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 $
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 2048 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
109 authentication combined with RSA host
110 authentication, RSA challenge-response authentication, or password
111 based authentication.
113 Regardless of the authentication type, the account is checked to
114 ensure that it is accessible. An account is not accessible if it is
117 or its group is listed in
119 \&. The definition of a locked account is system dependant. Some platforms
120 have their own account database (eg AIX) and some modify the passwd field (
122 on Solaris and UnixWare,
129 on FreeBSD and a leading
131 on Linux). If there is a requirement to disable password authentication
132 for the account while allowing still public-key, then the passwd field
133 should be set to something other than these values (eg
143 are disabled (thus completely disabling
148 .Ss SSH protocol version 2
149 Version 2 works similarly:
150 Each host has a host-specific key (RSA or DSA) used to identify the host.
151 However, when the daemon starts, it does not generate a server key.
152 Forward security is provided through a Diffie-Hellman key agreement.
153 This key agreement results in a shared session key.
155 The rest of the session is encrypted using a symmetric cipher, currently
156 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
157 The client selects the encryption algorithm
158 to use from those offered by the server.
159 Additionally, session integrity is provided
160 through a cryptographic message authentication code
161 (hmac-sha1 or hmac-md5).
163 Protocol version 2 provides a public key based
164 user (PubkeyAuthentication) or
165 client host (HostbasedAuthentication) authentication method,
166 conventional password authentication and challenge response based methods.
167 .Ss Command execution and data forwarding
168 If the client successfully authenticates itself, a dialog for
169 preparing the session is entered.
170 At this time the client may request
171 things like allocating a pseudo-tty, forwarding X11 connections,
172 forwarding TCP/IP connections, or forwarding the authentication agent
173 connection over the secure channel.
175 Finally, the client either requests a shell or execution of a command.
176 The sides then enter session mode.
177 In this mode, either side may send
178 data at any time, and such data is forwarded to/from the shell or
179 command on the server side, and the user terminal in the client side.
181 When the user program terminates and all forwarded X11 and other
182 connections have been closed, the server sends command exit status to
183 the client, and both sides exit.
186 can be configured using command-line options or a configuration file
188 .Xr sshd_config 5 ) .
189 Command-line options override values specified in the
193 rereads its configuration file when it receives a hangup signal,
195 by executing itself with the name and options it was started with, e.g.,
198 The options are as follows:
203 to use IPv4 addresses only.
207 to use IPv6 addresses only.
209 Specifies the number of bits in the ephemeral protocol version 1
210 server key (default 768).
212 When this option is specified,
214 will not detach and does not become a daemon.
215 This allows easy monitoring of
219 The server sends verbose debug output to the system
220 log, and does not put itself in the background.
221 The server also will not fork and will only process one connection.
222 This option is only intended for debugging for the server.
225 options increase the debugging level.
228 When this option is specified,
230 will send the output to the standard error instead of the system log.
231 .It Fl f Ar configuration_file
232 Specifies the name of the configuration file.
234 .Pa /etc/ssh/sshd_config .
236 refuses to start if there is no configuration file.
237 .It Fl g Ar login_grace_time
238 Gives the grace time for clients to authenticate themselves (default
240 If the client fails to authenticate the user within
241 this many seconds, the server disconnects and exits.
242 A value of zero indicates no limit.
243 .It Fl h Ar host_key_file
244 Specifies a file from which a host key is read.
245 This option must be given if
247 is not run as root (as the normal
248 host key files are normally not readable by anyone but root).
250 .Pa /etc/ssh/ssh_host_key
251 for protocol version 1, and
252 .Pa /etc/ssh/ssh_host_rsa_key
254 .Pa /etc/ssh/ssh_host_dsa_key
255 for protocol version 2.
256 It is possible to have multiple host key files for
257 the different protocol versions and host key algorithms.
265 from inetd because it needs to generate the server key before it can
266 respond to the client, and this may take tens of seconds.
267 Clients would have to wait too long if the key was regenerated every time.
268 However, with small key sizes (e.g., 512) using
272 .It Fl k Ar key_gen_time
273 Specifies how often the ephemeral protocol version 1 server key is
274 regenerated (default 3600 seconds, or one hour).
275 The motivation for regenerating the key fairly
276 often is that the key is not stored anywhere, and after about an hour
277 it becomes impossible to recover the key for decrypting intercepted
278 communications even if the machine is cracked into or physically
280 A value of zero indicates that the key will never be regenerated.
282 Can be used to give options in the format used in the configuration file.
283 This is useful for specifying options for which there is no separate
285 For full details of the options, and their values, see
288 Specifies the port on which the server listens for connections
290 Multiple port options are permitted.
291 Ports specified in the configuration file are ignored when a
292 command-line port is specified.
