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 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.
213 ChrootDirectory
215 Specifies the pathname of a directory to chroot(2) to after
216 authentication. At session startup sshd(8) checks that all com‐
217 ponents of the pathname are root-owned directories which are not
218 writable by any other user or group. After the chroot, sshd(8)
219 changes the working directory to the user's home directory.
220 Arguments to ChrootDirectory accept the tokens described in the
221 TOKENS section.
222 The ChrootDirectory must contain the necessary files and directo‐
224 ries to support the user's session. For an interactive session
225 this requires at least a shell, typically sh(1), and basic /dev
226 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
227 and tty(4) devices. For file transfer sessions using SFTP no
228 additional configuration of the environment is necessary if the
229 in-process sftp-server is used, though sessions which use logging
230 may require /dev/log inside the chroot directory on some operat‐
231 ing systems (see sftp-server(8) for details).
232 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.
237 The default is none, indicating not to chroot(2).
239 Ciphers
241 Specifies the ciphers allowed. Multiple ciphers must be comma-
242 separated. If the specified value begins with a ‘+’ 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 ‘-’ character, then the specified ciphers (including wildcards)
246 will be removed from the default set instead of replacing them.
247 The supported ciphers are:
249 3des-cbc
251 aes128-cbc
252 aes192-cbc
253 aes256-cbc
254 aes128-ctr
255 aes192-ctr
256 aes256-ctr
257 aes128-gcm@openssh.com
258 aes256-gcm@openssh.com
259 chacha20-poly1305@openssh.com
260 The default is:
262 chacha20-poly1305@openssh.com,
264 aes128-ctr,aes192-ctr,aes256-ctr,
265 aes128-gcm@openssh.com,aes256-gcm@openssh.com
266 The list of available ciphers may also be obtained using "ssh -Q
268 cipher".
269 ClientAliveCountMax
271 Sets the number of client alive messages which may be sent with‐
272 out sshd(8) receiving any messages back from the client. If this
273 threshold is reached while client alive messages are being sent,
274 sshd will disconnect the client, terminating the session. It is
275 important to note that the use of client alive messages is very
276 different from TCPKeepAlive. The client alive messages are sent
277 through the encrypted channel and therefore will not be spoofa‐
278 ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa‐
279 ble. The client alive mechanism is valuable when the client or
280 server depend on knowing when a connection has become inactive.
281 The default value is 3. If ClientAliveInterval is set to 15, and
283 ClientAliveCountMax is left at the default, unresponsive SSH
284 clients will be disconnected after approximately 45 seconds.
285 ClientAliveInterval
287 Sets a timeout interval in seconds after which if no data has
288 been received from the client, sshd(8) will send a message
289 through the encrypted channel to request a response from the
290 client. The default is 0, indicating that these messages will
291 not be sent to the client.
292 Compression
294 Specifies whether compression is enabled after the user has
295 authenticated successfully. The argument must be yes, delayed (a
296 legacy synonym for yes) or no. The default is yes.
297 DenyGroups
299 This keyword can be followed by a list of group name patterns,
300 separated by spaces. Login is disallowed for users whose primary
301 group or supplementary group list matches one of the patterns.
302 Only group names are valid; a numerical group ID is not recog‐
303 nized. By default, login is allowed for all groups. The
304 allow/deny directives are processed in the following order:
305 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
306 See PATTERNS in ssh_config(5) for more information on patterns.
308 DenyUsers
310 This keyword can be followed by a list of user name patterns,
311 separated by spaces. Login is disallowed for user names that
312 match one of the patterns. Only user names are valid; a numeri‐
313 cal user ID is not recognized. By default, login is allowed for
314 all users. If the pattern takes the form USER@HOST then USER and
315 HOST are separately checked, restricting logins to particular
316 users from particular hosts. HOST criteria may additionally con‐
317 tain addresses to match in CIDR address/masklen format. The
318 allow/deny directives are processed in the following order:
319 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
320 See PATTERNS in ssh_config(5) for more information on patterns.
322 DisableForwarding
324 Disables all forwarding features, including X11, ssh-agent(1),
325 TCP and StreamLocal. This option overrides all other forwarding-
326 related options and may simplify restricted configurations.
327 ExposeAuthInfo
329 Writes a temporary file containing a list of authentication meth‐
330 ods and public credentials (e.g. keys) used to authenticate the
331 user. The location of the file is exposed to the user session
332 through the SSH_USER_AUTH environment variable. The default is
333 no.
