1 SSHD(8) OpenBSD System Manager's Manual SSHD(8)
4 sshd - OpenSSH SSH daemon
7 sshd [-46DdeiqTt] [-b bits] [-C connection_spec]
8 [-c host_certificate_file] [-f config_file] [-g login_grace_time]
9 [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
12 sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
13 programs replace rlogin(1) and rsh(1), and provide secure encrypted
14 communications between two untrusted hosts over an insecure network.
16 sshd listens for connections from clients. It is normally started at
17 boot from /etc/rc. It forks a new daemon for each incoming connection.
18 The forked daemons handle key exchange, encryption, authentication,
19 command execution, and data exchange.
21 sshd can be configured using command-line options or a configuration file
22 (by default sshd_config(5)); command-line options override values
23 specified in the configuration file. sshd rereads its configuration file
24 when it receives a hangup signal, SIGHUP, by executing itself with the
25 name and options it was started with, e.g. /usr/sbin/sshd.
27 The options are as follows:
29 -4 Forces sshd to use IPv4 addresses only.
31 -6 Forces sshd to use IPv6 addresses only.
34 Specifies the number of bits in the ephemeral protocol version 1
35 server key (default 1024).
38 Specify the connection parameters to use for the -T extended test
39 mode. If provided, any Match directives in the configuration
40 file that would apply to the specified user, host, and address
41 will be set before the configuration is written to standard
42 output. The connection parameters are supplied as keyword=value
43 pairs. The keywords are ``user'', ``host'', and ``addr''. All
44 are required and may be supplied in any order, either with
45 multiple -C options or as a comma-separated list.
47 -c host_certificate_file
48 Specifies a path to a certificate file to identify sshd during
49 key exchange. The certificate file must match a host key file
50 specified using the -h option or the HostKey configuration
53 -D When this option is specified, sshd will not detach and does not
54 become a daemon. This allows easy monitoring of sshd.
56 -d Debug mode. The server sends verbose debug output to standard
57 error, and does not put itself in the background. The server
58 also will not fork and will only process one connection. This
59 option is only intended for debugging for the server. Multiple
60 -d options increase the debugging level. Maximum is 3.
62 -e When this option is specified, sshd will send the output to the
63 standard error instead of the system log.
66 Specifies the name of the configuration file. The default is
67 /etc/ssh/sshd_config. sshd refuses to start if there is no
71 Gives the grace time for clients to authenticate themselves
72 (default 120 seconds). If the client fails to authenticate the
73 user within this many seconds, the server disconnects and exits.
74 A value of zero indicates no limit.
77 Specifies a file from which a host key is read. This option must
78 be given if sshd is not run as root (as the normal host key files
79 are normally not readable by anyone but root). The default is
80 /etc/ssh/ssh_host_key for protocol version 1, and
81 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
82 /etc/ssh/ssh_host_rsa_key for protocol version 2. It is possible
83 to have multiple host key files for the different protocol
84 versions and host key algorithms.
86 -i Specifies that sshd is being run from inetd(8). sshd is normally
87 not run from inetd because it needs to generate the server key
88 before it can respond to the client, and this may take tens of
89 seconds. Clients would have to wait too long if the key was
90 regenerated every time. However, with small key sizes (e.g. 512)
91 using sshd from inetd may be feasible.
94 Specifies how often the ephemeral protocol version 1 server key
95 is regenerated (default 3600 seconds, or one hour). The
96 motivation for regenerating the key fairly often is that the key
97 is not stored anywhere, and after about an hour it becomes
98 impossible to recover the key for decrypting intercepted
99 communications even if the machine is cracked into or physically
100 seized. A value of zero indicates that the key will never be
104 Can be used to give options in the format used in the
105 configuration file. This is useful for specifying options for
106 which there is no separate command-line flag. For full details
107 of the options, and their values, see sshd_config(5).
110 Specifies the port on which the server listens for connections
111 (default 22). Multiple port options are permitted. Ports
112 specified in the configuration file with the Port option are
113 ignored when a command-line port is specified. Ports specified
114 using the ListenAddress option override command-line ports.
116 -q Quiet mode. Nothing is sent to the system log. Normally the
117 beginning, authentication, and termination of each connection is
120 -T Extended test mode. Check the validity of the configuration
121 file, output the effective configuration to stdout and then exit.
122 Optionally, Match rules may be applied by specifying the
123 connection parameters using one or more -C options.
125 -t Test mode. Only check the validity of the configuration file and
126 sanity of the keys. This is useful for updating sshd reliably as
127 configuration options may change.
