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 1.154 2001/11/07 22:12:01 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
56 (SSH Daemon) is the daemon program for
58 Together these programs replace rlogin and rsh, and
59 provide secure encrypted communications between two untrusted hosts
60 over an insecure network.
61 The programs are intended to be as easy to
62 install and use as possible.
65 is the daemon that listens for connections from clients.
66 It is normally started at boot from
69 daemon for each incoming connection.
70 The forked daemons handle
71 key exchange, encryption, authentication, command execution,
73 This implementation of
75 supports both SSH protocol version 1 and 2 simultaneously.
79 .Ss SSH protocol version 1
81 Each host has a host-specific RSA key
82 (normally 1024 bits) used to identify the host.
84 the daemon starts, it generates a server RSA key (normally 768 bits).
85 This key is normally regenerated every hour if it has been used, and
86 is never stored on disk.
88 Whenever a client connects the daemon responds with its public
90 The client compares the
91 RSA host key against its own database to verify that it has not changed.
92 The client then generates a 256 bit random number.
94 random number using both the host key and the server key, and sends
95 the encrypted number to the server.
96 Both sides then use this
97 random number as a session key which is used to encrypt all further
98 communications in the session.
99 The rest of the session is encrypted
100 using a conventional cipher, currently Blowfish or 3DES, with 3DES
101 being used by default.
102 The client selects the encryption algorithm
103 to use from those offered by the server.
105 Next, the server and the client enter an authentication dialog.
106 The client tries to authenticate itself using
110 authentication combined with RSA host
111 authentication, RSA challenge-response authentication, or password
112 based authentication.
114 Rhosts authentication is normally disabled
115 because it is fundamentally insecure, but can be enabled in the server
116 configuration file if desired.
117 System security is not improved unless
122 are disabled (thus completely disabling
128 .Ss SSH protocol version 2
130 Version 2 works similarly:
131 Each host has a host-specific key (RSA or DSA) used to identify the host.
132 However, when the daemon starts, it does not generate a server key.
133 Forward security is provided through a Diffie-Hellman key agreement.
134 This key agreement results in a shared session key.
136 The rest of the session is encrypted using a symmetric cipher, currently
137 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
138 The client selects the encryption algorithm
139 to use from those offered by the server.
140 Additionally, session integrity is provided
141 through a cryptographic message authentication code
142 (hmac-sha1 or hmac-md5).
144 Protocol version 2 provides a public key based
145 user (PubkeyAuthentication) or
146 client host (HostbasedAuthentication) authentication method,
147 conventional password authentication and challenge response based methods.
149 .Ss Command execution and data forwarding
151 If the client successfully authenticates itself, a dialog for
152 preparing the session is entered.
153 At this time the client may request
154 things like allocating a pseudo-tty, forwarding X11 connections,
155 forwarding TCP/IP connections, or forwarding the authentication agent
156 connection over the secure channel.
158 Finally, the client either requests a shell or execution of a command.
159 The sides then enter session mode.
160 In this mode, either side may send
161 data at any time, and such data is forwarded to/from the shell or
162 command on the server side, and the user terminal in the client side.
164 When the user program terminates and all forwarded X11 and other
165 connections have been closed, the server sends command exit status to
166 the client, and both sides exit.
169 can be configured using command-line options or a configuration
171 Command-line options override values specified in the
175 rereads its configuration file when it receives a hangup signal,
177 by executing itself with the name it was started as, i.e.,
180 The options are as follows:
183 Specifies the number of bits in the ephemeral protocol version 1
184 server key (default 768).
187 The server sends verbose debug output to the system
188 log, and does not put itself in the background.
189 The server also will not fork and will only process one connection.
190 This option is only intended for debugging for the server.
191 Multiple -d options increase the debugging level.
194 When this option is specified,
196 will send the output to the standard error instead of the system log.
197 .It Fl f Ar configuration_file
198 Specifies the name of the configuration file.
200 .Pa /etc/sshd_config .
202 refuses to start if there is no configuration file.
203 .It Fl g Ar login_grace_time
204 Gives the grace time for clients to authenticate themselves (default
206 If the client fails to authenticate the user within
207 this many seconds, the server disconnects and exits.
