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 1.142 2001/08/21 21:47:42 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
124 are disabled (thus completely disabling
130 .Ss SSH protocol version 2
132 Version 2 works similarly:
133 Each host has a host-specific key (RSA or DSA) used to identify the host.
134 However, when the daemon starts, it does not generate a server key.
135 Forward security is provided through a Diffie-Hellman key agreement.
136 This key agreement results in a shared session key.
138 The rest of the session is encrypted using a symmetric cipher, currently
139 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
140 The client selects the encryption algorithm
141 to use from those offered by the server.
142 Additionally, session integrity is provided
143 through a cryptographic message authentication code
144 (hmac-sha1 or hmac-md5).
146 Protocol version 2 provides a public key based
147 user (PubkeyAuthentication) or
148 client host (HostbasedAuthentication) authentication method,
149 conventional password authentication and challenge response based methods.
151 .Ss Command execution and data forwarding
153 If the client successfully authenticates itself, a dialog for
154 preparing the session is entered.
155 At this time the client may request
156 things like allocating a pseudo-tty, forwarding X11 connections,
157 forwarding TCP/IP connections, or forwarding the authentication agent
158 connection over the secure channel.
160 Finally, the client either requests a shell or execution of a command.
161 The sides then enter session mode.
162 In this mode, either side may send
163 data at any time, and such data is forwarded to/from the shell or
164 command on the server side, and the user terminal in the client side.
166 When the user program terminates and all forwarded X11 and other
167 connections have been closed, the server sends command exit status to
168 the client, and both sides exit.
171 can be configured using command-line options or a configuration
173 Command-line options override values specified in the
177 rereads its configuration file when it receives a hangup signal,
179 by executing itself with the name it was started as, i.e.,
182 The options are as follows:
185 Specifies the number of bits in the ephemeral protocol version 1
186 server key (default 768).
189 The server sends verbose debug output to the system
190 log, and does not put itself in the background.
191 The server also will not fork and will only process one connection.
192 This option is only intended for debugging for the server.
193 Multiple -d options increase the debugging level.
196 When this option is specified,
198 will send the output to the standard error instead of the system log.
199 .It Fl f Ar configuration_file
200 Specifies the name of the configuration file.
202 .Pa /etc/sshd_config .
204 refuses to start if there is no configuration file.
205 .It Fl g Ar login_grace_time
206 Gives the grace time for clients to authenticate themselves (default
208 If the client fails to authenticate the user within
209 this many seconds, the server disconnects and exits.
210 A value of zero indicates no limit.
211 .It Fl h Ar host_key_file
212 Specifies the file from which the host key is read (default
213 .Pa /etc/ssh_host_key ) .
214 This option must be given if
216 is not run as root (as the normal
217 host file is normally not readable by anyone but root).
218 It is possible to have multiple host key files for
219 the different protocol versions and host key algorithms.
223 is being run from inetd.
226 from inetd because it needs to generate the server key before it can
227 respond to the client, and this may take tens of seconds.
228 Clients would have to wait too long if the key was regenerated every time.
229 However, with small key sizes (e.g., 512) using
233 .It Fl k Ar key_gen_time
234 Specifies how often the ephemeral protocol version 1 server key is
235 regenerated (default 3600 seconds, or one hour).
236 The motivation for regenerating the key fairly
237 often is that the key is not stored anywhere, and after about an hour,
238 it becomes impossible to recover the key for decrypting intercepted
239 communications even if the machine is cracked into or physically
241 A value of zero indicates that the key will never be regenerated.
243 Specifies the port on which the server listens for connections
247 Nothing is sent to the system log.
248 Normally the beginning,
249 authentication, and termination of each connection is logged.
252 Only check the validity of the configuration file and sanity of the keys.
253 This is useful for updating
255 reliably as configuration options may change.
257 This option is used to specify the size of the field
260 structure that holds the remote host name.
261 If the resolved host name is longer than
263 the dotted decimal value will be used instead.
