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 1.161 2002/01/05 21:51:56 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
57 (SSH Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256 bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
123 are disabled (thus completely disabling
129 .Ss SSH protocol version 2
131 Version 2 works similarly:
132 Each host has a host-specific key (RSA or DSA) used to identify the host.
133 However, when the daemon starts, it does not generate a server key.
134 Forward security is provided through a Diffie-Hellman key agreement.
135 This key agreement results in a shared session key.
137 The rest of the session is encrypted using a symmetric cipher, currently
138 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
139 The client selects the encryption algorithm
140 to use from those offered by the server.
141 Additionally, session integrity is provided
142 through a cryptographic message authentication code
143 (hmac-sha1 or hmac-md5).
145 Protocol version 2 provides a public key based
146 user (PubkeyAuthentication) or
147 client host (HostbasedAuthentication) authentication method,
148 conventional password authentication and challenge response based methods.
150 .Ss Command execution and data forwarding
152 If the client successfully authenticates itself, a dialog for
153 preparing the session is entered.
154 At this time the client may request
155 things like allocating a pseudo-tty, forwarding X11 connections,
156 forwarding TCP/IP connections, or forwarding the authentication agent
157 connection over the secure channel.
159 Finally, the client either requests a shell or execution of a command.
160 The sides then enter session mode.
161 In this mode, either side may send
162 data at any time, and such data is forwarded to/from the shell or
163 command on the server side, and the user terminal in the client side.
165 When the user program terminates and all forwarded X11 and other
166 connections have been closed, the server sends command exit status to
167 the client, and both sides exit.
170 can be configured using command-line options or a configuration
172 Command-line options override values specified in the
176 rereads its configuration file when it receives a hangup signal,
178 by executing itself with the name it was started as, i.e.,
181 The options are as follows:
184 Specifies the number of bits in the ephemeral protocol version 1
185 server key (default 768).
188 The server sends verbose debug output to the system
189 log, and does not put itself in the background.
190 The server also will not fork and will only process one connection.
191 This option is only intended for debugging for the server.
192 Multiple -d options increase the debugging level.
195 When this option is specified,
197 will send the output to the standard error instead of the system log.
198 .It Fl f Ar configuration_file
199 Specifies the name of the configuration file.
201 .Pa /etc/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 names, 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 regardless of the group list.
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 names, 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 regardless of the user name.
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.
400 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour .
401 .It Cm ClientAliveInterval
402 Sets a timeout interval in seconds after which if no data has been received
405 will send a message through the encrypted
406 channel to request a response from the client.
408 is 0, indicating that these messages will not be sent to the client.
409 This option applies to protocol version 2 only.
410 .It Cm ClientAliveCountMax
411 Sets the number of client alive messages (see above) which may be
414 receiving any messages back from the client. If this threshold is
415 reached while client alive messages are being sent,
417 will disconnect the client, terminating the session. It is important
418 to note that the use of client alive messages is very different from
420 (below). The client alive messages are sent through the
421 encrypted channel and therefore will not be spoofable. The TCP keepalive
424 is spoofable. The client alive mechanism is valuable when the client or
425 server depend on knowing when a connection has become inactive.
427 The default value is 3. If
428 .Cm ClientAliveInterval
429 (above) is set to 15, and
430 .Cm ClientAliveCountMax
431 is left at the default, unresponsive ssh clients
432 will be disconnected after approximately 45 seconds.
434 This keyword can be followed by a number of group names, separated
436 Users whose primary group or supplementary group list matches
437 one of the patterns aren't allowed to log in.
442 wildcards in the patterns.
443 Only group names are valid; a numerical group ID is not recognized.
444 By default login is allowed regardless of the group list.
447 This keyword can be followed by a number of user names, separated
449 Login is disallowed for user names that match one of the patterns.
453 can be used as wildcards in the patterns.
454 Only user names are valid; a numerical user ID is not recognized.
455 By default login is allowed regardless of the user name.
457 Specifies whether remote hosts are allowed to connect to ports
458 forwarded for the client.
