3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: sshd.8,v 1.170 2002/02/28 20:46:10 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
57 (SSH Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256 bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
123 are disabled (thus completely disabling
129 .Ss SSH protocol version 2
131 Version 2 works similarly:
132 Each host has a host-specific key (RSA or DSA) used to identify the host.
133 However, when the daemon starts, it does not generate a server key.
134 Forward security is provided through a Diffie-Hellman key agreement.
135 This key agreement results in a shared session key.
137 The rest of the session is encrypted using a symmetric cipher, currently
138 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
139 The client selects the encryption algorithm
140 to use from those offered by the server.
141 Additionally, session integrity is provided
142 through a cryptographic message authentication code
143 (hmac-sha1 or hmac-md5).
145 Protocol version 2 provides a public key based
146 user (PubkeyAuthentication) or
147 client host (HostbasedAuthentication) authentication method,
148 conventional password authentication and challenge response based methods.
150 .Ss Command execution and data forwarding
152 If the client successfully authenticates itself, a dialog for
153 preparing the session is entered.
154 At this time the client may request
155 things like allocating a pseudo-tty, forwarding X11 connections,
156 forwarding TCP/IP connections, or forwarding the authentication agent
157 connection over the secure channel.
159 Finally, the client either requests a shell or execution of a command.
160 The sides then enter session mode.
161 In this mode, either side may send
162 data at any time, and such data is forwarded to/from the shell or
163 command on the server side, and the user terminal in the client side.
165 When the user program terminates and all forwarded X11 and other
166 connections have been closed, the server sends command exit status to
167 the client, and both sides exit.
170 can be configured using command-line options or a configuration
172 Command-line options override values specified in the
176 rereads its configuration file when it receives a hangup signal,
178 by executing itself with the name it was started as, i.e.,
181 The options are as follows:
184 Specifies the number of bits in the ephemeral protocol version 1
185 server key (default 768).
188 The server sends verbose debug output to the system
189 log, and does not put itself in the background.
190 The server also will not fork and will only process one connection.
191 This option is only intended for debugging for the server.
192 Multiple -d options increase the debugging level.
195 When this option is specified,
197 will send the output to the standard error instead of the system log.
198 .It Fl f Ar configuration_file
199 Specifies the name of the configuration file.
201 .Pa /etc/ssh/sshd_config .
203 refuses to start if there is no configuration file.
204 .It Fl g Ar login_grace_time
205 Gives the grace time for clients to authenticate themselves (default
207 If the client fails to authenticate the user within
208 this many seconds, the server disconnects and exits.
209 A value of zero indicates no limit.
210 .It Fl h Ar host_key_file
211 Specifies a file from which a host key is read.
212 This option must be given if
214 is not run as root (as the normal
215 host key files are normally not readable by anyone but root).
217 .Pa /etc/ssh/ssh_host_key
218 for protocol version 1, and
219 .Pa /etc/ssh/ssh_host_rsa_key
221 .Pa /etc/ssh/ssh_host_dsa_key
222 for protocol version 2.
223 It is possible to have multiple host key files for
224 the different protocol versions and host key algorithms.
228 is being run from inetd.
231 from inetd because it needs to generate the server key before it can
232 respond to the client, and this may take tens of seconds.
233 Clients would have to wait too long if the key was regenerated every time.
234 However, with small key sizes (e.g., 512) using
238 .It Fl k Ar key_gen_time
239 Specifies how often the ephemeral protocol version 1 server key is
240 regenerated (default 3600 seconds, or one hour).
241 The motivation for regenerating the key fairly
242 often is that the key is not stored anywhere, and after about an hour,
243 it becomes impossible to recover the key for decrypting intercepted
244 communications even if the machine is cracked into or physically
246 A value of zero indicates that the key will never be regenerated.
248 Can be used to give options in the format used in the configuration file.
249 This is useful for specifying options for which there is no separate
252 Specifies the port on which the server listens for connections
254 Multiple port options are permitted.
255 Ports specified in the configuration file are ignored when a
256 command-line port is specified.
259 Nothing is sent to the system log.
260 Normally the beginning,
261 authentication, and termination of each connection is logged.
264 Only check the validity of the configuration file and sanity of the keys.
265 This is useful for updating
267 reliably as configuration options may change.
269 This option is used to specify the size of the field
272 structure that holds the remote host name.
