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 1.148 2001/09/14 18:56:04 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
54 .Op Fl V Ar client_protocol_id
57 (SSH Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256 bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
123 are disabled (thus completely disabling
129 .Ss SSH protocol version 2
131 Version 2 works similarly:
132 Each host has a host-specific key (RSA or DSA) used to identify the host.
133 However, when the daemon starts, it does not generate a server key.
134 Forward security is provided through a Diffie-Hellman key agreement.
135 This key agreement results in a shared session key.
137 The rest of the session is encrypted using a symmetric cipher, currently
138 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
139 The client selects the encryption algorithm
140 to use from those offered by the server.
141 Additionally, session integrity is provided
142 through a cryptographic message authentication code
143 (hmac-sha1 or hmac-md5).
145 Protocol version 2 provides a public key based
146 user (PubkeyAuthentication) or
147 client host (HostbasedAuthentication) authentication method,
148 conventional password authentication and challenge response based methods.
150 .Ss Command execution and data forwarding
152 If the client successfully authenticates itself, a dialog for
153 preparing the session is entered.
154 At this time the client may request
155 things like allocating a pseudo-tty, forwarding X11 connections,
156 forwarding TCP/IP connections, or forwarding the authentication agent
157 connection over the secure channel.
159 Finally, the client either requests a shell or execution of a command.
160 The sides then enter session mode.
161 In this mode, either side may send
162 data at any time, and such data is forwarded to/from the shell or
163 command on the server side, and the user terminal in the client side.
165 When the user program terminates and all forwarded X11 and other
166 connections have been closed, the server sends command exit status to
167 the client, and both sides exit.
170 can be configured using command-line options or a configuration
172 Command-line options override values specified in the
176 rereads its configuration file when it receives a hangup signal,
178 by executing itself with the name it was started as, i.e.,
181 The options are as follows:
184 Specifies the number of bits in the ephemeral protocol version 1
185 server key (default 768).
188 The server sends verbose debug output to the system
189 log, and does not put itself in the background.
190 The server also will not fork and will only process one connection.
191 This option is only intended for debugging for the server.
192 Multiple -d options increase the debugging level.
195 When this option is specified,
197 will send the output to the standard error instead of the system log.
198 .It Fl f Ar configuration_file
199 Specifies the name of the configuration file.
201 .Pa /etc/sshd_config .
203 refuses to start if there is no configuration file.
204 .It Fl g Ar login_grace_time
205 Gives the grace time for clients to authenticate themselves (default
207 If the client fails to authenticate the user within
208 this many seconds, the server disconnects and exits.
209 A value of zero indicates no limit.
210 .It Fl h Ar host_key_file
211 Specifies the file from which the host key is read (default
212 .Pa /etc/ssh_host_key ) .
213 This option must be given if
215 is not run as root (as the normal
216 host file is normally not readable by anyone but root).
217 It is possible to have multiple host key files for
218 the different protocol versions and host key algorithms.
222 is being run from inetd.
225 from inetd because it needs to generate the server key before it can
226 respond to the client, and this may take tens of seconds.
227 Clients would have to wait too long if the key was regenerated every time.
228 However, with small key sizes (e.g., 512) using
232 .It Fl k Ar key_gen_time
233 Specifies how often the ephemeral protocol version 1 server key is
234 regenerated (default 3600 seconds, or one hour).
235 The motivation for regenerating the key fairly
236 often is that the key is not stored anywhere, and after about an hour,
237 it becomes impossible to recover the key for decrypting intercepted
238 communications even if the machine is cracked into or physically
240 A value of zero indicates that the key will never be regenerated.
242 Specifies the port on which the server listens for connections
246 Nothing is sent to the system log.
247 Normally the beginning,
248 authentication, and termination of each connection is logged.
251 Only check the validity of the configuration file and sanity of the keys.
252 This is useful for updating
254 reliably as configuration options may change.
256 This option is used to specify the size of the field
259 structure that holds the remote host name.
260 If the resolved host name is longer than
262 the dotted decimal value will be used instead.
263 This allows hosts with very long host names that
264 overflow this field to still be uniquely identified.
