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 1.159 2001/12/28 22:37:48 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 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
248 Multiple port options are permitted.
249 Ports specified in the configuration file are ignored when a
250 command-line port is specified.
253 Nothing is sent to the system log.
254 Normally the beginning,
255 authentication, and termination of each connection is logged.
258 Only check the validity of the configuration file and sanity of the keys.
259 This is useful for updating
261 reliably as configuration options may change.
263 This option is used to specify the size of the field
266 structure that holds the remote host name.
267 If the resolved host name is longer than
269 the dotted decimal value will be used instead.
270 This allows hosts with very long host names that
271 overflow this field to still be uniquely identified.
274 indicates that only dotted decimal addresses
275 should be put into the
279 is also be used to prevent
281 from making DNS requests unless the authentication
282 mechanism or configuration requires it.
283 Authentication mechanisms that may require DNS include
284 .Cm RhostsAuthentication ,
285 .Cm RhostsRSAAuthentication ,
286 .Cm HostbasedAuthentication
288 .Cm from="pattern-list"
289 option in a key file.
291 When this option is specified
293 will not detach and does not become a daemon.
294 This allows easy monitoring of
299 to use IPv4 addresses only.
303 to use IPv6 addresses only.
305 .Sh CONFIGURATION FILE
307 reads configuration data from
309 (or the file specified with
311 on the command line).
312 The file contains keyword-argument pairs, one per line.
315 and empty lines are interpreted as comments.
318 keywords and their meanings are as follows (note that
319 keywords are case-insensitive and arguments are case-sensitive):
321 .It Cm AFSTokenPassing
322 Specifies whether an AFS token may be forwarded to the server.
326 This keyword can be followed by a list of group names, separated
328 If specified, login is allowed only for users whose primary
329 group or supplementary group list matches one of the patterns.
334 wildcards in the patterns.
335 Only group names are valid; a numerical group ID is not recognized.
336 By default login is allowed regardless of the group list.
338 .It Cm AllowTcpForwarding
339 Specifies whether TCP forwarding is permitted.
342 Note that disabling TCP forwarding does not improve security unless
343 users are also denied shell access, as they can always install their
347 This keyword can be followed by a list of user names, separated
349 If specified, login is allowed only for users names that
350 match one of the patterns.
355 wildcards in the patterns.
356 Only user names are valid; a numerical user ID is not recognized.
357 By default login is allowed regardless of the user name.
358 If the pattern takes the form USER@HOST then USER and HOST
359 are separately checked, restricting logins to particular
360 users from particular hosts.
362 .It Cm AuthorizedKeysFile
363 Specifies the file that contains the public keys that can be used
364 for user authentication.
365 .Cm AuthorizedKeysFile
366 may contain tokens of the form %T which are substituted during connection
367 set-up. The following tokens are defined: %% is replaced by a literal '%',
368 %h is replaced by the home directory of the user being authenticated and
369 %u is replaced by the username of that user.
371 .Cm AuthorizedKeysFile
372 is taken to be an absolute path or one relative to the user's home
375 .Dq .ssh/authorized_keys
377 In some jurisdictions, sending a warning message before authentication
378 may be relevant for getting legal protection.
379 The contents of the specified file are sent to the remote user before
380 authentication is allowed.
381 This option is only available for protocol version 2.
383 .It Cm ChallengeResponseAuthentication
384 Specifies whether challenge response authentication is allowed.
385 All authentication styles from
391 Specifies the ciphers allowed for protocol version 2.
392 Multiple ciphers must be comma-separated.
394 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
395 .It Cm ClientAliveInterval
396 Sets a timeout interval in seconds after which if no data has been received
399 will send a message through the encrypted
400 channel to request a response from the client.
402 is 0, indicating that these messages will not be sent to the client.
403 This option applies to protocol version 2 only.
404 .It Cm ClientAliveCountMax
405 Sets the number of client alive messages (see above) which may be
408 receiving any messages back from the client. If this threshold is
409 reached while client alive messages are being sent,
411 will disconnect the client, terminating the session. It is important
412 to note that the use of client alive messages is very different from
414 (below). The client alive messages are sent through the
415 encrypted channel and therefore will not be spoofable. The TCP keepalive
418 is spoofable. The client alive mechanism is valuable when the client or
419 server depend on knowing when a connection has become inactive.
