1SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
2
4 sshd_config — OpenSSH SSH daemon configuration file
5
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-argu‐
9 ment pairs, one per line. For each keyword, the first obtained value
10 will be used. Lines starting with ‘#’ and empty lines are interpreted as
11 comments. Arguments may optionally be enclosed in double quotes (") in
12 order to represent arguments containing spaces.
13 The possible keywords and their meanings are as follows (note that key‐
15 words are case-insensitive and arguments are case-sensitive):
16 AcceptEnv
18 Specifies what environment variables sent by the client will be
19 copied into the session's environ(7). See SendEnv and SetEnv in
20 ssh_config(5) for how to configure the client. The TERM environ‐
21 ment variable is always accepted whenever the client requests a
22 pseudo-terminal as it is required by the protocol. Variables are
23 specified by name, which may contain the wildcard characters ‘*’
24 and ‘?’. 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.
30 AddressFamily
32 Specifies which address family should be used by sshd(8). Valid
33 arguments are any (the default), inet (use IPv4 only), or inet6
34 (use IPv6 only).
35 AllowAgentForwarding
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.
41 AllowGroups
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
50 AllowGroups.
51 See PATTERNS in ssh_config(5) for more information on patterns.
53 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 for‐
58 warding, 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.
63 AllowTcpForwarding
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.
72 AllowUsers
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
84 finally AllowGroups.
85 See PATTERNS in ssh_config(5) for more information on patterns.
87 AuthenticationMethods
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 lists of comma-separated 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.
96 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 authentica‐
100 tion. Only methods that are next in one or more lists are
101 offered at each stage, so for this example it would not be possi‐
102 ble to attempt password or keyboard-interactive authentication
103 before public key.
104 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 or pam. depending on
108 the server configuration. For example,
109 "keyboard-interactive:bsdauth" would restrict keyboard interac‐
110 tive authentication to the bsdauth device.
111 If the publickey method is listed more than once, sshd(8) veri‐
113 fies that keys that have been used successfully are not reused
114 for subsequent authentications. For example,
115 "publickey,publickey" requires successful authentication using
116 two different public keys.
117 Note that each authentication method listed should also be
119 explicitly enabled in the configuration.
120 The available authentication methods are: "gssapi-with-mic",
122 "hostbased", "keyboard-interactive", "none" (used for access to
123 password-less accounts when PermitEmptyPasswords is enabled),
124 "password" and "publickey".
125 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 oth‐
129 ers 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
132 target user is used.
133 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 authentica‐
138 tion continues using the usual AuthorizedKeysFile files. By
139 default, no AuthorizedKeysCommand is run.
140 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.
147 AuthorizedKeysFile
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 expan‐
153 sion, AuthorizedKeysFile is taken to be an absolute path or one
154 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".
158 AuthorizedPrincipalsCommand
160 Specifies a program to be used to generate the list of allowed
161 certificate principals as per AuthorizedPrincipalsFile. The pro‐
162 gram must be owned by root, not writable by group or others and
163 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.
167 The program should produce on standard output zero or more lines
169 of AuthorizedPrincipalsFile output. If either
170 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is speci‐
171 fied, then certificates offered by the client for authentication
172 must contain a principal that is listed. By default, no
173 AuthorizedPrincipalsCommand is run.
174 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.
182 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 ‘#’ are ignored.
191 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 princi‐
196 pals file – in this case, the username of the user must appear in
197 a certificate's principals list for it to be accepted.
198 Note that AuthorizedPrincipalsFile is only used when authentica‐
200 tion proceeds using a CA listed in TrustedUserCAKeys and is not
201 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).
204 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.
208 CASignatureAlgorithms
210 Specifies which algorithms are allowed for signing of certifi‐
211 cates by certificate authorities (CAs). The default is:
212 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
214 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
215 Certificates signed using other algorithms will not be accepted
217 for public key or host-based authentication.
218 ChallengeResponseAuthentication
220 Specifies whether challenge-response authentication is allowed
221 (e.g. via PAM or through authentication styles supported in
222 login.conf(5)) The default is yes.
223 ChrootDirectory
225 Specifies the pathname of a directory to chroot(2) to after
226 authentication. At session startup sshd(8) checks that all com‐
227 ponents of the pathname are root-owned directories which are not
228 writable by any other user or group. After the chroot, sshd(8)
229 changes the working directory to the user's home directory.
230 Arguments to ChrootDirectory accept the tokens described in the
231 TOKENS section.
232 The ChrootDirectory must contain the necessary files and directo‐
234 ries to support the user's session. For an interactive session
235 this requires at least a shell, typically sh(1), and basic /dev
236 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
237 and tty(4) devices. For file transfer sessions using SFTP no
238 additional configuration of the environment is necessary if the
239 in-process sftp-server is used, though sessions which use logging
240 may require /dev/log inside the chroot directory on some operat‐
241 ing systems (see sftp-server(8) for details).