267 indicates that only dotted decimal addresses
268 should be put into the
272 is also be used to prevent
274 from making DNS requests unless the authentication
275 mechanism or configuration requires it.
276 Authentication mechanisms that may require DNS include
277 .Cm RhostsAuthentication ,
278 .Cm RhostsRSAAuthentication ,
279 .Cm HostbasedAuthentication
281 .Cm from="pattern-list"
282 option in a key file.
284 When this option is specified
286 will not detach and does not become a daemon.
287 This allows easy monitoring of
292 to use IPv4 addresses only.
296 to use IPv6 addresses only.
298 .Sh CONFIGURATION FILE
300 reads configuration data from
302 (or the file specified with
304 on the command line).
305 The file contains keyword-argument pairs, one per line.
308 and empty lines are interpreted as comments.
311 keywords and their meanings are as follows (note that
312 keywords are case-insensitive and arguments are case-sensitive):
314 .It Cm AFSTokenPassing
315 Specifies whether an AFS token may be forwarded to the server.
319 This keyword can be followed by a list of group names, separated
321 If specified, login is allowed only for users whose primary
322 group or supplementary group list matches one of the patterns.
327 wildcards in the patterns.
328 Only group names are valid; a numerical group ID is not recognized.
329 By default login is allowed regardless of the group list.
331 .It Cm AllowTcpForwarding
332 Specifies whether TCP forwarding is permitted.
335 Note that disabling TCP forwarding does not improve security unless
336 users are also denied shell access, as they can always install their
340 This keyword can be followed by a list of user names, separated
342 If specified, login is allowed only for users names that
343 match one of the patterns.
348 wildcards in the patterns.
349 Only user names are valid; a numerical user ID is not recognized.
350 By default login is allowed regardless of the user name.
351 If the pattern takes the form USER@HOST then USER and HOST
352 are separately checked, restricting logins to particular
353 users from particular hosts.
355 .It Cm AuthorizedKeysFile
356 Specifies the file that contains the public keys that can be used
357 for user authentication.
358 .Cm AuthorizedKeysFile
359 may contain tokens of the form %T which are substituted during connection
360 set-up. The following tokens are defined: %% is replaced by a literal '%',
361 %h is replaced by the home directory of the user being authenticated and
362 %u is replaced by the username of that user.
364 .Cm AuthorizedKeysFile
365 is taken to be an absolute path or one relative to the user's home
368 .Dq .ssh/authorized_keys
370 In some jurisdictions, sending a warning message before authentication
371 may be relevant for getting legal protection.
372 The contents of the specified file are sent to the remote user before
373 authentication is allowed.
374 This option is only available for protocol version 2.
376 .It Cm ChallengeResponseAuthentication
377 Specifies whether challenge response authentication is allowed.
378 All authentication styles from
384 Specifies the ciphers allowed for protocol version 2.
385 Multiple ciphers must be comma-separated.
387 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
388 .It Cm ClientAliveInterval
389 Sets a timeout interval in seconds after which if no data has been received
392 will send a message through the encrypted
393 channel to request a response from the client.
395 is 0, indicating that these messages will not be sent to the client.
396 This option applies to protocol version 2 only.
397 .It Cm ClientAliveCountMax
398 Sets the number of client alive messages (see above) which may be
401 receiving any messages back from the client. If this threshold is
402 reached while client alive messages are being sent,
404 will disconnect the client, terminating the session. It is important
405 to note that the use of client alive messages is very different from
407 (below). The client alive messages are sent through the
408 encrypted channel and therefore will not be spoofable. The TCP keepalive
411 is spoofable. The client alive mechanism is valuable when the client or
412 server depend on knowing when a connection has become inactive.
414 The default value is 3. If
415 .Cm ClientAliveInterval
416 (above) is set to 15, and
417 .Cm Keepalive is left at the default, unresponsive ssh clients
418 will be disconnected after approximately 45 seconds.
420 This keyword can be followed by a number of group names, separated
422 Users whose primary group or supplementary group list matches
423 one of the patterns aren't allowed to log in.
