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 1.156 2001/12/06 13:30:06 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 the file from which the host key is read (default
212 .Pa /etc/ssh_host_key ) .
213 This option must be given if
215 is not run as root (as the normal
216 host file is normally not readable by anyone but root).
217 It is possible to have multiple host key files for
218 the different protocol versions and host key algorithms.
222 is being run from inetd.
225 from inetd because it needs to generate the server key before it can
226 respond to the client, and this may take tens of seconds.
227 Clients would have to wait too long if the key was regenerated every time.
228 However, with small key sizes (e.g., 512) using
232 .It Fl k Ar key_gen_time
233 Specifies how often the ephemeral protocol version 1 server key is
234 regenerated (default 3600 seconds, or one hour).
235 The motivation for regenerating the key fairly
236 often is that the key is not stored anywhere, and after about an hour,
237 it becomes impossible to recover the key for decrypting intercepted
238 communications even if the machine is cracked into or physically
240 A value of zero indicates that the key will never be regenerated.
242 Can be used to give options in the format used in the configuration file.
243 This is useful for specifying options for which there is no separate
246 Specifies the port on which the server listens for connections
250 Nothing is sent to the system log.
251 Normally the beginning,
252 authentication, and termination of each connection is logged.
255 Only check the validity of the configuration file and sanity of the keys.
256 This is useful for updating
258 reliably as configuration options may change.
260 This option is used to specify the size of the field
263 structure that holds the remote host name.
264 If the resolved host name is longer than
266 the dotted decimal value will be used instead.
267 This allows hosts with very long host names that
268 overflow this field to still be uniquely identified.
271 indicates that only dotted decimal addresses
272 should be put into the
276 is also be used to prevent
278 from making DNS requests unless the authentication
279 mechanism or configuration requires it.
280 Authentication mechanisms that may require DNS include
281 .Cm RhostsAuthentication ,
282 .Cm RhostsRSAAuthentication ,
283 .Cm HostbasedAuthentication
285 .Cm from="pattern-list"
286 option in a key file.
288 When this option is specified
290 will not detach and does not become a daemon.
291 This allows easy monitoring of
296 to use IPv4 addresses only.
300 to use IPv6 addresses only.
302 .Sh CONFIGURATION FILE
304 reads configuration data from
306 (or the file specified with
308 on the command line).
309 The file contains keyword-argument pairs, one per line.
312 and empty lines are interpreted as comments.
315 keywords and their meanings are as follows (note that
316 keywords are case-insensitive and arguments are case-sensitive):
318 .It Cm AFSTokenPassing
319 Specifies whether an AFS token may be forwarded to the server.
323 This keyword can be followed by a list of group names, separated
325 If specified, login is allowed only for users whose primary
326 group or supplementary group list matches one of the patterns.
331 wildcards in the patterns.
332 Only group names are valid; a numerical group ID is not recognized.
333 By default login is allowed regardless of the group list.
335 .It Cm AllowTcpForwarding
336 Specifies whether TCP forwarding is permitted.
339 Note that disabling TCP forwarding does not improve security unless
340 users are also denied shell access, as they can always install their
344 This keyword can be followed by a list of user names, separated
346 If specified, login is allowed only for users names that
347 match one of the patterns.
352 wildcards in the patterns.
353 Only user names are valid; a numerical user ID is not recognized.
354 By default login is allowed regardless of the user name.
355 If the pattern takes the form USER@HOST then USER and HOST
356 are separately checked, restricting logins to particular
357 users from particular hosts.
359 .It Cm AuthorizedKeysFile
360 Specifies the file that contains the public keys that can be used
361 for user authentication.
362 .Cm AuthorizedKeysFile
363 may contain tokens of the form %T which are substituted during connection
364 set-up. The following tokens are defined: %% is replaced by a literal '%',
365 %h is replaced by the home directory of the user being authenticated and
366 %u is replaced by the username of that user.
368 .Cm AuthorizedKeysFile
369 is taken to be an absolute path or one relative to the user's home
372 .Dq .ssh/authorized_keys
374 In some jurisdictions, sending a warning message before authentication
375 may be relevant for getting legal protection.
376 The contents of the specified file are sent to the remote user before
377 authentication is allowed.
