1 SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
4 sshd_config M-bM-^@M-^S OpenSSH SSH daemon configuration file
7 sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
8 specified with -f on the command line). The file contains keyword-
9 argument pairs, one per line. For each keyword, the first obtained value
10 will be used. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y and empty lines are interpreted as
11 comments. Arguments may optionally be enclosed in double quotes (") in
12 order to represent arguments containing spaces.
14 The possible keywords and their meanings are as follows (note that
15 keywords are case-insensitive and arguments are case-sensitive):
18 Specifies what environment variables sent by the client will be
19 copied into the session's environ(7). See SendEnv in
20 ssh_config(5) for how to configure the client. The TERM
21 environment variable is always sent whenever the client requests
22 a pseudo-terminal as it is required by the protocol. Variables
23 are specified by name, which may contain the wildcard characters
24 M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. Multiple environment variables may be separated by
25 whitespace or spread across multiple AcceptEnv directives. Be
26 warned that some environment variables could be used to bypass
27 restricted user environments. For this reason, care should be
28 taken in the use of this directive. The default is not to accept
29 any environment variables.
32 Specifies which address family should be used by sshd(8). Valid
33 arguments are any (the default), inet (use IPv4 only), or inet6
37 Specifies whether ssh-agent(1) forwarding is permitted. The
38 default is yes. Note that disabling agent forwarding does not
39 improve security unless users are also denied shell access, as
40 they can always install their own forwarders.
43 This keyword can be followed by a list of group name patterns,
44 separated by spaces. If specified, login is allowed only for
45 users whose primary group or supplementary group list matches one
46 of the patterns. Only group names are valid; a numerical group
47 ID is not recognized. By default, login is allowed for all
48 groups. The allow/deny directives are processed in the following
49 order: DenyUsers, AllowUsers, DenyGroups, and finally
52 See PATTERNS in ssh_config(5) for more information on patterns.
54 AllowStreamLocalForwarding
55 Specifies whether StreamLocal (Unix-domain socket) forwarding is
56 permitted. The available options are yes (the default) or all to
57 allow StreamLocal forwarding, no to prevent all StreamLocal
58 forwarding, local to allow local (from the perspective of ssh(1))
59 forwarding only or remote to allow remote forwarding only. Note
60 that disabling StreamLocal forwarding does not improve security
61 unless users are also denied shell access, as they can always
62 install their own forwarders.
65 Specifies whether TCP forwarding is permitted. The available
66 options are yes (the default) or all to allow TCP forwarding, no
67 to prevent all TCP forwarding, local to allow local (from the
68 perspective of ssh(1)) forwarding only or remote to allow remote
69 forwarding only. Note that disabling TCP forwarding does not
70 improve security unless users are also denied shell access, as
71 they can always install their own forwarders.
74 This keyword can be followed by a list of user name patterns,
75 separated by spaces. If specified, login is allowed only for
76 user names that match one of the patterns. Only user names are
77 valid; a numerical user ID is not recognized. By default, login
78 is allowed for all users. If the pattern takes the form
79 USER@HOST then USER and HOST are separately checked, restricting
80 logins to particular users from particular hosts. HOST criteria
81 may additionally contain addresses to match in CIDR
82 address/masklen format. The allow/deny directives are processed
83 in the following order: DenyUsers, AllowUsers, DenyGroups, and
86 See PATTERNS in ssh_config(5) for more information on patterns.
89 Specifies the authentication methods that must be successfully
90 completed for a user to be granted access. This option must be
91 followed by one or more comma-separated lists of authentication
92 method names, or by the single string any to indicate the default
93 behaviour of accepting any single authentication method. If the
94 default is overridden, then successful authentication requires
95 completion of every method in at least one of these lists.
97 For example, "publickey,password publickey,keyboard-interactive"
98 would require the user to complete public key authentication,
99 followed by either password or keyboard interactive
100 authentication. Only methods that are next in one or more lists
101 are offered at each stage, so for this example it would not be
102 possible to attempt password or keyboard-interactive
103 authentication before public key.
105 For keyboard interactive authentication it is also possible to
106 restrict authentication to a specific device by appending a colon
107 followed by the device identifier bsdauth, pam, or skey,
108 depending on the server configuration. For example,
109 "keyboard-interactive:bsdauth" would restrict keyboard
110 interactive authentication to the bsdauth device.
112 If the publickey method is listed more than once, sshd(8)
113 verifies that keys that have been used successfully are not
114 reused for subsequent authentications. For example,
115 "publickey,publickey" requires successful authentication using
116 two different public keys.
