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.
439 Specifies the maximum number of concurrent unauthenticated connections to the
442 Additional connections will be dropped until authentication succeeds or the
444 expires for a connection.
446 .It Cm PasswordAuthentication
447 Specifies whether password authentication is allowed.
450 Note that this option applies to both protocol version 1 and 2.
451 .It Cm PermitEmptyPasswords
452 When password authentication is allowed, it specifies whether the
453 server allows login to accounts with empty password strings.
456 .It Cm PermitRootLogin
457 Specifies whether the root can log in using
466 If this options is set to
468 only password authentication is disabled for root.
470 Root login with RSA authentication when the
473 specified will be allowed regardless of the value of this setting
474 (which may be useful for taking remote backups even if root login is
475 normally not allowed).
477 Specifies the file that contains the process identifier of the
481 .Pa /var/run/sshd.pid .
483 Specifies the port number that
487 Multiple options of this type are permitted.
493 when a user logs in interactively.
494 (On some systems it is also printed by the shell,
500 Specifies the protocol versions
503 The possible values are
507 Multiple versions must be comma-separated.
512 Random number generation uses other techniques.
513 .It Cm RhostsAuthentication
514 Specifies whether authentication using rhosts or /etc/hosts.equiv
516 Normally, this method should not be permitted because it is insecure.
517 .Cm RhostsRSAAuthentication
519 instead, because it performs RSA-based host authentication in addition
520 to normal rhosts or /etc/hosts.equiv authentication.
523 .It Cm RhostsRSAAuthentication
524 Specifies whether rhosts or /etc/hosts.equiv authentication together
525 with successful RSA host authentication is allowed.
528 .It Cm RSAAuthentication
529 Specifies whether pure RSA authentication is allowed.
532 Note that this option applies to protocol version 1 only.
534 Defines the number of bits in the server key.
535 The minimum value is 512, and the default is 768.
536 .It Cm SkeyAuthentication
539 authentication is allowed.
542 Note that s/key authentication is enabled only if
543 .Cm PasswordAuthentication
548 should check file modes and ownership of the
549 user's files and home directory before accepting login.
550 This is normally desirable because novices sometimes accidentally leave their
551 directory or files world-writable.
555 Configures an external subsystem (e.g. file transfer daemon).
556 Arguments should be a subsystem name and a command to execute upon subsystem request.
557 By default no subsystems are defined.
558 Note that this option applies to protocol version 2 only.
559 .It Cm SyslogFacility
560 Gives the facility code that is used when logging messages from
562 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
563 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
568 is used for interactive login sessions.
571 is not never for remote command execution.
574 .It Cm X11DisplayOffset
575 Specifies the first display number available for
580 from interfering with real X11 servers.
583 Specifies whether X11 forwarding is permitted.
586 Note that disabling X11 forwarding does not improve security in any
587 way, as users can always install their own forwarders.
589 Specifies the location of the
593 .Pa /usr/X11R6/bin/xauth .
596 When a user successfully logs in,
599 .Bl -enum -offset indent
601 If the login is on a tty, and no command has been specified,
602 prints last login time and
604 (unless prevented in the configuration file or by
605 .Pa $HOME/.hushlogin ;
610 If the login is on a tty, records login time.
614 if it exists, prints contents and quits
617 Changes to run with normal user privileges.
619 Sets up basic environment.
622 .Pa $HOME/.ssh/environment
625 Changes to user's home directory.
629 exists, runs it; else if
632 it; otherwise runs xauth.
635 files are given the X11
636 authentication protocol and cookie in standard input.
638 Runs user's shell or command.
640 .Sh AUTHORIZED_KEYS FILE FORMAT
642 .Pa $HOME/.ssh/authorized_keys
643 file lists the RSA keys that are
644 permitted for RSA authentication in SSH protocols 1.3 and 1.5
646 .Pa $HOME/.ssh/authorized_keys2
647 file lists the DSA keys that are
648 permitted for DSA authentication in SSH protocol 2.0.
649 Each line of the file contains one
650 key (empty lines and lines starting with a
654 Each line consists of the following fields, separated by
655 spaces: options, bits, exponent, modulus, comment.
657 is optional; its presence is determined by whether the line starts
658 with a number or not (the option field never starts with a number).
659 The bits, exponent, modulus and comment fields give the RSA key; the
660 comment field is not used for anything (but may be convenient for the
661 user to identify the key).
663 Note that lines in this file are usually several hundred bytes long
664 (because of the size of the RSA key modulus).
