1 SSHD(8) OpenBSD System Manager's Manual SSHD(8)
4 sshd - OpenSSH SSH daemon
7 sshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-f config_file]
8 [-g login_grace_time] [-h host_key_file] [-k key_gen_time]
9 [-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 com-
14 munications 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, com-
19 mand 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 speci-
23 fied in the configuration file. sshd rereads its configuration file when
24 it receives a hangup signal, SIGHUP, by executing itself with the name
25 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 out-
42 put. 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 multi-
45 ple -C options or as a comma-separated list.
47 -D When this option is specified, sshd will not detach and does not
48 become a daemon. This allows easy monitoring of sshd.
50 -d Debug mode. The server sends verbose debug output to the system
51 log, and does not put itself in the background. The server also
52 will not fork and will only process one connection. This option
53 is only intended for debugging for the server. Multiple -d op-
54 tions increase the debugging level. Maximum is 3.
56 -e When this option is specified, sshd will send the output to the
57 standard error instead of the system log.
60 Specifies the name of the configuration file. The default is
61 /etc/ssh/sshd_config. sshd refuses to start if there is no con-
65 Gives the grace time for clients to authenticate themselves (de-
66 fault 120 seconds). If the client fails to authenticate the user
67 within this many seconds, the server disconnects and exits. A
68 value of zero indicates no limit.
71 Specifies a file from which a host key is read. This option must
72 be given if sshd is not run as root (as the normal host key files
73 are normally not readable by anyone but root). The default is
74 /etc/ssh/ssh_host_key for protocol version 1, and
75 /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
76 tocol version 2. It is possible to have multiple host key files
77 for the different protocol versions and host key algorithms.
79 -i Specifies that sshd is being run from inetd(8). sshd is normally
80 not run from inetd because it needs to generate the server key
81 before it can respond to the client, and this may take tens of
82 seconds. Clients would have to wait too long if the key was re-
83 generated every time. However, with small key sizes (e.g. 512)
84 using sshd from inetd may be feasible.
87 Specifies how often the ephemeral protocol version 1 server key
88 is regenerated (default 3600 seconds, or one hour). The motiva-
89 tion for regenerating the key fairly often is that the key is not
90 stored anywhere, and after about an hour it becomes impossible to
91 recover the key for decrypting intercepted communications even if
92 the machine is cracked into or physically seized. A value of ze-
93 ro indicates that the key will never be regenerated.
96 Can be used to give options in the format used in the configura-
97 tion file. This is useful for specifying options for which there
98 is no separate command-line flag. For full details of the op-
99 tions, and their values, see sshd_config(5).
102 Specifies the port on which the server listens for connections
103 (default 22). Multiple port options are permitted. Ports speci-
104 fied in the configuration file with the Port option are ignored
105 when a command-line port is specified. Ports specified using the
106 ListenAddress option override command-line ports.
108 -q Quiet mode. Nothing is sent to the system log. Normally the be-
109 ginning, authentication, and termination of each connection is
112 -T Extended test mode. Check the validity of the configuration
113 file, output the effective configuration to stdout and then exit.
114 Optionally, Match rules may be applied by specifying the connec-
115 tion parameters using one or more -C options.
117 -t Test mode. Only check the validity of the configuration file and
118 sanity of the keys. This is useful for updating sshd reliably as
119 configuration options may change.
121 -u len This option is used to specify the size of the field in the utmp
122 structure that holds the remote host name. If the resolved host
123 name is longer than len, the dotted decimal value will be used
124 instead. This allows hosts with very long host names that over-
125 flow this field to still be uniquely identified. Specifying -u0
126 indicates that only dotted decimal addresses should be put into
127 the utmp file. -u0 may also be used to prevent sshd from making
128 DNS requests unless the authentication mechanism or configuration
129 requires it. Authentication mechanisms that may require DNS in-
130 clude RhostsRSAAuthentication, HostbasedAuthentication, and using
131 a from="pattern-list" option in a key file. Configuration op-
132 tions that require DNS include using a USER@HOST pattern in
133 AllowUsers or DenyUsers.
136 The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols
137 are supported by default, though this can be changed via the Protocol op-
138 tion in sshd_config(5). Protocol 2 supports both RSA and DSA keys; pro-
139 tocol 1 only supports RSA keys. For both protocols, each host has a
140 host-specific key, normally 2048 bits, used to identify the host.