118 Note that each authentication method listed should also be
119 explicitly enabled in the configuration.
121 The available authentication methods are: "gssapi-with-mic",
122 "hostbased", "keyboard-interactive", "none" (used for access to
123 password-less accounts when PermitEmptyPassword is enabled),
124 "password" and "publickey".
126 AuthorizedKeysCommand
127 Specifies a program to be used to look up the user's public keys.
128 The program must be owned by root, not writable by group or
129 others and specified by an absolute path. Arguments to
130 AuthorizedKeysCommand accept the tokens described in the TOKENS
131 section. If no arguments are specified then the username of the
134 The program should produce on standard output zero or more lines
135 of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a
136 key supplied by AuthorizedKeysCommand does not successfully
137 authenticate and authorize the user then public key
138 authentication continues using the usual AuthorizedKeysFile
139 files. By default, no AuthorizedKeysCommand is run.
141 AuthorizedKeysCommandUser
142 Specifies the user under whose account the AuthorizedKeysCommand
143 is run. It is recommended to use a dedicated user that has no
144 other role on the host than running authorized keys commands. If
145 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser
146 is not, then sshd(8) will refuse to start.
149 Specifies the file that contains the public keys used for user
150 authentication. The format is described in the AUTHORIZED_KEYS
151 FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile
152 accept the tokens described in the TOKENS section. After
153 expansion, AuthorizedKeysFile is taken to be an absolute path or
154 one relative to the user's home directory. Multiple files may be
155 listed, separated by whitespace. Alternately this option may be
156 set to none to skip checking for user keys in files. The default
157 is ".ssh/authorized_keys .ssh/authorized_keys2".
159 AuthorizedPrincipalsCommand
160 Specifies a program to be used to generate the list of allowed
161 certificate principals as per AuthorizedPrincipalsFile. The
162 program must be owned by root, not writable by group or others
163 and specified by an absolute path. Arguments to
164 AuthorizedPrincipalsCommand accept the tokens described in the
165 TOKENS section. If no arguments are specified then the username
166 of the target user is used.
168 The program should produce on standard output zero or more lines
169 of AuthorizedPrincipalsFile output. If either
170 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is
171 specified, then certificates offered by the client for
172 authentication must contain a principal that is listed. By
173 default, no AuthorizedPrincipalsCommand is run.
175 AuthorizedPrincipalsCommandUser
176 Specifies the user under whose account the
177 AuthorizedPrincipalsCommand is run. It is recommended to use a
178 dedicated user that has no other role on the host than running
179 authorized principals commands. If AuthorizedPrincipalsCommand
180 is specified but AuthorizedPrincipalsCommandUser is not, then
181 sshd(8) will refuse to start.
183 AuthorizedPrincipalsFile
184 Specifies a file that lists principal names that are accepted for
185 certificate authentication. When using certificates signed by a
186 key listed in TrustedUserCAKeys, this file lists names, one of
187 which must appear in the certificate for it to be accepted for
188 authentication. Names are listed one per line preceded by key
189 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
190 Empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are ignored.
192 Arguments to AuthorizedPrincipalsFile accept the tokens described
193 in the TOKENS section. After expansion, AuthorizedPrincipalsFile
194 is taken to be an absolute path or one relative to the user's
195 home directory. The default is none, i.e. not to use a
196 principals file M-bM-^@M-^S in this case, the username of the user must
197 appear in a certificate's principals list for it to be accepted.
199 Note that AuthorizedPrincipalsFile is only used when
200 authentication proceeds using a CA listed in TrustedUserCAKeys
201 and is not consulted for certification authorities trusted via
202 ~/.ssh/authorized_keys, though the principals= key option offers
203 a similar facility (see sshd(8) for details).
205 Banner The contents of the specified file are sent to the remote user
206 before authentication is allowed. If the argument is none then
207 no banner is displayed. By default, no banner is displayed.
209 ChallengeResponseAuthentication
210 Specifies whether challenge-response authentication is allowed
211 (e.g. via PAM or through authentication styles supported in
212 login.conf(5)) The default is yes.
215 Specifies the pathname of a directory to chroot(2) to after
216 authentication. At session startup sshd(8) checks that all
217 components of the pathname are root-owned directories which are
218 not writable by any other user or group. After the chroot,
219 sshd(8) changes the working directory to the user's home
220 directory. Arguments to ChrootDirectory accept the tokens
221 described in the TOKENS section.
