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 1.123 2001/05/19 16:46:19 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.
335 In some jurisdictions, sending a warning message before authentication
336 may be relevant for getting legal protection.
337 The contents of the specified file are sent to the remote user before
338 authentication is allowed.
339 This option is only available for protocol version 2.
341 .It Cm ChallengeResponseAuthentication
344 authentication is allowed.
345 Currently there is only support for
353 should check for new mail for interactive logins.
357 Specifies the ciphers allowed for protocol version 2.
358 Multiple ciphers must be comma-separated.
360 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
361 .It Cm ClientAliveInterval
362 Sets a timeout interval in seconds after which if no data has been received
365 will send a message through the encrypted
366 channel to request a response from the client.
368 is 0, indicating that these messages will not be sent to the client.
369 This option applies to protocol version 2 only.
370 .It Cm ClientAliveCountMax
371 Sets the number of client alive messages (see above) which may be
374 receiving any messages back from the client. If this threshold is
375 reached while client alive messages are being sent,
377 will disconnect the client, terminating the session. It is important
378 to note that the use of client alive messages is very different from
380 (below). The client alive messages are sent through the
381 encrypted channel and therefore will not be spoofable. The TCP keepalive
384 is spoofable. You want to use the client
385 alive mechanism when you are basing something important on
386 clients having an active connection to the server.
388 The default value is 3. If you set
389 .Cm ClientAliveInterval
390 (above) to 15, and leave this value at the default, unresponsive ssh clients
391 will be disconnected after approximately 45 seconds.
393 This keyword can be followed by a number of group names, separated
395 Users whose primary group or supplementary group list matches
396 one of the patterns aren't allowed to log in.
401 wildcards in the patterns.
402 Only group names are valid; a numerical group ID isn't recognized.
403 By default login is allowed regardless of the group list.
406 This keyword can be followed by a number of user names, separated
408 Login is disallowed for user names that match one of the patterns.
412 can be used as wildcards in the patterns.
413 Only user names are valid; a numerical user ID isn't recognized.
414 By default login is allowed regardless of the user name.
416 Specifies whether remote hosts are allowed to connect to ports
417 forwarded for the client.
424 .It Cm HostbasedAuthentication
425 Specifies whether rhosts or /etc/hosts.equiv authentication together
426 with successful public key client host authentication is allowed
427 (hostbased authentication).
428 This option is similar to
429 .Cm RhostsRSAAuthentication
430 and applies to protocol version 2 only.
434 Specifies the file containing the private host keys (default
435 .Pa /etc/ssh_host_key )
436 used by SSH protocol versions 1 and 2.
439 will refuse to use a file if it is group/world-accessible.
440 It is possible to have multiple host key files.
442 keys are used for version 1 and
446 are used for version 2 of the SSH protocol.
452 files will not be used in
453 .Cm RhostsAuthentication ,
454 .Cm RhostsRSAAuthentication
456 .Cm HostbasedAuthentication .
460 .Pa /etc/shosts.equiv
464 .It Cm IgnoreUserKnownHosts
467 should ignore the user's
468 .Pa $HOME/.ssh/known_hosts
470 .Cm RhostsRSAAuthentication
472 .Cm HostbasedAuthentication .
476 Specifies whether the system should send keepalive messages to the
478 If they are sent, death of the connection or crash of one
479 of the machines will be properly noticed.
480 However, this means that
481 connections will die if the route is down temporarily, and some people
483 On the other hand, if keepalives are not sent,
484 sessions may hang indefinitely on the server, leaving
486 users and consuming server resources.
490 (to send keepalives), and the server will notice
491 if the network goes down or the client host reboots.
492 This avoids infinitely hanging sessions.
494 To disable keepalives, the value should be set to
496 in both the server and the client configuration files.
497 .It Cm KerberosAuthentication
498 Specifies whether Kerberos authentication is allowed.
499 This can be in the form of a Kerberos ticket, or if
500 .Cm PasswordAuthentication
501 is yes, the password provided by the user will be validated through
503 To use this option, the server needs a
504 Kerberos servtab which allows the verification of the KDC's identity.
507 .It Cm KerberosOrLocalPasswd
508 If set then if password authentication through Kerberos fails then
509 the password will be validated via any additional local mechanism
514 .It Cm KerberosTgtPassing
515 Specifies whether a Kerberos TGT may be forwarded to the server.
518 as this only works when the Kerberos KDC is actually an AFS kaserver.
