1 SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5)
4 sshd_config - OpenSSH SSH daemon configuration file
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-
12 argument pairs, one per line. Lines starting with `#' and empty lines
13 are interpreted as comments. Arguments may optionally be enclosed in
14 double quotes (") in order to represent arguments containing spaces.
16 The possible keywords and their meanings are as follows (note that
17 keywords are case-insensitive and arguments are case-sensitive):
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
23 environment passing is only supported for protocol 2. Variables
24 are 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.
33 Specifies which address family should be used by sshd(8). Valid
34 arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
35 (use IPv6 only). The default is ``any''.
38 Specifies whether ssh-agent(1) forwarding is permitted. The
39 default is ``yes''. Note that disabling agent forwarding does
40 not improve security unless users are also denied shell access,
41 as they can always install their own forwarders.
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
53 See PATTERNS in ssh_config(5) for more information on patterns.
56 Specifies whether TCP forwarding is permitted. The available
57 options are ``yes'' or ``all'' to allow TCP forwarding, ``no'' to
58 prevent all TCP forwarding, ``local'' to allow local (from the
59 perspective of ssh(1)) forwarding only or ``remote'' to allow
60 remote forwarding only. The default is ``yes''. Note that
61 disabling TCP forwarding does not improve security unless users
62 are also denied shell access, as they can always install their
66 This keyword can be followed by a list of user name patterns,
67 separated by spaces. If specified, login is allowed only for
68 user names that match one of the patterns. Only user names are
69 valid; a numerical user ID is not recognized. By default, login
70 is allowed for all users. If the pattern takes the form
71 USER@HOST then USER and HOST are separately checked, restricting
72 logins to particular users from particular hosts. The allow/deny
73 directives are processed in the following order: DenyUsers,
74 AllowUsers, DenyGroups, and finally AllowGroups.
76 See PATTERNS in ssh_config(5) for more information on patterns.
79 Specifies the authentication methods that must be successfully
80 completed for a user to be granted access. This option must be
81 followed by one or more comma-separated lists of authentication
82 method names. Successful authentication requires completion of
83 every method in at least one of these lists.
85 For example, an argument of ``publickey,password
86 publickey,keyboard-interactive'' would require the user to
87 complete public key authentication, followed by either password
88 or keyboard interactive authentication. Only methods that are
89 next in one or more lists are offered at each stage, so for this
90 example, it would not be possible to attempt password or
91 keyboard-interactive authentication before public key.
93 For keyboard interactive authentication it is also possible to
94 restrict authentication to a specific device by appending a colon
95 followed by the device identifier ``bsdauth'', ``pam'', or
96 ``skey'', depending on the server configuration. For example,
97 ``keyboard-interactive:bsdauth'' would restrict keyboard
98 interactive authentication to the ``bsdauth'' device.
100 This option is only available for SSH protocol 2 and will yield a
101 fatal error if enabled if protocol 1 is also enabled. Note that
102 each authentication method listed should also be explicitly
103 enabled in the configuration. The default is not to require
104 multiple authentication; successful completion of a single
105 authentication method is sufficient.
107 AuthorizedKeysCommand
108 Specifies a program to be used to look up the user's public keys.
109 The program must be owned by root and not writable by group or
110 others. It will be invoked with a single argument of the
111 username being authenticated, and should produce on standard
112 output zero or more lines of authorized_keys output (see
113 AUTHORIZED_KEYS in sshd(8)). If a key supplied by
114 AuthorizedKeysCommand does not successfully authenticate and
115 authorize the user then public key authentication continues using
116 the usual AuthorizedKeysFile files. By default, no
117 AuthorizedKeysCommand is run.
119 AuthorizedKeysCommandUser
120 Specifies the user under whose account the AuthorizedKeysCommand
121 is run. It is recommended to use a dedicated user that has no
122 other role on the host than running authorized keys commands.
125 Specifies the file that contains the public keys that can be used
126 for user authentication. The format is described in the
127 AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
128 AuthorizedKeysFile may contain tokens of the form %T which are
129 substituted during connection setup. The following tokens are
130 defined: %% is replaced by a literal '%', %h is replaced by the
131 home directory of the user being authenticated, and %u is
132 replaced by the username of that user. After expansion,
133 AuthorizedKeysFile is taken to be an absolute path or one
134 relative to the user's home directory. Multiple files may be
135 listed, separated by whitespace. The default is
136 ``.ssh/authorized_keys .ssh/authorized_keys2''.
