1SSHD(8) BSD System Manager's Manual SSHD(8)
2
4 sshd — OpenSSH daemon
5
7 sshd [-46DdeGiqTtV] [-C connection_spec] [-c host_certificate_file]
8 [-E log_file] [-f config_file] [-g login_grace_time]
9 [-h host_key_file] [-o option] [-p port] [-u len]
10
12 sshd (OpenSSH Daemon) is the daemon program for ssh(1). It provides se‐
13 cure encrypted communications between two untrusted hosts over an inse‐
14 cure 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 -C connection_spec
34 Specify the connection parameters to use for the -T extended test
35 mode. If provided, any Match directives in the configuration
36 file that would apply are applied before the configuration is
37 written to standard output. The connection parameters are sup‐
38 plied as keyword=value pairs and may be supplied in any order,
39 either with multiple -C options or as a comma-separated list.
40 The keywords are “addr”, “user”, “host”, “laddr”, “lport”, and
41 “rdomain” and correspond to source address, user, resolved source
42 host name, local address, local port number and routing domain
43 respectively.
44 -c host_certificate_file
46 Specifies a path to a certificate file to identify sshd during
47 key exchange. The certificate file must match a host key file
48 specified using the -h option or the HostKey configuration direc‐
49 tive.
50 -D When this option is specified, sshd will not detach and does not
52 become a daemon. This allows easy monitoring of sshd.
53 -d Debug mode. The server sends verbose debug output to standard
55 error, and does not put itself in the background. The server
56 also will not fork(2) and will only process one connection. This
57 option is only intended for debugging for the server. Multiple
58 -d options increase the debugging level. Maximum is 3.
59 -E log_file
61 Append debug logs to log_file instead of the system log.
62 -e Write debug logs to 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 Parse and print configuration file. Check the validity of the
71 configuration file, output the effective configuration to stdout
72 and then exit. Optionally, Match rules may be applied by speci‐
73 fying the connection parameters using one or more -C options.
74 -g login_grace_time
76 Gives the grace time for clients to authenticate themselves (de‐
77 fault 120 seconds). If the client fails to authenticate the user
78 within this many seconds, the server disconnects and exits. A
79 value of zero indicates no limit.
80 -h host_key_file
82 Specifies a file from which a host key is read. This option must
83 be given if sshd is not run as root (as the normal host key files
84 are normally not readable by anyone but root). The default is
85 /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
86 /etc/ssh/ssh_host_rsa_key. It is possible to have multiple host
87 key files for the different host key algorithms.
88 -i Specifies that sshd is being run from inetd(8).
90 -o option
92 Can be used to give options in the format used in the configura‐
93 tion file. This is useful for specifying options for which there
94 is no separate command-line flag. For full details of the op‐
95 tions, and their values, see sshd_config(5).
96 -p port
98 Specifies the port on which the server listens for connections
99 (default 22). Multiple port options are permitted. Ports speci‐
100 fied in the configuration file with the Port option are ignored
101 when a command-line port is specified. Ports specified using the
102 ListenAddress option override command-line ports.
103 -q Quiet mode. Nothing is sent to the system log. Normally the be‐
105 ginning, authentication, and termination of each connection is
106 logged.
107 -T Extended test mode. Check the validity of the configuration
109 file, output the effective configuration to stdout and then exit.
110 Optionally, Match rules may be applied by specifying the connec‐
111 tion parameters using one or more -C options. This is similar to
112 the -G flag, but it includes the additional testing performed by
113 the -t flag.
114 -t Test mode. Only check the validity of the configuration file and
116 sanity of the keys. This is useful for updating sshd reliably as
117 configuration options may change.
118 -u len This option is used to specify the size of the field in the utmp
120 structure that holds the remote host name. If the resolved host
121 name is longer than len, the dotted decimal value will be used
122 instead. This allows hosts with very long host names that over‐
123 flow this field to still be uniquely identified. Specifying -u0
124 indicates that only dotted decimal addresses should be put into
125 the utmp file. -u0 may also be used to prevent sshd from making
126 DNS requests unless the authentication mechanism or configuration
127 requires it. Authentication mechanisms that may require DNS in‐
128 clude HostbasedAuthentication and using a from="pattern-list" op‐
129 tion in a key file. Configuration options that require DNS in‐
130 clude using a USER@HOST pattern in AllowUsers or DenyUsers.