129 -u len This option is used to specify the size of the field in the utmp
130 structure that holds the remote host name. If the resolved host
131 name is longer than len, the dotted decimal value will be used
132 instead. This allows hosts with very long host names that
133 overflow this field to still be uniquely identified. Specifying
134 -u0 indicates that only dotted decimal addresses should be put
135 into the utmp file. -u0 may also be used to prevent sshd from
136 making DNS requests unless the authentication mechanism or
137 configuration requires it. Authentication mechanisms that may
138 require DNS include RhostsRSAAuthentication,
139 HostbasedAuthentication, and using a from="pattern-list" option
140 in a key file. Configuration options that require DNS include
141 using a USER@HOST pattern in AllowUsers or DenyUsers.
144 The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to
145 use protocol 2 only, though this can be changed via the Protocol option
146 in sshd_config(5). Protocol 2 supports DSA, ECDSA and RSA keys; protocol
147 1 only supports RSA keys. For both protocols, each host has a host-
148 specific key, normally 2048 bits, used to identify the host.
150 Forward security for protocol 1 is provided through an additional server
151 key, normally 768 bits, generated when the server starts. This key is
152 normally regenerated every hour if it has been used, and is never stored
153 on disk. Whenever a client connects, the daemon responds with its public
154 host and server keys. The client compares the RSA host key against its
155 own database to verify that it has not changed. The client then
156 generates a 256-bit random number. It encrypts this random number using
157 both the host key and the server key, and sends the encrypted number to
158 the server. Both sides then use this random number as a session key
159 which is used to encrypt all further communications in the session. The
160 rest of the session is encrypted using a conventional cipher, currently
161 Blowfish or 3DES, with 3DES being used by default. The client selects
162 the encryption algorithm to use from those offered by the server.
164 For protocol 2, forward security is provided through a Diffie-Hellman key
165 agreement. This key agreement results in a shared session key. The rest
166 of the session is encrypted using a symmetric cipher, currently 128-bit
167 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
168 client selects the encryption algorithm to use from those offered by the
169 server. Additionally, session integrity is provided through a
170 cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64
173 Finally, the server and the client enter an authentication dialog. The
174 client tries to authenticate itself using host-based authentication,
175 public key authentication, challenge-response authentication, or password
178 Regardless of the authentication type, the account is checked to ensure
179 that it is accessible. An account is not accessible if it is locked,
180 listed in DenyUsers or its group is listed in DenyGroups . The
181 definition of a locked account is system dependant. Some platforms have
182 their own account database (eg AIX) and some modify the passwd field (
183 `*LK*' on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on
184 Tru64, a leading `*LOCKED*' on FreeBSD and a leading `!' on most
185 Linuxes). If there is a requirement to disable password authentication
186 for the account while allowing still public-key, then the passwd field
187 should be set to something other than these values (eg `NP' or `*NP*' ).
189 If the client successfully authenticates itself, a dialog for preparing
190 the session is entered. At this time the client may request things like
191 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP
192 connections, or forwarding the authentication agent connection over the
195 After this, the client either requests a shell or execution of a command.
196 The sides then enter session mode. In this mode, either side may send
197 data at any time, and such data is forwarded to/from the shell or command
198 on the server side, and the user terminal in the client side.
200 When the user program terminates and all forwarded X11 and other
201 connections have been closed, the server sends command exit status to the
202 client, and both sides exit.
205 When a user successfully logs in, sshd does the following:
207 1. If the login is on a tty, and no command has been specified,
208 prints last login time and /etc/motd (unless prevented in the
209 configuration file or by ~/.hushlogin; see the FILES section).
211 2. If the login is on a tty, records login time.
213 3. Checks /etc/nologin; if it exists, prints contents and quits
216 4. Changes to run with normal user privileges.
218 5. Sets up basic environment.
220 6. Reads the file ~/.ssh/environment, if it exists, and users are
221 allowed to change their environment. See the
222 PermitUserEnvironment option in sshd_config(5).
224 7. Changes to user's home directory.
226 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
227 runs it; otherwise runs xauth. The ``rc'' files are given the
228 X11 authentication protocol and cookie in standard input. See
231 9. Runs user's shell or command.
