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 1.165 2002/01/29 14:32:03 markus 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/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_host_key
218 for protocol version 1, and
219 .Pa /etc/ssh_host_rsa_key
221 .Pa /etc/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.
297 When this option is specified
299 will not detach and does not become a daemon.
300 This allows easy monitoring of
305 to use IPv4 addresses only.
309 to use IPv6 addresses only.
311 .Sh CONFIGURATION FILE
313 reads configuration data from
315 (or the file specified with
317 on the command line).
318 The file contains keyword-argument pairs, one per line.
321 and empty lines are interpreted as comments.
324 keywords and their meanings are as follows (note that
325 keywords are case-insensitive and arguments are case-sensitive):
327 .It Cm AFSTokenPassing
328 Specifies whether an AFS token may be forwarded to the server.
332 This keyword can be followed by a list of group name patterns, separated
334 If specified, login is allowed only for users whose primary
335 group or supplementary group list matches one of the patterns.
340 wildcards in the patterns.
341 Only group names are valid; a numerical group ID is not recognized.
342 By default, login is allowed for all groups.
344 .It Cm AllowTcpForwarding
345 Specifies whether TCP forwarding is permitted.
348 Note that disabling TCP forwarding does not improve security unless
349 users are also denied shell access, as they can always install their
353 This keyword can be followed by a list of user name patterns, separated
355 If specified, login is allowed only for users names that
356 match one of the patterns.
361 wildcards in the patterns.
362 Only user names are valid; a numerical user ID is not recognized.
363 By default, login is allowed for all users.
364 If the pattern takes the form USER@HOST then USER and HOST
365 are separately checked, restricting logins to particular
366 users from particular hosts.
368 .It Cm AuthorizedKeysFile
369 Specifies the file that contains the public keys that can be used
370 for user authentication.
371 .Cm AuthorizedKeysFile
372 may contain tokens of the form %T which are substituted during connection
373 set-up. The following tokens are defined: %% is replaced by a literal '%',
374 %h is replaced by the home directory of the user being authenticated and
375 %u is replaced by the username of that user.
377 .Cm AuthorizedKeysFile
378 is taken to be an absolute path or one relative to the user's home
381 .Dq .ssh/authorized_keys .
383 In some jurisdictions, sending a warning message before authentication
384 may be relevant for getting legal protection.
385 The contents of the specified file are sent to the remote user before
386 authentication is allowed.
387 This option is only available for protocol version 2.
389 .It Cm ChallengeResponseAuthentication
390 Specifies whether challenge response authentication is allowed.
391 All authentication styles from
397 Specifies the ciphers allowed for protocol version 2.
398 Multiple ciphers must be comma-separated.
402 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
403 aes192-cbc,aes256-cbc''
405 .It Cm ClientAliveInterval
406 Sets a timeout interval in seconds after which if no data has been received
409 will send a message through the encrypted
410 channel to request a response from the client.
412 is 0, indicating that these messages will not be sent to the client.
413 This option applies to protocol version 2 only.
414 .It Cm ClientAliveCountMax
415 Sets the number of client alive messages (see above) which may be
418 receiving any messages back from the client. If this threshold is
419 reached while client alive messages are being sent,
421 will disconnect the client, terminating the session. It is important
422 to note that the use of client alive messages is very different from
424 (below). The client alive messages are sent through the
425 encrypted channel and therefore will not be spoofable. The TCP keepalive
428 is spoofable. The client alive mechanism is valuable when the client or
429 server depend on knowing when a connection has become inactive.
431 The default value is 3. If
432 .Cm ClientAliveInterval
433 (above) is set to 15, and
434 .Cm ClientAliveCountMax
435 is left at the default, unresponsive ssh clients
436 will be disconnected after approximately 45 seconds.
438 This keyword can be followed by a list of group name patterns, separated
440 Login is disallowed for users whose primary group or supplementary
441 group list matches one of the patterns.
446 wildcards in the patterns.
447 Only group names are valid; a numerical group ID is not recognized.
448 By default, login is allowed for all groups.
451 This keyword can be followed by a list of user name patterns, separated
453 Login is disallowed for user names that match one of the patterns.
457 can be used as wildcards in the patterns.
