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 1.197 2003/05/20 12:09:32 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
49 .Op Fl f Ar config_file
50 .Op Fl g Ar login_grace_time
51 .Op Fl h Ar host_key_file
52 .Op Fl k Ar key_gen_time
59 (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 Rhosts authentication is normally disabled
118 because it is fundamentally insecure, but can be enabled in the server
119 configuration file if desired.
120 System security is not improved unless
125 are disabled (thus completely disabling
131 .Ss SSH protocol version 2
133 Version 2 works similarly:
134 Each host has a host-specific key (RSA or DSA) used to identify the host.
135 However, when the daemon starts, it does not generate a server key.
136 Forward security is provided through a Diffie-Hellman key agreement.
137 This key agreement results in a shared session key.
139 The rest of the session is encrypted using a symmetric cipher, currently
140 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
141 The client selects the encryption algorithm
142 to use from those offered by the server.
143 Additionally, session integrity is provided
144 through a cryptographic message authentication code
145 (hmac-sha1 or hmac-md5).
147 Protocol version 2 provides a public key based
148 user (PubkeyAuthentication) or
149 client host (HostbasedAuthentication) authentication method,
150 conventional password authentication and challenge response based methods.
152 .Ss Command execution and data forwarding
154 If the client successfully authenticates itself, a dialog for
155 preparing the session is entered.
156 At this time the client may request
157 things like allocating a pseudo-tty, forwarding X11 connections,
158 forwarding TCP/IP connections, or forwarding the authentication agent
159 connection over the secure channel.
161 Finally, the client either requests a shell or execution of a command.
162 The sides then enter session mode.
163 In this mode, either side may send
164 data at any time, and such data is forwarded to/from the shell or
165 command on the server side, and the user terminal in the client side.
167 When the user program terminates and all forwarded X11 and other
168 connections have been closed, the server sends command exit status to
169 the client, and both sides exit.
172 can be configured using command-line options or a configuration
174 Command-line options override values specified in the
178 rereads its configuration file when it receives a hangup signal,
180 by executing itself with the name it was started as, i.e.,
183 The options are as follows:
186 Specifies the number of bits in the ephemeral protocol version 1
187 server key (default 768).
190 The server sends verbose debug output to the system
191 log, and does not put itself in the background.
192 The server also will not fork and will only process one connection.
193 This option is only intended for debugging for the server.
196 options increase the debugging level.
199 When this option is specified,
201 will send the output to the standard error instead of the system log.
202 .It Fl f Ar configuration_file
203 Specifies the name of the configuration file.
205 .Pa /etc/ssh/sshd_config .
207 refuses to start if there is no configuration file.
208 .It Fl g Ar login_grace_time
209 Gives the grace time for clients to authenticate themselves (default
211 If the client fails to authenticate the user within
212 this many seconds, the server disconnects and exits.
213 A value of zero indicates no limit.
214 .It Fl h Ar host_key_file
215 Specifies a file from which a host key is read.
216 This option must be given if
218 is not run as root (as the normal
219 host key files are normally not readable by anyone but root).
221 .Pa /etc/ssh/ssh_host_key
222 for protocol version 1, and
223 .Pa /etc/ssh/ssh_host_rsa_key
225 .Pa /etc/ssh/ssh_host_dsa_key
226 for protocol version 2.
227 It is possible to have multiple host key files for
228 the different protocol versions and host key algorithms.
236 from inetd because it needs to generate the server key before it can
237 respond to the client, and this may take tens of seconds.
238 Clients would have to wait too long if the key was regenerated every time.
239 However, with small key sizes (e.g., 512) using
243 .It Fl k Ar key_gen_time
244 Specifies how often the ephemeral protocol version 1 server key is
245 regenerated (default 3600 seconds, or one hour).
246 The motivation for regenerating the key fairly
247 often is that the key is not stored anywhere, and after about an hour,
248 it becomes impossible to recover the key for decrypting intercepted
249 communications even if the machine is cracked into or physically
251 A value of zero indicates that the key will never be regenerated.
253 Can be used to give options in the format used in the configuration file.
254 This is useful for specifying options for which there is no separate
257 Specifies the port on which the server listens for connections
259 Multiple port options are permitted.
260 Ports specified in the configuration file are ignored when a
261 command-line port is specified.
264 Nothing is sent to the system log.
265 Normally the beginning,
266 authentication, and termination of each connection is logged.
