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 1.248 2009/03/26 08:38:39 sobrado Exp $
38 .Dd $Mdocdate: March 26 2009 $
43 .Nd OpenSSH SSH daemon
49 .Op Fl C Ar connection_spec
50 .Op Fl f Ar config_file
51 .Op Fl g Ar login_grace_time
52 .Op Fl h Ar host_key_file
53 .Op Fl k Ar key_gen_time
60 (OpenSSH Daemon) is the daemon program for
62 Together these programs replace
66 and provide secure encrypted communications between two untrusted hosts
67 over an insecure network.
70 listens for connections from clients.
71 It is normally started at boot from
74 daemon for each incoming connection.
75 The forked daemons handle
76 key exchange, encryption, authentication, command execution,
80 can be configured using command-line options or a configuration file
83 command-line options override values specified in the
86 rereads its configuration file when it receives a hangup signal,
88 by executing itself with the name and options it was started with, e.g.\&
91 The options are as follows:
96 to use IPv4 addresses only.
100 to use IPv6 addresses only.
102 Specifies the number of bits in the ephemeral protocol version 1
103 server key (default 1024).
104 .It Fl C Ar connection_spec
105 Specify the connection parameters to use for the
110 directives in the configuration file
111 that would apply to the specified user, host, and address will be set before
112 the configuration is written to standard output.
113 The connection parameters are supplied as keyword=value pairs.
119 All are required and may be supplied in any order, either with multiple
121 options or as a comma-separated list.
123 When this option is specified,
125 will not detach and does not become a daemon.
126 This allows easy monitoring of
130 The server sends verbose debug output to the system
131 log, and does not put itself in the background.
132 The server also will not fork and will only process one connection.
133 This option is only intended for debugging for the server.
136 options increase the debugging level.
139 When this option is specified,
141 will send the output to the standard error instead of the system log.
142 .It Fl f Ar config_file
143 Specifies the name of the configuration file.
145 .Pa /etc/ssh/sshd_config .
147 refuses to start if there is no configuration file.
148 .It Fl g Ar login_grace_time
149 Gives the grace time for clients to authenticate themselves (default
151 If the client fails to authenticate the user within
152 this many seconds, the server disconnects and exits.
153 A value of zero indicates no limit.
154 .It Fl h Ar host_key_file
155 Specifies a file from which a host key is read.
156 This option must be given if
158 is not run as root (as the normal
159 host key files are normally not readable by anyone but root).
161 .Pa /etc/ssh/ssh_host_key
162 for protocol version 1, and
163 .Pa /etc/ssh/ssh_host_rsa_key
165 .Pa /etc/ssh/ssh_host_dsa_key
166 for protocol version 2.
167 It is possible to have multiple host key files for
168 the different protocol versions and host key algorithms.
176 from inetd because it needs to generate the server key before it can
177 respond to the client, and this may take tens of seconds.
178 Clients would have to wait too long if the key was regenerated every time.
179 However, with small key sizes (e.g. 512) using
183 .It Fl k Ar key_gen_time
184 Specifies how often the ephemeral protocol version 1 server key is
185 regenerated (default 3600 seconds, or one hour).
186 The motivation for regenerating the key fairly
187 often is that the key is not stored anywhere, and after about an hour
188 it becomes impossible to recover the key for decrypting intercepted
189 communications even if the machine is cracked into or physically
191 A value of zero indicates that the key will never be regenerated.
193 Can be used to give options in the format used in the configuration file.
194 This is useful for specifying options for which there is no separate
196 For full details of the options, and their values, see
199 Specifies the port on which the server listens for connections
201 Multiple port options are permitted.
202 Ports specified in the configuration file with the
204 option are ignored when a command-line port is specified.
205 Ports specified using the
207 option override command-line ports.
210 Only fatal errors are sent to the system log.
211 Normally the beginning,
212 authentication, and termination of each connection is logged.
215 is given then nothing is sent to the system log.
218 Check the validity of the configuration file, output the effective configuration
219 to stdout and then exit.