458 Only user names are valid; a numerical user ID is not recognized.
459 By default, login is allowed for all users.
461 Specifies whether remote hosts are allowed to connect to ports
462 forwarded for the client.
465 binds remote port forwardings to the loopback addresss. This
466 prevents other remote hosts from connecting to forwarded ports.
468 can be used to specify that
470 should bind remote port forwardings to the wildcard address,
471 thus allowing remote hosts to connect to forwarded ports.
478 .It Cm HostbasedAuthentication
479 Specifies whether rhosts or /etc/hosts.equiv authentication together
480 with successful public key client host authentication is allowed
481 (hostbased authentication).
482 This option is similar to
483 .Cm RhostsRSAAuthentication
484 and applies to protocol version 2 only.
488 Specifies a file containing a private host key
491 .Pa /etc/ssh_host_key
492 for protocol version 1, and
493 .Pa /etc/ssh_host_rsa_key
495 .Pa /etc/ssh_host_dsa_key
496 for protocol version 2.
499 will refuse to use a file if it is group/world-accessible.
500 It is possible to have multiple host key files.
502 keys are used for version 1 and
506 are used for version 2 of the SSH protocol.
512 files will not be used in
513 .Cm RhostsAuthentication ,
514 .Cm RhostsRSAAuthentication
516 .Cm HostbasedAuthentication .
520 .Pa /etc/shosts.equiv
524 .It Cm IgnoreUserKnownHosts
527 should ignore the user's
528 .Pa $HOME/.ssh/known_hosts
530 .Cm RhostsRSAAuthentication
532 .Cm HostbasedAuthentication .
536 Specifies whether the system should send keepalive messages to the
538 If they are sent, death of the connection or crash of one
539 of the machines will be properly noticed.
540 However, this means that
541 connections will die if the route is down temporarily, and some people
543 On the other hand, if keepalives are not sent,
544 sessions may hang indefinitely on the server, leaving
546 users and consuming server resources.
550 (to send keepalives), and the server will notice
551 if the network goes down or the client host reboots.
552 This avoids infinitely hanging sessions.
554 To disable keepalives, the value should be set to
556 in both the server and the client configuration files.
557 .It Cm KerberosAuthentication
558 Specifies whether Kerberos authentication is allowed.
559 This can be in the form of a Kerberos ticket, or if
560 .Cm PasswordAuthentication
561 is yes, the password provided by the user will be validated through
563 To use this option, the server needs a
564 Kerberos servtab which allows the verification of the KDC's identity.
567 .It Cm KerberosOrLocalPasswd
568 If set then if password authentication through Kerberos fails then
569 the password will be validated via any additional local mechanism
574 .It Cm KerberosTgtPassing
575 Specifies whether a Kerberos TGT may be forwarded to the server.
578 as this only works when the Kerberos KDC is actually an AFS kaserver.
579 .It Cm KerberosTicketCleanup
580 Specifies whether to automatically destroy the user's ticket cache
584 .It Cm KeyRegenerationInterval
585 In protocol version 1, the ephemeral server key is automatically regenerated
586 after this many seconds (if it has been used).
587 The purpose of regeneration is to prevent
588 decrypting captured sessions by later breaking into the machine and
590 The key is never stored anywhere.
591 If the value is 0, the key is never regenerated.
592 The default is 3600 (seconds).
594 Specifies the local addresses
597 The following forms may be used:
599 .Bl -item -offset indent -compact
603 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
608 .Ar host No | Ar IPv4_addr No : Ar port
614 .Ar host No | Ar IPv6_addr Oc : Ar port
622 will listen on the address and all prior
624 options specified. The default is to listen on all local
627 options are permitted. Additionally, any
629 options must precede this option for non port qualified addresses.
630 .It Cm LoginGraceTime
631 The server disconnects after this time if the user has not
632 successfully logged in.
633 If the value is 0, there is no time limit.
634 The default is 600 (seconds).
636 Gives the verbosity level that is used when logging messages from
638 The possible values are:
639 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
640 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
641 and DEBUG3 each specify higher levels of debugging output.
642 Logging with a DEBUG level violates the privacy of users
643 and is not recommended.