264 This allows hosts with very long host names that
265 overflow this field to still be uniquely identified.
268 indicates that only dotted decimal addresses
269 should be put into the
273 When this option is specified
275 will not detach and does not become a daemon.
276 This allows easy monitoring of
281 to use IPv4 addresses only.
285 to use IPv6 addresses only.
287 .Sh CONFIGURATION FILE
289 reads configuration data from
291 (or the file specified with
293 on the command line).
294 The file contains keyword-argument pairs, one per line.
297 and empty lines are interpreted as comments.
300 keywords and their meanings are as follows (note that
301 keywords are case-insensitive and arguments are case-sensitive):
303 .It Cm AFSTokenPassing
304 Specifies whether an AFS token may be forwarded to the server.
308 This keyword can be followed by a list of group names, separated
310 If specified, login is allowed only for users whose primary
311 group or supplementary group list matches one of the patterns.
316 wildcards in the patterns.
317 Only group names are valid; a numerical group ID isn't recognized.
318 By default login is allowed regardless of the group list.
320 .It Cm AllowTcpForwarding
321 Specifies whether TCP forwarding is permitted.
324 Note that disabling TCP forwarding does not improve security unless
325 users are also denied shell access, as they can always install their
329 This keyword can be followed by a list of user names, separated
331 If specified, login is allowed only for users names that
332 match one of the patterns.
337 wildcards in the patterns.
338 Only user names are valid; a numerical user ID isn't recognized.
339 By default login is allowed regardless of the user name.
340 If the pattern takes the form USER@HOST then USER and HOST
341 are separately checked, allowing you to restrict logins to particular
342 users from particular hosts.
344 .It Cm AuthorizedKeysFile
345 Specifies the file that contains the public keys that can be used
346 for user authentication.
347 .Cm AuthorizedKeysFile
348 may contain tokens of the form %T which are substituted during connection
349 set-up. The following tokens are defined: %% is replaced by a literal '%',
350 %h is replaced by the home directory of the user being authenticated and
351 %u is replaced by the username of that user.
353 .Cm AuthorizedKeysFile
354 is taken to be an absolute path or one relative to the user's home
357 .Dq .ssh/authorized_keys
359 In some jurisdictions, sending a warning message before authentication
360 may be relevant for getting legal protection.
361 The contents of the specified file are sent to the remote user before
362 authentication is allowed.
363 This option is only available for protocol version 2.
365 .It Cm ChallengeResponseAuthentication
366 Specifies whether challenge response authentication is allowed.
367 All authentication styles from
373 Specifies the ciphers allowed for protocol version 2.
374 Multiple ciphers must be comma-separated.
376 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
377 .It Cm ClientAliveInterval
378 Sets a timeout interval in seconds after which if no data has been received
381 will send a message through the encrypted
382 channel to request a response from the client.
384 is 0, indicating that these messages will not be sent to the client.
385 This option applies to protocol version 2 only.
386 .It Cm ClientAliveCountMax
387 Sets the number of client alive messages (see above) which may be
390 receiving any messages back from the client. If this threshold is
391 reached while client alive messages are being sent,
393 will disconnect the client, terminating the session. It is important
394 to note that the use of client alive messages is very different from
396 (below). The client alive messages are sent through the
397 encrypted channel and therefore will not be spoofable. The TCP keepalive
400 is spoofable. You want to use the client
401 alive mechanism when you are basing something important on
402 clients having an active connection to the server.
404 The default value is 3. If you set
405 .Cm ClientAliveInterval
406 (above) to 15, and leave this value at the default, unresponsive ssh clients
407 will be disconnected after approximately 45 seconds.
409 This keyword can be followed by a number of group names, separated
411 Users whose primary group or supplementary group list matches
412 one of the patterns aren't allowed to log in.
417 wildcards in the patterns.
418 Only group names are valid; a numerical group ID isn't recognized.
419 By default login is allowed regardless of the group list.
422 This keyword can be followed by a number of user names, separated
424 Login is disallowed for user names that match one of the patterns.
428 can be used as wildcards in the patterns.
