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 1.183 2002/05/29 03:06:30 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.
393 By default, no banner is displayed.
395 .It Cm ChallengeResponseAuthentication
396 Specifies whether challenge response authentication is allowed.
397 All authentication styles from
403 Specifies the ciphers allowed for protocol version 2.
404 Multiple ciphers must be comma-separated.
408 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
409 aes192-cbc,aes256-cbc''
411 .It Cm ClientAliveInterval
412 Sets a timeout interval in seconds after which if no data has been received
415 will send a message through the encrypted
416 channel to request a response from the client.
418 is 0, indicating that these messages will not be sent to the client.
419 This option applies to protocol version 2 only.
420 .It Cm ClientAliveCountMax
421 Sets the number of client alive messages (see above) which may be
424 receiving any messages back from the client. If this threshold is
425 reached while client alive messages are being sent,
427 will disconnect the client, terminating the session. It is important
428 to note that the use of client alive messages is very different from
430 (below). The client alive messages are sent through the
431 encrypted channel and therefore will not be spoofable. The TCP keepalive
434 is spoofable. The client alive mechanism is valuable when the client or
435 server depend on knowing when a connection has become inactive.
437 The default value is 3. If
438 .Cm ClientAliveInterval
439 (above) is set to 15, and
440 .Cm ClientAliveCountMax
441 is left at the default, unresponsive ssh clients
442 will be disconnected after approximately 45 seconds.
444 This keyword can be followed by a list of group name patterns, separated
446 Login is disallowed for users whose primary group or supplementary
447 group list matches one of the patterns.
452 wildcards in the patterns.
453 Only group names are valid; a numerical group ID is not recognized.
454 By default, login is allowed for all groups.
457 This keyword can be followed by a list of user name patterns, separated
459 Login is disallowed for user names that match one of the patterns.
463 can be used as wildcards in the patterns.
464 Only user names are valid; a numerical user ID is not recognized.
465 By default, login is allowed for all users.
466 If the pattern takes the form USER@HOST then USER and HOST
467 are separately checked, restricting logins to particular
468 users from particular hosts.
470 Specifies whether remote hosts are allowed to connect to ports
471 forwarded for the client.
474 binds remote port forwardings to the loopback address. This
475 prevents other remote hosts from connecting to forwarded ports.
477 can be used to specify that
479 should bind remote port forwardings to the wildcard address,
480 thus allowing remote hosts to connect to forwarded ports.
487 .It Cm HostbasedAuthentication
488 Specifies whether rhosts or /etc/hosts.equiv authentication together
489 with successful public key client host authentication is allowed
490 (hostbased authentication).
491 This option is similar to
492 .Cm RhostsRSAAuthentication
493 and applies to protocol version 2 only.
497 Specifies a file containing a private host key
500 .Pa /etc/ssh/ssh_host_key
501 for protocol version 1, and
502 .Pa /etc/ssh/ssh_host_rsa_key
504 .Pa /etc/ssh/ssh_host_dsa_key
505 for protocol version 2.
508 will refuse to use a file if it is group/world-accessible.
509 It is possible to have multiple host key files.
511 keys are used for version 1 and
515 are used for version 2 of the SSH protocol.
521 files will not be used in
522 .Cm RhostsAuthentication ,
523 .Cm RhostsRSAAuthentication
525 .Cm HostbasedAuthentication .
529 .Pa /etc/shosts.equiv
533 .It Cm IgnoreUserKnownHosts
536 should ignore the user's
537 .Pa $HOME/.ssh/known_hosts
539 .Cm RhostsRSAAuthentication
541 .Cm HostbasedAuthentication .
545 Specifies whether the system should send TCP keepalive messages to the
547 If they are sent, death of the connection or crash of one
548 of the machines will be properly noticed.
549 However, this means that
550 connections will die if the route is down temporarily, and some people
552 On the other hand, if keepalives are not sent,
553 sessions may hang indefinitely on the server, leaving
555 users and consuming server resources.
559 (to send keepalives), and the server will notice
560 if the network goes down or the client host crashes.
