1SSHD(8)                   BSD System Manager's Manual                  SSHD(8)
2

NAME

4     sshd — OpenSSH SSH daemon
5

SYNOPSIS

7     sshd [-46DdeiqTt] [-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

DESCRIPTION

12     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these
13     programs replace rlogin and rsh, and provide secure encrypted communica‐
14     tions between two untrusted hosts over an insecure network.
15
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.
20
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.
26
27     The options are as follows:
28
29     -4      Forces sshd to use IPv4 addresses only.
30
31     -6      Forces sshd to use IPv6 addresses only.
32
33     -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
45     -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
51     -D      When this option is specified, sshd will not detach and does not
52             become a daemon.  This allows easy monitoring of sshd.
53
54     -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 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
60     -E log_file
61             Append debug logs to log_file instead of the system log.
62
63     -e      Write debug logs to standard error instead of the system log.
64
65     -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
70     -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
76     -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_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
81             /etc/ssh/ssh_host_rsa_key.  It is possible to have multiple host
82             key files for the different host key algorithms.
83
84     -i      Specifies that sshd is being run from inetd(8).
85
86     -o option
87             Can be used to give options in the format used in the configura‐
88             tion file.  This is useful for specifying options for which there
89             is no separate command-line flag.  For full details of the
90             options, and their values, see sshd_config(5).
91
92     -p port
93             Specifies the port on which the server listens for connections
94             (default 22).  Multiple port options are permitted.  Ports speci‐
95             fied in the configuration file with the Port option are ignored
96             when a command-line port is specified.  Ports specified using the
97             ListenAddress option override command-line ports.
98
99     -q      Quiet mode.  Nothing is sent to the system log.  Normally the
100             beginning, authentication, and termination of each connection is
101             logged.
102
103     -T      Extended test mode.  Check the validity of the configuration
104             file, output the effective configuration to stdout and then exit.
105             Optionally, Match rules may be applied by specifying the connec‐
106             tion parameters using one or more -C options.
107
108     -t      Test mode.  Only check the validity of the configuration file and
109             sanity of the keys.  This is useful for updating sshd reliably as
110             configuration options may change.
111
112     -u len  This option is used to specify the size of the field in the utmp
113             structure that holds the remote host name.  If the resolved host
114             name is longer than len, the dotted decimal value will be used
115             instead.  This allows hosts with very long host names that over‐
116             flow this field to still be uniquely identified.  Specifying -u0
117             indicates that only dotted decimal addresses should be put into
118             the utmp file.  -u0 may also be used to prevent sshd from making
119             DNS requests unless the authentication mechanism or configuration
120             requires it.  Authentication mechanisms that may require DNS
121             include HostbasedAuthentication and using a from="pattern-list"
122             option in a key file.  Configuration options that require DNS
123             include using a USER@HOST pattern in AllowUsers or DenyUsers.
124

AUTHENTICATION

126     The OpenSSH SSH daemon supports SSH protocol 2 only.  Each host has a
127     host-specific key, used to identify the host.  Whenever a client con‐
128     nects, the daemon responds with its public host key.  The client compares
129     the host key against its own database to verify that it has not changed.
130     Forward security is provided through a Diffie-Hellman key agreement.
131     This key agreement results in a shared session key.  The rest of the ses‐
132     sion is encrypted using a symmetric cipher, currently 128-bit AES, Blow‐
133     fish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.  The client
134     selects the encryption algorithm to use from those offered by the server.
135     Additionally, session integrity is provided through a cryptographic mes‐
136     sage authentication code (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-
137     sha2-256 or hmac-sha2-512).
138
139     Finally, the server and the client enter an authentication dialog.  The
140     client tries to authenticate itself using host-based authentication, pub‐
141     lic key authentication, GSSAPI authentication, challenge-response authen‐
142     tication, or password authentication.
143
144     Regardless of the authentication type, the account is checked to ensure
145     that it is accessible.  An account is not accessible if it is locked,
146     listed in DenyUsers or its group is listed in DenyGroups .  The defini‐
147     tion of a locked account is system dependent. Some platforms have their
148     own account database (eg AIX) and some modify the passwd field ( ‘*LK*’
149     on Solaris and UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a
150     leading ‘*LOCKED*’ on FreeBSD and a leading ‘!’ on most Linuxes).  If
151     there is a requirement to disable password authentication for the account
152     while allowing still public-key, then the passwd field should be set to
153     something other than these values (eg ‘NP’ or ‘*NP*’ ).
154
155     If the client successfully authenticates itself, a dialog for preparing
156     the session is entered.  At this time the client may request things like
157     allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con‐
158     nections, or forwarding the authentication agent connection over the
159     secure channel.
160
161     After this, the client either requests a shell or execution of a command.
162     The sides then enter session mode.  In this mode, either side may send
163     data at any time, and such data is forwarded to/from the shell or command
164     on the server side, and the user terminal in the client side.
165
166     When the user program terminates and all forwarded X11 and other connec‐
167     tions have been closed, the server sends command exit status to the
168     client, and both sides exit.
169