142 Forward security for protocol 1 is provided through an additional server
143 key, normally 768 bits, generated when the server starts. This key is
144 normally regenerated every hour if it has been used, and is never stored
145 on disk. Whenever a client connects, the daemon responds with its public
146 host and server keys. The client compares the RSA host key against its
147 own database to verify that it has not changed. The client then gener-
148 ates a 256-bit random number. It encrypts this random number using both
149 the host key and the server key, and sends the encrypted number to the
150 server. Both sides then use this random number as a session key which is
151 used to encrypt all further communications in the session. The rest of
152 the session is encrypted using a conventional cipher, currently Blowfish
153 or 3DES, with 3DES being used by default. The client selects the encryp-
154 tion algorithm to use from those offered by the server.
156 For protocol 2, forward security is provided through a Diffie-Hellman key
157 agreement. This key agreement results in a shared session key. The rest
158 of the session is encrypted using a symmetric cipher, currently 128-bit
159 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
160 client selects the encryption algorithm to use from those offered by the
161 server. Additionally, session integrity is provided through a crypto-
162 graphic message authentication code (hmac-md5, hmac-sha1, umac-64 or
165 Finally, the server and the client enter an authentication dialog. The
166 client tries to authenticate itself using host-based authentication, pub-
167 lic key authentication, challenge-response authentication, or password
170 Regardless of the authentication type, the account is checked to ensure
171 that it is accessible. An account is not accessible if it is locked,
172 listed in DenyUsers or its group is listed in DenyGroups . The defini-
173 tion of a locked account is system dependant. Some platforms have their
174 own account database (eg AIX) and some modify the passwd field ( `*LK*'
175 on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a
176 leading `*LOCKED*' on FreeBSD and a leading `!' on most Linuxes). If
177 there is a requirement to disable password authentication for the account
178 while allowing still public-key, then the passwd field should be set to
179 something other than these values (eg `NP' or `*NP*' ).
181 If the client successfully authenticates itself, a dialog for preparing
182 the session is entered. At this time the client may request things like
183 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con-
184 nections, or forwarding the authentication agent connection over the se-
187 After this, the client either requests a shell or execution of a command.
188 The sides then enter session mode. In this mode, either side may send
189 data at any time, and such data is forwarded to/from the shell or command
190 on the server side, and the user terminal in the client side.
192 When the user program terminates and all forwarded X11 and other connec-
193 tions have been closed, the server sends command exit status to the
194 client, and both sides exit.
197 When a user successfully logs in, sshd does the following:
199 1. If the login is on a tty, and no command has been specified,
200 prints last login time and /etc/motd (unless prevented in the
201 configuration file or by ~/.hushlogin; see the FILES section).
203 2. If the login is on a tty, records login time.
205 3. Checks /etc/nologin; if it exists, prints contents and quits
208 4. Changes to run with normal user privileges.
210 5. Sets up basic environment.
212 6. Reads the file ~/.ssh/environment, if it exists, and users are
213 allowed to change their environment. See the
214 PermitUserEnvironment option in sshd_config(5).
216 7. Changes to user's home directory.
218 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
219 runs it; otherwise runs xauth. The ``rc'' files are given the
220 X11 authentication protocol and cookie in standard input. See
223 9. Runs user's shell or command.
226 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
227 files but before starting the user's shell or command. It must not pro-
228 duce any output on stdout; stderr must be used instead. If X11 forward-
229 ing is in use, it will receive the "proto cookie" pair in its standard
230 input (and DISPLAY in its environment). The script must call xauth(1)
231 because sshd will not run xauth automatically to add X11 cookies.
