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 .Dd September 25, 1999
44 .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
58 (SSH Daemon) is the daemon program for
60 Together these programs replace rlogin and rsh, and
61 provide secure encrypted communications between two untrusted hosts
62 over an insecure network.
63 The programs are intended to be as easy to
64 install and use as possible.
67 is the daemon that listens for connections from clients.
68 It is normally started at boot from
71 daemon for each incoming connection.
72 The forked daemons handle
73 key exchange, encryption, authentication, command execution,
75 This implementation of
77 supports both SSH protocol version 1 and 2 simultaneously.
81 .Ss SSH protocol version 1
83 Each host has a host-specific RSA key
84 (normally 1024 bits) used to identify the host.
86 the daemon starts, it generates a server RSA key (normally 768 bits).
87 This key is normally regenerated every hour if it has been used, and
88 is never stored on disk.
90 Whenever a client connects the daemon responds with its public
92 The client compares the
93 RSA host key against its own database to verify that it has not changed.
94 The client then generates a 256 bit random number.
96 random number using both the host key and the server key, and sends
97 the encrypted number to the server.
98 Both sides then use this
99 random number as a session key which is used to encrypt all further
100 communications in the session.
101 The rest of the session is encrypted
102 using a conventional cipher, currently Blowfish or 3DES, with 3DES
103 being used by default.
104 The client selects the encryption algorithm
105 to use from those offered by the server.
107 Next, the server and the client enter an authentication dialog.
108 The client tries to authenticate itself using
112 authentication combined with RSA host
113 authentication, RSA challenge-response authentication, or password
114 based authentication.
116 Rhosts authentication is normally disabled
117 because it is fundamentally insecure, but can be enabled in the server
118 configuration file if desired.
119 System security is not improved unless
124 are disabled (thus completely disabling
130 .Ss SSH protocol version 2
132 Version 2 works similarly:
133 Each host has a host-specific key (RSA or DSA) used to identify the host.
134 However, when the daemon starts, it does not generate a server key.
135 Forward security is provided through a Diffie-Hellman key agreement.
136 This key agreement results in a shared session key.
138 The rest of the session is encrypted using a symmetric cipher, currently
139 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
140 The client selects the encryption algorithm
141 to use from those offered by the server.
142 Additionally, session integrity is provided
143 through a cryptographic message authentication code
144 (hmac-sha1 or hmac-md5).
146 Protocol version 2 provides a public key based
147 user (PubkeyAuthentication) or
148 client host (HostbasedAuthentication) authentication method,
149 conventional password authentication and challenge response based methods.
151 .Ss Command execution and data forwarding
153 If the client successfully authenticates itself, a dialog for
154 preparing the session is entered.
155 At this time the client may request
156 things like allocating a pseudo-tty, forwarding X11 connections,
157 forwarding TCP/IP connections, or forwarding the authentication agent
158 connection over the secure channel.
160 Finally, the client either requests a shell or execution of a command.
161 The sides then enter session mode.
162 In this mode, either side may send
163 data at any time, and such data is forwarded to/from the shell or
164 command on the server side, and the user terminal in the client side.
166 When the user program terminates and all forwarded X11 and other
167 connections have been closed, the server sends command exit status to
168 the client, and both sides exit.
171 can be configured using command-line options or a configuration
173 Command-line options override values specified in the
177 rereads its configuration file when it receives a hangup signal,
179 by executing itself with the name it was started as, i.e.,
182 The options are as follows:
185 Specifies the number of bits in the ephemeral protocol version 1
186 server key (default 768).
189 The server sends verbose debug output to the system
190 log, and does not put itself in the background.
191 The server also will not fork and will only process one connection.
192 This option is only intended for debugging for the server.
193 Multiple -d options increase the debugging level.
196 When this option is specified,
198 will send the output to the standard error instead of the system log.
199 .It Fl f Ar configuration_file
200 Specifies the name of the configuration file.
202 .Pa /etc/ssh/sshd_config .
204 refuses to start if there is no configuration file.
205 .It Fl g Ar login_grace_time
206 Gives the grace time for clients to authenticate themselves (default
208 If the client fails to authenticate the user within
209 this many seconds, the server disconnects and exits.
210 A value of zero indicates no limit.
211 .It Fl h Ar host_key_file
212 Specifies a file from which a host key is read.
213 This option must be given if
215 is not run as root (as the normal
216 host key files are normally not readable by anyone but root).
218 .Pa /etc/ssh/ssh_host_key
219 for protocol version 1, and
220 .Pa /etc/ssh/ssh_host_dsa_key
221 for protocol version 2.
222 It is possible to have multiple host key files for
223 the different protocol versions and host key algorithms.
