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 1.200 2003/10/08 08:27:36 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon
49 .Op Fl f Ar config_file
50 .Op Fl g Ar login_grace_time
51 .Op Fl h Ar host_key_file
52 .Op Fl k Ar key_gen_time
59 (SSH Daemon) is the daemon program for
61 Together these programs replace rlogin and rsh, and
62 provide secure encrypted communications between two untrusted hosts
63 over an insecure network.
64 The programs are intended to be as easy to
65 install and use as possible.
68 is the daemon that listens for connections from clients.
69 It is normally started at boot from
72 daemon for each incoming connection.
73 The forked daemons handle
74 key exchange, encryption, authentication, command execution,
76 This implementation of
78 supports both SSH protocol version 1 and 2 simultaneously.
81 .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 Regardless of the authentication type, the account is checked to
116 ensure that it is accessible. An account is not accessible if it is
119 or its group is listed in
121 \&. The definition of a locked account is system dependant. Some platforms
122 have their own account database (eg AIX) and some modify the passwd field (
128 on Tru64 and a leading
130 on Linux). If there is a requirement to disable password authentication
131 for the account while allowing still public-key, then the passwd field
132 should be set to something other than these values (eg
139 authentication is normally disabled
140 because it is fundamentally insecure, but can be enabled in the server
141 configuration file if desired.
142 System security is not improved unless
147 are disabled (thus completely disabling
152 .Ss SSH protocol version 2
153 Version 2 works similarly:
154 Each host has a host-specific key (RSA or DSA) used to identify the host.
155 However, when the daemon starts, it does not generate a server key.
156 Forward security is provided through a Diffie-Hellman key agreement.
157 This key agreement results in a shared session key.
159 The rest of the session is encrypted using a symmetric cipher, currently
160 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
161 The client selects the encryption algorithm
162 to use from those offered by the server.
163 Additionally, session integrity is provided
164 through a cryptographic message authentication code
165 (hmac-sha1 or hmac-md5).
167 Protocol version 2 provides a public key based
168 user (PubkeyAuthentication) or
169 client host (HostbasedAuthentication) authentication method,
170 conventional password authentication and challenge response based methods.
171 .Ss Command execution and data forwarding
172 If the client successfully authenticates itself, a dialog for
173 preparing the session is entered.
174 At this time the client may request
175 things like allocating a pseudo-tty, forwarding X11 connections,
176 forwarding TCP/IP connections, or forwarding the authentication agent
177 connection over the secure channel.
179 Finally, the client either requests a shell or execution of a command.
180 The sides then enter session mode.
181 In this mode, either side may send
182 data at any time, and such data is forwarded to/from the shell or
183 command on the server side, and the user terminal in the client side.
185 When the user program terminates and all forwarded X11 and other
186 connections have been closed, the server sends command exit status to
187 the client, and both sides exit.
190 can be configured using command-line options or a configuration file
192 .Xr sshd_config 5 ) .
193 Command-line options override values specified in the
197 rereads its configuration file when it receives a hangup signal,
199 by executing itself with the name it was started as, i.e.,
202 The options are as follows:
207 to use IPv4 addresses only.
211 to use IPv6 addresses only.
213 Specifies the number of bits in the ephemeral protocol version 1
214 server key (default 768).
216 When this option is specified,
218 will not detach and does not become a daemon.
219 This allows easy monitoring of
223 The server sends verbose debug output to the system
224 log, and does not put itself in the background.
225 The server also will not fork and will only process one connection.
226 This option is only intended for debugging for the server.
229 options increase the debugging level.
232 When this option is specified,
234 will send the output to the standard error instead of the system log.
235 .It Fl f Ar configuration_file
236 Specifies the name of the configuration file.
238 .Pa /etc/ssh/sshd_config .
240 refuses to start if there is no configuration file.
241 .It Fl g Ar login_grace_time
242 Gives the grace time for clients to authenticate themselves (default
244 If the client fails to authenticate the user within
245 this many seconds, the server disconnects and exits.
246 A value of zero indicates no limit.
247 .It Fl h Ar host_key_file
248 Specifies a file from which a host key is read.
249 This option must be given if
251 is not run as root (as the normal
252 host key files are normally not readable by anyone but root).
254 .Pa /etc/ssh/ssh_host_key
255 for protocol version 1, and
256 .Pa /etc/ssh/ssh_host_rsa_key
258 .Pa /etc/ssh/ssh_host_dsa_key
259 for protocol version 2.
260 It is possible to have multiple host key files for
261 the different protocol versions and host key algorithms.
269 from inetd because it needs to generate the server key before it can
270 respond to the client, and this may take tens of seconds.
