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] [-E log_file] [-f config_file]
9 [-g login_grace_time] [-h host_key_file] [-k key_gen_time]
10 [-o option] [-p port] [-u len]
13 sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
14 programs replace rlogin(1) and rsh(1), and provide secure encrypted
15 communications between two untrusted hosts over an insecure network.
17 sshd listens for connections from clients. It is normally started at
18 boot from /etc/rc. It forks a new daemon for each incoming connection.
19 The forked daemons handle key exchange, encryption, authentication,
20 command execution, and data exchange.
22 sshd can be configured using command-line options or a configuration file
23 (by default sshd_config(5)); command-line options override values
24 specified in the configuration file. sshd rereads its configuration file
25 when it receives a hangup signal, SIGHUP, by executing itself with the
26 name and options it was started with, e.g. /usr/sbin/sshd.
28 The options are as follows:
30 -4 Forces sshd to use IPv4 addresses only.
32 -6 Forces sshd to use IPv6 addresses only.
35 Specifies the number of bits in the ephemeral protocol version 1
36 server key (default 1024).
39 Specify the connection parameters to use for the -T extended test
40 mode. If provided, any Match directives in the configuration
41 file that would apply to the specified user, host, and address
42 will be set before the configuration is written to standard
43 output. The connection parameters are supplied as keyword=value
44 pairs. The keywords are ``user'', ``host'', ``laddr'',
45 ``lport'', and ``addr''. All are required and may be supplied in
46 any order, either with multiple -C options or as a comma-
49 -c host_certificate_file
50 Specifies a path to a certificate file to identify sshd during
51 key exchange. The certificate file must match a host key file
52 specified using the -h option or the HostKey configuration
55 -D When this option is specified, sshd will not detach and does not
56 become a daemon. This allows easy monitoring of sshd.
58 -d Debug mode. The server sends verbose debug output to standard
59 error, and does not put itself in the background. The server
60 also will not fork and will only process one connection. This
61 option is only intended for debugging for the server. Multiple
62 -d options increase the debugging level. Maximum is 3.
65 Append debug logs to log_file instead of the system log.
67 -e Write debug logs to standard error instead of the system log.
70 Specifies the name of the configuration file. The default is
71 /etc/ssh/sshd_config. sshd refuses to start if there is no
75 Gives the grace time for clients to authenticate themselves
76 (default 120 seconds). If the client fails to authenticate the
77 user within this many seconds, the server disconnects and exits.
78 A value of zero indicates no limit.
81 Specifies a file from which a host key is read. This option must
82 be given if sshd is not run as root (as the normal host key files
83 are normally not readable by anyone but root). The default is
84 /etc/ssh/ssh_host_key for protocol version 1, and
85 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
86 /etc/ssh/ssh_host_rsa_key for protocol version 2. It is possible
87 to have multiple host key files for the different protocol
88 versions and host key algorithms.
90 -i Specifies that sshd is being run from inetd(8). sshd is normally
91 not run from inetd because it needs to generate the server key
92 before it can respond to the client, and this may take tens of
93 seconds. Clients would have to wait too long if the key was
94 regenerated every time. However, with small key sizes (e.g. 512)
95 using sshd from inetd may be feasible.
98 Specifies how often the ephemeral protocol version 1 server key
99 is regenerated (default 3600 seconds, or one hour). The
100 motivation for regenerating the key fairly often is that the key
101 is not stored anywhere, and after about an hour it becomes
102 impossible to recover the key for decrypting intercepted
103 communications even if the machine is cracked into or physically
104 seized. A value of zero indicates that the key will never be
108 Can be used to give options in the format used in the
109 configuration file. This is useful for specifying options for
110 which there is no separate command-line flag. For full details
111 of the options, and their values, see sshd_config(5).
114 Specifies the port on which the server listens for connections
115 (default 22). Multiple port options are permitted. Ports
116 specified in the configuration file with the Port option are
117 ignored when a command-line port is specified. Ports specified
118 using the ListenAddress option override command-line ports.
120 -q Quiet mode. Nothing is sent to the system log. Normally the
121 beginning, authentication, and termination of each connection is
124 -T Extended test mode. Check the validity of the configuration
125 file, output the effective configuration to stdout and then exit.
126 Optionally, Match rules may be applied by specifying the
127 connection parameters using one or more -C options.
129 -t Test mode. Only check the validity of the configuration file and
130 sanity of the keys. This is useful for updating sshd reliably as
131 configuration options may change.