334 FingerprintHash
336 Specifies the hash algorithm used when logging key fingerprints.
337 Valid options are: md5 and sha256. The default is sha256.
338 ForceCommand
340 Forces the execution of the command specified by ForceCommand,
341 ignoring any command supplied by the client and ~/.ssh/rc if
342 present. The command is invoked by using the user's login shell
343 with the -c option. This applies to shell, command, or subsystem
344 execution. It is most useful inside a Match block. The command
345 originally supplied by the client is available in the
346 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
347 of internal-sftp will force the use of an in-process SFTP server
348 that requires no support files when used with ChrootDirectory.
349 The default is none.
350 GatewayPorts
352 Specifies whether remote hosts are allowed to connect to ports
353 forwarded for the client. By default, sshd(8) binds remote port
354 forwardings to the loopback address. This prevents other remote
355 hosts from connecting to forwarded ports. GatewayPorts can be
356 used to specify that sshd should allow remote port forwardings to
357 bind to non-loopback addresses, thus allowing other hosts to con‐
358 nect. The argument may be no to force remote port forwardings to
359 be available to the local host only, yes to force remote port
360 forwardings to bind to the wildcard address, or clientspecified
361 to allow the client to select the address to which the forwarding
362 is bound. The default is no.
363 GSSAPIAuthentication
365 Specifies whether user authentication based on GSSAPI is allowed.
366 The default is no.
367 GSSAPICleanupCredentials
369 Specifies whether to automatically destroy the user's credentials
370 cache on logout. The default is yes.
371 GSSAPIEnablek5users
373 Specifies whether to look at .k5users file for GSSAPI authentica‐
374 tion access control. Further details are described in ksu(1).
375 The default is no.
376 GSSAPIKeyExchange
378 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
379 key exchange doesn't rely on ssh keys to verify host identity.
380 The default is “no”.
381 GSSAPIStrictAcceptorCheck
383 Determines whether to be strict about the identity of the GSSAPI
384 acceptor a client authenticates against. If set to yes then the
385 client must authenticate against the host service on the current
386 hostname. If set to no then the client may authenticate against
387 any service key stored in the machine's default store. This
388 facility is provided to assist with operation on multi homed
389 machines. The default is yes.
390 GSSAPIStoreCredentialsOnRekey
392 Controls whether the user's GSSAPI credentials should be updated
393 following a successful connection rekeying. This option can be
394 used to accepted renewed or updated credentials from a compatible
395 client. The default is “no”.
396 For this to work GSSAPIKeyExchange needs to be enabled in the
398 server and also used by the client.
399 GSSAPIKexAlgorithms
401 The list of key exchange algorithms that are accepted by GSSAPI
402 key exchange. Possible values are
403 gss-gex-sha1-,
405 gss-group1-sha1-,
406 gss-group14-sha1-,
407 gss-group14-sha256-,
408 gss-group16-sha512-,
409 gss-nistp256-sha256-,
410 gss-curve25519-sha256-
411 The default is “gss-gex-sha1-,gss-group14-sha1-”. This option
413 only applies to protocol version 2 connections using GSSAPI.
414 HostbasedAcceptedKeyTypes
416 Specifies the key types that will be accepted for hostbased
417 authentication as a list of comma-separated patterns. Alter‐
418 nately if the specified value begins with a ‘+’ character, then
419 the specified key types will be appended to the default set
420 instead of replacing them. If the specified value begins with a
421 ‘-’ character, then the specified key types (including wildcards)
422 will be removed from the default set instead of replacing them.
423 The default for this option is:
424 ecdsa-sha2-nistp256-cert-v01@openssh.com,
426 ecdsa-sha2-nistp384-cert-v01@openssh.com,
427 ecdsa-sha2-nistp521-cert-v01@openssh.com,
428 ssh-ed25519-cert-v01@openssh.com,
429 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
430 ssh-rsa-cert-v01@openssh.com,
431 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
432 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
433 The list of available key types may also be obtained using "ssh
435 -Q key".
436 HostbasedAuthentication
438 Specifies whether rhosts or /etc/hosts.equiv authentication
439 together with successful public key client host authentication is
440 allowed (host-based authentication). The default is no.
