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 1.202 2004/08/26 16:00:55 markus 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 (SSH Daemon) is the daemon program for
61 Together these programs replace rlogin and rsh, and
62 provide secure encrypted communications between two untrusted hosts
63 over an insecure network.
64 The programs are intended to be as easy to
65 install and use as possible.
68 is the daemon that listens for connections from clients.
69 It is normally started at boot from
72 daemon for each incoming connection.
73 The forked daemons handle
74 key exchange, encryption, authentication, command execution,
76 This implementation of
78 supports both SSH protocol version 1 and 2 simultaneously.
81 .Ss SSH protocol version 1
82 Each host has a host-specific RSA key
83 (normally 1024 bits) used to identify the host.
85 the daemon starts, it generates a server RSA key (normally 768 bits).
86 This key is normally regenerated every hour if it has been used, and
87 is never stored on disk.
89 Whenever a client connects, the daemon responds with its public
91 The client compares the
92 RSA host key against its own database to verify that it has not changed.
93 The client then generates a 256-bit random number.
95 random number using both the host key and the server key, and sends
96 the encrypted number to the server.
97 Both sides then use this
98 random number as a session key which is used to encrypt all further
99 communications in the session.
100 The rest of the session is encrypted
101 using a conventional cipher, currently Blowfish or 3DES, with 3DES
102 being used by default.
103 The client selects the encryption algorithm
104 to use from those offered by the server.
106 Next, the server and the client enter an authentication dialog.
107 The client tries to authenticate itself using
109 authentication combined with RSA host
110 authentication, RSA challenge-response authentication, or password
111 based authentication.
113 Regardless of the authentication type, the account is checked to
114 ensure that it is accessible. An account is not accessible if it is
117 or its group is listed in
119 \&. The definition of a locked account is system dependant. Some platforms
120 have their own account database (eg AIX) and some modify the passwd field (
126 on Tru64 and a leading
128 on Linux). If there is a requirement to disable password authentication
129 for the account while allowing still public-key, then the passwd field
130 should be set to something other than these values (eg
140 are disabled (thus completely disabling
145 .Ss SSH protocol version 2
146 Version 2 works similarly:
147 Each host has a host-specific key (RSA or DSA) used to identify the host.
148 However, when the daemon starts, it does not generate a server key.
149 Forward security is provided through a Diffie-Hellman key agreement.
150 This key agreement results in a shared session key.
152 The rest of the session is encrypted using a symmetric cipher, currently
153 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
154 The client selects the encryption algorithm
155 to use from those offered by the server.
156 Additionally, session integrity is provided
157 through a cryptographic message authentication code
158 (hmac-sha1 or hmac-md5).
160 Protocol version 2 provides a public key based
161 user (PubkeyAuthentication) or
162 client host (HostbasedAuthentication) authentication method,
163 conventional password authentication and challenge response based methods.
164 .Ss Command execution and data forwarding
165 If the client successfully authenticates itself, a dialog for
166 preparing the session is entered.
167 At this time the client may request
168 things like allocating a pseudo-tty, forwarding X11 connections,
169 forwarding TCP/IP connections, or forwarding the authentication agent
170 connection over the secure channel.
172 Finally, the client either requests a shell or execution of a command.
173 The sides then enter session mode.
174 In this mode, either side may send
175 data at any time, and such data is forwarded to/from the shell or
176 command on the server side, and the user terminal in the client side.
178 When the user program terminates and all forwarded X11 and other
179 connections have been closed, the server sends command exit status to
180 the client, and both sides exit.
183 can be configured using command-line options or a configuration file
185 .Xr sshd_config 5 ) .
186 Command-line options override values specified in the
190 rereads its configuration file when it receives a hangup signal,
192 by executing itself with the name and options it was started with, e.g.,
195 The options are as follows:
200 to use IPv4 addresses only.
204 to use IPv6 addresses only.
206 Specifies the number of bits in the ephemeral protocol version 1
207 server key (default 768).
