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 1.170 2002/02/28 20:46:10 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/ssh/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/ssh_host_key
218 for protocol version 1, and
219 .Pa /etc/ssh/ssh_host_rsa_key
221 .Pa /etc/ssh/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.
296 Configuration options that require DNS include using a
302 When this option is specified
304 will not detach and does not become a daemon.
305 This allows easy monitoring of
310 to use IPv4 addresses only.
314 to use IPv6 addresses only.
316 .Sh CONFIGURATION FILE
318 reads configuration data from
319 .Pa /etc/ssh/sshd_config
320 (or the file specified with
322 on the command line).
323 The file contains keyword-argument pairs, one per line.
326 and empty lines are interpreted as comments.
329 keywords and their meanings are as follows (note that
330 keywords are case-insensitive and arguments are case-sensitive):
332 .It Cm AFSTokenPassing
333 Specifies whether an AFS token may be forwarded to the server.
337 This keyword can be followed by a list of group name patterns, separated
339 If specified, login is allowed only for users whose primary
340 group or supplementary group list matches one of the patterns.
345 wildcards in the patterns.
346 Only group names are valid; a numerical group ID is not recognized.
347 By default, login is allowed for all groups.
349 .It Cm AllowTcpForwarding
350 Specifies whether TCP forwarding is permitted.
353 Note that disabling TCP forwarding does not improve security unless
354 users are also denied shell access, as they can always install their
358 This keyword can be followed by a list of user name patterns, separated
360 If specified, login is allowed only for users names that
361 match one of the patterns.
366 wildcards in the patterns.
367 Only user names are valid; a numerical user ID is not recognized.
368 By default, login is allowed for all users.
369 If the pattern takes the form USER@HOST then USER and HOST
370 are separately checked, restricting logins to particular
371 users from particular hosts.
373 .It Cm AuthorizedKeysFile
374 Specifies the file that contains the public keys that can be used
375 for user authentication.
376 .Cm AuthorizedKeysFile
377 may contain tokens of the form %T which are substituted during connection
378 set-up. The following tokens are defined: %% is replaced by a literal '%',
379 %h is replaced by the home directory of the user being authenticated and
380 %u is replaced by the username of that user.
382 .Cm AuthorizedKeysFile
383 is taken to be an absolute path or one relative to the user's home
386 .Dq .ssh/authorized_keys .
388 In some jurisdictions, sending a warning message before authentication
389 may be relevant for getting legal protection.
390 The contents of the specified file are sent to the remote user before
391 authentication is allowed.
392 This option is only available for protocol version 2.
394 .It Cm ChallengeResponseAuthentication
395 Specifies whether challenge response authentication is allowed.
396 All authentication styles from
402 Specifies the ciphers allowed for protocol version 2.
403 Multiple ciphers must be comma-separated.
407 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
408 aes192-cbc,aes256-cbc''
410 .It Cm ClientAliveInterval
411 Sets a timeout interval in seconds after which if no data has been received
414 will send a message through the encrypted
415 channel to request a response from the client.
417 is 0, indicating that these messages will not be sent to the client.
418 This option applies to protocol version 2 only.
419 .It Cm ClientAliveCountMax
420 Sets the number of client alive messages (see above) which may be
423 receiving any messages back from the client. If this threshold is
424 reached while client alive messages are being sent,
426 will disconnect the client, terminating the session. It is important
427 to note that the use of client alive messages is very different from
429 (below). The client alive messages are sent through the
430 encrypted channel and therefore will not be spoofable. The TCP keepalive
433 is spoofable. The client alive mechanism is valuable when the client or
434 server depend on knowing when a connection has become inactive.
436 The default value is 3. If
437 .Cm ClientAliveInterval
438 (above) is set to 15, and
439 .Cm ClientAliveCountMax
440 is left at the default, unresponsive ssh clients
441 will be disconnected after approximately 45 seconds.
443 This keyword can be followed by a list of group name patterns, separated
445 Login is disallowed for users whose primary group or supplementary
446 group list matches one of the patterns.
451 wildcards in the patterns.
452 Only group names are valid; a numerical group ID is not recognized.
453 By default, login is allowed for all groups.
456 This keyword can be followed by a list of user name patterns, separated
458 Login is disallowed for user names that match one of the patterns.
462 can be used as wildcards in the patterns.