461 binds remote port forwardings to the loopback addresss. This
462 prevents other remote hosts from connecting to forwarded ports.
464 can be used to specify that
466 should bind remote port forwardings to the wildcard address,
467 thus allowing remote hosts to connect to forwarded ports.
474 .It Cm HostbasedAuthentication
475 Specifies whether rhosts or /etc/hosts.equiv authentication together
476 with successful public key client host authentication is allowed
477 (hostbased authentication).
478 This option is similar to
479 .Cm RhostsRSAAuthentication
480 and applies to protocol version 2 only.
484 Specifies a file containing a private host key
487 .Pa /etc/ssh_host_key
488 for protocol version 1, and
489 .Pa /etc/ssh_host_rsa_key
491 .Pa /etc/ssh_host_dsa_key
492 for protocol version 2.
495 will refuse to use a file if it is group/world-accessible.
496 It is possible to have multiple host key files.
498 keys are used for version 1 and
502 are used for version 2 of the SSH protocol.
508 files will not be used in
509 .Cm RhostsAuthentication ,
510 .Cm RhostsRSAAuthentication
512 .Cm HostbasedAuthentication .
516 .Pa /etc/shosts.equiv
520 .It Cm IgnoreUserKnownHosts
523 should ignore the user's
524 .Pa $HOME/.ssh/known_hosts
526 .Cm RhostsRSAAuthentication
528 .Cm HostbasedAuthentication .
532 Specifies whether the system should send keepalive messages to the
534 If they are sent, death of the connection or crash of one
535 of the machines will be properly noticed.
536 However, this means that
537 connections will die if the route is down temporarily, and some people
539 On the other hand, if keepalives are not sent,
540 sessions may hang indefinitely on the server, leaving
542 users and consuming server resources.
546 (to send keepalives), and the server will notice
547 if the network goes down or the client host reboots.
548 This avoids infinitely hanging sessions.
550 To disable keepalives, the value should be set to
552 in both the server and the client configuration files.
553 .It Cm KerberosAuthentication
554 Specifies whether Kerberos authentication is allowed.
555 This can be in the form of a Kerberos ticket, or if
556 .Cm PasswordAuthentication
557 is yes, the password provided by the user will be validated through
559 To use this option, the server needs a
560 Kerberos servtab which allows the verification of the KDC's identity.
563 .It Cm KerberosOrLocalPasswd
564 If set then if password authentication through Kerberos fails then
565 the password will be validated via any additional local mechanism
570 .It Cm KerberosTgtPassing
571 Specifies whether a Kerberos TGT may be forwarded to the server.
574 as this only works when the Kerberos KDC is actually an AFS kaserver.
575 .It Cm KerberosTicketCleanup
576 Specifies whether to automatically destroy the user's ticket cache
580 .It Cm KeyRegenerationInterval
581 In protocol version 1, the ephemeral server key is automatically regenerated
582 after this many seconds (if it has been used).
583 The purpose of regeneration is to prevent
584 decrypting captured sessions by later breaking into the machine and
586 The key is never stored anywhere.
587 If the value is 0, the key is never regenerated.
588 The default is 3600 (seconds).
590 Specifies the local addresses
593 The following forms may be used:
595 .Bl -item -offset indent -compact
599 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
604 .Ar host No | Ar IPv4_addr No : Ar port
610 .Ar host No | Ar IPv6_addr Oc : Ar port
618 will listen on the address and all prior
620 options specified. The default is to listen on all local
623 options are permitted. Additionally, any
625 options must precede this option for non port qualified addresses.
626 .It Cm LoginGraceTime
627 The server disconnects after this time if the user has not
628 successfully logged in.
629 If the value is 0, there is no time limit.
630 The default is 600 (seconds).
632 Gives the verbosity level that is used when logging messages from
634 The possible values are:
635 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
636 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
637 and DEBUG3 each specify higher levels of debugging output.
638 Logging with a DEBUG level violates the privacy of users
639 and is not recommended.
641 Specifies the available MAC (message authentication code) algorithms.
642 The MAC algorithm is used in protocol version 2
643 for data integrity protection.