271 Clients would have to wait too long if the key was regenerated every time.
272 However, with small key sizes (e.g., 512) using
276 .It Fl k Ar key_gen_time
277 Specifies how often the ephemeral protocol version 1 server key is
278 regenerated (default 3600 seconds, or one hour).
279 The motivation for regenerating the key fairly
280 often is that the key is not stored anywhere, and after about an hour
281 it becomes impossible to recover the key for decrypting intercepted
282 communications even if the machine is cracked into or physically
284 A value of zero indicates that the key will never be regenerated.
286 Can be used to give options in the format used in the configuration file.
287 This is useful for specifying options for which there is no separate
289 For full details of the options, and their values, see
292 Specifies the port on which the server listens for connections
294 Multiple port options are permitted.
295 Ports specified in the configuration file are ignored when a
296 command-line port is specified.
299 Nothing is sent to the system log.
300 Normally the beginning,
301 authentication, and termination of each connection is logged.
304 Only check the validity of the configuration file and sanity of the keys.
305 This is useful for updating
307 reliably as configuration options may change.
309 This option is used to specify the size of the field
312 structure that holds the remote host name.
313 If the resolved host name is longer than
315 the dotted decimal value will be used instead.
316 This allows hosts with very long host names that
317 overflow this field to still be uniquely identified.
320 indicates that only dotted decimal addresses
321 should be put into the
325 may also be used to prevent
327 from making DNS requests unless the authentication
328 mechanism or configuration requires it.
329 Authentication mechanisms that may require DNS include
330 .Cm RhostsRSAAuthentication ,
331 .Cm HostbasedAuthentication
333 .Cm from="pattern-list"
334 option in a key file.
335 Configuration options that require DNS include using a
341 .Sh CONFIGURATION FILE
343 reads configuration data from
344 .Pa /etc/ssh/sshd_config
345 (or the file specified with
347 on the command line).
348 The file format and configuration options are described in
351 When a user successfully logs in,
354 .Bl -enum -offset indent
356 If the login is on a tty, and no command has been specified,
357 prints last login time and
359 (unless prevented in the configuration file or by
360 .Pa $HOME/.hushlogin ;
365 If the login is on a tty, records login time.
369 if it exists, prints contents and quits
372 Changes to run with normal user privileges.
374 Sets up basic environment.
377 .Pa $HOME/.ssh/environment ,
378 if it exists, and users are allowed to change their environment.
380 .Cm PermitUserEnvironment
384 Changes to user's home directory.
388 exists, runs it; else if
391 it; otherwise runs xauth.
394 files are given the X11
395 authentication protocol and cookie in standard input.
397 Runs user's shell or command.
399 .Sh AUTHORIZED_KEYS FILE FORMAT
400 .Pa $HOME/.ssh/authorized_keys
401 is the default file that lists the public keys that are
402 permitted for RSA authentication in protocol version 1
403 and for public key authentication (PubkeyAuthentication)
404 in protocol version 2.
405 .Cm AuthorizedKeysFile
406 may be used to specify an alternative file.
408 Each line of the file contains one
409 key (empty lines and lines starting with a
413 Each RSA public key consists of the following fields, separated by
414 spaces: options, bits, exponent, modulus, comment.
415 Each protocol version 2 public key consists of:
416 options, keytype, base64 encoded key, comment.
418 is optional; its presence is determined by whether the line starts
419 with a number or not (the options field never starts with a number).
420 The bits, exponent, modulus and comment fields give the RSA key for
421 protocol version 1; the
422 comment field is not used for anything (but may be convenient for the
423 user to identify the key).
424 For protocol version 2 the keytype is
429 Note that lines in this file are usually several hundred bytes long
430 (because of the size of the public key encoding).
431 You don't want to type them in; instead, copy the
439 enforces a minimum RSA key modulus size for protocol 1
440 and protocol 2 keys of 768 bits.
442 The options (if present) consist of comma-separated option
444 No spaces are permitted, except within double quotes.
445 The following option specifications are supported (note
446 that option keywords are case-insensitive):
448 .It Cm from="pattern-list"
449 Specifies that in addition to public key authentication, the canonical name
450 of the remote host must be present in the comma-separated list of
456 The list may also contain
457 patterns negated by prefixing them with
459 if the canonical host name matches a negated pattern, the key is not accepted.
461 of this option is to optionally increase security: public key authentication
462 by itself does not trust the network or name servers or anything (but
463 the key); however, if somebody somehow steals the key, the key
464 permits an intruder to log in from anywhere in the world.
465 This additional option makes using a stolen key more difficult (name
466 servers and/or routers would have to be compromised in addition to
468 .It Cm command="command"
469 Specifies that the command is executed whenever this key is used for
471 The command supplied by the user (if any) is ignored.
472 The command is run on a pty if the client requests a pty;
473 otherwise it is run without a tty.