645 Specifies the available MAC (message authentication code) algorithms.
646 The MAC algorithm is used in protocol version 2
647 for data integrity protection.
648 Multiple algorithms must be comma-separated.
650 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
652 Specifies the maximum number of concurrent unauthenticated connections to the
655 Additional connections will be dropped until authentication succeeds or the
657 expires for a connection.
660 Alternatively, random early drop can be enabled by specifying
661 the three colon separated values
665 will refuse connection attempts with a probability of
668 if there are currently
671 unauthenticated connections.
672 The probability increases linearly and all connection attempts
673 are refused if the number of unauthenticated connections reaches
676 .It Cm PAMAuthenticationViaKbdInt
677 Specifies whether PAM challenge response authentication is allowed. This
678 allows the use of most PAM challenge response authentication modules, but
679 it will allow password authentication regardless of whether
680 .Cm PasswordAuthentication
684 .It Cm PasswordAuthentication
685 Specifies whether password authentication is allowed.
688 .It Cm PermitEmptyPasswords
689 When password authentication is allowed, it specifies whether the
690 server allows login to accounts with empty password strings.
693 .It Cm PermitRootLogin
694 Specifies whether root can login using
698 .Dq without-password ,
699 .Dq forced-commands-only
705 If this option is set to
707 password authentication is disabled for root.
709 If this option is set to
710 .Dq forced-commands-only
711 root login with public key authentication will be allowed,
714 option has been specified
715 (which may be useful for taking remote backups even if root login is
716 normally not allowed). All other authentication methods are disabled
719 If this option is set to
721 root is not allowed to login.
723 Specifies the file that contains the process identifier of the
727 .Pa /var/run/sshd.pid .
729 Specifies the port number that
733 Multiple options of this type are permitted.
739 should print the date and time when the user last logged in.
747 when a user logs in interactively.
748 (On some systems it is also printed by the shell,
754 Specifies the protocol versions
757 The possible values are
761 Multiple versions must be comma-separated.
764 .It Cm PubkeyAuthentication
765 Specifies whether public key authentication is allowed.
768 Note that this option applies to protocol version 2 only.
769 .It Cm RhostsAuthentication
770 Specifies whether authentication using rhosts or /etc/hosts.equiv
772 Normally, this method should not be permitted because it is insecure.
773 .Cm RhostsRSAAuthentication
775 instead, because it performs RSA-based host authentication in addition
776 to normal rhosts or /etc/hosts.equiv authentication.
779 This option applies to protocol version 1 only.
780 .It Cm RhostsRSAAuthentication
781 Specifies whether rhosts or /etc/hosts.equiv authentication together
782 with successful RSA host authentication is allowed.
785 This option applies to protocol version 1 only.
786 .It Cm RSAAuthentication
787 Specifies whether pure RSA authentication is allowed.
790 This option applies to protocol version 1 only.
792 Defines the number of bits in the ephemeral protocol version 1 server key.
793 The minimum value is 512, and the default is 768.
797 should check file modes and ownership of the
798 user's files and home directory before accepting login.
799 This is normally desirable because novices sometimes accidentally leave their
800 directory or files world-writable.
804 Configures an external subsystem (e.g., file transfer daemon).
805 Arguments should be a subsystem name and a command to execute upon subsystem
811 file transfer subsystem.
812 By default no subsystems are defined.
813 Note that this option applies to protocol version 2 only.
814 .It Cm SyslogFacility
815 Gives the facility code that is used when logging messages from
817 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
818 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
823 is used for interactive login sessions.
828 is never used for remote command execution.
829 Note also, that if this is enabled,
831 will be disabled because
833 does not know how to handle
836 .It Cm VerifyReverseMapping
839 should try to verify the remote host name and check that
840 the resolved host name for the remote IP address maps back to the
841 very same IP address.
844 .It Cm X11DisplayOffset
845 Specifies the first display number available for
850 from interfering with real X11 servers.
853 Specifies whether X11 forwarding is permitted.
856 Note that disabling X11 forwarding does not improve security in any
857 way, as users can always install their own forwarders.
