1SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
2
4 sshd_config — OpenSSH daemon configuration file
5
7 sshd(8) reads configuration data from /etc/gsissh/sshd_config (or the
8 file specified with -f on the command line). The file contains keyword-
9 argument pairs, one per line. For each keyword, the first obtained value
10 will be used. Lines starting with ‘#’ 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 groups directives are processed in the
49 following order: DenyGroups, AllowGroups.
50 See PATTERNS in ssh_config(5) for more information on patterns.
52 AllowStreamLocalForwarding
54 Specifies whether StreamLocal (Unix-domain socket) forwarding is
55 permitted. The available options are yes (the default) or all to
56 allow StreamLocal forwarding, no to prevent all StreamLocal for‐
57 warding, local to allow local (from the perspective of ssh(1))
58 forwarding only or remote to allow remote forwarding only. Note
59 that disabling StreamLocal forwarding does not improve security
60 unless users are also denied shell access, as they can always
61 install their own forwarders.
62 AllowTcpForwarding
64 Specifies whether TCP forwarding is permitted. The available
65 options are yes (the default) or all to allow TCP forwarding, no
66 to prevent all TCP forwarding, local to allow local (from the
67 perspective of ssh(1)) forwarding only or remote to allow remote
68 forwarding only. Note that disabling TCP forwarding does not
69 improve security unless users are also denied shell access, as
70 they can always install their own forwarders.
71 AllowUsers
73 This keyword can be followed by a list of user name patterns,
74 separated by spaces. If specified, login is allowed only for
75 user names that match one of the patterns. Only user names are
76 valid; a numerical user ID is not recognized. By default, login
77 is allowed for all users. If the pattern takes the form
78 USER@HOST then USER and HOST are separately checked, restricting
79 logins to particular users from particular hosts. HOST criteria
80 may additionally contain addresses to match in CIDR
81 address/masklen format. The allow/deny users directives are pro‐
82 cessed in the following order: DenyUsers, AllowUsers.
83 See PATTERNS in ssh_config(5) for more information on patterns.
85 AuthenticationMethods
87 Specifies the authentication methods that must be successfully
88 completed for a user to be granted access. This option must be
89 followed by one or more lists of comma-separated authentication
90 method names, or by the single string any to indicate the default
91 behaviour of accepting any single authentication method. If the
92 default is overridden, then successful authentication requires
93 completion of every method in at least one of these lists.
94 For example, "publickey,password publickey,keyboard-interactive"
96 would require the user to complete public key authentication,
97 followed by either password or keyboard interactive authentica‐
98 tion. Only methods that are next in one or more lists are
99 offered at each stage, so for this example it would not be possi‐
100 ble to attempt password or keyboard-interactive authentication
101 before public key.
102 For keyboard interactive authentication it is also possible to
104 restrict authentication to a specific device by appending a colon
105 followed by the device identifier bsdauth or pam. depending on
106 the server configuration. For example,
107 "keyboard-interactive:bsdauth" would restrict keyboard interac‐
108 tive authentication to the bsdauth device.
109 If the publickey method is listed more than once, sshd(8) veri‐
111 fies that keys that have been used successfully are not reused
112 for subsequent authentications. For example,
113 "publickey,publickey" requires successful authentication using
114 two different public keys.
115 Note that each authentication method listed should also be
117 explicitly enabled in the configuration.
118 The available authentication methods are: "gssapi-with-mic",
120 "hostbased", "keyboard-interactive", "none" (used for access to
121 password-less accounts when PermitEmptyPasswords is enabled),
122 "password" and "publickey".
123 AuthorizedKeysCommand
125 Specifies a program to be used to look up the user's public keys.
126 The program must be owned by root, not writable by group or oth‐
127 ers and specified by an absolute path. Arguments to
128 AuthorizedKeysCommand accept the tokens described in the TOKENS
129 section. If no arguments are specified then the username of the
130 target user is used.
131 The program should produce on standard output zero or more lines
133 of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a
134 key supplied by AuthorizedKeysCommand does not successfully
135 authenticate and authorize the user then public key authentica‐
136 tion continues using the usual AuthorizedKeysFile files. By
137 default, no AuthorizedKeysCommand is run.
138 AuthorizedKeysCommandUser
140 Specifies the user under whose account the AuthorizedKeysCommand
141 is run. It is recommended to use a dedicated user that has no
142 other role on the host than running authorized keys commands. If
143 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser
144 is not, then sshd(8) will refuse to start.
145 AuthorizedKeysFile
147 Specifies the file that contains the public keys used for user
148 authentication. The format is described in the AUTHORIZED_KEYS
149 FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile
150 accept the tokens described in the TOKENS section. After expan‐
151 sion, AuthorizedKeysFile is taken to be an absolute path or one
152 relative to the user's home directory. Multiple files may be
153 listed, separated by whitespace. Alternately this option may be
154 set to none to skip checking for user keys in files. The default
155 is ".ssh/authorized_keys .ssh/authorized_keys2".
156 AuthorizedPrincipalsCommand
158 Specifies a program to be used to generate the list of allowed
159 certificate principals as per AuthorizedPrincipalsFile. The pro‐
160 gram must be owned by root, not writable by group or others and
161 specified by an absolute path. Arguments to
162 AuthorizedPrincipalsCommand accept the tokens described in the
163 TOKENS section. If no arguments are specified then the username
164 of the target user is used.
165 The program should produce on standard output zero or more lines
167 of AuthorizedPrincipalsFile output. If either
168 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is speci‐
169 fied, then certificates offered by the client for authentication
170 must contain a principal that is listed. By default, no
171 AuthorizedPrincipalsCommand is run.
172 AuthorizedPrincipalsCommandUser
174 Specifies the user under whose account the
175 AuthorizedPrincipalsCommand is run. It is recommended to use a
176 dedicated user that has no other role on the host than running
177 authorized principals commands. If AuthorizedPrincipalsCommand
178 is specified but AuthorizedPrincipalsCommandUser is not, then
179 sshd(8) will refuse to start.