441 HostbasedUsesNameFromPacketOnly
443 Specifies whether or not the server will attempt to perform a
444 reverse name lookup when matching the name in the ~/.shosts,
445 ~/.rhosts, and /etc/hosts.equiv files during
446 HostbasedAuthentication. A setting of yes means that sshd(8)
447 uses the name supplied by the client rather than attempting to
448 resolve the name from the TCP connection itself. The default is
449 no.
450 HostCertificate
452 Specifies a file containing a public host certificate. The cer‐
453 tificate's public key must match a private host key already spec‐
454 ified by HostKey. The default behaviour of sshd(8) is not to
455 load any certificates.
456 HostKey
458 Specifies a file containing a private host key used by SSH. The
459 defaults are /etc/ssh/ssh_host_ecdsa_key,
460 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.
461 Note that sshd(8) will refuse to use a file if it is group/world-
463 accessible and that the HostKeyAlgorithms option restricts which
464 of the keys are actually used by sshd(8).
465 It is possible to have multiple host key files. It is also pos‐
467 sible to specify public host key files instead. In this case
468 operations on the private key will be delegated to an
469 ssh-agent(1).
470 HostKeyAgent
472 Identifies the UNIX-domain socket used to communicate with an
473 agent that has access to the private host keys. If the string
474 "SSH_AUTH_SOCK" is specified, the location of the socket will be
475 read from the SSH_AUTH_SOCK environment variable.
476 HostKeyAlgorithms
478 Specifies the host key algorithms that the server offers. The
479 default for this option is:
480 ecdsa-sha2-nistp256-cert-v01@openssh.com,
482 ecdsa-sha2-nistp384-cert-v01@openssh.com,
483 ecdsa-sha2-nistp521-cert-v01@openssh.com,
484 ssh-ed25519-cert-v01@openssh.com,
485 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
486 ssh-rsa-cert-v01@openssh.com,
487 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
488 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
489 The list of available key types may also be obtained using "ssh
491 -Q key".
492 IgnoreRhosts
494 Specifies that .rhosts and .shosts files will not be used in
495 HostbasedAuthentication.
496 /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The
498 default is yes.
499 IgnoreUserKnownHosts
501 Specifies whether sshd(8) should ignore the user's
502 ~/.ssh/known_hosts during HostbasedAuthentication and use only
503 the system-wide known hosts file /etc/ssh/known_hosts. The
504 default is no.
505 IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec‐
507 tion. Accepted values are af11, af12, af13, af21, af22, af23,
508 af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
509 cs6, cs7, ef, lowdelay, throughput, reliability, a numeric value,
510 or none to use the operating system default. This option may
511 take one or two arguments, separated by whitespace. If one argu‐
512 ment is specified, it is used as the packet class uncondition‐
513 ally. If two values are specified, the first is automatically
514 selected for interactive sessions and the second for non-interac‐
515 tive sessions. The default is af21 (Low-Latency Data) for inter‐
516 active sessions and cs1 (Lower Effort) for non-interactive ses‐
517 sions.
518 KbdInteractiveAuthentication
520 Specifies whether to allow keyboard-interactive authentication.
521 The argument to this keyword must be yes or no. The default is
522 to use whatever value ChallengeResponseAuthentication is set to
523 (by default yes).
524 KerberosAuthentication
526 Specifies whether the password provided by the user for
527 PasswordAuthentication will be validated through the Kerberos
528 KDC. To use this option, the server needs a Kerberos servtab
529 which allows the verification of the KDC's identity. The default
530 is no.
531 KerberosGetAFSToken
533 If AFS is active and the user has a Kerberos 5 TGT, attempt to
534 acquire an AFS token before accessing the user's home directory.
535 The default is no.
536 KerberosOrLocalPasswd
538 If password authentication through Kerberos fails then the pass‐
539 word will be validated via any additional local mechanism such as
540 /etc/passwd. The default is yes.
541 KerberosTicketCleanup
543 Specifies whether to automatically destroy the user's ticket
544 cache file on logout. The default is yes.
545 KerberosUniqueCCache
547 Specifies whether to store the acquired tickets in the per-ses‐
548 sion credential cache under /tmp/ or whether to use per-user cre‐
549 dential cache as configured in /etc/krb5.conf. The default value
550 no can lead to overwriting previous tickets by subseqent connec‐
551 tions to the same user account.