133 -u len This option is used to specify the size of the field in the utmp
134 structure that holds the remote host name. If the resolved host
135 name is longer than len, the dotted decimal value will be used
136 instead. This allows hosts with very long host names that
137 overflow this field to still be uniquely identified. Specifying
138 -u0 indicates that only dotted decimal addresses should be put
139 into the utmp file. -u0 may also be used to prevent sshd from
140 making DNS requests unless the authentication mechanism or
141 configuration requires it. Authentication mechanisms that may
142 require DNS include RhostsRSAAuthentication,
143 HostbasedAuthentication, and using a from="pattern-list" option
144 in a key file. Configuration options that require DNS include
145 using a USER@HOST pattern in AllowUsers or DenyUsers.
148 The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to
149 use protocol 2 only, though this can be changed via the Protocol option
150 in sshd_config(5). Protocol 2 supports DSA, ECDSA and RSA keys; protocol
151 1 only supports RSA keys. For both protocols, each host has a host-
152 specific key, normally 2048 bits, used to identify the host.
154 Forward security for protocol 1 is provided through an additional server
155 key, normally 768 bits, generated when the server starts. This key is
156 normally regenerated every hour if it has been used, and is never stored
157 on disk. Whenever a client connects, the daemon responds with its public
158 host and server keys. The client compares the RSA host key against its
159 own database to verify that it has not changed. The client then
160 generates a 256-bit random number. It encrypts this random number using
161 both the host key and the server key, and sends the encrypted number to
162 the server. Both sides then use this random number as a session key
163 which is used to encrypt all further communications in the session. The
164 rest of the session is encrypted using a conventional cipher, currently
165 Blowfish or 3DES, with 3DES being used by default. The client selects
166 the encryption algorithm to use from those offered by the server.
168 For protocol 2, forward security is provided through a Diffie-Hellman key
169 agreement. This key agreement results in a shared session key. The rest
170 of the session is encrypted using a symmetric cipher, currently 128-bit
171 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
172 client selects the encryption algorithm to use from those offered by the
173 server. Additionally, session integrity is provided through a
174 cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64,
175 umac-128, hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512).
177 Finally, the server and the client enter an authentication dialog. The
178 client tries to authenticate itself using host-based authentication,
179 public key authentication, challenge-response authentication, or password
182 Regardless of the authentication type, the account is checked to ensure
183 that it is accessible. An account is not accessible if it is locked,
184 listed in DenyUsers or its group is listed in DenyGroups . The
185 definition of a locked account is system dependant. Some platforms have
186 their own account database (eg AIX) and some modify the passwd field (
187 `*LK*' on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on
188 Tru64, a leading `*LOCKED*' on FreeBSD and a leading `!' on most
189 Linuxes). If there is a requirement to disable password authentication
190 for the account while allowing still public-key, then the passwd field
191 should be set to something other than these values (eg `NP' or `*NP*' ).
193 If the client successfully authenticates itself, a dialog for preparing
194 the session is entered. At this time the client may request things like
195 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP
196 connections, or forwarding the authentication agent connection over the
199 After this, the client either requests a shell or execution of a command.
200 The sides then enter session mode. In this mode, either side may send
201 data at any time, and such data is forwarded to/from the shell or command
202 on the server side, and the user terminal in the client side.
204 When the user program terminates and all forwarded X11 and other
205 connections have been closed, the server sends command exit status to the
206 client, and both sides exit.
209 When a user successfully logs in, sshd does the following:
211 1. If the login is on a tty, and no command has been specified,
212 prints last login time and /etc/motd (unless prevented in the
213 configuration file or by ~/.hushlogin; see the FILES section).
215 2. If the login is on a tty, records login time.
217 3. Checks /etc/nologin; if it exists, prints contents and quits
220 4. Changes to run with normal user privileges.
222 5. Sets up basic environment.
224 6. Reads the file ~/.ssh/environment, if it exists, and users are
225 allowed to change their environment. See the
226 PermitUserEnvironment option in sshd_config(5).
228 7. Changes to user's home directory.
230 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
231 runs it; otherwise runs xauth. The ``rc'' files are given the
232 X11 authentication protocol and cookie in standard input. See
235 9. Runs user's shell or command.