223 The ChrootDirectory must contain the necessary files and
224 directories to support the user's session. For an interactive
225 session this requires at least a shell, typically sh(1), and
226 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
227 stderr(4), and tty(4) devices. For file transfer sessions using
228 SFTP no additional configuration of the environment is necessary
229 if the in-process sftp-server is used, though sessions which use
230 logging may require /dev/log inside the chroot directory on some
231 operating systems (see sftp-server(8) for details).
233 For safety, it is very important that the directory hierarchy be
234 prevented from modification by other processes on the system
235 (especially those outside the jail). Misconfiguration can lead
236 to unsafe environments which sshd(8) cannot detect.
238 The default is none, indicating not to chroot(2).
241 Specifies the ciphers allowed. Multiple ciphers must be comma-
242 separated. If the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
243 then the specified ciphers will be appended to the default set
244 instead of replacing them. If the specified value begins with a
245 M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified ciphers (including wildcards)
246 will be removed from the default set instead of replacing them.
248 The supported ciphers are:
257 aes128-gcm@openssh.com
258 aes256-gcm@openssh.com
259 chacha20-poly1305@openssh.com
263 chacha20-poly1305@openssh.com,
264 aes128-ctr,aes192-ctr,aes256-ctr,
265 aes128-gcm@openssh.com,aes256-gcm@openssh.com
267 The list of available ciphers may also be obtained using "ssh -Q
271 Sets the number of client alive messages which may be sent
272 without sshd(8) receiving any messages back from the client. If
273 this threshold is reached while client alive messages are being
274 sent, sshd will disconnect the client, terminating the session.
275 It is important to note that the use of client alive messages is
276 very different from TCPKeepAlive. The client alive messages are
277 sent through the encrypted channel and therefore will not be
278 spoofable. The TCP keepalive option enabled by TCPKeepAlive is
279 spoofable. The client alive mechanism is valuable when the
280 client or server depend on knowing when a connection has become
283 The default value is 3. If ClientAliveInterval is set to 15, and
284 ClientAliveCountMax is left at the default, unresponsive SSH
285 clients will be disconnected after approximately 45 seconds.
288 Sets a timeout interval in seconds after which if no data has
289 been received from the client, sshd(8) will send a message
290 through the encrypted channel to request a response from the
291 client. The default is 0, indicating that these messages will
292 not be sent to the client.
295 Specifies whether compression is enabled after the user has
296 authenticated successfully. The argument must be yes, delayed (a
297 legacy synonym for yes) or no. The default is yes.
300 This keyword can be followed by a list of group name patterns,
301 separated by spaces. Login is disallowed for users whose primary
302 group or supplementary group list matches one of the patterns.
303 Only group names are valid; a numerical group ID is not
304 recognized. By default, login is allowed for all groups. The
305 allow/deny directives are processed in the following order:
306 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
308 See PATTERNS in ssh_config(5) for more information on patterns.
311 This keyword can be followed by a list of user name patterns,
312 separated by spaces. Login is disallowed for user names that
313 match one of the patterns. Only user names are valid; a
314 numerical user ID is not recognized. By default, login is
315 allowed for all users. If the pattern takes the form USER@HOST
316 then USER and HOST are separately checked, restricting logins to
317 particular users from particular hosts. HOST criteria may
318 additionally contain addresses to match in CIDR address/masklen
319 format. The allow/deny directives are processed in the following
320 order: DenyUsers, AllowUsers, DenyGroups, and finally
323 See PATTERNS in ssh_config(5) for more information on patterns.
326 Disables all forwarding features, including X11, ssh-agent(1),
327 TCP and StreamLocal. This option overrides all other forwarding-
328 related options and may simplify restricted configurations.
331 Writes a temporary file containing a list of authentication
332 methods and public credentials (e.g. keys) used to authenticate
333 the user. The location of the file is exposed to the user
334 session through the SSH_USER_AUTH environment variable. The
338 Specifies the hash algorithm used when logging key fingerprints.
339 Valid options are: md5 and sha256. The default is sha256.
342 Forces the execution of the command specified by ForceCommand,
343 ignoring any command supplied by the client and ~/.ssh/rc if
344 present. The command is invoked by using the user's login shell
345 with the -c option. This applies to shell, command, or subsystem
346 execution. It is most useful inside a Match block. The command
347 originally supplied by the client is available in the
348 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
349 of internal-sftp will force the use of an in-process SFTP server
350 that requires no support files when used with ChrootDirectory.
354 Specifies whether remote hosts are allowed to connect to ports
355 forwarded for the client. By default, sshd(8) binds remote port
356 forwardings to the loopback address. This prevents other remote
357 hosts from connecting to forwarded ports. GatewayPorts can be
358 used to specify that sshd should allow remote port forwardings to
359 bind to non-loopback addresses, thus allowing other hosts to
360 connect. The argument may be no to force remote port forwardings
361 to be available to the local host only, yes to force remote port
362 forwardings to bind to the wildcard address, or clientspecified
363 to allow the client to select the address to which the forwarding
364 is bound. The default is no.
