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'', ``laddr'',
44 ``lport'', and ``addr''. All are required and may be supplied in
45 any order, either with multiple -C options or as a comma-
48 -c host_certificate_file
49 Specifies a path to a certificate file to identify sshd during
50 key exchange. The certificate file must match a host key file
51 specified using the -h option or the HostKey configuration
54 -D When this option is specified, sshd will not detach and does not
55 become a daemon. This allows easy monitoring of sshd.
57 -d Debug mode. The server sends verbose debug output to standard
58 error, and does not put itself in the background. The server
59 also will not fork and will only process one connection. This
60 option is only intended for debugging for the server. Multiple
61 -d options increase the debugging level. Maximum is 3.
63 -e When this option is specified, sshd will send the output to the
64 standard error instead of the system log.
67 Specifies the name of the configuration file. The default is
68 /etc/ssh/sshd_config. sshd refuses to start if there is no
72 Gives the grace time for clients to authenticate themselves
73 (default 120 seconds). If the client fails to authenticate the
74 user within this many seconds, the server disconnects and exits.
75 A value of zero indicates no limit.
78 Specifies a file from which a host key is read. This option must
79 be given if sshd is not run as root (as the normal host key files
80 are normally not readable by anyone but root). The default is
81 /etc/ssh/ssh_host_key for protocol version 1, and
82 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
83 /etc/ssh/ssh_host_rsa_key for protocol version 2. It is possible
84 to have multiple host key files for the different protocol
85 versions and host key algorithms.
87 -i Specifies that sshd is being run from inetd(8). sshd is normally
88 not run from inetd because it needs to generate the server key
89 before it can respond to the client, and this may take tens of
90 seconds. Clients would have to wait too long if the key was
91 regenerated every time. However, with small key sizes (e.g. 512)
92 using sshd from inetd may be feasible.
95 Specifies how often the ephemeral protocol version 1 server key
96 is regenerated (default 3600 seconds, or one hour). The
97 motivation for regenerating the key fairly often is that the key
98 is not stored anywhere, and after about an hour it becomes
99 impossible to recover the key for decrypting intercepted
100 communications even if the machine is cracked into or physically
101 seized. A value of zero indicates that the key will never be
105 Can be used to give options in the format used in the
106 configuration file. This is useful for specifying options for
107 which there is no separate command-line flag. For full details
108 of the options, and their values, see sshd_config(5).
111 Specifies the port on which the server listens for connections
112 (default 22). Multiple port options are permitted. Ports
113 specified in the configuration file with the Port option are
114 ignored when a command-line port is specified. Ports specified
115 using the ListenAddress option override command-line ports.
117 -q Quiet mode. Nothing is sent to the system log. Normally the
118 beginning, authentication, and termination of each connection is
121 -T Extended test mode. Check the validity of the configuration
122 file, output the effective configuration to stdout and then exit.
123 Optionally, Match rules may be applied by specifying the
124 connection parameters using one or more -C options.
126 -t Test mode. Only check the validity of the configuration file and
127 sanity of the keys. This is useful for updating sshd reliably as
128 configuration options may change.
130 -u len This option is used to specify the size of the field in the utmp
131 structure that holds the remote host name. If the resolved host
132 name is longer than len, the dotted decimal value will be used
133 instead. This allows hosts with very long host names that
134 overflow this field to still be uniquely identified. Specifying
135 -u0 indicates that only dotted decimal addresses should be put
136 into the utmp file. -u0 may also be used to prevent sshd from
137 making DNS requests unless the authentication mechanism or
138 configuration requires it. Authentication mechanisms that may
139 require DNS include RhostsRSAAuthentication,
140 HostbasedAuthentication, and using a from="pattern-list" option
141 in a key file. Configuration options that require DNS include
142 using a USER@HOST pattern in AllowUsers or DenyUsers.
145 The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to
146 use protocol 2 only, though this can be changed via the Protocol option
147 in sshd_config(5). Protocol 2 supports DSA, ECDSA and RSA keys; protocol
148 1 only supports RSA keys. For both protocols, each host has a host-
149 specific key, normally 2048 bits, used to identify the host.