378 This option is only available for protocol version 2.
380 .It Cm ChallengeResponseAuthentication
381 Specifies whether challenge response authentication is allowed.
382 All authentication styles from
388 Specifies the ciphers allowed for protocol version 2.
389 Multiple ciphers must be comma-separated.
391 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
392 .It Cm ClientAliveInterval
393 Sets a timeout interval in seconds after which if no data has been received
396 will send a message through the encrypted
397 channel to request a response from the client.
399 is 0, indicating that these messages will not be sent to the client.
400 This option applies to protocol version 2 only.
401 .It Cm ClientAliveCountMax
402 Sets the number of client alive messages (see above) which may be
405 receiving any messages back from the client. If this threshold is
406 reached while client alive messages are being sent,
408 will disconnect the client, terminating the session. It is important
409 to note that the use of client alive messages is very different from
411 (below). The client alive messages are sent through the
412 encrypted channel and therefore will not be spoofable. The TCP keepalive
415 is spoofable. The client alive mechanism is valuable when the client or
416 server depend on knowing when a connection has become inactive.
418 The default value is 3. If
419 .Cm ClientAliveInterval
420 (above) is set to 15, and
421 .Cm ClientAliveCountMax
422 is left at the default, unresponsive ssh clients
423 will be disconnected after approximately 45 seconds.
425 This keyword can be followed by a number of group names, separated
427 Users whose primary group or supplementary group list matches
428 one of the patterns aren't allowed to log in.
433 wildcards in the patterns.
434 Only group names are valid; a numerical group ID is not recognized.
435 By default login is allowed regardless of the group list.
438 This keyword can be followed by a number of user names, separated
440 Login is disallowed for user names that match one of the patterns.
444 can be used as wildcards in the patterns.
445 Only user names are valid; a numerical user ID is not recognized.
446 By default login is allowed regardless of the user name.
448 Specifies whether remote hosts are allowed to connect to ports
449 forwarded for the client.
452 binds remote port forwardings to the loopback addresss. This
453 prevents other remote hosts from connecting to forwarded ports.
455 can be used to specify that
457 should bind remote port forwardings to the wildcard address,
458 thus allowing remote hosts to connect to forwarded ports.
465 .It Cm HostbasedAuthentication
466 Specifies whether rhosts or /etc/hosts.equiv authentication together
467 with successful public key client host authentication is allowed
468 (hostbased authentication).
469 This option is similar to
470 .Cm RhostsRSAAuthentication
471 and applies to protocol version 2 only.
475 Specifies the file containing the private host keys (default
476 .Pa /etc/ssh_host_key )
477 used by SSH protocol versions 1 and 2.
480 will refuse to use a file if it is group/world-accessible.
481 It is possible to have multiple host key files.
483 keys are used for version 1 and
487 are used for version 2 of the SSH protocol.
493 files will not be used in
494 .Cm RhostsAuthentication ,
495 .Cm RhostsRSAAuthentication
497 .Cm HostbasedAuthentication .
501 .Pa /etc/shosts.equiv
505 .It Cm IgnoreUserKnownHosts
508 should ignore the user's
509 .Pa $HOME/.ssh/known_hosts
511 .Cm RhostsRSAAuthentication
513 .Cm HostbasedAuthentication .
517 Specifies whether the system should send keepalive messages to the
519 If they are sent, death of the connection or crash of one
520 of the machines will be properly noticed.
521 However, this means that
522 connections will die if the route is down temporarily, and some people
524 On the other hand, if keepalives are not sent,
525 sessions may hang indefinitely on the server, leaving
527 users and consuming server resources.
531 (to send keepalives), and the server will notice
532 if the network goes down or the client host reboots.
533 This avoids infinitely hanging sessions.
535 To disable keepalives, the value should be set to
537 in both the server and the client configuration files.
538 .It Cm KerberosAuthentication
539 Specifies whether Kerberos authentication is allowed.
540 This can be in the form of a Kerberos ticket, or if
541 .Cm PasswordAuthentication
542 is yes, the password provided by the user will be validated through
544 To use this option, the server needs a
545 Kerberos servtab which allows the verification of the KDC's identity.
548 .It Cm KerberosOrLocalPasswd
549 If set then if password authentication through Kerberos fails then
550 the password will be validated via any additional local mechanism
555 .It Cm KerberosTgtPassing
556 Specifies whether a Kerberos TGT may be forwarded to the server.
559 as this only works when the Kerberos KDC is actually an AFS kaserver.
