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 1.125 2001/05/20 17:20:35 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
124 are disabled (thus completely disabling
130 .Ss SSH protocol version 2
132 Version 2 works similarly:
133 Each host has a host-specific DSA key 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, ie.
182 The options are as follows:
185 Specifies the number of bits in the ephemeral protocol version 1
186 server key (default 768).
190 The server sends verbose debug output to the system
191 log, and does not put itself in the background.
192 The server also will not fork and will only process one connection.
193 This option is only intended for debugging for the server.
194 Multiple -d options increase the debugging level.
197 When this option is specified,
199 will send the output to the standard error instead of the system log.
200 .It Fl f Ar configuration_file
201 Specifies the name of the configuration file.
203 .Pa /etc/sshd_config .
205 refuses to start if there is no configuration file.
206 .It Fl g Ar login_grace_time
207 Gives the grace time for clients to authenticate themselves (default
209 If the client fails to authenticate the user within
210 this many seconds, the server disconnects and exits.
211 A value of zero indicates no limit.
212 .It Fl h Ar host_key_file
213 Specifies the file from which the host key is read (default
214 .Pa /etc/ssh_host_key ) .
215 This option must be given if
217 is not run as root (as the normal
218 host file is normally not readable by anyone but root).
219 It is possible to have multiple host key files for
220 the different protocol versions and host key algorithms.
224 is being run from inetd.
227 from inetd because it needs to generate the server key before it can
228 respond to the client, and this may take tens of seconds.
229 Clients would have to wait too long if the key was regenerated every time.
230 However, with small key sizes (e.g., 512) using
234 .It Fl k Ar key_gen_time
235 Specifies how often the ephemeral protocol version 1 server key is
236 regenerated (default 3600 seconds, or one hour).
237 The motivation for regenerating the key fairly
238 often is that the key is not stored anywhere, and after about an hour,
239 it becomes impossible to recover the key for decrypting intercepted
240 communications even if the machine is cracked into or physically
242 A value of zero indicates that the key will never be regenerated.
244 Specifies the port on which the server listens for connections
248 Nothing is sent to the system log.
249 Normally the beginning,
250 authentication, and termination of each connection is logged.
252 This option is used to specify the size of the field
255 structure that holds the remote host name.
256 If the resolved host name is longer than
258 the dotted decimal value will be used instead.
259 This allows hosts with very long host names that
260 overflow this field to still be uniquely identified.
263 indicates that only dotted decimal addresses
264 should be put into the
268 When this option is specified
270 will not detach and does not become a daemon.
271 This allows easy monitoring of
276 to use IPv4 addresses only.
280 to use IPv6 addresses only.
282 .Sh CONFIGURATION FILE
284 reads configuration data from
286 (or the file specified with
288 on the command line).
289 The file contains keyword-value pairs, one per line.
292 and empty lines are interpreted as comments.
294 The following keywords are possible.
296 .It Cm AFSTokenPassing
297 Specifies whether an AFS token may be forwarded to the server.
301 This keyword can be followed by a list of group names, separated
303 If specified, login is allowed only for users whose primary
304 group or supplementary group list matches one of the patterns.
309 wildcards in the patterns.
310 Only group names are valid; a numerical group ID isn't recognized.
311 By default login is allowed regardless of the group list.
313 .It Cm AllowTcpForwarding
314 Specifies whether TCP forwarding is permitted.
317 Note that disabling TCP forwarding does not improve security unless
318 users are also denied shell access, as they can always install their
322 This keyword can be followed by a list of user names, separated
324 If specified, login is allowed only for users names that
325 match one of the patterns.
330 wildcards in the patterns.
331 Only user names are valid; a numerical user ID isn't recognized.
332 By default login is allowed regardless of the user name.
334 .It Cm AuthorizedKeysFile
335 Specifies the file that contains the public RSA keys that can be used
336 for RSA authentication in protocol version 1.
