1SSHD(8) BSD System Manager's Manual SSHD(8)
2
4 sshd — OpenSSH SSH daemon
5
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]
10
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.
15 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.
20 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.
26 The options are as follows:
28 -4 Forces sshd to use IPv4 addresses only.
30 -6 Forces sshd to use IPv6 addresses only.
32 -b bits
34 Specifies the number of bits in the ephemeral protocol version 1
35 server key (default 1024).
36 -c host_certificate_file
38 Specifies a path to a certificate file to identify sshd during
39 key exchange. The certificate file must match a host key file
40 specified using the --h option or the HostKey configuration
41 directive.
42 -C connection_spec
44 Specify the connection parameters to use for the -T extended test
45 mode. If provided, any Match directives in the configuration
46 file that would apply to the specified user, host, and address
47 will be set before the configuration is written to standard out‐
48 put. The connection parameters are supplied as keyword=value
49 pairs. The keywords are “user”, “host”, “laddr”, “lport”, and
50 “addr”. All are required and may be supplied in any order,
51 either with multiple -C options or as a comma-separated list.
52 -D When this option is specified, sshd will not detach and does not
54 become a daemon. This allows easy monitoring of sshd.
55 -d Debug mode. The server sends verbose debug output to the system
57 log, and does not put itself in the background. The server also
58 will not fork and will only process one connection. This option
59 is only intended for debugging for the server. Multiple -d
60 options increase the debugging level. Maximum is 3.
61 -e When this option is specified, sshd will send the output to the
63 standard error instead of the system log.
64 -f config_file
66 Specifies the name of the configuration file. The default is
67 /etc/ssh/sshd_config. sshd refuses to start if there is no con‐
68 figuration file.
69 -g login_grace_time
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.
75 -h host_key_file
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 ver‐
84 sions and host key algorithms.
85 -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.
92 -k key_gen_time
94 Specifies how often the ephemeral protocol version 1 server key
95 is regenerated (default 3600 seconds, or one hour). The motiva‐
96 tion for regenerating the key fairly often is that the key is not
97 stored anywhere, and after about an hour it becomes impossible to
98 recover the key for decrypting intercepted communications even if
99 the machine is cracked into or physically seized. A value of
100 zero indicates that the key will never be regenerated.
101 -o option
103 Can be used to give options in the format used in the configura‐
104 tion file. This is useful for specifying options for which there
105 is no separate command-line flag. For full details of the
106 options, and their values, see sshd_config(5).
107 -p port
109 Specifies the port on which the server listens for connections
110 (default 22). Multiple port options are permitted. Ports speci‐
111 fied in the configuration file with the Port option are ignored
112 when a command-line port is specified. Ports specified using the
113 ListenAddress option override command-line ports.
114 -q Quiet mode. Nothing is sent to the system log. Normally the
116 beginning, authentication, and termination of each connection is
117 logged.
118 -T Extended test mode. Check the validity of the configuration
120 file, output the effective configuration to stdout and then exit.
121 Optionally, Match rules may be applied by specifying the connec‐
122 tion parameters using one or more -C options.
123 -t Test mode. Only check the validity of the configuration file and
125 sanity of the keys. This is useful for updating sshd reliably as
126 configuration options may change.
127 -u len This option is used to specify the size of the field in the utmp
129 structure that holds the remote host name. If the resolved host
130 name is longer than len, the dotted decimal value will be used
131 instead. This allows hosts with very long host names that over‐
132 flow this field to still be uniquely identified. Specifying -u0
133 indicates that only dotted decimal addresses should be put into
134 the utmp file. -u0 may also be used to prevent sshd from making
135 DNS requests unless the authentication mechanism or configuration
136 requires it. Authentication mechanisms that may require DNS
137 include RhostsRSAAuthentication, HostbasedAuthentication, and
138 using a from="pattern-list" option in a key file. Configuration
139 options that require DNS include using a USER@HOST pattern in
140 AllowUsers or DenyUsers.
141
143 The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols
144 are supported by default, though this can be changed via the Protocol
145 option in sshd_config(5). Protocol 2 supports DSA, ECDSA and RSA keys;
146 protocol 1 only supports RSA keys. For both protocols, each host has a
147 host-specific key, normally 2048 bits, used to identify the host.
