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 1.172 2002/03/18 17:59:09 provos 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 .Cm UsePrivilegeSeparation
844 is specified, it will be disabled after authentication.
845 .It Cm UsePrivilegeSeparation
848 separated privileges by creating an unprivileged child process
849 to deal with incoming network traffic. After successful authentication,
850 another process will be created that has the privilege of the authenticated
851 user. The goal of privilege separation is to prevent privilege
852 escalation by containing any corruption within the unprivileged processes.
855 .It Cm VerifyReverseMapping
858 should try to verify the remote host name and check that
859 the resolved host name for the remote IP address maps back to the
860 very same IP address.
863 .It Cm X11DisplayOffset
864 Specifies the first display number available for
869 from interfering with real X11 servers.
872 Specifies whether X11 forwarding is permitted.
875 Note that disabling X11 forwarding does not improve security in any
876 way, as users can always install their own forwarders.
877 X11 forwarding is automatically disabled if
880 .It Cm X11UseLocalhost
883 should bind the X11 forwarding server to the loopback address or to
884 the wildcard address. By default,
886 binds the forwarding server to the loopback address and sets the
889 environment variable to
891 This prevents remote hosts from connecting to the fake display.
892 However, some older X11 clients may not function with this
897 to specify that the forwarding server should be bound to the wildcard
906 Specifies the location of the
910 .Pa /usr/X11R6/bin/xauth .
915 command-line arguments and configuration file options that specify time
916 may be expressed using a sequence of the form:
918 .Ar time Oo Ar qualifier Oc ,
922 is a positive integer value and
924 is one of the following:
926 .Bl -tag -width Ds -compact -offset indent
941 Each member of the sequence is added together to calculate
942 the total time value.
944 Time format examples:
946 .Bl -tag -width Ds -compact -offset indent
948 600 seconds (10 minutes)
952 1 hour 30 minutes (90 minutes)
955 When a user successfully logs in,
958 .Bl -enum -offset indent
960 If the login is on a tty, and no command has been specified,
961 prints last login time and
963 (unless prevented in the configuration file or by
964 .Pa $HOME/.hushlogin ;
969 If the login is on a tty, records login time.
973 if it exists, prints contents and quits
976 Changes to run with normal user privileges.
978 Sets up basic environment.
981 .Pa $HOME/.ssh/environment
984 Changes to user's home directory.
988 exists, runs it; else if
991 it; otherwise runs xauth.
994 files are given the X11
995 authentication protocol and cookie in standard input.
997 Runs user's shell or command.
999 .Sh AUTHORIZED_KEYS FILE FORMAT
1000 .Pa $HOME/.ssh/authorized_keys
1001 is the default file that lists the public keys that are
1002 permitted for RSA authentication in protocol version 1
1003 and for public key authentication (PubkeyAuthentication)
1004 in protocol version 2.
1005 .Cm AuthorizedKeysFile
1006 may be used to specify an alternative file.
1008 Each line of the file contains one
1009 key (empty lines and lines starting with a
1013 Each RSA public key consists of the following fields, separated by
1014 spaces: options, bits, exponent, modulus, comment.
1015 Each protocol version 2 public key consists of:
1016 options, keytype, base64 encoded key, comment.
1018 are optional; its presence is determined by whether the line starts
1019 with a number or not (the option field never starts with a number).
1020 The bits, exponent, modulus and comment fields give the RSA key for
1021 protocol version 1; the
1022 comment field is not used for anything (but may be convenient for the
1023 user to identify the key).
1024 For protocol version 2 the keytype is
1029 Note that lines in this file are usually several hundred bytes long
1030 (because of the size of the RSA key modulus).
1031 You don't want to type them in; instead, copy the
1038 The options (if present) consist of comma-separated option
1040 No spaces are permitted, except within double quotes.
1041 The following option specifications are supported (note
1042 that option keywords are case-insensitive):
1044 .It Cm from="pattern-list"
1045 Specifies that in addition to RSA authentication, the canonical name
1046 of the remote host must be present in the comma-separated list of
1051 serve as wildcards).
1052 The list may also contain
1053 patterns negated by prefixing them with
1055 if the canonical host name matches a negated pattern, the key is not accepted.
1057 of this option is to optionally increase security: RSA authentication
1058 by itself does not trust the network or name servers or anything (but
1059 the key); however, if somebody somehow steals the key, the key
1060 permits an intruder to log in from anywhere in the world.
1061 This additional option makes using a stolen key more difficult (name
1062 servers and/or routers would have to be compromised in addition to
1064 .It Cm command="command"
1065 Specifies that the command is executed whenever this key is used for
1067 The command supplied by the user (if any) is ignored.
1068 The command is run on a pty if the client requests a pty;
1069 otherwise it is run without a tty.