337 .Cm AuthorizedKeysFile
338 may contain tokens of the form %T which are substituted during connection
339 set-up. The following tokens are defined; %% is replaces by a literal '%',
340 %h is replaced by the home directory of the user being authenticated and
341 %u is replaced by the username of that user.
343 .Cm AuthorizedKeysFile
344 is taken to be an absolute path or one realtive to the user's home
347 .Dq .ssh/authorized_keys
348 .It Cm AuthorizedKeysFile2
349 Specifies the file that contains the public keys that can be used
350 for public key authentication in protocol version 2.
351 .Cm AuthorizedKeysFile2
352 may contain tokens of the form %T which are substituted during connection
353 set-up. The following tokens are defined; %% is replaces by a literal '%',
354 %h is replaced by the home directory of the user being authenticated and
355 %u is replaced by the username of that user.
357 .Cm AuthorizedKeysFile2
358 is taken to be an absolute path or one realtive to the user's home
361 .Dq .ssh/authorized_keys2
363 In some jurisdictions, sending a warning message before authentication
364 may be relevant for getting legal protection.
365 The contents of the specified file are sent to the remote user before
366 authentication is allowed.
367 This option is only available for protocol version 2.
369 .It Cm ChallengeResponseAuthentication
372 authentication is allowed.
373 Currently there is only support for
381 should check for new mail for interactive logins.
385 Specifies the ciphers allowed for protocol version 2.
386 Multiple ciphers must be comma-separated.
388 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
389 .It Cm ClientAliveInterval
390 Sets a timeout interval in seconds after which if no data has been received
393 will send a message through the encrypted
394 channel to request a response from the client.
396 is 0, indicating that these messages will not be sent to the client.
397 This option applies to protocol version 2 only.
398 .It Cm ClientAliveCountMax
399 Sets the number of client alive messages (see above) which may be
402 receiving any messages back from the client. If this threshold is
403 reached while client alive messages are being sent,
405 will disconnect the client, terminating the session. It is important
406 to note that the use of client alive messages is very different from
408 (below). The client alive messages are sent through the
409 encrypted channel and therefore will not be spoofable. The TCP keepalive
412 is spoofable. You want to use the client
413 alive mechanism when you are basing something important on
414 clients having an active connection to the server.
416 The default value is 3. If you set
417 .Cm ClientAliveInterval
418 (above) to 15, and leave this value at the default, unresponsive ssh clients
419 will be disconnected after approximately 45 seconds.
421 This keyword can be followed by a number of group names, separated
423 Users whose primary group or supplementary group list matches
424 one of the patterns aren't allowed to log in.
429 wildcards in the patterns.
430 Only group names are valid; a numerical group ID isn't recognized.
431 By default login is allowed regardless of the group list.
434 This keyword can be followed by a number of user names, separated
436 Login is disallowed for user names that match one of the patterns.
440 can be used as wildcards in the patterns.
441 Only user names are valid; a numerical user ID isn't recognized.
442 By default login is allowed regardless of the user name.
444 Specifies whether remote hosts are allowed to connect to ports
445 forwarded for the client.
452 .It Cm HostbasedAuthentication
453 Specifies whether rhosts or /etc/hosts.equiv authentication together
454 with successful public key client host authentication is allowed
455 (hostbased authentication).
456 This option is similar to
457 .Cm RhostsRSAAuthentication
458 and applies to protocol version 2 only.
462 Specifies the file containing the private host keys (default
463 .Pa /etc/ssh_host_key )
464 used by SSH protocol versions 1 and 2.
467 will refuse to use a file if it is group/world-accessible.
468 It is possible to have multiple host key files.
470 keys are used for version 1 and
474 are used for version 2 of the SSH protocol.
480 files will not be used in
481 .Cm RhostsAuthentication ,
482 .Cm RhostsRSAAuthentication
484 .Cm HostbasedAuthentication .
488 .Pa /etc/shosts.equiv
492 .It Cm IgnoreUserKnownHosts
495 should ignore the user's
496 .Pa $HOME/.ssh/known_hosts
498 .Cm RhostsRSAAuthentication
500 .Cm HostbasedAuthentication .
