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 1.246 2008/07/02 02:24:18 djm Exp $
39 .Dd $Mdocdate: July 2 2008 $
44 .Nd OpenSSH SSH daemon
50 .Op Fl C Ar connection_spec
51 .Op Fl f Ar config_file
52 .Op Fl g Ar login_grace_time
53 .Op Fl h Ar host_key_file
54 .Op Fl k Ar key_gen_time
61 (OpenSSH Daemon) is the daemon program for
63 Together these programs replace
67 and provide secure encrypted communications between two untrusted hosts
68 over an insecure network.
71 listens for connections from clients.
72 It is normally started at boot from
75 daemon for each incoming connection.
76 The forked daemons handle
77 key exchange, encryption, authentication, command execution,
81 can be configured using command-line options or a configuration file
84 command-line options override values specified in the
87 rereads its configuration file when it receives a hangup signal,
89 by executing itself with the name and options it was started with, e.g.\&
92 The options are as follows:
97 to use IPv4 addresses only.
101 to use IPv6 addresses only.
103 Specifies the number of bits in the ephemeral protocol version 1
104 server key (default 1024).
105 .It Fl C Ar connection_spec
106 Specify the connection parameters to use for the
111 directives in the configuration file
112 that would apply to the specified user, host, and address will be set before
113 the configuration is written to standard output.
114 The connection parameters are supplied as keyword=value pairs.
120 All are required and may be supplied in any order, either with multiple
122 options or as a comma-separated list.
124 When this option is specified,
126 will not detach and does not become a daemon.
127 This allows easy monitoring of
131 The server sends verbose debug output to the system
132 log, and does not put itself in the background.
133 The server also will not fork and will only process one connection.
134 This option is only intended for debugging for the server.
137 options increase the debugging level.
140 When this option is specified,
142 will send the output to the standard error instead of the system log.
143 .It Fl f Ar config_file
144 Specifies the name of the configuration file.
146 .Pa /etc/ssh/sshd_config .
148 refuses to start if there is no configuration file.
149 .It Fl g Ar login_grace_time
150 Gives the grace time for clients to authenticate themselves (default
152 If the client fails to authenticate the user within
153 this many seconds, the server disconnects and exits.
154 A value of zero indicates no limit.
155 .It Fl h Ar host_key_file
156 Specifies a file from which a host key is read.
157 This option must be given if
159 is not run as root (as the normal
160 host key files are normally not readable by anyone but root).
162 .Pa /etc/ssh/ssh_host_key
163 for protocol version 1, and
164 .Pa /etc/ssh/ssh_host_rsa_key
166 .Pa /etc/ssh/ssh_host_dsa_key
167 for protocol version 2.
168 It is possible to have multiple host key files for
169 the different protocol versions and host key algorithms.
177 from inetd because it needs to generate the server key before it can
178 respond to the client, and this may take tens of seconds.
179 Clients would have to wait too long if the key was regenerated every time.
180 However, with small key sizes (e.g. 512) using
184 .It Fl k Ar key_gen_time
185 Specifies how often the ephemeral protocol version 1 server key is
186 regenerated (default 3600 seconds, or one hour).
187 The motivation for regenerating the key fairly
188 often is that the key is not stored anywhere, and after about an hour
189 it becomes impossible to recover the key for decrypting intercepted
190 communications even if the machine is cracked into or physically
192 A value of zero indicates that the key will never be regenerated.
194 Can be used to give options in the format used in the configuration file.
195 This is useful for specifying options for which there is no separate
197 For full details of the options, and their values, see
200 Specifies the port on which the server listens for connections
202 Multiple port options are permitted.
203 Ports specified in the configuration file with the
205 option are ignored when a command-line port is specified.
206 Ports specified using the
208 option override command-line ports.
211 Nothing is sent to the system log.
212 Normally the beginning,
213 authentication, and termination of each connection is logged.
216 Check the validity of the configuration file, output the effective configuration
217 to stdout and then exit.
220 rules may be applied by specifying the connection parameters using one or more
225 Only check the validity of the configuration file and sanity of the keys.
226 This is useful for updating
228 reliably as configuration options may change.
230 This option is used to specify the size of the field
233 structure that holds the remote host name.
234 If the resolved host name is longer than
236 the dotted decimal value will be used instead.
237 This allows hosts with very long host names that
238 overflow this field to still be uniquely identified.
241 indicates that only dotted decimal addresses
242 should be put into the
246 may also be used to prevent
248 from making DNS requests unless the authentication
249 mechanism or configuration requires it.
250 Authentication mechanisms that may require DNS include
251 .Cm RhostsRSAAuthentication ,
252 .Cm HostbasedAuthentication ,
254 .Cm from="pattern-list"
255 option in a key file.
256 Configuration options that require DNS include using a
263 The OpenSSH SSH daemon supports SSH protocols 1 and 2.
264 Both protocols are supported by default,
265 though this can be changed via the
269 Protocol 2 supports both RSA and DSA keys;
270 protocol 1 only supports RSA keys.
272 each host has a host-specific key,
274 used to identify the host.
276 Forward security for protocol 1 is provided through
277 an additional server key,
279 generated when the server starts.
280 This key is normally regenerated every hour if it has been used, and
281 is never stored on disk.
282 Whenever a client connects, the daemon responds with its public
283 host and server keys.
284 The client compares the
285 RSA host key against its own database to verify that it has not changed.
286 The client then generates a 256-bit random number.
288 random number using both the host key and the server key, and sends
289 the encrypted number to the server.
290 Both sides then use this
291 random number as a session key which is used to encrypt all further
292 communications in the session.
293 The rest of the session is encrypted
294 using a conventional cipher, currently Blowfish or 3DES, with 3DES
295 being used by default.