131 -V Display the version number and exit.
133
135 The OpenSSH SSH daemon supports SSH protocol 2 only. Each host has a
136 host-specific key, used to identify the host. Whenever a client con‐
137 nects, the daemon responds with its public host key. The client compares
138 the host key against its own database to verify that it has not changed.
139 Forward secrecy is provided through a Diffie-Hellman key agreement. This
140 key agreement results in a shared session key. The rest of the session
141 is encrypted using a symmetric cipher. The client selects the encryption
142 algorithm to use from those offered by the server. Additionally, session
143 integrity is provided through a cryptographic message authentication code
144 (MAC).
145 Finally, the server and the client enter an authentication dialog. The
147 client tries to authenticate itself using host-based authentication, pub‐
148 lic key authentication, GSSAPI authentication, challenge-response authen‐
149 tication, or password authentication.
150 Regardless of the authentication type, the account is checked to ensure
152 that it is accessible. An account is not accessible if it is locked,
153 listed in DenyUsers or its group is listed in DenyGroups . The defini‐
154 tion of a locked account is system dependent. Some platforms have their
155 own account database (eg AIX) and some modify the passwd field ( ‘*LK*’
156 on Solaris and UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a
157 leading ‘*LOCKED*’ on FreeBSD and a leading ‘!’ on most Linuxes). If
158 there is a requirement to disable password authentication for the account
159 while allowing still public-key, then the passwd field should be set to
160 something other than these values (eg ‘NP’ or ‘*NP*’ ).
161 If the client successfully authenticates itself, a dialog for preparing
163 the session is entered. At this time the client may request things like
164 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con‐
165 nections, or forwarding the authentication agent connection over the se‐
166 cure channel.
167 After this, the client either requests an interactive shell or execution
169 or a non-interactive command, which sshd will execute via the user's
170 shell using its -c option. The sides then enter session mode. In this
171 mode, either side may send data at any time, and such data is forwarded
172 to/from the shell or command on the server side, and the user terminal in
173 the client side.
174 When the user program terminates and all forwarded X11 and other connec‐
176 tions have been closed, the server sends command exit status to the
177 client, and both sides exit.
178
180 When a user successfully logs in, sshd does the following:
181 1. If the login is on a tty, and no command has been specified,
183 prints last login time and /etc/motd (unless prevented in the
184 configuration file or by ~/.hushlogin; see the FILES section).
185 2. If the login is on a tty, records login time.
187 3. Checks /etc/nologin; if it exists, prints contents and quits
189 (unless root).
190 4. Changes to run with normal user privileges.
192 5. Sets up basic environment.
194 6. Reads the file ~/.ssh/environment, if it exists, and users are
196 allowed to change their environment. See the
197 PermitUserEnvironment option in sshd_config(5).
198 7. Changes to user's home directory.
200 8. If ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
202 is set, runs it; else if /etc/ssh/sshrc exists, runs it; oth‐
203 erwise runs xauth(1). The “rc” files are given the X11 au‐
204 thentication protocol and cookie in standard input. See
205 SSHRC, below.
206 9. Runs user's shell or command. All commands are run under the
208 user's login shell as specified in the system password data‐
209 base.
210
212 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
213 files but before starting the user's shell or command. It must not pro‐
214 duce any output on stdout; stderr must be used instead. If X11 forward‐
215 ing is in use, it will receive the "proto cookie" pair in its standard
216 input (and DISPLAY in its environment). The script must call xauth(1)
217 because sshd will not run xauth automatically to add X11 cookies.
218 The primary purpose of this file is to run any initialization routines
220 which may be needed before the user's home directory becomes accessible;
221 AFS is a particular example of such an environment.
222 This file will probably contain some initialization code followed by
224 something similar to:
225 if read proto cookie && [ -n "$DISPLAY" ]; then
227 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
228 # X11UseLocalhost=yes
229 echo add unix:`echo $DISPLAY |
230 cut -c11-` $proto $cookie
231 else
232 # X11UseLocalhost=no
233 echo add $DISPLAY $proto $cookie
234 fi | xauth -q -
235 fi
236 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
238 exist either, xauth is used to add the cookie.
