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 1.143 2001/08/22 16:30:02 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
54 .Op Fl V Ar client_protocol_id
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 Specifies the port on which the server listens for connections
246 Nothing is sent to the system log.
247 Normally the beginning,
248 authentication, and termination of each connection is logged.
251 Only check the validity of the configuration file and sanity of the keys.
252 This is useful for updating
254 reliably as configuration options may change.
256 This option is used to specify the size of the field
259 structure that holds the remote host name.
260 If the resolved host name is longer than
262 the dotted decimal value will be used instead.
263 This allows hosts with very long host names that
264 overflow this field to still be uniquely identified.
267 indicates that only dotted decimal addresses
268 should be put into the
272 When this option is specified
274 will not detach and does not become a daemon.
275 This allows easy monitoring of
280 to use IPv4 addresses only.
284 to use IPv6 addresses only.
286 .Sh CONFIGURATION FILE
288 reads configuration data from
290 (or the file specified with
292 on the command line).
293 The file contains keyword-argument pairs, one per line.
296 and empty lines are interpreted as comments.
299 keywords and their meanings are as follows (note that
300 keywords are case-insensitive and arguments are case-sensitive):
302 .It Cm AFSTokenPassing
303 Specifies whether an AFS token may be forwarded to the server.
307 This keyword can be followed by a list of group names, separated
309 If specified, login is allowed only for users whose primary
310 group or supplementary group list matches one of the patterns.
315 wildcards in the patterns.
316 Only group names are valid; a numerical group ID isn't recognized.
317 By default login is allowed regardless of the group list.
319 .It Cm AllowTcpForwarding
320 Specifies whether TCP forwarding is permitted.
323 Note that disabling TCP forwarding does not improve security unless
324 users are also denied shell access, as they can always install their
328 This keyword can be followed by a list of user names, separated
330 If specified, login is allowed only for users names that
331 match one of the patterns.
336 wildcards in the patterns.
337 Only user names are valid; a numerical user ID isn't recognized.
338 By default login is allowed regardless of the user name.
339 If the pattern takes the form USER@HOST then USER and HOST
340 are separately checked, allowing you to restrict logins to particular
341 users from particular hosts.
343 .It Cm AuthorizedKeysFile
344 Specifies the file that contains the public keys that can be used
345 for user authentication.
346 .Cm AuthorizedKeysFile
347 may contain tokens of the form %T which are substituted during connection
348 set-up. The following tokens are defined: %% is replaced by a literal '%',
349 %h is replaced by the home directory of the user being authenticated and
350 %u is replaced by the username of that user.
352 .Cm AuthorizedKeysFile
353 is taken to be an absolute path or one relative to the user's home
356 .Dq .ssh/authorized_keys
358 In some jurisdictions, sending a warning message before authentication
359 may be relevant for getting legal protection.
360 The contents of the specified file are sent to the remote user before
361 authentication is allowed.
362 This option is only available for protocol version 2.
364 .It Cm ChallengeResponseAuthentication
365 Specifies whether challenge response authentication is allowed.
366 All authentication styles from
372 Specifies the ciphers allowed for protocol version 2.
373 Multiple ciphers must be comma-separated.
375 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
376 .It Cm ClientAliveInterval
377 Sets a timeout interval in seconds after which if no data has been received
380 will send a message through the encrypted
381 channel to request a response from the client.
383 is 0, indicating that these messages will not be sent to the client.
384 This option applies to protocol version 2 only.
385 .It Cm ClientAliveCountMax
386 Sets the number of client alive messages (see above) which may be
389 receiving any messages back from the client. If this threshold is
390 reached while client alive messages are being sent,
392 will disconnect the client, terminating the session. It is important
393 to note that the use of client alive messages is very different from
395 (below). The client alive messages are sent through the
396 encrypted channel and therefore will not be spoofable. The TCP keepalive
399 is spoofable. You want to use the client
400 alive mechanism when you are basing something important on
401 clients having an active connection to the server.
403 The default value is 3. If you set
404 .Cm ClientAliveInterval
405 (above) to 15, and leave this value at the default, unresponsive ssh clients
406 will be disconnected after approximately 45 seconds.