222 rules may be applied by specifying the connection parameters using one or more
227 Only check the validity of the configuration file and sanity of the keys.
228 This is useful for updating
230 reliably as configuration options may change.
232 This option is used to specify the size of the field
235 structure that holds the remote host name.
236 If the resolved host name is longer than
238 the dotted decimal value will be used instead.
239 This allows hosts with very long host names that
240 overflow this field to still be uniquely identified.
243 indicates that only dotted decimal addresses
244 should be put into the
248 may also be used to prevent
250 from making DNS requests unless the authentication
251 mechanism or configuration requires it.
252 Authentication mechanisms that may require DNS include
253 .Cm RhostsRSAAuthentication ,
254 .Cm HostbasedAuthentication ,
256 .Cm from="pattern-list"
257 option in a key file.
258 Configuration options that require DNS include using a
265 The OpenSSH SSH daemon supports SSH protocols 1 and 2.
266 Both protocols are supported by default,
267 though this can be changed via the
271 Protocol 2 supports both RSA and DSA keys;
272 protocol 1 only supports RSA keys.
274 each host has a host-specific key,
276 used to identify the host.
278 Forward security for protocol 1 is provided through
279 an additional server key,
281 generated when the server starts.
282 This key is normally regenerated every hour if it has been used, and
283 is never stored on disk.
284 Whenever a client connects, the daemon responds with its public
285 host and server keys.
286 The client compares the
287 RSA host key against its own database to verify that it has not changed.
288 The client then generates a 256-bit random number.
290 random number using both the host key and the server key, and sends
291 the encrypted number to the server.
292 Both sides then use this
293 random number as a session key which is used to encrypt all further
294 communications in the session.
295 The rest of the session is encrypted
296 using a conventional cipher, currently Blowfish or 3DES, with 3DES
297 being used by default.
298 The client selects the encryption algorithm
299 to use from those offered by the server.
302 forward security is provided through a Diffie-Hellman key agreement.
303 This key agreement results in a shared session key.
304 The rest of the session is encrypted using a symmetric cipher, currently
305 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
306 The client selects the encryption algorithm
307 to use from those offered by the server.
308 Additionally, session integrity is provided
309 through a cryptographic message authentication code
310 (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
312 Finally, the server and the client enter an authentication dialog.
313 The client tries to authenticate itself using
314 host-based authentication,
315 public key authentication,
316 challenge-response authentication,
317 or password authentication.
319 Regardless of the authentication type, the account is checked to
320 ensure that it is accessible. An account is not accessible if it is
323 or its group is listed in
325 \&. The definition of a locked account is system dependant. Some platforms
326 have their own account database (eg AIX) and some modify the passwd field (
328 on Solaris and UnixWare,
335 on FreeBSD and a leading
338 If there is a requirement to disable password authentication
339 for the account while allowing still public-key, then the passwd field
340 should be set to something other than these values (eg
346 If the client successfully authenticates itself, a dialog for
347 preparing the session is entered.
348 At this time the client may request
349 things like allocating a pseudo-tty, forwarding X11 connections,
350 forwarding TCP connections, or forwarding the authentication agent
351 connection over the secure channel.
353 After this, the client either requests a shell or execution of a command.
354 The sides then enter session mode.
355 In this mode, either side may send
356 data at any time, and such data is forwarded to/from the shell or
357 command on the server side, and the user terminal in the client side.
359 When the user program terminates and all forwarded X11 and other
360 connections have been closed, the server sends command exit status to
361 the client, and both sides exit.
363 When a user successfully logs in,
366 .Bl -enum -offset indent
368 If the login is on a tty, and no command has been specified,
369 prints last login time and
371 (unless prevented in the configuration file or by
377 If the login is on a tty, records login time.
381 if it exists, prints contents and quits
384 Changes to run with normal user privileges.
386 Sets up basic environment.
389 .Pa ~/.ssh/environment ,
390 if it exists, and users are allowed to change their environment.
392 .Cm PermitUserEnvironment
396 Changes to user's home directory.
400 exists, runs it; else if
403 it; otherwise runs xauth.