209 When this option is specified,
211 will not detach and does not become a daemon.
212 This allows easy monitoring of
216 The server sends verbose debug output to the system
217 log, and does not put itself in the background.
218 The server also will not fork and will only process one connection.
219 This option is only intended for debugging for the server.
222 options increase the debugging level.
225 When this option is specified,
227 will send the output to the standard error instead of the system log.
228 .It Fl f Ar configuration_file
229 Specifies the name of the configuration file.
231 .Pa /etc/ssh/sshd_config .
233 refuses to start if there is no configuration file.
234 .It Fl g Ar login_grace_time
235 Gives the grace time for clients to authenticate themselves (default
237 If the client fails to authenticate the user within
238 this many seconds, the server disconnects and exits.
239 A value of zero indicates no limit.
240 .It Fl h Ar host_key_file
241 Specifies a file from which a host key is read.
242 This option must be given if
244 is not run as root (as the normal
245 host key files are normally not readable by anyone but root).
247 .Pa /etc/ssh/ssh_host_key
248 for protocol version 1, and
249 .Pa /etc/ssh/ssh_host_rsa_key
251 .Pa /etc/ssh/ssh_host_dsa_key
252 for protocol version 2.
253 It is possible to have multiple host key files for
254 the different protocol versions and host key algorithms.
262 from inetd because it needs to generate the server key before it can
263 respond to the client, and this may take tens of seconds.
264 Clients would have to wait too long if the key was regenerated every time.
265 However, with small key sizes (e.g., 512) using
269 .It Fl k Ar key_gen_time
270 Specifies how often the ephemeral protocol version 1 server key is
271 regenerated (default 3600 seconds, or one hour).
272 The motivation for regenerating the key fairly
273 often is that the key is not stored anywhere, and after about an hour
274 it becomes impossible to recover the key for decrypting intercepted
275 communications even if the machine is cracked into or physically
277 A value of zero indicates that the key will never be regenerated.
279 Can be used to give options in the format used in the configuration file.
280 This is useful for specifying options for which there is no separate
282 For full details of the options, and their values, see
285 Specifies the port on which the server listens for connections
287 Multiple port options are permitted.
288 Ports specified in the configuration file are ignored when a
289 command-line port is specified.
292 Nothing is sent to the system log.
293 Normally the beginning,
294 authentication, and termination of each connection is logged.
297 Only check the validity of the configuration file and sanity of the keys.
298 This is useful for updating
300 reliably as configuration options may change.
302 This option is used to specify the size of the field
305 structure that holds the remote host name.
306 If the resolved host name is longer than
308 the dotted decimal value will be used instead.
309 This allows hosts with very long host names that
310 overflow this field to still be uniquely identified.
313 indicates that only dotted decimal addresses
314 should be put into the
318 may also be used to prevent
320 from making DNS requests unless the authentication
321 mechanism or configuration requires it.
322 Authentication mechanisms that may require DNS include
323 .Cm RhostsRSAAuthentication ,
324 .Cm HostbasedAuthentication
326 .Cm from="pattern-list"
327 option in a key file.
328 Configuration options that require DNS include using a
334 .Sh CONFIGURATION FILE
336 reads configuration data from
337 .Pa /etc/ssh/sshd_config
338 (or the file specified with
340 on the command line).
341 The file format and configuration options are described in
344 When a user successfully logs in,
347 .Bl -enum -offset indent
349 If the login is on a tty, and no command has been specified,
350 prints last login time and
352 (unless prevented in the configuration file or by
353 .Pa $HOME/.hushlogin ;
358 If the login is on a tty, records login time.
362 if it exists, prints contents and quits
365 Changes to run with normal user privileges.
367 Sets up basic environment.
370 .Pa $HOME/.ssh/environment ,
371 if it exists, and users are allowed to change their environment.
373 .Cm PermitUserEnvironment
377 Changes to user's home directory.
381 exists, runs it; else if
384 it; otherwise runs xauth.