408 This keyword can be followed by a number of group names, separated
410 Users whose primary group or supplementary group list matches
411 one of the patterns aren't allowed to log in.
416 wildcards in the patterns.
417 Only group names are valid; a numerical group ID isn't recognized.
418 By default login is allowed regardless of the group list.
421 This keyword can be followed by a number of user names, separated
423 Login is disallowed for user names that match one of the patterns.
427 can be used as wildcards in the patterns.
428 Only user names are valid; a numerical user ID isn't recognized.
429 By default login is allowed regardless of the user name.
431 Specifies whether remote hosts are allowed to connect to ports
432 forwarded for the client.
439 .It Cm HostbasedAuthentication
440 Specifies whether rhosts or /etc/hosts.equiv authentication together
441 with successful public key client host authentication is allowed
442 (hostbased authentication).
443 This option is similar to
444 .Cm RhostsRSAAuthentication
445 and applies to protocol version 2 only.
449 Specifies the file containing the private host keys (default
450 .Pa /etc/ssh_host_key )
451 used by SSH protocol versions 1 and 2.
454 will refuse to use a file if it is group/world-accessible.
455 It is possible to have multiple host key files.
457 keys are used for version 1 and
461 are used for version 2 of the SSH protocol.
467 files will not be used in
468 .Cm RhostsAuthentication ,
469 .Cm RhostsRSAAuthentication
471 .Cm HostbasedAuthentication .
475 .Pa /etc/shosts.equiv
479 .It Cm IgnoreUserKnownHosts
482 should ignore the user's
483 .Pa $HOME/.ssh/known_hosts
485 .Cm RhostsRSAAuthentication
487 .Cm HostbasedAuthentication .
491 Specifies whether the system should send keepalive messages to the
493 If they are sent, death of the connection or crash of one
494 of the machines will be properly noticed.
495 However, this means that
496 connections will die if the route is down temporarily, and some people
498 On the other hand, if keepalives are not sent,
499 sessions may hang indefinitely on the server, leaving
501 users and consuming server resources.
505 (to send keepalives), and the server will notice
506 if the network goes down or the client host reboots.
507 This avoids infinitely hanging sessions.
509 To disable keepalives, the value should be set to
511 in both the server and the client configuration files.
512 .It Cm KerberosAuthentication
513 Specifies whether Kerberos authentication is allowed.
514 This can be in the form of a Kerberos ticket, or if
515 .Cm PasswordAuthentication
516 is yes, the password provided by the user will be validated through
518 To use this option, the server needs a
519 Kerberos servtab which allows the verification of the KDC's identity.
522 .It Cm KerberosOrLocalPasswd
523 If set then if password authentication through Kerberos fails then
524 the password will be validated via any additional local mechanism
529 .It Cm KerberosTgtPassing
530 Specifies whether a Kerberos TGT may be forwarded to the server.
533 as this only works when the Kerberos KDC is actually an AFS kaserver.
534 .It Cm KerberosTicketCleanup
535 Specifies whether to automatically destroy the user's ticket cache
539 .It Cm KeyRegenerationInterval
540 In protocol version 1, the ephemeral server key is automatically regenerated
541 after this many seconds (if it has been used).
542 The purpose of regeneration is to prevent
543 decrypting captured sessions by later breaking into the machine and
545 The key is never stored anywhere.
546 If the value is 0, the key is never regenerated.
547 The default is 3600 (seconds).
549 Specifies the local addresses
552 The following forms may be used:
554 .Bl -item -offset indent -compact
558 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
563 .Ar host No | Ar IPv4_addr No : Ar port
569 .Ar host No | Ar IPv6_addr Oc : Ar port
577 will listen on the address and all prior
579 options specified. The default is to listen on all local
582 options are permitted. Additionally, any
584 options must precede this option for non port qualified addresses.
585 .It Cm LoginGraceTime
586 The server disconnects after this time if the user has not
587 successfully logged in.