238 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
239 files but before starting the user's shell or command. It must not
240 produce any output on stdout; stderr must be used instead. If X11
241 forwarding is in use, it will receive the "proto cookie" pair in its
242 standard input (and DISPLAY in its environment). The script must call
243 xauth(1) because sshd will not run xauth automatically to add X11
246 The primary purpose of this file is to run any initialization routines
247 which may be needed before the user's home directory becomes accessible;
248 AFS is a particular example of such an environment.
250 This file will probably contain some initialization code followed by
251 something similar to:
253 if read proto cookie && [ -n "$DISPLAY" ]; then
254 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
255 # X11UseLocalhost=yes
256 echo add unix:`echo $DISPLAY |
257 cut -c11-` $proto $cookie
260 echo add $DISPLAY $proto $cookie
264 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
265 exist either, xauth is used to add the cookie.
267 AUTHORIZED_KEYS FILE FORMAT
268 AuthorizedKeysFile specifies the files containing public keys for public
269 key authentication; if none is specified, the default is
270 ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the
271 file contains one key (empty lines and lines starting with a `#' are
272 ignored as comments). Protocol 1 public keys consist of the following
273 space-separated fields: options, bits, exponent, modulus, comment.
274 Protocol 2 public key consist of: options, keytype, base64-encoded key,
275 comment. The options field is optional; its presence is determined by
276 whether the line starts with a number or not (the options field never
277 starts with a number). The bits, exponent, modulus, and comment fields
278 give the RSA key for protocol version 1; the comment field is not used
279 for anything (but may be convenient for the user to identify the key).
280 For protocol version 2 the keytype is ``ecdsa-sha2-nistp256'',
281 ``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-dss'' or
284 Note that lines in this file are usually several hundred bytes long
285 (because of the size of the public key encoding) up to a limit of 8
286 kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
287 kilobits. You don't want to type them in; instead, copy the
288 identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit
291 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
294 The options (if present) consist of comma-separated option
295 specifications. No spaces are permitted, except within double quotes.
296 The following option specifications are supported (note that option
297 keywords are case-insensitive):
300 Specifies that the listed key is a certification authority (CA)
301 that is trusted to validate signed certificates for user
304 Certificates may encode access restrictions similar to these key
305 options. If both certificate restrictions and key options are
306 present, the most restrictive union of the two is applied.
309 Specifies that the command is executed whenever this key is used
310 for authentication. The command supplied by the user (if any) is
311 ignored. The command is run on a pty if the client requests a
312 pty; otherwise it is run without a tty. If an 8-bit clean
313 channel is required, one must not request a pty or should specify
314 no-pty. A quote may be included in the command by quoting it
315 with a backslash. This option might be useful to restrict
316 certain public keys to perform just a specific operation. An
317 example might be a key that permits remote backups but nothing
318 else. Note that the client may specify TCP and/or X11 forwarding
319 unless they are explicitly prohibited. The command originally
320 supplied by the client is available in the SSH_ORIGINAL_COMMAND
321 environment variable. Note that this option applies to shell,
322 command or subsystem execution. Also note that this command may
323 be superseded by either a sshd_config(5) ForceCommand directive
324 or a command embedded in a certificate.
326 environment="NAME=value"
327 Specifies that the string is to be added to the environment when
328 logging in using this key. Environment variables set this way
329 override other default environment values. Multiple options of
330 this type are permitted. Environment processing is disabled by
331 default and is controlled via the PermitUserEnvironment option.
332 This option is automatically disabled if UseLogin is enabled.
335 Specifies that in addition to public key authentication, either
336 the canonical name of the remote host or its IP address must be
337 present in the comma-separated list of patterns. See PATTERNS in
338 ssh_config(5) for more information on patterns.
340 In addition to the wildcard matching that may be applied to
341 hostnames or addresses, a from stanza may match IP addresses
342 using CIDR address/masklen notation.
344 The purpose of this option is to optionally increase security:
345 public key authentication by itself does not trust the network or
346 name servers or anything (but the key); however, if somebody
347 somehow steals the key, the key permits an intruder to log in
348 from anywhere in the world. This additional option makes using a
349 stolen key more difficult (name servers and/or routers would have
350 to be compromised in addition to just the key).
353 Forbids authentication agent forwarding when this key is used for
357 Forbids TCP forwarding when this key is used for authentication.
358 Any port forward requests by the client will return an error.
359 This might be used, e.g. in connection with the command option.
361 no-pty Prevents tty allocation (a request to allocate a pty will fail).