367 Specifies whether user authentication based on GSSAPI is allowed.
370 GSSAPICleanupCredentials
371 Specifies whether to automatically destroy the user's credentials
372 cache on logout. The default is yes.
374 GSSAPIStrictAcceptorCheck
375 Determines whether to be strict about the identity of the GSSAPI
376 acceptor a client authenticates against. If set to yes then the
377 client must authenticate against the host service on the current
378 hostname. If set to no then the client may authenticate against
379 any service key stored in the machine's default store. This
380 facility is provided to assist with operation on multi homed
381 machines. The default is yes.
383 HostbasedAcceptedKeyTypes
384 Specifies the key types that will be accepted for hostbased
385 authentication as a comma-separated pattern list. Alternately if
386 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
387 specified key types will be appended to the default set instead
388 of replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
389 character, then the specified key types (including wildcards)
390 will be removed from the default set instead of replacing them.
391 The default for this option is:
393 ecdsa-sha2-nistp256-cert-v01@openssh.com,
394 ecdsa-sha2-nistp384-cert-v01@openssh.com,
395 ecdsa-sha2-nistp521-cert-v01@openssh.com,
396 ssh-ed25519-cert-v01@openssh.com,
397 ssh-rsa-cert-v01@openssh.com,
398 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
401 The list of available key types may also be obtained using "ssh
404 HostbasedAuthentication
405 Specifies whether rhosts or /etc/hosts.equiv authentication
406 together with successful public key client host authentication is
407 allowed (host-based authentication). The default is no.
409 HostbasedUsesNameFromPacketOnly
410 Specifies whether or not the server will attempt to perform a
411 reverse name lookup when matching the name in the ~/.shosts,
412 ~/.rhosts, and /etc/hosts.equiv files during
413 HostbasedAuthentication. A setting of yes means that sshd(8)
414 uses the name supplied by the client rather than attempting to
415 resolve the name from the TCP connection itself. The default is
419 Specifies a file containing a public host certificate. The
420 certificate's public key must match a private host key already
421 specified by HostKey. The default behaviour of sshd(8) is not to
422 load any certificates.
425 Specifies a file containing a private host key used by SSH. The
426 defaults are /etc/ssh/ssh_host_ecdsa_key,
427 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.
429 Note that sshd(8) will refuse to use a file if it is group/world-
430 accessible and that the HostKeyAlgorithms option restricts which
431 of the keys are actually used by sshd(8).
433 It is possible to have multiple host key files. It is also
434 possible to specify public host key files instead. In this case
435 operations on the private key will be delegated to an
439 Identifies the UNIX-domain socket used to communicate with an
440 agent that has access to the private host keys. If the string
441 "SSH_AUTH_SOCK" is specified, the location of the socket will be
442 read from the SSH_AUTH_SOCK environment variable.
445 Specifies the host key algorithms that the server offers. The
446 default for this option is:
448 ecdsa-sha2-nistp256-cert-v01@openssh.com,
449 ecdsa-sha2-nistp384-cert-v01@openssh.com,
450 ecdsa-sha2-nistp521-cert-v01@openssh.com,
451 ssh-ed25519-cert-v01@openssh.com,
452 ssh-rsa-cert-v01@openssh.com,
453 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
456 The list of available key types may also be obtained using "ssh
460 Specifies that .rhosts and .shosts files will not be used in
461 HostbasedAuthentication.
463 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
467 Specifies whether sshd(8) should ignore the user's
468 ~/.ssh/known_hosts during HostbasedAuthentication and use only
469 the system-wide known hosts file /etc/ssh/known_hosts. The
472 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
473 connection. Accepted values are af11, af12, af13, af21, af22,
474 af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3,
475 cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, a
476 numeric value, or none to use the operating system default. This
477 option may take one or two arguments, separated by whitespace.
478 If one argument is specified, it is used as the packet class
479 unconditionally. If two values are specified, the first is
480 automatically selected for interactive sessions and the second
481 for non-interactive sessions. The default is lowdelay for
482 interactive sessions and throughput for non-interactive sessions.