665 You don't want to type them in; instead, copy the
669 The options (if present) consists of comma-separated option
671 No spaces are permitted, except within double quotes.
672 The following option specifications are supported:
674 .It Cm from="pattern-list"
675 Specifies that in addition to RSA authentication, the canonical name
676 of the remote host must be present in the comma-separated list of
682 The list may also contain
683 patterns negated by prefixing them with
685 if the canonical host name matches a negated pattern, the key is not accepted.
687 of this option is to optionally increase security: RSA authentication
688 by itself does not trust the network or name servers or anything (but
689 the key); however, if somebody somehow steals the key, the key
690 permits an intruder to log in from anywhere in the world.
691 This additional option makes using a stolen key more difficult (name
692 servers and/or routers would have to be compromised in addition to
694 .It Cm command="command"
695 Specifies that the command is executed whenever this key is used for
697 The command supplied by the user (if any) is ignored.
698 The command is run on a pty if the connection requests a pty;
699 otherwise it is run without a tty.
700 A quote may be included in the command by quoting it with a backslash.
701 This option might be useful
702 to restrict certain RSA keys to perform just a specific operation.
703 An example might be a key that permits remote backups but nothing else.
704 Note that the client may specify TCP/IP and/or X11
705 forwarding unless they are explicitly prohibited.
706 .It Cm environment="NAME=value"
707 Specifies that the string is to be added to the environment when
708 logging in using this key.
709 Environment variables set this way
710 override other default environment values.
711 Multiple options of this type are permitted.
712 .It Cm no-port-forwarding
713 Forbids TCP/IP forwarding when this key is used for authentication.
714 Any port forward requests by the client will return an error.
715 This might be used, e.g., in connection with the
718 .It Cm no-X11-forwarding
719 Forbids X11 forwarding when this key is used for authentication.
720 Any X11 forward requests by the client will return an error.
721 .It Cm no-agent-forwarding
722 Forbids authentication agent forwarding when this key is used for
725 Prevents tty allocation (a request to allocate a pty will fail).
728 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
730 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
732 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
733 .Sh SSH_KNOWN_HOSTS FILE FORMAT
735 .Pa /etc/ssh_known_hosts ,
736 .Pa /etc/ssh_known_hosts2 ,
737 .Pa $HOME/.ssh/known_hosts ,
739 .Pa $HOME/.ssh/known_hosts2
740 files contain host public keys for all known hosts.
741 The global file should
742 be prepared by the administrator (optional), and the per-user file is
743 maintained automatically: whenever the user connects an unknown host
744 its key is added to the per-user file.
746 Each line in these files contains the following fields: hostnames,
747 bits, exponent, modulus, comment.
748 The fields are separated by spaces.
750 Hostnames is a comma-separated list of patterns ('*' and '?' act as
751 wildcards); each pattern in turn is matched against the canonical host
752 name (when authenticating a client) or against the user-supplied
753 name (when authenticating a server).
754 A pattern may also be preceded by
756 to indicate negation: if the host name matches a negated
757 pattern, it is not accepted (by that line) even if it matched another
760 Bits, exponent, and modulus are taken directly from the RSA host key; they
761 can be obtained, e.g., from
762 .Pa /etc/ssh_host_key.pub .
763 The optional comment field continues to the end of the line, and is not used.
767 and empty lines are ignored as comments.
769 When performing host authentication, authentication is accepted if any
770 matching line has the proper key.
771 It is thus permissible (but not
772 recommended) to have several lines or different host keys for the same
774 This will inevitably happen when short forms of host names
775 from different domains are put in the file.
777 that the files contain conflicting information; authentication is
778 accepted if valid information can be found from either file.
780 Note that the lines in these files are typically hundreds of characters
781 long, and you definitely don't want to type in the host keys by hand.
782 Rather, generate them by a script
784 .Pa /etc/ssh_host_key.pub
785 and adding the host names at the front.
787 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
790 .It Pa /etc/sshd_config
791 Contains configuration data for
793 This file should be writable by root only, but it is recommended
794 (though not necessary) that it be world-readable.
795 .It Pa /etc/ssh_host_key
796 Contains the private part of the host key.
797 This file should only be owned by root, readable only by root, and not
798 accessible to others.
801 does not start if this file is group/world-accessible.
802 .It Pa /etc/ssh_host_key.pub
803 Contains the public part of the host key.
804 This file should be world-readable but writable only by
806 Its contents should match the private part.
808 really used for anything; it is only provided for the convenience of
809 the user so its contents can be copied to known hosts files.
