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 1.213 2006/01/25 09:07:22 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 (OpenSSH 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.
66 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,
76 can be configured using command-line options or a configuration file
79 command-line options override values specified in the
82 rereads its configuration file when it receives a hangup signal,
84 by executing itself with the name and options it was started with, e.g.,
87 The options are as follows:
92 to use IPv4 addresses only.
96 to use IPv6 addresses only.
98 Specifies the number of bits in the ephemeral protocol version 1
99 server key (default 768).
101 When this option is specified,
103 will not detach and does not become a daemon.
104 This allows easy monitoring of
108 The server sends verbose debug output to the system
109 log, and does not put itself in the background.
110 The server also will not fork and will only process one connection.
111 This option is only intended for debugging for the server.
114 options increase the debugging level.
117 When this option is specified,
119 will send the output to the standard error instead of the system log.
120 .It Fl f Ar configuration_file
121 Specifies the name of the configuration file.
123 .Pa /etc/ssh/sshd_config .
125 refuses to start if there is no configuration file.
126 .It Fl g Ar login_grace_time
127 Gives the grace time for clients to authenticate themselves (default
129 If the client fails to authenticate the user within
130 this many seconds, the server disconnects and exits.
131 A value of zero indicates no limit.
132 .It Fl h Ar host_key_file
133 Specifies a file from which a host key is read.
134 This option must be given if
136 is not run as root (as the normal
137 host key files are normally not readable by anyone but root).
139 .Pa /etc/ssh/ssh_host_key
140 for protocol version 1, and
141 .Pa /etc/ssh/ssh_host_rsa_key
143 .Pa /etc/ssh/ssh_host_dsa_key
144 for protocol version 2.
145 It is possible to have multiple host key files for
146 the different protocol versions and host key algorithms.
154 from inetd because it needs to generate the server key before it can
155 respond to the client, and this may take tens of seconds.
156 Clients would have to wait too long if the key was regenerated every time.
157 However, with small key sizes (e.g., 512) using
161 .It Fl k Ar key_gen_time
162 Specifies how often the ephemeral protocol version 1 server key is
163 regenerated (default 3600 seconds, or one hour).
164 The motivation for regenerating the key fairly
165 often is that the key is not stored anywhere, and after about an hour
166 it becomes impossible to recover the key for decrypting intercepted
167 communications even if the machine is cracked into or physically
169 A value of zero indicates that the key will never be regenerated.
171 Can be used to give options in the format used in the configuration file.
172 This is useful for specifying options for which there is no separate
174 For full details of the options, and their values, see
177 Specifies the port on which the server listens for connections
179 Multiple port options are permitted.
180 Ports specified in the configuration file with the
182 option are ignored when a command-line port is specified.
183 Ports specified using the
185 option override command-line ports.
188 Nothing is sent to the system log.
189 Normally the beginning,
190 authentication, and termination of each connection is logged.
193 Only check the validity of the configuration file and sanity of the keys.
194 This is useful for updating
196 reliably as configuration options may change.
198 This option is used to specify the size of the field
201 structure that holds the remote host name.
202 If the resolved host name is longer than
204 the dotted decimal value will be used instead.
205 This allows hosts with very long host names that
206 overflow this field to still be uniquely identified.
209 indicates that only dotted decimal addresses
210 should be put into the
214 may also be used to prevent
216 from making DNS requests unless the authentication
217 mechanism or configuration requires it.
218 Authentication mechanisms that may require DNS include
219 .Cm RhostsRSAAuthentication ,
220 .Cm HostbasedAuthentication
222 .Cm from="pattern-list"
223 option in a key file.
224 Configuration options that require DNS include using a
231 This implementation of
233 supports both SSH protocol version 1 and 2 simultaneously.
236 .Sh SSH PROTOCOL VERSION 1
237 Each host has a host-specific RSA key
238 (normally 2048 bits) used to identify the host.
240 the daemon starts, it generates a server RSA key (normally 768 bits).