561 This avoids infinitely hanging sessions.
563 To disable keepalives, the value should be set to
565 .It Cm KerberosAuthentication
566 Specifies whether Kerberos authentication is allowed.
567 This can be in the form of a Kerberos ticket, or if
568 .Cm PasswordAuthentication
569 is yes, the password provided by the user will be validated through
571 To use this option, the server needs a
572 Kerberos servtab which allows the verification of the KDC's identity.
575 .It Cm KerberosOrLocalPasswd
576 If set then if password authentication through Kerberos fails then
577 the password will be validated via any additional local mechanism
582 .It Cm KerberosTgtPassing
583 Specifies whether a Kerberos TGT may be forwarded to the server.
586 as this only works when the Kerberos KDC is actually an AFS kaserver.
587 .It Cm KerberosTicketCleanup
588 Specifies whether to automatically destroy the user's ticket cache
592 .It Cm KeyRegenerationInterval
593 In protocol version 1, the ephemeral server key is automatically regenerated
594 after this many seconds (if it has been used).
595 The purpose of regeneration is to prevent
596 decrypting captured sessions by later breaking into the machine and
598 The key is never stored anywhere.
599 If the value is 0, the key is never regenerated.
600 The default is 3600 (seconds).
602 Specifies the local addresses
605 The following forms may be used:
607 .Bl -item -offset indent -compact
611 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
616 .Ar host No | Ar IPv4_addr No : Ar port
622 .Ar host No | Ar IPv6_addr Oc : Ar port
630 will listen on the address and all prior
632 options specified. The default is to listen on all local
635 options are permitted. Additionally, any
637 options must precede this option for non port qualified addresses.
638 .It Cm LoginGraceTime
639 The server disconnects after this time if the user has not
640 successfully logged in.
641 If the value is 0, there is no time limit.
642 The default is 600 (seconds).
644 Gives the verbosity level that is used when logging messages from
646 The possible values are:
647 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
648 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
649 and DEBUG3 each specify higher levels of debugging output.
650 Logging with a DEBUG level violates the privacy of users
651 and is not recommended.
653 Specifies the available MAC (message authentication code) algorithms.
654 The MAC algorithm is used in protocol version 2
655 for data integrity protection.
656 Multiple algorithms must be comma-separated.
658 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
660 Specifies the maximum number of concurrent unauthenticated connections to the
663 Additional connections will be dropped until authentication succeeds or the
665 expires for a connection.
668 Alternatively, random early drop can be enabled by specifying
669 the three colon separated values
673 will refuse connection attempts with a probability of
676 if there are currently
679 unauthenticated connections.
680 The probability increases linearly and all connection attempts
681 are refused if the number of unauthenticated connections reaches
684 .It Cm PAMAuthenticationViaKbdInt
685 Specifies whether PAM challenge response authentication is allowed. This
686 allows the use of most PAM challenge response authentication modules, but
687 it will allow password authentication regardless of whether
688 .Cm PasswordAuthentication
692 .It Cm PasswordAuthentication
693 Specifies whether password authentication is allowed.
696 .It Cm PermitEmptyPasswords
697 When password authentication is allowed, it specifies whether the
698 server allows login to accounts with empty password strings.
701 .It Cm PermitRootLogin
702 Specifies whether root can login using
706 .Dq without-password ,
707 .Dq forced-commands-only
713 If this option is set to
715 password authentication is disabled for root.
717 If this option is set to
718 .Dq forced-commands-only
719 root login with public key authentication will be allowed,
722 option has been specified
723 (which may be useful for taking remote backups even if root login is
724 normally not allowed). All other authentication methods are disabled
727 If this option is set to
729 root is not allowed to login.
731 Specifies the file that contains the process identifier of the
735 .Pa /var/run/sshd.pid .
737 Specifies the port number that
741 Multiple options of this type are permitted.
747 should print the date and time when the user last logged in.
755 when a user logs in interactively.