552 KerberosUseKuserok
554 Specifies whether to look at .k5login file for user's aliases.
555 The default is yes.
556 KexAlgorithms
558 Specifies the available KEX (Key Exchange) algorithms. Multiple
559 algorithms must be comma-separated. Alternately if the specified
560 value begins with a ‘+’ character, then the specified methods
561 will be appended to the default set instead of replacing them.
562 If the specified value begins with a ‘-’ character, then the
563 specified methods (including wildcards) will be removed from the
564 default set instead of replacing them. The supported algorithms
565 are:
566 curve25519-sha256
568 curve25519-sha256@libssh.org
569 diffie-hellman-group1-sha1
570 diffie-hellman-group14-sha1
571 diffie-hellman-group14-sha256
572 diffie-hellman-group16-sha512
573 diffie-hellman-group18-sha512
574 diffie-hellman-group-exchange-sha1
575 diffie-hellman-group-exchange-sha256
576 ecdh-sha2-nistp256
577 ecdh-sha2-nistp384
578 ecdh-sha2-nistp521
579 The default is:
581 curve25519-sha256,curve25519-sha256@libssh.org,
583 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
584 diffie-hellman-group-exchange-sha256,
585 diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
586 diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
587 The list of available key exchange algorithms may also be
589 obtained using "ssh -Q kex".
590 ListenAddress
592 Specifies the local addresses sshd(8) should listen on. The fol‐
593 lowing forms may be used:
594 ListenAddress hostname|address [rdomain domain]
596 ListenAddress hostname:port [rdomain domain]
597 ListenAddress IPv4_address:port [rdomain domain]
598 ListenAddress [hostname|address]:port [rdomain domain]
599 The optional rdomain qualifier requests sshd(8) listen in an
601 explicit routing domain. If port is not specified, sshd will
602 listen on the address and all Port options specified. The
603 default is to listen on all local addresses on the current
604 default routing domain. Multiple ListenAddress options are per‐
605 mitted. For more information on routing domains, see rdomain(4).
606 LoginGraceTime
608 The server disconnects after this time if the user has not suc‐
609 cessfully logged in. If the value is 0, there is no time limit.
610 The default is 120 seconds.
611 LogLevel
613 Gives the verbosity level that is used when logging messages from
614 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
615 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
616 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
617 higher levels of debugging output. Logging with a DEBUG level
618 violates the privacy of users and is not recommended.
619 MACs Specifies the available MAC (message authentication code) algo‐
621 rithms. The MAC algorithm is used for data integrity protection.
622 Multiple algorithms must be comma-separated. If the specified
623 value begins with a ‘+’ character, then the specified algorithms
624 will be appended to the default set instead of replacing them.
625 If the specified value begins with a ‘-’ character, then the
626 specified algorithms (including wildcards) will be removed from
627 the default set instead of replacing them.
628 The algorithms that contain "-etm" calculate the MAC after
630 encryption (encrypt-then-mac). These are considered safer and
631 their use recommended. The supported MACs are:
632 hmac-md5
634 hmac-md5-96
635 hmac-sha1
636 hmac-sha1-96
637 hmac-sha2-256
638 hmac-sha2-512
639 umac-64@openssh.com
640 umac-128@openssh.com
641 hmac-md5-etm@openssh.com
642 hmac-md5-96-etm@openssh.com
643 hmac-sha1-etm@openssh.com
644 hmac-sha1-96-etm@openssh.com
645 hmac-sha2-256-etm@openssh.com
646 hmac-sha2-512-etm@openssh.com
647 umac-64-etm@openssh.com
648 umac-128-etm@openssh.com
649 The default is:
651 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
653 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
654 hmac-sha1-etm@openssh.com,
655 umac-64@openssh.com,umac-128@openssh.com,
656 hmac-sha2-256,hmac-sha2-512,hmac-sha1
657 The list of available MAC algorithms may also be obtained using
659 "ssh -Q mac".
660 Match Introduces a conditional block. If all of the criteria on the
662 Match line are satisfied, the keywords on the following lines
663 override those set in the global section of the config file,
664 until either another Match line or the end of the file. If a
665 keyword appears in multiple Match blocks that are satisfied, only
666 the first instance of the keyword is applied.
667 The arguments to Match are one or more criteria-pattern pairs or
669 the single token All which matches all criteria. The available
670 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
671 and Address (with RDomain representing the rdomain(4) on which
672 the connection was received.)