242 For safety, it is very important that the directory hierarchy be
244 prevented from modification by other processes on the system
245 (especially those outside the jail). Misconfiguration can lead
246 to unsafe environments which sshd(8) cannot detect.
247 The default is none, indicating not to chroot(2).
249 Ciphers
251 Specifies the ciphers allowed. Multiple ciphers must be comma-
252 separated. If the specified list begins with a ‘+’ character,
253 then the specified ciphers will be appended to the default set
254 instead of replacing them. If the specified list begins with a
255 ‘-’ character, then the specified ciphers (including wildcards)
256 will be removed from the default set instead of replacing them.
257 If the specified list begins with a ‘^’ character, then the spec‐
258 ified ciphers will be placed at the head of the default set.
259 The supported ciphers are:
261 3des-cbc
263 aes128-cbc
264 aes192-cbc
265 aes256-cbc
266 aes128-ctr
267 aes192-ctr
268 aes256-ctr
269 aes128-gcm@openssh.com
270 aes256-gcm@openssh.com
271 chacha20-poly1305@openssh.com
272 The default is handled system-wide by crypto-policies(7). To see
274 the defaults and how to modify this default, see manual page
275 update-crypto-policies(8).
276 The list of available ciphers may also be obtained using "ssh -Q
278 cipher".
279 ClientAliveCountMax
281 Sets the number of client alive messages which may be sent with‐
282 out sshd(8) receiving any messages back from the client. If this
283 threshold is reached while client alive messages are being sent,
284 sshd will disconnect the client, terminating the session. It is
285 important to note that the use of client alive messages is very
286 different from TCPKeepAlive. The client alive messages are sent
287 through the encrypted channel and therefore will not be spoofa‐
288 ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa‐
289 ble. The client alive mechanism is valuable when the client or
290 server depend on knowing when a connection has become unrespon‐
291 sive.
292 The default value is 3. If ClientAliveInterval is set to 15, and
294 ClientAliveCountMax is left at the default, unresponsive SSH
295 clients will be disconnected after approximately 45 seconds.
296 ClientAliveInterval
298 Sets a timeout interval in seconds after which if no data has
299 been received from the client, sshd(8) will send a message
300 through the encrypted channel to request a response from the
301 client. The default is 0, indicating that these messages will
302 not be sent to the client.
303 Compression
305 Specifies whether compression is enabled after the user has
306 authenticated successfully. The argument must be yes, delayed (a
307 legacy synonym for yes) or no. The default is yes.
308 DenyGroups
310 This keyword can be followed by a list of group name patterns,
311 separated by spaces. Login is disallowed for users whose primary
312 group or supplementary group list matches one of the patterns.
313 Only group names are valid; a numerical group ID is not recog‐
314 nized. By default, login is allowed for all groups. The
315 allow/deny directives are processed in the following order:
316 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
317 See PATTERNS in ssh_config(5) for more information on patterns.
319 DenyUsers
321 This keyword can be followed by a list of user name patterns,
322 separated by spaces. Login is disallowed for user names that
323 match one of the patterns. Only user names are valid; a numeri‐
324 cal user ID is not recognized. By default, login is allowed for
325 all users. If the pattern takes the form USER@HOST then USER and
326 HOST are separately checked, restricting logins to particular
327 users from particular hosts. HOST criteria may additionally con‐
328 tain addresses to match in CIDR address/masklen format. The
329 allow/deny directives are processed in the following order:
330 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
331 See PATTERNS in ssh_config(5) for more information on patterns.
333 DisableForwarding
335 Disables all forwarding features, including X11, ssh-agent(1),
336 TCP and StreamLocal. This option overrides all other forwarding-
337 related options and may simplify restricted configurations.
338 ExposeAuthInfo
340 Writes a temporary file containing a list of authentication meth‐
341 ods and public credentials (e.g. keys) used to authenticate the
342 user. The location of the file is exposed to the user session
343 through the SSH_USER_AUTH environment variable. The default is
344 no.
345 FingerprintHash
347 Specifies the hash algorithm used when logging key fingerprints.
348 Valid options are: md5 and sha256. The default is sha256.
349 ForceCommand
351 Forces the execution of the command specified by ForceCommand,
352 ignoring any command supplied by the client and ~/.ssh/rc if
353 present. The command is invoked by using the user's login shell
354 with the -c option. This applies to shell, command, or subsystem
355 execution. It is most useful inside a Match block. The command
356 originally supplied by the client is available in the
357 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
358 of internal-sftp will force the use of an in-process SFTP server
359 that requires no support files when used with ChrootDirectory.