151 Forward security for protocol 1 is provided through an additional server
152 key, normally 768 bits, generated when the server starts. This key is
153 normally regenerated every hour if it has been used, and is never stored
154 on disk. Whenever a client connects, the daemon responds with its public
155 host and server keys. The client compares the RSA host key against its
156 own database to verify that it has not changed. The client then
157 generates a 256-bit random number. It encrypts this random number using
158 both the host key and the server key, and sends the encrypted number to
159 the server. Both sides then use this random number as a session key
160 which is used to encrypt all further communications in the session. The
161 rest of the session is encrypted using a conventional cipher, currently
162 Blowfish or 3DES, with 3DES being used by default. The client selects
163 the encryption algorithm to use from those offered by the server.
165 For protocol 2, forward security is provided through a Diffie-Hellman key
166 agreement. This key agreement results in a shared session key. The rest
167 of the session is encrypted using a symmetric cipher, currently 128-bit
168 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
169 client selects the encryption algorithm to use from those offered by the
170 server. Additionally, session integrity is provided through a
171 cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64,
172 hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512).
174 Finally, the server and the client enter an authentication dialog. The
175 client tries to authenticate itself using host-based authentication,
176 public key authentication, challenge-response authentication, or password
179 Regardless of the authentication type, the account is checked to ensure
180 that it is accessible. An account is not accessible if it is locked,
181 listed in DenyUsers or its group is listed in DenyGroups . The
182 definition of a locked account is system dependant. Some platforms have
183 their own account database (eg AIX) and some modify the passwd field (
184 `*LK*' on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on
185 Tru64, a leading `*LOCKED*' on FreeBSD and a leading `!' on most
186 Linuxes). If there is a requirement to disable password authentication
187 for the account while allowing still public-key, then the passwd field
188 should be set to something other than these values (eg `NP' or `*NP*' ).
190 If the client successfully authenticates itself, a dialog for preparing
191 the session is entered. At this time the client may request things like
192 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP
193 connections, or forwarding the authentication agent connection over the
196 After this, the client either requests a shell or execution of a command.
197 The sides then enter session mode. In this mode, either side may send
198 data at any time, and such data is forwarded to/from the shell or command
199 on the server side, and the user terminal in the client side.
201 When the user program terminates and all forwarded X11 and other
202 connections have been closed, the server sends command exit status to the
203 client, and both sides exit.
206 When a user successfully logs in, sshd does the following:
208 1. If the login is on a tty, and no command has been specified,
209 prints last login time and /etc/motd (unless prevented in the
210 configuration file or by ~/.hushlogin; see the FILES section).
212 2. If the login is on a tty, records login time.
214 3. Checks /etc/nologin; if it exists, prints contents and quits
217 4. Changes to run with normal user privileges.
219 5. Sets up basic environment.
221 6. Reads the file ~/.ssh/environment, if it exists, and users are
222 allowed to change their environment. See the
223 PermitUserEnvironment option in sshd_config(5).
225 7. Changes to user's home directory.
227 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
228 runs it; otherwise runs xauth. The ``rc'' files are given the
229 X11 authentication protocol and cookie in standard input. See
232 9. Runs user's shell or command.
235 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
236 files but before starting the user's shell or command. It must not
237 produce any output on stdout; stderr must be used instead. If X11
238 forwarding is in use, it will receive the "proto cookie" pair in its
239 standard input (and DISPLAY in its environment). The script must call
240 xauth(1) because sshd will not run xauth automatically to add X11
243 The primary purpose of this file is to run any initialization routines
244 which may be needed before the user's home directory becomes accessible;
245 AFS is a particular example of such an environment.
247 This file will probably contain some initialization code followed by
248 something similar to:
250 if read proto cookie && [ -n "$DISPLAY" ]; then
251 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
252 # X11UseLocalhost=yes
253 echo add unix:`echo $DISPLAY |
254 cut -c11-` $proto $cookie
257 echo add $DISPLAY $proto $cookie
261 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
262 exist either, xauth is used to add the cookie.
264 AUTHORIZED_KEYS FILE FORMAT
265 AuthorizedKeysFile specifies the files containing public keys for public
266 key authentication; if none is specified, the default is
267 ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the
268 file contains one key (empty lines and lines starting with a `#' are
269 ignored as comments). Protocol 1 public keys consist of the following
270 space-separated fields: options, bits, exponent, modulus, comment.