180 AuthorizedPrincipalsFile
182 Specifies a file that lists principal names that are accepted for
183 certificate authentication. When using certificates signed by a
184 key listed in TrustedUserCAKeys, this file lists names, one of
185 which must appear in the certificate for it to be accepted for
186 authentication. Names are listed one per line preceded by key
187 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
188 Empty lines and comments starting with ‘#’ are ignored.
189 Arguments to AuthorizedPrincipalsFile accept the tokens described
191 in the TOKENS section. After expansion, AuthorizedPrincipalsFile
192 is taken to be an absolute path or one relative to the user's
193 home directory. The default is none, i.e. not to use a princi‐
194 pals file – in this case, the username of the user must appear in
195 a certificate's principals list for it to be accepted.
196 Note that AuthorizedPrincipalsFile is only used when authentica‐
198 tion proceeds using a CA listed in TrustedUserCAKeys and is not
199 consulted for certification authorities trusted via
200 ~/.ssh/authorized_keys, though the principals= key option offers
201 a similar facility (see sshd(8) for details).
202 Banner The contents of the specified file are sent to the remote user
204 before authentication is allowed. If the argument is none then
205 no banner is displayed. By default, no banner is displayed.
206 CASignatureAlgorithms
208 The default is handled system-wide by crypto-policies(7). To see
209 the defaults and how to modify this default, see manual page
210 update-crypto-policies(8).
211 Specifies which algorithms are allowed for signing of certifi‐
213 cates by certificate authorities (CAs). Certificates signed
214 using other algorithms will not be accepted for public key or
215 host-based authentication.
216 ChallengeResponseAuthentication
218 Specifies whether challenge-response authentication is allowed
219 (e.g. via PAM or through authentication styles supported in
220 login.conf(5)) The default is yes.
221 ChrootDirectory
223 Specifies the pathname of a directory to chroot(2) to after
224 authentication. At session startup sshd(8) checks that all com‐
225 ponents of the pathname are root-owned directories which are not
226 writable by any other user or group. After the chroot, sshd(8)
227 changes the working directory to the user's home directory.
228 Arguments to ChrootDirectory accept the tokens described in the
229 TOKENS section.
230 The ChrootDirectory must contain the necessary files and directo‐
232 ries to support the user's session. For an interactive session
233 this requires at least a shell, typically sh(1), and basic /dev
234 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
235 and tty(4) devices. For file transfer sessions using SFTP no
236 additional configuration of the environment is necessary if the
237 in-process sftp-server is used, though sessions which use logging
238 may require /dev/log inside the chroot directory on some operat‐
239 ing systems (see sftp-server(8) for details).
240 For safety, it is very important that the directory hierarchy be
242 prevented from modification by other processes on the system
243 (especially those outside the jail). Misconfiguration can lead
244 to unsafe environments which sshd(8) cannot detect.
245 The default is none, indicating not to chroot(2).
247 Ciphers
249 The default is handled system-wide by crypto-policies(7). To see
250 the defaults and how to modify this default, see manual page
251 update-crypto-policies(8).
252 Specifies the ciphers allowed. Multiple ciphers must be comma-
254 separated. If the specified list begins with a ‘+’ character,
255 then the specified ciphers will be appended to the built-in
256 default set instead of replacing them. If the specified list
257 begins with a ‘-’ character, then the specified ciphers (includ‐
258 ing wildcards) will be removed from the built-in default set
259 instead of replacing them. If the specified list begins with a
260 ‘^’ character, then the specified ciphers will be placed at the
261 head of the built-in default set.
262 The supported ciphers are:
264 3des-cbc
266 aes128-cbc
267 aes192-cbc
268 aes256-cbc
269 aes128-ctr
270 aes192-ctr
271 aes256-ctr
272 aes128-gcm@openssh.com
273 aes256-gcm@openssh.com
274 chacha20-poly1305@openssh.com
275 The list of available ciphers may also be obtained using "ssh -Q
277 cipher".
278 ClientAliveCountMax
280 Sets the number of client alive messages which may be sent with‐
281 out sshd(8) receiving any messages back from the client. If this
282 threshold is reached while client alive messages are being sent,
283 sshd will disconnect the client, terminating the session. It is
284 important to note that the use of client alive messages is very
285 different from TCPKeepAlive. The client alive messages are sent
286 through the encrypted channel and therefore will not be spoofa‐
287 ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa‐
288 ble. The client alive mechanism is valuable when the client or
289 server depend on knowing when a connection has become unrespon‐
290 sive.
291 The default value is 3. If ClientAliveInterval is set to 15, and
293 ClientAliveCountMax is left at the default, unresponsive SSH
294 clients will be disconnected after approximately 45 seconds.
295 Setting a zero ClientAliveCountMax disables connection termina‐
296 tion.
297 ClientAliveInterval
299 Sets a timeout interval in seconds after which if no data has
300 been received from the client, sshd(8) will send a message
301 through the encrypted channel to request a response from the
302 client. The default is 0, indicating that these messages will
303 not be sent to the client.
304 Compression
306 Specifies whether compression is enabled after the user has
307 authenticated successfully. The argument must be yes, delayed (a
308 legacy synonym for yes) or no. The default is yes.
309 DenyGroups
311 This keyword can be followed by a list of group name patterns,
312 separated by spaces. Login is disallowed for users whose primary
313 group or supplementary group list matches one of the patterns.