360 The default is none.
361 GatewayPorts
363 Specifies whether remote hosts are allowed to connect to ports
364 forwarded for the client. By default, sshd(8) binds remote port
365 forwardings to the loopback address. This prevents other remote
366 hosts from connecting to forwarded ports. GatewayPorts can be
367 used to specify that sshd should allow remote port forwardings to
368 bind to non-loopback addresses, thus allowing other hosts to con‐
369 nect. The argument may be no to force remote port forwardings to
370 be available to the local host only, yes to force remote port
371 forwardings to bind to the wildcard address, or clientspecified
372 to allow the client to select the address to which the forwarding
373 is bound. The default is no.
374 GSSAPIAuthentication
376 Specifies whether user authentication based on GSSAPI is allowed.
377 The default is no.
378 GSSAPICleanupCredentials
380 Specifies whether to automatically destroy the user's credentials
381 cache on logout. The default is yes.
382 GSSAPIEnablek5users
384 Specifies whether to look at .k5users file for GSSAPI authentica‐
385 tion access control. Further details are described in ksu(1).
386 The default is no.
387 GSSAPIKeyExchange
389 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
390 key exchange doesn't rely on ssh keys to verify host identity.
391 The default is no.
392 GSSAPIStrictAcceptorCheck
394 Determines whether to be strict about the identity of the GSSAPI
395 acceptor a client authenticates against. If set to yes then the
396 client must authenticate against the host service on the current
397 hostname. If set to no then the client may authenticate against
398 any service key stored in the machine's default store. This
399 facility is provided to assist with operation on multi homed
400 machines. The default is yes.
401 GSSAPIStoreCredentialsOnRekey
403 Controls whether the user's GSSAPI credentials should be updated
404 following a successful connection rekeying. This option can be
405 used to accepted renewed or updated credentials from a compatible
406 client. The default is “no”.
407 For this to work GSSAPIKeyExchange needs to be enabled in the
409 server and also used by the client.
410 GSSAPIKexAlgorithms
412 The list of key exchange algorithms that are accepted by GSSAPI
413 key exchange. Possible values are
414 gss-gex-sha1-,
416 gss-group1-sha1-,
417 gss-group14-sha1-,
418 gss-group14-sha256-,
419 gss-group16-sha512-,
420 gss-nistp256-sha256-,
421 gss-curve25519-sha256-
422 The default is handled system-wide by crypto-policies(7). To see
424 the defaults and how to modify this default, see manual page
425 update-crypto-policies(8). This option only applies to protocol
426 version 2 connections using GSSAPI.
427 HostbasedAcceptedKeyTypes
429 Specifies the key types that will be accepted for hostbased
430 authentication as a list of comma-separated patterns. Alter‐
431 nately if the specified list begins with a ‘+’ character, then
432 the specified key types will be appended to the default set
433 instead of replacing them. If the specified list begins with a
434 ‘-’ character, then the specified key types (including wildcards)
435 will be removed from the default set instead of replacing them.
436 If the specified list begins with a ‘^’ character, then the spec‐
437 ified key types will be placed at the head of the default set.
438 The default for this option is:
439 ecdsa-sha2-nistp256-cert-v01@openssh.com,
441 ecdsa-sha2-nistp384-cert-v01@openssh.com,
442 ecdsa-sha2-nistp521-cert-v01@openssh.com,
443 ssh-ed25519-cert-v01@openssh.com,
444 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
445 ssh-rsa-cert-v01@openssh.com,
446 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
447 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
448 The list of available key types may also be obtained using "ssh
450 -Q key".
451 HostbasedAuthentication
453 Specifies whether rhosts or /etc/hosts.equiv authentication
454 together with successful public key client host authentication is
455 allowed (host-based authentication). The default is no.
456 HostbasedUsesNameFromPacketOnly
458 Specifies whether or not the server will attempt to perform a
459 reverse name lookup when matching the name in the ~/.shosts,
460 ~/.rhosts, and /etc/hosts.equiv files during
461 HostbasedAuthentication. A setting of yes means that sshd(8)
462 uses the name supplied by the client rather than attempting to
463 resolve the name from the TCP connection itself. The default is
464 no.
465 HostCertificate
467 Specifies a file containing a public host certificate. The cer‐
468 tificate's public key must match a private host key already spec‐
469 ified by HostKey. The default behaviour of sshd(8) is not to
470 load any certificates.
471 HostKey
473 Specifies a file containing a private host key used by SSH. The
474 defaults are /etc/ssh/ssh_host_ecdsa_key,
475 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.
476 Note that sshd(8) will refuse to use a file if it is group/world-
478 accessible and that the HostKeyAlgorithms option restricts which
479 of the keys are actually used by sshd(8).
480 It is possible to have multiple host key files. It is also pos‐
482 sible to specify public host key files instead. In this case
483 operations on the private key will be delegated to an
484 ssh-agent(1).
485 HostKeyAgent
487 Identifies the UNIX-domain socket used to communicate with an
488 agent that has access to the private host keys. If the string
489 "SSH_AUTH_SOCK" is specified, the location of the socket will be
490 read from the SSH_AUTH_SOCK environment variable.