271 Protocol 2 public key consist of: options, keytype, base64-encoded key,
272 comment. The options field is optional; its presence is determined by
273 whether the line starts with a number or not (the options field never
274 starts with a number). The bits, exponent, modulus, and comment fields
275 give the RSA key for protocol version 1; the comment field is not used
276 for anything (but may be convenient for the user to identify the key).
277 For protocol version 2 the keytype is ``ecdsa-sha2-nistp256'',
278 ``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-dss'' or
281 Note that lines in this file are usually several hundred bytes long
282 (because of the size of the public key encoding) up to a limit of 8
283 kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
284 kilobits. You don't want to type them in; instead, copy the
285 identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit
288 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
291 The options (if present) consist of comma-separated option
292 specifications. No spaces are permitted, except within double quotes.
293 The following option specifications are supported (note that option
294 keywords are case-insensitive):
297 Specifies that the listed key is a certification authority (CA)
298 that is trusted to validate signed certificates for user
301 Certificates may encode access restrictions similar to these key
302 options. If both certificate restrictions and key options are
303 present, the most restrictive union of the two is applied.
306 Specifies that the command is executed whenever this key is used
307 for authentication. The command supplied by the user (if any) is
308 ignored. The command is run on a pty if the client requests a
309 pty; otherwise it is run without a tty. If an 8-bit clean
310 channel is required, one must not request a pty or should specify
311 no-pty. A quote may be included in the command by quoting it
312 with a backslash. This option might be useful to restrict
313 certain public keys to perform just a specific operation. An
314 example might be a key that permits remote backups but nothing
315 else. Note that the client may specify TCP and/or X11 forwarding
316 unless they are explicitly prohibited. The command originally
317 supplied by the client is available in the SSH_ORIGINAL_COMMAND
318 environment variable. Note that this option applies to shell,
319 command or subsystem execution. Also note that this command may
320 be superseded by either a sshd_config(5) ForceCommand directive
321 or a command embedded in a certificate.
323 environment="NAME=value"
324 Specifies that the string is to be added to the environment when
325 logging in using this key. Environment variables set this way
326 override other default environment values. Multiple options of
327 this type are permitted. Environment processing is disabled by
328 default and is controlled via the PermitUserEnvironment option.
329 This option is automatically disabled if UseLogin is enabled.
332 Specifies that in addition to public key authentication, either
333 the canonical name of the remote host or its IP address must be
334 present in the comma-separated list of patterns. See PATTERNS in
335 ssh_config(5) for more information on patterns.
337 In addition to the wildcard matching that may be applied to
338 hostnames or addresses, a from stanza may match IP addresses
339 using CIDR address/masklen notation.
341 The purpose of this option is to optionally increase security:
342 public key authentication by itself does not trust the network or
343 name servers or anything (but the key); however, if somebody
344 somehow steals the key, the key permits an intruder to log in
345 from anywhere in the world. This additional option makes using a
346 stolen key more difficult (name servers and/or routers would have
347 to be compromised in addition to just the key).
350 Forbids authentication agent forwarding when this key is used for
354 Forbids TCP forwarding when this key is used for authentication.
355 Any port forward requests by the client will return an error.
356 This might be used, e.g. in connection with the command option.
358 no-pty Prevents tty allocation (a request to allocate a pty will fail).
361 Disables execution of ~/.ssh/rc.
364 Forbids X11 forwarding when this key is used for authentication.
365 Any X11 forward requests by the client will return an error.
367 permitopen="host:port"
368 Limit local ``ssh -L'' port forwarding such that it may only
369 connect to the specified host and port. IPv6 addresses can be
370 specified by enclosing the address in square brackets. Multiple
371 permitopen options may be applied separated by commas. No
372 pattern matching is performed on the specified hostnames, they
373 must be literal domains or addresses. A port specification of *
376 principals="principals"
377 On a cert-authority line, specifies allowed principals for
378 certificate authentication as a comma-separated list. At least
379 one name from the list must appear in the certificate's list of
380 principals for the certificate to be accepted. This option is
381 ignored for keys that are not marked as trusted certificate
382 signers using the cert-authority option.