138 AuthorizedPrincipalsFile
139 Specifies a file that lists principal names that are accepted for
140 certificate authentication. When using certificates signed by a
141 key listed in TrustedUserCAKeys, this file lists names, one of
142 which must appear in the certificate for it to be accepted for
143 authentication. Names are listed one per line preceded by key
144 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
145 Empty lines and comments starting with `#' are ignored.
147 AuthorizedPrincipalsFile may contain tokens of the form %T which
148 are substituted during connection setup. The following tokens
149 are defined: %% is replaced by a literal '%', %h is replaced by
150 the home directory of the user being authenticated, and %u is
151 replaced by the username of that user. After expansion,
152 AuthorizedPrincipalsFile is taken to be an absolute path or one
153 relative to the user's home directory.
155 The default is ``none'', i.e. not to use a principals file - in
156 this case, the username of the user must appear in a
157 certificate's principals list for it to be accepted. Note that
158 AuthorizedPrincipalsFile is only used when authentication
159 proceeds using a CA listed in TrustedUserCAKeys and is not
160 consulted for certification authorities trusted via
161 ~/.ssh/authorized_keys, though the principals= key option offers
162 a similar facility (see sshd(8) for details).
164 Banner The contents of the specified file are sent to the remote user
165 before authentication is allowed. If the argument is ``none''
166 then no banner is displayed. This option is only available for
167 protocol version 2. By default, no banner is displayed.
169 ChallengeResponseAuthentication
170 Specifies whether challenge-response authentication is allowed
171 (e.g. via PAM or though authentication styles supported in
172 login.conf(5)) The default is ``yes''.
175 Specifies the pathname of a directory to chroot(2) to after
176 authentication. All components of the pathname must be root-
177 owned directories that are not writable by any other user or
178 group. After the chroot, sshd(8) changes the working directory
179 to the user's home directory.
181 The pathname may contain the following tokens that are expanded
182 at runtime once the connecting user has been authenticated: %% is
183 replaced by a literal '%', %h is replaced by the home directory
184 of the user being authenticated, and %u is replaced by the
185 username of that user.
187 The ChrootDirectory must contain the necessary files and
188 directories to support the user's session. For an interactive
189 session this requires at least a shell, typically sh(1), and
190 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
191 stderr(4), arandom(4) and tty(4) devices. For file transfer
192 sessions using ``sftp'', no additional configuration of the
193 environment is necessary if the in-process sftp server is used,
194 though sessions which use logging do require /dev/log inside the
195 chroot directory (see sftp-server(8) for details).
197 In the special case when only sftp is used, not ssh nor scp, it
198 is possible to use ChrootDirectory %h or ChrootDirectory
199 /some/path/%u. The file system containing this directory must be
200 mounted with options nodev and either nosuid or noexec. The owner
201 of the directory should be the user. The ownership of the other
202 components of the path must fulfill the usual conditions. No adi-
203 tional files are required to be present in the directory.
205 The default is not to chroot(2).
208 Specifies the ciphers allowed for protocol version 2. Multiple
209 ciphers must be comma-separated. The supported ciphers are:
211 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
212 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
213 ``aes128-gcm@openssh.com'', ``aes256-gcm@openssh.com'',
214 ``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
215 ``cast128-cbc'', and ``chacha20-poly1305@openssh.com''.
219 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
220 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
221 chacha20-poly1305@openssh.com,
222 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
225 The list of available ciphers may also be obtained using the -Q
229 Sets the number of client alive messages (see below) which may be
230 sent without sshd(8) receiving any messages back from the client.
231 If this threshold is reached while client alive messages are
232 being sent, sshd will disconnect the client, terminating the
233 session. It is important to note that the use of client alive
234 messages is very different from TCPKeepAlive (below). The client
235 alive messages are sent through the encrypted channel and
236 therefore will not be spoofable. The TCP keepalive option
237 enabled by TCPKeepAlive is spoofable. The client alive mechanism
238 is valuable when the client or server depend on knowing when a
239 connection has become inactive.