LOGIN PROCESS

171     When a user successfully logs in, sshd does the following:
172
173           1.   If the login is on a tty, and no command has been specified,
174                prints last login time and /etc/motd (unless prevented in the
175                configuration file or by ~/.hushlogin; see the FILES section).
176
177           2.   If the login is on a tty, records login time.
178
179           3.   Checks /etc/nologin; if it exists, prints contents and quits
180                (unless root).
181
182           4.   Changes to run with normal user privileges.
183
184           5.   Sets up basic environment.
185
186           6.   Reads the file ~/.ssh/environment, if it exists, and users are
187                allowed to change their environment.  See the
188                PermitUserEnvironment option in sshd_config(5).
189
190           7.   Changes to user's home directory.
191
192           8.   If ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
193                is set, runs it; else if /etc/ssh/sshrc exists, runs it; oth‐
194                erwise runs xauth.  The “rc” files are given the X11 authenti‐
195                cation protocol and cookie in standard input.  See SSHRC,
196                below.
197
198           9.   Runs user's shell or command.  All commands are run under the
199                user's login shell as specified in the system password data‐
200                base.
201

SSHRC

203     If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
204     files but before starting the user's shell or command.  It must not pro‐
205     duce any output on stdout; stderr must be used instead.  If X11 forward‐
206     ing is in use, it will receive the "proto cookie" pair in its standard
207     input (and DISPLAY in its environment).  The script must call xauth(1)
208     because sshd will not run xauth automatically to add X11 cookies.
209
210     The primary purpose of this file is to run any initialization routines
211     which may be needed before the user's home directory becomes accessible;
212     AFS is a particular example of such an environment.
213
214     This file will probably contain some initialization code followed by
215     something similar to:
216
217        if read proto cookie && [ -n "$DISPLAY" ]; then
218                if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
219                        # X11UseLocalhost=yes
220                        echo add unix:`echo $DISPLAY |
221                            cut -c11-` $proto $cookie
222                else
223                        # X11UseLocalhost=no
224                        echo add $DISPLAY $proto $cookie
225                fi | xauth -q -
226        fi
227
228     If this file does not exist, /etc/ssh/sshrc is run, and if that does not
229     exist either, xauth is used to add the cookie.
230