673 The match patterns may consist of single entries or comma-sepa‐
675 rated lists and may use the wildcard and negation operators
676 described in the PATTERNS section of ssh_config(5).
677 The patterns in an Address criteria may additionally contain
679 addresses to match in CIDR address/masklen format, such as
680 192.0.2.0/24 or 2001:db8::/32. Note that the mask length pro‐
681 vided must be consistent with the address - it is an error to
682 specify a mask length that is too long for the address or one
683 with bits set in this host portion of the address. For example,
684 192.0.2.0/33 and 192.0.2.0/8, respectively.
685 Only a subset of keywords may be used on the lines following a
687 Match keyword. Available keywords are AcceptEnv,
688 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
689 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
690 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
691 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
692 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
693 Banner, ChrootDirectory, ClientAliveCountMax,
694 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
695 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
696 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
697 KbdInteractiveAuthentication, KerberosAuthentication,
698 KerberosUseKuserok, LogLevel, MaxAuthTries, MaxSessions,
699 PasswordAuthentication, PermitEmptyPasswords, PermitListen,
700 PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
701 PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
702 RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
703 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
704 X11MaxDisplays, X11Forwarding and X11UseLocalHost.
705 MaxAuthTries
707 Specifies the maximum number of authentication attempts permitted
708 per connection. Once the number of failures reaches half this
709 value, additional failures are logged. The default is 6.
710 MaxSessions
712 Specifies the maximum number of open shell, login or subsystem
713 (e.g. sftp) sessions permitted per network connection. Multiple
714 sessions may be established by clients that support connection
715 multiplexing. Setting MaxSessions to 1 will effectively disable
716 session multiplexing, whereas setting it to 0 will prevent all
717 shell, login and subsystem sessions while still permitting for‐
718 warding. The default is 10.
719 MaxStartups
721 Specifies the maximum number of concurrent unauthenticated con‐
722 nections to the SSH daemon. Additional connections will be
723 dropped until authentication succeeds or the LoginGraceTime
724 expires for a connection. The default is 10:30:100.
725 Alternatively, random early drop can be enabled by specifying the
727 three colon separated values start:rate:full (e.g. "10:30:60").
728 sshd(8) will refuse connection attempts with a probability of
729 rate/100 (30%) if there are currently start (10) unauthenticated
730 connections. The probability increases linearly and all connec‐
731 tion attempts are refused if the number of unauthenticated con‐
732 nections reaches full (60).
733 PasswordAuthentication
735 Specifies whether password authentication is allowed. The
736 default is yes.
737 PermitEmptyPasswords
739 When password authentication is allowed, it specifies whether the
740 server allows login to accounts with empty password strings. The
741 default is no.
742 PermitListen
744 Specifies the addresses/ports on which a remote TCP port forward‐
745 ing may listen. The listen specification must be one of the fol‐
746 lowing forms:
747 PermitListen port
749 PermitListen host:port
750 Multiple permissions may be specified by separating them with
752 whitespace. An argument of any can be used to remove all
753 restrictions and permit any listen requests. An argument of none
754 can be used to prohibit all listen requests. The host name may
755 contain wildcards as described in the PATTERNS section in
756 ssh_config(5). The wildcard ‘*’ can also be used in place of a
757 port number to allow all ports. By default all port forwarding
758 listen requests are permitted. Note that the GatewayPorts option
759 may further restrict which addresses may be listened on. Note
760 also that ssh(1) will request a listen host of “localhost” if no
761 listen host was specifically requested, and this this name is
762 treated differently to explicit localhost addresses of
763 “127.0.0.1” and “::1”.
764 PermitOpen
766 Specifies the destinations to which TCP port forwarding is per‐
767 mitted. The forwarding specification must be one of the follow‐
768 ing forms:
769 PermitOpen host:port
771 PermitOpen IPv4_addr:port
772 PermitOpen [IPv6_addr]:port
773 Multiple forwards may be specified by separating them with white‐
775 space. An argument of any can be used to remove all restrictions
776 and permit any forwarding requests. An argument of none can be
777 used to prohibit all forwarding requests. The wildcard ‘*’ can
778 be used for host or port to allow all hosts or ports, respec‐
779 tively. By default all port forwarding requests are permitted.