234 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
235 files but before starting the user's shell or command. It must not
236 produce any output on stdout; stderr must be used instead. If X11
237 forwarding is in use, it will receive the "proto cookie" pair in its
238 standard input (and DISPLAY in its environment). The script must call
239 xauth(1) because sshd will not run xauth automatically to add X11
242 The primary purpose of this file is to run any initialization routines
243 which may be needed before the user's home directory becomes accessible;
244 AFS is a particular example of such an environment.
246 This file will probably contain some initialization code followed by
247 something similar to:
249 if read proto cookie && [ -n "$DISPLAY" ]; then
250 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
251 # X11UseLocalhost=yes
252 echo add unix:`echo $DISPLAY |
253 cut -c11-` $proto $cookie
256 echo add $DISPLAY $proto $cookie
260 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
261 exist either, xauth is used to add the cookie.
263 AUTHORIZED_KEYS FILE FORMAT
264 AuthorizedKeysFile specifies the file containing public keys for public
265 key authentication; if none is specified, the default is
266 ~/.ssh/authorized_keys. Each line of the file contains one key (empty
267 lines and lines starting with a `#' are ignored as comments). Protocol 1
268 public keys consist of the following space-separated fields: options,
269 bits, exponent, modulus, comment. Protocol 2 public key consist of:
270 options, keytype, base64-encoded key, comment. The options field is
271 optional; its presence is determined by whether the line starts with a
272 number or not (the options field never starts with a number). The bits,
273 exponent, modulus, and comment fields give the RSA key for protocol
274 version 1; the comment field is not used for anything (but may be
275 convenient for the user to identify the key). For protocol version 2 the
276 keytype is ``ecdsa-sha2-nistp256'', ``ecdsa-sha2-nistp384'',
277 ``ecdsa-sha2-nistp521'', ``ssh-dss'' or ``ssh-rsa''.
279 Note that lines in this file are usually several hundred bytes long
280 (because of the size of the public key encoding) up to a limit of 8
281 kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
282 kilobits. You don't want to type them in; instead, copy the
283 identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit
286 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
289 The options (if present) consist of comma-separated option
290 specifications. No spaces are permitted, except within double quotes.
291 The following option specifications are supported (note that option
292 keywords are case-insensitive):
295 Specifies that the listed key is a certification authority (CA)
296 that is trusted to validate signed certificates for user
299 Certificates may encode access restrictions similar to these key
300 options. If both certificate restrictions and key options are
301 present, the most restrictive union of the two is applied.
304 Specifies that the command is executed whenever this key is used
305 for authentication. The command supplied by the user (if any) is
306 ignored. The command is run on a pty if the client requests a
307 pty; otherwise it is run without a tty. If an 8-bit clean
308 channel is required, one must not request a pty or should specify
309 no-pty. A quote may be included in the command by quoting it
310 with a backslash. This option might be useful to restrict
311 certain public keys to perform just a specific operation. An
312 example might be a key that permits remote backups but nothing
313 else. Note that the client may specify TCP and/or X11 forwarding
314 unless they are explicitly prohibited. The command originally
315 supplied by the client is available in the SSH_ORIGINAL_COMMAND
316 environment variable. Note that this option applies to shell,
317 command or subsystem execution. Also note that this command may
318 be superseded by either a sshd_config(5) ForceCommand directive
319 or a command embedded in a certificate.
321 environment="NAME=value"
322 Specifies that the string is to be added to the environment when
323 logging in using this key. Environment variables set this way
324 override other default environment values. Multiple options of
325 this type are permitted. Environment processing is disabled by
326 default and is controlled via the PermitUserEnvironment option.
327 This option is automatically disabled if UseLogin is enabled.
330 Specifies that in addition to public key authentication, either
331 the canonical name of the remote host or its IP address must be
332 present in the comma-separated list of patterns. See PATTERNS in
333 ssh_config(5) for more information on patterns.
335 In addition to the wildcard matching that may be applied to
336 hostnames or addresses, a from stanza may match IP addresses
337 using CIDR address/masklen notation.
339 The purpose of this option is to optionally increase security:
340 public key authentication by itself does not trust the network or
341 name servers or anything (but the key); however, if somebody
342 somehow steals the key, the key permits an intruder to log in
343 from anywhere in the world. This additional option makes using a
344 stolen key more difficult (name servers and/or routers would have
345 to be compromised in addition to just the key).
348 Forbids authentication agent forwarding when this key is used for
352 Forbids TCP forwarding when this key is used for authentication.
353 Any port forward requests by the client will return an error.
354 This might be used, e.g. in connection with the command option.
356 no-pty Prevents tty allocation (a request to allocate a pty will fail).