239
241 AuthorizedKeysFile specifies the files containing public keys for public
242 key authentication; if this option is not specified, the default is
243 ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the
244 file contains one key (empty lines and lines starting with a ‘#’ are ig‐
245 nored as comments). Public keys consist of the following space-separated
246 fields: options, keytype, base64-encoded key, comment. The options field
247 is optional. The supported key types are:
248 sk-ecdsa-sha2-nistp256@openssh.com
250 ecdsa-sha2-nistp256
251 ecdsa-sha2-nistp384
252 ecdsa-sha2-nistp521
253 sk-ssh-ed25519@openssh.com
254 ssh-ed25519
255 ssh-dss
256 ssh-rsa
257 The comment field is not used for anything (but may be convenient for the
259 user to identify the key).
260 Note that lines in this file can be several hundred bytes long (because
262 of the size of the public key encoding) up to a limit of 8 kilobytes,
263 which permits RSA keys up to 16 kilobits. You don't want to type them
264 in; instead, copy the id_dsa.pub, id_ecdsa.pub, id_ecdsa_sk.pub,
265 id_ed25519.pub, id_ed25519_sk.pub, or the id_rsa.pub file and edit it.
266 sshd enforces a minimum RSA key modulus size of 1024 bits.
268 The options (if present) consist of comma-separated option specifica‐
270 tions. No spaces are permitted, except within double quotes. The fol‐
271 lowing option specifications are supported (note that option keywords are
272 case-insensitive):
273 agent-forwarding
275 Enable authentication agent forwarding previously disabled by the
276 restrict option.
277 cert-authority
279 Specifies that the listed key is a certification authority (CA)
280 that is trusted to validate signed certificates for user authen‐
281 tication.
282 Certificates may encode access restrictions similar to these key
284 options. If both certificate restrictions and key options are
285 present, the most restrictive union of the two is applied.
286 command="command"
288 Specifies that the command is executed whenever this key is used
289 for authentication. The command supplied by the user (if any) is
290 ignored. The command is run on a pty if the client requests a
291 pty; otherwise it is run without a tty. If an 8-bit clean chan‐
292 nel is required, one must not request a pty or should specify
293 no-pty. A quote may be included in the command by quoting it
294 with a backslash.
295 This option might be useful to restrict certain public keys to
297 perform just a specific operation. An example might be a key
298 that permits remote backups but nothing else. Note that the
299 client may specify TCP and/or X11 forwarding unless they are ex‐
300 plicitly prohibited, e.g. using the restrict key option.
301 The command originally supplied by the client is available in the
303 SSH_ORIGINAL_COMMAND environment variable. Note that this option
304 applies to shell, command or subsystem execution. Also note that
305 this command may be superseded by a sshd_config(5) ForceCommand
306 directive.
307 If a command is specified and a forced-command is embedded in a
309 certificate used for authentication, then the certificate will be
310 accepted only if the two commands are identical.
311 environment="NAME=value"
313 Specifies that the string is to be added to the environment when
314 logging in using this key. Environment variables set this way
315 override other default environment values. Multiple options of
316 this type are permitted. Environment processing is disabled by
317 default and is controlled via the PermitUserEnvironment option.
318 expiry-time="timespec"
320 Specifies a time after which the key will not be accepted. The
321 time may be specified as a YYYYMMDD[Z] date or a YYYYMMD‐
322 DHHMM[SS][Z] time. Dates and times will be interpreted in the
323 system time zone unless suffixed by a Z character, in which case
324 they will be interpreted in the UTC time zone.
325 from="pattern-list"
327 Specifies that in addition to public key authentication, either
328 the canonical name of the remote host or its IP address must be
329 present in the comma-separated list of patterns. See PATTERNS in
330 ssh_config(5) for more information on patterns.
331 In addition to the wildcard matching that may be applied to host‐
333 names or addresses, a from stanza may match IP addresses using
334 CIDR address/masklen notation.
335 The purpose of this option is to optionally increase security:
337 public key authentication by itself does not trust the network or
338 name servers or anything (but the key); however, if somebody
339 somehow steals the key, the key permits an intruder to log in
340 from anywhere in the world. This additional option makes using a
341 stolen key more difficult (name servers and/or routers would have
342 to be compromised in addition to just the key).
343 no-agent-forwarding
345 Forbids authentication agent forwarding when this key is used for
346 authentication.
347 no-port-forwarding
349 Forbids TCP forwarding when this key is used for authentication.
350 Any port forward requests by the client will return an error.
351 This might be used, e.g. in connection with the command option.
352 no-pty Prevents tty allocation (a request to allocate a pty will fail).