148 Forward security for protocol 1 is provided through an additional server
150 key, normally 768 bits, generated when the server starts. This key is
151 normally regenerated every hour if it has been used, and is never stored
152 on disk. Whenever a client connects, the daemon responds with its public
153 host and server keys. The client compares the RSA host key against its
154 own database to verify that it has not changed. The client then gener‐
155 ates a 256-bit random number. It encrypts this random number using both
156 the host key and the server key, and sends the encrypted number to the
157 server. Both sides then use this random number as a session key which is
158 used to encrypt all further communications in the session. The rest of
159 the session is encrypted using a conventional cipher, currently Blowfish
160 or 3DES, with 3DES being used by default. The client selects the encryp‐
161 tion algorithm to use from those offered by the server.
162 For protocol 2, forward security is provided through a Diffie-Hellman key
164 agreement. This key agreement results in a shared session key. The rest
165 of the session is encrypted using a symmetric cipher, currently 128-bit
166 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
167 client selects the encryption algorithm to use from those offered by the
168 server. Additionally, session integrity is provided through a crypto‐
169 graphic message authentication code (hmac-md5, hmac-sha1, umac-64, hmac-
170 ripemd160, hmac-sha2-256 or hmac-sha2-512).
171 Finally, the server and the client enter an authentication dialog. The
173 client tries to authenticate itself using host-based authentication, pub‐
174 lic key authentication, challenge-response authentication, or password
175 authentication.
176 Regardless of the authentication type, the account is checked to ensure
178 that it is accessible. An account is not accessible if it is locked,
179 listed in DenyUsers or its group is listed in DenyGroups . The defini‐
180 tion of a locked account is system dependant. Some platforms have their
181 own account database (eg AIX) and some modify the passwd field ( ‘*LK*’
182 on Solaris and UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a
183 leading ‘*LOCKED*’ on FreeBSD and a leading ‘!’ on most Linuxes). If
184 there is a requirement to disable password authentication for the account
185 while allowing still public-key, then the passwd field should be set to
186 something other than these values (eg ‘NP’ or ‘*NP*’ ).
187 If the client successfully authenticates itself, a dialog for preparing
189 the session is entered. At this time the client may request things like
190 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con‐
191 nections, or forwarding the authentication agent connection over the
192 secure channel.
193 After this, the client either requests a shell or execution of a command.
195 The sides then enter session mode. In this mode, either side may send
196 data at any time, and such data is forwarded to/from the shell or command
197 on the server side, and the user terminal in the client side.
198 When the user program terminates and all forwarded X11 and other connec‐
200 tions have been closed, the server sends command exit status to the
201 client, and both sides exit.
202
204 When a user successfully logs in, sshd does the following:
205 1. If the login is on a tty, and no command has been specified,
207 prints last login time and /etc/motd (unless prevented in the
208 configuration file or by ~/.hushlogin; see the FILES section).
209 2. If the login is on a tty, records login time.
211 3. Checks /etc/nologin; if it exists, prints contents and quits
213 (unless root).
214 4. Changes to run with normal user privileges.
216 5. Sets up basic environment.
218 6. Reads the file ~/.ssh/environment, if it exists, and users are
220 allowed to change their environment. See the
221 PermitUserEnvironment option in sshd_config(5).
222 7. Changes to user's home directory.
224 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
226 runs it; otherwise runs xauth. The “rc” files are given the
227 X11 authentication protocol and cookie in standard input. See
228 SSHRC, below.
229 9. Runs user's shell or command.