273 If the resolved host name is longer than
275 the dotted decimal value will be used instead.
276 This allows hosts with very long host names that
277 overflow this field to still be uniquely identified.
280 indicates that only dotted decimal addresses
281 should be put into the
285 is also be used to prevent
287 from making DNS requests unless the authentication
288 mechanism or configuration requires it.
289 Authentication mechanisms that may require DNS include
290 .Cm RhostsAuthentication ,
291 .Cm RhostsRSAAuthentication ,
292 .Cm HostbasedAuthentication
294 .Cm from="pattern-list"
295 option in a key file.
296 Configuration options that require DNS include using a
302 When this option is specified
304 will not detach and does not become a daemon.
305 This allows easy monitoring of
310 to use IPv4 addresses only.
314 to use IPv6 addresses only.
316 .Sh CONFIGURATION FILE
318 reads configuration data from
319 .Pa /etc/ssh/sshd_config
320 (or the file specified with
322 on the command line).
323 The file contains keyword-argument pairs, one per line.
326 and empty lines are interpreted as comments.
329 keywords and their meanings are as follows (note that
330 keywords are case-insensitive and arguments are case-sensitive):
332 .It Cm AFSTokenPassing
333 Specifies whether an AFS token may be forwarded to the server.
337 This keyword can be followed by a list of group name patterns, separated
339 If specified, login is allowed only for users whose primary
340 group or supplementary group list matches one of the patterns.
345 wildcards in the patterns.
346 Only group names are valid; a numerical group ID is not recognized.
347 By default, login is allowed for all groups.
349 .It Cm AllowTcpForwarding
350 Specifies whether TCP forwarding is permitted.
353 Note that disabling TCP forwarding does not improve security unless
354 users are also denied shell access, as they can always install their
358 This keyword can be followed by a list of user name patterns, separated
360 If specified, login is allowed only for users names that
361 match one of the patterns.
366 wildcards in the patterns.
367 Only user names are valid; a numerical user ID is not recognized.
368 By default, login is allowed for all users.
369 If the pattern takes the form USER@HOST then USER and HOST
370 are separately checked, restricting logins to particular
371 users from particular hosts.
373 .It Cm AuthorizedKeysFile
374 Specifies the file that contains the public keys that can be used
375 for user authentication.
376 .Cm AuthorizedKeysFile
377 may contain tokens of the form %T which are substituted during connection
378 set-up. The following tokens are defined: %% is replaced by a literal '%',
379 %h is replaced by the home directory of the user being authenticated and
380 %u is replaced by the username of that user.
382 .Cm AuthorizedKeysFile
383 is taken to be an absolute path or one relative to the user's home
386 .Dq .ssh/authorized_keys .
388 In some jurisdictions, sending a warning message before authentication
389 may be relevant for getting legal protection.
390 The contents of the specified file are sent to the remote user before
391 authentication is allowed.
392 This option is only available for protocol version 2.
394 .It Cm ChallengeResponseAuthentication
395 Specifies whether challenge response authentication is allowed.
396 All authentication styles from
402 Specifies the ciphers allowed for protocol version 2.
403 Multiple ciphers must be comma-separated.
407 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
408 aes192-cbc,aes256-cbc''
410 .It Cm ClientAliveInterval
411 Sets a timeout interval in seconds after which if no data has been received
414 will send a message through the encrypted
415 channel to request a response from the client.
417 is 0, indicating that these messages will not be sent to the client.
418 This option applies to protocol version 2 only.
419 .It Cm ClientAliveCountMax
420 Sets the number of client alive messages (see above) which may be
423 receiving any messages back from the client. If this threshold is
424 reached while client alive messages are being sent,
426 will disconnect the client, terminating the session. It is important
427 to note that the use of client alive messages is very different from
429 (below). The client alive messages are sent through the
430 encrypted channel and therefore will not be spoofable. The TCP keepalive
433 is spoofable. The client alive mechanism is valuable when the client or
434 server depend on knowing when a connection has become inactive.
436 The default value is 3. If
437 .Cm ClientAliveInterval
438 (above) is set to 15, and
439 .Cm ClientAliveCountMax
440 is left at the default, unresponsive ssh clients
441 will be disconnected after approximately 45 seconds.