AUTHORIZED_KEYS FILE FORMAT

232     AuthorizedKeysFile specifies the files containing public keys for public
233     key authentication; if this option is not specified, the default is
234     ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.  Each line of the
235     file contains one key (empty lines and lines starting with a ‘#’ are
236     ignored as comments).  Public keys consist of the following space-sepa‐
237     rated fields: options, keytype, base64-encoded key, comment.  The options
238     field is optional.  The keytype is “ecdsa-sha2-nistp256”,
239     “ecdsa-sha2-nistp384”, “ecdsa-sha2-nistp521”, “ssh-ed25519”, “ssh-dss” or
240     “ssh-rsa”; the comment field is not used for anything (but may be conve‐
241     nient for the user to identify the key).
242
243     Note that lines in this file can be several hundred bytes long (because
244     of the size of the public key encoding) up to a limit of 8 kilobytes,
245     which permits DSA keys up to 8 kilobits and RSA keys up to 16 kilobits.
246     You don't want to type them in; instead, copy the id_dsa.pub,
247     id_ecdsa.pub, id_ed25519.pub, or the id_rsa.pub file and edit it.
248
249     sshd enforces a minimum RSA key modulus size of 1024 bits.
250
251     The options (if present) consist of comma-separated option specifica‐
252     tions.  No spaces are permitted, except within double quotes.  The fol‐
253     lowing option specifications are supported (note that option keywords are
254     case-insensitive):
255
256     agent-forwarding
257             Enable authentication agent forwarding previously disabled by the
258             restrict option.
259
260     cert-authority
261             Specifies that the listed key is a certification authority (CA)
262             that is trusted to validate signed certificates for user authen‐
263             tication.
264
265             Certificates may encode access restrictions similar to these key
266             options.  If both certificate restrictions and key options are
267             present, the most restrictive union of the two is applied.
268
269     command="command"
270             Specifies that the command is executed whenever this key is used
271             for authentication.  The command supplied by the user (if any) is
272             ignored.  The command is run on a pty if the client requests a
273             pty; otherwise it is run without a tty.  If an 8-bit clean chan‐
274             nel is required, one must not request a pty or should specify
275             no-pty.  A quote may be included in the command by quoting it
276             with a backslash.
277
278             This option might be useful to restrict certain public keys to
279             perform just a specific operation.  An example might be a key
280             that permits remote backups but nothing else.  Note that the
281             client may specify TCP and/or X11 forwarding unless they are
282             explicitly prohibited, e.g. using the restrict key option.
283
284             The command originally supplied by the client is available in the
285             SSH_ORIGINAL_COMMAND environment variable.  Note that this option
286             applies to shell, command or subsystem execution.  Also note that
287             this command may be superseded by a sshd_config(5) ForceCommand
288             directive.
289
290             If a command is specified and a forced-command is embedded in a
291             certificate used for authentication, then the certificate will be
292             accepted only if the two commands are identical.
293
294     environment="NAME=value"
295             Specifies that the string is to be added to the environment when
296             logging in using this key.  Environment variables set this way
297             override other default environment values.  Multiple options of
298             this type are permitted.  Environment processing is disabled by
299             default and is controlled via the PermitUserEnvironment option.
300
301     expiry-time="timespec"
302             Specifies a time after which the key will not be accepted.  The
303             time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS]
304             time in the system time-zone.
305
306     from="pattern-list"
307             Specifies that in addition to public key authentication, either
308             the canonical name of the remote host or its IP address must be
309             present in the comma-separated list of patterns.  See PATTERNS in
310             ssh_config(5) for more information on patterns.
311
312             In addition to the wildcard matching that may be applied to host‐
313             names or addresses, a from stanza may match IP addresses using
314             CIDR address/masklen notation.
315
316             The purpose of this option is to optionally increase security:
317             public key authentication by itself does not trust the network or