241 The default value is 3. If ClientAliveInterval (see below) is
242 set to 15, and ClientAliveCountMax is left at the default,
243 unresponsive SSH clients will be disconnected after approximately
244 45 seconds. This option applies to protocol version 2 only.
247 Sets a timeout interval in seconds after which if no data has
248 been received from the client, sshd(8) will send a message
249 through the encrypted channel to request a response from the
250 client. The default is 0, indicating that these messages will
251 not be sent to the client. This option applies to protocol
255 Specifies whether compression is allowed, or delayed until the
256 user has authenticated successfully. The argument must be
257 ``yes'', ``delayed'', or ``no''. The default is ``delayed''.
260 This keyword can be followed by a list of group name patterns,
261 separated by spaces. Login is disallowed for users whose primary
262 group or supplementary group list matches one of the patterns.
263 Only group names are valid; a numerical group ID is not
264 recognized. By default, login is allowed for all groups. The
265 allow/deny directives are processed in the following order:
266 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
268 See PATTERNS in ssh_config(5) for more information on patterns.
271 This keyword can be followed by a list of user name patterns,
272 separated by spaces. Login is disallowed for user names that
273 match one of the patterns. Only user names are valid; a
274 numerical user ID is not recognized. By default, login is
275 allowed for all users. If the pattern takes the form USER@HOST
276 then USER and HOST are separately checked, restricting logins to
277 particular users from particular hosts. The allow/deny
278 directives are processed in the following order: DenyUsers,
279 AllowUsers, DenyGroups, and finally AllowGroups.
281 See PATTERNS in ssh_config(5) for more information on patterns.
284 Forces the execution of the command specified by ForceCommand,
285 ignoring any command supplied by the client and ~/.ssh/rc if
286 present. The command is invoked by using the user's login shell
287 with the -c option. This applies to shell, command, or subsystem
288 execution. It is most useful inside a Match block. The command
289 originally supplied by the client is available in the
290 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
291 of ``internal-sftp'' will force the use of an in-process sftp
292 server that requires no support files when used with
296 Specifies whether remote hosts are allowed to connect to ports
297 forwarded for the client. By default, sshd(8) binds remote port
298 forwardings to the loopback address. This prevents other remote
299 hosts from connecting to forwarded ports. GatewayPorts can be
300 used to specify that sshd should allow remote port forwardings to
301 bind to non-loopback addresses, thus allowing other hosts to
302 connect. The argument may be ``no'' to force remote port
303 forwardings to be available to the local host only, ``yes'' to
304 force remote port forwardings to bind to the wildcard address, or
305 ``clientspecified'' to allow the client to select the address to
306 which the forwarding is bound. The default is ``no''.
309 Specifies whether user authentication based on GSSAPI is allowed.
310 The default is ``no''. Note that this option applies to protocol
313 GSSAPICleanupCredentials
314 Specifies whether to automatically destroy the user's credentials
315 cache on logout. The default is ``yes''. Note that this option
316 applies to protocol version 2 only.
318 HostbasedAuthentication
319 Specifies whether rhosts or /etc/hosts.equiv authentication
320 together with successful public key client host authentication is
321 allowed (host-based authentication). This option is similar to
322 RhostsRSAAuthentication and applies to protocol version 2 only.
323 The default is ``no''.
325 HostbasedUsesNameFromPacketOnly
326 Specifies whether or not the server will attempt to perform a
327 reverse name lookup when matching the name in the ~/.shosts,
328 ~/.rhosts, and /etc/hosts.equiv files during
329 HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
330 uses the name supplied by the client rather than attempting to
331 resolve the name from the TCP connection itself. The default is
335 Specifies a file containing a public host certificate. The
336 certificate's public key must match a private host key already
337 specified by HostKey. The default behaviour of sshd(8) is not to
338 load any certificates.
341 Specifies a file containing a private host key used by SSH. The
342 default is /etc/ssh/ssh_host_key for protocol version 1, and
343 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
344 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
345 protocol version 2. Note that sshd(8) will refuse to use a file
346 if it is group/world-accessible. It is possible to have multiple
347 host key files. ``rsa1'' keys are used for version 1 and
348 ``dsa'', ``ecdsa'', ``ed25519'' or ``rsa'' are used for version 2
349 of the SSH protocol. It is also possible to specify public host
350 key files instead. In this case operations on the private key
351 will be delegated to an ssh-agent(1).