233 The primary purpose of this file is to run any initialization routines
234 which may be needed before the user's home directory becomes accessible;
235 AFS is a particular example of such an environment.
237 This file will probably contain some initialization code followed by
238 something similar to:
240 if read proto cookie && [ -n "$DISPLAY" ]; then
241 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
242 # X11UseLocalhost=yes
243 echo add unix:`echo $DISPLAY |
244 cut -c11-` $proto $cookie
247 echo add $DISPLAY $proto $cookie
251 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
252 exist either, xauth is used to add the cookie.
254 AUTHORIZED_KEYS FILE FORMAT
255 AuthorizedKeysFile specifies the file containing public keys for public
256 key authentication; if none is specified, the default is
257 ~/.ssh/authorized_keys. Each line of the file contains one key (empty
258 lines and lines starting with a `#' are ignored as comments). Protocol 1
259 public keys consist of the following space-separated fields: options,
260 bits, exponent, modulus, comment. Protocol 2 public key consist of: op-
261 tions, keytype, base64-encoded key, comment. The options field is op-
262 tional; its presence is determined by whether the line starts with a num-
263 ber or not (the options field never starts with a number). The bits, ex-
264 ponent, modulus, and comment fields give the RSA key for protocol version
265 1; the comment field is not used for anything (but may be convenient for
266 the user to identify the key). For protocol version 2 the keytype is
267 ``ssh-dss'' or ``ssh-rsa''.
269 Note that lines in this file are usually several hundred bytes long (be-
270 cause of the size of the public key encoding) up to a limit of 8 kilo-
271 bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
272 kilobits. You don't want to type them in; instead, copy the
273 identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.
275 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
278 The options (if present) consist of comma-separated option specifica-
279 tions. No spaces are permitted, except within double quotes. The fol-
280 lowing option specifications are supported (note that option keywords are
284 Specifies that the command is executed whenever this key is used
285 for authentication. The command supplied by the user (if any) is
286 ignored. The command is run on a pty if the client requests a
287 pty; otherwise it is run without a tty. If an 8-bit clean chan-
288 nel is required, one must not request a pty or should specify no-
289 pty. A quote may be included in the command by quoting it with a
290 backslash. This option might be useful to restrict certain pub-
291 lic keys to perform just a specific operation. An example might
292 be a key that permits remote backups but nothing else. Note that
293 the client may specify TCP and/or X11 forwarding unless they are
294 explicitly prohibited. The command originally supplied by the
295 client is available in the SSH_ORIGINAL_COMMAND environment vari-
296 able. Note that this option applies to shell, command or subsys-
299 environment="NAME=value"
300 Specifies that the string is to be added to the environment when
301 logging in using this key. Environment variables set this way
302 override other default environment values. Multiple options of
303 this type are permitted. Environment processing is disabled by
304 default and is controlled via the PermitUserEnvironment option.
305 This option is automatically disabled if UseLogin is enabled.
308 Specifies that in addition to public key authentication, either
309 the canonical name of the remote host or its IP address must be
310 present in the comma-separated list of patterns. See PATTERNS in
311 ssh_config(5) for more information on patterns.
313 In addition to the wildcard matching that may be applied to host-
314 names or addresses, a from stanza may match IP addresses using
315 CIDR address/masklen notation.
317 The purpose of this option is to optionally increase security:
318 public key authentication by itself does not trust the network or
319 name servers or anything (but the key); however, if somebody
320 somehow steals the key, the key permits an intruder to log in
321 from anywhere in the world. This additional option makes using a
322 stolen key more difficult (name servers and/or routers would have
323 to be compromised in addition to just the key).