644 Multiple algorithms must be comma-separated.
646 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
648 Specifies the maximum number of concurrent unauthenticated connections to the
651 Additional connections will be dropped until authentication succeeds or the
653 expires for a connection.
656 Alternatively, random early drop can be enabled by specifying
657 the three colon separated values
661 will refuse connection attempts with a probability of
664 if there are currently
667 unauthenticated connections.
668 The probability increases linearly and all connection attempts
669 are refused if the number of unauthenticated connections reaches
672 .It Cm PAMAuthenticationViaKbdInt
673 Specifies whether PAM challenge response authentication is allowed. This
674 allows the use of most PAM challenge response authentication modules, but
675 it will allow password authentication regardless of whether
676 .Cm PasswordAuthentication
680 .It Cm PasswordAuthentication
681 Specifies whether password authentication is allowed.
684 .It Cm PermitEmptyPasswords
685 When password authentication is allowed, it specifies whether the
686 server allows login to accounts with empty password strings.
689 .It Cm PermitRootLogin
690 Specifies whether root can login using
694 .Dq without-password ,
695 .Dq forced-commands-only
701 If this option is set to
703 password authentication is disabled for root.
705 If this option is set to
706 .Dq forced-commands-only
707 root login with public key authentication will be allowed,
710 option has been specified
711 (which may be useful for taking remote backups even if root login is
712 normally not allowed). All other authentication methods are disabled
715 If this option is set to
717 root is not allowed to login.
719 Specifies the file that contains the process identifier of the
723 .Pa /var/run/sshd.pid .
725 Specifies the port number that
729 Multiple options of this type are permitted.
735 should print the date and time when the user last logged in.
743 when a user logs in interactively.
744 (On some systems it is also printed by the shell,
750 Specifies the protocol versions
753 The possible values are
757 Multiple versions must be comma-separated.
760 .It Cm PubkeyAuthentication
761 Specifies whether public key authentication is allowed.
764 Note that this option applies to protocol version 2 only.
765 .It Cm ReverseMappingCheck
768 should try to verify the remote host name and check that
769 the resolved host name for the remote IP address maps back to the
770 very same IP address.
773 .It Cm RhostsAuthentication
774 Specifies whether authentication using rhosts or /etc/hosts.equiv
776 Normally, this method should not be permitted because it is insecure.
777 .Cm RhostsRSAAuthentication
779 instead, because it performs RSA-based host authentication in addition
780 to normal rhosts or /etc/hosts.equiv authentication.
783 This option applies to protocol version 1 only.
784 .It Cm RhostsRSAAuthentication
785 Specifies whether rhosts or /etc/hosts.equiv authentication together
786 with successful RSA host authentication is allowed.
789 This option applies to protocol version 1 only.
790 .It Cm RSAAuthentication
791 Specifies whether pure RSA authentication is allowed.
794 This option applies to protocol version 1 only.
796 Defines the number of bits in the ephemeral protocol version 1 server key.
797 The minimum value is 512, and the default is 768.
801 should check file modes and ownership of the
802 user's files and home directory before accepting login.
803 This is normally desirable because novices sometimes accidentally leave their
804 directory or files world-writable.
808 Configures an external subsystem (e.g., file transfer daemon).
809 Arguments should be a subsystem name and a command to execute upon subsystem
815 file transfer subsystem.
816 By default no subsystems are defined.
817 Note that this option applies to protocol version 2 only.
818 .It Cm SyslogFacility
819 Gives the facility code that is used when logging messages from
821 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
822 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
827 is used for interactive login sessions.
832 is never used for remote command execution.
833 Note also, that if this is enabled,
835 will be disabled because
837 does not know how to handle
840 .It Cm X11DisplayOffset
841 Specifies the first display number available for
846 from interfering with real X11 servers.
849 Specifies whether X11 forwarding is permitted.
852 Note that disabling X11 forwarding does not improve security in any
853 way, as users can always install their own forwarders.
854 X11 forwarding is automatically disabled if
858 Specifies the location of the
862 .Pa /usr/X11R6/bin/xauth .