560 .It Cm KerberosTicketCleanup
561 Specifies whether to automatically destroy the user's ticket cache
565 .It Cm KeyRegenerationInterval
566 In protocol version 1, the ephemeral server key is automatically regenerated
567 after this many seconds (if it has been used).
568 The purpose of regeneration is to prevent
569 decrypting captured sessions by later breaking into the machine and
571 The key is never stored anywhere.
572 If the value is 0, the key is never regenerated.
573 The default is 3600 (seconds).
575 Specifies the local addresses
578 The following forms may be used:
580 .Bl -item -offset indent -compact
584 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
589 .Ar host No | Ar IPv4_addr No : Ar port
595 .Ar host No | Ar IPv6_addr Oc : Ar port
603 will listen on the address and all prior
605 options specified. The default is to listen on all local
608 options are permitted. Additionally, any
610 options must precede this option for non port qualified addresses.
611 .It Cm LoginGraceTime
612 The server disconnects after this time if the user has not
613 successfully logged in.
614 If the value is 0, there is no time limit.
615 The default is 600 (seconds).
617 Gives the verbosity level that is used when logging messages from
619 The possible values are:
620 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
622 Logging with level DEBUG violates the privacy of users
623 and is not recommended.
625 Specifies the available MAC (message authentication code) algorithms.
626 The MAC algorithm is used in protocol version 2
627 for data integrity protection.
628 Multiple algorithms must be comma-separated.
630 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
632 Specifies the maximum number of concurrent unauthenticated connections to the
635 Additional connections will be dropped until authentication succeeds or the
637 expires for a connection.
640 Alternatively, random early drop can be enabled by specifying
641 the three colon separated values
645 will refuse connection attempts with a probability of
648 if there are currently
651 unauthenticated connections.
652 The probability increases linearly and all connection attempts
653 are refused if the number of unauthenticated connections reaches
656 .It Cm PAMAuthenticationViaKbdInt
657 Specifies whether PAM challenge response authentication is allowed. This
658 allows the use of most PAM challenge response authentication modules, but
659 it will allow password authentication regardless of whether
660 .Cm PasswordAuthentication
664 .It Cm PasswordAuthentication
665 Specifies whether password authentication is allowed.
668 .It Cm PermitEmptyPasswords
669 When password authentication is allowed, it specifies whether the
670 server allows login to accounts with empty password strings.
673 .It Cm PermitRootLogin
674 Specifies whether root can login using
678 .Dq without-password ,
679 .Dq forced-commands-only
685 If this option is set to
687 password authentication is disabled for root.
689 If this option is set to
690 .Dq forced-commands-only
691 root login with public key authentication will be allowed,
694 option has been specified
695 (which may be useful for taking remote backups even if root login is
696 normally not allowed). All other authentication methods are disabled
699 If this option is set to
701 root is not allowed to login.
703 Specifies the file that contains the process identifier of the
707 .Pa /var/run/sshd.pid .
709 Specifies the port number that
713 Multiple options of this type are permitted.
719 should print the date and time when the user last logged in.
727 when a user logs in interactively.
728 (On some systems it is also printed by the shell,
734 Specifies the protocol versions
737 The possible values are
741 Multiple versions must be comma-separated.
744 .It Cm PubkeyAuthentication
745 Specifies whether public key authentication is allowed.
748 Note that this option applies to protocol version 2 only.
749 .It Cm ReverseMappingCheck
752 should try to verify the remote host name and check that
753 the resolved host name for the remote IP address maps back to the
754 very same IP address.
757 .It Cm RhostsAuthentication
758 Specifies whether authentication using rhosts or /etc/hosts.equiv
760 Normally, this method should not be permitted because it is insecure.
761 .Cm RhostsRSAAuthentication
763 instead, because it performs RSA-based host authentication in addition
764 to normal rhosts or /etc/hosts.equiv authentication.
767 This option applies to protocol version 1 only.
768 .It Cm RhostsRSAAuthentication
769 Specifies whether rhosts or /etc/hosts.equiv authentication together
770 with successful RSA host authentication is allowed.
773 This option applies to protocol version 1 only.
774 .It Cm RSAAuthentication
775 Specifies whether pure RSA authentication is allowed.
778 This option applies to protocol version 1 only.
780 Defines the number of bits in the ephemeral protocol version 1 server key.
781 The minimum value is 512, and the default is 768.
785 should check file modes and ownership of the
786 user's files and home directory before accepting login.
