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 1.193 2002/09/24 20:59:44 todd Exp $
38 .\" $FreeBSD: src/crypto/openssh/sshd.8,v 1.5.2.11 2003/02/05 19:31:31 trhodes Exp $
39 .\" $DragonFly: src/crypto/openssh/Attic/sshd.8,v 1.2 2003/06/17 04:24:36 dillon Exp $
40 .Dd September 25, 1999
45 .Nd OpenSSH SSH daemon
50 .Op Fl f Ar config_file
51 .Op Fl g Ar login_grace_time
52 .Op Fl h Ar host_key_file
53 .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.
194 Multiple -d options increase the debugging level.
197 When this option is specified,
199 will send the output to the standard error instead of the system log.
200 .It Fl f Ar configuration_file
201 Specifies the name of the configuration file.
203 .Pa /etc/ssh/sshd_config .
205 refuses to start if there is no configuration file.
206 .It Fl g Ar login_grace_time
207 Gives the grace time for clients to authenticate themselves (default
209 If the client fails to authenticate the user within
210 this many seconds, the server disconnects and exits.
211 A value of zero indicates no limit.
212 .It Fl h Ar host_key_file
213 Specifies a file from which a host key is read.
214 This option must be given if
216 is not run as root (as the normal
217 host key files are normally not readable by anyone but root).
219 .Pa /etc/ssh/ssh_host_key
220 for protocol version 1, and
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 .Pa /var/run/nologin ;
345 if one exists, it prints the contents and quits
348 Changes to run with normal user privileges.
350 Sets up basic environment.
353 .Pa $HOME/.ssh/environment
354 if it exists and users are allowed to change their environment.
356 .Cm PermitUserEnvironment
360 Changes to user's home directory.
364 exists, runs it; else if
371 files are given the X11
372 authentication protocol and cookie (if applicable) in standard input.
374 Runs user's shell or command.
376 .Sh AUTHORIZED_KEYS FILE FORMAT
377 .Pa $HOME/.ssh/authorized_keys
378 is the default file that lists the public keys that are
379 permitted for RSA authentication in protocol version 1
380 and for public key authentication (PubkeyAuthentication)
381 in protocol version 2.
382 .Cm AuthorizedKeysFile
383 may be used to specify an alternative file.
385 Each line of the file contains one
386 key (empty lines and lines starting with a
390 Each RSA public key consists of the following fields, separated by
391 spaces: options, bits, exponent, modulus, comment.
392 Each protocol version 2 public key consists of:
393 options, keytype, base64 encoded key, comment.
395 is optional; its presence is determined by whether the line starts
396 with a number or not (the options field never starts with a number).
397 The bits, exponent, modulus and comment fields give the RSA key for
398 protocol version 1; the
399 comment field is not used for anything (but may be convenient for the
400 user to identify the key).
401 For protocol version 2 the keytype is
406 Note that lines in this file are usually several hundred bytes long
407 (because of the size of the public key encoding).
408 You don't want to type them in; instead, copy the
416 enforces a minimum RSA key modulus size for protocol 1
417 and protocol 2 keys of 768 bits.
419 The options (if present) consist of comma-separated option
421 No spaces are permitted, except within double quotes.
422 The following option specifications are supported (note
423 that option keywords are case-insensitive):
425 .It Cm from="pattern-list"
426 Specifies that in addition to public key authentication, the canonical name
427 of the remote host must be present in the comma-separated list of
433 The list may also contain
434 patterns negated by prefixing them with
436 if the canonical host name matches a negated pattern, the key is not accepted.
438 of this option is to optionally increase security: public key authentication
439 by itself does not trust the network or name servers or anything (but
440 the key); however, if somebody somehow steals the key, the key
441 permits an intruder to log in from anywhere in the world.
442 This additional option makes using a stolen key more difficult (name
443 servers and/or routers would have to be compromised in addition to
445 .It Cm command="command"
446 Specifies that the command is executed whenever this key is used for
448 The command supplied by the user (if any) is ignored.
449 The command is run on a pty if the client requests a pty;
450 otherwise it is run without a tty.
451 If a 8-bit clean channel is required,
452 one must not request a pty or should specify
454 A quote may be included in the command by quoting it with a backslash.
455 This option might be useful
456 to restrict certain public keys to perform just a specific operation.
457 An example might be a key that permits remote backups but nothing else.
458 Note that the client may specify TCP/IP and/or X11
459 forwarding unless they are explicitly prohibited.