421 The default value is 3. If
422 .Cm ClientAliveInterval
423 (above) is set to 15, and
424 .Cm ClientAliveCountMax
425 is left at the default, unresponsive ssh clients
426 will be disconnected after approximately 45 seconds.
428 This keyword can be followed by a number of group names, separated
430 Users whose primary group or supplementary group list matches
431 one of the patterns aren't allowed to log in.
436 wildcards in the patterns.
437 Only group names are valid; a numerical group ID is not recognized.
438 By default login is allowed regardless of the group list.
441 This keyword can be followed by a number of user names, separated
443 Login is disallowed for user names that match one of the patterns.
447 can be used as wildcards in the patterns.
448 Only user names are valid; a numerical user ID is not recognized.
449 By default login is allowed regardless of the user name.
451 Specifies whether remote hosts are allowed to connect to ports
452 forwarded for the client.
455 binds remote port forwardings to the loopback addresss. This
456 prevents other remote hosts from connecting to forwarded ports.
458 can be used to specify that
460 should bind remote port forwardings to the wildcard address,
461 thus allowing remote hosts to connect to forwarded ports.
468 .It Cm HostbasedAuthentication
469 Specifies whether rhosts or /etc/hosts.equiv authentication together
470 with successful public key client host authentication is allowed
471 (hostbased authentication).
472 This option is similar to
473 .Cm RhostsRSAAuthentication
474 and applies to protocol version 2 only.
478 Specifies the file containing the private host keys (default
479 .Pa /etc/ssh_host_key )
480 used by SSH protocol versions 1 and 2.
483 will refuse to use a file if it is group/world-accessible.
484 It is possible to have multiple host key files.
486 keys are used for version 1 and
490 are used for version 2 of the SSH protocol.
496 files will not be used in
497 .Cm RhostsAuthentication ,
498 .Cm RhostsRSAAuthentication
500 .Cm HostbasedAuthentication .
504 .Pa /etc/shosts.equiv
508 .It Cm IgnoreUserKnownHosts
511 should ignore the user's
512 .Pa $HOME/.ssh/known_hosts
514 .Cm RhostsRSAAuthentication
516 .Cm HostbasedAuthentication .
520 Specifies whether the system should send keepalive messages to the
522 If they are sent, death of the connection or crash of one
523 of the machines will be properly noticed.
524 However, this means that
525 connections will die if the route is down temporarily, and some people
527 On the other hand, if keepalives are not sent,
528 sessions may hang indefinitely on the server, leaving
530 users and consuming server resources.
534 (to send keepalives), and the server will notice
535 if the network goes down or the client host reboots.
536 This avoids infinitely hanging sessions.
538 To disable keepalives, the value should be set to
540 in both the server and the client configuration files.
541 .It Cm KerberosAuthentication
542 Specifies whether Kerberos authentication is allowed.
543 This can be in the form of a Kerberos ticket, or if
544 .Cm PasswordAuthentication
545 is yes, the password provided by the user will be validated through
547 To use this option, the server needs a
548 Kerberos servtab which allows the verification of the KDC's identity.
551 .It Cm KerberosOrLocalPasswd
552 If set then if password authentication through Kerberos fails then
553 the password will be validated via any additional local mechanism
558 .It Cm KerberosTgtPassing
559 Specifies whether a Kerberos TGT may be forwarded to the server.
562 as this only works when the Kerberos KDC is actually an AFS kaserver.
563 .It Cm KerberosTicketCleanup
564 Specifies whether to automatically destroy the user's ticket cache
568 .It Cm KeyRegenerationInterval
569 In protocol version 1, the ephemeral server key is automatically regenerated
570 after this many seconds (if it has been used).
571 The purpose of regeneration is to prevent
572 decrypting captured sessions by later breaking into the machine and
574 The key is never stored anywhere.
575 If the value is 0, the key is never regenerated.
576 The default is 3600 (seconds).
578 Specifies the local addresses
581 The following forms may be used:
583 .Bl -item -offset indent -compact
587 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
592 .Ar host No | Ar IPv4_addr No : Ar port
598 .Ar host No | Ar IPv6_addr Oc : Ar port
606 will listen on the address and all prior
608 options specified. The default is to listen on all local
611 options are permitted. Additionally, any
613 options must precede this option for non port qualified addresses.