463 Only user names are valid; a numerical user ID is not recognized.
464 By default, login is allowed for all users.
465 If the pattern takes the form USER@HOST then USER and HOST
466 are separately checked, restricting logins to particular
467 users from particular hosts.
469 Specifies whether remote hosts are allowed to connect to ports
470 forwarded for the client.
473 binds remote port forwardings to the loopback addresss. This
474 prevents other remote hosts from connecting to forwarded ports.
476 can be used to specify that
478 should bind remote port forwardings to the wildcard address,
479 thus allowing remote hosts to connect to forwarded ports.
486 .It Cm HostbasedAuthentication
487 Specifies whether rhosts or /etc/hosts.equiv authentication together
488 with successful public key client host authentication is allowed
489 (hostbased authentication).
490 This option is similar to
491 .Cm RhostsRSAAuthentication
492 and applies to protocol version 2 only.
495 .It Cm GssapiAuthentication
496 Specifies whether authentication based on GSSAPI may be used, either using
497 the result of a successful key exchange, or using GSSAPI user
501 Note that this option applies to protocol version 2 only.
502 .It Cm GssapiKeyExchange
503 Specifies whether key exchange based on GSSAPI may be used. When using
504 GSSAPI key exchange the server need not have a host key.
507 Note that this option applies to protocol version 2 only.
508 .It Cm GssapiUseSessionCredCache
509 Specifies whether a unique credentials cache name should be generated per
510 session for storing delegated credentials.
513 Note that this option applies to protocol version 2 only.
515 Specifies a file containing a private host key
518 .Pa /etc/ssh/ssh_host_key
519 for protocol version 1, and
520 .Pa /etc/ssh/ssh_host_rsa_key
522 .Pa /etc/ssh/ssh_host_dsa_key
523 for protocol version 2.
526 will refuse to use a file if it is group/world-accessible.
527 It is possible to have multiple host key files.
529 keys are used for version 1 and
533 are used for version 2 of the SSH protocol.
539 files will not be used in
540 .Cm RhostsAuthentication ,
541 .Cm RhostsRSAAuthentication
543 .Cm HostbasedAuthentication .
547 .Pa /etc/shosts.equiv
551 .It Cm IgnoreUserKnownHosts
554 should ignore the user's
555 .Pa $HOME/.ssh/known_hosts
557 .Cm RhostsRSAAuthentication
559 .Cm HostbasedAuthentication .
563 Specifies whether the system should send TCP keepalive messages to the
565 If they are sent, death of the connection or crash of one
566 of the machines will be properly noticed.
567 However, this means that
568 connections will die if the route is down temporarily, and some people
570 On the other hand, if keepalives are not sent,
571 sessions may hang indefinitely on the server, leaving
573 users and consuming server resources.
577 (to send keepalives), and the server will notice
578 if the network goes down or the client host crashes.
579 This avoids infinitely hanging sessions.
581 To disable keepalives, the value should be set to
583 .It Cm KerberosAuthentication
584 Specifies whether Kerberos authentication is allowed.
585 This can be in the form of a Kerberos ticket, or if
586 .Cm PasswordAuthentication
587 is yes, the password provided by the user will be validated through
589 To use this option, the server needs a
590 Kerberos servtab which allows the verification of the KDC's identity.
593 .It Cm KerberosOrLocalPasswd
594 If set then if password authentication through Kerberos fails then
595 the password will be validated via any additional local mechanism
600 .It Cm KerberosTgtPassing
601 Specifies whether a Kerberos TGT may be forwarded to the server.
604 as this only works when the Kerberos KDC is actually an AFS kaserver.
605 .It Cm KerberosTicketCleanup
606 Specifies whether to automatically destroy the user's ticket cache
610 .It Cm KeyRegenerationInterval
611 In protocol version 1, the ephemeral server key is automatically regenerated
612 after this many seconds (if it has been used).
613 The purpose of regeneration is to prevent
614 decrypting captured sessions by later breaking into the machine and
616 The key is never stored anywhere.
617 If the value is 0, the key is never regenerated.
618 The default is 3600 (seconds).
620 Specifies the local addresses
623 The following forms may be used:
625 .Bl -item -offset indent -compact
629 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
634 .Ar host No | Ar IPv4_addr No : Ar port
640 .Ar host No | Ar IPv6_addr Oc : Ar port
648 will listen on the address and all prior
650 options specified. The default is to listen on all local
653 options are permitted. Additionally, any
655 options must precede this option for non port qualified addresses.