460 Note that this option applies to shell, command or subsystem execution.
461 .It Cm environment="NAME=value"
462 Specifies that the string is to be added to the environment when
463 logging in using this key.
464 Environment variables set this way
465 override other default environment values.
466 Multiple options of this type are permitted.
467 Environment processing is disabled by default and is
469 .Cm PermitUserEnvironment
471 This option is automatically disabled if
474 .It Cm no-port-forwarding
475 Forbids TCP/IP forwarding when this key is used for authentication.
476 Any port forward requests by the client will return an error.
477 This might be used, e.g., in connection with the
480 .It Cm no-X11-forwarding
481 Forbids X11 forwarding when this key is used for authentication.
482 Any X11 forward requests by the client will return an error.
483 .It Cm no-agent-forwarding
484 Forbids authentication agent forwarding when this key is used for
487 Prevents tty allocation (a request to allocate a pty will fail).
488 .It Cm permitopen="host:port"
491 port forwarding such that it may only connect to the specified host and
493 IPv6 addresses can be specified with an alternative syntax:
497 options may be applied separated by commas. No pattern matching is
498 performed on the specified hostnames, they must be literal domains or
502 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
504 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
506 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
508 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
509 .Sh SSH_KNOWN_HOSTS FILE FORMAT
511 .Pa /etc/ssh/ssh_known_hosts
513 .Pa $HOME/.ssh/known_hosts
514 files contain host public keys for all known hosts.
515 The global file should
516 be prepared by the administrator (optional), and the per-user file is
517 maintained automatically: whenever the user connects from an unknown host
518 its key is added to the per-user file.
520 Each line in these files contains the following fields: hostnames,
521 bits, exponent, modulus, comment.
522 The fields are separated by spaces.
524 Hostnames is a comma-separated list of patterns ('*' and '?' act as
525 wildcards); each pattern in turn is matched against the canonical host
526 name (when authenticating a client) or against the user-supplied
527 name (when authenticating a server).
528 A pattern may also be preceded by
530 to indicate negation: if the host name matches a negated
531 pattern, it is not accepted (by that line) even if it matched another
534 Bits, exponent, and modulus are taken directly from the RSA host key; they
535 can be obtained, e.g., from
536 .Pa /etc/ssh/ssh_host_key.pub .
537 The optional comment field continues to the end of the line, and is not used.
541 and empty lines are ignored as comments.
543 When performing host authentication, authentication is accepted if any
544 matching line has the proper key.
545 It is thus permissible (but not
546 recommended) to have several lines or different host keys for the same
548 This will inevitably happen when short forms of host names
549 from different domains are put in the file.
551 that the files contain conflicting information; authentication is
552 accepted if valid information can be found from either file.
554 Note that the lines in these files are typically hundreds of characters
555 long, and you definitely don't want to type in the host keys by hand.
556 Rather, generate them by a script
558 .Pa /etc/ssh/ssh_host_key.pub
559 and adding the host names at the front.
562 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
563 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
567 .It Pa /etc/ssh/sshd_config
568 Contains configuration data for
570 The file format and configuration options are described in
572 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key
573 These two files contain the private parts of the host keys.
574 These files should only be owned by root, readable only by root, and not
575 accessible to others.
578 does not start if this file is group/world-accessible.
579 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub
580 These two files contain the public parts of the host keys.
581 These files should be world-readable but writable only by
583 Their contents should match the respective private parts.
585 really used for anything; they are provided for the convenience of
586 the user so their contents can be copied to known hosts files.
587 These files are created using
589 .It Pa /etc/ssh/moduli
590 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
591 The file format is described in
597 during privilege separation in the pre-authentication phase.
598 The directory should not contain any files and must be owned by root
599 and not group or world-writable.
600 .It Pa /var/run/sshd.pid
601 Contains the process ID of the
603 listening for connections (if there are several daemons running
604 concurrently for different ports, this contains the process ID of the one
606 The content of this file is not sensitive; it can be world-readable.
607 .It Pa $HOME/.ssh/authorized_keys
608 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
609 This file must be readable by root (which may on some machines imply
610 it being world-readable if the user's home directory resides on an NFS
612 It is recommended that it not be accessible by others.
613 The format of this file is described above.
614 Users will place the contents of their
619 files into this file, as described in
621 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
622 These files are consulted when using rhosts with RSA host
623 authentication or protocol version 2 hostbased authentication
624 to check the public key of the host.