295 Nothing is sent to the system log.
296 Normally the beginning,
297 authentication, and termination of each connection is logged.
300 Only check the validity of the configuration file and sanity of the keys.
301 This is useful for updating
303 reliably as configuration options may change.
305 This option is used to specify the size of the field
308 structure that holds the remote host name.
309 If the resolved host name is longer than
311 the dotted decimal value will be used instead.
312 This allows hosts with very long host names that
313 overflow this field to still be uniquely identified.
316 indicates that only dotted decimal addresses
317 should be put into the
321 may also be used to prevent
323 from making DNS requests unless the authentication
324 mechanism or configuration requires it.
325 Authentication mechanisms that may require DNS include
326 .Cm RhostsRSAAuthentication ,
327 .Cm HostbasedAuthentication
329 .Cm from="pattern-list"
330 option in a key file.
331 Configuration options that require DNS include using a
337 .Sh CONFIGURATION FILE
339 reads configuration data from
340 .Pa /etc/ssh/sshd_config
341 (or the file specified with
343 on the command line).
344 The file format and configuration options are described in
347 When a user successfully logs in,
350 .Bl -enum -offset indent
352 If the login is on a tty, and no command has been specified,
353 prints last login time and
355 (unless prevented in the configuration file or by
361 If the login is on a tty, records login time.
365 if it exists, prints contents and quits
368 Changes to run with normal user privileges.
370 Sets up basic environment.
373 .Pa ~/.ssh/environment ,
374 if it exists, and users are allowed to change their environment.
376 .Cm PermitUserEnvironment
380 Changes to user's home directory.
384 exists, runs it; else if
387 it; otherwise runs xauth.
390 files are given the X11
391 authentication protocol and cookie in standard input.
393 Runs user's shell or command.
395 .Sh AUTHORIZED_KEYS FILE FORMAT
396 .Pa ~/.ssh/authorized_keys
397 is the default file that lists the public keys that are
398 permitted for RSA authentication in protocol version 1
399 and for public key authentication (PubkeyAuthentication)
400 in protocol version 2.
401 .Cm AuthorizedKeysFile
402 may be used to specify an alternative file.
404 Each line of the file contains one
405 key (empty lines and lines starting with a
409 Each RSA public key consists of the following fields, separated by
410 spaces: options, bits, exponent, modulus, comment.
411 Each protocol version 2 public key consists of:
412 options, keytype, base64 encoded key, comment.
414 is optional; its presence is determined by whether the line starts
415 with a number or not (the options field never starts with a number).
416 The bits, exponent, modulus and comment fields give the RSA key for
417 protocol version 1; the
418 comment field is not used for anything (but may be convenient for the
419 user to identify the key).
420 For protocol version 2 the keytype is
425 Note that lines in this file are usually several hundred bytes long
426 (because of the size of the public key encoding) up to a limit of
427 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
428 keys up to 16 kilobits.
429 You don't want to type them in; instead, copy the
437 enforces a minimum RSA key modulus size for protocol 1
438 and protocol 2 keys of 768 bits.
440 The options (if present) consist of comma-separated option
442 No spaces are permitted, except within double quotes.
443 The following option specifications are supported (note
444 that option keywords are case-insensitive):
446 .It Cm from="pattern-list"
447 Specifies that in addition to public key authentication, the canonical name
448 of the remote host must be present in the comma-separated list of
454 The list may also contain
455 patterns negated by prefixing them with
457 if the canonical host name matches a negated pattern, the key is not accepted.
459 of this option is to optionally increase security: public key authentication
460 by itself does not trust the network or name servers or anything (but
461 the key); however, if somebody somehow steals the key, the key
462 permits an intruder to log in from anywhere in the world.
463 This additional option makes using a stolen key more difficult (name
464 servers and/or routers would have to be compromised in addition to
466 .It Cm command="command"
467 Specifies that the command is executed whenever this key is used for
469 The command supplied by the user (if any) is ignored.
470 The command is run on a pty if the client requests a pty;
471 otherwise it is run without a tty.
472 If an 8-bit clean channel is required,
473 one must not request a pty or should specify
475 A quote may be included in the command by quoting it with a backslash.
476 This option might be useful
477 to restrict certain public keys to perform just a specific operation.
478 An example might be a key that permits remote backups but nothing else.
479 Note that the client may specify TCP/IP and/or X11
480 forwarding unless they are explicitly prohibited.
481 Note that this option applies to shell, command or subsystem execution.
482 .It Cm environment="NAME=value"
483 Specifies that the string is to be added to the environment when
484 logging in using this key.
