.
797 These two files are created using
799 .It Pa /var/run/sshd.pid
800 Contains the process ID of the
802 listening for connections (if there are several daemons running
803 concurrently for different ports, this contains the pid of the one
805 The contents of this file are not sensitive; it can be world-readable.
806 .It Pa $HOME/.ssh/authorized_keys
807 Lists the RSA keys that can be used to log into the user's account.
808 This file must be readable by root (which may on some machines imply
809 it being world-readable if the user's home directory resides on an NFS
811 It is recommended that it not be accessible by others.
812 The format of this file is described above.
813 Users will place the contents of their
815 files into this file, as described in
817 .It Pa $HOME/.ssh/authorized_keys2
818 Lists the DSA keys that can be used to log into the user's account.
819 This file must be readable by root (which may on some machines imply
820 it being world-readable if the user's home directory resides on an NFS
822 It is recommended that it not be accessible by others.
823 The format of this file is described above.
824 Users will place the contents of their
826 files into this file, as described in
828 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
829 These files are consulted when using rhosts with RSA host
830 authentication to check the public key of the host.
831 The key must be listed in one of these files to be accepted.
832 The client uses the same files
833 to verify that the remote host is the one we intended to connect.
834 These files should be writable only by root/the owner.
835 .Pa /etc/ssh_known_hosts
836 should be world-readable, and
837 .Pa $HOME/.ssh/known_hosts
838 can but need not be world-readable.
842 refuses to let anyone except root log in.
843 The contents of the file
844 are displayed to anyone trying to log in, and non-root connections are
846 The file should be world-readable.
847 .It Pa /etc/hosts.allow, /etc/hosts.deny
850 support, tcp-wrappers access controls may be defined here as described in
853 This file contains host-username pairs, separated by a space, one per
855 The given user on the corresponding host is permitted to log in
857 The same file is used by rlogind and rshd.
859 be writable only by the user; it is recommended that it not be
860 accessible by others.
862 If is also possible to use netgroups in the file.
864 name may be of the form +@groupname to specify all hosts or all users
868 this file is exactly the same as for
870 However, this file is
871 not used by rlogin and rshd, so using this permits access using SSH only.
873 This file is used during
876 In the simplest form, this file contains host names, one per line.
878 those hosts are permitted to log in without a password, provided they
879 have the same user name on both machines.
880 The host name may also be
881 followed by a user name; such users are permitted to log in as
883 user on this machine (except root).
884 Additionally, the syntax
886 can be used to specify netgroups.
887 Negated entries start with
890 If the client host/user is successfully matched in this file, login is
891 automatically permitted provided the client and server user names are the
893 Additionally, successful RSA host authentication is normally required.
894 This file must be writable only by root; it is recommended
895 that it be world-readable.
897 .Sy "Warning: It is almost never a good idea to use user names in"
899 Beware that it really means that the named user(s) can log in as
901 which includes bin, daemon, adm, and other accounts that own critical
902 binaries and directories.
903 Using a user name practically grants the user root access.
904 The only valid use for user names that I can think
905 of is in negative entries.
907 Note that this warning also applies to rsh/rlogin.
908 .It Pa /etc/shosts.equiv
909 This is processed exactly as
910 .Pa /etc/hosts.equiv .
911 However, this file may be useful in environments that want to run both
913 .It Pa $HOME/.ssh/environment
914 This file is read into the environment at login (if it exists).
915 It can only contain empty lines, comment lines (that start with
917 and assignment lines of the form name=value.
918 The file should be writable
919 only by the user; it need not be readable by anyone else.
921 If this file exists, it is run with /bin/sh after reading the
922 environment files but before starting the user's shell or command.
923 If X11 spoofing is in use, this will receive the "proto cookie" pair in
931 The primary purpose of this file is to run any initialization routines
932 which may be needed before the user's home directory becomes
933 accessible; AFS is a particular example of such an environment.
935 This file will probably contain some initialization code followed by
936 something similar to: "if read proto cookie; then echo add $DISPLAY
937 $proto $cookie | xauth -q -; fi".
939 If this file does not exist,
942 does not exist either, xauth is used to store the cookie.
944 This file should be writable only by the user, and need not be
945 readable by anyone else.
949 This can be used to specify
950 machine-specific login-time initializations globally.
951 This file should be writable only by root, and should be world-readable.
954 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
955 but with bugs removed and newer features re-added.
957 1.2.12 release, newer versions of the original ssh bore successively
958 more restrictive licenses, and thus demand for a free version was born.
960 This version of OpenSSH
963 has all components of a restrictive nature (i.e., patents)
964 directly removed from the source code; any licensed or patented components
968 has been updated to support SSH protocol 1.5 and 2, making it compatible with
969 all other SSH clients and servers.
971 contains added support for
973 authentication and ticket passing.
975 supports one-time password authentication with
979 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
980 Niels Provos, Theo de Raadt, and Dug Song.
982 The support for SSH protocol 2 was written by Markus Friedl.