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 1.189 2002/08/21 11:20:59 espie Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
48 .Op Fl f Ar config_file
49 .Op Fl g Ar login_grace_time
50 .Op Fl h Ar host_key_file
51 .Op Fl k Ar key_gen_time
57 (SSH Daemon) is the daemon program for
59 Together these programs replace rlogin and rsh, and
60 provide secure encrypted communications between two untrusted hosts
61 over an insecure network.
62 The programs are intended to be as easy to
63 install and use as possible.
66 is the daemon that listens for connections from clients.
67 It is normally started at boot from
70 daemon for each incoming connection.
71 The forked daemons handle
72 key exchange, encryption, authentication, command execution,
74 This implementation of
76 supports both SSH protocol version 1 and 2 simultaneously.
80 .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
111 authentication combined with RSA host
112 authentication, RSA challenge-response authentication, or password
113 based authentication.
115 Rhosts authentication is normally disabled
116 because it is fundamentally insecure, but can be enabled in the server
117 configuration file if desired.
118 System security is not improved unless
123 are disabled (thus completely disabling
129 .Ss SSH protocol version 2
131 Version 2 works similarly:
132 Each host has a host-specific key (RSA or DSA) used to identify the host.
133 However, when the daemon starts, it does not generate a server key.
134 Forward security is provided through a Diffie-Hellman key agreement.
135 This key agreement results in a shared session key.
137 The rest of the session is encrypted using a symmetric cipher, currently
138 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
139 The client selects the encryption algorithm
140 to use from those offered by the server.
141 Additionally, session integrity is provided
142 through a cryptographic message authentication code
143 (hmac-sha1 or hmac-md5).
145 Protocol version 2 provides a public key based
146 user (PubkeyAuthentication) or
147 client host (HostbasedAuthentication) authentication method,
148 conventional password authentication and challenge response based methods.
150 .Ss Command execution and data forwarding
152 If the client successfully authenticates itself, a dialog for
153 preparing the session is entered.
154 At this time the client may request
155 things like allocating a pseudo-tty, forwarding X11 connections,
156 forwarding TCP/IP connections, or forwarding the authentication agent
157 connection over the secure channel.
159 Finally, the client either requests a shell or execution of a command.
160 The sides then enter session mode.
161 In this mode, either side may send
162 data at any time, and such data is forwarded to/from the shell or
163 command on the server side, and the user terminal in the client side.
165 When the user program terminates and all forwarded X11 and other
166 connections have been closed, the server sends command exit status to
167 the client, and both sides exit.
170 can be configured using command-line options or a configuration
172 Command-line options override values specified in the
176 rereads its configuration file when it receives a hangup signal,
178 by executing itself with the name it was started as, i.e.,
181 The options are as follows:
184 Specifies the number of bits in the ephemeral protocol version 1
185 server key (default 768).
188 The server sends verbose debug output to the system
189 log, and does not put itself in the background.
190 The server also will not fork and will only process one connection.
191 This option is only intended for debugging for the server.
192 Multiple -d options increase the debugging level.
195 When this option is specified,
197 will send the output to the standard error instead of the system log.
198 .It Fl f Ar configuration_file
199 Specifies the name of the configuration file.
201 .Pa /etc/ssh/sshd_config .
203 refuses to start if there is no configuration file.
204 .It Fl g Ar login_grace_time
205 Gives the grace time for clients to authenticate themselves (default
207 If the client fails to authenticate the user within
208 this many seconds, the server disconnects and exits.
209 A value of zero indicates no limit.
210 .It Fl h Ar host_key_file
211 Specifies a file from which a host key is read.
212 This option must be given if
214 is not run as root (as the normal
215 host key files are normally not readable by anyone but root).
217 .Pa /etc/ssh/ssh_host_key
218 for protocol version 1, and
219 .Pa /etc/ssh/ssh_host_rsa_key
221 .Pa /etc/ssh/ssh_host_dsa_key
222 for protocol version 2.
223 It is possible to have multiple host key files for
224 the different protocol versions and host key algorithms.
228 is being run from inetd.
231 from inetd because it needs to generate the server key before it can
232 respond to the client, and this may take tens of seconds.
233 Clients would have to wait too long if the key was regenerated every time.
234 However, with small key sizes (e.g., 512) using
238 .It Fl k Ar key_gen_time
239 Specifies how often the ephemeral protocol version 1 server key is
240 regenerated (default 3600 seconds, or one hour).
