1SSHD_CONFIG(5)              BSD File Formats Manual             SSHD_CONFIG(5)
2

NAME

4     sshd_config — OpenSSH SSH daemon configuration file
5

SYNOPSIS

7     /etc/ssh/sshd_config
8

DESCRIPTION

10     sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
11     specified with -f on the command line).  The file contains keyword-argu‐
12     ment pairs, one per line.  Lines starting with ‘#’ and empty lines are
13     interpreted as comments.  Arguments may optionally be enclosed in double
14     quotes (") in order to represent arguments containing spaces.
15
16     The possible keywords and their meanings are as follows (note that key‐
17     words are case-insensitive and arguments are case-sensitive):
18
19     AcceptEnv
20             Specifies what environment variables sent by the client will be
21             copied into the session's environ(7).  See SendEnv in
22             ssh_config(5) for how to configure the client.  Note that envi‐
23             ronment passing is only supported for protocol 2.  Variables are
24             specified by name, which may contain the wildcard characters ‘*’
25             and ‘?’.  Multiple environment variables may be separated by
26             whitespace or spread across multiple AcceptEnv directives.  Be
27             warned that some environment variables could be used to bypass
28             restricted user environments.  For this reason, care should be
29             taken in the use of this directive.  The default is not to accept
30             any environment variables.
31
32     AddressFamily
33             Specifies which address family should be used by sshd(8).  Valid
34             arguments are “any”, “inet” (use IPv4 only), or “inet6” (use IPv6
35             only).  The default is “any”.
36
37     AllowAgentForwarding
38             Specifies whether ssh-agent(1) forwarding is permitted.  The
39             default is “yes”.  Note that disabling agent forwarding does not
40             improve security unless users are also denied shell access, as
41             they can always install their own forwarders.
42
43     AllowGroups
44             This keyword can be followed by a list of group name patterns,
45             separated by spaces.  If specified, login is allowed only for
46             users whose primary group or supplementary group list matches one
47             of the patterns.  Only group names are valid; a numerical group
48             ID is not recognized.  By default, login is allowed for all
49             groups.  The allow/deny directives are processed in the following
50             order: DenyUsers, AllowUsers, DenyGroups, and finally
51             AllowGroups.
52
53             See PATTERNS in ssh_config(5) for more information on patterns.
54
55     AllowTcpForwarding
56             Specifies whether TCP forwarding is permitted.  The default is
57             “yes”.  Note that disabling TCP forwarding does not improve secu‐
58             rity unless users are also denied shell access, as they can
59             always install their own forwarders.
60
61     AllowUsers
62             This keyword can be followed by a list of user name patterns,
63             separated by spaces.  If specified, login is allowed only for
64             user names that match one of the patterns.  Only user names are
65             valid; a numerical user ID is not recognized.  By default, login
66             is allowed for all users.  If the pattern takes the form
67             USER@HOST then USER and HOST are separately checked, restricting
68             logins to particular users from particular hosts.  The allow/deny
69             directives are processed in the following order: DenyUsers,
70             AllowUsers, DenyGroups, and finally AllowGroups.
71
72             See PATTERNS in ssh_config(5) for more information on patterns.
73
74     AuthorizedKeysFile
75             Specifies the file that contains the public keys that can be used
76             for user authentication.  AuthorizedKeysFile may contain tokens
77             of the form %T which are substituted during connection setup.
78             The following tokens are defined: %% is replaced by a literal
79             '%', %h is replaced by the home directory of the user being
80             authenticated, and %u is replaced by the username of that user.
81             After expansion, AuthorizedKeysFile is taken to be an absolute
82             path or one relative to the user's home directory.  The default
83             is “.ssh/authorized_keys”.
84
85     Banner  The contents of the specified file are sent to the remote user
86             before authentication is allowed.  If the argument is “none” then
87             no banner is displayed.  This option is only available for proto‐
88             col version 2.  By default, no banner is displayed.
89
90     ChallengeResponseAuthentication
91             Specifies whether challenge-response authentication is allowed
92             (e.g. via PAM or though authentication styles supported in
93             login.conf(5)) The default is “yes”.
94
95     ChrootDirectory
96             Specifies the pathname of a directory to chroot(2) to after
97             authentication.  All components of the pathname must be root-
98             owned directories that are not writable by any other user or
99             group.  After the chroot, sshd(8) changes the working directory
100             to the user's home directory.
101