656 .It Cm LoginGraceTime
657 The server disconnects after this time if the user has not
658 successfully logged in.
659 If the value is 0, there is no time limit.
660 The default is 600 (seconds).
662 Gives the verbosity level that is used when logging messages from
664 The possible values are:
665 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
666 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
667 and DEBUG3 each specify higher levels of debugging output.
668 Logging with a DEBUG level violates the privacy of users
669 and is not recommended.
671 Specifies the available MAC (message authentication code) algorithms.
672 The MAC algorithm is used in protocol version 2
673 for data integrity protection.
674 Multiple algorithms must be comma-separated.
676 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
678 Specifies the maximum number of concurrent unauthenticated connections to the
681 Additional connections will be dropped until authentication succeeds or the
683 expires for a connection.
686 Alternatively, random early drop can be enabled by specifying
687 the three colon separated values
691 will refuse connection attempts with a probability of
694 if there are currently
697 unauthenticated connections.
698 The probability increases linearly and all connection attempts
699 are refused if the number of unauthenticated connections reaches
702 .It Cm PAMAuthenticationViaKbdInt
703 Specifies whether PAM challenge response authentication is allowed. This
704 allows the use of most PAM challenge response authentication modules, but
705 it will allow password authentication regardless of whether
706 .Cm PasswordAuthentication
710 .It Cm PasswordAuthentication
711 Specifies whether password authentication is allowed.
714 .It Cm PermitEmptyPasswords
715 When password authentication is allowed, it specifies whether the
716 server allows login to accounts with empty password strings.
719 .It Cm PermitRootLogin
720 Specifies whether root can login using
724 .Dq without-password ,
725 .Dq forced-commands-only
731 If this option is set to
733 password authentication is disabled for root.
735 If this option is set to
736 .Dq forced-commands-only
737 root login with public key authentication will be allowed,
740 option has been specified
741 (which may be useful for taking remote backups even if root login is
742 normally not allowed). All other authentication methods are disabled
745 If this option is set to
747 root is not allowed to login.
749 Specifies the file that contains the process identifier of the
753 .Pa /var/run/sshd.pid .
755 Specifies the port number that
759 Multiple options of this type are permitted.
765 should print the date and time when the user last logged in.
773 when a user logs in interactively.
774 (On some systems it is also printed by the shell,
780 Specifies the protocol versions
783 The possible values are
787 Multiple versions must be comma-separated.
790 .It Cm PubkeyAuthentication
791 Specifies whether public key authentication is allowed.
794 Note that this option applies to protocol version 2 only.
795 .It Cm RhostsAuthentication
796 Specifies whether authentication using rhosts or /etc/hosts.equiv
798 Normally, this method should not be permitted because it is insecure.
799 .Cm RhostsRSAAuthentication
801 instead, because it performs RSA-based host authentication in addition
802 to normal rhosts or /etc/hosts.equiv authentication.
805 This option applies to protocol version 1 only.
806 .It Cm RhostsRSAAuthentication
807 Specifies whether rhosts or /etc/hosts.equiv authentication together
808 with successful RSA host authentication is allowed.
811 This option applies to protocol version 1 only.
812 .It Cm RSAAuthentication
813 Specifies whether pure RSA authentication is allowed.
816 This option applies to protocol version 1 only.
818 Defines the number of bits in the ephemeral protocol version 1 server key.
819 The minimum value is 512, and the default is 768.
823 should check file modes and ownership of the
824 user's files and home directory before accepting login.
825 This is normally desirable because novices sometimes accidentally leave their
826 directory or files world-writable.
830 Configures an external subsystem (e.g., file transfer daemon).
831 Arguments should be a subsystem name and a command to execute upon subsystem
837 file transfer subsystem.
838 By default no subsystems are defined.
839 Note that this option applies to protocol version 2 only.
840 .It Cm SyslogFacility
841 Gives the facility code that is used when logging messages from
843 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
844 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
849 is used for interactive login sessions.
854 is never used for remote command execution.
855 Note also, that if this is enabled,
857 will be disabled because
859 does not know how to handle
862 .It Cm VerifyReverseMapping
865 should try to verify the remote host name and check that
866 the resolved host name for the remote IP address maps back to the
867 very same IP address.
