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 1.137 2001/07/26 17:18:22 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
54 .Op Fl V Ar client_protocol_id
57 (SSH Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256 bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
124 are disabled (thus completely disabling
130 .Ss SSH protocol version 2
132 Version 2 works similarly:
133 Each host has a host-specific 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, i.e.,
182 The options are as follows:
185 Specifies the number of bits in the ephemeral protocol version 1
186 server key (default 768).
189 The server sends verbose debug output to the system
190 log, and does not put itself in the background.
191 The server also will not fork and will only process one connection.
192 This option is only intended for debugging for the server.
193 Multiple -d options increase the debugging level.
196 When this option is specified,
198 will send the output to the standard error instead of the system log.
199 .It Fl f Ar configuration_file
200 Specifies the name of the configuration file.
202 .Pa /etc/sshd_config .
204 refuses to start if there is no configuration file.
205 .It Fl g Ar login_grace_time
206 Gives the grace time for clients to authenticate themselves (default
208 If the client fails to authenticate the user within
209 this many seconds, the server disconnects and exits.
210 A value of zero indicates no limit.
211 .It Fl h Ar host_key_file
212 Specifies the file from which the host key is read (default
213 .Pa /etc/ssh_host_key ) .
214 This option must be given if
216 is not run as root (as the normal
217 host file is normally not readable by anyone but root).
218 It is possible to have multiple host key files for
219 the different protocol versions and host key algorithms.
223 is being run from inetd.
226 from inetd because it needs to generate the server key before it can
227 respond to the client, and this may take tens of seconds.
228 Clients would have to wait too long if the key was regenerated every time.
229 However, with small key sizes (e.g., 512) using
233 .It Fl k Ar key_gen_time
234 Specifies how often the ephemeral protocol version 1 server key is
235 regenerated (default 3600 seconds, or one hour).
236 The motivation for regenerating the key fairly
237 often is that the key is not stored anywhere, and after about an hour,
238 it becomes impossible to recover the key for decrypting intercepted
239 communications even if the machine is cracked into or physically
241 A value of zero indicates that the key will never be regenerated.
243 Specifies the port on which the server listens for connections
247 Nothing is sent to the system log.
248 Normally the beginning,
249 authentication, and termination of each connection is logged.
252 Only check the validity of the configuration file and sanity of the keys.
253 This is useful for updating
255 reliably as configuration options may change.
257 This option is used to specify the size of the field
260 structure that holds the remote host name.
261 If the resolved host name is longer than
263 the dotted decimal value will be used instead.
264 This allows hosts with very long host names that
265 overflow this field to still be uniquely identified.
268 indicates that only dotted decimal addresses
269 should be put into the
273 When this option is specified
275 will not detach and does not become a daemon.
276 This allows easy monitoring of
281 to use IPv4 addresses only.
285 to use IPv6 addresses only.
287 .Sh CONFIGURATION FILE
289 reads configuration data from
291 (or the file specified with
293 on the command line).
294 The file contains keyword-value pairs, one per line.
297 and empty lines are interpreted as comments.
299 The following keywords are possible.
301 .It Cm AFSTokenPassing
302 Specifies whether an AFS token may be forwarded to the server.
306 This keyword can be followed by a list of group names, separated
308 If specified, login is allowed only for users whose primary
309 group or supplementary group list matches one of the patterns.
314 wildcards in the patterns.
315 Only group names are valid; a numerical group ID isn't recognized.
316 By default login is allowed regardless of the group list.
318 .It Cm AllowTcpForwarding
319 Specifies whether TCP forwarding is permitted.
322 Note that disabling TCP forwarding does not improve security unless
323 users are also denied shell access, as they can always install their
327 This keyword can be followed by a list of user names, separated
329 If specified, login is allowed only for users names that
330 match one of the patterns.
335 wildcards in the patterns.
336 Only user names are valid; a numerical user ID isn't recognized.
337 By default login is allowed regardless of the user name.
338 If the pattern takes the form USER@HOST then USER and HOST
339 are separately checked, allowing you to restrict logins to particular
340 users from particular hosts.
342 .It Cm AuthorizedKeysFile
343 Specifies the file that contains the public RSA keys that can be used
344 for RSA authentication in protocol version 1.
345 .Cm AuthorizedKeysFile
346 may contain tokens of the form %T which are substituted during connection
347 set-up. The following tokens are defined; %% is replaces by a literal '%',
348 %h is replaced by the home directory of the user being authenticated and
349 %u is replaced by the username of that user.