387 files are given the X11
388 authentication protocol and cookie in standard input.
390 Runs user's shell or command.
392 .Sh AUTHORIZED_KEYS FILE FORMAT
393 .Pa $HOME/.ssh/authorized_keys
394 is the default file that lists the public keys that are
395 permitted for RSA authentication in protocol version 1
396 and for public key authentication (PubkeyAuthentication)
397 in protocol version 2.
398 .Cm AuthorizedKeysFile
399 may be used to specify an alternative file.
401 Each line of the file contains one
402 key (empty lines and lines starting with a
406 Each RSA public key consists of the following fields, separated by
407 spaces: options, bits, exponent, modulus, comment.
408 Each protocol version 2 public key consists of:
409 options, keytype, base64 encoded key, comment.
411 is optional; its presence is determined by whether the line starts
412 with a number or not (the options field never starts with a number).
413 The bits, exponent, modulus and comment fields give the RSA key for
414 protocol version 1; the
415 comment field is not used for anything (but may be convenient for the
416 user to identify the key).
417 For protocol version 2 the keytype is
422 Note that lines in this file are usually several hundred bytes long
423 (because of the size of the public key encoding).
424 You don't want to type them in; instead, copy the
432 enforces a minimum RSA key modulus size for protocol 1
433 and protocol 2 keys of 768 bits.
435 The options (if present) consist of comma-separated option
437 No spaces are permitted, except within double quotes.
438 The following option specifications are supported (note
439 that option keywords are case-insensitive):
441 .It Cm from="pattern-list"
442 Specifies that in addition to public key authentication, the canonical name
443 of the remote host must be present in the comma-separated list of
449 The list may also contain
450 patterns negated by prefixing them with
452 if the canonical host name matches a negated pattern, the key is not accepted.
454 of this option is to optionally increase security: public key authentication
455 by itself does not trust the network or name servers or anything (but
456 the key); however, if somebody somehow steals the key, the key
457 permits an intruder to log in from anywhere in the world.
458 This additional option makes using a stolen key more difficult (name
459 servers and/or routers would have to be compromised in addition to
461 .It Cm command="command"
462 Specifies that the command is executed whenever this key is used for
464 The command supplied by the user (if any) is ignored.
465 The command is run on a pty if the client requests a pty;
466 otherwise it is run without a tty.
467 If an 8-bit clean channel is required,
468 one must not request a pty or should specify
470 A quote may be included in the command by quoting it with a backslash.
471 This option might be useful
472 to restrict certain public keys to perform just a specific operation.
473 An example might be a key that permits remote backups but nothing else.
474 Note that the client may specify TCP/IP and/or X11
475 forwarding unless they are explicitly prohibited.
476 Note that this option applies to shell, command or subsystem execution.
477 .It Cm environment="NAME=value"
478 Specifies that the string is to be added to the environment when
479 logging in using this key.
480 Environment variables set this way
481 override other default environment values.
482 Multiple options of this type are permitted.
483 Environment processing is disabled by default and is
485 .Cm PermitUserEnvironment
487 This option is automatically disabled if
490 .It Cm no-port-forwarding
491 Forbids TCP/IP forwarding when this key is used for authentication.
492 Any port forward requests by the client will return an error.
493 This might be used, e.g., in connection with the
496 .It Cm no-X11-forwarding
497 Forbids X11 forwarding when this key is used for authentication.
498 Any X11 forward requests by the client will return an error.
499 .It Cm no-agent-forwarding
500 Forbids authentication agent forwarding when this key is used for
503 Prevents tty allocation (a request to allocate a pty will fail).
504 .It Cm permitopen="host:port"
507 port forwarding such that it may only connect to the specified host and
509 IPv6 addresses can be specified with an alternative syntax:
510 .Ar host Ns / Ns Ar port .
513 options may be applied separated by commas.