314 Only group names are valid; a numerical group ID is not recog‐
315 nized. By default, login is allowed for all groups. The
316 allow/deny groups directives are processed in the following
317 order: DenyGroups, AllowGroups.
318 See PATTERNS in ssh_config(5) for more information on patterns.
320 DenyUsers
322 This keyword can be followed by a list of user name patterns,
323 separated by spaces. Login is disallowed for user names that
324 match one of the patterns. Only user names are valid; a numeri‐
325 cal user ID is not recognized. By default, login is allowed for
326 all users. If the pattern takes the form USER@HOST then USER and
327 HOST are separately checked, restricting logins to particular
328 users from particular hosts. HOST criteria may additionally con‐
329 tain addresses to match in CIDR address/masklen format. The
330 allow/deny users directives are processed in the following order:
331 DenyUsers, AllowUsers.
332 See PATTERNS in ssh_config(5) for more information on patterns.
334 DisableForwarding
336 Disables all forwarding features, including X11, ssh-agent(1),
337 TCP and StreamLocal. This option overrides all other forwarding-
338 related options and may simplify restricted configurations.
339 ExposeAuthInfo
341 Writes a temporary file containing a list of authentication meth‐
342 ods and public credentials (e.g. keys) used to authenticate the
343 user. The location of the file is exposed to the user session
344 through the SSH_USER_AUTH environment variable. The default is
345 no.
346 FingerprintHash
348 Specifies the hash algorithm used when logging key fingerprints.
349 Valid options are: md5 and sha256. The default is sha256.
350 ForceCommand
352 Forces the execution of the command specified by ForceCommand,
353 ignoring any command supplied by the client and ~/.ssh/rc if
354 present. The command is invoked by using the user's login shell
355 with the -c option. This applies to shell, command, or subsystem
356 execution. It is most useful inside a Match block. The command
357 originally supplied by the client is available in the
358 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
359 of internal-sftp will force the use of an in-process SFTP server
360 that requires no support files when used with ChrootDirectory.
361 The default is none.
362 GatewayPorts
364 Specifies whether remote hosts are allowed to connect to ports
365 forwarded for the client. By default, sshd(8) binds remote port
366 forwardings to the loopback address. This prevents other remote
367 hosts from connecting to forwarded ports. GatewayPorts can be
368 used to specify that sshd should allow remote port forwardings to
369 bind to non-loopback addresses, thus allowing other hosts to con‐
370 nect. The argument may be no to force remote port forwardings to
371 be available to the local host only, yes to force remote port
372 forwardings to bind to the wildcard address, or clientspecified
373 to allow the client to select the address to which the forwarding
374 is bound. The default is no.
375 GSIAllowLimitedProxy
377 Specifies whether to accept limited proxy credentials for authen‐
378 tication. The default is no.
379 GSSAPIAuthentication
381 Specifies whether user authentication based on GSSAPI is allowed.
382 The default is yes.
383 GSSAPICleanupCredentials
385 Specifies whether to automatically destroy the user's credentials
386 cache on logout. The default is yes.
387 GSSAPICredentialsPath
389 If specified, the delegated GSSAPI credential is stored in the
390 given path, overwriting any existing credentials. Paths can be
391 specified with syntax similar to the AuthorizedKeysFile option
392 (i.e., accepting %h and %u tokens). When using this option, set‐
393 ting 'GssapiCleanupCredentials no' is recommended, so logging out
394 of one session doesn't remove the credentials in use by another
395 session of the same user. Currently only implemented for the GSI
396 mechanism.
397 GSSAPIDelegateCredentials
399 Specifies whether delegated credentials are stored in the user's
400 environment. The default is yes.
401 GSSAPIEnablek5users
403 Specifies whether to look at .k5users file for GSSAPI authentica‐
404 tion access control. Further details are described in ksu(1).
405 The default is no.
406 GSSAPIKeyExchange
408 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
409 key exchange doesn't rely on ssh keys to verify host identity.
410 The default is yes.
411 GSSAPIStrictAcceptorCheck
413 Determines whether to be strict about the identity of the GSSAPI
414 acceptor a client authenticates against. If set to yes then the
415 client must authenticate against the host service on the current
416 hostname. If set to no then the client may authenticate against
417 any service key stored in the machine's default store. This
418 facility is provided to assist with operation on multi homed
419 machines. The default is yes.
420 GSSAPIStoreCredentialsOnRekey
422 Controls whether the user's GSSAPI credentials should be updated
423 following a successful connection rekeying. This option can be
424 used to accepted renewed or updated credentials from a compatible
425 client. The default is “no”.
426 For this to work GSSAPIKeyExchange needs to be enabled in the
428 server and also used by the client.
429 GSSAPIKexAlgorithms
431 The default is handled system-wide by crypto-policies(7). To see
432 the defaults and how to modify this default, see manual page
433 update-crypto-policies(8).
434 The list of key exchange algorithms that are accepted by GSSAPI
436 key exchange. Possible values are
437 gss-gex-sha1-
439 gss-group1-sha1-
440 gss-group14-sha1-
441 gss-group14-sha256-
442 gss-group16-sha512-
443 gss-nistp256-sha256-
444 gss-curve25519-sha256-
445 This option only applies to connections using GSSAPI.
446 HostbasedAcceptedKeyTypes
448 Specifies the key types that will be accepted for hostbased
449 authentication as a list of comma-separated patterns. Alter‐
450 nately if the specified list begins with a ‘+’ character, then
451 the specified key types will be appended to the default set
452 instead of replacing them. If the specified list begins with a
453 ‘-’ character, then the specified key types (including wildcards)
454 will be removed from the default set instead of replacing them.
455 If the specified list begins with a ‘^’ character, then the spec‐
456 ified key types will be placed at the head of the default set.