491 HostKeyAlgorithms
493 Specifies the host key algorithms that the server offers. The
494 default is handled system-wide by crypto-policies(7). To see the
495 defaults and how to modify this default, see manual page
496 update-crypto-policies(8).
497 The list of available key types may also be obtained using "ssh
499 -Q key".
500 IgnoreRhosts
502 Specifies that .rhosts and .shosts files will not be used in
503 HostbasedAuthentication.
504 /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The
506 default is yes.
507 IgnoreUserKnownHosts
509 Specifies whether sshd(8) should ignore the user's
510 ~/.ssh/known_hosts during HostbasedAuthentication and use only
511 the system-wide known hosts file /etc/ssh/known_hosts. The
512 default is no.
513 IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec‐
515 tion. Accepted values are af11, af12, af13, af21, af22, af23,
516 af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
517 cs6, cs7, ef, lowdelay, throughput, reliability, a numeric value,
518 or none to use the operating system default. This option may
519 take one or two arguments, separated by whitespace. If one argu‐
520 ment is specified, it is used as the packet class uncondition‐
521 ally. If two values are specified, the first is automatically
522 selected for interactive sessions and the second for non-interac‐
523 tive sessions. The default is af21 (Low-Latency Data) for inter‐
524 active sessions and cs1 (Lower Effort) for non-interactive ses‐
525 sions.
526 KbdInteractiveAuthentication
528 Specifies whether to allow keyboard-interactive authentication.
529 The argument to this keyword must be yes or no. The default is
530 to use whatever value ChallengeResponseAuthentication is set to
531 (by default yes).
532 KerberosAuthentication
534 Specifies whether the password provided by the user for
535 PasswordAuthentication will be validated through the Kerberos
536 KDC. To use this option, the server needs a Kerberos servtab
537 which allows the verification of the KDC's identity. The default
538 is no.
539 KerberosGetAFSToken
541 If AFS is active and the user has a Kerberos 5 TGT, attempt to
542 acquire an AFS token before accessing the user's home directory.
543 The default is no.
544 KerberosOrLocalPasswd
546 If password authentication through Kerberos fails then the pass‐
547 word will be validated via any additional local mechanism such as
548 /etc/passwd. The default is yes.
549 KerberosTicketCleanup
551 Specifies whether to automatically destroy the user's ticket
552 cache file on logout. The default is yes.
553 KerberosUniqueCCache
555 Specifies whether to store the acquired tickets in the per-ses‐
556 sion credential cache under /tmp/ or whether to use per-user cre‐
557 dential cache as configured in /etc/krb5.conf. The default value
558 no can lead to overwriting previous tickets by subseqent connec‐
559 tions to the same user account.
560 KerberosUseKuserok
562 Specifies whether to look at .k5login file for user's aliases.
563 The default is yes.
564 KexAlgorithms
566 Specifies the available KEX (Key Exchange) algorithms. Multiple
567 algorithms must be comma-separated. Alternately if the specified
568 list begins with a ‘+’ character, then the specified methods will
569 be appended to the default set instead of replacing them. If the
570 specified list begins with a ‘-’ character, then the specified
571 methods (including wildcards) will be removed from the default
572 set instead of replacing them. If the specified list begins with
573 a ‘^’ character, then the specified methods will be placed at the
574 head of the default set. The supported algorithms are:
575 curve25519-sha256
577 curve25519-sha256@libssh.org
578 diffie-hellman-group1-sha1
579 diffie-hellman-group14-sha1
580 diffie-hellman-group14-sha256
581 diffie-hellman-group16-sha512
582 diffie-hellman-group18-sha512
583 diffie-hellman-group-exchange-sha1
584 diffie-hellman-group-exchange-sha256
585 ecdh-sha2-nistp256
586 ecdh-sha2-nistp384
587 ecdh-sha2-nistp521
588 The default is handled system-wide by crypto-policies(7). To see
590 the defaults and how to modify this default, see manual page
591 update-crypto-policies(8).
592 The list of available key exchange algorithms may also be
594 obtained using "ssh -Q kex".
595 ListenAddress
597 Specifies the local addresses sshd(8) should listen on. The fol‐
598 lowing forms may be used:
599 ListenAddress hostname|address [rdomain domain]
601 ListenAddress hostname:port [rdomain domain]
602 ListenAddress IPv4_address:port [rdomain domain]
603 ListenAddress [hostname|address]:port [rdomain domain]
604 The optional rdomain qualifier requests sshd(8) listen in an
606 explicit routing domain. If port is not specified, sshd will
607 listen on the address and all Port options specified. The
608 default is to listen on all local addresses on the current
609 default routing domain. Multiple ListenAddress options are per‐
610 mitted. For more information on routing domains, see rdomain(4).