406 files are given the X11
407 authentication protocol and cookie in standard input.
412 Runs user's shell or command.
419 runs it after reading the
420 environment files but before starting the user's shell or command.
421 It must not produce any output on stdout; stderr must be used
423 If X11 forwarding is in use, it will receive the "proto cookie" pair in
424 its standard input (and
431 will not run xauth automatically to add X11 cookies.
433 The primary purpose of this file is to run any initialization routines
434 which may be needed before the user's home directory becomes
435 accessible; AFS is a particular example of such an environment.
437 This file will probably contain some initialization code followed by
438 something similar to:
439 .Bd -literal -offset 3n
440 if read proto cookie && [ -n "$DISPLAY" ]; then
441 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
442 # X11UseLocalhost=yes
443 echo add unix:`echo $DISPLAY |
444 cut -c11-` $proto $cookie
447 echo add $DISPLAY $proto $cookie
452 If this file does not exist,
455 does not exist either, xauth is used to add the cookie.
456 .Sh AUTHORIZED_KEYS FILE FORMAT
457 .Cm AuthorizedKeysFile
458 specifies the file containing public keys for
459 public key authentication;
460 if none is specified, the default is
461 .Pa ~/.ssh/authorized_keys .
462 Each line of the file contains one
463 key (empty lines and lines starting with a
467 Protocol 1 public keys consist of the following space-separated fields:
468 options, bits, exponent, modulus, comment.
469 Protocol 2 public key consist of:
470 options, keytype, base64-encoded key, comment.
471 The options field is optional;
472 its presence is determined by whether the line starts
473 with a number or not (the options field never starts with a number).
474 The bits, exponent, modulus, and comment fields give the RSA key for
475 protocol version 1; the
476 comment field is not used for anything (but may be convenient for the
477 user to identify the key).
478 For protocol version 2 the keytype is
483 Note that lines in this file are usually several hundred bytes long
484 (because of the size of the public key encoding) up to a limit of
485 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
486 keys up to 16 kilobits.
487 You don't want to type them in; instead, copy the
495 enforces a minimum RSA key modulus size for protocol 1
496 and protocol 2 keys of 768 bits.
498 The options (if present) consist of comma-separated option
500 No spaces are permitted, except within double quotes.
501 The following option specifications are supported (note
502 that option keywords are case-insensitive):
504 .It Cm command="command"
505 Specifies that the command is executed whenever this key is used for
507 The command supplied by the user (if any) is ignored.
508 The command is run on a pty if the client requests a pty;
509 otherwise it is run without a tty.
510 If an 8-bit clean channel is required,
511 one must not request a pty or should specify
513 A quote may be included in the command by quoting it with a backslash.
514 This option might be useful
515 to restrict certain public keys to perform just a specific operation.
516 An example might be a key that permits remote backups but nothing else.
517 Note that the client may specify TCP and/or X11
518 forwarding unless they are explicitly prohibited.
519 The command originally supplied by the client is available in the
520 .Ev SSH_ORIGINAL_COMMAND
521 environment variable.
522 Note that this option applies to shell, command or subsystem execution.
523 .It Cm environment="NAME=value"
524 Specifies that the string is to be added to the environment when
525 logging in using this key.
526 Environment variables set this way
527 override other default environment values.
528 Multiple options of this type are permitted.
529 Environment processing is disabled by default and is
531 .Cm PermitUserEnvironment
533 This option is automatically disabled if
536 .It Cm from="pattern-list"
537 Specifies that in addition to public key authentication, either the canonical
538 name of the remote host or its IP address must be present in the
539 comma-separated list of patterns.
544 for more information on patterns.
546 In addition to the wildcard matching that may be applied to hostnames or
549 stanza may match IP addresses using CIDR address/masklen notation.
551 The purpose of this option is to optionally increase security: public key
552 authentication by itself does not trust the network or name servers or
553 anything (but the key); however, if somebody somehow steals the key, the key
554 permits an intruder to log in from anywhere in the world.