870 .It Cm X11DisplayOffset
871 Specifies the first display number available for
876 from interfering with real X11 servers.
879 Specifies whether X11 forwarding is permitted.
882 Note that disabling X11 forwarding does not improve security in any
883 way, as users can always install their own forwarders.
884 X11 forwarding is automatically disabled if
887 .It Cm X11UseLocalhost
890 should bind the X11 forwarding server to the loopback address or to
891 the wildcard address. By default,
893 binds the forwarding server to the loopback address and sets the
896 environment variable to
898 This prevents remote hosts from connecting to the fake display.
899 However, some older X11 clients may not function with this
904 to specify that the forwarding server should be bound to the wildcard
913 Specifies the location of the
917 .Pa /usr/X11R6/bin/xauth .
922 command-line arguments and configuration file options that specify time
923 may be expressed using a sequence of the form:
925 .Ar time Oo Ar qualifier Oc ,
929 is a positive integer value and
931 is one of the following:
933 .Bl -tag -width Ds -compact -offset indent
948 Each member of the sequence is added together to calculate
949 the total time value.
951 Time format examples:
953 .Bl -tag -width Ds -compact -offset indent
955 600 seconds (10 minutes)
959 1 hour 30 minutes (90 minutes)
962 When a user successfully logs in,
965 .Bl -enum -offset indent
967 If the login is on a tty, and no command has been specified,
968 prints last login time and
970 (unless prevented in the configuration file or by
971 .Pa $HOME/.hushlogin ;
976 If the login is on a tty, records login time.
980 if it exists, prints contents and quits
983 Changes to run with normal user privileges.
985 Sets up basic environment.
988 .Pa $HOME/.ssh/environment
991 Changes to user's home directory.
995 exists, runs it; else if
998 it; otherwise runs xauth.
1001 files are given the X11
1002 authentication protocol and cookie in standard input.
1004 Runs user's shell or command.
1006 .Sh AUTHORIZED_KEYS FILE FORMAT
1007 .Pa $HOME/.ssh/authorized_keys
1008 is the default file that lists the public keys that are
1009 permitted for RSA authentication in protocol version 1
1010 and for public key authentication (PubkeyAuthentication)
1011 in protocol version 2.
1012 .Cm AuthorizedKeysFile
1013 may be used to specify an alternative file.
1015 Each line of the file contains one
1016 key (empty lines and lines starting with a
1020 Each RSA public key consists of the following fields, separated by
1021 spaces: options, bits, exponent, modulus, comment.
1022 Each protocol version 2 public key consists of:
1023 options, keytype, base64 encoded key, comment.
1025 are optional; its presence is determined by whether the line starts
1026 with a number or not (the option field never starts with a number).
1027 The bits, exponent, modulus and comment fields give the RSA key for
1028 protocol version 1; the
1029 comment field is not used for anything (but may be convenient for the
1030 user to identify the key).
1031 For protocol version 2 the keytype is
1036 Note that lines in this file are usually several hundred bytes long
1037 (because of the size of the RSA key modulus).
1038 You don't want to type them in; instead, copy the
1045 The options (if present) consist of comma-separated option
1047 No spaces are permitted, except within double quotes.
1048 The following option specifications are supported (note
1049 that option keywords are case-insensitive):
1051 .It Cm from="pattern-list"
1052 Specifies that in addition to RSA authentication, the canonical name
1053 of the remote host must be present in the comma-separated list of
1058 serve as wildcards).
1059 The list may also contain
1060 patterns negated by prefixing them with
1062 if the canonical host name matches a negated pattern, the key is not accepted.
1064 of this option is to optionally increase security: RSA authentication
1065 by itself does not trust the network or name servers or anything (but
1066 the key); however, if somebody somehow steals the key, the key
1067 permits an intruder to log in from anywhere in the world.
1068 This additional option makes using a stolen key more difficult (name
1069 servers and/or routers would have to be compromised in addition to
1071 .It Cm command="command"
1072 Specifies that the command is executed whenever this key is used for
1074 The command supplied by the user (if any) is ignored.
1075 The command is run on a pty if the client requests a pty;
1076 otherwise it is run without a tty.
1077 If a 8-bit clean channel is required,
1078 one must not request a pty or should specify
1080 A quote may be included in the command by quoting it with a backslash.
1081 This option might be useful
1082 to restrict certain RSA keys to perform just a specific operation.