514 No pattern matching is performed on the specified hostnames,
515 they must be literal domains or addresses.
518 1024 33 12121...312314325 ylo@foo.bar
520 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
522 command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
524 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
525 .Sh SSH_KNOWN_HOSTS FILE FORMAT
527 .Pa /etc/ssh/ssh_known_hosts
529 .Pa $HOME/.ssh/known_hosts
530 files contain host public keys for all known hosts.
531 The global file should
532 be prepared by the administrator (optional), and the per-user file is
533 maintained automatically: whenever the user connects from an unknown host
534 its key is added to the per-user file.
536 Each line in these files contains the following fields: hostnames,
537 bits, exponent, modulus, comment.
538 The fields are separated by spaces.
540 Hostnames is a comma-separated list of patterns
545 wildcards); each pattern in turn is matched against the canonical host
546 name (when authenticating a client) or against the user-supplied
547 name (when authenticating a server).
548 A pattern may also be preceded by
550 to indicate negation: if the host name matches a negated
551 pattern, it is not accepted (by that line) even if it matched another
554 Bits, exponent, and modulus are taken directly from the RSA host key; they
555 can be obtained, e.g., from
556 .Pa /etc/ssh/ssh_host_key.pub .
557 The optional comment field continues to the end of the line, and is not used.
561 and empty lines are ignored as comments.
563 When performing host authentication, authentication is accepted if any
564 matching line has the proper key.
565 It is thus permissible (but not
566 recommended) to have several lines or different host keys for the same
568 This will inevitably happen when short forms of host names
569 from different domains are put in the file.
571 that the files contain conflicting information; authentication is
572 accepted if valid information can be found from either file.
574 Note that the lines in these files are typically hundreds of characters
575 long, and you definitely don't want to type in the host keys by hand.
576 Rather, generate them by a script
578 .Pa /etc/ssh/ssh_host_key.pub
579 and adding the host names at the front.
582 closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
583 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
587 .It Pa /etc/ssh/sshd_config
588 Contains configuration data for
590 The file format and configuration options are described in
592 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
593 These three files contain the private parts of the host keys.
594 These files should only be owned by root, readable only by root, and not
595 accessible to others.
598 does not start if this file is group/world-accessible.
599 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
600 These three files contain the public parts of the host keys.
601 These files should be world-readable but writable only by
603 Their contents should match the respective private parts.
605 really used for anything; they are provided for the convenience of
606 the user so their contents can be copied to known hosts files.
607 These files are created using
610 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
611 The file format is described in
617 during privilege separation in the pre-authentication phase.
618 The directory should not contain any files and must be owned by root
619 and not group or world-writable.
620 .It Pa /var/run/sshd.pid
621 Contains the process ID of the
623 listening for connections (if there are several daemons running
624 concurrently for different ports, this contains the process ID of the one
626 The content of this file is not sensitive; it can be world-readable.
627 .It Pa $HOME/.ssh/authorized_keys
628 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
629 This file must be readable by root (which may on some machines imply
630 it being world-readable if the user's home directory resides on an NFS
632 It is recommended that it not be accessible by others.
633 The format of this file is described above.
634 Users will place the contents of their
639 files into this file, as described in
641 .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
642 These files are consulted when using rhosts with RSA host
643 authentication or protocol version 2 hostbased authentication
644 to check the public key of the host.
645 The key must be listed in one of these files to be accepted.
646 The client uses the same files
647 to verify that it is connecting to the correct remote host.
648 These files should be writable only by root/the owner.
649 .Pa /etc/ssh/ssh_known_hosts
650 should be world-readable, and
651 .Pa $HOME/.ssh/known_hosts
652 can, but need not be, world-readable.
656 refuses to let anyone except root log in.