443 This keyword can be followed by a list of group name patterns, separated
445 Login is disallowed for users whose primary group or supplementary
446 group list matches one of the patterns.
451 wildcards in the patterns.
452 Only group names are valid; a numerical group ID is not recognized.
453 By default, login is allowed for all groups.
456 This keyword can be followed by a list of user name patterns, separated
458 Login is disallowed for user names that match one of the patterns.
462 can be used as wildcards in the patterns.
463 Only user names are valid; a numerical user ID is not recognized.
464 By default, login is allowed for all users.
465 If the pattern takes the form USER@HOST then USER and HOST
466 are separately checked, restricting logins to particular
467 users from particular hosts.
469 Specifies whether remote hosts are allowed to connect to ports
470 forwarded for the client.
473 binds remote port forwardings to the loopback addresss. This
474 prevents other remote hosts from connecting to forwarded ports.
476 can be used to specify that
478 should bind remote port forwardings to the wildcard address,
479 thus allowing remote hosts to connect to forwarded ports.
486 .It Cm HostbasedAuthentication
487 Specifies whether rhosts or /etc/hosts.equiv authentication together
488 with successful public key client host authentication is allowed
489 (hostbased authentication).
490 This option is similar to
491 .Cm RhostsRSAAuthentication
492 and applies to protocol version 2 only.
496 Specifies a file containing a private host key
499 .Pa /etc/ssh/ssh_host_key
500 for protocol version 1, and
501 .Pa /etc/ssh/ssh_host_rsa_key
503 .Pa /etc/ssh/ssh_host_dsa_key
504 for protocol version 2.
507 will refuse to use a file if it is group/world-accessible.
508 It is possible to have multiple host key files.
510 keys are used for version 1 and
514 are used for version 2 of the SSH protocol.
520 files will not be used in
521 .Cm RhostsAuthentication ,
522 .Cm RhostsRSAAuthentication
524 .Cm HostbasedAuthentication .
528 .Pa /etc/shosts.equiv
532 .It Cm IgnoreUserKnownHosts
535 should ignore the user's
536 .Pa $HOME/.ssh/known_hosts
538 .Cm RhostsRSAAuthentication
540 .Cm HostbasedAuthentication .
544 Specifies whether the system should send TCP keepalive messages to the
546 If they are sent, death of the connection or crash of one
547 of the machines will be properly noticed.
548 However, this means that
549 connections will die if the route is down temporarily, and some people
551 On the other hand, if keepalives are not sent,
552 sessions may hang indefinitely on the server, leaving
554 users and consuming server resources.
558 (to send keepalives), and the server will notice
559 if the network goes down or the client host crashes.
560 This avoids infinitely hanging sessions.
562 To disable keepalives, the value should be set to
564 .It Cm KerberosAuthentication
565 Specifies whether Kerberos authentication is allowed.
566 This can be in the form of a Kerberos ticket, or if
567 .Cm PasswordAuthentication
568 is yes, the password provided by the user will be validated through
570 To use this option, the server needs a
571 Kerberos servtab which allows the verification of the KDC's identity.
574 .It Cm KerberosOrLocalPasswd
575 If set then if password authentication through Kerberos fails then
576 the password will be validated via any additional local mechanism
581 .It Cm KerberosTgtPassing
582 Specifies whether a Kerberos TGT may be forwarded to the server.
585 as this only works when the Kerberos KDC is actually an AFS kaserver.
586 .It Cm KerberosTicketCleanup
587 Specifies whether to automatically destroy the user's ticket cache
591 .It Cm KeyRegenerationInterval
592 In protocol version 1, the ephemeral server key is automatically regenerated
593 after this many seconds (if it has been used).
594 The purpose of regeneration is to prevent
595 decrypting captured sessions by later breaking into the machine and
597 The key is never stored anywhere.
598 If the value is 0, the key is never regenerated.
599 The default is 3600 (seconds).
601 Specifies the local addresses
604 The following forms may be used:
606 .Bl -item -offset indent -compact
610 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
615 .Ar host No | Ar IPv4_addr No : Ar port
621 .Ar host No | Ar IPv6_addr Oc : Ar port
629 will listen on the address and all prior
631 options specified. The default is to listen on all local
634 options are permitted. Additionally, any
636 options must precede this option for non port qualified addresses.
637 .It Cm LoginGraceTime
638 The server disconnects after this time if the user has not
639 successfully logged in.
