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 1.199 2003/08/13 08:46:31 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.
82 .Ss SSH protocol version 1
84 Each host has a host-specific RSA key
85 (normally 1024 bits) used to identify the host.
87 the daemon starts, it generates a server RSA key (normally 768 bits).
88 This key is normally regenerated every hour if it has been used, and
89 is never stored on disk.
91 Whenever a client connects, the daemon responds with its public
93 The client compares the
94 RSA host key against its own database to verify that it has not changed.
95 The client then generates a 256 bit random number.
97 random number using both the host key and the server key, and sends
98 the encrypted number to the server.
99 Both sides then use this
100 random number as a session key which is used to encrypt all further
101 communications in the session.
102 The rest of the session is encrypted
103 using a conventional cipher, currently Blowfish or 3DES, with 3DES
104 being used by default.
105 The client selects the encryption algorithm
106 to use from those offered by the server.
108 Next, the server and the client enter an authentication dialog.
109 The client tries to authenticate itself using
113 authentication combined with RSA host
114 authentication, RSA challenge-response authentication, or password
115 based authentication.
117 Regardless of the authentication type, the account is checked to
118 ensure that it is accessible. An account is not accessible if it is
121 or its group is listed in
123 \&. The definition of a locked account is system dependant. Some platforms
124 have their own account database (eg AIX) and some modify the passwd field (
130 on Tru64 and a leading
132 on Linux). If there is a requirement to disable password authentication
133 for the account while allowing still public-key, then the passwd field
134 should be set to something other than these values (eg
140 Rhosts authentication is normally disabled
141 because it is fundamentally insecure, but can be enabled in the server
142 configuration file if desired.
143 System security is not improved unless
148 are disabled (thus completely disabling
154 .Ss SSH protocol version 2
156 Version 2 works similarly:
157 Each host has a host-specific key (RSA or DSA) used to identify the host.
158 However, when the daemon starts, it does not generate a server key.
159 Forward security is provided through a Diffie-Hellman key agreement.
160 This key agreement results in a shared session key.
162 The rest of the session is encrypted using a symmetric cipher, currently
163 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
164 The client selects the encryption algorithm
165 to use from those offered by the server.
166 Additionally, session integrity is provided
167 through a cryptographic message authentication code
168 (hmac-sha1 or hmac-md5).
170 Protocol version 2 provides a public key based
171 user (PubkeyAuthentication) or
172 client host (HostbasedAuthentication) authentication method,
173 conventional password authentication and challenge response based methods.
175 .Ss Command execution and data forwarding
177 If the client successfully authenticates itself, a dialog for
178 preparing the session is entered.
179 At this time the client may request
180 things like allocating a pseudo-tty, forwarding X11 connections,
181 forwarding TCP/IP connections, or forwarding the authentication agent
182 connection over the secure channel.
184 Finally, the client either requests a shell or execution of a command.
185 The sides then enter session mode.
186 In this mode, either side may send
187 data at any time, and such data is forwarded to/from the shell or
188 command on the server side, and the user terminal in the client side.
190 When the user program terminates and all forwarded X11 and other
191 connections have been closed, the server sends command exit status to
192 the client, and both sides exit.
195 can be configured using command-line options or a configuration
197 Command-line options override values specified in the
201 rereads its configuration file when it receives a hangup signal,
203 by executing itself with the name it was started as, i.e.,
206 The options are as follows:
209 Specifies the number of bits in the ephemeral protocol version 1
210 server key (default 768).
213 The server sends verbose debug output to the system
214 log, and does not put itself in the background.
215 The server also will not fork and will only process one connection.
216 This option is only intended for debugging for the server.
219 options increase the debugging level.
222 When this option is specified,
224 will send the output to the standard error instead of the system log.
225 .It Fl f Ar configuration_file
226 Specifies the name of the configuration file.
228 .Pa /etc/ssh/sshd_config .
230 refuses to start if there is no configuration file.
231 .It Fl g Ar login_grace_time
232 Gives the grace time for clients to authenticate themselves (default
234 If the client fails to authenticate the user within
235 this many seconds, the server disconnects and exits.
236 A value of zero indicates no limit.
237 .It Fl h Ar host_key_file
238 Specifies a file from which a host key is read.
239 This option must be given if
241 is not run as root (as the normal
242 host key files are normally not readable by anyone but root).
