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 1.234 2006/08/21 08:15:57 dtucker Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH daemon
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 rlogin and rsh, and
63 provide secure encrypted communications between two untrusted hosts
64 over an insecure network.
67 listens for connections from clients.
68 It is normally started at boot from
71 daemon for each incoming connection.
72 The forked daemons handle
73 key exchange, encryption, authentication, command execution,
77 can be configured using command-line options or a configuration file
80 command-line options override values specified in the
83 rereads its configuration file when it receives a hangup signal,
85 by executing itself with the name and options it was started with, e.g.\&
88 The options are as follows:
93 to use IPv4 addresses only.
97 to use IPv6 addresses only.
99 Specifies the number of bits in the ephemeral protocol version 1
100 server key (default 768).
102 When this option is specified,
104 will not detach and does not become a daemon.
105 This allows easy monitoring of
109 The server sends verbose debug output to the system
110 log, and does not put itself in the background.
111 The server also will not fork and will only process one connection.
112 This option is only intended for debugging for the server.
115 options increase the debugging level.
118 When this option is specified,
120 will send the output to the standard error instead of the system log.
121 .It Fl f Ar configuration_file
122 Specifies the name of the configuration file.
124 .Pa /etc/ssh/sshd_config .
126 refuses to start if there is no configuration file.
127 .It Fl g Ar login_grace_time
128 Gives the grace time for clients to authenticate themselves (default
130 If the client fails to authenticate the user within
131 this many seconds, the server disconnects and exits.
132 A value of zero indicates no limit.
133 .It Fl h Ar host_key_file
134 Specifies a file from which a host key is read.
135 This option must be given if
137 is not run as root (as the normal
138 host key files are normally not readable by anyone but root).
140 .Pa /etc/ssh/ssh_host_key
141 for protocol version 1, and
142 .Pa /etc/ssh/ssh_host_dsa_key
143 for protocol version 2.
144 It is possible to have multiple host key files for
145 the different protocol versions and host key algorithms.
153 from inetd because it needs to generate the server key before it can
154 respond to the client, and this may take tens of seconds.
155 Clients would have to wait too long if the key was regenerated every time.
156 However, with small key sizes (e.g. 512) using
160 .It Fl k Ar key_gen_time
161 Specifies how often the ephemeral protocol version 1 server key is
162 regenerated (default 3600 seconds, or one hour).
163 The motivation for regenerating the key fairly
164 often is that the key is not stored anywhere, and after about an hour
165 it becomes impossible to recover the key for decrypting intercepted
166 communications even if the machine is cracked into or physically
168 A value of zero indicates that the key will never be regenerated.
170 Can be used to give options in the format used in the configuration file.
171 This is useful for specifying options for which there is no separate
173 For full details of the options, and their values, see
176 Specifies the port on which the server listens for connections
178 Multiple port options are permitted.
179 Ports specified in the configuration file with the
181 option are ignored when a command-line port is specified.
182 Ports specified using the
184 option override command-line ports.
187 Nothing is sent to the system log.
188 Normally the beginning,
189 authentication, and termination of each connection is logged.
192 Only check the validity of the configuration file and sanity of the keys.
193 This is useful for updating
195 reliably as configuration options may change.
197 This option is used to specify the size of the field
200 structure that holds the remote host name.
201 If the resolved host name is longer than
203 the dotted decimal value will be used instead.
204 This allows hosts with very long host names that
205 overflow this field to still be uniquely identified.
208 indicates that only dotted decimal addresses
209 should be put into the
213 may also be used to prevent
215 from making DNS requests unless the authentication
216 mechanism or configuration requires it.
217 Authentication mechanisms that may require DNS include
218 .Cm RhostsRSAAuthentication ,
219 .Cm HostbasedAuthentication ,
221 .Cm from="pattern-list"
222 option in a key file.
223 Configuration options that require DNS include using a
230 The OpenSSH SSH daemon supports SSH protocols 1 and 2.
231 Both protocols are supported by default,
232 though this can be changed via the
236 Protocol 2 supports both RSA and DSA keys;
237 protocol 1 only supports RSA keys.
239 each host has a host-specific key,
241 used to identify the host.
243 Forward security for protocol 1 is provided through
244 an additional server key,
246 generated when the server starts.
247 This key is normally regenerated every hour if it has been used, and
248 is never stored on disk.
249 Whenever a client connects, the daemon responds with its public
250 host and server keys.
251 The client compares the
252 RSA host key against its own database to verify that it has not changed.
253 The client then generates a 256-bit random number.
255 random number using both the host key and the server key, and sends
256 the encrypted number to the server.
257 Both sides then use this
258 random number as a session key which is used to encrypt all further
259 communications in the session.