102             The pathname may contain the following tokens that are expanded
103             at runtime once the connecting user has been authenticated: %% is
104             replaced by a literal '%', %h is replaced by the home directory
105             of the user being authenticated, and %u is replaced by the user‐
106             name of that user.
107
108             The ChrootDirectory must contain the necessary files and directo‐
109             ries to support the user's session.  For an interactive session
110             this requires at least a shell, typically sh(1), and basic /dev
111             nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
112             arandom(4) and tty(4) devices.  For file transfer sessions using
113             “sftp”, no additional configuration of the environment is neces‐
114             sary if the in-process sftp server is used, though sessions which
115             use logging do require /dev/log inside the chroot directory (see
116             sftp-server(8) for details).
117
118             The default is not to chroot(2).
119
120     Ciphers
121             Specifies the ciphers allowed for protocol version 2.  Multiple
122             ciphers must be comma-separated.  The supported ciphers are
123             “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”,
124             “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour128”,
125             “arcfour256”, “arcfour”, “blowfish-cbc”, and “cast128-cbc”.  The
126             default is:
127
128                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
129                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
130                aes256-cbc,arcfour
131
132     ClientAliveCountMax
133             Sets the number of client alive messages (see below) which may be
134             sent without sshd(8) receiving any messages back from the client.
135             If this threshold is reached while client alive messages are
136             being sent, sshd will disconnect the client, terminating the ses‐
137             sion.  It is important to note that the use of client alive mes‐
138             sages is very different from TCPKeepAlive (below).  The client
139             alive messages are sent through the encrypted channel and there‐
140             fore will not be spoofable.  The TCP keepalive option enabled by
141             TCPKeepAlive is spoofable.  The client alive mechanism is valu‐
142             able when the client or server depend on knowing when a connec‐
143             tion has become inactive.
144
145             The default value is 3.  If ClientAliveInterval (see below) is
146             set to 15, and ClientAliveCountMax is left at the default, unre‐
147             sponsive SSH clients will be disconnected after approximately 45
148             seconds.  This option applies to protocol version 2 only.
149
150     ClientAliveInterval
151             Sets a timeout interval in seconds after which if no data has
152             been received from the client, sshd(8) will send a message
153             through the encrypted channel to request a response from the
154             client.  The default is 0, indicating that these messages will
155             not be sent to the client.  This option applies to protocol ver‐
156             sion 2 only.
157
158     Compression
159             Specifies whether compression is allowed, or delayed until the
160             user has authenticated successfully.  The argument must be “yes”,
161             “delayed”, or “no”.  The default is “delayed”.
162
163     DenyGroups
164             This keyword can be followed by a list of group name patterns,
165             separated by spaces.  Login is disallowed for users whose primary
166             group or supplementary group list matches one of the patterns.
167             Only group names are valid; a numerical group ID is not recog‐
168             nized.  By default, login is allowed for all groups.  The
169             allow/deny directives are processed in the following order:
170             DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
171
172             See PATTERNS in ssh_config(5) for more information on patterns.
173
174     DenyUsers
175             This keyword can be followed by a list of user name patterns,
176             separated by spaces.  Login is disallowed for user names that
177             match one of the patterns.  Only user names are valid; a numeri‐
178             cal user ID is not recognized.  By default, login is allowed for
179             all users.  If the pattern takes the form USER@HOST then USER and
180             HOST are separately checked, restricting logins to particular
181             users from particular hosts.  The allow/deny directives are pro‐
182             cessed in the following order: DenyUsers, AllowUsers, DenyGroups,
183             and finally AllowGroups.
184
185             See PATTERNS in ssh_config(5) for more information on patterns.
186
187     ForceCommand
188             Forces the execution of the command specified by ForceCommand,
189             ignoring any command supplied by the client and ~/.ssh/rc if
190             present.  The command is invoked by using the user's login shell
191             with the -c option.  This applies to shell, command, or subsystem
192             execution.  It is most useful inside a Match block.  The command
193             originally supplied by the client is available in the