640 If the value is 0, there is no time limit.
641 The default is 600 (seconds).
643 Gives the verbosity level that is used when logging messages from
645 The possible values are:
646 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
647 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
648 and DEBUG3 each specify higher levels of debugging output.
649 Logging with a DEBUG level violates the privacy of users
650 and is not recommended.
652 Specifies the available MAC (message authentication code) algorithms.
653 The MAC algorithm is used in protocol version 2
654 for data integrity protection.
655 Multiple algorithms must be comma-separated.
657 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
659 Specifies the maximum number of concurrent unauthenticated connections to the
662 Additional connections will be dropped until authentication succeeds or the
664 expires for a connection.
667 Alternatively, random early drop can be enabled by specifying
668 the three colon separated values
672 will refuse connection attempts with a probability of
675 if there are currently
678 unauthenticated connections.
679 The probability increases linearly and all connection attempts
680 are refused if the number of unauthenticated connections reaches
683 .It Cm PAMAuthenticationViaKbdInt
684 Specifies whether PAM challenge response authentication is allowed. This
685 allows the use of most PAM challenge response authentication modules, but
686 it will allow password authentication regardless of whether
687 .Cm PasswordAuthentication
691 .It Cm PasswordAuthentication
692 Specifies whether password authentication is allowed.
695 .It Cm PermitEmptyPasswords
696 When password authentication is allowed, it specifies whether the
697 server allows login to accounts with empty password strings.
700 .It Cm PermitRootLogin
701 Specifies whether root can login using
705 .Dq without-password ,
706 .Dq forced-commands-only
712 If this option is set to
714 password authentication is disabled for root.
716 If this option is set to
717 .Dq forced-commands-only
718 root login with public key authentication will be allowed,
721 option has been specified
722 (which may be useful for taking remote backups even if root login is
723 normally not allowed). All other authentication methods are disabled
726 If this option is set to
728 root is not allowed to login.
730 Specifies the file that contains the process identifier of the
734 .Pa /var/run/sshd.pid .
736 Specifies the port number that
740 Multiple options of this type are permitted.
746 should print the date and time when the user last logged in.
754 when a user logs in interactively.
755 (On some systems it is also printed by the shell,
761 Specifies the protocol versions
764 The possible values are
768 Multiple versions must be comma-separated.
771 .It Cm PubkeyAuthentication
772 Specifies whether public key authentication is allowed.
775 Note that this option applies to protocol version 2 only.
776 .It Cm RhostsAuthentication
777 Specifies whether authentication using rhosts or /etc/hosts.equiv
779 Normally, this method should not be permitted because it is insecure.
780 .Cm RhostsRSAAuthentication
782 instead, because it performs RSA-based host authentication in addition
783 to normal rhosts or /etc/hosts.equiv authentication.
786 This option applies to protocol version 1 only.
787 .It Cm RhostsRSAAuthentication
788 Specifies whether rhosts or /etc/hosts.equiv authentication together
789 with successful RSA host authentication is allowed.
792 This option applies to protocol version 1 only.
793 .It Cm RSAAuthentication
794 Specifies whether pure RSA authentication is allowed.
797 This option applies to protocol version 1 only.
799 Defines the number of bits in the ephemeral protocol version 1 server key.
800 The minimum value is 512, and the default is 768.
804 should check file modes and ownership of the
805 user's files and home directory before accepting login.
806 This is normally desirable because novices sometimes accidentally leave their
807 directory or files world-writable.
811 Configures an external subsystem (e.g., file transfer daemon).
812 Arguments should be a subsystem name and a command to execute upon subsystem
818 file transfer subsystem.
819 By default no subsystems are defined.
820 Note that this option applies to protocol version 2 only.
821 .It Cm SyslogFacility
822 Gives the facility code that is used when logging messages from
824 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
825 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
830 is used for interactive login sessions.
835 is never used for remote command execution.
836 Note also, that if this is enabled,
838 will be disabled because
840 does not know how to handle
843 .It Cm VerifyReverseMapping
846 should try to verify the remote host name and check that
847 the resolved host name for the remote IP address maps back to the
848 very same IP address.
851 .It Cm X11DisplayOffset
852 Specifies the first display number available for
857 from interfering with real X11 servers.