485 Environment variables set this way
486 override other default environment values.
487 Multiple options of this type are permitted.
488 Environment processing is disabled by default and is
490 .Cm PermitUserEnvironment
492 This option is automatically disabled if
495 .It Cm no-port-forwarding
496 Forbids TCP/IP forwarding when this key is used for authentication.
497 Any port forward requests by the client will return an error.
498 This might be used, e.g., in connection with the
501 .It Cm no-X11-forwarding
502 Forbids X11 forwarding when this key is used for authentication.
503 Any X11 forward requests by the client will return an error.
504 .It Cm no-agent-forwarding
505 Forbids authentication agent forwarding when this key is used for
508 Prevents tty allocation (a request to allocate a pty will fail).
509 .It Cm permitopen="host:port"
512 port forwarding such that it may only connect to the specified host and
514 IPv6 addresses can be specified with an alternative syntax:
515 .Ar host Ns / Ns Ar port .
518 options may be applied separated by commas.
519 No pattern matching is performed on the specified hostnames,
520 they must be literal domains or addresses.
523 1024 33 12121...312314325 ylo@foo.bar
525 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
527 command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
529 permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
530 .Sh SSH_KNOWN_HOSTS FILE FORMAT
532 .Pa /etc/ssh/ssh_known_hosts
534 .Pa ~/.ssh/known_hosts
535 files contain host public keys for all known hosts.
536 The global file should
537 be prepared by the administrator (optional), and the per-user file is
538 maintained automatically: whenever the user connects from an unknown host
539 its key is added to the per-user file.
541 Each line in these files contains the following fields: hostnames,
542 bits, exponent, modulus, comment.
543 The fields are separated by spaces.
545 Hostnames is a comma-separated list of patterns
550 wildcards); each pattern in turn is matched against the canonical host
551 name (when authenticating a client) or against the user-supplied
552 name (when authenticating a server).
553 A pattern may also be preceded by
555 to indicate negation: if the host name matches a negated
556 pattern, it is not accepted (by that line) even if it matched another
559 Alternately, hostnames may be stored in a hashed form which hides host names
560 and addresses should the file's contents be disclosed.
561 Hashed hostnames start with a
564 Only one hashed hostname may appear on a single line and none of the above
565 negation or wildcard operators may be applied.
567 Bits, exponent, and modulus are taken directly from the RSA host key; they
568 can be obtained, e.g., from
569 .Pa /etc/ssh/ssh_host_key.pub .
570 The optional comment field continues to the end of the line, and is not used.
574 and empty lines are ignored as comments.
576 When performing host authentication, authentication is accepted if any
577 matching line has the proper key.
578 It is thus permissible (but not
579 recommended) to have several lines or different host keys for the same
581 This will inevitably happen when short forms of host names
582 from different domains are put in the file.
584 that the files contain conflicting information; authentication is
585 accepted if valid information can be found from either file.
587 Note that the lines in these files are typically hundreds of characters
588 long, and you definitely don't want to type in the host keys by hand.
589 Rather, generate them by a script
591 .Pa /etc/ssh/ssh_host_key.pub
592 and adding the host names at the front.
595 closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
596 cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
600 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
605 .It Pa /etc/ssh/sshd_config
606 Contains configuration data for
608 The file format and configuration options are described in
610 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
611 These three files contain the private parts of the host keys.
612 These files should only be owned by root, readable only by root, and not
613 accessible to others.
616 does not start if this file is group/world-accessible.
617 .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
618 These three files contain the public parts of the host keys.
619 These files should be world-readable but writable only by
621 Their contents should match the respective private parts.
623 really used for anything; they are provided for the convenience of
624 the user so their contents can be copied to known hosts files.
625 These files are created using
628 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
629 The file format is described in
635 during privilege separation in the pre-authentication phase.
636 The directory should not contain any files and must be owned by root
637 and not group or world-writable.
638 .It Pa /var/run/sshd.pid
639 Contains the process ID of the
641 listening for connections (if there are several daemons running
642 concurrently for different ports, this contains the process ID of the one
644 The content of this file is not sensitive; it can be world-readable.
645 .It Pa ~/.ssh/authorized_keys
646 Lists the public keys (RSA or DSA) that can be used to log into the user's account.
647 This file must be readable by root (which may on some machines imply
648 it being world-readable if the user's home directory resides on an NFS
650 It is recommended that it not be accessible by others.
651 The format of this file is described above.
652 Users will place the contents of their
657 files into this file, as described in
659 .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
660 These files are consulted when using rhosts with RSA host
661 authentication or protocol version 2 hostbased authentication
662 to check the public key of the host.