457 The default for this option is:
458 ecdsa-sha2-nistp256-cert-v01@openssh.com,
460 ecdsa-sha2-nistp384-cert-v01@openssh.com,
461 ecdsa-sha2-nistp521-cert-v01@openssh.com,
462 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
463 ssh-ed25519-cert-v01@openssh.com,
464 sk-ssh-ed25519-cert-v01@openssh.com,
465 rsa-sha2-512-cert-v01@openssh.com,
466 rsa-sha2-256-cert-v01@openssh.com,
467 ssh-rsa-cert-v01@openssh.com,
468 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
469 sk-ecdsa-sha2-nistp256@openssh.com,
470 ssh-ed25519,sk-ssh-ed25519@openssh.com,
471 rsa-sha2-512,rsa-sha2-256,ssh-rsa
472 The list of available key types may also be obtained using "ssh
474 -Q HostbasedAcceptedKeyTypes".
475 HostbasedAuthentication
477 Specifies whether rhosts or /etc/hosts.equiv authentication
478 together with successful public key client host authentication is
479 allowed (host-based authentication). The default is no.
480 HostbasedUsesNameFromPacketOnly
482 Specifies whether or not the server will attempt to perform a
483 reverse name lookup when matching the name in the ~/.shosts,
484 ~/.rhosts, and /etc/hosts.equiv files during
485 HostbasedAuthentication. A setting of yes means that sshd(8)
486 uses the name supplied by the client rather than attempting to
487 resolve the name from the TCP connection itself. The default is
488 no.
489 HostCertificate
491 Specifies a file containing a public host certificate. The cer‐
492 tificate's public key must match a private host key already spec‐
493 ified by HostKey. The default behaviour of sshd(8) is not to
494 load any certificates.
495 HostKey
497 Specifies a file containing a private host key used by SSH. The
498 defaults are /etc/gsissh/ssh_host_ecdsa_key,
499 /etc/gsissh/ssh_host_ed25519_key and
500 /etc/gsissh/ssh_host_rsa_key.
501 Note that sshd(8) will refuse to use a file if it is group/world-
503 accessible and that the HostKeyAlgorithms option restricts which
504 of the keys are actually used by sshd(8).
505 It is possible to have multiple host key files. It is also pos‐
507 sible to specify public host key files instead. In this case
508 operations on the private key will be delegated to an
509 ssh-agent(1).
510 HostKeyAgent
512 Identifies the UNIX-domain socket used to communicate with an
513 agent that has access to the private host keys. If the string
514 "SSH_AUTH_SOCK" is specified, the location of the socket will be
515 read from the SSH_AUTH_SOCK environment variable.
516 HostKeyAlgorithms
518 The default is handled system-wide by crypto-policies(7). To see
519 the defaults and how to modify this default, see manual page
520 update-crypto-policies(8).
521 Specifies the host key algorithms that the server offers. The
523 list of available key types may also be obtained using "ssh -Q
524 HostKeyAlgorithms".
525 IgnoreRhosts
527 Specifies that .rhosts and .shosts files will not be used in
528 HostbasedAuthentication.
529 /etc/hosts.equiv and /etc/gsissh/shosts.equiv are still used.
531 The default is yes.
532 IgnoreUserKnownHosts
534 Specifies whether sshd(8) should ignore the user's
535 ~/.ssh/known_hosts during HostbasedAuthentication and use only
536 the system-wide known hosts file /etc/ssh/known_hosts. The
537 default is “no”.
538 Include
540 Include the specified configuration file(s). Multiple pathnames
541 may be specified and each pathname may contain glob(7) wildcards.
542 Files without absolute paths are assumed to be in /etc/ssh. An
543 Include directive may appear inside a Match block to perform con‐
544 ditional inclusion.
545 IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec‐
547 tion. Accepted values are af11, af12, af13, af21, af22, af23,
548 af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
549 cs6, cs7, ef, le, lowdelay, throughput, reliability, a numeric
550 value, or none to use the operating system default. This option
551 may take one or two arguments, separated by whitespace. If one
552 argument is specified, it is used as the packet class uncondi‐
553 tionally. If two values are specified, the first is automati‐
554 cally selected for interactive sessions and the second for non-
555 interactive sessions. The default is af21 (Low-Latency Data) for
556 interactive sessions and cs1 (Lower Effort) for non-interactive
557 sessions.
558 KbdInteractiveAuthentication
560 Specifies whether to allow keyboard-interactive authentication.
561 The argument to this keyword must be yes or no. The default is
562 to use whatever value ChallengeResponseAuthentication is set to
563 (by default yes).
564 KerberosAuthentication
566 Specifies whether the password provided by the user for
567 PasswordAuthentication will be validated through the Kerberos
568 KDC. To use this option, the server needs a Kerberos servtab
569 which allows the verification of the KDC's identity. The default
570 is no.
571 KerberosGetAFSToken
573 If AFS is active and the user has a Kerberos 5 TGT, attempt to
574 acquire an AFS token before accessing the user's home directory.
575 The default is no.
576 KerberosOrLocalPasswd
578 If password authentication through Kerberos fails then the pass‐
579 word will be validated via any additional local mechanism such as
580 /etc/passwd. The default is yes.
581 KerberosTicketCleanup
583 Specifies whether to automatically destroy the user's ticket
584 cache file on logout. The default is yes.
585 KerberosUniqueCCache
587 Specifies whether to store the acquired tickets in the per-ses‐
588 sion credential cache under /tmp/ or whether to use per-user cre‐
589 dential cache as configured in /etc/krb5.conf. The default value
590 no can lead to overwriting previous tickets by subseqent connec‐
591 tions to the same user account.