860 Specifies whether X11 forwarding is permitted.
863 Note that disabling X11 forwarding does not improve security in any
864 way, as users can always install their own forwarders.
865 X11 forwarding is automatically disabled if
868 .It Cm X11UseLocalhost
871 should bind the X11 forwarding server to the loopback address or to
872 the wildcard address. By default,
874 binds the forwarding server to the loopback address and sets the
877 environment variable to
879 This prevents remote hosts from connecting to the fake display.
880 However, some older X11 clients may not function with this
885 to specify that the forwarding server should be bound to the wildcard
894 Specifies the location of the
898 .Pa /usr/X11R6/bin/xauth .
903 command-line arguments and configuration file options that specify time
904 may be expressed using a sequence of the form:
906 .Ar time Oo Ar qualifier Oc ,
910 is a positive integer value and
912 is one of the following:
914 .Bl -tag -width Ds -compact -offset indent
929 Each member of the sequence is added together to calculate
930 the total time value.
932 Time format examples:
934 .Bl -tag -width Ds -compact -offset indent
936 600 seconds (10 minutes)
940 1 hour 30 minutes (90 minutes)
943 When a user successfully logs in,
946 .Bl -enum -offset indent
948 If the login is on a tty, and no command has been specified,
949 prints last login time and
951 (unless prevented in the configuration file or by
952 .Pa $HOME/.hushlogin ;
957 If the login is on a tty, records login time.
961 if it exists, prints contents and quits
964 Changes to run with normal user privileges.
966 Sets up basic environment.
969 .Pa $HOME/.ssh/environment
972 Changes to user's home directory.
976 exists, runs it; else if
979 it; otherwise runs xauth.
982 files are given the X11
983 authentication protocol and cookie in standard input.
985 Runs user's shell or command.
987 .Sh AUTHORIZED_KEYS FILE FORMAT
988 .Pa $HOME/.ssh/authorized_keys
989 is the default file that lists the public keys that are
990 permitted for RSA authentication in protocol version 1
991 and for public key authentication (PubkeyAuthentication)
992 in protocol version 2.
993 .Cm AuthorizedKeysFile
994 may be used to specify an alternative file.
996 Each line of the file contains one
997 key (empty lines and lines starting with a
1001 Each RSA public key consists of the following fields, separated by
1002 spaces: options, bits, exponent, modulus, comment.
1003 Each protocol version 2 public key consists of:
1004 options, keytype, base64 encoded key, comment.
1006 are optional; its presence is determined by whether the line starts
1007 with a number or not (the option field never starts with a number).
1008 The bits, exponent, modulus and comment fields give the RSA key for
1009 protocol version 1; the
1010 comment field is not used for anything (but may be convenient for the
1011 user to identify the key).
1012 For protocol version 2 the keytype is
1017 Note that lines in this file are usually several hundred bytes long
1018 (because of the size of the RSA key modulus).
1019 You don't want to type them in; instead, copy the
1026 The options (if present) consist of comma-separated option
1028 No spaces are permitted, except within double quotes.
1029 The following option specifications are supported (note
1030 that option keywords are case-insensitive):
1032 .It Cm from="pattern-list"
1033 Specifies that in addition to RSA authentication, the canonical name
1034 of the remote host must be present in the comma-separated list of
1039 serve as wildcards).
1040 The list may also contain
1041 patterns negated by prefixing them with
1043 if the canonical host name matches a negated pattern, the key is not accepted.
1045 of this option is to optionally increase security: RSA authentication
1046 by itself does not trust the network or name servers or anything (but
1047 the key); however, if somebody somehow steals the key, the key
1048 permits an intruder to log in from anywhere in the world.
1049 This additional option makes using a stolen key more difficult (name
1050 servers and/or routers would have to be compromised in addition to
1052 .It Cm command="command"
1053 Specifies that the command is executed whenever this key is used for
1055 The command supplied by the user (if any) is ignored.
1056 The command is run on a pty if the client requests a pty;
1057 otherwise it is run without a tty.
1058 If a 8-bit clean channel is required,
1059 one must not request a pty or should specify
1061 A quote may be included in the command by quoting it with a backslash.
1062 This option might be useful
1063 to restrict certain RSA keys to perform just a specific operation.
1064 An example might be a key that permits remote backups but nothing else.
1065 Note that the client may specify TCP/IP and/or X11
1066 forwarding unless they are explicitly prohibited.