208 A value of zero indicates no limit.
209 .It Fl h Ar host_key_file
210 Specifies the file from which the host key is read (default
211 .Pa /etc/ssh_host_key ) .
212 This option must be given if
214 is not run as root (as the normal
215 host file is normally not readable by anyone but root).
216 It is possible to have multiple host key files for
217 the different protocol versions and host key algorithms.
221 is being run from inetd.
224 from inetd because it needs to generate the server key before it can
225 respond to the client, and this may take tens of seconds.
226 Clients would have to wait too long if the key was regenerated every time.
227 However, with small key sizes (e.g., 512) using
231 .It Fl k Ar key_gen_time
232 Specifies how often the ephemeral protocol version 1 server key is
233 regenerated (default 3600 seconds, or one hour).
234 The motivation for regenerating the key fairly
235 often is that the key is not stored anywhere, and after about an hour,
236 it becomes impossible to recover the key for decrypting intercepted
237 communications even if the machine is cracked into or physically
239 A value of zero indicates that the key will never be regenerated.
241 Specifies the port on which the server listens for connections
245 Nothing is sent to the system log.
246 Normally the beginning,
247 authentication, and termination of each connection is logged.
250 Only check the validity of the configuration file and sanity of the keys.
251 This is useful for updating
253 reliably as configuration options may change.
255 This option is used to specify the size of the field
258 structure that holds the remote host name.
259 If the resolved host name is longer than
261 the dotted decimal value will be used instead.
262 This allows hosts with very long host names that
263 overflow this field to still be uniquely identified.
266 indicates that only dotted decimal addresses
267 should be put into the
271 is also be used to prevent
273 from making DNS requests unless the authentication
274 mechanism or configuration requires it.
275 Authentication mechanisms that may require DNS include
276 .Cm RhostsAuthentication ,
277 .Cm RhostsRSAAuthentication ,
278 .Cm HostbasedAuthentication
280 .Cm from="pattern-list"
281 option in a key file.
283 When this option is specified
285 will not detach and does not become a daemon.
286 This allows easy monitoring of
291 to use IPv4 addresses only.
295 to use IPv6 addresses only.
297 .Sh CONFIGURATION FILE
299 reads configuration data from
301 (or the file specified with
303 on the command line).
304 The file contains keyword-argument pairs, one per line.
307 and empty lines are interpreted as comments.
310 keywords and their meanings are as follows (note that
311 keywords are case-insensitive and arguments are case-sensitive):
313 .It Cm AFSTokenPassing
314 Specifies whether an AFS token may be forwarded to the server.
318 This keyword can be followed by a list of group names, separated
320 If specified, login is allowed only for users whose primary
321 group or supplementary group list matches one of the patterns.
326 wildcards in the patterns.
327 Only group names are valid; a numerical group ID is not recognized.
328 By default login is allowed regardless of the group list.
330 .It Cm AllowTcpForwarding
331 Specifies whether TCP forwarding is permitted.
334 Note that disabling TCP forwarding does not improve security unless
335 users are also denied shell access, as they can always install their
339 This keyword can be followed by a list of user names, separated
341 If specified, login is allowed only for users names that
342 match one of the patterns.
347 wildcards in the patterns.
348 Only user names are valid; a numerical user ID is not recognized.
349 By default login is allowed regardless of the user name.
350 If the pattern takes the form USER@HOST then USER and HOST
351 are separately checked, restricting logins to particular
352 users from particular hosts.
354 .It Cm AuthorizedKeysFile
355 Specifies the file that contains the public keys that can be used
356 for user authentication.
357 .Cm AuthorizedKeysFile
358 may contain tokens of the form %T which are substituted during connection
359 set-up. The following tokens are defined: %% is replaced by a literal '%',
360 %h is replaced by the home directory of the user being authenticated and
361 %u is replaced by the username of that user.
363 .Cm AuthorizedKeysFile
364 is taken to be an absolute path or one relative to the user's home
367 .Dq .ssh/authorized_keys
369 In some jurisdictions, sending a warning message before authentication
370 may be relevant for getting legal protection.
371 The contents of the specified file are sent to the remote user before
372 authentication is allowed.