318             name servers or anything (but the key); however, if somebody
319             somehow steals the key, the key permits an intruder to log in
320             from anywhere in the world.  This additional option makes using a
321             stolen key more difficult (name servers and/or routers would have
322             to be compromised in addition to just the key).
323
324     no-agent-forwarding
325             Forbids authentication agent forwarding when this key is used for
326             authentication.
327
328     no-port-forwarding
329             Forbids TCP forwarding when this key is used for authentication.
330             Any port forward requests by the client will return an error.
331             This might be used, e.g. in connection with the command option.
332
333     no-pty  Prevents tty allocation (a request to allocate a pty will fail).
334
335     no-user-rc
336             Disables execution of ~/.ssh/rc.
337
338     no-X11-forwarding
339             Forbids X11 forwarding when this key is used for authentication.
340             Any X11 forward requests by the client will return an error.
341
342     permitlisten="[host:]port"
343             Limit remote port forwarding with the ssh(1) -R option such that
344             it may only listen on the specified host (optional) and port.
345             IPv6 addresses can be specified by enclosing the address in
346             square brackets.  Multiple permitlisten options may be applied
347             separated by commas.  Hostnames may include wildcards as
348             described in the PATTERNS section in ssh_config(5).  A port spec‐
349             ification of * matches any port.  Note that the setting of
350             GatewayPorts may further restrict listen addresses.  Note that
351             ssh(1) will send a hostname of “localhost” if a listen host was
352             not specified when the forwarding was requested, and that this
353             name is treated differently to the explicit localhost addresses
354             “127.0.0.1” and “::1”.
355
356     permitopen="host:port"
357             Limit local port forwarding with the ssh(1) -L option such that
358             it may only connect to the specified host and port.  IPv6
359             addresses can be specified by enclosing the address in square
360             brackets.  Multiple permitopen options may be applied separated
361             by commas.  No pattern matching is performed on the specified
362             hostnames, they must be literal domains or addresses.  A port
363             specification of * matches any port.
364
365     port-forwarding
366             Enable port forwarding previously disabled by the restrict
367             option.
368
369     principals="principals"
370             On a cert-authority line, specifies allowed principals for cer‐
371             tificate authentication as a comma-separated list.  At least one
372             name from the list must appear in the certificate's list of prin‐
373             cipals for the certificate to be accepted.  This option is
374             ignored for keys that are not marked as trusted certificate sign‐
375             ers using the cert-authority option.
376
377     pty     Permits tty allocation previously disabled by the restrict
378             option.
379
380     restrict
381             Enable all restrictions, i.e. disable port, agent and X11 for‐
382             warding, as well as disabling PTY allocation and execution of
383             ~/.ssh/rc.  If any future restriction capabilities are added to
384             authorized_keys files they will be included in this set.
385
386     tunnel="n"
387             Force a tun(4) device on the server.  Without this option, the
388             next available device will be used if the client requests a tun‐
389             nel.
390
391     user-rc
392             Enables execution of ~/.ssh/rc previously disabled by the
393             restrict option.
394
395     X11-forwarding
396             Permits X11 forwarding previously disabled by the restrict
397             option.
398
399     An example authorized_keys file:
400
401        # Comments allowed at start of line
402        ssh-rsa AAAAB3Nza...LiPk== user@example.net
403        from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
404        AAAAB2...19Q== john@example.net
405        command="dump /home",no-pty,no-port-forwarding ssh-rsa
406        AAAAC3...51R== example.net
407        permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa
408        AAAAB5...21S==
409        permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
410        AAAAB5...21S==
411        tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
412        jane@example.net
413        restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
414        user@example.net
415        restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
416        user@example.net
417