354 Identifies the UNIX-domain socket used to communicate with an
355 agent that has access to the private host keys. If
356 ``SSH_AUTH_SOCK'' is specified, the location of the socket will
357 be read from the SSH_AUTH_SOCK environment variable.
360 Specifies that .rhosts and .shosts files will not be used in
361 RhostsRSAAuthentication or HostbasedAuthentication.
363 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
367 Specifies whether sshd(8) should ignore the user's
368 ~/.ssh/known_hosts during RhostsRSAAuthentication or
369 HostbasedAuthentication. The default is ``no''.
371 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
372 connection. Accepted values are ``af11'', ``af12'', ``af13'',
373 ``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'',
374 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'',
375 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
376 ``throughput'', ``reliability'', or a numeric value. This option
377 may take one or two arguments, separated by whitespace. If one
378 argument is specified, it is used as the packet class
379 unconditionally. If two values are specified, the first is
380 automatically selected for interactive sessions and the second
381 for non-interactive sessions. The default is ``lowdelay'' for
382 interactive sessions and ``throughput'' for non-interactive
385 KbdInteractiveAuthentication
386 Specifies whether to allow keyboard-interactive authentication.
387 The argument to this keyword must be ``yes'' or ``no''. The
388 default is to use whatever value ChallengeResponseAuthentication
389 is set to (by default ``yes'').
391 KerberosAuthentication
392 Specifies whether the password provided by the user for
393 PasswordAuthentication will be validated through the Kerberos
394 KDC. To use this option, the server needs a Kerberos servtab
395 which allows the verification of the KDC's identity. The default
399 If AFS is active and the user has a Kerberos 5 TGT, attempt to
400 acquire an AFS token before accessing the user's home directory.
401 The default is ``no''.
403 KerberosOrLocalPasswd
404 If password authentication through Kerberos fails then the
405 password will be validated via any additional local mechanism
406 such as /etc/passwd. The default is ``yes''.
408 KerberosTicketCleanup
409 Specifies whether to automatically destroy the user's ticket
410 cache file on logout. The default is ``yes''.
413 Specifies the available KEX (Key Exchange) algorithms. Multiple
414 algorithms must be comma-separated. The default is
416 curve25519-sha256@libssh.org,
417 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
418 diffie-hellman-group-exchange-sha256,
419 diffie-hellman-group-exchange-sha1,
420 diffie-hellman-group14-sha1,
421 diffie-hellman-group1-sha1
423 KeyRegenerationInterval
424 In protocol version 1, the ephemeral server key is automatically
425 regenerated after this many seconds (if it has been used). The
426 purpose of regeneration is to prevent decrypting captured
427 sessions by later breaking into the machine and stealing the
428 keys. The key is never stored anywhere. If the value is 0, the
429 key is never regenerated. The default is 3600 (seconds).
432 Specifies the local addresses sshd(8) should listen on. The
433 following forms may be used:
435 ListenAddress host|IPv4_addr|IPv6_addr
436 ListenAddress host|IPv4_addr:port
437 ListenAddress [host|IPv6_addr]:port
439 If port is not specified, sshd will listen on the address and all
440 prior Port options specified. The default is to listen on all
441 local addresses. Multiple ListenAddress options are permitted.
442 Additionally, any Port options must precede this option for non-
443 port qualified addresses.
446 The server disconnects after this time if the user has not
447 successfully logged in. If the value is 0, there is no time
448 limit. The default is 120 seconds.
451 Gives the verbosity level that is used when logging messages from
452 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
453 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
454 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
455 higher levels of debugging output. Logging with a DEBUG level
456 violates the privacy of users and is not recommended.
458 MACs Specifies the available MAC (message authentication code)
459 algorithms. The MAC algorithm is used in protocol version 2 for
460 data integrity protection. Multiple algorithms must be comma-
461 separated. The algorithms that contain ``-etm'' calculate the
462 MAC after encryption (encrypt-then-mac). These are considered
463 safer and their use recommended. The default is:
465 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
466 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
467 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
468 hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
469 hmac-md5-96-etm@openssh.com,
470 hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
471 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
472 hmac-sha1-96,hmac-md5-96
474 Match Introduces a conditional block. If all of the criteria on the
475 Match line are satisfied, the keywords on the following lines
476 override those set in the global section of the config file,
477 until either another Match line or the end of the file. If a
478 keyword appears in multiple Match blocks that are satisified,
479 only the first instance of the keyword is applied.