756 (On some systems it is also printed by the shell,
762 Specifies the protocol versions
765 The possible values are
769 Multiple versions must be comma-separated.
772 .It Cm PubkeyAuthentication
773 Specifies whether public key authentication is allowed.
776 Note that this option applies to protocol version 2 only.
777 .It Cm RhostsAuthentication
778 Specifies whether authentication using rhosts or /etc/hosts.equiv
780 Normally, this method should not be permitted because it is insecure.
781 .Cm RhostsRSAAuthentication
783 instead, because it performs RSA-based host authentication in addition
784 to normal rhosts or /etc/hosts.equiv authentication.
787 This option applies to protocol version 1 only.
788 .It Cm RhostsRSAAuthentication
789 Specifies whether rhosts or /etc/hosts.equiv authentication together
790 with successful RSA host authentication is allowed.
793 This option applies to protocol version 1 only.
794 .It Cm RSAAuthentication
795 Specifies whether pure RSA authentication is allowed.
798 This option applies to protocol version 1 only.
800 Defines the number of bits in the ephemeral protocol version 1 server key.
801 The minimum value is 512, and the default is 768.
805 should check file modes and ownership of the
806 user's files and home directory before accepting login.
807 This is normally desirable because novices sometimes accidentally leave their
808 directory or files world-writable.
812 Configures an external subsystem (e.g., file transfer daemon).
813 Arguments should be a subsystem name and a command to execute upon subsystem
819 file transfer subsystem.
820 By default no subsystems are defined.
821 Note that this option applies to protocol version 2 only.
822 .It Cm SyslogFacility
823 Gives the facility code that is used when logging messages from
825 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
826 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
831 is used for interactive login sessions.
836 is never used for remote command execution.
837 Note also, that if this is enabled,
839 will be disabled because
841 does not know how to handle
844 .Cm UsePrivilegeSeparation
845 is specified, it will be disabled after authentication.
846 .It Cm UsePrivilegeSeparation
849 separated privileges by creating an unprivileged child process
850 to deal with incoming network traffic. After successful authentication,
851 another process will be created that has the privilege of the authenticated
852 user. The goal of privilege separation is to prevent privilege
853 escalation by containing any corruption within the unprivileged processes.
856 .It Cm VerifyReverseMapping
859 should try to verify the remote host name and check that
860 the resolved host name for the remote IP address maps back to the
861 very same IP address.
864 .It Cm X11DisplayOffset
865 Specifies the first display number available for
870 from interfering with real X11 servers.
873 Specifies whether X11 forwarding is permitted.
876 Note that disabling X11 forwarding does not improve security in any
877 way, as users can always install their own forwarders.
878 X11 forwarding is automatically disabled if
881 .It Cm X11UseLocalhost
884 should bind the X11 forwarding server to the loopback address or to
885 the wildcard address. By default,
887 binds the forwarding server to the loopback address and sets the
890 environment variable to
892 This prevents remote hosts from connecting to the fake display.
893 However, some older X11 clients may not function with this
898 to specify that the forwarding server should be bound to the wildcard
907 Specifies the location of the
911 .Pa /usr/X11R6/bin/xauth .
916 command-line arguments and configuration file options that specify time
917 may be expressed using a sequence of the form:
919 .Ar time Oo Ar qualifier Oc ,
923 is a positive integer value and
925 is one of the following:
927 .Bl -tag -width Ds -compact -offset indent
942 Each member of the sequence is added together to calculate
943 the total time value.
945 Time format examples:
947 .Bl -tag -width Ds -compact -offset indent
949 600 seconds (10 minutes)
953 1 hour 30 minutes (90 minutes)
956 When a user successfully logs in,
959 .Bl -enum -offset indent
961 If the login is on a tty, and no command has been specified,
962 prints last login time and
964 (unless prevented in the configuration file or by
965 .Pa $HOME/.hushlogin ;
970 If the login is on a tty, records login time.
974 if it exists, prints contents and quits
977 Changes to run with normal user privileges.
979 Sets up basic environment.
982 .Pa $HOME/.ssh/environment
985 Changes to user's home directory.
989 exists, runs it; else if
992 it; otherwise runs xauth.