614 .It Cm LoginGraceTime
615 The server disconnects after this time if the user has not
616 successfully logged in.
617 If the value is 0, there is no time limit.
618 The default is 600 (seconds).
620 Gives the verbosity level that is used when logging messages from
622 The possible values are:
623 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
624 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
625 and DEBUG3 each specify higher levels of debugging output.
626 Logging with a DEBUG level violates the privacy of users
627 and is not recommended.
629 Specifies the available MAC (message authentication code) algorithms.
630 The MAC algorithm is used in protocol version 2
631 for data integrity protection.
632 Multiple algorithms must be comma-separated.
634 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
636 Specifies the maximum number of concurrent unauthenticated connections to the
639 Additional connections will be dropped until authentication succeeds or the
641 expires for a connection.
644 Alternatively, random early drop can be enabled by specifying
645 the three colon separated values
649 will refuse connection attempts with a probability of
652 if there are currently
655 unauthenticated connections.
656 The probability increases linearly and all connection attempts
657 are refused if the number of unauthenticated connections reaches
660 .It Cm PAMAuthenticationViaKbdInt
661 Specifies whether PAM challenge response authentication is allowed. This
662 allows the use of most PAM challenge response authentication modules, but
663 it will allow password authentication regardless of whether
664 .Cm PasswordAuthentication
668 .It Cm PasswordAuthentication
669 Specifies whether password authentication is allowed.
672 .It Cm PermitEmptyPasswords
673 When password authentication is allowed, it specifies whether the
674 server allows login to accounts with empty password strings.
677 .It Cm PermitRootLogin
678 Specifies whether root can login using
682 .Dq without-password ,
683 .Dq forced-commands-only
689 If this option is set to
691 password authentication is disabled for root.
693 If this option is set to
694 .Dq forced-commands-only
695 root login with public key authentication will be allowed,
698 option has been specified
699 (which may be useful for taking remote backups even if root login is
700 normally not allowed). All other authentication methods are disabled
703 If this option is set to
705 root is not allowed to login.
707 Specifies the file that contains the process identifier of the
711 .Pa /var/run/sshd.pid .
713 Specifies the port number that
717 Multiple options of this type are permitted.
723 should print the date and time when the user last logged in.
731 when a user logs in interactively.
732 (On some systems it is also printed by the shell,
738 Specifies the protocol versions
741 The possible values are
745 Multiple versions must be comma-separated.
748 .It Cm PubkeyAuthentication
749 Specifies whether public key authentication is allowed.
752 Note that this option applies to protocol version 2 only.
753 .It Cm ReverseMappingCheck
756 should try to verify the remote host name and check that
757 the resolved host name for the remote IP address maps back to the
758 very same IP address.
761 .It Cm RhostsAuthentication
762 Specifies whether authentication using rhosts or /etc/hosts.equiv
764 Normally, this method should not be permitted because it is insecure.
765 .Cm RhostsRSAAuthentication
767 instead, because it performs RSA-based host authentication in addition
768 to normal rhosts or /etc/hosts.equiv authentication.
771 This option applies to protocol version 1 only.
772 .It Cm RhostsRSAAuthentication
773 Specifies whether rhosts or /etc/hosts.equiv authentication together
774 with successful RSA host authentication is allowed.
777 This option applies to protocol version 1 only.
778 .It Cm RSAAuthentication
779 Specifies whether pure RSA authentication is allowed.
782 This option applies to protocol version 1 only.
784 Defines the number of bits in the ephemeral protocol version 1 server key.
785 The minimum value is 512, and the default is 768.
789 should check file modes and ownership of the
790 user's files and home directory before accepting login.
791 This is normally desirable because novices sometimes accidentally leave their
792 directory or files world-writable.
796 Configures an external subsystem (e.g., file transfer daemon).
797 Arguments should be a subsystem name and a command to execute upon subsystem
803 file transfer subsystem.
804 By default no subsystems are defined.
805 Note that this option applies to protocol version 2 only.
806 .It Cm SyslogFacility
807 Gives the facility code that is used when logging messages from
809 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
810 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
815 is used for interactive login sessions.