373 This option is only available for protocol version 2.
375 .It Cm ChallengeResponseAuthentication
376 Specifies whether challenge response authentication is allowed.
377 All authentication styles from
383 Specifies the ciphers allowed for protocol version 2.
384 Multiple ciphers must be comma-separated.
386 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
387 .It Cm ClientAliveInterval
388 Sets a timeout interval in seconds after which if no data has been received
391 will send a message through the encrypted
392 channel to request a response from the client.
394 is 0, indicating that these messages will not be sent to the client.
395 This option applies to protocol version 2 only.
396 .It Cm ClientAliveCountMax
397 Sets the number of client alive messages (see above) which may be
400 receiving any messages back from the client. If this threshold is
401 reached while client alive messages are being sent,
403 will disconnect the client, terminating the session. It is important
404 to note that the use of client alive messages is very different from
406 (below). The client alive messages are sent through the
407 encrypted channel and therefore will not be spoofable. The TCP keepalive
410 is spoofable. The client alive mechanism is valuable when the client or
411 server depend on knowing when a connection has become inactive.
413 The default value is 3. If
414 .Cm ClientAliveInterval
415 (above) is set to 15, and
416 .Cm ClientAliveCountMax
417 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.
469 .It Cm GssapiAuthentication
470 Specifies whether authentication based on GSSAPI may be used, either using
471 the result of a successful key exchange, or using GSSAPI user
475 Note that this option applies to protocol version 2 only.
476 .It Cm GssapiKeyExchange
477 Specifies whether key exchange based on GSSAPI may be used. When using
478 GSSAPI key exchange the server need not have a host key.
481 Note that this option applies to protocol version 2 only.
482 .It Cm GssapiUseSessionCredCache
483 Specifies whether a unique credentials cache name should be generated per
484 session for storing delegated credentials.
487 Note that this option applies to protocol version 2 only.
489 Specifies the file containing the private host keys (default
490 .Pa /etc/ssh_host_key )
491 used by SSH protocol versions 1 and 2.
494 will refuse to use a file if it is group/world-accessible.
495 It is possible to have multiple host key files.
497 keys are used for version 1 and
501 are used for version 2 of the SSH protocol.
507 files will not be used in
508 .Cm RhostsAuthentication ,
509 .Cm RhostsRSAAuthentication
511 .Cm HostbasedAuthentication .
515 .Pa /etc/shosts.equiv
519 .It Cm IgnoreUserKnownHosts
522 should ignore the user's
523 .Pa $HOME/.ssh/known_hosts
525 .Cm RhostsRSAAuthentication
527 .Cm HostbasedAuthentication .
531 Specifies whether the system should send keepalive messages to the
533 If they are sent, death of the connection or crash of one
534 of the machines will be properly noticed.
535 However, this means that
536 connections will die if the route is down temporarily, and some people
538 On the other hand, if keepalives are not sent,
539 sessions may hang indefinitely on the server, leaving
541 users and consuming server resources.
545 (to send keepalives), and the server will notice
546 if the network goes down or the client host reboots.
547 This avoids infinitely hanging sessions.
549 To disable keepalives, the value should be set to
551 in both the server and the client configuration files.
552 .It Cm KerberosAuthentication
553 Specifies whether Kerberos authentication is allowed.
554 This can be in the form of a Kerberos ticket, or if
555 .Cm PasswordAuthentication
556 is yes, the password provided by the user will be validated through
558 To use this option, the server needs a
559 Kerberos servtab which allows the verification of the KDC's identity.
562 .It Cm KerberosOrLocalPasswd
563 If set then if password authentication through Kerberos fails then
564 the password will be validated via any additional local mechanism
569 .It Cm KerberosTgtPassing
570 Specifies whether a Kerberos TGT may be forwarded to the server.
573 as this only works when the Kerberos KDC is actually an AFS kaserver.
574 .It Cm KerberosTicketCleanup
575 Specifies whether to automatically destroy the user's ticket cache
579 .It Cm KeyRegenerationInterval
580 In protocol version 1, the ephemeral server key is automatically regenerated
581 after this many seconds (if it has been used).
582 The purpose of regeneration is to prevent
583 decrypting captured sessions by later breaking into the machine and
585 The key is never stored anywhere.
586 If the value is 0, the key is never regenerated.
587 The default is 3600 (seconds).
589 Specifies the local addresses
592 The following forms may be used:
594 .Bl -item -offset indent -compact
598 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
603 .Ar host No | Ar IPv4_addr No : Ar port
609 .Ar host No | Ar IPv6_addr Oc : Ar port
617 will listen on the address and all prior
619 options specified. The default is to listen on all local
622 options are permitted. Additionally, any
624 options must precede this option for non port qualified addresses.