428 wildcards in the patterns.
429 Only group names are valid; a numerical group ID is not recognized.
430 By default login is allowed regardless of the group list.
433 This keyword can be followed by a number of user names, separated
435 Login is disallowed for user names that match one of the patterns.
439 can be used as wildcards in the patterns.
440 Only user names are valid; a numerical user ID is not recognized.
441 By default login is allowed regardless of the user name.
443 Specifies whether remote hosts are allowed to connect to ports
444 forwarded for the client.
447 binds remote port forwardings to the loopback addresss. This
448 prevents other remote hosts from connecting to forwarded ports.
450 can be used to specify that
452 should bind remote port forwardings to the wildcard address,
453 thus allowing remote hosts to connect to forwarded ports.
460 .It Cm HostbasedAuthentication
461 Specifies whether rhosts or /etc/hosts.equiv authentication together
462 with successful public key client host authentication is allowed
463 (hostbased authentication).
464 This option is similar to
465 .Cm RhostsRSAAuthentication
466 and applies to protocol version 2 only.
470 Specifies the file containing the private host keys (default
471 .Pa /etc/ssh_host_key )
472 used by SSH protocol versions 1 and 2.
475 will refuse to use a file if it is group/world-accessible.
476 It is possible to have multiple host key files.
478 keys are used for version 1 and
482 are used for version 2 of the SSH protocol.
488 files will not be used in
489 .Cm RhostsAuthentication ,
490 .Cm RhostsRSAAuthentication
492 .Cm HostbasedAuthentication .
496 .Pa /etc/shosts.equiv
500 .It Cm IgnoreUserKnownHosts
503 should ignore the user's
504 .Pa $HOME/.ssh/known_hosts
506 .Cm RhostsRSAAuthentication
508 .Cm HostbasedAuthentication .
512 Specifies whether the system should send keepalive messages to the
514 If they are sent, death of the connection or crash of one
515 of the machines will be properly noticed.
516 However, this means that
517 connections will die if the route is down temporarily, and some people
519 On the other hand, if keepalives are not sent,
520 sessions may hang indefinitely on the server, leaving
522 users and consuming server resources.
526 (to send keepalives), and the server will notice
527 if the network goes down or the client host reboots.
528 This avoids infinitely hanging sessions.
530 To disable keepalives, the value should be set to
532 in both the server and the client configuration files.
533 .It Cm KerberosAuthentication
534 Specifies whether Kerberos authentication is allowed.
535 This can be in the form of a Kerberos ticket, or if
536 .Cm PasswordAuthentication
537 is yes, the password provided by the user will be validated through
539 To use this option, the server needs a
540 Kerberos servtab which allows the verification of the KDC's identity.
543 .It Cm KerberosOrLocalPasswd
544 If set then if password authentication through Kerberos fails then
545 the password will be validated via any additional local mechanism
550 .It Cm KerberosTgtPassing
551 Specifies whether a Kerberos TGT may be forwarded to the server.
554 as this only works when the Kerberos KDC is actually an AFS kaserver.
555 .It Cm KerberosTicketCleanup
556 Specifies whether to automatically destroy the user's ticket cache
560 .It Cm KeyRegenerationInterval
561 In protocol version 1, the ephemeral server key is automatically regenerated
562 after this many seconds (if it has been used).
563 The purpose of regeneration is to prevent
564 decrypting captured sessions by later breaking into the machine and
566 The key is never stored anywhere.
567 If the value is 0, the key is never regenerated.
568 The default is 3600 (seconds).
570 Specifies the local addresses
573 The following forms may be used:
575 .Bl -item -offset indent -compact
579 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
584 .Ar host No | Ar IPv4_addr No : Ar port
590 .Ar host No | Ar IPv6_addr Oc : Ar port
598 will listen on the address and all prior
600 options specified. The default is to listen on all local
603 options are permitted. Additionally, any
605 options must precede this option for non port qualified addresses.
606 .It Cm LoginGraceTime
607 The server disconnects after this time if the user has not
608 successfully logged in.