481 The arguments to Match are one or more criteria-pattern pairs or
482 the single token All which matches all criteria. The available
483 criteria are User, Group, Host, LocalAddress, LocalPort, and
484 Address. The match patterns may consist of single entries or
485 comma-separated lists and may use the wildcard and negation
486 operators described in the PATTERNS section of ssh_config(5).
488 The patterns in an Address criteria may additionally contain
489 addresses to match in CIDR address/masklen format, e.g.
490 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
491 length provided must be consistent with the address - it is an
492 error to specify a mask length that is too long for the address
493 or one with bits set in this host portion of the address. For
494 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
496 Only a subset of keywords may be used on the lines following a
497 Match keyword. Available keywords are AcceptEnv,
498 AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
499 AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
500 AuthorizedKeysCommandUser, AuthorizedKeysFile,
501 AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
502 DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
503 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
504 KbdInteractiveAuthentication, KerberosAuthentication,
505 MaxAuthTries, MaxSessions, PasswordAuthentication,
506 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
507 PermitTunnel, PubkeyAuthentication, RekeyLimit,
508 RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
509 X11Forwarding and X11UseLocalHost.
512 Specifies the maximum number of authentication attempts permitted
513 per connection. Once the number of failures reaches half this
514 value, additional failures are logged. The default is 6.
517 Specifies the maximum number of open sessions permitted per
518 network connection. The default is 10.
521 Specifies the maximum number of concurrent unauthenticated
522 connections to the SSH daemon. Additional connections will be
523 dropped until authentication succeeds or the LoginGraceTime
524 expires for a connection. The default is 10:30:100.
526 Alternatively, random early drop can be enabled by specifying the
527 three colon separated values ``start:rate:full'' (e.g.
528 "10:30:60"). sshd(8) will refuse connection attempts with a
529 probability of ``rate/100'' (30%) if there are currently
530 ``start'' (10) unauthenticated connections. The probability
531 increases linearly and all connection attempts are refused if the
532 number of unauthenticated connections reaches ``full'' (60).
534 PasswordAuthentication
535 Specifies whether password authentication is allowed. The
539 When password authentication is allowed, it specifies whether the
540 server allows login to accounts with empty password strings. The
544 Specifies the destinations to which TCP port forwarding is
545 permitted. The forwarding specification must be one of the
549 PermitOpen IPv4_addr:port
550 PermitOpen [IPv6_addr]:port
552 Multiple forwards may be specified by separating them with
553 whitespace. An argument of ``any'' can be used to remove all
554 restrictions and permit any forwarding requests. An argument of
555 ``none'' can be used to prohibit all forwarding requests. By
556 default all port forwarding requests are permitted.
559 Specifies whether root can log in using ssh(1). The argument
560 must be ``yes'', ``without-password'', ``forced-commands-only'',
561 or ``no''. The default is ``yes''.
563 If this option is set to ``without-password'', password
564 authentication is disabled for root.
566 If this option is set to ``forced-commands-only'', root login
567 with public key authentication will be allowed, but only if the
568 command option has been specified (which may be useful for taking
569 remote backups even if root login is normally not allowed). All
570 other authentication methods are disabled for root.
572 If this option is set to ``no'', root is not allowed to log in.
575 Specifies whether tun(4) device forwarding is allowed. The
576 argument must be ``yes'', ``point-to-point'' (layer 3),
577 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits
578 both ``point-to-point'' and ``ethernet''. The default is ``no''.
581 Specifies whether pty(4) allocation is permitted. The default is
584 PermitUserEnvironment
585 Specifies whether ~/.ssh/environment and environment= options in
586 ~/.ssh/authorized_keys are processed by sshd(8). The default is
587 ``no''. Enabling environment processing may enable users to
588 bypass access restrictions in some configurations using
589 mechanisms such as LD_PRELOAD.
592 Specifies the file that contains the process ID of the SSH
593 daemon. The default is /var/run/sshd.pid.