227 is being run from inetd.
230 from inetd because it needs to generate the server key before it can
231 respond to the client, and this may take tens of seconds.
232 Clients would have to wait too long if the key was regenerated every time.
233 However, with small key sizes (e.g., 512) using
237 .It Fl k Ar key_gen_time
238 Specifies how often the ephemeral protocol version 1 server key is
239 regenerated (default 3600 seconds, or one hour).
240 The motivation for regenerating the key fairly
241 often is that the key is not stored anywhere, and after about an hour,
242 it becomes impossible to recover the key for decrypting intercepted
243 communications even if the machine is cracked into or physically
245 A value of zero indicates that the key will never be regenerated.
247 Can be used to give options in the format used in the configuration file.
248 This is useful for specifying options for which there is no separate
251 Specifies the port on which the server listens for connections
253 Multiple port options are permitted.
254 Ports specified in the configuration file are ignored when a
255 command-line port is specified.
258 Nothing is sent to the system log.
259 Normally the beginning,
260 authentication, and termination of each connection is logged.
263 Only check the validity of the configuration file and sanity of the keys.
264 This is useful for updating
266 reliably as configuration options may change.
268 This option is used to specify the size of the field
271 structure that holds the remote host name.
272 If the resolved host name is longer than
274 the dotted decimal value will be used instead.
275 This allows hosts with very long host names that
276 overflow this field to still be uniquely identified.
279 indicates that only dotted decimal addresses
280 should be put into the
284 is also be used to prevent
286 from making DNS requests unless the authentication
287 mechanism or configuration requires it.
288 Authentication mechanisms that may require DNS include
289 .Cm RhostsAuthentication ,
290 .Cm RhostsRSAAuthentication ,
291 .Cm HostbasedAuthentication
293 .Cm from="pattern-list"
294 option in a key file.
295 Configuration options that require DNS include using a
301 When this option is specified
303 will not detach and does not become a daemon.
304 This allows easy monitoring of
309 to use IPv4 addresses only.
313 to use IPv6 addresses only.
315 .Sh CONFIGURATION FILE
317 reads configuration data from
318 .Pa /etc/ssh/sshd_config
319 (or the file specified with
321 on the command line).
322 The file format and configuration options are described in
325 When a user successfully logs in,
328 .Bl -enum -offset indent
330 If the login is on a tty, and no command has been specified,
331 prints last login time and
333 (unless prevented in the configuration file or by
334 .Pa $HOME/.hushlogin ;
339 If the login is on a tty, records login time.
343 .Pa /var/run/nologin ;
344 if one exists, it prints the 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
370 files are given the X11
371 authentication protocol and cookie (if applicable) in standard input.
373 Runs user's shell or command.
375 .Sh AUTHORIZED_KEYS FILE FORMAT
376 .Pa $HOME/.ssh/authorized_keys
377 is the default file that lists the public keys that are
378 permitted for RSA authentication in protocol version 1
379 and for public key authentication (PubkeyAuthentication)
380 in protocol version 2.
381 .Cm AuthorizedKeysFile
382 may be used to specify an alternative file.
384 Each line of the file contains one
385 key (empty lines and lines starting with a
389 Each RSA public key consists of the following fields, separated by
390 spaces: options, bits, exponent, modulus, comment.
391 Each protocol version 2 public key consists of:
392 options, keytype, base64 encoded key, comment.
394 is optional; its presence is determined by whether the line starts
395 with a number or not (the options field never starts with a number).
396 The bits, exponent, modulus and comment fields give the RSA key for
397 protocol version 1; the
398 comment field is not used for anything (but may be convenient for the
399 user to identify the key).
400 For protocol version 2 the keytype is
405 Note that lines in this file are usually several hundred bytes long
406 (because of the size of the public key encoding).
407 You don't want to type them in; instead, copy the
415 enforces a minimum RSA key modulus size for protocol 1
416 and protocol 2 keys of 768 bits.
418 The options (if present) consist of comma-separated option
420 No spaces are permitted, except within double quotes.
421 The following option specifications are supported (note
422 that option keywords are case-insensitive):
424 .It Cm from="pattern-list"
425 Specifies that in addition to public key authentication, the canonical name
426 of the remote host must be present in the comma-separated list of
432 The list may also contain
433 patterns negated by prefixing them with
435 if the canonical host name matches a negated pattern, the key is not accepted.
437 of this option is to optionally increase security: public key authentication
438 by itself does not trust the network or name servers or anything (but
439 the key); however, if somebody somehow steals the key, the key
440 permits an intruder to log in from anywhere in the world.