354 no-user-rc
356 Disables execution of ~/.ssh/rc.
357 no-X11-forwarding
359 Forbids X11 forwarding when this key is used for authentication.
360 Any X11 forward requests by the client will return an error.
361 permitlisten="[host:]port"
363 Limit remote port forwarding with the ssh(1) -R option such that
364 it may only listen on the specified host (optional) and port.
365 IPv6 addresses can be specified by enclosing the address in
366 square brackets. Multiple permitlisten options may be applied
367 separated by commas. Hostnames may include wildcards as de‐
368 scribed in the PATTERNS section in ssh_config(5). A port speci‐
369 fication of * matches any port. Note that the setting of
370 GatewayPorts may further restrict listen addresses. Note that
371 ssh(1) will send a hostname of “localhost” if a listen host was
372 not specified when the forwarding was requested, and that this
373 name is treated differently to the explicit localhost addresses
374 “127.0.0.1” and “::1”.
375 permitopen="host:port"
377 Limit local port forwarding with the ssh(1) -L option such that
378 it may only connect to the specified host and port. IPv6 ad‐
379 dresses can be specified by enclosing the address in square
380 brackets. Multiple permitopen options may be applied separated
381 by commas. No pattern matching or name lookup is performed on
382 the specified hostnames, they must be literal host names and/or
383 addresses. A port specification of * matches any port.
384 port-forwarding
386 Enable port forwarding previously disabled by the restrict op‐
387 tion.
388 principals="principals"
390 On a cert-authority line, specifies allowed principals for cer‐
391 tificate authentication as a comma-separated list. At least one
392 name from the list must appear in the certificate's list of prin‐
393 cipals for the certificate to be accepted. This option is ig‐
394 nored for keys that are not marked as trusted certificate signers
395 using the cert-authority option.
396 pty Permits tty allocation previously disabled by the restrict op‐
398 tion.
399 no-touch-required
401 Do not require demonstration of user presence for signatures made
402 using this key. This option only makes sense for the FIDO au‐
403 thenticator algorithms ecdsa-sk and ed25519-sk.
404 verify-required
406 Require that signatures made using this key attest that they ver‐
407 ified the user, e.g. via a PIN. This option only makes sense for
408 the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
409 restrict
411 Enable all restrictions, i.e. disable port, agent and X11 for‐
412 warding, as well as disabling PTY allocation and execution of
413 ~/.ssh/rc. If any future restriction capabilities are added to
414 authorized_keys files, they will be included in this set.
415 tunnel="n"
417 Force a tun(4) device on the server. Without this option, the
418 next available device will be used if the client requests a tun‐
419 nel.
420 user-rc
422 Enables execution of ~/.ssh/rc previously disabled by the
423 restrict option.
424 X11-forwarding
426 Permits X11 forwarding previously disabled by the restrict op‐
427 tion.
428 An example authorized_keys file:
430 # Comments are allowed at start of line. Blank lines are allowed.
432 # Plain key, no restrictions
433 ssh-rsa ...
434 # Forced command, disable PTY and all forwarding
435 restrict,command="dump /home" ssh-rsa ...
436 # Restriction of ssh -L forwarding destinations
437 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
438 # Restriction of ssh -R forwarding listeners
439 permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
440 # Configuration for tunnel forwarding
441 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
442 # Override of restriction to allow PTY allocation
443 restrict,pty,command="nethack" ssh-rsa ...
444 # Allow FIDO key without requiring touch
445 no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ...
446 # Require user-verification (e.g. PIN or biometric) for FIDO key
447 verify-required sk-ecdsa-sha2-nistp256@openssh.com ...
448 # Trust CA key, allow touch-less FIDO if requested in certificate
449 cert-authority,no-touch-required,principals="user_a" ssh-rsa ...
450
452 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
453 public keys for all known hosts. The global file should be prepared by
454 the administrator (optional), and the per-user file is maintained auto‐
455 matically: whenever the user connects to an unknown host, its key is
456 added to the per-user file.
457 Each line in these files contains the following fields: marker (op‐
459 tional), hostnames, keytype, base64-encoded key, comment. The fields are
460 separated by spaces.
