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 1.186 2002/06/22 16:45:29 stevesk 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
355 Changes to user's home directory.
359 exists, runs it; else if
362 it; otherwise runs xauth.
365 files are given the X11
366 authentication protocol and cookie in standard input.
368 Runs user's shell or command.
370 .Sh AUTHORIZED_KEYS FILE FORMAT
371 .Pa $HOME/.ssh/authorized_keys
372 is the default file that lists the public keys that are
373 permitted for RSA authentication in protocol version 1
374 and for public key authentication (PubkeyAuthentication)
375 in protocol version 2.
376 .Cm AuthorizedKeysFile
377 may be used to specify an alternative file.
379 Each line of the file contains one
380 key (empty lines and lines starting with a
384 Each RSA public key consists of the following fields, separated by
385 spaces: options, bits, exponent, modulus, comment.
386 Each protocol version 2 public key consists of:
387 options, keytype, base64 encoded key, comment.
389 are optional; its presence is determined by whether the line starts
390 with a number or not (the option field never starts with a number).
391 The bits, exponent, modulus and comment fields give the RSA key for
392 protocol version 1; the
393 comment field is not used for anything (but may be convenient for the
394 user to identify the key).
395 For protocol version 2 the keytype is
400 Note that lines in this file are usually several hundred bytes long
401 (because of the size of the RSA key modulus).
402 You don't want to type them in; instead, copy the
410 enforces a minimum RSA key modulus size for protocol 1
411 and protocol 2 keys of 768 bits.
413 The options (if present) consist of comma-separated option
415 No spaces are permitted, except within double quotes.
416 The following option specifications are supported (note
417 that option keywords are case-insensitive):
419 .It Cm from="pattern-list"
420 Specifies that in addition to RSA authentication, the canonical name
421 of the remote host must be present in the comma-separated list of
427 The list may also contain
428 patterns negated by prefixing them with
430 if the canonical host name matches a negated pattern, the key is not accepted.
432 of this option is to optionally increase security: RSA authentication
433 by itself does not trust the network or name servers or anything (but
434 the key); however, if somebody somehow steals the key, the key
435 permits an intruder to log in from anywhere in the world.
436 This additional option makes using a stolen key more difficult (name
437 servers and/or routers would have to be compromised in addition to
439 .It Cm command="command"
440 Specifies that the command is executed whenever this key is used for
442 The command supplied by the user (if any) is ignored.
443 The command is run on a pty if the client requests a pty;
444 otherwise it is run without a tty.
445 If a 8-bit clean channel is required,
446 one must not request a pty or should specify
448 A quote may be included in the command by quoting it with a backslash.
449 This option might be useful
450 to restrict certain RSA keys to perform just a specific operation.
451 An example might be a key that permits remote backups but nothing else.
452 Note that the client may specify TCP/IP and/or X11
453 forwarding unless they are explicitly prohibited.
454 Note that this option applies to shell, command or subsystem execution.
455 .It Cm environment="NAME=value"
456 Specifies that the string is to be added to the environment when
457 logging in using this key.
458 Environment variables set this way
459 override other default environment values.
460 Multiple options of this type are permitted.
461 This option is automatically disabled if
464 .It Cm no-port-forwarding
465 Forbids TCP/IP forwarding when this key is used for authentication.
466 Any port forward requests by the client will return an error.
467 This might be used, e.g., in connection with the
470 .It Cm no-X11-forwarding
471 Forbids X11 forwarding when this key is used for authentication.
472 Any X11 forward requests by the client will return an error.
473 .It Cm no-agent-forwarding
474 Forbids authentication agent forwarding when this key is used for
477 Prevents tty allocation (a request to allocate a pty will fail).
478 .It Cm permitopen="host:port"
481 port forwarding such that it may only connect to the specified host and
483 IPv6 addresses can be specified with an alternative syntax:
487 options may be applied separated by commas. No pattern matching is
488 performed on the specified hostnames, they must be literal domains or
492 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
494 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
496 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
498 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
499 .Sh SSH_KNOWN_HOSTS FILE FORMAT
501 .Pa /etc/ssh/ssh_known_hosts ,
503 .Pa $HOME/.ssh/known_hosts
504 files contain host public keys for all known hosts.
505 The global file should
506 be prepared by the administrator (optional), and the per-user file is
507 maintained automatically: whenever the user connects from an unknown host
508 its key is added to the per-user file.
510 Each line in these files contains the following fields: hostnames,
511 bits, exponent, modulus, comment.
512 The fields are separated by spaces.
514 Hostnames is a comma-separated list of patterns ('*' and '?' act as
515 wildcards); each pattern in turn is matched against the canonical host
516 name (when authenticating a client) or against the user-supplied
517 name (when authenticating a server).
518 A pattern may also be preceded by
520 to indicate negation: if the host name matches a negated
521 pattern, it is not accepted (by that line) even if it matched another
524 Bits, exponent, and modulus are taken directly from the RSA host key; they
525 can be obtained, e.g., from
526 .Pa /etc/ssh/ssh_host_key.pub .
527 The optional comment field continues to the end of the line, and is not used.
531 and empty lines are ignored as comments.
533 When performing host authentication, authentication is accepted if any
534 matching line has the proper key.
535 It is thus permissible (but not
536 recommended) to have several lines or different host keys for the same
538 This will inevitably happen when short forms of host names
539 from different domains are put in the file.
541 that the files contain conflicting information; authentication is
542 accepted if valid information can be found from either file.
544 Note that the lines in these files are typically hundreds of characters
545 long, and you definitely don't want to type in the host keys by hand.
546 Rather, generate them by a script
548 .Pa /etc/ssh/ssh_host_key.pub
549 and adding the host names at the front.