555 This additional option makes using a stolen key more difficult (name
556 servers and/or routers would have to be compromised in addition to
558 .It Cm no-agent-forwarding
559 Forbids authentication agent forwarding when this key is used for
561 .It Cm no-port-forwarding
562 Forbids TCP forwarding when this key is used for authentication.
563 Any port forward requests by the client will return an error.
564 This might be used, e.g. in connection with the
568 Prevents tty allocation (a request to allocate a pty will fail).
570 Disables execution of
572 .It Cm no-X11-forwarding
573 Forbids X11 forwarding when this key is used for authentication.
574 Any X11 forward requests by the client will return an error.
575 .It Cm permitopen="host:port"
578 port forwarding such that it may only connect to the specified host and
580 IPv6 addresses can be specified with an alternative syntax:
581 .Ar host Ns / Ns Ar port .
584 options may be applied separated by commas.
585 No pattern matching is performed on the specified hostnames,
586 they must be literal domains or addresses.
590 device on the server.
591 Without this option, the next available device will be used if
592 the client requests a tunnel.
595 An example authorized_keys file:
596 .Bd -literal -offset 3n
597 # Comments allowed at start of line
598 ssh-rsa AAAAB3Nza...LiPk== user@example.net
599 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
600 AAAAB2...19Q== john@example.net
601 command="dump /home",no-pty,no-port-forwarding ssh-dss
602 AAAAC3...51R== example.net
603 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
605 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
608 .Sh SSH_KNOWN_HOSTS FILE FORMAT
610 .Pa /etc/ssh/ssh_known_hosts
612 .Pa ~/.ssh/known_hosts
613 files contain host public keys for all known hosts.
614 The global file should
615 be prepared by the administrator (optional), and the per-user file is
616 maintained automatically: whenever the user connects from an unknown host,
617 its key is added to the per-user file.
619 Each line in these files contains the following fields: hostnames,
620 bits, exponent, modulus, comment.
621 The fields are separated by spaces.
623 Hostnames is a comma-separated list of patterns
628 wildcards); each pattern in turn is matched against the canonical host
629 name (when authenticating a client) or against the user-supplied
630 name (when authenticating a server).
631 A pattern may also be preceded by
633 to indicate negation: if the host name matches a negated
634 pattern, it is not accepted (by that line) even if it matched another
636 A hostname or address may optionally be enclosed within
640 brackets then followed by
642 and a non-standard port number.
644 Alternately, hostnames may be stored in a hashed form which hides host names
645 and addresses should the file's contents be disclosed.
646 Hashed hostnames start with a
649 Only one hashed hostname may appear on a single line and none of the above
650 negation or wildcard operators may be applied.
652 Bits, exponent, and modulus are taken directly from the RSA host key; they
653 can be obtained, for example, from
654 .Pa /etc/ssh/ssh_host_key.pub .
655 The optional comment field continues to the end of the line, and is not used.
659 and empty lines are ignored as comments.
661 When performing host authentication, authentication is accepted if any
662 matching line has the proper key.
663 It is thus permissible (but not
664 recommended) to have several lines or different host keys for the same
666 This will inevitably happen when short forms of host names
667 from different domains are put in the file.
669 that the files contain conflicting information; authentication is
670 accepted if valid information can be found from either file.
672 Note that the lines in these files are typically hundreds of characters
673 long, and you definitely don't want to type in the host keys by hand.
674 Rather, generate them by a script
676 .Pa /etc/ssh/ssh_host_key.pub
677 and adding the host names at the front.
679 An example ssh_known_hosts file:
680 .Bd -literal -offset 3n
681 # Comments allowed at start of line
682 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
683 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
685 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
689 .Bl -tag -width Ds -compact
691 This file is used to suppress printing the last login time and
699 It does not suppress printing of the banner specified by
703 This file is used for host-based authentication (see
705 for more information).
706 On some machines this file may need to be
707 world-readable if the user's home directory is on an NFS partition,
711 Additionally, this file must be owned by the user,
712 and must not have write permissions for anyone else.