269 Only check the validity of the configuration file and sanity of the keys.
270 This is useful for updating
272 reliably as configuration options may change.
274 This option is used to specify the size of the field
277 structure that holds the remote host name.
278 If the resolved host name is longer than
280 the dotted decimal value will be used instead.
281 This allows hosts with very long host names that
282 overflow this field to still be uniquely identified.
285 indicates that only dotted decimal addresses
286 should be put into the
290 may also be used to prevent
292 from making DNS requests unless the authentication
293 mechanism or configuration requires it.
294 Authentication mechanisms that may require DNS include
295 .Cm RhostsAuthentication ,
296 .Cm RhostsRSAAuthentication ,
297 .Cm HostbasedAuthentication
299 .Cm from="pattern-list"
300 option in a key file.
301 Configuration options that require DNS include using a
307 When this option is specified
309 will not detach and does not become a daemon.
310 This allows easy monitoring of
315 to use IPv4 addresses only.
319 to use IPv6 addresses only.
321 .Sh CONFIGURATION FILE
323 reads configuration data from
324 .Pa /etc/ssh/sshd_config
325 (or the file specified with
327 on the command line).
328 The file format and configuration options are described in
331 When a user successfully logs in,
334 .Bl -enum -offset indent
336 If the login is on a tty, and no command has been specified,
337 prints last login time and
339 (unless prevented in the configuration file or by
340 .Pa $HOME/.hushlogin ;
345 If the login is on a tty, records login time.
349 if it exists, prints contents and quits
352 Changes to run with normal user privileges.
354 Sets up basic environment.
357 .Pa $HOME/.ssh/environment
358 if it exists and users are allowed to change their environment.
360 .Cm PermitUserEnvironment
364 Changes to user's home directory.
368 exists, runs it; else if
371 it; otherwise runs xauth.
374 files are given the X11
375 authentication protocol and cookie in standard input.
377 Runs user's shell or command.
379 .Sh AUTHORIZED_KEYS FILE FORMAT
380 .Pa $HOME/.ssh/authorized_keys
381 is the default file that lists the public keys that are
382 permitted for RSA authentication in protocol version 1
383 and for public key authentication (PubkeyAuthentication)
384 in protocol version 2.
385 .Cm AuthorizedKeysFile
386 may be used to specify an alternative file.
388 Each line of the file contains one
389 key (empty lines and lines starting with a
393 Each RSA public key consists of the following fields, separated by
394 spaces: options, bits, exponent, modulus, comment.
395 Each protocol version 2 public key consists of:
396 options, keytype, base64 encoded key, comment.
398 is optional; its presence is determined by whether the line starts
399 with a number or not (the options field never starts with a number).
400 The bits, exponent, modulus and comment fields give the RSA key for
401 protocol version 1; the
402 comment field is not used for anything (but may be convenient for the
403 user to identify the key).
404 For protocol version 2 the keytype is
409 Note that lines in this file are usually several hundred bytes long
410 (because of the size of the public key encoding).
411 You don't want to type them in; instead, copy the
419 enforces a minimum RSA key modulus size for protocol 1
420 and protocol 2 keys of 768 bits.
422 The options (if present) consist of comma-separated option
424 No spaces are permitted, except within double quotes.
425 The following option specifications are supported (note
426 that option keywords are case-insensitive):
428 .It Cm from="pattern-list"
429 Specifies that in addition to public key authentication, the canonical name
430 of the remote host must be present in the comma-separated list of
436 The list may also contain
437 patterns negated by prefixing them with
439 if the canonical host name matches a negated pattern, the key is not accepted.
441 of this option is to optionally increase security: public key authentication
442 by itself does not trust the network or name servers or anything (but
443 the key); however, if somebody somehow steals the key, the key
444 permits an intruder to log in from anywhere in the world.
445 This additional option makes using a stolen key more difficult (name
446 servers and/or routers would have to be compromised in addition to
448 .It Cm command="command"
449 Specifies that the command is executed whenever this key is used for
451 The command supplied by the user (if any) is ignored.
452 The command is run on a pty if the client requests a pty;
453 otherwise it is run without a tty.
454 If an 8-bit clean channel is required,
455 one must not request a pty or should specify
457 A quote may be included in the command by quoting it with a backslash.
458 This option might be useful
459 to restrict certain public keys to perform just a specific operation.