820 is never used for remote command execution.
821 Note also, that if this is enabled,
823 will be disabled because
825 does not know how to handle
828 .It Cm X11DisplayOffset
829 Specifies the first display number available for
834 from interfering with real X11 servers.
837 Specifies whether X11 forwarding is permitted.
840 Note that disabling X11 forwarding does not improve security in any
841 way, as users can always install their own forwarders.
842 X11 forwarding is automatically disabled if
846 Specifies the location of the
850 .Pa /usr/X11R6/bin/xauth .
855 command-line arguments and configuration file options that specify time
856 may be expressed using a sequence of the form:
858 .Ar time Oo Ar qualifier Oc ,
862 is a positive integer value and
864 is one of the following:
866 .Bl -tag -width Ds -compact -offset indent
881 Each member of the sequence is added together to calculate
882 the total time value.
884 Time format examples:
886 .Bl -tag -width Ds -compact -offset indent
888 600 seconds (10 minutes)
892 1 hour 30 minutes (90 minutes)
895 When a user successfully logs in,
898 .Bl -enum -offset indent
900 If the login is on a tty, and no command has been specified,
901 prints last login time and
903 (unless prevented in the configuration file or by
904 .Pa $HOME/.hushlogin ;
909 If the login is on a tty, records login time.
913 if it exists, prints contents and quits
916 Changes to run with normal user privileges.
918 Sets up basic environment.
921 .Pa $HOME/.ssh/environment
924 Changes to user's home directory.
928 exists, runs it; else if
931 it; otherwise runs xauth.
934 files are given the X11
935 authentication protocol and cookie in standard input.
937 Runs user's shell or command.
939 .Sh AUTHORIZED_KEYS FILE FORMAT
940 .Pa $HOME/.ssh/authorized_keys
941 is the default file that lists the public keys that are
942 permitted for RSA authentication in protocol version 1
943 and for public key authentication (PubkeyAuthentication)
944 in protocol version 2.
945 .Cm AuthorizedKeysFile
946 may be used to specify an alternative file.
948 Each line of the file contains one
949 key (empty lines and lines starting with a
953 Each RSA public key consists of the following fields, separated by
954 spaces: options, bits, exponent, modulus, comment.
955 Each protocol version 2 public key consists of:
956 options, keytype, base64 encoded key, comment.
958 are optional; its presence is determined by whether the line starts
959 with a number or not (the option field never starts with a number).
960 The bits, exponent, modulus and comment fields give the RSA key for
961 protocol version 1; the
962 comment field is not used for anything (but may be convenient for the
963 user to identify the key).
964 For protocol version 2 the keytype is
969 Note that lines in this file are usually several hundred bytes long
970 (because of the size of the RSA key modulus).
971 You don't want to type them in; instead, copy the
978 The options (if present) consist of comma-separated option
980 No spaces are permitted, except within double quotes.
981 The following option specifications are supported (note
982 that option keywords are case-insensitive):
984 .It Cm from="pattern-list"
985 Specifies that in addition to RSA authentication, the canonical name
986 of the remote host must be present in the comma-separated list of
992 The list may also contain
993 patterns negated by prefixing them with
995 if the canonical host name matches a negated pattern, the key is not accepted.
997 of this option is to optionally increase security: RSA authentication
998 by itself does not trust the network or name servers or anything (but
999 the key); however, if somebody somehow steals the key, the key
1000 permits an intruder to log in from anywhere in the world.
1001 This additional option makes using a stolen key more difficult (name
1002 servers and/or routers would have to be compromised in addition to
1004 .It Cm command="command"
1005 Specifies that the command is executed whenever this key is used for
1007 The command supplied by the user (if any) is ignored.
1008 The command is run on a pty if the client requests a pty;
1009 otherwise it is run without a tty.
1010 If a 8-bit clean channel is required,
1011 one must not request a pty or should specify
1013 A quote may be included in the command by quoting it with a backslash.
1014 This option might be useful
1015 to restrict certain RSA keys to perform just a specific operation.
1016 An example might be a key that permits remote backups but nothing else.
1017 Note that the client may specify TCP/IP and/or X11
1018 forwarding unless they are explicitly prohibited.