429 Only user names are valid; a numerical user ID isn't recognized.
430 By default login is allowed regardless of the user name.
432 Specifies whether remote hosts are allowed to connect to ports
433 forwarded for the client.
440 .It Cm HostbasedAuthentication
441 Specifies whether rhosts or /etc/hosts.equiv authentication together
442 with successful public key client host authentication is allowed
443 (hostbased authentication).
444 This option is similar to
445 .Cm RhostsRSAAuthentication
446 and applies to protocol version 2 only.
450 Specifies the file containing the private host keys (default
451 .Pa /etc/ssh_host_key )
452 used by SSH protocol versions 1 and 2.
455 will refuse to use a file if it is group/world-accessible.
456 It is possible to have multiple host key files.
458 keys are used for version 1 and
462 are used for version 2 of the SSH protocol.
468 files will not be used in
469 .Cm RhostsAuthentication ,
470 .Cm RhostsRSAAuthentication
472 .Cm HostbasedAuthentication .
476 .Pa /etc/shosts.equiv
480 .It Cm IgnoreUserKnownHosts
483 should ignore the user's
484 .Pa $HOME/.ssh/known_hosts
486 .Cm RhostsRSAAuthentication
488 .Cm HostbasedAuthentication .
492 Specifies whether the system should send keepalive messages to the
494 If they are sent, death of the connection or crash of one
495 of the machines will be properly noticed.
496 However, this means that
497 connections will die if the route is down temporarily, and some people
499 On the other hand, if keepalives are not sent,
500 sessions may hang indefinitely on the server, leaving
502 users and consuming server resources.
506 (to send keepalives), and the server will notice
507 if the network goes down or the client host reboots.
508 This avoids infinitely hanging sessions.
510 To disable keepalives, the value should be set to
512 in both the server and the client configuration files.
513 .It Cm KerberosAuthentication
514 Specifies whether Kerberos authentication is allowed.
515 This can be in the form of a Kerberos ticket, or if
516 .Cm PasswordAuthentication
517 is yes, the password provided by the user will be validated through
519 To use this option, the server needs a
520 Kerberos servtab which allows the verification of the KDC's identity.
523 .It Cm KerberosOrLocalPasswd
524 If set then if password authentication through Kerberos fails then
525 the password will be validated via any additional local mechanism
530 .It Cm KerberosTgtPassing
531 Specifies whether a Kerberos TGT may be forwarded to the server.
534 as this only works when the Kerberos KDC is actually an AFS kaserver.
535 .It Cm KerberosTicketCleanup
536 Specifies whether to automatically destroy the user's ticket cache
540 .It Cm KeyRegenerationInterval
541 In protocol version 1, the ephemeral server key is automatically regenerated
542 after this many seconds (if it has been used).
543 The purpose of regeneration is to prevent
544 decrypting captured sessions by later breaking into the machine and
546 The key is never stored anywhere.
547 If the value is 0, the key is never regenerated.
548 The default is 3600 (seconds).
550 Specifies the local addresses
553 The following forms may be used:
555 .Bl -item -offset indent -compact
559 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
564 .Ar host No | Ar IPv4_addr No : Ar port
570 .Ar host No | Ar IPv6_addr Oc : Ar port
578 will listen on the address and all prior
580 options specified. The default is to listen on all local
583 options are permitted. Additionally, any
585 options must precede this option for non port qualified addresses.
586 .It Cm LoginGraceTime
587 The server disconnects after this time if the user has not
588 successfully logged in.
589 If the value is 0, there is no time limit.
590 The default is 600 (seconds).
592 Gives the verbosity level that is used when logging messages from
594 The possible values are:
595 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
597 Logging with level DEBUG violates the privacy of users
598 and is not recommended.
600 Specifies the available MAC (message authentication code) algorithms.
601 The MAC algorithm is used in protocol version 2
602 for data integrity protection.