194             SSH_ORIGINAL_COMMAND environment variable.  Specifying a command
195             of “internal-sftp” will force the use of an in-process sftp
196             server that requires no support files when used with
197             ChrootDirectory.
198
199     GatewayPorts
200             Specifies whether remote hosts are allowed to connect to ports
201             forwarded for the client.  By default, sshd(8) binds remote port
202             forwardings to the loopback address.  This prevents other remote
203             hosts from connecting to forwarded ports.  GatewayPorts can be
204             used to specify that sshd should allow remote port forwardings to
205             bind to non-loopback addresses, thus allowing other hosts to con‐
206             nect.  The argument may be “no” to force remote port forwardings
207             to be available to the local host only, “yes” to force remote
208             port forwardings to bind to the wildcard address, or
209             “clientspecified” to allow the client to select the address to
210             which the forwarding is bound.  The default is “no”.
211
212     GSSAPIAuthentication
213             Specifies whether user authentication based on GSSAPI is allowed.
214             The default is “no”.  Note that this option applies to protocol
215             version 2 only.
216
217     GSSAPIKeyExchange
218             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
219             key exchange doesn't rely on ssh keys to verify host identity.
220             The default is “no”.  Note that this option applies to protocol
221             version 2 only.
222
223     GSSAPICleanupCredentials
224             Specifies whether to automatically destroy the user's credentials
225             cache on logout.  The default is “yes”.  Note that this option
226             applies to protocol version 2 only.
227
228     GSSAPIStrictAcceptorCheck
229             Determines whether to be strict about the identity of the GSSAPI
230             acceptor a client authenticates against. If “yes” then the client
231             must authenticate against the host service on the current host‐
232             name. If “no” then the client may authenticate against any ser‐
233             vice key stored in the machine's default store. This facility is
234             provided to assist with operation on multi homed machines.  The
235             default is “yes”.  Note that this option applies only to protocol
236             version 2 GSSAPI connections, and setting it to “no” may only
237             work with recent Kerberos GSSAPI libraries.
238
239     GSSAPIStoreCredentialsOnRekey
240             Controls whether the user's GSSAPI credentials should be updated
241             following a successful connection rekeying. This option can be
242             used to accepted renewed or updated credentials from a compatible
243             client. The default is “no”.
244
245     HostbasedAuthentication
246             Specifies whether rhosts or /etc/hosts.equiv authentication
247             together with successful public key client host authentication is
248             allowed (host-based authentication).  This option is similar to
249             RhostsRSAAuthentication and applies to protocol version 2 only.
250             The default is “no”.
251
252     HostbasedUsesNameFromPacketOnly
253             Specifies whether or not the server will attempt to perform a
254             reverse name lookup when matching the name in the ~/.shosts,
255             ~/.rhosts, and /etc/hosts.equiv files during
256             HostbasedAuthentication.  A setting of “yes” means that sshd(8)
257             uses the name supplied by the client rather than attempting to
258             resolve the name from the TCP connection itself.  The default is
259             “no”.
260
261     HostCertificate
262             Specifies a file containing a public host certificate.  The cer‐
263             tificate's public key must match a private host key already spec‐
264             ified by HostKey.  The default behaviour of sshd(8) is not to
265             load any certificates.
266
267     HostKey
268             Specifies a file containing a private host key used by SSH.  The
269             default is /etc/ssh/ssh_host_key for protocol version 1, and
270             /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro‐
271             tocol version 2.  Note that sshd(8) will refuse to use a file if
272             it is group/world-accessible.  It is possible to have multiple
273             host key files.  “rsa1” keys are used for version 1 and “dsa” or
274             “rsa” are used for version 2 of the SSH protocol.
275
276     IgnoreRhosts
277             Specifies that .rhosts and .shosts files will not be used in
278             RhostsRSAAuthentication or HostbasedAuthentication.
279
280             /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.  The
281             default is “yes”.
282
283     IgnoreUserKnownHosts
284             Specifies whether sshd(8) should ignore the user's
285             ~/.ssh/known_hosts during RhostsRSAAuthentication or
286             HostbasedAuthentication.  The default is “no”.
287
288     KerberosAuthentication
289             Specifies whether the password provided by the user for
290             PasswordAuthentication will be validated through the Kerberos
291             KDC.  To use this option, the server needs a Kerberos servtab