519 .It Cm KerberosTicketCleanup
520 Specifies whether to automatically destroy the user's ticket cache
524 .It Cm KeyRegenerationInterval
525 In protocol version 1, the ephemeral server key is automatically regenerated
526 after this many seconds (if it has been used).
527 The purpose of regeneration is to prevent
528 decrypting captured sessions by later breaking into the machine and
530 The key is never stored anywhere.
531 If the value is 0, the key is never regenerated.
532 The default is 3600 (seconds).
534 Specifies the local addresses
537 The following forms may be used:
539 .Bl -item -offset indent -compact
543 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
548 .Ar host No | Ar IPv4_addr No : Ar port
554 .Ar host No | Ar IPv6_addr Oc : Ar port
562 will listen on the address and all prior
564 options specified. The default is to listen on all local
567 options are permitted. Additionally, any
569 options must precede this option for non port qualified addresses.
570 .It Cm LoginGraceTime
571 The server disconnects after this time if the user has not
572 successfully logged in.
573 If the value is 0, there is no time limit.
574 The default is 600 (seconds).
576 Gives the verbosity level that is used when logging messages from
578 The possible values are:
579 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
581 Logging with level DEBUG violates the privacy of users
582 and is not recommended.
584 Specifies the available MAC (message authentication code) algorithms.
585 The MAC algorithm is used in protocol version 2
586 for data integrity protection.
587 Multiple algorithms must be comma-separated.
589 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
591 Specifies the maximum number of concurrent unauthenticated connections to the
594 Additional connections will be dropped until authentication succeeds or the
596 expires for a connection.
599 Alternatively, random early drop can be enabled by specifying
600 the three colon separated values
604 will refuse connection attempts with a probability of
607 if there are currently
610 unauthenticated connections.
611 The probability increases linearly and all connection attempts
612 are refused if the number of unauthenticated connections reaches
615 .It Cm PAMAuthenticationViaKbdInt
616 Specifies whether PAM challenge response authentication is allowed. This
617 allows the use of most PAM challenge response authentication modules, but
618 it will allow password authentication regardless of whether
619 .Cm PasswordAuthentication
623 .It Cm PasswordAuthentication
624 Specifies whether password authentication is allowed.
627 .It Cm PermitEmptyPasswords
628 When password authentication is allowed, it specifies whether the
629 server allows login to accounts with empty password strings.
632 .It Cm PermitRootLogin
633 Specifies whether root can login using
637 .Dq without-password ,
638 .Dq forced-commands-only
644 If this option is set to
646 password authentication is disabled for root.
648 If this option is set to
649 .Dq forced-commands-only
650 root login with public key authentication will be allowed,
653 option has been specified
654 (which may be useful for taking remote backups even if root login is
655 normally not allowed). All other authentication methods are disabled
658 If this option is set to
660 root is not allowed to login.
662 Specifies the file that contains the process identifier of the
666 .Pa /var/run/sshd.pid .
668 Specifies the port number that
672 Multiple options of this type are permitted.
678 should print the date and time when the user last logged in.
686 when a user logs in interactively.
687 (On some systems it is also printed by the shell,
693 Specifies the protocol versions
696 The possible values are
700 Multiple versions must be comma-separated.
703 .It Cm PubkeyAuthentication
704 Specifies whether public key authentication is allowed.
707 Note that this option applies to protocol version 2 only.
708 .It Cm ReverseMappingCheck
711 should try to verify the remote host name and check that
712 the resolved host name for the remote IP address maps back to the
713 very same IP address.
716 .It Cm RhostsAuthentication
717 Specifies whether authentication using rhosts or /etc/hosts.equiv
719 Normally, this method should not be permitted because it is insecure.
720 .Cm RhostsRSAAuthentication
722 instead, because it performs RSA-based host authentication in addition
723 to normal rhosts or /etc/hosts.equiv authentication.
726 This option applies to protocol version 1 only.
727 .It Cm RhostsRSAAuthentication
728 Specifies whether rhosts or /etc/hosts.equiv authentication together
729 with successful RSA host authentication is allowed.
732 This option applies to protocol version 1 only.
733 .It Cm RSAAuthentication
734 Specifies whether pure RSA authentication is allowed.
737 This option applies to protocol version 1 only.
739 Defines the number of bits in the ephemeral protocol version 1 server key.
740 The minimum value is 512, and the default is 768.
744 should check file modes and ownership of the
745 user's files and home directory before accepting login.