1083 An example might be a key that permits remote backups but nothing else.
1084 Note that the client may specify TCP/IP and/or X11
1085 forwarding unless they are explicitly prohibited.
1086 Note that this option applies to shell, command or subsystem execution.
1087 .It Cm environment="NAME=value"
1088 Specifies that the string is to be added to the environment when
1089 logging in using this key.
1090 Environment variables set this way
1091 override other default environment values.
1092 Multiple options of this type are permitted.
1093 This option is automatically disabled if
1096 .It Cm no-port-forwarding
1097 Forbids TCP/IP forwarding when this key is used for authentication.
1098 Any port forward requests by the client will return an error.
1099 This might be used, e.g., in connection with the
1102 .It Cm no-X11-forwarding
1103 Forbids X11 forwarding when this key is used for authentication.
1104 Any X11 forward requests by the client will return an error.
1105 .It Cm no-agent-forwarding
1106 Forbids authentication agent forwarding when this key is used for
1109 Prevents tty allocation (a request to allocate a pty will fail).
1110 .It Cm permitopen="host:port"
1113 port forwarding such that it may only connect to the specified host and
1115 IPv6 addresses can be specified with an alternative syntax:
1119 options may be applied separated by commas. No pattern matching is
1120 performed on the specified hostnames, they must be literal domains or
1124 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1126 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1128 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1130 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1131 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1133 .Pa /etc/ssh/ssh_known_hosts ,
1135 .Pa $HOME/.ssh/known_hosts
1136 files contain host public keys for all known hosts.
1137 The global file should
1138 be prepared by the administrator (optional), and the per-user file is
1139 maintained automatically: whenever the user connects from an unknown host
1140 its key is added to the per-user file.
1142 Each line in these files contains the following fields: hostnames,
1143 bits, exponent, modulus, comment.
1144 The fields are separated by spaces.
1146 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1147 wildcards); each pattern in turn is matched against the canonical host
1148 name (when authenticating a client) or against the user-supplied
1149 name (when authenticating a server).
1150 A pattern may also be preceded by
1152 to indicate negation: if the host name matches a negated
1153 pattern, it is not accepted (by that line) even if it matched another
1154 pattern on the line.
1156 Bits, exponent, and modulus are taken directly from the RSA host key; they
1157 can be obtained, e.g., from
1158 .Pa /etc/ssh/ssh_host_key.pub .
1159 The optional comment field continues to the end of the line, and is not used.
1163 and empty lines are ignored as comments.
1165 When performing host authentication, authentication is accepted if any
1166 matching line has the proper key.
1167 It is thus permissible (but not
1168 recommended) to have several lines or different host keys for the same
1170 This will inevitably happen when short forms of host names
1171 from different domains are put in the file.
1173 that the files contain conflicting information; authentication is
1174 accepted if valid information can be found from either file.
1176 Note that the lines in these files are typically hundreds of characters
1177 long, and you definitely don't want to type in the host keys by hand.
1178 Rather, generate them by a script
1180 .Pa /etc/ssh/ssh_host_key.pub
1181 and adding the host names at the front.
1184 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1185 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1189 .It Pa /etc/ssh/sshd_config
1190 Contains configuration data for
1192 This file should be writable by root only, but it is recommended
1193 (though not necessary) that it be world-readable.
1194 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1195 These three files contain the private parts of the host keys.
1196 These files should only be owned by root, readable only by root, and not
1197 accessible to others.
1200 does not start if this file is group/world-accessible.
1201 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1202 These three files contain the public parts of the host keys.
1203 These files should be world-readable but writable only by
1205 Their contents should match the respective private parts.
1207 really used for anything; they are provided for the convenience of
1208 the user so their contents can be copied to known hosts files.
1209 These files are created using
1212 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1213 .It Pa /var/run/sshd.pid
1214 Contains the process ID of the
1216 listening for connections (if there are several daemons running
1217 concurrently for different ports, this contains the pid of the one
1219 The content of this file is not sensitive; it can be world-readable.
1220 .It Pa $HOME/.ssh/authorized_keys
1221 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1222 This file must be readable by root (which may on some machines imply
1223 it being world-readable if the user's home directory resides on an NFS
1225 It is recommended that it not be accessible by others.
1226 The format of this file is described above.
1227 Users will place the contents of their
1232 files into this file, as described in
1234 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1235 These files are consulted when using rhosts with RSA host
1236 authentication or protocol version 2 hostbased authentication
1237 to check the public key of the host.