460 An example might be a key that permits remote backups but nothing else.
461 Note that the client may specify TCP/IP and/or X11
462 forwarding unless they are explicitly prohibited.
463 Note that this option applies to shell, command or subsystem execution.
464 .It Cm environment="NAME=value"
465 Specifies that the string is to be added to the environment when
466 logging in using this key.
467 Environment variables set this way
468 override other default environment values.
469 Multiple options of this type are permitted.
470 Environment processing is disabled by default and is
472 .Cm PermitUserEnvironment
474 This option is automatically disabled if
477 .It Cm no-port-forwarding
478 Forbids TCP/IP forwarding when this key is used for authentication.
479 Any port forward requests by the client will return an error.
480 This might be used, e.g., in connection with the
483 .It Cm no-X11-forwarding
484 Forbids X11 forwarding when this key is used for authentication.
485 Any X11 forward requests by the client will return an error.
486 .It Cm no-agent-forwarding
487 Forbids authentication agent forwarding when this key is used for
490 Prevents tty allocation (a request to allocate a pty will fail).
491 .It Cm permitopen="host:port"
494 port forwarding such that it may only connect to the specified host and
496 IPv6 addresses can be specified with an alternative syntax:
500 options may be applied separated by commas.
501 No pattern matching is performed on the specified hostnames,
502 they must be literal domains or addresses.
505 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
507 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
509 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
511 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
512 .Sh SSH_KNOWN_HOSTS FILE FORMAT
514 .Pa /etc/ssh/ssh_known_hosts
516 .Pa $HOME/.ssh/known_hosts
517 files contain host public keys for all known hosts.
518 The global file should
519 be prepared by the administrator (optional), and the per-user file is
520 maintained automatically: whenever the user connects from an unknown host
521 its key is added to the per-user file.
523 Each line in these files contains the following fields: hostnames,
524 bits, exponent, modulus, comment.
525 The fields are separated by spaces.
527 Hostnames is a comma-separated list of patterns
532 wildcards); each pattern in turn is matched against the canonical host
533 name (when authenticating a client) or against the user-supplied
534 name (when authenticating a server).
535 A pattern may also be preceded by
537 to indicate negation: if the host name matches a negated
538 pattern, it is not accepted (by that line) even if it matched another
541 Bits, exponent, and modulus are taken directly from the RSA host key; they
542 can be obtained, e.g., from
543 .Pa /etc/ssh/ssh_host_key.pub .
544 The optional comment field continues to the end of the line, and is not used.
548 and empty lines are ignored as comments.
550 When performing host authentication, authentication is accepted if any
551 matching line has the proper key.
552 It is thus permissible (but not
553 recommended) to have several lines or different host keys for the same
555 This will inevitably happen when short forms of host names
556 from different domains are put in the file.
558 that the files contain conflicting information; authentication is
559 accepted if valid information can be found from either file.
561 Note that the lines in these files are typically hundreds of characters
562 long, and you definitely don't want to type in the host keys by hand.
563 Rather, generate them by a script
565 .Pa /etc/ssh/ssh_host_key.pub
566 and adding the host names at the front.
569 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
570 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
574 .It Pa /etc/ssh/sshd_config
575 Contains configuration data for
577 The file format and configuration options are described in
579 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
580 These three files contain the private parts of the host keys.
581 These files should only be owned by root, readable only by root, and not
582 accessible to others.
585 does not start if this file is group/world-accessible.
586 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
587 These three files contain the public parts of the host keys.
588 These files should be world-readable but writable only by
590 Their contents should match the respective private parts.
592 really used for anything; they are provided for the convenience of
593 the user so their contents can be copied to known hosts files.
594 These files are created using
597 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
598 The file format is described in
604 during privilege separation in the pre-authentication phase.
605 The directory should not contain any files and must be owned by root
606 and not group or world-writable.
607 .It Pa /var/run/sshd.pid
608 Contains the process ID of the
610 listening for connections (if there are several daemons running
611 concurrently for different ports, this contains the process ID of the one
613 The content of this file is not sensitive; it can be world-readable.
614 .It Pa $HOME/.ssh/authorized_keys
615 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
616 This file must be readable by root (which may on some machines imply
617 it being world-readable if the user's home directory resides on an NFS
619 It is recommended that it not be accessible by others.
620 The format of this file is described above.
621 Users will place the contents of their
626 files into this file, as described in
628 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
629 These files are consulted when using rhosts with RSA host
630 authentication or protocol version 2 hostbased authentication
631 to check the public key of the host.
