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 1.235 2007/03/20 15:57:15 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
49 .Op Fl f Ar config_file
50 .Op Fl g Ar login_grace_time
51 .Op Fl h Ar host_key_file
52 .Op Fl k Ar key_gen_time
59 (OpenSSH Daemon) is the daemon program for
61 Together these programs replace
65 and provide secure encrypted communications between two untrusted hosts
66 over an insecure network.
69 listens for connections from clients.
70 It is normally started at boot from
73 daemon for each incoming connection.
74 The forked daemons handle
75 key exchange, encryption, authentication, command execution,
79 can be configured using command-line options or a configuration file
82 command-line options override values specified in the
85 rereads its configuration file when it receives a hangup signal,
87 by executing itself with the name and options it was started with, e.g.\&
90 The options are as follows:
95 to use IPv4 addresses only.
99 to use IPv6 addresses only.
101 Specifies the number of bits in the ephemeral protocol version 1
102 server key (default 768).
104 When this option is specified,
106 will not detach and does not become a daemon.
107 This allows easy monitoring of
111 The server sends verbose debug output to the system
112 log, and does not put itself in the background.
113 The server also will not fork and will only process one connection.
114 This option is only intended for debugging for the server.
117 options increase the debugging level.
120 When this option is specified,
122 will send the output to the standard error instead of the system log.
123 .It Fl f Ar config_file
124 Specifies the name of the configuration file.
126 .Pa /etc/ssh/sshd_config .
128 refuses to start if there is no configuration file.
129 .It Fl g Ar login_grace_time
130 Gives the grace time for clients to authenticate themselves (default
132 If the client fails to authenticate the user within
133 this many seconds, the server disconnects and exits.
134 A value of zero indicates no limit.
135 .It Fl h Ar host_key_file
136 Specifies a file from which a host key is read.
137 This option must be given if
139 is not run as root (as the normal
140 host key files are normally not readable by anyone but root).
142 .Pa /etc/ssh/ssh_host_key
143 for protocol version 1, and
144 .Pa /etc/ssh/ssh_host_rsa_key
146 .Pa /etc/ssh/ssh_host_dsa_key
147 for protocol version 2.
148 It is possible to have multiple host key files for
149 the different protocol versions and host key algorithms.
157 from inetd because it needs to generate the server key before it can
158 respond to the client, and this may take tens of seconds.
159 Clients would have to wait too long if the key was regenerated every time.
160 However, with small key sizes (e.g. 512) using
164 .It Fl k Ar key_gen_time
165 Specifies how often the ephemeral protocol version 1 server key is
166 regenerated (default 3600 seconds, or one hour).
167 The motivation for regenerating the key fairly
168 often is that the key is not stored anywhere, and after about an hour
169 it becomes impossible to recover the key for decrypting intercepted
170 communications even if the machine is cracked into or physically
172 A value of zero indicates that the key will never be regenerated.
174 Can be used to give options in the format used in the configuration file.
175 This is useful for specifying options for which there is no separate
177 For full details of the options, and their values, see
180 Specifies the port on which the server listens for connections
182 Multiple port options are permitted.
183 Ports specified in the configuration file with the
185 option are ignored when a command-line port is specified.
186 Ports specified using the
188 option override command-line ports.
191 Nothing is sent to the system log.
192 Normally the beginning,
193 authentication, and termination of each connection is logged.
196 Only check the validity of the configuration file and sanity of the keys.
197 This is useful for updating
199 reliably as configuration options may change.
201 This option is used to specify the size of the field
204 structure that holds the remote host name.
205 If the resolved host name is longer than
207 the dotted decimal value will be used instead.
208 This allows hosts with very long host names that
209 overflow this field to still be uniquely identified.
212 indicates that only dotted decimal addresses
213 should be put into the
217 may also be used to prevent
219 from making DNS requests unless the authentication
220 mechanism or configuration requires it.
221 Authentication mechanisms that may require DNS include
222 .Cm RhostsRSAAuthentication ,
223 .Cm HostbasedAuthentication ,
225 .Cm from="pattern-list"
226 option in a key file.
227 Configuration options that require DNS include using a
234 The OpenSSH SSH daemon supports SSH protocols 1 and 2.
235 Both protocols are supported by default,
236 though this can be changed via the
240 Protocol 2 supports both RSA and DSA keys;
241 protocol 1 only supports RSA keys.
243 each host has a host-specific key,
245 used to identify the host.
247 Forward security for protocol 1 is provided through
248 an additional server key,
250 generated when the server starts.
251 This key is normally regenerated every hour if it has been used, and
252 is never stored on disk.
253 Whenever a client connects, the daemon responds with its public
254 host and server keys.