SSH_KNOWN_HOSTS FILE FORMAT

419     The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
420     public keys for all known hosts.  The global file should be prepared by
421     the administrator (optional), and the per-user file is maintained auto‐
422     matically: whenever the user connects to an unknown host, its key is
423     added to the per-user file.
424
425     Each line in these files contains the following fields: markers
426     (optional), hostnames, keytype, base64-encoded key, comment.  The fields
427     are separated by spaces.
428
429     The marker is optional, but if it is present then it must be one of
430     “@cert-authority”, to indicate that the line contains a certification
431     authority (CA) key, or “@revoked”, to indicate that the key contained on
432     the line is revoked and must not ever be accepted.  Only one marker
433     should be used on a key line.
434
435     Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wild‐
436     cards); each pattern in turn is matched against the host name.  When sshd
437     is authenticating a client, such as when using HostbasedAuthentication,
438     this will be the canonical client host name.  When ssh(1) is authenticat‐
439     ing a server, this will be the host name given by the user, the value of
440     the ssh(1) HostkeyAlias if it was specified, or the canonical server
441     hostname if the ssh(1) CanonicalizeHostname option was used.
442
443     A pattern may also be preceded by ‘!’ to indicate negation: if the host
444     name matches a negated pattern, it is not accepted (by that line) even if
445     it matched another pattern on the line.  A hostname or address may
446     optionally be enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’
447     and a non-standard port number.
448
449     Alternately, hostnames may be stored in a hashed form which hides host
450     names and addresses should the file's contents be disclosed.  Hashed
451     hostnames start with a ‘|’ character.  Only one hashed hostname may
452     appear on a single line and none of the above negation or wildcard opera‐
453     tors may be applied.
454
455     The keytype and base64-encoded key are taken directly from the host key;
456     they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub.
457     The optional comment field continues to the end of the line, and is not
458     used.
459
460     Lines starting with ‘#’ and empty lines are ignored as comments.
461
462     When performing host authentication, authentication is accepted if any
463     matching line has the proper key; either one that matches exactly or, if
464     the server has presented a certificate for authentication, the key of the
465     certification authority that signed the certificate.  For a key to be
466     trusted as a certification authority, it must use the “@cert-authority”
467     marker described above.
468
469     The known hosts file also provides a facility to mark keys as revoked,
470     for example when it is known that the associated private key has been
471     stolen.  Revoked keys are specified by including the “@revoked” marker at
472     the beginning of the key line, and are never accepted for authentication
473     or as certification authorities, but instead will produce a warning from
474     ssh(1) when they are encountered.
475
476     It is permissible (but not recommended) to have several lines or differ‐
477     ent host keys for the same names.  This will inevitably happen when short
478     forms of host names from different domains are put in the file.  It is
479     possible that the files contain conflicting information; authentication
480     is accepted if valid information can be found from either file.
481
482     Note that the lines in these files are typically hundreds of characters
483     long, and you definitely don't want to type in the host keys by hand.
484     Rather, generate them by a script, ssh-keyscan(1) or by taking, for exam‐
485     ple, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the
486     front.  ssh-keygen(1) also offers some basic automated editing for
487     ~/.ssh/known_hosts including removing hosts matching a host name and con‐
488     verting all host names to their hashed representations.
489
490     An example ssh_known_hosts file:
491
492        # Comments allowed at start of line
493        closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
494        cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
495        # A hashed hostname
496        |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
497        AAAA1234.....=
498        # A revoked key
499        @revoked * ssh-rsa AAAAB5W...
500        # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
501        @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
502