746 This is normally desirable because novices sometimes accidentally leave their
747 directory or files world-writable.
751 Configures an external subsystem (e.g., file transfer daemon).
752 Arguments should be a subsystem name and a command to execute upon subsystem
758 file transfer subsystem.
759 By default no subsystems are defined.
760 Note that this option applies to protocol version 2 only.
761 .It Cm SyslogFacility
762 Gives the facility code that is used when logging messages from
764 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
765 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
770 is used for interactive login sessions.
773 is never used for remote command execution.
776 .It Cm X11DisplayOffset
777 Specifies the first display number available for
782 from interfering with real X11 servers.
785 Specifies whether X11 forwarding is permitted.
788 Note that disabling X11 forwarding does not improve security in any
789 way, as users can always install their own forwarders.
791 Specifies the location of the
795 .Pa /usr/X11R6/bin/xauth .
798 When a user successfully logs in,
801 .Bl -enum -offset indent
803 If the login is on a tty, and no command has been specified,
804 prints last login time and
806 (unless prevented in the configuration file or by
807 .Pa $HOME/.hushlogin ;
812 If the login is on a tty, records login time.
816 if it exists, prints contents and quits
819 Changes to run with normal user privileges.
821 Sets up basic environment.
824 .Pa $HOME/.ssh/environment
827 Changes to user's home directory.
831 exists, runs it; else if
834 it; otherwise runs xauth.
837 files are given the X11
838 authentication protocol and cookie in standard input.
840 Runs user's shell or command.
842 .Sh AUTHORIZED_KEYS FILE FORMAT
844 .Pa $HOME/.ssh/authorized_keys
845 file lists the RSA keys that are
846 permitted for RSA authentication in protocol version 1.
848 .Pa $HOME/.ssh/authorized_keys2
849 file lists the DSA and RSA keys that are
850 permitted for public key authentication (PubkeyAuthentication)
851 in protocol version 2.
853 Each line of the file contains one
854 key (empty lines and lines starting with a
858 Each RSA public key consists of the following fields, separated by
859 spaces: options, bits, exponent, modulus, comment.
860 Each protocol version 2 public key consists of:
861 options, keytype, base64 encoded key, comment.
863 are optional; its presence is determined by whether the line starts
864 with a number or not (the option field never starts with a number).
865 The bits, exponent, modulus and comment fields give the RSA key for
866 protocol version 1; the
867 comment field is not used for anything (but may be convenient for the
868 user to identify the key).
869 For protocol version 2 the keytype is
874 Note that lines in this file are usually several hundred bytes long
875 (because of the size of the RSA key modulus).
876 You don't want to type them in; instead, copy the
883 The options (if present) consist of comma-separated option
885 No spaces are permitted, except within double quotes.
886 The following option specifications are supported:
888 .It Cm from="pattern-list"
889 Specifies that in addition to RSA authentication, the canonical name
890 of the remote host must be present in the comma-separated list of
896 The list may also contain
897 patterns negated by prefixing them with
899 if the canonical host name matches a negated pattern, the key is not accepted.
901 of this option is to optionally increase security: RSA authentication
902 by itself does not trust the network or name servers or anything (but
903 the key); however, if somebody somehow steals the key, the key
904 permits an intruder to log in from anywhere in the world.
905 This additional option makes using a stolen key more difficult (name
906 servers and/or routers would have to be compromised in addition to
908 .It Cm command="command"
909 Specifies that the command is executed whenever this key is used for
911 The command supplied by the user (if any) is ignored.
912 The command is run on a pty if the connection requests a pty;
913 otherwise it is run without a tty.
914 Note that if you want a 8-bit clean channel,
915 you must not request a pty or should specify
917 A quote may be included in the command by quoting it with a backslash.
918 This option might be useful
919 to restrict certain RSA keys to perform just a specific operation.
920 An example might be a key that permits remote backups but nothing else.
921 Note that the client may specify TCP/IP and/or X11
922 forwarding unless they are explicitly prohibited.
923 .It Cm environment="NAME=value"
924 Specifies that the string is to be added to the environment when
925 logging in using this key.
926 Environment variables set this way
927 override other default environment values.