292             which allows the verification of the KDC's identity.  The default
293             is “no”.
294
295     KerberosGetAFSToken
296             If AFS is active and the user has a Kerberos 5 TGT, attempt to
297             acquire an AFS token before accessing the user's home directory.
298             The default is “no”.
299
300     KerberosOrLocalPasswd
301             If password authentication through Kerberos fails then the pass‐
302             word will be validated via any additional local mechanism such as
303             /etc/passwd.  The default is “yes”.
304
305     KerberosTicketCleanup
306             Specifies whether to automatically destroy the user's ticket
307             cache file on logout.  The default is “yes”.
308
309     KerberosUseKuserok
310             Specifies whether to look at .k5login file for user's aliases.
311             The default is “yes”.
312
313     KeyRegenerationInterval
314             In protocol version 1, the ephemeral server key is automatically
315             regenerated after this many seconds (if it has been used).  The
316             purpose of regeneration is to prevent decrypting captured ses‐
317             sions by later breaking into the machine and stealing the keys.
318             The key is never stored anywhere.  If the value is 0, the key is
319             never regenerated.  The default is 3600 (seconds).
320
321     ListenAddress
322             Specifies the local addresses sshd(8) should listen on.  The fol‐
323             lowing forms may be used:
324
325                   ListenAddress host|IPv4_addr|IPv6_addr
326                   ListenAddress host|IPv4_addr:port
327                   ListenAddress [host|IPv6_addr]:port
328
329             If port is not specified, sshd will listen on the address and all
330             prior Port options specified.  The default is to listen on all
331             local addresses.  Multiple ListenAddress options are permitted.
332             Additionally, any Port options must precede this option for non-
333             port qualified addresses.
334
335     LoginGraceTime
336             The server disconnects after this time if the user has not suc‐
337             cessfully logged in.  If the value is 0, there is no time limit.
338             The default is 120 seconds.
339
340     LogLevel
341             Gives the verbosity level that is used when logging messages from
342             sshd(8).  The possible values are: QUIET, FATAL, ERROR, INFO,
343             VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.
344             DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
345             higher levels of debugging output.  Logging with a DEBUG level
346             violates the privacy of users and is not recommended.
347
348     MACs    Specifies the available MAC (message authentication code) algo‐
349             rithms.  The MAC algorithm is used in protocol version 2 for data
350             integrity protection.  Multiple algorithms must be comma-sepa‐
351             rated.  The default is:
352
353                   hmac-md5,hmac-sha1,umac-64@openssh.com,
354                   hmac-ripemd160,hmac-sha1-96,hmac-md5-96
355
356     Match   Introduces a conditional block.  If all of the criteria on the
357             Match line are satisfied, the keywords on the following lines
358             override those set in the global section of the config file,
359             until either another Match line or the end of the file.
360
361             The arguments to Match are one or more criteria-pattern pairs.
362             The available criteria are User, Group, Host, and Address.  The
363             match patterns may consist of single entries or comma-separated
364             lists and may use the wildcard and negation operators described
365             in the PATTERNS section of ssh_config(5).
366
367             The patterns in an Address criteria may additionally contain
368             addresses to match in CIDR address/masklen format, e.g.
369             “192.0.2.0/24” or “3ffe:ffff::/32”.  Note that the mask length
370             provided must be consistent with the address - it is an error to
371             specify a mask length that is too long for the address or one
372             with bits set in this host portion of the address.  For example,
373             “192.0.2.0/33” and “192.0.2.0/8” respectively.
374
375             Only a subset of keywords may be used on the lines following a
376             Match keyword.  Available keywords are AllowAgentForwarding,
377             AllowTcpForwarding, Banner, ChrootDirectory, ForceCommand,
378             GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
379             KbdInteractiveAuthentication, KerberosAuthentication,
380             KerberosUseKuserok, MaxAuthTries, MaxSessions,
381             PubkeyAuthentication, AuthorizedKeysCommand,
382             AuthorizedKeysCommandRunAs, PasswordAuthentication,
383             PermitEmptyPasswords, PermitOpen, PermitRootLogin,
384             PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication,
385             X11DisplayOffset, X11Forwarding and X11UseLocalHost.
386
387     MaxAuthTries
388             Specifies the maximum number of authentication attempts permitted
389             per connection.  Once the number of failures reaches half this
390             value, additional failures are logged.  The default is 6.
391
392     MaxSessions