504 Specifies whether the system should send keepalive messages to the
506 If they are sent, death of the connection or crash of one
507 of the machines will be properly noticed.
508 However, this means that
509 connections will die if the route is down temporarily, and some people
511 On the other hand, if keepalives are not sent,
512 sessions may hang indefinitely on the server, leaving
514 users and consuming server resources.
518 (to send keepalives), and the server will notice
519 if the network goes down or the client host reboots.
520 This avoids infinitely hanging sessions.
522 To disable keepalives, the value should be set to
524 in both the server and the client configuration files.
525 .It Cm KerberosAuthentication
526 Specifies whether Kerberos authentication is allowed.
527 This can be in the form of a Kerberos ticket, or if
528 .Cm PasswordAuthentication
529 is yes, the password provided by the user will be validated through
531 To use this option, the server needs a
532 Kerberos servtab which allows the verification of the KDC's identity.
535 .It Cm KerberosOrLocalPasswd
536 If set then if password authentication through Kerberos fails then
537 the password will be validated via any additional local mechanism
542 .It Cm KerberosTgtPassing
543 Specifies whether a Kerberos TGT may be forwarded to the server.
546 as this only works when the Kerberos KDC is actually an AFS kaserver.
547 .It Cm KerberosTicketCleanup
548 Specifies whether to automatically destroy the user's ticket cache
552 .It Cm KeyRegenerationInterval
553 In protocol version 1, the ephemeral server key is automatically regenerated
554 after this many seconds (if it has been used).
555 The purpose of regeneration is to prevent
556 decrypting captured sessions by later breaking into the machine and
558 The key is never stored anywhere.
559 If the value is 0, the key is never regenerated.
560 The default is 3600 (seconds).
562 Specifies the local addresses
565 The following forms may be used:
567 .Bl -item -offset indent -compact
571 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
576 .Ar host No | Ar IPv4_addr No : Ar port
582 .Ar host No | Ar IPv6_addr Oc : Ar port
590 will listen on the address and all prior
592 options specified. The default is to listen on all local
595 options are permitted. Additionally, any
597 options must precede this option for non port qualified addresses.
598 .It Cm LoginGraceTime
599 The server disconnects after this time if the user has not
600 successfully logged in.
601 If the value is 0, there is no time limit.
602 The default is 600 (seconds).
604 Gives the verbosity level that is used when logging messages from
606 The possible values are:
607 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
609 Logging with level DEBUG violates the privacy of users
610 and is not recommended.
612 Specifies the available MAC (message authentication code) algorithms.
613 The MAC algorithm is used in protocol version 2
614 for data integrity protection.
615 Multiple algorithms must be comma-separated.
617 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
619 Specifies the maximum number of concurrent unauthenticated connections to the
622 Additional connections will be dropped until authentication succeeds or the
624 expires for a connection.
627 Alternatively, random early drop can be enabled by specifying
628 the three colon separated values
632 will refuse connection attempts with a probability of
635 if there are currently
638 unauthenticated connections.
639 The probability increases linearly and all connection attempts
640 are refused if the number of unauthenticated connections reaches
643 .It Cm PAMAuthenticationViaKbdInt
644 Specifies whether PAM challenge response authentication is allowed. This
645 allows the use of most PAM challenge response authentication modules, but
646 it will allow password authentication regardless of whether
647 .Cm PasswordAuthentication
651 .It Cm PasswordAuthentication
652 Specifies whether password authentication is allowed.
655 .It Cm PermitEmptyPasswords
656 When password authentication is allowed, it specifies whether the
657 server allows login to accounts with empty password strings.
660 .It Cm PermitRootLogin
661 Specifies whether root can login using
665 .Dq without-password ,
666 .Dq forced-commands-only
672 If this option is set to
674 password authentication is disabled for root.