714 permission for most machines is read/write for the user, and not
715 accessible by others.
718 This file is used in exactly the same way as
720 but allows host-based authentication without permitting login with
724 This directory is the default location for all user-specific configuration
725 and authentication information.
726 There is no general requirement to keep the entire contents of this directory
727 secret, but the recommended permissions are read/write/execute for the user,
728 and not accessible by others.
730 .It ~/.ssh/authorized_keys
731 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
732 The format of this file is described above.
733 The content of the file is not highly sensitive, but the recommended
734 permissions are read/write for the user, and not accessible by others.
738 directory, or the user's home directory are writable
739 by other users, then the file could be modified or replaced by unauthorized
743 will not allow it to be used unless the
745 option has been set to
748 .It ~/.ssh/environment
749 This file is read into the environment at login (if it exists).
750 It can only contain empty lines, comment lines (that start with
752 and assignment lines of the form name=value.
753 The file should be writable
754 only by the user; it need not be readable by anyone else.
755 Environment processing is disabled by default and is
757 .Cm PermitUserEnvironment
760 .It ~/.ssh/known_hosts
761 Contains a list of host keys for all hosts the user has logged into
762 that are not already in the systemwide list of known host keys.
763 The format of this file is described above.
764 This file should be writable only by root/the owner and
765 can, but need not be, world-readable.
768 Contains initialization routines to be run before
769 the user's home directory becomes accessible.
770 This file should be writable only by the user, and need not be
771 readable by anyone else.
775 Access controls that should be enforced by tcp-wrappers are defined here.
776 Further details are described in
780 This file is for host-based authentication (see
782 It should only be writable by root.
785 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
786 The file format is described in
796 refuses to let anyone except root log in.
797 The contents of the file
798 are displayed to anyone trying to log in, and non-root connections are
800 The file should be world-readable.
802 .It /etc/shosts.equiv
803 This file is used in exactly the same way as
805 but allows host-based authentication without permitting login with
808 .It /etc/ssh/ssh_host_key
809 .It /etc/ssh/ssh_host_dsa_key
810 .It /etc/ssh/ssh_host_rsa_key
811 These three files contain the private parts of the host keys.
812 These files should only be owned by root, readable only by root, and not
813 accessible to others.
816 does not start if these files are group/world-accessible.
818 .It /etc/ssh/ssh_host_key.pub
819 .It /etc/ssh/ssh_host_dsa_key.pub
820 .It /etc/ssh/ssh_host_rsa_key.pub
821 These three files contain the public parts of the host keys.
822 These files should be world-readable but writable only by
824 Their contents should match the respective private parts.
826 really used for anything; they are provided for the convenience of
827 the user so their contents can be copied to known hosts files.
828 These files are created using
831 .It /etc/ssh/ssh_known_hosts
832 Systemwide list of known host keys.
833 This file should be prepared by the
834 system administrator to contain the public host keys of all machines in the
836 The format of this file is described above.
837 This file should be writable only by root/the owner and
838 should be world-readable.
840 .It /etc/ssh/sshd_config
841 Contains configuration data for
843 The file format and configuration options are described in
849 it can be used to specify
850 machine-specific login-time initializations globally.
851 This file should be writable only by root, and should be world-readable.
857 during privilege separation in the pre-authentication phase.
858 The directory should not contain any files and must be owned by root
859 and not group or world-writable.
861 .It /var/run/sshd.pid
862 Contains the process ID of the
864 listening for connections (if there are several daemons running
865 concurrently for different ports, this contains the process ID of the one
867 The content of this file is not sensitive; it can be world-readable.
885 OpenSSH is a derivative of the original and free
886 ssh 1.2.12 release by Tatu Ylonen.
887 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
888 Theo de Raadt and Dug Song
889 removed many bugs, re-added newer features and
891 Markus Friedl contributed the support for SSH
892 protocol versions 1.5 and 2.0.
893 Niels Provos and Markus Friedl contributed support
894 for privilege separation.
896 System security is not improved unless
901 are disabled (thus completely disabling