611 LoginGraceTime
613 The server disconnects after this time if the user has not suc‐
614 cessfully logged in. If the value is 0, there is no time limit.
615 The default is 120 seconds.
616 LogLevel
618 Gives the verbosity level that is used when logging messages from
619 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
620 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
621 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
622 higher levels of debugging output. Logging with a DEBUG level
623 violates the privacy of users and is not recommended.
624 MACs Specifies the available MAC (message authentication code) algo‐
626 rithms. The MAC algorithm is used for data integrity protection.
627 Multiple algorithms must be comma-separated. If the specified
628 list begins with a ‘+’ character, then the specified algorithms
629 will be appended to the default set instead of replacing them.
630 If the specified list begins with a ‘-’ character, then the spec‐
631 ified algorithms (including wildcards) will be removed from the
632 default set instead of replacing them. If the specified list
633 begins with a ‘^’ character, then the specified algorithms will
634 be placed at the head of the default set.
635 The algorithms that contain "-etm" calculate the MAC after
637 encryption (encrypt-then-mac). These are considered safer and
638 their use recommended. The supported MACs are:
639 hmac-md5
641 hmac-md5-96
642 hmac-sha1
643 hmac-sha1-96
644 hmac-sha2-256
645 hmac-sha2-512
646 umac-64@openssh.com
647 umac-128@openssh.com
648 hmac-md5-etm@openssh.com
649 hmac-md5-96-etm@openssh.com
650 hmac-sha1-etm@openssh.com
651 hmac-sha1-96-etm@openssh.com
652 hmac-sha2-256-etm@openssh.com
653 hmac-sha2-512-etm@openssh.com
654 umac-64-etm@openssh.com
655 umac-128-etm@openssh.com
656 The default is handled system-wide by crypto-policies(7). To see
658 the defaults and how to modify this default, see manual page
659 update-crypto-policies(8).
660 The list of available MAC algorithms may also be obtained using
662 "ssh -Q mac".
663 Match Introduces a conditional block. If all of the criteria on the
665 Match line are satisfied, the keywords on the following lines
666 override those set in the global section of the config file,
667 until either another Match line or the end of the file. If a
668 keyword appears in multiple Match blocks that are satisfied, only
669 the first instance of the keyword is applied.
670 The arguments to Match are one or more criteria-pattern pairs or
672 the single token All which matches all criteria. The available
673 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
674 and Address (with RDomain representing the rdomain(4) on which
675 the connection was received).
676 The match patterns may consist of single entries or comma-sepa‐
678 rated lists and may use the wildcard and negation operators
679 described in the PATTERNS section of ssh_config(5).
680 The patterns in an Address criteria may additionally contain
682 addresses to match in CIDR address/masklen format, such as
683 192.0.2.0/24 or 2001:db8::/32. Note that the mask length pro‐
684 vided must be consistent with the address - it is an error to
685 specify a mask length that is too long for the address or one
686 with bits set in this host portion of the address. For example,
687 192.0.2.0/33 and 192.0.2.0/8, respectively.
688 Only a subset of keywords may be used on the lines following a
690 Match keyword. Available keywords are AcceptEnv,
691 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
692 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
693 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
694 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
695 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
696 Banner, ChrootDirectory, ClientAliveCountMax,
697 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
698 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
699 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
700 KbdInteractiveAuthentication, KerberosAuthentication,
701 KerberosUseKuserok, LogLevel, MaxAuthTries, MaxSessions,
702 PasswordAuthentication, PermitEmptyPasswords, PermitListen,
703 PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
704 PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
705 RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
706 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
707 X11MaxDisplays, X11Forwarding and X11UseLocalhost.
708 MaxAuthTries
710 Specifies the maximum number of authentication attempts permitted
711 per connection. Once the number of failures reaches half this
712 value, additional failures are logged. The default is 6.
713 MaxSessions
715 Specifies the maximum number of open shell, login or subsystem
716 (e.g. sftp) sessions permitted per network connection. Multiple
717 sessions may be established by clients that support connection
718 multiplexing. Setting MaxSessions to 1 will effectively disable
719 session multiplexing, whereas setting it to 0 will prevent all
720 shell, login and subsystem sessions while still permitting for‐
721 warding. The default is 10.
722 MaxStartups
724 Specifies the maximum number of concurrent unauthenticated con‐
725 nections to the SSH daemon. Additional connections will be
726 dropped until authentication succeeds or the LoginGraceTime
727 expires for a connection. The default is 10:30:100.
728 Alternatively, random early drop can be enabled by specifying the
730 three colon separated values start:rate:full (e.g. "10:30:60").
731 sshd(8) will refuse connection attempts with a probability of
732 rate/100 (30%) if there are currently start (10) unauthenticated
733 connections. The probability increases linearly and all connec‐
734 tion attempts are refused if the number of unauthenticated con‐
735 nections reaches full (60).