241 This key is normally regenerated every hour if it has been used, and
242 is never stored on disk.
244 Whenever a client connects, the daemon responds with its public
245 host and server keys.
246 The client compares the
247 RSA host key against its own database to verify that it has not changed.
248 The client then generates a 256-bit random number.
250 random number using both the host key and the server key, and sends
251 the encrypted number to the server.
252 Both sides then use this
253 random number as a session key which is used to encrypt all further
254 communications in the session.
255 The rest of the session is encrypted
256 using a conventional cipher, currently Blowfish or 3DES, with 3DES
257 being used by default.
258 The client selects the encryption algorithm
259 to use from those offered by the server.
261 Next, the server and the client enter an authentication dialog.
262 The client tries to authenticate itself using
264 authentication combined with RSA host
265 authentication, RSA challenge-response authentication, or password
266 based authentication.
268 Regardless of the authentication type, the account is checked to
269 ensure that it is accessible. An account is not accessible if it is
272 or its group is listed in
274 \&. The definition of a locked account is system dependant. Some platforms
275 have their own account database (eg AIX) and some modify the passwd field (
277 on Solaris and UnixWare,
284 on FreeBSD and a leading
286 on Linux). If there is a requirement to disable password authentication
287 for the account while allowing still public-key, then the passwd field
288 should be set to something other than these values (eg
294 System security is not improved unless
299 are disabled (thus completely disabling
304 .Sh SSH PROTOCOL VERSION 2
305 Version 2 works similarly:
306 Each host has a host-specific key (RSA or DSA) used to identify the host.
307 However, when the daemon starts, it does not generate a server key.
308 Forward security is provided through a Diffie-Hellman key agreement.
309 This key agreement results in a shared session key.
311 The rest of the session is encrypted using a symmetric cipher, currently
312 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
313 The client selects the encryption algorithm
314 to use from those offered by the server.
315 Additionally, session integrity is provided
316 through a cryptographic message authentication code
317 (hmac-sha1 or hmac-md5).
319 Protocol version 2 provides a public key based
320 user (PubkeyAuthentication) or
321 client host (HostbasedAuthentication) authentication method,
322 conventional password authentication and challenge response based methods.
323 .Sh COMMAND EXECUTION AND DATA FORWARDING
324 If the client successfully authenticates itself, a dialog for
325 preparing the session is entered.
326 At this time the client may request
327 things like allocating a pseudo-tty, forwarding X11 connections,
328 forwarding TCP connections, or forwarding the authentication agent
329 connection over the secure channel.
331 Finally, the client either requests a shell or execution of a command.
332 The sides then enter session mode.
333 In this mode, either side may send
334 data at any time, and such data is forwarded to/from the shell or
335 command on the server side, and the user terminal in the client side.
337 When the user program terminates and all forwarded X11 and other
338 connections have been closed, the server sends command exit status to
339 the client, and both sides exit.
340 .Sh CONFIGURATION FILE
342 reads configuration data from
343 .Pa /etc/ssh/sshd_config
344 (or the file specified with
346 on the command line).
347 The file format and configuration options are described in
350 When a user successfully logs in,
353 .Bl -enum -offset indent
355 If the login is on a tty, and no command has been specified,
356 prints last login time and
358 (unless prevented in the configuration file or by
364 If the login is on a tty, records login time.
368 if it exists, prints contents and quits
371 Changes to run with normal user privileges.
373 Sets up basic environment.
376 .Pa ~/.ssh/environment ,
377 if it exists, and users are allowed to change their environment.
379 .Cm PermitUserEnvironment
383 Changes to user's home directory.
387 exists, runs it; else if
390 it; otherwise runs xauth.
393 files are given the X11
394 authentication protocol and cookie in standard input.
396 Runs user's shell or command.
398 .Sh AUTHORIZED_KEYS FILE FORMAT
399 .Pa ~/.ssh/authorized_keys
400 is the default file that lists the public keys that are
401 permitted for RSA authentication in protocol version 1
402 and for public key authentication (PubkeyAuthentication)
403 in protocol version 2.