588 If the value is 0, there is no time limit.
589 The default is 600 (seconds).
591 Gives the verbosity level that is used when logging messages from
593 The possible values are:
594 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
596 Logging with level DEBUG violates the privacy of users
597 and is not recommended.
599 Specifies the available MAC (message authentication code) algorithms.
600 The MAC algorithm is used in protocol version 2
601 for data integrity protection.
602 Multiple algorithms must be comma-separated.
604 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
606 Specifies the maximum number of concurrent unauthenticated connections to the
609 Additional connections will be dropped until authentication succeeds or the
611 expires for a connection.
614 Alternatively, random early drop can be enabled by specifying
615 the three colon separated values
619 will refuse connection attempts with a probability of
622 if there are currently
625 unauthenticated connections.
626 The probability increases linearly and all connection attempts
627 are refused if the number of unauthenticated connections reaches
630 .It Cm PAMAuthenticationViaKbdInt
631 Specifies whether PAM challenge response authentication is allowed. This
632 allows the use of most PAM challenge response authentication modules, but
633 it will allow password authentication regardless of whether
634 .Cm PasswordAuthentication
638 .It Cm PasswordAuthentication
639 Specifies whether password authentication is allowed.
642 .It Cm PermitEmptyPasswords
643 When password authentication is allowed, it specifies whether the
644 server allows login to accounts with empty password strings.
647 .It Cm PermitRootLogin
648 Specifies whether root can login using
652 .Dq without-password ,
653 .Dq forced-commands-only
659 If this option is set to
661 password authentication is disabled for root.
663 If this option is set to
664 .Dq forced-commands-only
665 root login with public key authentication will be allowed,
668 option has been specified
669 (which may be useful for taking remote backups even if root login is
670 normally not allowed). All other authentication methods are disabled
673 If this option is set to
675 root is not allowed to login.
677 Specifies the file that contains the process identifier of the
681 .Pa /var/run/sshd.pid .
683 Specifies the port number that
687 Multiple options of this type are permitted.
693 should print the date and time when the user last logged in.
701 when a user logs in interactively.
702 (On some systems it is also printed by the shell,
708 Specifies the protocol versions
711 The possible values are
715 Multiple versions must be comma-separated.
718 .It Cm PubkeyAuthentication
719 Specifies whether public key authentication is allowed.
722 Note that this option applies to protocol version 2 only.
723 .It Cm ReverseMappingCheck
726 should try to verify the remote host name and check that
727 the resolved host name for the remote IP address maps back to the
728 very same IP address.
731 .It Cm RhostsAuthentication
732 Specifies whether authentication using rhosts or /etc/hosts.equiv
734 Normally, this method should not be permitted because it is insecure.
735 .Cm RhostsRSAAuthentication
737 instead, because it performs RSA-based host authentication in addition
738 to normal rhosts or /etc/hosts.equiv authentication.
741 This option applies to protocol version 1 only.
742 .It Cm RhostsRSAAuthentication
743 Specifies whether rhosts or /etc/hosts.equiv authentication together
744 with successful RSA host authentication is allowed.
747 This option applies to protocol version 1 only.
748 .It Cm RSAAuthentication
749 Specifies whether pure RSA authentication is allowed.
752 This option applies to protocol version 1 only.
754 Defines the number of bits in the ephemeral protocol version 1 server key.
755 The minimum value is 512, and the default is 768.
759 should check file modes and ownership of the
760 user's files and home directory before accepting login.
761 This is normally desirable because novices sometimes accidentally leave their
762 directory or files world-writable.
766 Configures an external subsystem (e.g., file transfer daemon).
767 Arguments should be a subsystem name and a command to execute upon subsystem
773 file transfer subsystem.
774 By default no subsystems are defined.
775 Note that this option applies to protocol version 2 only.
776 .It Cm SyslogFacility
777 Gives the facility code that is used when logging messages from
779 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
780 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
785 is used for interactive login sessions.
790 is never used for remote command execution.
791 Note also, that if this is enabled,
793 will be disabled because
795 does not know how to handle
798 .It Cm X11DisplayOffset
799 Specifies the first display number available for
804 from interfering with real X11 servers.