603 Multiple algorithms must be comma-separated.
605 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
607 Specifies the maximum number of concurrent unauthenticated connections to the
610 Additional connections will be dropped until authentication succeeds or the
612 expires for a connection.
615 Alternatively, random early drop can be enabled by specifying
616 the three colon separated values
620 will refuse connection attempts with a probability of
623 if there are currently
626 unauthenticated connections.
627 The probability increases linearly and all connection attempts
628 are refused if the number of unauthenticated connections reaches
631 .It Cm PAMAuthenticationViaKbdInt
632 Specifies whether PAM challenge response authentication is allowed. This
633 allows the use of most PAM challenge response authentication modules, but
634 it will allow password authentication regardless of whether
635 .Cm PasswordAuthentication
639 .It Cm PasswordAuthentication
640 Specifies whether password authentication is allowed.
643 .It Cm PermitEmptyPasswords
644 When password authentication is allowed, it specifies whether the
645 server allows login to accounts with empty password strings.
648 .It Cm PermitRootLogin
649 Specifies whether root can login using
653 .Dq without-password ,
654 .Dq forced-commands-only
660 If this option is set to
662 password authentication is disabled for root.
664 If this option is set to
665 .Dq forced-commands-only
666 root login with public key authentication will be allowed,
669 option has been specified
670 (which may be useful for taking remote backups even if root login is
671 normally not allowed). All other authentication methods are disabled
674 If this option is set to
676 root is not allowed to login.
678 Specifies the file that contains the process identifier of the
682 .Pa /var/run/sshd.pid .
684 Specifies the port number that
688 Multiple options of this type are permitted.
694 should print the date and time when the user last logged in.
702 when a user logs in interactively.
703 (On some systems it is also printed by the shell,
709 Specifies the protocol versions
712 The possible values are
716 Multiple versions must be comma-separated.
719 .It Cm PubkeyAuthentication
720 Specifies whether public key authentication is allowed.
723 Note that this option applies to protocol version 2 only.
724 .It Cm ReverseMappingCheck
727 should try to verify the remote host name and check that
728 the resolved host name for the remote IP address maps back to the
729 very same IP address.
732 .It Cm RhostsAuthentication
733 Specifies whether authentication using rhosts or /etc/hosts.equiv
735 Normally, this method should not be permitted because it is insecure.
736 .Cm RhostsRSAAuthentication
738 instead, because it performs RSA-based host authentication in addition
739 to normal rhosts or /etc/hosts.equiv authentication.
742 This option applies to protocol version 1 only.
743 .It Cm RhostsRSAAuthentication
744 Specifies whether rhosts or /etc/hosts.equiv authentication together
745 with successful RSA host authentication is allowed.
748 This option applies to protocol version 1 only.
749 .It Cm RSAAuthentication
750 Specifies whether pure RSA authentication is allowed.
753 This option applies to protocol version 1 only.
755 Defines the number of bits in the ephemeral protocol version 1 server key.
756 The minimum value is 512, and the default is 768.
760 should check file modes and ownership of the
761 user's files and home directory before accepting login.
762 This is normally desirable because novices sometimes accidentally leave their
763 directory or files world-writable.
767 Configures an external subsystem (e.g., file transfer daemon).
768 Arguments should be a subsystem name and a command to execute upon subsystem
774 file transfer subsystem.
775 By default no subsystems are defined.
776 Note that this option applies to protocol version 2 only.
777 .It Cm SyslogFacility
778 Gives the facility code that is used when logging messages from
780 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
781 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
786 is used for interactive login sessions.
791 is never used for remote command execution.
792 Note also, that if this is enabled,
794 will be disabled because
796 does not know how to handle
799 .It Cm X11DisplayOffset
800 Specifies the first display number available for
805 from interfering with real X11 servers.
808 Specifies whether X11 forwarding is permitted.
811 Note that disabling X11 forwarding does not improve security in any
812 way, as users can always install their own forwarders.
813 X11 forwarding is automatically disabled if
817 Specifies the location of the
821 .Pa /usr/X11R6/bin/xauth .