359 Disables execution of ~/.ssh/rc.
362 Forbids X11 forwarding when this key is used for authentication.
363 Any X11 forward requests by the client will return an error.
365 permitopen="host:port"
366 Limit local ``ssh -L'' port forwarding such that it may only
367 connect to the specified host and port. IPv6 addresses can be
368 specified by enclosing the address in square brackets. Multiple
369 permitopen options may be applied separated by commas. No
370 pattern matching is performed on the specified hostnames, they
371 must be literal domains or addresses.
373 principals="principals"
374 On a cert-authority line, specifies allowed principals for
375 certificate authentication as a comma-separated list. At least
376 one name from the list must appear in the certificate's list of
377 principals for the certificate to be accepted. This option is
378 ignored for keys that are not marked as trusted certificate
379 signers using the cert-authority option.
382 Force a tun(4) device on the server. Without this option, the
383 next available device will be used if the client requests a
386 An example authorized_keys file:
388 # Comments allowed at start of line
389 ssh-rsa AAAAB3Nza...LiPk== user@example.net
390 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
391 AAAAB2...19Q== john@example.net
392 command="dump /home",no-pty,no-port-forwarding ssh-dss
393 AAAAC3...51R== example.net
394 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
396 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
399 SSH_KNOWN_HOSTS FILE FORMAT
400 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
401 public keys for all known hosts. The global file should be prepared by
402 the administrator (optional), and the per-user file is maintained
403 automatically: whenever the user connects from an unknown host, its key
404 is added to the per-user file.
406 Each line in these files contains the following fields: markers
407 (optional), hostnames, bits, exponent, modulus, comment. The fields are
410 The marker is optional, but if it is present then it must be one of
411 ``@cert-authority'', to indicate that the line contains a certification
412 authority (CA) key, or ``@revoked'', to indicate that the key contained
413 on the line is revoked and must not ever be accepted. Only one marker
414 should be used on a key line.
416 Hostnames is a comma-separated list of patterns (`*' and `?' act as
417 wildcards); each pattern in turn is matched against the canonical host
418 name (when authenticating a client) or against the user-supplied name
419 (when authenticating a server). A pattern may also be preceded by `!' to
420 indicate negation: if the host name matches a negated pattern, it is not
421 accepted (by that line) even if it matched another pattern on the line.
422 A hostname or address may optionally be enclosed within `[' and `]'
423 brackets then followed by `:' and a non-standard port number.
425 Alternately, hostnames may be stored in a hashed form which hides host
426 names and addresses should the file's contents be disclosed. Hashed
427 hostnames start with a `|' character. Only one hashed hostname may
428 appear on a single line and none of the above negation or wildcard
429 operators may be applied.
431 Bits, exponent, and modulus are taken directly from the RSA host key;
432 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
433 optional comment field continues to the end of the line, and is not used.
435 Lines starting with `#' and empty lines are ignored as comments.
437 When performing host authentication, authentication is accepted if any
438 matching line has the proper key; either one that matches exactly or, if
439 the server has presented a certificate for authentication, the key of the
440 certification authority that signed the certificate. For a key to be
441 trusted as a certification authority, it must use the ``@cert-authority''
442 marker described above.
444 The known hosts file also provides a facility to mark keys as revoked,
445 for example when it is known that the associated private key has been
446 stolen. Revoked keys are specified by including the ``@revoked'' marker
447 at the beginning of the key line, and are never accepted for
448 authentication or as certification authorities, but instead will produce
449 a warning from ssh(1) when they are encountered.
451 It is permissible (but not recommended) to have several lines or
452 different host keys for the same names. This will inevitably happen when
453 short forms of host names from different domains are put in the file. It
454 is possible that the files contain conflicting information;
455 authentication is accepted if valid information can be found from either
458 Note that the lines in these files are typically hundreds of characters
459 long, and you definitely don't want to type in the host keys by hand.
460 Rather, generate them by a script, ssh-keyscan(1) or by taking
461 /etc/ssh/ssh_host_key.pub and adding the host names at the front.
462 ssh-keygen(1) also offers some basic automated editing for
463 ~/.ssh/known_hosts including removing hosts matching a host name and
464 converting all host names to their hashed representations.
466 An example ssh_known_hosts file:
468 # Comments allowed at start of line
469 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
470 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
472 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
475 @revoked * ssh-rsa AAAAB5W...