609 If the value is 0, there is no time limit.
610 The default is 600 (seconds).
612 Gives the verbosity level that is used when logging messages from
614 The possible values are:
615 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
617 Logging with level DEBUG violates the privacy of users
618 and is not recommended.
620 Specifies the available MAC (message authentication code) algorithms.
621 The MAC algorithm is used in protocol version 2
622 for data integrity protection.
623 Multiple algorithms must be comma-separated.
625 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
627 Specifies the maximum number of concurrent unauthenticated connections to the
630 Additional connections will be dropped until authentication succeeds or the
632 expires for a connection.
635 Alternatively, random early drop can be enabled by specifying
636 the three colon separated values
640 will refuse connection attempts with a probability of
643 if there are currently
646 unauthenticated connections.
647 The probability increases linearly and all connection attempts
648 are refused if the number of unauthenticated connections reaches
651 .It Cm PAMAuthenticationViaKbdInt
652 Specifies whether PAM challenge response authentication is allowed. This
653 allows the use of most PAM challenge response authentication modules, but
654 it will allow password authentication regardless of whether
655 .Cm PasswordAuthentication
659 .It Cm PasswordAuthentication
660 Specifies whether password authentication is allowed.
663 .It Cm PermitEmptyPasswords
664 When password authentication is allowed, it specifies whether the
665 server allows login to accounts with empty password strings.
668 .It Cm PermitRootLogin
669 Specifies whether root can login using
673 .Dq without-password ,
674 .Dq forced-commands-only
680 If this option is set to
682 password authentication is disabled for root.
684 If this option is set to
685 .Dq forced-commands-only
686 root login with public key authentication will be allowed,
689 option has been specified
690 (which may be useful for taking remote backups even if root login is
691 normally not allowed). All other authentication methods are disabled
694 If this option is set to
696 root is not allowed to login.
698 Specifies the file that contains the process identifier of the
702 .Pa /var/run/sshd.pid .
704 Specifies the port number that
708 Multiple options of this type are permitted.
714 should print the date and time when the user last logged in.
722 when a user logs in interactively.
723 (On some systems it is also printed by the shell,
729 Specifies the protocol versions
732 The possible values are
736 Multiple versions must be comma-separated.
739 .It Cm PubkeyAuthentication
740 Specifies whether public key authentication is allowed.
743 Note that this option applies to protocol version 2 only.
744 .It Cm ReverseMappingCheck
747 should try to verify the remote host name and check that
748 the resolved host name for the remote IP address maps back to the
749 very same IP address.
752 .It Cm RhostsAuthentication
753 Specifies whether authentication using rhosts or /etc/hosts.equiv
755 Normally, this method should not be permitted because it is insecure.
756 .Cm RhostsRSAAuthentication
758 instead, because it performs RSA-based host authentication in addition
759 to normal rhosts or /etc/hosts.equiv authentication.
762 This option applies to protocol version 1 only.
763 .It Cm RhostsRSAAuthentication
764 Specifies whether rhosts or /etc/hosts.equiv authentication together
765 with successful RSA host authentication is allowed.
768 This option applies to protocol version 1 only.
769 .It Cm RSAAuthentication
770 Specifies whether pure RSA authentication is allowed.
773 This option applies to protocol version 1 only.
775 Defines the number of bits in the ephemeral protocol version 1 server key.
776 The minimum value is 512, and the default is 768.
780 should check file modes and ownership of the
781 user's files and home directory before accepting login.
782 This is normally desirable because novices sometimes accidentally leave their
783 directory or files world-writable.
787 Configures an external subsystem (e.g., file transfer daemon).
788 Arguments should be a subsystem name and a command to execute upon subsystem
794 file transfer subsystem.
795 By default no subsystems are defined.
796 Note that this option applies to protocol version 2 only.
797 .It Cm SyslogFacility
798 Gives the facility code that is used when logging messages from
800 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
801 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
806 is used for interactive login sessions.
811 is never used for remote command execution.
812 Note also, that if this is enabled,
814 will be disabled because
816 does not know how to handle
819 .It Cm X11DisplayOffset
820 Specifies the first display number available for
825 from interfering with real X11 servers.