826 command-line arguments and configuration file options that specify time
827 may be expressed using a sequence of the form:
829 .Ar time Oo Ar qualifier Oc ,
833 is a positive integer value and
835 is one of the following:
837 .Bl -tag -width Ds -compact -offset indent
852 Each member of the sequence is added together to calculate
853 the total time value.
855 Time format examples:
857 .Bl -tag -width Ds -compact -offset indent
859 600 seconds (10 minutes)
863 1 hour 30 minutes (90 minutes)
866 When a user successfully logs in,
869 .Bl -enum -offset indent
871 If the login is on a tty, and no command has been specified,
872 prints last login time and
874 (unless prevented in the configuration file or by
875 .Pa $HOME/.hushlogin ;
880 If the login is on a tty, records login time.
884 if it exists, prints contents and quits
887 Changes to run with normal user privileges.
889 Sets up basic environment.
892 .Pa $HOME/.ssh/environment
895 Changes to user's home directory.
899 exists, runs it; else if
902 it; otherwise runs xauth.
905 files are given the X11
906 authentication protocol and cookie in standard input.
908 Runs user's shell or command.
910 .Sh AUTHORIZED_KEYS FILE FORMAT
911 .Pa $HOME/.ssh/authorized_keys
912 is the default file that lists the public keys that are
913 permitted for RSA authentication in protocol version 1
914 and for public key authentication (PubkeyAuthentication)
915 in protocol version 2.
916 .Cm AuthorizedKeysFile
917 may be used to specify an alternative file.
919 Each line of the file contains one
920 key (empty lines and lines starting with a
924 Each RSA public key consists of the following fields, separated by
925 spaces: options, bits, exponent, modulus, comment.
926 Each protocol version 2 public key consists of:
927 options, keytype, base64 encoded key, comment.
929 are optional; its presence is determined by whether the line starts
930 with a number or not (the option field never starts with a number).
931 The bits, exponent, modulus and comment fields give the RSA key for
932 protocol version 1; the
933 comment field is not used for anything (but may be convenient for the
934 user to identify the key).
935 For protocol version 2 the keytype is
940 Note that lines in this file are usually several hundred bytes long
941 (because of the size of the RSA key modulus).
942 You don't want to type them in; instead, copy the
949 The options (if present) consist of comma-separated option
951 No spaces are permitted, except within double quotes.
952 The following option specifications are supported (note
953 that option keywords are case-insensitive):
955 .It Cm from="pattern-list"
956 Specifies that in addition to RSA authentication, the canonical name
957 of the remote host must be present in the comma-separated list of
963 The list may also contain
964 patterns negated by prefixing them with
966 if the canonical host name matches a negated pattern, the key is not accepted.
968 of this option is to optionally increase security: RSA authentication
969 by itself does not trust the network or name servers or anything (but
970 the key); however, if somebody somehow steals the key, the key
971 permits an intruder to log in from anywhere in the world.
972 This additional option makes using a stolen key more difficult (name
973 servers and/or routers would have to be compromised in addition to
975 .It Cm command="command"
976 Specifies that the command is executed whenever this key is used for
978 The command supplied by the user (if any) is ignored.
979 The command is run on a pty if the connection requests a pty;
980 otherwise it is run without a tty.
981 Note that if you want a 8-bit clean channel,
982 you must not request a pty or should specify
984 A quote may be included in the command by quoting it with a backslash.
985 This option might be useful
986 to restrict certain RSA keys to perform just a specific operation.
987 An example might be a key that permits remote backups but nothing else.
988 Note that the client may specify TCP/IP and/or X11
989 forwarding unless they are explicitly prohibited.
990 .It Cm environment="NAME=value"
991 Specifies that the string is to be added to the environment when
992 logging in using this key.
993 Environment variables set this way
994 override other default environment values.