995 files are given the X11
996 authentication protocol and cookie in standard input.
998 Runs user's shell or command.
1000 .Sh AUTHORIZED_KEYS FILE FORMAT
1001 .Pa $HOME/.ssh/authorized_keys
1002 is the default file that lists the public keys that are
1003 permitted for RSA authentication in protocol version 1
1004 and for public key authentication (PubkeyAuthentication)
1005 in protocol version 2.
1006 .Cm AuthorizedKeysFile
1007 may be used to specify an alternative file.
1009 Each line of the file contains one
1010 key (empty lines and lines starting with a
1014 Each RSA public key consists of the following fields, separated by
1015 spaces: options, bits, exponent, modulus, comment.
1016 Each protocol version 2 public key consists of:
1017 options, keytype, base64 encoded key, comment.
1019 are optional; its presence is determined by whether the line starts
1020 with a number or not (the option field never starts with a number).
1021 The bits, exponent, modulus and comment fields give the RSA key for
1022 protocol version 1; the
1023 comment field is not used for anything (but may be convenient for the
1024 user to identify the key).
1025 For protocol version 2 the keytype is
1030 Note that lines in this file are usually several hundred bytes long
1031 (because of the size of the RSA key modulus).
1032 You don't want to type them in; instead, copy the
1040 enforces a minimum RSA key modulus size for protocol 1
1041 and protocol 2 keys of 768 bits.
1043 The options (if present) consist of comma-separated option
1045 No spaces are permitted, except within double quotes.
1046 The following option specifications are supported (note
1047 that option keywords are case-insensitive):
1049 .It Cm from="pattern-list"
1050 Specifies that in addition to RSA authentication, the canonical name
1051 of the remote host must be present in the comma-separated list of
1056 serve as wildcards).
1057 The list may also contain
1058 patterns negated by prefixing them with
1060 if the canonical host name matches a negated pattern, the key is not accepted.
1062 of this option is to optionally increase security: RSA authentication
1063 by itself does not trust the network or name servers or anything (but
1064 the key); however, if somebody somehow steals the key, the key
1065 permits an intruder to log in from anywhere in the world.
1066 This additional option makes using a stolen key more difficult (name
1067 servers and/or routers would have to be compromised in addition to
1069 .It Cm command="command"
1070 Specifies that the command is executed whenever this key is used for
1072 The command supplied by the user (if any) is ignored.
1073 The command is run on a pty if the client requests a pty;
1074 otherwise it is run without a tty.
1075 If a 8-bit clean channel is required,
1076 one must not request a pty or should specify
1078 A quote may be included in the command by quoting it with a backslash.
1079 This option might be useful
1080 to restrict certain RSA keys to perform just a specific operation.
1081 An example might be a key that permits remote backups but nothing else.
1082 Note that the client may specify TCP/IP and/or X11
1083 forwarding unless they are explicitly prohibited.
1084 Note that this option applies to shell, command or subsystem execution.
1085 .It Cm environment="NAME=value"
1086 Specifies that the string is to be added to the environment when
1087 logging in using this key.
1088 Environment variables set this way
1089 override other default environment values.
1090 Multiple options of this type are permitted.
1091 This option is automatically disabled if
1094 .It Cm no-port-forwarding
1095 Forbids TCP/IP forwarding when this key is used for authentication.
1096 Any port forward requests by the client will return an error.
1097 This might be used, e.g., in connection with the
1100 .It Cm no-X11-forwarding
1101 Forbids X11 forwarding when this key is used for authentication.
1102 Any X11 forward requests by the client will return an error.
1103 .It Cm no-agent-forwarding
1104 Forbids authentication agent forwarding when this key is used for
1107 Prevents tty allocation (a request to allocate a pty will fail).
1108 .It Cm permitopen="host:port"
1111 port forwarding such that it may only connect to the specified host and
1113 IPv6 addresses can be specified with an alternative syntax:
1117 options may be applied separated by commas. No pattern matching is
1118 performed on the specified hostnames, they must be literal domains or
1122 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1124 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1126 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1128 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1129 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1131 .Pa /etc/ssh/ssh_known_hosts ,
1133 .Pa $HOME/.ssh/known_hosts
1134 files contain host public keys for all known hosts.