928 Multiple options of this type are permitted.
929 .It Cm no-port-forwarding
930 Forbids TCP/IP forwarding when this key is used for authentication.
931 Any port forward requests by the client will return an error.
932 This might be used, e.g., in connection with the
935 .It Cm no-X11-forwarding
936 Forbids X11 forwarding when this key is used for authentication.
937 Any X11 forward requests by the client will return an error.
938 .It Cm no-agent-forwarding
939 Forbids authentication agent forwarding when this key is used for
942 Prevents tty allocation (a request to allocate a pty will fail).
943 .It Cm permitopen="host:port"
946 port forwarding such that it may only connect to the specified host and
949 options may be applied separated by commas. No pattern matching is
950 performed on the specified hostnames, they must be literal domains or
954 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
956 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
958 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
960 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
961 .Sh SSH_KNOWN_HOSTS FILE FORMAT
963 .Pa /etc/ssh_known_hosts ,
964 .Pa /etc/ssh_known_hosts2 ,
965 .Pa $HOME/.ssh/known_hosts ,
967 .Pa $HOME/.ssh/known_hosts2
968 files contain host public keys for all known hosts.
969 The global file should
970 be prepared by the administrator (optional), and the per-user file is
971 maintained automatically: whenever the user connects from an unknown host
972 its key is added to the per-user file.
974 Each line in these files contains the following fields: hostnames,
975 bits, exponent, modulus, comment.
976 The fields are separated by spaces.
978 Hostnames is a comma-separated list of patterns ('*' and '?' act as
979 wildcards); each pattern in turn is matched against the canonical host
980 name (when authenticating a client) or against the user-supplied
981 name (when authenticating a server).
982 A pattern may also be preceded by
984 to indicate negation: if the host name matches a negated
985 pattern, it is not accepted (by that line) even if it matched another
988 Bits, exponent, and modulus are taken directly from the RSA host key; they
989 can be obtained, e.g., from
990 .Pa /etc/ssh_host_key.pub .
991 The optional comment field continues to the end of the line, and is not used.
995 and empty lines are ignored as comments.
997 When performing host authentication, authentication is accepted if any
998 matching line has the proper key.
999 It is thus permissible (but not
1000 recommended) to have several lines or different host keys for the same
1002 This will inevitably happen when short forms of host names
1003 from different domains are put in the file.
1005 that the files contain conflicting information; authentication is
1006 accepted if valid information can be found from either file.
1008 Note that the lines in these files are typically hundreds of characters
1009 long, and you definitely don't want to type in the host keys by hand.
1010 Rather, generate them by a script
1012 .Pa /etc/ssh_host_key.pub
1013 and adding the host names at the front.
1016 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1017 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1021 .It Pa /etc/sshd_config
1022 Contains configuration data for
1024 This file should be writable by root only, but it is recommended
1025 (though not necessary) that it be world-readable.
1026 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1027 These three files contain the private parts of the host keys.
1028 These files should only be owned by root, readable only by root, and not
1029 accessible to others.
1032 does not start if this file is group/world-accessible.
1033 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1034 These three files contain the public parts of the host keys.
1035 These files should be world-readable but writable only by
1037 Their contents should match the respective private parts.
1039 really used for anything; they are provided for the convenience of
1040 the user so their contents can be copied to known hosts files.
1041 These files are created using
1044 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1045 .It Pa /var/run/sshd.pid
1046 Contains the process ID of the
1048 listening for connections (if there are several daemons running
1049 concurrently for different ports, this contains the pid of the one
1051 The content of this file is not sensitive; it can be world-readable.
1052 .It Pa $HOME/.ssh/authorized_keys
1053 Lists the RSA keys that can be used to log into the user's account.
1054 This file must be readable by root (which may on some machines imply
1055 it being world-readable if the user's home directory resides on an NFS
1057 It is recommended that it not be accessible by others.
1058 The format of this file is described above.
1059 Users will place the contents of their
1061 files into this file, as described in
1063 .It Pa $HOME/.ssh/authorized_keys2
1064 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1065 This file must be readable by root (which may on some machines imply
1066 it being world-readable if the user's home directory resides on an NFS
1068 It is recommended that it not be accessible by others.
1069 The format of this file is described above.
1070 Users will place the contents of their
1074 files into this file, as described in
1076 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1077 These files are consulted when using rhosts with RSA host
1078 authentication to check the public key of the host.
1079 The key must be listed in one of these files to be accepted.
1080 The client uses the same files
1081 to verify that it is connecting to the correct remote host.
1082 These files should be writable only by root/the owner.
1083 .Pa /etc/ssh_known_hosts
1084 should be world-readable, and
1085 .Pa $HOME/.ssh/known_hosts
1086 can but need not be world-readable.