260 The rest of the session is encrypted
261 using a conventional cipher, currently Blowfish or 3DES, with 3DES
262 being used by default.
263 The client selects the encryption algorithm
264 to use from those offered by the server.
267 forward security is provided through a Diffie-Hellman key agreement.
268 This key agreement results in a shared session key.
269 The rest of the session is encrypted using a symmetric cipher, currently
270 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
271 The client selects the encryption algorithm
272 to use from those offered by the server.
273 Additionally, session integrity is provided
274 through a cryptographic message authentication code
275 (hmac-sha1 or hmac-md5).
277 Finally, the server and the client enter an authentication dialog.
278 The client tries to authenticate itself using
279 host-based authentication,
280 public key authentication,
281 challenge-response authentication,
282 or password authentication.
284 Regardless of the authentication type, the account is checked to
285 ensure that it is accessible. An account is not accessible if it is
288 or its group is listed in
290 \&. The definition of a locked account is system dependant. Some platforms
291 have their own account database (eg AIX) and some modify the passwd field (
293 on Solaris and UnixWare,
300 on FreeBSD and a leading
302 on Linux). If there is a requirement to disable password authentication
303 for the account while allowing still public-key, then the passwd field
304 should be set to something other than these values (eg
310 If the client successfully authenticates itself, a dialog for
311 preparing the session is entered.
312 At this time the client may request
313 things like allocating a pseudo-tty, forwarding X11 connections,
314 forwarding TCP connections, or forwarding the authentication agent
315 connection over the secure channel.
317 After this, the client either requests a shell or execution of a command.
318 The sides then enter session mode.
319 In this mode, either side may send
320 data at any time, and such data is forwarded to/from the shell or
321 command on the server side, and the user terminal in the client side.
323 When the user program terminates and all forwarded X11 and other
324 connections have been closed, the server sends command exit status to
325 the client, and both sides exit.
327 When a user successfully logs in,
330 .Bl -enum -offset indent
332 If the login is on a tty, and no command has been specified,
333 prints last login time and
335 (unless prevented in the configuration file or by
341 If the login is on a tty, records login time.
345 .Pa /var/run/nologin ;
346 if one exists, it prints the contents and quits
349 Changes to run with normal user privileges.
351 Sets up basic environment.
354 .Pa ~/.ssh/environment ,
355 if it exists, and users are allowed to change their environment.
357 .Cm PermitUserEnvironment
361 Changes to user's home directory.
365 exists, runs it; else if
372 files are given the X11
373 authentication protocol and cookie in standard input.
378 Runs user's shell or command.
385 runs it after reading the
386 environment files but before starting the user's shell or command.
387 It must not produce any output on stdout; stderr must be used
389 If X11 forwarding is in use, it will receive the "proto cookie" pair in
390 its standard input (and
397 will not run xauth automatically to add X11 cookies.
399 The primary purpose of this file is to run any initialization routines
400 which may be needed before the user's home directory becomes
401 accessible; AFS is a particular example of such an environment.
403 This file will probably contain some initialization code followed by
404 something similar to:
405 .Bd -literal -offset 3n
406 if read proto cookie && [ -n "$DISPLAY" ]; then
407 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
408 # X11UseLocalhost=yes
409 echo add unix:`echo $DISPLAY |
410 cut -c11-` $proto $cookie
413 echo add $DISPLAY $proto $cookie
418 If this file does not exist,
421 does not exist either, xauth is used to add the cookie.
422 .Sh AUTHORIZED_KEYS FILE FORMAT
423 .Cm AuthorizedKeysFile
424 specifies the file containing public keys for
425 public key authentication;
426 if none is specified, the default is
427 .Pa ~/.ssh/authorized_keys .
428 Each line of the file contains one
429 key (empty lines and lines starting with a
433 Protocol 1 public keys consist of the following space-separated fields:
434 options, bits, exponent, modulus, comment.
435 Protocol 2 public key consist of:
436 options, keytype, base64-encoded key, comment.
437 The options field is optional;
438 its presence is determined by whether the line starts
439 with a number or not (the options field never starts with a number).
440 The bits, exponent, modulus, and comment fields give the RSA key for
441 protocol version 1; the
442 comment field is not used for anything (but may be convenient for the
443 user to identify the key).
444 For protocol version 2 the keytype is
449 Note that lines in this file are usually several hundred bytes long
450 (because of the size of the public key encoding) up to a limit of
451 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
452 keys up to 16 kilobits.
453 You don't want to type them in; instead, copy the
461 enforces a minimum RSA key modulus size for protocol 1
462 and protocol 2 keys of 768 bits.