326 Forbids authentication agent forwarding when this key is used for
330 Forbids TCP forwarding when this key is used for authentication.
331 Any port forward requests by the client will return an error.
332 This might be used, e.g. in connection with the command option.
334 no-pty Prevents tty allocation (a request to allocate a pty will fail).
337 Disables execution of ~/.ssh/rc.
340 Forbids X11 forwarding when this key is used for authentication.
341 Any X11 forward requests by the client will return an error.
343 permitopen="host:port"
344 Limit local ``ssh -L'' port forwarding such that it may only con-
345 nect to the specified host and port. IPv6 addresses can be spec-
346 ified with an alternative syntax: host/port. Multiple permitopen
347 options may be applied separated by commas. No pattern matching
348 is performed on the specified hostnames, they must be literal do-
352 Force a tun(4) device on the server. Without this option, the
353 next available device will be used if the client requests a tun-
356 An example authorized_keys file:
358 # Comments allowed at start of line
359 ssh-rsa AAAAB3Nza...LiPk== user@example.net
360 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
361 AAAAB2...19Q== john@example.net
362 command="dump /home",no-pty,no-port-forwarding ssh-dss
363 AAAAC3...51R== example.net
364 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
366 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
369 SSH_KNOWN_HOSTS FILE FORMAT
370 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
371 public keys for all known hosts. The global file should be prepared by
372 the administrator (optional), and the per-user file is maintained auto-
373 matically: whenever the user connects from an unknown host, its key is
374 added to the per-user file.
376 Each line in these files contains the following fields: hostnames, bits,
377 exponent, modulus, comment. The fields are separated by spaces.
379 Hostnames is a comma-separated list of patterns (`*' and `?' act as wild-
380 cards); each pattern in turn is matched against the canonical host name
381 (when authenticating a client) or against the user-supplied name (when
382 authenticating a server). A pattern may also be preceded by `!' to indi-
383 cate negation: if the host name matches a negated pattern, it is not ac-
384 cepted (by that line) even if it matched another pattern on the line. A
385 hostname or address may optionally be enclosed within `[' and `]' brack-
386 ets then followed by `:' and a non-standard port number.
388 Alternately, hostnames may be stored in a hashed form which hides host
389 names and addresses should the file's contents be disclosed. Hashed
390 hostnames start with a `|' character. Only one hashed hostname may ap-
391 pear on a single line and none of the above negation or wildcard opera-
394 Bits, exponent, and modulus are taken directly from the RSA host key;
395 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
396 optional comment field continues to the end of the line, and is not used.
398 Lines starting with `#' and empty lines are ignored as comments.
400 When performing host authentication, authentication is accepted if any
401 matching line has the proper key. It is thus permissible (but not recom-
402 mended) to have several lines or different host keys for the same names.
403 This will inevitably happen when short forms of host names from different
404 domains are put in the file. It is possible that the files contain con-
405 flicting information; authentication is accepted if valid information can
406 be found from either file.
408 Note that the lines in these files are typically hundreds of characters
409 long, and you definitely don't want to type in the host keys by hand.
410 Rather, generate them by a script or by taking /etc/ssh/ssh_host_key.pub
411 and adding the host names at the front.
413 An example ssh_known_hosts file:
415 # Comments allowed at start of line
416 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
417 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
419 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
424 This file is used to suppress printing the last login time and
425 /etc/motd, if PrintLastLog and PrintMotd, respectively, are en-
426 abled. It does not suppress printing of the banner specified by
430 This file is used for host-based authentication (see ssh(1) for
431 more information). On some machines this file may need to be
432 world-readable if the user's home directory is on an NFS parti-
433 tion, because sshd reads it as root. Additionally, this file
434 must be owned by the user, and must not have write permissions
435 for anyone else. The recommended permission for most machines is
436 read/write for the user, and not accessible by others.
439 This file is used in exactly the same way as .rhosts, but allows
440 host-based authentication without permitting login with
444 This directory is the default location for all user-specific con-
445 figuration and authentication information. There is no general
446 requirement to keep the entire contents of this directory secret,
447 but the recommended permissions are read/write/execute for the
448 user, and not accessible by others.