787 This is normally desirable because novices sometimes accidentally leave their
788 directory or files world-writable.
792 Configures an external subsystem (e.g., file transfer daemon).
793 Arguments should be a subsystem name and a command to execute upon subsystem
799 file transfer subsystem.
800 By default no subsystems are defined.
801 Note that this option applies to protocol version 2 only.
802 .It Cm SyslogFacility
803 Gives the facility code that is used when logging messages from
805 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
806 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
811 is used for interactive login sessions.
816 is never used for remote command execution.
817 Note also, that if this is enabled,
819 will be disabled because
821 does not know how to handle
824 .It Cm X11DisplayOffset
825 Specifies the first display number available for
830 from interfering with real X11 servers.
833 Specifies whether X11 forwarding is permitted.
836 Note that disabling X11 forwarding does not improve security in any
837 way, as users can always install their own forwarders.
838 X11 forwarding is automatically disabled if
842 Specifies the location of the
846 .Pa /usr/X11R6/bin/xauth .
851 command-line arguments and configuration file options that specify time
852 may be expressed using a sequence of the form:
854 .Ar time Oo Ar qualifier Oc ,
858 is a positive integer value and
860 is one of the following:
862 .Bl -tag -width Ds -compact -offset indent
877 Each member of the sequence is added together to calculate
878 the total time value.
880 Time format examples:
882 .Bl -tag -width Ds -compact -offset indent
884 600 seconds (10 minutes)
888 1 hour 30 minutes (90 minutes)
891 When a user successfully logs in,
894 .Bl -enum -offset indent
896 If the login is on a tty, and no command has been specified,
897 prints last login time and
899 (unless prevented in the configuration file or by
900 .Pa $HOME/.hushlogin ;
905 If the login is on a tty, records login time.
909 if it exists, prints contents and quits
912 Changes to run with normal user privileges.
914 Sets up basic environment.
917 .Pa $HOME/.ssh/environment
920 Changes to user's home directory.
924 exists, runs it; else if
927 it; otherwise runs xauth.
930 files are given the X11
931 authentication protocol and cookie in standard input.
933 Runs user's shell or command.
935 .Sh AUTHORIZED_KEYS FILE FORMAT
936 .Pa $HOME/.ssh/authorized_keys
937 is the default file that lists the public keys that are
938 permitted for RSA authentication in protocol version 1
939 and for public key authentication (PubkeyAuthentication)
940 in protocol version 2.
941 .Cm AuthorizedKeysFile
942 may be used to specify an alternative file.
944 Each line of the file contains one
945 key (empty lines and lines starting with a
949 Each RSA public key consists of the following fields, separated by
950 spaces: options, bits, exponent, modulus, comment.
951 Each protocol version 2 public key consists of:
952 options, keytype, base64 encoded key, comment.
954 are optional; its presence is determined by whether the line starts
955 with a number or not (the option field never starts with a number).
956 The bits, exponent, modulus and comment fields give the RSA key for
957 protocol version 1; the
958 comment field is not used for anything (but may be convenient for the
959 user to identify the key).
960 For protocol version 2 the keytype is
965 Note that lines in this file are usually several hundred bytes long
966 (because of the size of the RSA key modulus).
967 You don't want to type them in; instead, copy the
974 The options (if present) consist of comma-separated option
976 No spaces are permitted, except within double quotes.
977 The following option specifications are supported (note
978 that option keywords are case-insensitive):
980 .It Cm from="pattern-list"
981 Specifies that in addition to RSA authentication, the canonical name
982 of the remote host must be present in the comma-separated list of
988 The list may also contain
989 patterns negated by prefixing them with
991 if the canonical host name matches a negated pattern, the key is not accepted.
993 of this option is to optionally increase security: RSA authentication
994 by itself does not trust the network or name servers or anything (but
995 the key); however, if somebody somehow steals the key, the key
996 permits an intruder to log in from anywhere in the world.
997 This additional option makes using a stolen key more difficult (name
998 servers and/or routers would have to be compromised in addition to
1000 .It Cm command="command"
1001 Specifies that the command is executed whenever this key is used for
1003 The command supplied by the user (if any) is ignored.
1004 The command is run on a pty if the client requests a pty;
1005 otherwise it is run without a tty.