351 .Cm AuthorizedKeysFile
352 is taken to be an absolute path or one relative to the user's home
355 .Dq .ssh/authorized_keys
357 In some jurisdictions, sending a warning message before authentication
358 may be relevant for getting legal protection.
359 The contents of the specified file are sent to the remote user before
360 authentication is allowed.
361 This option is only available for protocol version 2.
363 .It Cm ChallengeResponseAuthentication
364 Specifies whether challenge response authentication is allowed.
365 All authentication styles from
373 should check for new mail for interactive logins.
377 Specifies the ciphers allowed for protocol version 2.
378 Multiple ciphers must be comma-separated.
380 .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
381 .It Cm ClientAliveInterval
382 Sets a timeout interval in seconds after which if no data has been received
385 will send a message through the encrypted
386 channel to request a response from the client.
388 is 0, indicating that these messages will not be sent to the client.
389 This option applies to protocol version 2 only.
390 .It Cm ClientAliveCountMax
391 Sets the number of client alive messages (see above) which may be
394 receiving any messages back from the client. If this threshold is
395 reached while client alive messages are being sent,
397 will disconnect the client, terminating the session. It is important
398 to note that the use of client alive messages is very different from
400 (below). The client alive messages are sent through the
401 encrypted channel and therefore will not be spoofable. The TCP keepalive
404 is spoofable. You want to use the client
405 alive mechanism when you are basing something important on
406 clients having an active connection to the server.
408 The default value is 3. If you set
409 .Cm ClientAliveInterval
410 (above) to 15, and leave this value at the default, unresponsive ssh clients
411 will be disconnected after approximately 45 seconds.
413 This keyword can be followed by a number of group names, separated
415 Users whose primary group or supplementary group list matches
416 one of the patterns aren't allowed to log in.
421 wildcards in the patterns.
422 Only group names are valid; a numerical group ID isn't recognized.
423 By default login is allowed regardless of the group list.
426 This keyword can be followed by a number of user names, separated
428 Login is disallowed for user names that match one of the patterns.
432 can be used as wildcards in the patterns.
433 Only user names are valid; a numerical user ID isn't recognized.
434 By default login is allowed regardless of the user name.
436 Specifies whether remote hosts are allowed to connect to ports
437 forwarded for the client.
444 .It Cm HostbasedAuthentication
445 Specifies whether rhosts or /etc/hosts.equiv authentication together
446 with successful public key client host authentication is allowed
447 (hostbased authentication).
448 This option is similar to
449 .Cm RhostsRSAAuthentication
450 and applies to protocol version 2 only.
454 Specifies the file containing the private host keys (default
455 .Pa /etc/ssh_host_key )
456 used by SSH protocol versions 1 and 2.
459 will refuse to use a file if it is group/world-accessible.
460 It is possible to have multiple host key files.
462 keys are used for version 1 and
466 are used for version 2 of the SSH protocol.
472 files will not be used in
473 .Cm RhostsAuthentication ,
474 .Cm RhostsRSAAuthentication
476 .Cm HostbasedAuthentication .
480 .Pa /etc/shosts.equiv
484 .It Cm IgnoreUserKnownHosts
487 should ignore the user's
488 .Pa $HOME/.ssh/known_hosts
490 .Cm RhostsRSAAuthentication
492 .Cm HostbasedAuthentication .
496 Specifies whether the system should send keepalive messages to the
498 If they are sent, death of the connection or crash of one
499 of the machines will be properly noticed.
500 However, this means that
501 connections will die if the route is down temporarily, and some people
503 On the other hand, if keepalives are not sent,
504 sessions may hang indefinitely on the server, leaving
506 users and consuming server resources.
510 (to send keepalives), and the server will notice
511 if the network goes down or the client host reboots.
512 This avoids infinitely hanging sessions.
514 To disable keepalives, the value should be set to
516 in both the server and the client configuration files.
517 .It Cm KerberosAuthentication
518 Specifies whether Kerberos authentication is allowed.
519 This can be in the form of a Kerberos ticket, or if
520 .Cm PasswordAuthentication
521 is yes, the password provided by the user will be validated through
523 To use this option, the server needs a
524 Kerberos servtab which allows the verification of the KDC's identity.
527 .It Cm KerberosOrLocalPasswd
528 If set then if password authentication through Kerberos fails then
529 the password will be validated via any additional local mechanism
534 .It Cm KerberosTgtPassing
535 Specifies whether a Kerberos TGT may be forwarded to the server.
538 as this only works when the Kerberos KDC is actually an AFS kaserver.