464 The options (if present) consist of comma-separated option
466 No spaces are permitted, except within double quotes.
467 The following option specifications are supported (note
468 that option keywords are case-insensitive):
470 .It Cm command="command"
471 Specifies that the command is executed whenever this key is used for
473 The command supplied by the user (if any) is ignored.
474 The command is run on a pty if the client requests a pty;
475 otherwise it is run without a tty.
476 If an 8-bit clean channel is required,
477 one must not request a pty or should specify
479 A quote may be included in the command by quoting it with a backslash.
480 This option might be useful
481 to restrict certain public keys to perform just a specific operation.
482 An example might be a key that permits remote backups but nothing else.
483 Note that the client may specify TCP and/or X11
484 forwarding unless they are explicitly prohibited.
485 The command originally supplied by the client is available in the
486 .Ev SSH_ORIGINAL_COMMAND
487 environment variable.
488 Note that this option applies to shell, command or subsystem execution.
489 .It Cm environment="NAME=value"
490 Specifies that the string is to be added to the environment when
491 logging in using this key.
492 Environment variables set this way
493 override other default environment values.
494 Multiple options of this type are permitted.
495 Environment processing is disabled by default and is
497 .Cm PermitUserEnvironment
499 This option is automatically disabled if
502 .It Cm from="pattern-list"
503 Specifies that in addition to public key authentication, the canonical name
504 of the remote host must be present in the comma-separated list of
507 of this option is to optionally increase security: public key authentication
508 by itself does not trust the network or name servers or anything (but
509 the key); however, if somebody somehow steals the key, the key
510 permits an intruder to log in from anywhere in the world.
511 This additional option makes using a stolen key more difficult (name
512 servers and/or routers would have to be compromised in addition to
519 for more information on patterns.
520 .It Cm no-agent-forwarding
521 Forbids authentication agent forwarding when this key is used for
523 .It Cm no-port-forwarding
524 Forbids TCP forwarding when this key is used for authentication.
525 Any port forward requests by the client will return an error.
526 This might be used, e.g. in connection with the
530 Prevents tty allocation (a request to allocate a pty will fail).
531 .It Cm no-X11-forwarding
532 Forbids X11 forwarding when this key is used for authentication.
533 Any X11 forward requests by the client will return an error.
534 .It Cm permitopen="host:port"
537 port forwarding such that it may only connect to the specified host and
539 IPv6 addresses can be specified with an alternative syntax:
540 .Ar host Ns / Ns Ar port .
543 options may be applied separated by commas.
544 No pattern matching is performed on the specified hostnames,
545 they must be literal domains or addresses.
549 device on the server.
550 Without this option, the next available device will be used if
551 the client requests a tunnel.
554 An example authorized_keys file:
555 .Bd -literal -offset 3n
556 # Comments allowed at start of line
557 ssh-rsa AAAAB3Nza...LiPk== user@example.net
558 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
559 AAAAB2...19Q== john@example.net
560 command="dump /home",no-pty,no-port-forwarding ssh-dss
561 AAAAC3...51R== example.net
562 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
564 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
567 .Sh SSH_KNOWN_HOSTS FILE FORMAT
569 .Pa /etc/ssh/ssh_known_hosts
571 .Pa ~/.ssh/known_hosts
572 files contain host public keys for all known hosts.
573 The global file should
574 be prepared by the administrator (optional), and the per-user file is
575 maintained automatically: whenever the user connects from an unknown host,
576 its key is added to the per-user file.
578 Each line in these files contains the following fields: hostnames,
579 bits, exponent, modulus, comment.
580 The fields are separated by spaces.
582 Hostnames is a comma-separated list of patterns
587 wildcards); each pattern in turn is matched against the canonical host
588 name (when authenticating a client) or against the user-supplied
589 name (when authenticating a server).
590 A pattern may also be preceded by
592 to indicate negation: if the host name matches a negated
593 pattern, it is not accepted (by that line) even if it matched another
595 A hostname or address may optionally be enclosed within
599 brackets then followed by
601 and a non-standard port number.
603 Alternately, hostnames may be stored in a hashed form which hides host names
604 and addresses should the file's contents be disclosed.
605 Hashed hostnames start with a
608 Only one hashed hostname may appear on a single line and none of the above
609 negation or wildcard operators may be applied.
611 Bits, exponent, and modulus are taken directly from the RSA host key; they
612 can be obtained, for example, from
613 .Pa /etc/ssh/ssh_host_key.pub .
614 The optional comment field continues to the end of the line, and is not used.
618 and empty lines are ignored as comments.
620 When performing host authentication, authentication is accepted if any
621 matching line has the proper key.
622 It is thus permissible (but not
623 recommended) to have several lines or different host keys for the same
625 This will inevitably happen when short forms of host names
626 from different domains are put in the file.