807 Specifies whether X11 forwarding is permitted.
810 Note that disabling X11 forwarding does not improve security in any
811 way, as users can always install their own forwarders.
812 X11 forwarding is automatically disabled if
816 Specifies the location of the
820 .Pa /usr/X11R6/bin/xauth .
825 command-line arguments and configuration file options that specify time
826 may be expressed using a sequence of the form:
828 .Ar time Oo Ar qualifier Oc ,
832 is a positive integer value and
834 is one of the following:
836 .Bl -tag -width Ds -compact -offset indent
851 Each member of the sequence is added together to calculate
852 the total time value.
854 Time format examples:
856 .Bl -tag -width Ds -compact -offset indent
858 600 seconds (10 minutes)
862 1 hour 30 minutes (90 minutes)
865 When a user successfully logs in,
868 .Bl -enum -offset indent
870 If the login is on a tty, and no command has been specified,
871 prints last login time and
873 (unless prevented in the configuration file or by
874 .Pa $HOME/.hushlogin ;
879 If the login is on a tty, records login time.
883 if it exists, prints contents and quits
886 Changes to run with normal user privileges.
888 Sets up basic environment.
891 .Pa $HOME/.ssh/environment
894 Changes to user's home directory.
898 exists, runs it; else if
901 it; otherwise runs xauth.
904 files are given the X11
905 authentication protocol and cookie in standard input.
907 Runs user's shell or command.
909 .Sh AUTHORIZED_KEYS FILE FORMAT
910 .Pa $HOME/.ssh/authorized_keys
911 is the default file that lists the public keys that are
912 permitted for RSA authentication in protocol version 1
913 and for public key authentication (PubkeyAuthentication)
914 in protocol version 2.
915 .Cm AuthorizedKeysFile
916 may be used to specify an alternative file.
918 Each line of the file contains one
919 key (empty lines and lines starting with a
923 Each RSA public key consists of the following fields, separated by
924 spaces: options, bits, exponent, modulus, comment.
925 Each protocol version 2 public key consists of:
926 options, keytype, base64 encoded key, comment.
928 are optional; its presence is determined by whether the line starts
929 with a number or not (the option field never starts with a number).
930 The bits, exponent, modulus and comment fields give the RSA key for
931 protocol version 1; the
932 comment field is not used for anything (but may be convenient for the
933 user to identify the key).
934 For protocol version 2 the keytype is
939 Note that lines in this file are usually several hundred bytes long
940 (because of the size of the RSA key modulus).
941 You don't want to type them in; instead, copy the
948 The options (if present) consist of comma-separated option
950 No spaces are permitted, except within double quotes.
951 The following option specifications are supported (note
952 that option keywords are case-insensitive):
954 .It Cm from="pattern-list"
955 Specifies that in addition to RSA authentication, the canonical name
956 of the remote host must be present in the comma-separated list of
962 The list may also contain
963 patterns negated by prefixing them with
965 if the canonical host name matches a negated pattern, the key is not accepted.
967 of this option is to optionally increase security: RSA authentication
968 by itself does not trust the network or name servers or anything (but
969 the key); however, if somebody somehow steals the key, the key
970 permits an intruder to log in from anywhere in the world.
971 This additional option makes using a stolen key more difficult (name
972 servers and/or routers would have to be compromised in addition to
974 .It Cm command="command"
975 Specifies that the command is executed whenever this key is used for
977 The command supplied by the user (if any) is ignored.
978 The command is run on a pty if the connection requests a pty;
979 otherwise it is run without a tty.
980 Note that if you want a 8-bit clean channel,
981 you must not request a pty or should specify
983 A quote may be included in the command by quoting it with a backslash.
984 This option might be useful
985 to restrict certain RSA keys to perform just a specific operation.
986 An example might be a key that permits remote backups but nothing else.
987 Note that the client may specify TCP/IP and/or X11
988 forwarding unless they are explicitly prohibited.
989 .It Cm environment="NAME=value"
990 Specifies that the string is to be added to the environment when
991 logging in using this key.