1019 Note that this option applies to shell, command or subsystem execution.
1020 .It Cm environment="NAME=value"
1021 Specifies that the string is to be added to the environment when
1022 logging in using this key.
1023 Environment variables set this way
1024 override other default environment values.
1025 Multiple options of this type are permitted.
1026 This option is automatically disabled if
1029 .It Cm no-port-forwarding
1030 Forbids TCP/IP forwarding when this key is used for authentication.
1031 Any port forward requests by the client will return an error.
1032 This might be used, e.g., in connection with the
1035 .It Cm no-X11-forwarding
1036 Forbids X11 forwarding when this key is used for authentication.
1037 Any X11 forward requests by the client will return an error.
1038 .It Cm no-agent-forwarding
1039 Forbids authentication agent forwarding when this key is used for
1042 Prevents tty allocation (a request to allocate a pty will fail).
1043 .It Cm permitopen="host:port"
1046 port forwarding such that it may only connect to the specified host and
1048 IPv6 addresses can be specified with an alternative syntax:
1052 options may be applied separated by commas. No pattern matching is
1053 performed on the specified hostnames, they must be literal domains or
1057 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1059 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1061 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1063 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1064 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1066 .Pa /etc/ssh_known_hosts ,
1068 .Pa $HOME/.ssh/known_hosts
1069 files contain host public keys for all known hosts.
1070 The global file should
1071 be prepared by the administrator (optional), and the per-user file is
1072 maintained automatically: whenever the user connects from an unknown host
1073 its key is added to the per-user file.
1075 Each line in these files contains the following fields: hostnames,
1076 bits, exponent, modulus, comment.
1077 The fields are separated by spaces.
1079 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1080 wildcards); each pattern in turn is matched against the canonical host
1081 name (when authenticating a client) or against the user-supplied
1082 name (when authenticating a server).
1083 A pattern may also be preceded by
1085 to indicate negation: if the host name matches a negated
1086 pattern, it is not accepted (by that line) even if it matched another
1087 pattern on the line.
1089 Bits, exponent, and modulus are taken directly from the RSA host key; they
1090 can be obtained, e.g., from
1091 .Pa /etc/ssh_host_key.pub .
1092 The optional comment field continues to the end of the line, and is not used.
1096 and empty lines are ignored as comments.
1098 When performing host authentication, authentication is accepted if any
1099 matching line has the proper key.
1100 It is thus permissible (but not
1101 recommended) to have several lines or different host keys for the same
1103 This will inevitably happen when short forms of host names
1104 from different domains are put in the file.
1106 that the files contain conflicting information; authentication is
1107 accepted if valid information can be found from either file.
1109 Note that the lines in these files are typically hundreds of characters
1110 long, and you definitely don't want to type in the host keys by hand.
1111 Rather, generate them by a script
1113 .Pa /etc/ssh_host_key.pub
1114 and adding the host names at the front.
1117 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1118 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1122 .It Pa /etc/sshd_config
1123 Contains configuration data for
1125 This file should be writable by root only, but it is recommended
1126 (though not necessary) that it be world-readable.
1127 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1128 These three files contain the private parts of the host keys.
1129 These files should only be owned by root, readable only by root, and not
1130 accessible to others.
1133 does not start if this file is group/world-accessible.
1134 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1135 These three files contain the public parts of the host keys.
1136 These files should be world-readable but writable only by
1138 Their contents should match the respective private parts.
1140 really used for anything; they are provided for the convenience of
1141 the user so their contents can be copied to known hosts files.
1142 These files are created using
1145 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1146 .It Pa /var/run/sshd.pid
1147 Contains the process ID of the
1149 listening for connections (if there are several daemons running
1150 concurrently for different ports, this contains the pid of the one
1152 The content of this file is not sensitive; it can be world-readable.
1153 .It Pa $HOME/.ssh/authorized_keys
1154 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1155 This file must be readable by root (which may on some machines imply
1156 it being world-readable if the user's home directory resides on an NFS
1158 It is recommended that it not be accessible by others.
1159 The format of this file is described above.
1160 Users will place the contents of their
1165 files into this file, as described in
1167 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1168 These files are consulted when using rhosts with RSA host
1169 authentication or protocol version 2 hostbased authentication
1170 to check the public key of the host.