995 Multiple options of this type are permitted.
996 .It Cm no-port-forwarding
997 Forbids TCP/IP forwarding when this key is used for authentication.
998 Any port forward requests by the client will return an error.
999 This might be used, e.g., in connection with the
1002 .It Cm no-X11-forwarding
1003 Forbids X11 forwarding when this key is used for authentication.
1004 Any X11 forward requests by the client will return an error.
1005 .It Cm no-agent-forwarding
1006 Forbids authentication agent forwarding when this key is used for
1009 Prevents tty allocation (a request to allocate a pty will fail).
1010 .It Cm permitopen="host:port"
1013 port forwarding such that it may only connect to the specified host and
1016 options may be applied separated by commas. No pattern matching is
1017 performed on the specified hostnames, they must be literal domains or
1021 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1023 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1025 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1027 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1028 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1030 .Pa /etc/ssh_known_hosts ,
1032 .Pa $HOME/.ssh/known_hosts
1033 files contain host public keys for all known hosts.
1034 The global file should
1035 be prepared by the administrator (optional), and the per-user file is
1036 maintained automatically: whenever the user connects from an unknown host
1037 its key is added to the per-user file.
1039 Each line in these files contains the following fields: hostnames,
1040 bits, exponent, modulus, comment.
1041 The fields are separated by spaces.
1043 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1044 wildcards); each pattern in turn is matched against the canonical host
1045 name (when authenticating a client) or against the user-supplied
1046 name (when authenticating a server).
1047 A pattern may also be preceded by
1049 to indicate negation: if the host name matches a negated
1050 pattern, it is not accepted (by that line) even if it matched another
1051 pattern on the line.
1053 Bits, exponent, and modulus are taken directly from the RSA host key; they
1054 can be obtained, e.g., from
1055 .Pa /etc/ssh_host_key.pub .
1056 The optional comment field continues to the end of the line, and is not used.
1060 and empty lines are ignored as comments.
1062 When performing host authentication, authentication is accepted if any
1063 matching line has the proper key.
1064 It is thus permissible (but not
1065 recommended) to have several lines or different host keys for the same
1067 This will inevitably happen when short forms of host names
1068 from different domains are put in the file.
1070 that the files contain conflicting information; authentication is
1071 accepted if valid information can be found from either file.
1073 Note that the lines in these files are typically hundreds of characters
1074 long, and you definitely don't want to type in the host keys by hand.
1075 Rather, generate them by a script
1077 .Pa /etc/ssh_host_key.pub
1078 and adding the host names at the front.
1081 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1082 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1086 .It Pa /etc/sshd_config
1087 Contains configuration data for
1089 This file should be writable by root only, but it is recommended
1090 (though not necessary) that it be world-readable.
1091 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1092 These three files contain the private parts of the host keys.
1093 These files should only be owned by root, readable only by root, and not
1094 accessible to others.
1097 does not start if this file is group/world-accessible.
1098 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1099 These three files contain the public parts of the host keys.
1100 These files should be world-readable but writable only by
1102 Their contents should match the respective private parts.
1104 really used for anything; they are provided for the convenience of
1105 the user so their contents can be copied to known hosts files.
1106 These files are created using
1109 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1110 .It Pa /var/run/sshd.pid
1111 Contains the process ID of the
1113 listening for connections (if there are several daemons running
1114 concurrently for different ports, this contains the pid of the one
1116 The content of this file is not sensitive; it can be world-readable.
1117 .It Pa $HOME/.ssh/authorized_keys
1118 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1119 This file must be readable by root (which may on some machines imply
1120 it being world-readable if the user's home directory resides on an NFS
1122 It is recommended that it not be accessible by others.
1123 The format of this file is described above.
1124 Users will place the contents of their
1129 files into this file, as described in
1131 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1132 These files are consulted when using rhosts with RSA host
1133 authentication or protocol version 2 hostbased authentication
1134 to check the public key of the host.
1135 The key must be listed in one of these files to be accepted.
1136 The client uses the same files
1137 to verify that it is connecting to the correct remote host.