736 PasswordAuthentication
738 Specifies whether password authentication is allowed. The
739 default is yes.
740 PermitEmptyPasswords
742 When password authentication is allowed, it specifies whether the
743 server allows login to accounts with empty password strings. The
744 default is no.
745 PermitListen
747 Specifies the addresses/ports on which a remote TCP port forward‐
748 ing may listen. The listen specification must be one of the fol‐
749 lowing forms:
750 PermitListen port
752 PermitListen host:port
753 Multiple permissions may be specified by separating them with
755 whitespace. An argument of any can be used to remove all
756 restrictions and permit any listen requests. An argument of none
757 can be used to prohibit all listen requests. The host name may
758 contain wildcards as described in the PATTERNS section in
759 ssh_config(5). The wildcard ‘*’ can also be used in place of a
760 port number to allow all ports. By default all port forwarding
761 listen requests are permitted. Note that the GatewayPorts option
762 may further restrict which addresses may be listened on. Note
763 also that ssh(1) will request a listen host of “localhost” if no
764 listen host was specifically requested, and this name is treated
765 differently to explicit localhost addresses of “127.0.0.1” and
766 “::1”.
767 PermitOpen
769 Specifies the destinations to which TCP port forwarding is per‐
770 mitted. The forwarding specification must be one of the follow‐
771 ing forms:
772 PermitOpen host:port
774 PermitOpen IPv4_addr:port
775 PermitOpen [IPv6_addr]:port
776 Multiple forwards may be specified by separating them with white‐
778 space. An argument of any can be used to remove all restrictions
779 and permit any forwarding requests. An argument of none can be
780 used to prohibit all forwarding requests. The wildcard ‘*’ can
781 be used for host or port to allow all hosts or ports, respec‐
782 tively. By default all port forwarding requests are permitted.
783 PermitRootLogin
785 Specifies whether root can log in using ssh(1). The argument
786 must be yes, prohibit-password, forced-commands-only, or no. The
787 default is prohibit-password.
788 If this option is set to prohibit-password (or its deprecated
790 alias, without-password), password and keyboard-interactive
791 authentication are disabled for root.
792 If this option is set to forced-commands-only, root login with
794 public key authentication will be allowed, but only if the
795 command option has been specified (which may be useful for taking
796 remote backups even if root login is normally not allowed). All
797 other authentication methods are disabled for root.
798 If this option is set to no, root is not allowed to log in.
800 PermitTTY
802 Specifies whether pty(4) allocation is permitted. The default is
803 yes.
804 PermitTunnel
806 Specifies whether tun(4) device forwarding is allowed. The argu‐
807 ment must be yes, point-to-point (layer 3), ethernet (layer 2),
808 or no. Specifying yes permits both point-to-point and ethernet.
809 The default is no.
810 Independent of this setting, the permissions of the selected
812 tun(4) device must allow access to the user.
813 PermitUserEnvironment
815 Specifies whether ~/.ssh/environment and environment= options in
816 ~/.ssh/authorized_keys are processed by sshd(8). Valid options
817 are yes, no or a pattern-list specifying which environment vari‐
818 able names to accept (for example "LANG,LC_*"). The default is
819 no. Enabling environment processing may enable users to bypass
820 access restrictions in some configurations using mechanisms such
821 as LD_PRELOAD.
822 PermitUserRC
824 Specifies whether any ~/.ssh/rc file is executed. The default is
825 yes.
826 PidFile
828 Specifies the file that contains the process ID of the SSH dae‐
829 mon, or none to not write one. The default is /var/run/sshd.pid.
830 Port Specifies the port number that sshd(8) listens on. The default
832 is 22. Multiple options of this type are permitted. See also
833 ListenAddress.
834 PrintLastLog
836 Specifies whether sshd(8) should print the date and time of the
837 last user login when a user logs in interactively. The default
838 is yes.
839 PrintMotd
841 Specifies whether sshd(8) should print /etc/motd when a user logs
842 in interactively. (On some systems it is also printed by the
843 shell, /etc/profile, or equivalent.) The default is yes.
844 PubkeyAcceptedKeyTypes
846 Specifies the key types that will be accepted for public key
847 authentication as a list of comma-separated patterns. Alter‐
848 nately if the specified list begins with a ‘+’ character, then
849 the specified key types will be appended to the default set
850 instead of replacing them. If the specified list begins with a
851 ‘-’ character, then the specified key types (including wildcards)
852 will be removed from the default set instead of replacing them.
853 If the specified list begins with a ‘^’ character, then the spec‐
854 ified key types will be placed at the head of the default set.
855 The default is handled system-wide by crypto-policies(7). To see
856 the defaults and how to modify this default, see manual page
857 update-crypto-policies(8).
858 The list of available key types may also be obtained using "ssh
860 -Q key".
861 PubkeyAuthentication
863 Specifies whether public key authentication is allowed. The
864 default is yes.