1070 If a 8-bit clean channel is required,
1071 one must not request a pty or should specify
1073 A quote may be included in the command by quoting it with a backslash.
1074 This option might be useful
1075 to restrict certain RSA keys to perform just a specific operation.
1076 An example might be a key that permits remote backups but nothing else.
1077 Note that the client may specify TCP/IP and/or X11
1078 forwarding unless they are explicitly prohibited.
1079 Note that this option applies to shell, command or subsystem execution.
1080 .It Cm environment="NAME=value"
1081 Specifies that the string is to be added to the environment when
1082 logging in using this key.
1083 Environment variables set this way
1084 override other default environment values.
1085 Multiple options of this type are permitted.
1086 This option is automatically disabled if
1089 .It Cm no-port-forwarding
1090 Forbids TCP/IP forwarding when this key is used for authentication.
1091 Any port forward requests by the client will return an error.
1092 This might be used, e.g., in connection with the
1095 .It Cm no-X11-forwarding
1096 Forbids X11 forwarding when this key is used for authentication.
1097 Any X11 forward requests by the client will return an error.
1098 .It Cm no-agent-forwarding
1099 Forbids authentication agent forwarding when this key is used for
1102 Prevents tty allocation (a request to allocate a pty will fail).
1103 .It Cm permitopen="host:port"
1106 port forwarding such that it may only connect to the specified host and
1108 IPv6 addresses can be specified with an alternative syntax:
1112 options may be applied separated by commas. No pattern matching is
1113 performed on the specified hostnames, they must be literal domains or
1117 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1119 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1121 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1123 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1124 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1126 .Pa /etc/ssh/ssh_known_hosts ,
1128 .Pa $HOME/.ssh/known_hosts
1129 files contain host public keys for all known hosts.
1130 The global file should
1131 be prepared by the administrator (optional), and the per-user file is
1132 maintained automatically: whenever the user connects from an unknown host
1133 its key is added to the per-user file.
1135 Each line in these files contains the following fields: hostnames,
1136 bits, exponent, modulus, comment.
1137 The fields are separated by spaces.
1139 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1140 wildcards); each pattern in turn is matched against the canonical host
1141 name (when authenticating a client) or against the user-supplied
1142 name (when authenticating a server).
1143 A pattern may also be preceded by
1145 to indicate negation: if the host name matches a negated
1146 pattern, it is not accepted (by that line) even if it matched another
1147 pattern on the line.
1149 Bits, exponent, and modulus are taken directly from the RSA host key; they
1150 can be obtained, e.g., from
1151 .Pa /etc/ssh/ssh_host_key.pub .
1152 The optional comment field continues to the end of the line, and is not used.
1156 and empty lines are ignored as comments.
1158 When performing host authentication, authentication is accepted if any
1159 matching line has the proper key.
1160 It is thus permissible (but not
1161 recommended) to have several lines or different host keys for the same
1163 This will inevitably happen when short forms of host names
1164 from different domains are put in the file.
1166 that the files contain conflicting information; authentication is
1167 accepted if valid information can be found from either file.
1169 Note that the lines in these files are typically hundreds of characters
1170 long, and you definitely don't want to type in the host keys by hand.
1171 Rather, generate them by a script
1173 .Pa /etc/ssh/ssh_host_key.pub
1174 and adding the host names at the front.
1177 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1178 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1182 .It Pa /etc/ssh/sshd_config
1183 Contains configuration data for
1185 This file should be writable by root only, but it is recommended
1186 (though not necessary) that it be world-readable.
1187 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1188 These three files contain the private parts of the host keys.
1189 These files should only be owned by root, readable only by root, and not
1190 accessible to others.
1193 does not start if this file is group/world-accessible.
1194 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1195 These three files contain the public parts of the host keys.
1196 These files should be world-readable but writable only by
1198 Their contents should match the respective private parts.
1200 really used for anything; they are provided for the convenience of
1201 the user so their contents can be copied to known hosts files.
1202 These files are created using
1205 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1206 .It Pa /var/run/sshd.pid
1207 Contains the process ID of the
1209 listening for connections (if there are several daemons running
1210 concurrently for different ports, this contains the pid of the one
1212 The content of this file is not sensitive; it can be world-readable.
1213 .It Pa $HOME/.ssh/authorized_keys
1214 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1215 This file must be readable by root (which may on some machines imply
1216 it being world-readable if the user's home directory resides on an NFS
1218 It is recommended that it not be accessible by others.
1219 The format of this file is described above.
1220 Users will place the contents of their
1225 files into this file, as described in
1227 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1228 These files are consulted when using rhosts with RSA host
1229 authentication or protocol version 2 hostbased authentication
1230 to check the public key of the host.