244 .Pa /etc/ssh/ssh_host_key
245 for protocol version 1, and
246 .Pa /etc/ssh/ssh_host_rsa_key
248 .Pa /etc/ssh/ssh_host_dsa_key
249 for protocol version 2.
250 It is possible to have multiple host key files for
251 the different protocol versions and host key algorithms.
259 from inetd because it needs to generate the server key before it can
260 respond to the client, and this may take tens of seconds.
261 Clients would have to wait too long if the key was regenerated every time.
262 However, with small key sizes (e.g., 512) using
266 .It Fl k Ar key_gen_time
267 Specifies how often the ephemeral protocol version 1 server key is
268 regenerated (default 3600 seconds, or one hour).
269 The motivation for regenerating the key fairly
270 often is that the key is not stored anywhere, and after about an hour,
271 it becomes impossible to recover the key for decrypting intercepted
272 communications even if the machine is cracked into or physically
274 A value of zero indicates that the key will never be regenerated.
276 Can be used to give options in the format used in the configuration file.
277 This is useful for specifying options for which there is no separate
280 Specifies the port on which the server listens for connections
282 Multiple port options are permitted.
283 Ports specified in the configuration file are ignored when a
284 command-line port is specified.
287 Nothing is sent to the system log.
288 Normally the beginning,
289 authentication, and termination of each connection is logged.
292 Only check the validity of the configuration file and sanity of the keys.
293 This is useful for updating
295 reliably as configuration options may change.
297 This option is used to specify the size of the field
300 structure that holds the remote host name.
301 If the resolved host name is longer than
303 the dotted decimal value will be used instead.
304 This allows hosts with very long host names that
305 overflow this field to still be uniquely identified.
308 indicates that only dotted decimal addresses
309 should be put into the
313 may also be used to prevent
315 from making DNS requests unless the authentication
316 mechanism or configuration requires it.
317 Authentication mechanisms that may require DNS include
318 .Cm RhostsRSAAuthentication ,
319 .Cm HostbasedAuthentication
321 .Cm from="pattern-list"
322 option in a key file.
323 Configuration options that require DNS include using a
329 When this option is specified
331 will not detach and does not become a daemon.
332 This allows easy monitoring of
337 to use IPv4 addresses only.
341 to use IPv6 addresses only.
343 .Sh CONFIGURATION FILE
345 reads configuration data from
346 .Pa /etc/ssh/sshd_config
347 (or the file specified with
349 on the command line).
350 The file format and configuration options are described in
353 When a user successfully logs in,
356 .Bl -enum -offset indent
358 If the login is on a tty, and no command has been specified,
359 prints last login time and
361 (unless prevented in the configuration file or by
362 .Pa $HOME/.hushlogin ;
367 If the login is on a tty, records login time.
371 if it exists, prints contents and quits
374 Changes to run with normal user privileges.
376 Sets up basic environment.
379 .Pa $HOME/.ssh/environment
380 if it exists and users are allowed to change their environment.
382 .Cm PermitUserEnvironment
386 Changes to user's home directory.
390 exists, runs it; else if
393 it; otherwise runs xauth.
396 files are given the X11
397 authentication protocol and cookie in standard input.
399 Runs user's shell or command.
401 .Sh AUTHORIZED_KEYS FILE FORMAT
402 .Pa $HOME/.ssh/authorized_keys
403 is the default file that lists the public keys that are
404 permitted for RSA authentication in protocol version 1
405 and for public key authentication (PubkeyAuthentication)
406 in protocol version 2.
407 .Cm AuthorizedKeysFile
408 may be used to specify an alternative file.
410 Each line of the file contains one
411 key (empty lines and lines starting with a
415 Each RSA public key consists of the following fields, separated by
416 spaces: options, bits, exponent, modulus, comment.
417 Each protocol version 2 public key consists of:
418 options, keytype, base64 encoded key, comment.
420 is optional; its presence is determined by whether the line starts
421 with a number or not (the options field never starts with a number).
422 The bits, exponent, modulus and comment fields give the RSA key for
423 protocol version 1; the
424 comment field is not used for anything (but may be convenient for the
425 user to identify the key).
426 For protocol version 2 the keytype is
431 Note that lines in this file are usually several hundred bytes long
432 (because of the size of the public key encoding).
433 You don't want to type them in; instead, copy the
441 enforces a minimum RSA key modulus size for protocol 1
442 and protocol 2 keys of 768 bits.