625 .It Cm LoginGraceTime
626 The server disconnects after this time if the user has not
627 successfully logged in.
628 If the value is 0, there is no time limit.
629 The default is 600 (seconds).
631 Gives the verbosity level that is used when logging messages from
633 The possible values are:
634 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
636 Logging with level DEBUG violates the privacy of users
637 and is not recommended.
639 Specifies the available MAC (message authentication code) algorithms.
640 The MAC algorithm is used in protocol version 2
641 for data integrity protection.
642 Multiple algorithms must be comma-separated.
644 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
646 Specifies the maximum number of concurrent unauthenticated connections to the
649 Additional connections will be dropped until authentication succeeds or the
651 expires for a connection.
654 Alternatively, random early drop can be enabled by specifying
655 the three colon separated values
659 will refuse connection attempts with a probability of
662 if there are currently
665 unauthenticated connections.
666 The probability increases linearly and all connection attempts
667 are refused if the number of unauthenticated connections reaches
670 .It Cm PAMAuthenticationViaKbdInt
671 Specifies whether PAM challenge response authentication is allowed. This
672 allows the use of most PAM challenge response authentication modules, but
673 it will allow password authentication regardless of whether
674 .Cm PasswordAuthentication
678 .It Cm PasswordAuthentication
679 Specifies whether password authentication is allowed.
682 .It Cm PermitEmptyPasswords
683 When password authentication is allowed, it specifies whether the
684 server allows login to accounts with empty password strings.
687 .It Cm PermitRootLogin
688 Specifies whether root can login using
692 .Dq without-password ,
693 .Dq forced-commands-only
699 If this option is set to
701 password authentication is disabled for root.
703 If this option is set to
704 .Dq forced-commands-only
705 root login with public key authentication will be allowed,
708 option has been specified
709 (which may be useful for taking remote backups even if root login is
710 normally not allowed). All other authentication methods are disabled
713 If this option is set to
715 root is not allowed to login.
717 Specifies the file that contains the process identifier of the
721 .Pa /var/run/sshd.pid .
723 Specifies the port number that
727 Multiple options of this type are permitted.
733 should print the date and time when the user last logged in.
741 when a user logs in interactively.
742 (On some systems it is also printed by the shell,
748 Specifies the protocol versions
751 The possible values are
755 Multiple versions must be comma-separated.
758 .It Cm PubkeyAuthentication
759 Specifies whether public key authentication is allowed.
762 Note that this option applies to protocol version 2 only.
763 .It Cm ReverseMappingCheck
766 should try to verify the remote host name and check that
767 the resolved host name for the remote IP address maps back to the
768 very same IP address.
771 .It Cm RhostsAuthentication
772 Specifies whether authentication using rhosts or /etc/hosts.equiv
774 Normally, this method should not be permitted because it is insecure.
775 .Cm RhostsRSAAuthentication
777 instead, because it performs RSA-based host authentication in addition
778 to normal rhosts or /etc/hosts.equiv authentication.
781 This option applies to protocol version 1 only.
782 .It Cm RhostsRSAAuthentication
783 Specifies whether rhosts or /etc/hosts.equiv authentication together
784 with successful RSA host authentication is allowed.
787 This option applies to protocol version 1 only.
788 .It Cm RSAAuthentication
789 Specifies whether pure RSA authentication is allowed.
792 This option applies to protocol version 1 only.
794 Defines the number of bits in the ephemeral protocol version 1 server key.
795 The minimum value is 512, and the default is 768.
799 should check file modes and ownership of the
800 user's files and home directory before accepting login.
801 This is normally desirable because novices sometimes accidentally leave their
802 directory or files world-writable.
806 Configures an external subsystem (e.g., file transfer daemon).
807 Arguments should be a subsystem name and a command to execute upon subsystem
813 file transfer subsystem.
814 By default no subsystems are defined.
815 Note that this option applies to protocol version 2 only.
816 .It Cm SyslogFacility
817 Gives the facility code that is used when logging messages from
819 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
820 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
825 is used for interactive login sessions.