231
233 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
234 files but before starting the user's shell or command. It must not pro‐
235 duce any output on stdout; stderr must be used instead. If X11 forward‐
236 ing is in use, it will receive the "proto cookie" pair in its standard
237 input (and DISPLAY in its environment). The script must call xauth(1)
238 because sshd will not run xauth automatically to add X11 cookies.
239 The primary purpose of this file is to run any initialization routines
241 which may be needed before the user's home directory becomes accessible;
242 AFS is a particular example of such an environment.
243 This file will probably contain some initialization code followed by
245 something similar to:
246 if read proto cookie && [ -n "$DISPLAY" ]; then
248 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
249 # X11UseLocalhost=yes
250 echo add unix:`echo $DISPLAY |
251 cut -c11-` $proto $cookie
252 else
253 # X11UseLocalhost=no
254 echo add $DISPLAY $proto $cookie
255 fi | xauth -q -
256 fi
257 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
259 exist either, xauth is used to add the cookie.
260
262 AuthorizedKeysFile specifies the file containing public keys for public
263 key authentication; if none is specified, the default is
264 ~/.ssh/authorized_keys. Each line of the file contains one key (empty
265 lines and lines starting with a ‘#’ are ignored as comments). Protocol 1
266 public keys consist of the following space-separated fields: options,
267 bits, exponent, modulus, comment. Protocol 2 public key consist of:
268 options, keytype, base64-encoded key, comment. The options field is
269 optional; its presence is determined by whether the line starts with a
270 number or not (the options field never starts with a number). The bits,
271 exponent, modulus, and comment fields give the RSA key for protocol ver‐
272 sion 1; the comment field is not used for anything (but may be convenient
273 for the user to identify the key). For protocol version 2 the keytype is
274 “ecdsa-sha2-nistp256”, “ecdsa-sha2-nistp384”, “ecdsa-sha2-nistp521”,
275 “ssh-dss” or “ssh-rsa”.
276 Note that lines in this file are usually several hundred bytes long
278 (because of the size of the public key encoding) up to a limit of 8 kilo‐
279 bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
280 kilobits. You don't want to type them in; instead, copy the
281 identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit
282 it.
283 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
285 2 keys of 768 bits.
286 The options (if present) consist of comma-separated option specifica‐
288 tions. No spaces are permitted, except within double quotes. The fol‐
289 lowing option specifications are supported (note that option keywords are
290 case-insensitive):
291 cert-authority
293 Specifies that the listed key is a certification authority (CA)
294 that is trusted to validate signed certificates for user authen‐
295 tication.
296 Certificates may encode access restrictions similar to these key
298 options. If both certificate restrictions and key options are
299 present, the most restrictive union of the two is applied.
300 command="command"
302 Specifies that the command is executed whenever this key is used
303 for authentication. The command supplied by the user (if any) is
304 ignored. The command is run on a pty if the client requests a
305 pty; otherwise it is run without a tty. If an 8-bit clean chan‐
306 nel is required, one must not request a pty or should specify
307 no-pty. A quote may be included in the command by quoting it
308 with a backslash. This option might be useful to restrict cer‐
309 tain public keys to perform just a specific operation. An exam‐
310 ple might be a key that permits remote backups but nothing else.
311 Note that the client may specify TCP and/or X11 forwarding unless
312 they are explicitly prohibited. The command originally supplied
313 by the client is available in the SSH_ORIGINAL_COMMAND environ‐
314 ment variable. Note that this option applies to shell, command
315 or subsystem execution. Also note that this command may be
316 superseded by either a sshd_config(5) ForceCommand directive or a
317 command embedded in a certificate.
318 environment="NAME=value"
320 Specifies that the string is to be added to the environment when
321 logging in using this key. Environment variables set this way
322 override other default environment values. Multiple options of
323 this type are permitted. Environment processing is disabled by
324 default and is controlled via the PermitUserEnvironment option.
325 This option is automatically disabled if UseLogin is enabled.
326 from="pattern-list"
328 Specifies that in addition to public key authentication, either
329 the canonical name of the remote host or its IP address must be
330 present in the comma-separated list of patterns. See PATTERNS in
331 ssh_config(5) for more information on patterns.