364 Disables execution of ~/.ssh/rc.
367 Forbids X11 forwarding when this key is used for authentication.
368 Any X11 forward requests by the client will return an error.
370 permitopen="host:port"
371 Limit local ``ssh -L'' port forwarding such that it may only
372 connect to the specified host and port. IPv6 addresses can be
373 specified by enclosing the address in square brackets. Multiple
374 permitopen options may be applied separated by commas. No
375 pattern matching is performed on the specified hostnames, they
376 must be literal domains or addresses. A port specification of *
379 principals="principals"
380 On a cert-authority line, specifies allowed principals for
381 certificate authentication as a comma-separated list. At least
382 one name from the list must appear in the certificate's list of
383 principals for the certificate to be accepted. This option is
384 ignored for keys that are not marked as trusted certificate
385 signers using the cert-authority option.
388 Force a tun(4) device on the server. Without this option, the
389 next available device will be used if the client requests a
392 An example authorized_keys file:
394 # Comments allowed at start of line
395 ssh-rsa AAAAB3Nza...LiPk== user@example.net
396 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
397 AAAAB2...19Q== john@example.net
398 command="dump /home",no-pty,no-port-forwarding ssh-dss
399 AAAAC3...51R== example.net
400 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
402 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
405 SSH_KNOWN_HOSTS FILE FORMAT
406 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
407 public keys for all known hosts. The global file should be prepared by
408 the administrator (optional), and the per-user file is maintained
409 automatically: whenever the user connects from an unknown host, its key
410 is added to the per-user file.
412 Each line in these files contains the following fields: markers
413 (optional), hostnames, bits, exponent, modulus, comment. The fields are
416 The marker is optional, but if it is present then it must be one of
417 ``@cert-authority'', to indicate that the line contains a certification
418 authority (CA) key, or ``@revoked'', to indicate that the key contained
419 on the line is revoked and must not ever be accepted. Only one marker
420 should be used on a key line.
422 Hostnames is a comma-separated list of patterns (`*' and `?' act as
423 wildcards); each pattern in turn is matched against the canonical host
424 name (when authenticating a client) or against the user-supplied name
425 (when authenticating a server). A pattern may also be preceded by `!' to
426 indicate negation: if the host name matches a negated pattern, it is not
427 accepted (by that line) even if it matched another pattern on the line.
428 A hostname or address may optionally be enclosed within `[' and `]'
429 brackets then followed by `:' and a non-standard port number.
431 Alternately, hostnames may be stored in a hashed form which hides host
432 names and addresses should the file's contents be disclosed. Hashed
433 hostnames start with a `|' character. Only one hashed hostname may
434 appear on a single line and none of the above negation or wildcard
435 operators may be applied.
437 Bits, exponent, and modulus are taken directly from the RSA host key;
438 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
439 optional comment field continues to the end of the line, and is not used.
441 Lines starting with `#' and empty lines are ignored as comments.
443 When performing host authentication, authentication is accepted if any
444 matching line has the proper key; either one that matches exactly or, if
445 the server has presented a certificate for authentication, the key of the
446 certification authority that signed the certificate. For a key to be
447 trusted as a certification authority, it must use the ``@cert-authority''
448 marker described above.
450 The known hosts file also provides a facility to mark keys as revoked,
451 for example when it is known that the associated private key has been
452 stolen. Revoked keys are specified by including the ``@revoked'' marker
453 at the beginning of the key line, and are never accepted for
454 authentication or as certification authorities, but instead will produce
455 a warning from ssh(1) when they are encountered.
457 It is permissible (but not recommended) to have several lines or
458 different host keys for the same names. This will inevitably happen when
459 short forms of host names from different domains are put in the file. It
460 is possible that the files contain conflicting information;
461 authentication is accepted if valid information can be found from either
464 Note that the lines in these files are typically hundreds of characters
465 long, and you definitely don't want to type in the host keys by hand.
466 Rather, generate them by a script, ssh-keyscan(1) or by taking
467 /etc/ssh/ssh_host_key.pub and adding the host names at the front.
468 ssh-keygen(1) also offers some basic automated editing for
469 ~/.ssh/known_hosts including removing hosts matching a host name and
470 converting all host names to their hashed representations.
472 An example ssh_known_hosts file:
474 # Comments allowed at start of line
475 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
476 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
478 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
481 @revoked * ssh-rsa AAAAB5W...