830 is never used for remote command execution.
831 Note also, that if this is enabled,
833 will be disabled because
835 does not know how to handle
838 .It Cm X11DisplayOffset
839 Specifies the first display number available for
844 from interfering with real X11 servers.
847 Specifies whether X11 forwarding is permitted.
850 Note that disabling X11 forwarding does not improve security in any
851 way, as users can always install their own forwarders.
852 X11 forwarding is automatically disabled if
856 Specifies the location of the
860 .Pa /usr/X11R6/bin/xauth .
865 command-line arguments and configuration file options that specify time
866 may be expressed using a sequence of the form:
868 .Ar time Oo Ar qualifier Oc ,
872 is a positive integer value and
874 is one of the following:
876 .Bl -tag -width Ds -compact -offset indent
891 Each member of the sequence is added together to calculate
892 the total time value.
894 Time format examples:
896 .Bl -tag -width Ds -compact -offset indent
898 600 seconds (10 minutes)
902 1 hour 30 minutes (90 minutes)
905 When a user successfully logs in,
908 .Bl -enum -offset indent
910 If the login is on a tty, and no command has been specified,
911 prints last login time and
913 (unless prevented in the configuration file or by
914 .Pa $HOME/.hushlogin ;
919 If the login is on a tty, records login time.
923 if it exists, prints contents and quits
926 Changes to run with normal user privileges.
928 Sets up basic environment.
931 .Pa $HOME/.ssh/environment
934 Changes to user's home directory.
938 exists, runs it; else if
941 it; otherwise runs xauth.
944 files are given the X11
945 authentication protocol and cookie in standard input.
947 Runs user's shell or command.
949 .Sh AUTHORIZED_KEYS FILE FORMAT
950 .Pa $HOME/.ssh/authorized_keys
951 is the default file that lists the public keys that are
952 permitted for RSA authentication in protocol version 1
953 and for public key authentication (PubkeyAuthentication)
954 in protocol version 2.
955 .Cm AuthorizedKeysFile
956 may be used to specify an alternative file.
958 Each line of the file contains one
959 key (empty lines and lines starting with a
963 Each RSA public key consists of the following fields, separated by
964 spaces: options, bits, exponent, modulus, comment.
965 Each protocol version 2 public key consists of:
966 options, keytype, base64 encoded key, comment.
968 are optional; its presence is determined by whether the line starts
969 with a number or not (the option field never starts with a number).
970 The bits, exponent, modulus and comment fields give the RSA key for
971 protocol version 1; the
972 comment field is not used for anything (but may be convenient for the
973 user to identify the key).
974 For protocol version 2 the keytype is
979 Note that lines in this file are usually several hundred bytes long
980 (because of the size of the RSA key modulus).
981 You don't want to type them in; instead, copy the
988 The options (if present) consist of comma-separated option
990 No spaces are permitted, except within double quotes.
991 The following option specifications are supported (note
992 that option keywords are case-insensitive):
994 .It Cm from="pattern-list"
995 Specifies that in addition to RSA authentication, the canonical name
996 of the remote host must be present in the comma-separated list of
1001 serve as wildcards).
1002 The list may also contain
1003 patterns negated by prefixing them with
1005 if the canonical host name matches a negated pattern, the key is not accepted.
1007 of this option is to optionally increase security: RSA authentication
1008 by itself does not trust the network or name servers or anything (but
1009 the key); however, if somebody somehow steals the key, the key
1010 permits an intruder to log in from anywhere in the world.
1011 This additional option makes using a stolen key more difficult (name
1012 servers and/or routers would have to be compromised in addition to
1014 .It Cm command="command"
1015 Specifies that the command is executed whenever this key is used for
1017 The command supplied by the user (if any) is ignored.
1018 The command is run on a pty if the client requests a pty;
1019 otherwise it is run without a tty.
1020 If a 8-bit clean channel is required,
1021 one must not request a pty or should specify
1023 A quote may be included in the command by quoting it with a backslash.
1024 This option might be useful
1025 to restrict certain RSA keys to perform just a specific operation.
1026 An example might be a key that permits remote backups but nothing else.
1027 Note that the client may specify TCP/IP and/or X11
1028 forwarding unless they are explicitly prohibited.