474 If an 8-bit clean channel is required,
475 one must not request a pty or should specify
477 A quote may be included in the command by quoting it with a backslash.
478 This option might be useful
479 to restrict certain public keys to perform just a specific operation.
480 An example might be a key that permits remote backups but nothing else.
481 Note that the client may specify TCP/IP and/or X11
482 forwarding unless they are explicitly prohibited.
483 Note that this option applies to shell, command or subsystem execution.
484 .It Cm environment="NAME=value"
485 Specifies that the string is to be added to the environment when
486 logging in using this key.
487 Environment variables set this way
488 override other default environment values.
489 Multiple options of this type are permitted.
490 Environment processing is disabled by default and is
492 .Cm PermitUserEnvironment
494 This option is automatically disabled if
497 .It Cm no-port-forwarding
498 Forbids TCP/IP forwarding when this key is used for authentication.
499 Any port forward requests by the client will return an error.
500 This might be used, e.g., in connection with the
503 .It Cm no-X11-forwarding
504 Forbids X11 forwarding when this key is used for authentication.
505 Any X11 forward requests by the client will return an error.
506 .It Cm no-agent-forwarding
507 Forbids authentication agent forwarding when this key is used for
510 Prevents tty allocation (a request to allocate a pty will fail).
511 .It Cm permitopen="host:port"
514 port forwarding such that it may only connect to the specified host and
516 IPv6 addresses can be specified with an alternative syntax:
517 .Ar host Ns / Ns Ar port .
520 options may be applied separated by commas.
521 No pattern matching is performed on the specified hostnames,
522 they must be literal domains or addresses.
525 1024 33 12121...312314325 ylo@foo.bar
527 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
529 command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
531 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
532 .Sh SSH_KNOWN_HOSTS FILE FORMAT
534 .Pa /etc/ssh/ssh_known_hosts
536 .Pa $HOME/.ssh/known_hosts
537 files contain host public keys for all known hosts.
538 The global file should
539 be prepared by the administrator (optional), and the per-user file is
540 maintained automatically: whenever the user connects from an unknown host
541 its key is added to the per-user file.
543 Each line in these files contains the following fields: hostnames,
544 bits, exponent, modulus, comment.
545 The fields are separated by spaces.
547 Hostnames is a comma-separated list of patterns
552 wildcards); each pattern in turn is matched against the canonical host
553 name (when authenticating a client) or against the user-supplied
554 name (when authenticating a server).
555 A pattern may also be preceded by
557 to indicate negation: if the host name matches a negated
558 pattern, it is not accepted (by that line) even if it matched another
561 Bits, exponent, and modulus are taken directly from the RSA host key; they
562 can be obtained, e.g., from
563 .Pa /etc/ssh/ssh_host_key.pub .
564 The optional comment field continues to the end of the line, and is not used.
568 and empty lines are ignored as comments.
570 When performing host authentication, authentication is accepted if any
571 matching line has the proper key.
572 It is thus permissible (but not
573 recommended) to have several lines or different host keys for the same
575 This will inevitably happen when short forms of host names
576 from different domains are put in the file.
578 that the files contain conflicting information; authentication is
579 accepted if valid information can be found from either file.
581 Note that the lines in these files are typically hundreds of characters
582 long, and you definitely don't want to type in the host keys by hand.
583 Rather, generate them by a script
585 .Pa /etc/ssh/ssh_host_key.pub
586 and adding the host names at the front.
589 closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
590 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
594 .It Pa /etc/ssh/sshd_config
595 Contains configuration data for
597 The file format and configuration options are described in
599 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
600 These three files contain the private parts of the host keys.
601 These files should only be owned by root, readable only by root, and not
602 accessible to others.
605 does not start if this file is group/world-accessible.
606 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
607 These three files contain the public parts of the host keys.
608 These files should be world-readable but writable only by
610 Their contents should match the respective private parts.
612 really used for anything; they are provided for the convenience of
613 the user so their contents can be copied to known hosts files.
614 These files are created using
617 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
618 The file format is described in
624 during privilege separation in the pre-authentication phase.
625 The directory should not contain any files and must be owned by root
626 and not group or world-writable.
627 .It Pa /var/run/sshd.pid
628 Contains the process ID of the
630 listening for connections (if there are several daemons running
631 concurrently for different ports, this contains the process ID of the one
633 The content of this file is not sensitive; it can be world-readable.
634 .It Pa $HOME/.ssh/authorized_keys
635 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
636 This file must be readable by root (which may on some machines imply
637 it being world-readable if the user's home directory resides on an NFS
639 It is recommended that it not be accessible by others.
640 The format of this file is described above.
641 Users will place the contents of their
646 files into this file, as described in
648 .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
649 These files are consulted when using rhosts with RSA host
650 authentication or protocol version 2 hostbased authentication
651 to check the public key of the host.