241 The motivation for regenerating the key fairly
242 often is that the key is not stored anywhere, and after about an hour,
243 it becomes impossible to recover the key for decrypting intercepted
244 communications even if the machine is cracked into or physically
246 A value of zero indicates that the key will never be regenerated.
248 Can be used to give options in the format used in the configuration file.
249 This is useful for specifying options for which there is no separate
252 Specifies the port on which the server listens for connections
254 Multiple port options are permitted.
255 Ports specified in the configuration file are ignored when a
256 command-line port is specified.
259 Nothing is sent to the system log.
260 Normally the beginning,
261 authentication, and termination of each connection is logged.
264 Only check the validity of the configuration file and sanity of the keys.
265 This is useful for updating
267 reliably as configuration options may change.
269 This option is used to specify the size of the field
272 structure that holds the remote host name.
273 If the resolved host name is longer than
275 the dotted decimal value will be used instead.
276 This allows hosts with very long host names that
277 overflow this field to still be uniquely identified.
280 indicates that only dotted decimal addresses
281 should be put into the
285 is also be used to prevent
287 from making DNS requests unless the authentication
288 mechanism or configuration requires it.
289 Authentication mechanisms that may require DNS include
290 .Cm RhostsAuthentication ,
291 .Cm RhostsRSAAuthentication ,
292 .Cm HostbasedAuthentication
294 .Cm from="pattern-list"
295 option in a key file.
296 Configuration options that require DNS include using a
302 When this option is specified
304 will not detach and does not become a daemon.
305 This allows easy monitoring of
310 to use IPv4 addresses only.
314 to use IPv6 addresses only.
316 .Sh CONFIGURATION FILE
318 reads configuration data from
319 .Pa /etc/ssh/sshd_config
320 (or the file specified with
322 on the command line).
323 The file format and configuration options are described in
326 When a user successfully logs in,
329 .Bl -enum -offset indent
331 If the login is on a tty, and no command has been specified,
332 prints last login time and
334 (unless prevented in the configuration file or by
335 .Pa $HOME/.hushlogin ;
340 If the login is on a tty, records login time.
344 if it exists, prints contents and quits
347 Changes to run with normal user privileges.
349 Sets up basic environment.
352 .Pa $HOME/.ssh/environment
353 if it exists and users are allowed to change their environment.
355 .Cm PermitUserEnvironment
359 Changes to user's home directory.
363 exists, runs it; else if
366 it; otherwise runs xauth.
369 files are given the X11
370 authentication protocol and cookie in standard input.
372 Runs user's shell or command.
374 .Sh AUTHORIZED_KEYS FILE FORMAT
375 .Pa $HOME/.ssh/authorized_keys
376 is the default file that lists the public keys that are
377 permitted for RSA authentication in protocol version 1
378 and for public key authentication (PubkeyAuthentication)
379 in protocol version 2.
380 .Cm AuthorizedKeysFile
381 may be used to specify an alternative file.
383 Each line of the file contains one
384 key (empty lines and lines starting with a
388 Each RSA public key consists of the following fields, separated by
389 spaces: options, bits, exponent, modulus, comment.
390 Each protocol version 2 public key consists of:
391 options, keytype, base64 encoded key, comment.
393 is optional; its presence is determined by whether the line starts
394 with a number or not (the options field never starts with a number).
395 The bits, exponent, modulus and comment fields give the RSA key for
396 protocol version 1; the
397 comment field is not used for anything (but may be convenient for the
398 user to identify the key).
399 For protocol version 2 the keytype is
404 Note that lines in this file are usually several hundred bytes long
405 (because of the size of the public key encoding).
406 You don't want to type them in; instead, copy the
414 enforces a minimum RSA key modulus size for protocol 1
415 and protocol 2 keys of 768 bits.
417 The options (if present) consist of comma-separated option
419 No spaces are permitted, except within double quotes.
420 The following option specifications are supported (note
421 that option keywords are case-insensitive):
423 .It Cm from="pattern-list"
424 Specifies that in addition to public key authentication, the canonical name
425 of the remote host must be present in the comma-separated list of
431 The list may also contain
432 patterns negated by prefixing them with
434 if the canonical host name matches a negated pattern, the key is not accepted.
436 of this option is to optionally increase security: public key authentication
437 by itself does not trust the network or name servers or anything (but
438 the key); however, if somebody somehow steals the key, the key
439 permits an intruder to log in from anywhere in the world.
440 This additional option makes using a stolen key more difficult (name
441 servers and/or routers would have to be compromised in addition to
443 .It Cm command="command"
444 Specifies that the command is executed whenever this key is used for
446 The command supplied by the user (if any) is ignored.
447 The command is run on a pty if the client requests a pty;
448 otherwise it is run without a tty.