255 The client compares the
256 RSA host key against its own database to verify that it has not changed.
257 The client then generates a 256-bit random number.
259 random number using both the host key and the server key, and sends
260 the encrypted number to the server.
261 Both sides then use this
262 random number as a session key which is used to encrypt all further
263 communications in the session.
264 The rest of the session is encrypted
265 using a conventional cipher, currently Blowfish or 3DES, with 3DES
266 being used by default.
267 The client selects the encryption algorithm
268 to use from those offered by the server.
271 forward security is provided through a Diffie-Hellman key agreement.
272 This key agreement results in a shared session key.
273 The rest of the session is encrypted using a symmetric cipher, currently
274 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
275 The client selects the encryption algorithm
276 to use from those offered by the server.
277 Additionally, session integrity is provided
278 through a cryptographic message authentication code
279 (hmac-sha1 or hmac-md5).
281 Finally, the server and the client enter an authentication dialog.
282 The client tries to authenticate itself using
283 host-based authentication,
284 public key authentication,
285 challenge-response authentication,
286 or password authentication.
288 Regardless of the authentication type, the account is checked to
289 ensure that it is accessible. An account is not accessible if it is
292 or its group is listed in
294 \&. The definition of a locked account is system dependant. Some platforms
295 have their own account database (eg AIX) and some modify the passwd field (
297 on Solaris and UnixWare,
304 on FreeBSD and a leading
306 on Linux). If there is a requirement to disable password authentication
307 for the account while allowing still public-key, then the passwd field
308 should be set to something other than these values (eg
314 If the client successfully authenticates itself, a dialog for
315 preparing the session is entered.
316 At this time the client may request
317 things like allocating a pseudo-tty, forwarding X11 connections,
318 forwarding TCP connections, or forwarding the authentication agent
319 connection over the secure channel.
321 After this, the client either requests a shell or execution of a command.
322 The sides then enter session mode.
323 In this mode, either side may send
324 data at any time, and such data is forwarded to/from the shell or
325 command on the server side, and the user terminal in the client side.
327 When the user program terminates and all forwarded X11 and other
328 connections have been closed, the server sends command exit status to
329 the client, and both sides exit.
331 When a user successfully logs in,
334 .Bl -enum -offset indent
336 If the login is on a tty, and no command has been specified,
337 prints last login time and
339 (unless prevented in the configuration file or by
345 If the login is on a tty, records login time.
349 if it exists, prints contents and quits
352 Changes to run with normal user privileges.
354 Sets up basic environment.
357 .Pa ~/.ssh/environment ,
358 if it exists, and users are allowed to change their environment.
360 .Cm PermitUserEnvironment
364 Changes to user's home directory.
368 exists, runs it; else if
371 it; otherwise runs xauth.
374 files are given the X11
375 authentication protocol and cookie in standard input.
380 Runs user's shell or command.
387 runs it after reading the
388 environment files but before starting the user's shell or command.
389 It must not produce any output on stdout; stderr must be used
391 If X11 forwarding is in use, it will receive the "proto cookie" pair in
392 its standard input (and
399 will not run xauth automatically to add X11 cookies.
401 The primary purpose of this file is to run any initialization routines
402 which may be needed before the user's home directory becomes
403 accessible; AFS is a particular example of such an environment.
405 This file will probably contain some initialization code followed by
406 something similar to:
407 .Bd -literal -offset 3n
408 if read proto cookie && [ -n "$DISPLAY" ]; then
409 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
410 # X11UseLocalhost=yes
411 echo add unix:`echo $DISPLAY |
412 cut -c11-` $proto $cookie
415 echo add $DISPLAY $proto $cookie
420 If this file does not exist,
423 does not exist either, xauth is used to add the cookie.
424 .Sh AUTHORIZED_KEYS FILE FORMAT
425 .Cm AuthorizedKeysFile
426 specifies the file containing public keys for
427 public key authentication;
428 if none is specified, the default is
429 .Pa ~/.ssh/authorized_keys .
430 Each line of the file contains one
431 key (empty lines and lines starting with a
435 Protocol 1 public keys consist of the following space-separated fields:
436 options, bits, exponent, modulus, comment.
437 Protocol 2 public key consist of:
438 options, keytype, base64-encoded key, comment.
439 The options field is optional;
440 its presence is determined by whether the line starts
441 with a number or not (the options field never starts with a number).
442 The bits, exponent, modulus, and comment fields give the RSA key for
443 protocol version 1; the
444 comment field is not used for anything (but may be convenient for the
445 user to identify the key).
446 For protocol version 2 the keytype is
451 Note that lines in this file are usually several hundred bytes long
452 (because of the size of the public key encoding) up to a limit of
453 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
454 keys up to 16 kilobits.
455 You don't want to type them in; instead, copy the
463 enforces a minimum RSA key modulus size for protocol 1
464 and protocol 2 keys of 768 bits.