592 KerberosUseKuserok
594 Specifies whether to look at .k5login file for user's aliases.
595 The default is yes.
596 KexAlgorithms
598 The default is handled system-wide by crypto-policies(7). To see
599 the defaults and how to modify this default, see manual page
600 update-crypto-policies(8).
601 Specifies the available KEX (Key Exchange) algorithms. Multiple
603 algorithms must be comma-separated. Alternately if the specified
604 list begins with a ‘+’ character, then the specified methods will
605 be appended to the built-in default set instead of replacing
606 them. If the specified list begins with a ‘-’ character, then
607 the specified methods (including wildcards) will be removed from
608 the built-in default set instead of replacing them. If the spec‐
609 ified list begins with a ‘^’ character, then the specified meth‐
610 ods will be placed at the head of the built-in default set. The
611 supported algorithms are:
612 curve25519-sha256
614 curve25519-sha256@libssh.org
615 diffie-hellman-group1-sha1
616 diffie-hellman-group14-sha1
617 diffie-hellman-group14-sha256
618 diffie-hellman-group16-sha512
619 diffie-hellman-group18-sha512
620 diffie-hellman-group-exchange-sha1
621 diffie-hellman-group-exchange-sha256
622 ecdh-sha2-nistp256
623 ecdh-sha2-nistp384
624 ecdh-sha2-nistp521
625 sntrup4591761x25519-sha512@tinyssh.org
626 The list of available key exchange algorithms may also be
628 obtained using "ssh -Q KexAlgorithms".
629 ListenAddress
631 Specifies the local addresses sshd(8) should listen on. The fol‐
632 lowing forms may be used:
633 ListenAddress hostname|address [rdomain domain]
635 ListenAddress hostname:port [rdomain domain]
636 ListenAddress IPv4_address:port [rdomain domain]
637 ListenAddress [hostname|address]:port [rdomain domain]
638 The optional rdomain qualifier requests sshd(8) listen in an
640 explicit routing domain. If port is not specified, sshd will
641 listen on the address and all Port options specified. The
642 default is to listen on all local addresses on the current
643 default routing domain. Multiple ListenAddress options are per‐
644 mitted. For more information on routing domains, see rdomain(4).
645 LoginGraceTime
647 The server disconnects after this time if the user has not suc‐
648 cessfully logged in. If the value is 0, there is no time limit.
649 The default is 120 seconds.
650 LogLevel
652 Gives the verbosity level that is used when logging messages from
653 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
654 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
655 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
656 higher levels of debugging output. Logging with a DEBUG level
657 violates the privacy of users and is not recommended.
658 MACs The default is handled system-wide by crypto-policies(7). To see
660 the defaults and how to modify this default, see manual page
661 update-crypto-policies(8).
662 Specifies the available MAC (message authentication code) algo‐
664 rithms. The MAC algorithm is used for data integrity protection.
665 Multiple algorithms must be comma-separated. If the specified
666 list begins with a ‘+’ character, then the specified algorithms
667 will be appended to the built-in default set instead of replacing
668 them. If the specified list begins with a ‘-’ character, then
669 the specified algorithms (including wildcards) will be removed
670 from the built-in default set instead of replacing them. If the
671 specified list begins with a ‘^’ character, then the specified
672 algorithms will be placed at the head of the built-in default
673 set.
674 The algorithms that contain "-etm" calculate the MAC after
676 encryption (encrypt-then-mac). These are considered safer and
677 their use recommended. The supported MACs are:
678 hmac-md5
680 hmac-md5-96
681 hmac-sha1
682 hmac-sha1-96
683 hmac-sha2-256
684 hmac-sha2-512
685 umac-64@openssh.com
686 umac-128@openssh.com
687 hmac-md5-etm@openssh.com
688 hmac-md5-96-etm@openssh.com
689 hmac-sha1-etm@openssh.com
690 hmac-sha1-96-etm@openssh.com
691 hmac-sha2-256-etm@openssh.com
692 hmac-sha2-512-etm@openssh.com
693 umac-64-etm@openssh.com
694 umac-128-etm@openssh.com
695 The list of available MAC algorithms may also be obtained using
697 "ssh -Q mac".
698 Match Introduces a conditional block. If all of the criteria on the
700 Match line are satisfied, the keywords on the following lines
701 override those set in the global section of the config file,
702 until either another Match line or the end of the file. If a
703 keyword appears in multiple Match blocks that are satisfied, only
704 the first instance of the keyword is applied.
705 The arguments to Match are one or more criteria-pattern pairs or
707 the single token All which matches all criteria. The available
708 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
709 and Address (with RDomain representing the rdomain(4) on which
710 the connection was received).
711 The match patterns may consist of single entries or comma-sepa‐
713 rated lists and may use the wildcard and negation operators
714 described in the PATTERNS section of ssh_config(5).
715 The patterns in an Address criteria may additionally contain
717 addresses to match in CIDR address/masklen format, such as
718 192.0.2.0/24 or 2001:db8::/32. Note that the mask length pro‐
719 vided must be consistent with the address - it is an error to
720 specify a mask length that is too long for the address or one
721 with bits set in this host portion of the address. For example,
722 192.0.2.0/33 and 192.0.2.0/8, respectively.
723 Only a subset of keywords may be used on the lines following a
725 Match keyword. Available keywords are AcceptEnv,
726 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
727 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
728 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
729 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
730 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
731 Banner, ChrootDirectory, ClientAliveCountMax,
732 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
733 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
734 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
735 Include, IPQoS, KbdInteractiveAuthentication,
736 KerberosAuthentication, KerberosUseKuserok, LogLevel,
737 MaxAuthTries, MaxSessions, PasswordAuthentication,
738 PermitEmptyPasswords, PermitListen, PermitOpen, PermitRootLogin,
739 PermitTTY, PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
740 PubkeyAuthentication, RekeyLimit, RevokedKeys, RDomain, SetEnv,
741 StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys,
742 X11DisplayOffset, X11MaxDisplays, X11Forwarding and
743 X11UseLocalhost.
744 MaxAuthTries
746 Specifies the maximum number of authentication attempts permitted
747 per connection. Once the number of failures reaches half this
748 value, additional failures are logged. The default is 6.
