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 1.208 2005/06/08 03:50:00 djm Exp $
39 .Dd September 25, 1999
44 .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
60 (SSH Daemon) is the daemon program for
62 Together these programs replace rlogin and rsh, and
63 provide secure encrypted communications between two untrusted hosts
64 over an insecure network.
65 The programs are intended to be as easy to
66 install and use as possible.
69 is the daemon that listens for connections from clients.
70 It is normally started at boot from
73 daemon for each incoming connection.
74 The forked daemons handle
75 key exchange, encryption, authentication, command execution,
77 This implementation of
79 supports both SSH protocol version 1 and 2 simultaneously.
82 .Ss SSH protocol version 1
83 Each host has a host-specific RSA key
84 (normally 2048 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
110 authentication combined with RSA host
111 authentication, RSA challenge-response authentication, or password
112 based authentication.
114 Regardless of the authentication type, the account is checked to
115 ensure that it is accessible. An account is not accessible if it is
118 or its group is listed in
120 \&. The definition of a locked account is system dependant. Some platforms
121 have their own account database (eg AIX) and some modify the passwd field (
127 on Tru64 and a leading
129 on Linux). If there is a requirement to disable password authentication
130 for the account while allowing still public-key, then the passwd field
131 should be set to something other than these values (eg
141 are disabled (thus completely disabling
146 .Ss SSH protocol version 2
147 Version 2 works similarly:
148 Each host has a host-specific key (RSA or DSA) used to identify the host.
149 However, when the daemon starts, it does not generate a server key.
150 Forward security is provided through a Diffie-Hellman key agreement.
151 This key agreement results in a shared session key.
153 The rest of the session is encrypted using a symmetric cipher, currently
154 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
155 The client selects the encryption algorithm
156 to use from those offered by the server.
157 Additionally, session integrity is provided
158 through a cryptographic message authentication code
159 (hmac-sha1 or hmac-md5).
161 Protocol version 2 provides a public key based
162 user (PubkeyAuthentication) or
163 client host (HostbasedAuthentication) authentication method,
164 conventional password authentication and challenge response based methods.
165 .Ss Command execution and data forwarding
166 If the client successfully authenticates itself, a dialog for
167 preparing the session is entered.
168 At this time the client may request
169 things like allocating a pseudo-tty, forwarding X11 connections,
170 forwarding TCP/IP connections, or forwarding the authentication agent
171 connection over the secure channel.
173 Finally, the client either requests a shell or execution of a command.
174 The sides then enter session mode.
175 In this mode, either side may send
176 data at any time, and such data is forwarded to/from the shell or
177 command on the server side, and the user terminal in the client side.
179 When the user program terminates and all forwarded X11 and other
180 connections have been closed, the server sends command exit status to
181 the client, and both sides exit.
184 can be configured using command-line options or a configuration file
186 .Xr sshd_config 5 ) .
187 Command-line options override values specified in the
191 rereads its configuration file when it receives a hangup signal,
193 by executing itself with the name and options it was started with, e.g.,
196 The options are as follows:
201 to use IPv4 addresses only.
205 to use IPv6 addresses only.
207 Specifies the number of bits in the ephemeral protocol version 1
208 server key (default 768).
210 When this option is specified,
212 will not detach and does not become a daemon.
213 This allows easy monitoring of
217 The server sends verbose debug output to the system
218 log, and does not put itself in the background.
219 The server also will not fork and will only process one connection.
220 This option is only intended for debugging for the server.
223 options increase the debugging level.
226 When this option is specified,
228 will send the output to the standard error instead of the system log.
229 .It Fl f Ar configuration_file
230 Specifies the name of the configuration file.
232 .Pa /etc/ssh/sshd_config .
234 refuses to start if there is no configuration file.
235 .It Fl g Ar login_grace_time
236 Gives the grace time for clients to authenticate themselves (default
238 If the client fails to authenticate the user within
239 this many seconds, the server disconnects and exits.
240 A value of zero indicates no limit.
241 .It Fl h Ar host_key_file
242 Specifies a file from which a host key is read.
243 This option must be given if
245 is not run as root (as the normal
246 host key files are normally not readable by anyone but root).
248 .Pa /etc/ssh/ssh_host_key
249 for protocol version 1, and
250 .Pa /etc/ssh/ssh_host_dsa_key
251 for protocol version 2.
252 It is possible to have multiple host key files for
253 the different protocol versions and host key algorithms.
261 from inetd because it needs to generate the server key before it can
262 respond to the client, and this may take tens of seconds.
263 Clients would have to wait too long if the key was regenerated every time.
264 However, with small key sizes (e.g., 512) using
268 .It Fl k Ar key_gen_time
269 Specifies how often the ephemeral protocol version 1 server key is
270 regenerated (default 3600 seconds, or one hour).