657 The contents of the file
658 are displayed to anyone trying to log in, and non-root connections are
660 The file should be world-readable.
661 .It Pa /etc/hosts.allow, /etc/hosts.deny
662 Access controls that should be enforced by tcp-wrappers are defined here.
663 Further details are described in
666 This file is used during
667 .Cm RhostsRSAAuthentication
669 .Cm HostbasedAuthentication
670 and contains host-username pairs, separated by a space, one per
672 The given user on the corresponding host is permitted to log in
674 The same file is used by rlogind and rshd.
676 be writable only by the user; it is recommended that it not be
677 accessible by others.
679 It is also possible to use netgroups in the file.
681 name may be of the form +@groupname to specify all hosts or all users
685 this file is exactly the same as for
687 However, this file is
688 not used by rlogin and rshd, so using this permits access using SSH only.
689 .It Pa /etc/hosts.equiv
690 This file is used during
691 .Cm RhostsRSAAuthentication
693 .Cm HostbasedAuthentication
695 In the simplest form, this file contains host names, one per line.
697 those hosts are permitted to log in without a password, provided they
698 have the same user name on both machines.
699 The host name may also be
700 followed by a user name; such users are permitted to log in as
702 user on this machine (except root).
703 Additionally, the syntax
705 can be used to specify netgroups.
706 Negated entries start with
709 If the client host/user is successfully matched in this file, login is
710 automatically permitted provided the client and server user names are the
712 Additionally, successful client host key authentication is required.
713 This file must be writable only by root; it is recommended
714 that it be world-readable.
716 .Sy "Warning: It is almost never a good idea to use user names in"
718 Beware that it really means that the named user(s) can log in as
720 which includes bin, daemon, adm, and other accounts that own critical
721 binaries and directories.
722 Using a user name practically grants the user root access.
723 The only valid use for user names that I can think
724 of is in negative entries.
726 Note that this warning also applies to rsh/rlogin.
727 .It Pa /etc/shosts.equiv
728 This is processed exactly as
729 .Pa /etc/hosts.equiv .
730 However, this file may be useful in environments that want to run both
732 .It Pa $HOME/.ssh/environment
733 This file is read into the environment at login (if it exists).
734 It can only contain empty lines, comment lines (that start with
736 and assignment lines of the form name=value.
737 The file should be writable
738 only by the user; it need not be readable by anyone else.
739 Environment processing is disabled by default and is
741 .Cm PermitUserEnvironment
744 If this file exists, it is run with
747 environment files but before starting the user's shell or command.
748 It must not produce any output on stdout; stderr must be used
750 If X11 forwarding is in use, it will receive the "proto cookie" pair in
751 its standard input (and
758 will not run xauth automatically to add X11 cookies.
760 The primary purpose of this file is to run any initialization routines
761 which may be needed before the user's home directory becomes
762 accessible; AFS is a particular example of such an environment.
764 This file will probably contain some initialization code followed by
765 something similar to:
767 if read proto cookie && [ -n "$DISPLAY" ]; then
768 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
769 # X11UseLocalhost=yes
770 echo add unix:`echo $DISPLAY |
771 cut -c11-` $proto $cookie
774 echo add $DISPLAY $proto $cookie
779 If this file does not exist,
782 does not exist either, xauth is used to add the cookie.
784 This file should be writable only by the user, and need not be
785 readable by anyone else.
786 .It Pa /etc/ssh/sshrc
789 This can be used to specify
790 machine-specific login-time initializations globally.
791 This file should be writable only by root, and should be world-readable.
813 .%T "SSH Protocol Architecture"
814 .%N draft-ietf-secsh-architecture-12.txt
816 .%O work in progress material
822 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
823 .%N draft-ietf-secsh-dh-group-exchange-02.txt
825 .%O work in progress material
828 OpenSSH is a derivative of the original and free
829 ssh 1.2.12 release by Tatu Ylonen.
830 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
831 Theo de Raadt and Dug Song
832 removed many bugs, re-added newer features and
834 Markus Friedl contributed the support for SSH
835 protocol versions 1.5 and 2.0.
836 Niels Provos and Markus Friedl contributed support
837 for privilege separation.