749 MaxSessions
751 Specifies the maximum number of open shell, login or subsystem
752 (e.g. sftp) sessions permitted per network connection. Multiple
753 sessions may be established by clients that support connection
754 multiplexing. Setting MaxSessions to 1 will effectively disable
755 session multiplexing, whereas setting it to 0 will prevent all
756 shell, login and subsystem sessions while still permitting for‐
757 warding. The default is 10.
758 MaxStartups
760 Specifies the maximum number of concurrent unauthenticated con‐
761 nections to the SSH daemon. Additional connections will be
762 dropped until authentication succeeds or the LoginGraceTime
763 expires for a connection. The default is 10:30:100.
764 Alternatively, random early drop can be enabled by specifying the
766 three colon separated values start:rate:full (e.g. "10:30:60").
767 sshd(8) will refuse connection attempts with a probability of
768 rate/100 (30%) if there are currently start (10) unauthenticated
769 connections. The probability increases linearly and all connec‐
770 tion attempts are refused if the number of unauthenticated con‐
771 nections reaches full (60).
772 PasswordAuthentication
774 Specifies whether password authentication is allowed. The
775 default is yes.
776 PermitEmptyPasswords
778 When password authentication is allowed, it specifies whether the
779 server allows login to accounts with empty password strings. The
780 default is no.
781 PermitListen
783 Specifies the addresses/ports on which a remote TCP port forward‐
784 ing may listen. The listen specification must be one of the fol‐
785 lowing forms:
786 PermitListen port
788 PermitListen host:port
789 Multiple permissions may be specified by separating them with
791 whitespace. An argument of any can be used to remove all
792 restrictions and permit any listen requests. An argument of none
793 can be used to prohibit all listen requests. The host name may
794 contain wildcards as described in the PATTERNS section in
795 ssh_config(5). The wildcard ‘*’ can also be used in place of a
796 port number to allow all ports. By default all port forwarding
797 listen requests are permitted. Note that the GatewayPorts option
798 may further restrict which addresses may be listened on. Note
799 also that ssh(1) will request a listen host of “localhost” if no
800 listen host was specifically requested, and this name is treated
801 differently to explicit localhost addresses of “127.0.0.1” and
802 “::1”.
803 PermitOpen
805 Specifies the destinations to which TCP port forwarding is per‐
806 mitted. The forwarding specification must be one of the follow‐
807 ing forms:
808 PermitOpen host:port
810 PermitOpen IPv4_addr:port
811 PermitOpen [IPv6_addr]:port
812 Multiple forwards may be specified by separating them with white‐
814 space. An argument of any can be used to remove all restrictions
815 and permit any forwarding requests. An argument of none can be
816 used to prohibit all forwarding requests. The wildcard ‘*’ can
817 be used for host or port to allow all hosts or ports respec‐
818 tively. Otherwise, no pattern matching or address lookups are
819 performed on supplied names. By default all port forwarding
820 requests are permitted.
821 PermitRootLogin
823 Specifies whether root can log in using ssh(1). The argument
824 must be yes, prohibit-password, forced-commands-only, or no. The
825 default is prohibit-password.
826 If this option is set to prohibit-password (or its deprecated
828 alias, without-password), password and keyboard-interactive
829 authentication are disabled for root.
830 If this option is set to forced-commands-only, root login with
832 public key authentication will be allowed, but only if the
833 command option has been specified (which may be useful for taking
834 remote backups even if root login is normally not allowed). All
835 other authentication methods are disabled for root.
836 If this option is set to no, root is not allowed to log in.
838 PermitTTY
840 Specifies whether pty(4) allocation is permitted. The default is
841 yes.
842 PermitTunnel
844 Specifies whether tun(4) device forwarding is allowed. The argu‐
845 ment must be yes, point-to-point (layer 3), ethernet (layer 2),
846 or no. Specifying yes permits both point-to-point and ethernet.
847 The default is no.
848 Independent of this setting, the permissions of the selected
850 tun(4) device must allow access to the user.
851 PermitUserEnvironment
853 Specifies whether ~/.ssh/environment and environment= options in
854 ~/.ssh/authorized_keys are processed by sshd(8). Valid options
855 are yes, no or a pattern-list specifying which environment vari‐
856 able names to accept (for example "LANG,LC_*"). The default is
857 no. Enabling environment processing may enable users to bypass
858 access restrictions in some configurations using mechanisms such
859 as LD_PRELOAD.
860 PermitUserRC
862 Specifies whether any ~/.ssh/rc file is executed. The default is
863 yes.
864 PidFile
866 Specifies the file that contains the process ID of the SSH dae‐
867 mon, or none to not write one. The default is
868 /var/run/gsisshd.pid.
869 Port Specifies the port number that sshd(8) listens on. The default
871 is 22. Multiple options of this type are permitted. See also
872 ListenAddress.
873 PrintLastLog
875 Specifies whether sshd(8) should print the date and time of the
876 last user login when a user logs in interactively. The default
877 is yes.
878 PrintMotd
880 Specifies whether sshd(8) should print /etc/motd when a user logs
881 in interactively. (On some systems it is also printed by the
882 shell, /etc/profile, or equivalent.) The default is yes.
883 PubkeyAcceptedKeyTypes
885 The default is handled system-wide by crypto-policies(7). To see
886 the defaults and how to modify this default, see manual page
887 update-crypto-policies(8).