332 In addition to the wildcard matching that may be applied to host‐
334 names or addresses, a from stanza may match IP addresses using
335 CIDR address/masklen notation.
336 The purpose of this option is to optionally increase security:
338 public key authentication by itself does not trust the network or
339 name servers or anything (but the key); however, if somebody
340 somehow steals the key, the key permits an intruder to log in
341 from anywhere in the world. This additional option makes using a
342 stolen key more difficult (name servers and/or routers would have
343 to be compromised in addition to just the key).
344 no-agent-forwarding
346 Forbids authentication agent forwarding when this key is used for
347 authentication.
348 no-port-forwarding
350 Forbids TCP forwarding when this key is used for authentication.
351 Any port forward requests by the client will return an error.
352 This might be used, e.g. in connection with the command option.
353 no-pty Prevents tty allocation (a request to allocate a pty will fail).
355 no-user-rc
357 Disables execution of ~/.ssh/rc.
358 no-X11-forwarding
360 Forbids X11 forwarding when this key is used for authentication.
361 Any X11 forward requests by the client will return an error.
362 permitopen="host:port"
364 Limit local ``ssh -L'' port forwarding such that it may only con‐
365 nect to the specified host and port. IPv6 addresses can be spec‐
366 ified with an alternative syntax: host/port. Multiple permitopen
367 options may be applied separated by commas. No pattern matching
368 is performed on the specified hostnames, they must be literal
369 domains or addresses. A port specification of * matches any
370 port.
371 principals="principals"
373 On a cert-authority line, specifies allowed principals for cer‐
374 tificate authentication as a comma-separated list. At least one
375 name from the list must appear in the certificate's list of prin‐
376 cipals for the certificate to be accepted. This option is
377 ignored for keys that are not marked as trusted certificate sign‐
378 ers using the cert-authority option.
379 tunnel="n"
381 Force a tun(4) device on the server. Without this option, the
382 next available device will be used if the client requests a tun‐
383 nel.
384 An example authorized_keys file:
386 # Comments allowed at start of line
388 ssh-rsa AAAAB3Nza...LiPk== user@example.net
389 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
390 AAAAB2...19Q== john@example.net
391 command="dump /home",no-pty,no-port-forwarding ssh-dss
392 AAAAC3...51R== example.net
393 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
394 AAAAB5...21S==
395 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
396 jane@example.net
397
399 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
400 public keys for all known hosts. The global file should be prepared by
401 the administrator (optional), and the per-user file is maintained auto‐
402 matically: whenever the user connects from an unknown host, its key is
403 added to the per-user file.
404 Each line in these files contains the following fields: markers
406 (optional), hostnames, bits, exponent, modulus, comment. The fields are
407 separated by spaces.
408 The marker is optional, but if it is present then it must be one of
410 “@cert-authority”, to indicate that the line contains a certification
411 authority (CA) key, or “@revoked”, to indicate that the key contained on
412 the line is revoked and must not ever be accepted. Only one marker
413 should be used on a key line.
414 Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wild‐
416 cards); each pattern in turn is matched against the canonical host name
417 (when authenticating a client) or against the user-supplied name (when
418 authenticating a server). A pattern may also be preceded by ‘!’ to indi‐
419 cate negation: if the host name matches a negated pattern, it is not
420 accepted (by that line) even if it matched another pattern on the line.
421 A hostname or address may optionally be enclosed within ‘[’ and ‘]’
422 brackets then followed by ‘:’ and a non-standard port number.
423 Alternately, hostnames may be stored in a hashed form which hides host
425 names and addresses should the file's contents be disclosed. Hashed
426 hostnames start with a ‘|’ character. Only one hashed hostname may
427 appear on a single line and none of the above negation or wildcard opera‐
428 tors may be applied.