652 The key must be listed in one of these files to be accepted.
653 The client uses the same files
654 to verify that it is connecting to the correct remote host.
655 These files should be writable only by root/the owner.
656 .Pa /etc/ssh/ssh_known_hosts
657 should be world-readable, and
658 .Pa $HOME/.ssh/known_hosts
659 can, but need not be, world-readable.
663 refuses to let anyone except root log in.
664 The contents of the file
665 are displayed to anyone trying to log in, and non-root connections are
667 The file should be world-readable.
668 .It Pa /etc/hosts.allow, /etc/hosts.deny
669 Access controls that should be enforced by tcp-wrappers are defined here.
670 Further details are described in
673 This file contains host-username pairs, separated by a space, one per
675 The given user on the corresponding host is permitted to log in
677 The same file is used by rlogind and rshd.
679 be writable only by the user; it is recommended that it not be
680 accessible by others.
682 It is also possible to use netgroups in the file.
684 name may be of the form +@groupname to specify all hosts or all users
688 this file is exactly the same as for
690 However, this file is
691 not used by rlogin and rshd, so using this permits access using SSH only.
692 .It Pa /etc/hosts.equiv
693 This file is used during
696 In the simplest form, this file contains host names, one per line.
698 those hosts are permitted to log in without a password, provided they
699 have the same user name on both machines.
700 The host name may also be
701 followed by a user name; such users are permitted to log in as
703 user on this machine (except root).
704 Additionally, the syntax
706 can be used to specify netgroups.
707 Negated entries start with
710 If the client host/user is successfully matched in this file, login is
711 automatically permitted provided the client and server user names are the
713 Additionally, successful RSA host authentication is normally required.
714 This file must be writable only by root; it is recommended
715 that it be world-readable.
717 .Sy "Warning: It is almost never a good idea to use user names in"
719 Beware that it really means that the named user(s) can log in as
721 which includes bin, daemon, adm, and other accounts that own critical
722 binaries and directories.
723 Using a user name practically grants the user root access.
724 The only valid use for user names that I can think
725 of is in negative entries.
727 Note that this warning also applies to rsh/rlogin.
728 .It Pa /etc/shosts.equiv
729 This is processed exactly as
730 .Pa /etc/hosts.equiv .
731 However, this file may be useful in environments that want to run both
733 .It Pa $HOME/.ssh/environment
734 This file is read into the environment at login (if it exists).
735 It can only contain empty lines, comment lines (that start with
737 and assignment lines of the form name=value.
738 The file should be writable
739 only by the user; it need not be readable by anyone else.
740 Environment processing is disabled by default and is
742 .Cm PermitUserEnvironment
745 If this file exists, it is run with
748 environment files but before starting the user's shell or command.
749 It must not produce any output on stdout; stderr must be used
751 If X11 forwarding is in use, it will receive the "proto cookie" pair in
752 its standard input (and
759 will not run xauth automatically to add X11 cookies.
761 The primary purpose of this file is to run any initialization routines
762 which may be needed before the user's home directory becomes
763 accessible; AFS is a particular example of such an environment.
765 This file will probably contain some initialization code followed by
766 something similar to:
768 if read proto cookie && [ -n "$DISPLAY" ]; then
769 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
770 # X11UseLocalhost=yes
771 echo add unix:`echo $DISPLAY |
772 cut -c11-` $proto $cookie
775 echo add $DISPLAY $proto $cookie
780 If this file does not exist,
783 does not exist either, xauth is used to add the cookie.
785 This file should be writable only by the user, and need not be
786 readable by anyone else.
787 .It Pa /etc/ssh/sshrc
790 This can be used to specify
791 machine-specific login-time initializations globally.
792 This file should be writable only by root, and should be world-readable.
814 .%T "SSH Protocol Architecture"
815 .%N draft-ietf-secsh-architecture-12.txt
817 .%O work in progress material
823 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
824 .%N draft-ietf-secsh-dh-group-exchange-02.txt
826 .%O work in progress material
829 OpenSSH is a derivative of the original and free
830 ssh 1.2.12 release by Tatu Ylonen.
831 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
832 Theo de Raadt and Dug Song
833 removed many bugs, re-added newer features and
835 Markus Friedl contributed the support for SSH
836 protocol versions 1.5 and 2.0.
837 Niels Provos and Markus Friedl contributed support
838 for privilege separation.