476 # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
477 @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
481 This file is used to suppress printing the last login time and
482 /etc/motd, if PrintLastLog and PrintMotd, respectively, are
483 enabled. It does not suppress printing of the banner specified
487 This file is used for host-based authentication (see ssh(1) for
488 more information). On some machines this file may need to be
489 world-readable if the user's home directory is on an NFS
490 partition, because sshd reads it as root. Additionally, this
491 file must be owned by the user, and must not have write
492 permissions for anyone else. The recommended permission for most
493 machines is read/write for the user, and not accessible by
497 This file is used in exactly the same way as .rhosts, but allows
498 host-based authentication without permitting login with
502 This directory is the default location for all user-specific
503 configuration and authentication information. There is no
504 general requirement to keep the entire contents of this directory
505 secret, but the recommended permissions are read/write/execute
506 for the user, and not accessible by others.
508 ~/.ssh/authorized_keys
509 Lists the public keys (DSA/ECDSA/RSA) that can be used for
510 logging in as this user. The format of this file is described
511 above. The content of the file is not highly sensitive, but the
512 recommended permissions are read/write for the user, and not
513 accessible by others.
515 If this file, the ~/.ssh directory, or the user's home directory
516 are writable by other users, then the file could be modified or
517 replaced by unauthorized users. In this case, sshd will not
518 allow it to be used unless the StrictModes option has been set to
522 This file is read into the environment at login (if it exists).
523 It can only contain empty lines, comment lines (that start with
524 `#'), and assignment lines of the form name=value. The file
525 should be writable only by the user; it need not be readable by
526 anyone else. Environment processing is disabled by default and
527 is controlled via the PermitUserEnvironment option.
530 Contains a list of host keys for all hosts the user has logged
531 into that are not already in the systemwide list of known host
532 keys. The format of this file is described above. This file
533 should be writable only by root/the owner and can, but need not
537 Contains initialization routines to be run before the user's home
538 directory becomes accessible. This file should be writable only
539 by the user, and need not be readable by anyone else.
543 Access controls that should be enforced by tcp-wrappers are
544 defined here. Further details are described in hosts_access(5).
547 This file is for host-based authentication (see ssh(1)). It
548 should only be writable by root.
551 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
552 Exchange". The file format is described in moduli(5).
558 If this file exists, sshd refuses to let anyone except root log
559 in. The contents of the file are displayed to anyone trying to
560 log in, and non-root connections are refused. The file should be
564 This file is used in exactly the same way as hosts.equiv, but
565 allows host-based authentication without permitting login with
568 /etc/ssh/ssh_host_key
569 /etc/ssh/ssh_host_dsa_key
570 /etc/ssh/ssh_host_ecdsa_key
571 /etc/ssh/ssh_host_rsa_key
572 These three files contain the private parts of the host keys.
573 These files should only be owned by root, readable only by root,
574 and not accessible to others. Note that sshd does not start if
575 these files are group/world-accessible.
577 /etc/ssh/ssh_host_key.pub
578 /etc/ssh/ssh_host_dsa_key.pub
579 /etc/ssh/ssh_host_ecdsa_key.pub
580 /etc/ssh/ssh_host_rsa_key.pub
581 These three files contain the public parts of the host keys.
582 These files should be world-readable but writable only by root.
583 Their contents should match the respective private parts. These
584 files are not really used for anything; they are provided for the
585 convenience of the user so their contents can be copied to known
586 hosts files. These files are created using ssh-keygen(1).
588 /etc/ssh/ssh_known_hosts
589 Systemwide list of known host keys. This file should be prepared
590 by the system administrator to contain the public host keys of
591 all machines in the organization. The format of this file is
592 described above. This file should be writable only by root/the
593 owner and should be world-readable.
596 Contains configuration data for sshd. The file format and
597 configuration options are described in sshd_config(5).
600 Similar to ~/.ssh/rc, it can be used to specify machine-specific
601 login-time initializations globally. This file should be
602 writable only by root, and should be world-readable.
605 chroot(2) directory used by sshd during privilege separation in
606 the pre-authentication phase. The directory should not contain
607 any files and must be owned by root and not group or world-
611 Contains the process ID of the sshd listening for connections (if
612 there are several daemons running concurrently for different
613 ports, this contains the process ID of the one started last).
614 The content of this file is not sensitive; it can be world-
618 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
619 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
620 sshd_config(5), inetd(8), sftp-server(8)
623 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
624 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
625 de Raadt and Dug Song removed many bugs, re-added newer features and
626 created OpenSSH. Markus Friedl contributed the support for SSH protocol
627 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
628 for privilege separation.
631 System security is not improved unless rshd, rlogind, and rexecd are
632 disabled (thus completely disabling rlogin and rsh into the machine).
634 OpenBSD 4.9 October 28, 2010 OpenBSD 4.9
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