595 Port Specifies the port number that sshd(8) listens on. The default
596 is 22. Multiple options of this type are permitted. See also
600 Specifies whether sshd(8) should print the date and time of the
601 last user login when a user logs in interactively. The default
605 Specifies whether sshd(8) should print /etc/motd when a user logs
606 in interactively. (On some systems it is also printed by the
607 shell, /etc/profile, or equivalent.) The default is ``yes''.
610 Specifies the protocol versions sshd(8) supports. The possible
611 values are `1' and `2'. Multiple versions must be comma-
612 separated. The default is `2'. Note that the order of the
613 protocol list does not indicate preference, because the client
614 selects among multiple protocol versions offered by the server.
615 Specifying ``2,1'' is identical to ``1,2''.
618 Specifies whether public key authentication is allowed. The
619 default is ``yes''. Note that this option applies to protocol
623 Specifies the maximum amount of data that may be transmitted
624 before the session key is renegotiated, optionally followed a
625 maximum amount of time that may pass before the session key is
626 renegotiated. The first argument is specified in bytes and may
627 have a suffix of `K', `M', or `G' to indicate Kilobytes,
628 Megabytes, or Gigabytes, respectively. The default is between
629 `1G' and `4G', depending on the cipher. The optional second
630 value is specified in seconds and may use any of the units
631 documented in the TIME FORMATS section. The default value for
632 RekeyLimit is ``default none'', which means that rekeying is
633 performed after the cipher's default amount of data has been sent
634 or received and no time based rekeying is done. This option
635 applies to protocol version 2 only.
638 Specifies revoked public keys. Keys listed in this file will be
639 refused for public key authentication. Note that if this file is
640 not readable, then public key authentication will be refused for
641 all users. Keys may be specified as a text file, listing one
642 public key per line, or as an OpenSSH Key Revocation List (KRL)
643 as generated by ssh-keygen(1). For more information on KRLs, see
644 the KEY REVOCATION LISTS section in ssh-keygen(1).
646 RhostsRSAAuthentication
647 Specifies whether rhosts or /etc/hosts.equiv authentication
648 together with successful RSA host authentication is allowed. The
649 default is ``no''. This option applies to protocol version 1
653 Specifies whether pure RSA authentication is allowed. The
654 default is ``yes''. This option applies to protocol version 1
658 Defines the number of bits in the ephemeral protocol version 1
659 server key. The minimum value is 512, and the default is 1024.
662 Specifies whether sshd(8) should check file modes and ownership
663 of the user's files and home directory before accepting login.
664 This is normally desirable because novices sometimes accidentally
665 leave their directory or files world-writable. The default is
666 ``yes''. Note that this does not apply to ChrootDirectory, whose
667 permissions and ownership are checked unconditionally.
670 Configures an external subsystem (e.g. file transfer daemon).
671 Arguments should be a subsystem name and a command (with optional
672 arguments) to execute upon subsystem request.
674 The command sftp-server(8) implements the ``sftp'' file transfer
677 Alternately the name ``internal-sftp'' implements an in-process
678 ``sftp'' server. This may simplify configurations using
679 ChrootDirectory to force a different filesystem root on clients.
681 By default no subsystems are defined. Note that this option
682 applies to protocol version 2 only.
685 Gives the facility code that is used when logging messages from
686 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
687 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
691 Specifies whether the system should send TCP keepalive messages
692 to the other side. If they are sent, death of the connection or
693 crash of one of the machines will be properly noticed. However,
694 this means that connections will die if the route is down
695 temporarily, and some people find it annoying. On the other
696 hand, if TCP keepalives are not sent, sessions may hang
697 indefinitely on the server, leaving ``ghost'' users and consuming
700 The default is ``yes'' (to send TCP keepalive messages), and the
701 server will notice if the network goes down or the client host
702 crashes. This avoids infinitely hanging sessions.
704 To disable TCP keepalive messages, the value should be set to
708 Specifies a file containing public keys of certificate
709 authorities that are trusted to sign user certificates for
710 authentication. Keys are listed one per line; empty lines and
711 comments starting with `#' are allowed. If a certificate is
712 presented for authentication and has its signing CA key listed in
713 this file, then it may be used for authentication for any user
714 listed in the certificate's principals list. Note that
715 certificates that lack a list of principals will not be permitted
716 for authentication using TrustedUserCAKeys. For more details on
717 certificates, see the CERTIFICATES section in ssh-keygen(1).