404 .Cm AuthorizedKeysFile
405 may be used to specify an alternative file.
407 Each line of the file contains one
408 key (empty lines and lines starting with a
412 Each RSA public key consists of the following fields, separated by
413 spaces: options, bits, exponent, modulus, comment.
414 Each protocol version 2 public key consists of:
415 options, keytype, base64 encoded key, comment.
417 is optional; its presence is determined by whether the line starts
418 with a number or not (the options field never starts with a number).
419 The bits, exponent, modulus and comment fields give the RSA key for
420 protocol version 1; the
421 comment field is not used for anything (but may be convenient for the
422 user to identify the key).
423 For protocol version 2 the keytype is
428 Note that lines in this file are usually several hundred bytes long
429 (because of the size of the public key encoding) up to a limit of
430 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
431 keys up to 16 kilobits.
432 You don't want to type them in; instead, copy the
440 enforces a minimum RSA key modulus size for protocol 1
441 and protocol 2 keys of 768 bits.
443 The options (if present) consist of comma-separated option
445 No spaces are permitted, except within double quotes.
446 The following option specifications are supported (note
447 that option keywords are case-insensitive):
449 .It Cm from="pattern-list"
450 Specifies that in addition to public key authentication, the canonical name
451 of the remote host must be present in the comma-separated list of
457 The list may also contain
458 patterns negated by prefixing them with
460 if the canonical host name matches a negated pattern, the key is not accepted.
462 of this option is to optionally increase security: public key authentication
463 by itself does not trust the network or name servers or anything (but
464 the key); however, if somebody somehow steals the key, the key
465 permits an intruder to log in from anywhere in the world.
466 This additional option makes using a stolen key more difficult (name
467 servers and/or routers would have to be compromised in addition to
469 .It Cm command="command"
470 Specifies that the command is executed whenever this key is used for
472 The command supplied by the user (if any) is ignored.
473 The command is run on a pty if the client requests a pty;
474 otherwise it is run without a tty.
475 If an 8-bit clean channel is required,
476 one must not request a pty or should specify
478 A quote may be included in the command by quoting it with a backslash.
479 This option might be useful
480 to restrict certain public keys to perform just a specific operation.
481 An example might be a key that permits remote backups but nothing else.
482 Note that the client may specify TCP and/or X11
483 forwarding unless they are explicitly prohibited.
484 Note that this option applies to shell, command or subsystem execution.
485 .It Cm environment="NAME=value"
486 Specifies that the string is to be added to the environment when
487 logging in using this key.
488 Environment variables set this way
489 override other default environment values.
490 Multiple options of this type are permitted.
491 Environment processing is disabled by default and is
493 .Cm PermitUserEnvironment
495 This option is automatically disabled if
498 .It Cm no-port-forwarding
499 Forbids TCP forwarding when this key is used for authentication.
500 Any port forward requests by the client will return an error.
501 This might be used, e.g., in connection with the
504 .It Cm no-X11-forwarding
505 Forbids X11 forwarding when this key is used for authentication.
506 Any X11 forward requests by the client will return an error.
507 .It Cm no-agent-forwarding
508 Forbids authentication agent forwarding when this key is used for
511 Prevents tty allocation (a request to allocate a pty will fail).
512 .It Cm permitopen="host:port"
515 port forwarding such that it may only connect to the specified host and
517 IPv6 addresses can be specified with an alternative syntax:
518 .Ar host Ns / Ns Ar port .
521 options may be applied separated by commas.
522 No pattern matching is performed on the specified hostnames,
523 they must be literal domains or addresses.
527 device on the server.
528 Without this option, the next available device will be used if
529 the client requests a tunnel.
532 1024 33 12121...312314325 ylo@foo.bar
534 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
536 command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
538 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
540 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
541 .Sh SSH_KNOWN_HOSTS FILE FORMAT
543 .Pa /etc/ssh/ssh_known_hosts
545 .Pa ~/.ssh/known_hosts
546 files contain host public keys for all known hosts.