429 Bits, exponent, and modulus are taken directly from the RSA host key;
431 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
432 optional comment field continues to the end of the line, and is not used.
433 Lines starting with ‘#’ and empty lines are ignored as comments.
435 When performing host authentication, authentication is accepted if any
437 matching line has the proper key; either one that matches exactly or, if
438 the server has presented a certificate for authentication, the key of the
439 certification authority that signed the certificate. For a key to be
440 trusted as a certification authority, it must use the “@cert-authority”
441 marker described above.
442 The known hosts file also provides a facility to mark keys as revoked,
444 for example when it is known that the associated private key has been
445 stolen. Revoked keys are specified by including the “@revoked” marker at
446 the beginning of the key line, and are never accepted for authentication
447 or as certification authorities, but instead will produce a warning from
448 ssh(1) when they are encountered.
449 It is permissible (but not recommended) to have several lines or differ‐
451 ent host keys for the same names. This will inevitably happen when short
452 forms of host names from different domains are put in the file. It is
453 possible that the files contain conflicting information; authentication
454 is accepted if valid information can be found from either file.
455 Note that the lines in these files are typically hundreds of characters
457 long, and you definitely don't want to type in the host keys by hand.
458 Rather, generate them by a script, ssh-keyscan(1) or by taking
459 /etc/ssh/ssh_host_key.pub and adding the host names at the front.
460 ssh-keygen(1) also offers some basic automated editing for
461 ~/.ssh/known_hosts including removing hosts matching a host name and con‐
462 verting all host names to their hashed representations.
463 An example ssh_known_hosts file:
465 # Comments allowed at start of line
467 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
468 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
469 # A hashed hostname
470 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
471 AAAA1234.....=
472 # A revoked key
473 @revoked * ssh-rsa AAAAB5W...
474 # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
475 @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
476
478 ~/.hushlogin
479 This file is used to suppress printing the last login time and
480 /etc/motd, if PrintLastLog and PrintMotd, respectively, are
481 enabled. It does not suppress printing of the banner specified
482 by Banner.
483 ~/.rhosts
485 This file is used for host-based authentication (see ssh(1) for
486 more information). On some machines this file may need to be
487 world-readable if the user's home directory is on an NFS parti‐
488 tion, because sshd reads it as root. Additionally, this file
489 must be owned by the user, and must not have write permissions
490 for anyone else. The recommended permission for most machines is
491 read/write for the user, and not accessible by others.
492 ~/.shosts
494 This file is used in exactly the same way as .rhosts, but allows
495 host-based authentication without permitting login with
496 rlogin/rsh.
497 ~/.ssh/
499 This directory is the default location for all user-specific con‐
500 figuration and authentication information. There is no general
501 requirement to keep the entire contents of this directory secret,
502 but the recommended permissions are read/write/execute for the
503 user, and not accessible by others.
504 ~/.ssh/authorized_keys
506 Lists the public keys (RSA/ECDSA/DSA) that can be used for log‐
507 ging in as this user. The format of this file is described
508 above. The content of the file is not highly sensitive, but the
509 recommended permissions are read/write for the user, and not
510 accessible by others.
511 If this file, the ~/.ssh directory, or the user's home directory
513 are writable by other users, then the file could be modified or
514 replaced by unauthorized users. In this case, sshd will not
515 allow it to be used unless the StrictModes option has been set to
516 “no”.
517 ~/.ssh/environment
519 This file is read into the environment at login (if it exists).
520 It can only contain empty lines, comment lines (that start with
521 ‘#’), and assignment lines of the form name=value. The file
522 should be writable only by the user; it need not be readable by
523 anyone else. Environment processing is disabled by default and
524 is controlled via the PermitUserEnvironment option.
525 ~/.ssh/known_hosts
527 Contains a list of host keys for all hosts the user has logged
528 into that are not already in the systemwide list of known host
529 keys. The format of this file is described above. This file
530 should be writable only by root/the owner and can, but need not
531 be, world-readable.