466 The options (if present) consist of comma-separated option
468 No spaces are permitted, except within double quotes.
469 The following option specifications are supported (note
470 that option keywords are case-insensitive):
472 .It Cm command="command"
473 Specifies that the command is executed whenever this key is used for
475 The command supplied by the user (if any) is ignored.
476 The command is run on a pty if the client requests a pty;
477 otherwise it is run without a tty.
478 If an 8-bit clean channel is required,
479 one must not request a pty or should specify
481 A quote may be included in the command by quoting it with a backslash.
482 This option might be useful
483 to restrict certain public keys to perform just a specific operation.
484 An example might be a key that permits remote backups but nothing else.
485 Note that the client may specify TCP and/or X11
486 forwarding unless they are explicitly prohibited.
487 The command originally supplied by the client is available in the
488 .Ev SSH_ORIGINAL_COMMAND
489 environment variable.
490 Note that this option applies to shell, command or subsystem execution.
491 .It Cm environment="NAME=value"
492 Specifies that the string is to be added to the environment when
493 logging in using this key.
494 Environment variables set this way
495 override other default environment values.
496 Multiple options of this type are permitted.
497 Environment processing is disabled by default and is
499 .Cm PermitUserEnvironment
501 This option is automatically disabled if
504 .It Cm from="pattern-list"
505 Specifies that in addition to public key authentication, the canonical name
506 of the remote host must be present in the comma-separated list of
509 of this option is to optionally increase security: public key authentication
510 by itself does not trust the network or name servers or anything (but
511 the key); however, if somebody somehow steals the key, the key
512 permits an intruder to log in from anywhere in the world.
513 This additional option makes using a stolen key more difficult (name
514 servers and/or routers would have to be compromised in addition to
521 for more information on patterns.
522 .It Cm no-agent-forwarding
523 Forbids authentication agent forwarding when this key is used for
525 .It Cm no-port-forwarding
526 Forbids TCP forwarding when this key is used for authentication.
527 Any port forward requests by the client will return an error.
528 This might be used, e.g. in connection with the
532 Prevents tty allocation (a request to allocate a pty will fail).
533 .It Cm no-X11-forwarding
534 Forbids X11 forwarding when this key is used for authentication.
535 Any X11 forward requests by the client will return an error.
536 .It Cm permitopen="host:port"
539 port forwarding such that it may only connect to the specified host and
541 IPv6 addresses can be specified with an alternative syntax:
542 .Ar host Ns / Ns Ar port .
545 options may be applied separated by commas.
546 No pattern matching is performed on the specified hostnames,
547 they must be literal domains or addresses.
551 device on the server.
552 Without this option, the next available device will be used if
553 the client requests a tunnel.
556 An example authorized_keys file:
557 .Bd -literal -offset 3n
558 # Comments allowed at start of line
559 ssh-rsa AAAAB3Nza...LiPk== user@example.net
560 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
561 AAAAB2...19Q== john@example.net
562 command="dump /home",no-pty,no-port-forwarding ssh-dss
563 AAAAC3...51R== example.net
564 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
566 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
569 .Sh SSH_KNOWN_HOSTS FILE FORMAT
571 .Pa /etc/ssh/ssh_known_hosts
573 .Pa ~/.ssh/known_hosts
574 files contain host public keys for all known hosts.
575 The global file should
576 be prepared by the administrator (optional), and the per-user file is
577 maintained automatically: whenever the user connects from an unknown host,
578 its key is added to the per-user file.
580 Each line in these files contains the following fields: hostnames,
581 bits, exponent, modulus, comment.
582 The fields are separated by spaces.
584 Hostnames is a comma-separated list of patterns
589 wildcards); each pattern in turn is matched against the canonical host
590 name (when authenticating a client) or against the user-supplied
591 name (when authenticating a server).
592 A pattern may also be preceded by
594 to indicate negation: if the host name matches a negated
595 pattern, it is not accepted (by that line) even if it matched another
597 A hostname or address may optionally be enclosed within
601 brackets then followed by
603 and a non-standard port number.
605 Alternately, hostnames may be stored in a hashed form which hides host names
606 and addresses should the file's contents be disclosed.
607 Hashed hostnames start with a
610 Only one hashed hostname may appear on a single line and none of the above
611 negation or wildcard operators may be applied.
613 Bits, exponent, and modulus are taken directly from the RSA host key; they
614 can be obtained, for example, from
615 .Pa /etc/ssh/ssh_host_key.pub .
616 The optional comment field continues to the end of the line, and is not used.
620 and empty lines are ignored as comments.
622 When performing host authentication, authentication is accepted if any
623 matching line has the proper key.
624 It is thus permissible (but not
625 recommended) to have several lines or different host keys for the same
627 This will inevitably happen when short forms of host names
628 from different domains are put in the file.