271 The motivation for regenerating the key fairly
272 often is that the key is not stored anywhere, and after about an hour
273 it becomes impossible to recover the key for decrypting intercepted
274 communications even if the machine is cracked into or physically
276 A value of zero indicates that the key will never be regenerated.
278 Can be used to give options in the format used in the configuration file.
279 This is useful for specifying options for which there is no separate
281 For full details of the options, and their values, see
284 Specifies the port on which the server listens for connections
286 Multiple port options are permitted.
287 Ports specified in the configuration file are ignored when a
288 command-line port is specified.
291 Nothing is sent to the system log.
292 Normally the beginning,
293 authentication, and termination of each connection is logged.
296 Only check the validity of the configuration file and sanity of the keys.
297 This is useful for updating
299 reliably as configuration options may change.
301 This option is used to specify the size of the field
304 structure that holds the remote host name.
305 If the resolved host name is longer than
307 the dotted decimal value will be used instead.
308 This allows hosts with very long host names that
309 overflow this field to still be uniquely identified.
312 indicates that only dotted decimal addresses
313 should be put into the
317 may also be used to prevent
319 from making DNS requests unless the authentication
320 mechanism or configuration requires it.
321 Authentication mechanisms that may require DNS include
322 .Cm RhostsRSAAuthentication ,
323 .Cm HostbasedAuthentication
325 .Cm from="pattern-list"
326 option in a key file.
327 Configuration options that require DNS include using a
333 .Sh CONFIGURATION FILE
335 reads configuration data from
336 .Pa /etc/ssh/sshd_config
337 (or the file specified with
339 on the command line).
340 The file format and configuration options are described in
343 When a user successfully logs in,
346 .Bl -enum -offset indent
348 If the login is on a tty, and no command has been specified,
349 prints last login time and
351 (unless prevented in the configuration file or by
357 If the login is on a tty, records login time.
361 .Pa /var/run/nologin ;
362 if one exists, it prints the contents and quits
365 Changes to run with normal user privileges.
367 Sets up basic environment.
370 .Pa ~/.ssh/environment ,
371 if it exists, and users are allowed to change their environment.
373 .Cm PermitUserEnvironment
377 Changes to user's home directory.
381 exists, runs it; else if
388 files are given the X11
389 authentication protocol and cookie (if applicable) in standard input.
391 Runs user's shell or command.
393 .Sh AUTHORIZED_KEYS FILE FORMAT
394 .Pa ~/.ssh/authorized_keys
395 is the default file that lists the public keys that are
396 permitted for RSA authentication in protocol version 1
397 and for public key authentication (PubkeyAuthentication)
398 in protocol version 2.
399 .Cm AuthorizedKeysFile
400 may be used to specify an alternative file.
402 Each line of the file contains one
403 key (empty lines and lines starting with a
407 Each RSA public key consists of the following fields, separated by
408 spaces: options, bits, exponent, modulus, comment.
409 Each protocol version 2 public key consists of:
410 options, keytype, base64 encoded key, comment.
412 is optional; its presence is determined by whether the line starts
413 with a number or not (the options field never starts with a number).
414 The bits, exponent, modulus and comment fields give the RSA key for
415 protocol version 1; the
416 comment field is not used for anything (but may be convenient for the
417 user to identify the key).
418 For protocol version 2 the keytype is
423 Note that lines in this file are usually several hundred bytes long
424 (because of the size of the public key encoding) up to a limit of
425 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
426 keys up to 16 kilobits.
427 You don't want to type them in; instead, copy the
435 enforces a minimum RSA key modulus size for protocol 1
436 and protocol 2 keys of 768 bits.
438 The options (if present) consist of comma-separated option
440 No spaces are permitted, except within double quotes.
441 The following option specifications are supported (note
442 that option keywords are case-insensitive):
444 .It Cm from="pattern-list"
445 Specifies that in addition to public key authentication, the canonical name
446 of the remote host must be present in the comma-separated list of
452 The list may also contain
453 patterns negated by prefixing them with
455 if the canonical host name matches a negated pattern, the key is not accepted.
457 of this option is to optionally increase security: public key authentication
458 by itself does not trust the network or name servers or anything (but
459 the key); however, if somebody somehow steals the key, the key
460 permits an intruder to log in from anywhere in the world.
461 This additional option makes using a stolen key more difficult (name
462 servers and/or routers would have to be compromised in addition to
464 .It Cm command="command"
465 Specifies that the command is executed whenever this key is used for
467 The command supplied by the user (if any) is ignored.
468 The command is run on a pty if the client requests a pty;
469 otherwise it is run without a tty.
470 If an 8-bit clean channel is required,
471 one must not request a pty or should specify
473 A quote may be included in the command by quoting it with a backslash.
474 This option might be useful
475 to restrict certain public keys to perform just a specific operation.