867 command-line arguments and configuration file options that specify time
868 may be expressed using a sequence of the form:
870 .Ar time Oo Ar qualifier Oc ,
874 is a positive integer value and
876 is one of the following:
878 .Bl -tag -width Ds -compact -offset indent
893 Each member of the sequence is added together to calculate
894 the total time value.
896 Time format examples:
898 .Bl -tag -width Ds -compact -offset indent
900 600 seconds (10 minutes)
904 1 hour 30 minutes (90 minutes)
907 When a user successfully logs in,
910 .Bl -enum -offset indent
912 If the login is on a tty, and no command has been specified,
913 prints last login time and
915 (unless prevented in the configuration file or by
916 .Pa $HOME/.hushlogin ;
921 If the login is on a tty, records login time.
925 if it exists, prints contents and quits
928 Changes to run with normal user privileges.
930 Sets up basic environment.
933 .Pa $HOME/.ssh/environment
936 Changes to user's home directory.
940 exists, runs it; else if
943 it; otherwise runs xauth.
946 files are given the X11
947 authentication protocol and cookie in standard input.
949 Runs user's shell or command.
951 .Sh AUTHORIZED_KEYS FILE FORMAT
952 .Pa $HOME/.ssh/authorized_keys
953 is the default file that lists the public keys that are
954 permitted for RSA authentication in protocol version 1
955 and for public key authentication (PubkeyAuthentication)
956 in protocol version 2.
957 .Cm AuthorizedKeysFile
958 may be used to specify an alternative file.
960 Each line of the file contains one
961 key (empty lines and lines starting with a
965 Each RSA public key consists of the following fields, separated by
966 spaces: options, bits, exponent, modulus, comment.
967 Each protocol version 2 public key consists of:
968 options, keytype, base64 encoded key, comment.
970 are optional; its presence is determined by whether the line starts
971 with a number or not (the option field never starts with a number).
972 The bits, exponent, modulus and comment fields give the RSA key for
973 protocol version 1; the
974 comment field is not used for anything (but may be convenient for the
975 user to identify the key).
976 For protocol version 2 the keytype is
981 Note that lines in this file are usually several hundred bytes long
982 (because of the size of the RSA key modulus).
983 You don't want to type them in; instead, copy the
990 The options (if present) consist of comma-separated option
992 No spaces are permitted, except within double quotes.
993 The following option specifications are supported (note
994 that option keywords are case-insensitive):
996 .It Cm from="pattern-list"
997 Specifies that in addition to RSA authentication, the canonical name
998 of the remote host must be present in the comma-separated list of
1003 serve as wildcards).
1004 The list may also contain
1005 patterns negated by prefixing them with
1007 if the canonical host name matches a negated pattern, the key is not accepted.
1009 of this option is to optionally increase security: RSA authentication
1010 by itself does not trust the network or name servers or anything (but
1011 the key); however, if somebody somehow steals the key, the key
1012 permits an intruder to log in from anywhere in the world.
1013 This additional option makes using a stolen key more difficult (name
1014 servers and/or routers would have to be compromised in addition to
1016 .It Cm command="command"
1017 Specifies that the command is executed whenever this key is used for
1019 The command supplied by the user (if any) is ignored.
1020 The command is run on a pty if the client requests a pty;
1021 otherwise it is run without a tty.
1022 If a 8-bit clean channel is required,
1023 one must not request a pty or should specify
1025 A quote may be included in the command by quoting it with a backslash.
1026 This option might be useful
1027 to restrict certain RSA keys to perform just a specific operation.
1028 An example might be a key that permits remote backups but nothing else.
1029 Note that the client may specify TCP/IP and/or X11
1030 forwarding unless they are explicitly prohibited.
1031 Note that this option applies to shell, command or subsystem execution.
1032 .It Cm environment="NAME=value"
1033 Specifies that the string is to be added to the environment when
1034 logging in using this key.
1035 Environment variables set this way
1036 override other default environment values.