539 .It Cm KerberosTicketCleanup
540 Specifies whether to automatically destroy the user's ticket cache
544 .It Cm KeyRegenerationInterval
545 In protocol version 1, the ephemeral server key is automatically regenerated
546 after this many seconds (if it has been used).
547 The purpose of regeneration is to prevent
548 decrypting captured sessions by later breaking into the machine and
550 The key is never stored anywhere.
551 If the value is 0, the key is never regenerated.
552 The default is 3600 (seconds).
554 Specifies the local addresses
557 The following forms may be used:
559 .Bl -item -offset indent -compact
563 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
568 .Ar host No | Ar IPv4_addr No : Ar port
574 .Ar host No | Ar IPv6_addr Oc : Ar port
582 will listen on the address and all prior
584 options specified. The default is to listen on all local
587 options are permitted. Additionally, any
589 options must precede this option for non port qualified addresses.
590 .It Cm LoginGraceTime
591 The server disconnects after this time if the user has not
592 successfully logged in.
593 If the value is 0, there is no time limit.
594 The default is 600 (seconds).
596 Gives the verbosity level that is used when logging messages from
598 The possible values are:
599 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
601 Logging with level DEBUG violates the privacy of users
602 and is not recommended.
604 Specifies the available MAC (message authentication code) algorithms.
605 The MAC algorithm is used in protocol version 2
606 for data integrity protection.
607 Multiple algorithms must be comma-separated.
609 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
611 Specifies the maximum number of concurrent unauthenticated connections to the
614 Additional connections will be dropped until authentication succeeds or the
616 expires for a connection.
619 Alternatively, random early drop can be enabled by specifying
620 the three colon separated values
624 will refuse connection attempts with a probability of
627 if there are currently
630 unauthenticated connections.
631 The probability increases linearly and all connection attempts
632 are refused if the number of unauthenticated connections reaches
635 .It Cm PAMAuthenticationViaKbdInt
636 Specifies whether PAM challenge response authentication is allowed. This
637 allows the use of most PAM challenge response authentication modules, but
638 it will allow password authentication regardless of whether
639 .Cm PasswordAuthentication
643 .It Cm PasswordAuthentication
644 Specifies whether password authentication is allowed.
647 .It Cm PermitEmptyPasswords
648 When password authentication is allowed, it specifies whether the
649 server allows login to accounts with empty password strings.
652 .It Cm PermitRootLogin
653 Specifies whether root can login using
657 .Dq without-password ,
658 .Dq forced-commands-only
664 If this option is set to
666 password authentication is disabled for root.
668 If this option is set to
669 .Dq forced-commands-only
670 root login with public key authentication will be allowed,
673 option has been specified
674 (which may be useful for taking remote backups even if root login is
675 normally not allowed). All other authentication methods are disabled
678 If this option is set to
680 root is not allowed to login.
682 Specifies the file that contains the process identifier of the
686 .Pa /var/run/sshd.pid .
688 Specifies the port number that
692 Multiple options of this type are permitted.
698 should print the date and time when the user last logged in.
706 when a user logs in interactively.
707 (On some systems it is also printed by the shell,
713 Specifies the protocol versions
716 The possible values are
720 Multiple versions must be comma-separated.
723 .It Cm PubkeyAuthentication
724 Specifies whether public key authentication is allowed.
727 Note that this option applies to protocol version 2 only.
728 .It Cm ReverseMappingCheck
731 should try to verify the remote host name and check that
732 the resolved host name for the remote IP address maps back to the
733 very same IP address.
736 .It Cm RhostsAuthentication
737 Specifies whether authentication using rhosts or /etc/hosts.equiv
739 Normally, this method should not be permitted because it is insecure.
740 .Cm RhostsRSAAuthentication
742 instead, because it performs RSA-based host authentication in addition
743 to normal rhosts or /etc/hosts.equiv authentication.
746 This option applies to protocol version 1 only.
747 .It Cm RhostsRSAAuthentication
748 Specifies whether rhosts or /etc/hosts.equiv authentication together
749 with successful RSA host authentication is allowed.