719 UseDNS Specifies whether sshd(8) should look up the remote host name and
720 check that the resolved host name for the remote IP address maps
721 back to the very same IP address. The default is ``yes''.
724 Specifies whether login(1) is used for interactive login
725 sessions. The default is ``no''. Note that login(1) is never
726 used for remote command execution. Note also, that if this is
727 enabled, X11Forwarding will be disabled because login(1) does not
728 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
729 is specified, it will be disabled after authentication.
731 UsePAM Enables the Pluggable Authentication Module interface. If set to
732 ``yes'' this will enable PAM authentication using
733 ChallengeResponseAuthentication and PasswordAuthentication in
734 addition to PAM account and session module processing for all
735 authentication types.
737 Because PAM challenge-response authentication usually serves an
738 equivalent role to password authentication, you should disable
739 either PasswordAuthentication or ChallengeResponseAuthentication.
741 If UsePAM is enabled, you will not be able to run sshd(8) as a
742 non-root user. The default is ``no''.
744 UsePrivilegeSeparation
745 Specifies whether sshd(8) separates privileges by creating an
746 unprivileged child process to deal with incoming network traffic.
747 After successful authentication, another process will be created
748 that has the privilege of the authenticated user. The goal of
749 privilege separation is to prevent privilege escalation by
750 containing any corruption within the unprivileged processes. The
751 default is ``yes''. If UsePrivilegeSeparation is set to
752 ``sandbox'' then the pre-authentication unprivileged process is
753 subject to additional restrictions.
756 Optionally specifies additional text to append to the SSH
757 protocol banner sent by the server upon connection. The default
761 Specifies the first display number available for sshd(8)'s X11
762 forwarding. This prevents sshd from interfering with real X11
763 servers. The default is 10.
766 Specifies whether X11 forwarding is permitted. The argument must
767 be ``yes'' or ``no''. The default is ``no''.
769 When X11 forwarding is enabled, there may be additional exposure
770 to the server and to client displays if the sshd(8) proxy display
771 is configured to listen on the wildcard address (see
772 X11UseLocalhost below), though this is not the default.
773 Additionally, the authentication spoofing and authentication data
774 verification and substitution occur on the client side. The
775 security risk of using X11 forwarding is that the client's X11
776 display server may be exposed to attack when the SSH client
777 requests forwarding (see the warnings for ForwardX11 in
778 ssh_config(5)). A system administrator may have a stance in
779 which they want to protect clients that may expose themselves to
780 attack by unwittingly requesting X11 forwarding, which can
781 warrant a ``no'' setting.
783 Note that disabling X11 forwarding does not prevent users from
784 forwarding X11 traffic, as users can always install their own
785 forwarders. X11 forwarding is automatically disabled if UseLogin
789 Specifies whether sshd(8) should bind the X11 forwarding server
790 to the loopback address or to the wildcard address. By default,
791 sshd binds the forwarding server to the loopback address and sets
792 the hostname part of the DISPLAY environment variable to
793 ``localhost''. This prevents remote hosts from connecting to the
794 proxy display. However, some older X11 clients may not function
795 with this configuration. X11UseLocalhost may be set to ``no'' to
796 specify that the forwarding server should be bound to the
797 wildcard address. The argument must be ``yes'' or ``no''. The
801 Specifies the full pathname of the xauth(1) program. The default
802 is /usr/X11R6/bin/xauth.
805 sshd(8) command-line arguments and configuration file options that
806 specify time may be expressed using a sequence of the form:
807 time[qualifier], where time is a positive integer value and qualifier is
808 one of the following:
817 Each member of the sequence is added together to calculate the total time
820 Time format examples:
822 600 600 seconds (10 minutes)
824 1h30m 1 hour 30 minutes (90 minutes)
828 Contains configuration data for sshd(8). This file should be
829 writable by root only, but it is recommended (though not
830 necessary) that it be world-readable.
836 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
837 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
838 de Raadt and Dug Song removed many bugs, re-added newer features and
839 created OpenSSH. Markus Friedl contributed the support for SSH protocol
840 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
841 for privilege separation.
843 OpenBSD 5.5 February 27, 2014 OpenBSD 5.5
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