1sshd_config(4) File Formats sshd_config(4)
2
6 sshd_config - sshd configuration file
7
9 /etc/ssh/sshd_config
10
13 The sshd(1M) daemon reads configuration data from /etc/ssh/sshd_config
14 (or the file specified with sshd -f on the command line). The file con‐
15 tains keyword-value pairs, one per line. A line starting with a hash
16 mark (#) and empty lines are interpreted as comments.
17 The sshd_config file supports the following keywords. Unless otherwise
20 noted, keywords and their arguments are case-insensitive.
21 AllowGroups
23 This keyword can be followed by a number of group names, separated
25 by spaces. If specified, login is allowed only for users whose pri‐
26 mary group or supplementary group list matches one of the patterns.
27 Asterisk (*) and question mark (?) can be used as wildcards in the
28 patterns. Only group names are valid; a numerical group ID is not
29 recognized. By default, login is allowed regardless of the primary
30 group.
31 AllowTcpForwarding
34 Specifies whether TCP forwarding is permitted. The default is yes.
36 Disabling TCP forwarding does not improve security unless users are
37 also denied shell access, as they can always install their own for‐
38 warders.
39 AllowUsers
42 This keyword can be followed by a number of user names, separated
44 by spaces. If specified, login is allowed only for user names that
45 match one of the patterns. Asterisk (*) and question mark (?) can
46 be used as wildcards in the patterns. Only user names are valid; a
47 numerical user ID is not recognized. By default login is allowed
48 regardless of the user name.
49 If a specified pattern takes the form user@host then user and host
51 are checked separately, restricting logins to particular users from
52 particular hosts.
53 AuthorizedKeysFile
56 Specifies the file that contains the public keys that can be used
58 for user authentication. AuthorizedKeysFile can contain tokens of
59 the form %T, which are substituted during connection set-up. The
60 following tokens are defined: %% is replaced by a literal %, %h is
61 replaced by the home directory of the user being authenticated and
62 %u is replaced by the username of that user. After expansion,
63 AuthorizedKeysFile is taken to be an absolute path or one relative
64 to the user's home directory. The default is .ssh/authorized_keys.
65 Banner
68 In some jurisdictions, sending a warning message before authentica‐
70 tion can be relevant for getting legal protection. The contents of
71 the specified file are sent to the remote user before authentica‐
72 tion is allowed. This option is only available for protocol version
73 2. By default, no banner is displayed.
74 ChrootDirectory
77 Specifies a path to chroot(2) to after authentication. This path,
79 and all its components, must be root-owned directories that are not
80 writable by any other user or group.
81 The server always tries to change to the user's home directory
83 locally under the chrooted environment but a failure to do so is
84 not considered an error. In addition, the path might contain the
85 following tokens that are expanded at runtime once the connecting
86 user has been authenticated: %% is replaced by a literal %, %h is
87 replaced by the home directory of the user being authenticated, and
88 %u is replaced by the username of that user.
89 The ChrootDirectory must contain the necessary files and directo‐
91 ries to support the user's session. For an interactive SSH session
92 this requires at least a user's shell, shared libraries needed by
93 the shell, dynamic linker, and possibly basic /dev nodes such as
94 null, zero, stdin, stdout, stderr, random, and tty. Additionally,
95 terminal databases are needed for screen oriented applications. For
96 file transfer sessions using sftp with the SSH protocol version 2,
97 no additional configuration of the environment is necessary if the
98 in-process sftp server is used. See Subsystem for details.
99 The default is not to chroot(2).
101 Ciphers
104 Specifies the ciphers allowed for protocol version 2. Cipher order‐
106 ing on the server side is not relevant. Multiple ciphers must be
107 comma separated.
108 Valid ciphers are: aes128-ctr, aes192-ctr, aes256-ctr, aes128-cbc,
110 aes192-cbc, aes256-cbc, arcfour, arcfour128, arcfour256, 3des-cbc,
111 and blowfish-cbc.
112 The default cipher list is:
114 aes128-ctr,aes192-ctr,aes256-ctr,arcfour128,
116 arcfour256,arcfour
117 Using CBC modes on the server side is not recommended due to poten‐
120 tial security issues in connection with the SSH protocol version 2.
121 ClientAliveCountMax
124 Sets the number of client alive messages, (see ClientAliveInter‐
126 val), that can be sent without sshd receiving any messages back
127 from the client. If this threshold is reached while client alive
128 messages are being sent, sshd disconnects the client, terminating
129 the session. The use of client alive messages is very different
130 from KeepAlive. The client alive messages are sent through the
131 encrypted channel and therefore are not spoofable. The TCP
132 keepalive option enabled by KeepAlive is spoofable. The client
133 alive mechanism is valuable when a client or server depend on know‐
134 ing when a connection has become inactive.