676 If this option is set to
677 .Dq forced-commands-only
678 root login with public key authentication will be allowed,
681 option has been specified
682 (which may be useful for taking remote backups even if root login is
683 normally not allowed). All other authentication methods are disabled
686 If this option is set to
688 root is not allowed to login.
690 Specifies the file that contains the process identifier of the
694 .Pa /var/run/sshd.pid .
696 Specifies the port number that
700 Multiple options of this type are permitted.
706 should print the date and time when the user last logged in.
714 when a user logs in interactively.
715 (On some systems it is also printed by the shell,
721 Specifies the protocol versions
724 The possible values are
728 Multiple versions must be comma-separated.
731 .It Cm PubkeyAuthentication
732 Specifies whether public key authentication is allowed.
735 Note that this option applies to protocol version 2 only.
736 .It Cm ReverseMappingCheck
739 should try to verify the remote host name and check that
740 the resolved host name for the remote IP address maps back to the
741 very same IP address.
744 .It Cm RhostsAuthentication
745 Specifies whether authentication using rhosts or /etc/hosts.equiv
747 Normally, this method should not be permitted because it is insecure.
748 .Cm RhostsRSAAuthentication
750 instead, because it performs RSA-based host authentication in addition
751 to normal rhosts or /etc/hosts.equiv authentication.
754 This option applies to protocol version 1 only.
755 .It Cm RhostsRSAAuthentication
756 Specifies whether rhosts or /etc/hosts.equiv authentication together
757 with successful RSA host authentication is allowed.
760 This option applies to protocol version 1 only.
761 .It Cm RSAAuthentication
762 Specifies whether pure RSA authentication is allowed.
765 This option applies to protocol version 1 only.
767 Defines the number of bits in the ephemeral protocol version 1 server key.
768 The minimum value is 512, and the default is 768.
772 should check file modes and ownership of the
773 user's files and home directory before accepting login.
774 This is normally desirable because novices sometimes accidentally leave their
775 directory or files world-writable.
779 Configures an external subsystem (e.g., file transfer daemon).
780 Arguments should be a subsystem name and a command to execute upon subsystem
786 file transfer subsystem.
787 By default no subsystems are defined.
788 Note that this option applies to protocol version 2 only.
789 .It Cm SyslogFacility
790 Gives the facility code that is used when logging messages from
792 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
793 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
798 is used for interactive login sessions.
801 is never used for remote command execution.
804 .It Cm X11DisplayOffset
805 Specifies the first display number available for
810 from interfering with real X11 servers.
813 Specifies whether X11 forwarding is permitted.
816 Note that disabling X11 forwarding does not improve security in any
817 way, as users can always install their own forwarders.
819 Specifies the location of the
823 .Pa /usr/X11R6/bin/xauth .
828 command-line arguments and configuration file options that specify time
829 may be expressed using a sequence of the form:
831 .Ar time Oo Ar qualifier Oc ,
835 is a positive integer value and
837 is one of the following:
839 .Bl -tag -width Ds -compact -offset indent
854 Each member of the sequence is added together to calculate
855 the total time value.
857 Time format examples:
859 .Bl -tag -width Ds -compact -offset indent
861 600 seconds (10 minutes)
865 1 hour 30 minutes (90 minutes)
869 When a user successfully logs in,
872 .Bl -enum -offset indent
874 If the login is on a tty, and no command has been specified,
875 prints last login time and
877 (unless prevented in the configuration file or by
878 .Pa $HOME/.hushlogin ;
883 If the login is on a tty, records login time.
887 if it exists, prints contents and quits
890 Changes to run with normal user privileges.
892 Sets up basic environment.
895 .Pa $HOME/.ssh/environment
898 Changes to user's home directory.
902 exists, runs it; else if
905 it; otherwise runs xauth.
908 files are given the X11
909 authentication protocol and cookie in standard input.
911 Runs user's shell or command.
913 .Sh AUTHORIZED_KEYS FILE FORMAT
914 .Pa $HOME/.ssh/authorized_keys
915 is the default file that lists the RSA keys that are
916 permitted for RSA authentication in protocol version 1.