888 Specifies the key types that will be accepted for public key
890 authentication as a list of comma-separated patterns. Alter‐
891 nately if the specified list begins with a ‘+’ character, then
892 the specified key types will be appended to the built-in default
893 set instead of replacing them. If the specified list begins with
894 a ‘-’ character, then the specified key types (including wild‐
895 cards) will be removed from the built-in default set instead of
896 replacing them. If the specified list begins with a ‘^’ charac‐
897 ter, then the specified key types will be placed at the head of
898 the built-in default set.
899 The list of available key types may also be obtained using "ssh
901 -Q PubkeyAcceptedKeyTypes".
902 PubkeyAuthOptions
904 Sets one or more public key authentication options. Two option
905 keywords are currently supported: none (the default; indicating
906 no additional options are enabled) and touch-required.
907 The touch-required option causes public key authentication using
909 a FIDO authenticator algorithm (i.e. ecdsa-sk or ed25519-sk) to
910 always require the signature to attest that a physically present
911 user explicitly confirmed the authentication (usually by touching
912 the authenticator). By default, sshd(8) requires user presence
913 unless overridden with an authorized_keys option. The
914 touch-required flag disables this override. This option has no
915 effect for other, non-authenticator public key types.
916 PubkeyAuthentication
918 Specifies whether public key authentication is allowed. The
919 default is yes.
920 RekeyLimit
922 Specifies the maximum amount of data that may be transmitted
923 before the session key is renegotiated, optionally followed a
924 maximum amount of time that may pass before the session key is
925 renegotiated. The first argument is specified in bytes and may
926 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
927 Megabytes, or Gigabytes, respectively. The default is between
928 ‘1G’ and ‘4G’, depending on the cipher. The optional second
929 value is specified in seconds and may use any of the units docu‐
930 mented in the TIME FORMATS section. The default value for
931 RekeyLimit is default none, which means that rekeying is per‐
932 formed after the cipher's default amount of data has been sent or
933 received and no time based rekeying is done.
934 RevokedKeys
936 Specifies revoked public keys file, or none to not use one. Keys
937 listed in this file will be refused for public key authentica‐
938 tion. Note that if this file is not readable, then public key
939 authentication will be refused for all users. Keys may be speci‐
940 fied as a text file, listing one public key per line, or as an
941 OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
942 For more information on KRLs, see the KEY REVOCATION LISTS sec‐
943 tion in ssh-keygen(1).
944 RDomain
946 Specifies an explicit routing domain that is applied after
947 authentication has completed. The user session, as well and any
948 forwarded or listening IP sockets, will be bound to this
949 rdomain(4). If the routing domain is set to %D, then the domain
950 in which the incoming connection was received will be applied.
951 SecurityKeyProvider
953 Specifies a path to a library that will be used when loading FIDO
954 authenticator-hosted keys, overriding the default of using the
955 built-in USB HID support.
956 SetEnv Specifies one or more environment variables to set in child ses‐
958 sions started by sshd(8) as “NAME=VALUE”. The environment value
959 may be quoted (e.g. if it contains whitespace characters). Envi‐
960 ronment variables set by SetEnv override the default environment
961 and any variables specified by the user via AcceptEnv or
962 PermitUserEnvironment.
963 StreamLocalBindMask
965 Sets the octal file creation mode mask (umask) used when creating
966 a Unix-domain socket file for local or remote port forwarding.
967 This option is only used for port forwarding to a Unix-domain
968 socket file.
969 The default value is 0177, which creates a Unix-domain socket
971 file that is readable and writable only by the owner. Note that
972 not all operating systems honor the file mode on Unix-domain
973 socket files.
974 StreamLocalBindUnlink
976 Specifies whether to remove an existing Unix-domain socket file
977 for local or remote port forwarding before creating a new one.
978 If the socket file already exists and StreamLocalBindUnlink is
979 not enabled, sshd will be unable to forward the port to the Unix-
980 domain socket file. This option is only used for port forwarding
981 to a Unix-domain socket file.
982 The argument must be yes or no. The default is no.
984 StrictModes
986 Specifies whether sshd(8) should check file modes and ownership
987 of the user's files and home directory before accepting login.
988 This is normally desirable because novices sometimes accidentally
989 leave their directory or files world-writable. The default is
990 yes. Note that this does not apply to ChrootDirectory, whose
991 permissions and ownership are checked unconditionally.
992 Subsystem
994 Configures an external subsystem (e.g. file transfer daemon).
995 Arguments should be a subsystem name and a command (with optional
996 arguments) to execute upon subsystem request.
997 The command sftp-server implements the SFTP file transfer subsys‐
999 tem.
1000 Alternately the name internal-sftp implements an in-process SFTP
1002 server. This may simplify configurations using ChrootDirectory
1003 to force a different filesystem root on clients.
1004 By default no subsystems are defined.
1006 SyslogFacility
1008 Gives the facility code that is used when logging messages from
1009 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
1010 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1011 The default is AUTH.
1012 TCPKeepAlive
1014 Specifies whether the system should send TCP keepalive messages
1015 to the other side. If they are sent, death of the connection or
1016 crash of one of the machines will be properly noticed. However,
1017 this means that connections will die if the route is down tempo‐
1018 rarily, and some people find it annoying. On the other hand, if
1019 TCP keepalives are not sent, sessions may hang indefinitely on
1020 the server, leaving "ghost" users and consuming server resources.
1021 The default is yes (to send TCP keepalive messages), and the
1023 server will notice if the network goes down or the client host
1024 crashes. This avoids infinitely hanging sessions.