385 Force a tun(4) device on the server. Without this option, the
386 next available device will be used if the client requests a
389 An example authorized_keys file:
391 # Comments allowed at start of line
392 ssh-rsa AAAAB3Nza...LiPk== user@example.net
393 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
394 AAAAB2...19Q== john@example.net
395 command="dump /home",no-pty,no-port-forwarding ssh-dss
396 AAAAC3...51R== example.net
397 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
399 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
402 SSH_KNOWN_HOSTS FILE FORMAT
403 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
404 public keys for all known hosts. The global file should be prepared by
405 the administrator (optional), and the per-user file is maintained
406 automatically: whenever the user connects from an unknown host, its key
407 is added to the per-user file.
409 Each line in these files contains the following fields: markers
410 (optional), hostnames, bits, exponent, modulus, comment. The fields are
413 The marker is optional, but if it is present then it must be one of
414 ``@cert-authority'', to indicate that the line contains a certification
415 authority (CA) key, or ``@revoked'', to indicate that the key contained
416 on the line is revoked and must not ever be accepted. Only one marker
417 should be used on a key line.
419 Hostnames is a comma-separated list of patterns (`*' and `?' act as
420 wildcards); each pattern in turn is matched against the canonical host
421 name (when authenticating a client) or against the user-supplied name
422 (when authenticating a server). A pattern may also be preceded by `!' to
423 indicate negation: if the host name matches a negated pattern, it is not
424 accepted (by that line) even if it matched another pattern on the line.
425 A hostname or address may optionally be enclosed within `[' and `]'
426 brackets then followed by `:' and a non-standard port number.
428 Alternately, hostnames may be stored in a hashed form which hides host
429 names and addresses should the file's contents be disclosed. Hashed
430 hostnames start with a `|' character. Only one hashed hostname may
431 appear on a single line and none of the above negation or wildcard
432 operators may be applied.
434 Bits, exponent, and modulus are taken directly from the RSA host key;
435 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
436 optional comment field continues to the end of the line, and is not used.
438 Lines starting with `#' and empty lines are ignored as comments.
440 When performing host authentication, authentication is accepted if any
441 matching line has the proper key; either one that matches exactly or, if
442 the server has presented a certificate for authentication, the key of the
443 certification authority that signed the certificate. For a key to be
444 trusted as a certification authority, it must use the ``@cert-authority''
445 marker described above.
447 The known hosts file also provides a facility to mark keys as revoked,
448 for example when it is known that the associated private key has been
449 stolen. Revoked keys are specified by including the ``@revoked'' marker
450 at the beginning of the key line, and are never accepted for
451 authentication or as certification authorities, but instead will produce
452 a warning from ssh(1) when they are encountered.
454 It is permissible (but not recommended) to have several lines or
455 different host keys for the same names. This will inevitably happen when
456 short forms of host names from different domains are put in the file. It
457 is possible that the files contain conflicting information;
458 authentication is accepted if valid information can be found from either
461 Note that the lines in these files are typically hundreds of characters
462 long, and you definitely don't want to type in the host keys by hand.
463 Rather, generate them by a script, ssh-keyscan(1) or by taking
464 /etc/ssh/ssh_host_key.pub and adding the host names at the front.
465 ssh-keygen(1) also offers some basic automated editing for
466 ~/.ssh/known_hosts including removing hosts matching a host name and
467 converting all host names to their hashed representations.
469 An example ssh_known_hosts file:
471 # Comments allowed at start of line
472 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
473 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
475 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
478 @revoked * ssh-rsa AAAAB5W...
479 # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
480 @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
484 This file is used to suppress printing the last login time and
485 /etc/motd, if PrintLastLog and PrintMotd, respectively, are
486 enabled. It does not suppress printing of the banner specified
490 This file is used for host-based authentication (see ssh(1) for
491 more information). On some machines this file may need to be
492 world-readable if the user's home directory is on an NFS
493 partition, because sshd reads it as root. Additionally, this
494 file must be owned by the user, and must not have write
495 permissions for anyone else. The recommended permission for most
496 machines is read/write for the user, and not accessible by
500 This file is used in exactly the same way as .rhosts, but allows
501 host-based authentication without permitting login with
505 This directory is the default location for all user-specific
506 configuration and authentication information. There is no
507 general requirement to keep the entire contents of this directory
508 secret, but the recommended permissions are read/write/execute
509 for the user, and not accessible by others.