917 .Cm AuthorizedKeysFile
918 may be used to specify an alternative file.
920 .Pa $HOME/.ssh/authorized_keys2
921 is the default file that lists the DSA and RSA keys that are
922 permitted for public key authentication (PubkeyAuthentication)
923 in protocol version 2.
924 .Cm AuthorizedKeysFile2
925 may be used to specify an alternative file.
927 Each line of the file contains one
928 key (empty lines and lines starting with a
932 Each RSA public key consists of the following fields, separated by
933 spaces: options, bits, exponent, modulus, comment.
934 Each protocol version 2 public key consists of:
935 options, keytype, base64 encoded key, comment.
937 are optional; its presence is determined by whether the line starts
938 with a number or not (the option field never starts with a number).
939 The bits, exponent, modulus and comment fields give the RSA key for
940 protocol version 1; the
941 comment field is not used for anything (but may be convenient for the
942 user to identify the key).
943 For protocol version 2 the keytype is
948 Note that lines in this file are usually several hundred bytes long
949 (because of the size of the RSA key modulus).
950 You don't want to type them in; instead, copy the
957 The options (if present) consist of comma-separated option
959 No spaces are permitted, except within double quotes.
960 The following option specifications are supported:
962 .It Cm from="pattern-list"
963 Specifies that in addition to RSA authentication, the canonical name
964 of the remote host must be present in the comma-separated list of
970 The list may also contain
971 patterns negated by prefixing them with
973 if the canonical host name matches a negated pattern, the key is not accepted.
975 of this option is to optionally increase security: RSA authentication
976 by itself does not trust the network or name servers or anything (but
977 the key); however, if somebody somehow steals the key, the key
978 permits an intruder to log in from anywhere in the world.
979 This additional option makes using a stolen key more difficult (name
980 servers and/or routers would have to be compromised in addition to
982 .It Cm command="command"
983 Specifies that the command is executed whenever this key is used for
985 The command supplied by the user (if any) is ignored.
986 The command is run on a pty if the connection requests a pty;
987 otherwise it is run without a tty.
988 Note that if you want a 8-bit clean channel,
989 you must not request a pty or should specify
991 A quote may be included in the command by quoting it with a backslash.
992 This option might be useful
993 to restrict certain RSA keys to perform just a specific operation.
994 An example might be a key that permits remote backups but nothing else.
995 Note that the client may specify TCP/IP and/or X11
996 forwarding unless they are explicitly prohibited.
997 .It Cm environment="NAME=value"
998 Specifies that the string is to be added to the environment when
999 logging in using this key.
1000 Environment variables set this way
1001 override other default environment values.
1002 Multiple options of this type are permitted.
1003 .It Cm no-port-forwarding
1004 Forbids TCP/IP forwarding when this key is used for authentication.
1005 Any port forward requests by the client will return an error.
1006 This might be used, e.g., in connection with the
1009 .It Cm no-X11-forwarding
1010 Forbids X11 forwarding when this key is used for authentication.
1011 Any X11 forward requests by the client will return an error.
1012 .It Cm no-agent-forwarding
1013 Forbids authentication agent forwarding when this key is used for
1016 Prevents tty allocation (a request to allocate a pty will fail).
1017 .It Cm permitopen="host:port"
1020 port forwarding such that it may only connect to the specified host and
1023 options may be applied separated by commas. No pattern matching is
1024 performed on the specified hostnames, they must be literal domains or
1028 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1030 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1032 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1034 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1035 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1037 .Pa /etc/ssh_known_hosts ,
1038 .Pa /etc/ssh_known_hosts2 ,
1039 .Pa $HOME/.ssh/known_hosts ,
1041 .Pa $HOME/.ssh/known_hosts2
1042 files contain host public keys for all known hosts.