630 that the files contain conflicting information; authentication is
631 accepted if valid information can be found from either file.
633 Note that the lines in these files are typically hundreds of characters
634 long, and you definitely don't want to type in the host keys by hand.
635 Rather, generate them by a script
637 .Pa /etc/ssh/ssh_host_key.pub
638 and adding the host names at the front.
640 An example ssh_known_hosts file:
641 .Bd -literal -offset 3n
642 # Comments allowed at start of line
643 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
644 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
646 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
650 .Bl -tag -width Ds -compact
652 This file is used to suppress printing the last login time and
660 It does not suppress printing of the banner specified by
664 This file is used for host-based authentication (see
666 for more information).
667 On some machines this file may need to be
668 world-readable if the user's home directory is on an NFS partition,
672 Additionally, this file must be owned by the user,
673 and must not have write permissions for anyone else.
675 permission for most machines is read/write for the user, and not
676 accessible by others.
679 This file is used in exactly the same way as
681 but allows host-based authentication without permitting login with
684 .It ~/.ssh/authorized_keys
685 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
686 The format of this file is described above.
687 The content of the file is not highly sensitive, but the recommended
688 permissions are read/write for the user, and not accessible by others.
692 directory, or the user's home directory are writable
693 by other users, then the file could be modified or replaced by unauthorized
697 will not allow it to be used unless the
699 option has been set to
701 The recommended permissions can be set by executing
702 .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
704 .It ~/.ssh/environment
705 This file is read into the environment at login (if it exists).
706 It can only contain empty lines, comment lines (that start with
708 and assignment lines of the form name=value.
709 The file should be writable
710 only by the user; it need not be readable by anyone else.
711 Environment processing is disabled by default and is
713 .Cm PermitUserEnvironment
716 .It ~/.ssh/known_hosts
717 Contains a list of host keys for all hosts the user has logged into
718 that are not already in the systemwide list of known host keys.
719 The format of this file is described above.
720 This file should be writable only by root/the owner and
721 can, but need not be, world-readable.
724 Contains initialization routines to be run before
725 the user's home directory becomes accessible.
726 This file should be writable only by the user, and need not be
727 readable by anyone else.
731 Access controls that should be enforced by tcp-wrappers are defined here.
732 Further details are described in
736 This file is for host-based authentication (see
738 It should only be writable by root.
741 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
742 The file format is described in
752 refuses to let anyone except root log in.
753 The contents of the file
754 are displayed to anyone trying to log in, and non-root connections are
756 The file should be world-readable.
758 .It /etc/shosts.equiv
759 This file is used in exactly the same way as
761 but allows host-based authentication without permitting login with
764 .It /etc/ssh/ssh_host_key
765 .It /etc/ssh/ssh_host_dsa_key
766 .It /etc/ssh/ssh_host_rsa_key
767 These three files contain the private parts of the host keys.
768 These files should only be owned by root, readable only by root, and not
769 accessible to others.
772 does not start if these files are group/world-accessible.
774 .It /etc/ssh/ssh_host_key.pub
775 .It /etc/ssh/ssh_host_dsa_key.pub
776 .It /etc/ssh/ssh_host_rsa_key.pub
777 These three files contain the public parts of the host keys.
778 These files should be world-readable but writable only by
780 Their contents should match the respective private parts.
782 really used for anything; they are provided for the convenience of
783 the user so their contents can be copied to known hosts files.
784 These files are created using
787 .It /etc/ssh/ssh_known_hosts
788 Systemwide list of known host keys.
789 This file should be prepared by the
790 system administrator to contain the public host keys of all machines in the
792 The format of this file is described above.
793 This file should be writable only by root/the owner and
794 should be world-readable.
796 .It /etc/ssh/sshd_config
797 Contains configuration data for
799 The file format and configuration options are described in
805 it can be used to specify
806 machine-specific login-time initializations globally.
807 This file should be writable only by root, and should be world-readable.
813 during privilege separation in the pre-authentication phase.
814 The directory should not contain any files and must be owned by root
815 and not group or world-writable.
817 .It /var/run/sshd.pid
818 Contains the process ID of the
820 listening for connections (if there are several daemons running
821 concurrently for different ports, this contains the process ID of the one
823 The content of this file is not sensitive; it can be world-readable.
841 OpenSSH is a derivative of the original and free
842 ssh 1.2.12 release by Tatu Ylonen.
843 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
844 Theo de Raadt and Dug Song
845 removed many bugs, re-added newer features and
847 Markus Friedl contributed the support for SSH
848 protocol versions 1.5 and 2.0.
849 Niels Provos and Markus Friedl contributed support
850 for privilege separation.
852 System security is not improved unless
857 are disabled (thus completely disabling