393             Specifies the maximum number of open sessions permitted per net‐
394             work connection.  The default is 10.
395
396     MaxStartups
397             Specifies the maximum number of concurrent unauthenticated con‐
398             nections to the SSH daemon.  Additional connections will be
399             dropped until authentication succeeds or the LoginGraceTime
400             expires for a connection.  The default is 10.
401
402             Alternatively, random early drop can be enabled by specifying the
403             three colon separated values “start:rate:full” (e.g. "10:30:60").
404             sshd(8) will refuse connection attempts with a probability of
405             “rate/100” (30%) if there are currently “start” (10) unauthenti‐
406             cated connections.  The probability increases linearly and all
407             connection attempts are refused if the number of unauthenticated
408             connections reaches “full” (60).
409
410     PasswordAuthentication
411             Specifies whether password authentication is allowed.  The
412             default is “yes”.
413
414     PermitEmptyPasswords
415             When password authentication is allowed, it specifies whether the
416             server allows login to accounts with empty password strings.  The
417             default is “no”.
418
419     PermitOpen
420             Specifies the destinations to which TCP port forwarding is per‐
421             mitted.  The forwarding specification must be one of the follow‐
422             ing forms:
423
424                   PermitOpen host:port
425                   PermitOpen IPv4_addr:port
426                   PermitOpen [IPv6_addr]:port
427
428             Multiple forwards may be specified by separating them with white‐
429             space.  An argument of “any” can be used to remove all restric‐
430             tions and permit any forwarding requests.  By default all port
431             forwarding requests are permitted.
432
433     PermitRootLogin
434             Specifies whether root can log in using ssh(1).  The argument
435             must be “yes”, “without-password”, “forced-commands-only”, or
436             “no”.  The default is “yes”.
437
438             If this option is set to “without-password”, password authentica‐
439             tion is disabled for root.
440
441             If this option is set to “forced-commands-only”, root login with
442             public key authentication will be allowed, but only if the
443             command option has been specified (which may be useful for taking
444             remote backups even if root login is normally not allowed).  All
445             other authentication methods are disabled for root.
446
447             If this option is set to “no”, root is not allowed to log in.
448
449     PermitTunnel
450             Specifies whether tun(4) device forwarding is allowed.  The argu‐
451             ment must be “yes”, “point-to-point” (layer 3), “ethernet” (layer
452             2), or “no”.  Specifying “yes” permits both “point-to-point” and
453             “ethernet”.  The default is “no”.
454
455     PermitUserEnvironment
456             Specifies whether ~/.ssh/environment and environment= options in
457             ~/.ssh/authorized_keys are processed by sshd(8).  The default is
458             “no”.  Enabling environment processing may enable users to bypass
459             access restrictions in some configurations using mechanisms such
460             as LD_PRELOAD.
461
462     PidFile
463             Specifies the file that contains the process ID of the SSH dae‐
464             mon.  The default is /var/run/sshd.pid.
465
466     Port    Specifies the port number that sshd(8) listens on.  The default
467             is 22.  Multiple options of this type are permitted.  See also
468             ListenAddress.
469
470     PrintLastLog
471             Specifies whether sshd(8) should print the date and time of the
472             last user login when a user logs in interactively.  The default
473             is “yes”.
474
475     PrintMotd
476             Specifies whether sshd(8) should print /etc/motd when a user logs
477             in interactively.  (On some systems it is also printed by the
478             shell, /etc/profile, or equivalent.)  The default is “yes”.
479
480     Protocol
481             Specifies the protocol versions sshd(8) supports.  The possible
482             values are ‘1’ and ‘2’.  Multiple versions must be comma-sepa‐
483             rated.  The default is ‘2’.  Note that the order of the protocol
484             list does not indicate preference, because the client selects
485             among multiple protocol versions offered by the server.  Specify‐
486             ing “2,1” is identical to “1,2”.
487
488     PubkeyAuthentication
489             Specifies whether public key authentication is allowed.  The
490             default is “yes”.  Note that this option applies to protocol ver‐
491             sion 2 only.
492
493     RevokedKeys
494             Specifies a list of revoked public keys.  Keys listed in this
495             file will be refused for public key authentication.  Note that if
496             this file is not readable, then public key authentication will be
497             refused for all users.
498
499     AuthorizedKeysCommand
500             Specifies a program to be used for lookup of the user's public
501             keys.  The program will be invoked with its first argument the