865 RekeyLimit
867 Specifies the maximum amount of data that may be transmitted
868 before the session key is renegotiated, optionally followed a
869 maximum amount of time that may pass before the session key is
870 renegotiated. The first argument is specified in bytes and may
871 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
872 Megabytes, or Gigabytes, respectively. The default is between
873 ‘1G’ and ‘4G’, depending on the cipher. The optional second
874 value is specified in seconds and may use any of the units docu‐
875 mented in the TIME FORMATS section. The default value for
876 RekeyLimit is default none, which means that rekeying is per‐
877 formed after the cipher's default amount of data has been sent or
878 received and no time based rekeying is done.
879 RevokedKeys
881 Specifies revoked public keys file, or none to not use one. Keys
882 listed in this file will be refused for public key authentica‐
883 tion. Note that if this file is not readable, then public key
884 authentication will be refused for all users. Keys may be speci‐
885 fied as a text file, listing one public key per line, or as an
886 OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
887 For more information on KRLs, see the KEY REVOCATION LISTS sec‐
888 tion in ssh-keygen(1).
889 RDomain
891 Specifies an explicit routing domain that is applied after
892 authentication has completed. The user session, as well and any
893 forwarded or listening IP sockets, will be bound to this
894 rdomain(4). If the routing domain is set to %D, then the domain
895 in which the incoming connection was received will be applied.
896 SetEnv Specifies one or more environment variables to set in child ses‐
898 sions started by sshd(8) as “NAME=VALUE”. The environment value
899 may be quoted (e.g. if it contains whitespace characters). Envi‐
900 ronment variables set by SetEnv override the default environment
901 and any variables specified by the user via AcceptEnv or
902 PermitUserEnvironment.
903 StreamLocalBindMask
905 Sets the octal file creation mode mask (umask) used when creating
906 a Unix-domain socket file for local or remote port forwarding.
907 This option is only used for port forwarding to a Unix-domain
908 socket file.
909 The default value is 0177, which creates a Unix-domain socket
911 file that is readable and writable only by the owner. Note that
912 not all operating systems honor the file mode on Unix-domain
913 socket files.
914 StreamLocalBindUnlink
916 Specifies whether to remove an existing Unix-domain socket file
917 for local or remote port forwarding before creating a new one.
918 If the socket file already exists and StreamLocalBindUnlink is
919 not enabled, sshd will be unable to forward the port to the Unix-
920 domain socket file. This option is only used for port forwarding
921 to a Unix-domain socket file.
922 The argument must be yes or no. The default is no.
924 StrictModes
926 Specifies whether sshd(8) should check file modes and ownership
927 of the user's files and home directory before accepting login.
928 This is normally desirable because novices sometimes accidentally
929 leave their directory or files world-writable. The default is
930 yes. Note that this does not apply to ChrootDirectory, whose
931 permissions and ownership are checked unconditionally.
932 Subsystem
934 Configures an external subsystem (e.g. file transfer daemon).
935 Arguments should be a subsystem name and a command (with optional
936 arguments) to execute upon subsystem request.
937 The command sftp-server implements the SFTP file transfer subsys‐
939 tem.
940 Alternately the name internal-sftp implements an in-process SFTP
942 server. This may simplify configurations using ChrootDirectory
943 to force a different filesystem root on clients.
944 By default no subsystems are defined.
946 SyslogFacility
948 Gives the facility code that is used when logging messages from
949 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
950 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
951 The default is AUTH.
952 TCPKeepAlive
954 Specifies whether the system should send TCP keepalive messages
955 to the other side. If they are sent, death of the connection or
956 crash of one of the machines will be properly noticed. However,
957 this means that connections will die if the route is down tempo‐
958 rarily, and some people find it annoying. On the other hand, if
959 TCP keepalives are not sent, sessions may hang indefinitely on
960 the server, leaving "ghost" users and consuming server resources.
961 The default is yes (to send TCP keepalive messages), and the
963 server will notice if the network goes down or the client host
964 crashes. This avoids infinitely hanging sessions.
965 To disable TCP keepalive messages, the value should be set to no.
967 TrustedUserCAKeys
969 Specifies a file containing public keys of certificate authori‐
970 ties that are trusted to sign user certificates for authentica‐
971 tion, or none to not use one. Keys are listed one per line;
972 empty lines and comments starting with ‘#’ are allowed. If a
973 certificate is presented for authentication and has its signing
974 CA key listed in this file, then it may be used for authentica‐
975 tion for any user listed in the certificate's principals list.
976 Note that certificates that lack a list of principals will not be
977 permitted for authentication using TrustedUserCAKeys. For more
978 details on certificates, see the CERTIFICATES section in
979 ssh-keygen(1).
980 UseDNS Specifies whether sshd(8) should look up the remote host name,
982 and to check that the resolved host name for the remote IP
983 address maps back to the very same IP address.