461 The marker is optional, but if it is present then it must be one of
463 “@cert-authority”, to indicate that the line contains a certification au‐
464 thority (CA) key, or “@revoked”, to indicate that the key contained on
465 the line is revoked and must not ever be accepted. Only one marker
466 should be used on a key line.
467 Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wild‐
469 cards); each pattern in turn is matched against the host name. When sshd
470 is authenticating a client, such as when using HostbasedAuthentication,
471 this will be the canonical client host name. When ssh(1) is authenticat‐
472 ing a server, this will be the host name given by the user, the value of
473 the ssh(1) HostkeyAlias if it was specified, or the canonical server
474 hostname if the ssh(1) CanonicalizeHostname option was used.
475 A pattern may also be preceded by ‘!’ to indicate negation: if the host
477 name matches a negated pattern, it is not accepted (by that line) even if
478 it matched another pattern on the line. A hostname or address may op‐
479 tionally be enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’ and
480 a non-standard port number.
481 Alternately, hostnames may be stored in a hashed form which hides host
483 names and addresses should the file's contents be disclosed. Hashed
484 hostnames start with a ‘|’ character. Only one hashed hostname may ap‐
485 pear on a single line and none of the above negation or wildcard opera‐
486 tors may be applied.
487 The keytype and base64-encoded key are taken directly from the host key;
489 they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub.
490 The optional comment field continues to the end of the line, and is not
491 used.
492 Lines starting with ‘#’ and empty lines are ignored as comments.
494 When performing host authentication, authentication is accepted if any
496 matching line has the proper key; either one that matches exactly or, if
497 the server has presented a certificate for authentication, the key of the
498 certification authority that signed the certificate. For a key to be
499 trusted as a certification authority, it must use the “@cert-authority”
500 marker described above.
501 The known hosts file also provides a facility to mark keys as revoked,
503 for example when it is known that the associated private key has been
504 stolen. Revoked keys are specified by including the “@revoked” marker at
505 the beginning of the key line, and are never accepted for authentication
506 or as certification authorities, but instead will produce a warning from
507 ssh(1) when they are encountered.
508 It is permissible (but not recommended) to have several lines or differ‐
510 ent host keys for the same names. This will inevitably happen when short
511 forms of host names from different domains are put in the file. It is
512 possible that the files contain conflicting information; authentication
513 is accepted if valid information can be found from either file.
514 Note that the lines in these files are typically hundreds of characters
516 long, and you definitely don't want to type in the host keys by hand.
517 Rather, generate them by a script, ssh-keyscan(1) or by taking, for exam‐
518 ple, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the
519 front. ssh-keygen(1) also offers some basic automated editing for
520 ~/.ssh/known_hosts including removing hosts matching a host name and con‐
521 verting all host names to their hashed representations.
522 An example ssh_known_hosts file:
524 # Comments allowed at start of line
526 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
527 # A hashed hostname
528 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
529 AAAA1234.....=
530 # A revoked key
531 @revoked * ssh-rsa AAAAB5W...
532 # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
533 @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
534
536 ~/.hushlogin
537 This file is used to suppress printing the last login time and
538 /etc/motd, if PrintLastLog and PrintMotd, respectively, are en‐
539 abled. It does not suppress printing of the banner specified by
540 Banner.
541 ~/.rhosts
543 This file is used for host-based authentication (see ssh(1) for
544 more information). On some machines this file may need to be
545 world-readable if the user's home directory is on an NFS parti‐
546 tion, because sshd reads it as root. Additionally, this file
547 must be owned by the user, and must not have write permissions
548 for anyone else. The recommended permission for most machines is
549 read/write for the user, and not accessible by others.
550 ~/.shosts
552 This file is used in exactly the same way as .rhosts, but allows
553 host-based authentication without permitting login with
554 rlogin/rsh.
555 ~/.k5login
557 ~/.k5users
558 These files enforce GSSAPI/Kerberos authentication access con‐
559 trol. Further details are described in ksu(1). The location of
560 the k5login file depends on the configuration option
561 k5login_directory in the krb5.conf(5).
562 ~/.ssh/
564 This directory is the default location for all user-specific con‐
565 figuration and authentication information. There is no general
566 requirement to keep the entire contents of this directory secret,
567 but the recommended permissions are read/write/execute for the
568 user, and not accessible by others.