1029 Note that this option applies to shell, command or subsystem execution.
1030 .It Cm environment="NAME=value"
1031 Specifies that the string is to be added to the environment when
1032 logging in using this key.
1033 Environment variables set this way
1034 override other default environment values.
1035 Multiple options of this type are permitted.
1036 This option is automatically disabled if
1039 .It Cm no-port-forwarding
1040 Forbids TCP/IP forwarding when this key is used for authentication.
1041 Any port forward requests by the client will return an error.
1042 This might be used, e.g., in connection with the
1045 .It Cm no-X11-forwarding
1046 Forbids X11 forwarding when this key is used for authentication.
1047 Any X11 forward requests by the client will return an error.
1048 .It Cm no-agent-forwarding
1049 Forbids authentication agent forwarding when this key is used for
1052 Prevents tty allocation (a request to allocate a pty will fail).
1053 .It Cm permitopen="host:port"
1056 port forwarding such that it may only connect to the specified host and
1058 IPv6 addresses can be specified with an alternative syntax:
1062 options may be applied separated by commas. No pattern matching is
1063 performed on the specified hostnames, they must be literal domains or
1067 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1069 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1071 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1073 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1074 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1076 .Pa /etc/ssh_known_hosts ,
1078 .Pa $HOME/.ssh/known_hosts
1079 files contain host public keys for all known hosts.
1080 The global file should
1081 be prepared by the administrator (optional), and the per-user file is
1082 maintained automatically: whenever the user connects from an unknown host
1083 its key is added to the per-user file.
1085 Each line in these files contains the following fields: hostnames,
1086 bits, exponent, modulus, comment.
1087 The fields are separated by spaces.
1089 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1090 wildcards); each pattern in turn is matched against the canonical host
1091 name (when authenticating a client) or against the user-supplied
1092 name (when authenticating a server).
1093 A pattern may also be preceded by
1095 to indicate negation: if the host name matches a negated
1096 pattern, it is not accepted (by that line) even if it matched another
1097 pattern on the line.
1099 Bits, exponent, and modulus are taken directly from the RSA host key; they
1100 can be obtained, e.g., from
1101 .Pa /etc/ssh_host_key.pub .
1102 The optional comment field continues to the end of the line, and is not used.
1106 and empty lines are ignored as comments.
1108 When performing host authentication, authentication is accepted if any
1109 matching line has the proper key.
1110 It is thus permissible (but not
1111 recommended) to have several lines or different host keys for the same
1113 This will inevitably happen when short forms of host names
1114 from different domains are put in the file.
1116 that the files contain conflicting information; authentication is
1117 accepted if valid information can be found from either file.
1119 Note that the lines in these files are typically hundreds of characters
1120 long, and you definitely don't want to type in the host keys by hand.
1121 Rather, generate them by a script
1123 .Pa /etc/ssh_host_key.pub
1124 and adding the host names at the front.
1127 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1128 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1132 .It Pa /etc/sshd_config
1133 Contains configuration data for
1135 This file should be writable by root only, but it is recommended
1136 (though not necessary) that it be world-readable.
1137 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1138 These three files contain the private parts of the host keys.
1139 These files should only be owned by root, readable only by root, and not
1140 accessible to others.
1143 does not start if this file is group/world-accessible.
1144 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1145 These three files contain the public parts of the host keys.
1146 These files should be world-readable but writable only by
1148 Their contents should match the respective private parts.
1150 really used for anything; they are provided for the convenience of
1151 the user so their contents can be copied to known hosts files.
1152 These files are created using
1155 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1156 .It Pa /var/run/sshd.pid
1157 Contains the process ID of the
1159 listening for connections (if there are several daemons running
1160 concurrently for different ports, this contains the pid of the one
1162 The content of this file is not sensitive; it can be world-readable.
1163 .It Pa $HOME/.ssh/authorized_keys
1164 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1165 This file must be readable by root (which may on some machines imply
1166 it being world-readable if the user's home directory resides on an NFS
1168 It is recommended that it not be accessible by others.
1169 The format of this file is described above.
1170 Users will place the contents of their
1175 files into this file, as described in
1177 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1178 These files are consulted when using rhosts with RSA host
1179 authentication or protocol version 2 hostbased authentication
1180 to check the public key of the host.