449 If a 8-bit clean channel is required,
450 one must not request a pty or should specify
452 A quote may be included in the command by quoting it with a backslash.
453 This option might be useful
454 to restrict certain public keys to perform just a specific operation.
455 An example might be a key that permits remote backups but nothing else.
456 Note that the client may specify TCP/IP and/or X11
457 forwarding unless they are explicitly prohibited.
458 Note that this option applies to shell, command or subsystem execution.
459 .It Cm environment="NAME=value"
460 Specifies that the string is to be added to the environment when
461 logging in using this key.
462 Environment variables set this way
463 override other default environment values.
464 Multiple options of this type are permitted.
465 Environment processing is disabled by default and is
467 .Cm PermitUserEnvironment
469 This option is automatically disabled if
472 .It Cm no-port-forwarding
473 Forbids TCP/IP forwarding when this key is used for authentication.
474 Any port forward requests by the client will return an error.
475 This might be used, e.g., in connection with the
478 .It Cm no-X11-forwarding
479 Forbids X11 forwarding when this key is used for authentication.
480 Any X11 forward requests by the client will return an error.
481 .It Cm no-agent-forwarding
482 Forbids authentication agent forwarding when this key is used for
485 Prevents tty allocation (a request to allocate a pty will fail).
486 .It Cm permitopen="host:port"
489 port forwarding such that it may only connect to the specified host and
491 IPv6 addresses can be specified with an alternative syntax:
495 options may be applied separated by commas. No pattern matching is
496 performed on the specified hostnames, they must be literal domains or
500 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
502 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
504 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
506 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
507 .Sh SSH_KNOWN_HOSTS FILE FORMAT
509 .Pa /etc/ssh/ssh_known_hosts ,
511 .Pa $HOME/.ssh/known_hosts
512 files contain host public keys for all known hosts.
513 The global file should
514 be prepared by the administrator (optional), and the per-user file is
515 maintained automatically: whenever the user connects from an unknown host
516 its key is added to the per-user file.
518 Each line in these files contains the following fields: hostnames,
519 bits, exponent, modulus, comment.
520 The fields are separated by spaces.
522 Hostnames is a comma-separated list of patterns ('*' and '?' act as
523 wildcards); each pattern in turn is matched against the canonical host
524 name (when authenticating a client) or against the user-supplied
525 name (when authenticating a server).
526 A pattern may also be preceded by
528 to indicate negation: if the host name matches a negated
529 pattern, it is not accepted (by that line) even if it matched another
532 Bits, exponent, and modulus are taken directly from the RSA host key; they
533 can be obtained, e.g., from
534 .Pa /etc/ssh/ssh_host_key.pub .
535 The optional comment field continues to the end of the line, and is not used.
539 and empty lines are ignored as comments.
541 When performing host authentication, authentication is accepted if any
542 matching line has the proper key.
543 It is thus permissible (but not
544 recommended) to have several lines or different host keys for the same
546 This will inevitably happen when short forms of host names
547 from different domains are put in the file.
549 that the files contain conflicting information; authentication is
550 accepted if valid information can be found from either file.
552 Note that the lines in these files are typically hundreds of characters
553 long, and you definitely don't want to type in the host keys by hand.
554 Rather, generate them by a script
556 .Pa /etc/ssh/ssh_host_key.pub
557 and adding the host names at the front.
560 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
561 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
565 .It Pa /etc/ssh/sshd_config
566 Contains configuration data for
568 The file format and configuration options are described in
570 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
571 These three files contain the private parts of the host keys.
572 These files should only be owned by root, readable only by root, and not
573 accessible to others.
576 does not start if this file is group/world-accessible.
577 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
578 These three files contain the public parts of the host keys.
579 These files should be world-readable but writable only by
581 Their contents should match the respective private parts.
583 really used for anything; they are provided for the convenience of
584 the user so their contents can be copied to known hosts files.
585 These files are created using
588 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
593 during privilege separation in the pre-authentication phase.
594 The directory should not contain any files and must be owned by root
595 and not group or world-writable.
596 .It Pa /var/run/sshd.pid
597 Contains the process ID of the
599 listening for connections (if there are several daemons running
600 concurrently for different ports, this contains the process ID of the one
602 The content of this file is not sensitive; it can be world-readable.
603 .It Pa $HOME/.ssh/authorized_keys
604 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
605 This file must be readable by root (which may on some machines imply
606 it being world-readable if the user's home directory resides on an NFS
608 It is recommended that it not be accessible by others.
609 The format of this file is described above.
610 Users will place the contents of their
615 files into this file, as described in
617 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
618 These files are consulted when using rhosts with RSA host
619 authentication or protocol version 2 hostbased authentication
620 to check the public key of the host.