476 An example might be a key that permits remote backups but nothing else.
477 Note that the client may specify TCP/IP and/or X11
478 forwarding unless they are explicitly prohibited.
479 Note that this option applies to shell, command or subsystem execution.
480 .It Cm environment="NAME=value"
481 Specifies that the string is to be added to the environment when
482 logging in using this key.
483 Environment variables set this way
484 override other default environment values.
485 Multiple options of this type are permitted.
486 Environment processing is disabled by default and is
488 .Cm PermitUserEnvironment
490 This option is automatically disabled if
493 .It Cm no-port-forwarding
494 Forbids TCP/IP forwarding when this key is used for authentication.
495 Any port forward requests by the client will return an error.
496 This might be used, e.g., in connection with the
499 .It Cm no-X11-forwarding
500 Forbids X11 forwarding when this key is used for authentication.
501 Any X11 forward requests by the client will return an error.
502 .It Cm no-agent-forwarding
503 Forbids authentication agent forwarding when this key is used for
506 Prevents tty allocation (a request to allocate a pty will fail).
507 .It Cm permitopen="host:port"
510 port forwarding such that it may only connect to the specified host and
512 IPv6 addresses can be specified with an alternative syntax:
513 .Ar host Ns / Ns Ar port .
516 options may be applied separated by commas.
517 No pattern matching is performed on the specified hostnames,
518 they must be literal domains or addresses.
521 1024 33 12121...312314325 ylo@foo.bar
523 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
525 command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
527 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
528 .Sh SSH_KNOWN_HOSTS FILE FORMAT
530 .Pa /etc/ssh/ssh_known_hosts
532 .Pa ~/.ssh/known_hosts
533 files contain host public keys for all known hosts.
534 The global file should
535 be prepared by the administrator (optional), and the per-user file is
536 maintained automatically: whenever the user connects from an unknown host
537 its key is added to the per-user file.
539 Each line in these files contains the following fields: hostnames,
540 bits, exponent, modulus, comment.
541 The fields are separated by spaces.
543 Hostnames is a comma-separated list of patterns
548 wildcards); each pattern in turn is matched against the canonical host
549 name (when authenticating a client) or against the user-supplied
550 name (when authenticating a server).
551 A pattern may also be preceded by
553 to indicate negation: if the host name matches a negated
554 pattern, it is not accepted (by that line) even if it matched another
557 Alternately, hostnames may be stored in a hashed form which hides host names
558 and addresses should the file's contents be disclosed.
559 Hashed hostnames start with a
562 Only one hashed hostname may appear on a single line and none of the above
563 negation or wildcard operators may be applied.
565 Bits, exponent, and modulus are taken directly from the RSA host key; they
566 can be obtained, e.g., from
567 .Pa /etc/ssh/ssh_host_key.pub .
568 The optional comment field continues to the end of the line, and is not used.
572 and empty lines are ignored as comments.
574 When performing host authentication, authentication is accepted if any
575 matching line has the proper key.
576 It is thus permissible (but not
577 recommended) to have several lines or different host keys for the same
579 This will inevitably happen when short forms of host names
580 from different domains are put in the file.
582 that the files contain conflicting information; authentication is
583 accepted if valid information can be found from either file.
585 Note that the lines in these files are typically hundreds of characters
586 long, and you definitely don't want to type in the host keys by hand.
587 Rather, generate them by a script
589 .Pa /etc/ssh/ssh_host_key.pub
590 and adding the host names at the front.
593 closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
594 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
598 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
603 .It Pa /etc/ssh/sshd_config
604 Contains configuration data for
606 The file format and configuration options are described in
608 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key
609 These two files contain the private parts of the host keys.
610 These files should only be owned by root, readable only by root, and not
611 accessible to others.
614 does not start if this file is group/world-accessible.
615 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub
616 These two files contain the public parts of the host keys.
617 These files should be world-readable but writable only by
619 Their contents should match the respective private parts.
621 really used for anything; they are provided for the convenience of
622 the user so their contents can be copied to known hosts files.
623 These files are created using
625 .It Pa /etc/ssh/moduli
626 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
627 The file format is described in
633 during privilege separation in the pre-authentication phase.
634 The directory should not contain any files and must be owned by root
635 and not group or world-writable.
636 .It Pa /var/run/sshd.pid
637 Contains the process ID of the
639 listening for connections (if there are several daemons running
640 concurrently for different ports, this contains the process ID of the one
642 The content of this file is not sensitive; it can be world-readable.
643 .It Pa ~/.ssh/authorized_keys
644 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
645 This file must be readable by root (which may on some machines imply
646 it being world-readable if the user's home directory resides on an NFS
648 It is recommended that it not be accessible by others.
649 The format of this file is described above.
650 Users will place the contents of their
655 files into this file, as described in
657 .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
658 These files are consulted when using rhosts with RSA host
659 authentication or protocol version 2 hostbased authentication
660 to check the public key of the host.