752 This option applies to protocol version 1 only.
753 .It Cm RSAAuthentication
754 Specifies whether pure RSA authentication is allowed.
757 This option applies to protocol version 1 only.
759 Defines the number of bits in the ephemeral protocol version 1 server key.
760 The minimum value is 512, and the default is 768.
764 should check file modes and ownership of the
765 user's files and home directory before accepting login.
766 This is normally desirable because novices sometimes accidentally leave their
767 directory or files world-writable.
771 Configures an external subsystem (e.g., file transfer daemon).
772 Arguments should be a subsystem name and a command to execute upon subsystem
778 file transfer subsystem.
779 By default no subsystems are defined.
780 Note that this option applies to protocol version 2 only.
781 .It Cm SyslogFacility
782 Gives the facility code that is used when logging messages from
784 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
785 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
790 is used for interactive login sessions.
795 is never used for remote command execution.
796 Note also, that if this is enabled,
798 will be disabled because
800 does not know how to handle
803 .It Cm X11DisplayOffset
804 Specifies the first display number available for
809 from interfering with real X11 servers.
812 Specifies whether X11 forwarding is permitted.
815 Note that disabling X11 forwarding does not improve security in any
816 way, as users can always install their own forwarders.
817 X11 forwarding is automatically disabled if
821 Specifies the location of the
825 .Pa /usr/X11R6/bin/xauth .
830 command-line arguments and configuration file options that specify time
831 may be expressed using a sequence of the form:
833 .Ar time Oo Ar qualifier Oc ,
837 is a positive integer value and
839 is one of the following:
841 .Bl -tag -width Ds -compact -offset indent
856 Each member of the sequence is added together to calculate
857 the total time value.
859 Time format examples:
861 .Bl -tag -width Ds -compact -offset indent
863 600 seconds (10 minutes)
867 1 hour 30 minutes (90 minutes)
870 When a user successfully logs in,
873 .Bl -enum -offset indent
875 If the login is on a tty, and no command has been specified,
876 prints last login time and
878 (unless prevented in the configuration file or by
879 .Pa $HOME/.hushlogin ;
884 If the login is on a tty, records login time.
888 if it exists, prints contents and quits
891 Changes to run with normal user privileges.
893 Sets up basic environment.
896 .Pa $HOME/.ssh/environment
899 Changes to user's home directory.
903 exists, runs it; else if
906 it; otherwise runs xauth.
909 files are given the X11
910 authentication protocol and cookie in standard input.
912 Runs user's shell or command.
914 .Sh AUTHORIZED_KEYS FILE FORMAT
915 .Pa $HOME/.ssh/authorized_keys
916 is the default file that lists the public keys that are
917 permitted for RSA authentication in protocol version 1
918 and for public key authentication (PubkeyAuthentication)
919 in protocol version 2.
920 .Cm AuthorizedKeysFile
921 may be used to specify an alternative file.
923 Each line of the file contains one
924 key (empty lines and lines starting with a
928 Each RSA public key consists of the following fields, separated by
929 spaces: options, bits, exponent, modulus, comment.
930 Each protocol version 2 public key consists of:
931 options, keytype, base64 encoded key, comment.
933 are optional; its presence is determined by whether the line starts
934 with a number or not (the option field never starts with a number).
935 The bits, exponent, modulus and comment fields give the RSA key for
936 protocol version 1; the
937 comment field is not used for anything (but may be convenient for the
938 user to identify the key).
939 For protocol version 2 the keytype is
944 Note that lines in this file are usually several hundred bytes long
945 (because of the size of the RSA key modulus).
946 You don't want to type them in; instead, copy the
953 The options (if present) consist of comma-separated option
955 No spaces are permitted, except within double quotes.
956 The following option specifications are supported:
958 .It Cm from="pattern-list"
959 Specifies that in addition to RSA authentication, the canonical name
960 of the remote host must be present in the comma-separated list of
966 The list may also contain
967 patterns negated by prefixing them with
969 if the canonical host name matches a negated pattern, the key is not accepted.
971 of this option is to optionally increase security: RSA authentication
972 by itself does not trust the network or name servers or anything (but
973 the key); however, if somebody somehow steals the key, the key
974 permits an intruder to log in from anywhere in the world.
975 This additional option makes using a stolen key more difficult (name
976 servers and/or routers would have to be compromised in addition to
978 .It Cm command="command"
979 Specifies that the command is executed whenever this key is used for
981 The command supplied by the user (if any) is ignored.
982 The command is run on a pty if the connection requests a pty;
983 otherwise it is run without a tty.