858 X11 forwarding is automatically disabled if
861 .It Cm X11UseLocalhost
864 should bind the X11 forwarding server to the loopback address or to
865 the wildcard address. By default,
867 binds the forwarding server to the loopback address and sets the
870 environment variable to
872 This prevents remote hosts from connecting to the fake display.
873 However, some older X11 clients may not function with this
878 to specify that the forwarding server should be bound to the wildcard
887 Specifies the location of the
891 .Pa /usr/X11R6/bin/xauth .
896 command-line arguments and configuration file options that specify time
897 may be expressed using a sequence of the form:
899 .Ar time Oo Ar qualifier Oc ,
903 is a positive integer value and
905 is one of the following:
907 .Bl -tag -width Ds -compact -offset indent
922 Each member of the sequence is added together to calculate
923 the total time value.
925 Time format examples:
927 .Bl -tag -width Ds -compact -offset indent
929 600 seconds (10 minutes)
933 1 hour 30 minutes (90 minutes)
936 When a user successfully logs in,
939 .Bl -enum -offset indent
941 If the login is on a tty, and no command has been specified,
942 prints last login time and
944 (unless prevented in the configuration file or by
945 .Pa $HOME/.hushlogin ;
950 If the login is on a tty, records login time.
954 if it exists, prints contents and quits
957 Changes to run with normal user privileges.
959 Sets up basic environment.
962 .Pa $HOME/.ssh/environment
965 Changes to user's home directory.
969 exists, runs it; else if
972 it; otherwise runs xauth.
975 files are given the X11
976 authentication protocol and cookie in standard input.
978 Runs user's shell or command.
980 .Sh AUTHORIZED_KEYS FILE FORMAT
981 .Pa $HOME/.ssh/authorized_keys
982 is the default file that lists the public keys that are
983 permitted for RSA authentication in protocol version 1
984 and for public key authentication (PubkeyAuthentication)
985 in protocol version 2.
986 .Cm AuthorizedKeysFile
987 may be used to specify an alternative file.
989 Each line of the file contains one
990 key (empty lines and lines starting with a
994 Each RSA public key consists of the following fields, separated by
995 spaces: options, bits, exponent, modulus, comment.
996 Each protocol version 2 public key consists of:
997 options, keytype, base64 encoded key, comment.
999 are optional; its presence is determined by whether the line starts
1000 with a number or not (the option field never starts with a number).
1001 The bits, exponent, modulus and comment fields give the RSA key for
1002 protocol version 1; the
1003 comment field is not used for anything (but may be convenient for the
1004 user to identify the key).
1005 For protocol version 2 the keytype is
1010 Note that lines in this file are usually several hundred bytes long
1011 (because of the size of the RSA key modulus).
1012 You don't want to type them in; instead, copy the
1019 The options (if present) consist of comma-separated option
1021 No spaces are permitted, except within double quotes.
1022 The following option specifications are supported (note
1023 that option keywords are case-insensitive):
1025 .It Cm from="pattern-list"
1026 Specifies that in addition to RSA authentication, the canonical name
1027 of the remote host must be present in the comma-separated list of
1032 serve as wildcards).
1033 The list may also contain
1034 patterns negated by prefixing them with
1036 if the canonical host name matches a negated pattern, the key is not accepted.
1038 of this option is to optionally increase security: RSA authentication
1039 by itself does not trust the network or name servers or anything (but
1040 the key); however, if somebody somehow steals the key, the key
1041 permits an intruder to log in from anywhere in the world.
1042 This additional option makes using a stolen key more difficult (name
1043 servers and/or routers would have to be compromised in addition to
1045 .It Cm command="command"
1046 Specifies that the command is executed whenever this key is used for
1048 The command supplied by the user (if any) is ignored.
1049 The command is run on a pty if the client requests a pty;
1050 otherwise it is run without a tty.
1051 If a 8-bit clean channel is required,
1052 one must not request a pty or should specify
1054 A quote may be included in the command by quoting it with a backslash.
1055 This option might be useful
1056 to restrict certain RSA keys to perform just a specific operation.
1057 An example might be a key that permits remote backups but nothing else.
1058 Note that the client may specify TCP/IP and/or X11
1059 forwarding unless they are explicitly prohibited.
1060 Note that this option applies to shell, command or subsystem execution.
1061 .It Cm environment="NAME=value"
1062 Specifies that the string is to be added to the environment when
1063 logging in using this key.