992 Environment variables set this way
993 override other default environment values.
994 Multiple options of this type are permitted.
995 .It Cm no-port-forwarding
996 Forbids TCP/IP forwarding when this key is used for authentication.
997 Any port forward requests by the client will return an error.
998 This might be used, e.g., in connection with the
1001 .It Cm no-X11-forwarding
1002 Forbids X11 forwarding when this key is used for authentication.
1003 Any X11 forward requests by the client will return an error.
1004 .It Cm no-agent-forwarding
1005 Forbids authentication agent forwarding when this key is used for
1008 Prevents tty allocation (a request to allocate a pty will fail).
1009 .It Cm permitopen="host:port"
1012 port forwarding such that it may only connect to the specified host and
1015 options may be applied separated by commas. No pattern matching is
1016 performed on the specified hostnames, they must be literal domains or
1020 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1022 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1024 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1026 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1027 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1029 .Pa /etc/ssh_known_hosts ,
1031 .Pa $HOME/.ssh/known_hosts
1032 files contain host public keys for all known hosts.
1033 The global file should
1034 be prepared by the administrator (optional), and the per-user file is
1035 maintained automatically: whenever the user connects from an unknown host
1036 its key is added to the per-user file.
1038 Each line in these files contains the following fields: hostnames,
1039 bits, exponent, modulus, comment.
1040 The fields are separated by spaces.
1042 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1043 wildcards); each pattern in turn is matched against the canonical host
1044 name (when authenticating a client) or against the user-supplied
1045 name (when authenticating a server).
1046 A pattern may also be preceded by
1048 to indicate negation: if the host name matches a negated
1049 pattern, it is not accepted (by that line) even if it matched another
1050 pattern on the line.
1052 Bits, exponent, and modulus are taken directly from the RSA host key; they
1053 can be obtained, e.g., from
1054 .Pa /etc/ssh_host_key.pub .
1055 The optional comment field continues to the end of the line, and is not used.
1059 and empty lines are ignored as comments.
1061 When performing host authentication, authentication is accepted if any
1062 matching line has the proper key.
1063 It is thus permissible (but not
1064 recommended) to have several lines or different host keys for the same
1066 This will inevitably happen when short forms of host names
1067 from different domains are put in the file.
1069 that the files contain conflicting information; authentication is
1070 accepted if valid information can be found from either file.
1072 Note that the lines in these files are typically hundreds of characters
1073 long, and you definitely don't want to type in the host keys by hand.
1074 Rather, generate them by a script
1076 .Pa /etc/ssh_host_key.pub
1077 and adding the host names at the front.
1080 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1081 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1085 .It Pa /etc/sshd_config
1086 Contains configuration data for
1088 This file should be writable by root only, but it is recommended
1089 (though not necessary) that it be world-readable.
1090 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1091 These three files contain the private parts of the host keys.
1092 These files should only be owned by root, readable only by root, and not
1093 accessible to others.
1096 does not start if this file is group/world-accessible.
1097 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1098 These three files contain the public parts of the host keys.
1099 These files should be world-readable but writable only by
1101 Their contents should match the respective private parts.
1103 really used for anything; they are provided for the convenience of
1104 the user so their contents can be copied to known hosts files.
1105 These files are created using
1108 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1109 .It Pa /var/run/sshd.pid
1110 Contains the process ID of the
1112 listening for connections (if there are several daemons running
1113 concurrently for different ports, this contains the pid of the one
1115 The content of this file is not sensitive; it can be world-readable.
1116 .It Pa $HOME/.ssh/authorized_keys
1117 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1118 This file must be readable by root (which may on some machines imply
1119 it being world-readable if the user's home directory resides on an NFS
1121 It is recommended that it not be accessible by others.
1122 The format of this file is described above.
1123 Users will place the contents of their
1128 files into this file, as described in
1130 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1131 These files are consulted when using rhosts with RSA host
1132 authentication or protocol version 2 hostbased authentication
1133 to check the public key of the host.
1134 The key must be listed in one of these files to be accepted.
1135 The client uses the same files
1136 to verify that it is connecting to the correct remote host.