625 The key must be listed in one of these files to be accepted.
626 The client uses the same files
627 to verify that it is connecting to the correct remote host.
628 These files should be writable only by root/the owner.
629 .Pa /etc/ssh/ssh_known_hosts
630 should be world-readable, and
631 .Pa $HOME/.ssh/known_hosts
632 can but need not be world-readable.
636 refuses to let anyone except root log in.
637 The contents of the file
638 are displayed to anyone trying to log in, and non-root connections are
640 The file should be world-readable.
641 .It Pa /etc/hosts.allow, /etc/hosts.deny
642 Access controls that should be enforced by tcp-wrappers are defined here.
643 Further details are described in
646 This file contains host-username pairs, separated by a space, one per
648 The given user on the corresponding host is permitted to log in
650 The same file is used by rlogind and rshd.
652 be writable only by the user; it is recommended that it not be
653 accessible by others.
655 If is also possible to use netgroups in the file.
657 name may be of the form +@groupname to specify all hosts or all users
661 this file is exactly the same as for
663 However, this file is
664 not used by rlogin and rshd, so using this permits access using SSH only.
665 .It Pa /etc/hosts.equiv
666 This file is used during
669 In the simplest form, this file contains host names, one per line.
671 those hosts are permitted to log in without a password, provided they
672 have the same user name on both machines.
673 The host name may also be
674 followed by a user name; such users are permitted to log in as
676 user on this machine (except root).
677 Additionally, the syntax
679 can be used to specify netgroups.
680 Negated entries start with
683 If the client host/user is successfully matched in this file, login is
684 automatically permitted provided the client and server user names are the
686 Additionally, successful RSA host authentication is normally required.
687 This file must be writable only by root; it is recommended
688 that it be world-readable.
690 .Sy "Warning: It is almost never a good idea to use user names in"
692 Beware that it really means that the named user(s) can log in as
694 which includes bin, daemon, adm, and other accounts that own critical
695 binaries and directories.
696 Using a user name practically grants the user root access.
697 The only valid use for user names that I can think
698 of is in negative entries.
700 Note that this warning also applies to rsh/rlogin.
701 .It Pa /etc/ssh/shosts.equiv
702 This is processed exactly as
703 .Pa /etc/hosts.equiv .
704 However, this file may be useful in environments that want to run both
706 .It Pa $HOME/.ssh/environment
707 This file is read into the environment at login (if it exists).
708 It can only contain empty lines, comment lines (that start with
710 and assignment lines of the form name=value.
711 The file should be writable
712 only by the user; it need not be readable by anyone else.
713 Environment processing is disabled by default and is
715 .Cm PermitUserEnvironment
718 If this file exists, it is run with
721 environment files but before starting the user's shell or command.
722 It must not produce any output on stdout; stderr must be used
724 If X11 forwarding is in use, it will receive the "proto cookie" pair in
725 its standard input (and
732 will not run xauth automatically to add X11 cookies.
734 The primary purpose of this file is to run any initialization routines
735 which may be needed before the user's home directory becomes
736 accessible; AFS is a particular example of such an environment.
738 This file will probably contain some initialization code followed by
739 something similar to:
741 if read proto cookie && [ -n "$DISPLAY" ]; then
742 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
743 # X11UseLocalhost=yes
744 echo add unix:`echo $DISPLAY |
745 cut -c11-` $proto $cookie
748 echo add $DISPLAY $proto $cookie
753 If this file does not exist,
756 does not exist either, xauth is used to add the cookie.
758 This file should be writable only by the user, and need not be
759 readable by anyone else.
760 .It Pa /etc/ssh/sshrc
763 This can be used to specify
764 machine-specific login-time initializations globally.
765 This file should be writable only by root, and should be world-readable.
768 OpenSSH is a derivative of the original and free
769 ssh 1.2.12 release by Tatu Ylonen.
770 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
771 Theo de Raadt and Dug Song
772 removed many bugs, re-added newer features and
774 Markus Friedl contributed the support for SSH
775 protocol versions 1.5 and 2.0.
776 Niels Provos and Markus Friedl contributed support
777 for privilege separation.
795 .%T "SSH Protocol Architecture"
796 .%N draft-ietf-secsh-architecture-12.txt
798 .%O work in progress material
804 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
805 .%N draft-ietf-secsh-dh-group-exchange-02.txt
807 .%O work in progress material