1037 Multiple options of this type are permitted.
1038 This option is automatically disabled if
1041 .It Cm no-port-forwarding
1042 Forbids TCP/IP forwarding when this key is used for authentication.
1043 Any port forward requests by the client will return an error.
1044 This might be used, e.g., in connection with the
1047 .It Cm no-X11-forwarding
1048 Forbids X11 forwarding when this key is used for authentication.
1049 Any X11 forward requests by the client will return an error.
1050 .It Cm no-agent-forwarding
1051 Forbids authentication agent forwarding when this key is used for
1054 Prevents tty allocation (a request to allocate a pty will fail).
1055 .It Cm permitopen="host:port"
1058 port forwarding such that it may only connect to the specified host and
1060 IPv6 addresses can be specified with an alternative syntax:
1064 options may be applied separated by commas. No pattern matching is
1065 performed on the specified hostnames, they must be literal domains or
1069 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1071 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1073 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1075 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1076 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1078 .Pa /etc/ssh_known_hosts ,
1080 .Pa $HOME/.ssh/known_hosts
1081 files contain host public keys for all known hosts.
1082 The global file should
1083 be prepared by the administrator (optional), and the per-user file is
1084 maintained automatically: whenever the user connects from an unknown host
1085 its key is added to the per-user file.
1087 Each line in these files contains the following fields: hostnames,
1088 bits, exponent, modulus, comment.
1089 The fields are separated by spaces.
1091 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1092 wildcards); each pattern in turn is matched against the canonical host
1093 name (when authenticating a client) or against the user-supplied
1094 name (when authenticating a server).
1095 A pattern may also be preceded by
1097 to indicate negation: if the host name matches a negated
1098 pattern, it is not accepted (by that line) even if it matched another
1099 pattern on the line.
1101 Bits, exponent, and modulus are taken directly from the RSA host key; they
1102 can be obtained, e.g., from
1103 .Pa /etc/ssh_host_key.pub .
1104 The optional comment field continues to the end of the line, and is not used.
1108 and empty lines are ignored as comments.
1110 When performing host authentication, authentication is accepted if any
1111 matching line has the proper key.
1112 It is thus permissible (but not
1113 recommended) to have several lines or different host keys for the same
1115 This will inevitably happen when short forms of host names
1116 from different domains are put in the file.
1118 that the files contain conflicting information; authentication is
1119 accepted if valid information can be found from either file.
1121 Note that the lines in these files are typically hundreds of characters
1122 long, and you definitely don't want to type in the host keys by hand.
1123 Rather, generate them by a script
1125 .Pa /etc/ssh_host_key.pub
1126 and adding the host names at the front.
1129 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1130 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1134 .It Pa /etc/sshd_config
1135 Contains configuration data for
1137 This file should be writable by root only, but it is recommended
1138 (though not necessary) that it be world-readable.
1139 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1140 These three files contain the private parts of the host keys.
1141 These files should only be owned by root, readable only by root, and not
1142 accessible to others.
1145 does not start if this file is group/world-accessible.
1146 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1147 These three files contain the public parts of the host keys.
1148 These files should be world-readable but writable only by
1150 Their contents should match the respective private parts.
1152 really used for anything; they are provided for the convenience of
1153 the user so their contents can be copied to known hosts files.
1154 These files are created using
1157 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1158 .It Pa /var/run/sshd.pid
1159 Contains the process ID of the
1161 listening for connections (if there are several daemons running
1162 concurrently for different ports, this contains the pid of the one
1164 The content of this file is not sensitive; it can be world-readable.
1165 .It Pa $HOME/.ssh/authorized_keys
1166 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1167 This file must be readable by root (which may on some machines imply
1168 it being world-readable if the user's home directory resides on an NFS
1170 It is recommended that it not be accessible by others.
1171 The format of this file is described above.
1172 Users will place the contents of their
1177 files into this file, as described in
1179 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1180 These files are consulted when using rhosts with RSA host
1181 authentication or protocol version 2 hostbased authentication
1182 to check the public key of the host.
1183 The key must be listed in one of these files to be accepted.
1184 The client uses the same files
1185 to verify that it is connecting to the correct remote host.