828 Specifies whether X11 forwarding is permitted.
831 Note that disabling X11 forwarding does not improve security in any
832 way, as users can always install their own forwarders.
833 X11 forwarding is automatically disabled if
837 Specifies the location of the
841 .Pa /usr/X11R6/bin/xauth .
846 command-line arguments and configuration file options that specify time
847 may be expressed using a sequence of the form:
849 .Ar time Oo Ar qualifier Oc ,
853 is a positive integer value and
855 is one of the following:
857 .Bl -tag -width Ds -compact -offset indent
872 Each member of the sequence is added together to calculate
873 the total time value.
875 Time format examples:
877 .Bl -tag -width Ds -compact -offset indent
879 600 seconds (10 minutes)
883 1 hour 30 minutes (90 minutes)
886 When a user successfully logs in,
889 .Bl -enum -offset indent
891 If the login is on a tty, and no command has been specified,
892 prints last login time and
894 (unless prevented in the configuration file or by
895 .Pa $HOME/.hushlogin ;
900 If the login is on a tty, records login time.
904 if it exists, prints contents and quits
907 Changes to run with normal user privileges.
909 Sets up basic environment.
912 .Pa $HOME/.ssh/environment
915 Changes to user's home directory.
919 exists, runs it; else if
922 it; otherwise runs xauth.
925 files are given the X11
926 authentication protocol and cookie in standard input.
928 Runs user's shell or command.
930 .Sh AUTHORIZED_KEYS FILE FORMAT
931 .Pa $HOME/.ssh/authorized_keys
932 is the default file that lists the public keys that are
933 permitted for RSA authentication in protocol version 1
934 and for public key authentication (PubkeyAuthentication)
935 in protocol version 2.
936 .Cm AuthorizedKeysFile
937 may be used to specify an alternative file.
939 Each line of the file contains one
940 key (empty lines and lines starting with a
944 Each RSA public key consists of the following fields, separated by
945 spaces: options, bits, exponent, modulus, comment.
946 Each protocol version 2 public key consists of:
947 options, keytype, base64 encoded key, comment.
949 are optional; its presence is determined by whether the line starts
950 with a number or not (the option field never starts with a number).
951 The bits, exponent, modulus and comment fields give the RSA key for
952 protocol version 1; the
953 comment field is not used for anything (but may be convenient for the
954 user to identify the key).
955 For protocol version 2 the keytype is
960 Note that lines in this file are usually several hundred bytes long
961 (because of the size of the RSA key modulus).
962 You don't want to type them in; instead, copy the
969 The options (if present) consist of comma-separated option
971 No spaces are permitted, except within double quotes.
972 The following option specifications are supported (note
973 that option keywords are case-insensitive):
975 .It Cm from="pattern-list"
976 Specifies that in addition to RSA authentication, the canonical name
977 of the remote host must be present in the comma-separated list of
983 The list may also contain
984 patterns negated by prefixing them with
986 if the canonical host name matches a negated pattern, the key is not accepted.
988 of this option is to optionally increase security: RSA authentication
989 by itself does not trust the network or name servers or anything (but
990 the key); however, if somebody somehow steals the key, the key
991 permits an intruder to log in from anywhere in the world.
992 This additional option makes using a stolen key more difficult (name
993 servers and/or routers would have to be compromised in addition to
995 .It Cm command="command"
996 Specifies that the command is executed whenever this key is used for
998 The command supplied by the user (if any) is ignored.
999 The command is run on a pty if the client requests a pty;
1000 otherwise it is run without a tty.
1001 If a 8-bit clean channel is required,
1002 one must not request a pty or should specify
1004 A quote may be included in the command by quoting it with a backslash.
1005 This option might be useful
1006 to restrict certain RSA keys to perform just a specific operation.
1007 An example might be a key that permits remote backups but nothing else.
1008 Note that the client may specify TCP/IP and/or X11
1009 forwarding unless they are explicitly prohibited.
1010 .It Cm environment="NAME=value"
1011 Specifies that the string is to be added to the environment when
1012 logging in using this key.
1013 Environment variables set this way
1014 override other default environment values.