1025 To disable TCP keepalive messages, the value should be set to no.
1027 TrustedUserCAKeys
1029 Specifies a file containing public keys of certificate authori‐
1030 ties that are trusted to sign user certificates for authentica‐
1031 tion, or none to not use one. Keys are listed one per line;
1032 empty lines and comments starting with ‘#’ are allowed. If a
1033 certificate is presented for authentication and has its signing
1034 CA key listed in this file, then it may be used for authentica‐
1035 tion for any user listed in the certificate's principals list.
1036 Note that certificates that lack a list of principals will not be
1037 permitted for authentication using TrustedUserCAKeys. For more
1038 details on certificates, see the CERTIFICATES section in
1039 ssh-keygen(1).
1040 UseDNS Specifies whether sshd(8) should look up the remote host name,
1042 and to check that the resolved host name for the remote IP
1043 address maps back to the very same IP address.
1044 If this option is set to no (the default) then only addresses and
1046 not host names may be used in ~/.ssh/authorized_keys from and
1047 sshd_config Match Host directives.
1048 UsePAM Enables the Pluggable Authentication Module interface. If set to
1050 yes this will enable PAM authentication using
1051 ChallengeResponseAuthentication and PasswordAuthentication in
1052 addition to PAM account and session module processing for all
1053 authentication types.
1054 Because PAM challenge-response authentication usually serves an
1056 equivalent role to password authentication, you should disable
1057 either PasswordAuthentication or ChallengeResponseAuthentication.
1058 If UsePAM is enabled, you will not be able to run sshd(8) as a
1060 non-root user. The default is no.
1061 PermitPAMUserChange
1063 If set to yes this will enable PAM authentication to change the
1064 name of the user being authenticated. The default is no.
1065 VersionAddendum
1067 Optionally specifies additional text to append to the SSH proto‐
1068 col banner sent by the server upon connection. The default is
1069 none.
1070 X11DisplayOffset
1072 Specifies the first display number available for sshd(8)'s X11
1073 forwarding. This prevents sshd from interfering with real X11
1074 servers. The default is 10.
1075 X11MaxDisplays
1077 Specifies the maximum number of displays available for sshd(8)'s
1078 X11 forwarding. This prevents sshd from exhausting local ports.
1079 The default is 1000.
1080 X11Forwarding
1082 Specifies whether X11 forwarding is permitted. The argument must
1083 be yes or no. The default is no.
1084 When X11 forwarding is enabled, there may be additional exposure
1086 to the server and to client displays if the sshd(8) proxy display
1087 is configured to listen on the wildcard address (see
1088 X11UseLocalhost), though this is not the default. Additionally,
1089 the authentication spoofing and authentication data verification
1090 and substitution occur on the client side. The security risk of
1091 using X11 forwarding is that the client's X11 display server may
1092 be exposed to attack when the SSH client requests forwarding (see
1093 the warnings for ForwardX11 in ssh_config(5)). A system adminis‐
1094 trator may have a stance in which they want to protect clients
1095 that may expose themselves to attack by unwittingly requesting
1096 X11 forwarding, which can warrant a no setting.
1097 Note that disabling X11 forwarding does not prevent users from
1099 forwarding X11 traffic, as users can always install their own
1100 forwarders.
1101 X11UseLocalhost
1103 Specifies whether sshd(8) should bind the X11 forwarding server
1104 to the loopback address or to the wildcard address. By default,
1105 sshd binds the forwarding server to the loopback address and sets
1106 the hostname part of the DISPLAY environment variable to
1107 localhost. This prevents remote hosts from connecting to the
1108 proxy display. However, some older X11 clients may not function
1109 with this configuration. X11UseLocalhost may be set to no to
1110 specify that the forwarding server should be bound to the wild‐
1111 card address. The argument must be yes or no. The default is
1112 yes.
1113 XAuthLocation
1115 Specifies the full pathname of the xauth(1) program, or none to
1116 not use one. The default is /usr/bin/xauth.
1117
1119 sshd(8) command-line arguments and configuration file options that spec‐
1120 ify time may be expressed using a sequence of the form: time[qualifier],
1121 where time is a positive integer value and qualifier is one of the fol‐
1122 lowing:
1123 ⟨none⟩ seconds
1125 s | S seconds
1126 m | M minutes
1127 h | H hours
1128 d | D days
1129 w | W weeks
1130 Each member of the sequence is added together to calculate the total time
1132 value.
1133 Time format examples:
1135 600 600 seconds (10 minutes)
1137 10m 10 minutes
1138 1h30m 1 hour 30 minutes (90 minutes)
1139
1141 Arguments to some keywords can make use of tokens, which are expanded at
1142 runtime:
1143 %% A literal ‘%’.
1145 %D The routing domain in which the incoming connection was
1146 received.
1147 %F The fingerprint of the CA key.
1148 %f The fingerprint of the key or certificate.
1149 %h The home directory of the user.
1150 %i The key ID in the certificate.
1151 %K The base64-encoded CA key.
1152 %k The base64-encoded key or certificate for authentication.
1153 %s The serial number of the certificate.
1154 %T The type of the CA key.
1155 %t The key or certificate type.
1156 %U The numeric user ID of the target user.
1157 %u The username.
1158 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1160 AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
1162 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1164 %k, %s, %T, %t, %U, and %u.
1165 AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
1167 ChrootDirectory accepts the tokens %%, %h, %U, and %u.
1169 RoutingDomain accepts the token %D.
1171
1173 /etc/gsissh/sshd_config
1174 Contains configuration data for sshd(8). This file should be
1175 writable by root only, but it is recommended (though not neces‐
1176 sary) that it be world-readable.
1177
1179 sftp-server(8), sshd(8)
1180
1182 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1183 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1184 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1185 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1186 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1187 for privilege separation.
1188BSD May 10, 2020 BSD