1043 The global file should
1044 be prepared by the administrator (optional), and the per-user file is
1045 maintained automatically: whenever the user connects from an unknown host
1046 its key is added to the per-user file.
1048 Each line in these files contains the following fields: hostnames,
1049 bits, exponent, modulus, comment.
1050 The fields are separated by spaces.
1052 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1053 wildcards); each pattern in turn is matched against the canonical host
1054 name (when authenticating a client) or against the user-supplied
1055 name (when authenticating a server).
1056 A pattern may also be preceded by
1058 to indicate negation: if the host name matches a negated
1059 pattern, it is not accepted (by that line) even if it matched another
1060 pattern on the line.
1062 Bits, exponent, and modulus are taken directly from the RSA host key; they
1063 can be obtained, e.g., from
1064 .Pa /etc/ssh_host_key.pub .
1065 The optional comment field continues to the end of the line, and is not used.
1069 and empty lines are ignored as comments.
1071 When performing host authentication, authentication is accepted if any
1072 matching line has the proper key.
1073 It is thus permissible (but not
1074 recommended) to have several lines or different host keys for the same
1076 This will inevitably happen when short forms of host names
1077 from different domains are put in the file.
1079 that the files contain conflicting information; authentication is
1080 accepted if valid information can be found from either file.
1082 Note that the lines in these files are typically hundreds of characters
1083 long, and you definitely don't want to type in the host keys by hand.
1084 Rather, generate them by a script
1086 .Pa /etc/ssh_host_key.pub
1087 and adding the host names at the front.
1090 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1091 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1095 .It Pa /etc/sshd_config
1096 Contains configuration data for
1098 This file should be writable by root only, but it is recommended
1099 (though not necessary) that it be world-readable.
1100 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1101 These three files contain the private parts of the host keys.
1102 These files should only be owned by root, readable only by root, and not
1103 accessible to others.
1106 does not start if this file is group/world-accessible.
1107 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1108 These three files contain the public parts of the host keys.
1109 These files should be world-readable but writable only by
1111 Their contents should match the respective private parts.
1113 really used for anything; they are provided for the convenience of
1114 the user so their contents can be copied to known hosts files.
1115 These files are created using
1118 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1119 .It Pa /var/run/sshd.pid
1120 Contains the process ID of the
1122 listening for connections (if there are several daemons running
1123 concurrently for different ports, this contains the pid of the one
1125 The content of this file is not sensitive; it can be world-readable.
1126 .It Pa $HOME/.ssh/authorized_keys
1127 Lists the RSA keys that can be used to log into the user's account.
1128 This file must be readable by root (which may on some machines imply
1129 it being world-readable if the user's home directory resides on an NFS
1131 It is recommended that it not be accessible by others.
1132 The format of this file is described above.
1133 Users will place the contents of their
1135 files into this file, as described in
1137 .It Pa $HOME/.ssh/authorized_keys2
1138 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1139 This file must be readable by root (which may on some machines imply
1140 it being world-readable if the user's home directory resides on an NFS
1142 It is recommended that it not be accessible by others.
1143 The format of this file is described above.
1144 Users will place the contents of their
1148 files into this file, as described in
1150 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1151 These files are consulted when using rhosts with RSA host
1152 authentication to check the public key of the host.
1153 The key must be listed in one of these files to be accepted.
1154 The client uses the same files
1155 to verify that it is connecting to the correct remote host.
1156 These files should be writable only by root/the owner.
1157 .Pa /etc/ssh_known_hosts
1158 should be world-readable, and
1159 .Pa $HOME/.ssh/known_hosts
1160 can but need not be world-readable.
1161 .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
1162 These files are consulted when using protocol version 2 hostbased
1163 authentication to check the public key of the host.
1164 The key must be listed in one of these files to be accepted.
1165 The client uses the same files
1166 to verify that it is connecting to the correct remote host.
1167 These files should be writable only by root/the owner.
1168 .Pa /etc/ssh_known_hosts2
1169 should be world-readable, and
1170 .Pa $HOME/.ssh/known_hosts2
1171 can but need not be world-readable.