296 The client selects the encryption algorithm
297 to use from those offered by the server.
300 forward security is provided through a Diffie-Hellman key agreement.
301 This key agreement results in a shared session key.
302 The rest of the session is encrypted using a symmetric cipher, currently
303 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
304 The client selects the encryption algorithm
305 to use from those offered by the server.
306 Additionally, session integrity is provided
307 through a cryptographic message authentication code
308 (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
310 Finally, the server and the client enter an authentication dialog.
311 The client tries to authenticate itself using
312 host-based authentication,
313 public key authentication,
314 challenge-response authentication,
315 or password authentication.
317 Regardless of the authentication type, the account is checked to
318 ensure that it is accessible. An account is not accessible if it is
321 or its group is listed in
323 \&. The definition of a locked account is system dependant. Some platforms
324 have their own account database (eg AIX) and some modify the passwd field (
326 on Solaris and UnixWare,
333 on FreeBSD and a leading
336 If there is a requirement to disable password authentication
337 for the account while allowing still public-key, then the passwd field
338 should be set to something other than these values (eg
344 If the client successfully authenticates itself, a dialog for
345 preparing the session is entered.
346 At this time the client may request
347 things like allocating a pseudo-tty, forwarding X11 connections,
348 forwarding TCP connections, or forwarding the authentication agent
349 connection over the secure channel.
351 After this, the client either requests a shell or execution of a command.
352 The sides then enter session mode.
353 In this mode, either side may send
354 data at any time, and such data is forwarded to/from the shell or
355 command on the server side, and the user terminal in the client side.
357 When the user program terminates and all forwarded X11 and other
358 connections have been closed, the server sends command exit status to
359 the client, and both sides exit.
361 When a user successfully logs in,
364 .Bl -enum -offset indent
366 If the login is on a tty, and no command has been specified,
367 prints last login time and
369 (unless prevented in the configuration file or by
375 If the login is on a tty, records login time.
379 .Pa /var/run/nologin ;
380 if one exists, it prints the contents and quits
383 Changes to run with normal user privileges.
385 Sets up basic environment.
388 .Pa ~/.ssh/environment ,
389 if it exists, and users are allowed to change their environment.
391 .Cm PermitUserEnvironment
395 Changes to user's home directory.
399 exists, runs it; else if
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 addressess 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
747 The recommended permissions can be set by executing
748 .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
750 .It ~/.ssh/environment
751 This file is read into the environment at login (if it exists).
752 It can only contain empty lines, comment lines (that start with
754 and assignment lines of the form name=value.
755 The file should be writable
756 only by the user; it need not be readable by anyone else.
757 Environment processing is disabled by default and is
759 .Cm PermitUserEnvironment
762 .It ~/.ssh/known_hosts
763 Contains a list of host keys for all hosts the user has logged into
764 that are not already in the systemwide list of known host keys.
765 The format of this file is described above.
766 This file should be writable only by root/the owner and
767 can, but need not be, world-readable.
770 Contains initialization routines to be run before
771 the user's home directory becomes accessible.
772 This file should be writable only by the user, and need not be
773 readable by anyone else.
777 Access controls that should be enforced by tcp-wrappers are defined here.
778 Further details are described in
782 This file is for host-based authentication (see
784 It should only be writable by root.
787 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
788 The file format is described in
798 refuses to let anyone except root log in.
799 The contents of the file
800 are displayed to anyone trying to log in, and non-root connections are
802 The file should be world-readable.
804 .It /etc/shosts.equiv
805 This file is used in exactly the same way as
807 but allows host-based authentication without permitting login with
810 .It /etc/ssh/ssh_host_key
811 .It /etc/ssh/ssh_host_dsa_key
812 .It /etc/ssh/ssh_host_rsa_key
813 These three files contain the private parts of the host keys.
814 These files should only be owned by root, readable only by root, and not
815 accessible to others.
818 does not start if these files are group/world-accessible.
820 .It /etc/ssh/ssh_host_key.pub
821 .It /etc/ssh/ssh_host_dsa_key.pub
822 .It /etc/ssh/ssh_host_rsa_key.pub
823 These three files contain the public parts of the host keys.
824 These files should be world-readable but writable only by
826 Their contents should match the respective private parts.
828 really used for anything; they are provided for the convenience of
829 the user so their contents can be copied to known hosts files.
830 These files are created using
833 .It /etc/ssh/ssh_known_hosts
834 Systemwide list of known host keys.
835 This file should be prepared by the
836 system administrator to contain the public host keys of all machines in the
838 The format of this file is described above.
839 This file should be writable only by root/the owner and
840 should be world-readable.
842 .It /etc/ssh/sshd_config
843 Contains configuration data for
845 The file format and configuration options are described in
851 it can be used to specify
852 machine-specific login-time initializations globally.
853 This file should be writable only by root, and should be world-readable.
859 during privilege separation in the pre-authentication phase.
860 The directory should not contain any files and must be owned by root
861 and not group or world-writable.
863 .It /var/run/sshd.pid
864 Contains the process ID of the
866 listening for connections (if there are several daemons running
867 concurrently for different ports, this contains the process ID of the one
869 The content of this file is not sensitive; it can be world-readable.
887 OpenSSH is a derivative of the original and free
888 ssh 1.2.12 release by Tatu Ylonen.
889 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
890 Theo de Raadt and Dug Song
891 removed many bugs, re-added newer features and
893 Markus Friedl contributed the support for SSH
894 protocol versions 1.5 and 2.0.
895 Niels Provos and Markus Friedl contributed support
896 for privilege separation.
898 System security is not improved unless
903 are disabled (thus completely disabling