1135 The global file should
1136 be prepared by the administrator (optional), and the per-user file is
1137 maintained automatically: whenever the user connects from an unknown host
1138 its key is added to the per-user file.
1140 Each line in these files contains the following fields: hostnames,
1141 bits, exponent, modulus, comment.
1142 The fields are separated by spaces.
1144 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1145 wildcards); each pattern in turn is matched against the canonical host
1146 name (when authenticating a client) or against the user-supplied
1147 name (when authenticating a server).
1148 A pattern may also be preceded by
1150 to indicate negation: if the host name matches a negated
1151 pattern, it is not accepted (by that line) even if it matched another
1152 pattern on the line.
1154 Bits, exponent, and modulus are taken directly from the RSA host key; they
1155 can be obtained, e.g., from
1156 .Pa /etc/ssh/ssh_host_key.pub .
1157 The optional comment field continues to the end of the line, and is not used.
1161 and empty lines are ignored as comments.
1163 When performing host authentication, authentication is accepted if any
1164 matching line has the proper key.
1165 It is thus permissible (but not
1166 recommended) to have several lines or different host keys for the same
1168 This will inevitably happen when short forms of host names
1169 from different domains are put in the file.
1171 that the files contain conflicting information; authentication is
1172 accepted if valid information can be found from either file.
1174 Note that the lines in these files are typically hundreds of characters
1175 long, and you definitely don't want to type in the host keys by hand.
1176 Rather, generate them by a script
1178 .Pa /etc/ssh/ssh_host_key.pub
1179 and adding the host names at the front.
1182 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1183 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1187 .It Pa /etc/ssh/sshd_config
1188 Contains configuration data for
1190 This file should be writable by root only, but it is recommended
1191 (though not necessary) that it be world-readable.
1192 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1193 These three files contain the private parts of the host keys.
1194 These files should only be owned by root, readable only by root, and not
1195 accessible to others.
1198 does not start if this file is group/world-accessible.
1199 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1200 These three files contain the public parts of the host keys.
1201 These files should be world-readable but writable only by
1203 Their contents should match the respective private parts.
1205 really used for anything; they are provided for the convenience of
1206 the user so their contents can be copied to known hosts files.
1207 These files are created using
1210 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1211 .It Pa /var/run/sshd.pid
1212 Contains the process ID of the
1214 listening for connections (if there are several daemons running
1215 concurrently for different ports, this contains the pid of the one
1217 The content of this file is not sensitive; it can be world-readable.
1218 .It Pa $HOME/.ssh/authorized_keys
1219 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1220 This file must be readable by root (which may on some machines imply
1221 it being world-readable if the user's home directory resides on an NFS
1223 It is recommended that it not be accessible by others.
1224 The format of this file is described above.
1225 Users will place the contents of their
1230 files into this file, as described in
1232 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1233 These files are consulted when using rhosts with RSA host
1234 authentication or protocol version 2 hostbased authentication
1235 to check the public key of the host.
1236 The key must be listed in one of these files to be accepted.
1237 The client uses the same files
1238 to verify that it is connecting to the correct remote host.
1239 These files should be writable only by root/the owner.
1240 .Pa /etc/ssh/ssh_known_hosts
1241 should be world-readable, and
1242 .Pa $HOME/.ssh/known_hosts
1243 can but need not be world-readable.
1245 If this file exists,
1247 refuses to let anyone except root log in.
1248 The contents of the file
1249 are displayed to anyone trying to log in, and non-root connections are
1251 The file should be world-readable.
1252 .It Pa /etc/hosts.allow, /etc/hosts.deny
1253 Access controls that should be enforced by tcp-wrappers are defined here.
1254 Further details are described in
1255 .Xr hosts_access 5 .