484 KbdInteractiveAuthentication
485 Specifies whether to allow keyboard-interactive authentication.
486 The argument to this keyword must be yes or no. The default is
487 to use whatever value ChallengeResponseAuthentication is set to
490 KerberosAuthentication
491 Specifies whether the password provided by the user for
492 PasswordAuthentication will be validated through the Kerberos
493 KDC. To use this option, the server needs a Kerberos servtab
494 which allows the verification of the KDC's identity. The default
498 If AFS is active and the user has a Kerberos 5 TGT, attempt to
499 acquire an AFS token before accessing the user's home directory.
502 KerberosOrLocalPasswd
503 If password authentication through Kerberos fails then the
504 password will be validated via any additional local mechanism
505 such as /etc/passwd. The default is yes.
507 KerberosTicketCleanup
508 Specifies whether to automatically destroy the user's ticket
509 cache file on logout. The default is yes.
512 Specifies the available KEX (Key Exchange) algorithms. Multiple
513 algorithms must be comma-separated. Alternately if the specified
514 value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified methods
515 will be appended to the default set instead of replacing them.
516 If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the
517 specified methods (including wildcards) will be removed from the
518 default set instead of replacing them. The supported algorithms
522 curve25519-sha256@libssh.org
523 diffie-hellman-group1-sha1
524 diffie-hellman-group14-sha1
525 diffie-hellman-group14-sha256
526 diffie-hellman-group16-sha512
527 diffie-hellman-group18-sha512
528 diffie-hellman-group-exchange-sha1
529 diffie-hellman-group-exchange-sha256
536 curve25519-sha256,curve25519-sha256@libssh.org,
537 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
538 diffie-hellman-group-exchange-sha256,
539 diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
540 diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
542 The list of available key exchange algorithms may also be
543 obtained using "ssh -Q kex".
546 Specifies the local addresses sshd(8) should listen on. The
547 following forms may be used:
549 ListenAddress hostname|address [rdomain domain]
550 ListenAddress hostname:port [rdomain domain]
551 ListenAddress IPv4_address:port [rdomain domain]
552 ListenAddress [hostname|address]:port [rdomain domain]
554 The optional rdomain qualifier requests sshd(8) listen in an
555 explicit routing domain. If port is not specified, sshd will
556 listen on the address and all Port options specified. The
557 default is to listen on all local addresses on the current
558 default routing domain. Multiple ListenAddress options are
559 permitted. For more information on routing domains, see
563 The server disconnects after this time if the user has not
564 successfully logged in. If the value is 0, there is no time
565 limit. The default is 120 seconds.
568 Gives the verbosity level that is used when logging messages from
569 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
570 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
571 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
572 higher levels of debugging output. Logging with a DEBUG level
573 violates the privacy of users and is not recommended.
575 MACs Specifies the available MAC (message authentication code)
576 algorithms. The MAC algorithm is used for data integrity
577 protection. Multiple algorithms must be comma-separated. If the
578 specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified
579 algorithms will be appended to the default set instead of
580 replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
581 character, then the specified algorithms (including wildcards)
582 will be removed from the default set instead of replacing them.
584 The algorithms that contain "-etm" calculate the MAC after
585 encryption (encrypt-then-mac). These are considered safer and
586 their use recommended. The supported MACs are:
596 hmac-md5-etm@openssh.com
597 hmac-md5-96-etm@openssh.com
598 hmac-sha1-etm@openssh.com
599 hmac-sha1-96-etm@openssh.com
600 hmac-sha2-256-etm@openssh.com
601 hmac-sha2-512-etm@openssh.com
602 umac-64-etm@openssh.com
603 umac-128-etm@openssh.com
607 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
608 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
609 hmac-sha1-etm@openssh.com,
610 umac-64@openssh.com,umac-128@openssh.com,
611 hmac-sha2-256,hmac-sha2-512,hmac-sha1
613 The list of available MAC algorithms may also be obtained using
616 Match Introduces a conditional block. If all of the criteria on the
617 Match line are satisfied, the keywords on the following lines
618 override those set in the global section of the config file,
619 until either another Match line or the end of the file. If a
620 keyword appears in multiple Match blocks that are satisfied, only
621 the first instance of the keyword is applied.
623 The arguments to Match are one or more criteria-pattern pairs or
624 the single token All which matches all criteria. The available
625 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
626 and Address (with RDomain representing the rdomain(4) on which
627 the connection was received.)
629 The match patterns may consist of single entries or comma-
630 separated lists and may use the wildcard and negation operators
631 described in the PATTERNS section of ssh_config(5).
633 The patterns in an Address criteria may additionally contain
634 addresses to match in CIDR address/masklen format, such as
635 192.0.2.0/24 or 2001:db8::/32. Note that the mask length
636 provided must be consistent with the address - it is an error to
637 specify a mask length that is too long for the address or one
638 with bits set in this host portion of the address. For example,
639 192.0.2.0/33 and 192.0.2.0/8, respectively.