810 These two files are created using
812 .It Pa /var/run/sshd.pid
813 Contains the process ID of the
815 listening for connections (if there are several daemons running
816 concurrently for different ports, this contains the pid of the one
818 The contents of this file are not sensitive; it can be world-readable.
819 .It Pa $HOME/.ssh/authorized_keys
820 Lists the RSA keys that can be used to log into the user's account.
821 This file must be readable by root (which may on some machines imply
822 it being world-readable if the user's home directory resides on an NFS
824 It is recommended that it not be accessible by others.
825 The format of this file is described above.
826 Users will place the contents of their
828 files into this file, as described in
830 .It Pa $HOME/.ssh/authorized_keys2
831 Lists the DSA keys that can be used to log into the user's account.
832 This file must be readable by root (which may on some machines imply
833 it being world-readable if the user's home directory resides on an NFS
835 It is recommended that it not be accessible by others.
836 The format of this file is described above.
837 Users will place the contents of their
839 files into this file, as described in
841 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
842 These files are consulted when using rhosts with RSA host
843 authentication to check the public key of the host.
844 The key must be listed in one of these files to be accepted.
845 The client uses the same files
846 to verify that the remote host is the one we intended to connect.
847 These files should be writable only by root/the owner.
848 .Pa /etc/ssh_known_hosts
849 should be world-readable, and
850 .Pa $HOME/.ssh/known_hosts
851 can but need not be world-readable.
855 refuses to let anyone except root log in.
856 The contents of the file
857 are displayed to anyone trying to log in, and non-root connections are
859 The file should be world-readable.
860 .It Pa /etc/hosts.allow, /etc/hosts.deny
863 support, tcp-wrappers access controls may be defined here as described in
866 This file contains host-username pairs, separated by a space, one per
868 The given user on the corresponding host is permitted to log in
870 The same file is used by rlogind and rshd.
872 be writable only by the user; it is recommended that it not be
873 accessible by others.
875 If is also possible to use netgroups in the file.
877 name may be of the form +@groupname to specify all hosts or all users
881 this file is exactly the same as for
883 However, this file is
884 not used by rlogin and rshd, so using this permits access using SSH only.
886 This file is used during
889 In the simplest form, this file contains host names, one per line.
891 those hosts are permitted to log in without a password, provided they
892 have the same user name on both machines.
893 The host name may also be
894 followed by a user name; such users are permitted to log in as
896 user on this machine (except root).
897 Additionally, the syntax
899 can be used to specify netgroups.
900 Negated entries start with
903 If the client host/user is successfully matched in this file, login is
904 automatically permitted provided the client and server user names are the
906 Additionally, successful RSA host authentication is normally required.
907 This file must be writable only by root; it is recommended
908 that it be world-readable.
910 .Sy "Warning: It is almost never a good idea to use user names in"
912 Beware that it really means that the named user(s) can log in as
914 which includes bin, daemon, adm, and other accounts that own critical
915 binaries and directories.
916 Using a user name practically grants the user root access.
917 The only valid use for user names that I can think
918 of is in negative entries.
920 Note that this warning also applies to rsh/rlogin.
921 .It Pa /etc/shosts.equiv
922 This is processed exactly as
923 .Pa /etc/hosts.equiv .
924 However, this file may be useful in environments that want to run both
926 .It Pa $HOME/.ssh/environment
927 This file is read into the environment at login (if it exists).
928 It can only contain empty lines, comment lines (that start with
930 and assignment lines of the form name=value.
931 The file should be writable
932 only by the user; it need not be readable by anyone else.
934 If this file exists, it is run with /bin/sh after reading the
935 environment files but before starting the user's shell or command.
936 If X11 spoofing is in use, this will receive the "proto cookie" pair in
944 The primary purpose of this file is to run any initialization routines
945 which may be needed before the user's home directory becomes
946 accessible; AFS is a particular example of such an environment.
948 This file will probably contain some initialization code followed by
949 something similar to: "if read proto cookie; then echo add $DISPLAY
950 $proto $cookie | xauth -q -; fi".
952 If this file does not exist,
955 does not exist either, xauth is used to store the cookie.
957 This file should be writable only by the user, and need not be
958 readable by anyone else.
962 This can be used to specify
963 machine-specific login-time initializations globally.
964 This file should be writable only by root, and should be world-readable.
968 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
969 but with bugs removed and newer features re-added.
971 1.2.12 release, newer versions of the original ssh bore successively
972 more restrictive licenses, and thus demand for a free version was born.
974 This version of OpenSSH
977 has all components of a restrictive nature (i.e., patents)
978 directly removed from the source code; any licensed or patented components
982 has been updated to support SSH protocol 1.5 and 2, making it compatible with
983 all other SSH clients and servers.
985 contains added support for
987 authentication and ticket passing.
989 supports one-time password authentication with
993 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
994 Niels Provos, Theo de Raadt, and Dug Song.
996 The support for SSH protocol 2 was written by Markus Friedl.