1238 The key must be listed in one of these files to be accepted.
1239 The client uses the same files
1240 to verify that it is connecting to the correct remote host.
1241 These files should be writable only by root/the owner.
1242 .Pa /etc/ssh/ssh_known_hosts
1243 should be world-readable, and
1244 .Pa $HOME/.ssh/known_hosts
1245 can but need not be world-readable.
1247 If this file exists,
1249 refuses to let anyone except root log in.
1250 The contents of the file
1251 are displayed to anyone trying to log in, and non-root connections are
1253 The file should be world-readable.
1254 .It Pa /etc/hosts.allow, /etc/hosts.deny
1255 Access controls that should be enforced by tcp-wrappers are defined here.
1256 Further details are described in
1257 .Xr hosts_access 5 .
1258 .It Pa $HOME/.rhosts
1259 This file contains host-username pairs, separated by a space, one per
1261 The given user on the corresponding host is permitted to log in
1263 The same file is used by rlogind and rshd.
1265 be writable only by the user; it is recommended that it not be
1266 accessible by others.
1268 If is also possible to use netgroups in the file.
1270 name may be of the form +@groupname to specify all hosts or all users
1272 .It Pa $HOME/.shosts
1274 this file is exactly the same as for
1276 However, this file is
1277 not used by rlogin and rshd, so using this permits access using SSH only.
1278 .It Pa /etc/hosts.equiv
1279 This file is used during
1282 In the simplest form, this file contains host names, one per line.
1284 those hosts are permitted to log in without a password, provided they
1285 have the same user name on both machines.
1286 The host name may also be
1287 followed by a user name; such users are permitted to log in as
1289 user on this machine (except root).
1290 Additionally, the syntax
1292 can be used to specify netgroups.
1293 Negated entries start with
1296 If the client host/user is successfully matched in this file, login is
1297 automatically permitted provided the client and server user names are the
1299 Additionally, successful RSA host authentication is normally required.
1300 This file must be writable only by root; it is recommended
1301 that it be world-readable.
1303 .Sy "Warning: It is almost never a good idea to use user names in"
1305 Beware that it really means that the named user(s) can log in as
1307 which includes bin, daemon, adm, and other accounts that own critical
1308 binaries and directories.
1309 Using a user name practically grants the user root access.
1310 The only valid use for user names that I can think
1311 of is in negative entries.
1313 Note that this warning also applies to rsh/rlogin.
1314 .It Pa /etc/shosts.equiv
1315 This is processed exactly as
1316 .Pa /etc/hosts.equiv .
1317 However, this file may be useful in environments that want to run both
1319 .It Pa $HOME/.ssh/environment
1320 This file is read into the environment at login (if it exists).
1321 It can only contain empty lines, comment lines (that start with
1323 and assignment lines of the form name=value.
1324 The file should be writable
1325 only by the user; it need not be readable by anyone else.
1326 .It Pa $HOME/.ssh/rc
1327 If this file exists, it is run with /bin/sh after reading the
1328 environment files but before starting the user's shell or command.
1329 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1337 The primary purpose of this file is to run any initialization routines
1338 which may be needed before the user's home directory becomes
1339 accessible; AFS is a particular example of such an environment.
1341 This file will probably contain some initialization code followed by
1342 something similar to:
1344 if read proto cookie; then
1345 echo add $DISPLAY $proto $cookie | xauth -q -
1349 If this file does not exist,
1352 does not exist either, xauth is used to store the cookie.
1354 This file should be writable only by the user, and need not be
1355 readable by anyone else.
1356 .It Pa /etc/ssh/sshrc
1359 This can be used to specify
1360 machine-specific login-time initializations globally.
1361 This file should be writable only by root, and should be world-readable.
1364 OpenSSH is a derivative of the original and free
1365 ssh 1.2.12 release by Tatu Ylonen.
1366 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1367 Theo de Raadt and Dug Song
1368 removed many bugs, re-added newer features and
1370 Markus Friedl contributed the support for SSH
1371 protocol versions 1.5 and 2.0.
1388 .%T "SSH Protocol Architecture"
1389 .%N draft-ietf-secsh-architecture-09.txt
1391 .%O work in progress material
1397 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1398 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1400 .%O work in progress material