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

NAME

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

LOGIN PROCESS

169     When a user successfully logs in, sshd does the following:
170
171           1.   If the login is on a tty, and no command has been specified,
172                prints last login time and /etc/motd (unless prevented in the
173                configuration file or by ~/.hushlogin; see the FILES section).
174
175           2.   If the login is on a tty, records login time.
176
177           3.   Checks /etc/nologin; if it exists, prints contents and quits
178                (unless root).
179
180           4.   Changes to run with normal user privileges.
181
182           5.   Sets up basic environment.
183
184           6.   Reads the file ~/.ssh/environment, if it exists, and users are
185                allowed to change their environment.  See the
186                PermitUserEnvironment option in sshd_config(5).
187
188           7.   Changes to user's home directory.
189
190           8.   If ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
191                is set, runs it; else if /etc/ssh/sshrc exists, runs it; oth‐
192                erwise runs xauth.  The “rc” files are given the X11 authenti‐
193                cation protocol and cookie in standard input.  See SSHRC,
194                below.
195
196           9.   Runs user's shell or command.  All commands are run under the
197                user's login shell as specified in the system password data‐
198                base.
199

SSHRC

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

AUTHORIZED_KEYS FILE FORMAT

230     AuthorizedKeysFile specifies the files containing public keys for public
231     key authentication; if this option is not specified, the default is
232     ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.  Each line of the
233     file contains one key (empty lines and lines starting with a ‘#’ are
234     ignored as comments).  Public keys consist of the following space-sepa‐
235     rated fields: options, keytype, base64-encoded key, comment.  The options
236     field is optional.  The supported key types are:
237
238           sk-ecdsa-sha2-nistp256@openssh.com
239           ecdsa-sha2-nistp256
240           ecdsa-sha2-nistp384
241           ecdsa-sha2-nistp521
242           sk-ssh-ed25519@openssh.com
243           ssh-ed25519
244           ssh-dss
245           ssh-rsa
246
247     The comment field is not used for anything (but may be convenient for the
248     user to identify the key).
249
250     Note that lines in this file can be several hundred bytes long (because
251     of the size of the public key encoding) up to a limit of 8 kilobytes,
252     which permits RSA keys up to 16 kilobits.  You don't want to type them
253     in; instead, copy the id_dsa.pub, id_ecdsa.pub, id_ecdsa_sk.pub,
254     id_ed25519.pub, id_ed25519_sk.pub, or the id_rsa.pub file and edit it.
255
256     sshd enforces a minimum RSA key modulus size of 1024 bits.
257
258     The options (if present) consist of comma-separated option specifica‐
259     tions.  No spaces are permitted, except within double quotes.  The fol‐
260     lowing option specifications are supported (note that option keywords are
261     case-insensitive):
262
263     agent-forwarding
264             Enable authentication agent forwarding previously disabled by the
265             restrict option.
266
267     cert-authority
268             Specifies that the listed key is a certification authority (CA)
269             that is trusted to validate signed certificates for user authen‐
270             tication.
271
272             Certificates may encode access restrictions similar to these key
273             options.  If both certificate restrictions and key options are
274             present, the most restrictive union of the two is applied.
275
276     command="command"
277             Specifies that the command is executed whenever this key is used
278             for authentication.  The command supplied by the user (if any) is
279             ignored.  The command is run on a pty if the client requests a
280             pty; otherwise it is run without a tty.  If an 8-bit clean chan‐
281             nel is required, one must not request a pty or should specify
282             no-pty.  A quote may be included in the command by quoting it
283             with a backslash.
284
285             This option might be useful to restrict certain public keys to
286             perform just a specific operation.  An example might be a key
287             that permits remote backups but nothing else.  Note that the
288             client may specify TCP and/or X11 forwarding unless they are
289             explicitly prohibited, e.g. using the restrict key option.
290
291             The command originally supplied by the client is available in the
292             SSH_ORIGINAL_COMMAND environment variable.  Note that this option
293             applies to shell, command or subsystem execution.  Also note that
294             this command may be superseded by a sshd_config(5) ForceCommand
295             directive.
296
297             If a command is specified and a forced-command is embedded in a
298             certificate used for authentication, then the certificate will be
299             accepted only if the two commands are identical.
300
301     environment="NAME=value"
302             Specifies that the string is to be added to the environment when
303             logging in using this key.  Environment variables set this way
304             override other default environment values.  Multiple options of
305             this type are permitted.  Environment processing is disabled by
306             default and is controlled via the PermitUserEnvironment option.
307
308     expiry-time="timespec"
309             Specifies a time after which the key will not be accepted.  The
310             time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS]
311             time in the system time-zone.
312
313     from="pattern-list"
314             Specifies that in addition to public key authentication, either