FILES

504     ~/.hushlogin
505             This file is used to suppress printing the last login time and
506             /etc/motd, if PrintLastLog and PrintMotd, respectively, are
507             enabled.  It does not suppress printing of the banner specified
508             by Banner.
509
510     ~/.rhosts
511             This file is used for host-based authentication (see ssh(1) for
512             more information).  On some machines this file may need to be
513             world-readable if the user's home directory is on an NFS parti‐
514             tion, because sshd reads it as root.  Additionally, this file
515             must be owned by the user, and must not have write permissions
516             for anyone else.  The recommended permission for most machines is
517             read/write for the user, and not accessible by others.
518
519     ~/.shosts
520             This file is used in exactly the same way as .rhosts, but allows
521             host-based authentication without permitting login with
522             rlogin/rsh.
523
524     ~/.k5login
525     ~/.k5users
526             These files enforce GSSAPI/Kerberos authentication access con‐
527             trol.  Further details are described in ksu(1).  The location of
528             the k5login file depends on the configuration option
529             k5login_directory in the krb5.conf(5).
530
531     ~/.ssh/
532             This directory is the default location for all user-specific con‐
533             figuration and authentication information.  There is no general
534             requirement to keep the entire contents of this directory secret,
535             but the recommended permissions are read/write/execute for the
536             user, and not accessible by others.
537
538     ~/.ssh/authorized_keys
539             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
540             for logging in as this user.  The format of this file is
541             described above.  The content of the file is not highly sensi‐
542             tive, but the recommended permissions are read/write for the
543             user, and not accessible by others.
544
545             If this file, the ~/.ssh directory, or the user's home directory
546             are writable by other users, then the file could be modified or
547             replaced by unauthorized users.  In this case, sshd will not
548             allow it to be used unless the StrictModes option has been set to
549             “no”.
550
551     ~/.ssh/environment
552             This file is read into the environment at login (if it exists).
553             It can only contain empty lines, comment lines (that start with
554             ‘#’), and assignment lines of the form name=value.  The file
555             should be writable only by the user; it need not be readable by
556             anyone else.  Environment processing is disabled by default and
557             is controlled via the PermitUserEnvironment option.
558
559     ~/.ssh/known_hosts
560             Contains a list of host keys for all hosts the user has logged
561             into that are not already in the systemwide list of known host
562             keys.  The format of this file is described above.  This file
563             should be writable only by root/the owner and can, but need not
564             be, world-readable.
565
566     ~/.ssh/rc
567             Contains initialization routines to be run before the user's home
568             directory becomes accessible.  This file should be writable only
569             by the user, and need not be readable by anyone else.
570
571     /etc/hosts.equiv
572             This file is for host-based authentication (see ssh(1)).  It
573             should only be writable by root.
574
575     /etc/ssh/moduli
576             Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
577             Exchange" key exchange method.  The file format is described in
578             moduli(5).  If no usable groups are found in this file then fixed
579             internal groups will be used.
580
581     /etc/motd
582             See motd(5).
583
584     /etc/nologin
585             If this file exists, sshd refuses to let anyone except root log
586             in.  The contents of the file are displayed to anyone trying to
587             log in, and non-root connections are refused.  The file should be
588             world-readable.
589
590     /etc/ssh/shosts.equiv
591             This file is used in exactly the same way as hosts.equiv, but
592             allows host-based authentication without permitting login with
593             rlogin/rsh.
594
595     /etc/ssh/ssh_host_ecdsa_key
596     /etc/ssh/ssh_host_ed25519_key
597     /etc/ssh/ssh_host_rsa_key
598             These files contain the private parts of the host keys.  These
599             files should only be owned by root, readable only by root, and
600             not accessible to others.  Note that sshd does not start if these
601             files are group/world-accessible.
602
603     /etc/ssh/ssh_host_ecdsa_key.pub
604     /etc/ssh/ssh_host_ed25519_key.pub
605     /etc/ssh/ssh_host_rsa_key.pub
606             These files contain the public parts of the host keys.  These
607             files should be world-readable but writable only by root.  Their
608             contents should match the respective private parts.  These files
609             are not really used for anything; they are provided for the con‐
610             venience of the user so their contents can be copied to known
611             hosts files.  These files are created using ssh-keygen(1).
612
613     /etc/ssh/ssh_known_hosts
614             Systemwide list of known host keys.  This file should be prepared
615             by the system administrator to contain the public host keys of
616             all machines in the organization.  The format of this file is
617             described above.  This file should be writable only by root/the
618             owner and should be world-readable.
619
620     /etc/ssh/sshd_config
621             Contains configuration data for sshd.  The file format and con‐
622             figuration options are described in sshd_config(5).
623
624     /etc/ssh/sshrc
625             Similar to ~/.ssh/rc, it can be used to specify machine-specific
626             login-time initializations globally.  This file should be
627             writable only by root, and should be world-readable.
628
629     /var/empty/sshd
630             chroot(2) directory used by sshd during privilege separation in
631             the pre-authentication phase.  The directory should not contain
632             any files and must be owned by root and not group or world-
633             writable.
634
635     /var/run/sshd.pid
636             Contains the process ID of the sshd listening for connections (if
637             there are several daemons running concurrently for different
638             ports, this contains the process ID of the one started last).
639             The content of this file is not sensitive; it can be world-read‐
640             able.
641

IPV6

643     IPv6 address can be used everywhere where IPv4 address. In all entries
644     must be the IPv6 address enclosed in square brackets. Note: The square
645     brackets are metacharacters for the shell and must be escaped in shell.
646

SEE ALSO

648     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
649     ssh-keyscan(1), chroot(2), login.conf(5), moduli(5), sshd_config(5),
650     inetd(8), sftp-server(8)
651

AUTHORS

653     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
654     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
655     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
656     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
657     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
658     for privilege separation.
659
660BSD                              June 20, 2019                             BSD
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