984 Note that if you want a 8-bit clean channel,
985 you must not request a pty or should specify
987 A quote may be included in the command by quoting it with a backslash.
988 This option might be useful
989 to restrict certain RSA keys to perform just a specific operation.
990 An example might be a key that permits remote backups but nothing else.
991 Note that the client may specify TCP/IP and/or X11
992 forwarding unless they are explicitly prohibited.
993 .It Cm environment="NAME=value"
994 Specifies that the string is to be added to the environment when
995 logging in using this key.
996 Environment variables set this way
997 override other default environment values.
998 Multiple options of this type are permitted.
999 .It Cm no-port-forwarding
1000 Forbids TCP/IP forwarding when this key is used for authentication.
1001 Any port forward requests by the client will return an error.
1002 This might be used, e.g., in connection with the
1005 .It Cm no-X11-forwarding
1006 Forbids X11 forwarding when this key is used for authentication.
1007 Any X11 forward requests by the client will return an error.
1008 .It Cm no-agent-forwarding
1009 Forbids authentication agent forwarding when this key is used for
1012 Prevents tty allocation (a request to allocate a pty will fail).
1013 .It Cm permitopen="host:port"
1016 port forwarding such that it may only connect to the specified host and
1019 options may be applied separated by commas. No pattern matching is
1020 performed on the specified hostnames, they must be literal domains or
1024 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1026 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1028 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1030 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1031 .Sh SSH_KNOWN_HOSTS FILE FORMAT
1033 .Pa /etc/ssh_known_hosts ,
1035 .Pa $HOME/.ssh/known_hosts
1036 files contain host public keys for all known hosts.
1037 The global file should
1038 be prepared by the administrator (optional), and the per-user file is
1039 maintained automatically: whenever the user connects from an unknown host
1040 its key is added to the per-user file.
1042 Each line in these files contains the following fields: hostnames,
1043 bits, exponent, modulus, comment.
1044 The fields are separated by spaces.
1046 Hostnames is a comma-separated list of patterns ('*' and '?' act as
1047 wildcards); each pattern in turn is matched against the canonical host
1048 name (when authenticating a client) or against the user-supplied
1049 name (when authenticating a server).
1050 A pattern may also be preceded by
1052 to indicate negation: if the host name matches a negated
1053 pattern, it is not accepted (by that line) even if it matched another
1054 pattern on the line.
1056 Bits, exponent, and modulus are taken directly from the RSA host key; they
1057 can be obtained, e.g., from
1058 .Pa /etc/ssh_host_key.pub .
1059 The optional comment field continues to the end of the line, and is not used.
1063 and empty lines are ignored as comments.
1065 When performing host authentication, authentication is accepted if any
1066 matching line has the proper key.
1067 It is thus permissible (but not
1068 recommended) to have several lines or different host keys for the same
1070 This will inevitably happen when short forms of host names
1071 from different domains are put in the file.
1073 that the files contain conflicting information; authentication is
1074 accepted if valid information can be found from either file.
1076 Note that the lines in these files are typically hundreds of characters
1077 long, and you definitely don't want to type in the host keys by hand.
1078 Rather, generate them by a script
1080 .Pa /etc/ssh_host_key.pub
1081 and adding the host names at the front.
1084 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1085 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1089 .It Pa /etc/sshd_config
1090 Contains configuration data for
1092 This file should be writable by root only, but it is recommended
1093 (though not necessary) that it be world-readable.
1094 .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1095 These three files contain the private parts of the host keys.
1096 These files should only be owned by root, readable only by root, and not
1097 accessible to others.
1100 does not start if this file is group/world-accessible.
1101 .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1102 These three files contain the public parts of the host keys.
1103 These files should be world-readable but writable only by
1105 Their contents should match the respective private parts.
1107 really used for anything; they are provided for the convenience of
1108 the user so their contents can be copied to known hosts files.
1109 These files are created using
1112 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1113 .It Pa /var/run/sshd.pid
1114 Contains the process ID of the
1116 listening for connections (if there are several daemons running
1117 concurrently for different ports, this contains the pid of the one
1119 The content of this file is not sensitive; it can be world-readable.
1120 .It Pa $HOME/.ssh/authorized_keys
1121 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1122 This file must be readable by root (which may on some machines imply
1123 it being world-readable if the user's home directory resides on an NFS
1125 It is recommended that it not be accessible by others.
1126 The format of this file is described above.
1127 Users will place the contents of their
1132 files into this file, as described in
1134 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1135 These files are consulted when using rhosts with RSA host
1136 authentication or protocol version 2 hostbased authentication
1137 to check the public key of the host.