1171 The key must be listed in one of these files to be accepted.
1172 The client uses the same files
1173 to verify that it is connecting to the correct remote host.
1174 These files should be writable only by root/the owner.
1175 .Pa /etc/ssh_known_hosts
1176 should be world-readable, and
1177 .Pa $HOME/.ssh/known_hosts
1178 can but need not be world-readable.
1180 If this file exists,
1182 refuses to let anyone except root log in.
1183 The contents of the file
1184 are displayed to anyone trying to log in, and non-root connections are
1186 The file should be world-readable.
1187 .It Pa /etc/hosts.allow, /etc/hosts.deny
1188 Access controls that should be enforced by tcp-wrappers are defined here.
1189 Further details are described in
1190 .Xr hosts_access 5 .
1191 .It Pa $HOME/.rhosts
1192 This file contains host-username pairs, separated by a space, one per
1194 The given user on the corresponding host is permitted to log in
1196 The same file is used by rlogind and rshd.
1198 be writable only by the user; it is recommended that it not be
1199 accessible by others.
1201 If is also possible to use netgroups in the file.
1203 name may be of the form +@groupname to specify all hosts or all users
1205 .It Pa $HOME/.shosts
1207 this file is exactly the same as for
1209 However, this file is
1210 not used by rlogin and rshd, so using this permits access using SSH only.
1211 .It Pa /etc/hosts.equiv
1212 This file is used during
1215 In the simplest form, this file contains host names, one per line.
1217 those hosts are permitted to log in without a password, provided they
1218 have the same user name on both machines.
1219 The host name may also be
1220 followed by a user name; such users are permitted to log in as
1222 user on this machine (except root).
1223 Additionally, the syntax
1225 can be used to specify netgroups.
1226 Negated entries start with
1229 If the client host/user is successfully matched in this file, login is
1230 automatically permitted provided the client and server user names are the
1232 Additionally, successful RSA host authentication is normally required.
1233 This file must be writable only by root; it is recommended
1234 that it be world-readable.
1236 .Sy "Warning: It is almost never a good idea to use user names in"
1238 Beware that it really means that the named user(s) can log in as
1240 which includes bin, daemon, adm, and other accounts that own critical
1241 binaries and directories.
1242 Using a user name practically grants the user root access.
1243 The only valid use for user names that I can think
1244 of is in negative entries.
1246 Note that this warning also applies to rsh/rlogin.
1247 .It Pa /etc/shosts.equiv
1248 This is processed exactly as
1249 .Pa /etc/hosts.equiv .
1250 However, this file may be useful in environments that want to run both
1252 .It Pa $HOME/.ssh/environment
1253 This file is read into the environment at login (if it exists).
1254 It can only contain empty lines, comment lines (that start with
1256 and assignment lines of the form name=value.
1257 The file should be writable
1258 only by the user; it need not be readable by anyone else.
1259 .It Pa $HOME/.ssh/rc
1260 If this file exists, it is run with /bin/sh after reading the
1261 environment files but before starting the user's shell or command.
1262 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1270 The primary purpose of this file is to run any initialization routines
1271 which may be needed before the user's home directory becomes
1272 accessible; AFS is a particular example of such an environment.
1274 This file will probably contain some initialization code followed by
1275 something similar to:
1277 if read proto cookie; then
1278 echo add $DISPLAY $proto $cookie | xauth -q -
1282 If this file does not exist,
1285 does not exist either, xauth is used to store the cookie.
1287 This file should be writable only by the user, and need not be
1288 readable by anyone else.
1292 This can be used to specify
1293 machine-specific login-time initializations globally.
1294 This file should be writable only by root, and should be world-readable.
1297 OpenSSH is a derivative of the original and free
1298 ssh 1.2.12 release by Tatu Ylonen.
1299 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1300 Theo de Raadt and Dug Song
1301 removed many bugs, re-added newer features and
1303 Markus Friedl contributed the support for SSH
1304 protocol versions 1.5 and 2.0.
1321 .%T "SSH Protocol Architecture"
1322 .%N draft-ietf-secsh-architecture-09.txt
1324 .%O work in progress material
1330 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1331 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1333 .%O work in progress material
andersk / openssh.git / blob