135 The default value is 3. If ClientAliveInterval is set to 15, and
137 ClientAliveCountMax is left at the default, unresponsive ssh
138 clients are disconnected after approximately 45 seconds.
139 ClientAliveInterval
142 Sets a timeout interval in seconds after which, if no data has been
144 received from the client, sshd sends a message through the
145 encrypted channel to request a response from the client. The
146 default is 0, indicating that these messages are not sent to the
147 client. This option applies only to protocol version 2.
148 Compression
151 Controls whether the server allows the client to negotiate the use
153 of compression. The default is yes.
154 DenyGroups
157 Can be followed by a number of group names, separated by spaces.
159 Users whose primary group matches one of the patterns are not
160 allowed to log in. Asterisk (*) and question mark (?) can be used
161 as wildcards in the patterns. Only group names are valid; a numeri‐
162 cal group ID is not recognized. By default, login is allowed
163 regardless of the primary group.
164 DenyUsers
167 Can be followed by a number of user names, separated by spaces.
169 Login is disallowed for user names that match one of the patterns.
170 Asterisk (*) and question mark (?) can be used as wildcards in the
171 patterns. Only user names are valid; a numerical user ID is not
172 recognized. By default, login is allowed regardless of the user
173 name.
174 If a specified pattern takes the form user@host then user and host
176 are checked separately, disallowing logins to particular users from
177 particular hosts.
178 GatewayPorts
181 Specifies whether remote hosts are allowed to connect to ports for‐
183 warded for the client. By default, sshd binds remote port forward‐
184 ings to the loopback address. This prevents other remote hosts from
185 connecting to forwarded ports. GatewayPorts can be used to specify
186 that sshd should bind remote port forwardings to the wildcard
187 address, thus allowing remote hosts to connect to forwarded ports.
188 The argument can be no to force remote port forwardings to be
190 available to the local host only, yes to force remote port forward‐
191 ings to bind to the wildcard address, or clientspecified to allow
192 the client to select the address to which the forwarding is bound.
193 The default is no. See also RemoteForward in ssh_config(4).
194 GSSAPIAuthentication
197 Enables/disables GSS-API user authentication. The default is yes.
199 Currently sshd authorizes client user principals to user accounts
201 as follows: if the principal name matches the requested user
202 account, then the principal is authorized. Otherwise, GSS-API
203 authentication fails.
204 GSSAPIKeyExchange
207 Enables/disables GSS-API-authenticated key exchanges. The default
209 is yes.
210 This option also enables the use of the GSS-API to authenticate the
212 user to server after the key exchange. GSS-API key exchange can
213 succeed but the subsequent authentication using the GSS-API fail if
214 the server does not authorize the user's GSS principal name to the
215 target user account.
216 Currently sshd authorizes client user principals to user accounts
218 as follows: if the principal name matches the requested user
219 account, then the principal is authorized. Otherwise, GSS-API
220 authentication fails.
221 GSSAPIStoreDelegatedCredentials
224 Enables/disables the use of delegated GSS-API credentials on the
226 server-side. The default is yes.
227 Specifically, this option, when enabled, causes the server to store
229 delegated GSS-API credentials in the user's default GSS-API creden‐
230 tial store (which for the Kerberos V mechanism means
231 /tmp/krb5cc_<uid>).
232 Note -
234 sshd does not take any steps to explicitly destroy stored dele‐
236 gated GSS-API credentials upon logout. It is the responsibility
237 of PAM modules to destroy credentials associated with a session.
238 HostbasedAuthentication
241 Specifies whether to try rhosts-based authentication with public
243 key authentication. The argument must be yes or no. The default is
244 no. This option applies to protocol version 2 only and is similar
245 to RhostsRSAAuthentication. See sshd(1M) for guidelines on setting
246 up host-based authentication.
247 HostbasedUsesNameFromPacketOnly
250 Controls which hostname is searched for in the files ~/.shosts,
252 /etc/shosts.equiv, and /etc/hosts.equiv. If this parameter is set
253 to yes, the server uses the name the client claimed for itself and
254 signed with that host's key. If set to no, the default, the server
255 uses the name to which the client's IP address resolves.
256 Setting this parameter to no disables host-based authentication
258 when using NAT or when the client gets to the server indirectly
259 through a port-forwarding firewall.