780 PermitRootLogin
782 Specifies whether root can log in using ssh(1). The argument
783 must be yes, prohibit-password, forced-commands-only, or no. The
784 default is prohibit-password.
785 If this option is set to prohibit-password (or its deprecated
787 alias, without-password), password and keyboard-interactive
788 authentication are disabled for root.
789 If this option is set to forced-commands-only, root login with
791 public key authentication will be allowed, but only if the
792 command option has been specified (which may be useful for taking
793 remote backups even if root login is normally not allowed). All
794 other authentication methods are disabled for root.
795 If this option is set to no, root is not allowed to log in.
797 PermitTTY
799 Specifies whether pty(4) allocation is permitted. The default is
800 yes.
801 PermitTunnel
803 Specifies whether tun(4) device forwarding is allowed. The argu‐
804 ment must be yes, point-to-point (layer 3), ethernet (layer 2),
805 or no. Specifying yes permits both point-to-point and ethernet.
806 The default is no.
807 Independent of this setting, the permissions of the selected
809 tun(4) device must allow access to the user.
810 PermitUserEnvironment
812 Specifies whether ~/.ssh/environment and environment= options in
813 ~/.ssh/authorized_keys are processed by sshd(8). Valid options
814 are yes, no or a pattern-list specifying which environment vari‐
815 able names to accept (for example "LANG,LC_*"). The default is
816 no. Enabling environment processing may enable users to bypass
817 access restrictions in some configurations using mechanisms such
818 as LD_PRELOAD.
819 PermitUserRC
821 Specifies whether any ~/.ssh/rc file is executed. The default is
822 yes.
823 PidFile
825 Specifies the file that contains the process ID of the SSH dae‐
826 mon, or none to not write one. The default is /var/run/sshd.pid.
827 Port Specifies the port number that sshd(8) listens on. The default
829 is 22. Multiple options of this type are permitted. See also
830 ListenAddress.
831 PrintLastLog
833 Specifies whether sshd(8) should print the date and time of the
834 last user login when a user logs in interactively. The default
835 is yes.
836 PrintMotd
838 Specifies whether sshd(8) should print /etc/motd when a user logs
839 in interactively. (On some systems it is also printed by the
840 shell, /etc/profile, or equivalent.) The default is yes.
841 PubkeyAcceptedKeyTypes
843 Specifies the key types that will be accepted for public key
844 authentication as a list of comma-separated patterns. Alter‐
845 nately if the specified value begins with a ‘+’ character, then
846 the specified key types will be appended to the default set
847 instead of replacing them. If the specified value begins with a
848 ‘-’ character, then the specified key types (including wildcards)
849 will be removed from the default set instead of replacing them.
850 The default for this option is:
851 ecdsa-sha2-nistp256-cert-v01@openssh.com,
853 ecdsa-sha2-nistp384-cert-v01@openssh.com,
854 ecdsa-sha2-nistp521-cert-v01@openssh.com,
855 ssh-ed25519-cert-v01@openssh.com,
856 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
857 ssh-rsa-cert-v01@openssh.com,
858 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
859 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
860 The list of available key types may also be obtained using "ssh
862 -Q key".
863 PubkeyAuthentication
865 Specifies whether public key authentication is allowed. The
866 default is yes.
867 RekeyLimit
869 Specifies the maximum amount of data that may be transmitted
870 before the session key is renegotiated, optionally followed a
871 maximum amount of time that may pass before the session key is
872 renegotiated. The first argument is specified in bytes and may
873 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
874 Megabytes, or Gigabytes, respectively. The default is between
875 ‘1G’ and ‘4G’, depending on the cipher. The optional second
876 value is specified in seconds and may use any of the units docu‐
877 mented in the TIME FORMATS section. The default value for
878 RekeyLimit is default none, which means that rekeying is per‐
879 formed after the cipher's default amount of data has been sent or
880 received and no time based rekeying is done.
881 RevokedKeys
883 Specifies revoked public keys file, or none to not use one. Keys
884 listed in this file will be refused for public key authentica‐
885 tion. Note that if this file is not readable, then public key
886 authentication will be refused for all users. Keys may be speci‐
887 fied as a text file, listing one public key per line, or as an
888 OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
889 For more information on KRLs, see the KEY REVOCATION LISTS sec‐
890 tion in ssh-keygen(1).