632 The key must be listed in one of these files to be accepted.
633 The client uses the same files
634 to verify that it is connecting to the correct remote host.
635 These files should be writable only by root/the owner.
636 .Pa /etc/ssh/ssh_known_hosts
637 should be world-readable, and
638 .Pa $HOME/.ssh/known_hosts
639 can, but need not be, world-readable.
643 refuses to let anyone except root log in.
644 The contents of the file
645 are displayed to anyone trying to log in, and non-root connections are
647 The file should be world-readable.
648 .It Pa /etc/hosts.allow, /etc/hosts.deny
649 Access controls that should be enforced by tcp-wrappers are defined here.
650 Further details are described in
653 This file contains host-username pairs, separated by a space, one per
655 The given user on the corresponding host is permitted to log in
657 The same file is used by rlogind and rshd.
659 be writable only by the user; it is recommended that it not be
660 accessible by others.
662 If is also possible to use netgroups in the file.
664 name may be of the form +@groupname to specify all hosts or all users
668 this file is exactly the same as for
670 However, this file is
671 not used by rlogin and rshd, so using this permits access using SSH only.
672 .It Pa /etc/hosts.equiv
673 This file is used during
676 In the simplest form, this file contains host names, one per line.
678 those hosts are permitted to log in without a password, provided they
679 have the same user name on both machines.
680 The host name may also be
681 followed by a user name; such users are permitted to log in as
683 user on this machine (except root).
684 Additionally, the syntax
686 can be used to specify netgroups.
687 Negated entries start with
690 If the client host/user is successfully matched in this file, login is
691 automatically permitted provided the client and server user names are the
693 Additionally, successful RSA host authentication is normally required.
694 This file must be writable only by root; it is recommended
695 that it be world-readable.
697 .Sy "Warning: It is almost never a good idea to use user names in"
699 Beware that it really means that the named user(s) can log in as
701 which includes bin, daemon, adm, and other accounts that own critical
702 binaries and directories.
703 Using a user name practically grants the user root access.
704 The only valid use for user names that I can think
705 of is in negative entries.
707 Note that this warning also applies to rsh/rlogin.
708 .It Pa /etc/shosts.equiv
709 This is processed exactly as
710 .Pa /etc/hosts.equiv .
711 However, this file may be useful in environments that want to run both
713 .It Pa $HOME/.ssh/environment
714 This file is read into the environment at login (if it exists).
715 It can only contain empty lines, comment lines (that start with
717 and assignment lines of the form name=value.
718 The file should be writable
719 only by the user; it need not be readable by anyone else.
720 Environment processing is disabled by default and is
722 .Cm PermitUserEnvironment
725 If this file exists, it is run with
728 environment files but before starting the user's shell or command.
729 It must not produce any output on stdout; stderr must be used
731 If X11 forwarding is in use, it will receive the "proto cookie" pair in
732 its standard input (and
739 will not run xauth automatically to add X11 cookies.
741 The primary purpose of this file is to run any initialization routines
742 which may be needed before the user's home directory becomes
743 accessible; AFS is a particular example of such an environment.
745 This file will probably contain some initialization code followed by
746 something similar to:
748 if read proto cookie && [ -n "$DISPLAY" ]; then
749 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
750 # X11UseLocalhost=yes
751 echo add unix:`echo $DISPLAY |
752 cut -c11-` $proto $cookie
755 echo add $DISPLAY $proto $cookie
760 If this file does not exist,
763 does not exist either, xauth is used to add the cookie.
765 This file should be writable only by the user, and need not be
766 readable by anyone else.
767 .It Pa /etc/ssh/sshrc
770 This can be used to specify
771 machine-specific login-time initializations globally.
772 This file should be writable only by root, and should be world-readable.
775 OpenSSH is a derivative of the original and free
776 ssh 1.2.12 release by Tatu Ylonen.
777 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
778 Theo de Raadt and Dug Song
779 removed many bugs, re-added newer features and
781 Markus Friedl contributed the support for SSH
782 protocol versions 1.5 and 2.0.
783 Niels Provos and Markus Friedl contributed support
784 for privilege separation.
802 .%T "SSH Protocol Architecture"
803 .%N draft-ietf-secsh-architecture-12.txt
805 .%O work in progress material
811 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
812 .%N draft-ietf-secsh-dh-group-exchange-02.txt
814 .%O work in progress material