441 This additional option makes using a stolen key more difficult (name
442 servers and/or routers would have to be compromised in addition to
444 .It Cm command="command"
445 Specifies that the command is executed whenever this key is used for
447 The command supplied by the user (if any) is ignored.
448 The command is run on a pty if the client requests a pty;
449 otherwise it is run without a tty.
450 If a 8-bit clean channel is required,
451 one must not request a pty or should specify
453 A quote may be included in the command by quoting it with a backslash.
454 This option might be useful
455 to restrict certain public keys to perform just a specific operation.
456 An example might be a key that permits remote backups but nothing else.
457 Note that the client may specify TCP/IP and/or X11
458 forwarding unless they are explicitly prohibited.
459 Note that this option applies to shell, command or subsystem execution.
460 .It Cm environment="NAME=value"
461 Specifies that the string is to be added to the environment when
462 logging in using this key.
463 Environment variables set this way
464 override other default environment values.
465 Multiple options of this type are permitted.
466 Environment processing is disabled by default and is
468 .Cm PermitUserEnvironment
470 This option is automatically disabled if
473 .It Cm no-port-forwarding
474 Forbids TCP/IP forwarding when this key is used for authentication.
475 Any port forward requests by the client will return an error.
476 This might be used, e.g., in connection with the
479 .It Cm no-X11-forwarding
480 Forbids X11 forwarding when this key is used for authentication.
481 Any X11 forward requests by the client will return an error.
482 .It Cm no-agent-forwarding
483 Forbids authentication agent forwarding when this key is used for
486 Prevents tty allocation (a request to allocate a pty will fail).
487 .It Cm permitopen="host:port"
490 port forwarding such that it may only connect to the specified host and
492 IPv6 addresses can be specified with an alternative syntax:
496 options may be applied separated by commas. No pattern matching is
497 performed on the specified hostnames, they must be literal domains or
501 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
503 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
505 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
507 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
508 .Sh SSH_KNOWN_HOSTS FILE FORMAT
510 .Pa /etc/ssh/ssh_known_hosts
512 .Pa $HOME/.ssh/known_hosts
513 files contain host public keys for all known hosts.
514 The global file should
515 be prepared by the administrator (optional), and the per-user file is
516 maintained automatically: whenever the user connects from an unknown host
517 its key is added to the per-user file.
519 Each line in these files contains the following fields: hostnames,
520 bits, exponent, modulus, comment.
521 The fields are separated by spaces.
523 Hostnames is a comma-separated list of patterns ('*' and '?' act as
524 wildcards); each pattern in turn is matched against the canonical host
525 name (when authenticating a client) or against the user-supplied
526 name (when authenticating a server).
527 A pattern may also be preceded by
529 to indicate negation: if the host name matches a negated
530 pattern, it is not accepted (by that line) even if it matched another
533 Bits, exponent, and modulus are taken directly from the RSA host key; they
534 can be obtained, e.g., from
535 .Pa /etc/ssh/ssh_host_key.pub .
536 The optional comment field continues to the end of the line, and is not used.
540 and empty lines are ignored as comments.
542 When performing host authentication, authentication is accepted if any
543 matching line has the proper key.
544 It is thus permissible (but not
545 recommended) to have several lines or different host keys for the same
547 This will inevitably happen when short forms of host names
548 from different domains are put in the file.
550 that the files contain conflicting information; authentication is
551 accepted if valid information can be found from either file.
553 Note that the lines in these files are typically hundreds of characters
554 long, and you definitely don't want to type in the host keys by hand.
555 Rather, generate them by a script
557 .Pa /etc/ssh/ssh_host_key.pub
558 and adding the host names at the front.
561 closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
562 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
566 .It Pa /etc/ssh/sshd_config
567 Contains configuration data for
569 The file format and configuration options are described in
571 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key
572 These two files contain the private parts of the host keys.
573 These files should only be owned by root, readable only by root, and not
574 accessible to others.
577 does not start if this file is group/world-accessible.
578 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub
579 These two files contain the public parts of the host keys.
580 These files should be world-readable but writable only by
582 Their contents should match the respective private parts.
584 really used for anything; they are provided for the convenience of
585 the user so their contents can be copied to known hosts files.
586 These files are created using
588 .It Pa /etc/ssh/moduli
589 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
590 The file format is described in
596 during privilege separation in the pre-authentication phase.
597 The directory should not contain any files and must be owned by root
598 and not group or world-writable.
599 .It Pa /var/run/sshd.pid
600 Contains the process ID of the
602 listening for connections (if there are several daemons running
603 concurrently for different ports, this contains the process ID of the one
605 The content of this file is not sensitive; it can be world-readable.
606 .It Pa $HOME/.ssh/authorized_keys
607 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
608 This file must be readable by root (which may on some machines imply
609 it being world-readable if the user's home directory resides on an NFS
611 It is recommended that it not be accessible by others.
612 The format of this file is described above.
613 Users will place the contents of their
618 files into this file, as described in
620 .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
621 These files are consulted when using rhosts with RSA host
622 authentication or protocol version 2 hostbased authentication
623 to check the public key of the host.