1006 If a 8-bit clean channel is required,
1007 one must not request a pty or should specify
1009 A quote may be included in the command by quoting it with a backslash.
1010 This option might be useful
1011 to restrict certain RSA keys to perform just a specific operation.
1012 An example might be a key that permits remote backups but nothing else.
1013 Note that the client may specify TCP/IP and/or X11
1014 forwarding unless they are explicitly prohibited.
1015 Note that this option applies to shell, command or subsystem execution.
1016 .It Cm environment="NAME=value"
1017 Specifies that the string is to be added to the environment when
1018 logging in using this key.
1019 Environment variables set this way
1020 override other default environment values.
1021 Multiple options of this type are permitted.
1022 This option is automatically disabled if
1025 .It Cm no-port-forwarding
1026 Forbids TCP/IP forwarding when this key is used for authentication.
1027 Any port forward requests by the client will return an error.
1028 This might be used, e.g., in connection with the
1031 .It Cm no-X11-forwarding
1032 Forbids X11 forwarding when this key is used for authentication.
1033 Any X11 forward requests by the client will return an error.
1034 .It Cm no-agent-forwarding
1035 Forbids authentication agent forwarding when this key is used for
1038 Prevents tty allocation (a request to allocate a pty will fail).
1039 .It Cm permitopen="host:port"
1042 port forwarding such that it may only connect to the specified host and
1044 IPv6 addresses can be specified with an alternative syntax:
1048 options may be applied separated by commas. No pattern matching is
1049 performed on the specified hostnames, they must be literal domains or
1053 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1055 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1057 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1059 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1060 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1062 .Pa /etc/ssh_known_hosts ,
1064 .Pa $HOME/.ssh/known_hosts
1065 files contain host public keys for all known hosts.
1066 The global file should
1067 be prepared by the administrator (optional), and the per-user file is
1068 maintained automatically: whenever the user connects from an unknown host
1069 its key is added to the per-user file.
1071 Each line in these files contains the following fields: hostnames,
1072 bits, exponent, modulus, comment.
1073 The fields are separated by spaces.
1075 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1076 wildcards); each pattern in turn is matched against the canonical host
1077 name (when authenticating a client) or against the user-supplied
1078 name (when authenticating a server).
1079 A pattern may also be preceded by
1081 to indicate negation: if the host name matches a negated
1082 pattern, it is not accepted (by that line) even if it matched another
1083 pattern on the line.
1085 Bits, exponent, and modulus are taken directly from the RSA host key; they
1086 can be obtained, e.g., from
1087 .Pa /etc/ssh_host_key.pub .
1088 The optional comment field continues to the end of the line, and is not used.
1092 and empty lines are ignored as comments.
1094 When performing host authentication, authentication is accepted if any
1095 matching line has the proper key.
1096 It is thus permissible (but not
1097 recommended) to have several lines or different host keys for the same
1099 This will inevitably happen when short forms of host names
1100 from different domains are put in the file.
1102 that the files contain conflicting information; authentication is
1103 accepted if valid information can be found from either file.
1105 Note that the lines in these files are typically hundreds of characters
1106 long, and you definitely don't want to type in the host keys by hand.
1107 Rather, generate them by a script
1109 .Pa /etc/ssh_host_key.pub
1110 and adding the host names at the front.
1113 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1114 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1118 .It Pa /etc/sshd_config
1119 Contains configuration data for
1121 This file should be writable by root only, but it is recommended
1122 (though not necessary) that it be world-readable.
1123 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1124 These three files contain the private parts of the host keys.
1125 These files should only be owned by root, readable only by root, and not
1126 accessible to others.
1129 does not start if this file is group/world-accessible.
1130 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1131 These three files contain the public parts of the host keys.
1132 These files should be world-readable but writable only by
1134 Their contents should match the respective private parts.
1136 really used for anything; they are provided for the convenience of
1137 the user so their contents can be copied to known hosts files.
1138 These files are created using
1141 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1142 .It Pa /var/run/sshd.pid
1143 Contains the process ID of the
1145 listening for connections (if there are several daemons running
1146 concurrently for different ports, this contains the pid of the one
1148 The content of this file is not sensitive; it can be world-readable.
1149 .It Pa $HOME/.ssh/authorized_keys
1150 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1151 This file must be readable by root (which may on some machines imply
1152 it being world-readable if the user's home directory resides on an NFS
1154 It is recommended that it not be accessible by others.
1155 The format of this file is described above.
1156 Users will place the contents of their
1161 files into this file, as described in
1163 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1164 These files are consulted when using rhosts with RSA host
1165 authentication or protocol version 2 hostbased authentication
1166 to check the public key of the host.