1186 These files should be writable only by root/the owner.
1187 .Pa /etc/ssh_known_hosts
1188 should be world-readable, and
1189 .Pa $HOME/.ssh/known_hosts
1190 can but need not be world-readable.
1192 If this file exists,
1194 refuses to let anyone except root log in.
1195 The contents of the file
1196 are displayed to anyone trying to log in, and non-root connections are
1198 The file should be world-readable.
1199 .It Pa /etc/hosts.allow, /etc/hosts.deny
1200 Access controls that should be enforced by tcp-wrappers are defined here.
1201 Further details are described in
1202 .Xr hosts_access 5 .
1203 .It Pa $HOME/.rhosts
1204 This file contains host-username pairs, separated by a space, one per
1206 The given user on the corresponding host is permitted to log in
1208 The same file is used by rlogind and rshd.
1210 be writable only by the user; it is recommended that it not be
1211 accessible by others.
1213 If is also possible to use netgroups in the file.
1215 name may be of the form +@groupname to specify all hosts or all users
1217 .It Pa $HOME/.shosts
1219 this file is exactly the same as for
1221 However, this file is
1222 not used by rlogin and rshd, so using this permits access using SSH only.
1223 .It Pa /etc/hosts.equiv
1224 This file is used during
1227 In the simplest form, this file contains host names, one per line.
1229 those hosts are permitted to log in without a password, provided they
1230 have the same user name on both machines.
1231 The host name may also be
1232 followed by a user name; such users are permitted to log in as
1234 user on this machine (except root).
1235 Additionally, the syntax
1237 can be used to specify netgroups.
1238 Negated entries start with
1241 If the client host/user is successfully matched in this file, login is
1242 automatically permitted provided the client and server user names are the
1244 Additionally, successful RSA host authentication is normally required.
1245 This file must be writable only by root; it is recommended
1246 that it be world-readable.
1248 .Sy "Warning: It is almost never a good idea to use user names in"
1250 Beware that it really means that the named user(s) can log in as
1252 which includes bin, daemon, adm, and other accounts that own critical
1253 binaries and directories.
1254 Using a user name practically grants the user root access.
1255 The only valid use for user names that I can think
1256 of is in negative entries.
1258 Note that this warning also applies to rsh/rlogin.
1259 .It Pa /etc/shosts.equiv
1260 This is processed exactly as
1261 .Pa /etc/hosts.equiv .
1262 However, this file may be useful in environments that want to run both
1264 .It Pa $HOME/.ssh/environment
1265 This file is read into the environment at login (if it exists).
1266 It can only contain empty lines, comment lines (that start with
1268 and assignment lines of the form name=value.
1269 The file should be writable
1270 only by the user; it need not be readable by anyone else.
1271 .It Pa $HOME/.ssh/rc
1272 If this file exists, it is run with /bin/sh after reading the
1273 environment files but before starting the user's shell or command.
1274 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1282 The primary purpose of this file is to run any initialization routines
1283 which may be needed before the user's home directory becomes
1284 accessible; AFS is a particular example of such an environment.
1286 This file will probably contain some initialization code followed by
1287 something similar to:
1289 if read proto cookie; then
1290 echo add $DISPLAY $proto $cookie | xauth -q -
1294 If this file does not exist,
1297 does not exist either, xauth is used to store the cookie.
1299 This file should be writable only by the user, and need not be
1300 readable by anyone else.
1304 This can be used to specify
1305 machine-specific login-time initializations globally.
1306 This file should be writable only by root, and should be world-readable.
1309 OpenSSH is a derivative of the original and free
1310 ssh 1.2.12 release by Tatu Ylonen.
1311 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1312 Theo de Raadt and Dug Song
1313 removed many bugs, re-added newer features and
1315 Markus Friedl contributed the support for SSH
1316 protocol versions 1.5 and 2.0.
1333 .%T "SSH Protocol Architecture"
1334 .%N draft-ietf-secsh-architecture-09.txt
1336 .%O work in progress material
1342 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1343 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1345 .%O work in progress material
andersk / openssh.git / blob