547 The global file should
548 be prepared by the administrator (optional), and the per-user file is
549 maintained automatically: whenever the user connects from an unknown host
550 its key is added to the per-user file.
552 Each line in these files contains the following fields: hostnames,
553 bits, exponent, modulus, comment.
554 The fields are separated by spaces.
556 Hostnames is a comma-separated list of patterns
561 wildcards); each pattern in turn is matched against the canonical host
562 name (when authenticating a client) or against the user-supplied
563 name (when authenticating a server).
564 A pattern may also be preceded by
566 to indicate negation: if the host name matches a negated
567 pattern, it is not accepted (by that line) even if it matched another
570 Alternately, hostnames may be stored in a hashed form which hides host names
571 and addresses should the file's contents be disclosed.
572 Hashed hostnames start with a
575 Only one hashed hostname may appear on a single line and none of the above
576 negation or wildcard operators may be applied.
578 Bits, exponent, and modulus are taken directly from the RSA host key; they
579 can be obtained, e.g., from
580 .Pa /etc/ssh/ssh_host_key.pub .
581 The optional comment field continues to the end of the line, and is not used.
585 and empty lines are ignored as comments.
587 When performing host authentication, authentication is accepted if any
588 matching line has the proper key.
589 It is thus permissible (but not
590 recommended) to have several lines or different host keys for the same
592 This will inevitably happen when short forms of host names
593 from different domains are put in the file.
595 that the files contain conflicting information; authentication is
596 accepted if valid information can be found from either file.
598 Note that the lines in these files are typically hundreds of characters
599 long, and you definitely don't want to type in the host keys by hand.
600 Rather, generate them by a script
602 .Pa /etc/ssh/ssh_host_key.pub
603 and adding the host names at the front.
606 closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
607 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
611 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
616 .It Pa /etc/ssh/sshd_config
617 Contains configuration data for
619 The file format and configuration options are described in
621 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
622 These three files contain the private parts of the host keys.
623 These files should only be owned by root, readable only by root, and not
624 accessible to others.
627 does not start if this file is group/world-accessible.
628 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
629 These three files contain the public parts of the host keys.
630 These files should be world-readable but writable only by
632 Their contents should match the respective private parts.
634 really used for anything; they are provided for the convenience of
635 the user so their contents can be copied to known hosts files.
636 These files are created using
639 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
640 The file format is described in
646 during privilege separation in the pre-authentication phase.
647 The directory should not contain any files and must be owned by root
648 and not group or world-writable.
649 .It Pa /var/run/sshd.pid
650 Contains the process ID of the
652 listening for connections (if there are several daemons running
653 concurrently for different ports, this contains the process ID of the one
655 The content of this file is not sensitive; it can be world-readable.
656 .It Pa ~/.ssh/authorized_keys
657 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
658 This file must be readable by root (which may on some machines imply
659 it being world-readable if the user's home directory resides on an NFS
661 It is recommended that it not be accessible by others.
662 The format of this file is described above.
663 Users will place the contents of their
668 files into this file, as described in
670 .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
671 These files are consulted when using rhosts with RSA host
672 authentication or protocol version 2 hostbased authentication
673 to check the public key of the host.
674 The key must be listed in one of these files to be accepted.
675 The client uses the same files
676 to verify that it is connecting to the correct remote host.
677 These files should be writable only by root/the owner.
678 .Pa /etc/ssh/ssh_known_hosts
679 should be world-readable, and
680 .Pa ~/.ssh/known_hosts
681 can, but need not be, world-readable.
686 This file is used to suppress printing the last login time and
694 It does not suppress printing of the banner specified by
699 refuses to let anyone except root log in.