1167 The key must be listed in one of these files to be accepted.
1168 The client uses the same files
1169 to verify that it is connecting to the correct remote host.
1170 These files should be writable only by root/the owner.
1171 .Pa /etc/ssh_known_hosts
1172 should be world-readable, and
1173 .Pa $HOME/.ssh/known_hosts
1174 can but need not be world-readable.
1176 If this file exists,
1178 refuses to let anyone except root log in.
1179 The contents of the file
1180 are displayed to anyone trying to log in, and non-root connections are
1182 The file should be world-readable.
1183 .It Pa /etc/hosts.allow, /etc/hosts.deny
1184 Access controls that should be enforced by tcp-wrappers are defined here.
1185 Further details are described in
1186 .Xr hosts_access 5 .
1187 .It Pa $HOME/.rhosts
1188 This file contains host-username pairs, separated by a space, one per
1190 The given user on the corresponding host is permitted to log in
1192 The same file is used by rlogind and rshd.
1194 be writable only by the user; it is recommended that it not be
1195 accessible by others.
1197 If is also possible to use netgroups in the file.
1199 name may be of the form +@groupname to specify all hosts or all users
1201 .It Pa $HOME/.shosts
1203 this file is exactly the same as for
1205 However, this file is
1206 not used by rlogin and rshd, so using this permits access using SSH only.
1207 .It Pa /etc/hosts.equiv
1208 This file is used during
1211 In the simplest form, this file contains host names, one per line.
1213 those hosts are permitted to log in without a password, provided they
1214 have the same user name on both machines.
1215 The host name may also be
1216 followed by a user name; such users are permitted to log in as
1218 user on this machine (except root).
1219 Additionally, the syntax
1221 can be used to specify netgroups.
1222 Negated entries start with
1225 If the client host/user is successfully matched in this file, login is
1226 automatically permitted provided the client and server user names are the
1228 Additionally, successful RSA host authentication is normally required.
1229 This file must be writable only by root; it is recommended
1230 that it be world-readable.
1232 .Sy "Warning: It is almost never a good idea to use user names in"
1234 Beware that it really means that the named user(s) can log in as
1236 which includes bin, daemon, adm, and other accounts that own critical
1237 binaries and directories.
1238 Using a user name practically grants the user root access.
1239 The only valid use for user names that I can think
1240 of is in negative entries.
1242 Note that this warning also applies to rsh/rlogin.
1243 .It Pa /etc/shosts.equiv
1244 This is processed exactly as
1245 .Pa /etc/hosts.equiv .
1246 However, this file may be useful in environments that want to run both
1248 .It Pa $HOME/.ssh/environment
1249 This file is read into the environment at login (if it exists).
1250 It can only contain empty lines, comment lines (that start with
1252 and assignment lines of the form name=value.
1253 The file should be writable
1254 only by the user; it need not be readable by anyone else.
1255 .It Pa $HOME/.ssh/rc
1256 If this file exists, it is run with /bin/sh after reading the
1257 environment files but before starting the user's shell or command.
1258 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1266 The primary purpose of this file is to run any initialization routines
1267 which may be needed before the user's home directory becomes
1268 accessible; AFS is a particular example of such an environment.
1270 This file will probably contain some initialization code followed by
1271 something similar to:
1273 if read proto cookie; then
1274 echo add $DISPLAY $proto $cookie | xauth -q -
1278 If this file does not exist,
1281 does not exist either, xauth is used to store the cookie.
1283 This file should be writable only by the user, and need not be
1284 readable by anyone else.
1288 This can be used to specify
1289 machine-specific login-time initializations globally.
1290 This file should be writable only by root, and should be world-readable.
1293 OpenSSH is a derivative of the original and free
1294 ssh 1.2.12 release by Tatu Ylonen.
1295 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1296 Theo de Raadt and Dug Song
1297 removed many bugs, re-added newer features and
1299 Markus Friedl contributed the support for SSH
1300 protocol versions 1.5 and 2.0.
1317 .%T "SSH Protocol Architecture"
1318 .%N draft-ietf-secsh-architecture-09.txt
1320 .%O work in progress material
1326 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1327 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1329 .%O work in progress material
andersk / openssh.git / blob