502             name of the user being authorized, and should produce on standard
503             output AuthorizedKeys lines (see AUTHORIZED_KEYS in sshd(8)).  By
504             default (or when set to the empty string) there is no Authorized‐
505             KeysCommand run.  If the AuthorizedKeysCommand does not success‐
506             fully authorize the user, authorization falls through to the
507             AuthorizedKeysFile.  Note that this option has an effect only
508             with PubkeyAuthentication turned on.
509
510     AuthorizedKeysCommandRunAs
511             Specifies the user under whose account the AuthorizedKeysCommand
512             is run. Empty string (the default value) means the user being
513             authorized is used.  “”
514
515     RhostsRSAAuthentication
516             Specifies whether rhosts or /etc/hosts.equiv authentication
517             together with successful RSA host authentication is allowed.  The
518             default is “no”.  This option applies to protocol version 1 only.
519
520     RSAAuthentication
521             Specifies whether pure RSA authentication is allowed.  The
522             default is “yes”.  This option applies to protocol version 1
523             only.
524
525     ServerKeyBits
526             Defines the number of bits in the ephemeral protocol version 1
527             server key.  The minimum value is 512, and the default is 1024.
528
529     ShowPatchLevel
530             Specifies whether sshd will display the patch level of the binary
531             in the identification string.  The patch level is set at compile-
532             time.  The default is “no”.  This option applies to protocol ver‐
533             sion 1 only.
534
535     StrictModes
536             Specifies whether sshd(8) should check file modes and ownership
537             of the user's files and home directory before accepting login.
538             This is normally desirable because novices sometimes accidentally
539             leave their directory or files world-writable.  The default is
540             “yes”.  Note that this does not apply to ChrootDirectory, whose
541             permissions and ownership are checked unconditionally.
542
543     Subsystem
544             Configures an external subsystem (e.g. file transfer daemon).
545             Arguments should be a subsystem name and a command (with optional
546             arguments) to execute upon subsystem request.
547
548             The command sftp-server(8) implements the “sftp” file transfer
549             subsystem.
550
551             Alternately the name “internal-sftp” implements an in-process
552             “sftp” server.  This may simplify configurations using
553             ChrootDirectory to force a different filesystem root on clients.
554
555             By default no subsystems are defined.  Note that this option
556             applies to protocol version 2 only.
557
558     SyslogFacility
559             Gives the facility code that is used when logging messages from
560             sshd(8).  The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
561             LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
562             The default is AUTH.
563
564     TCPKeepAlive
565             Specifies whether the system should send TCP keepalive messages
566             to the other side.  If they are sent, death of the connection or
567             crash of one of the machines will be properly noticed.  However,
568             this means that connections will die if the route is down tempo‐
569             rarily, and some people find it annoying.  On the other hand, if
570             TCP keepalives are not sent, sessions may hang indefinitely on
571             the server, leaving “ghost” users and consuming server resources.
572
573             The default is “yes” (to send TCP keepalive messages), and the
574             server will notice if the network goes down or the client host
575             crashes.  This avoids infinitely hanging sessions.
576
577             To disable TCP keepalive messages, the value should be set to
578             “no”.
579
580     TrustedUserCAKeys
581             Specifies a file containing public keys of certificate authori‐
582             ties that are trusted to sign user certificates for authentica‐
583             tion.  Keys are listed one per line; empty lines and comments
584             starting with ‘#’ are allowed.  If a certificate is presented for
585             authentication and has its signing CA key listed in this file,
586             then it may be used for authentication for any user listed in the
587             certificate's principals list.  Note that certificates that lack
588             a list of principals will not be permitted for authentication
589             using TrustedUserCAKeys.  For more details on certificates, see
590             the CERTIFICATES section in ssh-keygen(1).
591
592     UseDNS  Specifies whether sshd(8) should look up the remote host name and
593             check that the resolved host name for the remote IP address maps
594             back to the very same IP address.  The default is “yes”.
595
596     UseLogin
597             Specifies whether login(1) is used for interactive login ses‐
598             sions.  The default is “no”.  Note that login(1) is never used
599             for remote command execution.  Note also, that if this is