260 HostKey
263 Specifies the file containing the private host key used by SSH. The
265 default is /etc/ssh/ssh_host_key for protocol version 1, and
266 /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for proto‐
267 col version 2. sshd refuses to use a file if it is group/world-
268 accessible. It is possible to have multiple host key files. rsa1
269 keys are used for version 1 and dsa or rsa are used for version 2
270 of the SSH protocol.
271 IgnoreRhosts
274 Specifies that .rhosts and .shosts files are not used in authenti‐
276 cation. /etc/hosts.equiv and /etc/shosts.equiv are still used. The
277 default is yes. This parameter applies to both protocol versions 1
278 and 2.
279 IgnoreUserKnownHosts
282 Specifies whether sshd should ignore the user's
284 $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default
285 is no. This parameter applies to both protocol versions 1 and 2.
286 KbdInteractiveAuthentication
289 Specifies whether authentication by means of the "keyboard-interac‐
291 tive" authentication method (and PAM) is allowed. Defaults to yes.
292 (Deprecated: this parameter can only be set to yes.)
293 KeepAlive
296 Specifies whether the system should send keepalive messages to the
298 other side. If they are sent, death of the connection or crash of
299 one of the machines is properly noticed. However, this means that
300 connections die if the route is down temporarily, which can be an
301 annoyance. On the other hand, if keepalives are not sent, sessions
302 can hang indefinitely on the server, leaving ghost users and con‐
303 suming server resources.
304 The default is yes (to send keepalives), and the server notices if
306 the network goes down or the client host reboots. This avoids in‐
307 finitely hanging sessions.
308 To disable keepalives, the value should be set to no in both the
310 server and the client configuration files.
311 KeyRegenerationInterval
314 In protocol version 1, the ephemeral server key is automatically
316 regenerated after this many seconds (if it has been used). The pur‐
317 pose of regeneration is to prevent decrypting captured sessions by
318 later breaking into the machine and stealing the keys. The key is
319 never stored anywhere. If the value is 0, the key is never regener‐
320 ated. The default is 3600 (seconds).
321 ListenAddress
324 Specifies what local address sshd should listen on. The following
326 forms can be used:
327 ListenAddress host|IPv4_addr|IPv6_addr
329 ListenAddress host|IPv4_addr:port
330 ListenAddress [host|IPv6_addr]:port
331 If port is not specified, sshd listens on the address and all prior
333 Port options specified. The default is to listen on all local
334 addresses. Multiple ListenAddress options are permitted. Addition‐
335 ally, any Port options must precede this option for non-port quali‐
336 fied addresses.
337 The default is to listen on all local addresses. Multiple options
339 of this type are permitted. Additionally, the Ports options must
340 precede this option.
341 LoginGraceTime
344 The server disconnects after this time (in seconds) if the user has
346 not successfully logged in. If the value is 0, there is no time
347 limit. The default is 120 (seconds).
348 LogLevel
351 Gives the verbosity level that is used when logging messages from
353 sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE,
354 DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. DEBUG2 and
355 DEBUG3 each specify higher levels of debugging output. Logging with
356 level DEBUG violates the privacy of users and is not recommended.
357 LookupClientHostnames
360 Specifies whether or not to lookup the names of client's addresses.
362 Defaults to yes.
363 MACs
366 Specifies the available MAC (message authentication code) algo‐
368 rithms. The MAC algorithm is used in protocol version 2 for data
369 integrity protection. Multiple algorithms must be comma-separated.
370 The default is hmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96.
371 MaxStartups
374 Specifies the maximum number of concurrent unauthenticated connec‐
376 tions to the sshd daemon. Additional connections are dropped until
377 authentication succeeds or the LoginGraceTime expires for a connec‐
378 tion. The default is 10.
379 Alternatively, random early drop can be enabled by specifying the
381 three colon-separated values start:rate:full (for example,
382 10:30:60). Referring to this example, sshd refuse connection
383 attempts with a probability of rate/100 (30% in our example) if
384 there are currently 10 (from the start field) unauthenticated con‐
385 nections. The probability increases linearly and all connection
386 attempts are refused if the number of unauthenticated connections
387 reaches full (60 in our example).
388 PasswordAuthentication
391 Specifies whether password authentication is allowed. The default
393 is yes. This option applies to both protocol versions 1 and 2.
394 PermitEmptyPasswords
397 When password or keyboard-interactive authentication is allowed, it
399 specifies whether the server allows login to accounts with empty
400 password strings.