891 RDomain
893 Specifies an explicit routing domain that is applied after
894 authentication has completed. The user session, as well and any
895 forwarded or listening IP sockets, will be bound to this
896 rdomain(4). If the routing domain is set to %D, then the domain
897 in which the incoming connection was received will be applied.
898 SetEnv Specifies one or more environment variables to set in child ses‐
900 sions started by sshd(8) as “NAME=VALUE”. The environment value
901 may be quoted (e.g. if it contains whitespace characters). Envi‐
902 ronment variables set by SetEnv override the default environment
903 and any variables specified by the user via AcceptEnv or
904 PermitUserEnvironment.
905 ShowPatchLevel
907 Specifies whether sshd will display the patch level of the binary
908 in the identification string. The patch level is set at compile-
909 time. The default is “no”.
910 StreamLocalBindMask
912 Sets the octal file creation mode mask (umask) used when creating
913 a Unix-domain socket file for local or remote port forwarding.
914 This option is only used for port forwarding to a Unix-domain
915 socket file.
916 The default value is 0177, which creates a Unix-domain socket
918 file that is readable and writable only by the owner. Note that
919 not all operating systems honor the file mode on Unix-domain
920 socket files.
921 StreamLocalBindUnlink
923 Specifies whether to remove an existing Unix-domain socket file
924 for local or remote port forwarding before creating a new one.
925 If the socket file already exists and StreamLocalBindUnlink is
926 not enabled, sshd will be unable to forward the port to the Unix-
927 domain socket file. This option is only used for port forwarding
928 to a Unix-domain socket file.
929 The argument must be yes or no. The default is no.
931 StrictModes
933 Specifies whether sshd(8) should check file modes and ownership
934 of the user's files and home directory before accepting login.
935 This is normally desirable because novices sometimes accidentally
936 leave their directory or files world-writable. The default is
937 yes. Note that this does not apply to ChrootDirectory, whose
938 permissions and ownership are checked unconditionally.
939 Subsystem
941 Configures an external subsystem (e.g. file transfer daemon).
942 Arguments should be a subsystem name and a command (with optional
943 arguments) to execute upon subsystem request.
944 The command sftp-server implements the SFTP file transfer subsys‐
946 tem.
947 Alternately the name internal-sftp implements an in-process SFTP
949 server. This may simplify configurations using ChrootDirectory
950 to force a different filesystem root on clients.
951 By default no subsystems are defined.
953 SyslogFacility
955 Gives the facility code that is used when logging messages from
956 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
957 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
958 The default is AUTH.
959 TCPKeepAlive
961 Specifies whether the system should send TCP keepalive messages
962 to the other side. If they are sent, death of the connection or
963 crash of one of the machines will be properly noticed. However,
964 this means that connections will die if the route is down tempo‐
965 rarily, and some people find it annoying. On the other hand, if
966 TCP keepalives are not sent, sessions may hang indefinitely on
967 the server, leaving "ghost" users and consuming server resources.
968 The default is yes (to send TCP keepalive messages), and the
970 server will notice if the network goes down or the client host
971 crashes. This avoids infinitely hanging sessions.
972 To disable TCP keepalive messages, the value should be set to no.
974 TrustedUserCAKeys
976 Specifies a file containing public keys of certificate authori‐
977 ties that are trusted to sign user certificates for authentica‐
978 tion, or none to not use one. Keys are listed one per line;
979 empty lines and comments starting with ‘#’ are allowed. If a
980 certificate is presented for authentication and has its signing
981 CA key listed in this file, then it may be used for authentica‐
982 tion for any user listed in the certificate's principals list.
983 Note that certificates that lack a list of principals will not be
984 permitted for authentication using TrustedUserCAKeys. For more
985 details on certificates, see the CERTIFICATES section in
986 ssh-keygen(1).
987 UseDNS Specifies whether sshd(8) should look up the remote host name,
989 and to check that the resolved host name for the remote IP
990 address maps back to the very same IP address.
991 If this option is set to no (the default) then only addresses and
993 not host names may be used in ~/.ssh/authorized_keys from and
994 sshd_config Match Host directives.
995 UsePAM Enables the Pluggable Authentication Module interface. If set to
997 yes this will enable PAM authentication using
998 ChallengeResponseAuthentication and PasswordAuthentication in
999 addition to PAM account and session module processing for all
1000 authentication types.