511 ~/.ssh/authorized_keys
512 Lists the public keys (DSA/ECDSA/RSA) that can be used for
513 logging in as this user. The format of this file is described
514 above. The content of the file is not highly sensitive, but the
515 recommended permissions are read/write for the user, and not
516 accessible by others.
518 If this file, the ~/.ssh directory, or the user's home directory
519 are writable by other users, then the file could be modified or
520 replaced by unauthorized users. In this case, sshd will not
521 allow it to be used unless the StrictModes option has been set to
525 This file is read into the environment at login (if it exists).
526 It can only contain empty lines, comment lines (that start with
527 `#'), and assignment lines of the form name=value. The file
528 should be writable only by the user; it need not be readable by
529 anyone else. Environment processing is disabled by default and
530 is controlled via the PermitUserEnvironment option.
533 Contains a list of host keys for all hosts the user has logged
534 into that are not already in the systemwide list of known host
535 keys. The format of this file is described above. This file
536 should be writable only by root/the owner and can, but need not
540 Contains initialization routines to be run before the user's home
541 directory becomes accessible. This file should be writable only
542 by the user, and need not be readable by anyone else.
546 Access controls that should be enforced by tcp-wrappers are
547 defined here. Further details are described in hosts_access(5).
550 This file is for host-based authentication (see ssh(1)). It
551 should only be writable by root.
554 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
555 Exchange". The file format is described in moduli(5).
561 If this file exists, sshd refuses to let anyone except root log
562 in. The contents of the file are displayed to anyone trying to
563 log in, and non-root connections are refused. The file should be
567 This file is used in exactly the same way as hosts.equiv, but
568 allows host-based authentication without permitting login with
571 /etc/ssh/ssh_host_key
572 /etc/ssh/ssh_host_dsa_key
573 /etc/ssh/ssh_host_ecdsa_key
574 /etc/ssh/ssh_host_rsa_key
575 These files contain the private parts of the host keys. These
576 files should only be owned by root, readable only by root, and
577 not accessible to others. Note that sshd does not start if these
578 files are group/world-accessible.
580 /etc/ssh/ssh_host_key.pub
581 /etc/ssh/ssh_host_dsa_key.pub
582 /etc/ssh/ssh_host_ecdsa_key.pub
583 /etc/ssh/ssh_host_rsa_key.pub
584 These files contain the public parts of the host keys. These
585 files should be world-readable but writable only by root. Their
586 contents should match the respective private parts. These files
587 are not really used for anything; they are provided for the
588 convenience of the user so their contents can be copied to known
589 hosts files. These files are created using ssh-keygen(1).
591 /etc/ssh/ssh_known_hosts
592 Systemwide list of known host keys. This file should be prepared
593 by the system administrator to contain the public host keys of
594 all machines in the organization. The format of this file is
595 described above. This file should be writable only by root/the
596 owner and should be world-readable.
599 Contains configuration data for sshd. The file format and
600 configuration options are described in sshd_config(5).
603 Similar to ~/.ssh/rc, it can be used to specify machine-specific
604 login-time initializations globally. This file should be
605 writable only by root, and should be world-readable.
608 chroot(2) directory used by sshd during privilege separation in
609 the pre-authentication phase. The directory should not contain
610 any files and must be owned by root and not group or world-
614 Contains the process ID of the sshd listening for connections (if
615 there are several daemons running concurrently for different
616 ports, this contains the process ID of the one started last).
617 The content of this file is not sensitive; it can be world-
621 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
622 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
623 sshd_config(5), inetd(8), sftp-server(8)
626 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
627 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
628 de Raadt and Dug Song removed many bugs, re-added newer features and
629 created OpenSSH. Markus Friedl contributed the support for SSH protocol
630 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
631 for privilege separation.
634 System security is not improved unless rshd, rlogind, and rexecd are
635 disabled (thus completely disabling rlogin and rsh into the machine).
637 OpenBSD 5.2 June 18, 2012 OpenBSD 5.2
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