663 The key must be listed in one of these files to be accepted.
664 The client uses the same files
665 to verify that it is connecting to the correct remote host.
666 These files should be writable only by root/the owner.
667 .Pa /etc/ssh/ssh_known_hosts
668 should be world-readable, and
669 .Pa ~/.ssh/known_hosts
670 can, but need not be, world-readable.
675 This file is used to suppress printing the last login time and
683 It does not suppress printing of the banner specified by
688 refuses to let anyone except root log in.
689 The contents of the file
690 are displayed to anyone trying to log in, and non-root connections are
692 The file should be world-readable.
693 .It Pa /etc/hosts.allow, /etc/hosts.deny
694 Access controls that should be enforced by tcp-wrappers are defined here.
695 Further details are described in
698 This file is used during
699 .Cm RhostsRSAAuthentication
701 .Cm HostbasedAuthentication
702 and contains host-username pairs, separated by a space, one per
704 The given user on the corresponding host is permitted to log in
706 The same file is used by rlogind and rshd.
708 be writable only by the user; it is recommended that it not be
709 accessible by others.
711 It is also possible to use netgroups in the file.
713 name may be of the form +@groupname to specify all hosts or all users
717 this file is exactly the same as for
719 However, this file is
720 not used by rlogin and rshd, so using this permits access using SSH only.
721 .It Pa /etc/hosts.equiv
722 This file is used during
723 .Cm RhostsRSAAuthentication
725 .Cm HostbasedAuthentication
727 In the simplest form, this file contains host names, one per line.
729 those hosts are permitted to log in without a password, provided they
730 have the same user name on both machines.
731 The host name may also be
732 followed by a user name; such users are permitted to log in as
734 user on this machine (except root).
735 Additionally, the syntax
737 can be used to specify netgroups.
738 Negated entries start with
741 If the client host/user is successfully matched in this file, login is
742 automatically permitted provided the client and server user names are the
744 Additionally, successful client host key authentication is required.
745 This file must be writable only by root; it is recommended
746 that it be world-readable.
748 .Sy "Warning: It is almost never a good idea to use user names in"
750 Beware that it really means that the named user(s) can log in as
752 which includes bin, daemon, adm, and other accounts that own critical
753 binaries and directories.
754 Using a user name practically grants the user root access.
755 The only valid use for user names that I can think
756 of is in negative entries.
758 Note that this warning also applies to rsh/rlogin.
759 .It Pa /etc/shosts.equiv
760 This is processed exactly as
761 .Pa /etc/hosts.equiv .
762 However, this file may be useful in environments that want to run both
764 .It Pa ~/.ssh/environment
765 This file is read into the environment at login (if it exists).
766 It can only contain empty lines, comment lines (that start with
768 and assignment lines of the form name=value.
769 The file should be writable
770 only by the user; it need not be readable by anyone else.
771 Environment processing is disabled by default and is
773 .Cm PermitUserEnvironment
776 If this file exists, it is run with
779 environment files but before starting the user's shell or command.
780 It must not produce any output on stdout; stderr must be used
782 If X11 forwarding is in use, it will receive the "proto cookie" pair in
783 its standard input (and
790 will not run xauth automatically to add X11 cookies.
792 The primary purpose of this file is to run any initialization routines
793 which may be needed before the user's home directory becomes
794 accessible; AFS is a particular example of such an environment.
796 This file will probably contain some initialization code followed by
797 something similar to:
799 if read proto cookie && [ -n "$DISPLAY" ]; then
800 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
801 # X11UseLocalhost=yes
802 echo add unix:`echo $DISPLAY |
803 cut -c11-` $proto $cookie
806 echo add $DISPLAY $proto $cookie
811 If this file does not exist,
814 does not exist either, xauth is used to add the cookie.
816 This file should be writable only by the user, and need not be
817 readable by anyone else.
818 .It Pa /etc/ssh/sshrc
821 This can be used to specify
822 machine-specific login-time initializations globally.
823 This file should be writable only by root, and should be world-readable.
845 .%T "SSH Protocol Architecture"
846 .%N draft-ietf-secsh-architecture-12.txt
848 .%O work in progress material
854 .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
855 .%N draft-ietf-secsh-dh-group-exchange-02.txt
857 .%O work in progress material
860 OpenSSH is a derivative of the original and free
861 ssh 1.2.12 release by Tatu Ylonen.
862 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
863 Theo de Raadt and Dug Song
864 removed many bugs, re-added newer features and
866 Markus Friedl contributed the support for SSH
867 protocol versions 1.5 and 2.0.
868 Niels Provos and Markus Friedl contributed support
869 for privilege separation.