315             the canonical name of the remote host or its IP address must be
316             present in the comma-separated list of patterns.  See PATTERNS in
317             ssh_config(5) for more information on patterns.
318
319             In addition to the wildcard matching that may be applied to host‐
320             names or addresses, a from stanza may match IP addresses using
321             CIDR address/masklen notation.
322
323             The purpose of this option is to optionally increase security:
324             public key authentication by itself does not trust the network or
325             name servers or anything (but the key); however, if somebody
326             somehow steals the key, the key permits an intruder to log in
327             from anywhere in the world.  This additional option makes using a
328             stolen key more difficult (name servers and/or routers would have
329             to be compromised in addition to just the key).
330
331     no-agent-forwarding
332             Forbids authentication agent forwarding when this key is used for
333             authentication.
334
335     no-port-forwarding
336             Forbids TCP forwarding when this key is used for authentication.
337             Any port forward requests by the client will return an error.
338             This might be used, e.g. in connection with the command option.
339
340     no-pty  Prevents tty allocation (a request to allocate a pty will fail).
341
342     no-user-rc
343             Disables execution of ~/.ssh/rc.
344
345     no-X11-forwarding
346             Forbids X11 forwarding when this key is used for authentication.
347             Any X11 forward requests by the client will return an error.
348
349     permitlisten="[host:]port"
350             Limit remote port forwarding with the ssh(1) -R option such that
351             it may only listen on the specified host (optional) and port.
352             IPv6 addresses can be specified by enclosing the address in
353             square brackets.  Multiple permitlisten options may be applied
354             separated by commas.  Hostnames may include wildcards as
355             described in the PATTERNS section in ssh_config(5).  A port spec‐
356             ification of * matches any port.  Note that the setting of
357             GatewayPorts may further restrict listen addresses.  Note that
358             ssh(1) will send a hostname of “localhost” if a listen host was
359             not specified when the forwarding was requested, and that this
360             name is treated differently to the explicit localhost addresses
361             “127.0.0.1” and “::1”.
362
363     permitopen="host:port"
364             Limit local port forwarding with the ssh(1) -L option such that
365             it may only connect to the specified host and port.  IPv6
366             addresses can be specified by enclosing the address in square
367             brackets.  Multiple permitopen options may be applied separated
368             by commas.  No pattern matching or name lookup is performed on
369             the specified hostnames, they must be literal host names and/or
370             addresses.  A port specification of * matches any port.
371
372     port-forwarding
373             Enable port forwarding previously disabled by the restrict
374             option.
375
376     principals="principals"
377             On a cert-authority line, specifies allowed principals for cer‐
378             tificate authentication as a comma-separated list.  At least one
379             name from the list must appear in the certificate's list of prin‐
380             cipals for the certificate to be accepted.  This option is
381             ignored for keys that are not marked as trusted certificate sign‐
382             ers using the cert-authority option.
383
384     pty     Permits tty allocation previously disabled by the restrict
385             option.
386
387     no-touch-required
388             Do not require demonstration of user presence for signatures made
389             using this key.  This option only makes sense for the FIDO
390             authenticator algorithms ecdsa-sk and ed25519-sk.
391
392     verify-required
393             Require that signatures made using this key attest that they ver‐
394             ified the user, e.g. via a PIN.  This option only makes sense for
395             the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
396
397     restrict
398             Enable all restrictions, i.e. disable port, agent and X11 for‐
399             warding, as well as disabling PTY allocation and execution of
400             ~/.ssh/rc.  If any future restriction capabilities are added to
401             authorized_keys files they will be included in this set.
402
403     tunnel="n"
404             Force a tun(4) device on the server.  Without this option, the
405             next available device will be used if the client requests a tun‐
406             nel.
407
408     user-rc
409             Enables execution of ~/.ssh/rc previously disabled by the
410             restrict option.
411
412     X11-forwarding
413             Permits X11 forwarding previously disabled by the restrict
414             option.
415
416     An example authorized_keys file:
417
418        # Comments allowed at start of line
419        ssh-rsa AAAAB3Nza...LiPk== user@example.net
420        from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
421        AAAAB2...19Q== john@example.net
422        command="dump /home",no-pty,no-port-forwarding ssh-rsa
423        AAAAC3...51R== example.net
424        permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa
425        AAAAB5...21S==
426        permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
427        AAAAB5...21S==
428        tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
429        jane@example.net
430        restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
431        user@example.net
432        restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
433        user@example.net
434        no-touch-required sk-ecdsa-sha2-nistp256@openssh.com AAAAInN...Ko==
435        user@example.net
436