641 Only a subset of keywords may be used on the lines following a
642 Match keyword. Available keywords are AcceptEnv,
643 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
644 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
645 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
646 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
647 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
648 Banner, ChrootDirectory, ClientAliveCountMax,
649 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
650 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
651 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
652 KbdInteractiveAuthentication, KerberosAuthentication, LogLevel,
653 MaxAuthTries, MaxSessions, PasswordAuthentication,
654 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
655 PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
656 PubkeyAuthentication, RekeyLimit, RevokedKeys, RDomain,
657 StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys,
658 X11DisplayOffset, X11Forwarding and X11UseLocalHost.
661 Specifies the maximum number of authentication attempts permitted
662 per connection. Once the number of failures reaches half this
663 value, additional failures are logged. The default is 6.
666 Specifies the maximum number of open shell, login or subsystem
667 (e.g. sftp) sessions permitted per network connection. Multiple
668 sessions may be established by clients that support connection
669 multiplexing. Setting MaxSessions to 1 will effectively disable
670 session multiplexing, whereas setting it to 0 will prevent all
671 shell, login and subsystem sessions while still permitting
672 forwarding. The default is 10.
675 Specifies the maximum number of concurrent unauthenticated
676 connections to the SSH daemon. Additional connections will be
677 dropped until authentication succeeds or the LoginGraceTime
678 expires for a connection. The default is 10:30:100.
680 Alternatively, random early drop can be enabled by specifying the
681 three colon separated values start:rate:full (e.g. "10:30:60").
682 sshd(8) will refuse connection attempts with a probability of
683 rate/100 (30%) if there are currently start (10) unauthenticated
684 connections. The probability increases linearly and all
685 connection attempts are refused if the number of unauthenticated
686 connections reaches full (60).
688 PasswordAuthentication
689 Specifies whether password authentication is allowed. The
693 When password authentication is allowed, it specifies whether the
694 server allows login to accounts with empty password strings. The
698 Specifies the destinations to which TCP port forwarding is
699 permitted. The forwarding specification must be one of the
703 PermitOpen IPv4_addr:port
704 PermitOpen [IPv6_addr]:port
706 Multiple forwards may be specified by separating them with
707 whitespace. An argument of any can be used to remove all
708 restrictions and permit any forwarding requests. An argument of
709 none can be used to prohibit all forwarding requests. The
710 wildcard M-bM-^@M-^X*M-bM-^@M-^Y can be used for host or port to allow all hosts or
711 ports, respectively. By default all port forwarding requests are
715 Specifies whether root can log in using ssh(1). The argument
716 must be yes, prohibit-password, forced-commands-only, or no. The
717 default is prohibit-password.
719 If this option is set to prohibit-password (or its deprecated
720 alias, without-password), password and keyboard-interactive
721 authentication are disabled for root.
723 If this option is set to forced-commands-only, root login with
724 public key authentication will be allowed, but only if the
725 command option has been specified (which may be useful for taking
726 remote backups even if root login is normally not allowed). All
727 other authentication methods are disabled for root.
729 If this option is set to no, root is not allowed to log in.
732 Specifies whether pty(4) allocation is permitted. The default is
736 Specifies whether tun(4) device forwarding is allowed. The
737 argument must be yes, point-to-point (layer 3), ethernet (layer
738 2), or no. Specifying yes permits both point-to-point and
739 ethernet. The default is no.
741 Independent of this setting, the permissions of the selected
742 tun(4) device must allow access to the user.
744 PermitUserEnvironment
745 Specifies whether ~/.ssh/environment and environment= options in
746 ~/.ssh/authorized_keys are processed by sshd(8). The default is
747 no. Enabling environment processing may enable users to bypass
748 access restrictions in some configurations using mechanisms such
752 Specifies whether any ~/.ssh/rc file is executed. The default is
756 Specifies the file that contains the process ID of the SSH
757 daemon, or none to not write one. The default is
760 Port Specifies the port number that sshd(8) listens on. The default
761 is 22. Multiple options of this type are permitted. See also
765 Specifies whether sshd(8) should print the date and time of the
766 last user login when a user logs in interactively. The default
770 Specifies whether sshd(8) should print /etc/motd when a user logs
771 in interactively. (On some systems it is also printed by the
772 shell, /etc/profile, or equivalent.) The default is yes.