1256 .It Pa $HOME/.rhosts
1257 This file contains host-username pairs, separated by a space, one per
1259 The given user on the corresponding host is permitted to log in
1261 The same file is used by rlogind and rshd.
1263 be writable only by the user; it is recommended that it not be
1264 accessible by others.
1266 If is also possible to use netgroups in the file.
1268 name may be of the form +@groupname to specify all hosts or all users
1270 .It Pa $HOME/.shosts
1272 this file is exactly the same as for
1274 However, this file is
1275 not used by rlogin and rshd, so using this permits access using SSH only.
1276 .It Pa /etc/hosts.equiv
1277 This file is used during
1280 In the simplest form, this file contains host names, one per line.
1282 those hosts are permitted to log in without a password, provided they
1283 have the same user name on both machines.
1284 The host name may also be
1285 followed by a user name; such users are permitted to log in as
1287 user on this machine (except root).
1288 Additionally, the syntax
1290 can be used to specify netgroups.
1291 Negated entries start with
1294 If the client host/user is successfully matched in this file, login is
1295 automatically permitted provided the client and server user names are the
1297 Additionally, successful RSA host authentication is normally required.
1298 This file must be writable only by root; it is recommended
1299 that it be world-readable.
1301 .Sy "Warning: It is almost never a good idea to use user names in"
1303 Beware that it really means that the named user(s) can log in as
1305 which includes bin, daemon, adm, and other accounts that own critical
1306 binaries and directories.
1307 Using a user name practically grants the user root access.
1308 The only valid use for user names that I can think
1309 of is in negative entries.
1311 Note that this warning also applies to rsh/rlogin.
1312 .It Pa /etc/shosts.equiv
1313 This is processed exactly as
1314 .Pa /etc/hosts.equiv .
1315 However, this file may be useful in environments that want to run both
1317 .It Pa $HOME/.ssh/environment
1318 This file is read into the environment at login (if it exists).
1319 It can only contain empty lines, comment lines (that start with
1321 and assignment lines of the form name=value.
1322 The file should be writable
1323 only by the user; it need not be readable by anyone else.
1324 .It Pa $HOME/.ssh/rc
1325 If this file exists, it is run with /bin/sh after reading the
1326 environment files but before starting the user's shell or command.
1327 It must not produce any output on stdout; stderr must be used
1329 If X11 forwarding is in use, it will receive the "proto cookie" pair in
1330 its standard input (and
1332 in its environment).
1333 The script must call
1337 will not run xauth automatically to add X11 cookies.
1339 The primary purpose of this file is to run any initialization routines
1340 which may be needed before the user's home directory becomes
1341 accessible; AFS is a particular example of such an environment.
1343 This file will probably contain some initialization code followed by
1344 something similar to:
1346 if read proto cookie && [ -n "$DISPLAY" ]; then
1347 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
1348 # X11UseLocalhost=yes
1349 xauth add unix:`echo $DISPLAY |
1350 cut -c11-` $proto $cookie
1352 # X11UseLocalhost=no
1353 xauth add $DISPLAY $proto $cookie
1358 If this file does not exist,
1361 does not exist either, xauth is used to add the cookie.
1363 This file should be writable only by the user, and need not be
1364 readable by anyone else.
1365 .It Pa /etc/ssh/sshrc
1368 This can be used to specify
1369 machine-specific login-time initializations globally.
1370 This file should be writable only by root, and should be world-readable.
1373 OpenSSH is a derivative of the original and free
1374 ssh 1.2.12 release by Tatu Ylonen.
1375 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1376 Theo de Raadt and Dug Song
1377 removed many bugs, re-added newer features and
1379 Markus Friedl contributed the support for SSH
1380 protocol versions 1.5 and 2.0.
1381 Niels Provos and Markus Friedl contributed support
1382 for privilege separation.
1399 .%T "SSH Protocol Architecture"
1400 .%N draft-ietf-secsh-architecture-12.txt
1402 .%O work in progress material
1408 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1409 .%N draft-ietf-secsh-dh-group-exchange-02.txt
1411 .%O work in progress material