624 The key must be listed in one of these files to be accepted.
625 The client uses the same files
626 to verify that it is connecting to the correct remote host.
627 These files should be writable only by root/the owner.
628 .Pa /etc/ssh/ssh_known_hosts
629 should be world-readable, and
630 .Pa $HOME/.ssh/known_hosts
631 can but need not be world-readable.
635 refuses to let anyone except root log in.
636 The contents of the file
637 are displayed to anyone trying to log in, and non-root connections are
639 The file should be world-readable.
640 .It Pa /etc/hosts.allow, /etc/hosts.deny
641 Access controls that should be enforced by tcp-wrappers are defined here.
642 Further details are described in
645 This file contains host-username pairs, separated by a space, one per
647 The given user on the corresponding host is permitted to log in
649 The same file is used by rlogind and rshd.
651 be writable only by the user; it is recommended that it not be
652 accessible by others.
654 If is also possible to use netgroups in the file.
656 name may be of the form +@groupname to specify all hosts or all users
660 this file is exactly the same as for
662 However, this file is
663 not used by rlogin and rshd, so using this permits access using SSH only.
664 .It Pa /etc/hosts.equiv
665 This file is used during
668 In the simplest form, this file contains host names, one per line.
670 those hosts are permitted to log in without a password, provided they
671 have the same user name on both machines.
672 The host name may also be
673 followed by a user name; such users are permitted to log in as
675 user on this machine (except root).
676 Additionally, the syntax
678 can be used to specify netgroups.
679 Negated entries start with
682 If the client host/user is successfully matched in this file, login is
683 automatically permitted provided the client and server user names are the
685 Additionally, successful RSA host authentication is normally required.
686 This file must be writable only by root; it is recommended
687 that it be world-readable.
689 .Sy "Warning: It is almost never a good idea to use user names in"
691 Beware that it really means that the named user(s) can log in as
693 which includes bin, daemon, adm, and other accounts that own critical
694 binaries and directories.
695 Using a user name practically grants the user root access.
696 The only valid use for user names that I can think
697 of is in negative entries.
699 Note that this warning also applies to rsh/rlogin.
700 .It Pa /etc/ssh/shosts.equiv
701 This is processed exactly as
702 .Pa /etc/hosts.equiv .
703 However, this file may be useful in environments that want to run both
705 .It Pa $HOME/.ssh/environment
706 This file is read into the environment at login (if it exists).
707 It can only contain empty lines, comment lines (that start with
709 and assignment lines of the form name=value.
710 The file should be writable
711 only by the user; it need not be readable by anyone else.
712 Environment processing is disabled by default and is
714 .Cm PermitUserEnvironment
717 If this file exists, it is run with
720 environment files but before starting the user's shell or command.
721 It must not produce any output on stdout; stderr must be used
723 If X11 forwarding is in use, it will receive the "proto cookie" pair in
724 its standard input (and
731 will not run xauth automatically to add X11 cookies.
733 The primary purpose of this file is to run any initialization routines
734 which may be needed before the user's home directory becomes
735 accessible; AFS is a particular example of such an environment.
737 This file will probably contain some initialization code followed by
738 something similar to:
740 if read proto cookie && [ -n "$DISPLAY" ]; then
741 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
742 # X11UseLocalhost=yes
743 echo add unix:`echo $DISPLAY |
744 cut -c11-` $proto $cookie
747 echo add $DISPLAY $proto $cookie
752 If this file does not exist,
755 does not exist either, xauth is used to add the cookie.
757 This file should be writable only by the user, and need not be
758 readable by anyone else.
759 .It Pa /etc/ssh/sshrc
762 This can be used to specify
763 machine-specific login-time initializations globally.
764 This file should be writable only by root, and should be world-readable.
767 OpenSSH is a derivative of the original and free
768 ssh 1.2.12 release by Tatu Ylonen.
769 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
770 Theo de Raadt and Dug Song
771 removed many bugs, re-added newer features and
773 Markus Friedl contributed the support for SSH
774 protocol versions 1.5 and 2.0.
775 Niels Provos and Markus Friedl contributed support
776 for privilege separation.
794 .%T "SSH Protocol Architecture"
795 .%N draft-ietf-secsh-architecture-12.txt
797 .%O work in progress material
803 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
804 .%N draft-ietf-secsh-dh-group-exchange-02.txt
806 .%O work in progress material