628 that the files contain conflicting information; authentication is
629 accepted if valid information can be found from either file.
631 Note that the lines in these files are typically hundreds of characters
632 long, and you definitely don't want to type in the host keys by hand.
633 Rather, generate them by a script
635 .Pa /etc/ssh/ssh_host_key.pub
636 and adding the host names at the front.
638 An example ssh_known_hosts file:
639 .Bd -literal -offset 3n
640 # Comments allowed at start of line
641 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
642 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
644 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
648 .Bl -tag -width Ds -compact
650 This file is used to suppress printing the last login time and
658 It does not suppress printing of the banner specified by
662 This file is used for host-based authentication (see
664 for more information).
665 On some machines this file may need to be
666 world-readable if the user's home directory is on an NFS partition,
670 Additionally, this file must be owned by the user,
671 and must not have write permissions for anyone else.
673 permission for most machines is read/write for the user, and not
674 accessible by others.
677 This file is used in exactly the same way as
679 but allows host-based authentication without permitting login with
682 .It ~/.ssh/authorized_keys
683 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
684 The format of this file is described above.
685 The content of the file is not highly sensitive, but the recommended
686 permissions are read/write for the user, and not accessible by others.
690 directory, or the user's home directory are writable
691 by other users, then the file could be modified or replaced by unauthorized
695 will not allow it to be used unless the
697 option has been set to
699 The recommended permissions can be set by executing
700 .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
702 .It ~/.ssh/environment
703 This file is read into the environment at login (if it exists).
704 It can only contain empty lines, comment lines (that start with
706 and assignment lines of the form name=value.
707 The file should be writable
708 only by the user; it need not be readable by anyone else.
709 Environment processing is disabled by default and is
711 .Cm PermitUserEnvironment
714 .It ~/.ssh/known_hosts
715 Contains a list of host keys for all hosts the user has logged into
716 that are not already in the systemwide list of known host keys.
717 The format of this file is described above.
718 This file should be writable only by root/the owner and
719 can, but need not be, world-readable.
722 Contains initialization routines to be run before
723 the user's home directory becomes accessible.
724 This file should be writable only by the user, and need not be
725 readable by anyone else.
729 Access controls that should be enforced by tcp-wrappers are defined here.
730 Further details are described in
734 This file is for host-based authentication (see
736 It should only be writable by root.
739 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
740 The file format is described in
750 refuses to let anyone except root log in.
751 The contents of the file
752 are displayed to anyone trying to log in, and non-root connections are
754 The file should be world-readable.
756 .It /etc/shosts.equiv
757 This file is used in exactly the same way as
759 but allows host-based authentication without permitting login with
762 .It /etc/ssh/ssh_known_hosts
763 Systemwide list of known host keys.
764 This file should be prepared by the
765 system administrator to contain the public host keys of all machines in the
767 The format of this file is described above.
768 This file should be writable only by root/the owner and
769 should be world-readable.
771 .It /etc/ssh/ssh_host_key
772 .It /etc/ssh/ssh_host_dsa_key
773 .It /etc/ssh/ssh_host_rsa_key
774 These three files contain the private parts of the host keys.
775 These files should only be owned by root, readable only by root, and not
776 accessible to others.
779 does not start if these files are group/world-accessible.
781 .It /etc/ssh/ssh_host_key.pub
782 .It /etc/ssh/ssh_host_dsa_key.pub
783 .It /etc/ssh/ssh_host_rsa_key.pub
784 These three files contain the public parts of the host keys.
785 These files should be world-readable but writable only by
787 Their contents should match the respective private parts.
789 really used for anything; they are provided for the convenience of
790 the user so their contents can be copied to known hosts files.
791 These files are created using
794 .It /etc/ssh/sshd_config
795 Contains configuration data for
797 The file format and configuration options are described in
803 it can be used to specify
804 machine-specific login-time initializations globally.
805 This file should be writable only by root, and should be world-readable.
811 during privilege separation in the pre-authentication phase.
812 The directory should not contain any files and must be owned by root
813 and not group or world-writable.
815 .It /var/run/sshd.pid
816 Contains the process ID of the
818 listening for connections (if there are several daemons running
819 concurrently for different ports, this contains the process ID of the one
821 The content of this file is not sensitive; it can be world-readable.
838 OpenSSH is a derivative of the original and free
839 ssh 1.2.12 release by Tatu Ylonen.
840 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
841 Theo de Raadt and Dug Song
842 removed many bugs, re-added newer features and
844 Markus Friedl contributed the support for SSH
845 protocol versions 1.5 and 2.0.
846 Niels Provos and Markus Friedl contributed support
847 for privilege separation.
849 System security is not improved unless
854 are disabled (thus completely disabling