1173 If this file exists,
1175 refuses to let anyone except root log in.
1176 The contents of the file
1177 are displayed to anyone trying to log in, and non-root connections are
1179 The file should be world-readable.
1180 .It Pa /etc/hosts.allow, /etc/hosts.deny
1183 support, tcp-wrappers access controls may be defined here as described in
1184 .Xr hosts_access 5 .
1185 .It Pa $HOME/.rhosts
1186 This file contains host-username pairs, separated by a space, one per
1188 The given user on the corresponding host is permitted to log in
1190 The same file is used by rlogind and rshd.
1192 be writable only by the user; it is recommended that it not be
1193 accessible by others.
1195 If is also possible to use netgroups in the file.
1197 name may be of the form +@groupname to specify all hosts or all users
1199 .It Pa $HOME/.shosts
1201 this file is exactly the same as for
1203 However, this file is
1204 not used by rlogin and rshd, so using this permits access using SSH only.
1205 .It Pa /etc/hosts.equiv
1206 This file is used during
1209 In the simplest form, this file contains host names, one per line.
1211 those hosts are permitted to log in without a password, provided they
1212 have the same user name on both machines.
1213 The host name may also be
1214 followed by a user name; such users are permitted to log in as
1216 user on this machine (except root).
1217 Additionally, the syntax
1219 can be used to specify netgroups.
1220 Negated entries start with
1223 If the client host/user is successfully matched in this file, login is
1224 automatically permitted provided the client and server user names are the
1226 Additionally, successful RSA host authentication is normally required.
1227 This file must be writable only by root; it is recommended
1228 that it be world-readable.
1230 .Sy "Warning: It is almost never a good idea to use user names in"
1232 Beware that it really means that the named user(s) can log in as
1234 which includes bin, daemon, adm, and other accounts that own critical
1235 binaries and directories.
1236 Using a user name practically grants the user root access.
1237 The only valid use for user names that I can think
1238 of is in negative entries.
1240 Note that this warning also applies to rsh/rlogin.
1241 .It Pa /etc/shosts.equiv
1242 This is processed exactly as
1243 .Pa /etc/hosts.equiv .
1244 However, this file may be useful in environments that want to run both
1246 .It Pa $HOME/.ssh/environment
1247 This file is read into the environment at login (if it exists).
1248 It can only contain empty lines, comment lines (that start with
1250 and assignment lines of the form name=value.
1251 The file should be writable
1252 only by the user; it need not be readable by anyone else.
1253 .It Pa $HOME/.ssh/rc
1254 If this file exists, it is run with /bin/sh after reading the
1255 environment files but before starting the user's shell or command.
1256 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1264 The primary purpose of this file is to run any initialization routines
1265 which may be needed before the user's home directory becomes
1266 accessible; AFS is a particular example of such an environment.
1268 This file will probably contain some initialization code followed by
1269 something similar to:
1271 if read proto cookie; then
1272 echo add $DISPLAY $proto $cookie | xauth -q -
1276 If this file does not exist,
1279 does not exist either, xauth is used to store the cookie.
1281 This file should be writable only by the user, and need not be
1282 readable by anyone else.
1286 This can be used to specify
1287 machine-specific login-time initializations globally.
1288 This file should be writable only by root, and should be world-readable.
1291 OpenSSH is a derivative of the original and free
1292 ssh 1.2.12 release by Tatu Ylonen.
1293 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1294 Theo de Raadt and Dug Song
1295 removed many bugs, re-added newer features and
1297 Markus Friedl contributed the support for SSH
1298 protocol versions 1.5 and 2.0.
1315 .%T "SSH Protocol Architecture"
1316 .%N draft-ietf-secsh-architecture-07.txt
1318 .%O work in progress material
1324 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1325 .%N draft-ietf-secsh-dh-group-exchange-00.txt
1327 .%O work in progress material
andersk / openssh.git / blob