1015 Multiple options of this type are permitted.
1016 .It Cm no-port-forwarding
1017 Forbids TCP/IP forwarding when this key is used for authentication.
1018 Any port forward requests by the client will return an error.
1019 This might be used, e.g., in connection with the
1022 .It Cm no-X11-forwarding
1023 Forbids X11 forwarding when this key is used for authentication.
1024 Any X11 forward requests by the client will return an error.
1025 .It Cm no-agent-forwarding
1026 Forbids authentication agent forwarding when this key is used for
1029 Prevents tty allocation (a request to allocate a pty will fail).
1030 .It Cm permitopen="host:port"
1033 port forwarding such that it may only connect to the specified host and
1035 IPv6 addresses can be specified with an alternative syntax:
1039 options may be applied separated by commas. No pattern matching is
1040 performed on the specified hostnames, they must be literal domains or
1044 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1046 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1048 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1050 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1051 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1053 .Pa /etc/ssh_known_hosts ,
1055 .Pa $HOME/.ssh/known_hosts
1056 files contain host public keys for all known hosts.
1057 The global file should
1058 be prepared by the administrator (optional), and the per-user file is
1059 maintained automatically: whenever the user connects from an unknown host
1060 its key is added to the per-user file.
1062 Each line in these files contains the following fields: hostnames,
1063 bits, exponent, modulus, comment.
1064 The fields are separated by spaces.
1066 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1067 wildcards); each pattern in turn is matched against the canonical host
1068 name (when authenticating a client) or against the user-supplied
1069 name (when authenticating a server).
1070 A pattern may also be preceded by
1072 to indicate negation: if the host name matches a negated
1073 pattern, it is not accepted (by that line) even if it matched another
1074 pattern on the line.
1076 Bits, exponent, and modulus are taken directly from the RSA host key; they
1077 can be obtained, e.g., from
1078 .Pa /etc/ssh_host_key.pub .
1079 The optional comment field continues to the end of the line, and is not used.
1083 and empty lines are ignored as comments.
1085 When performing host authentication, authentication is accepted if any
1086 matching line has the proper key.
1087 It is thus permissible (but not
1088 recommended) to have several lines or different host keys for the same
1090 This will inevitably happen when short forms of host names
1091 from different domains are put in the file.
1093 that the files contain conflicting information; authentication is
1094 accepted if valid information can be found from either file.
1096 Note that the lines in these files are typically hundreds of characters
1097 long, and you definitely don't want to type in the host keys by hand.
1098 Rather, generate them by a script
1100 .Pa /etc/ssh_host_key.pub
1101 and adding the host names at the front.
1104 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1105 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1109 .It Pa /etc/sshd_config
1110 Contains configuration data for
1112 This file should be writable by root only, but it is recommended
1113 (though not necessary) that it be world-readable.
1114 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1115 These three files contain the private parts of the host keys.
1116 These files should only be owned by root, readable only by root, and not
1117 accessible to others.
1120 does not start if this file is group/world-accessible.
1121 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1122 These three files contain the public parts of the host keys.
1123 These files should be world-readable but writable only by
1125 Their contents should match the respective private parts.
1127 really used for anything; they are provided for the convenience of
1128 the user so their contents can be copied to known hosts files.
1129 These files are created using
1132 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1133 .It Pa /var/run/sshd.pid
1134 Contains the process ID of the
1136 listening for connections (if there are several daemons running
1137 concurrently for different ports, this contains the pid of the one
1139 The content of this file is not sensitive; it can be world-readable.
1140 .It Pa $HOME/.ssh/authorized_keys
1141 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1142 This file must be readable by root (which may on some machines imply
1143 it being world-readable if the user's home directory resides on an NFS
1145 It is recommended that it not be accessible by others.
1146 The format of this file is described above.
1147 Users will place the contents of their
1152 files into this file, as described in
1154 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1155 These files are consulted when using rhosts with RSA host
1156 authentication or protocol version 2 hostbased authentication
1157 to check the public key of the host.
1158 The key must be listed in one of these files to be accepted.
1159 The client uses the same files
1160 to verify that it is connecting to the correct remote host.