1001 Because PAM challenge-response authentication usually serves an
1003 equivalent role to password authentication, you should disable
1004 either PasswordAuthentication or ChallengeResponseAuthentication.
1005 If UsePAM is enabled, you will not be able to run sshd(8) as a
1007 non-root user. The default is no.
1008 VersionAddendum
1010 Optionally specifies additional text to append to the SSH proto‐
1011 col banner sent by the server upon connection. The default is
1012 none.
1013 X11DisplayOffset
1015 Specifies the first display number available for sshd(8)'s X11
1016 forwarding. This prevents sshd from interfering with real X11
1017 servers. The default is 10.
1018 X11MaxDisplays
1020 Specifies the maximum number of displays available for sshd(8)'s
1021 X11 forwarding. This prevents sshd from exhausting local ports.
1022 The default is 1000.
1023 X11Forwarding
1025 Specifies whether X11 forwarding is permitted. The argument must
1026 be yes or no. The default is no.
1027 When X11 forwarding is enabled, there may be additional exposure
1029 to the server and to client displays if the sshd(8) proxy display
1030 is configured to listen on the wildcard address (see
1031 X11UseLocalhost), though this is not the default. Additionally,
1032 the authentication spoofing and authentication data verification
1033 and substitution occur on the client side. The security risk of
1034 using X11 forwarding is that the client's X11 display server may
1035 be exposed to attack when the SSH client requests forwarding (see
1036 the warnings for ForwardX11 in ssh_config(5)). A system adminis‐
1037 trator may have a stance in which they want to protect clients
1038 that may expose themselves to attack by unwittingly requesting
1039 X11 forwarding, which can warrant a no setting.
1040 Note that disabling X11 forwarding does not prevent users from
1042 forwarding X11 traffic, as users can always install their own
1043 forwarders.
1044 X11UseLocalhost
1046 Specifies whether sshd(8) should bind the X11 forwarding server
1047 to the loopback address or to the wildcard address. By default,
1048 sshd binds the forwarding server to the loopback address and sets
1049 the hostname part of the DISPLAY environment variable to
1050 localhost. This prevents remote hosts from connecting to the
1051 proxy display. However, some older X11 clients may not function
1052 with this configuration. X11UseLocalhost may be set to no to
1053 specify that the forwarding server should be bound to the wild‐
1054 card address. The argument must be yes or no. The default is
1055 yes.
1056 XAuthLocation
1058 Specifies the full pathname of the xauth(1) program, or none to
1059 not use one. The default is /usr/bin/xauth.
1060
1062 sshd(8) command-line arguments and configuration file options that spec‐
1063 ify time may be expressed using a sequence of the form: time[qualifier],
1064 where time is a positive integer value and qualifier is one of the fol‐
1065 lowing:
1066 ⟨none⟩ seconds
1068 s | S seconds
1069 m | M minutes
1070 h | H hours
1071 d | D days
1072 w | W weeks
1073 Each member of the sequence is added together to calculate the total time
1075 value.
1076 Time format examples:
1078 600 600 seconds (10 minutes)
1080 10m 10 minutes
1081 1h30m 1 hour 30 minutes (90 minutes)
1082
1084 Arguments to some keywords can make use of tokens, which are expanded at
1085 runtime:
1086 %% A literal ‘%’.
1088 %D The routing domain in which the incoming connection was
1089 received.
1090 %F The fingerprint of the CA key.
1091 %f The fingerprint of the key or certificate.
1092 %h The home directory of the user.
1093 %i The key ID in the certificate.
1094 %K The base64-encoded CA key.
1095 %k The base64-encoded key or certificate for authentication.
1096 %s The serial number of the certificate.
1097 %T The type of the CA key.
1098 %t The key or certificate type.
1099 %U The numeric user ID of the target user.
1100 %u The username.
1101 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1103 AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
1105 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1107 %k, %s, %T, %t, %U, and %u.
1108 AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
1110 ChrootDirectory accepts the tokens %%, %h, %U, and %u.
1112 RoutingDomain accepts the token %D.
1114
1116 /etc/ssh/sshd_config
1117 Contains configuration data for sshd(8). This file should be
1118 writable by root only, but it is recommended (though not neces‐
1119 sary) that it be world-readable.
1120
1122 sftp-server(8), sshd(8)
1123
1125 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1126 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1127 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1128 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1129 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1130 for privilege separation.
1131BSD October 26, 2019 BSD