1064 Environment variables set this way
1065 override other default environment values.
1066 Multiple options of this type are permitted.
1067 This option is automatically disabled if
1070 .It Cm no-port-forwarding
1071 Forbids TCP/IP forwarding when this key is used for authentication.
1072 Any port forward requests by the client will return an error.
1073 This might be used, e.g., in connection with the
1076 .It Cm no-X11-forwarding
1077 Forbids X11 forwarding when this key is used for authentication.
1078 Any X11 forward requests by the client will return an error.
1079 .It Cm no-agent-forwarding
1080 Forbids authentication agent forwarding when this key is used for
1083 Prevents tty allocation (a request to allocate a pty will fail).
1084 .It Cm permitopen="host:port"
1087 port forwarding such that it may only connect to the specified host and
1089 IPv6 addresses can be specified with an alternative syntax:
1093 options may be applied separated by commas. No pattern matching is
1094 performed on the specified hostnames, they must be literal domains or
1098 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1100 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1102 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1104 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1105 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1107 .Pa /etc/ssh_known_hosts ,
1109 .Pa $HOME/.ssh/known_hosts
1110 files contain host public keys for all known hosts.
1111 The global file should
1112 be prepared by the administrator (optional), and the per-user file is
1113 maintained automatically: whenever the user connects from an unknown host
1114 its key is added to the per-user file.
1116 Each line in these files contains the following fields: hostnames,
1117 bits, exponent, modulus, comment.
1118 The fields are separated by spaces.
1120 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1121 wildcards); each pattern in turn is matched against the canonical host
1122 name (when authenticating a client) or against the user-supplied
1123 name (when authenticating a server).
1124 A pattern may also be preceded by
1126 to indicate negation: if the host name matches a negated
1127 pattern, it is not accepted (by that line) even if it matched another
1128 pattern on the line.
1130 Bits, exponent, and modulus are taken directly from the RSA host key; they
1131 can be obtained, e.g., from
1132 .Pa /etc/ssh_host_key.pub .
1133 The optional comment field continues to the end of the line, and is not used.
1137 and empty lines are ignored as comments.
1139 When performing host authentication, authentication is accepted if any
1140 matching line has the proper key.
1141 It is thus permissible (but not
1142 recommended) to have several lines or different host keys for the same
1144 This will inevitably happen when short forms of host names
1145 from different domains are put in the file.
1147 that the files contain conflicting information; authentication is
1148 accepted if valid information can be found from either file.
1150 Note that the lines in these files are typically hundreds of characters
1151 long, and you definitely don't want to type in the host keys by hand.
1152 Rather, generate them by a script
1154 .Pa /etc/ssh_host_key.pub
1155 and adding the host names at the front.
1158 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1159 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1163 .It Pa /etc/sshd_config
1164 Contains configuration data for
1166 This file should be writable by root only, but it is recommended
1167 (though not necessary) that it be world-readable.
1168 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1169 These three files contain the private parts of the host keys.
1170 These files should only be owned by root, readable only by root, and not
1171 accessible to others.
1174 does not start if this file is group/world-accessible.
1175 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1176 These three files contain the public parts of the host keys.
1177 These files should be world-readable but writable only by
1179 Their contents should match the respective private parts.
1181 really used for anything; they are provided for the convenience of
1182 the user so their contents can be copied to known hosts files.
1183 These files are created using
1186 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1187 .It Pa /var/run/sshd.pid
1188 Contains the process ID of the
1190 listening for connections (if there are several daemons running
1191 concurrently for different ports, this contains the pid of the one
1193 The content of this file is not sensitive; it can be world-readable.
1194 .It Pa $HOME/.ssh/authorized_keys
1195 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1196 This file must be readable by root (which may on some machines imply
1197 it being world-readable if the user's home directory resides on an NFS
1199 It is recommended that it not be accessible by others.
1200 The format of this file is described above.
1201 Users will place the contents of their
1206 files into this file, as described in
1208 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1209 These files are consulted when using rhosts with RSA host
1210 authentication or protocol version 2 hostbased authentication
1211 to check the public key of the host.