1137 These files should be writable only by root/the owner.
1138 .Pa /etc/ssh_known_hosts
1139 should be world-readable, and
1140 .Pa $HOME/.ssh/known_hosts
1141 can but need not be world-readable.
1143 If this file exists,
1145 refuses to let anyone except root log in.
1146 The contents of the file
1147 are displayed to anyone trying to log in, and non-root connections are
1149 The file should be world-readable.
1150 .It Pa /etc/hosts.allow, /etc/hosts.deny
1153 support, tcp-wrappers access controls may be defined here as described in
1154 .Xr hosts_access 5 .
1155 .It Pa $HOME/.rhosts
1156 This file contains host-username pairs, separated by a space, one per
1158 The given user on the corresponding host is permitted to log in
1160 The same file is used by rlogind and rshd.
1162 be writable only by the user; it is recommended that it not be
1163 accessible by others.
1165 If is also possible to use netgroups in the file.
1167 name may be of the form +@groupname to specify all hosts or all users
1169 .It Pa $HOME/.shosts
1171 this file is exactly the same as for
1173 However, this file is
1174 not used by rlogin and rshd, so using this permits access using SSH only.
1175 .It Pa /etc/hosts.equiv
1176 This file is used during
1179 In the simplest form, this file contains host names, one per line.
1181 those hosts are permitted to log in without a password, provided they
1182 have the same user name on both machines.
1183 The host name may also be
1184 followed by a user name; such users are permitted to log in as
1186 user on this machine (except root).
1187 Additionally, the syntax
1189 can be used to specify netgroups.
1190 Negated entries start with
1193 If the client host/user is successfully matched in this file, login is
1194 automatically permitted provided the client and server user names are the
1196 Additionally, successful RSA host authentication is normally required.
1197 This file must be writable only by root; it is recommended
1198 that it be world-readable.
1200 .Sy "Warning: It is almost never a good idea to use user names in"
1202 Beware that it really means that the named user(s) can log in as
1204 which includes bin, daemon, adm, and other accounts that own critical
1205 binaries and directories.
1206 Using a user name practically grants the user root access.
1207 The only valid use for user names that I can think
1208 of is in negative entries.
1210 Note that this warning also applies to rsh/rlogin.
1211 .It Pa /etc/shosts.equiv
1212 This is processed exactly as
1213 .Pa /etc/hosts.equiv .
1214 However, this file may be useful in environments that want to run both
1216 .It Pa $HOME/.ssh/environment
1217 This file is read into the environment at login (if it exists).
1218 It can only contain empty lines, comment lines (that start with
1220 and assignment lines of the form name=value.
1221 The file should be writable
1222 only by the user; it need not be readable by anyone else.
1223 .It Pa $HOME/.ssh/rc
1224 If this file exists, it is run with /bin/sh after reading the
1225 environment files but before starting the user's shell or command.
1226 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1234 The primary purpose of this file is to run any initialization routines
1235 which may be needed before the user's home directory becomes
1236 accessible; AFS is a particular example of such an environment.
1238 This file will probably contain some initialization code followed by
1239 something similar to:
1241 if read proto cookie; then
1242 echo add $DISPLAY $proto $cookie | xauth -q -
1246 If this file does not exist,
1249 does not exist either, xauth is used to store the cookie.
1251 This file should be writable only by the user, and need not be
1252 readable by anyone else.
1256 This can be used to specify
1257 machine-specific login-time initializations globally.
1258 This file should be writable only by root, and should be world-readable.
1261 OpenSSH is a derivative of the original and free
1262 ssh 1.2.12 release by Tatu Ylonen.
1263 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1264 Theo de Raadt and Dug Song
1265 removed many bugs, re-added newer features and
1267 Markus Friedl contributed the support for SSH
1268 protocol versions 1.5 and 2.0.
1285 .%T "SSH Protocol Architecture"
1286 .%N draft-ietf-secsh-architecture-09.txt
1288 .%O work in progress material
1294 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1295 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1297 .%O work in progress material