1231 The key must be listed in one of these files to be accepted.
1232 The client uses the same files
1233 to verify that it is connecting to the correct remote host.
1234 These files should be writable only by root/the owner.
1235 .Pa /etc/ssh/ssh_known_hosts
1236 should be world-readable, and
1237 .Pa $HOME/.ssh/known_hosts
1238 can but need not be world-readable.
1240 If this file exists,
1242 refuses to let anyone except root log in.
1243 The contents of the file
1244 are displayed to anyone trying to log in, and non-root connections are
1246 The file should be world-readable.
1247 .It Pa /etc/hosts.allow, /etc/hosts.deny
1248 Access controls that should be enforced by tcp-wrappers are defined here.
1249 Further details are described in
1250 .Xr hosts_access 5 .
1251 .It Pa $HOME/.rhosts
1252 This file contains host-username pairs, separated by a space, one per
1254 The given user on the corresponding host is permitted to log in
1256 The same file is used by rlogind and rshd.
1258 be writable only by the user; it is recommended that it not be
1259 accessible by others.
1261 If is also possible to use netgroups in the file.
1263 name may be of the form +@groupname to specify all hosts or all users
1265 .It Pa $HOME/.shosts
1267 this file is exactly the same as for
1269 However, this file is
1270 not used by rlogin and rshd, so using this permits access using SSH only.
1271 .It Pa /etc/hosts.equiv
1272 This file is used during
1275 In the simplest form, this file contains host names, one per line.
1277 those hosts are permitted to log in without a password, provided they
1278 have the same user name on both machines.
1279 The host name may also be
1280 followed by a user name; such users are permitted to log in as
1282 user on this machine (except root).
1283 Additionally, the syntax
1285 can be used to specify netgroups.
1286 Negated entries start with
1289 If the client host/user is successfully matched in this file, login is
1290 automatically permitted provided the client and server user names are the
1292 Additionally, successful RSA host authentication is normally required.
1293 This file must be writable only by root; it is recommended
1294 that it be world-readable.
1296 .Sy "Warning: It is almost never a good idea to use user names in"
1298 Beware that it really means that the named user(s) can log in as
1300 which includes bin, daemon, adm, and other accounts that own critical
1301 binaries and directories.
1302 Using a user name practically grants the user root access.
1303 The only valid use for user names that I can think
1304 of is in negative entries.
1306 Note that this warning also applies to rsh/rlogin.
1307 .It Pa /etc/shosts.equiv
1308 This is processed exactly as
1309 .Pa /etc/hosts.equiv .
1310 However, this file may be useful in environments that want to run both
1312 .It Pa $HOME/.ssh/environment
1313 This file is read into the environment at login (if it exists).
1314 It can only contain empty lines, comment lines (that start with
1316 and assignment lines of the form name=value.
1317 The file should be writable
1318 only by the user; it need not be readable by anyone else.
1319 .It Pa $HOME/.ssh/rc
1320 If this file exists, it is run with /bin/sh after reading the
1321 environment files but before starting the user's shell or command.
1322 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1330 The primary purpose of this file is to run any initialization routines
1331 which may be needed before the user's home directory becomes
1332 accessible; AFS is a particular example of such an environment.
1334 This file will probably contain some initialization code followed by
1335 something similar to:
1337 if read proto cookie; then
1338 echo add $DISPLAY $proto $cookie | xauth -q -
1342 If this file does not exist,
1345 does not exist either, xauth is used to store the cookie.
1347 This file should be writable only by the user, and need not be
1348 readable by anyone else.
1349 .It Pa /etc/ssh/sshrc
1352 This can be used to specify
1353 machine-specific login-time initializations globally.
1354 This file should be writable only by root, and should be world-readable.
1357 OpenSSH is a derivative of the original and free
1358 ssh 1.2.12 release by Tatu Ylonen.
1359 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1360 Theo de Raadt and Dug Song
1361 removed many bugs, re-added newer features and
1363 Markus Friedl contributed the support for SSH
1364 protocol versions 1.5 and 2.0.
1365 Niels Provos and Markus Friedl contributed support
1366 for privilege separation.
1383 .%T "SSH Protocol Architecture"
1384 .%N draft-ietf-secsh-architecture-09.txt
1386 .%O work in progress material
1392 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1393 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1395 .%O work in progress material