600             enabled, X11Forwarding will be disabled because login(1) does not
601             know how to handle xauth(1) cookies.  If UsePrivilegeSeparation
602             is specified, it will be disabled after authentication.
603
604     UsePAM  Enables the Pluggable Authentication Module interface.  If set to
605             “yes” this will enable PAM authentication using
606             ChallengeResponseAuthentication and PasswordAuthentication in
607             addition to PAM account and session module processing for all
608             authentication types.
609
610             Because PAM challenge-response authentication usually serves an
611             equivalent role to password authentication, you should disable
612             either PasswordAuthentication or ChallengeResponseAuthentication.
613
614             If UsePAM is enabled, you will not be able to run sshd(8) as a
615             non-root user.  The default is “no”.
616
617     UsePrivilegeSeparation
618             Specifies whether sshd(8) separates privileges by creating an
619             unprivileged child process to deal with incoming network traffic.
620             After successful authentication, another process will be created
621             that has the privilege of the authenticated user.  The goal of
622             privilege separation is to prevent privilege escalation by con‐
623             taining any corruption within the unprivileged processes.  The
624             default is “yes”.
625
626     X11DisplayOffset
627             Specifies the first display number available for sshd(8)'s X11
628             forwarding.  This prevents sshd from interfering with real X11
629             servers.  The default is 10.
630
631     X11Forwarding
632             Specifies whether X11 forwarding is permitted.  The argument must
633             be “yes” or “no”.  The default is “no”.
634
635             When X11 forwarding is enabled, there may be additional exposure
636             to the server and to client displays if the sshd(8) proxy display
637             is configured to listen on the wildcard address (see
638             X11UseLocalhost below), though this is not the default.  Addi‐
639             tionally, the authentication spoofing and authentication data
640             verification and substitution occur on the client side.  The
641             security risk of using X11 forwarding is that the client's X11
642             display server may be exposed to attack when the SSH client
643             requests forwarding (see the warnings for ForwardX11 in
644             ssh_config(5)).  A system administrator may have a stance in
645             which they want to protect clients that may expose themselves to
646             attack by unwittingly requesting X11 forwarding, which can war‐
647             rant a “no” setting.
648
649             Note that disabling X11 forwarding does not prevent users from
650             forwarding X11 traffic, as users can always install their own
651             forwarders.  X11 forwarding is automatically disabled if UseLogin
652             is enabled.
653
654     X11UseLocalhost
655             Specifies whether sshd(8) should bind the X11 forwarding server
656             to the loopback address or to the wildcard address.  By default,
657             sshd binds the forwarding server to the loopback address and sets
658             the hostname part of the DISPLAY environment variable to
659             “localhost”.  This prevents remote hosts from connecting to the
660             proxy display.  However, some older X11 clients may not function
661             with this configuration.  X11UseLocalhost may be set to “no” to
662             specify that the forwarding server should be bound to the wild‐
663             card address.  The argument must be “yes” or “no”.  The default
664             is “yes”.
665
666     XAuthLocation
667             Specifies the full pathname of the xauth(1) program.  The default
668             is /usr/bin/xauth.
669

TIME FORMATS

671     sshd(8) command-line arguments and configuration file options that spec‐
672     ify time may be expressed using a sequence of the form: time[qualifier],
673     where time is a positive integer value and qualifier is one of the fol‐
674     lowing:
675
676none⟩  seconds
677           s | S   seconds
678           m | M   minutes
679           h | H   hours
680           d | D   days
681           w | W   weeks
682
683     Each member of the sequence is added together to calculate the total time
684     value.
685
686     Time format examples:
687
688           600     600 seconds (10 minutes)
689           10m     10 minutes
690           1h30m   1 hour 30 minutes (90 minutes)
691

FILES

693     /etc/ssh/sshd_config
694             Contains configuration data for sshd(8).  This file should be
695             writable by root only, but it is recommended (though not neces‐
696             sary) that it be world-readable.
697

SEE ALSO

699     sshd(8)
700

AUTHORS

702     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
703     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
704     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
705     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
706     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
707     for privilege separation.
708
709BSD                              June 22, 2019                             BSD
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