401 If not set then the /etc/default/login PASSREQ value is used
403 instead.
404 PASSREQ=no is equivalent to PermitEmptyPasswords yes. PASSREQ=yes
406 is equivalent to PermitEmptyPasswords no. If neither PermitEmpty‐
407 Passwords or PASSREQ are set the default is no.
408 PermitRootLogin
411 Specifies whether the root can log in using ssh(1). The argument
413 must be yes, without-password, forced-commands-only, or no. with‐
414 out-password means that root cannot be authenticated using the
415 "password" or "keyboard-interactive" methods (see description of
416 KbdInteractiveAuthentication). forced-commands-only means that
417 authentication is allowed only for publickey (for SSHv2, or RSA,
418 for SSHv1) and only if the matching authorized_keys entry for root
419 has a command=<cmd> option.
420 In Solaris, the default /etc/ssh/sshd_config file is shipped with
422 PermitRootLogin set to no. If unset by the administrator, then CON‐
423 SOLE parameter from /etc/default/login supplies the default value
424 as follows: if the CONSOLE parameter is not commented out (it can
425 even be empty, that is, "CONSOLE="), then without-password is used
426 as default value. If CONSOLE is commented out, then the default for
427 PermitRootLogin is yes.
428 The without-password and forced-commands-only settings are useful
430 for, for example, performing remote administration and backups
431 using trusted public keys for authentication of the remote client,
432 without allowing access to the root account using passwords.
433 PermitUserEnvironment
436 Specifies whether a user's ~/.ssh/environment on the server side
438 and environment options in the AuthorizedKeysFile file are pro‐
439 cessed by sshd. The default is no. Enabling environment processing
440 can enable users to bypass access restrictions in some configura‐
441 tions using mechanisms such as LD_PRELOAD.
442 Environment setting from a relevant entry in AuthorizedKeysFile
444 file is processed only if the user was authenticated using the pub‐
445 lic key authentication method. Of the two files used, values of
446 variables set in ~/.ssh/environment are of higher priority.
447 PidFile
450 Allows you to specify an alternative to /var/run/sshd.pid, the
452 default file for storing the PID of the sshd listening for connec‐
453 tions. See sshd(1M).
454 Port
457 Specifies the port number that sshd listens on. The default is 22.
459 Multiple options of this type are permitted. See also ListenAd‐
460 dress.
461 PrintLastLog
464 Specifies whether sshd should display the date and time when the
466 user last logged in. The default is yes.
467 PrintMotd
470 Specifies whether sshd should display the contents of /etc/motd
472 when a user logs in interactively. (On some systems it is also dis‐
473 played by the shell or a shell startup file, such as /etc/profile.)
474 The default is yes.
475 Protocol
478 Specifies the protocol versions sshd should support in order of
480 preference. The possible values are 1 and 2. Multiple versions must
481 be comma-separated. The default is 2,1. This means that ssh tries
482 version 2 and falls back to version 1 if version 2 is not avail‐
483 able.
484 PubkeyAuthentication
487 Specifies whether public key authentication is allowed. The default
489 is yes. This option applies to protocol version 2 only.
490 RhostsAuthentication
493 Specifies whether authentication using rhosts or /etc/hosts.equiv
495 files is sufficient. Normally, this method should not be permitted
496 because it is insecure. RhostsRSAAuthentication should be used
497 instead, because it performs RSA-based host authentication in addi‐
498 tion to normal rhosts or /etc/hosts.equiv authentication. The
499 default is no. This parameter applies only to protocol version 1.
500 RhostsRSAAuthentication
503 Specifies whether rhosts or /etc/hosts.equiv authentication
505 together with successful RSA host authentication is allowed. The
506 default is no. This parameter applies only to protocol version 1.
507 RSAAuthentication
510 Specifies whether pure RSA authentication is allowed. The default
512 is yes. This option applies to protocol version 1 only.
513 ServerKeyBits
516 Defines the number of bits in the ephemeral protocol version 1
518 server key. The minimum value is 512, and the default is 768.
519 StrictModes
522 Specifies whether sshd should check file modes and ownership of the
524 user's files and home directory before accepting login. This is
525 normally desirable because novices sometimes accidentally leave
526 their directory or files world-writable. The default is yes.
527 Subsystem
530 Configures an external subsystem (for example, a file transfer dae‐
532 mon). Arguments should be a subsystem name and a command to execute
533 upon subsystem request. The command sftp-server(1M) implements the
534 sftp file transfer subsystem.