1067 Note that this option applies to shell, command or subsystem execution.
1068 .It Cm environment="NAME=value"
1069 Specifies that the string is to be added to the environment when
1070 logging in using this key.
1071 Environment variables set this way
1072 override other default environment values.
1073 Multiple options of this type are permitted.
1074 This option is automatically disabled if
1077 .It Cm no-port-forwarding
1078 Forbids TCP/IP forwarding when this key is used for authentication.
1079 Any port forward requests by the client will return an error.
1080 This might be used, e.g., in connection with the
1083 .It Cm no-X11-forwarding
1084 Forbids X11 forwarding when this key is used for authentication.
1085 Any X11 forward requests by the client will return an error.
1086 .It Cm no-agent-forwarding
1087 Forbids authentication agent forwarding when this key is used for
1090 Prevents tty allocation (a request to allocate a pty will fail).
1091 .It Cm permitopen="host:port"
1094 port forwarding such that it may only connect to the specified host and
1096 IPv6 addresses can be specified with an alternative syntax:
1100 options may be applied separated by commas. No pattern matching is
1101 performed on the specified hostnames, they must be literal domains or
1105 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1107 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1109 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1111 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1112 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1114 .Pa /etc/ssh/ssh_known_hosts ,
1116 .Pa $HOME/.ssh/known_hosts
1117 files contain host public keys for all known hosts.
1118 The global file should
1119 be prepared by the administrator (optional), and the per-user file is
1120 maintained automatically: whenever the user connects from an unknown host
1121 its key is added to the per-user file.
1123 Each line in these files contains the following fields: hostnames,
1124 bits, exponent, modulus, comment.
1125 The fields are separated by spaces.
1127 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1128 wildcards); each pattern in turn is matched against the canonical host
1129 name (when authenticating a client) or against the user-supplied
1130 name (when authenticating a server).
1131 A pattern may also be preceded by
1133 to indicate negation: if the host name matches a negated
1134 pattern, it is not accepted (by that line) even if it matched another
1135 pattern on the line.
1137 Bits, exponent, and modulus are taken directly from the RSA host key; they
1138 can be obtained, e.g., from
1139 .Pa /etc/ssh/ssh_host_key.pub .
1140 The optional comment field continues to the end of the line, and is not used.
1144 and empty lines are ignored as comments.
1146 When performing host authentication, authentication is accepted if any
1147 matching line has the proper key.
1148 It is thus permissible (but not
1149 recommended) to have several lines or different host keys for the same
1151 This will inevitably happen when short forms of host names
1152 from different domains are put in the file.
1154 that the files contain conflicting information; authentication is
1155 accepted if valid information can be found from either file.
1157 Note that the lines in these files are typically hundreds of characters
1158 long, and you definitely don't want to type in the host keys by hand.
1159 Rather, generate them by a script
1161 .Pa /etc/ssh/ssh_host_key.pub
1162 and adding the host names at the front.
1165 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1166 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1170 .It Pa /etc/ssh/sshd_config
1171 Contains configuration data for
1173 This file should be writable by root only, but it is recommended
1174 (though not necessary) that it be world-readable.
1175 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1176 These three files contain the private parts of the host keys.
1177 These files should only be owned by root, readable only by root, and not
1178 accessible to others.
1181 does not start if this file is group/world-accessible.
1182 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1183 These three files contain the public parts of the host keys.
1184 These files should be world-readable but writable only by
1186 Their contents should match the respective private parts.
1188 really used for anything; they are provided for the convenience of
1189 the user so their contents can be copied to known hosts files.
1190 These files are created using
1193 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1194 .It Pa /var/run/sshd.pid
1195 Contains the process ID of the
1197 listening for connections (if there are several daemons running
1198 concurrently for different ports, this contains the pid of the one
1200 The content of this file is not sensitive; it can be world-readable.
1201 .It Pa $HOME/.ssh/authorized_keys
1202 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1203 This file must be readable by root (which may on some machines imply
1204 it being world-readable if the user's home directory resides on an NFS
1206 It is recommended that it not be accessible by others.
1207 The format of this file is described above.
1208 Users will place the contents of their
1213 files into this file, as described in
1215 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1216 These files are consulted when using rhosts with RSA host
1217 authentication or protocol version 2 hostbased authentication
1218 to check the public key of the host.