621 The key must be listed in one of these files to be accepted.
622 The client uses the same files
623 to verify that it is connecting to the correct remote host.
624 These files should be writable only by root/the owner.
625 .Pa /etc/ssh/ssh_known_hosts
626 should be world-readable, and
627 .Pa $HOME/.ssh/known_hosts
628 can but need not be world-readable.
632 refuses to let anyone except root log in.
633 The contents of the file
634 are displayed to anyone trying to log in, and non-root connections are
636 The file should be world-readable.
637 .It Pa /etc/hosts.allow, /etc/hosts.deny
638 Access controls that should be enforced by tcp-wrappers are defined here.
639 Further details are described in
642 This file contains host-username pairs, separated by a space, one per
644 The given user on the corresponding host is permitted to log in
646 The same file is used by rlogind and rshd.
648 be writable only by the user; it is recommended that it not be
649 accessible by others.
651 If is also possible to use netgroups in the file.
653 name may be of the form +@groupname to specify all hosts or all users
657 this file is exactly the same as for
659 However, this file is
660 not used by rlogin and rshd, so using this permits access using SSH only.
661 .It Pa /etc/hosts.equiv
662 This file is used during
665 In the simplest form, this file contains host names, one per line.
667 those hosts are permitted to log in without a password, provided they
668 have the same user name on both machines.
669 The host name may also be
670 followed by a user name; such users are permitted to log in as
672 user on this machine (except root).
673 Additionally, the syntax
675 can be used to specify netgroups.
676 Negated entries start with
679 If the client host/user is successfully matched in this file, login is
680 automatically permitted provided the client and server user names are the
682 Additionally, successful RSA host authentication is normally required.
683 This file must be writable only by root; it is recommended
684 that it be world-readable.
686 .Sy "Warning: It is almost never a good idea to use user names in"
688 Beware that it really means that the named user(s) can log in as
690 which includes bin, daemon, adm, and other accounts that own critical
691 binaries and directories.
692 Using a user name practically grants the user root access.
693 The only valid use for user names that I can think
694 of is in negative entries.
696 Note that this warning also applies to rsh/rlogin.
697 .It Pa /etc/shosts.equiv
698 This is processed exactly as
699 .Pa /etc/hosts.equiv .
700 However, this file may be useful in environments that want to run both
702 .It Pa $HOME/.ssh/environment
703 This file is read into the environment at login (if it exists).
704 It can only contain empty lines, comment lines (that start with
706 and assignment lines of the form name=value.
707 The file should be writable
708 only by the user; it need not be readable by anyone else.
709 Environment processing is disabled by default and is
711 .Cm PermitUserEnvironment
714 If this file exists, it is run with /bin/sh after reading the
715 environment files but before starting the user's shell or command.
716 It must not produce any output on stdout; stderr must be used
718 If X11 forwarding is in use, it will receive the "proto cookie" pair in
719 its standard input (and
726 will not run xauth automatically to add X11 cookies.
728 The primary purpose of this file is to run any initialization routines
729 which may be needed before the user's home directory becomes
730 accessible; AFS is a particular example of such an environment.
732 This file will probably contain some initialization code followed by
733 something similar to:
735 if read proto cookie && [ -n "$DISPLAY" ]; then
736 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
737 # X11UseLocalhost=yes
738 xauth add unix:`echo $DISPLAY |
739 cut -c11-` $proto $cookie
742 xauth add $DISPLAY $proto $cookie
747 If this file does not exist,
750 does not exist either, xauth is used to add the cookie.
752 This file should be writable only by the user, and need not be
753 readable by anyone else.
754 .It Pa /etc/ssh/sshrc
757 This can be used to specify
758 machine-specific login-time initializations globally.
759 This file should be writable only by root, and should be world-readable.
762 OpenSSH is a derivative of the original and free
763 ssh 1.2.12 release by Tatu Ylonen.
764 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
765 Theo de Raadt and Dug Song
766 removed many bugs, re-added newer features and
768 Markus Friedl contributed the support for SSH
769 protocol versions 1.5 and 2.0.
770 Niels Provos and Markus Friedl contributed support
771 for privilege separation.
789 .%T "SSH Protocol Architecture"
790 .%N draft-ietf-secsh-architecture-12.txt
792 .%O work in progress material
798 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
799 .%N draft-ietf-secsh-dh-group-exchange-02.txt
801 .%O work in progress material