444 The options (if present) consist of comma-separated option
446 No spaces are permitted, except within double quotes.
447 The following option specifications are supported (note
448 that option keywords are case-insensitive):
450 .It Cm from="pattern-list"
451 Specifies that in addition to public key authentication, the canonical name
452 of the remote host must be present in the comma-separated list of
458 The list may also contain
459 patterns negated by prefixing them with
461 if the canonical host name matches a negated pattern, the key is not accepted.
463 of this option is to optionally increase security: public key authentication
464 by itself does not trust the network or name servers or anything (but
465 the key); however, if somebody somehow steals the key, the key
466 permits an intruder to log in from anywhere in the world.
467 This additional option makes using a stolen key more difficult (name
468 servers and/or routers would have to be compromised in addition to
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/IP and/or X11
484 forwarding unless they are explicitly prohibited.
485 Note that this option applies to shell, command or subsystem execution.
486 .It Cm environment="NAME=value"
487 Specifies that the string is to be added to the environment when
488 logging in using this key.
489 Environment variables set this way
490 override other default environment values.
491 Multiple options of this type are permitted.
492 Environment processing is disabled by default and is
494 .Cm PermitUserEnvironment
496 This option is automatically disabled if
499 .It Cm no-port-forwarding
500 Forbids TCP/IP forwarding when this key is used for authentication.
501 Any port forward requests by the client will return an error.
502 This might be used, e.g., in connection with the
505 .It Cm no-X11-forwarding
506 Forbids X11 forwarding when this key is used for authentication.
507 Any X11 forward requests by the client will return an error.
508 .It Cm no-agent-forwarding
509 Forbids authentication agent forwarding when this key is used for
512 Prevents tty allocation (a request to allocate a pty will fail).
513 .It Cm permitopen="host:port"
516 port forwarding such that it may only connect to the specified host and
518 IPv6 addresses can be specified with an alternative syntax:
522 options may be applied separated by commas.
523 No pattern matching is performed on the specified hostnames,
524 they must be literal domains or addresses.
527 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
529 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
531 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
533 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
534 .Sh SSH_KNOWN_HOSTS FILE FORMAT
536 .Pa /etc/ssh/ssh_known_hosts
538 .Pa $HOME/.ssh/known_hosts
539 files contain host public keys for all known hosts.
540 The global file should
541 be prepared by the administrator (optional), and the per-user file is
542 maintained automatically: whenever the user connects from an unknown host
543 its key is added to the per-user file.
545 Each line in these files contains the following fields: hostnames,
546 bits, exponent, modulus, comment.
547 The fields are separated by spaces.
549 Hostnames is a comma-separated list of patterns
554 wildcards); each pattern in turn is matched against the canonical host
555 name (when authenticating a client) or against the user-supplied
556 name (when authenticating a server).
557 A pattern may also be preceded by
559 to indicate negation: if the host name matches a negated
560 pattern, it is not accepted (by that line) even if it matched another
563 Bits, exponent, and modulus are taken directly from the RSA host key; they
564 can be obtained, e.g., from
565 .Pa /etc/ssh/ssh_host_key.pub .
566 The optional comment field continues to the end of the line, and is not used.
570 and empty lines are ignored as comments.
572 When performing host authentication, authentication is accepted if any
573 matching line has the proper key.
574 It is thus permissible (but not
575 recommended) to have several lines or different host keys for the same
577 This will inevitably happen when short forms of host names
578 from different domains are put in the file.
580 that the files contain conflicting information; authentication is
581 accepted if valid information can be found from either file.
583 Note that the lines in these files are typically hundreds of characters
584 long, and you definitely don't want to type in the host keys by hand.
585 Rather, generate them by a script
587 .Pa /etc/ssh/ssh_host_key.pub
588 and adding the host names at the front.
591 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
592 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
596 .It Pa /etc/ssh/sshd_config
597 Contains configuration data for
599 The file format and configuration options are described in
601 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
602 These three files contain the private parts of the host keys.
603 These files should only be owned by root, readable only by root, and not
604 accessible to others.
607 does not start if this file is group/world-accessible.
608 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
609 These three files contain the public parts of the host keys.
610 These files should be world-readable but writable only by
612 Their contents should match the respective private parts.
614 really used for anything; they are provided for the convenience of
615 the user so their contents can be copied to known hosts files.
616 These files are created using
619 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
620 The file format is described in
626 during privilege separation in the pre-authentication phase.
627 The directory should not contain any files and must be owned by root
628 and not group or world-writable.