1138 These files should be writable only by root/the owner.
1139 .Pa /etc/ssh_known_hosts
1140 should be world-readable, and
1141 .Pa $HOME/.ssh/known_hosts
1142 can but need not be world-readable.
1144 If this file exists,
1146 refuses to let anyone except root log in.
1147 The contents of the file
1148 are displayed to anyone trying to log in, and non-root connections are
1150 The file should be world-readable.
1151 .It Pa /etc/hosts.allow, /etc/hosts.deny
1154 support, tcp-wrappers access controls may be defined here as described in
1155 .Xr hosts_access 5 .
1156 .It Pa $HOME/.rhosts
1157 This file contains host-username pairs, separated by a space, one per
1159 The given user on the corresponding host is permitted to log in
1161 The same file is used by rlogind and rshd.
1163 be writable only by the user; it is recommended that it not be
1164 accessible by others.
1166 If is also possible to use netgroups in the file.
1168 name may be of the form +@groupname to specify all hosts or all users
1170 .It Pa $HOME/.shosts
1172 this file is exactly the same as for
1174 However, this file is
1175 not used by rlogin and rshd, so using this permits access using SSH only.
1176 .It Pa /etc/hosts.equiv
1177 This file is used during
1180 In the simplest form, this file contains host names, one per line.
1182 those hosts are permitted to log in without a password, provided they
1183 have the same user name on both machines.
1184 The host name may also be
1185 followed by a user name; such users are permitted to log in as
1187 user on this machine (except root).
1188 Additionally, the syntax
1190 can be used to specify netgroups.
1191 Negated entries start with
1194 If the client host/user is successfully matched in this file, login is
1195 automatically permitted provided the client and server user names are the
1197 Additionally, successful RSA host authentication is normally required.
1198 This file must be writable only by root; it is recommended
1199 that it be world-readable.
1201 .Sy "Warning: It is almost never a good idea to use user names in"
1203 Beware that it really means that the named user(s) can log in as
1205 which includes bin, daemon, adm, and other accounts that own critical
1206 binaries and directories.
1207 Using a user name practically grants the user root access.
1208 The only valid use for user names that I can think
1209 of is in negative entries.
1211 Note that this warning also applies to rsh/rlogin.
1212 .It Pa /etc/shosts.equiv
1213 This is processed exactly as
1214 .Pa /etc/hosts.equiv .
1215 However, this file may be useful in environments that want to run both
1217 .It Pa $HOME/.ssh/environment
1218 This file is read into the environment at login (if it exists).
1219 It can only contain empty lines, comment lines (that start with
1221 and assignment lines of the form name=value.
1222 The file should be writable
1223 only by the user; it need not be readable by anyone else.
1224 .It Pa $HOME/.ssh/rc
1225 If this file exists, it is run with /bin/sh after reading the
1226 environment files but before starting the user's shell or command.
1227 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1235 The primary purpose of this file is to run any initialization routines
1236 which may be needed before the user's home directory becomes
1237 accessible; AFS is a particular example of such an environment.
1239 This file will probably contain some initialization code followed by
1240 something similar to:
1242 if read proto cookie; then
1243 echo add $DISPLAY $proto $cookie | xauth -q -
1247 If this file does not exist,
1250 does not exist either, xauth is used to store the cookie.
1252 This file should be writable only by the user, and need not be
1253 readable by anyone else.
1257 This can be used to specify
1258 machine-specific login-time initializations globally.
1259 This file should be writable only by root, and should be world-readable.
1262 OpenSSH is a derivative of the original and free
1263 ssh 1.2.12 release by Tatu Ylonen.
1264 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1265 Theo de Raadt and Dug Song
1266 removed many bugs, re-added newer features and
1268 Markus Friedl contributed the support for SSH
1269 protocol versions 1.5 and 2.0.
1286 .%T "SSH Protocol Architecture"
1287 .%N draft-ietf-secsh-architecture-09.txt
1289 .%O work in progress material
1295 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1296 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1298 .%O work in progress material
andersk / openssh.git / blob