1212 The key must be listed in one of these files to be accepted.
1213 The client uses the same files
1214 to verify that it is connecting to the correct remote host.
1215 These files should be writable only by root/the owner.
1216 .Pa /etc/ssh_known_hosts
1217 should be world-readable, and
1218 .Pa $HOME/.ssh/known_hosts
1219 can but need not be world-readable.
1221 If this file exists,
1223 refuses to let anyone except root log in.
1224 The contents of the file
1225 are displayed to anyone trying to log in, and non-root connections are
1227 The file should be world-readable.
1228 .It Pa /etc/hosts.allow, /etc/hosts.deny
1229 Access controls that should be enforced by tcp-wrappers are defined here.
1230 Further details are described in
1231 .Xr hosts_access 5 .
1232 .It Pa $HOME/.rhosts
1233 This file contains host-username pairs, separated by a space, one per
1235 The given user on the corresponding host is permitted to log in
1237 The same file is used by rlogind and rshd.
1239 be writable only by the user; it is recommended that it not be
1240 accessible by others.
1242 If is also possible to use netgroups in the file.
1244 name may be of the form +@groupname to specify all hosts or all users
1246 .It Pa $HOME/.shosts
1248 this file is exactly the same as for
1250 However, this file is
1251 not used by rlogin and rshd, so using this permits access using SSH only.
1252 .It Pa /etc/hosts.equiv
1253 This file is used during
1256 In the simplest form, this file contains host names, one per line.
1258 those hosts are permitted to log in without a password, provided they
1259 have the same user name on both machines.
1260 The host name may also be
1261 followed by a user name; such users are permitted to log in as
1263 user on this machine (except root).
1264 Additionally, the syntax
1266 can be used to specify netgroups.
1267 Negated entries start with
1270 If the client host/user is successfully matched in this file, login is
1271 automatically permitted provided the client and server user names are the
1273 Additionally, successful RSA host authentication is normally required.
1274 This file must be writable only by root; it is recommended
1275 that it be world-readable.
1277 .Sy "Warning: It is almost never a good idea to use user names in"
1279 Beware that it really means that the named user(s) can log in as
1281 which includes bin, daemon, adm, and other accounts that own critical
1282 binaries and directories.
1283 Using a user name practically grants the user root access.
1284 The only valid use for user names that I can think
1285 of is in negative entries.
1287 Note that this warning also applies to rsh/rlogin.
1288 .It Pa /etc/shosts.equiv
1289 This is processed exactly as
1290 .Pa /etc/hosts.equiv .
1291 However, this file may be useful in environments that want to run both
1293 .It Pa $HOME/.ssh/environment
1294 This file is read into the environment at login (if it exists).
1295 It can only contain empty lines, comment lines (that start with
1297 and assignment lines of the form name=value.
1298 The file should be writable
1299 only by the user; it need not be readable by anyone else.
1300 .It Pa $HOME/.ssh/rc
1301 If this file exists, it is run with /bin/sh after reading the
1302 environment files but before starting the user's shell or command.
1303 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1311 The primary purpose of this file is to run any initialization routines
1312 which may be needed before the user's home directory becomes
1313 accessible; AFS is a particular example of such an environment.
1315 This file will probably contain some initialization code followed by
1316 something similar to:
1318 if read proto cookie; then
1319 echo add $DISPLAY $proto $cookie | xauth -q -
1323 If this file does not exist,
1326 does not exist either, xauth is used to store the cookie.
1328 This file should be writable only by the user, and need not be
1329 readable by anyone else.
1333 This can be used to specify
1334 machine-specific login-time initializations globally.
1335 This file should be writable only by root, and should be world-readable.
1338 OpenSSH is a derivative of the original and free
1339 ssh 1.2.12 release by Tatu Ylonen.
1340 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1341 Theo de Raadt and Dug Song
1342 removed many bugs, re-added newer features and
1344 Markus Friedl contributed the support for SSH
1345 protocol versions 1.5 and 2.0.
1362 .%T "SSH Protocol Architecture"
1363 .%N draft-ietf-secsh-architecture-09.txt
1365 .%O work in progress material
1371 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1372 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1374 .%O work in progress material
andersk / openssh.git / blob