1138 The key must be listed in one of these files to be accepted.
1139 The client uses the same files
1140 to verify that it is connecting to the correct remote host.
1141 These files should be writable only by root/the owner.
1142 .Pa /etc/ssh_known_hosts
1143 should be world-readable, and
1144 .Pa $HOME/.ssh/known_hosts
1145 can but need not be world-readable.
1147 If this file exists,
1149 refuses to let anyone except root log in.
1150 The contents of the file
1151 are displayed to anyone trying to log in, and non-root connections are
1153 The file should be world-readable.
1154 .It Pa /etc/hosts.allow, /etc/hosts.deny
1157 support, tcp-wrappers access controls may be defined here as described in
1158 .Xr hosts_access 5 .
1159 .It Pa $HOME/.rhosts
1160 This file contains host-username pairs, separated by a space, one per
1162 The given user on the corresponding host is permitted to log in
1164 The same file is used by rlogind and rshd.
1166 be writable only by the user; it is recommended that it not be
1167 accessible by others.
1169 If is also possible to use netgroups in the file.
1171 name may be of the form +@groupname to specify all hosts or all users
1173 .It Pa $HOME/.shosts
1175 this file is exactly the same as for
1177 However, this file is
1178 not used by rlogin and rshd, so using this permits access using SSH only.
1179 .It Pa /etc/hosts.equiv
1180 This file is used during
1183 In the simplest form, this file contains host names, one per line.
1185 those hosts are permitted to log in without a password, provided they
1186 have the same user name on both machines.
1187 The host name may also be
1188 followed by a user name; such users are permitted to log in as
1190 user on this machine (except root).
1191 Additionally, the syntax
1193 can be used to specify netgroups.
1194 Negated entries start with
1197 If the client host/user is successfully matched in this file, login is
1198 automatically permitted provided the client and server user names are the
1200 Additionally, successful RSA host authentication is normally required.
1201 This file must be writable only by root; it is recommended
1202 that it be world-readable.
1204 .Sy "Warning: It is almost never a good idea to use user names in"
1206 Beware that it really means that the named user(s) can log in as
1208 which includes bin, daemon, adm, and other accounts that own critical
1209 binaries and directories.
1210 Using a user name practically grants the user root access.
1211 The only valid use for user names that I can think
1212 of is in negative entries.
1214 Note that this warning also applies to rsh/rlogin.
1215 .It Pa /etc/shosts.equiv
1216 This is processed exactly as
1217 .Pa /etc/hosts.equiv .
1218 However, this file may be useful in environments that want to run both
1220 .It Pa $HOME/.ssh/environment
1221 This file is read into the environment at login (if it exists).
1222 It can only contain empty lines, comment lines (that start with
1224 and assignment lines of the form name=value.
1225 The file should be writable
1226 only by the user; it need not be readable by anyone else.
1227 .It Pa $HOME/.ssh/rc
1228 If this file exists, it is run with /bin/sh after reading the
1229 environment files but before starting the user's shell or command.
1230 If X11 spoofing is in use, this will receive the "proto cookie" pair in
1238 The primary purpose of this file is to run any initialization routines
1239 which may be needed before the user's home directory becomes
1240 accessible; AFS is a particular example of such an environment.
1242 This file will probably contain some initialization code followed by
1243 something similar to:
1245 if read proto cookie; then
1246 echo add $DISPLAY $proto $cookie | xauth -q -
1250 If this file does not exist,
1253 does not exist either, xauth is used to store the cookie.
1255 This file should be writable only by the user, and need not be
1256 readable by anyone else.
1260 This can be used to specify
1261 machine-specific login-time initializations globally.
1262 This file should be writable only by root, and should be world-readable.
1265 OpenSSH is a derivative of the original and free
1266 ssh 1.2.12 release by Tatu Ylonen.
1267 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1268 Theo de Raadt and Dug Song
1269 removed many bugs, re-added newer features and
1271 Markus Friedl contributed the support for SSH
1272 protocol versions 1.5 and 2.0.
1289 .%T "SSH Protocol Architecture"
1290 .%N draft-ietf-secsh-architecture-07.txt
1292 .%O work in progress material
1298 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1299 .%N draft-ietf-secsh-dh-group-exchange-01.txt
1301 .%O work in progress material