450 ~/.ssh/authorized_keys
451 Lists the public keys (RSA/DSA) that can be used for logging in
452 as this user. The format of this file is described above. The
453 content of the file is not highly sensitive, but the recommended
454 permissions are read/write for the user, and not accessible by
457 If this file, the ~/.ssh directory, or the user's home directory
458 are writable by other users, then the file could be modified or
459 replaced by unauthorized users. In this case, sshd will not al-
460 low it to be used unless the StrictModes option has been set to
464 This file is read into the environment at login (if it exists).
465 It can only contain empty lines, comment lines (that start with
466 `#'), and assignment lines of the form name=value. The file
467 should be writable only by the user; it need not be readable by
468 anyone else. Environment processing is disabled by default and
469 is controlled via the PermitUserEnvironment option.
472 Contains a list of host keys for all hosts the user has logged
473 into that are not already in the systemwide list of known host
474 keys. The format of this file is described above. This file
475 should be writable only by root/the owner and can, but need not
479 Contains initialization routines to be run before the user's home
480 directory becomes accessible. This file should be writable only
481 by the user, and need not be readable by anyone else.
485 Access controls that should be enforced by tcp-wrappers are de-
486 fined here. Further details are described in hosts_access(5).
489 This file is for host-based authentication (see ssh(1)). It
490 should only be writable by root.
493 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
494 Exchange". The file format is described in moduli(5).
500 If this file exists, sshd refuses to let anyone except root log
501 in. The contents of the file are displayed to anyone trying to
502 log in, and non-root connections are refused. The file should be
506 This file is used in exactly the same way as hosts.equiv, but al-
507 lows host-based authentication without permitting login with
510 /etc/ssh/ssh_host_key
511 /etc/ssh/ssh_host_dsa_key
512 /etc/ssh/ssh_host_rsa_key
513 These three files contain the private parts of the host keys.
514 These files should only be owned by root, readable only by root,
515 and not accessible to others. Note that sshd does not start if
516 these files are group/world-accessible.
518 /etc/ssh/ssh_host_key.pub
519 /etc/ssh/ssh_host_dsa_key.pub
520 /etc/ssh/ssh_host_rsa_key.pub
521 These three files contain the public parts of the host keys.
522 These files should be world-readable but writable only by root.
523 Their contents should match the respective private parts. These
524 files are not really used for anything; they are provided for the
525 convenience of the user so their contents can be copied to known
526 hosts files. These files are created using ssh-keygen(1).
528 /etc/ssh/ssh_known_hosts
529 Systemwide list of known host keys. This file should be prepared
530 by the system administrator to contain the public host keys of
531 all machines in the organization. The format of this file is de-
532 scribed above. This file should be writable only by root/the
533 owner and should be world-readable.
536 Contains configuration data for sshd. The file format and con-
537 figuration options are described in sshd_config(5).
540 Similar to ~/.ssh/rc, it can be used to specify machine-specific
541 login-time initializations globally. This file should be
542 writable only by root, and should be world-readable.
545 chroot(2) directory used by sshd during privilege separation in
546 the pre-authentication phase. The directory should not contain
547 any files and must be owned by root and not group or world-
551 Contains the process ID of the sshd listening for connections (if
552 there are several daemons running concurrently for different
553 ports, this contains the process ID of the one started last).
554 The content of this file is not sensitive; it can be world-read-
558 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
559 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
560 sshd_config(5), inetd(8), sftp-server(8)
563 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
564 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
565 de Raadt and Dug Song removed many bugs, re-added newer features and cre-
566 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
567 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
568 for privilege separation.
571 System security is not improved unless rshd, rlogind, and rexecd are dis-
572 abled (thus completely disabling rlogin and rsh into the machine).
574 OpenBSD 4.6 March 26, 2009 9
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