661 The key must be listed in one of these files to be accepted.
662 The client uses the same files
663 to verify that it is connecting to the correct remote host.
664 These files should be writable only by root/the owner.
665 .Pa /etc/ssh/ssh_known_hosts
666 should be world-readable, and
667 .Pa ~/.ssh/known_hosts
668 can, but need not be, world-readable.
673 This file is used to suppress printing the last login time and
681 It does not suppress printing of the banner specified by
686 refuses to let anyone except root log in.
687 The contents of the file
688 are displayed to anyone trying to log in, and non-root connections are
690 The file should be world-readable.
691 .It Pa /etc/hosts.allow, /etc/hosts.deny
692 Access controls that should be enforced by tcp-wrappers are defined here.
693 Further details are described in
696 This file is used during
697 .Cm RhostsRSAAuthentication
699 .Cm HostbasedAuthentication
700 and contains host-username pairs, separated by a space, one per
702 The given user on the corresponding host is permitted to log in
704 The same file is used by rlogind and rshd.
706 be writable only by the user; it is recommended that it not be
707 accessible by others.
709 It is also possible to use netgroups in the file.
711 name may be of the form +@groupname to specify all hosts or all users
715 this file is exactly the same as for
717 However, this file is
718 not used by rlogin and rshd, so using this permits access using SSH only.
719 .It Pa /etc/hosts.equiv
720 This file is used during
721 .Cm RhostsRSAAuthentication
723 .Cm HostbasedAuthentication
725 In the simplest form, this file contains host names, one per line.
727 those hosts are permitted to log in without a password, provided they
728 have the same user name on both machines.
729 The host name may also be
730 followed by a user name; such users are permitted to log in as
732 user on this machine (except root).
733 Additionally, the syntax
735 can be used to specify netgroups.
736 Negated entries start with
739 If the client host/user is successfully matched in this file, login is
740 automatically permitted provided the client and server user names are the
742 Additionally, successful client host key authentication is required.
743 This file must be writable only by root; it is recommended
744 that it be world-readable.
746 .Sy "Warning: It is almost never a good idea to use user names in"
748 Beware that it really means that the named user(s) can log in as
750 which includes bin, daemon, adm, and other accounts that own critical
751 binaries and directories.
752 Using a user name practically grants the user root access.
753 The only valid use for user names that I can think
754 of is in negative entries.
756 Note that this warning also applies to rsh/rlogin.
757 .It Pa /etc/ssh/shosts.equiv
758 This is processed exactly as
759 .Pa /etc/hosts.equiv .
760 However, this file may be useful in environments that want to run both
762 .It Pa ~/.ssh/environment
763 This file is read into the environment at login (if it exists).
764 It can only contain empty lines, comment lines (that start with
766 and assignment lines of the form name=value.
767 The file should be writable
768 only by the user; it need not be readable by anyone else.
769 Environment processing is disabled by default and is
771 .Cm PermitUserEnvironment
774 If this file exists, it is run with
777 environment files but before starting the user's shell or command.
778 It must not produce any output on stdout; stderr must be used
780 If X11 forwarding is in use, it will receive the "proto cookie" pair in
781 its standard input (and
788 will not run xauth automatically to add X11 cookies.
790 The primary purpose of this file is to run any initialization routines
791 which may be needed before the user's home directory becomes
792 accessible; AFS is a particular example of such an environment.
794 This file will probably contain some initialization code followed by
795 something similar to:
797 if read proto cookie && [ -n "$DISPLAY" ]; then
798 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
799 # X11UseLocalhost=yes
800 echo add unix:`echo $DISPLAY |
801 cut -c11-` $proto $cookie
804 echo add $DISPLAY $proto $cookie
809 If this file does not exist,
812 does not exist either, xauth is used to add the cookie.
814 This file should be writable only by the user, and need not be
815 readable by anyone else.
816 .It Pa /etc/ssh/sshrc
819 This can be used to specify
820 machine-specific login-time initializations globally.
821 This file should be writable only by root, and should be world-readable.
843 .%T "SSH Protocol Architecture"
844 .%N draft-ietf-secsh-architecture-12.txt
846 .%O work in progress material
852 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
853 .%N draft-ietf-secsh-dh-group-exchange-02.txt
855 .%O work in progress material
858 OpenSSH is a derivative of the original and free
859 ssh 1.2.12 release by Tatu Ylonen.
860 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
861 Theo de Raadt and Dug Song
862 removed many bugs, re-added newer features and
864 Markus Friedl contributed the support for SSH
865 protocol versions 1.5 and 2.0.
866 Niels Provos and Markus Friedl contributed support
867 for privilege separation.