629 .It Pa /var/run/sshd.pid
630 Contains the process ID of the
632 listening for connections (if there are several daemons running
633 concurrently for different ports, this contains the process ID of the one
635 The content of this file is not sensitive; it can be world-readable.
636 .It Pa $HOME/.ssh/authorized_keys
637 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
638 This file must be readable by root (which may on some machines imply
639 it being world-readable if the user's home directory resides on an NFS
641 It is recommended that it not be accessible by others.
642 The format of this file is described above.
643 Users will place the contents of their
648 files into this file, as described in
650 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
651 These files are consulted when using rhosts with RSA host
652 authentication or protocol version 2 hostbased authentication
653 to check the public key of the host.
654 The key must be listed in one of these files to be accepted.
655 The client uses the same files
656 to verify that it is connecting to the correct remote host.
657 These files should be writable only by root/the owner.
658 .Pa /etc/ssh/ssh_known_hosts
659 should be world-readable, and
660 .Pa $HOME/.ssh/known_hosts
661 can, but need not be, world-readable.
665 refuses to let anyone except root log in.
666 The contents of the file
667 are displayed to anyone trying to log in, and non-root connections are
669 The file should be world-readable.
670 .It Pa /etc/hosts.allow, /etc/hosts.deny
671 Access controls that should be enforced by tcp-wrappers are defined here.
672 Further details are described in
675 This file contains host-username pairs, separated by a space, one per
677 The given user on the corresponding host is permitted to log in
679 The same file is used by rlogind and rshd.
681 be writable only by the user; it is recommended that it not be
682 accessible by others.
684 If is also possible to use netgroups in the file.
686 name may be of the form +@groupname to specify all hosts or all users
690 this file is exactly the same as for
692 However, this file is
693 not used by rlogin and rshd, so using this permits access using SSH only.
694 .It Pa /etc/hosts.equiv
695 This file is used during
698 In the simplest form, this file contains host names, one per line.
700 those hosts are permitted to log in without a password, provided they
701 have the same user name on both machines.
702 The host name may also be
703 followed by a user name; such users are permitted to log in as
705 user on this machine (except root).
706 Additionally, the syntax
708 can be used to specify netgroups.
709 Negated entries start with
712 If the client host/user is successfully matched in this file, login is
713 automatically permitted provided the client and server user names are the
715 Additionally, successful RSA host authentication is normally required.
716 This file must be writable only by root; it is recommended
717 that it be world-readable.
719 .Sy "Warning: It is almost never a good idea to use user names in"
721 Beware that it really means that the named user(s) can log in as
723 which includes bin, daemon, adm, and other accounts that own critical
724 binaries and directories.
725 Using a user name practically grants the user root access.
726 The only valid use for user names that I can think
727 of is in negative entries.
729 Note that this warning also applies to rsh/rlogin.
730 .It Pa /etc/shosts.equiv
731 This is processed exactly as
732 .Pa /etc/hosts.equiv .
733 However, this file may be useful in environments that want to run both
735 .It Pa $HOME/.ssh/environment
736 This file is read into the environment at login (if it exists).
737 It can only contain empty lines, comment lines (that start with
739 and assignment lines of the form name=value.
740 The file should be writable
741 only by the user; it need not be readable by anyone else.
742 Environment processing is disabled by default and is
744 .Cm PermitUserEnvironment
747 If this file exists, it is run with
750 environment files but before starting the user's shell or command.
751 It must not produce any output on stdout; stderr must be used
753 If X11 forwarding is in use, it will receive the "proto cookie" pair in
754 its standard input (and
761 will not run xauth automatically to add X11 cookies.
763 The primary purpose of this file is to run any initialization routines
764 which may be needed before the user's home directory becomes
765 accessible; AFS is a particular example of such an environment.
767 This file will probably contain some initialization code followed by
768 something similar to:
770 if read proto cookie && [ -n "$DISPLAY" ]; then
771 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
772 # X11UseLocalhost=yes
773 echo add unix:`echo $DISPLAY |
774 cut -c11-` $proto $cookie
777 echo add $DISPLAY $proto $cookie
782 If this file does not exist,
785 does not exist either, xauth is used to add the cookie.
787 This file should be writable only by the user, and need not be
788 readable by anyone else.
789 .It Pa /etc/ssh/sshrc
792 This can be used to specify
793 machine-specific login-time initializations globally.
794 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.