1219 The key must be listed in one of these files to be accepted.
1220 The client uses the same files
1221 to verify that it is connecting to the correct remote host.
1222 These files should be writable only by root/the owner.
1223 .Pa /etc/ssh/ssh_known_hosts
1224 should be world-readable, and
1225 .Pa $HOME/.ssh/known_hosts
1226 can but need not be world-readable.
1228 If this file exists,
1230 refuses to let anyone except root log in.
1231 The contents of the file
1232 are displayed to anyone trying to log in, and non-root connections are
1234 The file should be world-readable.
1235 .It Pa /etc/hosts.allow, /etc/hosts.deny
1236 Access controls that should be enforced by tcp-wrappers are defined here.
1237 Further details are described in
1238 .Xr hosts_access 5 .
1239 .It Pa $HOME/.rhosts
1240 This file contains host-username pairs, separated by a space, one per
1242 The given user on the corresponding host is permitted to log in
1244 The same file is used by rlogind and rshd.
1246 be writable only by the user; it is recommended that it not be
1247 accessible by others.
1249 If is also possible to use netgroups in the file.
1251 name may be of the form +@groupname to specify all hosts or all users
1253 .It Pa $HOME/.shosts
1255 this file is exactly the same as for
1257 However, this file is
1258 not used by rlogin and rshd, so using this permits access using SSH only.
1259 .It Pa /etc/hosts.equiv
1260 This file is used during
1263 In the simplest form, this file contains host names, one per line.
1265 those hosts are permitted to log in without a password, provided they
1266 have the same user name on both machines.
1267 The host name may also be
1268 followed by a user name; such users are permitted to log in as
1270 user on this machine (except root).
1271 Additionally, the syntax
1273 can be used to specify netgroups.
1274 Negated entries start with
1277 If the client host/user is successfully matched in this file, login is
1278 automatically permitted provided the client and server user names are the
1280 Additionally, successful RSA host authentication is normally required.
1281 This file must be writable only by root; it is recommended
1282 that it be world-readable.
1284 .Sy "Warning: It is almost never a good idea to use user names in"
1286 Beware that it really means that the named user(s) can log in as
1288 which includes bin, daemon, adm, and other accounts that own critical
1289 binaries and directories.
1290 Using a user name practically grants the user root access.
1291 The only valid use for user names that I can think
1292 of is in negative entries.
1294 Note that this warning also applies to rsh/rlogin.
1295 .It Pa /etc/shosts.equiv
1296 This is processed exactly as
1297 .Pa /etc/hosts.equiv .
1298 However, this file may be useful in environments that want to run both
1300 .It Pa $HOME/.ssh/environment
1301 This file is read into the environment at login (if it exists).
1302 It can only contain empty lines, comment lines (that start with
1304 and assignment lines of the form name=value.
1305 The file should be writable
1306 only by the user; it need not be readable by anyone else.
1307 .It Pa $HOME/.ssh/rc
1308 If this file exists, it is run with /bin/sh after reading the
1309 environment files but before starting the user's shell or command.
1310 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1318 The primary purpose of this file is to run any initialization routines
1319 which may be needed before the user's home directory becomes
1320 accessible; AFS is a particular example of such an environment.
1322 This file will probably contain some initialization code followed by
1323 something similar to:
1325 if read proto cookie; then
1326 echo add $DISPLAY $proto $cookie | xauth -q -
1330 If this file does not exist,
1333 does not exist either, xauth is used to store the cookie.
1335 This file should be writable only by the user, and need not be
1336 readable by anyone else.
1337 .It Pa /etc/ssh/sshrc
1340 This can be used to specify
1341 machine-specific login-time initializations globally.
1342 This file should be writable only by root, and should be world-readable.
1345 OpenSSH is a derivative of the original and free
1346 ssh 1.2.12 release by Tatu Ylonen.
1347 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1348 Theo de Raadt and Dug Song
1349 removed many bugs, re-added newer features and
1351 Markus Friedl contributed the support for SSH
1352 protocol versions 1.5 and 2.0.
1369 .%T "SSH Protocol Architecture"
1370 .%N draft-ietf-secsh-architecture-09.txt
1372 .%O work in progress material
1378 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1379 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1381 .%O work in progress material