774 PubkeyAcceptedKeyTypes
775 Specifies the key types that will be accepted for public key
776 authentication as a comma-separated pattern list. Alternately if
777 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
778 specified key types will be appended to the default set instead
779 of replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
780 character, then the specified key types (including wildcards)
781 will be removed from the default set instead of replacing them.
782 The default for this option is:
784 ecdsa-sha2-nistp256-cert-v01@openssh.com,
785 ecdsa-sha2-nistp384-cert-v01@openssh.com,
786 ecdsa-sha2-nistp521-cert-v01@openssh.com,
787 ssh-ed25519-cert-v01@openssh.com,
788 ssh-rsa-cert-v01@openssh.com,
789 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
792 The list of available key types may also be obtained using "ssh
796 Specifies whether public key authentication is allowed. The
800 Specifies the maximum amount of data that may be transmitted
801 before the session key is renegotiated, optionally followed a
802 maximum amount of time that may pass before the session key is
803 renegotiated. The first argument is specified in bytes and may
804 have a suffix of M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate Kilobytes,
805 Megabytes, or Gigabytes, respectively. The default is between
806 M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, depending on the cipher. The optional second
807 value is specified in seconds and may use any of the units
808 documented in the TIME FORMATS section. The default value for
809 RekeyLimit is default none, which means that rekeying is
810 performed after the cipher's default amount of data has been sent
811 or received and no time based rekeying is done.
814 Specifies revoked public keys file, or none to not use one. Keys
815 listed in this file will be refused for public key
816 authentication. Note that if this file is not readable, then
817 public key authentication will be refused for all users. Keys
818 may be specified as a text file, listing one public key per line,
819 or as an OpenSSH Key Revocation List (KRL) as generated by
820 ssh-keygen(1). For more information on KRLs, see the KEY
821 REVOCATION LISTS section in ssh-keygen(1).
824 Specifies an explicit routing domain that is applied after
825 authentication has completed. The user session, as well and any
826 forwarded or listening IP sockets, will be bound to this
827 rdomain(4). If the routing domain is set to %D, then the domain
828 in which the incoming connection was received will be applied.
831 Sets the octal file creation mode mask (umask) used when creating
832 a Unix-domain socket file for local or remote port forwarding.
833 This option is only used for port forwarding to a Unix-domain
836 The default value is 0177, which creates a Unix-domain socket
837 file that is readable and writable only by the owner. Note that
838 not all operating systems honor the file mode on Unix-domain
841 StreamLocalBindUnlink
842 Specifies whether to remove an existing Unix-domain socket file
843 for local or remote port forwarding before creating a new one.
844 If the socket file already exists and StreamLocalBindUnlink is
845 not enabled, sshd will be unable to forward the port to the Unix-
846 domain socket file. This option is only used for port forwarding
847 to a Unix-domain socket file.
849 The argument must be yes or no. The default is no.
852 Specifies whether sshd(8) should check file modes and ownership
853 of the user's files and home directory before accepting login.
854 This is normally desirable because novices sometimes accidentally
855 leave their directory or files world-writable. The default is
856 yes. Note that this does not apply to ChrootDirectory, whose
857 permissions and ownership are checked unconditionally.
860 Configures an external subsystem (e.g. file transfer daemon).
861 Arguments should be a subsystem name and a command (with optional
862 arguments) to execute upon subsystem request.
864 The command sftp-server implements the SFTP file transfer
867 Alternately the name internal-sftp implements an in-process SFTP
868 server. This may simplify configurations using ChrootDirectory
869 to force a different filesystem root on clients.
871 By default no subsystems are defined.
874 Gives the facility code that is used when logging messages from
875 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
876 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
880 Specifies whether the system should send TCP keepalive messages
881 to the other side. If they are sent, death of the connection or
882 crash of one of the machines will be properly noticed. However,
883 this means that connections will die if the route is down
884 temporarily, and some people find it annoying. On the other
885 hand, if TCP keepalives are not sent, sessions may hang
886 indefinitely on the server, leaving "ghost" users and consuming
889 The default is yes (to send TCP keepalive messages), and the
890 server will notice if the network goes down or the client host
891 crashes. This avoids infinitely hanging sessions.
893 To disable TCP keepalive messages, the value should be set to no.
896 Specifies a file containing public keys of certificate
897 authorities that are trusted to sign user certificates for
898 authentication, or none to not use one. Keys are listed one per
899 line; empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are allowed. If
900 a certificate is presented for authentication and has its signing
901 CA key listed in this file, then it may be used for
902 authentication for any user listed in the certificate's
903 principals list. Note that certificates that lack a list of
904 principals will not be permitted for authentication using
905 TrustedUserCAKeys. For more details on certificates, see the
906 CERTIFICATES section in ssh-keygen(1).
908 UseDNS Specifies whether sshd(8) should look up the remote host name,
909 and to check that the resolved host name for the remote IP
910 address maps back to the very same IP address.