SSH_KNOWN_HOSTS FILE FORMAT

438     The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
439     public keys for all known hosts.  The global file should be prepared by
440     the administrator (optional), and the per-user file is maintained auto‐
441     matically: whenever the user connects to an unknown host, its key is
442     added to the per-user file.
443
444     Each line in these files contains the following fields: markers
445     (optional), hostnames, keytype, base64-encoded key, comment.  The fields
446     are separated by spaces.
447
448     The marker is optional, but if it is present then it must be one of
449     “@cert-authority”, to indicate that the line contains a certification
450     authority (CA) key, or “@revoked”, to indicate that the key contained on
451     the line is revoked and must not ever be accepted.  Only one marker
452     should be used on a key line.
453
454     Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wild‐
455     cards); each pattern in turn is matched against the host name.  When sshd
456     is authenticating a client, such as when using HostbasedAuthentication,
457     this will be the canonical client host name.  When ssh(1) is authenticat‐
458     ing a server, this will be the host name given by the user, the value of
459     the ssh(1) HostkeyAlias if it was specified, or the canonical server
460     hostname if the ssh(1) CanonicalizeHostname option was used.
461
462     A pattern may also be preceded by ‘!’ to indicate negation: if the host
463     name matches a negated pattern, it is not accepted (by that line) even if
464     it matched another pattern on the line.  A hostname or address may
465     optionally be enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’
466     and a non-standard port number.
467
468     Alternately, hostnames may be stored in a hashed form which hides host
469     names and addresses should the file's contents be disclosed.  Hashed
470     hostnames start with a ‘|’ character.  Only one hashed hostname may
471     appear on a single line and none of the above negation or wildcard opera‐
472     tors may be applied.
473
474     The keytype and base64-encoded key are taken directly from the host key;
475     they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub.
476     The optional comment field continues to the end of the line, and is not
477     used.
478
479     Lines starting with ‘#’ and empty lines are ignored as comments.
480
481     When performing host authentication, authentication is accepted if any
482     matching line has the proper key; either one that matches exactly or, if
483     the server has presented a certificate for authentication, the key of the
484     certification authority that signed the certificate.  For a key to be
485     trusted as a certification authority, it must use the “@cert-authority”
486     marker described above.
487
488     The known hosts file also provides a facility to mark keys as revoked,
489     for example when it is known that the associated private key has been
490     stolen.  Revoked keys are specified by including the “@revoked” marker at
491     the beginning of the key line, and are never accepted for authentication
492     or as certification authorities, but instead will produce a warning from
493     ssh(1) when they are encountered.
494
495     It is permissible (but not recommended) to have several lines or differ‐
496     ent host keys for the same names.  This will inevitably happen when short
497     forms of host names from different domains are put in the file.  It is
498     possible that the files contain conflicting information; authentication
499     is accepted if valid information can be found from either file.
500
501     Note that the lines in these files are typically hundreds of characters
502     long, and you definitely don't want to type in the host keys by hand.
503     Rather, generate them by a script, ssh-keyscan(1) or by taking, for exam‐
504     ple, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the
505     front.  ssh-keygen(1) also offers some basic automated editing for
506     ~/.ssh/known_hosts including removing hosts matching a host name and con‐
507     verting all host names to their hashed representations.
508
509     An example ssh_known_hosts file:
510
511        # Comments allowed at start of line
512        closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
513        cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
514        # A hashed hostname
515        |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
516        AAAA1234.....=
517        # A revoked key
518        @revoked * ssh-rsa AAAAB5W...
519        # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
520        @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
521