569 ~/.ssh/authorized_keys
571 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
572 for logging in as this user. The format of this file is de‐
573 scribed above. The content of the file is not highly sensitive,
574 but the recommended permissions are read/write for the user, and
575 not accessible by others.
576 If this file, the ~/.ssh directory, or the user's home directory
578 are writable by other users, then the file could be modified or
579 replaced by unauthorized users. In this case, sshd will not al‐
580 low it to be used unless the StrictModes option has been set to
581 “no”.
582 ~/.ssh/environment
584 This file is read into the environment at login (if it exists).
585 It can only contain empty lines, comment lines (that start with
586 ‘#’), and assignment lines of the form name=value. The file
587 should be writable only by the user; it need not be readable by
588 anyone else. Environment processing is disabled by default and
589 is controlled via the PermitUserEnvironment option.
590 ~/.ssh/known_hosts
592 Contains a list of host keys for all hosts the user has logged
593 into that are not already in the systemwide list of known host
594 keys. The format of this file is described above. This file
595 should be writable only by root/the owner and can, but need not
596 be, world-readable.
597 ~/.ssh/rc
599 Contains initialization routines to be run before the user's home
600 directory becomes accessible. This file should be writable only
601 by the user, and need not be readable by anyone else.
602 /etc/hosts.equiv
604 This file is for host-based authentication (see ssh(1)). It
605 should only be writable by root.
606 /etc/ssh/moduli
608 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
609 Exchange" key exchange method. The file format is described in
610 moduli(5). If no usable groups are found in this file then fixed
611 internal groups will be used.
612 /etc/motd
614 See motd(5).
615 /etc/nologin
617 If this file exists, sshd refuses to let anyone except root log
618 in. The contents of the file are displayed to anyone trying to
619 log in, and non-root connections are refused. The file should be
620 world-readable.
621 /etc/ssh/shosts.equiv
623 This file is used in exactly the same way as hosts.equiv, but al‐
624 lows host-based authentication without permitting login with
625 rlogin/rsh.
626 /etc/ssh/ssh_host_ecdsa_key
628 /etc/ssh/ssh_host_ed25519_key
629 /etc/ssh/ssh_host_rsa_key
630 These files contain the private parts of the host keys. These
631 files should only be owned by root, readable only by root, and
632 not accessible to others. Note that sshd does not start if these
633 files are group/world-accessible.
634 /etc/ssh/ssh_host_ecdsa_key.pub
636 /etc/ssh/ssh_host_ed25519_key.pub
637 /etc/ssh/ssh_host_rsa_key.pub
638 These files contain the public parts of the host keys. These
639 files should be world-readable but writable only by root. Their
640 contents should match the respective private parts. These files
641 are not really used for anything; they are provided for the con‐
642 venience of the user so their contents can be copied to known
643 hosts files. These files are created using ssh-keygen(1).
644 /etc/ssh/ssh_known_hosts
646 Systemwide list of known host keys. This file should be prepared
647 by the system administrator to contain the public host keys of
648 all machines in the organization. The format of this file is de‐
649 scribed above. This file should be writable only by root/the
650 owner and should be world-readable.
651 /etc/ssh/sshd_config
653 Contains configuration data for sshd. The file format and con‐
654 figuration options are described in sshd_config(5).
655 /etc/ssh/sshrc
657 Similar to ~/.ssh/rc, it can be used to specify machine-specific
658 login-time initializations globally. This file should be
659 writable only by root, and should be world-readable.
660 /usr/share/empty.sshd
662 chroot(2) directory used by sshd during privilege separation in
663 the pre-authentication phase. The directory should not contain
664 any files and must be owned by root and not group or world-
665 writable.
666 /var/run/sshd.pid
668 Contains the process ID of the sshd listening for connections (if
669 there are several daemons running concurrently for different
670 ports, this contains the process ID of the one started last).
671 The content of this file is not sensitive; it can be world-read‐
672 able.
673
675 IPv6 address can be used everywhere where IPv4 address. In all entries
676 must be the IPv6 address enclosed in square brackets. Note: The square
677 brackets are metacharacters for the shell and must be escaped in shell.
678
680 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
681 ssh-keyscan(1), chroot(2), login.conf(5), moduli(5), sshd_config(5),
682 inetd(8), sftp-server(8)
683
685 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
686 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
687 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
688 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
689 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
690 for privilege separation.
691BSD February 10, 2023 BSD