984 If this option is set to no (the default) then only addresses and
986 not host names may be used in ~/.ssh/authorized_keys from and
987 sshd_config Match Host directives.
988 UsePAM Enables the Pluggable Authentication Module interface. If set to
990 yes this will enable PAM authentication using
991 ChallengeResponseAuthentication and PasswordAuthentication in
992 addition to PAM account and session module processing for all
993 authentication types.
994 Because PAM challenge-response authentication usually serves an
996 equivalent role to password authentication, you should disable
997 either PasswordAuthentication or ChallengeResponseAuthentication.
998 If UsePAM is enabled, you will not be able to run sshd(8) as a
1000 non-root user. The default is no.
1001 VersionAddendum
1003 Optionally specifies additional text to append to the SSH proto‐
1004 col banner sent by the server upon connection. The default is
1005 none.
1006 X11DisplayOffset
1008 Specifies the first display number available for sshd(8)'s X11
1009 forwarding. This prevents sshd from interfering with real X11
1010 servers. The default is 10.
1011 X11MaxDisplays
1013 Specifies the maximum number of displays available for sshd(8)'s
1014 X11 forwarding. This prevents sshd from exhausting local ports.
1015 The default is 1000.
1016 X11Forwarding
1018 Specifies whether X11 forwarding is permitted. The argument must
1019 be yes or no. The default is no.
1020 When X11 forwarding is enabled, there may be additional exposure
1022 to the server and to client displays if the sshd(8) proxy display
1023 is configured to listen on the wildcard address (see
1024 X11UseLocalhost), though this is not the default. Additionally,
1025 the authentication spoofing and authentication data verification
1026 and substitution occur on the client side. The security risk of
1027 using X11 forwarding is that the client's X11 display server may
1028 be exposed to attack when the SSH client requests forwarding (see
1029 the warnings for ForwardX11 in ssh_config(5)). A system adminis‐
1030 trator may have a stance in which they want to protect clients
1031 that may expose themselves to attack by unwittingly requesting
1032 X11 forwarding, which can warrant a no setting.
1033 Note that disabling X11 forwarding does not prevent users from
1035 forwarding X11 traffic, as users can always install their own
1036 forwarders.
1037 X11UseLocalhost
1039 Specifies whether sshd(8) should bind the X11 forwarding server
1040 to the loopback address or to the wildcard address. By default,
1041 sshd binds the forwarding server to the loopback address and sets
1042 the hostname part of the DISPLAY environment variable to
1043 localhost. This prevents remote hosts from connecting to the
1044 proxy display. However, some older X11 clients may not function
1045 with this configuration. X11UseLocalhost may be set to no to
1046 specify that the forwarding server should be bound to the wild‐
1047 card address. The argument must be yes or no. The default is
1048 yes.
1049 XAuthLocation
1051 Specifies the full pathname of the xauth(1) program, or none to
1052 not use one. The default is /usr/bin/xauth.
1053
1055 sshd(8) command-line arguments and configuration file options that spec‐
1056 ify time may be expressed using a sequence of the form: time[qualifier],
1057 where time is a positive integer value and qualifier is one of the fol‐
1058 lowing:
1059 ⟨none⟩ seconds
1061 s | S seconds
1062 m | M minutes
1063 h | H hours
1064 d | D days
1065 w | W weeks
1066 Each member of the sequence is added together to calculate the total time
1068 value.
1069 Time format examples:
1071 600 600 seconds (10 minutes)
1073 10m 10 minutes
1074 1h30m 1 hour 30 minutes (90 minutes)
1075
1077 Arguments to some keywords can make use of tokens, which are expanded at
1078 runtime:
1079 %% A literal ‘%’.
1081 %D The routing domain in which the incoming connection was
1082 received.
1083 %F The fingerprint of the CA key.
1084 %f The fingerprint of the key or certificate.
1085 %h The home directory of the user.
1086 %i The key ID in the certificate.
1087 %K The base64-encoded CA key.
1088 %k The base64-encoded key or certificate for authentication.
1089 %s The serial number of the certificate.
1090 %T The type of the CA key.
1091 %t The key or certificate type.
1092 %U The numeric user ID of the target user.
1093 %u The username.
1094 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1096 AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
1098 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1100 %k, %s, %T, %t, %U, and %u.
1101 AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
1103 ChrootDirectory accepts the tokens %%, %h, %U, and %u.
1105 RoutingDomain accepts the token %D.
1107
1109 /etc/ssh/sshd_config
1110 Contains configuration data for sshd(8). This file should be
1111 writable by root only, but it is recommended (though not neces‐
1112 sary) that it be world-readable.
1113
1115 sftp-server(8), sshd(8)
1116
1118 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1119 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1120 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1121 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1122 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1123 for privilege separation.
1124BSD December 31, 2019 BSD