FILES

523     ~/.hushlogin
524             This file is used to suppress printing the last login time and
525             /etc/motd, if PrintLastLog and PrintMotd, respectively, are
526             enabled.  It does not suppress printing of the banner specified
527             by Banner.
528
529     ~/.rhosts
530             This file is used for host-based authentication (see ssh(1) for
531             more information).  On some machines this file may need to be
532             world-readable if the user's home directory is on an NFS parti‐
533             tion, because sshd reads it as root.  Additionally, this file
534             must be owned by the user, and must not have write permissions
535             for anyone else.  The recommended permission for most machines is
536             read/write for the user, and not accessible by others.
537
538     ~/.shosts
539             This file is used in exactly the same way as .rhosts, but allows
540             host-based authentication without permitting login with
541             rlogin/rsh.
542
543     ~/.k5login
544     ~/.k5users
545             These files enforce GSSAPI/Kerberos authentication access con‐
546             trol.  Further details are described in ksu(1).  The location of
547             the k5login file depends on the configuration option
548             k5login_directory in the krb5.conf(5).
549
550     ~/.ssh/
551             This directory is the default location for all user-specific con‐
552             figuration and authentication information.  There is no general
553             requirement to keep the entire contents of this directory secret,
554             but the recommended permissions are read/write/execute for the
555             user, and not accessible by others.
556
557     ~/.ssh/authorized_keys
558             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
559             for logging in as this user.  The format of this file is
560             described above.  The content of the file is not highly sensi‐
561             tive, but the recommended permissions are read/write for the
562             user, and not accessible by others.
563
564             If this file, the ~/.ssh directory, or the user's home directory
565             are writable by other users, then the file could be modified or
566             replaced by unauthorized users.  In this case, sshd will not
567             allow it to be used unless the StrictModes option has been set to
568             “no”.
569
570     ~/.ssh/environment
571             This file is read into the environment at login (if it exists).
572             It can only contain empty lines, comment lines (that start with
573             ‘#’), and assignment lines of the form name=value.  The file
574             should be writable only by the user; it need not be readable by
575             anyone else.  Environment processing is disabled by default and
576             is controlled via the PermitUserEnvironment option.
577
578     ~/.ssh/known_hosts
579             Contains a list of host keys for all hosts the user has logged
580             into that are not already in the systemwide list of known host
581             keys.  The format of this file is described above.  This file
582             should be writable only by root/the owner and can, but need not
583             be, world-readable.
584
585     ~/.ssh/rc
586             Contains initialization routines to be run before the user's home
587             directory becomes accessible.  This file should be writable only
588             by the user, and need not be readable by anyone else.
589
590     /etc/hosts.equiv
591             This file is for host-based authentication (see ssh(1)).  It
592             should only be writable by root.
593
594     /etc/ssh/moduli
595             Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
596             Exchange" key exchange method.  The file format is described in
597             moduli(5).  If no usable groups are found in this file then fixed
598             internal groups will be used.
599
600     /etc/motd
601             See motd(5).
602
603     /etc/nologin
604             If this file exists, sshd refuses to let anyone except root log
605             in.  The contents of the file are displayed to anyone trying to
606             log in, and non-root connections are refused.  The file should be
607             world-readable.
608
609     /etc/ssh/shosts.equiv
610             This file is used in exactly the same way as hosts.equiv, but
611             allows host-based authentication without permitting login with
612             rlogin/rsh.
613
614     /etc/ssh/ssh_host_ecdsa_key
615     /etc/ssh/ssh_host_ed25519_key
616     /etc/ssh/ssh_host_rsa_key
617             These files contain the private parts of the host keys.  These
618             files should only be owned by root, readable only by root, and
619             not accessible to others.  Note that sshd does not start if these
620             files are group/world-accessible.
621
622     /etc/ssh/ssh_host_ecdsa_key.pub
623     /etc/ssh/ssh_host_ed25519_key.pub
624     /etc/ssh/ssh_host_rsa_key.pub
625             These files contain the public parts of the host keys.  These
626             files should be world-readable but writable only by root.  Their
627             contents should match the respective private parts.  These files
628             are not really used for anything; they are provided for the con‐
629             venience of the user so their contents can be copied to known
630             hosts files.  These files are created using ssh-keygen(1).
631
632     /etc/ssh/ssh_known_hosts
633             Systemwide list of known host keys.  This file should be prepared
634             by the system administrator to contain the public host keys of
635             all machines in the organization.  The format of this file is
636             described above.  This file should be writable only by root/the
637             owner and should be world-readable.
638
639     /etc/ssh/sshd_config
640             Contains configuration data for sshd.  The file format and con‐
641             figuration options are described in sshd_config(5).
642
643     /etc/ssh/sshrc
644             Similar to ~/.ssh/rc, it can be used to specify machine-specific
645             login-time initializations globally.  This file should be
646             writable only by root, and should be world-readable.
647
648     /var/empty/sshd
649             chroot(2) directory used by sshd during privilege separation in
650             the pre-authentication phase.  The directory should not contain
651             any files and must be owned by root and not group or world-
652             writable.
653
654     /var/run/sshd.pid
655             Contains the process ID of the sshd listening for connections (if
656             there are several daemons running concurrently for different
657             ports, this contains the process ID of the one started last).
658             The content of this file is not sensitive; it can be world-read‐
659             able.
660

IPV6

662     IPv6 address can be used everywhere where IPv4 address. In all entries
663     must be the IPv6 address enclosed in square brackets. Note: The square
664     brackets are metacharacters for the shell and must be escaped in shell.
665

SEE ALSO

667     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
668     ssh-keyscan(1), chroot(2), login.conf(5), moduli(5), sshd_config(5),
669     inetd(8), sftp-server(8)
670

AUTHORS

672     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
673     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
674     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
675     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
676     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
677     for privilege separation.
678
679BSD                             August 27, 2020                            BSD
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