552 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
553 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
557 .It Pa /etc/ssh/sshd_config
558 Contains configuration data for
560 The file format and configuration options are described in
562 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
563 These three files contain the private parts of the host keys.
564 These files should only be owned by root, readable only by root, and not
565 accessible to others.
568 does not start if this file is group/world-accessible.
569 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
570 These three files contain the public parts of the host keys.
571 These files should be world-readable but writable only by
573 Their contents should match the respective private parts.
575 really used for anything; they are provided for the convenience of
576 the user so their contents can be copied to known hosts files.
577 These files are created using
580 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
585 during privilege separation in the pre-authentication phase.
586 The directory should not contain any files and must be owned by root
587 and not group or world-writable.
588 .It Pa /var/run/sshd.pid
589 Contains the process ID of the
591 listening for connections (if there are several daemons running
592 concurrently for different ports, this contains the process ID of the one
594 The content of this file is not sensitive; it can be world-readable.
595 .It Pa $HOME/.ssh/authorized_keys
596 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
597 This file must be readable by root (which may on some machines imply
598 it being world-readable if the user's home directory resides on an NFS
600 It is recommended that it not be accessible by others.
601 The format of this file is described above.
602 Users will place the contents of their
607 files into this file, as described in
609 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
610 These files are consulted when using rhosts with RSA host
611 authentication or protocol version 2 hostbased authentication
612 to check the public key of the host.
613 The key must be listed in one of these files to be accepted.
614 The client uses the same files
615 to verify that it is connecting to the correct remote host.
616 These files should be writable only by root/the owner.
617 .Pa /etc/ssh/ssh_known_hosts
618 should be world-readable, and
619 .Pa $HOME/.ssh/known_hosts
620 can but need not be world-readable.
624 refuses to let anyone except root log in.
625 The contents of the file
626 are displayed to anyone trying to log in, and non-root connections are
628 The file should be world-readable.
629 .It Pa /etc/hosts.allow, /etc/hosts.deny
630 Access controls that should be enforced by tcp-wrappers are defined here.
631 Further details are described in
634 This file contains host-username pairs, separated by a space, one per
636 The given user on the corresponding host is permitted to log in
638 The same file is used by rlogind and rshd.
640 be writable only by the user; it is recommended that it not be
641 accessible by others.
643 If is also possible to use netgroups in the file.
645 name may be of the form +@groupname to specify all hosts or all users
649 this file is exactly the same as for
651 However, this file is
652 not used by rlogin and rshd, so using this permits access using SSH only.
653 .It Pa /etc/hosts.equiv
654 This file is used during
657 In the simplest form, this file contains host names, one per line.
659 those hosts are permitted to log in without a password, provided they
660 have the same user name on both machines.
661 The host name may also be
662 followed by a user name; such users are permitted to log in as
664 user on this machine (except root).
665 Additionally, the syntax
667 can be used to specify netgroups.
668 Negated entries start with
671 If the client host/user is successfully matched in this file, login is
672 automatically permitted provided the client and server user names are the
674 Additionally, successful RSA host authentication is normally required.
675 This file must be writable only by root; it is recommended
676 that it be world-readable.
678 .Sy "Warning: It is almost never a good idea to use user names in"
680 Beware that it really means that the named user(s) can log in as
682 which includes bin, daemon, adm, and other accounts that own critical
683 binaries and directories.
684 Using a user name practically grants the user root access.
685 The only valid use for user names that I can think
686 of is in negative entries.
688 Note that this warning also applies to rsh/rlogin.
689 .It Pa /etc/shosts.equiv
690 This is processed exactly as
691 .Pa /etc/hosts.equiv .
692 However, this file may be useful in environments that want to run both
694 .It Pa $HOME/.ssh/environment
695 This file is read into the environment at login (if it exists).
696 It can only contain empty lines, comment lines (that start with
698 and assignment lines of the form name=value.
699 The file should be writable
700 only by the user; it need not be readable by anyone else.
702 If this file exists, it is run with /bin/sh after reading the
703 environment files but before starting the user's shell or command.
704 It must not produce any output on stdout; stderr must be used
706 If X11 forwarding is in use, it will receive the "proto cookie" pair in
707 its standard input (and
714 will not run xauth automatically to add X11 cookies.
716 The primary purpose of this file is to run any initialization routines
717 which may be needed before the user's home directory becomes
718 accessible; AFS is a particular example of such an environment.
720 This file will probably contain some initialization code followed by
721 something similar to:
723 if read proto cookie && [ -n "$DISPLAY" ]; then
724 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
725 # X11UseLocalhost=yes
726 xauth add unix:`echo $DISPLAY |
727 cut -c11-` $proto $cookie
730 xauth add $DISPLAY $proto $cookie
735 If this file does not exist,
738 does not exist either, xauth is used to add the cookie.
740 This file should be writable only by the user, and need not be
741 readable by anyone else.
742 .It Pa /etc/ssh/sshrc
745 This can be used to specify
746 machine-specific login-time initializations globally.
747 This file should be writable only by root, and should be world-readable.
750 OpenSSH is a derivative of the original and free
751 ssh 1.2.12 release by Tatu Ylonen.
752 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
753 Theo de Raadt and Dug Song
754 removed many bugs, re-added newer features and
756 Markus Friedl contributed the support for SSH
757 protocol versions 1.5 and 2.0.
758 Niels Provos and Markus Friedl contributed support
759 for privilege separation.
777 .%T "SSH Protocol Architecture"
778 .%N draft-ietf-secsh-architecture-12.txt
780 .%O work in progress material
786 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
787 .%N draft-ietf-secsh-dh-group-exchange-02.txt
789 .%O work in progress material