912 If this option is set to no (the default) then only addresses and
913 not host names may be used in ~/.ssh/authorized_keys from and
914 sshd_config Match Host directives.
916 UsePAM Enables the Pluggable Authentication Module interface. If set to
917 yes this will enable PAM authentication using
918 ChallengeResponseAuthentication and PasswordAuthentication in
919 addition to PAM account and session module processing for all
920 authentication types.
922 Because PAM challenge-response authentication usually serves an
923 equivalent role to password authentication, you should disable
924 either PasswordAuthentication or ChallengeResponseAuthentication.
926 If UsePAM is enabled, you will not be able to run sshd(8) as a
927 non-root user. The default is no.
930 Optionally specifies additional text to append to the SSH
931 protocol banner sent by the server upon connection. The default
935 Specifies the first display number available for sshd(8)'s X11
936 forwarding. This prevents sshd from interfering with real X11
937 servers. The default is 10.
940 Specifies whether X11 forwarding is permitted. The argument must
941 be yes or no. The default is no.
943 When X11 forwarding is enabled, there may be additional exposure
944 to the server and to client displays if the sshd(8) proxy display
945 is configured to listen on the wildcard address (see
946 X11UseLocalhost), though this is not the default. Additionally,
947 the authentication spoofing and authentication data verification
948 and substitution occur on the client side. The security risk of
949 using X11 forwarding is that the client's X11 display server may
950 be exposed to attack when the SSH client requests forwarding (see
951 the warnings for ForwardX11 in ssh_config(5)). A system
952 administrator may have a stance in which they want to protect
953 clients that may expose themselves to attack by unwittingly
954 requesting X11 forwarding, which can warrant a no setting.
956 Note that disabling X11 forwarding does not prevent users from
957 forwarding X11 traffic, as users can always install their own
961 Specifies whether sshd(8) should bind the X11 forwarding server
962 to the loopback address or to the wildcard address. By default,
963 sshd binds the forwarding server to the loopback address and sets
964 the hostname part of the DISPLAY environment variable to
965 localhost. This prevents remote hosts from connecting to the
966 proxy display. However, some older X11 clients may not function
967 with this configuration. X11UseLocalhost may be set to no to
968 specify that the forwarding server should be bound to the
969 wildcard address. The argument must be yes or no. The default
973 Specifies the full pathname of the xauth(1) program, or none to
974 not use one. The default is /usr/X11R6/bin/xauth.
977 sshd(8) command-line arguments and configuration file options that
978 specify time may be expressed using a sequence of the form:
979 time[qualifier], where time is a positive integer value and qualifier is
980 one of the following:
982 M-bM-^_M-(noneM-bM-^_M-) seconds
989 Each member of the sequence is added together to calculate the total time
992 Time format examples:
994 600 600 seconds (10 minutes)
996 1h30m 1 hour 30 minutes (90 minutes)
999 Arguments to some keywords can make use of tokens, which are expanded at
1002 %% A literal M-bM-^@M-^X%M-bM-^@M-^Y.
1003 %D The routing domain in which the incoming connection was
1005 %F The fingerprint of the CA key.
1006 %f The fingerprint of the key or certificate.
1007 %h The home directory of the user.
1008 %i The key ID in the certificate.
1009 %K The base64-encoded CA key.
1010 %k The base64-encoded key or certificate for authentication.
1011 %s The serial number of the certificate.
1012 %T The type of the CA key.
1013 %t The key or certificate type.
1016 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, and %u.
1018 AuthorizedKeysFile accepts the tokens %%, %h, and %u.
1020 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1021 %k, %s, %T, %t, and %u.
1023 AuthorizedPrincipalsFile accepts the tokens %%, %h, and %u.
1025 ChrootDirectory accepts the tokens %%, %h, and %u.
1027 RoutingDomain accepts the token %D.
1030 /etc/ssh/sshd_config
1031 Contains configuration data for sshd(8). This file should be
1032 writable by root only, but it is recommended (though not
1033 necessary) that it be world-readable.
1036 sftp-server(8), sshd(8)
1039 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1040 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1041 de Raadt and Dug Song removed many bugs, re-added newer features and
1042 created OpenSSH. Markus Friedl contributed the support for SSH protocol
1043 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1044 for privilege separation.
1046 OpenBSD 6.2 February 16, 2018 OpenBSD 6.2
projects / FreeBSD / FreeBSD.git / blob