1181 The key must be listed in one of these files to be accepted.
1182 The client uses the same files
1183 to verify that it is connecting to the correct remote host.
1184 These files should be writable only by root/the owner.
1185 .Pa /etc/ssh_known_hosts
1186 should be world-readable, and
1187 .Pa $HOME/.ssh/known_hosts
1188 can but need not be world-readable.
1190 If this file exists,
1192 refuses to let anyone except root log in.
1193 The contents of the file
1194 are displayed to anyone trying to log in, and non-root connections are
1196 The file should be world-readable.
1197 .It Pa /etc/hosts.allow, /etc/hosts.deny
1198 Access controls that should be enforced by tcp-wrappers are defined here.
1199 Further details are described in
1200 .Xr hosts_access 5 .
1201 .It Pa $HOME/.rhosts
1202 This file contains host-username pairs, separated by a space, one per
1204 The given user on the corresponding host is permitted to log in
1206 The same file is used by rlogind and rshd.
1208 be writable only by the user; it is recommended that it not be
1209 accessible by others.
1211 If is also possible to use netgroups in the file.
1213 name may be of the form +@groupname to specify all hosts or all users
1215 .It Pa $HOME/.shosts
1217 this file is exactly the same as for
1219 However, this file is
1220 not used by rlogin and rshd, so using this permits access using SSH only.
1221 .It Pa /etc/hosts.equiv
1222 This file is used during
1225 In the simplest form, this file contains host names, one per line.
1227 those hosts are permitted to log in without a password, provided they
1228 have the same user name on both machines.
1229 The host name may also be
1230 followed by a user name; such users are permitted to log in as
1232 user on this machine (except root).
1233 Additionally, the syntax
1235 can be used to specify netgroups.
1236 Negated entries start with
1239 If the client host/user is successfully matched in this file, login is
1240 automatically permitted provided the client and server user names are the
1242 Additionally, successful RSA host authentication is normally required.
1243 This file must be writable only by root; it is recommended
1244 that it be world-readable.
1246 .Sy "Warning: It is almost never a good idea to use user names in"
1248 Beware that it really means that the named user(s) can log in as
1250 which includes bin, daemon, adm, and other accounts that own critical
1251 binaries and directories.
1252 Using a user name practically grants the user root access.
1253 The only valid use for user names that I can think
1254 of is in negative entries.
1256 Note that this warning also applies to rsh/rlogin.
1257 .It Pa /etc/shosts.equiv
1258 This is processed exactly as
1259 .Pa /etc/hosts.equiv .
1260 However, this file may be useful in environments that want to run both
1262 .It Pa $HOME/.ssh/environment
1263 This file is read into the environment at login (if it exists).
1264 It can only contain empty lines, comment lines (that start with
1266 and assignment lines of the form name=value.
1267 The file should be writable
1268 only by the user; it need not be readable by anyone else.
1269 .It Pa $HOME/.ssh/rc
1270 If this file exists, it is run with /bin/sh after reading the
1271 environment files but before starting the user's shell or command.
1272 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1280 The primary purpose of this file is to run any initialization routines
1281 which may be needed before the user's home directory becomes
1282 accessible; AFS is a particular example of such an environment.
1284 This file will probably contain some initialization code followed by
1285 something similar to:
1287 if read proto cookie; then
1288 echo add $DISPLAY $proto $cookie | xauth -q -
1292 If this file does not exist,
1295 does not exist either, xauth is used to store the cookie.
1297 This file should be writable only by the user, and need not be
1298 readable by anyone else.
1302 This can be used to specify
1303 machine-specific login-time initializations globally.
1304 This file should be writable only by root, and should be world-readable.
1307 OpenSSH is a derivative of the original and free
1308 ssh 1.2.12 release by Tatu Ylonen.
1309 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1310 Theo de Raadt and Dug Song
1311 removed many bugs, re-added newer features and
1313 Markus Friedl contributed the support for SSH
1314 protocol versions 1.5 and 2.0.
1331 .%T "SSH Protocol Architecture"
1332 .%N draft-ietf-secsh-architecture-09.txt
1334 .%O work in progress material
1340 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1341 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1343 .%O work in progress material
andersk / gssapi-openssh.git / blob