482 # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
483 @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
487 This file is used to suppress printing the last login time and
488 /etc/motd, if PrintLastLog and PrintMotd, respectively, are
489 enabled. It does not suppress printing of the banner specified
493 This file is used for host-based authentication (see ssh(1) for
494 more information). On some machines this file may need to be
495 world-readable if the user's home directory is on an NFS
496 partition, because sshd reads it as root. Additionally, this
497 file must be owned by the user, and must not have write
498 permissions for anyone else. The recommended permission for most
499 machines is read/write for the user, and not accessible by
503 This file is used in exactly the same way as .rhosts, but allows
504 host-based authentication without permitting login with
508 This directory is the default location for all user-specific
509 configuration and authentication information. There is no
510 general requirement to keep the entire contents of this directory
511 secret, but the recommended permissions are read/write/execute
512 for the user, and not accessible by others.
514 ~/.ssh/authorized_keys
515 Lists the public keys (DSA/ECDSA/RSA) that can be used for
516 logging in as this user. The format of this file is described
517 above. The content of the file is not highly sensitive, but the
518 recommended permissions are read/write for the user, and not
519 accessible by others.
521 If this file, the ~/.ssh directory, or the user's home directory
522 are writable by other users, then the file could be modified or
523 replaced by unauthorized users. In this case, sshd will not
524 allow it to be used unless the StrictModes option has been set to
528 This file is read into the environment at login (if it exists).
529 It can only contain empty lines, comment lines (that start with
530 `#'), and assignment lines of the form name=value. The file
531 should be writable only by the user; it need not be readable by
532 anyone else. Environment processing is disabled by default and
533 is controlled via the PermitUserEnvironment option.
536 Contains a list of host keys for all hosts the user has logged
537 into that are not already in the systemwide list of known host
538 keys. The format of this file is described above. This file
539 should be writable only by root/the owner and can, but need not
543 Contains initialization routines to be run before the user's home
544 directory becomes accessible. This file should be writable only
545 by the user, and need not be readable by anyone else.
549 Access controls that should be enforced by tcp-wrappers are
550 defined here. Further details are described in hosts_access(5).
553 This file is for host-based authentication (see ssh(1)). It
554 should only be writable by root.
557 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
558 Exchange". The file format is described in moduli(5).
564 If this file exists, sshd refuses to let anyone except root log
565 in. The contents of the file are displayed to anyone trying to
566 log in, and non-root connections are refused. The file should be
570 This file is used in exactly the same way as hosts.equiv, but
571 allows host-based authentication without permitting login with
574 /etc/ssh/ssh_host_key
575 /etc/ssh/ssh_host_dsa_key
576 /etc/ssh/ssh_host_ecdsa_key
577 /etc/ssh/ssh_host_rsa_key
578 These files contain the private parts of the host keys. These
579 files should only be owned by root, readable only by root, and
580 not accessible to others. Note that sshd does not start if these
581 files are group/world-accessible.
583 /etc/ssh/ssh_host_key.pub
584 /etc/ssh/ssh_host_dsa_key.pub
585 /etc/ssh/ssh_host_ecdsa_key.pub
586 /etc/ssh/ssh_host_rsa_key.pub
587 These files contain the public parts of the host keys. These
588 files should be world-readable but writable only by root. Their
589 contents should match the respective private parts. These files
590 are not really used for anything; they are provided for the
591 convenience of the user so their contents can be copied to known
592 hosts files. These files are created using ssh-keygen(1).
594 /etc/ssh/ssh_known_hosts
595 Systemwide list of known host keys. This file should be prepared
596 by the system administrator to contain the public host keys of
597 all machines in the organization. The format of this file is
598 described above. This file should be writable only by root/the
599 owner and should be world-readable.
602 Contains configuration data for sshd. The file format and
603 configuration options are described in sshd_config(5).
606 Similar to ~/.ssh/rc, it can be used to specify machine-specific
607 login-time initializations globally. This file should be
608 writable only by root, and should be world-readable.
611 chroot(2) directory used by sshd during privilege separation in
612 the pre-authentication phase. The directory should not contain
613 any files and must be owned by root and not group or world-
617 Contains the process ID of the sshd listening for connections (if
618 there are several daemons running concurrently for different
619 ports, this contains the process ID of the one started last).
620 The content of this file is not sensitive; it can be world-
624 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
625 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
626 sshd_config(5), inetd(8), sftp-server(8)
629 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
630 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
631 de Raadt and Dug Song removed many bugs, re-added newer features and
632 created OpenSSH. Markus Friedl contributed the support for SSH protocol
633 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
634 for privilege separation.
637 System security is not improved unless rshd, rlogind, and rexecd are
638 disabled (thus completely disabling rlogin and rsh into the machine).
640 OpenBSD 5.4 June 27, 2013 OpenBSD 5.4
projects / FreeBSD / releng / 10.0.git / blob