1161 These files should be writable only by root/the owner.
1162 .Pa /etc/ssh_known_hosts
1163 should be world-readable, and
1164 .Pa $HOME/.ssh/known_hosts
1165 can but need not be world-readable.
1167 If this file exists,
1169 refuses to let anyone except root log in.
1170 The contents of the file
1171 are displayed to anyone trying to log in, and non-root connections are
1173 The file should be world-readable.
1174 .It Pa /etc/hosts.allow, /etc/hosts.deny
1177 support, tcp-wrappers access controls may be defined here as described in
1178 .Xr hosts_access 5 .
1179 .It Pa $HOME/.rhosts
1180 This file contains host-username pairs, separated by a space, one per
1182 The given user on the corresponding host is permitted to log in
1184 The same file is used by rlogind and rshd.
1186 be writable only by the user; it is recommended that it not be
1187 accessible by others.
1189 If is also possible to use netgroups in the file.
1191 name may be of the form +@groupname to specify all hosts or all users
1193 .It Pa $HOME/.shosts
1195 this file is exactly the same as for
1197 However, this file is
1198 not used by rlogin and rshd, so using this permits access using SSH only.
1199 .It Pa /etc/hosts.equiv
1200 This file is used during
1203 In the simplest form, this file contains host names, one per line.
1205 those hosts are permitted to log in without a password, provided they
1206 have the same user name on both machines.
1207 The host name may also be
1208 followed by a user name; such users are permitted to log in as
1210 user on this machine (except root).
1211 Additionally, the syntax
1213 can be used to specify netgroups.
1214 Negated entries start with
1217 If the client host/user is successfully matched in this file, login is
1218 automatically permitted provided the client and server user names are the
1220 Additionally, successful RSA host authentication is normally required.
1221 This file must be writable only by root; it is recommended
1222 that it be world-readable.
1224 .Sy "Warning: It is almost never a good idea to use user names in"
1226 Beware that it really means that the named user(s) can log in as
1228 which includes bin, daemon, adm, and other accounts that own critical
1229 binaries and directories.
1230 Using a user name practically grants the user root access.
1231 The only valid use for user names that I can think
1232 of is in negative entries.
1234 Note that this warning also applies to rsh/rlogin.
1235 .It Pa /etc/shosts.equiv
1236 This is processed exactly as
1237 .Pa /etc/hosts.equiv .
1238 However, this file may be useful in environments that want to run both
1240 .It Pa $HOME/.ssh/environment
1241 This file is read into the environment at login (if it exists).
1242 It can only contain empty lines, comment lines (that start with
1244 and assignment lines of the form name=value.
1245 The file should be writable
1246 only by the user; it need not be readable by anyone else.
1247 .It Pa $HOME/.ssh/rc
1248 If this file exists, it is run with /bin/sh after reading the
1249 environment files but before starting the user's shell or command.
1250 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1258 The primary purpose of this file is to run any initialization routines
1259 which may be needed before the user's home directory becomes
1260 accessible; AFS is a particular example of such an environment.
1262 This file will probably contain some initialization code followed by
1263 something similar to:
1265 if read proto cookie; then
1266 echo add $DISPLAY $proto $cookie | xauth -q -
1270 If this file does not exist,
1273 does not exist either, xauth is used to store the cookie.
1275 This file should be writable only by the user, and need not be
1276 readable by anyone else.
1280 This can be used to specify
1281 machine-specific login-time initializations globally.
1282 This file should be writable only by root, and should be world-readable.
1285 OpenSSH is a derivative of the original and free
1286 ssh 1.2.12 release by Tatu Ylonen.
1287 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1288 Theo de Raadt and Dug Song
1289 removed many bugs, re-added newer features and
1291 Markus Friedl contributed the support for SSH
1292 protocol versions 1.5 and 2.0.
1309 .%T "SSH Protocol Architecture"
1310 .%N draft-ietf-secsh-architecture-09.txt
1312 .%O work in progress material
1318 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1319 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1321 .%O work in progress material
andersk / openssh.git / blob