535 Alternately, the name internal-sftp implements an in-process sftp
537 server. This can simplify configurations using ChrootDirectory to
538 force a different filesystem root on clients.
539 By default, no subsystems are defined. This option applies to pro‐
541 tocol version 2 only.
542 SyslogFacility
545 Gives the facility code that is used when logging messages from
547 sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1,
548 LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, and LOCAL7. The default is
549 AUTH.
550 UseOpenSSLEngine
553 Specifies whether sshd should use the OpenSSL PKCS#11 engine for
555 offloading cryptographic operations to the Cryptographic Framework.
556 Cryptographic operations are accelerated according to the available
557 installed plug-ins. When no suitable plug-ins are present this
558 option does not have an effect. The default is yes.
559 VerifyReverseMapping
562 Specifies whether sshd should try to verify the remote host name
564 and check that the resolved host name for the remote IP address
565 maps back to the very same IP address. (A yes setting means "ver‐
566 ify".) Setting this parameter to no can be useful where DNS servers
567 might be down and thus cause sshd to spend much time trying to
568 resolve the client's IP address to a name. This feature is useful
569 for Internet-facing servers. The default is no.
570 X11DisplayOffset
573 Specifies the first display number available for sshd's X11 for‐
575 warding. This prevents sshd from interfering with real X11 servers.
576 The default is 10.
577 X11Forwarding
580 Specifies whether X11 forwarding is permitted. The default is yes.
582 Disabling X11 forwarding does not improve security in any way, as
583 users can always install their own forwarders.
584 When X11 forwarding is enabled, there can be additional exposure to
586 the server and to client displays if the sshd proxy display is con‐
587 figured to listen on the wildcard address (see X11UseLocalhost).
588 However, this is not the default. Additionally, the authentication
589 spoofing and authentication data verification and substitution
590 occur on the client side. The security risk of using X11 forwarding
591 is that the client's X11 display server can be exposed to attack
592 when the ssh client requests forwarding (see the warnings for For‐
593 wardX11 in ssh_config(4)). A system administrator who wants to pro‐
594 tect clients that expose themselves to attack by unwittingly
595 requesting X11 forwarding, should specify a no setting.
596 Disabling X11 forwarding does not prevent users from forwarding X11
598 traffic, as users can always install their own forwarders.
599 X11UseLocalhost
602 Specifies whether sshd should bind the X11 forwarding server to the
604 loopback address or to the wildcard address. By default, sshd binds
605 the forwarding server to the loopback address and sets the hostname
606 part of the DISPLAY environment variable to localhost. This pre‐
607 vents remote hosts from connecting to the proxy display. However,
608 some older X11 clients might not function with this configuration.
609 X11UseLocalhost can be set to no to specify that the forwarding
610 server should be bound to the wildcard address. The argument must
611 be yes or no. The default is yes.
612 XAuthLocation
615 Specifies the location of the xauth(1) program. The default is
617 /usr/X11/bin/xauth and sshd attempts to open it when X11 forwarding
618 is enabled.
619 Time Formats
622 sshd command-line arguments and configuration file options that specify
623 time can be expressed using a sequence of the form: time[qualifier,]
624 where time is a positive integer value and qualifier is one of the fol‐
625 lowing:
626 <none> seconds
628 s | S seconds
631 m | M minutes
634 h | H hours
637 d | D days
640 w | weeks
643 Each element of the sequence is added together to calculate the total
647 time value. For example:
648 600 600 seconds (10 minutes)
650 10m 10 minutes
653 1h30m 1 hour, 30 minutes (90 minutes)
656
659 /etc/ssh/sshd_config Contains configuration data for sshd. This file
660 should be writable by root only, but it is rec‐
661 ommended (though not necessary) that it be
662 world-readable.
663
666 See attributes(5) for descriptions of the following attributes:
667 ┌─────────────────────────────┬─────────────────────────────┐
672 │ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
673 ├─────────────────────────────┼─────────────────────────────┤
674 │Availability │SUNWsshu │
675 ├─────────────────────────────┼─────────────────────────────┤
676 │Interface Stability │Uncommitted │
677 └─────────────────────────────┴─────────────────────────────┘
678
680 login(1), sshd(1M), chroot(2), ssh_config(4), attributes(5), ker‐
681 beros(5)
682
684 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
685 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
686 Theo de Raadt, and Dug Song removed many bugs, re-added recent fea‐
687 tures, and created OpenSSH. Markus Friedl contributed the support for
688 SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl con‐
689 tributed support for privilege separation.
690SunOS 5.11 26 Mar 2009 sshd_config(4)