532 ~/.ssh/rc
534 Contains initialization routines to be run before the user's home
535 directory becomes accessible. This file should be writable only
536 by the user, and need not be readable by anyone else.
537 /etc/hosts.allow
539 /etc/hosts.deny
540 Access controls that should be enforced by tcp-wrappers are
541 defined here. Further details are described in hosts_access(5).
542 /etc/hosts.equiv
544 This file is for host-based authentication (see ssh(1)). It
545 should only be writable by root.
546 /etc/ssh/moduli
548 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
549 Exchange". The file format is described in moduli(5).
550 /etc/motd
552 See motd(5).
553 /etc/nologin
555 If this file exists, sshd refuses to let anyone except root log
556 in. The contents of the file are displayed to anyone trying to
557 log in, and non-root connections are refused. The file should be
558 world-readable.
559 /etc/ssh/shosts.equiv
561 This file is used in exactly the same way as hosts.equiv, but
562 allows host-based authentication without permitting login with
563 rlogin/rsh.
564 /etc/ssh/ssh_host_key
566 /etc/ssh/ssh_host_dsa_key
567 /etc/ssh/ssh_host_ecdsa_key
568 /etc/ssh/ssh_host_rsa_key
569 These three files contain the private parts of the host keys.
570 These files should only be owned by root, readable only by root,
571 and not accessible to others. Note that sshd does not start if
572 these files are group/world-accessible.
573 /etc/ssh/ssh_host_key.pub
575 /etc/ssh/ssh_host_dsa_key.pub
576 /etc/ssh/ssh_host_ecdsa_key.pub
577 /etc/ssh/ssh_host_rsa_key.pub
578 These three files contain the public parts of the host keys.
579 These files should be world-readable but writable only by root.
580 Their contents should match the respective private parts. These
581 files are not really used for anything; they are provided for the
582 convenience of the user so their contents can be copied to known
583 hosts files. These files are created using ssh-keygen(1).
584 /etc/ssh/ssh_known_hosts
586 Systemwide list of known host keys. This file should be prepared
587 by the system administrator to contain the public host keys of
588 all machines in the organization. The format of this file is
589 described above. This file should be writable only by root/the
590 owner and should be world-readable.
591 /etc/ssh/sshd_config
593 Contains configuration data for sshd. The file format and con‐
594 figuration options are described in sshd_config(5).
595 /etc/ssh/sshrc
597 Similar to ~/.ssh/rc, it can be used to specify machine-specific
598 login-time initializations globally. This file should be
599 writable only by root, and should be world-readable.
600 /var/empty/sshd
602 chroot(2) directory used by sshd during privilege separation in
603 the pre-authentication phase. The directory should not contain
604 any files and must be owned by root and not group or world-
605 writable.
606 /var/run/sshd.pid
608 Contains the process ID of the sshd listening for connections (if
609 there are several daemons running concurrently for different
610 ports, this contains the process ID of the one started last).
611 The content of this file is not sensitive; it can be world-read‐
612 able.
613
615 SSH_USE_STRONG_RNG
616 The reseeding of the OpenSSL random generator is usually done
617 from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari‐
618 able is set to value other than 0 the OpenSSL random generator is
619 reseeded from /dev/random. The number of bytes read is defined
620 by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This set‐
621 ting is not recommended on the computers without the hardware
622 random generator because insufficient entropy causes the connec‐
623 tion to be blocked until enough entropy is available.
624
626 IPv6 address can be used everywhere where IPv4 address. In all entries
627 must be the IPv6 address enclosed in square brackets. Note: The square
628 brackets are metacharacters for the shell and must be escaped in shell.
629
631 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
632 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
633 sshd_config(5), inetd(8), sftp-server(8)
634
636 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
637 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
638 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
639 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
640 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
641 for privilege separation.
642
644 System security is not improved unless rshd, rlogind, and rexecd are dis‐
645 abled (thus completely disabling rlogin and rsh into the machine).
646BSD June 21, 2019 BSD