1087 .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
1088 These files are consulted when using protocol version 2 hostbased
1089 authentication to check the public key of the host.
1090 The key must be listed in one of these files to be accepted.
1091 The client uses the same files
1092 to verify that it is connecting to the correct remote host.
1093 These files should be writable only by root/the owner.
1094 .Pa /etc/ssh_known_hosts2
1095 should be world-readable, and
1096 .Pa $HOME/.ssh/known_hosts2
1097 can but need not be world-readable.
1099 If this file exists,
1101 refuses to let anyone except root log in.
1102 The contents of the file
1103 are displayed to anyone trying to log in, and non-root connections are
1105 The file should be world-readable.
1106 .It Pa /etc/hosts.allow, /etc/hosts.deny
1109 support, tcp-wrappers access controls may be defined here as described in
1110 .Xr hosts_access 5 .
1111 .It Pa $HOME/.rhosts
1112 This file contains host-username pairs, separated by a space, one per
1114 The given user on the corresponding host is permitted to log in
1116 The same file is used by rlogind and rshd.
1118 be writable only by the user; it is recommended that it not be
1119 accessible by others.
1121 If is also possible to use netgroups in the file.
1123 name may be of the form +@groupname to specify all hosts or all users
1125 .It Pa $HOME/.shosts
1127 this file is exactly the same as for
1129 However, this file is
1130 not used by rlogin and rshd, so using this permits access using SSH only.
1131 .It Pa /etc/hosts.equiv
1132 This file is used during
1135 In the simplest form, this file contains host names, one per line.
1137 those hosts are permitted to log in without a password, provided they
1138 have the same user name on both machines.
1139 The host name may also be
1140 followed by a user name; such users are permitted to log in as
1142 user on this machine (except root).
1143 Additionally, the syntax
1145 can be used to specify netgroups.
1146 Negated entries start with
1149 If the client host/user is successfully matched in this file, login is
1150 automatically permitted provided the client and server user names are the
1152 Additionally, successful RSA host authentication is normally required.
1153 This file must be writable only by root; it is recommended
1154 that it be world-readable.
1156 .Sy "Warning: It is almost never a good idea to use user names in"
1158 Beware that it really means that the named user(s) can log in as
1160 which includes bin, daemon, adm, and other accounts that own critical
1161 binaries and directories.
1162 Using a user name practically grants the user root access.
1163 The only valid use for user names that I can think
1164 of is in negative entries.
1166 Note that this warning also applies to rsh/rlogin.
1167 .It Pa /etc/shosts.equiv
1168 This is processed exactly as
1169 .Pa /etc/hosts.equiv .
1170 However, this file may be useful in environments that want to run both
1172 .It Pa $HOME/.ssh/environment
1173 This file is read into the environment at login (if it exists).
1174 It can only contain empty lines, comment lines (that start with
1176 and assignment lines of the form name=value.
1177 The file should be writable
1178 only by the user; it need not be readable by anyone else.
1179 .It Pa $HOME/.ssh/rc
1180 If this file exists, it is run with /bin/sh after reading the
1181 environment files but before starting the user's shell or command.
1182 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1190 The primary purpose of this file is to run any initialization routines
1191 which may be needed before the user's home directory becomes
1192 accessible; AFS is a particular example of such an environment.
1194 This file will probably contain some initialization code followed by
1195 something similar to:
1197 if read proto cookie; then
1198 echo add $DISPLAY $proto $cookie | xauth -q -
1202 If this file does not exist,
1205 does not exist either, xauth is used to store the cookie.
1207 This file should be writable only by the user, and need not be
1208 readable by anyone else.
1212 This can be used to specify
1213 machine-specific login-time initializations globally.
1214 This file should be writable only by root, and should be world-readable.
1217 OpenSSH is a derivative of the original and free
1218 ssh 1.2.12 release by Tatu Ylonen.
1219 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1220 Theo de Raadt and Dug Song
1221 removed many bugs, re-added newer features and
1223 Markus Friedl contributed the support for SSH
1224 protocol versions 1.5 and 2.0.
1241 .%T "SSH Protocol Architecture"
1242 .%N draft-ietf-secsh-architecture-07.txt
1244 .%O work in progress material
1250 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1251 .%N draft-ietf-secsh-dh-group-exchange-00.txt
1253 .%O work in progress material
andersk / openssh.git / blob