700 The contents of the file
701 are displayed to anyone trying to log in, and non-root connections are
703 The file should be world-readable.
704 .It Pa /etc/hosts.allow, /etc/hosts.deny
705 Access controls that should be enforced by tcp-wrappers are defined here.
706 Further details are described in
709 This file is used during
710 .Cm RhostsRSAAuthentication
712 .Cm HostbasedAuthentication
713 and contains host-username pairs, separated by a space, one per
715 The given user on the corresponding host is permitted to log in
717 The same file is used by rlogind and rshd.
719 be writable only by the user; it is recommended that it not be
720 accessible by others.
722 It is also possible to use netgroups in the file.
724 name may be of the form +@groupname to specify all hosts or all users
728 this file is exactly the same as for
730 However, this file is
731 not used by rlogin and rshd, so using this permits access using SSH only.
732 .It Pa /etc/hosts.equiv
733 This file is used during
734 .Cm RhostsRSAAuthentication
736 .Cm HostbasedAuthentication
738 In the simplest form, this file contains host names, one per line.
740 those hosts are permitted to log in without a password, provided they
741 have the same user name on both machines.
742 The host name may also be
743 followed by a user name; such users are permitted to log in as
745 user on this machine (except root).
746 Additionally, the syntax
748 can be used to specify netgroups.
749 Negated entries start with
752 If the client host/user is successfully matched in this file, login is
753 automatically permitted provided the client and server user names are the
755 Additionally, successful client host key authentication is required.
756 This file must be writable only by root; it is recommended
757 that it be world-readable.
759 .Sy "Warning: It is almost never a good idea to use user names in"
761 Beware that it really means that the named user(s) can log in as
763 which includes bin, daemon, adm, and other accounts that own critical
764 binaries and directories.
765 Using a user name practically grants the user root access.
766 The only valid use for user names that I can think
767 of is in negative entries.
769 Note that this warning also applies to rsh/rlogin.
770 .It Pa /etc/shosts.equiv
771 This is processed exactly as
772 .Pa /etc/hosts.equiv .
773 However, this file may be useful in environments that want to run both
775 .It Pa ~/.ssh/environment
776 This file is read into the environment at login (if it exists).
777 It can only contain empty lines, comment lines (that start with
779 and assignment lines of the form name=value.
780 The file should be writable
781 only by the user; it need not be readable by anyone else.
782 Environment processing is disabled by default and is
784 .Cm PermitUserEnvironment
787 If this file exists, it is run with
790 environment files but before starting the user's shell or command.
791 It must not produce any output on stdout; stderr must be used
793 If X11 forwarding is in use, it will receive the "proto cookie" pair in
794 its standard input (and
801 will not run xauth automatically to add X11 cookies.
803 The primary purpose of this file is to run any initialization routines
804 which may be needed before the user's home directory becomes
805 accessible; AFS is a particular example of such an environment.
807 This file will probably contain some initialization code followed by
808 something similar to:
810 if read proto cookie && [ -n "$DISPLAY" ]; then
811 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
812 # X11UseLocalhost=yes
813 echo add unix:`echo $DISPLAY |
814 cut -c11-` $proto $cookie
817 echo add $DISPLAY $proto $cookie
822 If this file does not exist,
825 does not exist either, xauth is used to add the cookie.
827 This file should be writable only by the user, and need not be
828 readable by anyone else.
829 .It Pa /etc/ssh/sshrc
832 This can be used to specify
833 machine-specific login-time initializations globally.
834 This file should be writable only by root, and should be world-readable.
856 .%T "SSH Protocol Architecture"
857 .%N draft-ietf-secsh-architecture-12.txt
859 .%O work in progress material
865 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
866 .%N draft-ietf-secsh-dh-group-exchange-02.txt
868 .%O work in progress material
871 OpenSSH is a derivative of the original and free
872 ssh 1.2.12 release by Tatu Ylonen.
873 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
874 Theo de Raadt and Dug Song
875 removed many bugs, re-added newer features and
877 Markus Friedl contributed the support for SSH
878 protocol versions 1.5 and 2.0.
879 Niels Provos and Markus Friedl contributed support
880 for privilege separation.